Convergent functional genomics in addiction research - a translational approach to study candidate genes and gene networks

Invited Expert Opinion- Bioinformatic and Limitation Directives to Help Adopt Genetic Addiction Risk Screening and Identify Preaddictive Reward Dysregulation: Required Analytic Evidence to Induce Dopamine Homeostatsis

Addiction, albeit some disbelievers like Mark Lewis [1], is a chronic, relapsing brain disease, resulting in unwanted loss of control over both substance and non- substance behavioral addictions leading to serious adverse consequences [2]. Addiction scientists and clinicians face an incredible challenge in combatting the current opioid and alcohol use disorder (AUD) pandemic throughout the world. Provisional data from the Centers for Disease Control and Prevention (CDC) shows that from July 2021-2022, over 100,000 individuals living in the United States (US) died from a drug overdose, and 77,237 of those deaths were related to opioid use [3]. This number is expected to rise, and according to the US Surgeon General it is highly conceivable that by 2025 approximately 165,000 Americans will die from an opioid overdose. Alcohol abuse, according to data from the World Health Organization (WHO), results in 3 million deaths worldwide every year, which represents 5.3% of all deaths globally [4].

Functional Characterization and Molecular Marker Development of the Proenkephalin as Biomarker of Food Addiction in Food Habit Domestication of Mandarin Fish (Siniperca Chuatsi)

Proenkephalin (PENK), as the precursor of endogenous opioid enkephalin (ENK), is widely present in the nervous system and plays an important role in animal food addiction and rewarding behavior. In our study, we intend to study the functional characterization and molecular marker development of the penk gene related to food habit domestication of mandarin fish. We found that the penk gene of mandarin fish had three types of endogenous opioid peptide sequences. Compared with other tissues, penk mRNA was highly expressed in the whole brain. Intracerebroventricular (ICV) injection of lysine or methionine significantly increased the expression of penk mRNA. The expression of penk mRNA in the brain of mandarin fish that could be easily domesticated from eating live prey fish to artificial diets was significantly higher than those that could not. After feeding with high-carbohydrate artificial diets, the expression of penk mRNA showed no significant difference between mandarin fish with hypophagia and those that still ate normally. A total of four single nucleotide polymorphisms (SNP) loci related to easy domestication toward eating artificial diets were screened from the mandarin fish population. Additionally, the TT genotype at one of the loci was significantly correlated with the food habit domestication of mandarin fish.

Using hierarchical similarity to examine the genetics of Behçet's disease

OBJECTIVE

Behçet's disease (BD) is a multisystem inflammatory disease that affects patients along the historic silk road. Thus far, the pathogenesis of the disease has proved elusive due to the complex genetic interactions of the disease. In this paper, we seek to clarify the genetic factors of the disease while also uncovering other diseases of interest that present with a similar genotype as BD.

RESULTS

To do this, we employ a convergent functional genomics approach by leveraging the hierarchical similarity tool available in Geneweaver. Through our analysis, we were able to ascertain 7 BD consensus genes and 16 autoimmune diseases with genetic overlap with BD. The results of our study will inform further research into the pathogenesis of Behçet's disease.

Intracranial self-stimulation and concomitant behaviors following systemic methamphetamine administration in Hnrnph1 mutant mice

RATIONALE

Methamphetamine (MA) addiction is a major public health issue in the USA, with a poorly understood genetic component. We previously identified heterogeneous nuclear ribonucleoprotein H1 (Hnrnph1; H1) as a quantitative trait gene underlying sensitivity to MA-induced behavioral sensitivity. Mice heterozygous for a frameshift deletion in the first coding exon of H1 (H1^+/-) showed reduced MA phenotypes including oral self-administration, locomotor activity, dopamine release, and dose-dependent differences in MA conditioned place preference. However, the effects of H1^+/- on innate and MA-modulated reward sensitivity are not known.

OBJECTIVES

We examined innate reward sensitivity and facilitation by MA in H1^+/- mice via intracranial self-stimulation (ICSS).

METHODS

We used intracranial self-stimulation (ICSS) of the medial forebrain bundle to assess shifts in reward sensitivity following acute, ascending doses of MA (0.5-4.0 mg/kg, i.p.) using a within-subjects design. We also assessed video-recorded behaviors during ICSS testing sessions.

RESULTS

H1^+/- mice displayed reduced normalized maximum response rates in response to MA. H1^+/- females had lower normalized M50 values compared to wild-type females, suggesting enhanced reward facilitation by MA. Finally, regardless of genotype, there was a dose-dependent reduction in distance to the response wheel following MA administration, providing an additional measure of MA-induced reward-driven behavior.

