The DNA-mismatch repair enzyme hMSH2 modulates UV-B-induced cell cycle arrest and apoptosis in melanoma cells

TREX2 deficiency suppresses spontaneous and genotoxin-associated mutagenesis

TREX2, a 3'-5' exonuclease, is a part of the DNA damage tolerance (DDT) pathway that stabilizes replication forks (RFs) by ubiquitinating PCNA along with the ubiquitin E3 ligase RAD18 and other DDT factors. Mismatch repair (MMR) corrects DNA polymerase errors, including base mismatches and slippage. Here we demonstrate that TREX2 deletion reduces mutations in cells upon exposure to genotoxins, including those that cause base lesions and DNA polymerase slippage. Importantly, we show that TREX2 generates most of the spontaneous mutations in MMR-mutant cells derived from mice and people. TREX2-induced mutagenesis is dependent on the nuclease and DNA-binding attributes of TREX2. RAD18 deletion also reduces spontaneous mutations in MMR-mutant cells, albeit to a lesser degree. Inactivation of both MMR and TREX2 additively increases RF stalls, while it decreases DNA breaks, consistent with a synthetic phenotype.

Exogenous melatonin enhances soybean (Glycine max (L.) Merr.) seedling tolerance to saline-alkali stress by regulating antioxidant response and DNA damage repair

Saline-alkali (SA) stress induces excessive reactive oxygen species (ROS) accumulation in plant cells, resulting in oxidative damages of membranes, lipids, proteins, and nucleic acids. Melatonin has antioxidant protection effects in living organisms and thus has received a lot of attention. This study aimed to investigate the effect and regulating mechanism of melatonin treatment on soybean tolerance to SA stress. In this study, cultivars Heihe 49 (HH49, SA-tolerant) and Henong 95 (HN95, SA-sensitive) were pot-cultured in SA soil, then treated with MT (0-300 μM) at V1 stage. SA stress induced ROS accumulation and DNA damage in the seedling roots of both cultivars, causing G1/S arrest in HN95 and G2/M arrest in HH49. Melatonin treatment enhanced the activity of antioxidant enzymes in soybean seedling roots and reduced ROS accumulation. Additionally, melatonin treatment upregulated DNA damage repair genes, thus enhancing the reduction of DNA oxidative damage under SA stress. The effects of melatonin treatment were manifested as decreased RAPD polymorphism, 8-hydroxy-2'-deoxyguanine (8-OH-dG) level, and relative density of apurinic sites (AP-sites). Meanwhile, melatonin treatment partially alleviated the SA-induced G1/S arrest in HN95 and G2/M arrest in HH49, thus enhancing soybean seedling tolerance to SA stress.

Identification of Distinct Molecular Subtypes of Endometrioid Adenocarcinoma

Endometrial carcinoma (EC) is one of the most common gynecological cancers worldwide. Endometrioid adenocarcinoma (EAC) is the major form of EC, accounting for 75–80% of cases. Currently, there is no molecular classification system for EAC, so there are no corresponding targeted treatments. In this study, we identified two distinct molecular subtypes of EAC with different gene expression patterns and clinicopathologic characteristics. Subtype I EAC cases, accounting for the majority of cases (56%), were associated with an earlier stage, a more well-differentiated grade, a lower tumor invasion rate, and a more favorable prognosis, and the median tumor necrosis percent (15%) was also significantly higher in subtype I EAC. In contrast, subtype II EAC represents high-grade EAC, with a higher tumor invasion rate and tumor weight. The up-regulated genes in subtype I EAC were associated with the immune response, defense response, cell motion, and cell motility pathway, whereas the up-regulated genes in subtype II EAC were associated with the cell cycle, DNA replication, and RNA processing pathways. Additionally, we identified three potential subtype-specific biomarkers, comprising MDM2 (MDM2 proto-oncogene) for subtype I, and MSH2 (mutS homolog 2) and MSH6 (mutS homolog 6) for subtype II.

