Tandem duplications and large-scale deletions of mitochondrial DNA are early molecular events of human aging process

Large-scale deletions and tandem duplications of mtDNA, which were originally identified in the patients with KSS or CPEO, have recently been found, although with lower abundance, in various tissues of aged individuals. By use of PCR techniques with back-to-back primers, we demonstrated for the first time that small tandem duplications occur in the D-loop of mtDNA in an age-dependent manner in human tissues. A total of 10 types of such tandem duplications were identified and confirmed by primer-shift PCR and DNA sequencing. Based on the sequence characteristics of the junction sites, we classified these small tandem duplications into 3 groups. Most of the tandem duplications were found to occur at hot spots containing poly C runs, and the number of C residues exhibited wide variations in type I, II, V, VI, VII, and vIII duplications. These observations suggest that these tandem duplications may be generated through similar recombination mechanisms. Among them, type I, II, III, IV and IX duplications were found to occur more frequently and abundantly in aging human tissues. They were not detectable in muscle, testis or skin tissues of young subjects or blood cells from subjects of any age. On the other hand, we also analyzed the samples for the aging-associated 4977-bp and 7436-bp mtDNA deletions. The results showed that these two deletions and some of the small tandem duplications could occur alone or in different combinations in human tissues in the aging process. From our data, no clear association between tandem duplications and large-scale deletions of mtDNA could be established. However, one common and important observation is that the incidence and abundance of some of the tandem duplications as well as the large-scale deletions were increased in an age-dependent manner. On the basis of these findings and data reported from this and other laboratories, we propose that tandem duplications and large-scale deletions of mtDNA are early molecular events of the human aging process.