Genetic Factors Underlying Risk for Methamphetamine Intake and Associated Traits

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Risk as a population measure can be assessed in animal models in the absence of drug exposure through a number of strategies, such as via the use of selectively bred animal lines. However, modelling risk is not a straightforward process, even in non-human models, since risk is not a unitary construct; thus, multiple genetic and non-genetic factors must be considered, and risk factors likely vary across individuals. Methamphetamine (MA) has powerful euphoric effects that encourage use; but, after many years of research, broadly effective medications have not been identified. We have developed a genetic animal model comprised of lines of mice selectively bred for high and low voluntary MA drinking (MAHDR and MALDR), with the goal of identifying genetic risk and protective factors for MA use. These lines differ for multiple MA traits critically relevant to MA use disorders. We mapped a region on mouse chromosome 10, that accounts for >50% of the genetic variance in MA consumption, and obtained data that provide evidence for the trace amine-associated receptor 1 gene, Taar1, as a quantitative trait gene for MA intake. This application is focused on Taar1 in risk for MA use, and on the discovery of genetic modifiers of the increased risk associated with a Taar1 mutation (Taar1m1J) that codes for a non-functional receptor; TAAR1. This will be accomplished through the identification of individual differences in the transcriptome that impact the effect of the Taar1m1J/m1J genotype on MA intake. Our findings will guide the examination of novel mechanisms for the treatment of MA use disorders. Three aims are proposed. In Aim 1, CRISPR-Cas9 Taar1m1J allele replaced mice, in which TAAR1 function has been restored specifically in MAHDR mice (MAHDR-Taar1+/+) will be compared to non-replaced controls (MAHDR-Taar1 m1J/m1J). MA-related traits that reliably differentiate the MAHDR and MALDR selected lines, which differ in Taar1 genotype, will be examined (i.e., MAHDR are all Taar1m1J/m1J and MALDR are Taar1+/+ or Taar1+/m1J). Studies with saccharin (as a tastant and a natural reward) and quinine (as a tastant) will also be performed. Behavioral assessment of the effectiveness of the allele swap will be tested using a TAAR1-specific agonist. In Aim 2, RNA-Seq data will be used in gene expression network analyses in recombinant inbred strains of mice (BXD RI) that all possess the Taar1m1J/m1J genotype. The goal is to identify genetic modifiers that reduce the impact of the Taar1m1J/m1J genotype on MA intake, because they could lead to new treatments. The nucleus accumbens (NAc) medial shell will be initially studied due for its critical role in drug reward. In Aim 3, RNA-Seq results, data base searches (DrugBank, Broad Institute Connectivity Map, PubChem at NCBI) and published results will be used to nominate neural targets to manipulate for their impact on MA intake and ultimately to identify novel treatments. The long-term goal of this research is to identify novel treatments that could reduce MA use, based on matching to individual genetic susceptibility “markers”, comprised of specific constellations of genes.