Skip Navigation


Human Molecular Genetics Advance Access originally published online on March 9, 2007
Human Molecular Genetics 2007 16(9):1072-1077; doi:10.1093/hmg/ddm055
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
16/9/1072    most recent
ddm055v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrow Search for citing articles in:
ISI Web of Science (1)
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Hubner, R. A.
Right arrow Articles by Houlston, R. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Hubner, R. A.
Right arrow Articles by Houlston, R. S.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

MTHFR C677T has differential influence on risk of MSI and MSS colorectal cancer

Richard A. Hubner*, Steven Lubbe, Ian Chandler and Richard S. Houlston

Section of Cancer Genetics, Institute of Cancer Rsearch, 15 Cotswold Road, Sutton, Surrey SM2 5NG, UK

* To whom correspondence should be addressed at: Tel: +44 2087224385; Fax: +44 2087224359; Email: Richard.Hubner{at}icr.ac.uk

Received January 12, 2007; Revised February 14, 2007; Accepted March 2, 2007


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
The majority of colorectal cancer (CRC) exhibiting the micosatellite instability (MSI) phenotype is due to hypermethylation of the hMLH1 gene promoter. We aimed to test the hypothesis that polymorphisms in genes coding for enzymes involved in folate metabolism play a role in altered promoter-specific hypermethylation and thus predispose to MSI CRC. Analysis of MSI was performed in 1685 CRCs, and polymorphism genotypes were determined in germline DNA for all cases and 2692 cancer-free controls. MSI was observed in 171 cancers (10.1%). Compared to homozygous wild-type individuals, those with MTHFR 677TT genotype were more likely to have MSI than microsatellite stable (MSS) CRC [odds ratio (OR) 1.90; 95% confidence interval (CI): 1.09–3.31]. When MTHFR C677T genotype frequencies in MSS CRC cases were compared to controls, individuals with homozygous variant genotype were at 19% reduced risk of cancer compared to wild type (OR = 0.81; 95% CI: 0.65–1.02). Conversely, when MSI CRC cases were compared to controls, individuals with one or two MTHFR 677T alleles were at 42% increased cancer risk (OR = 1.42; 95% CI: 1.02–1.96). Our observations indicate that MTHFR 677TT homozygous individuals are more likely to develop MSI CRC than those with wild-type genotype, and this common polymorphism has differential influences on MSI and MSS CRC risk. Stratification by MSI status should aid future studies investigating the complex relationships between genotype, environmental factors and CRC risk.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
Colorectal cancer (CRC) develops along at least two biologically distinct pathways characterized by different genetic abnormalities (1). The chromosome instability pathway is typified by chromosomal abnormalities resulting in loss of heterozygosity and tumour suppressor gene inactivation, while cancers exhibiting microsatellite instability (MSI) have defective DNA mismatch repair (2). Compared to microsatellite stable (MSS) CRCs, those exhibiting MSI are more likely to be proximally located, have altered chemo-sensitivity, and carry a more favourable prognosis (3,4).

Disruption of the normal patterns of DNA methylation is an established epigenetic feature of carcinogenesis, resulting in both global hypomethylation, and hypermethylation of CpG islands in specific regions of gene promoters (5–7). Global hypomethylation leads to chromosomal instability and activation of proto-oncogenes, whereas promoter-specific hypermethylation suppresses transcription, resulting in gene silencing and loss of tumour suppressor gene function (6,8). The majority of MSI CRCs result from promoter-specific hypermethylation of the hMLH1 gene, while CRCs developing via chromosomal instability exhibit globally reduced DNA methylation (7,9). Germline variants have been demonstrated to alter global DNA methylation levels within normal cells (10). An important unanswered question, however, is whether inherited factors, either alone or in combination with relevant environmental factors, can similarly influence promoter-specific hypermethylation.

