GABAA receptor subunit genes lie in clusters on chromosomes 4p (γ1, α2, α4, β1), 5q (γ2, α1, α6, β2), 15 (β3, α5, γ3), and X (ε, α3, θ), as well as individually on chromosomes 1p (δ), 3q (ρ3), 5q (π, outside of the cluster), and 6q (ρ1, ρ2) (Enoch, 2008). Linkage and association studies have implicated variation in several GABAA subunit genes in a variety of behavioral phenotypes related to alcohol, including dependence diagnosis (Cui et al., 2012) and symptomatology (Lind et al., 2008a), subjective intoxication and response (Lind et al., 2008b) and electroencephalographic measures (Edenberg et al., 2004) among others. Among GABAA receptor subunit genes, markers and haplotypes in the α2 subunit gene GABRA2 have been most frequently identified with variation in alcohol response and dependence (Cui et al., 2012) and phenotypes related to other psychoactive substances (Agrawal et al., 2006), as well as externalizing conduct (Dick et al., 2006). However, there have also been studies that were unable to confirm effects of GABRA2 polymorphisms on alcohol dependence (Drgon et al., 2006; Matthews et al., 2007; Onori et al., 2010).
