BR Biomedical Reports 2049-9434 2049-9442 D.A. Spandidos 10.3892/br.2013.134 br-01-05-0781 Articles Association of methylenetetrahydrofolate reductase C677T and A1298C polymorphisms with colorectal cancer risk: A meta-analysis ZHAOMENGMENG12 LIXUELIAN12 XINGCHENGZHONG3 ZHOUBAOSEN12 Department of Epidemiology, School of Public Health, China Medical University, Shenyang, Liaoning 110001, P.R. China Key Laboratory of Cancer Etiology and Intervention, University of Liaoning Province, Shenyang, Liaoning 110001, P.R. China Department of Anorectal Surgery, The First Affiliated Hospital of China Medical University, Shenyang, Liaoning 110001, P.R. China Correspondence to: Professor Baosen Zhou, Department of Epidemiology, School of Public Health, China Medical University, 92 Beier Road, Heping District, Shenyang, Liaoning 110001, P.R. China, E-mail: bszhou@mail.cmu.edu.cn 9 2013 15 07 2013 1 5 781 791 19 04 2013 08 07 2013 Copyright © 2013, Spandidos Publications 2013 This is an open-access article licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. The article may be redistributed, reproduced, and reused for non-commercial purposes, provided the original source is properly cited.

Colorectal cancer (CRC) is one of the most common types of cancer worldwide and a leading cause of cancer-related mortality. This meta-analysis was conducted to determine the effect of methylenetetrahydrofolate reductase (MTHFR) mutants on the risk of CRC. A literature search was conducted on PubMed, Medline and the China National Knowledge Infrastructure (CNKI) databases. Eligible studies were collected based on rigorous criteria of inclusion. The pooled odds ratios (ORs) with 95% confidence intervals (CIs) were calculated by the fixed- or random-effects model. After all the studies were pooled, the OR of CRC for individuals carrying the MTHFR 677TT genotype, compared to the CC genotype, was 0.89 (95% CI: 0.82–0.97). When analyzed by ethnicity, Asians with the MTHFR 1298CC genotype exhibited a decreased risk of CRC (OR=0.69; 95% CI: 0.54–0.89). In a mixed population, a significantly reduced risk of CRC was observed among carriers of the 677TT (OR=0.86; 95% CI: 0.76–0.96) and the 1298CC (OR=0.82; 95% CI: 0.69–0.98) genotypes, compared to the wild-type homozygous genotype. In the subgroup of colon cancer, the OR of 677TT vs. CC+CT was 0.83 (95% CI: 0.72–0.96) and the OR of 1298CC vs. AA+AC was 0.81 (95% CI: 0.69–0.96). In the rectal cancer subgroup, the OR of 677TT vs. CC+CT was 0.86 (95% CI: 0.77–0.97). Therefore, this meta-analysis suggested that the MTHFR 677T and 1298C alleles were associated with a low risk of CRC.

 methylenetetrahydrofolate reductase polymorphism colorectal cancer meta-analysis Introduction

Colorectal cancer (CRC) is one of the most common types of cancer worldwide (1). It is the fourth leading cause of cancer-related mortality among males and the third among females. Furthermore, the mortality rate of CRC in developed countries was shown to be higher compared to that in developing countries (3).

Folate is a type of water-soluble B vitamin and an essential nutrient required for human metabolism. Folate plays a key role in the formation of S-adenosylmethionine, which is the universal methyl donor for DNA methylation, as well as in the formation of purine and thymidine for DNA synthesis (4). Folate deficiency increases the risk of tumorigenesis through one of the following mechanisms: by leading to aberrant DNA methylation, which may in turn lead to an altered expression of critical tumor suppressor genes and proto-oncogenes; or by causing imbalances in the pools of nucleotide precursors, leading to DNA strand breaks and mutations and disruption of DNA integrity and repair (4,5).

