POLG-related Disease Research Articles

Zebrafish polg2 knock-out recapitulates human POLG-disorders; implications for drug treatment.

The human mitochondrial DNA polymerase gamma is a holoenzyme, involved in mitochondrial DNA (mtDNA) replication and maintenance, composed of a catalytic subunit (POLG) and a dimeric accessory subunit (POLG2) conferring processivity. Mutations in POLG or POLG2 cause POLG-related diseases in humans, leading to a subset of Mendelian-inherited mitochondrial disorders characterized by mtDNA depletion (MDD) or accumulation of multiple deletions, presenting multi-organ defects and often leading to premature death at a young age. Considering the paucity of POLG2 models, we have generated a stable zebrafish polg2 mutant line (polg2ia304) by CRISPR/Cas9 technology, carrying a 10-nucleotide deletion with frameshift mutation and premature stop codon. Zebrafish polg2 homozygous mutants present slower development and decreased viability compared to wild type siblings, dying before the juvenile stage. Mutants display a set of POLG-related phenotypes comparable to the symptoms of human patients affected by POLG-related diseases, including remarkable MDD, altered mitochondrial network and dynamics, and reduced mitochondrial respiration. Histological analyses detected morphological alterations in high-energy demanding tissues, along with a significant disorganization of skeletal muscle fibres. Consistent with the last finding, locomotor assays highlighted a decreased larval motility. Of note, treatment with the Clofilium tosylate drug, previously shown to be effective in POLG models, could partially rescue MDD in Polg2 mutant animals. Altogether, our results point at zebrafish as an effective model to study the etiopathology of human POLG-related disorders linked to POLG2, and a suitable platform to screen the efficacy of POLG-directed drugs in POLG2-associated forms.

Astrocytic pathology in Alpers’ syndrome

Refractory epilepsy is the main neurological manifestation of Alpers’ syndrome, a severe childhood-onset mitochondrial disease caused by bi-allelic pathogenic variants in the mitochondrial DNA (mtDNA) polymerase gamma gene (POLG). The pathophysiological mechanisms underpinning neuronal hyperexcitabilty leading to seizures in Alpers’ syndrome remain unknown. However, pathological changes to reactive astrocytes are hypothesised to exacerbate neural dysfunction and seizure-associated cortical activity in POLG-related disease. Therefore, we sought to phenotypically characterise astrocytic pathology in Alpers’ syndrome. We performed a detailed quantitative investigation of reactive astrocytes in post-mortem neocortical tissues from thirteen patients with Alpers’ syndrome, eight neurologically normal controls and five sudden unexpected death in epilepsy (SUDEP) patients, to control for generalised epilepsy-associated astrocytic pathology. Immunohistochemistry to identify glial fibrillary acidic protein (GFAP)-reactive astrocytes revealed striking reactive astrogliosis localised to the primary visual cortex of Alpers’ syndrome tissues, characterised by abnormal-appearing hypertrophic astrocytes. Phenotypic characterisation of individual GFAP-reactive astrocytes demonstrated decreased abundance of mitochondrial oxidative phosphorylation (OXPHOS) proteins and altered expression of key astrocytic proteins including Kir4.1 (subunit of the inwardly rectifying K+ ion channel), AQP4 (astrocytic water channel) and glutamine synthetase (enzyme that metabolises glutamate). These phenotypic astrocytic changes were typically different from the pathology observed in SUDEP tissues, suggesting alternative mechanisms of astrocytic dysfunction between these epilepsies. Crucially, our findings provide further evidence of occipital lobe involvement in Alpers’ syndrome and support the involvement of reactive astrocytes in the pathogenesis of POLG-related disease.

Model systems informing mechanisms and drug discovery: a systematic review of POLG-related disease models

