Human cells bearing homozygous mutations in the DNA mismatch repair genes hMLH1 or hMSH2 are fully proficient in transcription-coupled nucleotide excision repair

DNA repair and genomic stability in lungs affected by acute injury

Acute pulmonary injury, or acute respiratory distress syndrome, has a high incidence in elderly individuals and high mortality in its most severe degree, becoming a challenge to public health due to pathophysiological complications and increased economic burden. Acute pulmonary injury can develop from sepsis, septic shock, and pancreatitis causing reduction of alveolar airspace due to hyperinflammatory response. Oxidative stress acts directly on the maintenance of inflammation, resulting in tissue injury, as well as inducing DNA damages. Once the DNA is damaged, enzymatic DNA repair mechanisms act on lesions in order to maintain genomic stability and, consequently, contribute to cell viability and homeostasis. Although palliative treatment based on mechanical ventilation and antibiotic using have a kind of efficacy, therapies based on modulation of DNA repair and genomic stability could be effective for improving repair and recovery of lung tissue in patients with acute pulmonary injury.

DNA mismatch repair preferentially safeguards actively transcribed genes

DNA mismatch repair (MMR) is an evolutionally conserved genome maintenance pathway and is well known for its role in maintaining replication fidelity by correcting biosynthetic errors generated during DNA replication. However, recent studies have shown that MMR preferentially protects actively transcribed genes from mutation during both DNA replication and transcription. This review describes the recent discoveries in this area. Potential mechanisms by which MMR safeguards actively transcribed genes are also discussed.

H3K36me3-mediated mismatch repair preferentially protects actively transcribed genes from mutation

Histone H3 trimethylation at lysine 36 (H3K36me3) is an important histone mark involved in both transcription elongation and DNA mismatch repair (MMR). It is known that H3K36me3 recruits the mismatch-recognition protein MutSα to replicating chromatin via its physical interaction with MutSα's PWWP domain, but the exact role of H3K36me3 in transcription is undefined. Using ChIP combined with whole-genome DNA sequencing analysis, we demonstrate here that H3K36me3, together with MutSα, is involved in protecting against mutation, preferentially in actively transcribed genomic regions. We found that H3K36me3 and MutSα are much more co-enriched in exons and actively transcribed regions than in introns and nontranscribed regions. The H3K36me3-MutSα co-enrichment correlated with a much lower mutation frequency in exons and actively transcribed regions than in introns and nontranscribed regions. Correspondingly, depleting H3K36me3 or disrupting the H3K36me3-MutSα interaction elevated the spontaneous mutation frequency in actively transcribed genes, but it had little influence on the mutation frequency in nontranscribed or transcriptionally inactive regions. Similarly, H₂O₂-induced mutations, which mainly cause base oxidations, preferentially occurred in actively transcribed genes in MMR-deficient cells. The data presented here suggest that H3K36me3-mediated MMR preferentially safeguards actively transcribed genes not only during replication by efficiently correcting mispairs in early replicating chromatin but also during transcription by directly or indirectly removing DNA lesions associated with a persistently open chromatin structure.

DNA mismatch repair and its many roles in eukaryotic cells

DNA mismatch repair (MMR) is an important DNA repair pathway that plays critical roles in DNA replication fidelity, mutation avoidance and genome stability, all of which contribute significantly to the viability of cells and organisms. MMR is widely-used as a diagnostic biomarker for human cancers in the clinic, and as a biomarker of cancer susceptibility in animal model systems. Prokaryotic MMR is well-characterized at the molecular and mechanistic level; however, MMR is considerably more complex in eukaryotic cells than in prokaryotic cells, and in recent years, it has become evident that MMR plays novel roles in eukaryotic cells, several of which are not yet well-defined or understood. Many MMR-deficient human cancer cells lack mutations in known human MMR genes, which strongly suggests that essential eukaryotic MMR components/cofactors remain unidentified and uncharacterized. Furthermore, the mechanism by which the eukaryotic MMR machinery discriminates between the parental (template) and the daughter (nascent) DNA strand is incompletely understood and how cells choose between the EXO1-dependent and the EXO1-independent subpathways of MMR is not known. This review summarizes recent literature on eukaryotic MMR, with emphasis on the diverse cellular roles of eukaryotic MMR proteins, the mechanism of strand discrimination and cross-talk/interactions between and co-regulation of MMR and other DNA repair pathways in eukaryotic cells. The main conclusion of the review is that MMR proteins contribute to genome stability through their ability to recognize and promote an appropriate cellular response to aberrant DNA structures, especially when they arise during DNA replication. Although the molecular mechanism of MMR in the eukaryotic cell is still not completely understood, increased used of single-molecule analyses in the future may yield new insight into these unsolved questions.

