Mutation and expression of the XPA gene in revertants and hybrids of a xeroderma pigmentosum cell line

Physical and functional interaction between DDB and XPA in nucleotide excision repair

Damaged DNA-binding protein (DDB), consisting of DDB1 and DDB2 subunits recognizes a wide spectrum of DNA lesions. DDB is dispensable for in vitro nucleotide excision repair (NER) reaction, but stimulates this reaction especially for cyclobutane pyrimidine dimer (CPD). Here we show that DDB directly interacts with XPA, one of core NER factors, mainly through DDB2 subunit and the amino-acid residues between 185 and 226 in XPA are important for the interaction. Interestingly, the point mutation causing the substitution from Arg-207 to Gly, which was previously identified in a XP-A revertant cell-line XP129, diminished the interaction with DDB in vitro and in vivo. In a defined system containing R207G mutant XPA and other core NER factors, DDB failed to stimulate the excision of CPD, although the mutant XPA was competent for the basal NER reaction. Moreover, in vivo experiments revealed that the mutant XPA is recruited to damaged DNA sites with much less efficiency compared with wild-type XPA and fails to support the enhancement of CPD repair by ectopic expression of DDB2 in SV40-transformed human cells. These results suggest that the physical interaction between DDB and XPA plays an important role in the DDB-mediated NER reaction.

The interdependence of skin aging, skin cancer, and DNA repair capacity: a novel perspective with therapeutic implications

The human body is constantly exposed to exogenous and endogenous insults that threaten its genomic integrity and that lead to changes at the molecular, biochemical, and cellular levels. As a major interface between the environment and the internal milieu, our skin is especially subject to such events. Common insults include but are not limited to infectious agents, environmental pollutions and toxins, carcinogens, and ultraviolet (UV) irradiation. It is estimated that there are thousands of DNA alterations in each cell daily. Therefore, if not efficiently repaired, our genome would rapidly be destroyed. This review focuses predominantly on UV-induced DNA damage in human skin, protective molecular responses to UV damage, and the consequences of these opposing forces for aging and photocarcinogenesis.

Sequential binding of UV DNA damage binding factor and degradation of the p48 subunit as early events after UV irradiation

The UV-damaged DNA binding protein complex (UV-DDB) is implicated in global genomic nucleotide excision repair (NER) in mammalian cells. The complex consists of a heterodimer of p127 and p48. UV-DDB is defective in one complementation group (XP-E) of the heritable, skin cancer-prone disorder xeroderma pigmentosum. Upon UV irradiation of primate cells, UV-DDB associates tightly with chromatin, concomitant with the loss of extractable binding activity. We report here that an early event after UV, but not ionizing, radiation is the transient dose-dependent degradation of the small subunit, p48. Treatment of human cells with the proteasomal inhibitor NIP-L3VS blocks this UV-induced degradation of p48. In XP-E cell lines with impaired UV-DDB binding, p48 is resistant to degradation. UV-mediated degradation of p48 occurs independently of the expression of p53 and the cell's proficiency for NER, but recovery of p48 levels at later times (12 h and thereafter) is dependent upon the capacity of the cell to repair non-transcribed DNA. In addition, we find that the p127 subunit of UV-DDB binds in vivo to p300, a histone acetyltransferase. The data support a functional connection between UV-DDB binding activity, proteasomal degradation of p48 and chromatin remodeling during early steps of NER.

The xeroderma pigmentosum group C gene leads to selective repair of cyclobutane pyrimidine dimers rather than 6-4 photoproducts

We investigated the contribution of the xeroderma pigmentosum group C (XPC) gene to DNA repair. We stably transfected XPC cells (XP4PA-SV-EB) with XPC cDNA and selected a partially corrected (XP4PA-SE1) and a fully corrected (XP4PA-SE2) clone. Cell survival after UVC (254 nm) exposure was low for XP4PA-SV-EB, intermediate for XP4PA-SE1, and normal for XP4PA-SE2 cells. XP4PA-SV-EB cells had undetectable XPC mRNA and protein levels. XP4PA-SE1 cells had 130% of normal mRNA but 25% of normal protein levels, whereas XP4PA-SE2 cells had an 18-fold mRNA overexpression and normal XPC protein levels compared with normal cells. We measured cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts (6-4PP) by using specific mAbs and the ELISA technique. XP4PA-SV-EB cells had no detectable removal of CPD or 6-4PP from their global genome by 24 h after 30 J/m(2) UVC exposure. The partially corrected XP4PA-SE1 cells had normal repair of CPD but minimal repair of 6-4PP by 24 h, whereas the fully corrected XP4PA-SE2 cells regained normal CPD and 6-4PP repair capacities. We also exposed pRSVcat plasmid to UVC (to induce CPD and 6-4PP), to UVC + photolyase (to leave only 6-4PP on the plasmid), or to UVB + acetophenone (to induce only CPD). Host cell reactivation of UVB + acetophenone-, but not of UVC + photolyase-treated plasmids was normal in XP4PA-SE1 cells. Thus, increasing XPC gene expression leads to selective repair of CPD in the global genome. Undetectable XPC protein is associated with no repair of CPD or 6-4PP, detectable but subnormal XPC protein levels reconstitute CPD but not 6-4PP repair, and normal XPC protein levels fully reconstitute both CPD and 6-4PP repair.

