Human genetic polymorphism impact in a mouse model

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DESCRIPTION (provided by applicant): This CEBRA application addresses a high-risk, high-impact question: Can human genetic variations be modeled by transgenic mouse models? This is a question in urgent need of examination, because studies have shown that small genetic variations, such as single nucleotide polymorphism (SNP), are the primary genetic factors for drug addiction vulnerability and other behavioral changes. However, there needs to be an approach to address causal relationship between genetic variations and phenotypic changes. This CEBRA application addresses this question, using genetically modified mouse models to reproduce human genetic polymorphisms, allowing rigorous testing for resulting phenotypic differences to assess the impact of genetic changes. The focus is on a highly relevant, well-documented human genetic variation - a functional SNP in the human mu opioid receptor gene. The mu opioid receptor is a key component of the endogenous opioid system, mediating the physiological effects of opioid drugs including morphine and heroin. The A118G SNP that we and others have identified is common, leads to a single amino acid change, and appears to alter receptor function by increasing beta-endorphin sensitivity and receptor expression level in cells. Recent human studies suggest that this genetic variant may be of functional importance in vivo: it is associated with alterations of the HPA-axis in functional studies in humans, and is implicated as an important contributor to addictions. Thus, the A118G SNP is an ideal genetic polymorphism to test the feasibility of mouse modeling for human genetic variations - so as to determine the causal relationship between a genetic change and subsequent functional alterations in behavior and in vivo physiology. This is the goal of our CEBRA proposal. Aim 1. Generating a mouse model for the human genetic polymorphism in the mu opioid receptor. We will model the human A118G polymorphism in the mouse. The homologous recombination-based gene targeting approach will be used to "knock-in" the human polymorphic variations into the mu opioid receptor gene in mouse, thus creating a mouse model of this common human genetic variation. Aim 2. Characterize impact of the mu opioid receptor polymorphism in vivo. We will characterize the mouse model generated in Aim 1 for impact of genetic polymorphism in vivo. We will examine the effects of the polymorphism on receptor expression and ligand sensitivity. We will also explore any potential impact on opioid-regulated behaviors. Because of the high-risk high-impact nature, we choose to apply under the CEBRA funding mechanism. The field of drug abuse research needs to know whether human genetic vulnerability factors can be modeled and studied in transgenic mice. The outcome from our proposed studies will serve as a guiding example for future strategic considerations by drug abuse researchers.