Geographical and Ethnic Distributions of the MTHFR C677T, A1298C and MTRR A66G Gene Polymorphisms in Chinese Populations: A Meta-Analysis

Abstract

Background

The geographical and ethnic distributions of the polymorphic methylenetetrahydrofolate reductase (MTHFR) mutations (C677T and A1298C) and methionine synthase reductase (MTRR) mutation (A66G) remain heterogeneous in China. The goal of this study was to estimate the pooled frequencies of the alleles and associated genotypes of these gene polymorphisms among healthy populations in Mainland China.

Objective and Methods

We systematically reviewed published epidemiological studies on the distributions of 3 genetic variants in Chinese healthy populations living in Mainland China through a meta-analysis. The relevant electronic databases were searched. All of the raw data of the eligible citations were extracted. The frequency estimates were stratified by geography, ethnicity and sex.

Results

Sixty-six studies were identified with a total of 92277 study participants. The meta-analysis revealed that the frequencies of the MTHFR C677T, A1298C, and MTRR A66G gene polymorphisms varied significantly between different ethnic groups and along geographical gradients. The frequencies of the 677T allele and 677TT genotype increased along the southern-central-northern direction across Mainland China (all Pvalues≤0.001). The frequencies of the 1298C, 1298CC, 66G and 66GG genotypes decreased along the south-central-north direction across the country (all Pvalues≤0.001).

Conclusions

Our meta-analysis strongly indicates significant geographical and ethnic variations in the frequencies of the C677T, A1298C, and A66G gene polymorphisms in the folate metabolism pathway among Chinese populations.

About the Authors:

Xingmin Wang

Contributed equally to this work with: Xingmin Wang, Jinjian Fu, Qianxi Li

Affiliation: Department of Gynecology, Liuzhou Municipal Maternity and Child Healthcare Hospital, Liuzhou, Guangxi, China

Jinjian Fu

Contributed equally to this work with: Xingmin Wang, Jinjian Fu, Qianxi Li

Affiliation: Department of Gynecology, Liuzhou Municipal Maternity and Child Healthcare Hospital, Liuzhou, Guangxi, China

Qianxi Li

Contributed equally to this work with: Xingmin Wang, Jinjian Fu, Qianxi Li

Affiliation: Department of Gynecology, Liuzhou Municipal Maternity and Child Healthcare Hospital, Liuzhou, Guangxi, China

Dingyuan Zeng

* E-mail: [email protected]

Affiliation: Department of Gynecology, Liuzhou Municipal Maternity and Child Healthcare Hospital, Liuzhou, Guangxi, China

Introduction

Multiple epidemiological studies have demonstrated that homocysteine is an important biomarker with biological functions in the folate metabolism pathway. Hyperhomocysteinemia (HHCY) is a medical health problem characterized by elevated homocysteine concentrations in the plasma that has been identified as a key pathophysiological risk factor for a series of adverse events, including neural-tube defects, vascular dementia, pregnancy complications, cancers and psychiatric disorders [1–4]. Previous studies have revealed that the regulation of the plasma levels of homocysteine are quite complex and involve both environmental factors (such as folate acid and vitamin B₁₂ intake) and hereditary components [5]. However, how a number of genes and hereditary determinants might contribute to HHCY remains unclear. Mutations in some key genes encoding homocysteine-metabolizing enzymes, such as methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C and methionine synthase reductase (MTRR) A66G, may contribute to the risk of the development of hyperhomocysteinemia and thus leas to clinical disorders [6].

The enzyme MTHFR catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyletetrahydrofolate, which is the carbon donor for the methylation of homocysteine to methionine [7]. The C677T polymorphism is a point mutation at position 677 of the MTHFR gene that causes the substitution of alanine with valine, which leads to a reduction in enzyme activity and causes mild to moderate hyperhomocysteinemia and reduces plasma folate levels. Genome-wide association studies (GWASs) have confirmed the association between the MTHFR C677T genotype and homocysteine levels in healthy populations [8]. Along with those investigations, several studies have proposed that double 677CT/1298AC heterozygosity can result in a reduction in enzymatic activity that represents an important risk factor for congenital anomalies, particularly in patients with low blood folate and vitamin B₁₂ concentrations [9].

The frequency distributions of MTHFR and MTRR polymorphisms, especially C677T, vary substantially between different regional and ethnic groups. For example, the frequency of the 677T allele has been found to be highest in north India (16.7%) and lowest was in east India (1.1%). Moreover, the highest frequency of the 677TT genotype has been found in the Rajput population (7.8%), and this genotype is absent in the Kom, Meitei, Paite, Thadou, Kabui, Munda, Oraon and Naikda population groups in India [10]. A number of studies have investigated the C677T and A1298C in MTHFR and A66G in MTRR polymorphisms in different ethnic and geographical regions in Chinese general populations, however, the results have been irreproducible and inconclusive [11].

Accurate information about the geographical and ethnic distributions of the alleles and associated genotypes of MTHFR and MTRR in Mainland China will enable the design of proper interventions (e.g., folic acid supplementation) in the general population to reduce the rates of some medical diseases [12].

We conducted this comprehensive meta-analysis that integrated multiple studies to provide an overall assessment of the key polymorphisms in the major folate pathway genes among general Chinese populations. Sex-stratified and northern-central-southern gradients in the heterozygosities and allele frequencies were also assessed.

Materials and Methods

Literature database

The following major electronic literature databases were searched in September 2015 without language restrictions: PubMed, the Chinese National Knowledge Infrastructure (CNKI), the Chinese Wanfang Database, the Chinese VIP Database, and Google Scholar. The keywords and medical subject headings “MTHFR”, “MTRR”, “methylenetetrahydrofolate reductase”, “methionine synthase reductase”, “folate pathway’, “polymorphisms” or “SNP”, and “Chinese” or “China” were used to scan for potentially relevant studies.

