Characterization of the mouse Xpf DNA repair gene and differential expression during spermatogenesis

Divergence and conservation of the meiotic recombination machinery

Sexually reproducing eukaryotes use recombination between homologous chromosomes to promote chromosome segregation during meiosis. Meiotic recombination is almost universally conserved in its broad strokes, but specific molecular details often differ considerably between taxa, and the proteins that constitute the recombination machinery show substantial sequence variability. The extent of this variation is becoming increasingly clear because of recent increases in genomic resources and advances in protein structure prediction. We discuss the tension between functional conservation and rapid evolutionary change with a focus on the proteins that are required for the formation and repair of meiotic DNA double-strand breaks. We highlight phylogenetic relationships on different time scales and propose that this remarkable evolutionary plasticity is a fundamental property of meiotic recombination that shapes our understanding of molecular mechanisms in reproductive biology.

ERCC polymorphisms and prognosis of patients with osteosarcoma

Osteosarcoma is the most common primary bone malignancy in children and teenagers, and its clinical outcome remains poor. Previous studies have investigated the association between excision repair cross-complementing (ERCC) and prognosis of osteosarcoma patients, but their results were inconsistent. We aimed to clarify the associations between ERCC polymorphisms and osteosarcoma prognosis by using meta-analysis. We searched relevant studies in PubMed, Embase, coupled with Chinese National Knowledge Infrastructure (CNKI) in human osteosarcoma published prior to April, 2014. Hazard ratios (HR) together with their 95 % confidence intervals (95 % CI) were used to measure the relationship between ERCC mutations and prognosis in patients with osteosarcoma. Pooled results showed that polymorphism of ERCC2 Lys751Gln was associated with the overall survival of osteosarcoma (GG vs. AA, HR = 0.40; 95 % CI 0.18-0.86), and ERCC5 His46His mutation was associated with the event-free survival of osteosarcoma (CC vs. TT, HR = 0.37; 95 % CI 0.15, 0.93). In addition, there is no evidence of association on ERCC1 Asn118Asn, ERCC1 Gln504Lys, and ERCC2 Asp312Asn polymorphisms with prognosis in osteosarcoma. In summary, the ERCC2 Lys751Gln and ERCC5 His46His polymorphisms might influence osteosarcoma prognosis.

Laryngeal cancer risk and common single nucleotide polymorphisms in nucleotide excision repair pathway genes ERCC1, ERCC2, ERCC3, ERCC4, ERCC5 and XPA

Because the molecular mechanisms underlying the development of laryngeal cancer are not well understood, we conducted a case-control study to determine the association between eight common SNPs in NER pathway genes and risk of laryngeal cancer, and the association between genetic polymorphisms and environmental factors. A 1:1 matched case-control study of 176 cases and 176 controls was conducted. Laryngeal cancer cases were more likely to smoke and drink (all P values<0.05). Subjects with the ERCC1 rs11615 CC genotype and C allele had an increased risk of laryngeal cancer. Similarly, individuals with the ERCC5 rs17655 GG genotype and G allele had an increased risk of laryngeal cancer. Gene-gene interaction analysis showed that subjects carrying ERCC1 rs11615 C allele and XPG/ERCC5 rs17655 G allele had a greatly increased risk of breast cancer. Stratified analysis revealed that the interaction between polymorphisms of ERCC1 rs11615 and ERCC5 rs17655 and smoking on cancer risk was statistically significant, and ERCC1 rs11615 polymorphisms also had a significant interaction with drinking habit. In conclusion, our study suggests that ERCC1 rs11615 and ERCC5 rs17655 polymorphisms are associated with increased risk of laryngeal cancer, and that they confer more risk among smokers and drinkers.

Association of single nucleotide polymorphisms of nucleotide excision repair genes with laryngeal cancer risk and interaction with cigarette smoking and alcohol drinking

We investigated the potential association of 12 single nucleotide polymorphisms (SNPs) of nucleotide excision repair (NER) genes with risk of laryngeal cancer. A ratio of 1:1 matched case-control study was conducted. Conditional regression analysis indicated that subjects with ERCC1 rs11615 CC and C allele had an increased risk of laryngeal cancer compared with the TT genotype. Individuals with the ERCC5 rs17655 GG and G allele had a moderately increased risk of laryngeal cancer when compared with the CC genotype. By stratified analysis, ERCC1 rs11615 CC genotype and C allele were significantly associated with greatly increased risk of laryngeal cancer in ever smokers. ERCC1 rs11615 and ERCC5 rs17655 polymorphisms were associated with a moderately increased risk of this cancer in ever drinkers. In summary, we suggest that genetic variations in ERCC1 rs11615 and ERCC5 rs17655 are associated with laryngeal cancer risk in a Chinese population, especially in ever smokers and drinkers. Our finding could be helpful in identifying people at high risk for the disease for early intervention.

