| Gastric cancer is the fourth most common cancer in incidence and second leading cause of cancer death worldwide. In recent years,gastric cancer is the leading causes of cancer and cancer-related deaths in China, and more than one-thirds of gastric cancer cases occur in China. Gastric cancer remains a highly lethal disease in both men and women, and it remains a significant cancer burden in China.Many environmental factors, including Helicobacter pylori (HP) infection, a high level of alcohol consumption, use of tobacco products and intake of dietary carcinogens, appear to be important components of gastric cancer risk. In addition, chronic infection in stomach and lifestyle factors, especially dietary factors, are thought to be important in modifying the risk of gastric cancer. However, not all of those who have been exposed to the risk factors will develop gastric cancer, suggesting the inter-individual differences in susceptibility. These differences may in part be caused by genetic variation, such as single nucleotide polymorphism (SNP).Accumulating evidence show that the crucial DNA repair gene, XRCC1, and the important novel tumor suppressor gene, SMAD4, play an important role in development of gastric cancer. This study aims to evaluate the association between the polymorphisms of XRCC1 and SMAD4 genes and genetic susceptibility of gastric cancer in a Chinese population; and will provides the evidence to clarify the mechanism and the clues to carry out the personalized prevention and intervention of gastric cancer.Part IPolymorphisms in the promoter region of XRCC1 gene are associated with risk of gastric cancerA wide diversity of DNA damage in human cells could be induced by endogenous and exogenous agents. If not repaired, such DNA damage can cause mutations and genomic instability, leading to cellular malignant transformation. The base excision repair pathway is an important mechanism that repairs DNA base damage and single-strand breaks. X-ray crosscomplementing gene 1 (XRCC1) is one of the major DNA repair proteins involved in the base excision repair (BER)pathway, responsible for a scaffolding protein that directly associates with other proteins such as DNA polymeraseβ, PARP (ADP-ribose polymerase), and DNA ligase III in a complex, to facilitate the processes of BER or single-strand break repair. XRCC1-mediated pathways repair damage to DNA bases, from oxidation or covalent binding of non-bulky electrophiles, and to the deoxyribose phosphate backbone. Recently, accumulating evidence shows that polymorphic variations in DNA repair genes can explain inter-individual differences in risk of cancer. Gene expression was suggested to be regulated by promoter activity,suggesting the variants in promoter region may effect the expression,eventually influence the susceptibility to cancer. In the present study, we evaluated the association between the XRCC1 polymorphisms in the promoter region and gastric cancer risk in a case-control study.To investigate the distribution of four polymorphisms (i.e., -77T>C, -1450GGCCins>del, -1889T>C, and -1991C>G) of XRCC1 gene in gastric cancer cases and cancer-free controls and the association between variant genotypes of XRCC1 gene and individual's susceptibility to gastric cancer, we conducted a case-control study of 332 patients with newly diagnosed and pathohistologically confirmed gastric cancer and 332 cancer-free controls frequency matched on age and sex. We genotyped these four polymorphisms by polymerase chain reaction-restrictive fragment length polymorphism (PCR-RFLP) method. The associations between XRCC1 polymorphisms and gastric cancer risk were estimated by computing the odds ratios (ORs) and 95% confidence intervals (CIs) from logistic regression analyses. Application of the electrophoretic mobility shift experiment (EMSA) and the luciferase reporter gene transfection evaluated the impact of XRCC1 -1450GGCCins>del on DNA-protein binding affinity and transcriptional activities of XRCC1 gene.We found that the variant XRCC1 -1450GGCCdel/del and GGCCins/del+del/del genotypes were associated with a significantly decreased risk of gastric cancer (adjusted OR=0.36, 95% CI=0.21-0.64 for GGCCdel/del and 0.42, 0.30-0.58 for GGCCins/del+del/del) compared with the GGCCins/ins wild-type homozygote. In addition, for the XRCC1 -1991C>G polymorphism, individuals with the variants GG and CG+GG genotypes had a 1.61-fold (95% CI=1.04-2.49) and 1.21-fold (95% CI=0.87-1.69) increased risks, respectively, compared with those with the CC genotype. An increased risk of gastric cancer was associated with the variant XRCC1 -77TC genotype compared with the TT genotype (OR=1.53, 95% CI=1.01–2.33). Haplotype analysis showed that compared to the most frequent haplotype CCIT,the haplotypes CCDT, GCDT, CTDT, and CCDC were associated with the decreased risk of gastric cancer ("I"represented the -1450GGCCins allele;"D"represented the -1450GGCCdel allele). The EMSA results showed that the XRCC1 -1450GGCCins allele had stronger DNA-protein binding activity than the -1450 GGCCdel allele. The luciferase reporter gene transfection assay showed that the relative luciferase activity levels of the variant -1450GGCCins allele was higher than those with the -1450GGCCdel allele.Taken together, these results showed that the XRCC1 promoter polymorphisms were associated with the genetic susceptibility of gastric cancer. Especially, the SNP -1450GGCCins>del in the XRCC1 promoter region was associated with development of gastric cancer owing to the decreased transcriptional activity of GGCCins-allele containing promoter with higher affinity to transcription factor binding. These findings suggested that the polymorphisms in the XRCC1 promoter region may modulate the risk of gastric cancer.Part IIGenetic variants of SMAD4 gene and risk of gastric cancerSMAD4, as one member of SMADs family, is apparently common to all of the ligand-specific SMAD pathways and appears to mediate the actions of the other SMADs. The SMAD4 (also known as DPC4) gene is a novel tumor suppressor gene associated with gastric cancer that encodes a central mediator of transforming growth factor-β(TGF-β). To determine whether the SMAD4 polymorphisms are associated with risk of gastric cancer, we conducted a molecular epidemiologic case-control study of 322 gastric cancer patients and 351 cancer-free controls to assess the associations between the SMAD4 tagging single nucleotide polymorphisms (tSNPs) and gastric cancer risk.This case-control study included 322 histologically confirmed incident gastric cancer patients and 351 cancer-free controls frequency matched for age and sex. The study design and methods were similar to Part I described previously. In the single-locus analysis, a significantly decreased risk genotypes for gastric cancer were observed: the SNP2 rs8084630AA genotype (adjusted OR=0.11, 95% CI=0.04-0.33) compared with the GG genotype; the SNP3 rs17663887TC genotype (0.40, 0.22-0.75) compared with the TT genotype; and the SNP5 rs12456284AG genotype (2.97, 1.51-5.84) and GG genotype (3.30, 1.67-6.52) compared with the AA genotype. In the combined analyses of these three tSNPs, we found that the combined genotypes with 4-6 risk alleles (i.e., SNP2 G,SNP3 Tå'ŒSNP5 G alleles) were associated with a statistically significantly increased risk of gastric cancer compared with those with 1-3 risk alleles (adjusted OR=1.67, 95% CI=1.13-2.45), and this increased risk was more pronounced among subgroups of age <65 (1.77, 1.08-2.88), women (1.91, 1.00-3.64), smokers (1.99, 1.21-3.27), and drinkers (1.64, 1.05-2.55). Haplotype-based association analysis revealed that the decreased risk of gastric cancer was significantly associated the haplotype AGTGAA(OR=0.58, 95% CI=0.40-0.85), compared with the haplotype GATCGG.These findings suggested that genetic variants in the SMAD4 gene may modulate the risk of gastric cancer in a Chinese population. Further larger and functional studies are warranted to validate these results. |
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