Introduction Pathogenic variants in the gene encoding the catalytic subunit of DNA polymerase gamma (POLG), comprise an important single-gene cause of inherited mitochondrial disorders. Clinical manifestations are now recognised as an array of overlapping clinical features rather than discrete syndromes as originally conceptualised. Animal and cellular models have been used to address numerous scientific questions, from basic science to the development and assessment of novel therapies. Here, we sought to perform a systematic review of the existing models used in mitochondrial research and their effectiveness in recapitulating POLG-related disease. Methods Four databases were searched from inception to May 31, 2022: MEDLINE, Scopus, Web of Science, and Cochrane Review. Original articles available in English, reporting the use of a model system designed to recapitulate POLG­-related disease, or related pathogenicity, were eligible for inclusion. Risk of bias and the methodological quality of articles were assessed by an adapted version of the Cochrane Risk of Bias Tool, with the quality of evidence synthesized across each model. Results A total of 55 articles, including seven model organisms (Human, yeast [Saccharomyces cerevisiae and Schizosaccharomyces pombe], Drosophila, Mouse, Nematoda, and Zebrafish) with 258 distinct variants were included. Of these, 66% (N=38) of articles recapitulated mitochondrial DNA (mtDNA) depletion and 42% (N=23) recapitulated POLG-related disease. Thirty-three percent of articles (N=18/55) utilised tissue-specific models of POLG-related dysfunction, while 13% (N=7) investigated the effect of potential therapeutics in POLG-related mitochondrial disorders. Discussion The available evidence supporting the ability of models for POLG-related disease to recapitulate molecular mechanisms and phenotype is limited, inconsistent and of poor methodologic quality. Further success in examining and translating novel therapies into effective treatments will be enhanced by the availability of more robust models that better recapitulate the entire spectrum of POLG-related disease. PROSPERO registration: CRD42021234883

Nutritional Interventions for Patients with Mitochondrial POLG-Related Diseases: A Systematic Review on Efficacy and Safety.

Ketogenic diet is recommended as a treatment to reduce seizure frequency in patients with intractable epilepsy. The evidence and safety results are sparse for diet interventions in patients with pathogenic polymerase gamma (POLG) variants and intractable epilepsy. The aim of this systematic review is to summarize the efficacy of diet treatment on seizure frequency, clinical symptoms, and potential deleterious effect of liver involvement in patients with mitochondrial diseases caused by pathogenic POLG variants. Literature was searched in PubMed, Embase; and Cochrane in April 2022; no filter restrictions were imposed. The reference lists of retrieved studies were checked for additional literature. Eligibility criteria included verified pathogenic POLG variant and diet treatment. Overall, 880 studies were identified, providing eight case-reports representing nine patients eligible for inclusion. In eight of nine cases, clinical symptoms were improved; six out of nine cases reported improvements in seizure frequency. However, increasing levels of liver enzymes after initiating ketogenic diet were found in four of the nine cases, with one case revealing decreased levels of liver enzymes after initiating long-chain triglyceride restriction. Viewed together, the studies imply that ketogenic diet can have a positive impact on seizure frequency, but may induce progression of liver impairment in patients with pathogenic POLG variants.

Neuropathic Pain as Main Manifestation of POLG-Related Disease: A Case Report

Mutations in nuclear-encoded genes that are involved in mitochondrial DNA replication and maintenance (e.g., POLG) have been associated with chronic progressive external ophthalmoplegia (CPEO) phenotype. These nuclear genome mutations may lead to multiple mitochondrial DNA deletions or mitochondrial DNA depletion. On the other hand, primary genetic defects of mitochondrial DNA (such as single large-scale deletion or point mutations) have also been associated with the CPEO phenotype. Chronic progressive external ophthalmoplegia (CPEO) may be a manifestation of specific syndromes that, when clinically recognized, prompt clinicians to investigate specific genetic defects. Thus, CPEO, as part of Kearns Sayre syndrome, suggests the presence of a large-scale deletion of mitochondrial DNA. However, in pure CPEO or CPEO plus phenotypes, it is more difficult to know whether causative genetic defects affect the nuclear or mitochondrial DNA. Here, we present a patient with a long-standing history of CPEO plus phenotype, in whom the sequencing of mitochondrial DNA from skeletal muscle was normal, and no other genetic defect was suspected at first. At the time of our evaluation, the presence of polyneuropathy and neuropathic pain prompted us to investigate nuclear genetic defects and, specifically, mutations in the POLG gene. Thus, the sequencing of the POLG gene revealed p.Thr251Ile and p.Pro587Leu mutations in one allele, and p.Ala467Thr mutation in another allele. Although one would expect that mutations in POLG lead to multiple mitochondrial DNA deletions or depletion (loss of copies), the absence of mitochondrial DNA abnormalities in tissue may be explained by heteroplasmy, a lack or no significant involvement of biopsied tissue, or a sampling bias. So, the absence of secondary mitochondrial DNA alterations should not discourage clinicians from further investigating mutations in nuclear-encoded genes. Lastly, mitochondrial point mutations and single mitochondrial DNA deletions very rarely cause CPEO associated with polyneuropathy and neuropathic pain, and POLG-related disease should be considered in this scenario, instead.

