Functional characterisation of gene variants of oxytocin and vasopressin receptors implicated in psychological disorders

Abstract

Oxytocin (OXT) and arginine vasopressin (AVP) are well known for their classical roles in initiation of myoepithelial contractions during labour and lactation, and regulation of blood pressure and osmolality, respectively. However, there is also substantial evidence supporting central roles of these neuropeptides in neuronal and cognitive functioning. Indeed, their altered signalling has been implicated in several psychological disorders, social impairments, and behavioural traits. OXT and AVP elicit their effects through interaction with cognate G protein-coupled receptors (GPCRs). The OXT receptor (OXTR) and two AVP receptor subtypes (AVPR1a and AVPR1b) are expressed in many brain regions. Several studies have highlighted the potential association of genetic variants of these receptors with behavioural/social disorders. Yet, the impacts of these variants on receptor function are often unknown. Through literature research, a selection of naturally occurring OXTR and AVPR1a/1b variants (11 and 7 variants, respectively) associated with a variety of psychological disorders, were identified. In the case of the OXTR, a range of variants linked to birth disorders (a group of disorders in which OXTR variants/ disrupted OXT function has a very well characterised role) were included for comparison. The variants were first analysed using in silico methods to predict their effects on receptor function. This was followed by in vitro characterisation of receptor expression (by receptor ELISA assay), receptor signalling (by inositol phosphate accumulation assay) and ligand binding (by radiolabelled ligand binding assay). This in vitro analysis demonstrated that OXTR variants V45L, A63V, M133V, H173R, W203R, G221S, A238T, I266V, T273R, T273M and V281M, AVPR1a variant F136L and AVPR1b variants K65N, G191R, R364H and R364L, result in severe or partial reduction in receptor function and, therefore, these variants may contribute to the pathophysiologies of psychological/birth disorders that have been indicated in genetic association studies. The nature of dysfunction for each receptor was then further characterised. For the OXTR the majority are believed to be Class II and IV variants, while Class IV variants seem to predominate for AVPR1a/b.

From comparison of the in silico prediction outcomes and in vitro analyses, it was iv apparent that the ability of variant effect prediction (VEP) programs to successfully predict the functional consequences of GPCR variants was variable and, in some cases, unreliable. The VEP program SIFT appeared most reliable for future in silico analysis of OXTR, while LRT, Mutation Assessor and Mutation test, have the highest predictive power for future in silico analysis of AVPR1a but none of the VEP programs appear to be accurate for the in silico analysis of AVPR1b variants.