The methylenetetrahydrofolate reductase (MTHFR) is an enzyme that is crucial in the metabolism of folate (6). MTHFR catalyzes the irreversible conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the primary methyl donor for the remethylation of homocysteine to methionine. The gene encoding MTHFR is located on chromosome 1p36.3 (7). Two common single-nucleotide polymorphisms of MTHFR are MTHFR C677T (Ala222Val, rs1801133) and A1298C (Glu429Ala, rs1801131). The C677T variant enhances enzyme thermolability and is associated with decreased activity of the MTHFR enzyme (8). The A1298C variant (Glu429Ala, rs1801131) is a missense mutation leading to reduced MTHFR enzyme activity (9,10). The homozygous genotypes of MTHFR C677T and A1298C are associated with higher homocysteine levels, which may lead to DNA hypomethylation and increased cancer prevalence. However, the decreasing enzyme activity results in higher 5,10-methylenetetrahydrofolate and thymidine levels and, thus, increased DNA synthesis and repair. Therefore, MTHFR polymorphisms are regarded as a protective factor against tumor development (8,11).

It was first reported by Chen et al(10) that the homozygote of the MTHFR 677T allele was associated with a reduced CRC risk. Previous studies reported that the MTHFR 677T or 1298C allele exerted a protective effect compared to the wild-type genotypes (12,13). However, other studies reported opposing results, i.e., that mutant genotypes were associated with an increased risk of developing CRC (14,15).

A meta-analysis of all published studies was conducted to determine the effect of MTHFR mutants on CRC risk. In a subgroup analysis, the study subjects were classified by ethnicity and tumor location to provide comprehensive evidence on the association of MTHFR C677T and A1298C with CRC.

 Materials and methods Study identification and selection

A literature search was conducted on PubMed, Medline and China National Knowledge Infrastructure (January, 1991-September, 2012) databases, using the following keywords and subject terms: 'MTHFR', 'polymorphism' and 'colon cancer' or 'rectal cancer'. All the studies in our meta-analysis were required to meet the following inclusion criteria: i) case-control studies; ii) raw data to calculate odds ratios (ORs) with 95% confidence intervals (95% CIs); iii) in case of the same results published in multiple studies, the most recent publication or the largest sample was considered. A given study was excluded from this meta-analysis when: i) the genotype or allele frequencies were not reported, ii) the study design was not case-control, iii) the association between MTHFR polymorphisms and colorectal adenoma was investigated.

 Data extraction

Data were carefully and independently collected according to the genotypes MTHFR C677T or A1298C. Two authors extracted the following information from the eligible studies: first author's name, publication year, country, ethnicity of participants and number of cases and controls. In our study, ethnicities were classified as European and American, Asian, African and mixed.

 Statistical analysis

The association between MTHFR C677T and A1298C gene polymorphisms and CRC risk was assessed by using the codominant (677CT vs. CC; 677TT vs. CC; 1298AC vs. AA; 1298CC vs. AA), the dominant (677CT+TT vs. CC; 1298AC+CC vs. AA) and the recessive (677TT vs. CC+CT; 1298CC vs. AA+AC) models. The same procedures were applied for the MTHFR A1298C genotype. Subgroup analyses were performed by tumor location and ethnicity of the control groups.

The strength of association of the MTHFR gene polymorphisms with CRC was measured by the ORs (1) together with the 95% CIs. The significance of the pooled ORs was determined by the Z-test and P<0.05 was considered to indicate a statistically significant difference. The Chi-square test was first used to assess whether the distribution of genotypes among controls conformed to the Hardy-Weinberg equilibrium (HWE), with P<0.05 considered a departure from HWE. The Q-test was used to assess heterogeneity among the studies. When P<0.05, the heterogeneity was considered to indicate a statistically significant difference. The I2 index was used to quantify the percentage of the total variation among studies when heterogeneity was calculated. The I2 value ranged from 0 to 100%, with 25, 50 and 75% expressing low, moderate and high heterogeneity, respectively. When I2<50%, a fixed-effects model (the Mantel-Haenszel method) was applied to estimate the pooled results. Otherwise, the random-effects model (the DerSimonian-Laird method) was used.