Nicotinamide Riboside and Metformin Ameliorate Mitophagy Defect in Induced Pluripotent Stem Cell-Derived Astrocytes With POLG Mutations.

Mitophagy specifically recognizes and removes damaged or superfluous mitochondria to maintain mitochondrial homeostasis and proper neuronal function. Defective mitophagy and the resulting accumulation of damaged mitochondria occur in several neurodegenerative diseases. Previously, we showed mitochondrial dysfunction in astrocytes with POLG mutations, and here, we examined how POLG mutations affect mitophagy in astrocytes and how this can be ameliorated pharmacologically. Using induced pluripotent stem cell (iPSC)-derived astrocytes carrying POLG mutations, we found downregulation of mitophagy/autophagy-related genes using RNA sequencing-based KEGG metabolic pathway analysis. We confirmed a deficit in mitochondrial autophagosome formation under exogenous stress conditions and downregulation of the mitophagy receptor p62, reduced lipidation of LC3B-II, and decreased expression of lysosome protein lysosomal-associated membrane protein 2A (LAMP2A). These changes were regulated by the PINK1/Parkin pathway and AKT/mTOR/AMPK/ULK1 signaling pathways. Importantly, we found that double treatment with nicotinamide riboside (NR) and metformin rescued mitophagy defects and mitochondrial dysfunction in POLG-mutant astrocytes. Our findings reveal that impaired mitophagy is involved in the observed mitochondrial dysfunction caused by POLG mutations in astrocytes, potentially contributing to the phenotype in POLG-related diseases. This study also demonstrates the therapeutic potential of NR and metformin in these incurable mitochondrial diseases.

DNA polymerase gamma mutations that impair holoenzyme stability cause catalytic subunit depletion.

Mutations in POLG, encoding POLγA, the catalytic subunit of the mitochondrial DNA polymerase, cause a spectrum of disorders characterized by mtDNA instability. However, the molecular pathogenesis of POLG-related diseases is poorly understood and efficient treatments are missing. Here, we generate the PolgA449T/A449T mouse model, which reproduces the A467T change, the most common human recessive mutation of POLG. We show that the mouse A449T mutation impairs DNA binding and mtDNA synthesis activities of POLγ, leading to a stalling phenotype. Most importantly, the A449T mutation also strongly impairs interactions with POLγB, the accessory subunit of the POLγ holoenzyme. This allows the free POLγA to become a substrate for LONP1 protease degradation, leading to dramatically reduced levels of POLγA in A449T mouse tissues. Therefore, in addition to its role as a processivity factor, POLγB acts to stabilize POLγA and to prevent LONP1-dependent degradation. Notably, we validated this mechanism for other disease-associated mutations affecting the interaction between the two POLγ subunits. We suggest that targeting POLγA turnover can be exploited as a target for the development of future therapies.

A characteristic occipital epileptiform EEG pattern in ADCK3-related mitochondrial disease.

ADCK3-related disease is a mitochondrial disorder associated with an abnormality of coenzyme Q10 metabolism. Ataxia and epilepsy are common, and the phenotype overlaps with other mitochondrial encephalopathies, particularly POLG-related disease. CoQ10 supplementation may be beneficial. We have noted a remarkable epileptiform pattern in ADCK3-related encephalopathy, and since EEG studies in this rare condition are limited, we wished to assess the evolution of EEG characteristics in patients with this disorder. All EEG recordings of the four known patients from Mid-Norway were systematically reviewed. EEG graphoelements were classified according to the standardized computer-based organized reporting of EEG (SCORE) and international glossary terms. The evolution of EEG features was assessed. A total of 96 recordings spanning over 15-32 years were available, with a mean of 24 per patient (range: 17-28). Altogether, 50 digital recordings were reviewed, including four long-term and 46 selected paper segments. In three patients, EEG showed prominent bilateral asynchronous and synchronous epileptiform discharges in occipital and posterior-temporal regions. This intense activity included multiple epileptiform graphoelements, which occurred continuously, nearly continuously or in prolonged runs. The findings remained stable over many years. Although the number of patients is small, we suggest that interictal EEG findings of continuous/nearly continuous bi-occipital spike-waves may serve as a biomarker for this potentially treatable condition. This peculiar EEG pattern might help to differentiate ADCK3-related disease from the more common POLG-related disease, which is usually characterized by lateralized or focal slowing with more sporadic epileptiform elements of similar topography.

