Association Between MTHFR C677T Polymorphism and

CLINICAL STUDY

Association Between MTHFR C677T Polymorphism andCongenital Heart Disease

A PRISMA-Compliant Meta-Analysis

Peng-Fei Liu,1* MD, Bing Ding,1* MD, Jun-Yi Zhang,1 MD, Xiao-Fei Mei,1 MD, Fei Li,1 MD,

Peng Wu,1 MD, Chun-Hao Mei,1 MD, Ya-Feng Zhou,1 PhD and Tan Chen,1 PhD

SummaryMany published studies have evaluated the association between the 5,10-methylenetetrahydrofolate reduc-

tase (MTHFR) C677T (rs1801133) polymorphism and the risk of congenital heart disease (CHD); however, the

specific conclusion is still controversial.

To get a more accurate conclusion, we used a meta-analysis to evaluate the association between the

MTHFR gene C677T polymorphism and the risk of CHD.

Based on the design-based search strategy, a comprehensive literature search was conducted on PubMed,

OVID, Cochrane Library, Embase, Wanfang, CNKI, and Web of Science. We selected the Newcastle-Ottawa

Scale (NOS) to assess the quality of the included studies. We performed a heterogeneity test on the results of

the study and calculated the combined odds ratios (ORs) and its corresponding 95% confidence intervals (95%

CIs) under a random- or fixed-effect model. Subgroup analyses were conducted by ethnicity, source of controls,

sample size, and genotyping method. Sensitivity analysis was used to insure authenticity of this meta-analysis

result. Egger’s test and Begg’s funnel plot were performed to detect publication bias.

Eventually, our meta-analysis included 15 eligible studies. We observed a significant correlation between

the MTHFR C677T polymorphism and the development of CHD in the recessive model (OR: 1.35, 95% CI:

1.06-1.71, P = 0.006) for the overall population. In subgroups stratified by ethnicity and source of controls,

subgroup analyses indicated similar associations in Asians and hospital-based groups, but not for Caucasians

and population-based groups. Egger’s test and Begg’s funnel plot demonstrated no significant publication bias

in our study.

Our analysis identified that MTHFR C677T allele is a risk genetic for CHD development, especially in

Asians compared with Caucasians.

(Int Heart J 2020; 61: 553-561)

Key words: Single nucleotide polymorphism, rs1801133

Congenital heart disease (CHD) is a common vari-

ety of structural abnormality in newborns, with a

prevalence of approximately 1:100 live births.1,2)

Many CHDs are caused by the interaction of genetic and

environmental factors, although the exact cause for it is

quite complex and still unclear.3) Folic acid plays a very

important role in the development of the cardiovascular

system.4) It is known that the lack of folic acid leads to

hyperhomocysteinemia, which has been described as a

possible risk factor for CHD.5,6)

The 5,10-methylenetetrahydrofolate reductase

(MTHFR) encoded by the MTHFR gene located at 1p36.3

is an important enzyme in homocysteine metabolism, con-

verting 5,10-methylenetetrahydrofolate into 5-

methyltetrahydrofolate in the process of methyltetrahydro-

folate.7) The MTHFR C677T mutation has been proved to

reduce the activity of enzyme and increase the level of

plasma homocysteine.7)

The relationship between the MTHFR gene polymor-

phism and the risk of CHD was first analyzed by Wen-

strom et al.8) In recent years, several articles have been

published on the relationship between the polymorphism

(C677T) and the risk of CHD. However, the conclusion is

still inconsistent. Thus, to provide a more comprehensive

result, we performed the present meta-analysis based on

15 trials9-23) to evaluate the association between MTHFR

From the 1Department of Cardiology, The First Affiliated Hospital of Soochow University, Suzhou, China.

*These authors contributed equally to this work.

This work was supported by grants from the National Natural Science Foundation of China (81873486,81770327), Natural Scientific Fund of Jiangsu prov-

ince (BK20161226), Jiangsu Province’s Key Provincial Talents Program (ZDRCA2016043), Jiangsu Province’s 333 High-Level Talents Project (BRA

2017539), and Jiangsu Provincial Medical Innovation Team (NO.CXTDA2017009). The funders had no roles in study design, data collection and analysis, de-

cision to publish, or preparation of the manuscript.