Publication bias was visually investigated in a funnel plot of log (OR) against its standard error (SE). An asymmetric plot suggested possible publication bias. The degree of asymmetry was assessed via the Egger's test (P<0.05 was considered publication bias). A sensitivity analysis was performed by omitting each study in turn to assess the stability of the results.

All the analyses were performed with Stata software version 11.0 (StataCorp, College Station, TX, USA). All the P-values were two-sided.

 Results Selection of studies

A total of 302 studies were identified during the literature search and 232 were excluded due to departures from the inclusion criteria. Eventually, 70 studies were included (1,2,6,10,12–77). One study, conducted by Lee et al(41), consisted of three individual case-control studies and was handled as three populations; furthermore, the studies by Keku et al(12) and Lima et al(28), included two populations each. This meta-analysis included a larger population compared to previous meta-analyses. Of the 74 case-control studies included in the 70 publications, 74 studies investigated MTHFR C677T (29,783 cases and 41,772 controls) and 39 investigated MTHFR A1298C (13,285 cases and 20,164 controls). In total, 33 populations were from Europe and America; 31 were from Asia; 4 were from Africa and the remaining were mixed populations. In addition, a subanalysis was conducted by tumor location. Twenty-one of the 74 studies provided detailed data on colon and 14 on rectal cancer. The characteristics of the studies are listed in Table I.

 Meta-analysis

The results of the meta-analysis are presented in Table II. There was obvious heterogeneity for all the models, except for the MTHFR A1298C additive model (CA vs. AA, P=0.307, I2=9.2%) and the random-effects model was used to estimate the pooled data. The meta-analysis of the 74 populations demonstrated that MTHFR C677T was associated with reduced CRC risk under the homozygote (TT vs. CC: OR=0.89; 95% CI: 0.82–0.97; P=0.009) (Fig. 1) and recessive (TT vs. CC+CT: OR=0.89; 95% CI: 0.82–0.96; P=0.003) models (Fig. 2). However, the MTHFR 1298C allele was not associated with a significantly decreased risk of CRC.

The association of MTHFR C677T and A1298C polymorphisms with CRC was further stratified by ethnicity. As shown in Table II, no significant association was observed between MTHFR C677T and the risk of CRC under any genetic models, in any of the populations. However, Asians carrying the MTHFR 1298CC genotype exhibited a reduced risk of CRC. The OR of CC vs. AA was 0.69 (95% CI: 0.54–0.89) and the OR of CC vs. AA+AC was 0.69 (95% CI: 0.54–0.88). In the mixed populations, the pooled analysis demonstrated that carriers of MTHFR 677TT and 1298CC were more common among CRC patients than among controls when compared to individuals with wild-type genotypes (677TT vs. CC: OR=0.86; 95% CI: 0.76–0.96; TT vs. CC+CT: OR=0.85; 95% CI: 0.76–0.95) (1298CC vs. AA: OR=0.82; 95% CI: 0.69–0.98; and CC vs. AA+AC: OR=0.83; 95% CI: 0.70–0.99).

Furthermore, the participants were stratified by tumor location. A significantly decreased risk of CRC was observed under the recessive model of MTHFR 677TT (OR=0.83; 95% CI: 0.72–0.96) and 1298CC (OR=0.81; 95% CI: 0.69–0.95) in the colon cancer group. In addition, the stratified analysis revealed that MTHFR C677T was associated with reduced risk of rectal cancer. The OR under the recessive model was 0.86 (95% CI: 0.77–0.97). The main results are presented in Table II.

 Sensitivity analysis and publication bias

The elimination of each individual study imparted no qualitative difference on the pooled OR values, indicating that the final results of the meta-analysis were relatively stable. The publication bias of the studies was determined by a funnel plot and the Egger's test. The shapes of the funnel plot for each comparison indicated no obvious asymmetry (Figs. 3 and 4) and the Egger's test was then used to provide statistical evidence for the funnel plot symmetry. No significant publication bias was detected in the studies. The results are presented in Table III.