POLG-associated ataxias can represent a substantial part of recessive and sporadic ataxias in adults

ObjectiveWe aimed to analyze prevalence, clinical, and genetic characteristics of the POLG-associated ataxias in a cohort of recessive and sporadic ataxias in adults with previously excluded acquired ataxias. MethodsWe did a retrospective analysis of the medical records of 74 patients older than 18 years referred to the Research Center of Neurology between 2012 and 2019 with progressive sporadic or autosomal recessive ataxia with onset before 50 years of age. A stepwise approach in genetic testing was used. All patients with genetically confirmed POLG-associated disorders underwent clinical, biochemical, electrophysiological, and neuroimaging assessments. ResultsIn our cohort of 74 adult patients with autosomal recessive and sporadic ataxias, POLG-related disease was identified in 11 individuals (14.9 %). The median age of onset was 30 years. One patient had a positive family history. The core clinical syndrome included external ophthalmoparesis, cerebellar signs, and sensory neuropathy. In all patients, the Montreal Cognitive Assessment score was less than 26. All but 3 patients had specific brain MRI changes. Mutation spectrum of the POLG gene in our cohort is discussed. ConclusionOur study shows that POLG-associated ataxias comprise a significant part of the recessive and sporadic ataxias in adults in the Russian population after excluding acquired causes of ataxic disorders. We suggest first screening patients with specific clinical and (or) neuroimaging features for the population-specific common POLG mutations, followed by the NGS panel testing where necessary. In future clinical studies, thorough cognitive and neuropsychiatric profiling is needed to complete the phenotype of the POLG-related disorders.

Efficient clofilium tosylate-mediated rescue of POLG-related disease phenotypes in zebrafish

The DNA polymerase gamma (Polg) is a nuclear-encoded enzyme involved in DNA replication in animal mitochondria. In humans, mutations in the POLG gene underlie a set of mitochondrial diseases characterized by mitochondrial DNA (mtDNA) depletion or deletion and multiorgan defects, named POLG disorders, for which an effective therapy is still needed. By applying antisense strategies, ENU- and CRISPR/Cas9-based mutagenesis, we have generated embryonic, larval-lethal and adult-viable zebrafish Polg models. Morphological and functional characterizations detected a set of phenotypes remarkably associated to POLG disorders, including cardiac, skeletal muscle, hepatic and gonadal defects, as well as mitochondrial dysfunctions and, notably, a perturbed mitochondria-to-nucleus retrograde signaling (CREB and Hypoxia pathways). Next, taking advantage of preliminary evidence on the candidate molecule Clofilium tosylate (CLO), we tested CLO toxicity and then its efficacy in our zebrafish lines. Interestingly, at well tolerated doses, the CLO drug could successfully rescue mtDNA and Complex I respiratory activity to normal levels, even in mutant phenotypes worsened by treatment with Ethidium Bromide. In addition, the CLO drug could efficiently restore cardio-skeletal parameters and mitochondrial mass back to normal values. Altogether, these evidences point to zebrafish as a valuable vertebrate organism to faithfully phenocopy multiple defects detected in POLG patients. Moreover, this model represents an excellent platform to screen, at the whole-animal level, candidate molecules with therapeutic effects in POLG disorders.

Elevated cerebrospinal fluid protein in POLG-related epilepsy: Diagnostic and prognostic implications.