Folate coenzymes, acting as acceptors or donors of one-carbon units, influence both methionine synthesis and DNA methylation (11). The methylenetetrahydrofolate reductase (MTHFR) enzyme occupies a pivotal position in folate metabolism, catalysing the irreversible conversion of 5,10-methyleneTHF to 5-methylTHF (Fig. 1). The MTHFR product, 5-methylTHF, is the methyl group donor for the remethylation of homocysteine to methionine, in a reaction catalysed by methionine synthase (MTR). Methionine is the precursor of S-adenosylmethionine (SAM) which is the universal methyl donor for methylation of DNA, RNA and proteins, and reduced levels of methionine results in aberrant DNA methlyation (11). The MTR enzyme may become inactive due to oxidation of its vitamin B12 cofactor, and restoration of MTR activity is dependent on reductive remethylation of vitamin B12 by methionine synthase reductase (MTRR) (12). The genes coding for all three enzymes have polymorphic forms, and in some cases the genetic variants have been shown to impact on the function of the expressed proteins. The common functional MTHFR 677TT variant results in a thermolabile enzyme with 35% of wild-type activity, and has been shown to be associated with reduced global DNA methylation in peripheral leucocytes, lower plasma folate levels and reduced CRC risk (10,13,14). The MTR A2756G polymorphism is associated with reduced plasma homocysteine levels, and cancers in individuals with GG genotype have been reported to have significantly reduced methylation of tumour suppressor gene CpG islands (15). Variant alleles of the MTRR A66G polymorphism generate an enzyme with lower affinity for MTR, and are associated with a reduced risk of colorectal adenoma recurrence (16,17). Thus, through altered one-carbon metabolism, it is conceivable that these genetic variants may also influence promoter-specific hypermethylation.


Figure 1
View larger version (11K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 1. Schematic representation of folate metabolism. DHF, dihydrofolate; dUMP, deoxyuridine monophosphate; dTMP, deoxythymidine monophoshate; SAH, S-adenosylhomocysteine.

 
We aimed to test the hypothesis that folate metabolism gene polymorphisms influence the likelihood of developing MSI versus MSS CRC in a study of 1685 CRC cases. We further investigated the role of MTHFR C677T by comparing genotype frequencies between 2695 cancer-free controls and MSI and MSS CRC cases separately. All cases and controls were British Caucasians.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
A total of 1685 CRCs were available for analysis of MSI. We used the BAT25 and BAT26 mononucleotide microsatellite markers which have been demonstrated to achieve 93% specificity and a 99% negative predictive value, and in which the distribution of allele sizes in MSI and MSS tumours are sufficiently different for MSI status to be determined solely on the basis of analysis of tumour DNA (18–20). In our study, BAT25 and BAT26 were concordant in 1628 (97%) cancers (Table 1). Novel alleles were detected at either or both loci in 171 (10.1%) cancers and these were assigned MSI status, while in 1478, no novel alleles at either marker were detected and these were assigned MSS status. Patients with MSI CRC were younger and more likely to have a family history of CRC than those with MSS CRC (Table 2). MSI CRCs were more likely to be proximally located, poorly differentiated, mucinous and earlier stage. Allele frequencies for all polymorphisms were consistent with previous reports in Caucasian populations, and genotype frequencies were in Hardy–Weinberg equilibrium (17,21). MTHFR C677T genotype was associated with stage at diagnosis, with TT homozygote individuals more likely to present with earlier stage disease than wild types (P = 0.003).


View this table:
[in this window]
[in a new window]

  		Table 1. Results of BAT25 and BAT26 mononucleotide microsatellite marker analysis

 


View this table:
[in this window]
[in a new window]

  		Table 2. Baseline characteristics of MSI and MSS cancers

 
Compared to patients with homozygous wild-type genotype, those with MTHFR 677TT genotype were at greater probability of developing MSI rather than MSS CRC [odds ratio (OR) = 1.90; 95% confidence interval (CI): 1.09–3.31], whereas heterozygotes were at intermediate probability (OR = 1.29; 95% CI: 0.88–1.88) (Table 3). Restricting the analysis to proximal colon cancer cases only (n = 530), the probabilities of MSI cancer for MTHFR 677TT and CT genotypes compared to CC were further increased (OR = 1.97; 95% CI: 0.98–3.95 and OR = 1.47; 95% CI: 0.94–2.31 respectively). Overall, possession of one or more MTHFR 677T alleles resulted in a 39% increase in likelihood of MSI in all CRC cases (OR = 1.39; 95% CI: 0.97–1.99) and a 55% increase in likelihood of MSI in proximal colon cancer cases (OR = 1.55; 95% CI: 1.00–2.39). MTHFR A1298C genotype, however, did not significantly influence likelihood of MSI CRC, and similarly no influences of MTR A2756G or MTRR A66G were observed.