Address for correspondence: Tan Chen, PhD or Ya-Feng Zhou, PhD, Department of Cardiology, The First Affiliated Hospital of Soochow University, 188

Shizi Road, Suzhou, Jiangsu, 215006, China. E-mail: [email protected] or [email protected]

Received for publication July 26, 2019. Revised and accepted November 27, 2019.

Released in advance online on J-STAGE May 15, 2020.

doi: 10.1536/ihj.19-389

All rights reserved by the International Heart Journal Association.

553

Int Heart J

May 2020554 LIU, ET AL

Figure　1.　The PRISMA flow diagram of the study selection and exclusion.

Table　I.　Characteristics of the Studies Included for Meta-Analysis

Author Year Country Ethnicity PolymorphismSource of

controls

Genotyping

method

NOS

score

HWE

test

Yan, et al.9) 2003 China Asian MTHFR677 (rs1801133) HB PCR-RFLP 7 0.075

Storti, et al.10) 2003 Italy Caucasian MTHFR677 (rs1801133) HB Direct sequencing 8 0.277

Shaw, et al.11) 2005 America Caucasian MTHFR677 (rs1801133) PB Direct sequencing 9 0.235

Li, et al.12) 2005 China Asian MTHFR677 (rs1801133) HB PCR-RFLP 7 0.684

Zhu, et al.13) 2006 China Asian MTHFR677 (rs1801133) PB PCR-RFLP 7 0.513

van Beynum, et al.14) 2006 Netherlands Caucasian MTHFR677 (rs1801133) PB Direct sequencing 8 0.277

van Driel, et al.15) 2008 Netherlands Caucasian MTHFR677 (rs1801133) PB Direct sequencing 7 0.263

Kuehl, et al.16) 2010 America Caucasian MTHFR677 (rs1801133) PB Direct sequencing 7 0.911

Obermann-Borst, et al.17) 2011 Netherlands Caucasian MTHFR677 (rs1801133) PB Direct sequencing 8 0.682

Wang, et al.18) 2013 China Asian MTHFR677 (rs1801133) HB Direct sequencing 7 0.168

Sahiner, et al.19) 2014 Turkey Asian MTHFR677 (rs1801133) HB Direct sequencing 8 0.059

Li, et al.20) 2015 China Asian MTHFR677 (rs1801133) HB Direct sequencing 9 0.779

Jiang, et al.21) 2015 China Asian MTHFR677 (rs1801133) HB PCR-RFLP 8 0.586

Wang, et al.22) 2016 China Asian MTHFR677 (rs1801133) HB PCR-RFLP 7 0.949

Wang, et al.23) 2018 China Asian MTHFR677 (rs1801133) HB Direct sequencing 7 0.271

Case-control design was used in all the included studies. PCR-RFLP indicates polymerase chain reaction-restriction fragment length polymor-

phism; Year, publication year; NOS, Newcastle-Ottawa Scale; HWE, Hardy-Weinberg equilibrium; HB, hospital-based; and PB, popula-

tion-based.

polymorphism (C677T) and CHD susceptibility. Methods

This updated meta-analysis was organized according

to the Preferred Reporting Items for Systematic Reviews

Int Heart J

May 2020 555MTHFR AND CONGENITAL HEART DISEASE

Table　II.　The Results of Newcastle-Ottawa Scale

Selection Comparability Exposure

Yan, et al.9) ★★★ ★★ ★★Storti, et al.10) ★★★★ ★★ ★★Shaw, et al.11) ★★★★ ★★ ★★★Li, et al.12) ★★★ ★★ ★★Zhu, et al.13) ★★★ ★★ ★★Beynum, et al.14) ★★★ ★★ ★★★Driel, et al.15) ★★★★ ★ ★★Gardemann, et al.16) ★★★ ★★ ★★Obermann-Borst, et al.17) ★★★★ ★★ ★★Wang, et al.18) ★★★ ★★ ★★Sahiner, et al.19) ★★★★ ★★ ★★Li, et al.20) ★★★★ ★★ ★★★Jiang, et al.21) ★★★★ ★★ ★★Wang, et al.22) ★★★ ★★ ★★Wang, et al.23) ★★★ ★★ ★★

Table　III.　MTHFR C677T Polymorphism Genotype Distribution and Allele Frequency in Cases and Controls

Author

Genotype (n) Allele frequency (n, %)