Epilepsy is common in individuals with mutations in POLG, the gene encoding the catalytic subunit of the mitochondrial DNA polymerase gamma. Early recognition and aggressive seizure management are crucial for patient survival. Disruption of the blood-brain barrier (BBB) is implicated in various neurological disorders including epilepsy. The aim of this study was to assess whether POLG-related disease is associated with BBB dysfunction and what clinical implications this has for patients. Our retrospective study used data from 83 patients with pathogenic POLG mutations from 4 countries--Norway, Sweden, Finland, and the United Kingdom. Data were collected using a structured questionnaire. We used the presence of raised cerebrospinal fluid (CSF) protein and a raised CSF/serum ratio of albumin (Q-alb) to evaluate the integrity of the blood-CSF barrier. Raised CSF protein was found in 70% of patients (n = 58/83) and appeared to be associated with the most severe phenotypes. In those in whom it was measured, the Q-alb ratio was markedly elevated (n = 18). The majority of those with epilepsy (n = 50/66, 76%) had raised CSF protein, and this preceded seizure debut in 75% (n = 15/20). The median survival time from symptom onset for those with raised CSF protein was decreased (13 months) compared to those with normal CSF protein (32 months). Our results indicate that there is disruption of the BBB in POLG-related disease, as evidenced by a raised CSF protein and Q-alb ratio. We also find that raised CSF protein is a common finding in patients with POLG disease. Our data suggest that the presence of BBB dysfunction predicts a poorer outcome, and elevated CSF protein may therefore be an additional biomarker both for early diagnosis and to identify those at high risk of developing epilepsy.

Defining how mitochondrial DNA stress regulates innate immune responses

Abstract Polymerase gamma (pol γ) is nuclear-encoded protein responsible for mtDNA replication. POLG mutations are recognized as a significant cause of primary mitochondrial diseases and are implicated in a range of disorders. Several mouse models of POLG-related disease have been reported, the most well studied is the POLG-mutator mouse. Animals homozygous for the mutant alleles exhibit elevated mtDNA instability, and POLG-mutator mice exhibit pathology that mirrors various aspects of human POLG-related diseases. The literature has few descriptions about the immune system of POLG-mutator mice or patients with POLG-related disorders, despite the fact that aberrant immune responses have been implicated in the pathology of diseases with similar phenotypes. We have found that mitochondrial dysfunction, resulting from POLG mutation, alters innate immune responses at baseline and after challenge, which we hypothesize may exacerbate pathology in POLG-related mitochondrial disorders. To explore this hypothesis, we have stimulated macrophages from WT and POLG-mutator mice with various innate immune stimuli, and have uncovered that POLG-mutator macrophages display alterations in type I interferon responses, but strikingly, exhibit profound decreases in inflammasome-dependent interleukin-1 beta (IL-1b) secretion. Ongoing work will determine how mitochondrial dysfunction impairs inflammasome activation and IL-1b secretion in order to appreciate how aberrant innate immune responses influence POLG-related disease progression.

The presence of anaemia negatively influences survival in patients with POLG disease.

Mitochondria play an important role in iron metabolism and haematopoietic cell homeostasis. Recent studies in mice showed that a mutation in the catalytic subunit of polymerase gamma (POLG) was associated with haematopoietic dysfunction including anaemia. The aim of this study was to analyse the frequency of anaemia in a large cohort of patients with POLG related disease. We conducted a multi-national, retrospective study of 61 patients with confirmed, pathogenic biallelic POLG mutations from six centres, four in Norway and two in the United Kingdom. Clinical, laboratory and genetic data were collected using a structured questionnaire. Anaemia was defined as an abnormally low haemoglobin value adjusted for age and sex. Univariate survival analysis was performed using log-rank test to compare differences in survival time between categories. Anaemia occurred in 67% (41/61) of patients and in 23% (14/61) it was already present at clinical presentation. The frequency of anaemia in patients with early onset disease including Alpers syndrome and myocerebrohepatopathy spectrum (MCHS) was high (72%) and 35% (8/23) of these had anaemia at presentation. Survival analysis showed that the presence of anaemia was associated with a significantly worse survival (P=0.004). Our study reveals that anaemia can be a feature of POLG-related disease. Further, we show that its presence is associated with significantly worse prognosis either because anaemia itself is impacting survival or because it reflects the presence of more serious disease. In either case, our data suggests anaemia is a marker for negative prognosis.

The clinical spectrum and natural history of early-onset diseases due to DNA polymerase gamma mutations.

