4.5. CONCLUSIONS AND FUTURE PROSPECTS
Using a transgenic mouse model harboring chromosomally integrated lacZ reporter genes that can be recovered into E. coli and inspected for a wide range of different mutations, it has been demonstrated that mutations accumulate with age in a variety of organs and tissues, albeit at different rates. For example, in brain there was no evidence for mutation accumulation, while in liver mutation accumulation was clearly occurring. This organ-specificity in the rate of mutation accumulation was confirmed by studies on additional organs (M. Dolle, in preparation). Our results with mouse mutants deficient for cell cycle checkpoints have thus far revealed that TP53 does not seem to play a role of importance in mutation accumulation in vivo. Nucleotide excision repair, on the other hand, appeared to influence the rate of mutation accumulation in the liver at young age.
This increased mutation accumulation coincides with the reported early appearance of liver tumors in this mouse model (82).
How could somatic mutation loads of the magnitude given above be responsible for the adverse effects associated with the aging process in mammals? First, they could explain the well known age-related increase in the incidence of cancer. The increased rate of mutation accumulation observed in the XPA-/- animals, which coincided with an increased frequency of tumors in the same organ (liver) underscores this possibility. Second, somatic mutations could be responsible for the loss of cells in various organs, such as the brain. In this respect, empirically determined in vivo mutation frequencies are always underestimated due to the elimination of cells with a high mutation load. This could explain our observation of a lack of mutation accumulation in the brain in both normal and XPA-deficient animals. Finally, random somatic mutations, especially large genome rearrangements, could lead to impaired cell functioning rather than cell death or cell transformation.
In this view, the accumulation of random mutations would result in a mosaic of cells at various levels of deficiency. It is conceivable that, especially large structural alterations at mutational hotspots could gradually impair genome functioning. Indeed, rather than a catalog of useful genes interspersed with functionless DNA, each chromosome is now viewed as a complex information organelle with sophisticated maintenance and control systems. Destabilization of these structures by DNA mutations may lead to changes in gene expression, for example, by influencing patterns of DNA methylation and conformation. At the current state of technology a proposed role of somatic DNA alterations in aging is a testable hypothesis. Assuming that the rate of mutation accumulation and the type of mutations that accumulate are determined by the various DNA repair systems, identification of the genes involved in these processes should permit the identification of longevity-associated DNA repair genotypes.
Transgenic mouse modeling of these genotypes and monitoring of genome stability using chromosomally integrated reporter genes will then allow for the direct testing of these potential molecular determinants of aging for their phenotypic consequences, in terms of markers for cellular senescence, pathophysiological variables and lifespan. This concept is not new (85), but its practical implementation has now become a concrete rather than an abstract possibility.