Mutant C. elegans mitofusin leads to selective removal of mtDNA heteroplasmic deletions at different rates across generations

Deleterious and intact mitochondrial DNA (mtDNA) mutations frequently co-exist (heteroplasmy). Such mutations likely survive and are inherited due to complementation via the intra-cellular mitochondrial network. Hence, we hypothesized that compromised mitochondrial fusion would hamper such complementation, thereby affecting heteroplasmy inheritance. To test this hypothesis, we assessed heteroplasmy levels in three Caenorhabditis elegans strains carrying different heteroplasmic mtDNA deletions (ΔmtDNA) in the background of mutant mitofusin. Firstly, these animals displayed severe embryonic lethality and developmental delay. Strikingly, these phenotypes were relieved during subsequent generations in association with complete ΔmtDNA removal. Moreover, the rates of deletion loss negatively correlated with the size of mtDNA deletions, suggesting that mitochondrial fusion is essential and sensitive to the nature of the heteroplasmic mtDNA mutations. While introducing the ΔmtDNA into a fzo-1;pdr-1 (PARKIN ortholog) double mutant, we observed skew in the mendelian distribution of progeny, in contrast to normal distribution in the ΔmtDNA;fzo-1 mutant, and severely reduced brood size. Notably, the ΔmtDNA was lost across generations in association with improved phenotypes. This underlines the importance of cross-talk between mitochondrial fusion and mitophagy in modulating the inheritance of mtDNA heteroplasmy. Finally, while investigating heteroplasmy patterns in three Charcot-Marie-Tooth disease type 2A pedigrees, which carry a mutated mitofusin 2, we found a single potentially deleterious heteroplasmic mutation, whose levels were selectively reduced in the patient versus healthy maternal relatives. Taken together our findings show that when mitochondrial fusion is compromised, deleterious heteroplasmic mutations cannot evade natural selection, while inherited from generation to generation.