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Keywords: Bayesian false discovery probability, male infertility, meta-analysis, Methylenetetrahydrofolate reductase, polymorphism
Abstract
Background:
18 previous meta-analyses have been published on the methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C polymorphisms with male infertility risk. However, results of the previous meta-analyses were still inconsistent. Moreover, their meta-analyses did not assess false-positive report probabilities except one study. Furthermore, many new studies have been published, and therefore an updated meta-analysis and re-analysis of systematic previous meta-analyses were performed to further explore these issues.
Objectives:
To determine the association between MTHFR C677T and A1298C polymorphisms and male infertility risk.
Methods:
Crude odds ratios and their 95% confidence intervals were used to assess the association between MTHFR C677T and A1298C polymorphisms and male infertility risk. We used the Bayesian false discovery probability (BFDP) to assess the credibility of statistically significant associations.
Results:
Fifty-nine studies were included concerning the MTHFR C677T and 28 studies were found on the MTHFR A1298C with male infertility risk. Overall, the MTHFR C677T was associated with increased male infertility risk in overall populations, Africans, East Asians, West Asians, South Asians, azoospermia, and Oligoasthenoteratozoospermia (OAT). In further sensitivity analysis and BFDP test, the positive results were only considered as “noteworthy” in the overall population (TT vs CC: BFDP = 0.294, CT + TT vs CC: BFDP = 0.300, T vs C: BFDP = 0.336), East Asians (TT vs CC: BFDP = 0.089, TT vs CT + CC: BFDP = 0.020, T vs C: BFDP < 0.001), West Asians (TT vs CC: BFDP = 0.584), hospital-based studies (TT vs CC: BFDP = 0.726, TT vs CT + CC: BFDP = 0.126), and OAT (TT vs CT + CC: BFDP = 0.494) for MTHFR C677T. In addition, a significantly increased male infertility risk was found in East Asians and population-based studies for MTHFR A1298C. However, we did not find that the positive results were considered as “noteworthy” in the overall and all subgroup analyses for MTHFR A1298C.
Conclusions:
In summary, this study indicates that the MTHFR C677T is associated with increased male infertility risk in East Asians, West Asians, and OAT. No significant association was observed on the MTHFR A1298C with male infertility risk.
1. Introduction
Infertility, defined as the inability to conceive after one year of regular unprotected sexual intercourse by the World Health Organization, has been a major health problem which is multifactorial in nature and affected approximately 15% to 20% of all couples trying for pregnancy.[1–3] Male factors infertility accounts for 40% to 50% about the cases of infertility.[4–5] The etiological factors of male infertility are multifactorial syndrome with a very complex pathogenesis, involving lifestyle, organic diseases, genetic factors, environmental risk factors, and their interactions.[6–8]
Folate play much essential roles for the maintenance of genome integrity in Deoxyribonucleic acid synthesis, repair and methylation.[9] Methylenetetrahydrofolate reductase (MTHFR) gene has the chromosomal locus 1p36.6 and is 2.2 kb in length with a total of 11 exons, which is involved in folate and homocysteine metabolism. A change of C to T at nucleotide 677 in MTHFR C677T (Ala222Val, rs1801133) results in an amino acid substance change of an alanine to valine, and this substance is associated with reduced enzyme activity that leads to reduced plasma folate levels.[10,11] The MTHFR A1298C polymorphism, marked as rs1801131 in the NCBI database, is located at exon 7 and results in a 1298A-C mutation resulting in a glu429-to-ala (E429A) substitution at codon 429,[12] is also associated with decreased enzyme activity.[13,14]
To date, sixty-six studies have been published on the MTHFR C677T and A1298C polymorphisms with male infertility risk. However, the results of these studies were still contradictory. In addition, 15 previous meta-analyses[15,17,19–29,31,32] have been reported on the MTHFR C677T polymorphism with male infertility risk (as shown in Table 1). Among these publications, two studies[32] investigated this issue in Caucasians, two studies[22,27] in Asians, one study[25] in Chinese population, and 11 studies[15,17,19–21,23,24,26,28,29,31] in overall populations. Moreover, ten previous meta-analyses[15,16,18,20,24–27,30,32] have also been published on the MTHFR A1298C polymorphism with male infertility risk (as shown in Table 2). However, the previous meta-analysis results still inconsistent. Moreover, their meta-analyses did not assess false-positive report probabilities except Liu et al[26] by using the Benjamini-Hochberg methods, which control for false discovery rate, furthermore, many new studies have been published, and therefore an updated and high quality meta-analysis were performed to further explore the issues. For all we know, this is the first meta-analysis to further investigate the positive result using a Bayesian method.
Table 1.
Results of previous meta-analysis between MTHFR C677T polymorphism with male infertility risk.
| CT vs. CC | TT vs. CC | (CT + TT) vs. CC | TT vs. (CC + CT) | T vs. C | ||||||||||
| First author/year | Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | Whether performed assessment of literature quality | Whether performed P adjust |
| Ullah[32] 2019 | Low income | 8 (NA) | NA | NA | NA | NA | 1.87 (0.96, 3.64) | NA | NA | NA | NA | NA | No | No |
| Middle income | 13 (NA) | NA | NA | NA | NA | 1.38 (1.02, 1.88) | NA | NA | NA | NA | NA | |||
| High income | 9 (NA) | NA | NA | NA | NA | 1.26 (0.92, 1.71) | NA | NA | NA | NA | NA | |||
| Shi[27] 2019 | Asian | 20 (4734/3967) | 1.35 (1.22, 1.49) | NA/38 | 2.08 (1.79, 2.44) | NA/44 | 1.49 (1.35, 1.64) | NA/50 | 1.67 (1.49, 1.89) | NA/27 | 1.43 (1.33, 1.52) | NA/49 | Yes | No |
| East Asian | 13 (3013/2571) | 1.45 (1.28, 1.67) | NA/20 | 2.13 (1.82, 2.50) | NA/17 | 1.61 (1.43, 1.75) | NA/28 | 1.67 (1.45, 1.89) | NA/3 | 1.45 (1.35, 1.56) | NA/20 | |||
| South/West Asia | 7 (1721/1396) | 1.22 (1.03, 1.43) | NA/53 | 1.89 (1.35, 2.63) | NA/68 | 1.32 (1.12, 1.54) | NA/65 | 1.78 (1.29, 2.14) | NA/57 | 1.33 (1.16, 1.52) | NA/71 | |||
| Hong[19] 2017 | Overall | 15 (3853/3613) | 1.34 (1.03, 1.74) | <.001/80 | 1.86 (1.36, 2.54) | 0.009/55 | 1.46 (1.05, 2.04) | <.001/89 | 1.42 (1.19, 1.70) | .03/49 | 1.38 (1.18, 1.63) | .0007/66 | Yes | No |
| Caucasian | 2 (NA) | NA | NA | NA | NA | NA | NA | NA | NA | 1.23 (0.85, 1.70) | .10/63 | |||
| East-asian | 5 (NA) | NA | NA | NA | NA | NA | NA | NA | NA | 1.39 (1.20, 1.61) | .23/29 | |||
| Middle-estern | 2 (NA) | NA | NA | NA | NA | NA | NA | NA | NA | 1.30 (1.05, 1.63) | .78/0 | |||
| Indian | 3 (NA) | NA | NA | NA | NA | NA | NA | NA | NA | 1.25 (0.74, 2.13) | .0003/88 | |||
| Mixed-race | 1 (NA) | NA | NA | NA | NA | NA | NA | NA | NA | 1.96 (1.35, 2.85) | .001/63 | |||
| Rai[22] 2017 | Asian | 17 (4392/3667) | 1.40 (1.18, 1.62) | .005/52.7 | 2.10 (1.61, 2.61) | .02/47.4 | 1.53 (1.30, 1.77) | .005/53.7 | 1.70 (1.38, 2.10) | .03/43.7 | 1.99 (1.58, 2.51) | <.001/89.4 | No | No |
| Ren[25] 2017 | Chinese | 9 (1713/1104) | NA | NA | 2.08 (1.68, 2.58) | NA/35 | 1.51 (1.30, 1.77) | NA/29 | 1.58 (1.31, 1.90) | NA/0.0 | 1.47 (1.32, 1.63) | NA/42 | Yes | No |
| Yang et al.[15] 2016 | Overall | 21 (4505/4024) | 1.21 (1.04, 1.41) | .001/54.7 | 1.63 (1.22, 2.18) | <.001/69.4 | 1.29 (1.09, 1.54) | <.001/68.6 | 1.46 (1.16, 1.85) | <.001/60.3 | 1.26 (1.10, 1.46) | <.001/76.1 | No | No |
