POLG-related mitochondrial disorders …

Amy Goldstein MD

Neurogenetic Disorders

Updated 09.12.2024
Released 09.15.2014
Expires For CME 09.12.2027

POLG-related mitochondrial disorders

Author: Amy Goldstein MD

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Editor: Deepa S Rajan MD

POLG-related mitochondrial disorders

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Introduction

Overview

POLG-related disease is a variable condition that is best described as falling into a spectrum, with those most severely affected having early-onset progressive and severe neurologic concerns and liver failure and those less severely affected having eye muscle weakness later in life. The classic phenotypes caused by POLG pathogenic variants include Alpers-Huttenlocher syndrome; myocerebrohepatopathy; myoclonic epilepsy, myopathy, sensory ataxia; ataxia neuropathy spectrum disorder; autosomal recessive progressive external ophthalmoplegia; and autosomal dominant progressive external ophthalmoplegia. Unfortunately, only supportive treatments are available, although clinical trials are ongoing. There are no FDA-approved therapies for POLG-related disorders.

Key points

	• Polymerase gamma is the only human polymerase able to replicate mitochondrial DNA, and pathogenic variants in POLG are responsible for a host of symptoms that result from mitochondrial DNA depletion (ie, a reduction in mtDNA copy number) or multiple mtDNA deletions.
	• Alpers-Huttenlocher syndrome generally presents between 2 and 4 years of age with rapidly progressive and medically intractable epilepsy. A second peak of disease presentation occurs between 17 and 24 years of age with a female predominance.
	• Alpers-Huttenlocher syndrome usually causes progressive encephalopathy associated with refractory epilepsy, cortical visual loss, pyramidal signs, movement disorders, and neuropathy. Brain MRI changes include generalized atrophy, cerebellar atrophy, thalamic/pulvinar lesions, the “peri-Rolandic sign,” and stroke-like lesions usually in the posterior/occipital lobe.
	• Hepatic involvement is common in Alpers-Huttenlocher syndrome, but the onset may be delayed years to decades after the first clinical symptoms. Fulminant liver failure may occur due to exposure to valproic acid or other hepatically cleared medications, or due to other stressors such as fever/infection.
	• There are over 300 pathogenic variants in POLG that are expressed mainly in autosomal recessive but also in dominant inheritance patterns. Most of these are point mutations but deletions also exist. In those disorders caused by the recessively expressed alleles, the number of potential combinations is nearly limitless, which may explain the variability in the spectrum of clinical presentations. There may be some consistent genotype-phenotype correlations that can aid in prognosis, and online tools are available.
	• POLG-related disorders are now recognized as a continuum rather than six or more separate clinically defined syndromes, and the age of onset is now used as the most important prognostic factor according to recent literature.

Historical note and terminology

The original description of what is now known as Alpers-Huttenlocher syndrome was made by Alfons Maria Jakob (50). The following year Bernard Alpers, a student of Jakob’s, published a clinical-pathological report of a 4-month-old girl with typical development who developed intractable seizures in the context of a 1-month illness (01). Alpers’ description led to the recognition of the disease and fostered further reports, although initial descriptions of this disease likely occurred earlier (10). The eponym of Alpers disease was given in 1963, and it was later renamed Alpers poliodystrophy (37). Hints as to the pathophysiology of this disorder did not exist until 1972, when ultrastructural studies showed giant and disorganized mitochondria in neurons from patients with the disorder (89). In 1976 Peter Huttenlocher first reported the hepatic features of the disease and elevated CSF protein, and he suggested that it was a monoallelic and autosomal recessive disorder based on recurrence in family members (47). Several reports suggested this disorder was linked to abnormalities in metabolism, such as abnormal pyruvate metabolism, citric acid cycle dysfunction, electron transport chain dysfunction, or isolated cytochrome c oxidase activity (84; 85; 28; 112). However, these biochemical findings provided only secondary evidence of mitochondrial dysfunction and, in retrospect, did not identify the primary cause of the illness.

The first extensive review of the clinical features, electrophysiology, and pathology of this disorder described the course of 32 patients with distinctive liver and brain pathology. Other important features in the manuscript described typical early development followed by an insidious onset of developmental delay, failure to thrive, bouts of vomiting, and pronounced hypotonia (37). Typically, the clinical course became rapidly progressive soon after the onset of seizures. Liver involvement was variable; in some patients, it preceded seizure onset, and in others, it occurred at the terminal stages of the disease (22). The postmortem liver findings demonstrated a characteristic combination of pathogenic features, and examination of the cerebral cortex revealed variability but a constant involvement of the calcarine cortex with microscopic changes, including spongiosis, neuronal loss, and astrocytosis that involved all cortical layers (37; 75).

In 1996, POLG was characterized and cloned as the gene encoding for polymerase gamma, the only polymerase involved in mtDNA replication (87; 59). This discovery ushered in the molecular era of mitochondrial DNA depletion disorders. However, the clinical implications for POLG were not yet known. A few years later, biochemical studies provided evidence that mtDNA depletion was involved in Alpers-Huttenlocher syndrome (72). In 2001, the first firm evidence of a human illness, progressive external ophthalmoplegia (PEO) linked to autosomal recessive pathogenic variants in POLG, was published (105). In retrospect, a report two years earlier described the first nuclear gene disorder causing progressive external ophthalmoplegia with multiple mitochondrial DNA deletions—the disorder known as mitochondrial neurogastrointestinal encephalomyopathy. The importance of this discovery is that pathologic variants in the TYMP gene, which encodes for thymidine phosphorylase, cause alterations in nucleotide pools, resulting in mtDNA replication infidelity and subsequent mtDNA depletion (77). More than 70 years passed between Alpers’ first description of Alpers-Huttenlocher syndrome and Naviaux’s 2004 description of pathogenic variants in POLG in two unrelated families with Alpers-Huttenlocher syndrome (71). These data provided the full pathophysiology of the phenotype, including the identification of the genetic etiology and the physiologic changes associated with reduced mtDNA content, also known as mtDNA depletion. Within the next four years, a number of publications outlined the full spectrum and clinical descriptions of POLG disorders, including descriptions of both dominant and recessive pathogenic variants in 61 patients that can cause a wide spectrum of clinical symptoms (111). A review article was published by Saneto and colleagues in 2013 (90), followed by GeneReviews articles, which are updated every few years; the last update in February 2024 reflects the current understanding of overlapping phenotypes and considers age of onset a correlation with symptoms and overall prognosis (15).

POLG-related disorders are now recognized as a continuum of overlapping clinical phenotypes, all named before the molecular basis of these disorders was known. The onset of symptoms ranges from infancy to late adulthood. Establishing the diagnosis requires two (biallelic) pathogenic variants for autosomal recessive disease and one pathogenic variant for autosomal dominant disease. Also important to note that some pathogenic variants have been reclassified over time and that previously published pathogenic variants have been reclassified to benign and vice versa. In addition, in order to prove biallelic inheritance, parental testing needs to confirm parents are each carriers because some variants can coexist on one allele (in cis) and be inherited from the same parent and, thus, are not inherited in an autosomal recessive manner (in trans).

The expressivity of POLG disease varies by age of onset and disease severity. The clinical presentation is influenced by a combination of factors that include the specific gene mutation(s), region of the mutation within the gene, genetic background and epigenetic effects, environmental factors, and the age of onset. The neurologic features include encephalopathy and/or dementia, seizures, migrainous headache, visual loss, pyramidal and extrapyramidal motor dysfunction, movement disorders (ataxia, myoclonus, chorea, and dystonia), and neuropathy. Systemic features include gastrointestinal and bladder dysfunction, dysautonomia, hepatic dysfunction, gonadal failure, and other endocrinopathies (46). The POLG disorders can be classified into several recognizable and described phenotypes, yet symptoms overlap among individuals, even those with identical mutations (27; 45; 46; 103; 14; 111; 07; 97; 41). Some affected individuals will present with a classically recognized syndrome, whereas others can have some, but not all, of the signs and symptoms of the classic syndromes. Over the past several decades, there have been numerous publications describing the genetic variants, epilepsy, and other symptoms associated with POLG-related disorders.

Given the growing clinical phenotypes in the literature, all with significant overlap, a study of 155 patients affected by POLG pathogenic variants was published to demonstrate the spectrum of clinical features in the largest known cohort of patients (41). The authors recognized a continuum of symptoms and stratified patients into one of three groups based on age of onset of symptoms to identify clinical phenotype and prognosis. Those presenting before 12 years of age had liver involvement (87%), seizures (84%), and feeding difficulties (84%) as main symptoms. Those between 12 to 40 years old had ataxia (90%), peripheral neuropathy (84%), and seizures (71%) as main symptoms. Those presenting after age 40 years had ptosis (95%), progressive external ophthalmoplegia (89%), and ataxia (58%) as major symptoms. Poor prognosis was associated with younger age of onset, epilepsy, and autosomal recessive disease from compound heterozygote variants.

