Genetic Polymorphisms Of DNA Double-strand Break Repair Pathway Genes And Glioma Susceptibility

Gliomas are the most common type of primary intracerebral neoplasm in Chinaas well as in the West, comprising more than40%of primary brain tumors in humansand, despite improvements in clinical care over the last20years, remain associatedwith considerable morbidity. Although the etiology of gliomas is largely unknown,exposure to ionizing radiation (IR) and genetic alterations are unequivocallyassociated with an increased risk of gliomas. Ionizing radiation has been shown toinduce various types of DNA damage, including both single-strand breaks anddouble-strand breaks (DSBs). Single nucleotide polymorphisms (SNPs), the mostcommon type of DNA sequence variations in the human genome, contribute to humanphenotypic differences. Recently，genome-wide association studies (GWAS) usinghigh-throughput technologies have identified several single nucleotidepolymorphisms (SNPs) that contribute to the gliomas at5p15.33(TERT),8q24.21(CCDC26),9p21.3(CDKN2A-CDKN2B),20q13.33(RTEL1),11q23.3(PHLDB1),and two independent signals at7p11.2(EGFR). There may be many more SNPs thatare associated with an increased risk for glioma. Genetic variations in DNAdouble-strand break repair genes can influence the ability of a cell to repair damagedDNA and alter an individual’s susceptibility to cancer. Recent evidence suggests thatseveral single nucleotide polymorphisms (SNPs) in the DSB repair pathway genesmay be prognostic biomarkers for GBM survival and modulategamma-radiation-induced mutagen sensitivity in glioma patients. Genetic variants inDSB repair pathway genes have been extensively studied in multiple cancers.However, few studies have specifically identified any association between geneticvariations in the DSB repair pathway genes and the risk of gliomas. Here wegenotyped10potentially functional single nucleotide polymorphisms (SNPs) in7DNA double-strand break repair pathway genes (XRCC3, BRCA2, RAG1, XRCC5, LIG4, XRCC4and ATM) in a case-control study including384glioma patients and384cancer-free controls in a Chinese Han population, and further evaluate theirgene-gene and gene-environment interactions in the development of glioma.Genotypes were determined using the OpenArray platform.PartⅠ Polymorphisms of non-homologous end-joining (NHEJ)genes and risk of glioma.DNA double-strand breaks (DSBs) can be generated during V(D)Jrecombination, class-switch recombination at the immunoglobulin heavy chain (IgH)locus or meiosis and result from a variety of factors including ionizing radiation andreactive oxygen species. Inadequacy or defects in DSB repair can lead to large-scaleloss of genetic information and can have disastrous consequences such as genomicinstability, immunodeficiency, radiosensitivity, cell death and oncogenictransformation. DSBs are sensed by the MRN (MRE11, RAD50, and NBS1) complex,which catalyzes activation of ATM. Two major pathways have evolved in mammaliancells to repair DSBs: non-homologous end-joining (NHEJ) and homologousrecombination (HR). The central components of the NHEJ pathway are Ku70(XRCC6), Ku80(XRCC5), DNA-dependent protein kinase catalytic subunit(DNA-PKcs), XRCC4and DNA ligase IV (LIG4) proteins. NHEJ is a multistepprocess initiated by the XRCC5/XRCC6dimer (also known as Ku80/Ku70) whichimmediately binds to both broken ends of DNA and recruits the DNA-dependentprotein kinase catalytic subunit (DNA-PKcs) forming the trimeric DNA-PKholoenzyme. Finally, the DNA ligase IV-XRCC4complex in vivo carries out theligation step to complete repair.We studied whether polymorphisms in non-homologous end-joining (NHEJ)repair genes are associated with an increased risk of glioma development. Wegenotyped4potentially functional single nucleotide polymorphisms (SNPs) in4non-homologous end-joining (NHEJ) pathway genes (RAG1, XRCC5, LIG4and XRCC4) in a case-control study including384glioma patients and384cancer-freecontrols in a Chinese Han population. Genotypes were determined using theOpenArray platform.In the single-locus analysis there was a significant association