Impaired spermatogenesis and elevated spontaneous tumorigenesis in xeroderma pigmentosum group A gene (Xpa)-deficient mice

We have reported that xeroderma pigmentosum group A (Xpa) gene-knockout mice [Xpa (-/-) mice] are deficient in nucleotide excision repair (NER) and highly sensitive to UV-induced skin carcinogenesis. Although xeroderma pigmentosum group A patients show growth retardation, immature sexual development, and neurological abnormalities as well as a high incidence of UV-induced skin tumors, Xpa (-/-) mice were physiologically and behaviorally normal. In the present study, we kept Xpa (-/-) mice for 2 years under specific pathogen-free (SPF) conditions and found that the testis diminished in an age-dependent manner, and degenerating seminiferous tubules and no spermatozoa were detected in the 24-month-old Xpa (-/-) mice. In addition, a higher incidence of spontaneous tumorigenesis was observed in the 24-month-old Xpa (-/-) mice compared to Xpa (+/+) controls. Xpa (-/-) mice provide a useful model for investigating the aging and internal tumor formation in XPA patients.

Structural and functional relationships of the XPF/MUS81 family of proteins

Proteins belonging to the XPF/MUS81 family play important roles in the repair of DNA lesions caused by UV-light or DNA cross-linking agents. Most eukaryotes have four family members that assemble into two distinct heterodimeric complexes, XPF-ERCC1 and MUS81-EME1. Each complex contains one catalytic and one noncatalytic subunit and exhibits endonuclease activity with a variety of 3'-flap or fork DNA structures. The catalytic subunits share a characteristic core containing an excision repair cross complementation group 4 (ERCC4) nuclease domain and a tandem helix-hairpin-helix (HhH)(2) domain. Diverged domains are present in the noncatalytic subunits and may be required for substrate targeting. Vertebrates possess two additional family members, FANCM and Fanconi anemia-associated protein 24 kDa (FAAP24), which possess inactive nuclease domains. Instead, FANCM contains a functional Superfamily 2 (SF2) helicase domain that is required for DNA translocation. Determining how these enzymes recognize specific DNA substrates and promote key repair reactions is an important challenge for the future.

ERCC1/XPF limits L1 retrotransposition

Retrotransposons are currently active in the human and mouse genomes contributing to novel disease mutations and genomic variation via de novo insertions. However, little is known about the interactions of non-long terminal repeat (non-LTR) retrotransposons with the host DNA repair machinery. Based on the model of retrotransposition for the human and mouse LINE-1 element, one likely intermediate is an extension of cDNA that is heterologous to the genomic target, a flap intermediate. To determine whether a human flap endonuclease could recognize and process this potential intermediate, the genetic requirement for the ERCC1/XPF heterodimer during LINE-1 retrotransposition was characterized. Reduction of XPF in human cells increased retrotransposition whereas complementation of ERCC1-deficiency in hamster cells reduced retrotransposition. These results demonstrate for the first time that DNA repair enzymes act to limit non-LTR retrotransposition and may provide insight into the genetic instability phenotypes of ercc1 and xpf individuals.

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

Deletion of genes implicated in protecting the integrity of male germ cells has differential effects on the incidence of DNA breaks and germ cell loss

Background

Infertility affects ∼20% of couples in Europe and in 50% of cases the problem lies with the male partner. The impact of damaged DNA originating in the male germ line on infertility is poorly understood but may increase miscarriage. Mouse models allow us to investigate how deficiencies in DNA repair/damage response pathways impact on formation and function of male germ cells. We have investigated mice with deletions of ERCC1 (excision repair cross-complementing gene 1), MSH2 (MutS homolog 2, involved in mismatch repair pathway), and p53 (tumour suppressor gene implicated in elimination of germ cells with DNA damage).

Principal Findings

We demonstrate for the first time that depletion of ERCC1 or p53 from germ cells results in an increased incidence of unrepaired DNA breaks in pachytene spermatocytes and increased numbers of caspase-3 positive (apoptotic) germ cells. Sertoli cell-only tubules were detected in testes from mice lacking expression of ERCC1 or MSH2 but not p53. The number of sperm recovered from epididymes was significantly reduced in mice lacking testicular ERCC1 and 40% of sperm contained DNA breaks whereas the numbers of sperm were not different to controls in adult Msh2 −/− or p53 −/− mice nor did they have significantly compromised DNA.

Conclusions

These data have demonstrated that deletion of Ercc1, Msh2 and p53 can have differential but overlapping affects on germ cell function and sperm production. These findings increase our understanding of the ways in which gene mutations can have an impact on male fertility.

DNA repair in mammalian embryos

Mammalian cells have developed complex mechanisms to identify DNA damage and activate the required response to maintain genome integrity. Those mechanisms include DNA damage detection, DNA repair, cell cycle arrest and apoptosis which operate together to protect the conceptus from DNA damage originating either in parental gametes or in the embryo's somatic cells. DNA repair in the newly fertilized preimplantation embryo is believed to rely entirely on the oocyte's machinery (mRNAs and proteins deposited and stored prior to ovulation). DNA repair genes have been shown to be expressed in the early stages of mammalian development. The survival of the embryo necessitates that the oocyte be sufficiently equipped with maternal stored products and that embryonic gene expression commences at the correct time. A Medline based literature search was performed using the keywords 'DNA repair' and 'embryo development' or 'gametogenesis' (publication dates between 1995 and 2006). Mammalian studies which investigated gene expression were selected. Further articles were acquired from the citations in the articles obtained from the preliminary Medline search. This paper reviews mammalian DNA repair from gametogenesis to preimplantation embryos to late gestational stages.

