Knock-in mice with a chimeric human/murine p53 gene develop normally and show wild-type p53 responses to DNA damaging agents: a new biomedical research tool

The high prevalence and great diversity of p53 tumor suppressor gene mutati ons in human tumors call for development of therapeutic molecules that resc ue function of aberrant p53 protein. P53 mutations also offer new approache s to the study of the origins of mutations in human cancer. An experimental mouse model with a genetically modified but normal functioning p53 gene ha rboring the human rather than the murine core domain, would be of considera ble benefit to research on both cancer therapeutics and etiology; however, it is uncertain whether such mice would permit biological functions of p53 to be retained. Using a Cre/lox P gene-targeting approach, we have construc ted a human p53 knock-in (hupki) mouse strain in which exons 4-9 of the end ogenous mouse p53 allele were replaced with the homologous, normal human p5 3 gene sequence, The chimeric p53 allele (p53(KI)) is properly spliced, tra nscribed in various tissues at levels equivalent to wild-type mice, and yie lds cDNA with the anticipated sequence, that is, with a core domain matchin g that of humans. The hupki p53 protein binds to p53 consensus sequences in gel mobility shift assays and accumulates in the nucleus of hupki fibrobla sts in response to UV irradiation, as is characteristic of wild-type p53, I nduction of various p53-regulated genes in spleen of gamma -irradiated homo zygous hupki mice (p53(KI/KI)), and the kinetics of p53-dependent apoptosis in thymocytes are similar to results with wild-type (p53(+/+)) mice, furth er indicating normal p53 pathway function in the hupki strain, The mice are phenotypically normal and do not develop spontaneous tumors at an early ag e, in contrast to knock-out (p53(-/-)) strains with a defective p53 gene. T he chimeric (p53(KI)) allele thus appears to provide a biological equivalen t to the endogenous murine (p53(+)) gene. This strain is a unique tool for examining in vivo spontaneous and induced mutations in human p53 gene seque nces for comparison with published human tumor p53 mutation spectra, In add ition, the hupki strain paves the way for mouse models in pre-clinical test ing of pharmaceuticals designed to modulate DNA-binding activity of human p 53.