| Caucasian | 13 (NA) | 1.13 (0.90, 1.42) | NA | 1.38 (0.84, 2.27) | NA | 1.17 (0.90, 1.51) | NA | 1.30 (0.86, 1.98) | NA | 1.16 (0.92, 1.45) | NA | |||
| Asian | 8 (NA) | 1.32 (1.14–1.53) | NA | 1.90 (1.54, 2.35) | NA | 1.47 (1.25, 1.73) | NA | 1.63 (1.36, 1.96) | NA | 1.40 (1.24, 1.59) | NA | |||
| Zhu[17] 2016 | Overall | 26 (5659/5528) | 1.09 (1.00, 1.19) | .008/45 | 1.83 (1.48, 2.26) | <.001/74 | 1.19 (1.04, 1.36) | .0002/57 | 1.54 (1.27, 1.88) | .002/51 | 1.23 (1.10,1.37) | <.001/66 | No | No |
| Asian | 16 (NA) | 1.22 (1.10, 1.36) | .08/36 | 2.43 (2.08, 2.83) | .13/30 | 1.36 (1.19, 1.56) | .04/42 | 1.81 (1.47, 2.23) | .08/36 | 1.37 (1.27, 1.47) | .01/51 | |||
| Caucasian | 10 (NA) | 0.89 (0.77, 1.03) | .28/17 | 1.08 (0.83, 1.41) | .04/49 | 0.93 (0.81, 1.06) | .15/32 | 1.14 (0.92, 1.41) | .05/48 | 0.99 (0.89, 1.09) | .03/51 | |||
| Azoo∗ | 12 (NA) | 1.67 (1.36, 2.06) | .14/31 | 1.17 (1.02, 1.35) | .10/36 | 1.26 (1.10, 1.44) | .04/42 | 1.50 (1.25, 1.82) | .3/15 | 1.25 (1.14, 1.38) | .03/49 | |||
| OAT† | 14 (NA) | 1.01 (0.90, 1.14) | .05/41 | 1.41 (1.00, 1.99) | <.001/69 | 1.10 (0.91, 1.33) | .15/32 | 1.43 (1.04, 1.98) | <.001/69 | 1.17 (0.98, 1.40) | <.001/76 | |||
| Gong[23] 2015 | Overall | 26 (5575/5447) | NA | NA | 1.76 (1.53, 2.01) | NA/54.0 | 1.34 (1.23, 1.46) | NA/68.2 | 1.60 (1.41, 1.81) | NA/36.9 | 1.32 (1.24, 1.41) | NA/71.5 | No | No |
| Asian | 10 (NA) | NA | NA | NA | NA | 1.54 (1.35, 1.76) | NA/0.0 | NA | NA | NA | NA | |||
| Caucasian | 11 (NA) | NA | NA | NA | NA | 1.19 (1.05, 1.36) | NA/48.1 | NA | NA | NA | NA | |||
| Azoo∗ | 1412/3532 | NA | NA | NA | NA | 1.36 (1.18, 1.55) | NA/49.1 | NA | NA | NA | NA | |||
| OAT† | 615/1865 | NA | NA | NA | NA | 1.35 (1.11, 1.64) | NA/44.7 | NA | NA | NA | NA | |||
| Liu[26] 2015 | Overall | 32 (NA) | 1.17 (1.03, 1.33) | <.001/NA | 1.62 (1.29, 2.04) | <.001/NA | 1.26 (1.10, 1.45) | <.001/NA | 1.47 (1.23, 1.77) | .002/NA | 1.25 (1.12, 1.40) | <.001/NA | Yes | Yes (FDR) |
| Asian | 17 (NA) | 1.28 (1.13, 1.46) | .153/NA | 2.15 (1.67, 2.75) | .036/NA | 1.44 (1.24, 1.66) | .019/NA | 1.79 (1.48, 2.16) | .193/NA | 1.42 (1.27, 1.60) | .005/NA | |||
| Azoo∗ | 14 (NA) | 1.21 (1.04, 1.41) | .298/NA | 1.64 (1.12, 2.42) | .002/NA | 1.31 (1.06, 1.61) | .015/NA | 1.49 (1.07, 2.06) | .011/NA | 1.27 (1.05, 1.54) | <.001/NA | |||
| OAT† | 16 (NA) | 1.17 (0.96, 1.44) | .001/NA | 1.52 (1.12, 2.06) | .008/NA | 1.25 (1.01, 1.55) | <.001/NA | 1.43 (1.13, 1.82) | .098/NA | 1.24 (1.05, 1.47) | <.001/NA | |||
| Nikzad[28] 2015 | Overall | 23 (5174/5253) | NA | NA | 1.44 (1.09, 1.89) | <.001/66 | 1.21 (1.06, 1.39) | <.001/60 | 1.38 (1.14, 1.68) | .017/43 | 1.21 (1.08, 1.36) | < .001/67 | No | No |
| Weiner[29] 2014 | Overall | 17 (2972/3436) | NA | NA | NA | NA | 1.05 (0.99, 1.11) | NA | NA | NA | NA | NA | No | No |
| Azoo∗ | 6 (NA) | NA | NA | NA | NA | 1.18 (0.92, 1.51) | NA | NA | NA | NA | NA | |||
| Gupta[31] 2013 | Overall | 13 (3094/2877) | NA | NA | NA | NA | 1.31 (1.17, 1.46) | NA | NA | NA | 1.30 (1.20, 1.41) | NA | No | No |
| Azoo∗ | NA | NA | NA | NA | NA | 1.65 (1.36, 1.99) | NA | NA | NA | 1.52 (1.32, 1.17) | NA | |||
| OAT† | NA | NA | NA | NA | NA | 1.08 (0.88, 1.34) | NA | NA | NA | 1.43 (1.18, 1.73) | NA | |||
| Wu[21] 2012 | Overall | 10 (2275/1958) | 1.11 (0.86, 1.43) | .004/NA | 1.39 (0.93, 2.07) | <.001/NA | 1.15 (0.89, 1.49) | <.001/NA | 1.34 (0.99, 1.81) | .012/NA | 1.17 (0.95, 1.43) | <.001/NA | No | No |
| Asian | 6 (NA) | 1.70 (0.97, 1.72) | .017/NA | 1.79 (1.08, 2.96) | .003/NA | 1.42 (1.02, 1.96) | .001/NA | 1.50 (1.21, 1.86) | .055/NA | 1.36 (1.06, 1.75) | <.001/NA | |||
| Caucasian | 3 (NA) | 0.71 (0.45, 1.12) | .932/NA | 0.71 (0.45, 1.12) | .522/NA | 0.75 (0.55, 1.01) | .774/NA | 0.81 (0.53, 1.23) | .534/NA | 0.81 (0.65, 1.01) | .534/NA | |||
| Azoo∗ | 5 (NA) | 1.45 (1.18, 1.79) | .340/NA | 1.89 (1.43, 2.51) | .308/NA | 1.55 (1.28, 1.88) | .257/NA | 1.51 (1.17, 1.95) | .333/NA | 1.38 (1.20, 1.57) | .155/NA | |||
| OAT† | 7 (NA) | 0.90 (0.74, 1.08) | .062/NA | 1.02 (0.78, 1.32) | .064/NA | 0.91 (0.69, 1.19) | .031/NA | 1.08 (0.85, 1.38) | .119/NA | 0.96 (0.78, 1.18) | .017/NA | |||
| Wei[24] 2012 | Overall | 11 (2217/2312) | 1.22 (0.96, 1.56) | .001/NA | 1.40 (0.98, 2.00) | .001/NA | 1.26 (0.97, 1.65) | <.001/NA | 1.28 (1.00, 1.64) | .09/NA | NA | NA | No | No |
| Caucasian | 5 (635/611) | 1.17 (0.67, 2.06) | .001/NA | 1.18 (0.60, 2.34) | .01/NA | 1.17 (0.65, 2.12) | <.001/NA | 1.09 (0.70, 1.71) | .15/NA | NA | NA | |||
| Asian | 6 (1582/1701) | 1.26 (0.98, 1.62) | .03/NA | 1.57 (1.05, 2.37) | .02/NA | 1.34 (1.01, 1.77) | .004/NA | 1.40 (1.05, 1.86) | .17/NA | NA | NA | |||
| Tüttelmann[20] 2007 | Overall | 8 (1843/1791) | NA | NA | NA | NA | 1.39 (1.15, 1.69) | NA | NA | NA | NA | NA | No | No |
Table 2.
Results of previous meta-analysis between MTHFR A1298C polymorphism with male infertility risk.
| AC vs. AA | CC vs. AA | (AC + CC) vs. AA | CC vs. (AA + AC) | C vs. A | ||||||||||
| First author | Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | OR (95% CI) | Ph/I2 (%) | Whether performed assessment of literature quality | Whether performed P adjust |
| Shi[27] 2019 | Asian | 12 (2673/2328) | 1.20 (1.08, 1.37) | NA/27 | 1.64 (1.08, 2.56) | NA/ 58 | 1.27 (1.14, 1.43) | NA/46 | 1.61 (1.27, 2.04) | NA/50 | 1.22 (1.05, 1.41) | NA/57 | Yes | No |
| East Asian | 7 (1759/1586) | 1.35 (1.16, 1.56) | NA/0 | 2.17 (1.11, 4.17) | NA/ 65 | 1.43 (1.25, 1.67) | NA/38 | 2.04 (1.47, 2.86) | NA/59 | 1.37 (1.12, 1.67) | NA/56 | |||
| South/West Asia | 5 (878/742) | 0.96 (0.78, 1.19) | NA/0 | 1.14 (0.78, 1.67) | NA/0 | 1.00 (0.82,1.22) | NA/0 | 1.20 (0.84, 1.72) | NA/0 | 1.03 (0.77, 1.20) | NA/0 | |||
| Ullah[32] 2019 | Low income | 6 (NA) | NA | NA | NA | NA | NA | NA | 1.71 (1.19, 2.47) | NA | NA | NA | No | No |
| Middle income | 10 (NA) | NA | NA | NA | NA | NA | NA | 1.02 (0.81, 1.28) | NA | NA | NA | |||
| High income | 4 (NA) | NA | NA | NA | NA | NA | NA | 0.86 (0.62, 1.19) | NA | NA | NA | |||
| Zhang[16] 2017 | Overall | 20 (4293/4507) | 1.02 (0.93, 1.12) | .165/NA | 1.01 (0.85, 1.20) | .100/NA | 1.02 (0.93, 1.12) | .157/NA | 1.01 (0.86, 1.19) | .111/NA | 1.02 (0.95, 1.09) | .148/NA | Yes | No |
| Caucasian | 15 (NA) | 0.95 (0.85, 1.06) | .142/NA | 0.94 (0.78, 1.14) | .063/NA | 0.95 (0.86, 1.06) | .177/NA | 0.96 (0.80, 1.14) | .056/NA | 0.96 (0.89, 1.04) | .202/NA | |||
| Asian | 5 (NA) | 1.20 (1.01, 1.44) | .994/NA | 1.41 (0.93, 2.15) | .860/NA | 1.23 (1.04, 1.45) | .996/NA | 1.33 (0.88, 2.02) | .846/NA | 1.20 (1.04, 1.39) | .985/NA | |||
| Ren[25] 2017 | Chinese | 3 (540/457) | NA | NA | 1.34 (0.66, 2.71) | NA/0.0 | 1.27 (0.95, 1.65) | NA/0.0 | 1.44 (0.72, 2.88) | NA/9 | 1.22 (0.97, 1.53) | NA/0.0 | Yes | No |
| Yang[15] 2016 | Overall | 13 (2785/3094) | 1.02 (0.91, 1.14) | .216/22.5 | 1.29 (1.03, 1.62) | .330/11.5 | 1.06 (0.95, 1.18) | .224/21.7 | 1.29 (1.03, 1.60) | .345/10.0 | 1.08 (0.99, 1.18) | .294/15.0 | No | No |
| Caucasian | 8 (NA) | 0.90 (0.78, 1.04) | NA | 1.31 (1.00, 1.72) | NA | 0.95 (0.83, 1.09) | NA | 1.35 (1.04, 1.74) | NA | 1.02 (0.92, 1.14) | NA | |||
| Asian | 5 (NA) | 1.24 (1.03, 1.48) | NA | 1.24 (0.82, 1.88) | NA | 1.24 (1.04, 1.47) | NA | 1.16 (0.77, 1.75) | NA | 1.19 (1.03, 1.37) | NA | |||
| Liu[26] 2015 | Overall | 17 (NA) | NA | NA | NA | NA | NA | NA | 1.11 (0.87, 1.41) | NA | NA | NA | Yes | Yes (FDR) |
| Gupta[30] 2013 | Overall | 10 (2734/2737) | NA | NA | NA | NA | 1.05 (0.89, 1.23) | .058/45.4 | NA | NA | NA | NA | No | No |
| Azoo∗ | NA | NA | NA | NA | NA | 0.97 (0.79, 1.18) | .697/0.0 | NA | NA | NA | NA | |||
| OAT† | NA | NA | NA | NA | NA | 0.96 (0.74, 1.24) | .006/66.6 | NA | NA | NA | NA | |||
| Shen[18] 2012 | Overall | 7 (1633/1735) | 1.10 (0.95, 1.27) | .855/NA | 1.29 (0.97, 1.72) | .087/NA | 1.13 (0.98, 1.30) | .578/NA | 1.26 (0.95, 1.65) | .119/NA | 1.12 (1.00, 1.26) | .215/NA | No | No |