CONCLUSIONS

H1^+/- mice displayed a complex ICSS phenotype following MA, displaying indications of both blunted reward magnitude (lower normalized maximum response rates) and enhanced reward sensitivity specific to H1^+/- females (lower normalized M50 values).

Association of the alcohol dehydrogenase gene polymorphism rs1789891 with gray matter brain volume, alcohol consumption, alcohol craving and relapse risk

Alcohol metabolizing enzymes, such as the alcohol dehydrogenases and the aldehyde dehydrogenases, regulate the levels of acetaldehyde in the blood and play an important role in the development and maintenance of alcohol addiction. Recent genome-wide systematic searches found associations between a single nucleotide polymorphism (rs1789891, risk allele: A, protective allele: C) in the alcohol dehydrogenase gene cluster and the risk of alcohol dependence. The current study investigated the effect of this single nucleotide polymorphism on alcohol consumption, craving for alcohol, relapse risk and brain gray matter volume. Alcohol-dependent patients (n = 74) and controls (n = 43) were screened, genotyped and underwent magnetic resonance imaging scanning, and relapse data were collected during 3 months following the experiment. Alcohol-dependent A allele carriers reported increased alcohol craving and higher alcohol consumption compared with the group of alcohol-dependent individuals homozygous for the C allele, which displayed craving values similar to the control group. Further, follow-up data indicated that A allele carriers relapsed earlier to heavy drinking compared with individuals with two C alleles. Analyses of gray matter volume indicated a significant genotype difference in the patient group: individuals with two C alleles had reduced gray matter volume in the left and right superior, middle and inferior temporal gyri. Findings of the current study further support the relevance of genetic variants in alcohol metabolizing enzymes to addictive behavior, brain tissue volume and relapse risk. Genotype-dependent differences in acetaldehyde formation, implicated by earlier studies, might be the biological substrate of the genotype differences.

Animal models of addiction

In recent years, animal models in psychiatric research have been criticized for their limited translational value to the clinical situation. Failures in clinical trials have thus often been attributed to the lack of predictive power of preclinical animal models. Here, I argue that animal models of voluntary drug intake—under nonoperant and operant conditions—and addiction models based on the Diagnostic and Statistical Manual of Mental Disorders are crucial and informative tools for the identification of pathological mechanisms, target identification, and drug development. These models provide excellent face validity, and it is assumed that the neurochemical and neuroanatomical substrates involved in drug-intake behavior are similar in laboratory rodents and humans. Consequently, animal models of drug consumption and addiction provide predictive validity. This predictive power is best illustrated in alcohol research, in which three approved medications—acamprosate, naltrexone, and nalmefene—were developed by means of animal models and then successfully translated into the clinical situation.

[A pharmacogenetic analysis of dopaminergic and opioidergic genes in opioid addicts treated with the combination of naltrexone and guanfacine]

AIM

To evaluate an effect of opioid receptor and dopamine system gene polymorphisms on the efficacy of combined treatment with oral naltrexone and guanfacine in a randomized double blinded double dummy placebo controlled clinical trial.

MATERIAL AND METHODS

Three hundred and one patients with opioid dependence were randomized into 4 treatment groups: naltrexone 50 mg/day + guanfacine 1 mg/day (N+G); naltrexone + placebo guanfacine (N+GP); placebo naltrexone + guanfacine (NP+G); double placebo (NP+GP). The primary outcome was treatment retention. All enrolled participants were genotyped for polymorphisms in the following genes: mu- (OPRM1), kappa-opioid receptors (OPRK1), catechol-O-methyltransferase (COMT), dopamine receptors types 2 (DRD2) and 4 (DRD4), dopamine-beta-hydroxylase, and dopamine transporter (SLC6A3, DAT1) and alpha-2-adrenoreceptor (ADRA2A) a pharmacological target of guanfacine.

