Our study showed that maternal FAS, and offspring MTRR gene polymorphisms were significantly associated with the risk of CHD and its subtypes of Chinese descent. Additionally, there were statistically significant interactions between maternal FAS and offspring MTRR gene polymorphisms on the risk of CHD in offspring. As far as we know, this is the first time that the association of maternal FAS, offspring MTRR gene polymorphisms, and their interactions with the risk of CHD has been exhaustively explored, which highlight the importance of periconception FAS and may help provide new clues for future etiological studies and interventions in CHD.
In this study, we observed that maternal FAS significantly reduced the risk of CHD (OR = 0.55) in the offspring. The results of previous studies on this topic were consistent with our findings. For example, Wang et al.[21] analyzed the data from the birth defect surveillance system in a district of Beijing and found that periconception maternal FAS could significantly reduce the risk of CHD in their offspring. Additionally, case-control studies from the United States[22], New Zealand[23], and a birth cohort study from Gansu Province, China[24] also observed a protective effect of maternal FAS against multiple types of CHD. It is well known that maternal FAS during pregnancy has been the main preventive measure to reduce the risk of neural tube malformations in offspring[4]. In recent years, more and more studies have shown that maternal FAS also has a preventive effect on CHD in offspring[7, 9]. Besides, there were studies claiming that folic acid can prevent birth defects such as cleft lip and palate, as well as cardiovascular diseases and cancers[5, 6]. The present study also found that the protective effect of maternal FAS on CHD subtypes such as ASD (OR = 0.25), VSD (OR = 0.42) and CTD (OR = 0.23) was stronger than that of total CHD, and the same results were observed in the studies of Morikawa et al.[13] and Qu et al.[25]. Folic acid played an irreplaceable role in one-carbon metabolism and was essential for the synthesis of many substances such as proteins, deoxyribonucleic acid, neurotransmitters and phospholipids[26–28]. Folic acid deficiency could affect this process, which could lead to the development of a variety of diseases. Therefore, our findings emphasized that women of childbearing age should raise their awareness of FAS and ensure adequate intake of folic acid during periconception to prevent CHD in offspring.
This study also assessed the association of MTRR gene polymorphisms with the risk of CHD. The results suggested that polymorphisms of the MTRR gene at rs162048, rs1802059, rs10380 and rs1801394 were significantly associated with the risk of CHD and its subtypes. Previous studies reached the same conclusion[29–31], and proposed that MTRR gene polymorphism might be associated with acyanotic CHD[32]. It was worth noting that there were also some studies suggesting that the association between MTRR gene polymorphism and CHD was not statistically significant. For example, a case-control study from the Netherlands did not find a significant association between offspring MTRR rs1801394 polymorphism and the development of heart defects in children[19]. The inconsistent results may be explained by racial differences, differences in study design, and the presence of bias. In addition, several studies confirmed that MTRR gene polymorphisms were also associated with a variety of cancers[33–35], reproductive disorders[36] and cleft lip and palate[37]. However, most of the previous studies only focused on two loci (i.e., rs1801394 and rs1532268). Our study was the first to explore the relationship between five polymorphisms (rs162048, rs1802059, rs10380, rs1801394 and rs3776455) of MTRR gene and the risk of CHD and its subtypes, which provided new clues for further research on MTRR gene variants associated with CHD. MTRR gene was one of the key regulatory enzymes involved in homocysteine metabolism pathway, and homocysteine was an independent risk factor for CHD[14], so there was a high possibility that MTRR gene polymorphisms might be associated with CHD. MTRR maintained sufficient levels of activated cobalamin, which served as a cofactor for MTR. In the process of MTR catalyzed re-methylation of homocysteine to methionine, cobalamin acted as an intermediated methyl carrier between methyltetrahydrofolate and homocysteine. The cobalamin cofactor cycled between the cob (I) alamin and methyl cob (III) alamin, but the cob (I) alamin could be oxidized to the unactivated cob (II) alamin form, and for it to regain activity, cob (II) needed to be converted to the methyl cob (III) alamin form by obtaining a methyl donor for S-adenosylmethionine catalyzed by MTRR. This cycle ensured the activity of MTR, and MTRR acted as a “companion” that played an important role in keeping MTR in an active state[38]. Therefore, MTRR gene mutation might change the concentration of homocysteine and thus affected the normal development of embryonic heart, and studies confirmed this speculation[14]. However, the specific mechanism will still require further studies to elucidate.
In this study, we also found an interaction between maternal FAS and offspring MTRR gene polymorphisms at rs1802059 in the development of CHD. This study showed that children who carried the variant genotype and with maternal FAS had a significantly lower risk of developing CHD compared with children who carried the variant genotype but without maternal FAS. This suggested that maternal FAS might help to offset some of the risk of developing CHD due to variations in the MTRR gene in offspring. The interaction between maternal FAS and offspring MTRR gene polymorphisms in CHD had not been investigated before, and this study was the first to find this association. Previous literatures indicated that the insufficient activity of MTRR and MTR caused by MTRR gene variation will decrease the utilization rate of folic acid and increase the level of Hcy, which might eventually induce vascular endothelial injury and cardiovascular diseases[39, 40]. Additionally, insufficient maternal folic acid intake may affect the methylation of genes related to fetal growth and development[41–43]. We speculate that this might be a mechanism of interaction between maternal FAS and offspring MTRR gene polymorphisms. However, the exact mechanism remained unclear and required further study. More and more evidence show that the interaction of gene and environment could affect CHD, so the influence of interaction would be one of the future research directions of CHD.
There were some limitations in this study. First, selection bias was hard to avoid. This study was a hospital-based case-control study, and our target population only included those children with CHD who were born successfully and survived. We did not know the cases of termination of pregnancy due to CHD or death after birth due to severe CHD, which may lead to a series of problems, such as the representativeness of the sample. Second, this study classified the exposure to maternal FAS as "yes" and "no", and did not investigate other ways of folic acid intake, such as dietary conditions, so that the true folic acid level of each pregnant woman during the perinatal period could not be obtained, which may affect the study results to some extent. Third, residual confounding was of concern, even though we used multivariate logistic regression to control for the effect of confounding. Fourth, previous studies shown MTRR gene polymorphisms at rs1532268 was associated with the risk of CHD[15]. However, in this study, the distribution of this locus in the control population did not meet the HWE test, so we did not analyze this locus. In addition, as this study included only Han Chinese, more studies in other populations need to generalize the findings. These limitations suggest that future studies should conduct prospective cohort studies with larger sample sizes in different ethnic populations.