DNA Mismatch Repair Gene Variants in Sporadic Solid Cancers

The phenotypic effects of single nucleotide polymorphisms (SNPs) in the development of sporadic solid cancers are still scarce. The aim of this review was to summarise and analyse published data on the associations between SNPs in mismatch repair genes and various cancers. The mismatch repair system plays a unique role in the control of the genetic integrity and it is often inactivated (germline and somatic mutations and hypermethylation) in cancer patients. Here, we focused on germline variants in mismatch repair genes and found the outcomes rather controversial: some SNPs are sometimes ascribed as protective, while other studies reported their pathological effects. Regarding the complexity of cancer as one disease, we attempted to ascertain if particular polymorphisms exert the effect in the same direction in the development and treatment of different malignancies, although it is still not straightforward to conclude whether polymorphisms always play a clear positive role or a negative one. Most recent and robust genome-wide studies suggest that risk of cancer is modulated by variants in mismatch repair genes, for example in colorectal cancer. Our study shows that rs1800734 in MLH1 or rs2303428 in MSH2 may influence the development of different malignancies. The lack of functional studies on many DNA mismatch repair SNPs as well as their interactions are not explored yet. Notably, the concerted action of more variants in one individual may be protective or harmful. Further, complex interactions of DNA mismatch repair variations with both the environment and microenvironment in the cancer pathogenesis will deserve further attention.

MSH2 and MSH6 in Mismatch Repair System Account for Soybean (Glycine max (L.) Merr.) Tolerance to Cadmium Toxicity by Determining DNA Damage Response

Our aim was to investigate DNA mismatch repair (MMR) genes regulating cadmium tolerance in two soybean cultivars. Cultivars Liaodou 10 (LD10, Cd-sensitive) and Shennong 20 (SN20, Cd-tolerant) seedlings were grown hydroponically on Murashige and Skoog (MS) media containing 0-2.5 mg·L-1 Cd for 4 days. Cd stress induced less random amplified polymorphism DNA (RAPD) polymorphism in LD10 than in SN20 roots, causing G1/S arrest in LD10 and G2/M arrest in SN20 roots. Virus-induced gene silencing (VIGS) of MLH1 in LD10-TRV-MLH1 plantlets showed markedly diminished G1/S arrest but enhanced root length/area under Cd stress. However, an increase in G1/S arrest and reduction of G2/M arrest occurred in SN20-TRV-MSH2 and SN20-TRV-MSH6 plantlets with decreased root length/area under Cd stress. Taken together, we conclude that the low expression of MSH2 and MSH6, involved in the G2/M arrest, results in Cd-induced DNA damage recognition bypassing the MMR system to activate G1/S arrest with the assistance of MLH1. This then leads to repressed root growth in LD10, explaining the intervarietal difference in Cd tolerance in soybean.

New Phenotypes of Potato Co-induced by Mismatch Repair Deficiency and Somatic Hybridization

As plants are sessile they need a very efficient system for repairing damage done by external or internal mutagens to their DNA. Mismatch repair (MMR) is one of the systems that maintain genome integrity and prevent homeologous recombination. In all eukaryotes mismatches are recognized by evolutionary conserved MSH proteins often acting as heterodimers, the constant component of which is MSH2. Changes affecting the function of MSH2 gene may induce a 'mutator' phenotype and microsatellite instability (MSI), as is demonstrated in MSH2 knock-out and silenced lines of Arabidopsis thaliana. The goal of this study was to screen for 'mutator' phenotypes in somatic hybrids between potato cvs. 'Delikat' and 'Désirée' and MMR deficient Solanum chacoense transformed using antisense (AS) or dominant negative mutant (DN) AtMSH2 genes. The results demonstrate that first generation fusion hybrids have a range of morphological abnormalities caused by uniparental MMR deficiency; these mutant phenotypes include: dwarf or gigantic plants; bushiness; curled, small, large or abnormal leaves; a deterioration in chloroplast structure; small deep-purple tubers and early dehiscent flowers. Forty percent of the viable somatic hybrids planted in a greenhouse, (10 out of 25 genotypes) had mutant phenotypes accompanied by MSI. The majority of the hybrids with 'mutator' phenotypes cultured on media containing kanamycin developed roots so sustaining the presence of selectable marker gene nptII, from the initial constructs. Here for the first time, MMR deficiency combined with somatic hybridization, are used to induce new phenotypes in plants, which supports the role of MMR deficiency in increasing introgressions between two related species.

MutS-Homolog2 silencing generates tetraploid meiocytes in tomato (Solanum lycopersicum)