Cases Controls Cases Controls

Total CC CT TT Total CC CT TT C T RAF C T RAF

Yan, et al.9) 207 58 97 52 103 24 57 22 213 201 0.49 105 101 0.49

Storti, et al.10) 128 20 55 53 200 40 108 52 95 161 0.63 188 212 0.53

Shaw, et al.11) 138 16 68 54 434 52 202 180 100 176 0.64 306 562 0.65

Li, et al.12) 210 61 94 55 102 25 57 20 216 204 0.49 107 97 0.48

Zhu, et al.13) 106 27 22 57 103 24 57 22 76 136 0.64 105 101 0.49

van Beynum, et al.14) 144 20 66 58 220 18 104 98 106 182 0.63 140 300 0.68

van Driel, et al.15) 191 27 103 61 251 25 107 119 157 225 0.59 157 345 0.69

Kuehl, et al.16) 107 10 33 64 290 32 124 134 53 161 0.75 188 392 0.68

Obermann-Borst, et al.17) 140 9 66 65 183 15 76 92 84 196 0.70 106 260 0.71

Wang, et al.18) 160 25 76 59 188 35 100 53 126 194 0.61 170 206 0.55

Sahiner, et al.19) 136 14 53 69 93 7 39 47 81 191 0.70 53 133 0.72

Li, et al.20) 150 41 78 31 150 25 66 59 160 140 0.47 116 184 0.61

Jiang, et al.21) 100 16 46 38 100 11 48 41 78 122 0.61 70 130 0.65

Wang, et al.22) 147 60 73 14 168 35 84 49 193 101 0.34 154 182 0.54

Wang, et al.23) 193 60 68 65 234 31 120 83 188 198 0.51 182 286 0.61

Case-control design was used in all the included studies. RAF indicates risk allele frequency.

and Meta-Analyses (PRISMA).24) Because our articles

were based on previously published studies, patients’ in-

formed consent and ethical approval were not required.

Search strategy: This systematic literature search was

conducted by the first two investigators until March 31,

2019 in the PubMed, OVID, Cochrane Library, Embase,

Wanfang, CNKI, and Web of Science without language

limitation. For the literature search, we used a combina-

tion of the following items: methylenetetrahydrofolate re-

ductase or MTHFR and CHD or CHD or birth defects

and polymorphism or Single Nucleotide Polymorphism

(SNP). Furthermore, reference listings of studies included

in our meta-analysis were manually searched for possible

eligible articles.

Inclusion and exclusion criteria: All eligible studies in-

cluded in this study needed to follow the inclusion crite-

ria: studies with case-control designs, report of the asso-

ciation between the MTHFR C677T polymorphism and

the risk of CHD, and articles with sufficient data. The ex-

clusion criteria were as follows: letters, case reports, meta-

analysis, review articles, articles without control group, ar-

ticles with abstract only, and studies without detailed

genotype data.

Data extraction: From each study, the first two research-

ers independently extracted all useful data. Any potential

conflicts were discussed with other authors. The extraction

of study data included the following: first author, publica-

tion year, country of origin, ethnicity, number of cases

and controls, genotype frequency, source of controls,

genotyping method, and Hardy-Weinberg equilibrium

(HWE). We investigated the quality of each study based

on the nine-point Newcastle-Ottawa Scale (NOS).25)

Statistical analysis: In each of the included study, we ex-

amined HWE to assess bias in genotype distribution. We

calculated odds ratios (ORs) and 95% confidence intervals

(95% CIs) for analyses of the C677T polymorphism and

the risk of CHD. The pooled ORs and 95% CIs were cal-

culated in five genetic models: allele model (T versus C),

heterozygote model (TC versus CC), homozygote model

(TT versus CC), dominant model (TT + TC versus CC),

and recessive model (TT versus TC + CC). We used the I2

test to quantify the proportion of the total variation caused

by the heterogeneity. The range of I2 is between 0% and

100%. A value of 0% indicates that no heterogeneity was

observed, a large value indicates an increase in heteroge-

neity, 25% is considered low, 50% is considered moder-

ate, and 75% is considered high heterogeneity. When I2 >

50%, a random-effect model26) should be taken. Otherwise,

the fixed-effect model27) was then adopted. The present

analysis included a wide variety of study designs such as

ethnicity, source of controls, sample size, and genotyping

method. Sensitivity analysis was conducted to detect

whether omitting each study involved in this meta-analysis

will alter the stability of the results. Publication bias was

assessed by drawing Begg’s funnel plot. We also assessed

funnel plot asymmetry via Egger’s test, and P > 0.05

means that there was no statistically significant bias of

publication.28) Our meta-analysis was performed using the

Cochrane Collaboration Review Manager 5.3 software and

Int Heart J


Figure　2.　Forest plot from the meta-analysis on the association between the MTHFR C677T (rs1801133) polymorphism and the risk of CHD.