Influence of a pre-stimulation with chronic low-dose UVB on stress response mechanisms in human skin fibroblasts

Exposure to solar ultraviolet type B (UVB), through the induction of cyclobutane pyrimidine dimer (CPD), is the major risk factor for cutaneous cancer. Cells respond to UV-induced CPD by triggering the DNA damage response (DDR) responsible for signaling DNA repair, programmed cell death and cell cycle arrest. Underlying mechanisms implicated in the DDR have been extensively studied using single acute UVB irradiation. However, little is known concerning the consequences of chronic low-dose of UVB (CLUV) on the DDR. Thus, we have investigated the effect of a CLUV pre-stimulation on the different stress response pathways. We found that CLUV pre-stimulation enhances CPD repair capacity and leads to a cell cycle delay but leave residual unrepaired CPD. We further analyzed the consequence of the CLUV regimen on general gene and protein expression. We found that CLUV treatment influences biological processes related to the response to stress at the transcriptomic and proteomic levels. This overview study represents the first demonstration that human cells respond to chronic UV irradiation by modulating their genotoxic stress response mechanisms.

Multi-Scale Genomic, Transcriptomic and Proteomic Analysis of Colorectal Cancer Cell Lines to Identify Novel Biomarkers

Selecting colorectal cancer (CRC) patients likely to respond to therapy remains a clinical challenge. The objectives of this study were to establish which genes were differentially expressed with respect to treatment sensitivity and relate this to copy number in a panel of 15 CRC cell lines. Copy number variations of the identified genes were assessed in a cohort of CRCs. IC50's were measured for 5-fluorouracil, oxaliplatin, and BEZ-235, a PI3K/mTOR inhibitor. Cell lines were profiled using array comparative genomic hybridisation, Illumina gene expression analysis, reverse phase protein arrays, and targeted sequencing of KRAS hotspot mutations. Frequent gains were observed at 2p, 3q, 5p, 7p, 7q, 8q, 12p, 13q, 14q, and 17q and losses at 2q, 3p, 5q, 8p, 9p, 9q, 14q, 18q, and 20p. Frequently gained regions contained EGFR, PIK3CA, MYC, SMO, TRIB1, FZD1, and BRCA2, while frequently lost regions contained FHIT and MACROD2. TRIB1 was selected for further study. Gene enrichment analysis showed that differentially expressed genes with respect to treatment response were involved in Wnt signalling, EGF receptor signalling, apoptosis, cell cycle, and angiogenesis. Stepwise integration of copy number and gene expression data yielded 47 candidate genes that were significantly correlated. PDCD6 was differentially expressed in all three treatment responses. Tissue microarrays were constructed for a cohort of 118 CRC patients and TRIB1 and MYC amplifications were measured using fluorescence in situ hybridisation. TRIB1 and MYC were amplified in 14.5% and 7.4% of the cohort, respectively, and these amplifications were significantly correlated (p≤0.0001). TRIB1 protein expression in the patient cohort was significantly correlated with pERK, Akt, and Caspase 3 expression. In conclusion, a set of candidate predictive biomarkers for 5-fluorouracil, oxaliplatin, and BEZ235 are described that warrant further study. Amplification of the putative oncogene TRIB1 has been described for the first time in a cohort of CRC patients.