View this table:
[in this window]
[in a new window]

  		Table 3. Genotype frequencies of MSI and MSS cancers

 
Comparison of MTHFR C677T genotype frequencies in the case-control analysis, indicated that when MSI and MSS CRCs were combined and compared to controls, individuals with MTHFR 677TT genotype were at a non-significant reduced risk of CRC compared to homozygous wild-type individuals (OR = 0.87; 95% CI: 0.70–1.07) (Table 4). However, after stratification by MSI status, differential influences of MTHFR C677T genotype on risk of MSI and MSS CRC were observed. When MSI CRC cases were compared to controls, individuals with both homozygote variant and heterozygote genotypes were at an increased risk of CRC compared to homozygous wild-type individuals (OR = 1.43; 95% CI: 0.87–2.34 and OR = 1.42; 95% CI: 1.01–1.99, respectively). Overall, individuals with one or two MTHFR 677T alleles were at 42% increased risk of MSI CRC compared to wild type (OR = 1.42; 95% CI: 1.02–1.96). Conversely, when MSS CRC cases were compared to controls, individuals with MTHFR 677TT genotype were at 19% reduced risk of CRC compared to wild type (OR = 0.81; 95% CI: 0.65–1.02).


View this table:
[in this window]
[in a new window]

  		Table 4. Case-control analysis of MTHFR C677T genotype and risk of MSI and MSS CRC

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
MATERIALS AND METHODS
 REFERENCES
 
Our study indicates that, compared to wild-type individuals, carriers of the MTHFR 677TT genotype are more likely to develop MSI than MSS CRC. Furthermore, the MTHFR 677TT genotype had differential influences on risk of MSI and MSS CRC, predisposing to the former and protecting against the latter. This apparent dichotomy can be resolved if one considers the mechanisms that may underlie these influences. The majority of MSI CRC in this study will have occurred as a consequence of hypermethylation of the hMLH1 gene promoter, and thus the increased risk of MSI CRC in MTHFR 677TT individuals is likely to have arisen indirectly through an association of this genotype with increased promoter-specific methylation. Although reduced production of 5-methylTHF as a consequence of TT genotype might be expected to lead to reduced promoter-specific methylation, similar to the reduced global methylation levels observed with this genotype, previous studies have also provided evidence of increased promoter-specific methylation in TT homozygotes (22,23). Firstly, tumour samples from CRCs exhibiting the MSI phenotype have been shown to have higher levels of the MTHFR substrate 5,10-methylenetetrahydrofolate compared to MSS tumours, which would be expected to occur in individuals with reduced enzyme activity genotypes (22). Secondly, in proximal colon cancers an increased hypermethylation of tumour suppressor genes, including hMLH1, has been associated directly with low activity MTHFR genotypes (23). Conversely, the reduced risk of MSS CRC in MTHFR 677TT homozygotes in our study is more likely due to increased channelling of folate intermediates towards DNA synthesis, aberrations of which are known to play a major role in colorectal carcinogenesis (11). If MSI and MSS CRC are considered together, then the reduction in risk of MSS CRC will predominate since this is the more frequent form, however failure to differentiate between these two molecular subgroups may in part explain the variable associations between MTHFR 677TT genotype and CRC risk reported in some studies (24).

Unlike MTHFR C677T, the MTHFR A1298C polymorphism genotype did not have a significant influence on the likelihood of developing MSI rather than MSS CRC in our study. Although the A1298C polymorphism has also been associated with reduced enzyme function, discrepancies in disease-gene associations between these two variants have been frequently reported and the effects of C677T appear to predominate in most studies (25,26).

The hereditary non-polyposis colorectal cancer (HNPCC) syndrome is due to germline mutations in mismatch repair genes, and results in early-onset CRC which displays the MSI phenotype. It is possible that some of the CRC cases included in our study had undiagnosed HNPCC, and the mean age of MSI CRC cases in our study was younger than might be expected (27,28). In HNPCC cases MSI status will not be influenced by the MTHFR genotype, and thus inclusion of HNPCC cases would tend to diminish the overall effect of MTHFR genotype on MSI status. It is therefore unlikely that the potential inclusion of HNPCC cases could account for the association of MTHFR 677TT genotype and MSI CRC observed in this study.

