A variant that appears most likely to be a biological contributor to nicotine dependence is rs16969968 because this polymorphism changes an amino acid from aspartate to asparagine at position 398 (D398N) in the α5 nicotinic receptor protein. This amino acid is highly conserved across species, and an in vitro model that inserts this single amino acid change into an α5 nicotinic receptor subunit alters nicotinic receptor function [15]. This work supports the hypothesis that the genetic variant rs16969968 causes biological changes that alter the risk of developing nicotine dependence and heavy smoking. Though this amino acid change is the most compelling candidate for the underlying biologic risk responsible for this genetic association, we cannot rule out the role of other correlated genetic variants. A second biologic mechanism that is associated with heavy smoking and nicotine dependence includes different levels of expression for CHRNA5 messenger RNA (mRNA) in the brain [30]. Similar changes in mRNA expression are seen in lung tissue [31-32].
