Zhx2 Is a Candidate Gene Underlying Oxymorphone Metabolite Brain Concentration Associated with State-Dependent Oxycodone Reward

Abstract

Understanding the pharmacogenomics of opioid metabolism and behavior is vital to therapeutic success, as mutations can dramatically alter therapeutic efficacy and addiction liability. We found robust, sex-dependent BALB/c substrain differences in oxycodone behaviors and whole brain concentration of oxycodone metabolites. BALB/cJ females showed robust statedependent oxycodone reward learning as measured via conditioned place preference when compared to the closely related BALB/cByJ substrain. Accordingly, BALB/cJ females also showed a robust increase in brain concentration of the inactive metabolite noroxycodone and the active metabolite oxymorphone compared to BALB/cByJ mice. Oxymorphone is a highly potent, full agonist at the mu opioid receptor that could enhance drug-induced interoception and state-dependent oxycodone reward learning. Quantitative trait locus (QTL) mapping in a BALB/c F2 reduced complexity cross revealed one major QTL on chromosome 15 underlying brain oxymorphone concentration that explained 32% of the female variance. BALB/cJ and BALB/cByJ differ by fewer than 10,000 variants which can greatly facilitate candidate gene/variant identification. Hippocampal and striatal cis-expression QTL (eQTL) and exon-level eQTL analysis identified Zhx2, a candidate gene coding for a transcriptional repressor with a private BALB/cJ retroviral insertion that reduces Zhx2 expression and sex-dependent dysregulation of CYP enzymes. Whole brain proteomics corroborated the Zhx2 eQTL and identified upregulated CYP2D11 that could increase brain oxymorphone in BALB/cJ females. To summarize, Zhx2 is a highly promising candidate gene underlying brain oxycodone metabolite levels. Future studies will validate Zhx2 and its site of action using reciprocal gene editing and tissue-specific viral manipulations in BALB/c substrains.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

“…In addition, the authors suggest that chronic stress 'primes' the opioid system in females and this would promote opioid-associated learning, again a gender-specific response that may account for the known sex differences in opioid use, abuse and in the effects of stress since these changes are not seen in males and the absence of such changes in these parameters may reduce the capacity to support opioid-mediated learning. Beierle et al (2022) found robust sex-dependent substrain differences when comparing the effects of oxycodone in BALB/cj and BALB/cByj mice. In CPP, the female BALB/cj mice spent more time in the side associated with oxycodone than the BALB/cByj mice, showing enhanced sensitivity to oxycodone, whereas the male strains did not differ.…”

Atp1a2 and Kcnj9 are candidate genes underlying sensitivity to oxycodone-induced locomotor activation and withdrawal-induced anxiety-like behaviors in C57BL/6 substrains

“…Genetic regulation of gene expression (eQTLs) provides a functional link and causal support for quantitative trait genes/variants underlying behavior 23,43,44,51 . Using a striatal RNA-seq dataset from 23 F2 mice that had undergone OXY-CPP testing, two weeks of high-dose OXY treatment (20 mg/kg, i.p.…”

“…Finally, Phillips and colleagues exploited the reduced genetic complexity between DBA/2J substrains to identify a functional missense mutation in Taar1 that influences methamphetamine-induced hyperthermia, negative reinforcement, and toxicity [38][39][40][41][42] . Our lab also extended the RCC approach to BALB/c substrains, where we quickly identified candidate genes for nociceptive sensitivity, brain weight, and opioid behavioral and molecular traits 43,44 .…”