The Study On The Relationships Between The Interaction Of Folate Metabolism Enzymes And Environment Factors On Genetic Susceptibility Of Gastric Cancer In Chinese Population

Although the incidence rate of gastric cancer (GC) in the world wasdeclined in recent years, it has been the second most common cause ofcancer-related deaths in the world. China was in the high incidence areas,41％incidence cases were from China in 2002, and GC becomes one ofthe most important healthy threaten to Chinese. Epidemiology, molecularbiology and genetics studies have identified that many environment andgenetic factors may increase the risk of GC. Epidemiology studies havedemonstrated that high consumption of vegetables and fruits in dietaryare associated with decreased risk of GC. Folate is one of the importantconstituents in fruits and vegetables, folic acid is essential for both thesynthesis of nucleotide precursors of DNA and cellular methylationreactions and therefore low folate intake has been associated with anincreased risk for a number of gastrointestinal cancers. Homocysteine(Hcy) is the median product of folate metabolism and the level of plasmatotal Hcy may reflect the level of folate metabolism indirectly. Recently,accumulative evidences suggest that hyperhomocysteinemia (elevatedtHcy in blood circulation) might be a risk factor for carcinogenesis,several studies reported that hyperhomocysteinemia were associated withincreased risk of live cancer, breast cancer and colorectal cancer. Theenzymes those involved in the folate metabolism pathway are obviouscandidates for screening the genetic variants associated with folate metabolism. These important enzymes include RFC1, MTHFR,thymidylate synthase (TS), methionine synthase reductase (MTRR) andmethylene-tetrahydrofolate dehydrogenase (MTHFD). The singlenucleotide polymorphisms (SNPs) variants in these genes may cause thedisorders of the enzymes. The SNPs in the folate metabolism enzymeswhich are associated with the serum folic acid and Hcy levels indicatedtheir play important roles in GC, so seeking for the functional SNPs andtagSNPs in folate metabolism genes is a new topic.To test this hypothesis, we conducted a case-control study toevaluate the association of vegetables and fruits consumption, plasmatHcy levels and SNPs of folate metabolism genes with gastric cancer riskin Chinese people by cross-sectional study, serology epidemiology andmolecular epidemiology study methods, to clarify the mechanism and riskgenotypes of GC, and to be considered as a biomarker for screen the highrisks and susceptible people.PART 1: Association study on the plasma total homocysteine levels,dietary habits and the risk of gastric cancerA population-based case-control study was conducted, including 391histologically-confirmed adenocarcinoma GC cases and 608 age and sexfrequency-matched cancer-free controls. The plasma tHcy concentrationwas measured with enzymatic biochemical assay of homocysteine onmicrotiter plates using crude lysate containing recombinant methionineγ-lyase. The relationship between different tHcy levels and risk of GCwas analyzed and smoking, drinking and H.pylori infection status werealso evaluated together with tHcy levels on the risk of GC. The average tHcy levels in GC cases (12.9(±5.7)μmol/L) was significant higher thanthat in controls (11.6(±6.5)μmol/L) (P=0.002). In addition, accordingto the quartile levels (7.9, 10.1, 13.7μmol/L) in the controls, the risks ofGC were increased 67％(adjusted OR=1.67, 95％CI=1.12-2.48), 98％(adjusted OR=1.98, 95％CI=1.33-2.94) and 112％(adjusted OR=2.12,95％CI=1.44-3.15) compared with the lowest quartile of tHcy(≤7.9μmol/L), respectively, and the trend of increase was significant(x~2=15.78, P＜0.001). The subjects with the tHcy＞15.0μmol/L couldincrease 49％GC risk compared with the subject with thetHcy≤15.0μmol/L. The difference of tHcy levels between the twovegetable intake groups in the normal controls were in the borderlinesignificant (P=0.061). The crossover analyses indicated that smoker ordrinker together with plasma tHcy＞15.0μmol/L respectively couldincrease the GC risk compared with the effect on GC risk of each factor,but the interaction were not significant. These findings support thehypothesis that the high levels of plasma tHcy were associated with theincreased risk of GC. Further large and genetics studies on theenvironment and genetic factors are