Mitochondrial genetic diseases are complex disorders that impair cellular energy production, leading to diverse clinical manifestations across multiple organ systems. These diseases arise from mutations in either mitochondrial DNA or nuclear DNA. Among nuclear DNA-related cases, mutations in POLG and POLG2, which encode subunits of mitochondrial DNA polymerase γ, are particularly significant, causing conditions such as Alpers-Huttenlocher syndrome and progressive external ophthalmoplegia. Model organisms have been instrumental in elucidating POLG-related disease mechanisms and advancing therapeutic strategies. Saccharomyces cerevisiae (budding yeast) provided insights into fundamental mitochondrial functions, while Caenorhabditis elegans (roundworm) helped explore POLG's roles in multicellular organisms. Drosophila melanogaster (fruit fly) has been pivotal in studying neurological aspects, and Mus musculus (mouse) models contributed to understanding systemic effects in mammals. Recently, Danio rerio (zebrafish) has emerged as a promising vertebrate model for drug screening, due to its optical transparency and genetic tractability. Each model system offers unique advantages, collectively bridging the gap between basic research and clinical applications. This review will examine in vivo models used in POLG disorder research, highlighting their contributions to understanding disease mechanisms and therapeutic advancements.

The nuclear-encoded enzyme polymerase gamma (Pol-γ) is crucial in the replication of the mitochondrial genome (mtDNA), which in turn is vital for mitochondria and hence numerous metabolic processes and energy production in eukaryotic cells. Variants in the POLG gene, which encodes the catalytic subunit of Pol-γ, can significantly impair Pol-γ enzyme function. Pol-γ-associated disorders are referred to as POLG-spectrum disorders (POLG-SDs) and are mainly autosomal-recessively inherited. Clinical manifestations include muscle weakness and fatigue, and severe forms of the disease can lead to premature death in infancy, childhood, and early adulthood, often associated with seizures, liver failure, or intractable epilepsy. Here, we analyzed fibroblasts from a compound heterozygous patient with the established pathogenic variant c.2419C>T; p.(Arg807Cys) and a previously undescribed variant c.678G>C; p.(Gln226His) with a clinical manifestation compatible with POLG-SDs, sensory ataxic neuropathy, and infantile muscular atrophy. We conducted a battery of functional studies for Pol-γ and mitochondrial dysfunction on the patient's fibroblasts, to test whether the novel variant c.678G>C; p.(Gln226His) may be causative in human disease. We analyzed skin-derived fibroblasts in comparison to a first-degree relative (the mother of the patient), an asymptomatic carrier harboring only the established c.2419C>T; p.(Arg807Cys) mutation. Assessments of mitochondrial function included measurements of mtDNA content, mRNA levels of mitochondrial genes, mitochondrial mass, and mitochondrial morphology. A 13-year-old male presented with symptoms starting at three years of age, including muscle weakness and atrophy in the lower extremities and facial muscles, which later extended to the upper limbs, voice, and back muscles, without further progression. The patient also reported fatigue and muscle pain after physical activity, with no sensory deficits. Extensive diagnostic tests such as electromyography, nerve conduction studies, muscle biopsy, and MRI were unremarkable. Exome sequencing revealed that he carried the compound heterozygous variants in POLG c.678G>C; p.(Gln226His) and c.2419C>T; p.(Arg807Cys), but no other potential genetic pathogenic causes. In comparison to a first-degree relative (his mother) who only carried the c.2419C>T; p.(Arg807Cys) pathogenic mutation, in vitro analyses revealed a significant reduction in mtDNA content (~50%) and mRNA levels of mtDNA-encoded proteins. Mitochondrial mass was reduced by approximately 20%, and mitochondrial interconnectivity within cells was impaired, as determined by fluorescence microscopy and mitochondrial staining. Our findings suggest that the c.678G>C; p.(Gln226His) variant, in conjunction with the c.2419C>T; p.(Arg807Cys) mutation, may compromise mtDNA replication and mitochondrial function and could result in clinically significant mitochondriopathy. As this study is based on one patient compared to a first-degree relative (but with an identical mitochondrial genome), the pathogenicity of c.678G>C; p.(Gln226His) of POLG should be confirmed in future studies, in particular, in conjunction with other POLG-variants.

A six-year-old child presented with an acute onset of refractory epileptic seizures during a coronavirus disease 2019 (COVID-19) infection. As her clinical condition progressed, she developed super-refractory status epilepticus, resulting in significant cognitive and motor impairments. Genetic analysis revealed a homozygous mutation in the DNA Polymerase Gamma, Catalytic Subunit (POLG) gene (c.1399G>A; p.Ala467Thr), confirming a diagnosis of Alpers-Huttenlocher syndrome. The clinical course was characterized by refractory seizures and developmental regression, and it ultimately culminated in liver failure and multiorgan dysfunction, resulting in death. This case underscores the critical importance of early genetic evaluation in children with unexplained refractory seizures, particularly for detecting underlying mitochondrial disorders such as POLG-related syndromes. Mitochondrial function is highly sensitive to physiological and environmental stressors, including viral infections. Pathogens such as hepatitis viruses, influenza virus, HIV, respiratory syncytial virus (RSV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can exacerbate mitochondrial dysfunction. Therefore, identifying genetic vulnerabilities in these patients is essential for optimizing management strategies and potentially mitigating rapid clinical decline.

Following the standardized nomenclature proposed by the American Clinical Neurophysiology Society (ACNS), rhythmic high-amplitude delta activity with superimposed spikes (RHADS) can be reported as an extreme delta brush (EDB). The clinical implications of similar electrographic patterns being reported as RHADS versus EDB are important to highlight. We aim to review the electrographic characteristics of RHADS, evaluate whether RHADS is seen in other neurological disorders, and identify the similar and unique characteristics between RHADS and EDB to ultimately determine the most accurate way to differentiate and report these patterns. We believe that the differentiation of RHADS and EDB is important as there is a vast difference in the diagnostic approach and the medical management of associated underlying etiologies. We conducted an extensive search on MEDLINE and Pubmed utilizing various combinations of keywords. Searching for "gamma polymerase and EEG", or "RHADS" or "Alpers syndrome and EEG" or "EEG" AND "Alpers-Huttenlocher syndrome". Three articles were found to be focused on the description of "RHADS" pattern in Alpers Syndrome. No publication to date were found when searching for the terms "EDB" AND "children", AND "infant" AND "adolescent" excluding "encephalitis" and "neonate". Although RHADS and EDB appear as similar EEG patterns, meticulous analysis can differentiate them. RHADS is not exclusive to patients with Alpers-Huttenlocher syndrome and may manifest in regions beyond the posterior head region. Reactivity to eye-opening and response to anesthesia can be two other elements that help in the differentiation of these patterns. RHADS is not exclusive to patients with AHS and may manifest in regions beyond the posterior head region. Reactivity to eye-opening and response to anesthesia are features that help in the differentiation of these patterns. 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.