Prenatal nutrition, epigenetics and schizophrenia risk: can we test causal effects?

Abstract

We posit that maternal prenatal nutrition can influence offspring schizophrenia risk via epigenetic effects. In this article, we consider evidence that prenatal nutrition is linked to epigenetic outcomes in offspring and schizophrenia in offspring, and that schizophrenia is associated with epigenetic changes. We focus upon one-carbon metabolism as a mediator of the pathway between perturbed prenatal nutrition and the subsequent risk of schizophrenia. Although post-mortem human studies demonstrate DNA methylation changes in brains of people with schizophrenia, such studies cannot establish causality. We suggest a testable hypothesis that utilizes a novel two-step Mendelian randomization approach, to test the component parts of the proposed causal pathway leading from prenatal nutritional exposure to schizophrenia. Applied here to a specific example, such an approach is applicable for wider use to strengthen causal inference of the mediating role of epigenetic factors linking exposures to health outcomes in population-based studies.

Figure 1. Potential mediating mechanisms linking prenatal nutrition with later schizophrenia development

Prenatal nutrition may exert its influence on schizophrenia risk via a number of pathways (influencing the kinetics of one-carbon metabolism, epigenetic perturbation of the fetal epigenome or via interfering with neurodevelopment). These possible mediating mechanisms are not mutually exclusive and may be interconnected through some or all of the routes depicted.

Figure 2. Simplified one-carbon metabolism

One-carbon metabolism is critical for sustaining many normal cellular functions, including DNA methylation to facilitate normal cell division, growth and differentiation. In this regard, adequate micronutrient availability, including the availability of folate, either through diet or as folic acid supplementation, may be particularly important at certain stages of the life course, particularly prenatally, when the body is undergoing rapid cell division and growth. Technical detail: in one-carbon metabolism, folate from the diet or folic acid supplements are metabolized to DHF via a reductase enzyme in the liver and converted to a series of THF compounds. At both these stages vitamin B₃ is a necessary cofactor. Via the addition of a carbon donor (serine or glycine) THF forms 5,10 MTHF, the methylated substrate used by the enzyme MTHFR to produce 5-MTHF, where vitamin B₂ is a necessary cofactor. 5-MTHF is used in the reaction to recycle homocysteine to methionine, via single-carbon donation as a result of the interaction between vitamin B₁₂, a necessary cofactor, and MS. Here, choline is synthesized to betaine to facilitate methyl donation to convert homocysteine to methionine via a BMHT. Methylation reactions, including DNA methylation, are then facilitated by the conversion of methionine to SAM, the key methyl donor involved in methylation, via interaction with MAT. Unused methionine in the form of (an inhibitor of DNMTs) is hydrolyzed back to homocysteine via an enyzmatic reaction with SAHH. 5,10 MTHF: 5,10-methylenetetrahydrofolate; 5-MTHF: 5-methyltetrahydrfolic acid; BMHT: Betaine-homocysteine methyltransferase; DHF: Dihydrofolate; DNMT: DNA methyltransferase; MS: Methionine synthase; THF: Tetrahydrofolate.

Figure 3. Mendelian randomization

Using a genetic variant as a proxy to establish whether prenatal exposure to folate/homocysteine is causally related to schizophrenia. Maternal MTHFR 677C>T (rs1801133) genotype is used here as a proxy for folate/homocysteine exposure in utero. Genotype is independent of U. Genotype will only influence schizophrenia if the exposure→schizophrenia association is causal. U: Unmeasured confounders.

Figure 4. The principle of two-step epigenetic Mendelian randomization

The principle of Mendelian randomization is applied to assess the mediating role of DNA methylation. Genetic variants can be used in a two-step framework. (A) In step 1, a genetic variant is used to proxy for the environmental exposure of interest. (B) In step 2, a separate genetic variant is used to proxy for DNA methylation.

Figure 5. Two-step epigenetic Mendelian randomization applied to prenatal nutrition and schizophrenia risk

(A) In step 1, maternal MTHFR 677C>T (rs1801133) genotype acts as a proxy for prenatal folate/homocysteine influences on fetal DNA methylation. MTHFR genotype will only influence DNA methylation if folate/homocysteine is causally related to methylation levels at the locus (CpG) studied. (B) In step 2, a separate genetic variant that correlates with offspring DNA methylation levels (commonly a cis variant in close proximity to the CpG site being measured) is utilized. The offspring genotype will only be associated with schizophrenia if the relationship between DNA methylation and schizophrenia is causal. U: Unmeasured confounders.