Regulation of the DDB2 gene by p53 and the role of DDB2 in cisplatin-damaged DNA repair

Ultraviolet radiation (UV) from the sun and benzo-[a]-pyrene from tobacco damage DNA, and if left unrepaired, can lead to cancer. Nucleotide excision repair (NER) is a versatile repair system that removes UV lesions as well as other bulky lesions such as benzo-[a]-pyrene adducts. The UV-damaged DNA binding protein (UV-DDB) is a heterodimer of the DDB1 and DDB2 proteins. UV-DDB has a strong affinity for bulky DNA lesions and is part of the NER machinery. UV-DDB is critical for the global genomic repair (GGR) subpathway of NER and suppresses mutagenesis of nontranscribed DNA. Unlike human fibroblasts, rodent fibroblasts are deficient in UV-DDB activity and GGR. We cloned the mouse DDB2 gene and found that it is proficient for conferring UV-DDB to rodent cells. We used Northern blots to confirm that the levels of DDB2 mRNA are very low in rodent fibroblasts and conclude that the UV-DDB and GGR deficiency is due to decreased expression of the DDB2 gene. In human cells, DDB2 transcription is regulated by the p53 tumor suppressor. We identified a p53 response element in the human DDB2 gene, and using electrophoretic mobility shift assays we found that both mouse and human p53 protein bind to this element. Furthermore, a reporter construct containing the element was transcriptionally activated by p53. In contrast, the cognate site in the mouse DDB2 gene was deficient for p53 binding and activation. This lack of p53 regulation in rodent cells might be understood from the fact that rodents are nocturnal animals rarely exposed to the sun, and they have fur to shield away UV. These findings have implications for better rodent models of skin cancer through introduction of a functional p53 element in the rodent DDB2 gene. We also found that expression of DDB2 reduces sensitivity to cisplatin, a drug commonly used in chemotherapy. A common failure of chemotherapy is drug resistance. Perhaps UV-DDB is upregulated in tumors refractory to cisplatin treatment. We provide data for the role of DDB2 in suppressing cisplatin-induced mutagenesis, which could be important for the maintenance of genomic integrity in normal cells during chemotherapy.