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Carcinogenesis Advance Access originally published online on March 13, 2008
Carcinogenesis 2008 29(9):1765-1773; doi:10.1093/carcin/bgn074
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Associations of dietary methyl donor intake with MLH1 promoter hypermethylation and related molecular phenotypes in sporadic colorectal cancer

Stefan de Vogel1,2,*, Brenda W.C. Bongaerts1, Kim A.D. Wouters2, Arnold D.M. Kester3, Leo J. Schouten1, Anton F.P.M. de Goeij2, Adriaan P. de Bruïne2, R. Alexandra Goldbohm4, Piet A. van den Brandt1, Manon van Engeland2 and Matty P. Weijenberg1

1 Department of Epidemiology, GROW—School for Oncology and Developmental Biology, Maastricht University, Maastricht, The Netherlands
2 Department of Pathology, GROW—School for Oncology and Developmental Biology, Maastricht University, Maastricht, The Netherlands
3 Department of Methodology and Statistics, Maastricht University, PO BOX 616, 6200MD Maastricht, The Netherlands
4 Department of Prevention and Health, TNO Quality of Life, Leiden, The Netherlands

* To whom correspondence should be addressed. Tel: +31 43 3882236; Fax: +31 43 3884128; Email: stefan.devogel{at}epid.unimaas.nl


   Abstract
Top
 Abstract
Introduction
 Subjects and methods
 Results
 Discussion
 Funding
 References
 
Intake of dietary factors that serve as methyl group donors may influence promoter hypermethylation in colorectal carcinogenesis. We investigated whether dietary folate, vitamin B2 and vitamin B6, methionine and alcohol were associated with mutL homologue 1 (MLH1) hypermethylation and the related molecular phenotypes of MLH1 protein expression, microsatellite instability (MSI) and BRAF mutations in patients with colorectal carcinomas. Within the Netherlands Cohort Study on diet and cancer (n = 120 852), 648 cases (367 men and 281 women) and 4059 subcohort members were available for data analyses from a follow-up period between 2.3 and 7.3 years after baseline. Gender-specific adjusted incidence rate ratios (RRs) were calculated over categories of dietary intake in case-cohort analyses. The intakes of folate, vitamin B2, methionine and alcohol were not associated with risk of tumors showing MLH1 hypermethylation, those lacking MLH1 protein expression or with MSI. Among men, we observed strong positive associations between folate and BRAF-mutated tumors (RR = 3.04 for the highest versus lowest tertile of intake, Ptrend = 0.03) and between vitamin B6 and tumors showing MLH1 hypermethylation (highest versus lowest tertile: RR = 3.23, Ptrend = 0.03). Among women, the relative risks of tumors with BRAF mutations or MLH1 hypermethylation were also increased in the highest tertiles of folate and vitamin B6 intake, respectively, but these did not reach statistical significance. The positive associations between folate intake and tumors harboring BRAF mutations and between vitamin B6 intake and those showing MLH1 hypermethylation were most pronounced among men and may suggest that these vitamins enhance colorectal cancer risk through genetic as well as epigenetic aberrations.

Abbreviations: CI, confidence interval; CRC, colorectal cancer; FFQ, food frequency questionnaire; MLH1, mutL homologue 1; MSI, microsatellite instability; MSP, methylation-specific PCR; PALGA, Pathologisch Anatomisch Landelijk Geautomatiseerd Archief; PCR, polymerase chain reaction; RR, rate ratio


   Introduction
Top
 Abstract
 Introduction
Subjects and methods
 Results
 Discussion
 Funding
 References
 
It has been hypothesized that two forms of genetic instability may contribute to the carcinogenesis of sporadic colorectal cancer (CRC). Whereas microsatellite instability (MSI) occurs in ~15% of the sporadic CRCs, chromosomal instability would account for the remaining 85% (1). Sporadic tumors with MSI present a distinct molecular phenotype and may develop predominantly as a consequence of hypermethylation of the mismatch repair gene mutL homologue 1 (MLH1) (2–4). Furthermore, mutations in the BRAF oncogene are strongly associated with MLH1 promoter methylation and MSI (5) and are an additional type of molecular alteration characterizing this phenotype.

