Rewriting nuclear epigenetic scripts in mitochondrial diseases as a strategy for heteroplasmy control

Mitochondrial diseases, caused by mutations in nuclear or mitochondrial DNA (mtDNA), have limited treatment options. For mtDNA mutations, reducing the mutant-to-wild-type mtDNA ratio (heteroplasmy shift) is a promising strategy, though it currently faces challenges. Previous research showed that severe mitochondrial dysfunction triggers an adaptive nuclear epigenetic response, through changes in DNA methylation, absent or less important for subtle mitochondrial impairment. Therefore, we hypothesized that targeting nuclear DNA methylation could impair cells with high-mutant mtDNA load while sparing those with lower levels, reducing overall heteroplasmy. Using cybrid models harboring two disease-causing mtDNA mutations-m.13513 G > A and m.8344 A > G-at varying heteroplasmies, we discovered that both the mutation type and load distinctly shape the nuclear DNA methylome. We found this methylation pattern critical for the survival of high-heteroplasmy cells but not for low-heteroplasmy ones. Treatment with FDA-approved DNA methylation inhibitors selectively impacted high-heteroplasmy cybrids and reduced heteroplasmy. These findings were validated in cultured cells and xenografts. Our findings highlight nuclear DNA methylation as a key regulator of heteroplasmic cell survival and a potential therapeutic target for mitochondrial diseases.