Heteroplasmic shifts in tumor mitochondrial genomes reveal tissue-specific signals of relaxed and positive selection.

Although mitochondrial genomes (mtDNA) accumulate elevated levels of mutations in cancer cells, the origin and functional impact of these mutations remain controversial. Here, we queried whole-genome sequence data from 1,916 patients across 24 cancer types to characterize patterns of mtDNA mutations and elucidate the selective constraints driving their fate. Given that mitochondrial genomes are polyploid, cells with advantageous levels of mtDNA mutations can be selected for depending on their cellular environment. Therefore, we tracked changes in per-cell abundances of mtDNA mutations from normal to tumor cells in the same patient. Tumor mitochondrial genomes show distinct mutational patterns and are disproportionately enriched for protein-altering changes. Moreover, protein-altering mtDNA variants that are initially present at low frequencies in normal cells preferentially expand in the altered tumor environment, suggesting selective advantage. We also perform these analyses with attention to the cancer's tissue of origin, which revealed tissue-specific differences in selective signals. The mitochondrial genomes in renal chromophobe and thyroid cancers show particularly strong signals of positive selection, indicated by higher proportions and per-cell abundances of truncating variants. Dramatic tumor- and tissue-specific variations in selective pressures suggest that cancer cells with advantageous levels of damaged mitochondrial genomes will selectively proliferate to facilitate the tumorigenic process.
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