Human telomeres are hypersensitive to UV-induced DNA Damage and refractory to repair

Telomeric repeats preserve genome integrity by stabilizing chromosomes, a function that appears to be important for both cancer and aging. In view of this critical role in genomic integrity, the telomere's own integrity should be of paramount importance to the cell. Ultraviolet light (UV), the preeminent risk factor in skin cancer development, induces mainly cyclobutane pyrimidine dimers (CPD) which are both mutagenic and lethal. The human telomeric repeat unit (5′TTAGGG/CCCTAA3′) is nearly optimal for acquiring UV-induced CPD, which form at dipyrimidine sites. We developed a ChIP–based technique, immunoprecipitation of DNA damage (IPoD), to simultaneously study DNA damage and repair in the telomere and in the coding regions of p53, 28S rDNA, and mitochondrial DNA. We find that human telomeres in vivo are 7-fold hypersensitive to UV-induced DNA damage. In double-stranded oligonucleotides, this hypersensitivity is a property of both telomeric and non-telomeric repeats; in a series of telomeric repeat oligonucleotides, a phase change conferring UV-sensitivity occurs above 4 repeats. Furthermore, CPD removal in the telomere is almost absent, matching the rate in mitochondria known to lack nucleotide excision repair. Cells containing persistent high levels of telomeric CPDs nevertheless proliferate, and chronic UV irradiation of cells does not accelerate telomere shortening. Telomeres are therefore unique in at least three respects: their biophysical UV sensitivity, their prevention of excision repair, and their tolerance of unrepaired lesions. Utilizing a lesion-tolerance strategy rather than repair would prevent double-strand breaks at closely-opposed excision repair sites on opposite strands of a damage-hypersensitive repeat.

Telomeres consist of a repeated sequence located at each end of each chromosome. This repeated sequence is required for chromosomal stability and integrity, a function important for both cancer and aging. The DNA sequence of human telomeres is 5–10 kb of a repeated double-strand hexamer (5′TTAGGG/5′CCCTAA). In theory, this sequence is nearly optimal for acquiring UV-induced DNA damage. We developed a novel technique, the immunoprecipitation of DNA damage (IPoD), to study DNA damage induction and repair in the telomere and in coding regions (p53, 28S rDNA, and mitochondrial DNA). We find that human telomeres are hypersensitive to UV-induced DNA photoproducts and that the removal of those DNA photoproducts is almost absent. Cells containing persistent high levels of telomeric DNA damage nevertheless proliferate and chronic UV irradiation of cells does not accelerate telomere shortening. Telomeres are therefore unique in at least three respects: their biophysical UV sensitivity, their prevention of excision repair, and their tolerance of unrepaired lesions.

UV damage and DNA repair in malignant melanoma and nonmelanoma skin cancer

Exposition of the skin with solar ultraviolet radiation (UV) is the main cause of skin cancer development. The consistently increasing incidences of melanocytic and nonmelanocytic skin tumors are believed to be at least in part associated with recreational sun exposure. Epidemiological data indicate that excessive or cumulative sunlight exposition takes place years and decades before the resulting malignancies arise. The most important defense mechanisms that protect human skin against UV radiation involve melanin synthesis and active repair mechanisms. DNA is the major target of direct or indirect UV-induced cellular damage. Low pigmentation capacity in white Caucasians and rare congenital defects in DNA repair are mainly responsible for protection failures. The important function of nucleotide excision DNA repair (NER) to protect against skin cancer becomes obvious by the rare genetic disease xeroderma pigmentosum, in which diverse NER genes are mutated. In animal models, it has been demonstrated that UVB is more effective to induce skin cancer than UVA. UV-induced DNA photoproducts are able to cause specific mutations (UV-signature) in susceptible genes for squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). In SCC development, UV-signature mutations in the p513 tumor suppressor gene are the most common event, as precancerous lesions reveal approximately 80% and SCCs > 90% UV-specific p53 mutations. Mutations in Hedgehog pathway related genes, especially PTCH1, are well known to represent the most significant pathogenic event in BCC. However, specific UV-induced mutations can be found only in approximately 50% of sporadic BCCs. Thus, cumulative UVB radiation can not be considered to be the single etiologic risk factor for BCC development. During the last decades, experimental animal models, including genetically engineered mice, the Xiphophorus hybrid fish, the south american oppossum and human skin xenografts, have further elucidated the important role of the DNA repair system in the multi-step process of UV-induced melanomagenesis. An increasing body of evidence now indicates that nucleotide excision repair is not the only DNA repair pathway that is involved in UV-induced tumorigenesis of melanoma and nonmelanoma skin cancer. An interesting new perspective in DNA damage and repair research lies in the participation of mammalian mismatch repair (MMR) in UV damage correction. As MMR enzyme hMSH2 displays a p53 target gene, is induced by UVB radiation and is involved in NER pathways, studies have now been initiated to elucidate the physiological and pathophysiological role of MMR in malignant melanoma and nonmelanoma skin cancer development.

