Expression of the p48 xeroderma pigmentosum gene is p53-dependent and is involved in global genomic repair

Abstract

In human cells, efficient global genomic repair of DNA damage induced by ultraviolet radiation requires the p53 tumor suppressor, but the mechanism has been unclear. The p48 gene is required for expression of an ultraviolet radiation-damaged DNA binding activity and is disrupted by mutations in the subset of xeroderma pigmentosum group E cells that lack this activity. Here, we show that p48 mRNA levels strongly depend on basal p53 expression and increase further after DNA damage in a p53-dependent manner. Furthermore, like p53(-/-) cells, xeroderma pigmentosum group E cells are deficient in global genomic repair. These results identify p48 as the link between p53 and the nucleotide excision repair apparatus.

Figure 1

UV induces p48 transcription by a p53-dependent pathway. Primary human fibroblast cell lines, either p53^+/+ (WI38) or p53^−/− (041 mut), were treated with 10 J/m² UV radiation and were harvested after different time intervals. The level of p53 protein was measured by immunoblot (IB) of protein extract. The levels of p48, p21, p125, actin mRNA, and 28S rRNA were measured by Northern blot (NB) of total RNA. The levels of 28S rRNA confirmed equal transfer of total RNA to the membrane.

Figure 2

IR induces p48 transcription by a p53-dependent pathway. Cells were exposed to 2 Gy of IR and were analyzed 0, 1, 2, 4, and 8 h later for levels of p53 protein, p48, p21, p125, actin mRNA, and 28S rRNA.

Figure 3

Expression of p53 induces p48 transcription and the activity of UV-damaged DNA binding protein. We used p53^−/− (041 mut) cells that had been stably transfected with a control vector not containing p53 (p53⁻) or with a vector containing wild-type p53 cDNA under the control of a tetracycline regulated promoter (p53⁺) (2). Cells were analyzed for levels of p53 protein, p48, p21, p125, actin mRNA, and 28S rRNA at different times after inducing p53 expression by removal of tetracycline. UV-DDB was measured by an electrophoretic mobility-shift assay (EMSA) with UV-damaged DNA probe.

Figure 4

GGR and TCR in XPE cells. DDB⁻ (XP2RO) and DDB⁺ (XP89TO) cells were analyzed at different times after exposure to 10 J/m² UV. GGR of 6-4 photoproducts (A) and CPDs (B) were measured by an immunoblot assay with monoclonal antibodies specific for 6-4 photoproducts or CPDs. TCR of CPDs on the transcribed strand (C) and nontranscribed strand (D) of the dihydrofolate reductase gene were measured by restored resistance to T4 endonuclease V, which specifically cleaves CPDs. GGR and TCR also are shown for other primary fibroblast cell lines: wild type (WI38), p53^−/− (041 mut), and XPC (XP10BE), as determined in previous experiments (2, 3, 34, 35). To facilitate comparison, repair in wild type (dotted lines) and XPE (solid lines) are shown over the entire time course.

Figure 5

Model for photoproduct recognition in nucleotide excision repair. (i) Damage recognition. CPDs (square) are recognized by UV-DDB when p48 transcription increases in a p53-dependent manner. Although 6-4 photoproducts (triangle) can be recognized by UV-DDB (32), they may distort the DNA sufficiently to be recognized directly by XPC/HR23B. Thus, the absence of UV-DDB in XPE cells causes only a delay in the global repair of 6-4 photoproducts rather than an absolute deficiency. Either photoproduct is recognized in the transcribed strand of expressed genes by the arrest of RNA polymerase II (RNAP II) translocation (31). (ii) Preparation for repair coupling. XPC/HR23B is postulated to recognize UV-DDB bound to DNA, perhaps replacing it at the lesion site and then partially opening the DNA. A complex of the Cockayne Syndrome proteins, CSA/CSB, is postulated to recognize and displace the arrested RNA polymerase II, opening the DNA at that site. (iii) Recruitment of core repair apparatus. XPC/HR23B recruits the core nucleotide excision repair apparatus for GGR whereas CSA/CSB recruits the core apparatus for TCR. The helicases in TFIIH create a bubble of unwound DNA and, along with XPA/RPA, may participate in lesion verification before permitting the structure-specific nucleases XPF/ERCC1 and XPG to make incisions on each side of the lesion.