Inclusion/exclusion criteria

The identified studies were eligible for inclusion if they met the following criteria: (1) published in Chinese or English, (2) the study participants were general Chinese populations who lived in Mainland China, (3) the evaluation of data related to any or all of the polymorphisms in MTHFR or MTRR in general Chinese populations, and (4) included data regarding genotype/allele counts of the C677T, A1298C and A66G polymorphisms among the population for the estimation of the frequencies and 95% confidence intervals (95% CIs). Studies were excluded if they met the following criteria: (1) reviews, lectures, editorials or correspondence letters, (2) the study participants were evaluated in terms of folate pathway gene polymorphisms associated with relevant diseases, (3) duplicated studies were eliminated, and only recently published studies were ultimately selected, and (4) if the same data were published in English and Chinese, only the English-language articles were included.

Data extraction

Two authors (XM Wang and JJ Fu) independently extracted the following information: the first author’s name, publication year, investigated location, ethnic groups, age, sample source, sample size, genotyping method, genotype distribution, frequency, and 95% CI.

Statistical analysis

The Hardy-Weinberg equilibrist (HWEs) were evaluated to determine whether the MTHFR C677T and A1298C and MTRR A66G genotype distributions were in genetic equilibrium (threshold set to 0.05) [13]. Meta-analyses of the prevalences of the allele frequencies (e.g., C677T: TT vs. total genotypes) and allele contrasts (e.g., C677T: T vs. total C+T) were performed using Stata statistical software version 13.0 (Stata corporation LP, College Station, Texas, USA). A random-effects model was used to account for the pooled relevant genotype frequencies and their corresponding 95% CIs. Stratified analyses were performed according to the northern-central-southern gradient, ethnicity, and sex. The heterogeneity among the studies was evaluated with the Cochrane chi-square (x²) and quantified with the I² statistic [14–15]. Publication bias was evaluated using Begg’s funnel plots and Egger’s test (significant at P≤0.1) [16–17].

Results

Characteristics of the studies

In total, 495 articles were identified, of which 471 potentially relevant citations were included for further evaluation. Eventually, 68 articles (66 on the C677T, 51 on the A1298C, and 43 on the A66G polymorphisms) with a total of 92277 participants met the inclusion criteria [18–85] (Fig 1). The main characteristics of the studies on the MTHFR C677T and A1298C and MTRR A66G polymorphisms are listed in Tables 1–3, respectively. In the majority of the studies, buccal cells were obtained and tested with real-time polymerase chain reaction (RT-PCR); otherwise, blood samples were tested with restriction fragment length polymorphism (RFLP) analysis. The genotype frequencies indicated that all of the polymorphisms were in HWE in all of the samples.

[Figure omitted. See PDF.]

Fig 1. Flow chart of the study selection process.

https://doi.org/10.1371/journal.pone.0152414.g001

[Figure omitted. See PDF.]

Table 1. Distribution of the MTHFR C677T polymorphism among populations in China.

https://doi.org/10.1371/journal.pone.0152414.t001

[Figure omitted. See PDF.]

Table 2. Distribution of the MTHFR A1298C polymorphism among populations in China.

https://doi.org/10.1371/journal.pone.0152414.t002

[Figure omitted. See PDF.]

Table 3. Distribution of the MTRR A66G polymorphism among populations in China.

https://doi.org/10.1371/journal.pone.0152414.t003

Pooled frequencies of the allele genotypes of the three gene polymorphisms in the Chinese general population

Table 4 illustrates the summarized national estimates of the 677TT and 677T frequencies among healthy populations from 1998 to 2015. Taking all populations together, the frequencies of the 677TT genotype and the 677T allele in the healthy Chinese population were 20% (18%-23%) and 42% (38%-45%), respectively (S1 and S2 Files). Overall, the combined estimated frequencies of the 1298CC genotype and the 1298C allele in the healthy Chinese population were 5% (4%-5%) and 20% (18%-22%), respectively(S3 and S4 Files). The average frequencies of the 66GG genotype and the 66G allele in the healthy Chinese population were 7% (6%-7%) and 26% (25%-28%), respectively(S5 and S6 Files).

[Figure omitted. See PDF.]

Table 4. Summarized prevalence with 95% confidence intervals of genetic polymorphisms in the folate pathway among Chinese populations.

https://doi.org/10.1371/journal.pone.0152414.t004

Geographical distributions of the three polymorphisms in the folate pathway

The allele and genotype frequencies of the three polymorphisms according to geographical region are given in Table 5. The genotype frequencies of the MTHFR C677T and 677T alleles and the 677TT genotype frequency exhibited increases in the southern-central-northern direction in Mainland China. The frequencies of the 677T allele and the 677TT genotype increased from lower values (5% and 17%, respectively) in Guangxi, to intermediate values (12% and 32%, respectively) in Anhui, to higher values (39% and 62%, respectively) in Shandong. Taken together, the frequencies of the 677TT genotype and the 677T allele along the geographical gradient were 7% (5%-8%) and 25% (23%-27%) in southern, 19% (16%-21%) and 41% (36%-45%) in central, and 28% (25%-31%) and 53% (51%-55%) in northern China, respectively. There were significant geographical gradients in the variations in the frequencies of the 677T allele and 677TT genotype (both P values≤0.001).

[Figure omitted. See PDF.]

Table 5. Summarized prevalence with 95% confidence intervals of genetic polymorphisms in the folate pathway with geographical distribution among Chinese populations.

https://doi.org/10.1371/journal.pone.0152414.t005

The frequency of the MTHFR A1298C polymorphism exhibited the reverse trend; i.e., this frequency decreasing from southern to central to northern China. The pooled geographical gradient frequencies of the 1298C allele and 1298CC genotype were found to be 28% (24%-31%) and 7% (5%-9%) in southern, 18% (17%-19%) and 4% (3%-4%) in central, and 17% (16%-19%) and 3% (2%-3%) in northern China, respectively (Table 5). There were significant geographical gradients in the frequencies of the 1298C allele and 1298CC genotype (both P values≤0.001).

The mean frequencies of the MTRR 66G allele and 66GG genotype decreased from 29% (28%-30%) and 8% (6%-10%) in southern China, to 25% (23%-27%) and 6% (6%-7%) in central China, and 24% (23%-25%) and 6% (5%-6%) in northern China (Table 5) in a pattern similar to that observed in the gradients of the MTHFR 1298C allele and 1298CC genotype frequencies (both P values≤0.001).