 Discussion

The first study to evaluate the association between the MTHFR C677T polymorphism and CRC was conducted by Chen et al(10). The findings of that study suggested that the MTHFR C677T mutation affected enzyme activity and was involved in abnormal methylation as well as DNA synthesis, leading to colorectal tumorigenesis. Similar results were subsequently reported by Ma et al(16), Slattery et al(70) and Le Marchand et al(77). In addition, Le Marchand et al(77) observed that the MTHFR 1298C allele was weakly protective against CRC. However, Guimarães et al(69) reported that the carriers of the combined variants MTHFR 1298AC+CC and 677CT+TT exhibited an increased risk of CRC, whether in isolation or in combination. According to Shannon et al(14) and Prasad et al(63), the MTHFR polymorphism C677T was a risk factor for CRC development. Furthermore, several other published studies failed to support an effect of MTHFR gene polymorphisms on CRC risk (43,49,60), due to statistically non-significant results.

The conflicting conclusions among the studies mentioned above may be attributed to several causes. First, the sample sizes of the populations included in several studies were relatively small (19,27,69), which may result in false-positive or false-negative outcomes. Second, the eligibility criteria for inclusion of control subjects differed among the studies. Certain studies were hospital-based (35,45,50), whereas others were population-based (12,13,47,63). Therefore, some controls were non-cancer cases, whereas others were healthy individuals. The inclusion of individuals from different ethnic backgrounds should also be considered.

As regards the conflicting results, we performed a meta-analysis to elucidate the association of MTHFR C677T and A1298C polymorphisms with CRC risk and to provide a comprehensive assessment. In this meta-analysis, the pooled results indicated that the homozygous variant of MTHFR C677T polymorphism exerted a protective effect against CRC development (OR=0.89; 95% CI: 0.82–0.97). However, when analysis was performed by ethnicity, this effect was not observed in all the subgroups, except for the mixed population (OR=0.86; 95%CI: 0.76–0.96). Additionally, a significant association was observed between MTHFR 1298CC and CRC in the mixed population (OR=0.82; 95% CI: 0.69–0.98). When this analysis was restricted by limiting studies to Asian populations, the MTHFR 1298CC genotype exhibited a decreased risk of CRC, with an OR of 0.69 (95% CI: 0.54–0.89). In the subanalysis by tumor location it was demonstrated that individuals with the MTHFR 677TT genotype exhibited a decreased risk of colon and rectal cancer, with an OR of 0.83 (95% CI: 0.72–0.96) and 0.86 (95% CI: 0.77–0.97), respectively. It was also confirmed that the MTHFR A1298C polymorphism is involved in colon cancer development (CC vs. AA+AC: OR=0.81; 95% CI: 0.69–0.96). Our findings in this meta-analysis were consistent with the results reported by the majority of the published studies.

According to Begg's funnel plots and Egger's test, there was no significant publication bias in the present meta-analysis. However, there was obvious heterogeneity, which was a potential problem when interpreting the results of the meta-analysis. Several sources of heterogeneity should be considered. First, different selection criteria of cases and controls, as well as gender and age distribution, may affect between-study heterogeneity. Second, there was some diversity among the studies regarding design, sample size and family history. Ethnic variations were also crucial as it was demonstrated that the heterogeneity was decreased when analyzed by ethnicity in this meta-analysis. The different ethnicities were distinguished according to geography; however, potential confounding factors, such as genetic background, lifestyle and dietary habits, could not be excluded.