Another retrospective review of 40 pediatric patients in France discovered three patterns of clinical course and survival (88). There were no genotype-phenotype correlations. The first category of neurologic symptoms included 24 patients needing urgent neurointensive care for epilepsy management; other neurologic symptoms included dystonia, cerebellar ataxia, and peripheral neuropathy. The neuropathy mimics Guillain-Barre syndrome and chronic inflammatory demyelinating polyneuropathy given the presence of elevated CSF protein and diffuse enhancement of nerve roots on postgadolinium contrast MRI spine of both cranial nerve roots and cauda equina seen in six patients. The second category of symptoms is gastrointestinal or digestive disorders and includes vomiting, gastroparesis, and chronic intestinal pseudo-obstruction, present later in age and with longer survival. Patients in this group did not develop epilepsy. The third group, hepatopathy, had the earliest age of onset and shortest survival (death by 3 months to 10 years old), frequently precipitated by valproate administration for seizures prior to genetic diagnosis. Of their total cohort of 40 patients, only six survived.

Clinical manifestations

Presentation and course

Pathogenic variants in POLG are expressed in both dominant and recessive fashion (27; 45; 46; 103; 14; 111; 07; 97). Most of the dominant mutations cause symptoms with onset in the later adult years, and these variants reside in the catalytic residues of the polymerase domain of POLG (33). Symptoms caused by recessive mutations can present at any age and represent the most common disease inheritance pattern in POLG diseases (111; 16; 93; 15). Disease-causing mutations occur in all exons and can produce many different clinical presentations that do not necessarily cluster within distinct phenotypic syndromes (105; 63; 18; 35; 14; 44). There are over a dozen common mutations, and most patients with autosomal recessive disease have compound heterozygote mutations, and the remainder are homozygotes. Over 300 pathogenic variants have been reported in the Human DNA Polymerase Gamma Mutation Database; 14 cause autosomal dominant progressive external ophthalmoplegia, with the remainder listed as autosomal recessive.

Pathogenic mutations in POLG

(From: National Institute of Environmental Health Sciences.)

The infantile and early childhood presentations of POLG disorders include childhood myocerebrohepatopathy syndrome and Alpers-Huttenlocher syndrome, with childhood myocerebrohepatopathy being a distinct and much less common form of Alpers-Huttenlocher syndrome (71; 93). The onset of childhood myocerebrohepatopathy is usually by 6 months of life, which is much younger than what is seen in Alpers-Huttenlocher syndrome. Infants with childhood myocerebrohepatopathy syndrome usually present with rapidly progressive liver failure and lactic acidosis. An encephalopathy, with or without seizures, may occur later in the disease. The liver involvement in childhood myocerebrohepatopathy syndrome is often the most devastating clinical manifestation, but the hepatic pathology found in Alpers-Huttenlocher syndrome is usually lacking (75).

Though Alpers-Huttenlocher syndrome and childhood myocerebrohepatopathy syndrome are sometimes confused with each other because they both cause rapidly progressive disease and early death, Alpers-Huttenlocher syndrome generally presents between 2 and 4 years of age with progressively medically intractable seizures and associated devastating encephalopathy (01; 37; 72; 111). Infants and children with Alpers-Huttenlocher syndrome are generally healthy until disease onset, although some have identified nonspecific developmental delays. If affected individuals live long enough, they can develop a distinctive pathologically defined liver disease (37; 71). However, liver involvement may be early and variable in the course of the Alpers-Huttenlocher syndrome, thus, making the distinction between Alpers-Huttenlocher syndrome and childhood myocerebrohepatopathy difficult in some rare cases (37).

Most children with Alpers-Huttenlocher syndrome present between 2 and 4 years of age, with a range of 3 months to 8 years (72; 29; 18; Naviaux and Nguyen 2005; 75). The age of Alpers-Huttenlocher syndrome onset is bimodal, with a second peak onset between 17 and 24 years of age and a range of 10 and 27 years (38; 103; 108). In older patients, the illness may be less fulminant, with a long time between the first presentation and the ultimate diagnosis. The later age of onset and this second peak of Alpers-Huttenlocher syndrome onset have been confirmed by several reports (37; 38; 103; 104; 108). Many patients in the older-aged onset group have homozygous recessive mutations, most frequently the p.A467T or p.W748S transitions in the linker region. However, a compound heterozygous p.T851A and p.R1047W has been reported in the late-onset group, so the homozygote state alone cannot serve as the complete explanation for later disease onset (108). Therefore, the mutation location may, in part, be responsible for alterations in polymerase gamma activity and phenotype. However, there are clearly other modifying factors, as well.

Age of onset is influenced, in part, by the specific mutations within the POLG gene or, rarely, pathologic mutations in related genes (36). Changes within the POLG nucleotide sequence can induce changes in the enzyme activity of polymerase gamma, called ecogenetic structural nucleotide variant (ESNV) changes, which can influence age of onset, severity of disease, and the specific organ involvement (111; 93; 90). The environmental influence of concurrent viral illness can also induce onset of symptoms in POLG disorders as well as the stepwise progression of these disorders. The full set of mechanisms for this variability remains unknown, but the role of immunity and inflammation has been explored. A study in patients with POLG disorders showed immunodeficiency traits with delayed interferon response, an early replicative advantage to viruses, an augmented proinflammatory reaction (with elevations of IL-6), subacute loss of GABA interneurons, and liver damage (53).

Organ involvement. The primary organs involved in Alpers-Huttenlocher syndrome are the brain, liver, peripheral nervous system, and gastrointestinal tract. These organs are primarily post-mitotic at birth, require large expenditures of energy, and are prone to oxidative damage. Although the cells within these organs are not mitotic, the mitochondria within them are constantly replicating and require additional copies of mtDNA as they replicate. This requirement likely accounts for the predilection of mtDNA diseases to preferentially affect these organs.

Central nervous system (CNS) involvement. Seizures are the most significant CNS manifestation of Alpers-Huttenlocher syndrome. In about one-half of patients, seizures are the heralding symptom. Once seizures appear, the tempo of the disease tends to become rapidly progressive. Death usually occurs within four years of onset of symptoms (103; 104; 108). However, the disease progression is slower in other cases (103; 93). The reason for the variation in disease progression is not known, but in some cases, seems to be related to specific genotypes.

The EEG findings and seizure semiology can vary among patients and can change as the disease progresses in any one patient. Some patients have febrile convulsions, which may indicate comorbid seizure networks (92). Focal seizures may present as visual and tactile hallucinations, nausea and vomiting, dysautonomia, headache, nystagmus, and auras, but focal motor seizures and generalized seizures have also been reported (110; 92). In one large study of 229 patients with seizures, the seizure description was focal motor in 64% or myoclonus at onset in 58%, which progressed to generalized status epilepticus in 49% (02). Thirty-four percent of patients had episodes of focal motor status with or without alteration of awareness. The EEG during the initial period of seizure onset often includes occipital predominant epileptiform discharges with concomitant focal slowing.

Alpers-Huttenlocher syndrome (EEG)

EEG findings in an Alpers-Huttenlocher syndrome patient demonstrating occipital spike and polyspike morphology. (Contributed by Dr. Bruce H Cohen.)

Epileptiform wave morphologies are described as spike/polyspikes and may be unilateral or bilateral over the posterior head region (102; 23; 110; 92). As the disease progresses, most patients have repeated episodes of status epilepticus and epilepsia partialis continua (72; 29; 71; 35; 109; 45; 103; 92). Myoclonic jerks noted clinically do not always correlate with EEG changes. Over time, seizures become more resistant to medical therapy and individual seizures last longer. Myoclonus and myoclonic seizures become more prominent and almost continuous, and both are refractory to medical treatment. Death is not rare in the setting of persistent seizures and myoclonus. In any patient with an epileptic encephalopathy without adequate explanation, Alpers-Huttenlocher syndrome should be considered and valproate avoided until genetic testing of POLG is normal (06; 71; 18; 74; 92). Seizures and the lack of effective medical control are major causes of morbidity and mortality. The predominance of neuronal loss in the calcarine and striate cortex causes visual loss and then blindness. Visual loss may be transient early in the course of Alpers-Huttenlocher syndrome but usually becomes permanent at some point during the disease progression. Microscopic evaluation demonstrates spongiosis, neuronal loss, and astrocytosis that progresses into the depth of the cortex (37). In the terminal phases of the disease, the entire cortex becomes a thin gliotic scar with resultant cerebral atrophy. These pathological changes likely correlate with the evolving encephalopathy as the disease progresses. Medically intractable seizures produce hippocampal sclerosis. The cerebellum is variably involved, often with Purkinje cell dropout and Bergman gliosis with preserved granular cell layer (71). The ataxia that develops during the course of the illness likely relates to the loss of Purkinje cells.