Dietary factors that serve as methyl group donors, such as folate and methionine, potentially have an effect at the level of DNA methylation. A low folate status may decrease genomic DNA methylation, which in turn presumably contributes to the process of carcinogenesis (6). A specific form of aberrant methylation that is frequently observed in carcinogenesis involves CpG island promoter hypermethylation of, for example, DNA repair genes (7). It may be hypothesized that deficient status or low intake of methyl donors could also lead to increased frequencies of this type of aberrant DNA methylation. In this respect, in a pilot study (n = 122 patients), we previously observed a weak association between relatively low folate intake in combination with high alcohol consumption and increased promoter hypermethylation of at least one of six of the studied CRC genes (8). Conversely, it was recently suggested that folate supplementation of 4.6 mg/day in combination with vitamin B12 during 6 months may be associated with increased levels of promoter hypermethylation in colorectal mucosa, although this association was borderline significant (9). These results seem contradictive and it is obviously important to further study the potential effect of relatively high methyl donor intake on the occurrence of promoter hypermethylation.

Other dietary factors, such as vitamin B2 and vitamin B6, are also involved in the folate-mediated one-carbon metabolism (Figure 1). These vitamins may therefore modulate the bioavailability of methyl groups and thereby DNA methylation as well (10). Flavin adenine dinucleotide, a metabolite of vitamin B2, is the cofactor for methylenetetrahydrofolate reductase (MTHFR), the enzyme that converts 5,10-methylenetetrahydrofolate into 5-methyl-tetrahydrofolate and thereby enhances DNA methylation. Low vitamin B2 status was previously observed to be associated with increased tHcy concentration, which possibly results in lower availability of methyl groups needed for DNA methylation (11). Interestingly, it was suggested that patients with the MTHFR C677T and A1298C polymorphisms, which are associated with lower activity of this enzyme, tended to have a higher level of MLH1 promoter methylation in colon carcinomas (12). Subjects with the homozygous MTHFR 677TT variant also had lower serum folate levels (13) and may be at increased risk of developing tumors harboring MSI (13,14). Vitamin B6 is involved in the conversion of tetrahydrofolate into 5,10-methylenetetrahydrofolate, which is one of the steps of the folate cycle and therefore essential for the subsequent supply of methyl groups. Conversely, high alcohol intake reduces the bioavailability of folate and such a disruption of the one-carbon metabolism may affect DNA methylation in, for example, the colonic mucosa (15,16). Long-term alcohol consumption may also result in an increased risk of tumors harboring MSI (17,18).


Figure 1
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  		Fig. 1. The role of folate, vitamin B2, vitamin B6 and alcohol in the synthesis and methylation of DNA. Dietary factors are given in italics. Methyl donors are folate and methionine; factors that may modulate the bioavailability of methyl groups are vitamin B2, vitamin B6 and alcohol. FAD, flavin adenine dinucleotide; MTR, methionine synthase; MTRR, methionine synthase reductase.

 
Since folate plays an important role in both DNA synthesis and DNA methylation, one would expect that adequate folate status or sufficient folate intake can also reduce the risk of tumors harboring gene mutations. Opposite to this, however, we have previously observed that men in the third tertile of folate intake may be at increased risk of CRCs with truncating APC mutations (19). The relation between diet and BRAF mutations has been studied, but no associations were observed with folate, vitamin B6, vitamin B12 and methionine (20).

Here, we investigate associations between dietary folate, vitamin B2, vitamin B6, methionine and alcohol in relation to MLH1 promoter methylation and the associated molecular characteristics of absence of MLH1 protein expression, MSI and BRAF mutations in CRCs. This allows us to establish which of these end points associated with the MSI phenotype is most sensitive to dietary exposure and whether the effect of methyl donor intake through folate and methionine may be modulated by vitamin B2 and vitamin B6 or alcohol. We also describe the occurrence and overlap between MLH1 promoter methylation, MLH1 protein expression, MSI and BRAF mutations. This study is carried out within the Netherlands Cohort Study on diet and cancer among a large group of unselected CRC patients.