The frequencies of the MTHFR C677T, A1298C, and MTRR A66G polymorphisms by ethnicity

The allele and genotype distributions of MTHFR and MTRR by ethnicity are presented in Table 6. The distributions of the MTHFR 677T allele and the 677TT genotype exhibited ethnic variations (with both P values≤0.001). The 677T allele frequencies in the minority groups (e.g., Miao, Zhuang, She, Shui, etc.) and Chinese Han were 28% (25%-31%) and 45% (41%-49%), respectively. The 677TT genotype frequencies in the minority groups and Chinese Han were 5% (4%-6%) and 22% (20%-25%), respectively.

[Figure omitted. See PDF.]

Table 6. Summarized prevalence with 95% confidence intervals of genetic polymorphisms in the folate pathway with ethnicity distribution among Chinese populations.

https://doi.org/10.1371/journal.pone.0152414.t006

In contrast to C677T, the distribution of the A1298C polymorphism by ethnicity demonstrated the reverse trend: the 1298C allele was much more common among the minority groups [26%, (23%-30%)] than the Chinese Han [19% (17%-20%); P value≤0.001]. The 1298CC genotype exhibited similar variability with frequencies of 7% (5%-9%) in the minority groups and 4% (3%-5%) in the Chinese Han (P value ≤0.001).

The frequencies of the MTRR 66G allele and 66GG genotype varied by ethnic group and geographical location. The frequency of the 66G allele was slightly higher among the minority groups [35% (35%-36%)] compared with 25% (24%-26%) among the Chinese Han group (P value ≤0.001). The frequencies of the 66GG genotype were 10% (8%-12%) in the minority groups and 6% (5%-7%) in the Chinese Han group, which were similar to those of the MTHFR A1298C polymorphism (P value ≤0.001).

The frequencies of MTHFR C677T, A1298C and MTRR A66G polymorphisms by sex

Table 7 provides the pooled frequencies of the variant alleles and genotypes of MTHFR C677T and A1298C and MTRR A66G according to sex. A total of 88255 samples with reported C677T polymorphisms were obtained. Based on all these samples, we did not find any difference between the males [19% (12%-25%)] and females [21% (19%-24%)] in terms of 677TTgenotype frequency.

[Figure omitted. See PDF.]

Table 7. Summarized prevalence with 95% confidence intervals of genetic polymorphisms in the folate pathway with sex distribution among Chinese populations.

https://doi.org/10.1371/journal.pone.0152414.t007

Only 41 studies reported the frequency of the MTHFR A1298C polymorphism and included 82532 females of reproductive age. The 1298C allele and 1298CC genotype frequencies in females were 19% (18%-21%) and 4% (4%-5%), respectively. Among the 43 articles that reported on the MTRR A66G polymorphism, 42 studies included 84416 females. The 66G allele and 66GG genotype frequencies in females were 26% (25%-27%) and 7% (6%-7%), respectively.

Publication bias

Tables 4–7 presents information related to heterogeneity and publication bias. We noted significant heterogeneity within the studies and the subgroups (all P values were ≤0.001, I² = 100.0).

Discussion

Methylenetetrahydrofolate reductase (MTHFR) (C677T and A1298C) and methionine synthase reductase (MTRR) mutations (A66G) cause mild hyperhomocysteinemia and low folate level and are associated with several disorders. The geographical and ethnic distributions of these alleles and the associated genotypes are important to study worldwide.

The frequencies of the MTHFR C677T and A1298C and MTRR A66G polymorphism in 68 epidemiological studies covering 23 provinces in Mainland China were pooled and investigated in the present study. Currently, there is a lack of national data regarding the prevalences of gene polymorphisms in the folate metabolism pathway in healthy general populations in China. We documented distinctive geographical and ethnic variations in the frequencies of the C677T and A1298C polymorphisms of the MTHFR gene and the A66G polymorphisms of the MTRR gene among nation-wide samples in China.

Worldwide data have revealed that significant heterogeneities in the frequencies of the T allele and TT homozygosity exist in every population and even with racial groups. One investigations conducted in Texas reported that the frequency of the 677T was lowest among African-Americans (11.9%), followed by in Caucasians (32.7%) and Ashkenazi Jews (47.7%), and the highest frequency exists among the Hispanic population (47.9%) [86]. In the Chinese Han, the frequencies of the 677T allele have been found to be lowest in Hainan (24.0%) followed by Hubei (40.3%) and Jiangsu (43.5%), and the highest frequency has been observed in Shandong (63.1%) [51].

Population genetic comparisons provide an appropriate method for picturing geographical and ethnic variations and can suggest that environmental factors may exert selective pressures on genetic mutations. A north-to-south increase in the frequency of the 677T allele has been observed in Europe [87]. North-to-south increases in dietary folate intake have also been encountered in European populations [88]. Thus adequate folic acid intakes have presumed enabled increase in the MTHFR 677T frequency in these populations [89]. Economic and dietary habits might have played important roles in the spread of the 677T allele worldwide. For example, the frequency of the 677T allele is high in the USA with an average frequency of 36.2% in Texas [86]. Another study conducted in India observed the highest frequency of the 677T allele among the Sindhi population (23.8%). In contrast, the 677T allele is absent in the Kom, Thadou and Munda populations, and its average frequency is 10.1% across all 23 populations in India [10]. The low frequencies of the 677T allele among the tribal groups (i.e., the Kom, Thadou and Munda populations) may have been influenced by folate deficiencies because the majority of the population in India has a vegetarian diets that is low in vitamin B₁₂ [10]. The populations of America carried higher frequencies of the 677T allele, which may be related to abundant nutritional statuses and particularly with folic acid and vitamin B₁₂ supplementation, which are associated with low levels of homocysteinemia. Across all 23 of the studied provinces, we observed increases in the 677T allele and 677TT genotype frequencies in the southern-central-northern direction across Mainland China. Because high 677T allele and 677TT genotype frequencies were observed in the northern populations, we assumed that the folic acid intakes are greater in the northern populations than in the southern populations; however, the opposite pattern has been observed in nutritional studies. One such nutritional investigation revealed that the geometric mean of the blood folate concentration is lower in the northern populations than the southern populations [90].

