POLG-related disorders exhibit marked phenotypic heterogeneity and frequent clinical overlap, often leading to delayed diagnosis. A precise delineation of their clinical spectrum, natural history, and the identification of reliable biomarkers is essential to improve diagnostic accuracy and guide therapeutic development. We analyzed a cohort of 34 patients with confirmed pathogenic POLG variants, assessing clinical phenotypes, molecular findings, and biomarkers (plasma growth differentiation factor-15 [GDF15] in 16, plasma neurofilament light chain [NF-L] in 14, and mitochondrial DNA [mtDNA] copy number in muscle in 16). Thirty four patients (0.6-71 years) from 33 families were included. Juvenile/adult onset (12-40 years) was the most common presentation (62%). The predominant phenotypic categories were ataxia-neuropathy spectrum ([ANS], 44%), autosomal recessive PEO-plus (arPEO-plus, 26%), and autosomal dominant PEO-plus ([adPEO-plus], 15%), with frequent phenotypic overlap. Recessive inheritance accounted for 74% of cases, with the most common variants being p.([Thr251Ile; Pro587Leu]) paired on 1 allele, p.(Ala467Thr), and p.(Trp748Ser). Dominant variants were associated with milder, primarily myopathic phenotypes. The most common dominant variant was p.(Tyr955Cys). No clear genotype-phenotype correlations were identified among recessive variants. Compared with previously reported cohorts, our patients exhibited a lower prevalence of seizures, hepatopathy, and stroke-like episodes. GDF15 was elevated in 87.5% of patients, with a mean level of 3,315 pg/mL (±1,559.79), showing no significant differences between myopathic and ANS phenotypes, supporting its role as a general biomarker of mitochondrial dysfunction. NF-L was elevated in 78.6% of tested individuals but did not correlate with phenotype or clinical severity (as per Newcastle Mitochondrial Disease Adult Scale score).On average, muscle mtDNA copy number in patients was 76% of that observed in controls, with no differences by phenotype or inheritance pattern. All but 1 patient exhibited multiple mtDNA deletions, likely representing the primary mechanism of oxidative phosphorylation dysfunction rather than mtDNA depletion. POLG-related disorders demonstrate extensive clinical variability with no consistent genotype-phenotype correlation. GDF15 and NF-L may serve as useful, though nonspecific, biomarkers of mitochondrial and neuroaxonal dysfunction, respectively. Prospective studies incorporating advanced molecular profiling are essential to establish reliable outcome measures and inform future therapeutic strategies.

Mitochondria are critical modulators of antiviral tolerance through the release of mitochondrial RNA and DNA (mtDNA and mtRNA) fragments into the cytoplasm after infection, activating virus sensors and type-I interferon (IFN-I) response1-4. The relevance of these mechanisms for mitochondrial diseases remains understudied. Here we investigated mitochondrial recessive ataxia syndrome (MIRAS), which is caused by a common European founder mutation in DNA polymerase gamma (POLG1)5. Patients homozygous for the MIRAS variant p.W748S show exceptionally variable ages of onset and symptoms5, indicating that unknown modifying factors contribute to disease manifestation. We report that the mtDNA replicase POLG1 has a role in antiviral defence mechanisms to double-stranded DNA and positive-strand RNA virus infections (HSV-1, TBEV and SARS-CoV-2), and its p.W748S variant dampens innate immune responses. Our patient and knock-in mouse data show that p.W748S compromises mtDNA replisome stability, causing mtDNA depletion, aggravated by virus infection. Low mtDNA and mtRNA release into the cytoplasm and a slow IFN response in MIRAS offer viruses an early replicative advantage, leading to an augmented pro-inflammatory response, a subacute loss of GABAergic neurons and liver inflammation and necrosis. A population databank of around 300,000 Finnish individuals6 demonstrates enrichment of immunodeficient traits in carriers of the POLG1 p.W748S mutation. Our evidence suggests that POLG1 defects compromise antiviral tolerance, triggering epilepsy and liver disease. The finding has important implications for the mitochondrial disease spectrum, including epilepsy, ataxia and parkinsonism. POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. Early-onset disease (prior to age 12 years): Liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. Juvenile/adult-onset form (age 12-40 years: Disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years): Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis. Establishing the diagnosis of a POLG-related disorder relies on clinical findings and the identification of biallelic POLG pathogenic variants on molecular genetic testing for all phenotypes except autosomal dominant progressive external ophthalmoplegia (adPEO), for which identification of a heterozygous POLG pathogenic variant on molecular genetic testing is diagnostic. Treatment of manifestations: Clinical management is largely supportive and involves standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support; and standard treatment of liver failure, epilepsy, movement abnormalities, sleep disorders, vision, and hearing issues. Surveillance: Evaluations by a multidisciplinary team of health care providers based on clinical findings; routine evaluation of growth, nutrition, oral intake, and respiratory status; monitoring of liver enzymes every three months or as clinically indicated; monitoring of epilepsy with repeat liver function tests after introduction of any new anti-seizure medication. Agents/circumstances to avoid: Valproic acid (Depakene®) and sodium divalproate (divalproex) (Depakote®) because of the risk of precipitating and/or accelerating liver disease. Early-onset and juvenile/adult-onset POLG-related disorders are typically caused by biallelic pathogenic variants and inherited in an autosomal recessive manner. Late-onset PEO may be caused by a heterozygous POLG pathogenic variant and inherited in an autosomal dominant manner. Autosomal recessive inheritance: If both parents are known to be heterozygous for a POLG pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting biallelic pathogenic variants and being affected, a 50% chance of being heterozygous, and a 25% chance of inheriting neither of the familial POLG pathogenic variants. Heterozygous sibs of a proband with an autosomal recessive POLG-related disorder are typically asymptomatic. Once the POLG pathogenic variants have been identified in an affected family member, testing for at-risk family members is possible. Autosomal dominant inheritance: Most individuals with PEO caused by a heterozygous POLG pathogenic variant (i.e., adPEO) have an affected parent, although age of onset and severity of presentation can vary greatly from generation to generation. Each child of an individual with POLG-related adPEO has a 50% chance of inheriting the pathogenic variant. Once the POLG pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing for POLG-related disorders is possible.