Predicting cisplatin and trabectedin drug sensitivity in ovarian and colon cancers

Molecular profiling of markers involved in the activity of chemotherapeutic agents can shed light on the successes and failures of treatment in patients and can also provide a basis for individualization of therapy. Toward those ends, we have used reverse-phase protein lysate microarrays to evaluate expression of protein components of the nucleotide excision repair (NER) pathways. Those pathways strongly influence the anticancer activities of numerous drugs, including those that are the focus here, cisplatin and ecteinascidin 743 (Et-743; Yondelis, Trabectedin). Cisplatin is generally more active in cell types deficient in NER, whereas Et-743 tends to be less active in those cells. We measured protein expression and sensitivity to those drugs in 17 human ovarian and colon cancer cell lines (13 of them from the NCI-60 panel) and five xeroderma pigmentosum (XP) patient cell types, each containing a different NER defect. Of the NER proteins giving reliable signals, XPF and XPG showed the highest correlations of protein expression with drug activity across all three tissue-of-origin groups. When we compared protein expression data with mRNA expression data from Affymetrix U133A chips, we found no consistent correlation between the two across the cell lines studied, which reinforces the conclusion that protein measurements can give more interpretable mechanistic information than can transcript measurements. The work reported here provides motivation for larger proteomic studies with more cell types focused on potential biomarkers in additional pharmacologically pertinent pathways.

Reduced host cell reactivation of oxidative DNA damage in human cells deficient in the mismatch repair gene hMSH2

Germ line mutations in the mismatch repair (MMR) genes hMSH2 and hMLH1 account for approximately 98% of hereditary non-polyposis colorectal cancers. In addition, there is increasing evidence for an involvement of MMR gene expression in the response of cells to UV-induced skin cancer. The link between MMR and skin cancer suggests an involvement of MMR gene expression in the response of skin cells to UV-induced DNA damage. In this report, we have used two reporter gene assays to examine the role of hMSH2 and hMLH1 in the repair of oxidative DNA damage induced by UVA light and DNA damage caused by methylene blue plus visible light (MB+VL). UVA and MB+VL produce 8-hydroxyguanines in DNA that are repaired by base excision repair (BER). AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase (beta-gal) reporter gene under the control of the human cytomegalovirus (CMV) immediate-early promoter. We show a reduced host cell reactivation for beta-gal expression of UVA-treated and MB+VL-treated AdHCMVlacZ in hMSH2-deficient LoVo human colon adenocarcinoma cells compared to their hMSH2-proficient counterpart SW480 cells, but not in hMLH1-deficient HCT116 human colon adenocarcinoma cells compared to hMLH1-proficient HCT116-chr3 cells. We have also reported previously that enhanced expression of the undamaged AdHCMVlacZ reporter gene is induced by the pre-treatment of cells with lower levels of the DNA-damaging agent and to higher expression levels in transcription-coupled repair (TCR)-deficient compared to TCR-proficient cells. Here we show that pre-treatment of cells with UVA or MB+VL enhanced expression of the undamaged reporter gene to a higher level in LoVo compared to SW480 cells but there was little or no difference in HCT116 compared to HCT116-chr3 cells. These results suggest a substantial involvement of hMSH2 but little or no involvement of hMLH1 in the repair of UVA- and MB+VL-induced oxidative DNA damage by BER.

