Associations between polymorphisms in folate-metabolizing genes and pancreatic cancer risk in Japanese subjects

Pancreatic cancer is often diagnosed at an advanced stage and has the poorest 5-year survival rate of any cancer. There is no effective screening method to identify apparently healthy individuals who are at risk for pancreatic cancer. The etiology of pancreatic cancer remains largely unknown, with only cigarette smoking and long standing diabetes being established as risk factors [1]. The role of diet in carcinogenesis has been recognized for a long time. Evidence on the diet-pancreatic cancer associations from epidemiologic studies remains elusive, in part because of wide variation in dietary habits and the difficulty of accurate diet measurement. In nutrient-based studies, there has been much research interest in folate, a water soluble vitamin B that is abundant in green leafy vegetables, citrus fruit, legumes and cereals [2]. Folate plays a vital role in maintaining health because it is closely involved in two vital cellular processes, DNA methylation and DNA synthesis [2]. Epidemiologic studies have linked folate deficiency to a variety of conditions, including cardiovascular diseases and cancer [3‐5]. For pancreatic cancer, previous studies have shown mixed findings on the association between dietary folate intake and pancreatic cancer risk [6‐9], although a 2014 meta-analysis reported an inverse association [10]. One possible reason for the inconsistent findings is that folate status is determined by both dietary folate intake and folate metabolism, making it difficult to accurately quantify folate intake. Of numerous genes involved in the folate metabolizing pathway, MTHFR has been the most extensively studied [11, 12]. MTHFR irreversibly converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the predominant form of folate in the circulation. Two variants in the MTHFR, namely C677T (rs1801133) and A1298C (rs1801131), have been the focus of most studies [11] because the variations are associated with a small change in protein structure. Compared with the CC genotype (wild-type), the TT genotype (minor homozygotes) of C677T has approximately 35 % lower enzyme activity and 10 % lower levels of methylhydrolate folate [11]. The 677 T variant has been associated with numerous conditions, including elevated homocysteine, spinal bifida, colon cancer, and Down syndrome [3] As for MTHFR A1298C, the homozygous CC genotype has approximately 60 % of normal MTHFR activity [12].

Evidence supporting the associations between folate metabolizing gene polymorphisms and pancreatic cancer has been inconclusive. To date, five case-control studies have examined the association between the polymorphisms in MTHFR, MTR, or MTRR genes and pancreatic cancer risk, with conflicting results [13‐17]. Of them, two case-control studies conducted in Japan found no main effects for polymorphisms in MTHFR (rs1801133, rs1801131), MTR (rs1805087) or MTRR (rs1801394) genes [15, 17]. However, rs162049 in the MTRR gene, which encodes enzymes responsible for DNA methylation, has been shown to be associated with the susceptibility to pancreatic cancer in one previous Japanese case-control study [17]. The significance of these polymorphisms in the folate metabolizing pathway on pancreatic cancer remains to be determined.

Given that substantial evidence from epidemiologic and laboratory research supports an important role of folate in carcinogenesis [18], we genotyped several genetic polymorphisms in the folate metabolizing pathway and examined their associations with pancreatic cancer risk in Japanese subjects. We hypothesized that the variant MTHFR C667T and A1298C genotypes resulting in decreased enzyme activity are associated with an increased risk for pancreatic cancer. Additionally, we performed analyses stratified by cigarette smoking and alcohol drinking to address the possibility that these two lifestyle factors may modify the associations between genetic polymorphisms and pancreatic cancer risk.

The distribution of genotypes for all SNPs among control subjects did not deviate from HWE. The selected characteristics of cases and controls are presented in Table 1. Compared with control subjects, the cases were more likely to have a history of diabetes and to be current smokers. The proportion of current drinkers is higher in control subjects than in case patients. The daily ethanol intake was 24.1 g/d among case patients and 17.5 g/d among control subjects. The median of daily folate intake was 338.9 μg for case patients and 359.5 μg for control subjects. High intake of dietary folate was inversely associated with pancreatic cancer risk, with OR of 0.52 (95 % CI: 0.33-0.82) among individuals falling into the highest quartile when compare with those falling into the lowest quartile.