| Asian | 5 (NA) | 1.11 (0.94, 1.31) | .919/NA | 1.18 (0.84, 1.66) | .435/NA | 1.12 (0.96, 1.31) | .917/NA | 1.14 (0.82, 1.59) | .415/NA | 1.10 (0.97, 1.25) | .841/NA | |||
| Caucasian | 2 (NA) | 1.06 (0.77, 1.45) | .206/NA | 1.55 (0.42, 5.72) | .012/NA | 1.15 (0.85, 1.55) | .052/NA | 1.52 (0.49, 4.71) | .023/NA | 0.81 (0.65, 1.01) | .011/NA | |||
| Azoo∗ | 4 (NA) | 1.01 (0.78, 1.31) | .840/NA | 1.66 (1.01, 2.73) | .124/NA | 1.08 (0.85, 1.38) | .965/NA | 1.67 (1.03, 2.71) | .078/NA | 1.14 (0.94, 1.38) | .625/NA | |||
| OAT† | 5 (NA) | 1.10 (0.91, 1.34) | .401/NA | 1.15 (0.82, 1.63) | .140/NA | 1.12 (0.93, 1.34) | .177/NA | 1.12 (0.81, 1.56) | .290/NA | 1.09 (0.95, 1.26) | .079/NA | |||
| Wei[24] 2012 | Overall | 7 (1633/1735) | 1.30 (0.87, 1.95) | .09/NA | 1.10 (0.95, 1.27) | .86/NA | 1.13 (0.98, 1.30) | .58/NA | 1.26 (0.95, 1.65) | .12/N1 | NA | NA | No | No |
| Caucasian | 2 (406/346) | 1.55 (0.42, 5.72) | .01/NA | 1.06 (0.77, 1.45) | .21/NA | 1.15 (0.85, 1.55) | .05/NA | 1.54 (0.94, 2.54) | .02/N1 | NA | NA | |||
| Asian | 5 (1227/1389) | 1.16 (0.82, 1.64) | .44/NA | 1.11 (0.94, 1.31) | .92/NA | 1.12 (0.96, 1.32) | .92/NA | 1.14 (0.82, 1.59) | .42/NA | NA | NA | |||
| Tüttelmann[20] 2007 | Overall | 2 (539/525) | NA | NA | NA | NA | 0.97 (0.54, 1.74) | NA | NA | NA | NA | NA | No | No |
2. Materials and methods
2.1. Search strategy
The eligible studies were searched (the deadline was April 9, 2020) to used three databases (PubMed, CNKI, and WangFang). Retrieval strategy was designed by the following keywords (methylenetetrahydrofolate reductase OR MTHFR) AND (polymorphism OR mutation OR variant) AND (infertility OR azoospermia OR oligoasthenoteratozoospermia OR oligozoospermia OR subinfertility). Language did not be restrict in this study. We send emails to the corresponding authors if data of a few studies did not be collect by full-text. In addition, the previous meta-analyses were also carefully examined by reference lists.
2.2. Inclusion and exclusion criteria
The inclusion criteria as following:
-
(1)
human case-control or cohort studies (Infertility was defined as conception failure after at least 1 year of regular unprotected sexual intercourse among couples; Controls were healthy without a history of infertility, and had one child at least with normal sperm parameters. In addition, Cases and controls should be comparable),
-
(2)
studies on the MTHFR C677T and A1298C polymorphisms and male infertility risk,
-
(3)
If more than one study had been published using the same case series, we selected one study including the maximum sample size, and
-
(4)
the genotype data or odds ratios (ORs) and their 95% confidence intervals (CIs) provided.
The exclusion criteria as following:
-
(1)
data not listed,
-
(2)
not human case–control or cohort studies, and
-
(3)
reviews, meta-analyses, conference abstracts, letters, and editorials.
2.3. Data extraction and quality score assessment
Two authors independently extracted data from selected studies including the following information:
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(1)
first author's name,
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(2)
year of publication,
-
(3)
country,
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(4)
ethnicity,
-
(5)
source of controls,
-
(6)
sample size, and
-
(7)
genotype distribution of male infertility cases and controls.
Two investigators assessed independently the quality of eligible articles. The literature quality assessment criteria was shown in supplemental Table 1. The biggest score value is eleven by the quality assessment; scoring ≥ 5 were considered as high quality studies. A third author adjudicated inconsistent scores.
2.4. Statistical analysis
We evaluated the association between the MTHFR C677T and A1298C polymorphisms and male infertility risk by pooled the crude ORs and their 95% CIs. The pooled ORs with the corresponding 95% CIs were performed by the following genetic models: a dominant model: (CT + TT) vs. CC for the MTHFR C677T polymorphism and (AC + CC) vs. AA for the MTHFR A1298C polymorphism, a recessive model: TT vs (CC + CT) for the C677T and (AC + CC) vs AA for the A1298C, a heterozygote model: CT vs. CC for the C677T and AC vs. AA for the A1298C, a homozygote model: TT vs CC for the C677T and CC vs. AA for the A1298C, and an allele model: T vs. C for the C677T and C vs. A for the A1298C. Heterogeneity among studies was checked according to the Cochran Q[94] and I2 value[95]. The P > .10 and/or I2 < 50% indicate a lack of heterogeneity among studies, hence, the pooled crude ORs was calculated using a fixed-effects model (Mantel–Haenszel method)[96]; otherwise, a random-effect model (DerSimonian and Laird method) was applied[97]. A meta-regression analysis was used to explore sources of heterogeneity[98] if heterogeneity among studies was significant. Subgroup analyses were conducted according to ethnicity, source of controls and type of male infertility. Sensitivity analyses were also performed to estimate the robustness of the pooled results. We used the following methods to perform the sensitivity analyses: excluded the studies of Hardy-Weinberg dis-equilibrium (HWD) and quality scores < 5. Hardy-Weinberg equilibrium (HWE) was calculated by chi-square goodness-of-fit test, and significant deviation was considered in control groups if the P value < .05. The publication bias was assessed to using Begg funnel[99] and Egger test.[100] Last, a Bayesian false discovery probability (BFDP: a cutoff value was set up to be a level of 0.8 and a prior probability of 0.001)[101] was used to evaluate positive results whether were noteworthy or not. All statistical analyses were conducted using STATA version 12.0 (STATA Corporation, College Station, TX).
3. Results
3.1. Study characteristics
A flowchart of study selection is listed in Figure 1. Overall, we retrieved 243 publications by several databases. Among these publications, sixty-six articles were selected after filtering titles, abstracts, and full texts. In addition, the sample size of four publications[54,75,86,92] overlapped with those of another four publications.[2,57,61,88] Therefore, sixty-two publications were involved in the final analysis. Table 3 lists the main characteristics of the selected studies. Fifty-nine studies[2,28,29,31–43,46–53,55–63,65–74,76–85,87–91] were included concerning the MTHFR C677T polymorphism (11,767 male infertility cases and 10,591 controls; two studies on Africans, thirteen on Caucasians, twenty-seven on East Asians, seven on West Asians, eight on South Asians, and two mixed populations; fifty-three hospital-based studies and six population-based studies; twenty-four azoospermia studies and thirty-seven Oligoasthenoteratozoospermia (OAT) studies) with male infertility risk. Twenty-eight studies were found on the MTHFR A1298C polymorphis[2,29,30,32,37,38,41,43–45,49,50,52,53,55–59,61,63–65,72,73,80,81,91] (5,976 male infertility cases and 5,774 controls; four studies on South Asians, 7 on West Asians, nine on East Asians, six on Caucasians, one on Africans, and one mixed populations; twenty-five hospital-based studies and three population-based studies; twelve azoospermia studies and twelve OAT studies) with male infertility risk. In addition, HWD of controls was observed in six studies[2,32,43,69,76,80] for C677T polymorphism and six studies[32,44,49,63,81,91] for A1298C polymorphism.
Figure 1.
OA_ Guidelines Flow Diagram Study selection flowchart in the current meta-analysis.
Table 3.
Characteristics of studies included in the current meta-analysis.