A: allele; B: heterozygote; C: homozygote; D: dominant; E: recessive. CHD, congenital heart disease; CI, confidence interval; OR, odds ratio.

Stata version 15.0 (StataCorp, College Station, TX, USA). Results

Study characteristics: Our systematic research strategy

Int Heart J


Figure　3.　Subgroup meta-analysis by ethnicity of the relationship between the MTHFR C677T polymorphism and the risk of CHD. A: allele; B: heterozygote; C: homozygote; D: dominant; E: recessive.

Int Heart J


Table　IV.　Subgroup Analyses of the Association between MTHFR C677T Polymorphism and Risk of CHD

Subgroup Number Odds ratio 95% Confidential interval P value I2 (%)

Allele model

Total 15 1.08 (0.89-1.32) 0.000 81.1

ethnicity

Asian 9 1.15 (0.86-1.52) 0.000 84.0

Caucasian 6 1.00 (0.77-1.31) 0.001 76.7

Source of control

HB 9 1.17 (0.89-1.52) 0.258 82.9

PB 6 0.97 (0.73-1.31) 0.860 80.0

Sample size

≥ 300 12 1.14 (0.92-1.40) 0.230 81.6

< 300 3 0.88 (0.54-1.43) 0.600 77.7

Genotyping method

PCR-RFLP 5 1.08 (0.68-1.70) 0.749 87.8

Direct sequencing 10 1.09 (0.88-1.35) 0.445 78.1

Heterozygote model

Total 15 0.90 (0.65-1.22) 0.000 82.1

ethnicity

Asian 9 0.84 (0.52-1.37) 0.000 85.1

Caucasian 6 0.97 (0.65-1.44) 0.000 78.4

Source of control

HB 9 0.94 (0.65-1.37) 0.760 76.1

PB 6 0.82 (0.46-1.44) 0.484 78.3

Sample size

≥ 300 12 1.00 (0.74-1.36) 0.985 90.4

< 300 3 0.53 (0.17-1.67) 0.279 60.9

Genotyping method

PCR-RFLP 5 0.73 (0.30-1.76) 0.483 89.8


Homozygote model

Total 15 1.25 (0.86-1.83) 0.000 76.9

ethnicity

Asian 9 1.43 (0.83-2.48) 0.000 81.5

Caucasian 6 1.03 (0.63-1.67) 0.015 64.5

Source of control

HB 9 1.45 (0.84-2.49) 0.182 81.5

PB 6 1.02 (0.62-1.68) 0.941 65.5

Sample size

≥ 300 12 1.34 (0.88-2.04) 0.178 78.6

< 300 3 0.94 (0.40-2.21) 0.883 66.2

Genotyping method

PCR-RFLP 5 0.94 (0.54-3.08) 0.558 85.4


Dominant model

Total 15 0.99 (0.73-1.34) 0.000 83.1

ethnicity

Asian 9 1.00 (0.63-1.60) 0.000 86.0

Caucasian 6 0.98 (0.66-1.46) 0.000 80.8

Source of control

HB 9 1.08 (0.72-1.60) 0.721 81.5

PB 6 0.88 (0.53-1.47) 0.626 87.4

Sample size

≥ 300 12 1.09 (0.80-1.50) 0.573 81.5

< 300 3 0.65 (0.25-1.68) 0.369 88.3

Genotyping method

PCR-RFLP 5 0.89 (0.38-2.07) 0.791 90.2


Recessive model

Total 15 1.35 (1.06-1.71) 0.006 54.8

ethnicity

Asian 9 1.57 (1.16-2.12) 0.018 56.7

Caucasian 6 1.06 (0.78-1.43) 0.288 19.2

Source of control

HB 9 1.48 (1.06-2.08) 0.023 66.3

PB 6 1.16 (0.88-1.52) 0.291 0.0

Sample size

≥ 300 12 1.34 (1.01-1.78) 0.041 63.9

< 300 3 1.29 (0.83-2.02) 0.253 0.0

Genotyping method

PCR-RFLP 5 1.52 (1.09-2.12) 0.013 36.5


PCR-RFLP indicates polymerase chain reaction-restriction fragment length polymorphism; HB, hospital-based; and PB, population-based.