The present study has certain limitations. First, the criteria for inclusion of controls differed among the studies. The controls in certain studies were selected from healthy individuals, whereas the controls in other studies were selected from non-cancer cases. Second, our results were based on unadjusted OR values, which may lead to relatively low power of the estimation of the real association. The gender and age distribution of the participants, the dietary pattern, the folate status, alcohol consumption and other risk factors may affect between-study heterogeneity. An analysis should be conducted to obtain adjusted ORs for other covariates, such as age, gender and folate status, provided more individual study data are available. Third, the subgroup analyses had insufficient statistical power to detect the association. The gene-gene and gene-environment interactions may affect the association between MTHFR polymorphisms and CRC. According to the study by Keku et al(12), the combination of MTHFR 677CC and 1298AA genotypes exhibited an increased risk of CRC. In addition, Ma et al(16) and Kim et al(67) demonstrated that a high folate intake was associated with reduced risk of CRC and high alcohol consumption was associated with an increased risk of CRC. The gene-gene and gene-environment interactions could not be examined due to unavailability of individual data.

Despite the limitations described above, our meta-analysis also has certain advantages. The substantial number of cases and controls were pooled from different studies, which provided more reliable evidence on the association between MTHFR polymorphisms and the risk of CRC. In addition, the study subjects were classified into colon and rectal cancer groups, in order to exclude certain confounding factors. The pooled data clearly demonstrated that MTHFR C677T significantly affected carcinogenesis in the colon and rectum, whereas A1298C appeared to be mainly associated with rectal tumorigenesis. This finding may be attributed to different carcinogenic mechanisms underlying colon and rectal cancer.

In conclusion, the results of this meta-analysis indicated a significant association of the MTHFR C677T and A1298C polymorphisms with the risk of CRC. Particularly, the MTHFR 677T and 1298C alleles were associated with a low risk of CRC.

 Acknowledgements

This study was supported by grants no. 81102194 and no. 81272293 from the National Natural Science Foundation of China, grant no. LS2010168 from the Liaoning Provincial Department of Education and grant no. 00726 from the China Medical Board. The authors are grateful to all the participants in this study.

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Forest plot for the association between methylenetetrahydrofolate reductase C677T polymorphism and colorectal cancer risk for the TT genotype compared to the CC genotype in the overall population. OR, odds ratio; CI, confidence interval.

Forest plot for the association between methylenetetrahydrofolate reductase C677T polymorphism and colorectal cancer risk for the TT genotype compared to the CC+CT genotype in the overall population. OR, odds ratio; CI, confidence interval.

Funnel plot with pseudo 95% confidence limits on the association between methylenetetrahydrofolate reductase C677T polymorphism and colorectal cancer risk. SE, standard error; OR, odds ratio.

Funnel plot with pseudo 95% confidence limits on the association between methylenetetrahydrofolate reductase A1298C polymorphism and colorectal cancer risk. SE, standard error; OR, odds ratio.

Characteristics of the studies included in the meta-analysis.

