Molecular Psychiatry (2015) 20, 640–646 © 2015 Macmillan Publishers Limited All rights reserved 1359-4184/15 www.nature.com/mp ORIGINAL ARTICLE The oxytocin receptor gene (OXTR) is associated with autism spectrum disorder: a meta-analysis D LoParo and ID Waldman The oxytocin receptor gene (OXTR) has been studied as a risk factor for autism spectrum disorder (ASD) owing to converging evidence from multiple levels of analysis that oxytocin (OXT) has an important role in the regulation of affiliative behavior and social bonding in both nonhuman mammals and humans. Inconsistency in the effect sizes of the OXTR variants included in association studies render it unclear whether OXTR is truly associated with ASD, and, if so, which OXTR single-nucleotide polymorphisms (SNPs) are associated. Thus, a meta-analytic review of extant studies is needed to determine whether OXTR shows association with ASD, and to elucidate which specific SNPs have a significant effect on ASD. The current meta-analysis of 16 OXTR SNPs included 3941 individuals with ASD from 11 independent samples, although analyses of each individual SNP included a subset of this total. We found significant associations between ASD and the SNPs rs7632287, rs237887, rs2268491 and rs2254298. OXTR was also significantly associated with ASD in a gene-based test. The current meta-analysis is the largest and most comprehensive investigation of the association of OXTR with ASD and the findings suggest directions for future studies of the etiology of ASD. Molecular Psychiatry (2015) 20, 640–646; doi:10.1038/mp.2014.77; published online 5 August 2014 INTRODUCTION sizes.8 Nevertheless, several genes and genomic regions have Autism spectrum disorder (ASD) refers to a group of neuro- shown relatively consistent association with ASD. developmental disorders characterized by developmental delays, Geneticists have used several methodologies in an attempt to impaired functioning in social and communicative skills and uncover the sources of heritability in ASD. Genome-wide linkage the presence of restricted, repetitive behavior.1 Researchers have studies have identified dozens of genomic regions likely to harbor – 2,3 autism risk genes.9 Several genome-wide association studies estimated the heritability of ASD at 55 80%, indicating that – fl (GWAS) of ASD also have been conducted,10 12 although repli- genetic in uences are responsible for most of its etiology. 8,13 Neuropeptide genes have been investigated in relation to ASD cated results have yet to emerge. An area of recent growth is owing to evidence that oxytocin (OXT) and arginine vasopressin the exploration of copy-number variants in individuals with have an important role in the regulation of affiliative behavior and ASD. Recent studies have shown that large, rare copy-number social bonding in both nonhuman mammals and humans4,5 and variants occur more frequently in ASD individuals than in controls14 and that 5–11% of ASD individuals have at least one are differentially expressed in the blood of autistic individuals in 14 comparison with non-autistic individuals.6,7 The oxytocin receptor rare or de novo copy-number variant, most of which were unique to each individual.14,15 Beyond copy-number variants, gene (OXTR) is the neuropeptide gene most frequently studied for 16–18 association with ASD. This gene contains many single-nucleotide recent exome sequencing studies found that de novo point polymorphisms (SNPs), and each association study of OXTR and mutations (single-base substitutions) were more frequent in indi- viduals with ASD, particularly in genes involved in cell structure ASD has analyzed a different set of these variants, leading to and protein binding in the brain.8 Although these genome-wide, considerable heterogeneity in the literature. Further, inconsistency ‘hypothesis-free’ approaches have begun to yield results, many in the effect sizes of the OXTR variants renders it unclear whether researchers also study specific genes based on their hypothesized OXTR is truly associated with ASD, and, if so, which OXTR SNPs are role in the etiology of ASD. associated. Thus, we conducted a meta-analytic review of the Genes in several biological pathways have been examined current literature to determine whether OXTR and its constituent for association with ASD, with a particular focus on pathways SNPs are associated with ASD. involved in neuronal and cortical functioning. For example, studies have found that ASD is significantly associated with the genes for 9,19 Genetics of ASD hepatocyte growth factor and its receptor, MET, the gene that 20 fi codes for the protein Reelin, and genes in serotonergic, Although there has been considerable research devoted to nding 21 specific genes that underlie ASD, most of its heritability remains