Systems genetics of intravenous cocaine self-administration in the BXD recombinant inbred mouse panel

Abstract

Rationale: Cocaine addiction is a major public health problem with a substantial genetic basis for which the biological mechanisms remain largely unknown. Systems genetics is a powerful method for discovering novel mechanisms underlying complex traits, and intravenous drug self-administration (IVSA) is the gold standard for assessing volitional drug use in preclinical studies. We have integrated these approaches to identify novel genes and networks underlying cocaine use in mice.

Methods: Mice from 39 BXD strains acquired cocaine IVSA (0.56 mg/kg/infusion). Mice from 29 BXD strains completed a full dose-response curve (0.032-1.8 mg/kg/infusion). We identified independent genetic correlations between cocaine IVSA and measures of environmental exploration and cocaine sensitization. We identified genome-wide significant quantitative trait loci (QTL) on chromosomes 7 and 11 associated with shifts in the dose-response curve and on chromosome 16 associated with sessions to acquire cocaine IVSA. Using publicly available gene expression data from the nucleus accumbens, midbrain, and prefrontal cortex of drug-naïve mice, we identified Aplp1 and Cyfip2 as positional candidates underlying the behavioral QTL on chromosomes 7 and 11, respectively. A genome-wide significant trans-eQTL linking Fam53b (a GWAS candidate for human cocaine dependence) on chromosome 7 to the cocaine IVSA behavioral QTL on chromosome 11 was identified in the midbrain; Fam53b and Cyfip2 were co-expressed genome-wide significantly in the midbrain. This finding indicates that cocaine IVSA studies using mice can identify genes involved in human cocaine use.

Conclusions: These data provide novel candidate genes underlying cocaine IVSA in mice and suggest mechanisms driving human cocaine use.

Keywords: Addiction; Aplp1; Cocaine sensitization; Cyfip2; Fam53b; Genetic correlation; Light dark box; Open field; QTL; eQTL.

Figure 1. Intravenous cocaine self-administration in the BXD recombinant inbred panel

(a) Active presses, inactive presses, and cocaine infusions (mean ± SE) during the first seven 120-min sessions of cocaine IVSA testing in 39 BXD strains. Number of active presses was significantly greater than number of inactive presses on all sessions (p < .01). (b) Number of cocaine infusions at each dose (mean of final two 120 min sessions) along the dose-response curve in 29 BXD strains. Red circles represent the mean of all BXD strains at each dose and black diamonds represent individual strain means. (c) Frequency distribution of the peak of the dose-response curve for individual BXD strains. (d, e) Active and inactive lever presses (mean ± SE) and cocaine intake (mean ± SD) across the dose-response curve. (f) Principal components associated with cocaine IVSA infusions at all doses along the dose-response curve.

Figure 2. Genetic correlations

(a, b) Cocaine sensitization was positively genetically correlated with cocaine IVSA infusions at the two lowest cocaine doses. (c) Distance traveled over 60 min in a novel open field was positively genetically correlated with cocaine IVSA infusions at the lowest dose. (d, e, f) Percentage of time in the light side of a light/dark box during a single 10 min session was positively genetically correlated with cocaine infusions at the three lowest cocaine doses. After adjusting for the effects of the other two phenotypes, each of these measures explained unique genetic variance on cocaine IVSA infusions at the lowest cocaine dose: cocaine sensitization (26.2%, p < .01), light/dark box (29.8%, p < .001), and open field (11.7%, p < .05). Data points in all panels represent BXD strain means.

Figure 3. QTL associated with the number of sessions to meet cocaine IVSA acquisition criteria

(a, b) Genome-wide significant QTL on chromosome 16 associated with sessions to reach cocaine IVSA acquisition criteria (2 LOD CI: 83.407 - 88.783 Mb). (c) BXD strains carrying the D2 allele at the QTL peak required significantly more sessions to reach acquisition criteria (M = 5.32, SD = 1.32) relative to BXD strains carrying the B6 allele (M = 3.40, SD = 1.18). This effect accounted for 38% of the variance on the sessions to acquire cocaine IVSA measure.

Figure 4. QTL associated with infusions on the ascending limb of the dose response curve

(a, b) Genome-wide significant QTL on chromosome 11 associated with infusions on the ascending limb of the dose response curve (IAL) (2 LOD CI: 46.00 - 50.39 Mb). (c) IAL factor scores of BXD mice carrying the B6 allele at the QTL peak differed significantly from scores of BXD mice carrying the D2 allele at the QTL peak, and this effect accounted for 54% of the variance on the IAL factor. (d, e) BXD strains carrying the B6 allele at the QTL peak infused significantly more cocaine on the ascending limb of the dose response curve relative to strains carrying the D2 allele at the QTL peak. This effect was significantly reversed on the descending limb, indicating a leftward shift of the dose response curve in BXD strains carrying the B6 allele at the QTL peak. Data points represent group means ± SE.

Figure 5. QTL associated with consistency of cocaine intake across the ascending limb of the dose response curve

(a, b) Genome-wide significant QTL on chromosome 7 associated with consistency of cocaine intake across the ascending limb of the dose response curve (CAL) (2 LOD CI: 31.206 - 31.467 Mb). (c) CAL index scores of BXD strains carrying the B6 allele at the QTL peak differed significantly from scores of BXD strains carrying the D2 allele at the QTL peak, and this effect accounted for 60% of the variance on the CAL index. (d, e) BXD strains carrying the B6 allele at the QTL peak infused significantly more cocaine on the ascending and descending limbs of the dose response curve relative to strains carrying the D2 allele at the QTL peak. Collectively, these observations indicate an upward shift of the dose response curve in BXD strains carrying the B6 allele at the QTL peak. Data points represent group means ± SE.

Figure 6. Combined allelic effects on the cocaine IVSA dose response curve of the behavioral QTL on chromosome 7 and 11

BXD strains carrying the B6 allele at both Chr 11 @ 47.93 Mb (IAL QTL peak) and Chr 7 @ 31.22 Mb (CAL QTL peak) infused significantly more cocaine (a) and made significantly more active lever presses (b) on the ascending limb of the dose response curve relative to BXD strains carrying the D2 allele at both QTL. The combined allelic effects were significantly greater than the independent effects on the two lowest cocaine doses (p < .05). Data points represent group means ± SE.

Figure 7. Fam53b maps to a cocaine IVSA behavioral QTL on chromosome 11

(a) Fam53b is a human GWAS candidate for cocaine dependence (Gelernter et al. 2014). Midbrain expression of Fam53b (Chr 7 @ 139.95 Mb) maps genome-wide significantly to the IAL behavioral QTL located on Chr 11. (b) Fam53b and the top gene candidate for the IAL behavioral QTL (Cyfip2) were co-expressed genome-wide significantly in the midbrain. Collectively, these data suggest a functional relationship between Cyfip2 and Fam53b in the context of cocaine use.