Reanalysis of mtDNA mutations of human primordial germ cells (PGCs) reveals significant contamination with NUMTs, and challenges predominantly purifying selection in late PGCs
The resilience of the mitochondrial genome (mtDNA) to a high mutational pressure depends, in part, on negative purifying selection in the germline. A paradigm in the field has been that such selection, at least in part, takes place in primordial germ cells (PGCs). Specifically, Floros et al. (Nature Cell Biology 20: 144-51) reported an increase in the synonymity of mtDNA mutations (a sign of purifying selection) between pooled early-stage and late-stage PGCs. We re-analyzed Floros et al. pooled PGC data and noticed that their mutational dataset was significantly contaminated with single nucleotide variants (SNVs) derived from a nuclear sequence of mtDNA origin (NUMT) located on chromosome 5. Contamination was caused by co-amplification of the NUMT sequence by cross-specific PCR primers. Importantly, when we removed NUMT-derived SNVs, the evidence of purifying selection was abolished. In addition to pooled PGCs, Floros et al. reported the analysis of single late-stage PGCs, which were amplified with different sets of PCR primers that cannot amplify the NUMT sequence. Accordingly, we found no NUMT-derived SNVs among single PGCs mutations. Interestingly, single PGC mutations show a decrease of synonymity with increased intracellular mutant fraction. This pattern is incompatible with predominantly negative selection. This suggests that germline selection of mtDNA mutations is a complex phenomenon and that the part of this process that takes place in PGCs may be predominantly positive. However counterintuitive, positive germline selection of detrimental mtDNA mutations has been reported previously and potentially may be evolutionarily advantageous.