Targeting the clonal expansion of mitochondrial DNA mutations in ageing and disease

Abstract

The clonal expansion of mitochondrial DNA (mtDNA) mutations is believed to mediate the presentation and severity of clinical symptoms of a wide range of inherited and age-related diseases. The mechanisms of clonal expansion are not fully understood, however the ability of cells to maintain their normal function in lower levels of mtDNA mutational heteroplasmy highlights the importance of understanding clonal expansion in causing the progression of disease and might offer an avenue for potential therapeutic intervention. The origin and molecular nature of clonal expansion is explored in a range of mitochondrial myopathies in this thesis, using immunofluorescence techniques and single cell molecular studies. The data indicate that in skeletal muscle, clonal expansion originates in a perinuclear niche and spreads transversely throughout a muscle fibre, regardless of how the mtDNA mutation originally arose, and the size of deletion identified. The natural history of clonal expansion was explored in vivo using the heterozygous PolgA ‘mutator’ mouse model. A constant level of mtDNA mutations with increasing age was observed in these mice, but a progressive increase in mitochondrial respiratory chain dysfunction. These findings indicate that the mtDNA mutations were clonally expanding to higher heteroplasmy levels with ageing. The process of clonal expansion was manipulated by treatment with nicotinamide riboside (NR), in an attempt to increase mitochondrial biogenesis. NR supplementation resulted in increased mitochondrial protein mass, although not a consistent increase in mtDNA copy number. NR was able to attenuate age-associated respiratory chain complex IV dysfunction, however had no detectable effect on complex I. NR treatment had a complex effect on the mice, additionally showing an anti-inflammatory response on transcriptional analysis, and protection from age-associated weight gain. These effects are similar to those of a calorie restricted diet. NR supplementation it is unlikely to reverse the clonal expansion of mtDNA mutations and prevent mitochondrial disease but has shown evidence of slowing the process in colon. Mitochondrial disease is complex and heterogeneous, affecting a wide range of tissues in a variety of ways. Whilst NR supplementation has shown benefit, it is likely that a complex intervention would be required to better target the clonal expansion of mtDNA mutations.