Most studies on rare variants in the field of addiction have been conducted in nicotine dependent groups. Genome wide association studies (GWAS) have identified common variants within the cholinergic nicotinic receptor genes to be associated with nicotine addiction [7, 10, 32]. Rare variants within the same gene cluster (CHRNA5, CHRNB2, CHRNA3 and CHRNA4) are also associated with nicotine addiction with OR as low as 0.29 observed [17, 36, 37]. As MOR has an important role in mediating the rewarding effects of drugs of abuse, we were interested to see whether rare variants within OPRM1 would be associated with addiction to opioids or cocaine. In order to address this we combined cocaine addicted individuals and heroin addicted individuals together to create a ‘drug-addicted’ cohort.
