p53-mediated DNA repair responses to UV radiation: Studies of mouse cells lacking p53, p21, and/or gadd45 genes

Human cells lacking functional p53 exhibit a partial deficiency in nucleoti de excision repair (NER), the pathway for repair of UV-induced DNA damage. The global genomic repair (GGR) subpathway of NER, but not transcription-co upled repair (TCR), is mainly affected by p53 loss or inactivation. We have utilized mouse embryo fibroblasts (MEFs) lacking p53 genes or downstream e ffector genes of the p53 pathway, gadd45 (Gadd45a) or p21 (Cdkn1a), as well as MEFs lacking both gadd45 and p21 genes to address the potential contrib ution of these downstream effecters to p53-associated DNA repair. Loss of p 53 or gadd45 had a pronounced effect on GGR, while p21 loss had only a marg inal effect, determined by measurements of repair synthesis (unscheduled DN A synthesis), by immunoassays to detect removal of UV photoproducts from ge nomic DNA, and by assays determining strand-specific removal of CPDs from t he mouse dhfr gene. Taken together, the evidence suggests a role for Gadd45 , but relatively little role for p21, in DNA repair responses to UV radiati on. Recent evidence suggests that Gadd45 hinds to UV-damaged chromatin and may affect lesion accessibility. MEFs lacking p53 or gadd45 genes exhibited decreased colony-forming ability after UV radiation and cisplatin compared to wildtype MEFs, indicating their sensitivity to DNA damage. We provide e vidence that Gadd45 affects chromatin remodelling of templates concurrent w ith DNA repair, thus indicating that Gadd45 may participate in the coupling between chromatin assembly and DNA repair.