Photoageing-associated mitochondrial DNA length mutations in human skin

It has recently been suggested that mitochondrial DNA (mtDNA) mutations are important contributors to human ageing and degenerative diseases. Using PCR techniques, we demonstrated three types of mtDNA length mutations, a 4977 bp deletion, a 7436 bp deletion and tandem duplications, in normal human skin tissues. We found that these mutations started to appear in the third decade of life, and the age at which the mutations could be detected in sun-exposed skin was usually younger than in non-exposed skin. Moreover, the incidences of these deletions and tandem duplications of mtDNA in sun-exposed skin were all significantly higher than those in non-exposed skin (P<0.05). The 4977 bp deletion was the most prevalant mtDNA mutation in human skin, and the 7436 bp deletion was the least frequent among the three types of mtDNA mutations examined. We first demonstrated the existence of tandem duplications with sizes of about 260 bp, 200 bp and 150 bp in the D-loop region of mtDNA in the skin of elderly individuals. Among the three tandem duplications, the 200-bp duplication was found to occur most frequently in ageing skin. The tandem duplications were found to coexist with either or both of the deletions in some elderly individuals. The frequency of occurrence of mtDNA deletions and tandem duplications in skin was found to increase in an age-dependent manner. However, the incidence of tandem duplications was not well correlated with the age of the subject. Using a semiquantitative PCR method, we found that the proportion of the 4977 bp-deleted mtDNA in the skin was significantly increased with advancing age (regression analysis: r=0.4, P<0.05). In the same individual, exposed skin harboured the 4977 bp-deleted mtDNA more frequently and usually more abundantly than non-exposed skin. Taken together, these findings demonstrate that deletions and/ or tandem duplications of mtDNA occur, alone or in combination, in human skin tissues with increasing frequency during ageing. Moreover, the incidence and proportion of mtDNA deletions and tandem duplications in exposed skin were usually higher than in non-exposed skin. We therefore suggest that the stochastic induction and accumulation of these mtDNA mutations may be greatly enhanced by light exposure and that photoageing plays a dominant role in human skin ageing.