MTHFR 677C/T (case/control) MTHFR 1298A/C (case/control)
First author Year Country Location CC CT TT P-value AA AC CC P-value Refs.
Europe/America
Chen et al 1996 USA NR 67/280 64/263 13/84 0.08 10
Ma et al 1997 USA NR 92/145 92/132 18/49 0.04 16
Ryan et al 2001 Ireland NR 49/439 73/326 14/83 0.05 17
Keku et al 2002 USA Colon 144/265 140/223 24/51 0.68 156/237 132/236 21/68 0.44 12
Shannon et al 2002 Australia Colon 249/533 197/560 55/114 0.05 14
Sachse et al 2002 UK NR 238/271 199/272 53/49 0.09 18
Heijmans et al 2003 Netherlands NR 7/399 7/329 4/65 0.81 13
Pufulete et al 2003 UK NR 36/41 19/29 8/6 0.78 34/47 22/26 7/3 0.80 19
Plaschke et al 2003 Germany NR 133/149 120/159 34/38 0.65 134/154 124/151 29/41 0.67 20
Toffoli et al 2003 Italy Colon 93/83 145/140 38/56 0.83 122/133 129/121 25/25 0.74 21
Ulvik et al 2004 Norway NR 1,103/1,092 899/886 157/212 0.10 22
Landi et al 2005 Spain NR 128/109 158/139 64/61 0.17 189/170 146/127 25/22 0.79 23
Koushik et al 2006 USA NR 166/355 145/327 38/112 0.01 154/389 166/332 33/85 0.26 24
Van Guelpen et al 2006 Sweden NR 123/212 85/161 12/42 0.17 85/189 103/173 32/50 0.29 25
Battistelli et al 2006 Italy NR 32/30 40/51 21/19 0.75 26
Osian et al 2007 Romania NR 38/47 25/17 6/3 0.38 33/41 32/25 4/1 0.19 27
Lima et al 2007 Brazil NR 36/143 46/127 15/30 0.82 68/191 28/93 6/16 0.30 28
Theodoratou et al 2008 Scotland NR 447/439 441/455 111/116 0.91 465/462 425/445 106/102 0.73 29
Sharp et al 2008 Scotland NR 117/170 111/177 23/47 0.93 105/177 111/157 29/60 0.01 30
Eklöf et al 2008 Sweden NR 123/212 85/160 12/42 0.15 31
Küry et al 2008 France NR 435/457 452/515 136/149 0.84 484/577 432/443 107/101 0.23 32
Derwinger et al 2009 Sweden NR 273/167 216/107 55/25 0.19 33
de Vogel et al 2009 Netherlands NR 318/876 320/750 51/167 0.72 299/735 275/774 110/258 0.02 34
Gallegos-Arreola et al 2009 Mexico NR 124/59 126/79 119/32 0.54 36
Fernández-Peralta et al 2010 Spain NR 89/44 52/50 2/9 0.32 84/57 53/44 6/2 0.05 35
Komlosi et al 2010 Hungary Colon 208/216 196/186 68/59 0.01 37
Rectum 190/226 231/194 58/58
Karpinski et al 2010 Poland NR 74/71 97/55 15/14 0.49 38
Eussen et al 2010 Europe NR 567/1,019 608/1,076 154/271 0.61 605/1,099 574/1,007 151/259 0.22 40
Pardini et al 2011 Czech NR 317/613 307/627 42/136 0.18 281/583 309/638 76/156 0.35 1
Vossen et al 2011 Germany Colon 454/795 502/807 122/209 0.85 39
Rectum 283/795 321/807 80/209
Lee et al 2012 USA/NHS NR 89/165 66/140 20/48 0.04 72/181 82/136 21/38 0.11 41
Lee et al 2012 USA/HPFS NR 72/140 69/127 17/51 0.02 73/147 73/133 7/32 0.81 41
Lee et al 2012 USA/PHS NR 89/159 94/124 15/50 0.00 101/167 100/154 12/44 0.36 41
Asia
Park et al 1999 Korea NR 65/140 107/246 28/74 0.05 42
Matsuo et al 2002 Japan Colon 23/81 39/124 10/36 0.30 50/157 19/75 3/9 0.02 43
Rectum 16/81 42/124 12/36 44/157 25/75 0/9
Huang et al 2003 China NR 36/40 40/33 6/9 0.58 44