RESULTS

The efficacy of the combination of naltrexone and guanfacine was comparable to naltrexone monotherapy. Regardless of treatment, several gene polymorphisms were associated with higher chance to complete the treatment program: allele Т DRD4 - 521 С/Т (rs1800955) (р=0.039; OR (95% CI)=3.7 (1.1-12.7); log-rank test: р=0.01); allele С DRD2 С957Т (rs6277) (р=0.03; HR=0.6 (0.34-0.95); genotype combination: DRD4 VNTR (LL) + OPRM1 A118G (rs1799971) (AA), р=0.051; DRD2 C957T (ТТ) + OPRM1 (rs1074287) (СС), р=0.025; DRD2 - 141С (II) + OPRM1 (rs510769) (АА), р=0.035; DBH Fau(СС) + OPRM1 (rs1074287) (СС), р=0.0497. Regardless of treatment several polymorphisms were associated with high risk of relapse: allele Т (rs510769) OPRM1 (р=0.053), allele А (rs1799971, A118G) OPRM1 (р=0.056), allele S exon III 48 bp DRD4 VNTR (р=0.001; HR=3.1 (ДИ 95% 1.57-6.18); genotype combinations: DRD4 - 521 С/Т (ТТ) + DRD2 Nco I (TT), р=0.026; DRD4 -521 С/Т (ТТ) + DRD2 -141 С (II), р=0.011; DRD4 - 521 С/Т (ТТ) + OPRM1 A118G (rs1799971) (AA), р=0.011; DRD2 Nco I(ТТ) + ADRA2A (СС), р=0.012; DRD2 Nco I(ТТ) + OPRM1 A118G (AA), р=0.02. The effects dependent on the treatment group were as follows: 1) in the N+G group, patients with the DRD4 -521 С/Т TT genotype had higher probability of completion of treatment program in comparison with other genotypes (CC and CT) (log-rank test: p=0.002); 2) in NP + GP group, patients with the OPRM1 rs510769 T allele had higher risk of relapse compared to the genotype GG (p=0.008) (FDR p<0.0125).

CONCLUSION

The additive effect of opioid receptor genes and dopaminergic system genes on outcomes of treatment opioid dependence with oral naltrexone and guanfacine was shown. Pharmacological effects of naltrexone and guanfacine were associated with genetic variants of the DRD4 - 521C/T polymorphism, since its effect was shown only in the N+G group. The effect of the OPRM1 rs510769 polymorphism was demonstrated in the double placebo group that was associated with personality traits (temperament, character) and determined compliance. Genetic analysis is useful for determining potential responders to treatment of opioid dependence; genotyping can increase the efficacy of pharmacotherapy.

Applicability of gene expression and systems biology to develop pharmacogenetic predictors; antipsychotic-induced extrapyramidal symptoms as an example

Pharmacogenetics has been driven by a candidate gene approach. The disadvantage of this approach is that is limited by our current understanding of the mechanisms by which drugs act. Gene expression could help to elucidate the molecular signatures of antipsychotic treatments searching for dysregulated molecular pathways and the relationships between gene products, especially protein-protein interactions. To embrace the complexity of drug response, machine learning methods could help to identify gene-gene interactions and develop pharmacogenetic predictors of drug response. The present review summarizes the applicability of the topics presented here (gene expression, network analysis and gene-gene interactions) in pharmacogenetics. In order to achieve this, we present an example of identifying genetic predictors of extrapyramidal symptoms induced by antipsychotic.

Quantification of alcohol drinking patterns in mice

The use of mice in alcohol research provides an excellent model system for a better understanding of the genetics and neurobiology of alcohol addiction. Almost 60 years ago, alcohol researchers began to test strains of mice for alcohol preference and intake. In particular, various voluntary alcohol drinking paradigms in the home cage were developed. In mouse models of voluntary oral alcohol consumption, animals have concurrent access to water and either one or several concentrated alcohol solutions in their home cages. Although these models have high face validity, many experimental conditions require a more precise monitoring of alcohol consumption in mice in order to capture the role of specific strains or genes, or any other manipulation on alcohol drinking behavior. Therefore, we have developed a fully automated, highly precise monitoring system for alcohol drinking in mice in the home cage. This system is now commercially available. We show that this drinkometer system allows for detecting differences in drinking behavior (i) in transgenic mice, (ii) following alcohol deprivation, and (iii) following stress applications that are usually not detected by classical home-cage drinking paradigms. In conclusion, our drinkometer system allows disturbance-free and high resolution monitoring of alcohol drinking behavior. In particular, micro-drinking and circadian drinking patterns can be monitored in genetically modified and inbred strains of mice after environmental and pharmacological manipulation, and therefore this system represents an improvement in measuring behavioral features that are of relevance for the development of alcohol use disorders.

