A novel MT-ATP6 variant associated with complicated ataxia in two unrelated Italian patients: case report and functional studies.

Background : The MT-ATP6 gene is a mitochondrial gene which encodes for the intramembrane subunit 6 (or A) of the mitochondrial ATP synthase. The latter corresponds to mitochondrial complex V, which intervenes in the last step of oxidative phosphorylation to produce cellular ATP through aerobic metabolism. Although classically associated with the NARP syndrome, recent evidences point out to an important role of MT-ATP6 pathogenic variants in complicated adult onset ataxias. Methods : We described two unrelated patients with adult onset cerebellar ataxia associated with severe optic atrophy and mild cognitive impairment. We performed the sequencing of the whole mitochondrial DNA. We employed patients’ primary fibroblasts and cytoplasmic hybrids (cybrids), generated from patients-derived cells, to assess the activity of respiratory chain, oxygen consumption rate (OCR), ATP production and mitochondrial membrane potential. Results : In both patients, we identified the same novel m.8777T>C variant in MT-ATP6 with variable degree of heteroplasmy in different tissues. We identify an additional novel variant in MT-ATP6 , m.8879G>T, with high percentage of heteroplasmy, in the patients with the most severe phenotype. A significant reduction in complex V activity, OCR and ATP production was observed in cybrid clones homoplasmic for the m.8777T>C variant, while no such defects were detected in m.8879G>T homoplasmic clones. In addition, fibroblasts with high heteroplasmic levels for the m.8777T>C variant showed increased polarization of mitochondrial membranes. Conclusions : We describe a new pathogenic mtDNA variant in MT-ATP6 associated with adult onset ataxia, supporting the value of mtDNA screening within the diagnostic workflow of selected patients with late onset ataxias.