Mutation

With the development of tests based on selectable endogenous marker genes, it became possible to assess mutation frequencies at these loci in T-cells from human and animal donors (for a review see Vijg, 2000). Using the hypoxanthine phosphoribosyl transferase (HPRT) locus test, investigators have shown that mutation frequencies at this locus increase with donor age. For example, results obtained with this assay suggest that mutation frequencies in humans increase with age from about 2 × 10^-6 in young individuals to about 1 x 10^-5 in middle aged and old individuals. In mice mutation frequencies have been reported from about 5 x 10^-6 in young animals to about 3 x 10^-5 in middle aged mice (Dempsey et al., 1993). However, in both mice and humans these values could be underestimates, due to the loss of HPRT mutants in vivo or in vitro. Indeed, results from Grist et al. (1992), who assayed the HLA locus (using immunoselection for mutationally lost HLA antigen) in human lymphocytes, indicate mutant frequencies two to three times higher. Values higher than HPRT were also found with other assays involving selectable target genes. The discrepancy has been explained in terms of the inability of the HPRT test to detect mitotic recombination events (HPRT is X-linked) and a relatively strong in vivo selection against mutants.

In mice subjected to caloric restriction, the only intervention demonstrated to increase life span (Masoro, 1993), HPRT mutation frequencies were found to increase with age at a significantly slower rate than in the animals that fed at will. Dempsey et al. (1993) studied both types of animals for twelve months. They saw an age-associated accumulation of mutations in the mice that fed at will, but not in the mice that experienced calorie restriction. Their conclusion was that the great majority of endogenous mutations are related to diet. However, it is not clear if this effect of diet is the result of intake of multiple exogenous mutagens or is related to the metabolism of food.

The results of Dempsey et al. suggest that the level of accumulated somatic mutations reflects biological rather than chronological age. This conclusion was further strengthened by Odagiri et al. (1998), who demonstrated accelerated accumulation of mutations in peripheral blood lymphocytes of so-called senescence-accelerated mice (SAM). Although the SAM model is not generally accepted as a mouse model of accelerated aging, these findings nevertheless demonstrate a link between somatic mutation rate and physiological decline. Interestingly, the HPRT test has been used on tubular epithelial cells of kidney tissue from human donors two to ninety-four years old. The mutation frequencies found were much higher than the values for blood lymphocytes and also increased with age (Martin et al., 1996). The high mutation frequency in the kidney cells could reflect a relatively slow turnover as compared to T-cells.