Molecular Biology of Aging

Molecular biology can be loosely defined as the study of biology at the molecular level. However, the term is usually used in a more limited sense to mean the study of macromolecules such as proteins, DNA, and RNA, and their specific roles in living systems. This use of the term came into being in the 1960s, largely as the result of the elucidation of the structure of DNA in 1953, and the realization, beginning in 1961, of how this structure encodes information for synthesizing proteins. These findings made it possible to turn the emphasis in biochemistry away from elucidating metabolic pathways in cells, and towards understanding how chromosomes direct and regulate the functioning of the cell. The next breakthroughs arrived through the development of technologies to isolate and sequence specific genes through gene cloning—made possible by the discovery, characterization, and use of special enzymes, that break DNA only at very specific sites.

The application of molecular biology to the study of aging did not begin in earnest until the 1980s. Most early research on aging focused on developing animal models for studying aging, describing various aging processes, and characterizing age-related changes in both humans and animal models. Of particular interest was the need to identify and quantitate age-related changes in gene expression. This became possible once genes could be cloned and used as probes to measure the amount of specific messenger RNA molecules present in the cell under any given condition, such as a specific age, nutritional status, or disease status. Such measurements provide information about the rate of transcription of any given gene into messenger RNA, as well as the rate of translation of this RNA into protein. For example, this technology has been used to identify age-related changes in liver gene expression, and to determine which of these changes are delayed by caloric restriction, an intervention known to slow down aging and extend both mean and maximum life span in rodents.

The ability to measure the relative quantities of specific messenger RNA molecules in the cell made a quantum leap forward in the late 1990s with the development of technologies to carry out this analysis using fluorescent tags and thousands of gene probes attached to small glass plates or filters. These microarrays are able to do thousands of measurements in a single experiment, and thus markedly speed up the rate of research.

The ability to clone genes and manipulate DNA sequences led also to a new approach to understanding how genes function in cells through the creation of genetically altered mice. This area of research is now called functional genomics. Rather than relying on random mutation to generate interesting mutants, the early 1990s saw the exploitation of techniques for introducing new genes into mice and overexpressing these genes. Alternatively, genes, or parts of genes, can be "knocked out" to look at the impact of loss of expression of any given gene or part of a gene. This has been of particular importance in developing mouse models of age-related human diseases and syndromes such as Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, Werner's syndrome, and muscular dystrophy. Genetically altered mice are also useful for studying normal cell function. For example, the introduction and continued expression of the telomerase gene in human cells in culture was shown to prevent the proliferation block that normally occurs after about fifty to eighty cell divisions of human cells, thereby showing that telomere shortening could cause this replicative senescence. An important remaining challenge is to develop better strategies for turning these transgenes "on" and "off" at will.

Finally, it is now known through gene sequencing studies that some base differences occur frequently in human DNA, and these are called single nucleotide polymorphisms (SNPs). SNPs presumably occur in all genes as frequently as one per thousand base pairs of DNA. Thus, different individuals contain varying amounts of slightly different forms of any given gene; these different forms of genes are called alleles. The best example of this is the human apolipoprotein E gene, of which there are at least three major alleles: apoE2, apoE3, and apoE4. The frequency of these three alleles in any given population varies with age, suggesting that certain alleles are risk factors for aging, presumably by increasing susceptibility to age-related diseases. For example, the E4 allele of the apoE gene increases the risk for developing Alzheimer's disease. It is suspected that the existence of these SNPs in critical genes at varying frequencies in each individual is at least partly responsible for the very different aging patterns seen among individuals in a population. Examples of critical genes might include genes for DNA repair enzymes, antioxidant enzymes, tumor suppressor proteins, and signal transduction proteins.

The highly mutable human mitochondrial genome is known to contain at least eleven different SNPs, but it is not yet known how these SNPs impact aging. Mitochondrial mutations and SNPs have been implicated in a wide range of age-related pathologies and diseases in humans, including deafness, muscle weakness, diabetes, and cardiomyopathy. Determining the frequency and biological impact of mitochondrial SNPs and other SNPs in human populations promises to be an intense area of research in the twenty-first century.