XRCC5 as a risk gene for alcohol dependence: evidence from a genome-wide gene-set-based analysis and follow-up studies in Drosophila and humans

Genetic factors have as large role as environmental factors in the etiology of alcohol dependence (AD). Although genome-wide association studies (GWAS) enable systematic searches for loci not hitherto implicated in the etiology of AD, many true findings may be missed owing to correction for multiple testing. The aim of the present study was to circumvent this limitation by searching for biological system-level differences, and then following up these findings in humans and animals. Gene-set-based analysis of GWAS data from 1333 cases and 2168 controls identified 19 significantly associated gene-sets, of which 5 could be replicated in an independent sample. Clustered in these gene-sets were novel and previously identified susceptibility genes. The most frequently present gene, ie in 6 out of 19 gene-sets, was X-ray repair complementing defective repair in Chinese hamster cells 5 (XRCC5). Previous human and animal studies have implicated XRCC5 in alcohol sensitivity. This phenotype is inversely correlated with the development of AD, presumably as more alcohol is required to achieve the desired effects. In the present study, the functional role of XRCC5 in AD was further validated in animals and humans. Drosophila mutants with reduced function of Ku80-the homolog of mammalian XRCC5-due to RNAi silencing showed reduced sensitivity to ethanol. In humans with free access to intravenous ethanol self-administration in the laboratory, the maximum achieved blood alcohol concentration was influenced in an allele-dose-dependent manner by genetic variation in XRCC5. In conclusion, our convergent approach identified new candidates and generated independent evidence for the involvement of XRCC5 in alcohol dependence.

Addiction and reward-related genes show altered expression in the postpartum nucleus accumbens

Motherhood involves a switch in natural rewards, whereby offspring become highly rewarding. Nucleus accumbens (NAC) is a key CNS region for natural rewards and addictions, but to date no study has evaluated on a large scale the events in NAC that underlie the maternal change in natural rewards. In this study we utilized microarray and bioinformatics approaches to evaluate postpartum NAC gene expression changes in mice. Modular Single-set Enrichment Test (MSET) indicated that postpartum (relative to virgin) NAC gene expression profile was significantly enriched for genes related to addiction and reward in five of five independently curated databases (e.g., Malacards, Phenopedia). Over 100 addiction/reward related genes were identified and these included: Per1, Per2, Arc, Homer2, Creb1, Grm3, Fosb, Gabrb3, Adra2a, Ntrk2, Cry1, Penk, Cartpt, Adcy1, Npy1r, Htr1a, Drd1a, Gria1, and Pdyn. ToppCluster analysis found maternal NAC expression profile to be significantly enriched for genes related to the drug action of nicotine, ketamine, and dronabinol. Pathway analysis indicated postpartum NAC as enriched for RNA processing, CNS development/differentiation, and transcriptional regulation. Weighted Gene Coexpression Network Analysis (WGCNA) identified possible networks for transcription factors, including Nr1d1, Per2, Fosb, Egr1, and Nr4a1. The postpartum state involves increased risk for mental health disorders and MSET analysis indicated postpartum NAC to be enriched for genes related to depression, bipolar disorder (BPD), and schizophrenia. Mental health related genes included: Fabp7, Grm3, Penk, and Nr1d1. We confirmed via quantitative PCR Nr1d1, Per2, Grm3, Penk, Drd1a, and Pdyn. This study indicates for the first time that postpartum NAC involves large scale gene expression alterations linked to addiction and reward. Because the postpartum state also involves decreased response to drugs, the findings could provide insights into how to mitigate addictions.

The alcohol deprivation effect model for studying relapse behavior: a comparison between rats and mice

Understanding the psychological mechanisms and underlying neurobiology of relapse behavior is essential for improving the treatment of addiction. Because the neurobiology of relapse behavior cannot be well studied in patients, we must rely on appropriate animal models. The alcohol deprivation effect (ADE) is a phenomenon in laboratory animals that models a relapse-like drinking situation, providing excellent face and predictive validity. In rodents, relapse-like behavior is largely influenced by the genetic make-up of an animal. It is not clear which other factors are responsible for variability of this behavior, but there seems to be no correlation between levels of baseline alcohol intake and the occurrence, duration, and robustness of the ADE. Rats that undergo long-term alcohol drinking for several months with repeated deprivation phases develop a compulsive drinking behavior during a relapse situation, characterized by insensitivity to taste adulteration with quinine, a loss of circadian drinking patterns during relapse-like drinking, and a shift toward drinking highly concentrated alcohol solutions to rapidly increase blood alcohol concentrations and achieve intoxication. Some mouse strains also exhibit an ADE, but this is usually of shorter duration than in rats. However, compulsive drinking in mice during a relapse situation has yet to be demonstrated. We extend our review section with original data showing that during long-term alcohol consumption, mice show a decline in alcohol intake, and the ADE fades with repeated deprivation phases. Furthermore, anti-relapse compounds that produce reliable effects on the ADE in rats produce paradoxical effects in mice. We conclude that the rat provides a better model system to study alcohol relapse and putative anti-relapse compounds.