GABAergic and glutamatergic pathways. Researchers have also unexplained.8 ASD is likely a highly polygenic condition, with no investigated biological factors thought to be particularly involved one gene expected to explain a substantial proportion of its in social and communicative functioning, such as OXT. variance.8 Further, variants associated with ASD are often rare or even de novo mutations (spontaneous mutations that neither Oxytocin and social behavior parent possessed or transmitted), making the reliable identifica- OXT is a peptide constituted by oligopepetide chains of nine tion of associated genes difficult, especially without large sample amino acids.22 The gene that codes for the OXT receptor, OXTR, Psychology Department, Emory University, Atlanta, GA, USA. Correspondence: D LoParo, Psychology Department, Emory University, 36 Eagle Row, Atlanta, GA 30322, USA. E-mail: [email protected] Received 12 December 2013; revised 8 May 2014; accepted 17 June 2014; published online 5 August 2014 OXTR is associated with autism spectrum disorder D LoParo and ID Waldman 641 spans 19.2 kilobases on chromosome 3 and is expressed in association studies of OXTR SNPs with ASD,23,45–53 comprising 13 humans throughout the body in reproductive structures22 as well independent samples, 11 of which used family-based designs. as in neural regions, such as the frontal cortex, amygdala, Family-based designs examine the over-transmission of a putative hypothalamus and olfactory nucleus.5,23 high-risk allele at an SNP from heterozygous parents to their Nonhuman mammal research has established a role for OXT in affected offspring, under the assumption that if an allele is social behavior. OXT receptors are expressed in neural regions transmitted more often than expected by chance (i.e., 50%) then it related to social behavior, pair-bonding, social memory and social is associated with the disorder. Each of these studies tested a aggression in nonhuman mammals.24 In particular, OXT neurons different set of OXTR SNPs, and no SNP has been found to be in the hypothalamus project to the nucleus accumbens and significantly associated with ASD in every study in which it has posterior pituitary in prairie voles.25 Researchers have found that been examined, rendering a qualitative synthesis of this literature OXT seems to promote approach behavior by inhibiting difficult. Also, no studies to date have performed a gene-based instinctual avoidance of closeness and defensive behavior.5,26 test to evaluate the association between OXTR and ASD. Gene- Disrupting oxytocin receptor functioning also seems to disrupt based tests combine significance levels across multiple SNPs and social behavior. For example, OXTR knockout mice have impaired the linkage disequilibrium among them, thus reducing type I error social memory and recognition,27 impaired mother-offspring and increasing statistical power.54 Owing to the considerable interaction28 and increased aggression.28 This evidence suggests heterogeneity across studies in the SNPs tested and their evidence that OXT has a causal role in the development and maintenance of for association, a meta-analysis is needed to determine whether social functioning in rodent species. These findings have OXTR as a whole and specific OXTR SNPs are associated with ASD. prompted researchers to investigate the role that OXT and OXTR have in human social behavior. The present meta-analysis OXT in humans has been shown to impact the processing of The primary aims of the current meta-analysis are to evaluate the social stimuli. For example, intranasal OXT administration has been main effects of OXTR and variants therein on ASD, and to test for shown to increase calmness and decrease anxiety in response to heterogeneity in the effects of these variants across studies. a social stressor,29 increase ability to infer affective states from 30 Unfortunately, too few studies currently exist to reliably explore viewing the eye regions of emotional faces and increase social variables that may explain between-study heterogeneity. None- referencing to eye regions.31 Further, OXT in humans increases fi 32 theless, characterizing the magnitude of heterogeneity will facili- af liative behavior and trust in social situations. It is worth tate the future examination of study-level moderators as samples noting that commonly used methods for measuring peripheral increase. We also use the extant data to perform a gene-based test oxytocin can be unreliable, and it is not known how much OXT of association between OXTR and ASD. reaches the brain after intranasal administration,33 thus these results should be interpreted with caution. Nonetheless, findings in the human literature suggest that OXT has a complex but METHODS fi facilitative effect on social