Involvement of mismatch repair in transcription-coupled nucleotide excision repair

Nucleotide excision repair (NER) is a versatile repair pathway to remove a variety of DNA distorting lesions. NER operate via two subpathways, that are global genome repair (GGR) and transcription coupled nucleotide excision repair (TCR). GGR removes DNA damage from the genome over all, whilst TCR is selectively directed to DNA lesions in the transcribed strand of expressed genes. The damage recognition step in GGR and TCR is also different. In GGR, the XPC-HR23B complex is an essential factor to recruit proteins for subsequent process. In TCR, a stalled RNA polymerase II is a presumed trigger to initiate TCR machinery in concert with Cockayne syndrome (CS) proteins. Mismatch repair (MMR) keeps fidelity of DNA replication through correcting replication errors. A distinctive feature of MMR pathway is that this repair is directed exclusively to the newly synthesized strand. This characteristic contributes to mediation of cytotoxity by methylating agents, and MMR deficient cells are more resistant to methylating agents than MMR proficient cells. The interaction between MMR and NER has been reported by several investigators. However, the most controversial problem is the role of MMR in TCR TCR in E. coli requires the participation of the MutS and MutL MMR proteins. On the contrary, TCR in yeast is independent of the yeast MutS and MutL homologues. To date, in mammalian cells, there are conflicting evidences for the association of MMR with TCR pathway. The aim of this article is to provide a brief overview of the recent literature on this subject.

Optimal conditions and specific characteristics of Vent exo- DNA polymerase in ligation-mediated polymerase chain reaction protocols

An optimized procedure for the ligation-mediated polymerase chain reaction (PCR) technique using Thermococcus litoralis exo- DNA polymerase (Vent exo-) was developed. The optimal dosage of Vent exo- at the primer extension and PCR amplification steps as well as the optimal DNA quantity to use were established. We showed that Vent exo- can efficiently create the blunt-ended termini required for subsequent linker ligation. Vent exo- proves to be more efficient than Pyrococcus furiosus exo- (Pfu exo-) for this task. Vent exo- resolves highly GC-rich sequence substantially better than Thermus aquaticus DNA polymerase (Taq) and with a similar efficiency as Pfu exo-. The DNA/DNA polymerase activity ratio is significantly higher for Vent exo- than for Pfu exo-, which is reflected by the sensibility of Vent exo- in efficiently amplifying genomic DNA. Furthermore, the range of efficiency of Vent exo- demonstrates the importance of conducting evaluative testing to identify the optimal dosage of use of this polymerase to obtain successful PCR amplification. Optimal MgSO4 concentrations to use with Vent exo- were established. Our results show that Vent exo- DNA polymerase produces bands of uniform and strong intensity and can efficiently be used for the analysis of DNA in living cells by ligation-mediated PCR.

SW480, a p53 Double-mutant Cell Line Retains Proficiency for Some p53 Functions

During certain types of cellular stress, the p53 tumor suppressor protein binds to DNA and transactivates a variety of genes that regulate critical responses including apoptosis, cell cycle checkpoints, differentiation, and angiogenesis. In addition, functional p53 is known to be required for efficient nucleotide excision repair (NER) of bulky DNA adducts generated through exposure to environmental mutagens such as UV light. Nonetheless, we previously showed that the model p53-mutated human adenocarcinoma strain SW480 is proficient in the removal of UV-induced cyclobutane pyrimidine dimers (CPD) via NER. We undertook the present study to begin probing the molecular basis for this unexpected repair phenotype. Cytogenetic analysis indicated that SW480 is stable at the chromosomal level, i.e. manifests a karyotypic profile very similar to that revealed for this line as far back as 14 years ago. After fluorescence in situ hybridization (FISH), using a probe complementary to the p53 gene, we found that 98% of the SW480 interphase nuclei contains three copies of the gene, later revealed to be localized on intact short arms of three chromosomes 17. DNA sequence analysis further showed that all three p53 copies in SW480 carry two point mutations (R273H and P309S), and levels of the corresponding mutated p53 protein are about 20-fold higher than in the closely related p53 wild-type strain LoVo. Using an electrophoretic mobility shift assay (EMSA), we demonstrated that R273H/P309S p53 is able to bind with wild-type affinity to its consensus DNA sequence in vitro. Analysis of p21(Cip1/WAF1) expression and in vivo footprinting by ligation-mediated PCR (LMPCR) showed that, in wild-type LoVo cells, an exposure to cellular stress (e.g. UV or ionizing radiation) is necessary for p53 activation of the p21(Cip1/WAF1) promoter. In contrast, the R273H/P309S-mutated p53 protein in SW480 constitutively activates p21(Cip1/WAF1) in the absence of stress through an unknown mechanism. A similar phenomenon whereby mutated p53 in SW480 is able to induce NER-related proteins might explain the normal DNA repair phenotype previously observed in this strain. For now we conclude that, in general, results obtained using SW480 as a p53-deficient cell line should be interpreted very cautiously.

