A meta-analysis of studies in thousands of twin pairs has shown that the heritability of alcoholism is around 50%, and the heritability of cocaine and opiate addiction is around 60 – 70% (7). Therefore genetic and environmental influences on the development of addictive disorders are equally important. Preclinical studies indicate that GABRA2, the gene that encodes the GABAA α2 receptor subunit, may play a role in drug dependence (8–11). In human studies, the GABRA2 distal haplotype block has been robustly associated with alcoholism, at least in Caucasians (12–21). The first findings came from the Collaborative Study on the Genetics of Alcoholism (COGA) (12). A subsequent re-analysis of the dataset showed that the association signal derived from alcoholics with comorbid illicit drug dependence (13). However, another study found that the association was strongest in alcoholics without drug dependence (19). The two published studies in African Americans were negative (22,23). HapMap data shows that, unlike Caucasian and Asian populations who have only two common GABRA2 yin yang haplotypes, African populations have two unique haplotypes in addition to the yin yang haplotypes (24).