Table 2 shows the ORs of pancreatic cancer in relation to individual polymorphisms in the following genes: MTHFR (rs1801133, rs1801131), MTRR (rs1801394), and MTR (rs1805087). Overall no significant associations were noted between any single genotype and risk of pancreatic cancer. For rs1801133, compared with individuals with the CC genotype of MTHFR C677T, the OR for those with the CT genotype and TT genotype was 0.87 (0.62–1.22) and 0.99 (0.65–1.51), respectively. For rs1801131, individuals with the CC genotype had approximately 1.2-fold increased risk compared with those with the AA genotype, but the association was not statistically significant.

Table 3 shows the associations of pancreatic cancer with folate metabolizing gene polymorphisms by smoking and drinking status. There were no significant associations in either never smokers or current smokers. Similarly, there were no significant associations in either never drinkers or current drinkers.

Table 4 shows the joint effects of smoking, drinking and MTHFR genotypes on pancreatic cancer risk. No significant interactions were observed for C667T genotypes and cigarette smoking; the OR of pancreatic cancer was 1.94 (1.05–3.57) for individuals who had the TT genotype and were ever smokers. Similarly, no significant interactions were noted for C677T genotypes and alcohol drinking; the OR of pancreatic cancer was 1.05 (95 % CI: 0.62-1.78) for individuals who had the TT genotype and were ever smokers.

In this genetic case-control association study, we found no statistically significant associations between polymorphisms in folate metabolizing gene pathways, including MTHFR C677T and A1298C, and the risk of pancreatic cancer. Previous studies on the associations of MTHFR (C677T) with pancreatic cancer risk have shown inconsistent results [13‐16]. A hospital-based case-control study, conducted at the MD Anderson Cancer Center in the United States, found that individuals with the TT variant genotype had a 2-fold increased risk for pancreatic cancer when compared with individuals with the CC genotype [13]. A stronger association was reported in a case-control study of Chinese population, in which the OR was 5.12 for individuals with the TT genotype compared with individuals with the CC genotype [14]. By contrast, two previous case-control studies in Japan did not find significant main effects for MTHFR C677T and A1298C genotypes [15, 17]. Our null findings were in agreement with results of these two previous studies. Our null findings were also consistent with the conclusion of a recent meta-analysis, which included all previous studies and showed no significant main effects for both MTHFR C677T and A1298C genotypes [22]. Furthermore, a recent study has sought to examine 37 genes and 834 SNPs related to one-carbon metabolism. There were no significant associations for any of the SNPs after correction for multiple comparisons [23]. However, in the case-control study by Ohnami et al, rs162049 (intronic SNP) in the MTRR gene showed significant associations after multiple testing [17]. One strength of their study is that functional tests were performed to collaborate the SNP-phenotype association. Unfortunately, we did not genotype rs162049 in the MTRR gene, but further studies needs to replicate their positive finding.