A DNA repair gene of Caenorhabditis elegans: a homolog of human XPF

The xeroderma pigmentosum complementation group F (XPF) protein is a structure-specific endonuclease in a complex with ERCC1 and is essential for nucleotide excision repair (NER). We report a single cDNA of Caenorhabditis elegans (C. elegans) encoding highly similar protein to human XPF and other XPF members. We propose to name the corresponding C. elegans gene xpf. Messenger RNA for C. elegans xpf is 5'-tagged with a SL2 splice leader, suggesting an operon-like expression for xpf. Using RNAi, we showed that loss of C. elegans xpf function caused hypersensitivity to ultra-violet (UV) irradiation, as observed in enhanced germ cell apoptosis and increased embryonic lethality. This study suggests that C. elegans xpf is conserved in evolution and plays a role in the repair of UV-damaged DNA in C. elegans.

Stress defense in murine embryonic stem cells is superior to that of various differentiated murine cells

A very small number of embryonic stem (ES) cells gives rise to all tissues of the embryo proper. This means that ES cells should be equipped with highly efficient mechanisms to defend themselves against various stresses and to prevent or repair DNA damage. One of these mechanisms is a high activity of a verapamil-sensitive multidrug efflux pump. Because reactive oxygen species are a major source of DNA damage, we further tested the idea that murine ES cells might differ from their more differentiated counterparts by high levels of antioxidant defense and good DNA strand break repair capacity. This was confirmed by comparing cellular peroxide levels, total antioxidant capacity, and activity of radiation-induced strand break repair between murine ES cells and embryoid bodies or embryonic fibroblasts. Using microarrays and confirmation by reverse transcription-polymerase chain reaction, we identified several candidate antioxidant and stress-resistance genes that become downregulated during differentiation of ES cells into embryoid bodies.

DNA repair gene Ercc1 is essential for normal spermatogenesis and oogenesis and for functional integrity of germ cell DNA in the mouse

Ercc1 is essential for nucleotide excision repair (NER) but, unlike other NER proteins, Ercc1 and Xpf are also involved in recombination repair pathways. Ercc1 knockout mice have profound cell cycle abnormalities in the liver and die before weaning. Subsequently Xpa and Xpc knockouts have proved to be good models for the human NER deficiency disease, xeroderma pigmentosum, leading to speculation that the recombination, rather than the NER deficit is the key to the Ercc1 knockout phenotype. To investigate the importance of the recombination repair functions of Ercc1 we studied spermatogenesis and oogenesis in Ercc1-deficient mice. Male and female Ercc1-deficient mice were both infertile. Ercc1 was expressed at a high level in the testis and the highest levels of Ercc1 protein occurred in germ cells following meiotic crossing over. However, in Ercc1 null males some germ cell loss occurred prior to meiotic entry and there was no evidence that Ercc1 was essential for meiotic crossing over. An increased level of DNA strand breaks and oxidative DNA damage was found in Ercc1-deficient testis and increased apoptosis was noted in male germ cells. We conclude that the repair functions of Ercc1 are required in both male and female germ cells at all stages of their maturation. The role of endogenous oxidative DNA damage and the reason for the sensitivity of the germ cells to Ercc1 deficiency are discussed.

The human XPG gene: gene architecture, alternative splicing and single nucleotide polymorphisms

Defects in the XPG DNA repair endonuclease gene can result in the cancer-prone disorders xeroderma pigmentosum (XP) or the XP-Cockayne syndrome complex. While the XPG cDNA sequence was known, determination of the genomic sequence was required to understand its different functions. In cells from normal donors, we found that the genomic sequence of the human XPG gene spans 30 kb, contains 15 exons that range from 61 to 1074 bp and 14 introns that range from 250 to 5763 bp. Analysis of the splice donor and acceptor sites using an information theory-based approach revealed three splice sites with low information content, which are components of the minor (U12) spliceosome. We identified six alternatively spliced XPG mRNA isoforms in cells from normal donors and from XPG patients: partial deletion of exon 8, partial retention of intron 8, two with alternative exons (in introns 1 and 6) and two that retained complete introns (introns 3 and 9). The amount of alternatively spliced XPG mRNA isoforms varied in different tissues. Most alternative splice donor and acceptor sites had a relatively high information content, but one has the U12 spliceosome sequence. A single nucleotide polymorphism has allele frequencies of 0.74 for 3507G and 0.26 for 3507C in 91 donors. The human XPG gene contains multiple splice sites with low information content in association with multiple alternatively spliced isoforms of XPG mRNA.