| Case | Control | |||||||||||||||||
| Azoospermia | OAT | Total | ||||||||||||||||
| First Author/Year | Country | Ethnicity | SC | Sample size | CC | CT | TT | CC | CT | TT | CC | CT | TT | CC | CT | TT | HWE | Quality score |
| MTHFR C667T | ||||||||||||||||||
| Bezold[33] 2001 | German | Caucasian | HB | 255/200 | – | – | – | – | – | – | 114 | 93 | 48 | 92 | 89 | 19 | 0.705 | 5 |
| Stuppia[34] 2003 | Italy | Caucasian | HB | 93/105 | 8 | 6 | 7 | 29 | 31 | 12 | 37 | 37 | 19 | 33 | 43 | 29 | 0.066 | 4 |
| Ebisch[35] 2003 | Netherlands | Caucasian | PB | 77/113 | – | – | – | – | – | – | 42 | 28 | 7 | 50 | 48 | 15 | 0.522 | 4 |
| Singh[36] 2005 | India | South Asian | PB | 151/200 | – | – | – | – | – | – | 105 | 40 | 6 | 163 | 37 | 0 | 0.149 | 6 |
| Park[37] 2005 | Korea | East Asian | HB | 373/396 | 75 | 164 | 47 | 28 | 40 | 17 | 105 | 205 | 63 | 145 | 200 | 51 | 0.161 | 5 |
| Lee[38] 2006 | Korea | East Asian | HB | 360/325 | 44 | 100 | 30 | 71 | 81 | 34 | 115 | 181 | 64 | 118 | 166 | 41 | 0.138 | 5 |
| Paracchini[39] 2006 | Italy | Caucasian | HB | 59/46 | – | – | – | – | – | – | 11 | 32 | 16 | 18 | 21 | 7 | 0.83 | 4 |
| A[40] 2007 | China | East Asian | HB | 355/252 | 83 | 97 | 48 | 47 | 63 | 17 | 130 | 160 | 65 | 128 | 95 | 29 | 0.085 | 5 |
| Dhillon[41] 2007 | India | South Asian | HB | 179/200 | – | – | – | 81 | 77 | 21 | 81 | 77 | 21 | 70 | 100 | 30 | 0.556 | 5 |
| Sun[42] 2007 | China | East Asian | NR | 182/53 | – | – | – | 22 | 75 | 52 | 27 | 86 | 69 | 15 | 28 | 10 | 0.63 | 3 |
| Zhang[73] 2007 | China | East Asian | HB | 165/132 | – | – | – | – | – | – | 41 | 93 | 31 | 48 | 60 | 24 | 0.492 | 4 |
| Ravel[43] 2009 | French | Caucasian | HB | 250/113 | 33 | 31 | 6 | 85 | 70 | 25 | 118 | 101 | 31 | 49 | 52 | 31 | 0.024 | 3 |
| Yang[72] 2010 | China | East Asian | HB | 131/293 | – | – | – | 34 | 55 | 42 | 34 | 55 | 42 | 98 | 142 | 53 | 0.901 | 4 |
| Đorđević[61] 2010 | Serbia | Caucasian | HB | 52/56 | – | – | – | – | – | – | 22 | 24 | 6 | 23 | 26 | 7 | 0.934 | 5 |
| Zhang[82] 2010 | China | East Asian | HB | 491/430 | – | – | – | – | – | – | 43 | 253 | 195 | 87 | 213 | 130 | 0.988 | 5 |
| Gava[45] 2011 | Brazil | Mixed | HB | 156/233 | 27 | 15 | 7 | 54 | 45 | 8 | 81 | 60 | 15 | 167 | 53 | 13 | 0.003 | 3 |
| Safarinejad[46] 2011 | Iran | West Asian | HB | 164/328 | – | – | – | 58 | 80 | 26 | 58 | 80 | 26 | 144 | 148 | 36 | 0.826 | 7 |
| Liu[47] 2011 | China | East Asian | HB | 75/72 | – | – | – | 27 | 38 | 10 | 27 | 38 | 10 | 40 | 28 | 4 | 0.753 | 3 |
| Qiu[48] 2011 | China | East Asian | HB | 271/180 | 42 | 66 | 50 | 33 | 46 | 34 | 75 | 112 | 84 | 63 | 85 | 32 | 0.72 | 4 |
| Murphy[64] 2011 | Swede | Caucasian | HB | 153/184 | – | – | – | – | – | – | 73 | 63 | 13 | 94 | 73 | 15 | 0.876 | 6 |
| Kumar[67] 2011 | India | South Asian | HB | 100/100 | – | – | – | – | – | – | 86 | 14 | 0 | 81 | 19 | 0 | 0.294 | 4 |
| Gupta[31] 2011 | India | South Asian | HB | 522/315 | 49 | 15 | 4 | 144 | 46 | 10 | 378 | 116 | 28 | 251 | 58 | 6 | 0.229 | 5 |
| Vani[49] 2012 | India | South Asian | HB | 206/230 | – | – | – | – | – | – | 158 | 42 | 6 | 188 | 42 | 0 | 0.128 | 4 |
| Eloualid[50] 2012 | Morocco | African | HB | 344/690 | 65 | 37 | 8 | 134 | 88 | 12 | 199 | 125 | 20 | 351 | 286 | 53 | 0.611 | 6 |
| Chellat[69] 2012 | Algeria | Mixed | HB | 74/84 | 20 | 19 | 7 | 11 | 14 | 3 | 31 | 33 | 10 | 36 | 38 | 10 | 0.995 | 3 |
| Liu[68] 2012 | China | East Asian | HB | 75/72 | – | – | – | 27 | 38 | 10 | 27 | 38 | 10 | 40 | 28 | 4 | 0.753 | 3 |
| Stangler[66] 2013 | Slovene | Caucasian | PB | 100/111 | – | – | – | – | – | – | 29 | 51 | 20 | 47 | 50 | 14 | 0.902 | 6 |
| Camprubi[71] 2013 | Spain | Caucasian | HB | 107/25 | – | – | – | 42 | 36 | 14 | 47 | 43 | 17 | 8 | 15 | 2 | 0.172 | 3 |
| Pei J[75] 2013 | China | East Asian | HB | 290/90 | – | – | – | – | – | – | 39 | 138 | 113 | 24 | 47 | 19 | 0.651 | 4 |
| Balkan[81] 2014 | Turkey | West Asian | NR | 108/125 | 57 | 40 | 11 | – | – | – | 57 | 40 | 11 | 78 | 36 | 11 | 0.032 | 3 |
| Mfady[51] 2014 | Jordan | West Asian | HB | 150/150 | – | – | – | – | – | – | 67 | 63 | 20 | 74 | 67 | 9 | 0.221 | 5 |
| Naqvi[52] 2014 | India | South Asian | HB | 637/364 | 34 | 11 | 4 | 413 | 143 | 33 | 447 | 154 | 36 | 275 | 79 | 10 | 0.145 | 7 |
| Li SS[54] 2014 | China | East Asian | HB | 82/133 | – | – | – | – | – | – | 14 | 36 | 32 | 36 | 61 | 36 | 0.34 | 4 |
| Weiner[29] 2014 | Russia | Caucasian | PB | 271/301 | 49 | 41 | 8 | 40 | 31 | 11 | 129 | 116 | 26 | 153 | 115 | 33 | 0.113 | 7 |
| Vardarli[62] 2014 | Turkey | Caucasian | HB | 100/50 | 23 | 22 | 5 | 21 | 22 | 7 | 44 | 44 | 12 | 30 | 20 | 0 | 0.077 | 4 |
| Hussein[77] 2014 | Egypt | African | HB | 107/107 | 64 | 35 | 8 | – | – | – | 64 | 35 | 8 | 62 | 32 | 13 | 0.012 | 3 |
| Ng[78] 2014 | Canada | Caucasian | NR | 39/19 | 10 | 10 | 2 | 12 | 4 | 1 | 22 | 14 | 3 | 8 | 5 | 3 | 0.219 | 2 |
| Ni W[56] 2015 | China | East Asian | PB | 296/204 | – | – | – | – | – | – | 117 | 135 | 44 | 84 | 94 | 26 | 0.97 | 7 |
| Gurkan[57] 2015 | Turkey | West Asian | HB | 137/134 | 41 | 25 | 9 | 29 | 24 | 9 | 70 | 49 | 18 | 71 | 55 | 8 | 0.533 | 5 |
| Li XY[58] 2015 | China | East Asian | HB | 162/120 | 36 | 49 | 15 | 25 | 28 | 9 | 61 | 77 | 24 | 48 | 54 | 18 | 0.661 | 5 |
| Kurzawski[59] 2015 | Poland | Caucasian | HB | 284/352 | – | – | – | – | – | – | 143 | 113 | 28 | 166 | 150 | 36 | 0.806 | 5 |
| Kim[60] 2015 | Korea | East Asian | HB | 85/246 | 30 | 44 | 11 | – | – | – | 30 | 44 | 11 | 87 | 106 | 53 | 0.057 | 4 |
| Nikzad[28] 2015 | Iran | West Asian | HB | 242/255 | 47 | 49 | 11 | 62 | 60 | 13 | 109 | 109 | 24 | 144 | 98 | 13 | 0.48 | 5 |
| Karimian[53] 2016 | Iran | West Asian | HB | 118/132 | – | – | – | 51 | 59 | 8 | 51 | 59 | 8 | 77 | 52 | 3 | 0.087 | 4 |
| Irfan[79] 2016 | Pakistan | South Asian | PB | 437/218 | 36 | 18 | 3 | 249 | 118 | 3 | 285 | 136 | 16 | 187 | 30 | 1 | 0.862 | 9 |
| Najafipour[74] 2017 | Iran | West Asian | HB | 280/120 | 25 | 34 | 11 | 88 | 89 | 33 | 113 | 123 | 44 | 66 | 43 | 11 | 0.31 | 4 |
| Ma FF[86] 2017 | China | East Asian | HB | 140/96 | 40 | 30 | 4 | 36 | 22 | 8 | 76 | 52 | 12 | 44 | 44 | 8 | 0.514 | 4 |
| Hu[70] 2017 | China | East Asian | HB | 186/131 | – | – | – | 68 | 80 | 38 | 68 | 80 | 38 | 72 | 41 | 18 | 0.005 | 3 |
| Wang Y[83] 2018 | China | East Asian | HB | 76/95 | – | – | – | 14 | 34 | 11 | 15 | 37 | 24 | 24 | 54 | 17 | 0.163 | 3 |
| Hu LL[84] 2018 | China | East Asian | HB | 145/88 | – | – | – | 23 | 60 | 62 | 23 | 60 | 62 | 11 | 48 | 29 | 0.194 | 4 |
| Zhou SH[85] 2018 | China | East Asian | HB | 145/88 | 3 | 18 | 8 | 4 | 13 | 11 | 15 | 90 | 40 | 11 | 48 | 29 | 0.194 | 4 |
| Cai[63] 2018 | China | East Asian | HB | 90/90 | – | – | – | 13 | 40 | 37 | 13 | 40 | 37 | 26 | 47 | 17 | 0.602 | 3 |
| Zuo YJ[80] 2018 | China | East Asian | HB | 154/294 | – | – | – | 33 | 59 | 62 | 33 | 59 | 62 | 95 | 138 | 61 | 0.406 | 4 |
| Ullah[32] 2019 | Pakistan | South Asian | HB | 232/114 | – | – | – | – | – | – | 169 | 53 | 10 | 99 | 12 | 3 | 0.003 | 3 |
| Xie C[88] 2019 | China | East Asian | HB | 167/78 | – | – | – | 23 | 82 | 62 | 23 | 82 | 62 | 33 | 39 | 6 | 0.229 | 4 |
| Suo F[89] 2019 | China | East Asian | HB | 715/572 | – | – | – | 126 | 326 | 264 | 126 | 326 | 264 | 134 | 272 | 166 | 0.272 | 6 |
