Objective: To investigate the molecular consequences of a variant of unknown significance (VUS) in POLG2.

Background: Mitochondrial DNA (mtDNA) is replicated by DNA polymerase gamma, which is composed of a catalytic and an accessory subunit. The latter one is encoded by the DNA polymerase-gamma2 (POLG2) gene. In a few cases, pathogenic variants in POLG2 have been shown to cause a mitochondrial disease with variable phenotypic presentations ranging from adult-onset ataxia with progressive external ophthalmoplegia (PEO) to severe, infancy-onset phenotypes including delayed psychomotor development, lactic acidosis, and liver disease.

Method: We performed exome sequencing in three members of a family with adult-onset ataxia. To interpret pathogenicity of a novel missense variant in POLG2, we carried out deep mtDNA sequencing, tested for mtDNA deletions by quantitative and long-range PCR, and established primary fibroblast lines from two mutation carriers to investigate mitochondrial integrity by immunocytochemistry, immunoblotting, and flow cytometry.

Results: The index patient, a 55-year-old woman, presented with signs of vertigo at the age of 30 years, followed by the development of cerebellar ataxia and progressive ophthalmoplegia. Her brother and mother had onset of ataxia at ages of 40 and 50 years, respectively. By exome sequencing, we identified a segregating, heterozygous, novel missense variant (c.1270T>C [p.Ser424Pro], NM_007215.3) in POLG2 in all three affected family members. Initially, this was a VUS. We found a higher number of mtDNA sequence variants in blood cells from the mutation carriers compared to matched healthy controls. However, large mtDNA deletions were not detected in the blood samples. In POLG2-mutant fibroblasts, we demonstrated an impaired mitochondrial network, a decreased mitochondrial membrane potential, and decreased TOMM20 protein levels in comparison to healthy controls. A muscular biopsy was not available.

Conclusion: Although we could not demonstrate mtDNA deletions, we provide multiple lines of evidence that the novel POLG2 variant is associated with impaired mitochondrial integrity suggesting its pathogenicity in respect to the mitochondrial phenotype (ataxia and progressive external ophthalmoplegia) in this family. Our work highlights the importance of functional characterization of VUS to enable meaningful genetic diagnosis counselling.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/a-novel-polg2-variant-leads-to-disrupted-mitochondrial-integrity-in-a-family-with-cerebellar-ataxia-and-progressive-ophthalmoplegia/