Genetics: Longevity Assurance

Much of what is understood about longevity comes from studies in model organisms such as baker's yeast (Saccharomyces cerevisiae), nematode worms (Caenorhabditis elegans), and fruit flies (Drosophila melanogaster). Genetic screens for long-lived mutants have identified numerous longevity genes, many of which function in a conserved signaling pathway that regulates somatic maintenance and survival in response to environmental stress. In many species, including C. elegans and yeast, this regulatory pathway appears to be responsible for the longevity associated with calorie restriction.

Baker's yeast. The aging process and its regulation are better understood for yeast than for any other organism except, perhaps, nematode worms. There are two ways to define longevity in budding yeast. The more common measure is replicative life span, which is the number of offspring, or daughter cells, that a mother cell produces before she dies. Chronological life span is the length of time a population of nondividing yeast cells remains viable when deprived of nutrients. More than twelve yeast longevity genes have been identified. Most of these affect replicative life span, including genes for a sugar-processing enzyme, hexokinase 1 (HXK1); cyclic adenosine monophosphate production (CDC25); and the silent information regulator 2 (SIR2) (Defossez et al., 2001). Variants of these genes extend life span up to twofold by mimicking the effect of low food supply.

Unlike other model organisms, the precise mechanism by which many yeast longevity genes extend life span is known. In 1997, David Sinclair and Leonard Guarente discovered that circular DNA molecules known as ERCs are a primary cause of yeast aging (Sinclair and Guarente, 1997). ERCs are excised from the ribosomal DNA (a highly repetitive region of the yeast genome) by homologous DNA recombination about midway through a yeast cell's life span. ERCs then replicate each cell cycle until they reach toxic quantities (about one thousand per cell). The variants of most longevity genes that extend replicative life span (e.g., HXK2, SIR2, CDC25, FOB1, and NPT1) do so by stabilizing the ribosomal DNA locus, thus delaying the formation of ERCs.

One of the most interesting yeast longevity genes is SIR2. In 1999, Guarente and colleagues discovered that cells with additional copies of SIR2 enjoy a life span extension of 30 percent (Kaeberlein et al., 1999). SIR2 binds at various regions of the genome, including the ribosomal DNA, where it suppresses the formation of ERCs. SIR2 has been shown to encode a type of enzyme known as histone deacetylase (HDAC). HDACs rearrange DNA into a more compact chromatin structure. SIR2 activity is dependent on the availability of a key metabolite, nicotinamide adenine dinucleotide, which may explain how metabolic activity is coupled to longevity in this organism.

Longevity genes that regulate chronological life span include the gene for adenylate cyclase (CYR1) and a protein kinase signaling protein (SCH9) (Longo, 1999). Deletion of either of these genes increases resistance to oxidants and extends life span by up to threefold. SCH9 is considered a public longevity gene because a related worm gene, akt-1, also regulates life span and stress resistance in that organism.

Nematode worms. In 1988, Thomas Johnson and colleagues isolated the longevity gene age-1 from the nematode worm C. elegans, the first from any species. Mutations in age-1 extend life span by about 50 percent. In 1993, Cynthia Kenyon and colleagues showed that worm life span could be doubled by mutating a gene called daf-2. More than ten longevity genes have now been identified in C. elegans (Braeckman et al., 2001).

The life cycle of C. elegans comprises four larval stages prior to the adult stage. In harsh conditions such as starvation or crowding, larvae often enter a developmentally arrested but resistant form called dauer. The majority of longevity genes in C. elegans encode components of an insulin-like growth factor (IGF-1) signaling pathway that regulates dauer development (see Table 1). Loss-of-function mutations in dauer formation (daf) genes extend the life span by allowing worms to reach maturity and retain some of the traits of dauers, including resistance to heat and oxidative stress.

Not all longevity genes in C. elegans are associated with loss-of-function mutations. The C. elegans sir-2 gene is a relative of the yeast SIR2 longevity gene. In 2001, Tissenbaum and Guarente reported that additional copies of sir-2 extended life span in worms by 30 percent. This extension did not occur when the daf-16 gene was mutated, which suggests that sir-2 regulates the dauer pathway via daf-16. Sir-2 is now considered a significant public longevity gene whose relatives likely regulate longevity in a variety of organisms (Kenyon, 2001).

Certain variants of another C. elegans gene, clk-1, slow development and extend life span up to 50 percent (Wong et al., 1995). Worms engineered to possess longevity variants of both clk-1 and daf-2 live up to five times longer than normal. The clk-1 gene is implicated in the biosynthesis of coenzyme Q, a component of the mitochondrial electron transport chain. The electron transport chain is a primary source of free radicals that can damage DNA, lipids, and proteins. It was originally thought that clk-1 increased longevity by reducing free radicals, but recent findings suggest that increased longevity may be attributable to the increased expression of a catalase gene, ctl-1, that helps detoxify free radicals (Taub et al., 1999).

Fruit flies. The fruit fly Drosophila melanogaster has been used since the 1970s to study the relationship between genetics and longevity, but only recently has there been a concerted effort to identify individual longevity genes in this organism. During winter, Drosophila egg development is arrested by downregulating the production of juvenile hormone, which, like worm development, appears to be regulated by an insulin-like growth factor (IGF) signaling pathway (Gems and Partridge, 2001). Mutations in the insulin receptor substrate (IRS) gene, chico, and in the insulin/IGF-1 receptor (InR) gene allow flies to live up to 80 percent longer than normal by apparently invoking diapausal survival mechanisms. In Drosophila, the insulin/IGF-1 pathway also regulates body size, and many long-lived mutants are small. It is not yet known how the other two Drosophila longevity genes, indy (I'm not dead yet) and methusela, extend life span.

Mice. Although large-scale genetic screens for long-lived mice have not been undertaken because of the cost and labor involved, some longevity mutants have identified in preexisting laboratory stocks of mice, some of which live 60 percent longer than normal mice (Bartke et al., 2001). Snell and Ames dwarf strains of mice are both long-lived and carry spontaneous mutations in the Pit-1 and Prop-1 genes, respectively, which are required for the proper development of pituitary cells that produce growth hormone, prolactin, and thyroid hormone, among others. Two other long-lived mouse strains have defects in growth hormone metabolism (i.e., little mice and mice with a targeted disruption of the growth homone receptor gene). All of these mice are small and have very low levels of insulin-like growth factor 1 (IGF-1), which has prompted speculation that an insulin/IGF-1 signaling pathway regulates body size and longevity in mice, as it does in flies.

In 1999, Pier Giuseppe Pelicci and colleagues reported that mice lacking the p66^shc gene are not small but live one-third longer than normal animals (Migliaccio et al., 1999). p66^shc encodes a signaling protein that promotes cell death after environmental stress and also seems to promote metabolic activities that generate free radicals.