| Shao LJ[90] 2019 | China | East Asian | HB | 167/65 | – | – | – | 52 | 71 | 44 | 52 | 71 | 44 | 30 | 28 | 7 | 0.903 | 4 |
| Song N[91] 2019 | China | East Asian | HB | 100/100 | – | – | – | – | – | – | 31 | 46 | 23 | 32 | 52 | 16 | 0.501 | 4 |
| Xu JJ[92] 2019 | China | East Asian | HB | 104/108 | – | – | – | – | – | – | 38 | 41 | 29 | 50 | 44 | 14 | 0.386 | 4 |
| MTHFR A1298C | ||||||||||||||||||
| Park[37] 2005 | Korea | East Asian | HB | 373/396 | – | – | – | – | – | – | 237 | 118 | 18 | 269 | 111 | 16 | 0.294 | 5 |
| Lee[38] 2006 | Korea | East Asian | HB | 360/325 | 109 | 57 | 8 | 113 | 63 | 10 | 222 | 120 | 18 | 213 | 98 | 14 | 0.526 | 5 |
| Dhillon[41] 2007 | India | South Asian | HB | 179/200 | – | – | – | 90 | 80 | 9 | 90 | 80 | 9 | 103 | 84 | 13 | 0.451 | 5 |
| Zhang[73] 2007 | China | East Asian | HB | 165/132 | – | – | – | – | – | – | 90 | 65 | 15 | 85 | 45 | 2 | 0.142 | 4 |
| Ravel[43] 2009 | French | Caucasian | HB | 250/113 | 34 | 28 | 7 | 97 | 66 | 18 | 131 | 94 | 25 | 54 | 46 | 13 | 0.501 | 4 |
| Farcas[65] 2009 | Romania | Caucasian | HB | 66/67 | – | – | – | – | – | – | 35 | 29 | 2 | 39 | 26 | 2 | 0.34 | 4 |
| Singh[44] 2010 | India | South Asian | HB | 151/141 | 66 | 76 | 9 | – | – | – | 66 | 76 | 9 | 64 | 74 | 2 | 0.0002 | 3 |
| Zhang[82] 2010 | China | East Asian | HB | 491/430 | – | – | – | – | – | – | 224 | 220 | 47 | 270 | 150 | 10 | 0.039 | 4 |
| Gava[45] 2011 | Brazil | Mixed | HB | 156/233 | 26 | 14 | 9 | 45 | 48 | 14 | 71 | 62 | 23 | 130 | 89 | 14 | 0.811 | 4 |
| Safarinejad[46] 2011 | Iran | West Asian | HB | 164/328 | – | – | – | 75 | 70 | 19 | 75 | 70 | 19 | 149 | 141 | 38 | 0.599 | 7 |
| Murphy[64] 2011 | Swede | Caucasian | HB | 153/184 | – | – | – | – | – | – | 58 | 77 | 11 | 87 | 62 | 27 | 0.007 | 5 |
| Eloualid[50] 2012 | Morocco | African | HB | 344/690 | 67 | 39 | 4 | 138 | 83 | 13 | 205 | 122 | 17 | 370 | 303 | 17 | < 0.001 | 5 |
| Gupta[30] 2013 | India | South Asian | HB | 611/136 | – | – | – | – | – | – | 165 | 320 | 126 | 27 | 74 | 35 | 0.283 | 7 |
| Stangler[66] 2013 | Slovene | Caucasian | PB | 100/111 | – | – | – | – | – | – | 44 | 35 | 21 | 48 | 50 | 13 | 0.997 | 6 |
| Weiner[29] 2014 | Russia | Caucasian | PB | 275/349 | 37 | 54 | 8 | 42 | 32 | 9 | 126 | 125 | 23 | 142 | 142 | 30 | 0.52 | 7 |
| Mfady[51] 2014 | Jordan | West Asian | HB | 150/150 | – | – | – | – | – | – | 71 | 61 | 18 | 59 | 75 | 16 | 0.273 | 5 |
| Vardarli[62] 2014 | Turkey | West Asian | HB | 100/50 | 21 | 23 | 6 | 24 | 18 | 8 | 45 | 41 | 14 | 19 | 22 | 9 | 0.556 | 4 |
| Balkan[81] 2014 | Turkey | West Asian | NR | 108/125 | 47 | 42 | 19 | – | – | – | 47 | 42 | 19 | 45 | 56 | 24 | 0.383 | 4 |
| Li SS[54] 2014 | China | East Asian | HB | 82/133 | – | – | – | – | – | – | 49 | 29 | 4 | 88 | 36 | 9 | 0.059 | 4 |
| Ni W[56] 2015 | China | East Asian | PB | 296/204 | – | – | – | – | – | – | 181 | 106 | 9 | 137 | 62 | 5 | 0.514 | 7 |
| Gurkan[57] 2015 | Turkey | West Asian | HB | 137/134 | 34 | 34 | 7 | 29 | 25 | 8 | 63 | 59 | 15 | 49 | 66 | 19 | 0.668 | 5 |
| Li XY[58] 2015 | China | East Asian | HB | 162/120 | 66 | 31 | 3 | 35 | 23 | 4 | 101 | 54 | 7 | 80 | 38 | 2 | 0.29 | 5 |
| Kurzawski[59] 2015 | Poland | Caucasian | HB | 284/352 | – | – | – | – | – | – | 128 | 130 | 26 | 156 | 156 | 40 | 0.916 | 5 |
| Kim[60] 2015 | Korea | East Asian | HB | 85/246 | 52 | 28 | 5 | – | – | – | 52 | 28 | 5 | 184 | 56 | 6 | 0.486 | 4 |
| Karimian[53] 2016 | Iran | West Asian | HB | 118/132 | – | – | – | 59 | 44 | 15 | 59 | 44 | 15 | 70 | 48 | 14 | 0.194 | 4 |
| Najafipour[74] 2017 | Iran | West Asian | HB | 280/120 | 27 | 30 | 13 | 102 | 114 | 22 | 129 | 116 | 35 | 57 | 50 | 13 | 0.683 | 4 |
| Ullah[32] 2019 | Pakistan | South Asian | HB | 235/109 | – | – | – | – | – | – | 59 | 133 | 43 | 47 | 59 | 3 | 0.002 | 3 |
| Xu JJ[92] 2019 | China | East Asian | HB | 104/108 | – | – | – | – | – | – | 77 | 14 | 13 | 78 | 15 | 15 | < 0.001 | 4 |
3.2. Quantitative synthesis
3.2.1. MTHFR C677T polymorphism
Table 4 shows the results of the association between the MTHFR C677T polymorphism and male infertility risk. Overall, a significantly increased male infertility risk (CT vs CC: OR = 1.27, 95% CI: 1.15–1.40, Ph < .001, I2 = 54.1%; TT vs CC: OR = 1.74, 95% CI: 1.47–2.07, Ph < .001, I2 = 65.3%; CT + TT vs CC: OR = 1.38, 95% CI: 1.24–1.54, Ph < .001, I2 = 66.6%; TT vs CC + CT: OR = 1.52, 95% CI: 1.33–1.74, Ph < .001, I2 = 56.4%; T vs C: OR = 1.33, 95% CI: 1.22–1.45, Ph < .001, I2 = 73.1%) was observed in all eligible studies.
Table 4.
The results of the association of MTHFR C667T polymorphism with male infertility.
| CT vs. CC | TT vs. CC | (CT + TT) vs. CC | TT vs. (CC + CT) | T vs. C | ||||||||||||
| Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP |
| Overall | 59 (11767/10591) | 1.27 (1.15–1.40)∗ | <.001/54.1 | 0.106 | 1.74 (1.47–2.07)∗ | <.001/65.3 | <0.001 | 1.38 (1.24–1.54)∗ | <.001/66.6 | 0.001 | 1.52 (1.33–1.74)∗ | <.001/56.4 | <0.001 | 1.33 (1.22–1.45)∗ | <.001/73.1 | <0.001 |
| Ethnicity | ||||||||||||||||
| African | 2 (451/797) | 0.82 (0.64–1.04) | .340/0.0 | – | 0.65 (0.40–1.04) | .843/0.0 | – | 0.78 (0.62–0.99) | .507/0.0 | 0.998 | 0.70 (0.44–1.11) | .661/0.0 | – | 0.80 (0.67–0.97) | .818/0.0 | 0.998 |
| Caucasian | 13 (1836/1689) | 0.99 (0.86–1.15) | .235/20.7 | – | 1.06 (0.71–1.57)∗ | .002/62.1 | – | 1.01 (0.83–1.24)∗ | .032/46.8 | – | 1.04 (0.74–1.47)∗ | .006/56.6 | – | 1.03 (0.86–1.24)∗ | .001/64.4 | – |
| East Asian | 27 (5587/4803) | 1.37 (1.21–1.56)∗ | .038/35.2 | 0.111 | 2.07 (1.70–2.51)∗ | <.001/57.0 | <0.001 | 1.57 (1.37–1.80)∗ | .001/52.1 | <0.001 | 1.70 (1.44–1.96)∗ | .001/51.5 | <0.001 | 1.45 (1.31–1.60)∗ | <.001/63.2 | <0.001 |
| West Asian | 7 (1199/1244) | 1.36 (1.14–1.61) | .471/0.0 | 0.928 | 2.15 (1.60–2.90) | .879/0.0 | 0.031 | 1.47 (1.25–1.74) | .653/0.0 | 0.268 | 1.86 (1.40–2.48) | .823/0.0 | 0.479 | 1.42 (1.26–1.62) | .895/0.0 | 0.012 |
| South Asian | 8 (2,464/1,741) | – | <.001/77.3 | – | 2.70 (1.14–6.40)∗ | .002/71.6 | 0.997 | – | <.001/80.6 | – | 2.42 (1.14–5.13)∗ | .011/63.9 | 0.997 | – | <.001/82.8 | – |
| Source of controls | ||||||||||||||||
| HB | 53 (10435/9444) | 1.25 (1.13–1.38)∗ | <.001/50.2 | 0.408 | 1.77 (1.48–2.12)∗ | <.001/65.3 | <0.001 | 1.37 (1.23–1.53)∗ | <.001/64.5 | 0.002 | 1.54 (1.34–1.77)∗ | <.001/56.5 | <0.001 | 1.33 (1.22–1.45)∗ | <.001/71.4 | <0.001 |
| PB | 6 (1,332/1,147) | – | .001/75.5 | – | 1.50 (0.79–2.86)∗ | .15/64.4 | – | – | <.001/ 81.4 | – | 1.31 (0.76–2.24)∗ | .049/ 55.0 | – | – | <.001/85.0 | – |
| Infertility type | ||||||||||||||||
| Azoospermia | 24 (2,241/4,952) | 1.27 (1.13–1.42) | .101/28.1 | 0.619 | 1.45 (1.09–1.93)∗ | .001/55.7 | 0.995 | 1.30 (1.11–1.53)∗ | .003/50.1 | 0.981 | 1.29 (1.00–1.66)∗ | .002/51.6 | 0.999 | 1.23 (1.07–1.42)∗ | <.001/65.4 | 0.993 |
| OAT∗ | 37 (5670/7062) | 1.25 (1.09–1.44)∗ | <.001/58.0 | 0.986 | 1.75 (1.39–2.19)∗ | <.001/63.4 | 0.049 | 1.37 (1.18–1.59)∗ | <.001/67.9 | 0.619 | 1.59 (1.33–1.89)∗ | <.001/52.1 | 0.009 | 1.35 (1.20–1.52)∗ | <.001/73.7 | 0.047 |