between gliomasand the LIG4rs1805388(Ex2+54C>T, Thr9Ile) TC genotype (adjusted OR,1.62;95%CI,1.20-2.18), as well as the TT genotype (adjusted OR,3.27;95%CI,1.87-5.71). We also found that the homozygous variant genotype (GG) of XRCC4rs1805377(IVS7-1A>G, splice-site) was associated with a significantly increased riskof gliomas (OR,1.77;95%CI,1.12-2.80).In conclusion, genetic variants of the genes involved in the non-homologousend-joining (NHEJ) pathway may play a role in the etiology of glioma.PartⅡ Polymorphisms of homologous recombination (HR) genesand risk of glioma.Homologous recombination (HR) provides an important error-free mechanismfor DNA double-strand breaks (DSBs) in mammalian cells, which involves numerousproteins. RAD51interacts with other important repair proteins, including BRCA1,BRCA2, XRCC2, and XRCC3and plays a central role in the HR activation throughthe use of sister-chromatid sequences as a template for precise repair. HR requires the5’-3’ resection of dsDNA to generate single-stranded DNA (ssDNA)-dsDNA junctions,which is initiated by MRN complex (a heterotrimeric protein complex consisting ofMRE11, RAD50and NBS1). The ssDNA overhang becomes coated with theheterotrimetic replication protein A (RPA). RAD51recombinase play a central role inthe in error-free homologous recombination. RAD51protein binds ssDNA to formthe functional presynaptic complex, a step that is facilitated by RAD51paralogs (suchas RAD51B, RAD51C, RAD51D, XRCC2and XRCC3), BRCA1/2and RAD54. Theresulting nucleoprotein filament catalyses homologous pairing and strand invasioninto an intact homologous DNA molecule in the cell. Following strand invasion, the second end capture mediated by RAD52leads to formation of double Hollidayjunctions. Subsequently, Holliday junctions are resolved resulting in either crossoveror non-crossover gene conversion products.We studied whether polymorphisms in Homologous recombination (HR) genesare associated with an increased risk of glioma development. We genotyped6potentially functional single nucleotide polymorphisms (SNPs) in3homologousrecombination genes (XRCC3，BRCA2，ATM) in a case-control study including384glioma patients and384cancer-free controls in a Chinese Han population. Genotypeswere determined using the OpenArray platform.No significant association was found between XRCC3(rs861539，rs1799794，rs1799796)，BRCA2(rs1799943，rs15869), ATM (rs189037) polymorphisms and therisk of glioma.Part III DNA double-strand break repair pathway gene-gene andgene-environment interactions in the development of gliomaThe LIG4–XRCC4complex plays a fundamental role in DNA non-homologousend-joining and is present in all eukaryotes. It has been demonstrated that XRCC4can stimulate LIG4activity and is required to stabilize LIG4. Thus, we estimated thecombined effect of LIG4and XRCC4genes on glioma risk. Since tobacco is awell-confirmed inducer of DNA damage, in particular DSBs, we further evaluated theinteraction between the LIG4rs1805388and XRCC4rs1805377polymorphisms andtobacco smoking with respect to the risk of gliomas.35.4%of the cases and19.3%of the controls had variant genotypes at both loci(LIG4rs1805388CT+TT and XRCC4rs1805377AG+GG). In comparison with thereference combination of LIG4rs1805388CC and XRCC4rs1805377AA, thecombination of the LIG4rs1805388CT+TT genotype together with XRCC4rs1805377AG+GG genotype was found to be significantly associated with glioma(adjusted OR,2.22;95%CI,1.49-3.30). Furthermore, significantmore-than-multiplicative (0.009) and more-than additive (0.005) gene-gene interactions of these two loci (LIG4rs1805388CT+TT and XRCC4rs1805377AG+GG) were found in relation to the risk of gliomas. We also detected a significantadditive (Pinteraction=0.013) and multiplicative interaction (Pinteraction=0.046) effectbetween LIG4rs1805388and tobacco smoking for the development of gliomas.Compared with never-smokers carrying the wild-type genotype of LIG4rs1805388,those ever-smokers with variant-containing genotype of LIG4rs1805388polymorphism had a significantly increased risk to develop gliomas (adjusted OR,1.67;95%CI,1.11-2.50).