Increased susceptibility to mild neonatal stress in MTHFR deficient mice

Highlights

• The effect of mild neonatal stress (NS) was amplified in MTHFR deficient mice.

• NS and MTHFR deficiency act synergistically to suppress exploration in female mice.

• Stress associated variables were sensitive to genotype in female mice.

• Behavior in a social context presented an interaction between MTHFR and NS in male mice.

Abstract

Early life stress is shown to have a life-span outcome on human and animal behavior, increasing the risk for psychopathology. The gene methylenetetrahydrofolate reductase (MTHFR), which encodes for a key enzyme in one carbon metabolism, shows a high prevalence of polymorphism in patients with developmental disorders. Here we examined the hypothesis that MTHFR deficiency results in an increased susceptibility of the developing brain to mild neonatal stress (NS). Mild NS failed to alter corticosterone levels in young and adult Wt mice. However, an elevated level of corticosterone was found in the MTHFR deficient-NS female, exemplifying enhanced sensitivity to NS. Behavioral phenotyping of Wt and MTHFR deficient mice provides evidence that the effect of mild NS may be amplified by the MTHFR deficient genotype. Distinct behavioral characteristics were altered in male and female mice. In general, three patterns of influence on mice behavior were observed: (1) an additive suppressive effect of NS and MTHFR deficiency on exploration and activity was evident in females; (2) stress related parameters were significantly sensitive to genotype in females, presenting an interaction between genotype and sex; (3) various aspects of behavior in a social setting were modified preferably in males by genotype, NS and the interaction between the two, while females exhibited a smaller effect that was restricted to NS with no genotype effect. Overall, our results support an interaction between mild NS, the MTHFR genotype and sex. We suggest using this animal model to study the molecular mechanism linking these two risk factors and their involvement in neurodevelopmental disorders such as schizophrenia and autism.

Introduction

Polymorphism in genes encoding enzymes of one carbon (C1) metabolism was found to increase the risk for neuropsychiatric disorders such as schizophrenia and autism spectrum disorders. One gene of this cycle, methylenetetrahydrofolate reductase (MTHFR), shows a high prevalence of polymorphism in autistic patients and/or mothers [1], [2], [3]. Similarly, the maternal MTHFR 677C>T polymorphism was associated with increased risk for schizophrenia in their children [4]. It is possible that the low activity of MTHFR combined with other genetic and environmental factors contributes to the developmental outcome.

Additional factors can interfere with the brain's developmental program, the most significant being those associated with the immediate environment of the fetus and the newborn. Adverse stimuli during sensitive periods when the neuron-endocrine circuit of the stress response is being established have long-lasting impacts on the response to stressors in later life. Effects of maternal care and maternal separation on various aspects of the stress response and cognitive function were described more than 40 years ago [5], [6], [7]. Initial evidence from the last decade has suggested that stable modifications of the DNA, such as DNA methylation, demethylation, and acetylation, play important roles in the maintenance of the long-term effects of early life events on the regulation of gene expression [8], [9].

MTHFR deficient mice (MTHFR^+/−) exhibit alterations in anxiety, activity, and social behavior [10], [11]. Moreover, the MTHFR deficient genotype interacts with early life exposure to GABA potentiating drugs to modify mice behavior and the levels of proteins associated with synapse function and plasticity [11].

We hypothesize that the metabolic stress induced by the MTHFR deficient genotype results in the increased susceptibility of the developing brain to mild NS. To examine this hypothesis, male and female MTHFR^+/+ and MTHFR^+/− newborns were subjected to daily mild stress during early postnatal life. Behavioral outcome and corticosterone level were tested at 3 months of age. A significant interaction between the MTHFR genotype, mild NS and gender was found to modulate adult mice behavior in the contexts of anxiety-related behavior and social settings.