The Debrisoquine Metabolic Phenotype and DNA-Based Assays: Implications of Misclassification for the Association of Lung Cancer and the Debrisoquine Metabolic Phenotype

Debrisoquine is an antihypertensive drug that is metabolized by cytochrome P4502 D6. Deficient metabolism is inherited as an autosomal recessive condition. We previously reported in a case-control study that extensive metabolizers of debrisoquine were at greater risk of lung cancer compared to poor and intermediate metabolizers. Cloning of the gene that encodes P4502 D6(CYP2D6) led to the identification of both wild-type and mutant forms of the gene. Subsequently, a DNA-restriction fragment length polymorphism (RFLP) was identified, and a Southern hybridization-based test was developed in an attempt to define the genotype. When the DNA-RFLP test was applied to stored DNA from our study subjects there was neither a significant association with the metabolic phenotype nor an association with lung cancer. Further work has demonstrated that the wild-type gene, which was characterized by a 29-kb allele, can also contain mutations that result in nonfunctional or absent proteins. When these mutations are present, individuals exhibit the poor or intermediate metabolizer phenotype in spite of the presence of the 29-kb putative wild-type allele. Sequence determination of the mutants led to the development of techniques to exploit the polymerase chain reaction, which, together with Southern analysis, have been reported to detect as many as 95% of poor metabolizers. This technique is being used to examine the association of the extensive metabolizer genotype with lung cancer in the subjects from the case-control study. Preliminary results indicate a weak association between the homozygous wild-type genotype and lung cancer; in contrast, the extensive metabolizer phenotype is strongly associated with lung cancer in this subset. Employing this polymerase chain reaction method only, misclassification in the genotype assignment continues to occur, and work is in progress to identify further mutations that may account for subjects who are phenotypically poor metabolizers but possess "wild-type" alleles. The phenotyping approach is currently more sensitive, while the genotyping method may be more specific with regard to detection of the deficient metabolizer state in the context of population studies. Increasing use of genotyping is anticipated in future studies.