Is there selection against somatic mtDNA mutations?

Abstract

Mitochondrial DNA (mtDNA) mutations are implicated in a number of human diseases, ranging from rare maternally inherited neuromuscular disorders of infancy to late onset neurodegenerative disease and cancer. While much is known of the mechanisms by which damaged mitochondria are selectively degraded in the lysosome through a mitochondrial-specific form of autophagy termed mitophagy, it remains unclear whether mitophagy limits the accumulation of harmful mtDNA mutations. To address this matter, we created a Drosophila strain that expresses a mitochondrial DNA polymerase lacking proofreading ability. This strain has up to 55-fold elevation in mtDNA point mutation frequency and a number of accompanying phenotypes, including reduced lifespan, a locomotor deficit, and mitochondrial dysfunction. I hypothesized that mitophagy would selectively remove harmful mutations, and that less deleterious mutations would persist in mtDNA mutator flies. Surprisingly, my analysis revealed that deleterious mutations were overrepresented in mutator flies. I propose two models to explain this finding: 1) Mitochondria that bear deleterious mtDNA mutations may undergo less oxidative damage, thereby evading mitophagy. 2) Cells that stochastically acquire a deleterious mtDNA mutation induce compensatory mitochondrial biogenesis, leading to the amplification of mutant genomes. The overrepresentation of deleterious mtDNA mutations present in mutator flies suggests that mitophagy is either incapable of selecting against harmful mtDNA mutations or is insufficient to overcome a positive selective force acting in favor of harmful mtDNA mutations. To test whether mitophagy combats the accumulation of deleterious mutations, I employed a novel proteomic approach developed in our lab to identify the turnover rate of cellular proteins. I hypothesized that mutator flies would exhibit faster mitochondrial protein turnover to clear mitochondria damaged by mutant mtDNA, but instead discovered that the rates of mitochondrial protein turnover were unchanged between wild-type and mutator flies. I also tested whether genetic manipulations including knockout and ubiquitous overexpression of the mitophagy-promoting factor parkin influence selection against mtDNA mutations, but I found that genetic perturbations to parkin did not alter the frequency, spectrum, or pathogenicity of mtDNA mutations. Taken together, my findings suggest that mitophagy does not select against deleterious mtDNA point mutations in somatic tissues.