Strand-specific removal of cyclobutane pyrimidine dimers from the p53 gene in the epidermis of UVB-irradiated hairless mice.

Removal of UVB-induced cyclobutane pyrimidine dimers (CPD) from each of the two strands of the transcriptionally active p53 tumor suppressor gene and the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene was determined in the epidermis of the hairless mouse using the CPD-specific enzyme T4 endonuclease V. Mice were exposed to a single dose of UVB (2 kJ/m2) and kept in darkness for up to 24 h. About 80% of the CPD were removed from the transcribed strand of the p53 and HPRT genes within 24 h. Most rapid removal was observed during the first 4 h. In contrast, very little removal of CPD from the nontranscribed strand of the p53 and the HPRT genes was observed in 24 h. The same low level of repair was observed in the inactive c-mos proto-oncogene. The efficient repair of the transcribed strand compared to the nontranscribed strand of transcriptionally active genes in the epidermis of the hairless mouse resembles the repair of CPD in cultured rodent cells. Moreover, the selective removal of CPD from the transcribed strand of the p53 gene correlates well with the known strand bias of u.v.-induced mutations at dipyrimidine sites in the p53 gene of u.v.-induced mouse skin tumors.