A new pathogenic POLG variant

The Y831C Mutation of the POLG Gene in Dementia

BACKGROUND

The POLG gene encodes the catalytic subunit of DNA polymerase γ, which is crucial for mitochondrial DNA (mtDNA) repair and replication. Gene mutation alters the stability of mtDNA and is associated with several clinical presentations, such as dysarthria and ophthalmoplegia (SANDO), progressive external ophthalmoplegia (PEO), spinocerebellar ataxia and epilepsy (SCAE), Alpers syndrome, and sensory ataxic neuropathy. Recent evidence has also indicated that POLG mutations may be involved in some neurodegenerative disorders, although systematic screening is currently lacking.

METHODS

To investigate the frequency of POLG gene mutations in neurodegenerative disorders, we screened a group of 33 patients affected by neurodegenerative diseases, including Parkinson's disease, some atypical parkinsonisms, and dementia of different types.

RESULTS

Mutational analysis revealed the presence of the heterozygous Y831C mutation in two patients, one with frontotemporal dementia and one with Lewy body dementia. The allele frequency of this mutation reported by the 1000 Genomes Project in the healthy population is 0.22%, while in our group of patients, it was 3.03%, thus showing a statistically significant difference between the two groups.

CONCLUSIONS

Our results may expand the genotype-phenotype spectrum associated with mutations in the POLG gene and strengthen the hypothesis of a pathogenic role of the Y831C mutation in neurodegeneration.

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

The effects of nuclear DNA mutations on mitochondrial function

ABSTRACT

The multiple functions of mitochondria, including adenosine triphosphate synthesis, are controlled by the coordination of both the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA) genomes. Mitochondrial disorders manifest because of impairment of energy metabolism. This article focuses on mutations in two nuclear genes and their effect on mitochondrial function. Mutations in the polymerase gamma, or POLG, gene are associated with multisystemic disease processes, including Alpers Syndrome, a severe childhood-onset syndrome. Mutations in the OPA1 gene are associated with autosomal dominant optic atrophy and other neurologic, musculoskeletal, and ophthalmologic symptoms. When assessing for disorders affecting energy metabolism, sequencing of both the mtDNA genome and the nDNA whole exome sequencing is necessary.

[Status epilepticus in a pregnant patient with a previously unrecognized POLG-associated disease]

POLG-associated diseases are rare causes of pharmacoresistant epilepsy and status epilepticus, especially in adult patients. Phenotypic and genotypic variability in these conditions causes the complexity of their diagnosis. In the study, we report a case of a 33-year-old female patient who developed recurrent convulsive status epilepticus with focal clonic onset at the week 22/23 of pregnancy. Intensive anti-seizure therapy was administered, including the use of valproic acid, as well as the treatment of somatic complications. Given the acute onset, the semiology of seizures, the presence of psychopathological symptoms, autoimmune etiology of the disease was initially suspected. A month after the withdrawal of valproic acid, the patient began to show signs of toxic hepatitis, which eventually led to death. According to the results of whole-exome sequencing obtained later, the patient was a carrier of a pathogenic homozygous variant c.2243G>C (p.W748S) in the POLG gene. The presented case highlights the importance of molecular genetic testing and the risk associated with valproic acid hepatotoxicity in patients with cryptogenic epileptic status.