   Subjects and methods
Top
 Abstract
 Introduction
 Subjects and methods
Results
 Discussion
 Funding
 References
 
Study population
The participants in this study are incident colon and rectal cancer patients and subcohort members from the Netherlands Cohort Study, which has been described in detail elsewhere (21). Briefly, the study was initiated in 1986 and includes 58 279 men and 62 573 women aged 55–69 years at baseline, who originated from 204 Dutch municipalities with computerized population registers. At baseline, participants completed a self-administered food frequency questionnaire (FFQ) that also provided information about age, sex and other risk factors for cancer. The entire cohort was being monitored for cancer occurrence by annual record linkage to the Netherlands Cancer Registry (nine cancer registries in the Netherlands) and to PALGA (Pathologisch Anatomisch Landelijk Geautomatiseerd Archief), a nationwide network and registry of histopathology and cytopathology reports (22). Accumulation of person-time in the cohort has been estimated through biennial vital status follow-up of a subcohort of 5000 men and women who were randomly selected after baseline exposure measurement. Cases with prevalent cancer other than non-melanoma skin cancer were excluded from this subcohort, which left 4774 men and women eligible for analysis.

In 1986, the PALGA registry was not implemented in some of the municipalities included in the study but reached full coverage by the end of 1988. Incomplete coverage may introduce selection bias, and in addition, possible preclinical disease may have affected exposure status. For these reasons, we excluded the first 2.3 years of follow-up from the analyses. A total of 101 subcohort members were either deceased or diagnosed with cancer other than non-melanoma skin cancer within this period, leaving 4673 men and women for analysis. From 1989 to 1994, 925 incident cases were identified with histologically confirmed CRC, of whom 815 could also be linked to a PALGA report of the lesion. The PALGA database was used to identify and locate tumor tissue in Dutch pathology laboratories. CRC was classified according to disease site as follows: colon, i.e. proximal colon (ICD-O-1 codes 153.0, 153.1, 153.4, 153.5, 153.6) and distal colon (153.2, 153.3, 153.7), rectosigmoid (154.0), rectum (154.1) or ICD-O-1 codes 153.8 and 153.9 if information of the disease site was not available.

Tissue samples
Tumor material of the CRC patients was collected after approval by the ethical review boards of Maastricht University, the Netherlands Cancer Registry and PALGA (23). In addition, all pathology laboratories in the Netherlands agreed to make relevant tissue samples available from PALGA upon request. Of the 815 tissue samples that were scattered over 54 pathology laboratories in the Netherlands, 734 samples (90%) could be traced and were retrieved between August 1999 and December 2001 (23).

MLH1 promoter methylation
DNA methylation in the CpG islands of the MLH1 gene was determined by chemical modification of genomic DNA with sodium bisulfite and subsequent methylation-specific polymerase chain reaction (PCR) (MSP, described in detail elsewhere (24)). In brief, 500 ng of DNA was denatured by NaOH and modified by sodium bisulfite. DNA samples were then purified using Wizard DNA purification resin (Promega, Leiden, The Netherlands), treated again with NaOH, precipitated with ethanol and resuspended in H2O.

To facilitate MSP analysis on DNA retrieved from formalin-fixed, paraffin-embedded tissue, DNA was first amplified with flanking PCR primers (described in ref. 8) that amplify bisulfite-modified DNA but do not preferentially amplify methylated or unmethylated DNA. The resulting fragment was used as a template for the MSP reaction (8,25).

All PCRs were performed with controls for unmethylated alleles (DNA from normal lymphocytes), methylated alleles [normal lymphocyte DNA treated in vitro with SssI methyltransferase (New England Biolabs, Ipswich, MA)] and a control without DNA. Ten microliters of each MSP reaction was directly loaded onto non-denaturing 6% polyacrylamide gels, stained with ethidium bromide and visualized under UV illumination. MSP analyses of MLH1 were successfully performed for 686 (93%) of 734 patients. Reproducibility was high, with duplo analyses performed on a random subset of 72 samples yielding the same result in 89% of the cases.

MLH1 protein expression status
Immunohistochemical analyses were performed and scored on 4 µm sections of formalin-fixed, paraffin-embedded cancer tissue and adjacent normal tissue using a monoclonal antibody against MLH1, as previously described (17). Staining was evaluated with normal cells as internal control. Two investigators reviewed the immunohistochemical staining profiles independently and discrepancies were re-examined and discussed with a pathologist until consensus was reached. MLH1 protein expression status was determined successfully in 721 (98%) of the 734 patients.

Microsatellite instability
MSI was determined by a pentaplex PCR using the MSI markers BAT-26, BAT-25, NR-21, NR-22 and NR-24, as described in detail by Suraweera et al. (26). MSI analyses were successful on 662 (90%) of the 734 available samples. The reproducibility was 100% since all the 53 duplo analyses showed identical results.