Most patients are healthy before the onset of symptoms, but some can demonstrate nonspecific neurologic symptoms, such as developmental delay, clumsiness, migraine headaches, progressive ataxia, or mild medically controlled seizures before severe seizure onset (35; 103; 108; 92). Migraine headaches can be associated with visual hallucination auras due to occipital lobe involvement (35; 108).

Other nervous system manifestations have been reported in Alpers-Huttenlocher syndrome. Abnormal eye movements and delayed acquisition of smooth pursuits (delayed maturation of cranial nerves and eye movement networks within the cortex and cerebellum) are due to central and peripheral neuronal dysfunction. The peripheral nervous system is involved in most patients. In some series, peripheral neuropathy was universal, suggesting that neuropathy is a common peripheral nerve involvement in Alpers-Huttenlocher syndrome (103). Evaluating sensory neuropathy in young children is difficult and often escapes identification until muscle stretch reflexes are lost. The objective loss of pain and temperature, vibratory sensation, and light touch sensation are usually late findings. In older patients with recessive POLG mutations, sensory peripheral neuropathy was demonstrated to be due to loss of the peripheral axonal branches of the dorsal root ganglia neurons, secondary to neuronal death (58; 44). The same mechanism is likely occurring in younger children, but rigorous testing in the younger population of Alpers-Huttenlocher syndrome has not been reported, and performing electrodiagnostic studies (electromyography and nerve conduction studies) is not necessary or advised. However, sensory neuropathy has been reported as an early finding in younger patients (92). MRI imaging of the spine with contrast can show diffuse enhancement of nerve roots, especially of cranial nerves or at the cauda equina, the appearance like Guillain Barre syndrome, and chronic inflammatory demyelinating polyneuropathy, demonstrating the inflammatory nature of the pathophysiology (88). Ataxia can be an early symptom and is universally seen during the course of the disease due to cerebellar, sensory nerve ascending pathways or cortical tract involvement (102; 106; 109; 92; 94; 86).

Hepatic involvement. The specific histologic findings in Alpers-Huttenlocher syndrome require that Wilson disease be excluded as a possibility, and at least three of the eight following findings must be present (75):

	1. Bridging fibrosis or cirrhosis
	2. Bile ductular proliferation
	3. Collapse of liver cell plates
	4. Hepatocyte dropout or focal necrosis, with or without portal inflammation
	5. Microvesicular steatosis
	6. Oncocytic change (mitochondrial proliferation associated with intensely eosinophilic cytoplasm in scattered hepatocytes and not affected by steatosis)
	7. Regenerative nodules
	8. Parenchymal disease or disorganization of the normal lobular architecture

The first association of the use of valproic acid causing rapid onset of liver disease in a child with Alpers-Huttenlocher syndrome was in 1992, and this report soon led to the understanding that the clinical variables associated with valproate-induced liver failure were in fact the clinical features of Alpers-Huttenlocher syndrome (06). Because of the intractable nature of the epilepsy, it is rare for any anticonvulsant to be effective, and one by one, they are each discarded for a trial of another anticonvulsant. The physician, running out of options, will then initiate treatment with valproic acid or divalproex sodium, which often improves seizure control, but often results in liver failure. Therefore, valproic acid and divalproex sodium likely alter the natural history of Alpers-Huttenlocher syndrome and may give the impression of a rapidly progressive disease. Since gaining the ability to rapidly identify patients with Alpers-Huttenlocher syndrome and the acceptance that valproate is contraindicated in this disorder, an analysis of seizure frequency, duration, and medication use (without valproic acid exposure) needs to be performed to understand the natural history of disease progression without this variable. Valproic acid exposure inducing severe liver dysfunction has now become one of the defining features of Alpers-Huttenlocher syndrome (06; 71; 74; 110; 92).

Liver dysfunction occurs without valproate exposure, and it is part of the typical natural history of this illness. For example, the early reports of both Huttenlocher and Harding described liver involvement without valproic acid exposure (47; 37; 38). Furthermore, the histochemical changes induced by valproic acid are identical to that found without exposure (37; 74). The pathological features in the liver in Alpers-Huttenlocher syndrome are distinctive and differ from other chemically induced or toxic liver disorders (09; 95). As with other organ involvement in Alpers-Huttenlocher syndrome, the expression of liver dysfunction varies between patients. Regardless, exposure to valproic acid seems to be the catalyst for liver dysfunction, regardless of the specific genotype or timing of liver involvement. The mechanism of liver alterations remains elusive. If patients live long enough, most will eventually develop liver involvement and subsequent liver failure.

In situations in which the patient is placed on valproate, liver dysfunction usually begins within 2 to 3 months, and is usually heralded by hypoglycemia, decreased albumin synthesis, reduced synthesis of coagulation factors, and mild to moderate elevation in the transaminase values. Early recognition of liver dysfunction and discontinuation of valproate may prevent the progression to fulminant liver failure (67; 110). One study demonstrated that the use of intravenous levocarnitine reversed the course of early valproate-induced hepatic failure (08). Rotig and colleagues reported that valproate administration was responsible for four of nine deaths of children with neurologic phenotypes, but five of nine receiving valproate did not develop hepatic dysfunction (88). They also noted that of those with hepatic presentations, one had a liver transplant at 15 years old with good survival, as previously seen in the literature (48; 05; 51). In general, POLG remains a contraindication to liver transplantation (80), and valproate remains contraindicated in POLG-related disease (19).

Hypoglycemia can be an early symptom of Alpers-Huttenlocher syndrome. The precise etiology remains unclear but likely reflects impaired gluconeogenesis. Onset of hypoglycemic symptoms as early as 1 year of life with a normal liver biopsy has been described (92). Slightly older patients have been described with fasting hypoglycemia (68). Unexplainably, the hypoglycemic events may resolve over time.

Gastrointestinal involvement. Dysfunction of the autonomic nervous system and, possibly, the smooth muscle in the gastrointestinal tract are the cause of gastrointestinal involvement in Alpers-Huttenlocher syndrome, with patients developing esophageal dysmotility, delayed gastric emptying, and intestinal dysmotility that all worsen as the disease progresses. These children often require placement of a gastric tube (G-tube) for nutrition in the mid-stage of the disorder. The use of continuous G-tube feedings is often required because normal bolus feedings are not tolerated due to poor gastric and intestinal motility. Complete gastrointestinal dysmotility may occur, and the use of jejunal tube feedings and, eventually, total parental nutrition is common. A complete loss of the longitudinal muscularis propria external muscle layer in the gastrointestinal tract was found in two patients with Alpers-Huttenlocher syndrome at autopsy, presenting initially at the age of 2 and 13 years, respectively (Saneto, unpublished data). MtDNA depletion within the muscularis propria was demonstrated in one patient with POLG mutations compatible with Alpers-Huttenlocher syndrome in an infant who died at 20 days of life (30). A similar pathological finding has been reported in mitochondrial neurogastrointestinal encephalopathy, another mtDNA depletion disease (30). The reason for the specific nature of the loss of the external muscle layer of the gastrointestinal tract is unknown, but the mtDNA depletion within the smooth muscle of the muscularis propria of both diseases may be responsible.

Pancreatitis may also occur in Alpers-Huttenlocher syndrome, with the mechanism remaining unknown. Although pancreatitis is a possible side effect of valproic acid in patients with epilepsy, and it is tempting to invoke pancreatitis with valproic acid exposure, Harding reported pancreatitis in patients not exposed to valproic acid (37; 03).

Other phenotypes associated with mutations in POLG – the POLG spectrum disorders. There are other phenotypes that are caused by mutations in POLG. These phenotypes serve a purpose for general classification and, to some extent, are distinct in that each has a general age range, set of syndromic features, and severity. However, with any single patient, the delineation between phenotypes may not be clear and, over time, could shift from one phenotype to another as the illness progresses or fails to progress as initially predicted.

The childhood myocerebrohepatopathy spectrum disorders represent the most severe and rarest phenotype caused by mutations in POLG. These patients present between the first few months of life up to about 3 years of age. The clinical features include loss of cognitive milestones, failure to thrive, lactic acidosis, and myopathy. Liver failure often occurs, as well as renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Seizures are not common--at least early in the disease course. These disorders are universally fatal, with a more rapid course than Alpers-Huttenlocher syndrome (111).