Worldwide epidemiological data have revealed that the frequency of A1298C homozygosity varies from continent to continent. The frequencies of the 1298C allele range from 18% to 70% in East Asia, 17% to 44% in Asia, 24% to 40% in Europe, 0% to 15% in South America and 14.7% in North America [91]. The present data revealed variation in the frequency of the 1298C allele within China. In contrast to the distribution of 677T, the frequency of the 1298C allele was found to be the lowest in northern China [18% (17%-19%)], intermediate in central China [18% (17%-19%)], and highest in southern China [28% (24%-31%)]. The mean frequency of the 1298C allele was 20% (18%-22%).

Based on all 8 of the investigated minority ethnic populations (e.g., the She, Xibo, and Uygur), the minority ethnic populations seemed to carry greater 1298C allele frequencies than the Chinese Han population. Notably, the frequency of the 1298C allele has been reported to vary between different ethnic populations worldwide, and the lowest frequency has been found in Indians (10%) [92] followed by the Chinese (18.4%) [51] and Tamils (35%) [93], and the highest frequency has been observed in the Lebanese [94].

Although A1298C homozygotes do not exhibit elevated blood homocysteinemialevels, many investigations have revealed that compound heterozygotes for C677T/ A1298C may be at risk for hyperhomocysteinemia and low folate levels, which can contribute to many disorders, such as neutral tube defects [6] and abortions [95].

Because lifestyle and environmental factors, such as folate supplementation, vary across different ethnic populations and may influence the frequencies of the C677T and A1298C alleles, these factors cannot be ruled out when considering the influences of environmental-genetic interactions on the distributions of MTHFR gene polymorphisms.

Our pooled data revealed that the frequencies of the 66G allele and 66GG genotype exhibited variations across geographical gradients and ethnic populations. Globally, the distributions of the MTRR 66G allele and 66GG genotype frequencies also exhibit geographical and ethnic variations. For example, the frequencies of the 66G allele have been reported to be 58% in the Yadav, 62% in the Scheduled Castes, and 71% in the rural Sunni Muslim population in Uttar Pradesh in India [96,97]. Our study observed a 66GG genotype frequency of 7% across Mainland China, which is much lower than those in Brazil (23%), Australia (10%), and Ireland (17.5%) [98–100]. MTRR is involved in the homocysteine and folate metabolic pathway via its activation of methionine synthase via reductive methylation and is consequently a critical determinant of homocysteinemia levels [101]. Therefore, the MTRR A66G mutation may indirectly contribute to many medical disorders, such as neural tube defects and congenital heart disease [102], due to its key role in the folate metabolism pathway. However, due to limited sample sizes and the lower frequency of studies of the A66G polymorphisms in MTRR, no solid evidence has been found to relate the MTRR A66G variant with the risks of diseases. Long-term data and larger sample sizes are necessary to determine the real connections between the distribution of the A66G variant and the risks of diseases.

Conclusions

In conclusion, our meta-analysis revealed significant geographical variations in the frequencies of the MTHFR C677T and A1298C and MTRR A66G polymorphisms in the folate metabolism pathway between different ethnic populations in China. Our findings provide an overall picture of these three genetic polymorphisms in the folate metabolism pathway among the general populations in Mainland China, and these evidence-based genomic data should be integrated into medical and public health practices.

Supporting Information

[Figure omitted. See PDF.]

S1 File. The average frequencies of the 677TT genotype in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s001

(TXT)

S2 File. The average frequencies of the 677T allele in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s002

(TXT)

S3 File. The average frequencies of the 1298CC genotype in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s003

(TXT)

S4 File. The average frequencies of the 1298C allele in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s004

(TXT)

S5 File. The average frequencies of the 66GG genotype in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s005

(TXT)

S6 File. The average frequencies of the 66G allele in the healthy Chinese population.

https://doi.org/10.1371/journal.pone.0152414.s006

(TXT)

Author Contributions

Conceived and designed the experiments: DYZ. Performed the experiments: XMW JJF. Analyzed the data: QXL JJF. Contributed reagents/materials/analysis tools: XMW JJF QXL DYZ. Wrote the paper: XMW JJF. Guided the writing: QXL. Revised the manuscript: DYZ.

Citation: Wang X, Fu J, Li Q, Zeng D (2016) Geographical and Ethnic Distributions of the MTHFR C677T, A1298C and MTRR A66G Gene Polymorphisms in Chinese Populations: A Meta-Analysis. PLoS ONE 11(4): e0152414. https://doi.org/10.1371/journal.pone.0152414

References

1. Brustolin S, Giugliani R, Félix TM.Genetics of homocysteine metabolism and associated disorders.Braz J Med Bir Res 2010 43: 1–7.

2. Hogeveen M, Blom HJ, den Heijer M. Maternal homocysteine and small-for-gestational-age offspring: systematic review and meta-analysis. Am J ClinNutr2012 95: 130–6.

3. Martí-Carvajal AJ, Solà I, Lathyris D, Karakitsiou DE, Simancas-Racines D. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane DatabeseSyst Rev 2013 31: CD006612.

4. Ferretti A, Parisi P, Villa MP.The role of hyperhomocysteinemia in neurological features associated with celiac disease. Med Hypotheses 2013 81: 524–31. pmid:23891042

5. Yakub M, Moti N, Parveen S, Chaudhry B, AzamI ,Iqbal MP. Polymorphisms in MTHFR, MS and CBS genes and homocysteine levels in a Pakistani population.PLoS One 2012 7(3): e33222. pmid:22470444

6. Zhang T, Lou J, Zhong R, Wu J, Zou L, Yu S, et al. Genetic variants in the folate pathway and the risk of neural tube defects: a meta-analysis of the published literature. PLoS One 2013 8(4): e59570. pmid:23593147

7. Goyette P, Sumner JS, Milos R, Duncan AM, Rosenblatt DS, Matthews RG, et al. Human methylenetetrahydrofolate reductase: isolation of cDNA, mapping and mutation identification. Nat Genet 1994 7(4): 195–200.