The MTHFR 677TT genotype has been consistently associated with low plasma folate and high plasma homocysteine, levels of which increase under conditions of folate deficiency and are considered a highly sensitive indicator of folate depletion (14,29). Through Mendelian Randomization MTHFR 677TT genotype can be considered a proxy for lifelong low folate status (30). As such our results suggest that inadequate dietary folate results in an increased CRC risk primarily through an increase in risk of MSI CRC, whereas high dietary folate may reduce CRC risk primarily through reductions in MSS CRC. Thus, the association between dietary folate intake and CRC risk will be dependent on the proportion of MSI and MSS CRC in the study population as well as MTHFR C677T genotype frequencies. If these two factors are not considered then results will likely be misleading, and this may in part explain the discrepancies in results of studies reporting on the relationship between dietary folate intake and CRC risk (31,32). Furthermore, since the MSI phenotype is more common in colon cancer than rectal cancer, this may account for the increased propensity for studies of colon cancer cases alone to detect an increased cancer risk in individuals with low dietary folate (31).

Our observation of an association between MTHFR C677T genotype and MSI status is not without precedent. Some studies that have investigated this relationship have also observed similar findings, however all have been based on small sample sets prohibiting definitive conclusions (27,33,34). Pooling data from our study and all five previous studies, which provide an additional 1526 CRC cases, lend further support to our findings with MTHFR 677TT carriers being 1.7 times (95% CI 1.2–2.4, P = 0.002) more likely to develop MSI than MSS CRCs (27,28,33–36).

In summary, our results suggest that homozygous carriers of the MTHFR 677TT variant are more likely to develop MSI CRC than individuals with the more common CC genotype, and that this polymorphism has differential influences on risk of MSI and MSS CRC. By considering MSI and MSS CRC separately, future studies should be better equipped to unravel the complex relationships between genotype, folate status and CRC risk.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
REFERENCES
 
Study population
CRC cases were ascertained through 140 centres participating in the National Study of Colorectal Cancer Genetics (NSCCG). Details of the NSCCG study design are available online (37). All cases had histologically proven colorectal adenocarcinoma confirmed by central review. Further clinicopathological information was obtained from hospital records, and detailed family histories were ascertained through a previously validated questionnaire. Controls were recruited as part of ongoing National Cancer Research Network genetic epidemiological studies, and the UK Study of Breast Cancer Genetics, all established within the UK. Controls were the spouses or unrelated friends of patients with malignancies, and none had a personal history of malignancy at the time of ascertainment. The study was performed with informed consent and ethical review board approval in accordance with the tenets of the Declaration of Helsinki.

Analysis of microsatellite instability
Ten-micron sections were cut from blocks of formalin fixed paraffin embedded tumours, lightly stained with toluidine blue and regions containing at least 60% tumour micro-dissected. DNA was extracted using the QIAamp DNA Mini kit (Qiagen, Crawley, UK) according to the manufacturers instructions. Tumour DNA samples were analysed for MSI using the mononucleotide microsatellite loci BAT25 and BAT26. Target DNA sequences were amplified by PCR using 32P end-labelled primers. Mismatch repair status was assigned by presence or absence of novel alleles at autoradiography. Samples showing novel alleles at either BAT26 or BAT25 or both were assigned MSI (corresponding to a high level of instability, MSI-H), whereas those without novel alleles at both loci were assigned MSS (38). Only tumours with unambiguous genotypes were assigned MSI status.

Genotyping
Constitutional DNA was extracted from EDTA venous blood samples using a standard salt extraction procedure, and quantified by PicoGreen (Invitrogen, Paisley, UK). MTHFR C677T, MTHFR A1298C, MTR A2756G and MTRR A66G genotypes were generated using either Taqman technology implemented on an ABI 7900HT sequence detection system (Applied Biosystems, Foster City, USA), or customized Illumina Sentrix Bead Arrays according to the manufacturers instructions (Illumina, San Diego, USA). To ensure quality of genotyping, a series of duplicate samples were genotyped with 100% concordance. Details of all PCR primer sequences and reaction conditions are available on request.