DNA polymerase gamma (POLG)-related disorders are a group of rare neurodegenerative mitochondrial diseases caused by pathogenic variants in POLG, the gene encoding POLG. Patients may experience a range of signs and symptoms, including seizures, vision loss, myopathy, neuropathy, developmental impairment or regression, and liver failure. The diseases follow a progressive, degenerative course, with most affected individuals dying within 3 months-12 years of diagnosis. At present, there are no effective treatments for POLG-related disorders. In this study we report the interim 6-month data from a long term open-label, single arm phase 2 trial, in which we assessed the safety and efficacy of combination therapy with deoxycytidine and deoxythymidine (dC/dT) in children with POLG-related disorders. dC/dT was given enterally in powder form, dissolved in water. The primary outcome measures included Newcastle Mitochondrial Disease Scale (NMDS) score, serum growth differentiation factor 15 (GDF-15; a biomarker of mitochondrial dysfunction), electroencephalography (EEG), seizure diary, and blood and urine tests to assess end organ and mitochondrial function. Secondary outcome measures included recording of all adverse events to evaluate the safety of the intervention. The trial is registered with ClinicalTrials.gov, NCT04802707 (https://clinicaltrials.gov/ct2/show/NCT04802707). Data were collected from 14 October, 2021 to 13 December, 2023. We present 6-month interim data from the first ten people with POLG-related disorders enrolled in the trial, six with Alpers-Huttenlocher syndrome, two with ataxia-neuropathy spectrum, and two who do not fit into a classical POLG-related phenotype. During the 6 months of treatment, NMDS score improved from a mean of 27.3 at baseline to 20.7 at 6 months (estimated difference 6.0; 95% CI 2.5-∞). GDF-15 values remained stable or decreased in all patients; the mean decreased from 1031 pg/ml to 729 pg/ml (estimated difference 200; 95% CI 12-∞). 8/10 patients had abnormal baseline EEG; improvement in EEG was seen in 5 of these 8. There were no significant changes in other blood and urine testing. Regarding adverse events, two patients experienced diarrhea that spontaneously resolved. dC/dT is a promising treatment option for people with POLG-related disorders. Further research is needed to assess the long-term safety and efficacy in POLG-related disorders, as well as safety and efficacy in other mitochondrial DNA depletion disorders. This study was primarily funded by the Liam Foundation, with additional funding from the Savoy Foundation, Grand Défi Pierre Lavoie Foundation, and Fonds de Recherche du Québec - Santé.

Tropical ataxic neuropathy (TAN) is characterised by ataxic polyneuropathy, degeneration of the posterior columns and pyramidal tracts, optic atrophy, and sensorineural hearing loss. It has been attributed to nutritional/toxic etiologies, but evidence for the same has been equivocal. TAN shares common clinical features with inherited neuropathies and mitochondrial disorders, it may be hypothesised that genetic abnormalities may underlie the pathophysiology of TAN. This study aimed to establish evidence for mitochondrial dysfunction by adopting an integrated biochemical and multipronged genetic analysis. Patients (n = 65) with chronic progressive ataxic neuropathy with involvement of visual and/or auditory pathways underwent deep phenotyping, genetic studies including mitochondrial DNA (mtDNA) deletion analysis, mtDNA and clinical exome sequencing (CES), and respiratory chain complex (RCC) assay. The phenotypic characteristics included dysfunction of visual (n = 14), auditory (n = 12) and visual + auditory pathways (n = 29). Reduced RCC activity was present in 13 patients. Mitochondrial DNA deletions were noted in five patients. Sequencing of mtDNA (n = 45) identified a homoplasmic variant (MT-ND6) and a heteroplasmic variant (MT-COI) in one patient each. CES (n = 45) revealed 55 variants in nuclear genes that are associated with neuropathy (n = 27), deafness (n = 7), ataxia (n = 4), and mitochondrial phenotypes (n = 5) in 36 patients. This study provides preliminary evidence that TAN is associated with a spectrum of genetic abnormalities, including those associated with mitochondrial dysfunction, which is in contradistinction from the prevailing hypothesis that TAN is related to dietary toxins. Analysing the functional relevance of these genetic variants may improve the understanding of the pathogenesis of TAN.