The most prominent features of myoclonic epilepsy myopathy sensory ataxia are epilepsy, myopathy, and ataxia, but without ophthalmoplegia. These patients may appear similar to patients with myoclonic epilepsy with ragged-red fibers but do not have ragged-red fibers. The first symptoms typically occur in the late teens. The presence of clear myopathic features without ophthalmoplegia distinguishes these patients from the other POLG spectrum illnesses. The myoclonic epilepsy, myopathy, sensory ataxia phenotype includes earlier epitomic phenotypes, including mitochondrial recessive ataxia syndrome and spinocerebellar ataxia with epilepsy syndromes, although predominant myopathic features are not present in mitochondrial recessive ataxia syndrome and spinocerebellar ataxia with epilepsy. In these disorders, sensory polyneuropathy may present as ataxia, sometimes as the first sign of the disease. Epilepsy develops later in the course of the illness, often starting as focal seizures in the dominant arm and then becoming more generalized. As with Alpers-Huttenlocher syndrome, the seizures may be refractory to therapy and accompanied by progressive encephalopathy (107).

Ataxia neuropathy spectrum disorders were previously described in the literature as “sensory ataxia, neuropathy, dysarthria, and ophthalmoplegia” syndrome and encompass several described clinical syndromes that include ataxia, neuropathy, bulbar dysfunction, and progressive external ophthalmoplegia, but without a predominant myopathy. The median age of onset of ataxia neuropathy syndrome is in the late teenage years. This category also includes many of the patients previously classified as having mitochondrial recessive ataxia syndrome and spinocerebellar ataxia with epilepsy syndromes, but with ophthalmoplegia and the absence of a predominant myopathy, although cramps may be present in one-quarter of patients. An epileptic encephalopathy is often, but not always, a feature of ataxia neuropathy syndrome. The encephalopathy is similar to that seen in Alpers-Huttenlocher syndrome but is generally less severe. The neuropathy can mimic Charcot-Marie-Tooth disorder (83). Other features may include myoclonus, cortical blindness, hearing loss, and liver dysfunction (25; 35; 103; 111; 16). The liver dysfunction is variable, with some patients having no dysfunction, others having asymptomatic elevated enzymes and mild synthetic dysfunction, and a third group that may progress to florid liver failure (103; 111). Psychiatric disturbance, primarily mood disorders, was present in over one-half of patients in one series and may often be severe (35).

Autosomal dominant progressive external ophthalmoplegia presents as an adult-onset disorder with progressive external ophthalmoplegia, usually along with a weakness of a generalized myopathy (105). Patients with autosomal dominant progressive external ophthalmoplegia are likely part of the syndrome previously referred to as ‘‘chronic progressive external ophthalmoplegia plus.” These patients can have a variable degree of sensorineural hearing loss, axonal neuropathy, ataxia, mood disorder, parkinsonism, hypogonadism, and cataracts (57; 63; 79).

Autosomal recessive progressive external ophthalmoplegia generally presents in adulthood with CPEO and is also often referred to as chronic progressive external ophthalmoplegia plus or as a mild variant of Kearns-Sayre syndrome. Other variable signs can include depression (mood disorder), premature ovarian failure, parkinsonism, ataxia, and a mitochondrial neurogastrointestinal encephalomyopathy-like phenotype. Some mutations associated with autosomal recessive progressive external ophthalmoplegia are also found in the more severe POLG-related disorders, including ataxia neuropathy syndrome (105; 57; 63; 79).

Prognosis and complications

The disease progression is variable in both timing and rapidity. Because of the myriad pathologic mutation combinations, modifying genetic factor, and environmental influences, it is not possible to predict the rate of progression. There will be loss of neurologic function culminating in dementia, spastic tetraparesis from corticospinal tract involvement, movement disorders from extrapyramidal track involvement, cortical visual loss, and, ultimately, death. The rate of neurodegeneration varies and is marked by periods of stability. The degree of dementia is often difficult to assess because of frequent seizures and high doses of anticonvulsants that can cloud the sensorium. The reported life expectancy from onset of first symptoms ranges from 3 months to 12 years, but this varies on both the underlying illness and intensity of medical intervention (16). Based on age of presentation, those under 12 years have the worst prognosis, those between 12 and 40 years have a better prognosis than early-onset, and those older than 40 years at disease onset have the best prognosis.

Prognosis is based on genotype-phenotype correlations, and previously published cases are available in a publicly available database, which can be accessed at the following website: https://mitomap.org/polg/. The POLG Pathogenicity Prediction Server aims to provide the most current available information regarding the pathogenicity of POLG variants based on a clustering model and was first published by Nurminen and colleagues (78).

Clinical vignette

The following case involves a summary of events that occurred in a patient with Alpers-Huttenlocher syndrome, and the case illustrates the spectrum of clinical manifestations that can occur in this illness.

The child was healthy until 3 years of age when his parents noted several staring spells over the course of a month. He became increasingly irritable and distractible during this time, as well. One evening it was noted that the right side of his face and right shoulder were twitching.

He was admitted to the hospital for evaluation of what was thought to be a partial seizure. His examination showed a distant effect without language output, but he would intermittently follow commands and had a nonlateralized motor examination. The head CT was normal. The brain MRI was normal except for mild enhancement of the left leptomeninges. The EEG showed intermittent left hemispheric slowing. CSF was acellular with normal glucose and moderately elevated protein. He was diagnosed with epilepsia partialis continua and treated with increasing dosages of phenobarbital and lorazepam. Over the next few days, the epilepsia partialis continua resolved, and the child was discharged on oral medication. The parents noted in retrospect that during the month of staring spells, he had lost some of his expressive language skills, and these did not recover over the next few months. He then had several hospitalizations for prolonged motor seizures, and they were treated with several other anticonvulsants, including fosphenytoin, phenytoin, levetiracetam, and lamotrigine. During this period, the child’s gait became mildly spastic and ataxic. When the seizures were well controlled, he regained some language function. His parents thought his personality was also returning to normal. A repeat brain MRI scan 3 months into the onset of the illness revealed cortical atrophy.

One month later, the child was readmitted with status epilepticus. After using increasing dosages of benzodiazepine and barbiturates to try to control the seizure, he was given a loading dose of sodium valproate. His seizures soon subsided, and he was discharged four days later on oral valproic acid and phenobarbital. Because of the general concern of liver toxicity with valproic acid use, the family was instructed to report any nausea, vomiting, confusion, or other mental changes. Three weeks later the child appeared to be lethargic, and alanine transaminase was found to be 1.5 times the upper limit of normal with normal liver synthetic studies, including ammonia, bilirubin, cholesterol, albumin, and prothrombin time. He was admitted to the hospital for evaluation and observation. The following morning his glucose (while on intravenous fluids containing 5% dextrose) was 42 mg/dL, and his alanine transaminase was 800 u/L. Over the next several days he developed florid liver failure. IV levocarnitine was instituted at a dose of 300 mg per kg per day, and POLG gene testing was ordered. The child was evaluated by the liver transplant service, deemed a good transplant candidate, and received a perfect human leukocyte antigen-matched liver within 3 days. Over the following week, the child recovered and returned to his baseline neurologic state, although his seizures continued as the valproic acid was not restarted after the liver transplant. Genetic testing revealed known pathogenic variants and diagnostic mutations in POLG, and the diagnosis of Alpers-Huttenlocher syndrome was confirmed.

Over the next two years, the child had numerous hospitalizations for status epilepticus. He lost his ability to swallow and became fully G-tube-fed. The examination showed a steady loss of pyramidal function with increasing spasticity, chorea, myoclonus, and dystonia. He became cortically blind and lost communication skills. He was admitted with aspiration pneumonia, and, at that time, his parents declined ventilation, so he was treated with mask BiPAP therapy. After consultation with the ethics team and the palliative care service, the family decided not to have a tracheostomy placed and favored home-based palliative care services. The child died peacefully from hypoventilation after what was likely an aspiration event. Of note, liver transplantation is generally contraindicated in POLG-related disorders due to the poor prognosis following liver transplantation, especially in younger patients with epilepsy.

Biological basis

Etiology and pathogenesis

Ultimately, the triggering factor in disease manifestation is the reduction in mitochondrial function due to a loss of mtDNA-produced proteins, tRNA and rRNA. The reduction in mtDNA products compromises the manufacture of ATP and, therefore, critical energy production required for cellular function. Once a critical nadir of ATP is reached, cellular function and survival are compromised. Depending on the energy demands of the cell, organs become dysfunctional and disease symptoms are manifested.

The unique physiological properties of the mitochondria, in part, give rise to the varied phenotypic manifestations of POLG diseases. The sole mtDNA replicase is POLG. When mutations occur in the nuclear-encoded POLG gene, the integrity of the replication, editing, and repair of mtDNA is compromised. The portion of the POLG gene that is mutated plays a large role in POLG malfunctions. Mutations in the mtDNA binding site inhibit POLG action in an autosomal dominant inheritance pattern. Mutations in the linker region that produced disease are always autosomal recessive and often produce milder disease. Likewise, mutations in the exonuclease region produce multiple deletions within the mtDNA molecule and less severe disease that is often only expressed later in life. For unclear reasons, both autosomal dominant and recessive diseases can occur in this type of POLG mutation. When a combination of POLG regions is mutated, a more severe phenotype is most often produced in an autosomal recessive manner and is the result of mtDNA depletion (reduced ability to replicate functional mtDNA).

