Chromosome instability contributes to loss of heterozygosity in mice lacking p53

The p53 tumor suppressor protein participates in multiple cellular processe s including cell cycle checkpoints and programmed cell death. In cell lines , loss of p53 function is associated with increased genetic instability inc luding aneuploidy, gene amplification, and point mutation. Although similar genetic instability often accompanies the progression of malignancy in tum ors, its role in tumor initiation in normal cells is not clear. To study wh ether or not loss of p53 leads to genetic instability in normal cells in vi ve, we have examined mechanisms of loss of heterozygosity (LOH) at the Aprt (adenine phosphoribsyltransferase) and flanking loci in normal fibroblasts and T lymphocytes of p53-deficient mice. Somatic cell variants that arose in vivo as a consequence of genetic or epigenetic alterations abolishing Ap rt function were selected and expanded in vitro by virtue of their resistan ce to 2,6-diaminopurine (DAP). We observed that p53 null mice produced abou t three times as many DAP-resistant fibroblast colonies than wild-type mice , but the frequency of DAP-resistant T lymphocyte colonies was not signific antly changed. Mitotic recombination, but not point mutation, partly accoun ted for the increase in the frequency of DAP-resistant fibroblasts. Most si gnificantly, chromosome loss/duplication and interstitial deletion, which w ere extremely rare events in the wild-type mice, represented a significant proportion of LOH events in both fibroblasts and T lymphocytes of p53 null mice. Also, increased interstitial deletion was observed in fibroblasts of p53 heterozygous mice. These data suggest that increased genetic variation, including chromosome instability, starts at the initiation stage of tumori genesis when functional p53 is absent or reduced.