Interaction of genetic predisposition and epigenetic factors in the development of anxiety

It is becoming increasingly clear by current research that the continuum of physiological anxiety up to psychopathology is not merely dependent on genes, but is orchestrated by the interplay of genetic predisposition, gene x environment and epigenetic interactions. To consider this interplay, we here took advantage of the rigid genetic predisposition of a selectively bred mouse model exhibiting high anxiety-related behavior (HAB) and tested whether and how enriched environment, a manipulation of housing conditions, is capable of rescuing the genetically driven high anxiety phenotype via gene x environment and/or epigenetic interactions. Indeed, enriched environment exerts a significant anxiolytic effect on HABs of both sexes indicating for the first time that even a rigid genetic predisposition of high anxiety can be rescued by beneficial environmental stimuli. Thereby, a reduced neophobia and a bigger behavioral repertoire of HABs (e.g. social interactions) have been observed with a stronger anxiolysis in males than in females. The behavioral shift is accompanied by an attenuated release of corticosterone after application of a mild stressor. A hyperreactive hypothalamic-pituitary-adrenal (HPA) axis and amygdala constitute the most common symptoms of anxiety disorders, and decreased corticosterone release seems to entail a reduced release of noradrenaline from locus caeruleus (LC) to the medial prefrontal cortex (mPFC), thereby increasing the top-down control of mPFC on amygdala. This would entail less activation of amygdala and thus HPA axis, a consequence we indeed can observe as decreased neuronal activity flow through the amygdala of enriched housed (EE) compared to standard housed (SE) HABs. We suggest that corticotropin-releasing hormone receptor 1 (Crhr1) is critically involved in this phenomenon since (i) HABs compared to low anxiety-related behavior (LAB) mice exhibit higher Crhr1 mRNA in the basolateral amygdala (BLA), (ii) this overexpression can be significantly reduced when HABs are housed in enriched environment and (iii) a bilateral application of a CRHR1 antagonist in the BLA of SE HABs induced a significant anxiolytic effect. Subsequent pyrosequencing identified that enriched environment increased methylation at a CpG site in the promoter of Crhr1, which is located next to a transcription factor binding site (TFB) of the epigenetic transcription factor Yin Yang 1 (YY1), whose mRNA levels are indeed decreased in EE HABs. In silico analysis identified Nr4a1 and D3Ertd300e as critical co-transcription factors, whereas Nr4a1 seems to be regulated by the quantity of available glucocorticoid receptor (GR) and D3Ertd300e positively regulates YY1. Thus, we hypothesize that reduced corticosterone release decreases the availability and thus binding of corticosterone to GR in the BLA. This, in turn decreases the binding affinity of Nr4a1 to D3Ertd300e, which then cannot positively regulate YY1 to decrease or even prevent methylation at the identified CpG site of Crhr1. This would finally result in a differentially methylated region (DMR) with higher methylation levels in EE HABs, which underlies the observed gene expression differences. The identified DMR might therefore be used as a biomarker for high or pathological anxiety. This hypothesized mechanism highlights the possibility that even a rigid genetic predisposition modeling pathological anxiety might be rescued by an epigenetic process that seems to be triggered by beneficial environmental stimuli, thereby raising the exciting possibility for new treatment strategies, which can be utilized complementary to already existing ones.