AGE-DEPENDENT INCREASE IN DELETED MITOCHONDRIAL-DNA IN THE HUMAN HEART - POSSIBLE CONTRIBUTORY FACTOR TO PRESBYCARDIA

Cardiac function deteriorates with age, and endogenous damage to mitochondrial DNA (mt DNA) is believed to be a major contributory factor to aging. Mitochondria occupy a pivotal position in energy metabolism, and mitochondria have their own DNA, which encodes 13 subunits of the mitochondrial energy transducing system. Other subunits are encoded by nuclear DNA. DNA has been shown to have a high mutation rate, and genetic mutation might primarily be ascribed to mtDNA mutation in the energy transducing system. Recent advances in gene technology, especially in polymerase chain reactions (PCR), permit us to analyze mtDNA mutations in a small quantity of tissue. We devised rapid and accurate methods to detect mitochondrial mutations - the primer shift PCR method, PCR-Southern method, the modified primer shift PCR method, and the asymmetric PCR method. With these methods, we analyzed myocardial mtDNA in human cadavers of various ages (from 3 years old to 97 years old, mean 57 years old). The 7.4 kb deletion of mtDNA was commonly detected in elderly subjects, and the proportion of deleted mtDNA to normal mtDNA increased with age. Deleted mtDNA was observed in all subjects that were over 70 years old. The mutation was based on the directly repeated sequence: 5'-CATCAA-CAACCG-3', which exists in both the adenosine triphosphatase 6 gene and the displacement loop (D-loop) region. Replication impairment occurred at that directly repeated sequence, which caused the elimination of genomes between the adenosine triphosphatase 6 gene and the D-loop region and resulted in a 7.4 kb deletion. Accordingly, some subunits of the mitochondrial energy transducing system could not be biosynthesized by the deleted mtDNA fragments, which led to inhibition of energy transduction. A chronic ischemia-like state is induced in the myocardium, which might contribute to the genesis of aging heart (presbycardia).