The Genetics of Addiction: Where Do We Go From Here?

Behavioral phenotypes revealed during reversal learning are linked with novel genetic loci in diversity outbred mice

Impulsive behavior and impulsivity are heritable phenotypes that are strongly associated with risk for substance use disorders. Identifying the neurogenetic mechanisms that influence impulsivity may also reveal novel biological insights into addiction vulnerability. Our past studies using the BXD and Collaborative Cross (CC) recombinant inbred mouse panels have revealed that behavioral indicators of impulsivity measured in a reversal-learning task are heritable and are genetically correlated with aspects of intravenous cocaine self-administration. Genome-wide linkage studies in the BXD panel revealed a quantitative trait locus (QTL) on chromosome 10, but we expect to identify additional QTL by testing in a population with more genetic diversity. To this end, we turned to Diversity Outbred (DO) mice; 392 DO mice (156 males, 236 females) were phenotyped using the same reversal learning test utilized previously. Our primary indicator of impulsive responding, a measure that isolates the relative difficulty mice have with reaching performance criteria under reversal conditions, revealed a genome-wide significant QTL on chromosome 7 (max LOD score = 8.73, genome-wide corrected p<0.05). A measure of premature responding akin to that implemented in the 5-choice serial reaction time task yielded a suggestive QTL on chromosome 17 (max LOD score = 9.14, genome-wide corrected <0.1). Candidate genes were prioritized (2900076A07Rik, Wdr73 and Zscan2) based upon expression QTL data we collected in DO and CC mice and analyses using publicly available gene expression and phenotype databases. These findings may advance understanding of the genetics that drive impulsive behavior and enhance risk for substance use disorders.

Addiction-Associated Genetic Variants Implicate Brain Cell Type- and Region-Specific Cis-Regulatory Elements in Addiction Neurobiology

Recent large genome-wide association studies have identified multiple confident risk loci linked to addiction-associated behavioral traits. Most genetic variants linked to addiction-associated traits lie in noncoding regions of the genome, likely disrupting cis-regulatory element (CRE) function. CREs tend to be highly cell type-specific and may contribute to the functional development of the neural circuits underlying addiction. Yet, a systematic approach for predicting the impact of risk variants on the CREs of specific cell populations is lacking. To dissect the cell types and brain regions underlying addiction-associated traits, we applied stratified linkage disequilibrium score regression to compare genome-wide association studies to genomic regions collected from human and mouse assays for open chromatin, which is associated with CRE activity. We found enrichment of addiction-associated variants in putative CREs marked by open chromatin in neuronal (NeuN+) nuclei collected from multiple prefrontal cortical areas and striatal regions known to play major roles in reward and addiction. To further dissect the cell type-specific basis of addiction-associated traits, we also identified enrichments in human orthologs of open chromatin regions of female and male mouse neuronal subtypes: cortical excitatory, D1, D2, and PV. Last, we developed machine learning models to predict mouse cell type-specific open chromatin, enabling us to further categorize human NeuN+ open chromatin regions into cortical excitatory or striatal D1 and D2 neurons and predict the functional impact of addiction-associated genetic variants. Our results suggest that different neuronal subtypes within the reward system play distinct roles in the variety of traits that contribute to addiction.SIGNIFICANCE STATEMENT We combine statistical genetic and machine learning techniques to find that the predisposition to for nicotine, alcohol, and cannabis use behaviors can be partially explained by genetic variants in conserved regulatory elements within specific brain regions and neuronal subtypes of the reward system. Our computational framework can flexibly integrate open chromatin data across species to screen for putative causal variants in a cell type- and tissue-specific manner for numerous complex traits.

Early life mental health and problematic drinking in mid-adulthood: evidence from two British birth cohorts

PURPOSE

Accumulating evidence suggests that externalising problems are consistently associated with alcohol use behaviours, but findings are inconsistent regarding the role of internalising problems. We investigate whether externalising and internalising problems are associated with problematic drinking in mid-adulthood, and whether potential associations are modified by age, sex and cohort.

METHODS

The National Child Development Study (NCDS58, n = 17,633) and 1970 British Cohort Study (BCS70, n = 17,568) recruited new-borns in Great Britain in a single week in 1958 and 1970. Mental health was assessed with the Rutter Behaviour Questionnaire at ages 7, 11, and 16 in NCDS58 and ages 5, 10 and 16 in BCS70. Problematic drinking was measured with the CAGE questionnaire at age 33 in NCDS58 and age 34 in BCS70, and the AUDIT scale at age 44/45 in NCDS58 and age 46 in BCS70. Latent scores of externalising and internalising problems were added chronologically into lagged logistic regression models.