8. Lange LA, Croteat-Chonka DC, Marvelle AF, Qin L, Gaulton KJ, Kuzawa CW, et al. Genome-wide association study of homocysteine levels in Filipinos provides evidence for CPS1 in women and a stronger MTHFR effect. Hum Mol Genet 2010 19(10): 2050–8. pmid:20154341

9. van der Put NM, Gabreels F, Stevens EM, Smeitink JA, Trijbels FJ, Eskes TK,et al. A second common mutation in the methylenetetrahydrofolate reductase gene: an additional risk factor for neural-tube defects? Am J Hum Genet 1998 62(5): 1044–1051. pmid:9545395

10. Saraswathy KN, Asghar M, Samtani R, Murry B, Mondal PR, Ghosh PK, et al. Spectrum of MTHFR gene SNPs C677T and A1298C: a study among 23 population groups of India. MolBiol Rep 2012 39(4): 5025–31.

11. Yan ZM, Lu YQ, Li Y, Chen X,Xu SZ. Polymorphisms of MTHFR and MTRR among the Han nationality women in Qionghai city. Hai Nan Yi Xue Yuan XueBao2013 19(1): 18–20.

12. Chen PL, Li WT, Wang J, Jiang YD, Wu P, Chen T, et al. Association between MTHFR gene polymorphisms (C677T, A1298C) and genetic susceptibility to prostate cancer: a meta-analysis. Genet Mol Res 2015 14(4):19191–202. pmid:26782572

13. Ott J. Utility programs for analysis of genetic linkage, Program, HWE version 1.10. New York: Columbia University; 1988.

14. Higgins JP, Thompson SG.Quantifying heterogeneity in a meta-analysis. Stat Med 2002 21(11): 1539–1558. pmid:12111919

15. Higgins JP, Thompson SG, Deeks JJ, Altman DG.Measuring inconsistency in meta-analyses. BMJ 2003 327(7414): 557–560. pmid:12958120

16. Begg CB, Mazumdar M. Operating characteristics of a rank correlation test for publication bias. Biometrics 1994 50(4): 1088–1101. pmid:7786990

17. Egger M, Davey Smith G, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. BMJ 1997 315(7109): 629–634. pmid:9310563

18. Yu JM, Chen BB, Zhang GY, Fu SB, Li P. The 677C>T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene in five Chinese ethnic groups. Hum Hered 2000 50(4): 268–270. pmid:10782023

19. Xue YL, Yu JM, Wan Q, Fu SB, Cheng F, Huang CB, et al. Distribution of a common methylenetetrahydrofolate mutation in six Chinese population groups. Anthropol Anz 2000 58(3):253–257. pmid:11082782

20. Pei LJ, Zhu HP, Shen WY, Zhao RB, Dao JJ, Li Z, et al. Comparing the distribution of genetic polymorphism of MTHFR thermolabile between Mongolian population and Hans of China. Yi Chuan 2000 22(6): 369–371.

21. Zhu HP, Zhao RB, Dao JJ, Gao Q. Genetic polymorphism of homocysteine metabolic enzymes among populations of minority ethnicities in north China. ZhongGuo You Sheng You Yu 2000 11(2):51–53.

22. Yang BH, Liu YZ. Determination of methylenetetrahydrofolate reductase genotypes of the residents of Han nationality in South Anhui by Polymerase-Chain-Reaction-restriction fragment length polymorphism. Wan Nan Yi Xue Yuan XueBao 2001 20(4):308–309.

23. Chen SQ, Zuo BS, Deng WY, Fang MH, Yao QJ.Genetic polymorphysim of MTHFR thermolabile in the Hans of Guangdong.ZhongGuo You Sheng Yu Yi ChuanZaZhi 2002 10(4):22–23.

24. Liang JN, Chen SQ. Detection of the methylenetetrahydrofolate reductase gene in 143 individuals. Guangzhou Yi Xue Yuan XueBao 2002 30(1):38–40.

25. Sun WP, Wan Q, Su MQ. Two genetic polymorphysims of 5,10- methylenetetrahydrofolate reductase among the Han people in Xi’an area. Di Si Jun Yi Da XueXueBao 2003 24(7):634–637.

26. Shen LP, Chou XQ, Zeng XY, Wei JH, Zhong QA. Association between the gene polymorphism of methylenetetrahydrofolate reductase and pregnant outcomes in Guangxi healthy women of reproductive age.ZhongGuo Lin Chuang Kang Fu 2005 9(27): 8–9.

27. Xiao Y, Shan KR, Li Y, Zhao Y, Qi XL, Xie Y, et al. Comparing the distribution of genetic polymorphism of MTHFR reductase between Han populations of Libo and Miao of Leishan in Guizhou. ZhongGuo You Sheng Yu Yi ChuanZaZhi 2005 13(5):14–16.

28. Xiao Y, Shan KR, Li Y, Zhao Y, Guan ZZ, et al. 5,10- methylenetetra- hydrofolate reductase polymorphism in three nationalities in Guizhou Province. ActaNatropologicaSinica 2005 24(4): 315–318.

29. Xiao Y, Shan KR, Li Y, Zhao Y, Qi XL, Xie Y, et al. 5,10- methylenetetrahydrofolate reductase polymorphism in three nationalities of Guizhou in China. Zhong Hua Yi Xue Yi ChuanXueZaZhi 2005 22(2): 219–221.

30. Zhang CS, Pan XD, Liu CW. Genetic polymorphism of methylenetetrahy- drofolate reductase in Han people in Qingdao. Qi Lu Yi XueZaZhi 2005 30(4): 310–312.

31. Li AF, Xu YM, Zheng SF, Song B. Detection of polymorphism of methylenetetrahydrofolate reductase of Han population in Henan. ZhongGuo You Sheng Yu Yi ChuanZaZhi 2007 15(10): 19–20.

32. Dai XJ, Zhong HJ, Jiao HY, Peng L, Chen YT, Dang J, et al.Study of the genetic polymorphism of MTHFR C677T between Hui and Han population in Ningxia. Ning Xia Yi Xue Yuan XueBao 2008 30(2): 152–154.

33. Mao RF, Fan YH, Chen F, Sun DL, Bai J, Fu SB, et al. Methylenetetrahydrofolate reductase gene polymorphism in 13 Chinese ethnic populations. Cell BiochemFunct 2008 26: 352–358.

