Sex-specific gene-environment interactions underlying ASD-like behaviors.

The male bias in the incidence of autism spectrum disorders (ASDs) is one of the most notable characteristics of this group of neurodevelopmental disorders. The etiology of this sex bias is far from known, but pivotal for understanding the etiology of ASDs in general. Here we investigate whether a "three-hit" (genetic load × environmental factor × sex) theory of autism may help explain the male predominance. We found that LPS-induced maternal immune activation caused male-specific deficits in certain social responses in the contactin-associated protein-like 2 (Cntnap2) mouse model for ASD. The three "hits" had cumulative effects on ultrasonic vocalizations at postnatal day 3. Hits synergistically affected social recognition in adulthood: only mice exposed to all three hits showed deficits in this aspect of social behavior. In brains of the same mice we found a significant three-way interaction on corticotropin-releasing hormone receptor-1 (Crhr1) gene expression, in the left hippocampus specifically, which co-occurred with epigenetic alterations in histone H3 N-terminal lysine 4 trimethylation (H3K4me3) over the Crhr1 promoter. Although it is highly likely that multiple (synergistic) interactions may be at work, change in the expression of genes in the hypothalamic-pituitary-adrenal/stress system (e.g., Crhr1) is one of them. The data provide proof-of-principle that genetic and environmental factors interact to cause sex-specific effects that may help explain the male bias in ASD incidence.