Lack of a relationship between the polymorphism of debrisoquine oxidation and lung cancer

Bioinformatics Research on Inter-racial Difference in Drug Metabolism I. Analysis on Frequencies of Mutant Alleles and Poor Metabolizers on CYP2D6 and CYP2C19

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. In the present study, the frequencies of mutant alleles and PMs in each race were analyzed based on information from published studies, considering the genetic polymorphisms of CYP2D6 and CYP2C19 as the causal factors of racial and inter-individual differences in pharmacokinetics. As a result, it was shown that there were racial differences in the frequencies of each mutant allele and PMs. The frequencies of PMs on CYP2D6 are 1.9% of Asians and 7.7% of Caucasians, and those of PMs on CYP2C19 are 15.8% of Asians and 2.2% of Caucasians. Based on the results, it was suggested that there would be racial differences in the frequencies of PM subjects whose blood concentrations might be higher for drugs metabolized by these enzymes. Additionally, it was suggested that enzyme activities would vary according to the number of functional alleles even in subjects judged to be extensive metabolizers (EMs). In the bridging study, genetic information regarding CYP2D6 and CYP2C19 of the subjects will help extrapolate foreign clinical data to a domestic population.

The molecular epidemiology of lung cancer

One in ten tobacco smokers develops bronchogenic carcinoma over a lifetime. The study of susceptibility of an individual and a population to lung cancer traditionally has been limited to the study of tobacco smoke dose and family history of cancer. New insights into lung carcinogenesis have made the study of molecular markers of risk possible in human populations in the emerging field of molecular epidemiology. This review summarizes data addressing the relationships of human lung cancer to polymorphisms of phase I procarcinogen-activating and phase II-deactivating enzymes and intermediate biomarkers of DNA mutation, such as DNA adducts, oncogene and tumor suppressor gene mutation, and polymorphisms. These parameters are reviewed as they relate to tobacco smoke exposure, procarcinogen metabolizing polymorphisms, and the presence of lung cancer. Problem areas in biomarker validation, such as cross-sectional data interpretation; tissue source, race, statistical power, and ethical implications are addressed.

Interaction between dose and susceptibility to environmental cancer: a short review

Increased risk of environmentally induced cancer is associated with various types of exposures and host factors, including differences in carcinogen metabolism. Since many carcinogenic compounds require metabolic activation to enable them to react with cellular macromolecules, individual features of carcinogen metabolism may play an essential role in the development of environmental cancer. In this context, cigarette smoking has often been the main type of carcinogenic exposure examined in human studies. Increasing attention has recently been paid to the dose level at which individual susceptibility may be observed. Present studies on increased risk of smoking-related lung cancer associated with phenotypic or genotypic variation of the genes encoding for CYP1A1 or CYP2D6 enzymes are summarized. Similarly, higher risks of lung or bladder cancer seen at various levels of smoking in association with polymorphism of the glutathione S-transferase gene GSTM1 or NAT1 and NAT2 genes involved in N-acetylation are reviewed. Finally, the influence of CYP2E1, GSTM1, or the combined at-risk genotype on the risk of hepatocellular carcinoma in smokers is briefly discussed.

Interactions between genetic predisposition and environmental toxicants for development of lung cancer

Significant interindividual variations in health outcome may be caused by the inheritance of variant polymorphic genes, such as CYP2D6 and CYP2E1 for activation, and GSTM1 and GSTT1 for detoxification of chemicals. However, mechanistic studies linking the inheritance of predisposing genes with genotoxic effects towards cancer have yet to be systematically conducted. We have studied 54 lung cancer patients and 50 matched normal controls, who have been cigarette smokers, to elucidate the role of polymorphic genes in cancer. Our data indicates that the inheritance of unfavorable CYP2D6, CYP2E1, and GSTT1 genes in strongly correlated with the smoking-related lung cancer. For heavy cigarette smokers (> 30 pack-years), the smoking habit is the strongest predictor of lung cancer risk irrespective of the inheritance of unfavorable metabolizing genes. For moderate to light smokers (< 30 pack-years), the genetic predisposition plays an important role for the risk (odds ratio = 3.46; 95% Cl = 0.46-40.2). Using a subgroup of the study population, we observed that cigarette smokers having the defective GST genes have significantly more chromosome aberrations as determined by the fluorescence-in-situ-hybridization (FISH) technique than smokers with the normal GST genes (P < 0.001). In conclusion, our study provides data to indicate that individuals who have inherited unfavorable metabolizing genes have increased body burden of toxicants to cause increased genetic damage and to have increased risk for cancer. Studies like ours can be used to understand the basis for interindividual variations in cancer outcome, to identify high risk individuals and to assess health risk.

The sparteine/debrisoquine (CYP2D6) oxidation polymorphism and the risk of lung cancer: a meta-analysis

OBJECTIVE

To examine the association between the sparteine/debrisoquine (CYP2D6) oxidation polymorphism and the risk of lung cancer.

METHOD

Meta-analysis of case-control studies using a random effects model. The "Main outcome measure" was the odds ratio for the risk of lung cancer, using extensive metabolisers as the reference group.

RESULTS

Thirteen studies were identified. The studies were too heterogeneous to be pooled the size of the odds ratio increased with the sample size. When the analysis was restricted to the largest studies, there was no difference in risk between poor and extensive metabolisers (odds ratio 0.95, 95% confidence interval 0.68-1.33).

CONCLUSION

No association was found between the CYP2D6 oxidation polymorphism and lung cancer risk when sample size bias was taken into account.