MSH2 is the core protein of MutS-homolog family involved in recognition and repair of the errors in the DNA. While other members of MutS-homolog family reportedly regulate mitochondrial stability, meiosis, and fertility, MSH2 is believed to participate mainly in mismatch repair. The search for polymorphism in MSH2 sequence in tomato accessions revealed both synonymous and nonsynonymous SNPs; however, SIFT algorithm predicted that none of the SNPs influenced MSH2 protein function. The silencing of MSH2 gene expression by RNAi led to phenotypic abnormalities in highly silenced lines, particularly in the stamens with highly reduced pollen formation. MSH2 silencing exacerbated formation of UV-B-induced thymine dimers and blocked light-induced repair of the dimers. The MSH2 silencing also affected the progression of male meiosis to a varying degree with either halt of meiosis at zygotene stage or formation of diploid tetrads. The immunostaining of male meiocytes with centromere localized CENPC (centromere protein C) antibody showed the presence of 48 univalent along with 24 bivalent chromosomes suggesting abnormal tetraploid meiosis. The mitotic cells of root tips of silenced lines showed diploid nuclei but lacked intervening cell plates leading to cells with syncytial nuclei. Thus, we speculate that tetraploid pollen mother cells may have arisen due to the fusion of syncytial nuclei before the onset of meiosis. It is likely that in addition to mismatch repair (MMR), MSH2 may have an additional role in regulating ploidy stability.

DNA mismatch repair and the DNA damage response

This review discusses the role of DNA mismatch repair (MMR) in the DNA damage response (DDR) that triggers cell cycle arrest and, in some cases, apoptosis. Although the focus is on findings from mammalian cells, much has been learned from studies in other organisms including bacteria and yeast [1,2]. MMR promotes a DDR mediated by a key signaling kinase, ATM and Rad3-related (ATR), in response to various types of DNA damage including some encountered in widely used chemotherapy regimes. An introduction to the DDR mediated by ATR reveals its immense complexity and highlights the many biological and mechanistic questions that remain. Recent findings and future directions are highlighted.

Excision of translesion synthesis errors orchestrates responses to helix-distorting DNA lesions

In addition to correcting mispaired nucleotides, DNA mismatch repair (MMR) proteins have been implicated in mutagenic, cell cycle, and apoptotic responses to agents that induce structurally aberrant nucleotide lesions. Here, we investigated the mechanistic basis for these responses by exposing cell lines with single or combined genetic defects in nucleotide excision repair (NER), postreplicative translesion synthesis (TLS), and MMR to low-dose ultraviolet light during S phase. Our data reveal that the MMR heterodimer Msh2/Msh6 mediates the excision of incorrect nucleotides that are incorporated by TLS opposite helix-distorting, noninstructive DNA photolesions. The resulting single-stranded DNA patches induce canonical Rpa-Atr-Chk1-mediated checkpoints and, in the next cell cycle, collapse to double-stranded DNA breaks that trigger apoptosis. In conclusion, a novel MMR-related DNA excision repair pathway controls TLS a posteriori, while initiating cellular responses to environmentally relevant densities of genotoxic lesions. These results may provide a rationale for the colorectal cancer tropism in Lynch syndrome, which is caused by inherited MMR gene defects.

FANCJ localization by mismatch repair is vital to maintain genomic integrity after UV irradiation

Nucleotide excision repair (NER) is critical for the repair of DNA lesions induced by UV radiation, but its contribution in replicating cells is less clear. Here, we show that dual incision by NER endonucleases, including XPF and XPG, promotes the S-phase accumulation of the BRCA1 and Fanconi anemia-associated DNA helicase FANCJ to sites of UV-induced damage. FANCJ promotes replication protein A phosphorylation and the arrest of DNA synthesis following UV irradiation. Interaction defective mutants of FANCJ reveal that BRCA1 binding is not required for FANCJ localization, whereas interaction with the mismatch repair (MMR) protein MLH1 is essential. Correspondingly, we find that FANCJ, its direct interaction with MLH1, and the MMR protein MSH2 function in a common pathway in response to UV irradiation. FANCJ-deficient cells are not sensitive to killing by UV irradiation, yet we find that DNA mutations are significantly enhanced. Thus, we considered that FANCJ deficiency could be associated with skin cancer. Along these lines, in melanoma we found several somatic mutations in FANCJ, some of which were previously identified in hereditary breast cancer and Fanconi anemia. Given that, mutations in XPF can also lead to Fanconi anemia, we propose collaborations between Fanconi anemia, NER, and MMR are necessary to initiate checkpoint activation in replicating human cells to limit genomic instability.

DNA mismatch repair system: repercussions in cellular homeostasis and relationship with aging

The mechanisms that concern DNA repair have been studied in the last years due to their consequences in cellular homeostasis. The diverse and damaging stimuli that affect DNA integrity, such as changes in the genetic sequence and modifications in gene expression, can disrupt the steady state of the cell and have serious repercussions to pathways that regulate apoptosis, senescence, and cancer. These altered pathways not only modify cellular and organism longevity, but quality of life (“health-span”). The DNA mismatch repair system (MMR) is highly conserved between species; its role is paramount in the preservation of DNA integrity, placing it as a necessary focal point in the study of pathways that prolong lifespan, aging, and disease. Here, we review different insights concerning the malfunction or absence of the DNA-MMR and its impact on cellular homeostasis. In particular, we will focus on DNA-MMR mechanisms regulated by known repair proteins MSH2, MSH6, PMS2, and MHL1, among others.

