Gene Study Helps Unravel Biology of Alcoholism

A new genomewide association study (GWAS) has found several mutations linked to increased susceptibility for developing alcohol dependence, bringing scientists a step closer to understanding the complex biological mechanisms of alcohol use disorders.

Two single nucleotide polymorphisms (SNPs), both located on the chromosomal region 2p35, had the highest degree of association with alcohol dependence in a relatively homogenous patient population. These SNPs are located near the PECR gene, which encodes an enzyme (peroxisomal trans-2-enoyl-coA reductase) involved in fatty-acid metabolism, particularly when the body's energy supply is switched from glucose to fat.

A third SNP with a slightly lower association with alcohol dependence is located within the PECR gene. These three variants are “in strong linkage disequilibrium, or LD, meaning that the same variants at different loci almost always appear together,” Marcella Rietschel, M.D., the senior author of the study, told Psychiatric News. “So it is very likely—but still not certain—that the PECR gene is involved [in alcohol dependence].”

Rietschel is a professor of genetic epidemiology in psychiatry at the Central Institute of Mental Health Mannheim at the University of Heidelberg, Germany.

This chromosomal region has been implicated in alcohol dependence in previous research. The PECR gene is expressed most heavily in the liver, but very little in the brain. “As alcohol does not act only on the brain, alcohol dependence can be modulated by many factors whose primary target is not the brain,” Rietschel said. “The best known genetic variants modulating alcohol addiction are variants in the genes metabolizing alcohol, like variants in the alcohol dehydrogenase gene clusters.”

Indeed, this study confirmed several other SNPs associated with alcohol dependence, including those located in the ADH1C gene coding for one of the alcohol-metabolizing alcohol dehydrogenases, as well as the CDH13 gene coding for a cell-adhesion protein known as T-cadherin. Both genes have been implicated in alcohol dependence in previous studies.

Despite the strong evidence, these variants and their impact on alcohol dependence need to be replicated by additional studies. “The uncertainty is a problem encountered in many GWA studies: A SNP is found to be associated but is not functional itself, so one cannot be sure if this SNP is itself involved in the regulation of genes or if it is only in LD with other causal SNPs, which can be quite far away or even in other genes,” said Rietschel. Since the three top SNPs discovered in this study are close to and in the PECR gene, “this gene definitely merits further investigation.”

The researchers first performed a genomic scan for more than 500,000 SNPs, using a sample of 487 alcohol-dependent patients and 1,358 controls. The GWAS identified 121 SNPs that were likely candidates for genetic association.

The patients selected for the GWAS were German men with a DSM-IV diagnosis of alcohol dependence, whose condition was severe enough to require hospitalization for treatment or prevention of alcohol withdrawal. In addition, the subjects all had an onset of alcohol dependence before age 28; early-onset alcohol dependence has been shown to have a stronger hereditary component. Because alcohol dependence is a multifactorial disorder with multiple phenotypes and genotypes, the researchers narrowed their sampling to clinically similar patients to reduce the heterogeneity.

Because of the vast number of mutations existing in every person and the large number scanned in a GWAS, scientists face the challenge of weeding out too many potential false-positive “hits,” or variants that appear to be significantly associated with a disease when they are, in fact, random coincidences. To minimize false hits and maximize true disease-associated mutations, strategies such as stringent statistical criteria and replication studies are often used in genetic association studies.

Here, the researchers used a method called convergent functional genomics. This approach combines gene-expression data from animal models, evidence from human genetic association studies, and findings from human tissues such as brain tissue in autopsies to help prioritize investigation on the most promising candidate genes or the most likely biological pathways. In this study, 19 SNPs were identified after the human GWAS findings were compared with homologous, over-expressed genes in rats that were“ alcoholic” strains.

Armed with 121 candidate SNPs from the GWAS and 19 SNPs derived from convergent functional genomics analysis, the researchers performed a replication study of 1,024 male patients with alcohol dependence and 996 age-matched controls. The replication study confirmed that 15 SNPs have a significant association with alcohol dependence.

This study was funded by grants from the German government and the European Commission.

An abstract of “Genome-Wide Association Study of Alcohol Dependence” is posted at<archpsyc.ama-assn.org/cgi/content/abstract/66/7/773>.▪