DDB2 gene disruption leads to skin tumors and resistance to apoptosis after exposure to ultraviolet light but not a chemical carcinogen

Abstract

Mutations in the human DDB2 gene give rise to xeroderma pigmentosum group E, a disease characterized by increased skin tumorigenesis in response to UV-irradiation. Cell strains derived from xeroderma pigmentosum group E individuals also have enhanced resistance to UV-irradiation due to decreased p53-mediated apoptosis. To further address the precise function(s) of DDB2 and the consequence of non-naturally occurring DDB2 mutations, we generated mice with a disruption of the gene. The mice exhibited significantly enhanced skin carcinogenesis in response to UV-irradiation, and cells from the DDB2 ؊/؊ mice were abnormally resistant to killing by the radiation and had diminished UVinduced, p53-mediated apoptosis. Notably, the cancer-prone phenotype and the resistance to cellular killing were not observed after exposure to the chemical carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA), to which mice carrying defective nucleotide excision repair genes respond with enhanced tumors and cell killing. Although cells from heterozygous DDB2 ؉/؊ mice appeared normal, these mice had enhanced skin carcinogenesis after UVirradiation, so that XP-E heterozygotes might be at risk for carcinogenesis. In sum, these results demonstrate that DDB2 is well conserved between humans and mice and functions as a tumor suppressor, at least in part, by controlling p53-mediated apoptosis after UV-irradiation.T he disease xeroderma pigmentosum (XP) group E is one of eight subgroups of the cancer-prone syndrome, XP, and it has been associated with mutations in the DDB2 gene (1-4) that codes for the smaller subunit of the heterodimeric damagespecific DNA binding protein, DDB (5, 6). DDB strongly binds to DNA damages caused by UV light, including (6-4) photoproducts and trans,syn-cyclobutane pyrimidine dimers, yet it does not have a strong affinity for cis,syn-cyclobutane pyrimidine dimers, the most abundant UV photoproduct (5, 7-11).The binding to DNA damages might suggest a direct role for DDB in nucleotide excision repair (NER) and DNA repair assays with rodent Chinese hamster ovary cell lines that lack DDB2 expression combined with ectopic overexpression of DDB2 cDNA led to the conclusion that DDB2 is a repair factor involved in global genomic repair, a subpathway of NER. However, the removal from DNA of many bulky lesions, including (6-4) photoproducts, occurs in the absence of DDB (8, 12, 13), and DDB2 is induced by UV-irradiation at a time by which NER has been completed. Moreover, the DDB2 promoter resembles that of a cell cycleregulated gene (1, 14), and DDB2 is part of the COP9 signalosome complex (15, 16). Instead, DDB2 has been proposed to be a transcription transactivator through its stimulation of E2F1 (17), and we recently observed that DDB2 controls p53-mediated apoptosis after UV-irradiation of human diploid fibroblasts and that XP group E (XP-E) primary skin fibroblasts are abnormally resistant to killing by . In sum, the functions of DDB are apparently varied and remain undefined.The absence of DDB2 from Chinese hamster ovary cel...

Tumor-prone phenotype of the DDB2-deficient mice

“…A recent study on DDB2À/À mice reported high incidences of UV-induced skin carcinogenesis (Itoh et al, 2004). In agreement with that study, we observed a high susceptibility to skin carcinogenesis by UV irradiation.…”

“…We, independently, have generated a strain of DDB2-deficient mice. Consistent with the observations of Itoh et al (2004), our DDB2-deficient mice were highly susceptible to skin carcinogenesis following UV irradiation. We observed that, while a significant difference in repair could be detected in the skin, the mouse embryonic fibroblasts (MEFs) from the DDB2-deficient mice exhibit only a marginal deficiency in the repair of cyclobutane pyrimidine dimers.…”

“…The tumors were aggressive, as the tumor-bearing mice died within 3 weeks after tumor development, others were killed within 4 days after the appearance of the tumors. Our data on the heterozygotes are different from the previous report showing higher incidences of UVinduced carcinogenesis in the DDB2 þ /À mice (Itoh et al, 2004). We think that the difference resulted from the difference in the UV irradiation protocols.…”

“…The interaction between TDG and CBP/p300 is believed to play roles both in repair and transcription. Itoh et al (2004) studied the function of the DDB2 subunit of DDB using a knockout strain of mice; and demonstrated that the DDB2À/À mice developed skin malignancies at a higher rate compared to the wild-type littermates following UV irradiation. Also, the MEFs lacking DDB2 were somewhat deficient in activating the level of p53 after UV irradiation.…”

“…Recently, Itoh et al (2004) generated a strain of DDB2À/À mice and demonstrated a role of DDB2 in preventing UV-induced skin carcinogenesis. We, independently, have generated a strain of DDB2-deficient mice.…”