Exploratory locomotion, a predictor of addiction vulnerability, is oligogenic in rats selected for this phenotype

Artificially selected model organisms can reveal hidden features of the genetic architecture of the complex disorders that they model. Addictions are disease phenotypes caused by different intermediate phenotypes and pathways and thereby are potentially highly polygenic. High responder (bHR) and low responder (bLR) rat lines have been selectively bred (b) for exploratory locomotion (EL), a behavioral phenotype correlated with novelty-seeking, impulsive response to reward, and vulnerability to addiction, and is inversely correlated with spontaneous anxiety and depression-like behaviors. The rapid response to selection indicates loci of large effect for EL. Using exome sequencing of HR and LR rats, we identified alleles in gene-coding regions that segregate between the two lines. Quantitative trait locus (QTL) analysis in F2 rats derived from a bHR × bLR intercross confirmed that these regions harbored genes affecting EL. The combined effects of the seven genome-wide significant QTLs accounted for approximately one-third of the total variance in EL, and two-thirds of the variance attributable to genetic factors, consistent with an oligogenic architecture of EL estimated both from the phenotypic distribution of F2 animals and rapid response to selection. Genetic association in humans linked APBA2, the ortholog of the gene at the center of the strongest QTL, with substance use disorders and related behavioral phenotypes. Our finding is also convergent with molecular and animal behavioral studies implicating Apba2 in locomotion. These results provide multilevel evidence for genes/loci influencing EL. They shed light on the genetic architecture of oligogenicity in animals artificially selected for a phenotype modeling a more complex disorder in humans.