34. Chen F, Wang Y, Xun YM, Zheng H, Song B, He Y, et al. Polymorphisms of MTHFR among Han population in Henan area. Zheng Zhou Da XueXueBao (Yi Xue Ban) 2009 44(2): 317–320.

35. Shan KR, Xiao Y, Chen Z. He Y, Zhang T, Li Y, et al. Study of gene polymorphism of CBS and MTHFR in Miao of Guizhou.ZhongGuo You Sheng Yu Yi ChuanZaZhi2009 17(4): 16–18.

36. Chen YX, Wu YJ, Shen LL. Polymorphism of methylenetetrahydrofolate reductase and cystathionine beta-synthase in child-bearing women. ZhongGuo Gong Gong Wei Sheng 2010 26(8): 958–959.

37. He XM, Zhang Q, Yang Q, Zheng FF, You FZ, Lao HH, et al. Study on the methylenetetrahydrofolate reductase and methionine synthase reductase polymorphism. ZhongGuoJi Hua Sheng Yu XueZaZhi 2010 1: 13–18.

38. Jiang HO. Detection of genetic polymorphism of MTHFR of Han population in Huaihua.ZhongGuo You Sheng Yu Yi ChuanZaZhi 2010 18(5): 18–19.

39. Zhang QF, Lin YL, Xue YY, He XM.Determination of capacity of folate utilization of pregnant and lying-in women.ZhongGuo Re Dai Yi Xue 2010 10(11): 1346–1351.

40. Zhang L, Yin RX, Liu WY, Miao L, Wu DF, Aung LHH, et al. Association of methylenetetrahydrofolate reductase C677T polymorphism and serum lipid levels in the Guangxi Bai Ku Yao and Han populations. Lipids Health Dis 2010 9(10):123.

41. Lao HH, He XM. Study on methylenetetrahydrofolate reductase and methionine synthase reductase gene polymorphism among the Han and Li women in Hainan Province. ZhongGuoJi Hua Sheng Yu XueZaZhi 2011 19(11): 655–657.

42. Zhang Y, Zhu ZY, Wang W, Liu L, Bi J, Wang DC, et al. Polymorphism of MTHFR and MTRR among Han women in Sichuan. ZhongGuoXun Zheng Yi XueZaZhi 2012 12(6): 631–634.

43. Wu HZ, Zhao YL, Wang YJ, Wu SW. Polymorphism in the 5,10- methylenetetrahydrofolate reductase gene in Bengbu Han population. BaoTou Yi Xue Yuan XueBao 2011 27(2): 4–7.

44. He YX, Gong JM, Shen Y, Jia LT, Li XP, Wang Y, et al. The study of folic acid usage in women of child bearing age in Henan province. Shi Yong Jian Yan Yi Shi ZaZhi2012 4(1): 6–10.

45. Yang Y, Lu YQ, Rui XY, Cao XY, Xu XF, Tao Y. Study on the MTHFR and MTRR polymorphism among the Han women in Zhenjiang city. Nan Jing Yi Ke Da XueXueBao 2012 32(9): 1250–1253.

46. Cong YY, Lu YQ, Rui XY, Cao XL, Wang QY. Study on the methylenetetrahydrofolate reductase and methionine synthase gene polymorphism among the Han women in Zibo city. Xian Dai Fu Chan KeJin Zhan 2012 21(10): 779–781.

47. Zhang YL, Lu YQ, Li HF, Rui XY, Zhang LJ, Wu CY, et al. Methylenetetrahydrofolate reductase and methionine synthase gene polymorphisms in ethnic Han women from Linyi. Zhong Hua Yi Xue Yi ChuanXueZaZhi 2012 29(6): 705–708.

48. Cheng HB, Chen YX, Wu YJ, Zhang XY. Study on level of plasma homocysteine and polymorphism of methylenetetrahydrofolate reductase in old population. Chang Zhi Yi Xue Yuan XueBao 2012 26(6): 401–403.

49. Gao LJ, Lu YQ, Rui XY, Lin XB. Polymorphism distribution of methylenetetrahydrofolate reductase and methionine synthase reductase among the Han women in Huizhou.Zhong Shan Da XueXueBao (Medical Science) 2013 34(1): 140–143.

50. Du LL, Lu ZP, Liu CW, Luo H, Hu CY, Yang Y, et al. Association between the MTHFR C677T polymorphism and serum lipid levels and longevity in the long-lived cohort in Guangxi, Hongshuihe river basin. ZhongGuo Dong Mai Zhou Yang Ying Hua ZaZhi 2013 24(4): 325–330.

51. Yang BY, Liu YY, Li YF, Fan SJ, Zhi XY, Lu XX, et al. Geographical distribution of MTHFR C677T, A1289C and MTRR A66G gene polymorphisms in China: findings from 15357 adults of Han nationality. PLoS One 2013 8(3):e57917. pmid:23472119

52. Wang LN, Pu XM. Polymorphism of 5,10-methylenetetrahydrofolate reductase gene in Xinjiang Uygur and Han nationality. Zhong Hua Pi Fu KeZaZhi 2012 45(3): 178–180.

53. Xiu X, Mou YY, Tao HJ, Zhao JH, Lian WJ. Folic acid usage in women of child bearing age in Weifang. Shan Dong Yi Yao 2013 53(9): 31–33.

54. Chen YX, Cheng HB, Wu YJ, Zhang XY. Study on polymorphism of methylenetetrahydrofolate reductase gene and concerntration of homocysteine in Changzhi population. Shan Xi Yi Ke Da XueXueBao 2013 44(2): 130–133.

55. Wang WP, Lu YQ, Rui XY, Yi NH. Study on the distribution of gene polymorphism of methylenetetrahydrofolate reductase and methionine synthase reductase among the Han women in Hubei province. ZhongGuo Fu You Bao Jian 2013 28(1): 140–142.

56. Guo H, Hu XW, Wang B. Analysis of factors impacting the gene polymorphism of methylenetetrahydrofolate reductase and methionine synthase reductase. Jian Yan Yi Xue Yu Lin Chuang 2013 10(13): 1655–1657.

57. Zhang T, Xie Y, Li Y, Shan KR, Guan ZZ. Methylenetetrahydrofolate reductase polymorphism in three nationality in Guizhou. Chongqing Medicine 2013 42(28): 3413–5.