The phenotype of disease is usually correlated with the degree of mtDNA depletion within the mitochondrion. Due to the heterogeneity of numbers of mitochondria per cell and mtDNA copies per mitochondrion, differing symptoms arise at distinctive times within tissues as mtDNA is depleted or compromised in integrity. The complexity of the phenotype of POLG mutations is due to the position of the mutation, the degree of mtDNA depletion, and unclear factors, including environmental and toxin exposure.

Polymerase gamma 1 protein (pol γ) is the only known DNA polymerase in the mammalian mitochondria and is responsible for mtDNA replication and repair (52; 33; 56). Pol γ is synthesized in the cell nucleus and transported to its inner membrane location. In the inner membrane, it associates with other nuclear-encoded proteins that make up the mtDNA replisome and nucleoids. Pol γ has three distinct DNA activities: a 5’->3’ DNA polymerase, a 3’->5’ exonuclease, and a 5’-deoxyribose phosphate lyase activity. The exonuclease activity (catalytic domain) is located within the N-terminal regions and is connected by a linker region to the C-terminal domain, which contains the 5’-3’ polymerase activity. The polymerase region has three subdomains named palm, fingers, and thumb. The C-terminal domain contains the 5’-3’-polymerase activity as well as the 5’-deoxyribose phosphate lyase activity. The exact location of the 5’-deoxyribose phosphate lyase activity within the C-terminal region is unknown (62). There are three motifs within the exonuclease region, I, II, and III, and three within the polymerase region, A, B, and C, which are essential for full pol γ activity (52; 33). Within the linker region, there are two regions required for full activity: the accessory protein interacting domain (AID) located toward the N-terminal part of the linker region and the intrinsic processivity subdomain (IP) located distally in the linker region (toward the C-terminal).

Functional and structural domains of the polymerase gamma protein

The N-terminal domain (NTD) region contains the mitochondrial targeting sequence. The thumb subdomains (T) of the protein are located between the exonuclease and linker region, as well as within the polymerase region. The exonucle...

Holoenzyme. The holoenzyme of pol γ is a heterotrimer comprised of one molecule of pol γ associated with two molecules of POLG2 (87; 59). POLG2, the 55 kDa accessory protein enhances the processivity (the average number of nucleotides added by the enzyme per association-disassociation with the template DNA) of the holoenzyme (60). One site of POLG2 association is with the linker region at the AID subdomain of the catalytic subunit (60). This interaction increases the DNA-binding affinity of the holoenzyme. The second POLG2 protein makes limited contact with pol γ and is labeled the distal accessory site (60). Interaction of POLG2 to this distal site enhances the polymerization rate of the holoenzyme (60). The replisome complex also comprises the mtDNA helicase PEO1 (Twinkle), a single-stranded DNA binding protein, mtSSB, and a number of accessory proteins and transcription factors (33).

POLG genetics. Although there are some pathogenic variants in POLG that are expressed with autosomal dominant disease, POLG-related disorders are autosomal recessive disorders. Both copies of POLG are expressed in mammalian cells, and monoallelic expression of wild-type POLG is sufficient to avoid disease (13). Homozygote dominant POLG mutations, or even a dominant POLG mutation with a recessive mutation, have not been described, suggesting that these possible mutation combinations are likely embryonic lethal. Although Alpers-Huttenlocher syndrome–producing mutations are distributed along the entire length of the POLG gene, mutations tend to cluster within three distinct regions within the gene, referred to as the polymerase, linker, and exonuclease domains (24). Thus far, no dominant mutations have been reported to cause Alpers-Huttenlocher syndrome. When found in other POLG disorders, dominant mutations occur in the polymerase region. Recessive mutations within the polymerase region decrease polymerase activity, alter DNA-binding affinity, and lower the catalytic efficiency (24). Mutations in the linker region of the AID mediate interactions with POLG2 and destabilize the pol γ–DNA complex. When mutations are found in the linker intrinsic processivity region, they induce decreases in processivity (12). Mutations within the exonuclease region, specifically in the finger subdomain, confer partitioning of the DNA substrate between the polymerase and exonuclease sites and diminish the fidelity of the polymerase (24). Predictive genotype-phenotype and disease severity algorithms have been suggested (26).

Recessive mutations in POLG occur as homozygous or compound heterozygous mutations. In general, compound heterozygous mutations usually induce a more severe phenotype, whereas homozygous recessive mutations are associated with milder and later-onset disease (74; 33). Both types of recessive mutations are associated with early-childhood and juvenile-onset Alpers-Huttenlocher syndrome. However, for reasons that are not understood, either compound heterozygous or homozygous mutations can be found in patients with the severe early-onset phenotype and milder juvenile-onset Alpers-Huttenlocher syndromes. The reasons for such disparate genotype/phenotype variation within a single syndrome, Alpers-Huttenlocher syndrome, and the larger group of POLG syndromes are not completely understood.

A partial explanation for the phenotypic variability within Alpers-Huttenlocher syndrome may be the location of the mutation within the gene. The compound heterozygote mutations are usually associated with more severe early-onset or childhood disease. The combination of mutations within two distinct regions of POLG may cause a compounding of the reduction in enzyme activity and, thereby, more severely compromising mtDNA replication (24). The most common example is patients with a mutation within the linker region and another mutation in the polymerase region, p.A467T/p.W748S. This genotype is expressed as decreased survival and increased incidence of liver failure compared with patients having the homozygous mutations p.A467T/p.A467T or p.W748S/p.W748S (33; 11; 100). Unfortunately, this attractive hypothesis cannot fully explain why these same homozygous mutations can also give rise to early-onset Alpers-Huttenlocher syndrome or why the same compound heterozygote mutations can lead to other distinct POLG syndromes with a wide range of disease onset and severity (33; 07). Although it is likely that combinations of mutations within distinct regions of POLG do, in part, determine Alpers-Huttenlocher syndrome phenotype, there are other yet unknown mechanisms involved.

Epidemiology

The true prevalence of Alpers-Huttenlocher syndrome is unknown. Due to high mortality and early death, epidemiology parameters of incidence and prevalence are not informative. However, Alpers-Huttenlocher syndrome is relatively uncommon. It is estimated that the minimum birth frequency of children who will develop mitochondrial disease is approximately 1 in 5000 (96). Of this population, up to 25% will develop POLG disease (Chinnery and Zeviani 2007). Crude estimates from published reports suggest that up to 30% of patients with recessive POLG mutations have Alpers-Huttenlocher syndrome; however, accurate data are not available. A best estimate of prevalence is between 1 in 51,000 to about 1 in 100,000 (17; 70). Clearly, more reliable population studies are needed to more reliably estimate the incidence and prevalence of Alpers-Huttenlocher syndrome. POLG pathogenic variants are thought to be the most common cause of inherited mitochondrial disease, and it is estimated that 2% of the population harbors a POLG pathogenic variant (86).

Population estimates of common POLG mutations. The most common mutation reported inducing Alpers-Huttenlocher syndrome and associated POLG spectrum disorders is the p.A467T mutation. The second most common mutation in Alpers-Huttenlocher syndrome and POLG spectrum disorders is the p.W748S mutation. Elegant population studies have demonstrated that both mutations arose from a common ancestor within various populations of Northern European descent (34). The carrier frequency of p.A467T is estimated to be as high as 0.6% in Belgium and 1% in Norway, and the p.W748S frequency is estimated to be 1:125 in Finland (107; 35; 34; 109). Other Alpers-Huttenlocher syndrome founder gene mutations have not been discovered in other ethnic populations. Whether a founder effect is exclusive to the Northern European ethnic population is not known, as most of the early mutational research has centered on the North European population. POLG mutations inducing Alpers-Huttenlocher syndrome have been found in many other ethnic groups (32; 92; 69).

As more patients are described with POLG mutations, it is possible that more “hot spot” regions of POLG will be uncovered. Using crude estimates of the gene frequency of the most common mutations, members of these Northern European populations have about a 2% risk of carrying a pathogenic (heterozygote) mutation, suggesting a disease frequency of about 1:10,000.

Gender. Males and females seem to be equally affected with Alpers-Huttenlocher syndrome (105; 103; 111; 93).