Statistical analysis
Baseline characteristics of MSI and MSS CRCs were compared using the {chi}2 and t-tests for categorical and continuous variables, respectively. Genotype frequencies were tested for departure from Hardy–Weinberg equilibrium using the {chi}2 test. The relationship between genotype and risk of MSI versus MSS CRC was assessed by means of ORs and 95% CIs. Homozygote variant and heterozygote genotypes were compared to homozygote wild-type genotype both separately and combined. Unadjusted and adjusted ORs were calculated by logistic regression. Variables for age, family history (defined as ≥1 first-degree relative with diagnosis of CRC), cancer location (proximal or distal to the splenic flexure), stage and grade were included in adjusted analyses since these variables were found to independently influence MSI status. Missing values were treated as a separate category for each variable in multivariate analyses. For the case-control analysis, variables for age and sex were included in adjusted analyses. The relationship between MSI status and MTHFR 677TT genotype in this and published studies were formerly evaluated by pooling data using standard meta-analytical techniques.

Statistical analyses were undertaken using STATA, version 8.2 (Stata Corporation, College Station, TX, USA). All tests were two sided, and a P-value < 0.05 was considered significant.


   ACKNOWLEDGEMENTS
 
This work was supported by grants from Cancer Research UK, RAH is in receipt of a clinical research training fellowship from Cancer Research UK.

Conflict of Interest statement. We declare that we have no conflict of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 

  1. Lengauer C., Kinzler K.W., Vogelstein B. Genetic instability in colorectal cancers. Nature (1997) 386:623–627.[CrossRef][Medline]

  2. Fearon E.R., Vogelstein B. A genetic model for colorectal tumorigenesis. Cell (1990) 61:759–767.[CrossRef][ISI][Medline]

  3. Jover R., Zapater P., Castells A., Llor X., Andreu M., Cubiella J., Pinol V., Xicola R.M., Bujanda L., Rene J.M., et al. Mismatch repair status in the prediction of benefit from adjuvant fluorouracil chemotherapy in colorectal cancer. Gut (2006) 55:848–855.[Abstract/Free Full Text]

  4. Popat S., Hubner R., Houlston R.S. Systematic review of microsatellite instability and colorectal cancer prognosis. J. Clin. Oncol. (2005) 23:609–618.[Abstract/Free Full Text]

  5. Das P.M., Singal R. DNA methylation and cancer. J. Clin. Oncol (2004) 22:4632–4642.[Abstract/Free Full Text]

  6. Jones P.A., Baylin S.B. The fundamental role of epigenetic events in cancer. Nat. Rev. Genet. (2002) 3:415–428.[ISI][Medline]

  7. Ehrlich M. DNA methylation in cancer: too much, but also too little. Oncogene (2002) 21:5400–5413.[CrossRef][ISI][Medline]

  8. Eden A., Gaudet F., Waghmare A., Jaenisch R. Chromosomal instability and tumors promoted by DNA hypomethylation. Science (2003) 300:455.[Free Full Text]

  9. Liu B., Nicolaides N.C., Markowitz S., Willson J.K., Parsons R.E., Jen J., Papadopolous N., Peltomaki P., de la Chapelle A., Hamilton S.R., et al. Mismatch repair gene defects in sporadic colorectal cancers with microsatellite instability. Nat. Genet. (1995) 9:48–55.[CrossRef][ISI][Medline]

  10. Stern L.L., Mason J.B., Selhub J., Choi S.W. Genomic DNA hypomethylation, a characteristic of most cancers, is present in peripheral leukocytes of individuals who are homozygous for the C677T polymorphism in the methylenetetrahydrofolate reductase gene. Cancer Epidemiol. Biomarkers Prev. (2000) 9:849–853.[Abstract/Free Full Text]

  11. Choi S.W., Mason J.B. Folate and carcinogenesis: an integrated scheme. J. Nutr. (2000) 130:129–132.[Abstract/Free Full Text]