Ectopically expressed human tumor biomarker MutS homologue 2 is a novel endogenous ligand that is recognized by human γδ T cells to induce innate anti-tumor/virus immunity

Human (h) MutS homologue 2, a nuclear protein, is a critical element of the DNA mismatch repair system. Our previous studies suggest that hMSH2 might be a protein ligand for TCRγδ. Here, we show that hMSH2 is ectopically expressed on a large panel of epithelial tumor cells. We found that hMSH2 interacts with both TCRγδ and NKG2D and contributes to Vδ2 T cell-mediated cytolysis of tumor cells. Moreover, recombinant human MSH2 protein stimulates the proliferation and IFN-γ secretion of Vδ2 T cells in vitro. Finally, hMSH2 expression is induced on the cell surface of Epstein-Barr virus-transformed lymphoblastoid cell lines, and the induction increases the sensitivity of these lymphoblastoid cell lines to γδ T cell-mediated cytolysis. Our data suggest that hMSH2 functions as a tumor-associated or virus infection-related antigen recognized by both Vδ2 TCR and NKG2D, and it plays a role in eliciting the immune responses of γδ T cells against tumor- and virus-infected cells. The recognition of ectopic surface-expressing endogenous antigen by TCRγδ and NKG2D may be an important mechanism of innate immune response to carcinogenesis and viral infection.

Involvement of nucleotide excision and mismatch repair mechanisms in double strand break repair

Living organisms are constantly threatened by environmental DNA-damaging agents, including UV and ionizing radiation (IR). Repair of various forms of DNA damage caused by IR is normally thought to follow lesion-specific repair pathways with distinct enzymatic machinery. DNA double strand break is one of the most serious kinds of damage induced by IR, which is repaired through double strand break (DSB) repair mechanisms, including homologous recombination (HR) and non-homologous end joining (NHEJ). However, recent studies have presented increasing evidence that various DNA repair pathways are not separated, but well interlinked. It has been suggested that non-DSB repair mechanisms, such as Nucleotide Excision Repair (NER), Mismatch Repair (MMR) and cell cycle regulation, are highly involved in DSB repairs. These findings revealed previously unrecognized roles of various non-DSB repair genes and indicated that a successful DSB repair requires both DSB repair mechanisms and non-DSB repair systems. One of our recent studies found that suppressed expression of non-DSB repair genes, such as XPA, RPA and MLH1, influenced the yield of IR induced micronuclei formation and/or chromosome aberrations, suggesting that these genes are highly involved in DSB repair and DSB-related cell cycle arrest, which reveals new roles for these gene products in the DNA repair network. In this review, we summarize current progress on the function of non-DSB repair-related proteins, especially those that participate in NER and MMR pathways, and their influence on DSB repair. In addition, we present our developing view that the DSB repair mechanisms are more complex and are regulated by not only the well known HR/NHEJ pathways, but also a systematically coordinated cellular network.

Regulation of plant MSH2 and MSH6 genes in the UV-B-induced DNA damage response

Deleterious effects of UV-B radiation on DNA include the formation of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs). These lesions must be repaired to maintain the integrity of DNA and provide genetic stability. Of the several repair systems involved in the recognition and removal of UV-B-induced lesions in DNA, the focus in the present study was on the mismatch repair system (MMR). The contribution of MutSα (MSH2-MSH6) to UV-induced DNA lesion repair and cell cycle regulation was investigated. MSH2 and MSH6 genes in Arabidopsis and maize are up-regulated by UV-B, indicating that MMR may have a role in UV-B-induced DNA damage responses. Analysis of promoter sequences identified MSH6 as a target of the E2F transcription factors. Using electrophoretic mobility shift assays, MSH6 was experimentally validated as an E2F target gene, suggesting an interaction between MMR genes and the cell cycle control. Mutations in MSH2 or MSH6 caused an increased accumulation of CPDs relative to wild-type plants. In addition, msh2 mutant plants showed a different expression pattern of cell cycle marker genes after the UV-B treatment when compared with wild-type plants. Taken together, these data provide evidence that plant MutSα is involved in a UV-B-induced DNA damage response pathway.

