MUTATIONS IN MITOCHONDRIAL-DNA ACCUMULATE DIFFERENTIALLY IN 3 DIFFERENT HUMAN TISSUES DURING AGING

In 60 human tissue samples (encompassing skeletal muscle, heart and ki dney) obtained from subjects aged from under 1 to 90 years, we used qu antitative PCR procedures to quantify mitochondrial DNA (mtDNA) molecu les carrying the 4977 bp deletion (mtDNA(4977)) and 3243 A-->G base su bstitution, In addition, the prevalence of multiple mtDNA deletions wa s assessed in a semi-quantitative manner, For all three tissues, the c orrelations between the accumulation of the particular mtDNA mutations and age of the subject are highly significant, However, differential extents of accumulation of the two specific mutations in the various t issues were observed, Thus, the mean abundance (percentage of mutant m tDNA out of total mtDNA) of mtDNA(4977) in a subset of age-matched adu lts is substantially higher in skeletal muscle than in heart and kidne y, However, the mean abundance of the 3243 A-->G mutation in skeletal muscle was found to be lower than that in heart and kidney. Visualisat ion of arrays of PCR products arising from multiple mtDNA deletions in DNA extracted from adult skeletal muscle, was readily made after 30 c ycles of PCR, By contrast, in DNA extracted from adult heart or kidney , amplification for 35 cycles of PCR was required to detect multiple m tDNA deletions, Although such multiple deletions are less abundant in heart and kidney than in skeletal muscle, in all tissue extracts there are unique patterns of bands, even from different tissues of the same subject. The differential accumulation of mtDNA(4977), other mtDNA de letions and the 3243 A-->G mutation in the three tissues analysed pres umably reflects different metabolic and senescence characteristics of these various tissues.