Abnormal regulation of DDB2 gene expression in xeroderma pigmentosum group E strains

Abstract

A damage-speci®c DNA binding protein (DDB) activity is absent from a subset (DDB 7 ) of cells from individuals initially classi®ed as group E of xeroderma pigmentosum (XP), a hereditary, photosensitive disease with a high incidence of skin malignancies. In these cases, mutations have been identi®ed in the DDB2 gene (DDB2 7 ) that codes for the small subunit, p48, of the DDB heterodimer. In four DDB2 7 strains, neither p48 nor DDB activity were observed before or after UVirradiation, despite an unusually strong up-regulation of DDB2 mRNA levels after UV-irradiation. In a ®fth strain, XP82TO, p48 was detectable and both DDB2 mRNA and p48 levels were more up-regulated after UVirradiation than in normal primary cells. Moreover, DDB activity also became apparent after irradiation. XP82TO showed very mild clinical manifestations compared with the other DDB 7 patients. These results, coupled with our ®ndings that most, if not all DDB + cells classi®ed as XP-E were misclassi®ed, suggests a direct correlation between DDB2 levels and the XP-E phenotype. Oncogene (2001) 20, 7041 ± 7050.

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

“…Therefore, levels of DDB2 protein in the mouse cells were estimated from damaged DNA binding assays (Fig. 3B). [In human diploid fibroblasts, levels of DDB2 protein are limiting for the DDB heterodimer and paralleled damaged DNA binding activity (1,3).] Consistent with the observations in normal human cells (1,18), DDB activity decreased on irradiation and then reappeared after DNA repair was largely completed in the mouse DDB2 ϩ/ϩ cells (Fig.…”

Impaired Regulation of Tumor Suppressor p53 Caused by Mutations in the Xeroderma Pigmentosum DDB2 Gene: Mutual Regulatory Interactions between p48DDB2 and p53

“…RT-PCR was performed as described previously (27). The following DDB2 primers were used for semiquantitative RT-PCR: S5 (nucleotides ϩ871 to ϩ890) and AS6 (nucleotides ϩ1151 to ϩ1170) and S2 and AS3.…”

“…Since nucleotide excision repair (NER) is all but complete 48 h after UV irradiation (13), and since NER reconstitution studies have shown that DDB is not required for NER in vitro (1,35), the direct function, if any, of DDB in DNA repair is unclear. In XP-E cell strains, p48 DDB2 protein and DDB activity are undetectable after UV irradiation (except in strain XP82TO), yet DDB2 mRNA levels are abnormally high both before (2-to 4-fold) and after (11-to 37-fold) UV irradiation (27). However, the regulation of DDB2 induction after UV damage is poorly understood.…”

“…Expression of the DDB2 gene is perturbed by UV irradiation. In a human normal diploid fibroblast strain, IMR-90, after an initial reduction in DDB activity, there is a two-to fourfold increase in DDB2 mRNA levels 36 h after UV irradiation followed by a peaking of p48 DDB2 protein and DDB activity 48 h after irradiation (27,37). Since nucleotide excision repair (NER) is all but complete 48 h after UV irradiation (13), and since NER reconstitution studies have shown that DDB is not required for NER in vitro (1,35), the direct function, if any, of DDB in DNA repair is unclear.…”

“…For example, XP-E patients suffer from skin cancers without typical xerosis, and cell strains from XP-E patients are not abnormally sensitive to UV and have normal levels of DNA repair synthesis in vivo (26,27). XP-E is caused by mutations in the DDB2 gene (26,27,37), which encodes the small subunit, p48 DDB2 , of a heterodimeric damage-specific DNA binding protein (DDB) that also contains p127 DDB1 (10). Expression of the DDB2 gene is perturbed by UV irradiation.…”