In present study, we aimed to identify the interaction between maternal exposure to Al and Fe during pregnancy and CHDs in offspring, measured in maternal and cord blood simultaneously. The results showed that the Al exposed in MB and CB were higher in the CHDs groups than in the non-CHDs groups (Table 1). Maternal exposure to Al has a significant association with the risk of CHDs in the offspring in logistic analysis model, and higher concentrations of Al may be at increased risk of CHDs. The current study was consistent for CHDs subtypes, including the septal defects, PDA, and isolated CHDs in CB (Tables 4 and 5). Finally, our findings suggested that mothers with high Al merge high Fe upped the ante for CHDs compared to mothers with low blood Al and low Fe meanwhile.
4.1. Associations between Al and CHDs
Aluminum is the third enriched element in the crust. In addition, environmental Al pollution is common due to the vigorous exploitation of Al or for smelting, use in cookers, utensils, and toys, Al agents to treat gastrointestinal diseases, Al salt to purify water, and as a food additive [32, 33]. Al is a hazardous element to the human body. It can accumulate and exert teratogenic, carcinogenic, and mutagenic effects. However, there is a lack of understanding of maternal Al exposure and the risk of congenital disabilities, especially CHDs. A previous study [29] reported that accumulation of Al in a healthy person would be probably 30–50 mg/kg, and the levels of Al in serum are likely 1–3 μg/L under normal circumstances [34]. The current findings showed that the presence of Al (median=2.117 mg/L) in MB was higher than that reported in a normal person (1–3 μg/L). However, the median Al levels in CB (1.654 mg/L) were significantly less than MB collected at birth (2.117 mg/L), indicating the presence of Al in CB, but the placenta might provide a physical barrier against metallic elements (Table 1). Animal experiments revealed that Al is teratogenic to the nervous system, and the differentiation and formation of yolk blood vessels, the heart, and the neurons are sensitive to Al toxicity [15]. The development of the tube is sensitive to Al toxicity, which provides clues for the effect of Al on human embryonic heart development. In previous studies in China [17, 33], they evaluated hair samples by ICP-MS, and find a statistically significant difference in hair Al content and the occurrence of total CHDs in the offspring (aOR=2.32, 95% CI: 1.72–3.13). Thus, it could be deduced that Al is involved in the occurrence of CHD through oxidative stress. A Polish study showed that Al in maternal serum (mean±SD: 250.3±176.2 µg/L) and amniotic fluid (144.8±54.1 µg/L) in women who have a birth defects baby. Moreover, a high level of Al is the best predictor of birth defects (β coefficient=-0.28; P=0.02) [17]. Accumulating evidence, including our recent study, showed that high Al in MB significantly increases the risk of birth with CHDs, such as hair, amniotic fluid, or cord blood. Notably, in this study, the risk of Al exposure was related to fetal CHDs occurrence in a dose-dependent manner with elevated aluminum levels (≥2.408 mg/L in the cord blood), supporting the theory that maternal Al exposure increases the risk of CHDs in offspring.
4.2. Associations between Fe and CHDs
Comparatively, Fe have received great attention for their role in the occurrence of CHDs event. Fe has an important role in the growth and development of the embryo in many animal studies [22, 35, 36], especially for cardiovascular development [19]. However, molecular mechanisms underlying this relationship are ill-defined and the evidence on the correlations between Fe nutritional status and CHDs among humans has been controversial in the past decade. Some studies have pointed out that maternal Fe deficiency is a risk factor for CHDs [27, 28]. Moreover, by interacting with other nutrients (such as folate), low ferritin status may alter the utilization of folate despite adequate intake [37]. A meta-analysis study suggested that maternal folate deficiency reduce the risk of CHDs [38]. Our previous study also showed that pregnant women who did not take a folate supplement and had low folic acid intake from dietary increased the rate of CHDs by two-fold in their offspring [39]. Hence in the current study, we considered pregnant women’s folate intake as a confounder. Among iron-deficient women of childbearing age, iron demands are exceedingly high pre-existing during pregnancy, and pregnant women were found more susceptible to iron deficiency. The resulting of that may induce tissue hypoxia and stress response, this in turn affects subsequent early and late consequences for fetal cardiac development in general[40]. However, recent studies showed that Fe status overload could also cause heart defects [26] . To the best of our knowledge, one research in China explored that maternal Fe nutritional in low status were correlated with the rate of offspring’s CHDs [27]. Our study classifies the exposure of Fe status both in MB and CB directly and found that the association of the high concentration of Fe in the MB and CB did not differ statistically in CHDs and their subtypes adjusting hypertensive disorders during pregnancy, diabetes, cesarean delivery, folic acid supplement and average energy intake of dietary during pregnancy. The mechanisms underlying these associations between Fe deficiency and the development remain unclear and needs an in-depth investigation.
4.3. Interactions between trace elements
To the best of our knowledge, none of the previous studies have systematically explored the trace Al and Fe elements interactions on CHDs. However, this negative finding could be due to the low sample size. Since the scarcity of research on the co-exposed effects of mental elements for offspring’s heart development, fetal susceptibility to environmental exposure is warranted to further studies. The current study no significant differences were observed between categorical variables in CHDs and the subtypes in MB. However, in CB, high Al in addition to high Fe exist a stronger correction on fetal heart development than high Al alone in total CHDs, multiple CHDs, septal defects and PDA. The potential pathogenic mechanisms of Al-induced CHD are as follows. First, aluminum accumulates in lysosomes, causing damage to cardiomyocytes. Second, transferrin can bind to approximately 90% of the aluminum circulating in the blood[35, 41-43], thereby disrupting the normal synthesis of transferrin receptors and ferritin, then increasing free iron levels in cells and activation abnormal reactive oxygen species (ROS) which lead to increased lipid peroxidation, DNA damage, and even cell apoptosis ([36, 44, 45]. Third, Al binds to nuclear chromatin, affecting the metabolism of phosphorus and calcium and the normal physiological functions of cells, which in turn interferes with cytoskeleton polymerization [46]. The current study also demonstrates that simultaneous high levels of iron exposure may enhance the effects of aluminum exposure.
The current study has several advantages, which makes our results effective. Frist, maternal and fetal biomarkers were identified during pregnancy, and the demographic, pregnancy, and birth data were collected from clinical records, which are not susceptible to cause the recall bias. Second, we had age-matched controls and their radius of a residential area within 2 km, which ensured the representation of the sampling framework. Third, we evaluated the two elements (Al/ Fe) both in MB and CB, as well as their co-correlation with CHDs adjusted for a set of potential confounding variables
Nevertheless, the present study has some limitations. First, our cohort cannot be considered representative of the pregnant women in China because the sample size was small, and hence, the current findings may not be extended to adult pregnant women. However, the prevalence of CHDs (9.3‰ live births) in current population was similar to the reported prevalence in other study in China [47]. Second, the small sample size is a major limitation of our study, which limits the statistical power of the association and interaction studies of CHDs subtypes in this study. Nonetheless, a possible pathogenic role of Al in CHDs and a synergistic effect with Fe in pathogenicity were observed.