We also evaluated the influence of the
We also evaluated the influence of the number of CYP3A variant Clarithromycin (n=0 to 3) on the modifying effects on MDI and PDI. No clear allele dosage effect could be observed (data not shown).
Discussion Prior reports for all four populations showed that an increase in prenatal Hg was not adversely associated with MDI (Davidson et al., 2008, Llop et al., 2012, Valent et al., 2013). In this study, we found that for a doubling of cord blood THg concentrations, children carrying a high activity allele of CYP3A obtained higher MDI scores, and not children carrying low activity CYP3A alleles for whom coefficients obtained were near null. One explanation for improving scores with increasing THg might be that cord blood THg acts as a surrogate of fish nutrients that are beneficial for brain development, such as fatty acids, vitamins or selenium. Our findings suggest that the higher activity CYP3A alleles may modify the extent to which the benefits of such nutrients outweigh the adverse influences of mercury. Which nutrients would fall under a positive influence of CYP3A enzyme activity remains largely speculative; but a potential nutrient to be explored could be the vitamin D, which metabolism has been recently related with the human liver microsomal CYP3A4 (Cheng et al., 2016) and that has been associated with improved mental and psychomotor development in the Spanish cohort (Morales et al., 2012). Nevertheless, we observed similar results for the associations between CYP3A and neurodevelopmental scores in the sensitivity analysis by adjusting the models by fish consumption, which would indicate the existence of some residual confounding, and a more in-depth analysis evaluating specific fish-related biomarkers is needed to pinpoint which fish nutrients may influence the associations observed. We did not find any evidence of allele-dependent dosage effect for the CYP3A genes, implying that other polymorphisms in CYP3A or other genes explain the population differences. It is interesting to note that no effect of CYP3A genotype was found when maternal hair THg was used as marker of prenatal MeHg exposure. This could be due to differences in maternal hair sampling across the cohorts (hair length or pregnancy period). Hair THg levels have been shown to correlate highly with Hg levels in critical target tissues, such as the brain (Cernichiari et al., 1995) but we obtained strongest associations for cord blood THg which reflect more recent exposure in the third trimester of pregnancy. We speculate that, in this study, cord blood THg better reflect the fetal MeHg dose and thus is more relevant for studying interaction with the child\'s CYP3A genotype for neurodevelopmental outcomes. In the populations we studied, children carrying high activity CYP3A alleles showed higher MDI scores as THg concentrations increased. This is consistent with an experimental study in fruit flies, where the transgenic expression of CYP6g1, or its human homolog CYP3A4, was seen to confer tolerance to MeHg toxicity during development (Rand et al., 2012). However, the mechanism for this tolerance to toxicity remains uncertain. In humans, the CYP3A enzymes are known to be involved in the metabolism of xenobiotics primarily in the liver but also in extrahepatic tissues including placenta, kidney, intestines, and lung (Pavek and Dvorak, 2008). Consistent with the notion that CYP3A genes afford MeHg protection during development, we observed that the strongest association occurred between MeHg and CYP3A7, which is the CYP3A isoform that is expressed exclusively in the fetal liver (Stevens et al., 2003). Polymorphisms in the CYP3A genes are known to alter the metabolism of some xenobiotics (Pavek and Dvorak, 2008). Since cord blood THg concentrations were not seen to vary as a function of CYP3A genotype, it appears that CYP3A enzymes have little influence on MeHg toxicokinetics. An alternative explanation for the beneficial effects of more active CYP3A genotypes may relate to metabolism and clearance of toxic by-products of a MeHg insult. In this regard, CYP3A4 catalyzes the reduction of α- and β-unsaturated aldehydes, notably, 4-hydroxynonenal (4-HNE) (Amunom et al., 2011), a common endogenous product of lipid peroxidation resulting from metal toxicity (Valko et al., 2005). Localized expression of CYP3A activity, e.g. in the brain, may therefore have a neuroprotective function, and accordingly, reduced CYP activity, via MeHg inhibition, could leave the brain vulnerable. In support of this notion, inhibition of CYP3A4 or CYP2D6 activity in SH-SY5Y human neuroblastoma cells has been shown to enhance the neurotoxicity of MPP+ (1-methyl-4-phenylpyridinium), a neurotoxic derivative of MPTP ((1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and an agent used to induce dopaminergic neuron death in models of Parkinson\'s disease (Mann and Tyndale, 2010). Resolving mechanisms by which CYP enzymes alleviate toxic effects specific to MeHg will require further studies.