THAP5 is a DNA-binding transcriptional repressor that is regulated in melanoma cells during DNA damage-induced cell death

THAP5 was originally isolated as a specific interactor and substrate of the mitochondrial pro-apoptotic Omi/HtrA2 protease. It is a human zinc finger protein characterized by a restricted pattern of expression and the lack of orthologs in mouse and rat. The biological function of THAP5 is unknown but our previous studies suggest it could regulate G2/M transition in kidney cells and could be involved in human cardiomyocyte cell death associated with coronary artery disease (CAD). In this report, we expanded our studies on the properties and function of THAP5 in human melanoma cells. THAP5 was expressed in primary human melanocytes as well as in all melanoma cell lines that were tested. THAP5 protein level was significantly induced by UV irradiation or cisplatin treatment, conditions known to cause DNA damage. The induction of THAP5 correlated with a significant increase in apoptotic cell death. In addition, we show that THAP5 is a nuclear protein that could recognize and bind a specific DNA motif. THAP5 could also repress the transcription of a reporter gene in a heterologous system. Our work suggests that THAP5 is a DNA-binding protein and a transcriptional repressor. Furthermore, THAP5 has a pro-apoptotic function and it was induced in melanoma cells under conditions that promoted cell death.

Solar radiation induced skin damage: review of protective and preventive options

PURPOSE

Solar energy has a number of short- and long-term detrimental effects on skin that can result in several skin disorders. The aim of this review is to summarise current knowledge on endogenous systems within the skin for protection from solar radiation and present research findings to date, on the exogenous options for such skin photoprotection.

RESULTS

Endogenous systems for protection from solar radiation include melanin synthesis, epidermal thickening and an antioxidant network. Existing lesions are eliminated via repair mechanisms. Cells with irreparable damage undergo apoptosis. Excessive and chronic sun exposure however can overwhelm these mechanisms leading to photoaging and the development of cutaneous malignancies. Therefore exogenous means are a necessity. Exogenous protection includes sun avoidance, use of photoprotective clothing and sufficient application of broad-spectrum sunscreens as presently the best way to protect the skin. However other strategies that may enhance currently used means of protection are being investigated. These are often based on the endogenous protective response to solar light such as compounds that stimulate pigmentation, antioxidant enzymes, DNA repair enzymes, non-enzymatic antioxidants.

CONCLUSION

More research is needed to confirm the effectiveness of new alternatives to photoprotection such as use of DNA repair and antioxidant enzymes and plant polyphenols and to find an efficient way for their delivery to the skin. New approaches to the prevention of skin damage are important especially for specific groups of people such as (young) children, photosensitive people and patients on immunosuppressive therapy. Changes in public awareness on the subject too must be made.

Role of MutS homolog 2 (MSH2) in intestinal myofibroblast proliferation during Crohn's disease stricture formation

Tissue remodeling and mesenchymal cell accumulation accompanies chronic inflammatory disorders involving joints, lung, vasculature, and bowel. Chronic inflammation may alter DNA-mismatch repair (MMR) systems in mesenchymal cells, but is not defined in Crohn's disease (CD) and its associated intestinal remodeling and stricture formation. We determined whether DNA-MMR alteration plays a role in the pathogenesis of CD tissue remodeling. Control and CD bowel tissues were used to generate primary cultures of muscularis mucosa myofibroblasts, which were assessed directly or following stimulation with TNF-alpha/LPS or H2O2. MutS homolog (MSH)2, MSH3, and MSH6 expression in tissues and myofibroblasts was determined. Immunohistochemical staining revealed an increased expression of MSH2 in CD muscularis mucosa and submucosal tissues compared with controls or uninvolved CD tissue, and MSH2 expression was increased in CD myofibroblasts compared with control cells. TNF-alpha/LPS and H2O2 further enhanced MSH2 expression in both control and CD cells, which were decreased by simvastatin. There were no significant changes in MSH3 and MSH6 expression. Proliferating cell nuclear antigen and Ki67 staining of CD tissue revealed increased proliferation in the muscularis mucosa and submucosa of chronically inflamed tissues, and enhanced proliferation was seen in CD myofibroblasts compared with controls. Simvastatin reversed the effects of inflammatory stress on the DNA-MMR and inhibited proliferation of control and CD myofibroblasts. Gene silencing with MSH2 siRNA selectively decreased CD myofibroblast proliferation. These data demonstrate a potential role for MSH2 in the pathogenesis of nonneoplastic mesenchymal cell accumulation and intestinal remodeling in CD chronic inflammation.

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.