Analyzing tumor suppressor activities in the murine small intestine.

We have used mice deficient in a series of genes with either known or potential tumor suppressive activity to determine the phenotype of loss of function of these genes in the mouse. We have tested a series of endpoints that derive from the hypothesis that loss of an apoptotic program would be predicted to fail to delete cells carrying DNA damage, that this would lead to increased clonogenic survival and thereby to an increased mutation burden and tumor predisposition. For p53 deficiency, we show that loss of the apoptotic program does not translate into an increase in spontaneous mutation rate. However, p53 deficiency can lead to increased clonogenic survival, although this is highly damage-type dependent. Furthermore, p53 deficiency weakly accelerates tumorigenesis associated with inactivation of the adenomatous polyposis coli gene, Apc. We have also analyzed mice mutant for the mismatch repair genes Msh2, Mlh1, and Pms2, describing circumstances in which all of these strains show defective apoptosis, increased clonogenic survival, and increased mutation rate. However, these effects are highly drug-type dependent and the pattern of dependency argues strongly that mutation rate increases as a direct result from loss of the apoptotic program. We have also identified a new role for p53 by intercrossing the p53 and Msh2 mutants, so demonstrating that heterozygosity for p53 accelerates microsatellite instability. Finally, we have analyzed mice mutant for Mbd4 and show that this gene functions in vivo as a tumor suppressor.