In subgroup analyses by ethnicity and source of controls, a significantly increased male infertility risk was found in Africans (CT + TT vs CC: OR = 0.78, 95% CI: 0.62–0.99, Ph = .507, I2 = 0.0%; T vs C: OR = 0.80, 95% CI: 0.67–0.97, Ph = .818, I2 = 0.0%), East Asians (CT vs CC: OR = 1.37, 95% CI: 1.21–1.56, Ph = .038, I2 = 35.2%, Fig. 2; TT vs CC: OR = 2.07, 95% CI: 1.70–2.51, Ph < .001, I2 = 57.0%; CT + TT vs CC: OR = 1.57, 95% CI: 1.37–1.80, Ph = .001, I2 = 52.1%; TT vs CC + CT: OR = 1.70, 95% CI: 1.44–1.96, Ph = .001, I2 = 51.5%; T vs C: OR = 1.45, 95% CI: 1.31–1.60, Ph < .001, I2 = 63.2%), West Asians (CT vs CC: OR = 1.36, 95% CI: 1.14–1.61, Ph = .471, I2 = 0.0%; TT vs. CC: OR = 2.15, 95% CI: 1.60–2.90, Ph = .879, I2 = 0.0%, Fig. 3; CT + TT vs. CC: OR = 1.47, 95% CI: 1.25–1.74, Ph = .653, I2 = 0.0%; TT vs CC + CT: OR = 1.86, 95% CI: 1.40–2.48, Ph = .823, I2 = 0.0%; T vs C: OR = 1.42, 95% CI: 1.26–1.62, Ph = .895, I2 = 0.0%), South Asians (TT vs CC: OR = 2.70, 95% CI: 1.14–6.40, Ph = .002, I2 = 71.6%; TT vs CC + CT: OR = 2.42, 95% CI: 1.14–5.13, Ph = .011, I2 = 63.9%), and hospital-based studies (CT vs CC: OR = 1.25, 95% CI: 1.13–1.38, Ph < .001, I2 = 50.2%; TT vs CC: OR = 1.77, 95% CI: 1.48–2.12, Ph < .001, I2 = 65.3%; CT + TT vs CC: OR = 1.37, 95% CI: 1.23–1.53, Ph < .001, I2 = 64.5%; TT vs CC + CT: OR = 1.54, 95% CI: 1.34–1.77, Ph < .001, I2 = 56.5%; T vs C: OR = 1.33, 95% CI: 1.22–1.45, Ph < .001, I2 = 71.4%). In subgroup analysis by infertility type, the MTHFR C677T polymorphism was also associated with increased azoospermia (CT vs. CC: OR = 1.27, 95% CI: 1.13–1.42, Ph = .101, I2 = 28.1%; TT vs CC: OR = 1.45, 95% CI: 1.09–1.93, Ph = .001, I2 = 55.7%; (CT + TT) vs. CC: OR = 1.30, 95% CI: 1.11–1.53, Ph = .003, I2 = 50.1%; TT vs (CC + CT): OR = 1.29, 95% CI: 1.00–1.66, Ph = .002, I2 = 51.6%; T vs C: OR = 1.23, 95% CI: 1.07–1.42, Ph < .001, I2 = 65.4%) and OAT risk (CT vs CC: OR = 1.25, 95% CI: 1.09–1.44, Ph < .001, I2 = 58.0%; TT vs. CC: OR = 1.75, 95% CI: 1.39–2.19, Ph < .001, I2 = 63.4%; CT + TT vs CC: OR = 1.37, 95% CI: 1.18–1.59, Ph < .001, I2 = 67.9%; TT vs (CC + CT): OR = 1.59, 95% CI: 1.33–1.89, Ph < .001, I2 = 52.1%; T vs C: OR = 1.35, 95% CI: 1.20–1.52, Ph < .001, I2 = 73.7%).
Figure 2.
Forest plot of MTHFR C677T polymorphism and male infertile risk in East Asians (CT vs CC).
Figure 3.
Forest plot of MTHFR C677T polymorphism and male infertile risk in West Asians (TT vs CC).
Obvious heterogeneity was observed in the current meta-analysis, as also shown in Table 2. I2 > 75% was found in South Asians (CT vs. CC: I2 = 77.3%, (CT + TT) vs. CC: I2 = 80.6%, T vs. C: I2 = 82.8%) and population-based studies (CT vs. CC: I2 = 75.5%, (CT + TT) vs CC: I2 = 81.4%, T vs C: I2 = 85.0%). Then, a meta-regression analysis method was applied to explore the sources of heterogeneity and the results indicate that ethnicity (TT vs CC: P = .014; TT vs (CC + CT): P = .008; T vs C: P = .021) and HWE (TT vs CC: P = .041; TT vs (CC + CT): P = .020) were sources of heterogeneity.
The results of sensitivity analysis were shown in Table 3. It is not clear whether the MTHFR C677T polymorphism is associated with increased male infertility risk in South Asians. The results did not pool because I2 > 75% was observed in any genetic model. Another results did not change, such as overall population, Africans, East Asians, West Asians, and so on.
No significant publication bias was found by Begg funnel plot shape (supplemental Figs. 1, –5) and Egger test (CT vs. CC: P = .418,TT vs CC: P = .203, CT + TT vs CC: P = .274, CT + TT vs CC: P = .179, T vs C: P = .402) in the overall analysis.
An BFDP test was used to further investigate significant associations in this study, as shown in Tables 4 and 5. Significantly increased male infertility risk was considered as “noteworthy” in the overall population (CT vs CC: BFDP = 0.106, TT vs CC: BFDP < 0.001, CT + TT vs. CC: BFDP = 0.001, TT vs. CT + CC: BFDP < 0.001, T vs. C: BFDP < 0.001), East Asians (CT vs. CC: BFDP = 0.111, TT vs. CC: BFDP < 0.001, CT + TT vs. CC: BFDP < 0.001, TT vs CT + CC: BFDP < 0.001, T vs C: BFDP < 0.001), West Asians (TT vs. CC: BFDP = 0.031, CT + TT vs CC: BFDP = 0.268, TT vs. CT + CC: BFDP = 0.479, T vs. C: BFDP = 0.012), hospital-based studies (CT vs CC: BFDP = 0.408, TT vs. CC: BFDP < 0.001, CT + TT vs. CC: BFDP = 0.002, TT vs. CT + CC: BFDP < 0.001, T vs C: BFDP < 0.001), azoospermia (CT vs. CC: BFDP = 0.619), and OAT (TT vs. CC: BFDP = 0.049, CT + TT vs CC: BFDP = 0.619, TT vs CT + CC: BFDP = 0.009, T vs. C: BFDP = 0.047) for MTHFR C677T polymorphism.
Table 5.
The results of sensitivity analysis between MTHFR C667T polymorphism with male infertility.
| CT vs. CC | TT vs. CC | (CT + TT) vs. CC | TT vs. (CC + CT) | T vs. C | ||||||||||||
| Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | OR (95% CI) | Ph/I2 | OR (95% CI) | Ph/I2 | BFDP | ||
| Quality score ≥ 5 and HWE | ||||||||||||||||
| Overall | 23 (6827/6355) | 1.22 (1.06–1.41)∗ | <.001/65.1 | 0.995 | 1.60 (1.29–1.97)∗ | <.001/59.6 | 0.294 | 1.31 (1.13–1.52)∗ | <.001/72.1 | 0.937 | 1.41 (1.21–1.64)∗ | .035/37.9 | 0.300 | 1.27 (1.14–1.41)∗ | <.001/72.4 | 0.336 |
| African | 1 (344/690) | 0.77 (0.59–1.01) | – | – | 0.67 (0.39–1.15) | – | – | 0.75 (0.58–0.98) | – | 0.998 | 0.74 (0.44–1.26) | – | – | 0.80 (0.64–0.98) | – | 0.998 |
| Caucasian | 6 (1111/1202) | 1.03 (0.87–1.23) | .405/1.7 | – | 1.24 (0.95–1.64) | .188/33.0 | – | 1.07 (0.91–1.26) | .426/0.0 | – | 1.23 (0.95–1.59) | .167/36.0 | – | 1.09 (0.96–1.24) | .271/21.7 | – |
| East Asian | 7 (2753/2299) | 1.37 (1.13–1.68)∗ | .046/53.1 | 0.985 | 1.77 (1.39–2.24)∗ | .092/44.8 | 0.089 | 1.47 (1.19–1.81)∗ | .016/61.7 | 0.903 | 1.43 (1.25–1.64) | .864/ 0.0 | 0.020 | 1.33 (1.22–1.44) | .148/ 36.7 | <0.001 |
| West Asian | 4 (693/867) | 1.22 (0.99–1.52) | .397/0.0 | – | 2.16 (1.50–3.11) | .898/0.0 | 0.584 | 1.36 (1.11–1.66) | .584/0.0 | 0.985 | 1.93 (1.37–2.73) | .743/0.0 | 0.861 | 1.36 (1.17–1.59) | .876/0.0 | 0.829 |
| South Asian | 5 (1926/1297) | – | <.001/83.3 | – | – | <.001/78.5 | – | – | <.001/86.4 | – | 2.35 (0.97–5.68)∗ | .007/ 71.7 | – | – | <.001/88.5 | – |
| HB | 18 (5572/5321) | 1.15 (1.00–1.33)∗ | .001/59.8 | 0.999 | 1.59 (1.27–2.00)∗ | <.001/60.4 | 0.726 | 1.23 (1.06–1.44)∗ | <.001/69.1 | 0.996 | 1.43 (1.22–1.66)∗ | .083/ 33.5 | 0.126 | 1.21 (1.09–1.35)∗ | <.001/67.4 | 0.969 |
| PB | 5 (1255/1034) | 1.57 (1.07–2.30)∗ | .005/73.4 | 0.996 | 1.86 (0.92–3.76)∗ | .024/64.3 | – | – | .001/79.6 | – | 1.53 (0.83–2.81)∗ | .049/58.2 | – | – | <.001/84.6 | – |
| Azoospermia | 11 (1352/3370) | 1.31 (1.06–1.64)∗ | .021/52.3 | 0.997 | 1.79 (1.30–2.48)∗ | .074/41.3 | 0.927 | 1.40 (1.12–1.77)∗ | .004/ 61.3 | 0.991 | 1.48 (1.20–1.82) | .144/31.9 | 0.873 | 1.34 (1.12–1.61)∗ | .001/65.2 | 0.981 |
| OAT∗ | 14 (3191/4470) | 1.19 (0.98–1.44)∗ | <.001/66.0 | – | 1.46 (1.14–1.87)∗ | .045/ 42.7 | 0.984 | 1.25 (1.03–1.53)∗ | <.001/69.9 | 0.998 | 1.38 (1.19–1.60) | .226/ 21.0 | 0.494 | 1.24 (1.07–1.44)∗ | <.001/70.1 | 0.993 |
However, the positive results by sensitivity analysis (Table 5) were only considered as “noteworthy” in the overall population (TT vs. CC: BFDP = 0.294, CT + TT vs. CC: BFDP = 0.300, T vs. C: BFDP = 0.336), East Asians (TT vs. CC: BFDP = 0.089, TT vs. CT + CC: BFDP = 0.020, T vs. C: BFDP < 0.001), West Asians (TT vs. CC: BFDP = 0.584), hospital-based studies (TT vs. CC: BFDP = 0.726, TT vs. CT + CC: BFDP = 0.126), and OAT (TT vs. CT + CC: BFDP = 0.494) for MTHFR C677T polymorphism.