"It's the genes, stupid". Molecular bases and clinical consequences of genetic cytochrome P450 2D6 polymorphism

In this review we highlight the information available on the genetic polymorphism of cytochrome P4502D6 expression in man. An absent function of this enzyme is observed in 7-10 percent of the Caucasian population which are referred to as Poor metabolizers as opposed to the remainder of the population (Extensive Metabolizers). More than 30 widely used drugs have been identified as substrates for CYP2D6. Disposition and action of these compounds depend on the individual phenotype. Both the molecular bases of the variable enzyme activity and the consequences for drug therapy are outlined. While mutations on the DNA level have been investigated in great detail larger scale clinical trials are lacking and information on therapeutic consequences of CYP2D6 mediated polymorphic drug oxidation is restricted to case reports. Besides these implications for drug metabolism several lines of evidence indicate that CYP2D6 could be involved in biotransformation of endogenous compounds.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

The CYP1A1 gene and cancer susceptibility

A close correlation between cigarette smoking associated lung cancer incidence and an Msp I restriction fragment length polymorphism (RFLP) of the human P-450 1A1 (CYP1A1) gene was found in a Japanese population in terms of genotype frequency and cigarette dose. A Val/Ile codon difference in the primary structure of the CYP1A1 protein (Val-, Ile-type) was in linkage disequilibrium with the Msp I RFLP. A synergistic increase in susceptibility to lung cancer was found when combining genotyping of CYP1A1 and the Mu-class of glutathione S-transferase (GST1). Interindividual variability in the genetic make-up of carcinogen metabolizing enzymes may thus be a key host factor to explain the differences in susceptibility to chemical carcinogenesis among individuals.

Cancer risk related to genetic polymorphisms in carcinogen metabolism and DNA repair

Chemical carcinogenesis involves metabolism in the body of the carcinogen to the ultimate carcinogen and its interaction with DNA. There is considerable interindividual variation in the metabolic ability to activate as well as detoxify the carcinogens and in the ability to repair the carcinogen-DNA adducts. In many cases such differences occur as genetic polymorphisms and form the basis for variation in susceptibility to carcinogens and thereby to cancer risk. The activation mechanism is particularly related to the cytochromes P-450 (CYPs), and four of these are known to activate carcinogens: CYP1A1, CYP1A2, CYP2E1, and CYP3A4. Increased cancer risk has been related to polymorphisms in the CYPs and other activating enzymes. The DNA repair mechanisms show considerable complexity, and deficient repair mechanisms in certain human disorders are clearly related to increased cancer risk. Yet, there is no unambiguous epidemiological evidence available for cancer risk among individuals in general. In vivo methods have to be refined and developed for use in epidemiological studies.

The Role of Pharmacogenetics in Cancer Chemotherapy and the Development of Malignancy

The role of pharmacogenetics in the area of cancer chemotherapy and the development of malignancy has not been well defined. Only four chemotherapeutic agents have been evaluated for toxicity or clinical response based on genetic differences in metabolism. These include 5-fluorouracil, 6-mercaptopurine, amonafide, and cyclophosphamide. Severe toxicity of 5-fluorouracil and amonafide due to individual differences in drug metabolism has been reported in the literature. Tumor response in leukemic children may be associated with genetic differences in metabolism of 6-mercaptorpurine. The development of malignancy may be secondary to an individual's ability to detoxify carcinogens found in the environment. For example, the incidence of bladder cancer appears higher in subjects who have occupational exposure to aromatic amines and the slow acetylator phenotype. Some evidence also exists that smokers who are very extensive metabolizers of debrisoquin may be more prone to developing lung cancer. Strong evidence for an association between other cancer types and pharmacogenetics requires more study.

Mutant genes of cytochrome P-450IID6, glutathione S-transferase class Mu, and arylamine N-acetyltransferase in lung cancer patients

Epidemiological studies suggested a protective effect of certain phenotypes of polymorphic foreign-compound-metabolizing enzymes in some types of cancer. Poor metabolizers (PM) of debrisoquine 4-hydroxylase (cytochrome P-450IID6, CYP2D6) were found to be underrepresented among patients with lung cancer. Recent advances in molecular genetic characterization of CYP2D6, glutathione S-transferase (GST) class Mu, and arylamine N-acetyltransferase enabled genotypical determination of mutant alleles in lung cancer patients. Restriction fragment length polymorphism (RFLP) with a cDNA gene probe of CYP2D6 was analyzed in 79 lung cancer patients who were phenotyped with debrisoquine. Mutant alleles were detected by allele-specific polymerase chain reaction (PCR). In the same individuals, genotype of GST class Mu was analyzed by PCR and correlated with ex vivo activity of glutathione conjugation towards trans-stilbene oxide. RFLP patterns allowed discrimination between the slow and fast genotype of N-acetyltransferase as well as the heterozygotes. Three phenotypical PMs of debrisoquine (3.8%) were confirmed by PCR and RFLP. No PM could be unambiguously recognized only by RFLP patterns. The PMs were characterized by PCR and RFLP as carriers of the 29B/29B (n = 1), 29A/29B (n = 1), and 29A/44 (n = 1) mutant alleles. Higher debrisoquine hydroxylase activities were found in the homozygous EMs, who possess two active genes, as compared to heterozygous EMs, who have only one active gene. The patients with phenotypically impaired GST Mu activity were confirmed as such by PCR. A complete correspondence between phenotyping of N-acetyltransferase (with caffeine) and genotyping was found. The new genetic techniques proved to be powerful tools for molecular-epidemiological studies aimed at establishing host factors of cancer susceptibility.

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.