The findings reported here have limitations. Because the nonsynonymous rs16969968 variant in CHRNA5 is associated with changes in nicotinic receptor function,4,16 we hypothesized that other coding variants will have a similar effect, but functional studies will need to confirm this. Noncoding variants have been previously associated with changes in CHRNA5 mRNA expression levels in the brain35-37 as well as nicotine dependence and lung cancer. Including these non-coding variants in the analyses of our primary sample did not appreciably alter the associations with the coding variants. Because replication analyses were based on exome chip data, the contribution of these non-coding variants could not be further tested. In addition, the majority of rare variants are also not available on the exome chip, and therefore typing of these rare variants is required for replication. Another limitation is that our analysis is restricted to a single gene. Other nicotinic receptors contribute to nicotine dependence.30,38