3.2.2. MTHFR A1298C polymorphism
Table 6 shows the results of meta-analysis on the association between the MTHFR A1298C polymorphism and male infertility risk. No significantly increased male infertility risk was found in all eligible studies. In subgroup analyses by ethnicity and source of controls, a significantly increased male infertility risk was found in East Asians (AC vs. AA: OR = 1.37, 95% CI: 1.20–1.56, Ph = 0.515, I2 = 0.0%; CC vs. AA: OR = 1.88, 95% CI: 1.10–3.20, Ph = 0.006, I2 = 62.7%; (AC + CC) vs. AA: OR = 1.42, 95% CI: 1.25–1.62, Ph = 0.106, I2 = 39.3%; CC vs. (AA + AC): OR = 1.69, 95% CI: 1.04–2.75, Ph = 0.020, I2 = 55.8%; C vs. A: OR = 1.35, 95% CI: 1.13–1.60, Ph = 0.016, I2 = 57.3%) and population-based studies (C vs. A: OR = 1.53, 95% CI: 1.28–1.83, Ph = 0.767, I2 = 0.0%). Moreover, no significant association was observed in subgroup analysis by infertility type.
Table 6.
Meta-analysis of the association of MTHFR A1298C polymorphism with male infertility.
| AC vs AA | CC vs AA | (AC + CC) vs. AA | CC vs. (AA + AC) | C vs. A | ||||||||||||
| Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | BFDP | OR (95% CI) | Ph/I2 | BFDP |
| Overall | 28 (5,976/5,774 | 1.08 (0.96–1.22)∗ | .002/48.6 | – | 1.28 (0.99–1.67)∗ | <.001/63.5 | – | 1.11 (0.98–1.26)∗ | <.001/59.3 | – | 1.25 (0.99–1.58)∗ | <.001/58.4 | – | 1.11 (0.99–1.24)∗ | <.001/66.5 | – |
| Ethnicity | ||||||||||||||||
| South Asian | 4 (1,176/585) | 1.08 (0.75–1.55)∗ | .065/58.5 | – | – | <.001/87.2 | – | – | .004/77.3 | – | – | <.001/83.6 | – | – | <.001/87.9 | – |
| West Asian | 7 (1057/1039) | 0.86 (0.71–1.04) | .702/0.0 | – | 0.91 (0.69–1.21) | .816/ 0.0 | – | 0.87 (0.73–1.04) | .601/ 0.0 | – | 0.99 (0.76–1.29) | .940/ 0.0 | – | 0.93 (0.81–1.06) | .646/ 0.0 | – |
| East Asian | 9 (2123/2094) | 1.37 (1.20–1.56) | .515/0.0 | 0.111 | 1.88 (1.10–3.20)∗ | .006/ 62.7 | 0.996 | 1.42 (1.25–1.62) | .106/ 39.3 | 0.012 | 1.69 (1.04–2.75)∗ | .020/ 55.8 | 0.997 | 1.35 (1.13–1.60)∗ | .016/ 57.3 | 0.949 |
| Caucasian | 6 (1120/1133) | 1.06 (0.89–1.26) | .158/37.3 | – | 0.88 (0.65–1.17) | .551/0.0 | – | 1.02 (0.87–1.21) | .541/0.0 | – | 0.86 (0.65–1.13) | .159/37.2 | – | 0.98 (0.86–1.11) | .822/0.0 | – |
| Source of controls | ||||||||||||||||
| HB | 25 (5,306/5145) | 1.09 (0.96–1.24)∗ | .001/52.1 | – | 1.30 (0.97–1.74)∗ | <.001/66.7 | – | 1.11 (0.97–1.28)∗ | <.001/62.9 | – | 1.26 (0.97–1.63)∗ | <.001/61.2 | – | 1.11 (0.99–1.25)∗ | <.001/69.4 | – |
| PB | 3 (670/629) | 1.05 (0.83–1.33) | .309/14.7 | – | 1.15 (0.75–1.77) | .356/3.1 | – | 1.08 (0.86–1.35) | .471/ 0.0 | – | 1.19 (0.79–1.80) | .219/ 34.1 | – | 1.53 (1.28–1.83) | .767/ 0.0 | 0.139 |
| Infertility type | ||||||||||||||||
| Azoospermia | 12 (1,140/2,610) | 1.01 (0.86–1.18) | .316/13.1 | – | 1.21 (0.91–1.61) | .117/ 34.2 | – | 1.04 (0.90–1.21) | .212/ 23.5 | – | 1.21 (0.92–1.58) | .131/ 32.4 | – | 1.06 (0.95–1.19) | .109/35.1 | – |
| OAT∗ | 12 (1,664/2,759) | 0.98 (0.86–1.12) | .198/25.0 | – | 1.16 (0.91–1.47) | .248/19.9 | – | 1.01 (0.89–1.15) | .141/31.3 | – | 1.17 (0.93–1.47) | .375/7.2 | – | 1.04 (0.94–1.15) | .173/27.2 | – |
Obvious heterogeneity was observed in the current meta-analysis, as also shown in Table 6.
The results indicate that quality score of the eligible studies (AC vs. AA: P = .038, CC vs. AA: P = .013, (AC + CC) vs. AA: P = .009, CC vs. (AA +AC): P = .024, C vs. A: P = .003) was source of heterogeneity by a meta-regression analysis method.
The results of sensitivity analysis was shown in Table 7 indicating that the results are stable except in West Asians. Significant increased male infertility risk was observed in West Asians (AC vs AA: OR = 0.79, 95% CI: 0.62–1.00, Ph = .586, I2 = 0.0%).
Table 7.
The results of sensitivity analysis between MTHFR A1298C polymorphism with male infertility.
| AC vs AA | CC vs AA | (AC + CC) vs AA | CC vs (AA + AC) | C vs A | ||||||||||||
| Variable | n (Cases/Controls) | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP | OR (95% CI) | Ph/I2 (%) | BFDP |
| Quality score ≥ 5 and HWE | ||||||||||||||||
| Overall | 13 (3198/2895) | 1.00 (0.89–1.11) | .371/7.5 | – | 0.92 (0.76–1.12) | .562/0.0 | – | 1.00 (0.89–1.11) | .296/14.7 | – | 0.96 (0.80–1.16) | .689/0.0 | – | 0.99 (0.91–1.08) | .292/15.1 | – |
| South Asian | 2 (790/336) | 0.90 (0.66–1.23) | .182/43.8 | – | 0.64 (0.40–1.02) | .581/0.0 | – | 0.85 (0.55–1.32)∗ | .148/52.2 | – | 0.75 (0.51–1.11) | .975/ 0.0 | – | 0.86 (0.70–1.06) | .257/ 22.1 | – |
| West Asian | 4 (559/737) | 0.79 (0.62–1.00) | .586/0.0 | 0.999 | 0.83 (0.58–1.18) | .786/0.0 | – | 0.80 (0.64–1.00) | .579/ 0.0 | 0.999 | 0.94 (0.68–1.31) | .862/ 0.0 | – | 0.87 (0.74–1.03) | .681/ 0.0 | – |
| East Asian | 4 (1191/1045) | 1.20 (1.00–1.45) | .973/0.0 | 0.999 | 1.36 (0.88–2.11) | .834/0.0 | – | 1.22 (1.03–1.46) | .985/ 0.0 | 0.998 | 1.28 (0.83–1.98) | .822/ 0.0 | – | 1.19 (1.03–1.38) | .981/ 0.0 | 0.998 |
| Caucasian | 3 (658/777) | 0.97 (0.78–1.21) | .701/0.0 | – | 0.96 (0.67–1.37) | .248/28.3 | – | 0.97 (0.79–1.20) | 1.000/ 0.0 | – | 1.04 (0.62–1.75)∗ | .114/ 53.9 | – | 0.98 (0.84–1.15) | .546/ 0.0 | – |
| HB | 10 (2528/2266) | 0.98 (0.86–1.11) | .321/13.2 | – | 0.87 (0.70–1.08) | .608/0.0 | – | 0.97 (0.86–1.10) | .214/ 24.9 | – | 0.92 (0.75–1.12) | .844/0.0 | – | 0.97 (0.88–1.06) | .273/ 18.5 | – |
| Azoospermia | 5 (556/1018) | 1.03 (0.82–1.29) | .292/19.2 | – | 0.86 (0.57–1.30) | .700/0.0 | – | 1.01 (0.82–1.26) | .267/ 23.1 | – | 0.88 (0.60–1.30) | .848/0.0 | – | 0.98 (0.83–1.16) | .403/0.5 | – |
| OAT∗ | 6 (736/1421) | 1.01 (0.84–1.23) | .427/0.0 | – | 1.03 (0.73–1.46) | .531/0.0 | – | 1.03 (0.85–1.23) | .349/10.4 | – | 1.06 (0.76–1.49) | .659/0.0 | – | 1.03 (0.89–1.19) | .370/7.3 | – |
Significant publication was observed by the Begg funnel plot shape (Figures not shown) and Egger test (CC vs. AA: P = 0.032; CC vs. (AA + AC): P = .024) in the overall analysis. Supplemental Figs.6 –7, list the Begg's funnel plots by the trim and fill method. Notably, log OR and 95% CI did not change.