  12. Leclerc D., Wilson A., Dumas R., Gafuik C., Song D., Watkins D., Heng H.H., Rommens J.M., Scherer S.W., Rosenblatt D.S., et al. Cloning and mapping of a cDNA for methionine synthase reductase, a flavoprotein defective in patients with homocystinuria. Proc. Natl. Acad. Sci. USA (1998) 95:3059–3064.[Abstract/Free Full Text]

  13. Frosst P., Blom H.J., Milos R., Goyette P., Sheppard C.A., Matthews R.G., Boers G.J., den Heijer M., Kluijtmans L.A., van den Heuvel L.P., et al. A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase. Nat. Genet. (1995) 10:111–113.[CrossRef][ISI][Medline]

  14. Ma J., Stampfer M.J., Giovannucci E., Artigas C., Hunter D.J., Fuchs C., Willett W.C., Selhub J., Hennekens C.H., Rozen R. Methylenetetrahydrofolate reductase polymorphism, dietary interactions, and risk of colorectal cancer. Cancer Res. (1997) 57:1098–1102.[Abstract/Free Full Text]

  15. Paz M.F., Avila S., Fraga M.F., Pollan M., Capella G., Peinado M.A., Sanchez-Cespedes M., Herman J.G., Esteller M. Germ-line variants in methyl-group metabolism genes and susceptibility to DNA methylation in normal tissues and human primary tumors. Cancer Res. (2002) 62:4519–4524.[Abstract/Free Full Text]

  16. Olteanu H., Munson T., Banerjee R. Differences in the efficiency of reductive activation of methionine synthase and exogenous electron acceptors between the common polymorphic variants of human methionine synthase reductase. Biochemistry (2002) 41:13378–13385.[CrossRef][Medline]

  17. Hubner R.A., Muir K.R., Liu J.F., Sellick G.S., Logan R.F., Grainge M., Armitage N., Chau I., Houlston R.S. Folate metabolism polymorphisms influence risk of colorectal adenoma recurrence. Cancer Epidemiol. Biomarkers Prev. (2006) 15:1607–1613.[Abstract/Free Full Text]

  18. Hoang J.M., Cottu P.H., Thuille B., Salmon R.J., Thomas G., Hamelin R. BAT-26, an indicator of the replication error phenotype in colorectal cancers and cell lines. Cancer Res. (1997) 57:300–303.[Abstract/Free Full Text]

  19. Zhou X.P., Hoang J.M., Li Y.J., Seruca R., Carneiro F., Sobrinho-Simoes M., Lothe R.A., Gleeson C.M., Russell S.E., Muzeau F., et al. Determination of the replication error phenotype in human tumors without the requirement for matching normal DNA by analysis of mononucleotide repeat microsatellites. Genes Chromosomes Cancer (1998) 21:101–107.[CrossRef][ISI][Medline]

  20. Stone J.G., Tomlinson I.P., Houlston R.S. Optimising methods for determining RER status in colorectal cancers. Cancer Lett. (2000) 149:15–20.[CrossRef][ISI][Medline]

  21. Ogino S., Wilson R.B. Genotype and haplotype distributions of MTHFR677C > T and 1298A > C single nucleotide polymorphisms: a meta-analysis. J. Hum. Genet. (2003) 48:1–7.[CrossRef][ISI][Medline]

  22. Kawakami K., Ruszkiewicz A., Bennett G., Moore J., Watanabe G., Iacopetta B. The folate pool in colorectal cancers is associated with DNA hypermethylation and with a polymorphism in methylenetetrahydrofolate reductase. Clin. Cancer Res. (2003) 9:5860–5865.[Abstract/Free Full Text]

  23. Oyama K., Kawakami K., Maeda K., Ishiguro K., Watanabe G. The association between methylenetetrahydrofolate reductase polymorphism and promoter methylation in proximal colon cancer. Anticancer Res. (2004) 24:649–654.[ISI][Medline]

  24. Hubner R.A., Houlston R.S. MTHFR C677T and colorectal cancer risk: a meta-analysis of 25 populations. Int. J. Cancer (2007) 120:1027–1035.[CrossRef][ISI][Medline]

  25. Weisberg I., Tran P., Christensen B., Sibani S., Rozen R. A second genetic polymorphism in methylenetetrahydrofolate reductase (MTHFR) associated with decreased enzyme activity. Mol. Genet. Metab. (1998) 64:169–172.[CrossRef][ISI][Medline]

