COGA’s brain function data (see, 3. Brain Function) have also been paired with the project’s functional genomics pipeline (see, 5. Functional Genomics) to provide mechanistic insights. In an example of this, several variants within KCNJ6 (encoding the GIRK2 G‐protein coupled inwardly rectifying potassium channel) were identified as genome‐wide significant in our family‐based GWAS of a frontal theta EEG phenotype75 (an endophenotype for AUD14). While the recent use of GWAS to identify the underlying genetic components of AUD has been promising, there are several limitations of GWAS that must be considered.
FROM GENE DISCOVERY TO POLYGENICITY: POLYGENIC AND WITHIN‐FAMILY APPROACHES TO ILLUMINATE MECHANISMS OF GENETIC RISK
This may reflect boththe limited sample sizes and the clinical and genetic heterogeneity of thedisease. As noted above, the functional ADH1B polymorphism isnot represented on GWAS platforms; GABA-receptor genes are often nominallysignificant but well below genome-wide significance in these studies. Thus, thegenes and SNPs found through GWAS have had little overlap with previous findingsbased on candidate genes/pathways and linkage analyses. Like many other complex traits, alcoholism appears to be clinically and etiologicaly hetrogenous[13]. This implies that there might be several steps and intermediate conditions in the development of AUD.
- The initial approach was to use linkage studies in extended and densely affected pedigrees,11, 15, 16 where the key question was whether large sections of chromosomes (measured in centiMorgans, approximately 1 Mb) co‐segregated with AUD within families at a level greater than expected by chance (i.e., observed vs. expected identity‐by‐descent).
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- The causes of AUD are complex and can involve a variety of factors, including early exposure to alcohol use, peer group pressure, and living with other mental health conditions.
- The Diagnostic and Statistical Manual of Mental Disorders (DSM-5; [1)]) defines alcohol use disorder (AUD) as a single spectrum of problematic use and clinically significant impairment based on endorsement of 2 or more of 11 criteria assessing behavioral and physical manifestations of heavy alcohol use1.
- Information about the underlying genetic factors that influence risk to AUD can be derived from multiple levels of AUD including amounts of drinks (Alcohol consumption), severity and symptoms of alcohol abuse and dependence.
- The genes with the clearest contribution to the risk for alcoholism andalcohol consumption are alcohol dehydrogenase 1B (ADH1B) andaldehyde dehydrogenase 2 (ALDH2; mitochondrial aldehydedehydrogenase), two genes central to the metabolism of alcohol (Figure 1)20.
Immune Function Genes, Genetics, and the Neurobiology of Addiction
Exposure to parental relationship discord and parental divorce mediated, in part, the transmission of genetic risk for alcohol problems from parents to children to predict earlier ages regular drinking and intoxication, greater lifetime maximum drinks and more lifetime AUD criteria. Of note, these effects were observed in the European but not African ancestry families, underscoring the need for further empirical attention to nature of nurture processes in samples of non‐European ancestry. It is likely that, as for most complex diseases, alcohol dependence and AUDsare due to variations in hundreds of genes, interacting with different socialenvironments. An additional challenge in the search for genetic variants that affectthe risk for AUDs is that there is extensive clinical heterogeneity among thosemeeting criteria.
Within- and cross-ancestry fine mapping
Some of these genes have been identified, including twogenes of alcohol metabolism, ADH1B and ALDH2,that have the strongest known affects on risk for alcoholism. Studies arerevealing other genes in which variants impact risk for alcoholism or relatedtraits, including GABRA2, CHRM2,KCNJ6, and AUTS2. As larger samples areassembled and more variants analyzed, a much fuller picture of the many genesand pathways that impact risk will be discovered. This finding celebrities with fetal alcohol syndrome suggests that variants of a gene or genes within this region reduced the risk of becoming alcoholic. ADH alleles are known to affect the risk for alcoholism; however, the known protective alleles occur at high frequency in Asian populations but are rare in the Caucasian population that makes up most of the COGA sample (Edenberg 2000). Therefore, these analyses may have identified a new protective ADH allele or another protective gene located nearby.
Family, twin, and adoption studies have shown that alcoholism definitely has a genetic component. In 1990, Blum et al. proposed an association between the A1 allele of the DRD2 gene and alcoholism. The DRD2 gene was the first candidate gene that showed promise of an association with alcoholism. There is a growing body of scientific evidence that shows alcoholism has a genetic component. According to the American Academy of Child & Adolescent Psychiatry, children of alcoholics are four times more likely than other children to become alcoholics. With many people affected by alcohol abuse and alcoholism, it is imperative to develop and promote effective recovery treatment programs.
PRS for phenome-wide associations
Drug use and addiction represent a public health crisis, characterized by high social, emotional, and financial costs to families, communities, and society. We performed gene-based association analysis for PAU or AUD in multiple ancestries using MAGMA implemented in FUMA78. Bonferroni corrections for the number of genes tested (range from 18,390 to 19,002 in different ancestries) were used to determine GWS genes.
- Importantly, the prevalence of the various isoforms of ADH and ALDH differs among ethnicities and populations.
- Hence, Vrieze et al. (2013) found that substance use phenotypes, including those pertaining to alcohol use, and behavioral disinhibition share a genetic etiology, and that measured genetic variants contribute to their heritability.
- “Substance use disorders and mental disorders often co-occur, and we know that the most effective treatments help people address both issues at the same time.
- Both probands and family members were characterized with age‐appropriate assessments, including a standardized diagnostic instrument designed by COGA, the Semi‐Structured Assessment for the Genetics of Alcoholism (SSAGA),10, 11 administered by trained interviewers.
- Therefore, additional markers within these regions of interest were analyzed in the same people.
Recent findings:
These findings will further our understanding of the genetic etiology of AUD, and will also promote the advancement of “Post-GWAS” approaches seeking to better understand the mechanisms through which genetic variation leads to increased AUD risk. It is hoped that such information will ultimately lead to improved prevention and treatment efforts. In the context of AUD, GCTA could be applied to the subsets of previously discussed SNPs that reached genome-wide significance and were correlated with alcohol-dependent phenotypes.