PurposeMutations in POLG, the most common single-gene cause of inherited mitochondrial disease, are diagnostically challenging owing to clinical heterogeneity and overlap between syndromes. We aimed to improve the clinical recognition of POLG-related disorders in the pediatric population.MethodsWe performed a multinational, phenotype: genotype study using patients from three centers, two Norwegian and one from the United Kingdom. Patients with age at onset <12 years and confirmed pathogenic biallelic POLG mutations were considered eligible.ResultsA total of 27 patients were identified with a median age at onset of 11 months (range 0.6-80.4). The majority presented with global developmental delay (n=24/24, 100%), hypotonia (n=22/23, 96%) and faltering growth (n=24/27, 89%). Epilepsy was common, but notably absent in patients with the myocerebrohepatopathy spectrum phenotype. We identified two novel POLG gene mutations.ConclusionOur data suggest that POLG-related disease should be suspected in any child presenting with diffuse neurological symptoms. Full POLG sequencing is recommended since targeted screening may miss mutations. Finally, we simplify the classification of POLG-related disease in children using epilepsy as the crucial defining element; we show that Alpers and myocerebrohepatopathy spectrum follow different outcomes and that they manifest different degrees of respiratory chain dysfunction.

Oxidative stress-induced apoptosis in peripheral blood lymphocytes from patients with POLG-related disorders

BackgroundPOLG-related disorders are a group of heterogeneous diseases characterized by an overlapping clinical presentations and associated with mutations in the POLG gene. POLG codes for the catalytic subunit of mitochondrial polymerase gamma (POLG), essential for mitochondrial DNA (mtDNA) replication and repair.Studies on mutator POLG mice showed an increase in oxidative stress and apoptosis.In this regard we analysed the involvement of POLG mutations in the apoptotic regulation, evaluating apoptosis in peripheral blood lymphocytes (PBLs) from patients with POLG-related diseases. MethodsCells were cultured under basal conditions and with 2-deoxy-d-ribose (dRib), a reducing sugar that induces apoptosis by oxidative stress. Apoptosis rate was assessed by flow cytometry. Phosphatidylserine translocation, mitochondrial membrane depolarization and caspase 3 activation were also analysed. ResultsOur data showed higher percentages of apoptosis after dRib treatment in patients with POLG mutations than in controls, while under basal culture conditions, apoptosis levels were similar in the two groups. ConclusionsCells with POLG mutations are more sensitive than control cells to oxidative stress-induced apoptosis, confirming that mtDNA mutations may have a role in mitochondrial apoptosis pathway. We also suggest that redox state homeostasis may play a crucial role in phenotypic expression of POLG-related diseases.

Solid organ transplantation in primary mitochondrial disease: Proceed with caution.

Solid organ transplants are rarely performed in both adult and pediatric patients with primary mitochondrial disease. Poor outcomes have been described in case reports and small case series. It is unclear whether the underlying genetic disease has a significant impact on post-transplant morbidity and mortality. Data were obtained for 35 patients from 17 Mitochondrial Disease Centers across North America, the United Kingdom and Australia. Patient outcomes were noted after liver, kidney or heart transplantation. Excluding patients with POLG-related disease, post-transplant survival approached or met outcomes seen in non-mitochondrial disease transplant patients. The majority of mitochondrial disease patients did not have worsening of their mitochondrial disease within 90-days post-transplant. Post-transplant complications, including organ rejection, were not a common occurrence and were generally treatable. Many patients did not have a mitochondrial disease considered or diagnosed prior to transplantation. In conclusion, patients with mitochondrial disease in this cohort generally tolerated solid-organ transplantation. Such patients may not need to be excluded from transplant solely for their mitochondrial diagnosis; additional caution may be needed for patients with POLG-related disease. Transplant teams should be aware of mitochondrial disease as an etiology for organ-failure and consider appropriate consultation in patients without a known cause of their symptoms.

Combined use of Saccharomyces cerevisiae, Caenorhabditis elegans and patient fibroblasts leads to the identification of clofilium tosylate as a potential therapeutic chemical against POLG-related diseases.

Mitochondria are organelles that have their own DNA (mitochondrial DNA, mtDNA) whose maintenance is necessary for the majority of ATP production in eukaryotic cells. Defects in mtDNA maintenance or integrity are responsible for numerous diseases. The DNA polymerase γ (POLG) ensures proper mtDNA replication and repair. Mutations in POLG are a major cause of mitochondrial disorders including hepatic insufficiency, Alpers syndrome, progressive external ophthalmoplegia, sensory neuropathy and ataxia. Mutations in POLG are also associated with parkinsonism. To date, no effective therapy is available. Based on the conservation of mitochondrial function from yeast to human, we used Saccharomyces cerevisiae and Caenorhabditis elegans as first pass filters to identify a chemical that suppresses mtDNA instability in cultured fibroblasts of a POLG-deficient patient. We showed that this unsuspected compound, clofilium tosylate (CLO), belonging to a class of anti-arrhythmic agents, prevents mtDNA loss of all yeast mitochondrial polymerase mutants tested, improves behavior and mtDNA content of polg-1-deficient worms and increases mtDNA content of quiescent POLG-deficient fibroblasts. Furthermore, the mode of action of the drug seems conserved as CLO increases POLG steady-state level in yeast and human cells. Two other anti-arrhythmic agents (FDA-approved) sharing common pharmacological properties and chemical structure also show potential benefit for POLG deficiency in C. elegans. Our findings provide evidence of the first mtDNA-stabilizing compound that may be an effective pharmacological alternative for the treatment of POLG-related diseases.

