A role for BCL2L13 and autophagy in germline purifying selection of mtDNA

Author summaryWe have addressed the role of autophagy on purifying selection of mtDNA by mating different autophagy-deficient mouse models, including knockouts of Parkin, Bcl2l13, Ulk1, and Ulk2, to female mice with high levels of the pathogenic tRNA(Ala) gene mutation. We identified a robust effect of Bcl2l13 on the selection process, weaker effects of Ulk1 and Ulk2, whereas Parkin had no statistically significant impact. Our study thus provides strong experimental evidence for distinctive roles of autophagy in germline purifying selection of mtDNA. Furthermore, the experimental strategy and statistical methods we have developed in the manuscript provide a novel framework for future studies of the enigmatic purifying selection process in mammals. Mammalian mitochondrial DNA (mtDNA) is inherited uniparentally through the female germline without undergoing recombination. This poses a major problem as deleterious mtDNA mutations must be eliminated to avoid a mutational meltdown over generations. At least two mechanisms that can decrease the mutation load during maternal transmission are operational: a stochastic bottleneck for mtDNA transmission from mother to child, and a directed purifying selection against transmission of deleterious mtDNA mutations. However, the molecular mechanisms controlling these processes remain unknown. In this study, we systematically tested whether decreased autophagy contributes to purifying selection by crossing the C5024T mouse model harbouring a single pathogenic heteroplasmic mutation in the tRNA(Ala) gene of the mtDNA with different autophagy-deficient mouse models, including knockouts of Parkin, Bcl2l13, Ulk1, and Ulk2. Our study reveals a statistically robust effect of knockout of Bcl2l13 on the selection process, and weaker evidence for the effect of Ulk1 and potentially Ulk2, while no statistically significant impact is seen for knockout of Parkin. This points at distinctive roles of these players in germline purifying selection. Overall, our approach provides a framework for investigating the roles of other important factors involved in the enigmatic process of purifying selection and guides further investigations for the role of BCL2L13 in the elimination of non-synonymous mutations in protein-coding genes.

Automated summary

In this study, the role of autophagy in germline purifying selection of mtDNA was investigated by mating different autophagy-deficient mouse models with mice carrying a pathogenic tRNA(Ala) gene mutation. The results showed that Bcl2l13 had a significant effect on the selection process, while Ulk1 and Ulk2 had weaker effects, and Parkin had no statistically significant impact. This study provides experimental evidence for the distinct roles of autophagy in germline purifying selection of mtDNA and establishes a framework for future studies on this process.