Debrisoquine oxidation phenotype and susceptibility to lung cancer

Involvement of cytochrome P450, glutathione S-transferase, and epoxide hydrolase in the metabolism of aflatoxin B1 and relevance to risk of human liver cancer

In recent years there has been considerable interest in the effect of variations in activities of xenobiotic-metabolizing enzymes on cancer incidence. This interest has accelerated with the development of methods for analyzing genetic polymorphisms. However, progress in epidemiology has been slow and the contributions of polymorphisms to risks from individual chemicals and mixtures are often controversial. A series of studies is presented to show the complexities encountered with a single chemical, aflatoxin B1 (AFB1). AFB1 is oxidized by human cytochrome P450 enzymes to several products. Only one of these, the 8,9-exo-epoxide, appears to be mutagenic and the others are detoxication products. P450 3A4, which can both activate and detoxicate AFB1, is found in the liver and the small intestine. In the small intestine, the first contact after oral exposure, epoxidation would not lead to liver cancer. The (nonenzymatic) half-life of the epoxide has been determined to be approximately 1 sec at 23 degrees C and neutral pH. Although the half-life is short, AFB1-8,9-exo-epoxide does react with DNA and glutathione S-transferase. Levels of these conjugates have been measured and combined with the rate of hydrolysis in a kinetic model to predict constants for binding of the epoxide with DNA and glutathione S-transferase. A role for epoxide hydrolase in alteration of AFB1 hepatocarcinogenesis has been proposed, although experimental evidence is lacking. Some inhibition of microsome-generated genotoxicity was observed with rat epoxide hydrolase; further information on the extent of contribution of this enzyme to AFB1 metabolism is not yet available.

DNA haplotype dependency of debrisoquine 4-hydroxylase (CYP2D6) expression among extensive metabolisers

Deficient debrisoquine/sparteine type oxidation is inherited as an autosomal recessive trait. Of all Caucasians, 5-10% are poor metabolisers, due to the absence of cytochrome P4502D6. Extensive metabolisers (EMs) exhibit highly variable metabolic activity. We investigated the relationship between CYP2D6 activity and genotypes of the CYP2D locus in a large set of French Caucasian families. Genotypes concern both common mutations affecting the enzyme activity and linked BamHI polymorphisms of the locus. We found, like other authors, that in EMs part of the heterogeneity is explained by a subgroup of individuals heterozygous for a mutant allele. However, a second level of heterogeneity was detected among individuals not carrying mutations, and this was related to a polymorphic BamHI-defined DNA haplotype. Different combinations of haplotypes are associated with differences in CYP2D6 metabolic activity. This finding might help to clarify the conflicting data on the relation between CYP2D6 activity and susceptibility to lung cancer.

Debrisoquine/sparteine hydroxylation genotype and phenotype: analysis of common mutations and alleles of CYP2D6 in a European population

Four different mutations of the cytochrome P450 CYP2D6 gene associated with the poor metabolizer phenotype (PM) of the debrisoquine/sparteine polymorphism were analyzed by Xba I restriction fragment length polymorphism (RFLP) analysis and a polymerase chain reaction (PCR)-based DNA amplification method in DNA of 394 healthy European subjects; 341 of these were phenotyped by sparteine or debrisoquine administration and urinary metabolic ratios (MR). Our study demonstrates the efficiency of the PCR-test for phenotype prediction; 96.4% of individuals were correctly predicted, i.e., 100% of the extensive metabolizers (EMs) and 86.0% of the poor metabolizers (PMs). In contrast, Xba I RFLP analysis was far less informative, predicting the phenotype in only 26.8% of PMs. By combining both DNA tests, the prediction rate of the PM phenotype increased to 90.6%. A point mutation at a splice-site consensus sequence termed D6-B represented the most common mutant CYP2D6 gene and accounted for more than 75% of mutant alleles. In addition, other known mutations such as D6-D (14%), D6-A (5%), and the rare D6-C mutation bring the identified mutant alleles to greater than 95% of all mutant PM-alleles. Most of Xba I 44-kb alleles were confirmed as mutant alleles carrying the D6-B mutation. However, 9.7% did not have this mutation and may express a functional CYP2D6 gene. Moreover, all Xba I 16 + 9-kb alleles contained the D6-B mutation. Heterozygous EM individuals had a significantly higher MR when compared to homozygous EMs. Genotyping provides an important advantage for investigations of the influence of CYP2D6 activity on drug therapy and its association with certain diseases.

Is there a link between debrisoquine oxidation phenotype and lung cancer susceptibility?

A link between debrisoquine oxidation phenotype and lung cancer susceptibility has been evoked by some authors but not confirmed by others. For this reason, we compared the frequency of debrisoquine poor metabolizer phenotype in 91 Belgian patients with lung cancer to that studied in 167 healthy Belgian subjects. No significant difference was observed in our study. These results do not support the existence of this link.