BRAF mutations
The common V600E BRAF mutation in exon 15 was analyzed by a semi-nested PCR and subsequent restriction fragment length polymorphism analyses as previously described (27). BRAF mutation status could be analyzed successfully in 697 (95%) of the 734 tissue samples. The analyses were repeated on 33 randomly drawn samples and could be succesfully reproduced in 88% of these cases.

Food frequency questionnaire
The self-administered questionnaire was a 150-item semi-quantitative FFQ, which concentrated on habitual consumption of food and beverages during the year preceding the start of the study, and also contained questions about body weight, body length, smoking status, physical activity and family history of CRC. Daily mean nutrient intakes were calculated as the cumulated product of the frequencies and portion sizes of all food items and their tabulated nutrient contents from the Dutch Food Composition Table (NEVO table, 1986 (28)). The validity and reproducibility of the FFQ were determined (29,30). Questionnaire data were key entered twice for all incident cases in the cohort and for all subcohort members in a blinded manner with respect to case–subcohort status. This was done in order to minimize observer bias in coding and interpretation of the data.

Folate data were derived from a validated liquid chromatography trienzyme method (31) used to analyze the 125 most important Dutch foods contributing to folate intake (32). Mean daily intakes of all other relevant nutrients were calculated using the computerized Dutch Food Composition Table (28). Dietary supplement data were also obtained via the FFQ. However, the use of B vitamin supplements was low (7%) and folic acid was generally not included in these supplements in the Netherlands in the late 1980s. Therefore, folic acid supplement use most probably plays a very minor role in our study population, and supplement use was not further accounted for in the analyses.

Statistical analyses
The occurrence and relative overlap of MLH1 hypermethylation with any of the three other characteristics—absence of MLH1 protein expression, MSI and BRAF mutations—were calculated and tested with Chi-square tests. Dietary factors and other baseline characteristics were summarized for men and women separately for subcohort members and CRC cases by calculating means and standard deviations for continuous variables and distributions of the categorical variables. Differences in dietary folate, vitamin B2, vitamin B6, methionine and alcohol intake were tested between CRC cases and the cases without CRC in the subcohort using Student’s t-tests or Chi-square tests where appropriate. This comparison was also made for the four other molecular characteristics.

Cox proportional hazards regression models were used to estimate multivariate-adjusted incidence rate ratios (RRs) and corresponding 95% confidence intervals (CIs) over tertiles of folate, vitamin B2, vitamin B6 and methionine intake and over categories of alcohol intake, using the lowest intake categories as reference. Standard errors of the RRs were estimated using the robust Huber–White sandwich estimator to account for additional variance introduced by sampling from the cohort (33). The proportional hazards assumption was tested using the scaled Schoenfeld residuals (34). Tests for dose–response trends over the different categories of intake were estimated by fitting the ordinal exposure variables as continuous variables and evaluated using the Wald test. For each end point, a model was used that included all these five dietary variables. Folate, vitamin B2, vitamin B6 and methionine were adjusted for total energy intake by calculating nutrient residuals from the regression of nutrient intake on total energy intake, as described by Willet et al. (35). These nutrient residuals are uncorrelated with total energy intake, and the effect of the variation in nutrient intake can subsequently be estimated independently of a potential effect of energy intake. The RRs were calculated for all colorectal tumors, for tumors with MLH1 hypermethylation, absence of MLH1 protein expression, MSI or BRAF mutations. Because tumors presenting MSI usually occur more frequently among women and may have a different etiology than in men, we have chosen to present gender-specific results. In addition to these analyses, we also performed overall analyses for men and women combined in order to create larger subgroups and thereby to reduce the probability of reporting chance findings.

Tests for heterogeneity were performed to evaluate differences between subtypes of tumors (e.g. MLH1 methylated versus unmethylated) using the competing risks procedure in Stata. However, the standard error for the difference of the log Hazard Ratios from this procedure assumes independence of both estimated Hazard Ratios, which would underestimate that standard error and thus overestimate the P values for their difference. Therefore, these P values and the associated CIs were estimated based on a bootstrapping method that was developed for the case-cohort design (36). For each bootstrap sample, X subcohort members were randomly drawn from the subcohort of X subjects and Y cases from the total of Y cases outside the subcohort, both with replacement, out of the dataset of X + Y observations. The log HRs were obtained from this sample using Stata’s competing risks procedure and recalculated for each bootstrap replication. The CI and P value of the differences in hazard ratio of the subtypes were then calculated from the replicated statistics. Each bootstrap analysis was based on 1000 replications.

