A translational systems biology approach in both animals and humans identifies a functionally related module of accumbal genes involved in the regulation of reward processing and binge drinking in males

On behalf of the IMAGEN consortium.
[Background]: The mesolimbic dopamine system, composed primarily of dopaminergic neurons in the ventral tegmental area that project to striatal structures, is considered to be the key mediator of reinforcement-related mechanisms in the brain. Prompted by a genome-wide association meta-analysis implicating the Ras-specific guanine nucleotide-releasing factor 2 (RASGRF2) gene in the regulation of alcohol intake in men, we have recently shown that male Rasgrf2–/– mice exhibit reduced ethanol intake and preference accompanied by a perturbed mesolimbic dopamine system. We therefore propose that these mice represent a valid model to further elucidate the precise genes and mechanisms regulating mesolimbic dopamine functioning. [Methods]: Transcriptomic data from the nucleus accumbens (NAcc) of male Rasgrf2–/– mice and wild-type controls were analyzed by weighted gene coexpression network analysis (WGCNA). We performed follow-up genetic association tests in humans using a sample of male adolescents from the IMAGEN study characterized for binge drinking (n = 905) and ventral striatal activation during an fMRI reward task (n = 608). [Results]: The WGCNA analyses using accumbal transcriptomic data revealed 37 distinct “modules,” or functionally related groups of genes. Two of these modules were significantly associated with Rasgrf2 knockout status: M5 (p < 0.001) and M6 (p < 0.001). In follow-up translational analyses we found that human orthologues for the M5 module were significantly (p < 0.01) enriched with genetic association signals for binge drinking in male adolescents. Furthermore, the most significant locus, originating from the EH-domain containing 4 (EHD4) gene (p < 0.001), was also significantly associated with altered ventral striatal activity in male adolescents performing an fMRI reward task (pempirical < 0.001). [Limitations]: It was not possible to determine the extent to which the M5 module was dysregulated in Rasgrf2–/– mice by perturbed mesolimbic dopamine signalling or by the loss of Rasgrf2 function in the NAcc. [Conclusion]: Taken together, our findings indicate that the accumbal M5 module, initially identified as being dysregulated in male Rasgrf2–/– mice, is also relevant for human alcohol-related phenotypes potentially through the modulation of reinforcement mechanisms in the NAcc. We therefore propose that the genes comprising this module represent important candidates for further elucidation within the context of alcohol-related phenotypes.
This work was supported by the European Union-funded FP6 Integrated Project IMAGEN (reinforcement-­related behaviour in normal brain function and psychopathology) (LSHM-CT2007-037286), the FP7 projects IMAGEMEND, MATRICS and the Innovative Medicine Initiative Project EU-AIMS (115300-2), as well as the Medical Research Council Programme grant “Developmental pathways into adolescent substance abuse” (93558). Further support was provided by the Swedish Funding Agency FORMAS, the German Bundesministerium für Bildung und Forschung (NGFN Plus; FKZ: 01GS08152 and eMED „Alcoholism“) and the Deutsche Forschungsgemeinschaft (DFG): Reinhart-Koselleck Award SP 383/5-1, European Research Council and FP7 SynSys (to JAG) and Ecole des Neurosciences de Paris-Ile de France (ENP) and City of Paris (to MM), Grants PS09/01979 and RD06/0020/0000 from MINECO, Spain (to ES).