Genetic determinants of cytochrome P450 2A6 activity and biomarkers of tobacco smoke exposure in relation to risk of lung cancer development in the Shanghai cohort study

Cytochrome P450 2A6 (CYP2A6) catalyzes nicotine metabolism and contributes to the metabolism of the tobacco-specific lung carcinogen, NNK. Genetic variation in CYP2A6 may affect smoking behavior and contribute to lung cancer risk. A nested case-control study of 325 lung cancer cases and 356 controls was conducted within a prospective cohort of 18,244 Chinese men in Shanghai, China. Quantified were 4 allelic variants of CYP2A6 [*1(+51A), *4, *7, and *9] and urinary total nicotine, total cotinine, total trans-3'-hydroxycotinine (3HC) and total NNAL (an NNK metabolite). Calculated were total nicotine equivalents (TNE), the sum of total nicotine, total cotinine and total 3HC and the total 3HC:total cotinine ratio as a measure of CYP2A6 activity. The nicotine metabolizer status (normal, intermediate, slow and poor) was determined by CYP2A6 genotypes. The smoking-adjusted odds ratios (95% confidence intervals) of lung cancer for the highest vs lowest quartile of total nicotine, total cotinine, total 3HC, TNE and total NNAL were 3.03 (1.80-5.10), 4.70 (2.61-8.46), 4.26 (2.37-7.68), 4.71 (2.61-8.52), and 3.15 (1.86-5.33) (all P-trend<0.001), respectively. Among controls CYP2A6 poor metabolizers had a 78% lower total 3HC:total cotinine ratio and 72% higher total nicotine (P(trend)0.002). Poor metabolizers had an odds ratio of 0.64 (95% confidence interval=0.43-0.97) for lung cancer, which was statistically nonsignificant (odds ratio=0.74, 95% confidence interval=0.48-1.15) after adjustment for urinary TNE and smoking intensity and duration. The lower lung cancer risk observed in CYP2A6 poor metabolizers is partially explained by the strong influence of CYP2A6 genetic polymorphisms on nicotine uptake and metabolism. What's new? Why do some smokers develop lung cancer, while others do not? The answer may lie in genetic variation that affects nicotine metabolism. These authors asked whether genetic variants of a nicotine-metabolizing enzyme, Cytochrome P450 2A6, impacted risk of lung cancer. More than 80 SNPs have been observed in the gene, CYP2A6, and some variants cause decreased enzyme activity and changes in nicotine metabolism. People with these variants, it turns out, do tend to smoke less and take in less nicotine than other smokers, and their lung cancer risk may be reduced even after correcting for nicotine intake.