Mouse model ofOPRM1(A118G) polymorphism has sex-specific effects on drug-mediated behavior

Abstract

A single nucleotide polymorphism (SNP) in the human -opioid receptor gene (OPRM1 A118G) has been widely studied for its association in a variety of drug addiction and pain sensitivity phenotypes; however, the extent of these adaptations and the mechanisms underlying these associations remain elusive. To clarify the functional mechanisms linking the OPRM1 A118G SNP to addiction and analgesia phenotypes, we derived a mouse model possessing the equivalent nucleotide/amino acid substitution in the Oprm1 gene. Mice harboring this SNP (A112G) demonstrated several phenotypic similarities to humans carrying the A118G SNP, including reduced mRNA expression and morphine-mediated antinociception. We found additional phenotypes associated with this SNP including significant reductions of receptor protein levels, morphine-mediated hyperactivity, and the development of locomotor sensitization in mice harboring the G112 allele. In addition, we found sex-specific reductions in the rewarding properties of morphine and the aversive components of naloxone-precipitated morphine withdrawal. Further cross-species analysis will allow us to investigate mechanisms and adaptations present in humans carrying this SNP.analgesia ͉ morphine ͉ -opioid receptor ͉ sex differences ͉ SNP M OPR (-opioid receptors) are integrally involved in the modulation of several pathways including pain, stress, and drug reward. Genetic mutations of the MOPR alter endogenous and exogenous opioidergic function, thus influencing behavior. A single nucleotide polymorphism (SNP) in exon 1 of the -opioid receptor gene (OPRM1), in which an adenine-toguanine substitution (A118G) exchanges an asparagine for an aspartic acid at a putative N-glycosylation site (N40D), is common in persons of European (15Ϫ30%) and Asian ancestry (49-60%), with lower prevalence in African American and Hispanic populations (1-3). The A118G SNP has been associated with an altered vulnerability to opioid addiction (4-6), a decreased response to opioid-induced analgesia (7,8), and an enhanced response to therapies for alcohol (9, 10) and nicotine addiction (7, 11). However, some association studies report divergent effects (12, 13), and sex-specific associations (14-16), underscoring the need to understand the functional significance of this SNP.Examination of the A118G variant in heterologous expression systems has yielded inconsistent results. Initial in vitro studies indicated that expression of the human G118 MOPR variant in AV-12 cells increases the binding affinity of ␤-endorphin to 3-fold higher than that of the human A118 MOPR and results in higher potency for activation of G protein-coupled potassium channels (17), suggesting a gain of function of the receptor. However, other studies report no differences in agonist binding, functional coupling, or desensitization (18). Using an allelic expression assay, Zhang and colleagues (19) found a 1.5-fold reduction in allele-specific mRNA expression in postmortem brain tissue and also a 10-fold reduction in protein levels in CHO cells expressi...

Human Mu Opioid Receptor ( OPRM1 A118G) polymorphism is associated with brain mu-opioid receptor binding potential in smokers

“…Consistent with preclinical data (18), smokers homozygous for the wild-type OPRM1 A allele exhibited higher levels of MOR BP ND in the bilateral amygdala, left thalamus, and left anterior cingulate cortex compared with those with the G allele. Among G allele carriers, the extent of subjective reward difference (denicotinized versus nicotine cigarette) was associated significantly with MOR BP ND difference in right amygdala, caudate, anterior cingulate cortex, and thalamus.…”

“…The observation of a relative reduction in MOR BP ND in multiple regions among OPRM1 G allele carriers is consistent with observations from a knock-in mouse model, in which mice possess the A112G polymorphism that corresponds to the same amino acid change in humans (18). At the molecular level, the 112G mice exhibit reduced mRNA expression and MOR protein, as well as decreased [ 3 H]DAMGO binding in thalamus (18).…”

“…Although the minor (G) allele was originally thought to be a "gain-of-function" variant, on the basis of increased affinity of MOR agonists (15), other data suggest that the G allele is associated with reduced mRNA and protein expression (16,17). Further, knock-in mice homozygous for the equivalent (112G) allele exhibit reduced MOR mRNA and protein levels in multiple brain regions, providing additional support for a "loss-offunction" phenotype (18).…”

Modeling Neuropsychiatric Disease-Relevant Human SNPs in Mice

“…The best-characterized polymorphism in this gene is a missense mutation in exon 1, involving an A-G substitution at position 118. Owing to the high sequence similarity between mouse and human at the nucleotide (86.9%), and amino-acid level (92.3%), a knock-in mouse was developed that possessed the mouseequivalent SNP of the human A118G SNP in the murine Oprm1 gene (Mague et al, 2009). In a complimentary approach, a second mouse line for this SNP was generated that expressed humanized receptors with and without the A118G variant (Ramchandani et al, 2010).…”

Neuroimaging in Psychiatric Pharmacogenetics Research: The Promise and Pitfalls

“…j Functional consequences uncertain, but suspected to alter gene expression (Wilcox et al, 2013). k G allele associated with reduced mRNA and protein levels and reduced binding affinity (Mague et al, 2009). l Functional consequences uncertain, but may code a splice variant (Zhang et al, 2004).…”