Effect of DNA lesions on transcription elongation

Decreased UV sensitivity, mismatch repair activity and abnormal cell cycle checkpoints in skin cancer cell lines derived from UVB-irradiated XPA-deficient mice

Xeroderma pigmentosum group A gene (XPA)-deficient mice are defective in nucleotide excision repair (NER) and are therefore highly sensitive to ultraviolet (UV)-induced skin carcinogenesis. We established cell lines from skin cancers of UVB-irradiated XPA-deficient mice to investigate the phenotypic changes occurring during skin carcinogenesis. As anticipated, the skin cancer cell lines were devoid of NER activity but were less sensitive to killing by UV-irradiation than the XPA(-/-) fibroblast cell line. The lines were also more resistant to 6-thioguanine (6-TG) than XPA(-/-) and XPA(+/+) fibroblasts, which was suggestive of a mismatch repair (MMR) defect. Indeed, in vitro mismatch binding and MMR activity were impaired in several of these cell lines. Moreover, these cell lines displayed cell cycle checkpoint derangements following UV-irradiation and 6-TG exposure. The above findings suggest that MMR downregulation may help cells escape killing by UVB, as was seen previously for methylating agents and cisplatin, and thus that MMR deficient clones are selected for during the tumorigenic transformation of XPA(-/-) cells.

Genomic heterogeneity of nucleotide excision repair

Nucleotide excision repair (NER) is one of the major cellular pathways that removes bulky DNA adducts and helix-distorting lesions. The biological consequences of defective NER in humans include UV-light-induced skin carcinogenesis and extensive neurodegeneration. Understanding the mechanism of the NER process is of great importance as the number of individuals diagnosed with skin cancer has increased considerably in recent years, particularly in the United States. Rapid progress made in the DNA repair field since the early 1980s has revealed the complexity of NER, which operates differently in different genomic regions. The genomic heterogeneity of repair seems to be governed by the functional compartmentalization of chromatin into transcriptionally active and inactive domains in the nucleus. Two sub-pathways of NER remove UV-induced photolesions: (I) Global Genome Repair (GGR) and (II) Transcription Coupled Repair (TCR). GGR is a random process that occurs slowly, while the TCR, which is tightly linked to RNA polymerase II transcription, is highly specific and efficient. The efficiency of these pathways is important in avoiding cancer and genomic instability. Studies with cell lines derived from Cockayne syndrome (CS) and Xeroderma pigmentosum (XP) group C patients, that are defective in the NER sub-pathways, have yielded valuable information regarding the genomic heterogeneity of DNA repair. This review deals with the complexity of repair heterogeneity, its mechanism and interacting molecular pathways as well as its relevance in the maintenance of genomic integrity.

Proneness to UV-induced apoptosis in human fibroblasts defective in transcription coupled repair is associated with the lack of Mdm2 transactivation

The apoptotic response and the level of expression of p53 and of three genes transcriptionally activated by p53 (Mdm2, p21 and bax) were investigated in UV-sensitive cells from patients with xeroderma pigmentosum (XP) or Cockayne syndrome (CS). These disorders are due to different genetic defects affecting transcription-coupled repair (TCR) and/or global genome repair (GGR), the nucleotide excision repair subpathways which remove UV-induced lesions from the transcribed strand of active genes or from the rest of the genome, respectively. After 20 J/m2 UV light, normal and GGR-defective XP-C fibroblasts showed rapid increase in p53, late induction of Mdm2 and no evidence of apoptosis even 96 h after irradiation. In contrast, in XP-A (defective in GGR and TCR), CS-A and CS-B (defective only in TCR) fibroblasts, the p53 increase was not followed by Mdm2 induction and the persistence of high levels of p53, due to the lack of its degradation by Mdm2, was associated with the appearance of apoptosis. Besides indicating that the persistence of DNA damage in the transcribed strand of active genes leads to apoptosis, these findings provide the first evidence that the lack of activation of Mdm2 plays a key role in the cascade of events leading to apoptosis. Oncogene (2000).

Decreased DNA repair efficiency by loss or disruption of p53 function preferentially affects removal of cyclobutane pyrimidine dimers from non-transcribed strand and slow repair sites in transcribed strand

The tumor suppressor protein p53 plays a central role in modulating the cellular responses to DNA damage. Several recent studies, undertaken with the whole genomic DNA or full-length gene segments, have shown that p53 is involved in nucleotide excision repair and it selectively influences the adduct removal from the non-transcribed strand in the genome. In this study, we have analyzed the damage induction at nucleotide resolution by ligase-mediated polymerase chain reaction and compared the repair of ultraviolet radiation-induced cyclobutane pyrimidine dimers within exon 8 of p53 gene in normal and Li-Fraumeni syndrome fibroblasts as well as in normal and human papillomavirus 16 E6 and E7 protein-expressing human mammary epithelial cells. The results demonstrate that (i) loss or disruption of p53 function decreases efficiency of DNA repair, by preferentially affecting the repair of non-transcribed strand and of intrinsically slow repair sites in transcribed strand; (ii) mutant p53 protein affects DNA repair, at least of non-transcribed strand, in a dominant negative manner; and (iii) pRb does not have an effect on the repair of DNA damage within transcribed or non-transcribed strand. The overall data suggest that p53 could regulate excision repair or related events through direct protein-protein interaction.

Structural characterization of RNA polymerase II complexes arrested by a cyclobutane pyrimidine dimer in the transcribed strand of template DNA

We have characterized the properties of immunopurified transcription complexes arrested at a specifically located cyclobutane pyrimidine dimer (CPD) using enzymatic probes and an in vitro transcription system with purified RNA polymerase II (RNAP II) and initiation factors. To help understand how RNAP II distinguishes between a natural impediment and a lesion in the DNA to initiate a repair event, we have compared the conformation of RNAP II complexes arrested at a CPD with complexes arrested at a naturally occurring elongation impediment. The footprint of RNAP II arrested at a CPD, using exonuclease III and T4 DNA polymerase's 3'-->5' exonuclease, covers approximately 35 base pairs and is asymmetrically located around the dimer. A similar footprint is observed when RNAP II is arrested at the human histone H3.3 arrest site. Addition of elongation factor SII to RNAP II arrested at a CPD produced shortened transcripts of discrete lengths up to 25 nucleotides shorter than those seen without SII. After addition of photolyase and exposure to visible light, some of the transcripts could be reelongated beyond the dimer, suggesting that SII-mediated transcript cleavage accompanied significant RNAP II backup, thereby providing access of the repair enzyme to the arresting CPD.