Yin et al 2004 Japan Colon 154/278 180/367 48/133 0.53 236/515 128/244 18/19 0.11 15
Rectum 110/278 144/367 36/133 192/515 90/244 8/19
Kim et al 2004 Korea Colon 24/83 67/109 20/33 0.77 45
Rectum 62/83 55/109 15/33
Matsuo et al 2005 Japan NR 106/289 114/348 36/134 0.10 163/479 85/257 9/31 0.63 46
Jiang et al 2005 China Colon 19/134 31/143 3/62 0.03 36/226 17/103 0/6 0.03 47
Rectum 32/134 28/143 12/62 57/226 13/103 1/6
Miao et al 2005 China NR 53/133 87/201 58/86 0.52 48
Otani et al 2005 Japan NR 32/51 49/114 25/57 0.68 73/156 32/63 1/5 0.64 49
Wang et al 2006 India Colon 53/255 6/36 0/0 0.26 32/105 22/135 5/51 50
Rectum 204/255 37/36 2/0 109/105 108/135 26/51
Chang et al 2007 China NR 85/92 86/87 24/16 0.47 120/127 65/55 10/13 0.05 51
Zeybek et al 2007 Turkey NR 18/64 27/65 7/15 0.80 52
Jin et al 2007 China Colon 99/211 117/325 35/136 0.60 53
Rectum 82/211 92/325 20/136
Cao et al 2008 China Colon 30/121 53/183 22/66 0.82 66/239 38/119 1/13 0.70 2
Rectum 79/121 101/183 30/66 138/239 67/119 5/13
Mokarram et al 2008 Iran Colon 64/40 80/31 7/10 0.31 54
Zhang et al 2008 China NR 97/91 136/139 67/69 0.26 56
Haghighi et al 2009 Iran NR 117/94 68/80 49/83 0.00 55
Promthet et al 2010 Thailand Colon 104/94 26/31 0/5 0.24 43/54 84/71 3/5 0.00 57
Cui et al 2010 Korea NR 622/540 923/863 284/297 0.13 6
Naghibalhossaini et al 2010 Iran Colon 64/150 80/68 7/13 0.16 58
Chandy et al 2010 India NR 74/66 25/19 1/1 0.78 22/22 70/50 8/14 0.11 60
Zhu et al 2010 China NR 88/50 102/53 26/8 0.23 61
Yang et al 2010 China NR 58/62 61/75 22/28 0.52 75
Kim et al 2011 Korea NR 30/15 30/21 7/17 0.13 44/36 22/16 1/1 0.61 59
Zhu et al 2011 China NR 29/49 42/41 15/10 0.74 76
Kang et al 2011 Korea NR 87/145 134/238 34/65 0.04 62
Prasad and Wilkhoo 2011 India NR 97/228 12/12 1/1 0.07 63
Sameer et al 2011 India Colon 23/121 7/27 6/12 0.00 64
Rectum 36/121 11/27 3/12
Li et al 2011 China NR 68/55 54/64 15/26 0.33 88/76 47/60 2/9 0.53 65
Yin et al 2012 China NR 124/139 167/178 79/53 0.74 66
Kim et al 2012 Korea Colon 121/205 185/289 57/162 0.00 67
Rectum 109/205 164/289 57/162
Africa
Keku et al 2002 USA NR 198/264 43/59 3/6 0.21 157/217 78/99 8/13 0.69 12
Lima et al 2007 Brazil NR 4/143 5/127 1/30 0.82 5/191 4/93 1/16 0.30 28
El Awady et al 2009 Egypt NR 6/44 23/20 6/4 0.41 5/26 21/37 9/15 0.09 68
Guimarães et al 2011 Brazil Colon 17/92 28/79 9/17 0.99 39/127 12/49 3/12 0.02 69
Rectum 31/92 22/79 6/17 28/127 26/49 5/12
Mixed
Slattery et al 1999 USA Colon 673/827 655/787 139/207 0.34 70
Curtin et al 2004 USA Colon 734/887 724/858 150/227 0.37 757/929 698/827 153/216 0.12 71
Le Marchand et al 2005 USA Colon 295/987 246/779 56/255 0.00 77
Rectum 99/987 90/779 31/255
Murtaugh et al 2007 USA Rectum 357/466 301/392 84/112 0.04 360/436 317/424 65/110 0.65 72
Iacopetta et al 2009 Australia NR 382/428 386/429 82/101 0.67 73
Reeves et al 2009 Australia NR 105/101 83/91 18/19 0.82 92/86 89/98 25/27 0.91 74

