Because early-onset smoking is a strong risk factor for heavy smoking in adulthood and age at onset of smoking is a heritable characteristic, we must consider the possibility that a shared genetic factor could lead to early-onset smoking and heavy smoking in adulthood. However, rs16969968 does not seem to be a shared genetic factor between early-onset smoking and heavy smoking. Specifically, no association was seen between dichotomized age at onset and rs16969968 in this sample of 67 128 smokers (P = .77). This suggests that a direct relationship between rs16969968 and age at onset is unlikely to explain the observed association between smoking quantity and rs16969968 in early-onset smokers.