58. Huang GX, Lu YQ, Li Y, Wang X, Xie HY, Chen CB. Polymorphisms of MTHFR and MTRR among Han and Li nationality women in Lingshui county, Hainan. China Tropical Medicine 2013 13(12): 1446–8.

59. Luo XL, Lu YQ, Guo WZ, Li FC, Wei ZG, Fu M, et al. Geographical distribution of MTHFR C677T, A1298C and MTRR A66G gene polymorphisms among the Han gestational age women in Songzi city. J of Pub Health and Prev Med 2014 25(2): 29–32.

60. Wang FX, Ding LL, Zou AM, Chen BL. Study on the Distribution of gene polymorphism of folate metabolism-related enzyme gene among the Han women in Shaanxi Province. J Mod Lab Med 2014 29(1): 35–8.

61. Hao YY, Zhao H, Lu YQ, Li Y, Ma EHB, Yang Q. Comparison of single nucleotide polymorphisms of MTHFR gene and MTRR gene between Uighur women and Han women. ZhongGuo Fu You Bao Jian 2014 29: 3495–7.

62. Yan Q, Lu YQ, Li Y, Shi CH, Jiang L, Yu CT. Polymorphisms of MTHFR and MTRR in women of Chinese Han population in Yantai city. JOURNAL OF SHANDONG UNIVERSITY (HEALTH SCIENCES) 2014 52(1): 79–85.

63. Xing JF, Jia LT, Yuan EW, Zhang X, Meng XY, Hu YF.Genetic polymorphism impact lowering homocysteine with folic acid in childbearing age women. J Medical Forum 2014 35(5): 10–1.

64. Hu XW, Wang WP. Polymorphisms of two loci in MTFHR gene and the relationships with MTRR gene polymorphism.ZhongGuo Fu You Bao Jian 2015 25: 4336–9.

65. Jia XP, Lu YQ, Ma SJ, Fang ZD, Zhou F, Yang Q. Genetic polymorphism of methylenetetrahydrofolate reductase and methionine synthase in Han ethnic women in Pixian county of Chengdu city. J of Wannan Medical College 2015 34(4): 317–21.

66. Huang QH, Lu YQ, Ma SJ, Xue Y, Yang Q. Distribution of gene polymorphisms of methylenetetrahydrofolate reductase and methionine synthase among the Han women in Jingang Town. Chin J FamPlann 2015 23(1): 15–7.

67. Li JH, Lu YQ, Ma SJ, Zhao YJ, Yang Q. Distribution of gene polymorphisms of methylenetetrahydrofolate reductase and methionine synthase among the Han women in Langfang City. Basic& Clinical Medicine 2014 11(11): 1564–5.

68. Xiang CG, Lu YQ, Ma SJ, Xue Y, Xin YL, You RF, et al. Analysis of MTHFR and MTRR gene polymorphisms of Han women in 656 cases. Shan Dong Yi Yao 2015 55(14): 22–5.

69. Jiang W, Lu YQ, Li Y, Shen YC, Huang WT, Qin D, et al.A comparison of single nucleotide polymorphisms of MTHFR and MTRR in women between Chinese Zhuang and Han population in Nanning City. Guang Xi Yi Xue 2014 36(11): 1517–9.

70. Ouyang QQ, Zhao W, Li S, Yu DY. Genetic polymorphism of methylene-tetrahydrofolate reductase c667t in han women of childbearing age in Qingdao. ZhongGuo You Sheng Yu Yi ChuanZaZhi 2014 22(6):6–7.

71. Chen XL, Lu YQ, Ma SJ, Tang YP, Yang Q. Study on the distribution of gene polymorphism of methylenetetrahydrofolate reductase and methionine synthase among Han women in Jiyuan City. Chinese Journal of Women and Children Health 2014 5(4): 17–20.

72. Ma LM, Lu YQ, Li Y, Gao H, Chen YQ, Liu TT, et al. Genetic polymorphisms of MTHFR and MTRR among the Han women in Shangzhi City. PractPrev Med 2015 22(3): 289–91.

73. Tang HY, Lu YQ, Li Y, Li DQ, Yang Q. Polymorphisms of MTHFR and MTRR among the Han women in Shouguang City. China Medical Herald 2014 11(23): 8–11.

74. Tian Y, Hu YZ, Lu YQ, Ma SJ, Yang Q. Polymorphism analysis of MTHFR and MTRR among women in Wuxi. Xin Yi Xue2014 45(4): 253–8.

75. Lu GR, Lu YQ, Ma SJ, Ma AJ, Wang P, Wang XL, et al. Polymorphisms of MTHFR and MTRR gene among the Han gestational women in Xinxiang City. Henan Medical Research 2014 23(7): 7–10.

76. Jiao FY, Lu YQ, Li Y, Sun CY, Yu HP, Zhang WW, et al. Gene polymorphisms of MTHFR and MTRR among the women in Muping district. MolDiagnTher 2014 6(2): 105–9.

77. Gao X, Lu YQ, Ma SJ, Yang Q. Geographical distribution of MTHFR C677T, A1298C and MTRR A66G gene polymorphisms among the Han gestational age women in ZhangJiakou city. ZhongGuo You Sheng Yu Yi ChuanZaZhi 2014 22(11): 35–7.

78. Luo SQ, Qiu P, Yan TZ, Yang FH, Liang YT, Chang RN, et al. Study on the MTHFR and MTRR polymorphism among the Miao nationality women in Liuzhou city. ZhongGuo You Sheng Yu Yi ChuanZaZhi 2015 23(9): 20–3.

79. Yu YH, Lu YQ, Li Y, Tang P, Song JX, Feng TL, et al. Polymorphisms of MTHFR and MTRR among the Han women in Yanbian Prefecture. Chin J FamPlann2015 23(9): 591–4.

80. Li XX, Lu YQ, Ma SJ, Gong MX, Yuan YQ, Huang YX, et al.The distribution of gene polymorphism of methylenetetrahydrofolate reductase and methionine synthase among Han women in Zhangjiagang city. Basic & Clinical Medicine 2015 35(9): 1228–31.