XPC lymphoblastoid cells defective in the hMutSalpha DNA mismatch repair complex exhibit normal sensitivity to UVC radiation and normal transcription-coupled excision repair of DNA cyclobutane pyrimidine dimers

Nucleotide excision (NER) is generally considered to comprise two partially distinct subpathways. Global genomic repair (GGR) removes damage from the genome overall and transcription-coupled repair (TCR) selectively excises damage from transcribed DNA. Cells from individuals belonging to xeroderma pigmentosum (XP) complementation group C are defective in GGR but retain a functional TCR pathway. DNA mismatch repair (MMR) corrects replication errors but can also process DNA damage. It has been suggested that the essential hMutSalpha and hMutLalpha MMR protein complexes are also required for effective excision of UV-induced cyclobutane pyrimidine dimers (CPD) by TCR. We have combined an MMR and an XPC defect in a human lymphoblastoid cell line. The MMR-defective XPC cells were defective in the hMutSalpha mismatch recognition complex that comprises hMSH2 and hMSH6. They were not detectably more sensitive to killing by UV than their MMR proficient counterparts and were able to excise CPDs from an actively transcribed DNA strand. We conclude efficient TCR does not depend on a functional hMutSalpha complex.

Detection of an involvement of the human mismatch repair genes hMLH1 and hMSH2 in nucleotide excision repair is dependent on UVC fluence to cells

There is conflicting evidence for the role of the mismatch repair (MMR) genes hMLH1 and hMSH2 in the transcription-coupled repair (TCR) pathway of nucleotide excision repair. In the present work, we have examined the role of these MMR genes in nucleotide excision repair using two reporter gene assays. AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase reporter gene under the control of the human cytomegalovirus immediate early promoter. We have reported previously a reduced host cell reactivation (HCR) for beta-galactosidase expression of UVC-irradiated AdHCMVlacZ in TCR-deficient Cockayne syndrome (CS) fibroblasts compared with normal fibroblasts, indicating that HCR depends, at least in part, on TCR. In addition, we have reported that UVC-enhanced expression of the undamaged reporter gene is induced at lower UVC fluences to cells and at higher levels after low UVC fluences in TCR-deficient compared with normal human fibroblasts, suggesting that persistent damage in active genes triggers increased activity from the human cytomegalovirus-driven reporter construct. We have examined HCR and UV-enhanced expression of the reporter gene in hMLH1-deficient HCT116 human colon adenocarcinoma cells and HCT116-chr3 cells (the MMR-proficient counterpart of HCT116) as well as hMSH2-deficient LoVo human colon adenocarcinoma cells and their hMSH2-proficient counterpart SW480 cells. We show a greater UV-enhanced expression of the undamaged reporter gene after low UVC exposure in HCT116 compared with HCT116-chr3 cells and in LoVo compared with SW480 cells. We show also a reduced HCR in HCT116 compared with HCT116-chr3 cells and in LoVo compared with SW480 cells. However, the reduction in HCR was less or absent when cells were pretreated with UVC. These results suggest that detection of an involvement of hMLH1 and hMSH2 in TCR is dependent on UVC (254 nm) fluence to cells.

