Mitochondria

What happens to the mitochondrial genome as it gets older has now been extensively studied and documented. It is known that mtDNA mutations can compromise the mitochondria function in many ways: they can disrupt both transcription and the translation of encoded proteins; they can produce nonfunctional ribosomal RNA, (RNA), transfer RNA (+RNA) and proteins; and they can impair mtDNA replication. The mitochondrial genome has a great ability to mutate during the life span, producing a heterogeneous array of somatic mutations. The mutation rate for the mitochondrial genome is ten to twenty times larger than for nuclear DNA study by Khrapko et al. (1997) showed a several-hundred-fold higher rate of somatic mutations both in vivo and in vitro in human mtDNA than in nuclear DNA. This increased rate is due to both a high spontaneous mutation rate and the sensitivity of the mitochondrial genome to exogenous environmental mutagens. Five different types of mtDNA mutations have been shown to be age-associated: point mutations, deletions, additions, duplications and rearrangements. One problem that has been raised by some experts concerning studies of the age-related accumulation of specific mutations in human mtDNA is that although the level of a specific mtDNA mutation increases substantially with age, any of these age-associated mutations affects no more than 1 percent of the organelle mtDNA molecules. However, a large number of specific mutations are likely to occur at each of the 16,569 nucleotide positions within the mitochondrial genome during a lifetime, so that even if each mutation is found at a low level, the increasing accumulation of a large number of mutations will eventually reach a critical level, leading to nonfunctional mitochondria. Furthermore, the load of mtDNA mutations is usually underestimated, since most of the mutations are only detectable using the polymerase chain reaction (PCR). This technique, routinely used to estimate the relative proportions of age-associated mutant DNA may give biased results, as it is dependent on the choice of primers and PCR conditions selected by the individual conducting the study.

One of the most reported mtDNA mutations is the so-called common deletion. Initially identified by Cortopassi and Arnheim (1990), the accumulation of mtDNA molecules exhibiting a 4,977 base pairs deletion increases with age. This deletion occurs between two thirteen base pairs sequence repeats, removing almost five kilobase pairs of mtDNA that encodes six essential polypeptides of the respiratory chain as well as five tRNAs. This deletion was subsequently shown by many other investigators to increase with age in many different tissues. Many other age-associated mtDNA mutations have been identified Khrapko et al. (1999) used long PCR techniques in single cell cardiomyocyte from elderly patients to show that multiple mutations coexist in various tissues of aged individuals and that single mutations occur within individual cells. A large age-dependent accumulation of specific mutations in a critical control region for mtDNA replication has been shown in human fibroblasts (Michikawa et al., 1999).

The incidence of mutant mtDNA has been found to correlate with oxidative damage to mtDNA. Adachi et al. (1993) provided the first evidence that ROS is responsible for the occurrence of mtDNA deletions. A large number of DNA base modifications resulting from oxidative stress have been reported—the one that has been the most widely studied is oxidized nucleotide 8-OH-dG (8-hydroxy-deoxyguanosine). This specific product of oxidative damage to DNA has been shown to accumulate with age, and it correlates with an increase of mtDNA 7.4 kilobase pairs deletion (Mecocci et al., 1993).

As mitochondrial respiration and oxidative phosphorylation gradually uncouple from each other, the activity of the mitochondrial respiratory chain gradually declines. The immediate consequence of a decline of respiratory functions is a decline of ATP synthesis, which will further elevate ROS generation. As the production of ROS species in mitochondria increases, the oxidative damage is reflected by an increasing number of mtDNA mutations. Therefore, respiratory enzymes will incorporate the defective mtDNA-encoded subunits and show impaired respiratory function. This vicious circle operates in an age-dependent manner and plays an important role in aging. This scenario can be also amplified by exogenous factors—many types of mtDNA mutations occur more frequently in sun-exposed skin and mtDNA deletions in the human lung are significantly increased by cigarette smoking, suggesting that ROS resulting from environmental factors play a role in promoting mtDNA damage during aging. Although the mitochondrial free-radical theory of aging has gained prominence, it is important to remember that aging is a multifactorial biological process and that many other cellular components are involved.

Ultrastructural changes are also seen in the mitochondria of aged individuals. The mitochondria become larger and less numerous and they exhibit vacuolization, cristae rupture, and accumulations of occlusions.