81. Wang SY, Lu YQ, Ma SJ, Huang JX, Yang KY, Xiong M, et al. Relationship of plasma homocysteine with gene polymorphisms of MTHFR and MTRR among Han women in Xiangtan City. Tianjin Med J 2015 42(12): 1205–9.

82. Wu WQ, Zhou YZ, Lu YQ, Ma SJ, Yang Q. Gene polymorphisms of MTHFR and MTRR in the women of Han population in Suzhou City. Jiangsu Med J 2015 41(1): 71–3.

83. Mao WC, Lu YQ, Xue Y, Yang Q, Chen SF, Tian M, et al. Study on gene polymorphism of methylenetetrahydrofolate reductase and methionine synthase among the Han, Miao and Tujia women in Guizhou Province. Chin J FamPlann 2015 23(5): 310–3.

84. Cui HL, Lu YQ, Ma SJ, Xue Y, Wang T, Duan GH, et al.Geographical distribution of MTHFR and MTRR gene polymorphisms among the Han women in Zhengzhou city. J Cent South Univ (Med Sci) 2015 40(7): 710–4.

85. Liu XL, Lu YQ, Li Y, Yang YQ, Yang Q. Polymorphisms of MTHFR and MTRR among the Han women in Yinchuan City. Journal of Ningxia Medical University 2014 36(11): 1216–20.

86. Rady PL, Szucs S, Grady J, Hudnall SP, Kellner LH, Nitowsky H, et al. Genetic polymorphisms of methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR) in ethnic populations in Texas; a report of a novel MTHFR polymorphic site, G1793A. Am J Med Genet 2002 107(2):162–8. pmid:11807892

87. Wilcken B, Barnforth F, Li Z, Zhu H, Ritvanen A, Renlund M, et al. Geographical and ethnic variantion of the 677 C>T allele of 5,10- methylenetetrahy- drofolate reductase (MTHFR): findings from over 7000 newborns from 16 areas world wide. J Med Genet 2003 40(8): 619–625. pmid:12920077

88. Guéant-Rodriguez RM, Guéant JL, Debard R, Thirion S, Hong LX, Bronowicki JP, et al. Prevalence of methylenetetrahydrofolate reductase 677T and 1298C alleles and folate status: a comparative study in Mexican, West Africa, and European populations. Am J ClinNutr 2006 83(3): 701–707.

89. Reyes-Engel A, Muñoz E, Gaitan MJ, Fabre E, Gallo M, Dieguez JL, et al. Implications on human fertility of the 677C>T and 1298 A>C polymorphisms of the MTHFR gene: consequences of a possible genetic selection. Mol Hum Reprod 2002 8(10): 952–957. pmid:12356947

90. Hao L, Ma J, Stampfer MJ, Ren A, Tian Y, Tang Y, et al. Geographical, seasonal and gender differences in folate status among Chinese adults. J Nutr 2003 133(11):3630–3635. pmid:14608086

91. Amouzou EK, Chabi NW, Adjalla CE, Rodriguez-Guéant RM, Feillet F, Villaume C, et al. High prevalence of hyperhomocysteinemia related to folate deficiency and the 677C-->T mutation of the gene encoding methylenetetrahydrofolate reductase in coastal West Africa. Am J Clin Nutr 2004 79(4):619–24.

92. Markan S, Sachdeva M, Sehrawat BS, Kumari S, Jain S, Khullar M. MTHFR 677CT/ MTHFR1298CC genotypes are associated with increased risk of hypertension in Indians. Mol Cell Biochem 2007 302(1–2): 125–131. pmid:17333388

93. Angeline T, Jeyaraj N, Granito S,Tsongalis GJ. Prevalence of MTHFR gene polymorphisms (C677T and A1298C) among Tamillians.ExpMolPathol 2004 77(2): 85–88.

94. Sabbagh AS, Mahfoud Z, Taher A, Zaatari G, Daher R, Mahfouz RA.High prevalence of MTHFR gene A1298C polymorphism in Lebanon. Genetic Test 2008 12(1): 75–80.

95. Callejón G, Mayor-Olea A, Jiménez AJ, Gaitán MJ, Palomares AR, Martínez F, et al. Genotypes of the C677T and A1298C polymorphisms of the MTHFR gene as a cause of human spontaneous embryo loss. Hum Reprod 2007 22(12): 3249–54. pmid:17965025

96. Rai V, Yadav U, Kumar P. MTRR A66G polymorphism among two caste groups of Uttar Pradesh (India). Indian J Med Sci 2012 66(5–6): 136–140. pmid:23806987

97. Rai V, Yadav U, Kumar P, Yadav SK. Analysis of methionine synthase reductase polymorphism (A66G) in Indian Muslim population. Indian J Hum Genet 2013 19(2): 183–7. pmid:24019620

98. Barbosa PR, Stabler SP, Trentin R, Carvalho FR, Luchessi AD, Hirata RD, et al. Evaluation of nutritional and genetic determinants of total homocysteine, methylmalonic acid and S-adenosylmethionine/S-adenosylhomocysteine values in Brazilian childbearing-age women. ClinChimActra 2008 388(1–2): 139–47.

99. Szvetko AL, Fowdar J, Nelson J, Colson N, Tajouri L, Csurhes PA, et al. No association between MTHFR A1298C and MTRR A66G polymorphisms, and MS in Australian cohort. J NeurolSci 2007 252(1):49–52.

100. Gaughan DJ, Kluijtmans LA, Barbaux S, McMaster D, Young IS, Yarnell JW, et al. The methionine synthase reductase (MTRR) A66G polymorphism is a novel genetic determinant of plasma homocysteine concentrations. Atherosclerosis 2001 157(2): 451–6. pmid:11472746

101. Yamada K, Gravel RA, Toraya T, Matthews RG. Human methionine synthase reductase is a molecular chaperone for human methionine synthase. Proc Natl AcadSci USA 2006 103(25): 9476–9481.

102. Cai BX, Zhang T, Zhong R, Zou L,Zhu B, Chen W, et al. Genetic variant in MTRR, but not MTR, is associated with risk of congenital heart disease: an integrated meta-analysis. PLoS One 2014 9(3): e89609. pmid:24595101