Identification and characterisation of novel polymorphisms in the CYP2A locus: implications for nicotine metabolism

The polymorphic human cytochrome p450 2A6 (CYP2A6) metabolises a number of drugs, activates a variety of precarcinogens and constitutes the major nico tine C-oxidase, A relationship between CYP2A6 genotype and smoking habits, as well as incidence of lung cancer, has been proposed. Two defective allel es have hitherto been identified, one of which is very common in Asian popu lations. Among Caucasians, an additional defective and frequently distribut ed allele (CYP2A6*3) has been suggested to play a protective role against n icotine addiction and cigarette consumption. Here, we have re-evaluated the genotyping method used for the CYP2A6*3 allele and found that a gene conve rsion in the 3' flanking region of 30-40% of CYP2A6*1 alleles results in ge notype misclassification. In fact, no true CYP2A6*3 alleles were found amon g 100 Spaniards and 96 Chinese subjects. Tn one Spanish poor metaboliser of the CYP2A6 probe drug coumarin, we found two novel defective alleles, One, CYP2A6*5, encoded an unstable enzyme having a G479L substitution and the o ther was found to carry a novel type of CYP2A6 gene deletion (CYP2A6*4D). T he results imply the presence of numerous defective as well as active CYP2A 6 alleles as a consequence of CYP2A6/CYP2A7 gene conversion events. We conc lude that molecular epidemiological studies concerning CYP2A6 require valid ated genotyping methods for accurate detection of all known defective CYP2A 6 alleles, (C) 1999 Federation of European Biochemical Societies.