The covariates included in the multivariate analyses were those found to influence the RR by >10% or those observed to be associated with CRC in previous studies. This applied for the variables age, family history of CRC, smoking, body mass index and dietary intakes of energy, meat, fat, fiber, vitamin C, iron and calcium. After excluding subjects with missing information on these covariates or subjects who only partly filled out the questionnaire, 4059 subcohort members remained for statistical analyses as well as 648 CRC cases, irrespective of the available molecular analyses.

We determined possible interactions between dietary intakes of methyl donors (folate and methionine) and the potentially ‘modulating’ factors vitamin B2, vitamin B6 and alcohol for each of the individual end points. This was done by first testing, in separate models, the gender-specific interaction terms between folate and methionine with vitamin B2, vitamin B6 and alcohol, i.e. folate x methionine, folate x B2, folate x B6, folate x alcohol, methionine x B2, methionine x B6 and methionine x alcohol. The Cox proportional hazard analyses without the interaction terms were subsequently stratified by low or high intake of folate, vitamin B2, vitamin B6 and methionine using the median intakes as cutoff values to define both strata within each variable. The strata used for alcohol intake were 1, abstainers; 2, subjects with intake <30 g/day and 3, subjects who consumed ≥30 g/day.

All statistical analyses were performed with the Stata statistical software package (version 9.1).


   Results
Top
 Abstract
 Introduction
 Subjects and methods
 Results
Discussion
 Funding
 References
 
Among the CRC patients in our study, we first assessed the occurrence of MLH1 hypermethylation, MLH1 protein expression, MSI and BRAF mutations. The analyses of these four characteristics were complete in 648 patients. The frequencies and percentages as well as the overlap between these molecular phenotypes are shown in Table I and Figure 2. In 152 (22.1%) of the patients with successful MLH1 MSP analyses, promoter methylation of this gene was found. The percentages of tumors that lacked the MLH1 protein, those with MSI as determined by the MSI pentaplex assay or those harboring BRAF mutations were 8.5, 12.7 and 16.1%, respectively. Although the associations between MLH1 hypermethylation and the three other aberrations individually were highly significant (all Chi-square tests had P < 0.001), the overlap between the four molecular phenotypes, which was based on patients with all molecular analyses available, was not complete. The highest relative overlap was observed with MLH1 protein expression since of the 55 tumors lacking the MLH1 protein, 41 (74.6%) also showed MLH1 hypermethylation. The overlap between MSI and MLH1 hypermethylation was lower, with 50 (65.8%) of 76 tumors harboring MSI also having MLH1 hypermethylation. The lowest percentage of overlap was observed between BRAF mutations and MLH1 hypermethylation, as 48 (47.5%) of 101 BRAF-mutated tumors had a hypermethylated MLH1 gene.


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  		Table I. Frequencies of molecular phenotypes and overlap of MLH1 promoter methylation with MLH1 expression, MSI and BRAF mutations

 


Figure 2
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  		Fig. 2. Overlap between MLH1 hypermethylation, absence of MLH1 protein expression, MSI and BRAF mutations in tumors showing at least one of these four aberrations. In total, there were 238 tumors harboring at least one aberration. Numbers are based on tumors with complete analyses of all four molecular characteristics (tumors of 648 cases in total). The sizes of the different areas in this figure do not exactly reflect the numbers of the applicable subsets.

 
We then explored dietary intakes and other baseline characteristics of subcohort members and cancer cases with MLH1 promoter methylation, absence of MLH1 protein expression, MSI or BRAF mutations among men and women (Table II). The percentage of male patients with a family history of CRC was substantially lower among men with a hypermethylated MLH1 gene compared with all CRC tumors combined in men. It was also clear that tumors with MLH1 hypermethylation, without MLH1 protein expression, with MSI or with BRAF mutations all occurred more often in the proximal colon when compared with the total group of tumors in both men and women. The intakes of folate, vitamin B2, vitamin B6, methionine and alcohol were similar among cases in the different subgroups compared with subcohort members.