  26. Chen J., Ma J., Stampfer M.J., Palomeque C., Selhub J., Hunter D.J. Linkage disequilibrium between the 677C > T and 1298A > C polymorphisms in human methylenetetrahydrofolate reductase gene and their contributions to risk of colorectal cancer. Pharmacogenetics (2002) 12:339–342.[CrossRef][ISI][Medline]

  27. Shannon B., Gnanasampanthan S., Beilby J., Iacopetta B. A polymorphism in the methylenetetrahydrofolate reductase gene predisposes to colorectal cancers with microsatellite instability. Gut (2002) 50:520–524.[Abstract/Free Full Text]

  28. Eaton A.M., Sandler R., Carethers J.M., Millikan R.C., Galanko J., Keku T.O. 5,10-methylenetetrahydrofolate reductase 677 and 1298 polymorphisms, folate intake, and microsatellite instability in colon cancer. Cancer Epidemiol. Biomarkers Prev. (2005) 14:2023–2029.[Abstract/Free Full Text]

  29. Kim Y.I. Folate and carcinogenesis: evidence, mechanisms, and implications. J. Nutr. Biochem. (1999) 10:66–88.[CrossRef][ISI][Medline]

  30. Davey Smith G., Ebrahim S. Mendelian randomization’: can genetic epidemiology contribute to understanding environmental determinants of disease? Int. J. Epidemiol. (2003) 32:1–22.[Abstract/Free Full Text]

  31. Sanjoaquin M.A., Allen N., Couto E., Roddam A.W., Key T.J. Folate intake and colorectal cancer risk: a meta-analytical approach. Int. J. Cancer (2005) 113:825–828.[CrossRef][ISI][Medline]

  32. Van Guelpen B., Hultdin J., Johansson I., Hallmans G., Stenling R., Riboli E., Winkvist A., Palmqvist R. Low folate levels may protect against colorectal cancer. Gut (2006) 55:1461–1466.[Abstract/Free Full Text]

  33. Clarizia A.D., Bastos-Rodrigues L., Pena H.B., Anacleto C., Rossi B., Soares F.A., Lopes A., Rocha J.C., Caballero O., Camargo A., et al. Relationship of the methylenetetrahydrofolate reductase C677T polymorphism with microsatellite instability and promoter hypermethylation in sporadic colorectal cancer. Genet. Mol. Res. (2006) 5:315–322.[Medline]

  34. Chang S.C., Lin P.C., Lin J.K., Yang S.H., Wang H.S., Fen-Yau Li A. Role of MTHFR polymorphisms and folate levels in different phenotypes of sporadic colorectal cancers. In: Int. J. Colorectal. Dis (2006) [Epub ahead of print].

  35. Toffoli G., Gafa R., Russo A., Lanza G., Dolcetti R., Sartor F., Libra M., Viel A., Boiocchi M. Methylenetetrahydrofolate reductase 677 C->T polymorphism and risk of proximal colon cancer in north Italy. Clin. Cancer Res. (2003) 9:743–748.[Abstract/Free Full Text]

  36. Plaschke J., Schwanebeck U., Pistorius S., Saeger H.D., Schackert H.K. Methylenetetrahydrofolate reductase polymorphisms and risk of sporadic and hereditary colorectal cancer with or without microsatellite instability. Cancer Lett. (2003) 191:179–185.[CrossRef][ISI][Medline]

  37. National Study of Colorectal Cancer Genetics Trial. http://www.ncrn.org.uk/portfolio/data.asp?ID=1269(last accessed January 2006).

  38. Perucho M., et al. A National Cancer Institute workshop on microsatellite instability for cancer detection and familial predisposition: development of international criteria for the determination of microsatellite instability in colorectal cancer. Cancer Res. (1999) 58:5248–5257. 1998, 59, 249–256Correspondence re: C.R. Boland.


Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?



This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
16/9/1072    most recent
ddm055v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrow Search for citing articles in:
ISI Web of Science (1)
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Hubner, R. A.
Right arrow Articles by Houlston, R. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Hubner, R. A.
Right arrow Articles by Houlston, R. S.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?