Folate is essential for one-carbon metabolism, a pathway required by DNA synthesis, methylation, and repair. Low dietary and circulating folate and polymorphic variation in this pathway are associated with increased risk of colorectal adenoma and cancer.
We genotyped 882 single nucleotide polymorphisms (SNP) in 82 one-carbon metabolism genes for 1,331 cases of advanced colorectal adenoma, identified by sigmoidoscopy at baseline, and 1,501 controls from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO). We evaluated associations between one-carbon genes and adenoma risk in all subjects and stratified by folate intake. We applied the Adaptive Rank Truncated Product (ARTP) method to assess statistical significance at the gene and pathway levels.
Folate intake was inversely associated with advanced colorectal adenoma risk [odds ratio (OR) by quartile = 0.85, P = 1.9 × 10(-5)]. We found no statistically significant associations between one-carbon genes and adenoma risk in all subjects. As hypothesized, we observed a statistically significant pathway-level association (P = 0.038) in the lowest quartile of folate; no significant associations were found in higher quartiles. Several genes including adenosine deaminase (ADA) and cysteine dioxygenase (CDO1) contributed to this signal (gene-level P = 0.001 and 0.0073, respectively). The most statistically significant SNP was rs244072 in ADA (P = 2.37 × 10(-5)).
Stratification by dietary folate and application of the ARTP method revealed statistically significant pathway- and gene-level associations between one-carbon metabolism genes and risk of advanced colorectal adenoma, which were not apparent in analysis of the entire population. Folate intake may interact with associations between common variants in one-carbon metabolism genes and colorectal adenoma risk.
"Figure S1 shows the study selection process. A total of 27 studies were retrieved based on the search criteria for CRC/CRA susceptibility related to the MTR A2756G polymorphism –. The main study characteristics were summarized in Table 1. "
[Show abstract][Hide abstract] ABSTRACT: Methionine synthase (MTR), which plays a central role in maintaining adequate intracellular folate, methionine and normal homocysteine concentrations, was thought to be involved in the development of colorectal cancer (CRC) and colorectal adenoma (CRA) by affecting DNA methylation. However, studies on the association between MTR A2756G polymorphism and CRC/CRA remain conflicting. We conducted a meta-analysis of 27 studies, including 13465 cases and 20430 controls for CRC, and 4844 cases and 11743 controls for CRA. Potential sources of heterogeneity and publication bias were also systematically explored. Overall, the summary odds ratio of G variant for CRC was 1.03 (95% CI: 0.96-1.09) and 1.05 (95% CI: 0.99-1.12) for CRA. No significant results were observed in heterozygous and homozygous when compared with wild genotype for these polymorphisms. In the stratified analyses according to ethnicity, source of controls, sample size, sex, and tumor site, no evidence of any gene-disease association was obtained. Results from the meta-analysis of four studies on MTR stratified according to smoking and alcohol drinking status showed an increased CRC risk in heavy smokers (OR = 2.06, 95% CI: 1.32-3.20) and heavy drinkers (OR = 2.00, 95% CI: 1.28-3.09) for G allele carriers. This meta-analysis suggests that the MTR A2756G polymorphism is not associated with CRC/CRA susceptibility and that gene-environment interaction may exist.
PLoS ONE 04/2013; 8(4):e60508. DOI:10.1371/journal.pone.0060508 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent genome-wide association studies (GWASs) designed to detect the main effects of genetic markers have had considerable success with many findings validated by replication studies. However, relatively few findings of gene-gene or gene-environment interactions have been successfully reproduced. Besides the main issues associated with insufficient sample sizes in current studies, a complication is that interactions that rank high based on p-values often correspond to extreme forms of joint effects that are biologically less plausible. To reduce false positives and to increase power, we develop various gene-environment/gene-gene tests based on biologically more plausible constraints using bivariate isotonic regressions for case-control data. We extend our method to exploit gene-environment or gene-gene independence information, integrating the approach proposed by Chatterjee and Carroll. We propose appropriate nonparametric and parametric permutation procedures for evaluating the significance of the tests. Simulations show that our method gains power over traditional unconstrained methods by reducing the sizes of alternative parameter spaces. We apply our method to several real-data examples, including an analysis of bladder cancer data to detect interactions between the NAT2 gene and smoking. We also show that the proposed method is computationally feasible for large-scale problems by applying it to the National Cancer Institute (NCI) lung cancer GWAS data.
Journal of the American Statistical Association 12/2012; 107(500). DOI:10.1080/01621459.2012.726892 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Abstract The importance of proper consumption of dietary folate for human health has been highlighted by an extensive number of publications over several decades. Fortification of grain products with folic acid was initiated with the specific intent to prevent neural tube defects, and the scope of this endeavor is unique in that its target population (women of the periconceptional period) is many times smaller than the population it affects (everyone who ingests fortified grain products). Folate fortification has been wildly successful in terms of its goal; since its inception, the incidence of neural tube defects has markedly decreased. In the wake of this public health triumph, it is important to catalog both the serendipitous benefits and potential side effects of folic acid supplementation. The vitamin is generally regarded as a harmless nutrient based on studies evaluating the safe upper limits of folate intake. In recent years, however, a concern has been raised with respect to a potential downside to folate supplementation; namely, its proposed ability to enhance proliferation of malignant tumors. The current review summarizes the available literature on the effects of folate supplementation and the molecular mechanisms by which high doses of folate may have negative consequences on human health, especially with regard to cancer.
Clinical Chemistry and Laboratory Medicine 12/2012; 51(3):1-10. DOI:10.1515/cclm-2012-0561 · 2.71 Impact Factor
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