Given the lower enzymatic activity in individuals with the variant TT genotype of MTHFR C677T, a key enzyme in the folate metabolizing pathway, we hypothesized that the TT genotype carriers have an increased risk of pancreatic cancer. Our results, however, showed that case patients had a similar distribution of the TT genotypes with that of control subjects, and neither CT nor TT genotypes were significantly associated with the risk when compared with CC genotypes. The reason for our null findings on the associations between MTHFR genotypes and pancreatic cancer risk is not clear, but the differences observed in minor allele frequencies among ethnic groups may in part account for the conflicting results. The genotype frequencies of MTHFR C677T differed between our case-control study and the previous case-control study that involved a US population [13]. Even though there may be no main effects for the genotypes themselves, previous studies have shown that the MTHFR 677C → T polymorphism is only associated with increased coronary heart disease risk in a setting of low folate intake [4]. This finding suggested that the effect modification by dietary folate intake is important when interpreting the genotype-disease associations. If nutritional folate status is poor, the 677 T variant might promote the misincorporation of uracil into DNA, leading to genomic instability, a hallmark of cancer [3]. This mechanism may, in part, account for the increased cancer risk among individuals with the 677 T variant of MTHFR. On the other hand, if folate intake is adequate, the 677 T variant of MTHFR preferentially routes one-carbon units to DNA synthesis at the expense of methionine, which is involved in DNA methylation [3]. To address the possible effect modification by dietary folate intake, we evaluated their associations in low- versus high- dietary intake group, and found no significant effect modifications (data not shown). One possible reason is that most subjects in our study had adequate dietary folate intake because the estimated amount was comparable to 240 μg per day for Japanese adults recommended by the government.

In addition to dietary folate intake, other lifestyle factors that may modify the associations between MTHFR C677T and pancreatic cancer risk are smoking and drinking. Because alcohol is known as a folate antagonist and because smoking may impair folate status, several studies have evaluated the joint effects of the MTHFR polymorphisms with cigarette smoking or alcohol consumption in relation to pancreatic cancer risk [13‐15]. In one hospital-based case-control study conducted in Japan, the risk of pancreatic cancer increased by 4.5-fold among heavy drinkers with the MTHFR 677 CC genotype [15]. Similarly, another two case-control studies found that individuals with the 677 T variant combined with heavy smoking or drinking had significantly increased risk for pancreatic cancer [13, 14]. In contrast to their findings, we failed to observe the synergistic effect of the genotypes with either cigarette smoking or heavy alcohol consumption. It is worth noting that the analysis of gene-environment interaction was limited by a small sample size in our study, as well as in those previous studies. So the probability that the positive interaction was due to chance cannot be ruled out.

Our study has several limitations. First, we had sufficient number of cases required to detect significant associations for the main effect of genotypes based on sample size estimation; however, we did not have sufficient power to detect significant associations in the analyses examining the synergistic effect of genotypes and exposure of interest. Assuming a dominant genetic model, a dichotomous exposure prevalence of 10 %, a relative risk for a genotype of 1.5, a relative risk for exposure of 1.5, and 1:1 case-control ratio, we need thousands of cases and controls to detect multiplicative interactions with relative risk of 2.0 [24]. Second, environmental exposure (folate) assessment or environmental exposure (folate)-pancreatic cancer association could have been affected by selection or recall bias inherent in case-control studies; however, the genotype-pancreatic cancer association was generally not affected by bias due to differential environmental exposure assessment between cases and control subjects. Third, the lack of the data on plasma folate levels did not allow us to address the relationship among MTHFR genotypes, plasma levels and pancreatic cancer risk. It has been shown that DNA methylation was affected by genotype among only those with lower plasma folate levels [25]. Moreover, it is expected that multifactorial interactions may exist among MTHFR genotypes, dietary folate, circulating folate level, alcohol, and/or other relevant nutrients including vitamin B2 and B12. Further studies need to develop a novel analytical method addressing the complex gene-environment interaction pathways involved in folate-derived carcinogenesis.

In conclusion, our case-control study suggests that there is no association between genetic polymorphisms in folate-metabolizing genes and the risk of pancreatic cancer in Japanese subjects. The results may not be generalizable in view of limited sample size. Our study is also limited to address effect modification by alcohol, smoking or folate intake (gene-environment interaction) because of inadequate statistical power.

BMI, body mass index; CI, confidence interval; FFQ, food frequency questionnaire; HWE, Hardy-Weinberg equilibrium; MTHFR, methylenetetrahydrofolate; MTR, 5-methyltetrahydrofolate-homocysteine methyltransferase; MTRR, 5-methyltetrahydrofolate-homocysteine methyltransferase reductase; ORs, odds ratios.