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  		Table II. Baseline dietary intake (mean ± SD) and other characteristics of cancer cases and subcohort members from the Netherlands Cohort Study on diet and cancer

 
We subsequently estimated the associations between intakes of folate, vitamin B2, vitamin B6, methionine and alcohol on the one hand, with colorectal carcinoma risk with or without MLH1 hypermethylation, absence of MLH1 protein expression, MSI or BRAF mutations on the other hand. Among men, folate intake was not associated with overall CRC, with tumors showing MLH1 hypermethylation, lacking MLH1 protein expression or with MSI. However, it was positively associated with colorectal tumors harboring BRAF mutations in men (RR = 3.04, CI = 1.13–8.20, Ptrend = 0.03 for the highest versus the lowest tertile of intake, Table III). Conversely, there was an inverse association between methionine intake and risk of BRAF-mutated tumors (highest versus lowest tertile: RR = 0.28, CI = 0.09–0.86, Ptrend = 0.02). Both vitamin B2 and alcohol were not associated with any of the end points studied in men. However, dietary intake of vitamin B6 was positively associated with overall CRC in men (RR = 1.54, CI = 1.01–2.36, Ptrend = 0.06 for the highest versus the lowest tertile of intake). The association between vitamin B6 and tumors with MLH1 hypermethylation in men was even stronger (RR = 3.23, CI = 1.15–9.06, Ptrend = 0.03).


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  		Table III. Associations between dietary factors and MLH1 hypermethylation, absence of the MLH1 protein, MSI and BRAF mutations in colorectal tumors among men

 
Among women, dietary intakes of folate, vitamin B2, vitamin B6, methionine and alcohol were not associated with overall CRC or any of the end points studied (Table IV). Although the RRs of tumors with BRAF mutations and those with MLH1 hypermethylation were also increased in the highest tertiles of folate and vitamin B6 intake, respectively, these associations were not statistically significant among women.


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  		Table IV. Associations between dietary factors and MLH1 hypermethylation, absence of the MLH1 protein, MSI and BRAF mutations in colorectal tumors among women

 
Increased relative risks could be observed in the highest category of alcohol intake for most of the end points; however, trends were not statistically significant and the numbers of cases in the highest category were low for both men and women.

When performing overall analyses for men and women together (data not shown), we observed that, although borderline significant, the highest tertile of folate intake was associated with colorectal tumors harboring BRAF mutations (RR = 1.85, CI = 0.92–3.73, Ptrend = 0.08). There clearly were positive associations between vitamin B6 intake and overall CRC (RR = 1.51, CI = 1.11–2.04, Ptrend = 0.01) and risk of tumors with MLH1 promoter methylation (RR = 2.32, CI = 1.21–4.43, Ptrend = 0.01). The tests for heterogeneity did not show significant differences between tumors with or without MLH1 hypermethylation, MLH1 protein expression, MSI or BRAF mutations (data not shown).

After testing potential interactions, it appeared that there was no interaction between folate and methionine, vitamin B2 or alcohol in either men or women. However, among men, we observed an interaction between folate and vitamin B6 for overall CRC. In this respect, stratified analyses revealed that relatively high vitamin B6 intake was associated with an increased risk of overall CRC among men who had a folate intake in the category above the median (RR = 1.96, CI = 1.06–3.60, Ptrend = 0.02 for the highest versus the lowest tertile of B6 intake, data not shown), whereas the RR was not increased among subjects with folate intake below the median of the distribution (Pinteraction = 0.06). Similarly, men having tumors with MLH1 hypermethylation also had higher RRs in the third tertile of vitamin B6 intake in the subgroup of high folate intake (RR = 3.36, CI = 0.81–13.93, Ptrend = 0.08, data not shown), which was not observed among men with low folate intake. However, the interaction term for this end point was not statistically significant. Among women, an interaction between vitamin B6 and folate was not observed, though it was present for men and women combined for overall CRC (Pinteraction = 0.07). We observed no interactions between methionine and any of the other dietary variables under study.


   Discussion
Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
Funding
 References
 
In this study, we investigated the associations between dietary folate, vitamin B2, vitamin B6, methionine, alcohol and CRC risk while accounting for MLH1 promoter methylation, MLH1 protein expression, MSI and BRAF mutations. It was observed that the highest tertiles of folate, vitamin B2, vitamin B6 and methionine intake and low alcohol intake were not associated with decreased CRC risk, irrespective of the presence of MLH1 hypermethylation, the MLH1 protein or MSI. On the other hand, we observed a positive association between dietary folate and BRAF mutations in men. Moreover, vitamin B6 increased overall CRC risk and especially MLH1 hypermethylated tumors among men.