Ecogenetic structural nucleotide variants (ESNV). One possible emerging mechanism for phenotypic variability is the presence of silent nucleotide polymorphisms or ESNVs that alter disease expression depending on environmental or epigenetic factors. When present, a particular POLG genotype will remain clinically silent unless specific conditions are present that can either lead to disease or modify the phenotype. The polymorphic nature of POLG suggests that ESNVs may exist (93). Two ENSVs have been described in the Northern European population: p.E1143G and p.Q1236H (45; 11; 98). p.E1143G has a frequency of approximately 4.5%, and p.Q1236H has a frequency of approximately 8.6%, of the Northern European population. These variants are almost nonexistent in the Asian, sub-Saharan African, or African-American populations (90). The presence of the ESNV p.E1134G has been demonstrated to increase the catalytic rate for incoming nucleotides as well as to increase intrinsic stability in in vitro studies (12). When present in cis with certain pathological mutations, especially with the p.W748S mutation, p.E1143G can lessen the disease severity and delay the age of onset by enhancing enzyme activity (45; 11). This same ESNV can also increase the liver’s sensitivity to valproic acid exposure, which appears counterintuitive to the disease-lessening effect. Thus, the p.E1143G ESNV can be a disease modifier by either enhancing enzyme activity or compromising liver function, depending on other genetic factors and environmental exposures. As with p.E1143G, the presence of p.Q1236H can induce liver failure with exposure to valproic acid. It is estimated that the presence of either p.E1143G or p.Q1236H increases valproic acid sensitivity more than 20-fold (39). However, as p.Q1236H was originally reported as a neutral polymorphism and has a known prevalence of 8.3% in the European population, 14.8% to 15.9% in the Finnish population, and about 1% in Africans and Asians, it is not thought to contribute to valproic acid-induced hepatotoxicity (49).

A question arises as to whether either ENSV is wholly or partially responsible for hastening liver failure from exposure to valproic acid in Alpers-Huttenlocher syndrome. We have preliminary evidence that both p.E1143G and p.Q1236H are likely independent of other, yet unknown, POLG mutations that induce liver failure. For example, six previously reported patients with Alpers-Huttenlocher syndrome and liver failure did not have either ESNV (92). There are rare reports of long-term use of valproic acid exposure with POLG mutation(s) without liver failure (103). Whether other valproic acid-sensitive ENSVs exist, and the full understanding of valproic acid-induced liver failure, remain unknown.

Prevention

Mutations inducing POLG diseases are uncommon in the population. As with most autosomal recessive disorders, the expression of these diseases is rare. The finding of an individual within a family is likely unique within that family. However, once an individual is identified, common Mendelian genetics would dictate the probability of another family member being affected. Once an individual is identified, genetic counseling is recommended for family planning.

There is an autosomal dominant pattern of inheritance in the condition of autosomal dominant progressive external ophthalmoplegia. Individuals with this form of POLG disease would herald more prompt genetic counseling as multiple individuals within that family have a higher probability of having the gene mutation and, hence, a higher likelihood of expressing the disease.

Currently, we do not have any preventative medications to ameliorate the symptoms of POLG disease. Management of disease is purely symptomatic. Patients with POLG disease have been treated with investigational medications in multi-institutional trials, including the following medications: vincerinone (EPI-743; Bioelectron Technologies), cysteamine bitartrate (RP-103; Horizon Pharmaceuticals), elamipretide (SS-31, MTP-131; Stealth Biotherapeutics), and omaveloxolone (RTA-408; Reata Pharmaceuticals) (B Cohen, personal observation). Results of these trials have not yet been published.

Elamipretide (Stealth Biotherapeutics) is in a phase 3 trial for patients with replisome defects, including adults with POLG, as the previous phase 3 trial did not meet primary endpoints (improvement in 6-minute walk test) but suggested possible improvement in a subset of patients (54).

Omaveloxolone (Reata Pharmaceuticals) did not meet primary endpoints in a clinical trial of adults with primary mitochondrial myopathy (65); however, it was approved by the FDA for Friedreich ataxia (64).

Pekeles and colleagues published the results of an open-label, single-arm, phase 2 clinical trial on the safety and efficacy of deoxynucleoside therapy in a cohort of patients after a 6-month treatment window (82). These interim data include reporting on the first 10 patients with POLG-related disorders given oral therapy of deoxycytidine and deoxythymidine and outcome measures of the Newcastle Mitochondrial Disease Scale, DGF-15, EEG, seizure diary, additional lab studies, and adverse events. This study remains ongoing and is registered at clinicaltrials.gov: NCT04802707.

Differential diagnosis

In addition to POLG, there are several other genes associated with the Alpers-Huttenlocher syndrome phenotype. Mutations in other genes that are part of the mtDNA replisome, which includes POLG, POLG2, and C10orf2 (previously referred to as PEO1 or TWINKLE), can cause a similar illness. Two siblings have been described with compound heterozygote mutations in the mtDNA helicase PEO1. Autopsy in one of the siblings who died during a status epilepticus event revealed that the liver changes did not meet the strict criteria of the histochemical and structural findings in Alpers-Huttenlocher syndrome. However, it is likely that the young age of this patient at death prevented full expression of the liver changes, as the other features of Alpers-Huttenlocher syndrome were present. There are also genes that serve as mtDNA maintenance genes by regulating the deoxynucleotide triphosphate pools and performing other functions. Although beyond the scope of this article, these genes (and their general phenotype) include TP1 (mitochondrial neurogastrointestinal encephalomyopathy), TK2 (myopathy), DGUOK (infantile hepatocerebral syndrome), SUCLA1 (infantile lactic acidosis), SUCLG2 (infantile encephalomyopathy), ANT1 (progressive external ophthalmoplegia, myopathy), RRM2B (encephalomyopathy), and MPV17 (hepatocerebral syndrome). The phenotypes of these disorders are distinct from Alpers-Huttenlocher syndrome, although they may have some overlapping features with Alpers-Huttenlocher syndrome and POLG spectrum disorders, and they do share in common the depletion of wild mtDNA as the basis of the pathophysiology (15).

POLG should be a consideration in refractory epilepsy, myoclonic epilepsy, and epilepsia partialis continua. In addition, there are patients ultimately diagnosed with POLG who were thought to have autoimmune encephalitis or acute disseminated encephalomyelitis at presentation, even in those who responded well to immunomodulation therapies. Likewise, patients who present with FIRES (febrile infection-related epilepsy syndrome) or NORSE (new-onset refractory status epilepticus) should undergo genetic testing to rule out entities such as POLG.

Diagnostic workup

Genetic testing. Sequencing and deletion studies of POLG should be performed if POLG-related disorder is suspected. The use of gene sequencing has more importance in these syndromes than in other biochemical disorders because there are no specific or sensitive biochemical markers for POLG disease. Disease-causing variants in POLG remain the most sensitive and specific methods of confirming the diagnosis of POLG-related disorders.

Electron transport chain enzymology. Depression of electron transport chain enzymatic activities in muscle and liver are not specific for Alpers-Huttenlocher syndrome; this is an insensitive method of diagnosis, especially early in the course of the illness. Reports have demonstrated a variable pattern of electron transport chain defects, ranging from completely normal to single and multiple electron transport chain deficiencies (74; 20).

The variability of electron transport chain complex enzyme activity is likely related to the stage of disease and the tissue tested. As the disease progresses, abnormal electron transport chain enzyme deficiencies become more pronounced, as the catalytic subunits of the various complexes encoded by mtDNA are either not produced or have abnormal protein function. Although electron transport chain enzymology may be abnormal, the lack of sensitivity and specificity suggests electron transport chain activity should not be used to screen or diagnose Alpers-Huttenlocher syndrome.

Mitochondrial DNA content. The mtDNA copy number in Alpers-Huttenlocher syndrome patients is 3% to 40% of normal, but this is also a function of what tissue is tested, the specific mutation, and the stage of the illness (71; 27; 97; 99). The use of mtDNA content may seem useful for diagnosis, but it is not sensitive or specific for Alpers-Huttenlocher syndrome because mtDNA content may be normal early in the disease process. Muscle and liver mtDNA depletion may lag behind disease symptoms. Some mutations impair the fidelity of the transcript and render the mtDNA as a functionless copy instead of altering the mtDNA content. In this case, the mtDNA content will be normal. If mtDNA content is analyzed, it should be measured in liver and muscle tissue, as POLG mutations do not always induce mtDNA depletion in blood (21). As the disease progresses, almost all Alpers-Huttenlocher syndrome patients will begin to demonstrate mtDNA depletion. Rare exceptions have been reported in patients with Alpers-Huttenlocher syndrome who had mtDNA deletions without mtDNA depletion (55). Therefore, the finding of mtDNA depletion may be helpful in diagnosis, but its absence cannot be used to exclude Alpers-Huttenlocher syndrome.

Electroencephalogram and seizures.

Alpers-Huttenlocher syndrome (EEG)

EEG findings in an Alpers-Huttenlocher syndrome patient demonstrating occipital spike and polyspike morphology. (Contributed by Dr. Bruce H Cohen.)

