Severe Somatic Bottleneck in Mitochondrial DNA of Human Hair

Heteroplasmy is the presence of variable mitochondrial DNA (mtDNA) within the same individual. The dynamics of heteroplasmy allele frequency among tissues of the human body is not well understood. Here we determined allele frequency at heteroplasmic sites from 5-10 hairs from each of 11 humans using next-generation sequencing. We observed high variance in heteroplasmic allele frequency among separate hairs from the same individual—much higher than that for blood and buccal tissues. Our population genetic modeling determined the somatic bottleneck during embryonic follicle development of separate hairs to be only 7.07 (95% CI 3.4-23.6) mtDNA segregating units. This bottleneck is much more drastic than somatic bottlenecks for blood and buccal tissues (136 and 458 units, respectively), as well as more drastic than, or comparable to, the germline bottleneck (equal to 7-10 or 25-32 units, depending on the study). We demonstrated that hair undergoes additional genetic drift before and after the divergence of mtDNA lineages of individual hair follicles. Additionally, we showed a positive correlation between donor’s age and variance in heteroplasmy allele frequency in hair. These findings have important implications for forensics and for our understanding of mtDNA dynamics in the human body.