Patricia F. Fitzpatrick Dimond Ph.D. Technical Editor of Clinical OMICs President of BioInsight Communications
Is Screening for Addictions Useful?
As researchers continue to find numerous genes that may increase the risk for drug and alcohol dependence, a complex and multifactorial picture continues to emerge. Genetic differences in individual physiology, drug- or alcohol-induced changes in epigenetic gene regulation and expression, and developmental and environmental factors all contribute to the development and persistence of chronic substance abuse. All evidence indicates that save for a very few well-validated genetic variations, reliable screening for substance dependence that identifies at-risk individuals remains a long way off.
Twin and adoption studies have confirmed that genes contribute to the development of alcohol dependence (AD) with heritability estimates ranging from 50–60% for both men and women. Studies examining drug dependence among twins suggested that among identical twins, shared genes influence the risk of both alcohol and drug dependence.
For alcoholism, the most significant and durable identifiable genetic risk factors involve genes linked to ethanol metabolism. Genetic variations in enzymes, such as variants of the alcohol dehydrogenase (ALDH) gene, allow buildup of the toxic metabolite acetaldehyde, producing the unpleasant effects associated with hangovers. Among the 40% of Asian Individuals bearing the ALDH2-2 mutation, homozygous individuals experience severe nausea and vomiting and an intense skin flush with low alcohol consumption, reducing the risk of alcohol abuse to almost zero. Heterozygous individuals with one copy of the mutated gene experience more minor symptoms such as skin flush.
But other unambiguous associations between genes and substance abuse that might allow for screening and prevention remain elusive and function in multiple pathways. Genes involved in alcohol metabolism as well as in the transmission of nerve cell signals and modulation of nerve cell activity (e.g., γ-aminobutyric acid [GABA] and acetylcholinergic neurotransmission and the endogenous opioid and cannabinoid systems) have been implicated in alcohol addiction, as well as drug abuse.
Beyond identification of addiction-associated genes or alleles, drug addiction involves potentially life-long behavioral abnormalities caused by repeated exposure to a drug of abuse. The persistence of long-lasting changes in gene expression within particular regions of the brain may contribute to the addiction phenotype. Research over the past decade has demonstrated a crucial role for epigenetic mechanisms in driving lasting changes in gene expression in diverse tissues, investigators say.
The association between a variant in the CYP2E1 gene that encodes a member of the cytochrome P450 superfamily of enzymes and increased sensitivity to alcohol illustrates the biochemical complexities of addiction among different groups of individuals. CYP2E1, induced by chronic alcohol consumption, metabolizes ethanol to acetaldehyde at elevated ethanol concentrations. In addition, CYP2E1- dependent ethanol oxidation may occur in tissues such as the brain where ADHL activity is low. The enzyme also produces reactive oxygen species, including hydroxyethyl, superoxide anion, and hydroxyl radicals, all of which increase the risk of tissue damage.
Researchers at the University of North Carolina reported in Alcoholism, Clinical and Experimental Research in 2011 that combined linkage and association studies indicated that sequence changes in or near CYP2E1 affect the level of response to alcohol, providing a predictor of risk of alcoholism. The investigators performed a family-based genome-wide linkage analysis using sibling pairs that underwent an alcohol challenge where the level of response to alcohol was measured with the Subjective High Assessment Scale. They said their findings implicated the 10q terminal (10qter) region, and CYP2E1 mapped to this region.
Complex Processes Involved
“It turns out that a specific version or allele of CYP2E1 makes people more sensitive to alcohol, and we are now exploring whether it is because it generates more (of these) free radicals” said Kirk Wilhelmsen, M.D., Ph.D., associate professor, departments of genetics and neurology at the Bowles Center for Alcohol Studies at the University of North Carolina in Chapel Hill. “This finding is interesting because it hints at a totally new mechanism of how we perceive alcohol when we drink. The conventional model basically says that alcohol affects how neurotransmitters, the molecules that communicate between neurons, do their job. But our findings suggest it is even more complex than that.”
But as David Goldman, M.D., chief of the lab of neurogenetics at the National Institute on Alcohol Abuse and Alcoholism, commented in Clinical Pharmacology and Therapeutics in 2009, “Even if one gene is important, as now appears to be the case for this metabolic gene CYP2E1, there may be multiple functional variants in the gene. We probably don’t even know the identities of the players.”
