Ageing-associated large-scale deletions of mitochondrial DNA in human hair follicles

Hair follicles plucked from the bi-temporal region of the scalp of 433 Chinese subjects of different ages were used for the examination of ageing-associated mutations of human mitochondrial DNA (mtDNA). By use of PCR techniques, we detected the 4977 bp and 7436 bp deletions of mtDNA in hair follicles from aged individuals. The frequencies of occurrence of both mtDNA deletions were found to increase with age of the subject. Moreover, we employed a semi-quantitative PCR method to determine the proportion of the 4977 bp deleted mtDNA (dmtDNA) in hair follicles. The results showed that the average proportion of the 4977 bp dmtDNA in hair follicles were 0.05% ± 0.01%. 0.00%, 0.55% ± 0.05%, 0.52% ± 0.24%, 0.65% ± 0.17%, 1.33% ± 0.25%, and 1.89% ± 0.81% for the subjects in the age groups of 21-30, 31-40, 41-50, 51-60. 61-70, 71-80, and 81-99, respectively. Furthermore, we screened all the subjects harboring the 4977 bp and/or 7436 bp deletions for tandem duplications in the D-loop region of mtDNA by PCR with back-to-back primers. The results showed that none of the previously reported tandem duplications were present in all the hair follicles examined. This indicates that tandem duplications do not predispose to large-scale deletions of mtDNA. However, the data suggest that mtDNA deletions occur and accumulate in hair follicles during human ageing. As hair follicles can be easily and non-invasively obtained from the human, we suggest that the age-dependent accumulation of dmtDNAs in hair follicles may be used for the monitoring of human ageing process.