Expression and nucleotide excision repair of a UV-irradiated reporter gene in unirradiated human cells

It has been suggested that reactivation of damaged reporter genes introduced into cultured mammalian cells reflects transcription-coupled nucleotide excision repair. To evaluate this possibility directly, we introduced a UV-irradiated shuttle vector, pCMV beta, into unirradiated human cells and compared expression of the reporter gene (lacZ) with repair of cyclobutane pyrimidine dimers (CPDs). Expression of the irradiated reporter gene was more UV resistant in XPC cells, which are deficient in global genome repair, than in CSB cells, which are deficient in transcription-coupled repair. These results are consistent with the idea that repair of the reporter gene is primarily dependent upon transcription-coupled repair. However, when the plasmid DNA was analyzed for removal of CPDs, no clear evidence was obtained for transcription-coupled repair either in XPC cells or in cells with normal repair capacity.

Mutational analysis of a function of xeroderma pigmentosum group A (XPA) protein in strand-specific DNA repair

To analyze the function of the xeroderma pigmentosum group A (XPA) protein in strand-specific DNA repair, we examined repair of UV-induced cyclobutane pyrimidine dimer (CPD) in transcribed and non-transcribed strands of the dihydrofolate reductase gene of xeroderma pigmentosum group A (XP-A) cell line (XP12ROSV) which was transfected with various types of mutant XPA cDNA. The transfectant overexpressing mutant XPA with a defect in the interaction with either ERCC1, replication protein A (RPA), or general transcription factor TFIIH, showed more or less decreased repair of CPD in each strand in parallel, while in the transfectant overexpressing R207G (Arg207to Gly) mutant XPA derived from XP129, a UV-resistant XP12ROSV revertant, the rate of CPD repair was almost normal in each strand. We also examined the dose responses of the XPA protein on CPD repair in each strand by the modulation of the expression levels of wild-type or R207G mutant XPA using an inducible expression system, LacSwitchtrade mark promoter. There were good correlations between the rate of CPD repair in each strand and the amount of XPA protein produced in these Lac cells. Our results indicate that the XPA protein is equally important for the CPD repair in both transcribed and non-transcribed strands and that the R207G mutation found in XP129 may not be responsible for a selective defect in CPD repair in the non-transcribed strand in XP129.

Expression of wild-type p53 is required for efficient global genomic nucleotide excision repair in UV-irradiated human fibroblasts

We have shown previously that Li-Fraumeni syndrome fibroblasts homozygous for p53 mutations are deficient in the removal of UV-induced cyclobutane pyrimidine dimers from genomic DNA, but still proficient in the transcription-coupled repair pathway (Ford, J. M., and Hanawalt, P. C. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 8876-8880). We have now utilized monoclonal antibodies specific for cyclobutane pyrimidine dimers or 6-4 photoproducts, respectively, to measure their repair in UV-irradiated human fibroblasts. Cells homozygous for p53 mutations were deficient in the repair of both photoproducts, whereas cells heterozygous for mutant p53 exhibited normal repair of 6-4 photoproducts, but decreased initial rates of removal of cyclobutane pyrimidine dimers, compared with normal cells. The specificity of the effect of wild-type p53 on nucleotide excision repair was demonstrated in a p53 homozygous mutant cell line containing a tetracycline-regulated wild-type p53 gene. Wild-type p53 expression and activity were suppressed in the presence of tetracycline, whereas withdrawal of tetracycline resulted in the induction of p53 expression, cell cycle checkpoint activation, and DNA damage-induced apoptosis. The regulated expression of wild-type p53 resulted in the recovery of normal levels of repair of both cyclobutane pyrimidine dimers and 6-4 photoproducts in genomic DNA, but did not alter the transcription-coupled repair of cyclobutane pyrimidine dimers. Therefore, the wild-type p53 gene product is an important determinant of nucleotide excision repair activity in human cells.