And as Hart and Kranzler, of the department of psychiatry, Perelman School of Medicine, University of Pennsylvania, pointed out in Alcoholism: Clinical and Experimental Research in June 2015, many studies have tried to identify specific genetic variants associated with susceptibility to alcohol dependence. These studies, primarily linkage or candidate gene based, have been “mostly unsuccessful” in identifying replicable risk loci, they said. “While findings from AD GWAS and post-GWAS analyses have greatly increased understanding of the genetic etiology of AD, larger samples will be necessary to detect loci in addition to those that encode alcohol-metabolizing enzymes, which may only be possible through consortium-based efforts.”
Post-GWAS approaches, such as increasingly common genome-wide meta-analysis to studying the genetic influences on AD, could greatly increase our knowledge of both the genetic architecture of AD and the specific genes and pathways that influence risk, they explained.
Some GWAS studies with relatively large samples confirm genotypic features of alcohol dependence, mostly verifying well-established gene variations. Gelemter and colleagues at Yale University reported in Molecular Psychiatry in 2013 a GWAS study of alcohol-dependent individuals in European-American (EA) and African-Americans (AA). The sample for discovery and replication consisted of 16,087 individuals, the largest sample for alcohol dependence GWAS to date.
The investigators said that numerous genome-wide significant (GWS) associations were discovered, many of which were previously unrecognized. Most associations were population specific but, in several cases, were GWS in EAs and AAs for different SNPs at the same locus, showing biological convergence across populations.
Li et al. performed a meta-analysis of addiction candidate gene association studies and GWAS to investigate possible functional mechanisms associated with addiction susceptibility. They published their paper in Genomics in 2011.
From meta-data retrieved from 212 publications on candidate gene association studies and five GWAS reports, the investigators could link a total of 843 haplotypes to addiction susceptibility.
The researchers mapped the SNPs in these haplotypes to functional and regulatory elements in the genome and estimated the magnitude of the contributions of different molecular mechanisms to their effects on addiction susceptibility. In addition to SNPs in coding regions, the data suggested that haplotypes in gene regulatory regions may also contribute to addiction susceptibility.
In comparing the lists of genes identified by association studies and those identified by molecular biological studies of drug-regulated genes, the authors observed significantly higher participation in the same gene interaction networks than expected by chance, despite little overlap between the two gene lists.
Effects of Repeated Drug Exposures
In addition to functional gene variants, recent data suggest that repeated exposure to drugs of abuse induces changes within the brain’s reward regions in three major modes of epigenetic regulation: histone modifications such as acetylation and methylation, DNA methylation, and non-coding RNAs.
Eric Nestler, M.D., Ph.D., professor and chair of neuroscience at Mt. Sinai Hospital in New York and colleagues have studied aberrant patterns of transcriptional and epigenetic regulation in the brain in response to repeated drug exposure. One object of these studies is to discover potential drug targets and interventions for addictive disorders.
Writing in Nature Neuroscience last year, Dr. Nestler and colleagues noted that induction of histone acetylation in the nucleus accumbens (NAc), a key brain reward region, promotes cocaine-induced alterations in gene expression. Acetylated histones, proteins that organize chromatin into nucleosomes and ultimately higher order structures, represent a type of epigenetic marker within chromatin. Histone deacetylases (HDAC) are enzymes that regulate the acetylation of part of histone molecules, histone tails. Histone tail modifications, such as acetylation and methylation, have emerged as critical components of chromatin dynamics, and play an integral role in chromatin-based processes like gene regulation.
However, Dr. Nester and colleagues say that the functional specificity of different HDAC isoforms in the development and maintenance of cocaine-induced plasticity remains poorly understood, and previous studies of HDAC inhibitors report conflicting effects on cocaine-elicited behavioral adaptations.
The authors demonstrated that specific and prolonged blockade of the HDAC1 isoform in NAc of mice increased global levels of histone acetylation, but also induced repressive histone methylation and antagonized cocaine-induced changes in behavior. This antagonistic effect of HDAC1 blockage was mediated in part the authors said, through a chromatin-mediated suppression of GABAA receptor subunit expression and inhibitory tone on NC neurons.
These findings suggest a new mechanism by which prolonged and selective HDAC inhibition can alter behavioral and molecular adaptations to cocaine and inform the development of therapeutics for cocaine dependence.
Most scientists agree that identifying all the contributing factors to addictions will be not be achieved without integrating genetic information with epigenetic, metabolic, and pharmacogenetics information enabling an understanding of interacting effects of multiple biological mechanisms involved in tolerance, craving, anxiety, dysphoria, executive cognitive function, and reward.
This article was originally published in the December 2015 issue of Clinical OMICs. For more content like this and details on how to get a free subscription to this digital publication, go to www.clinicalomics.com.