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.