needed to confirm our findings.PART 2: Reduced folate carrier gene RFC1 G80A polymorphism wasassociated with an increased risk of gastroesophageal cancers in aChinese populationLow folate intake has been associated with an increased risk of GC.Reduced folate carrier (SLC19A1) RFC1 is an essential folate transporter and functions as a bidirectional anion exchanger, taking up folatecofactors and exporting various organic anions. A G80A polymorphismin RFC1 gene has been shown to be associated with alterations in folateand homocysteine metabolism in healthy individuals. We obtained twotagSNPs of RFC1 gene from HapMap. In this study, we evaluated theassociations of the G80A and two tagSNPs polymorphism of RFC1 withGC risk in a case-control study of 553 GC cases and 593 cancer-freecontrols in a Chinese population. We found that the difference of G80Agenotypes between the cases and controls was significant (x~2=8.811, P=0.012), while those of two tagSNPs were not significant. Logisticregression analyse showed that 80GA and 80AA were associated with theincreased GC risk comparaed with the 80GG genotype (adjusted OR=0.89, 95％CI=0.67-1.17 for 80GA genotype; adjusted OR=1.39, 95％CI=1.01-1.90 for 80AA genotype). In the variant of the two tagSNPs,compared with widetypes (AA and GG), the variant heterozygotegenotypes (AG and GA) and variant homozygote genotypes (GG and AA)were not associated with the GC risk. In the haplotype analyse, comparedwith the high frequence haplotype in the control group AGG (46.0％), theAGA and AAG haplotypes were associated with the GC risk (adjustedOR=1.76, 95％CI=1.25-2.46 for AGA; adjusted OR=3.33,95=CI=1.19-9.32 for AAG). Stratified analyses indicated that subjectscarrying RFC1 rs2838956 AG/GG genotype were associated with anincreased GC risk among female and non-drinkers, while subjectscarrying RFC1 rs1051266GG/GA genotype were associated with anincreased GC risk among individuals with tHcy＞15.0μmol/L. Ourfindings suggest possible involvement of RFC1 variant in thesusceptibility of GC. PART 3: Polymorphisms of methylenetetrahydrofolatedehydrogenase, plasma homocysteine levels, and risk of gastriccancer in a high risk Chinese populationAccumulative evidence suggests that folate has a protective effect on GC.The methylenetetrahydrofolate dehydrogenase (MTHFD) plays animportant role in folate and homocysteine (Hcy) metabolisms andpolymorphisms of MTHFD may result in disturbance of thefolate-mediated homocysteine pathway. The aim of this study is to testthe hypothesis that genetic variants of MTHFD and plasma Hcy levels areassociated with risk of GC and modulated by genotypes ofmethylene-tetrahydrofolate reductase (MTHFR). We genotyped G1958Aand T401C in MTHFD, C677T in MTHFR and detected total plasma Hcy(tHcy) levels in a case-control study of 589 GC cases and 635 cancer-freecontrols in a high-risk Chinese population. The variant genotypes ofMTHFD 1958AA and 401CC were associated with a significantlyincreased risk of GC (adjusted OR=2.05, 95％CI=1.34-3.13 for 1958AA;adjusted OR=1.43, 95％CI=1.14-1.80 for 401CC), compared with1958GG/GA and 401TT/TC genotypes, respectively. Both of the effectswere more evident in the subjects carrying MTHFR 677CT/TT genotypes.We analyzed the association between the plasma tHcy and the G1958Aand T401C variant in MTHFD, the difference of the levels of tHcy in thedifferent genotypes were not significant in the cases and controls, buttogether with variance of the genotype from wide type to variant type, thelevels of tHcy had an increased trend, but they were not signicant. In thecrossover analyse, we found that the subjects with 401TT/TC genotypestogether with 1958AA variant genotype were associated with increased risk of GC. The subjects with 677CT/TT and MTHFD1958AA and401CC genotypes were associated with increased GC risk. Stratifiedanalyses indicated that subjects carrying MTHFD 1958AA genotypeattained an increased GC risk among men, smokers, non-drinkers andindividuals with H.pylori infection, while subjects carrying MTHFD401CC genotype were associated with an increased GC risk among men,smokers, non-drinkers and individuals with tHcy＞15.0μmol/L. Thestrong associations between MTHFD variants and the plasma tHcy levelsand GC risk suggest, for the first time, a possible gene-environmentinteraction between genetic variants of folate metabolizing genes andhigh tHcy levels in gastric carcinogenesis.