The debrisoquine metabolic phenotype and DNA-based assays: implications of misclassification for the association of lung cancer and the debrisoquine metabolic phenotype.

Abstract

Debrisoquine is an antihypertensive drug that is metabolized by cytochrome P4502D6. 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 P4502D6 (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.

DNA haplotype-dependent differences in the amino acid sequence of debrisoquine 4-hydroxylase (CYP2D6): evidence for two major allozymes in extensive metabolisers

“…Several reports claim somewhat controversially that there is an association between CYP2D6 polymorphism and certain diseases, such as lung cancer and Parkinson's disease (Ayesh et al 1984;Caporaso et al 1992;Smith et al 1992a;Wolf et al 1992;Armstrong et al 1992;Smith et al 1992b). We have shown (Mura et al 1993) that the EM phenotype is genetically heterogeneous, with variable phenotypic expression and the present study demonstrates that the distinct metabolic profiles correspond to distinct allozyme patterns.…”

Phenoconversion of Cytochrome P450 Metabolism: A Systematic Review

“…Three papers evaluating the effect of cancer on the occurrence of phenoconversion were identified. In lung cancer patients, CYP2D6 genotype–phenotype mismatches had already been reported more than 25 years ago [ 54 ]. However, these mismatches were not recognized as phenoconversion at that time.…”

Use of antiarrhythmic drugs in elderly patients

“…Procainamide is acetylated in the liver to form NAPA and is subsequently excreted by the kidneys through active tubular secretion, which can be inhibited by cimetidine and trimethoprim. Quinidine is metabolized by CYP3A4, but it is also a potent inhibitor of CYP2D6 and has complex interactions with propafenone metabolism [95]. Drugs that inhibit the CYP system, such as cimetidine, ketoconazole, and macrolide antibiotics, also inhibit clearance of Class I antiarrhythmic drugs.…”

“…Quinidine is metabolized by CYP3A4, but it is also a potent inhibitor of CYP2D6 and has complex interactions with propafenone metabolism. [95] Drugs that inhibit the CYP system, such as cimetidine, ketoconazole, and macrolide antibiotics, also inhibit clearance of Class I antiarrhythmic drugs. Grapefruit juice inhibits intestinal wall CYP3A4 and CYP1A2 and potentiates the effects of drugs that are cleared by these enzymes.…”