Int Heart J


Figure　4.　Sensitivity analysis of the association between the MTHFR C677T polymorphism and the risk of CHD. A: allele; B: heterozygote; C: homozygote; D: dominant; E: recessive.

found 639 potentially relevant studies. After removing du-

plicated articles, 329 studies were left for screening and

303 of studies were excluded. Of the remaining 26 stud-

ies, 4 did not provide sufficient data, 3 were unrelated to

MTHFR, and 4 were excluded due to unmatched study

design. Figure 1 shows the specific process of the study

inclusion and exclusion. Finally, a total of 15 studies were

included in this meta-analysis between the C677T poly-

morphism of the MTHFR gene and the risk of CHD, of

which 2257 cases and 2819 controls were compared. The

main characteristics of each study in our meta-analysis are

presented in Table I. The races of these articles were

Asian (n = 9) and Caucasian (n = 6). Our meta-analysis

included six population-based studies3,6-10) and nine

hospital-based studies.1,2,4,5,11-15) The sample sizes for all in-

volved eligible articles ranged from 200 to 572. All stud-

ies involved are consistent with the HWE test. The results

of NOS for all the included studies are shown in Table II.

The NOS scores for all eligible studies in this meta-

analysis exceeded 6 points, indicating that our analysis is

of good quality. Table III shows the genotype distribution

and allele frequency of all included studies.

Int Heart J


Quantitative synthesis: The present meta-analysis exam-

ined five genetic models of the MTHFR C677T polymor-

phism and the risk of CHD. In all eligible studies in the

random-effect model, a significantly increased risk of

CHD was found only in the recessive genetic model (OR

= 1.35, 95% CI = 1.06-1.71, P < 0.001, I2 = 54.8%, Phetero-

geneity = 0.006) in the overall population. However, our

study did not find significant association under allele

model (OR = 1.08, 95% CI = 0.89-1.32, P = 0.413, I2 =

81.1%, Pheterogeneity < 0.001), homozygote model (OR =

1.25, 95% CI = 0.86-1.83, P = 0.241, I2 = 76.9%, Pheterogene-

ity < 0.001), heterozygote model (OR = 0.90, 95% CI =

0.65-1.22, P = 0.487, I2 = 82.1%, Pheterogeneity < 0.001), and

dominant model (OR = 0.99, 95% CI = 0.73-1.34, P =

0.960, I2 = 83.1%, Pheterogeneity < 0.001) (Figure 2). Because

a high degree of heterogeneity was found in the study, we

use subgroup analysis to investigate the sources of hetero-

geneity. In the subgroup analysis of different ethnicity

(Figure 3), remarkable association was also found in

Asians under recessive model (OR = 1.35, 95% CI =

1.06-1.71, P = 0.014, I2 = 56.7%, Pheterogeneity = 0.006).

However, we found no statistical association in Cauca-

sians in recessive model (OR = 1.06, 95% CI = 0.78-1.43,

P = 0.288, I2 = 19.2%, Pheterogeneity = 0.713). We also con-

ducted subgroup analysis based on source of controls,

sample size, and genotyping method. We obtained similar

results in the subgroup analysis under recessive model. In

the source of control subgroup analysis, we found a sig-

nificant relationship in hospital-based controls (OR =

1.48, 95% CI = 1.06-2.08, P = 0.023, I2 = 66.3%,

Pheterogeneity = 0.003), while there is no relevant influence in

population-based controls (OR = 1.16, 95% CI = 0.88-

1.52, P = 0.291, I2 = 0.0%, Pheterogeneity = 0.291). In addi-

tion, in the subgroup analysis of sample size, an obvious

association between C677T mutation and the risk of CHD

was discovered in �300 group (OR = 1.34, 95% CI =

1.01-1.78, P = 0.041, I2 = 63.9%, Pheterogeneity =

0.001); however, the relationship disappeared in <300

group (OR = 1.29, 95% CI = 0.83-2.02, P = 0.253, I2 =

0.0%, Pheterogeneity = 0.822). In addition, this association was

revealed to be stronger in polymerase chain reaction-

restriction fragment length polymorphism group (OR =

1.52, 95% CI = 1.09-2.12, P = 0.013, I2 = 36.5%,

Pheterogeneity = 0.178) nor direct sequencing group (OR =

1.26, 95% CI = 0.91-1.75, P = 0.253, I2 = 62.2%,

Pheterogeneity = 0.005). The detailed results of our subgroup

analysis were listed in Table IV.