Well-described and specific EEG findings and seizure semiology early in the disease course may suggest Alpers-Huttenlocher syndrome as a diagnosis. Severe seizures with occipital lobe predominance of epileptiform discharges that evolve into status epilepticus and/or epilepsia partialis continua, combined with psychomotor delay, occur frequently in Alpers-Huttenlocher syndrome and do not in other illnesses; therefore, these findings should signal the possibility of Alpers-Huttenlocher syndrome (23; 110; 92). Only a few epileptic syndromes, including Panayiotopoulos syndrome; Gastaut syndrome; Lafora disease; celiac disease; mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; myoclonic epilepsy with ragged-red fibers; epilepsy with bilateral occipital calcifications; idiopathic photosensitive occipital epilepsy; and malformations of cortical development have a posterior head predominance of spike/polyspike and wave epileptiform discharges (101). This combination of epileptiform location and seizure semiology should also signal the clinician to confirm that POLG is normal before starting valproic acid.

Neuroimaging. Neuroimaging findings are nonspecific but can be helpful in diagnosis. Head CT and brain MRI may be normal early in the course of the disease, but MR imaging changes reflect acute and chronic pathological changes as the disease progresses. The occipital location of EEG abnormalities and neuronal loss/gliosis can sometimes be seen on brain MRI as hyperintensity on T-weighted and FLAIR sequences in the occipital regions that suggest mitochondrial dysfunction. When the seizures are uncontrolled, T2-weighted and FLAIR hyperintensities are often prominent in the thalami and basal ganglia (110), as well as the inferior olivary nuclei (40). There may also be a transient resolution of MRI changes with normalization of the signal changes (91). However, as the disease progresses, MRI imaging demonstrates cortical atrophy reflecting the pathological changes in the basal ganglia and brainstem induced by the disease (37; 55). Some patients have cerebellar atrophy, which corresponds to the prominent Purkinje cell loss seen in autopsy cases (37). In a systematic review of neuroimaging in 136 patients with POLG-related epilepsy, stroke-like lesions in the cortex were found in 43%, in addition to lesions in the thalamus (37%), basal ganglia (14%), cerebellum (17%), as well as generalized atrophy (28%). Of note, some patients had normal imaging results, emphasizing that a normal brain MRI does not exclude POLG-related disease (02). Additional studies on MRI findings include a review of 13 children with epilepsy (including epilepsia partialis continua) and POLG pathogenic variants (31). The most common findings include abnormal signal changes in the thalamus (77%) and perirolandic cortex (54%), in addition to rapid volume loss (77%) on subsequent imaging. Hikmat and colleagues found the following in the cohort of POLG with status epilepticus: MRI brain was abnormal in 55 of 86 (64%) patients at status epilepticus onset, with focal lesions of the cortex and subcortical areas the most common finding (55 of 77, 67%) (42). Some had thalamic lesions (n=36 of 77, 47%) or generalized cerebral atrophy (n=10 of 30, 33%).

The diagnosis of Alpers-Huttenlocher syndrome takes a great deal of clinical acumen. The clinical expression of signs and symptoms varies in timing, intensity, and severity. Clinical suspicion is needed early in the course of diagnosis as the sequence of symptoms may be distinctive for each patient. However, the constellation of clinical symptoms will eventually be expressed, and POLG sequencing will confirm the diagnosis.

Management

Treatment of manifestations. Once the diagnosis of a POLG-related disorder is established, it is important to perform a systemic initial evaluation to determine the extent of neurologic and systemic involvement.

Table 1. Initial Evaluation for Patients with POLG-related disorders

Brain MRI; early studies may appear normal Electroencephalogram (EEG); consider video-EEG if subclinical seizures are suspected Ophthalmologic examination Audiology Gastroenterology Physical medicine and rehabilitation Physical therapy, occupational therapy, or speech therapy Psychotherapy (mood disorders prevalent)
	• Nutritional assessment • Swallow study if any signs of bulbar dysfunction are present • Liver evaluation (blood)
		- Glucose - Ammonia - Amino acids - Bilirubin (conjugated and unconjugated) - Albumin - Cholesterol - Carnitine studies - Coagulation studies (prothrombin time-INR) - Ultrasound - Enzymes (ALT, GGT, AST)
Routine laboratory: complete blood count, comprehensive metabolic panel, urine analysis Mitochondrial biomarkers (if available): GDF-15, FGF-21, glutathione (total, reduced, oxidized) Inflammatory markers: cytokine studies Pulmonary function tests Polysomnogram Electrocardiogram
Note: Not all testing is necessary for every patient, and this list should serve as a guide.

Because the illness requires different types of therapy and generally involves different organ systems, it is reasonable to assemble a treatment team.

Table 2. Treatment Team for Patients with POLG-related disorders

• Primary care provider
• Medical genetics
• Neurology
• (+/- epileptologist)
• Physical, occupational, or speech therapy
• Gastroenterologist (+/- hepatologist)
• Pulmonologist (+/- sleep specialist)
• Physiatrist
• Ophthalmologist
• Palliative care medicine

Treatment is limited to symptom management and supportive care. It is critical that the family be informed about the critical nature of this illness as soon as the family is able to absorb the diagnosis. The global perspective of care should be palliative-oriented, even if death is not imminent. Quality-of-life issues are important to discuss, specifically whether to institute invasive therapy, such as a tracheostomy, before major clinical changes occur.

Alpers-Huttenlocher syndrome ultimately progresses to fatal encephalopathy or liver failure. Because variable levels of intensive treatment are available, these options should be discussed openly with the family. Supportive care could include the placement of a gastrostomy feeding tube for medication, hydration, and/or nutrition. Different levels of ventilator support may include less invasive treatments such as continuous positive airway pressure (CPAP), bilevel positive airway pressure (BiPAP), assisted nasal ventilation, intubation and/or placement of a tracheostomy, and use of mechanical ventilation. These issues are complex because the maximal degree of medical support that is acceptable can vary among parents. In addition, a procedure that is minimally invasive, such as the institution of BiPAP, can lead to a quality-of-life improvement. However, such interventions may ultimately escalate towards tracheostomy placement and mechanical ventilation because each decision requires painful reflection, and the parents may hope that the escalation will reverse the progression of the disease or have an equivalent restoration of the quality of life that previous, less invasive interventions formerly provided. Nonetheless, as the disease progresses, the benefits of more invasive interventions may have a diminishing impact on the child’s quality of life. Involvement of palliative care services can maintain the process of iterative discussions about the family’s goals, support the family and care team with these discussions, and help implement the decisions. In larger hospitals, rehabilitation units are often where family members learn to care for their loved ones with new gastrostomy tubes and ventilatory support. Most patients utilize rehabilitative services that help to maintain neurologic function for as long as possible, including occupational, physical, and/or speech therapies.

Consultations with a gastroenterologist and nutrition therapy early in the disease course are necessary to address feeding and nutritional issues and to assess and manage liver dysfunction. Surgical placement of a gastric feeding tube, when oral feeding becomes impaired, can maintain nutritional status or prevent aspiration of oral feedings. However, given the ultimate course of the illness, some families may wish to forego the placement of a feeding tube.

Hypoventilation is common in Alpers-Huttenlocher syndrome and can exist long before anyone has observed frank apnea or clinical signs of hypercarbia. Assessment of both daytime and nocturnal ventilatory function can be performed for evidence of central and/or obstructive apnea. Depending on the severity of the findings, the use of CPAP or BiPAP may improve the quality of life in these children. Tracheostomy placement with or without varying use of artificial ventilation is a consideration for some cases, but families may wish to allow the natural course of the illness to proceed without these invasive and usually lifelong approaches.

Attempts should be made to control the seizures as best as possible. However, refractory epilepsy, especially epilepsia partialis continua, may be impossible to control with any treatment, and the side effects of treatment may outweigh any clinical benefit. Epilepsia partialis continua may be confused with movement disorder as it may not correlate with EEG changes, and epilepsia partialis continua may also warrant imaging as it can indicate a new stroke-like lesion. There is no evidence that newer anticonvulsants, such as felbamate, lamotrigine, topiramate, oxcarbazepine, or levetiracetam, offer a better therapeutic benefit over the older medications (phenobarbital, phenytoin, carbamazepine, primidone); however, the newer medications tend to be less sedating, may require less processing by the liver, and may have fewer drug-drug interactions. Valproic acid and sodium divalproate should be avoided (06; 92; 19). Because other anticonvulsants have also been implicated in accelerating liver deterioration, it is reasonable to monitor liver transaminase levels every 2 to 4 weeks after introducing any new medications.