NR, not reported; P-value is for Hardy-Weinberg equilibrium.

Association of MTHFR C677T and A1298C polymorphisms with CRC risk.

TT vs. CC CT vs. CC TT vs. CC+CT TT+CT vs. CC
C677T OR (95% CI) P-value OR (95% CI) P-value OR (95% CI) P-value OR (95% CI) P-value
Total 0.89 (0.45–0.97) 0.009 1.02 (0.97–1.07) 0.378 0.89 (0.82–0.96) 0.003 1.00 (0.95–1.05) 0.889
Ethnicity
 European 0.90 (0.80–1.02) 0.088 1.02 (0.95–1.09) 0.661 0.90 (0.80–1.01) 0.075 0.99 (0.93–1.06) 0.831
 Asian 0.86 (0.73–1.02) 0.087 1.03 (0.93–1.13) 0.583 0.87 (0.75–1.01) 0.059 1.00 (0.90–1.11) 0.981
 African 1.96 (0.66–5.85) 0.225 1.75 (0.81–3.78) 0.153 1.49 (0.85–2.62) 0.165 1.80 (0.82.3.94) 0.143
 Mixed 0.86 (0.76–0.96) 0.010 1.02 (0.95–1.10) 0.557 0.85 (0.76–0.95) 0.004 0.99 (0.92–1.06) 0.703
Tumor location
 Colon 0.86 (0.73–1.02) 0.079 1.08 (0.97–1.20) 0.161 0.83 (0.72–0.96) 0.014 1.03 (0.93–1.15) 0.566
 Rectum 0.88 (0.73–1.07) 0.208 1.07 (0.98–1.16) 0.129 0.86 (0.77–0.97) 0.014 1.01 (0.89–1.15) 0.880
CC vs. AA AC vs. AA CC vs. AA+AC CC+AC vs. AA
A1298C OR (95% CI) P-value OR (95% CI) P-value OR (95% CI) P-value OR (95% CI) P-value
Total 0.92 (0.81–1.05) 0.202 1.03 (0.98–1.08) 0.221 0.90(0.80–1.01) 0.086 1.02 (0.96–1.08) 0.619
Ethnicity
 European 1.01 (0.91–1.11) 0.864 1.04 (0.98–1.11) 0.169 0.99 (0.90–1.09) 0.841 1.04 (0.98–1.10) 0.208
 Asian 0.69 (0.54–0.89) 0.004 1.00 (0.89–1.11) 0.984 0.69 (0.54–0.88) 0.002 0.96 (0.86–1.07) 0.466
 African 1.92 (0.70–5.31) 0.207 1.33 (0.96–1.83) 0.082 1.50 (0.70–3.20) 0.294 1.45 (0.94–2.23) 0.091
 Mixed 0.82 (0.69–0.98) 0.033 0.98 (0.88–1.10) 0.730 0.83 (0.70–0.99) 0.035 0.95 (0.85–1.05) 0.316
Tumor location
 Colon 0.81 (0.59–1.12) 0.200 1.03 (0.94–1.13) 0.535 0.81 (0.69–0.95) 0.012 0.99 (0.91–1.08) 0.872
 Rectum 0.85 (0.50–1.42) 0.530 1.00 (0.79–1.26) 0.980 0.85 (0.55–1.32) 0.474 0.95 (0.74–1.23) 0.719

CRC, colorectal cancer; OR, odds ratio; CI, confidence interval.

Results of Egger's test for the models.

Polymorphism Model T-value P-value 95% CI of intercept value
MTHFR C677T
TT vs. CC 1.04 0.303 −0.3457011 1.096597
CT vs. CC 1.86 0.067 −0.0274857 0.7770219
TT vs. CC+CT 0.62 0.535 −0.485652 0.9266581
TT+CT vs. CC 1.66 0.102 −0.1176829 1.27792
MTHFR A1298C
CC vs. AA −0.52 0.605 −0.9875751 0.5834467
AC vs. AA 1.57 0.125 −0.1645624 1.29779
CC vs. AA+AC −1.02 0.312 −1.094435 0.3594053
CC+AC vs. AA 1.13 0.266 −0.3669942 1.292081

CI, confidence interval.