An BFDP test was also applied to further investigate significant associations between MTHFR A1298C and male infertility risk, as shown in Tables 4 and 5. Significantly increased male infertility risk was considered as “noteworthy” in the East Asians (AC vs AA: BFDP = 0.111, AC + CC vs AA: BFDP = 0.012) and population-based studies (C vs A: BFDP = 0.139). However, we did not find that the positive results of sensitivity analysis were considered as “noteworthy” in the overall and all subgroup analyses.
4. Discussion
In 2001, Bezold et al.[33] first investigated the association between the MTHFR C667T polymorphism and male infertility risk. In 2005, Park et al.[37] first explored the MTHFR A1298C polymorphism with male infertility risk. Since then a lot of case–control studies have investigated the associations but the results are still inconsistent. Here, an updated and high quality meta-analysis was carried out to explore the above two gene polymorphism with male infertility risk.
Overall, the MTHFR C677T polymorphism was associated with increased male infertility risk in overall populations, Africans, East Asians, West Asians, South Asians, hospital-based studies, azoospermia and OAT. In addition, a significantly increased male infertility risk was also found in East Asians and population-based studies for the MTHFR A1298C polymorphism. The pooled data was analyzed using five different genetic models and several subgroup analyses in this study. Under the circumstances, the P-value must be adjusted to explain the multiple comparisons.[93] In addition, random error and bias were common in the studies with small sample sizes so that the results were unreliable, especially in molecular epidemiological studies. Wakefield et al.[101] in 2007 proposed a more precise Bayesian measure of false discovery in genetic epidemiology studies, for determining the “noteworthiness” of the positive association. Hence, we used BFDP test to assess the false discovery in the current meta-analysis. Finally, the positive results by sensitivity analysis were only considered as “noteworthy” in the overall population and OAT for MTHFR C677T polymorphism. We did not find that the positive results of sensitivity analysis were considered as “noteworthy” in the overall and all subgroup analyses for MTHFR A1298C.
Based on biochemical properties described for MTHFR C677T and A1298C polymorphisms, we expected that the two genes were associated with risk of male infertility risk risk in all races. However, we only observed that MTHFR C677T is associated with increased male infertility risk in East Asians and West Asians, but not other races (such as Caucasians and Africans). Moreover, no significant association was observed on MTHFR A1298C polymorphism with male infertility risk in any race. Hence, an ethnic variant in the frequency of MTHFR C677T polymorphism was demonstrated in different populations. The frequency of the 677T allele ranges from 30.5 to 42% among Asian population, from 32.2 to 44% in Caucasians. African population shows a lower frequency of T allele, ranging from 6 to 10.3%.[102,103] These results indicated that the same genes may play different roles in different races and countries, because infertility is a complicated multigenetic disease, and different genetic backgrounds and environmental factor (smoking or life style) may contribute to the discrepancy. Another possible explanation for the difference suggested the influence of the genetic variant might be masked by the presence of other as-yet unidentified causal genes involved in male infertility. The current studies demonstrated a clear north-to-south gradient in the effect of the MTHFR C677T variant in the determination of hyperhomocysteinemia, suggesting that diet is a relevant environmental agent, being the presence of folates in the food higher in the South of Europe than in the North. In addition, there was also the presence of folates in the food higher in the Caucasians than in the Asians. Obvious heterogeneity was observed in the current meta-analysis, as also shown in Tables 2 and 4. Ethnicity and HWE were sources of heterogeneity for MTHFR C677T polymorphism and quality score of the eligible studies was source of heterogeneity by a meta-regression analysis method. HWD may be genotyping errors and selection bias in molecular epidemiological studies. Small sample studies were easier to accept if there were positive reports as they tend to yield false-positive results because they may be not rigorous and are often of low-quality. Supplemental Fig. 3, indicated that the asymmetry of the funnel plot was caused by studies of low-quality small samples. Therefore, we performed a sensitivity analysis restricted to studies that only included high-quality articles and controls in HWE.
15 previous meta-analyses[15,17,19–29,31,32] have been reported on the MTHFR C677T polymorphism with male infertility risk (as shown in Table 6). Yang et al.[15] and Wei et al.[24] showed that the MTHFR C677T polymorphism was associated with a significantly increased male infertility risk in the overall and Asian populations. Zhu et al.[17] suggested the MTHFR C677T polymorphism is capable of causing male infertility susceptibility, especially in Asians, azoospermia and OAT. Hong et al[19] demonstrated that the MTHFR C677T polymorphism is associated with male infertility in East-asian populations, Middle-eastern populations, and mixed-race. Tüttelmann et al[20] and Nikzad et al[28] indicated that the MTHFR C677T polymorphism is associated with male infertility in overall populations. Wu et al.[21] supported that the MTHFR C677T polymorphism was capable of causing male infertility susceptibility in Asians and azoospermia. Gong et al[23] and Liu et al.[26]indicated that the MTHFR polymorphism was associated with an increased risk of male infertility in overall populations, especially in Asians and Caucasians and subgroups of azoospermia and OAT. Weiner et al.[29] suggested that the MTHFR C677T polymorphism was associated with an increased risk of male infertility in overall populations and subgroup of azoospermia. Gupta et al[31] supported that the MTHFR C677T polymorphism was associated with an increased risk of male infertility in overall populations and subgroups of azoospermia and OAT. Ullah et al[32] indicated that the MTHFR C677T polymorphism was associated with an increased risk of male infertility in Caucasians for middle income countries. Rai et al[22] and Shi et al[27] supported an association between C677T polymorphism and male infertility in Asians. Ren et al[25] suggested that the MTHFR C667T polymorphism may contribute to the genetic susceptibility to male infertility in the Chinese population. In addition, ten previous meta-analyses[15,16,18,20,24–27,30,32] have also been published on the MTHFR A1298C polymorphism with male infertility risk (as shown in Table 7). Among these publications, one study[32] investigated this issue in Caucasians, one study[27] in Asians, one study[25] in Chinese population, and seven studies[15,16,18,20,24,26,30] in overall populations. Ullah et al[32] indicated that the MTHFR A1298C polymorphism was associated with an increased risk of male infertility in Caucasians for low income countries. Shi et al[27] supported that MTHFR A1298C polymorphism was the risk factor with susceptibility to male infertility in Asians, especially in East Asians. Ren et al[25] demonstrated that MTHFR A1298C polymorphism may be unrelated to male infertility risk in Chinese population. Yang et al[15] suggested that there was a significant association between the A1298C polymorphism and male infertility risk in the Asian, Caucasian, and overall groups. Zhang et al[16] indicated that the MTHFR A1298C polymorphism may be a potential risk factor for male infertility, especially in the Asian population. Shen et al[18] and supported that the MTHFR A1298C polymorphism was capable of causing male infertility susceptibility, especially azoospermia. Tüttelmann et al,[20] Wei et al,[24] Gupta et al,[30]and Liu et al[26] indicated that the MTHFR A1298C polymorphism was not associated with male infertility susceptibility. However, quality assessment of the eligible studies was not performed in 13 previous meta-analyses.[15,17,18,20–24,28–32] In addition, the false-positive report probabilities of statistically significant association and statistical power was not evaluated in all previous meta-analyses except the study of Liu et al.[26] Moreover, many new studies have been published, therefore, an updated meta-analysis should be carried out.
This study has several advantages over previous meta-analyses.[15–32] First, the sample size was much larger, 59 studies on MTHFR C677T (11,767 male infertility cases and 10,591 controls) and 28 studies on MTHFR A1298C (5,976 male infertility cases and 5,774 controls) were identified in overall population. Second, this is the first meta-analysis to explore a false-positive report probability by BFDP method. Third, an important sensitivity analysis was performed on studies that were high-quality and HWE. Although we have put considerable effort and resources into testing possible associations between MTHFR C677T and A1298C polymorphisms and male infertility risk, there are still some limitations inherited from the published studies. First, the controls were not uniformly defined. Second, no data were extracted on exploring interaction between gene and environment.
In summary, this study indicates that the MTHFR C677T polymorphism is associated with increased male infertility risk in East Asians, West Asians, and OAT. Other significant association should be interpreted with caution and may most likely result from false-positive results, rather than from true associations or biological factors.
Author contributions
Conceptualization: Xiao-Feng He and Xiang-Hua Ye.
Data curation: Li-Juan Han and Xiao-Feng He.
Formal analysis: Xiao-Feng He.
Investigation: Li-Juan Han and Xiang-Hua Ye.
Methodology: Li-Juan Han and Xiao-Feng He.
Resources: Xiao-Feng He and Xiang-Hua Ye.
Software: Xiao-Feng He
Supervision: Xiao-Feng He and Xiang-Hua Ye.
Validation: Xiao-Feng He and Xiang-Hua Ye.
Visualization: Xiao-Feng He
Writing – original draft: Li-Juan Han
Writing – review & editing: Xiao-Feng He and Xiang-Hua Ye.
Supplementary Material
Supplementary Material
Supplementary Material
Supplementary Material
Supplementary Material
Supplementary Material
Supplementary Material
Supplementary Material
Footnotes
Abbreviations: BFDP = Bayesian false discovery probability, CIs = confidence intervals, HWD = Hardy-Weinberg dis-equilibrium, HWE = Hardy-Weinberg equilibrium, MTHFR = methylenetetrahydrofolate reductase, OAT = oligoasthenoteratozoospermia, ORs = odds ratios.
How to cite this article: Han LJ, He XF, Ye XH. Methylenetetrahydrofolate reductase C677T and A1298C polymorphisms and male infertility risk: an updated meta-analysis. Medicine. 2020;99:51(e23662).
This is a meta-analysis, hence, ethical approval was waived or not necessary
This study was designed by Xiao-Feng He and Xiang-Hua Ye. Li-Juan Han and Xiao-Feng He did the literature search, study quality assessment, and data extraction. Xiao-Feng He performed the statistical analysis and drafted the tables and figures. Li-Juan Han wrote the first draft of this analysis, and Xiao-FH and XHY helped to finish the final version. All authors approved the conclusions of our study.
The authors have no funding and conflicts of interest to disclose.
The datasets generated during and/or analyzed during the current study are publicly available.
Azoospermia.
Including oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia. NA = not available.
Azoospermia, 2 Including oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia.
1Including Oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia. HB = hospital-based studies, PB = population-based studies.
Including Oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia.
Including Oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia.
Including Oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia.
Including Oligoasthenoteratozoospermia (OAT), severe OAT, oligozoospermia, and teratozoospermia.
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