DNA mismatch repair protein Msh6 is required for optimal levels of ultraviolet-B-induced apoptosis in primary mouse fibroblasts

Recent data support a role for DNA mismatch repair in the cellular response to some forms of exogenous DNA damage beyond that of DNA repair; cells with defective DNA mismatch repair have partial or complete failure to undergo apoptosis and/or G2M arrest following specific types of damage. We propose that the DNA mismatch repair Msh2/Msh6 heterodimer, responsible for the detection of DNA damage, promotes apoptosis in normal cells, thus protecting mammals from ultraviolet-induced malignant transformation. Using primary mouse embryonic fibroblasts derived from Msh6+/+ and Msh6-/- mice, we compare the response of DNA-mismatch repair-proficient and -deficient cells to ultraviolet B radiation. In the wild-type mouse embryonic fibroblasts, ultraviolet-B-induced increases in Msh6 protein levels were not dependent on p53. Msh6-/- mouse embryonic fibroblasts were significantly less sensitive to the cytotoxic effects of ultraviolet B radiation. Further comparison of the Msh6+/+ and Msh6-/- mouse embryonic fibroblasts revealed that Msh6-/- mouse embryonic fibroblasts undergo significantly less apoptosis following ultraviolet B irradiation, thus indicating that ultraviolet-B-induced apoptosis is partially Msh6 dependent. These data support a role for Msh6 in protective cellular responses of primary cells to ultraviolet-B-induced mutagenesis and, hence, the prevention of skin cancer.

Cockayne syndrome group B cellular and biochemical functions

The devastating genetic disorder Cockayne syndrome (CS) arises from mutations in the CSA and CSB genes. CS is characterized by progressive multisystem degeneration and is classified as a segmental premature-aging syndrome. The CS complementation group B (CSB) protein is at the interface of transcription and DNA repair and is involved in transcription-coupled and global genome-DNA repair, as well as in general transcription. Recent structure-function studies indicate a process-dependent variation in the molecular mechanism employed by CSB and provide a starting ground for a description of the mechanisms and their interplay.

DNA mismatch repair proteins: potential guardians against genomic instability and tumorigenesis induced by ultraviolet photoproducts

In addition to their established role in repairing post-replicative DNA errors, DNA mismatch repair proteins contribute to cell cycle arrest and apoptosis in response to a wide range of exogenous DNA damage (e.g., alkylation-induced lesions). The role of DNA mismatch repair in response to ultraviolet-induced DNA damage has been historically controversial. Recent data, however, suggest that DNA mismatch repair proteins probably do not contribute to the removal of ultraviolet-induced DNA damage, but may be important in suppressing mutagenesis, effecting apoptosis, and suppressing tumorigenesis following exposure to ultraviolet radiation.

UV wavelength-dependent regulation of transcription-coupled nucleotide excision repair in p53-deficient human cells

Nucleotide excision repair (NER) prevents skin cancer by eliminating highly genotoxic cyclobutane pyrimidine dimers (CPDs) induced in DNA by the UVB component of sunlight. NER consists of two distinct but overlapping subpathways, i.e., global NER, which removes CPD from the genome overall, and transcription-coupled NER (TCNER), which removes CPD uniquely from the transcribed strand of active genes. Previous investigations have clearly established that the p53 tumor suppressor plays a crucial role in the NER process. Here we used the ligation-mediated PCR technique to demonstrate, at nucleotide resolution along two chromosomal genes in human cells, that the requirement for functional p53 in TCNER, but not in global NER, depends on incident UV wavelength. Indeed, relative to an isogenic p53 wild-type counterpart, p53-deficient human lymphoblastoid strains were shown to remove CPD significantly less efficiently along both the transcribed and nontranscribed strands of the c-jun and hprt loci after exposure to polychromatic UVB (290-320 nm). However, in contrast, after irradiation with 254-nm UV, p53 deficiency engendered less efficient CPD repair only along the nontranscribed strands of these target genes. The revelation of this intriguing wavelength-dependent phenomenon reconciles an apparent conflict between previous studies which used either UVB or 254-nm UV to claim, respectively, that p53 is required for, or plays no role whatsoever in, TCNER of CPD. Furthermore, our finding highlights a major caveat in experimental photobiology by providing a prominent example where the extensively used "nonsolar" model mutagen 254-nm UV does not accurately replicate the effects of environmentally relevant UVB.