Sensitivity analysis: We carried out a sensitivity analysis

to determine if the omission of each study would under-

mine the results of this meta-analysis. Figure 4 shows that

the results of the changes were not obtained after omitting

each study, indicating the stability of our consequence.

Publication bias: When performing a meta-analysis, it’s

no doubt that publication bias is another common problem

to be resolved. Therefore, we used Egger’s test and con-

structed the Begg’s funnel plot to solve this problem. Ob-

viously, from the funnel plot (Supplemental Figure), all

the 15 studies were symmetrically distributed on the two

sides, which indicated no evidence of publication bias in

our meta-analysis (allele model: P = 0.183; heterozygote

model: P = 0.330; homozygote model: P = 0.542; domi-

nant model: P = 0.643; recessive model: P = 0.118).

Discussion

In general, many case-control articles focusing on the

association between the C677T polymorphism and the

risk of CHD have been reported, but the conclusion re-

mains unclear. Due to the conflicting results and small

sample size of individual studies, we performed this pre-

sent meta-analysis to obtain a more accurate relationship

between the MTHFR gene C677T polymorphism and the

risk of CHD.

We included a total of 15 eligible studies9-23) in our

meta-analysis. All of the results revealed that the MTHFR

C677T polymorphism increased the risk of CHD. Because

of a high heterogeneity, we conducted group analysis ac-

cording to ethnicity, sample size, source of control, and

genotyping method. Analysis by ethnicity group showed

an increased susceptibility of CHD and variant alleles in

Asian population of C677T polymorphism under the re-

cessive model. In contrast, no significant association was

found in the Caucasian population. Moreover, the in-

creased risk of the recessive model was also observed in

stratified analyses by source of controls, sample size, and

genotyping method. Among the regarded studies, no evi-

dence showed publication bias in our meta-analysis.

At present, the pathogenesis of CHD has not been

fully recognized. However, studies have indicated that ge-

netic factors are associated with the pathogenesis of coro-

nary heart disease. 3 ) The synthesis of 5-

methyltetrahydrofolate is affected by the MTHFR enzyme

activity, and then, the process of remethylation of homo-

cysteine to methionine is affected, which finally induces

hyperhomocysteinemia.29) In addition, it has been found

that the MTHFR C677T mutation can reduce the activity

of enzyme and increase plasma homocysteine.7) Hyperho-

mocysteinemia can cause neural crest cells to initiate

apoptosis and has been shown to be toxic to heart cells in

animal model experiments.30,31) These previous studies

provide a good explanation for our results that MTHFR

C677T polymorphism is relevant to increase the risk of

CHD. Folic acid has the effect of lowering the concentra-

tion of homocysteine in the human body.32) In addition, a

published study indicated that proper supplementation

with folic acid significantly reduced the risk of CHD.33)

Therefore, supplementing folic acid during pregnancy

should be considered.

Our meta-analysis did have several limitations. First,

all 15 studies were collected only in Chinese and English,

so relevant studies performed in other languages and pos-

sible unpublished articles may be missed. Second, there

were no studies including Africans, which may lead to se-

lection bias. Third, almost all studies did not classify their

cases by types of CHD, which may have various etiolo-

gies. Fourth, several gene polymorphisms may act to-

gether, which may increase the incidence of CHD. In ad-

dition, some confounding factors, such as gender, age,

physical conditions, and living environments, may affect

the final results.

Based on our analysis, we found that the MTHFR C

677T polymorphism is relevant to increase the risk of

Int Heart J


CHD in recessive genetic model. The association was

more significant in Asians compared with Caucasians. In

addition, gene-gene and gene-environment interactions

need further study.

Disclosure

Conflicts of interest: None.

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Association Between MTHFR C677T Polymorphism and