Exploring a causal role of DNA methylation in the relationship between maternal vitamin B12 during pregnancy and child's IQ at age 8, cognitive performance and educational attainment: a two-step Mendelian randomization study

Abstract

An adequate intake of vitamin B12 during pregnancy plays an important role in offspring neurodevelopment, potentially via epigenetic processes. We used a two-step Mendelian randomization approach to assess whether DNA methylation plays a mediating and causal role in associations between maternal vitamin B12 status and offspring's cognition. Firstly, we estimated the causal effect of maternal vitamin B12 levels on cord blood DNA methylation using the maternal FUT2 genotypes rs492602:A > G and rs1047781:A > T as proxies for circulating vitamin B12 levels in the Avon Longitudinal Study of Parents and Children (ALSPAC) and we tested the observed associations in a replication cohort. Secondly, we estimated the causal effect of DNA methylation on IQ using the offspring genotype at sites close to the methylated CpG site as a proxy for DNA methylation in ALSPAC and in a replication sample. The first step Mendelian randomization estimated that maternal vitamin B12 had a small causal effect on DNA methylation in offspring at three CpG sites, which was replicated for one of the sites. The second step Mendelian randomization found weak evidence of a causal effect of DNA methylation at two of these sites on childhood performance IQ which was replicated for one of the sites. The findings support a causal effect of maternal vitamin B12 levels on cord blood DNA methylation, and a causal effect of vitamin B12-responsive DNA methylation changes on children's cognition. Some limitations were identified and future studies using a similar approach should aim to overcome such issues.

Figure 1

Diagram showing the two-step Mendelian randomization approach used in this study. Step 1: The maternal FUT2 genotype is used as instrumental variable (IV) for circulating vitamin B₁₂ levels in pregnancy (exposure) as its effect on cord blood CpG methylation (outcome) is not vulnerable to confounders. Step 2: The offspring genotype at a cis-SNP around the CpG whose methylation is affected by maternal FUT2 genotype is used as IV for CpG methylation to estimate the effect of cord blood methylation (exposure) on childhood IQ (outcome) as it is free from confounder biases.

Figure 2

Flow diagram showing the two steps of the analysis conducted.

Figure 3

Manhattan plots showing the results of the methylome-wide association study (MWAS) for maternal FUT2 genotype in cord blood in the Avon Longitudinal Study of Parents and Children (ALSPAC), adjusted for batch, cell composition, child’s genotype, mother’s body mass index (BMI), mother’s education, mother’s age at conception, smoking during pregnancy and parity.