Status epilepticus is common in POLG, and in a large multinational retrospective study of 195 patients of whom 130 had epilepsy, status epilepticus was identified in 97 of them (77%), with a median age of onset of 7 years (42). Status epilepticus was the presenting symptom of the disease in 40 patients (43%) and developed during the course of the disease in 53 (57%). Seizure semiology included convulsive status in 94% and epilepsia partialis continua in 67%. Associated comorbidities included liver impairment (78%), ataxia (69%), and stroke-like episodes (57%). Status epilepticus was typically refractory or super-refractory in 66%, and the presence of seizures was associated with significantly higher mortality compared to those without seizures.

Treatment for status epilepticus is known to be refractory, and most patients need several antiseizure medications. In the study by Hikmat and colleagues, six was the median number of antiseizure medications used during a single status epilepticus episode (range two to 11) with 62% of patients on more than six antiseizure medications (42). Treatments included antiseizure medications, anesthesia medications, immunomodulators, and ketogenic diet. Ketogenic diet was started during status epilepticus in 13 patients and reported to be effective in two older patients. Immunomodulators given during status epilepticus consisted of prednisolone (16), immunoglobulin (four), ACTH (two), plasmapheresis (one), tacrolimus (one), with no difference in seizure control or long-term survival; however, the authors acknowledge that giving these agents late and under treatment-resistant conditions does not prove lack of efficacy and that prospective trials are warranted. There are insufficient data for vagus nerve stimulation and transcranial direct current stimulation in POLG-related epilepsy, although transcranial direct current stimulation was used successfully in one teenager with focal motor status (73) but, unfortunately, not in another (66).

Movement disorders are common. Some, such as chorea and athetosis, may cause pain or psychological stress. Muscle relaxants and pain medications, including narcotics, would be advised in this circumstance. Some movement disorders can be treated with dopaminergic medication, such as levodopa-carbidopa or tetrabenazine. Therefore, a trial of either of these medications can be considered, if medically indicated.

Visual loss is common in Alpers-Huttenlocher syndrome and is usually attributable to the destruction of the calcarine cortex. This occurs at variable states of the illness. There is no specific therapy available.

Standard treatment for liver failure can include small frequent meals or continuous feeding to compensate for impaired gluconeogenesis. In addition, reducing dietary protein to a minimum, use of non-absorbable sugars to create an osmotic diarrhea, and the use of conjugating agents to treat hyperammonemia may be helpful. Because levocarnitine may have some benefit in the setting of liver failure, and because of its low toxicity, some recommend its use from the time of diagnosis (08). The use of mitochondrial supplements, including vitamins and cofactors, has not been proven to be helpful specifically in POLG spectrum disorders, but most clinicians and families prefer to start some combination of these supplements (81). An updated paper on mitochondrial supplements based on preclinical data in animal models suggests a core cocktail followed by additional components based on genetic etiology or symptoms (04). For POLG these would include folinic acid (see below) and N-acetylcysteine for the hepatopathy that might be aided by improved glutathione levels and, therefore, improved the ability to handle oxidative stress (similar to treatment of Tylenol overdose).

Reports suggest that CSF folate deficiency can occur in POLG-related disorders, so some advocate testing the CSF for folate deficiency, which requires a lumbar puncture and treatment with folinic acid (calcium leucovorin) if there is a deficiency (39). From a practical standpoint, CSF folate deficiency can occur at any point in the disease process, and monitoring CSF folate a few times a year is not practical; therefore, initiating empiric treatment at the time of diagnosis is reasonable. In a mouse model of POLG disease, forced exercise resulted in clear clinical, biochemical, and pathological benefits and clearly delayed disease manifestations. Whether exercise can help modify the disease in humans remains to be proven, and it would certainly be difficult to institute as a therapy in most patients.

There are no guidelines available to suggest the best frequency for monitoring common laboratory values, and there is no one biomarker for diagnosis, prognosis, or management of POLG-related disorders. Testing should be guided by clinical features, and the proposed schedule should be modified if the clinical course is stable. Blood counts, electrolytes, and liver enzymes are reasonable to check every few months. Once there is an elevation in liver enzymes, it is reasonable to begin monitoring liver function (preprandial serum glucose concentration), ammonium, albumin, bilirubin (free and conjugated), cholesterol, and prothrombin time/INR every few months. There is no clear evidence that either lactic acid or plasma amino acids assist in management, but some clinicians find value in these tests. Because of the potential for liver disease, it is reasonable to monitor the plasma concentration of free and total carnitine occasionally, even in those on levocarnitine supplementation. Other testing should be performed as clinically necessary, such as liver ultrasound, EEG, brainstem auditory evoked potential, swallowing evaluation, polysomnography, and pulmonary function tests. However, because subclinical hypoventilation is common, it is reasonable to obtain a baseline polysomnogram and repeat this test every year or so or if clinical suspicion arises. Finally, subclinical seizures can mimic an encephalopathic state, so any acute change in sensorium, if lasting more than a few hours, should be evaluated with an EEG or a continuous EEG, if available. There seems to be little value in using brain MRI or head CT results to monitor the progression of the disease.

Media

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Contributors

All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.

Author

Amy Goldstein MD

Dr. Goldstein of University of Pennsylvania Perelman School of Medicine and Children’s Hospital of Pittsburgh has no relevant financial relationships to disclose.
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Editor

Deepa S Rajan MD

Dr. Rajan of UPMC Children's Hospital of Pittsburgh has no relevant financial relationships to disclose.
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Former Authors

Bruce H Cohen MD
Russell P Saneto PhD DO
Tyler Reimschisel MD

Patient Profile

Age range of presentation: Alpers-Huttenlocher syndrome: 2 to 44 years (two peaks, before 4 years, and 12 to 16 years)

ANS, SANDO, MIRAS, MEMSA: 13 to 44 years

arPEO: 19 to 64 years

adPEO: 45 to 65+ years

Myocerebrohepatopathy: 1 to 23 months
Sex preponderance: male=female in younger groups

females 60%: males 40% after age 12
Heredity: autosomal dominant

autosomal recessive

heredity typical

sporadic mutations occur
Population groups selectively affected: Australia

Central Europe

New Zealand

Northwestern Europe

United States
Occupation groups selectively affected: None

ICD & OMIM codes

ICD-10: Alpers disease: G31.81
ICD-11: Mitochondrial DNA depletion syndromes: 5C53.20
OMIM: Mitochondrial DNA depletion syndrome 1 (MNGIE type); MTDPS1: #603041

Mitochondrial DNA depletion syndrome 4A (Alpers type); MTDPS4A: #203700

Mitochondrial DNA depletion syndrome 4B (MNGIE type); MTDPS4B: #613662

Polymerase, DNA, gamma; POLG: #174763

Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis; SANDO: #607459

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POLG-related mitochondrial disorders

Associated Disorders

Ataxia
Axonal sensorimotor neuropathy
Cardiomyopathy
Cataract
Chorea
- Cortical blindness
- Dementia
- Depression
- Diabetes
- Exercise intolerance
- Focal motor seizures
- Gastrointestinal dysmotility
- Generalized seizures
- Liver failure
- Migraine
- Myoclonus
- Parkinsonism
- Primary ovarian failure
- Primary testicular failure
- Proximal myopathy
- Psychosis
- Ptosis and external ophthalmoparesis (PEO or CPEO)
- Retinopathy
- Sensorineural deafness
- Sensory neuropathy/ganglionopathy
- Stroke-like episodes

Differential Diagnosis

Mutations in POLG, POLG2, and C10orf2 (previously referred to as PEO1 or TWINKLE)
Mutations in TP1, TK2, DGUOK, SUCLA1, SUCLG2, ANT1, RRM2B, and MPV17

Also Relevant

Ataxia
Epilepsia partialis continua of Kozhevnikov
Epilepsy in mitochondrial disorders
Febrile seizures
Myoclonic seizures
- Myoclonus
- Myoclonus epilepsy with ragged-red fibers
- Valproic acid

POLG-related mitochondrial disorders

Introduction

Overview

Key points

Historical note and terminology

Clinical manifestations

Presentation and course

Prognosis and complications

Clinical vignette

Biological basis

Etiology and pathogenesis

Epidemiology

Prevention

Differential diagnosis

Diagnostic workup

Electroencephalogram and seizures.

Management

Table 1. Initial Evaluation for Patients with POLG-related disorders

Table 2. Treatment Team for Patients with POLG-related disorders

Special considerations

Pregnancy

Anesthesia

Media

References

Contributors

Author

Editor

Former Authors

Patient Profile

ICD & OMIM codes

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Also in This Category

Wilson disease

White matter abnormalities in the brain

Hypermethioninemia

Multiple acyl-CoA dehydrogenase deficiency

Medium-chain acyl-CoA dehydrogenase deficiency

Long-chain fatty acid oxidation defects

Nonketotic hyperglycinemia

Leukodystrophies

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