The use of long PCR to confirm three common alleles at the CYP2A6 locus and the relationship between genotype and smoking habit

Long PCR follow ed by nested PCR has previously been used to determine CYP2 A6 160H alleles; but the method proved unreliable. We have optimized this a pproach in a DNA bank of 1032 subjects rage range 59-74 years) to give reli able results, yielding indirect molecular evidence and very strong statisti cal evidence of hitherto unrecognized common alleles (designated O) recalci trant to the long PCR. Coding three alleles (160L, 160H and O) and an appro ach to association analysis originally developed to deal with null alleles implicit in ABO blood group phenotyping, the contribution of 160H (function ally null) to reduced smoking habit has been clearly measured for the first time, unconfounded by alleles null to the long PCR. The most significant f indings (p < 0.01) are that the possession of a 160H allele, compared with not possessing a 160H allele, is associated with a mean age of starting reg ular smoking 3 Sears later (95% CI+/-1.93 years, average start age 20-21 ye ars rather than 17-18 years); and that the average likelihood of quitting s moking at any time is 1.75 fold (95% CI. 1.17-2.61) for those possessing an 160H allele compared with those who hare no 160H allele. This suggests tha t a smoking subject with a genotype predicted to confer 50% of the ability to eliminate nicotine via the CYP2A6 pathways has almost twice the likeliho od of quitting smoking.