We studied four different aberrations that have been suggested previously to be associated with the MSI pathway. Despite the strong and statistically significant correlations between these characteristics, they were not always concurrently present in tumors. Whereas the percentages of overlap between absence of MLH1 protein expression and MLH1 methylation and between MSI and MLH1 methylation were relatively high (74.6 and 65.8%, respectively), less than half of the tumors with BRAF mutations (47.5%) appeared to have MLH1 methylation. In this respect, it is appreciated that individual techniques were used for each end point, each having specific sensitivities, which may partly account for the observed incomplete overlap. However, an additional, even more obvious, reason may lie in potential differences in tumor biology. For example, within the subset of sporadic CRCs showing MSI, there may be differences in the frequency of MLH1 hypermethylation depending on the localization of the tumor. In this respect, MLH1 hypermethylation was observed less frequently in distal sporadic CRCs with MSI compared with proximal MSI cancers, suggesting that an epigenetic pathway has played a smaller role in the development of distal MSI cancers (37). Next to MLH1 hypermethylation, MLH1 mutations may result in loss of the MLH1 protein as well. Although such mutations are mainly observed in tumors of hereditary non-polyposis colorectal cancer patients with MSI (38), it has been suggested that they may also occur in sporadic colorectal carcinomas and that MSI may be present concurrently (39). The occurrence of MLH1 hypermethylation, lack of MLH1 protein expression or BRAF mutations may apparently differ among tumors that develop through the MSI pathway. In addition to the observed incomplete overlap in our study, this is an important reason to study these molecular phenotypes separately in relation to methyl donor intake. From our results, it appeared that the end point of MSI is least related to the intake of the studied methyl donors but that there rather may be an effect on methylation and mutations that are associated with MSI. Moreover, the relation between folate intake and MSI in CRC was investigated previously (18), but no association was observed in that particular study.

Promoter hypermethylation and global hypomethylation are alternative types of aberrant methylation, and it has been suggested that both these patterns may contribute separately to the process of colorectal carcinogenesis (40,41). Moreover, they may also specifically affect DNA stability, given the observed strong associations between promoter hypermethylation and MSI and between global hypomethylation and chromosomal instability in colorectal carcinomas (42,43). However, the relative contribution of methyl donor intake to either hypermethylation or hypomethylation is unknown. Promoter hypermethylation may occur in normal colorectal mucosa of patients with hyperplastic polyposis (44) or CRC (45), suggesting that dietary methyl donors may play a role in the prevention or initiation of neoplastic formation. Moreover, folic acid supplementation may decrease global hypomethylation in the normal-appearing colonic mucosa of patients with adenomas (46). Furthermore, in a pilot study, we previously observed an indication for an inverse association between relatively high folate intake in combination with low alcohol intake and promoter methylation in colorectal tumors (8). However, this association was only weak, and the conclusion in this study was based on an outcome measure defined as hypermethylation of at least one of six genes. Others observed positive associations between high alcohol consumption and risk of MSI-H tumors (17,18,20), indicating that disruption of the folate metabolism may lead to CRC, possibly partly through increasing promoter hypermethylation.

Although the above suggests a protective effect of adequate folate status or intake on aberrant methylation, it has been hypothesized that its actual influence may also differ depending on the stage of tumor development in colorectal carcinogenesis. Although folate deficiency might increase the risk of neoplastic transformation of normal tissue by inducing genomic hypomethylation, it may have an inhibitory effect on progression of neoplasms to cancer. Conversely, folate supplementation may prevent aberrant DNA methylation in normal tissue but could promote established lesions by increasing promoter hypermethylation (47). Results of a recent study suggest that folic acid and vitamin B12 supplementation in subjects with a history of colorectal adenoma may indeed increase promoter hypermethylation, although this association was statistically borderline significant (9). Moreover, folic acid supplementation was associated with increased risk of advanced lesions or recurrence of multiple adenomas in patients with available follow-up information (48). Interestingly, the participants in that study also had a recent history of colorectal adenomas, and possibly, undetected neoplasms were present in these subjects, which may have had a growth advantage in the presence of high concentrations of folic acid. However, one might wonder whether intake of dietary folate has a similar effect as supplementation with folic acid. Nevertheless, our data revealed the strongest increased risk among patients in the highest tertile of vitamin B6 intake of tumors with MLH1 hypermethylation, suggesting that vitamin B6 may have had a tumor-promoting effect by increasing promoter methylation. We also observed that the increased RRs in the highest tertiles of vitamin B6 of overall CRC and tumors harboring MLH1 hypermethylation were present only among men who also had a relatively high folate intake. The latter association suggests that the transfer of methyl donors provided by folate may at least partly depend on the availability of vitamin B6 as a ‘modulating’ factor and that the combination of relatively high vitamin B6 and folate intake may increase promoter methylation and thereby enhance the development of tumors with a methylation-associated phenotype. Our results therefore contribute to the hypothesis that relatively high methyl donor intake potentially increases aberrant promoter hypermethylation rather than preventing it.