RESULTS

Externalising and internalising problems were associated in opposite directions with problematic drinking in mid-adulthood. Externalising was a risk factor (OR [95% CI] ranging from 1.06 [1.03, 1.10] to 1.11 [1.07, 1.15] for different ages), and internalising was a protective factor (OR [95% CI] ranging from 0.95 [0.92, 0.99] to 0.90 [0.86, 0.94] for different ages). Associations between early life mental health and mid-adulthood problematic drinking did not differ by developmental timing but were stronger in males.

CONCLUSION

Our study provides new insights on links of externalising and internalising difficulties with alcohol use and has implications for public policy in the UK.

Cocaine-Induced Sensitization is Linked to Distal Chromosome 6 Region in Congenic Mouse Model

OBJECTIVE

Previously we mapped QTL Eac2 to mouse Chr6 and identified the first gene (Grm7) as accounting for alcohol consumption in a mammalian model. Despite the central role of glutamate receptors in addiction, the effects of Grm7 gene variants are not well known. Here we test the hypothesis that genetic variation of the distal mouse Chr6 Eac2 region, location of Grm7, controls cocaine-induced locomotor sensitization.

METHOD

C57BL/6By background and B6.C6.327.54 congenic mice were subjected to whole-genome SNP genotyping. Isogeneic (C57BL/6ByXB6.C6.327.54)F2 mice homozygous for SNPs in the BALB/c-type Eac2 region were selected to create a subcongenic strain (B6By.C6.108-120). In a 2-strain x 2-sex 2-treatment factorial design (n = 6-10) C57BL/6By and B6By.C6.108-120 mice received repeated daily cocaine or saline intraperitoneal injections, and locomotor activity was recorded for 90 minutes immediately after injection.

RESULTS

C57BL/6By females with the G/G genotype of SNP rs3723352 of Grm7 responded to cocaine with significantly higher activity and greater cocaine-induced sensitization than those with the BALB/cJ-type T/T genotype in the congenic strain.

CONCLUSION

The results are consistent with a large body of accumulated mechanistic evidence for a role of the mGlu7 receptor in the control of neurobiological responses to cocaine, and are consistent with the hypotheses that (1) natural variants of the Grm7 gene show pleiotropy and can modulate cocaine-induced behaviors in addition to alcohol consumption, (2) interactions between mGluR7 expression, estrogen receptors, and estradiol may explain phenotypic variation in females. Heritable variation of GRM7 may affect vulnerability to substance abuse in women.

Commentary: Perspectives on alcohol-related gene and environment interplay in diverse populations

BACKGROUND AND OBJECTIVES

Racial/ethnic groups comprise more than 20% of the U.S. population, but many experience disproportionately high risk for alcohol misuse, often resulting in higher rates of alcohol-associated consequences. Completion of mapping the human genome has launched rapidly evolving research methods aimed at improved understanding of genetic contribution to disease. Despite decades of research on the influence of genetic and environmental risks on alcohol use disorders and outcomes, few studies have included racial/ethnic subpopulations in sufficient numbers to allow for proper statistical analysis.

METHODS

The papers in this special issue help to elucidate current knowledge on the etiology of genetic and environmental contributors and potential moderators of alcohol use and associated problems among racial/ethnic populations. The lack of racial/ethnic diversity across many genetic studies contributes to challenges in interpretation of findings and eventually applications to precision medicine.

RESULTS

Proposed approaches to overcome disparities in racial/ethnic participant recruitment in genetic studies include methods to address population stratification in allele frequency, improve transparency in subjects' consenting to participate, and engaging interdisciplinary research teams and community involvement to improve recruitment of racial/ethnic minorities.

DISCUSSION AND CONCLUSIONS

The reviews presented underscore various gaps in our knowledge of the genetic influences on alcohol use disorders due to the failure to include racially and ethnically diverse populations in genetic and epigenetic study samples. New directions are suggested to overcome the resulting research challenges and ultimately to inform future personalized intervention approaches for racial/ethnic populations.

SCIENTIFIC SIGNIFICANCE

Inclusion of heterogeneous populations in genomic research will provide a better comprehension of possible unique genetic factors in the broader general population that may be missed due to exclusion of unique and common variants that may be present in racial/ethnic populations. (Am J Addict 2017;26:526-531).