A diagnostic flow chart for POLG-related diseases based on signs sensitivity and specificity

ObjectiveDiseases due to mutations of POLG gene, encoding the mitochondrial DNA polymerase, are reputed to have very diverse clinical presentations and have been proposed to cause up to 25% adult...

POLG: a cause of intractable epilepsy

Occasionally our Journal Club is driven by the identification of a single clinical story or diagnosis encountered in routine clinical practice which sparks interest and identifies a need to develop knowledge of the disorder and a better understanding of what we are able to offer our patient(s). This month’s article is an example of this dynamic and reflects a prolonged diagnostic process in a patient who eventually was found to have a form of mitochondrial disease. This is a commonly proposed, but rarely substantiated, diagnostic consideration when faced with a patient with multiple, seemingly unrelated, neurological symptoms and a series of negative tests and continues to be a favourite of clinico pathological conferences. The ambiguous acronyms, diagnostic challenges, and clinical profiles of these disorders do not result in straightforward clinical management and continue to present a significant challenge for neurologists. However, our understanding of this group of disorders is continuing to evolve and although, for the time being, mitochondrial diseases remain incurable, there are important ways in which initial front line management can influence disease course and prognosis and are, therefore, of relevance to general neurologists. Mutations in the nuclear DNA encoded POLG gene on chromosome 15 have now emerged as the commonest cause of mitochondrial disease, and account for up to 25 % of cases. First described in 2001, there are now over 150 reported pathogenic mutations in POLG and the carrier frequency of the commonest mutation, A467T, is as high as 1 % in selected European populations. The most striking clinical enigma of POLG-related disease is that identical mutations can exhibit different Mendelian inheritance patterns and produce a range of phenotypes. As a result, a continuous spectrum of clinical features has been reported within families and case series, although several distinct phenotypes have been described. The commonest dominantly inherited phenotype is autosomal dominant progressive external opthalmoplegia (adPEO). Recessive phenotypes include Alpers-Huttenlocher syndrome (childhood-onset severe epilepsy and liver failure), syndrome of sensory ataxic neuropathy with opthalmoparesis (SANDO), and mitochondrial recessive ataxia syndrome (MIRAS). Furthermore, perhaps the most important clinical management point to emerge from experience to date is that in POLG disease patients with epilepsy, sodium valproate can cause fatal liver failure and should be avoided. In this month’s Journal Club we discuss three papers on POLG disease and their implications for clinical practice. The first paper explores the phenotype of the commonest POLG mutation; the second examines the sensory neuropathy of POLG disease, which is often overlooked; and the last asks how many undiagnosed POLG disease patients may be out there in our paediatric intractable epilepsy population.

Clinical and Molecular Features of POLG-Related Mitochondrial Disease

The inability to replicate mitochondrial genomes (mtDNA) by the mitochondrial DNA polymerase (pol γ) leads to a subset of mitochondrial diseases. Many mutations in POLG, the gene that encodes pol γ, have been associated with mitochondrial diseases such as myocerebrohepatopathy spectrum (MCHS) disorders, Alpers-Huttenlocher syndrome, myoclonic epilepsy myopathy sensory ataxia (MEMSA), ataxia neuropathy spectrum (ANS), and progressive external ophthalmoplegia (PEO). This chapter explores five important topics in POLG-related disease: (1) clinical symptoms that identify and distinguish POLG-related diseases, (2) molecular characterization of defects in polymerase activity by POLG disease variants, (3) the importance of holoenzyme formation in disease presentation, (4) the role of pol γ exonuclease activity and mutagenesis in disease and aging, and (5) novel approaches to therapy and avoidance of toxicity based on primary research in pol γ replication.