Several other studies showed inverse associations between vitamin B6 intake and CRC risk. However, in these studies, the levels of vitamin B6 intake were generally higher and the distributions sometimes even lie largely above the median intake of the highest tertile of the Dutch population included in our study (49–52). One could hypothesize that if vitamin B6 is protective above a certain threshold of intake, this may have been a reason why we did not observe an inverse association. However, vitamin B6 was also associated with decreased CRC risk in a recent study in a Japanese population, in which intake of vitamin B6 was comparable with that in our study (53). There is one study that has previously demonstrated a positive association between vitamin B6 intake and CRC risk among women who had a much higher vitamin B6 intake than in our study population (54). Apparently, a potential positive association between vitamin B6 and CRC should be taken into account and obviously needs further attention in future studies.

Another interesting finding in our study is the positive association between dietary folate and BRAF mutations among men, which was not expected considering the importance of folate for nucleotide synthesis. We have previously observed that folate may increase the risk of tumors harboring truncating APC mutations in men (19). BRAF and APC mutations are inversely correlated in this study population, which was also the case in a previous study (55), indicating that relatively high folate intake may give a growth advantage to mutated tumors independent of the type of mutation. Moreover, folate was not associated with MLH1 hypermethylation in men, suggesting that it exerts a specific effect on tumors with gene mutations.

A limitation of our study is that some of the molecular subgroups present relatively small numbers of cases, and the possibility that some of the gender-specific results are partly based on chance can therefore not be excluded. However, overall analyses, with subgroups then containing substantial numbers of cases (at least 30) within tertiles of intake, showed similar positive associations of folate with BRAF mutations and of vitamin B6 with MLH1 hypermethylation. Furthermore, residual confounding may have been present for the associations observed, although methyl donor intake was comparable between molecular subgroups and associations were further adjusted for a number of potential confounders or known risk factors of CRC. We consider it unlikely that changes in dietary intake over time have influenced disease outcome since the reproducibility of the baseline FFQ over a period of 5 years was relatively high (30) and the follow-up of 7.3 years was only 2.3 years longer in the current study.

In this Dutch population, relatively high intakes of folate, vitamin B2, vitamin B6 and methionine or low alcohol intake were not associated with a decreased risk of CRC or with tumors harboring MLH1 hypermethylation, lack of MLH1 protein expression or MSI. The occurrence of MSI does not seem to be sensitive to methyl donor intake; however, folate increased the risk of BRAF mutations, whereas vitamin B6 increased the risk of tumors with MLH1 hypermethylation among men. This may indicate that dietary folate and vitamin B6 have different effects but may both enhance colorectal carcinogenesis by exerting an effect on genetic or epigenetic alterations.


   Funding
Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 Funding
References
 
Dutch Cancer Society (UM2004-3171 and UM99-1980).


   Acknowledgments
 
We thank Dr M. Brink for the collection of tissue samples; Dr M. Brink and P. Wark for molecular analyses; S. van de Crommert, H. Brants, J. Nelissen, C. de Zwart, M. Moll, W. van Dijk, M. Jansen and A. Pisters for data management; H. van Montfort, T. van Moergastel, L. van den Bosch and R. Schmeitz for programming assistance and Dr A. Volovics for statistical advice.

Conflict of Interest Statement: None declared.


   References
Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 Funding
 References
 

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Received January 25, 2008; revised March 3, 2008; accepted March 6, 2008.


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