Fluorescent light-induced chromatid breaks distinguish Alzheimer disease cells from normal cells in tissue culture

The neurodegeneration and amyloid deposition of sporadic Alzheimer disease (AD) also occur in familial AD and in all trisomy-21 Down syndrome (DS) patients, suggesting a common pathogenetic mechanism. We investigated whether defective processing of damaged DNA might be that mechanism, as postulated for the neurodegeneration in xeroderma pigmentosum, a disease with defective repair not only of UV radiation-induced, but also of some oxygen free radical-induced, DNA lesions. We irradiated AD and DS skin fibroblasts or blood lymphocytes with fluorescent light, which is known to cause free radical-induced DNA damage. The cells were then treated with either beta-cytosine arabinoside (araC) or caffeine, and chromatid breaks were quantified. At least 28 of 31 normal donors and 10 of 11 donors with nonamyloid neurodegenerations gave normal test results. All 12 DS, 11 sporadic AD, and 16 familial AD patients tested had abnormal araC and caffeine tests, as did XP-A cells. In one of our four AD families, an abnormal caffeine test was found in all 10 afflicted individuals (including 3 asymptomatic when their skin biopsies were obtained) and in 8 of 11 offspring at a 50% risk for AD. Our tests could prove useful in predicting inheritance of familial AD and in supporting, or rendering unlikely, the diagnosis of sporadic AD in patients suspected of having the disease.

Fanconi anaemia in Italy: high prevalence of complementation group A in two geographic clusters

Cell fusion studies using lymphoblastoid cell lines from Fanconi anaemia (FA) patients have identified five complementation groups (FA-A to FA-E) among European FA patients. In Italy, of the 45 FA families referred to the Italian Registry of Fanconi Anaemia (RIAF), 15 took part in a project for the identification of complementation groups. Since three immortalized lymphoblast lines were resistant to a cross-linking agent, we analysed only 12 patients by complementation analysis and found that 11 belong to complementation group A. Four and seven families came from two geographic clusters in the Veneto and Campania regions, respectively, which are thought to consist of aggregates of related families in reproductive isolation. The clinical characteristics of the patients showed both intra- and interfamilial heterogeneity, although overall the disease had a relatively mild course. Since the populations in both Veneto and Campania are likely to represent genetic isolates, our finding predicts linkage disequilibrium for markers flanking the FAA gene. DNAs from these FA families may thus be utilized for positional cloning of this gene through haplotype disequilibrium mapping.

The levels of repair of endonuclease III-sensitive sites, 6-4 photoproducts and cyclobutane pyrimidine dimers differ in a point mutant for RAD14, the Saccharomyces cerevisiae homologue of the human gene defective in XPA patients

In the accompanying paper we demonstrated that endonuclease III-sensitive sites in the MAT alpha and HML alpha loci of Saccharomyces cerevisiae are repaired by the Nucleotide Excision Repair (NER) pathway. In the current report we investigated the repair of endonuclease III sites, 6-4 photoproducts and cyclobutane pyrimidine dimers (CPDs) in a rad14-2 point mutant and in a rad14 deletion mutant. The RAD14 gene is the yeast homologue of the human gene that complements the defect in cells from xeroderma pigmentosum (XP) patients belonging to complementation group A. In the point mutant we observed normal repair of endonuclease III site (i.e. as wild type), but no removal of CPDs at the MAT alpha and HML alpha loci. Similar experiments were undertaken using the recently created rad14 deletion mutant. Here, neither endonuclease III sites nor CPDs were repaired in MATa or HMRa. Thus the point mutant appears to produce a gene product that permits the repair of endonuclease III sites, but prevents the repair of CPDs. Previously it was found that in the genome overall, repair of 6-4 photoproducts was less impaired that repair of CPDs in the point mutant. The deletion mutant repairs neither CPDs nor 6-4 photoproducts in the genome overall. This finding is consistent with the RAD14 protein being involved in lesion recognition in yeast. A logical interpretation is that the rad14-2 point mutant produces a modified protein that enables the cell to repair endonuclease III sites and 6-4 photoproducts much more efficiently than CPDs. This modified protein may aid studies designed to elucidate the role of the RAD14 protein in lesion recognition.