Background: Vitamin B12 is an essential nutrient and it is required in one-carbon metabolism, the biochemical pathway that leads to DNA methylation. Maternal vitamin B12 and more generally the one-carbon metabolism are associated with neurodevelopment although the evidence is scarce (e.g. Bhate et al., 2008, Bonilla et al., 2012). Maternal vitamin B12 status seems to affect offspring's DNA methylation (McKay et al., 2012) suggesting a role for DNA methylation in the association between vitamin B12 and neruocognitive development.

Aim: To examine the causal association between vitamin B12 during pregnancy and offspring cognitive development, using genetic variants previously associated with one-carbon metabolism as proxies for vitamin B12 in a two-way Mendelian Randomization study. To assess the role of DNA methylation as a mediator in the association.

Hypotheses: We hypothesize that alleles which increase the levels of vitamin B12 during pregnancy will be associated with better cognitive performance. We hypothesize that DNA methylation mediates this association.

Exposure variables: Maternal and offspring genotypes for polymorphisms associated with vitamin B12 status and DNA methylation. Maternal vitamin B12 intake during pregnancy and cord blood levels.

Outcome variables: SCDC, WISC, WASI, CCC, DAWBA conduct problems.

Confounders and mediators: Maternal education, social class, age at delivery, parity, smoking during pregnancy, alcohol and folate assumption during pregnancy, infections during pregnancy. Child's date of birth, birth weight, sex, gestational age. Breastfeeding duration. Child's DNA methylation status.

Bhate V, Deshpande S, Bhat D, Joshi N, Ladkat R, Watve S, Fall C, de Jager CA, Refsum H, and Yajnik C. Vitamin B12 status of pregnant Indian women and cognitive function in their 9-year-old children. Food Nutr Bull. (2008) 29: 249-254.

Bonilla C, Lawlor DA, Taylor AE, Gunnell DJ, Ben-Shlomo Y, Ness AR, Timpson NJ, St Pourcain B, Ring SM, Emmett PM, Smith AD, Refsum H, Pennell CE, Brion MJ, Smith GD, Lewis SJ. Vitamin B-12 status during pregnancy and child's IQ at age 8: a Mendelian randomization study in the Avon longitudinal study of parents and children. PLoS One. (2012) 7:e51084. doi: 10.1371/journal.pone.0051084.

McKay JA, Groom A, Potter C, Coneyworth LJ, Ford D, Mathers JC, Relton CL. Genetic and non-genetic influences during pregnancy on infant global and site specific DNA methylation: role for folate gene variants and vitamin B12. PLoS One. (2012) 7:e33290. doi: 10.1371/journal.pone.0033290.