Lack of relation between genetic polymorphism of cytochrome P-450IID6 and sporadic idiopathic Parkinson's disease

We investigated genetic polymorphism of the cytochrome P-450 CYP2D6 gene from white patients with idiopathic Parkinson's disease (IPD). The mutations of the CYP2D6 gene associated with the poor metabolizer (PM) phenotype of the debrisoquine/sparteine polymorphism were analyzed in DNA of 130 IPD patients by a polymerase chain reaction (PCR)-based DNA amplification combined with Xba I restriction fragment length polymorphism (RFLP) analysis in 105 patients. Another mutation located in exon 6 was analyzed by Hha I RFLP in 94 IPD patients. The frequencies of the different CYP2D6 gene mutations were compared to the frequencies in sex- and age-matched white control population with chronic bronchitis. The rate of genotypically defined PM and the frequencies of the different mutations were not significantly different in IPD patients and controls. These results fail to confirm the previously reported results concerning CYP2D6 gene mutations in IPD. These equivocal results might be related to methodologic problems. However, other hypotheses have been suggested: impairment of neuronal CYP 2D6 expression, transient modification of CYP 2D6 phenotype, or linkage of CYP2D6 gene to the candidate gene locus directly involved in IPD.

An additional allelic variant of the CYP2D6 gene causing impaired metabolism of sparteine

The identification of a novel CYP2D6 allele from a healthy Caucasian poor metabolizer was achieved by using a previously described polymerase chain reaction/single-strand conformation polymorphism strategy. Among the four point mutations that this allele carries, a missense mutation in exon 1 (212 G-->A or D6-H) seems to be responsible for the loss of CYP2D6 function. Although the mutation D6-H has a low prevalence in a randomly selected population of healthy Caucasians, its identification should further increase the phenotype prediction rate by genotyping.

An efficient strategy for detection of known and new mutations of the CYP2D6 gene using single strand conformation polymorphism analysis

To detect mutations in the cytochrome P450 CYP2D6 gene (CYP2D6), we developed a strategy based on single-strand conformation polymorphism (SSCP) analysis of the gene amplified by polymerase chain reaction (PCR). The efficiency of the method was evaluated by analysing DNA samples from extensive metabolizers (EM) and poor metabolizers (PM) of debrisoquine. Haplotypes, alleles and mutations of CYP2D6 had previously been characterized in each individual using PCR assays, Xba I restriction fragment length polymorphism (RFLP) and sequencing. PCR-SSCP results were in complete agreement with those obtained using established methods. All previously characterized mutations were associated with particular shifts in the electrophoretic mobility of DNA fragments allowing their identification. We further tested the efficiency of PCR-SSCP for detecting new CYP2D6 mutations. DNA from a PM subject presumed to carry an unknown non-functional mutant allele of CYP2D6 was amplified and bands with aberrant migration patterns were observed on SSCP gels. Sequence analysis of the corresponding DNA fragments revealed the causative mutations. In this way, a novel non-functional allele of the gene, carrying three previously reported mutations and a new mutation in the third exon which results in a premature termination codon, was characterized. Finally, CYP2D6 SSCP analysis was performed on DNA amplified with fluorescent primers and an automated DNA sequencer was used for SSCP analysis of products. We conclude that the PCR-SSCP approach is a powerful method of identifying simultaneously known and new mutations of the CYP2D6 gene.

A nonsense mutation in the cytochrome P450 CYP2D6 gene identified in a Caucasian with an enzyme deficiency

A novel mutation that generates a stop codon in the third exon of the gene encoding the cytochrome P-450 CYP2D6 was identified in a Caucasian having a deficiency of the isozyme, by means of single strand conformation polymorphism analysis of DNA fragments amplified by the polymerase chain reaction, followed by selective sequencing.

A novel CYP2D6 allele with an abolished splice recognition site associated with the poor metabolizer phenotype

A novel loss-of function allele of the CYP2D6 gene was characterized in a PM individual using exon-by-exon PCR-SSCP analysis. This allele, we termed CYP2D6(F), harbours four mutations including a new mutation (D6-F) which abolishes the splice acceptor site of the 1st intron and results in a premature stop codon. DNA samples from a large population of healthy unrelated volunteers were tested for D6-F using a PCR-assay we developed for the specific identification of the mutation in genomic DNA. The prevalence of D6-F was very low. However, its identification combined with that of the previously reported gene inactivating mutations would further increase the phenotype prediction rate by genotyping.

Genetic polymorphism of cytochrome P450 2D6 in idiopathic Parkinson disease and diffuse Lewy body disease

We investigated genetic polymorphism of the cytochrome P450 CYP 2D6 gene in 105 caucasian patients with idiopathic Parkinson's disease (IPD) and 15 patients with diffuse Lewy body disease (DLBD). The mutations of the CYP 2D6 gene associated with the poor metabolizer (PM) phenotype of the debrisoquine/sparteine polymorphism were analyzed in DNA by a polymerase chain reaction (PCR)-based DNA amplification combined with Xba I restriction fragment length polymorphism (RFLP) analysis. The rate of genotypically defined PM and the frequencies of the mutation D6-B were not significantly different in IPD and DLBD patients. This study fails to find a relationship between CYP 2D6 impairment and neuropathological lesions diffusion in IPD and DLBD. This study cannot exclude involvement of neuronal expression of CYP 2D6.

Correlation between N-acetyltransferase activity and NAT2 genotype in Chinese males

Eighty-four healthy Chinese male control subjects derived from an occupation-based case-control study of bladder cancer were evaluated for hepatic N-acetyltransferase activity by dapsone and for NAT2 genotype using allele-specific amplification of peripheral leukocyte DNA by the polymerase chain reaction. Fifty-nine percent of the overall variation in acetylation activity was explained by genotype (p < 0.0001). The remaining variation in acetylation was not associated with dapsone N-hydroxylation activity, age, current smoking status, or weight in the study population, or within any genotype subgroup. Although acetylation activity in the homozygous mutant group did not overlap with the other genotype categories, there was moderate overlap in acetylation between the heterozygous mutant and wildtype groups, and substantial variation in acetylation within them. Considering all subjects with the identical NAT2 genotype as phenotypically similar and all subjects with differing NAT2 genotypes as phenotypically distinct may result in misclassification of metabolic risk factors in epidemiological investigations. As such, it would seem prudent, where possible, to collect both acetylation phenotype and NAT2 genotype data, since the advantages and limitations of these two sources of information complement, and serve to assess the accuracy of each other.

Debrisoquine oxidation polymorphism: phenotypic consequences of a 3-base-pair deletion in exon 5 of the CYP2D6 gene

A mutant allele of the CYP2D6 gene (CYP2D6* C) characterized by a 3-base-pair deletion in exon 5 (mutation D6-C) and carried by a Xba I 29 kb restriction fragment (haplotype 29-C) was previously presumed to be associated with the debrisoquine poor metabolizer phenotype on the basis of in vitro enzymatic criteria. In order to determine whether D6-C was related to a deficient CYP2D6 activity in vivo, we first analyzed the CYP2D6 gene in the family of a carrier of the haplotype 29-C by combining Xba I-restriction-fragment-length polymorphism analysis and specific CYP2D6 mutation detection by polymerase chain reaction assays. Moreover, each member of the family was phenotyped using debrisoquine as probe drug. Secondly, we used polymerase chain reaction assays to test for the CYP2D6* C mutation DNA samples from 146 unrelated healthy volunteers with the extensive metabolizer phenotype and previously identified as heterozygous carrier of one of the haplotypes known to be associated with the poor metabolizer phenotype. All family members were extensive metabolizers and three were compound heterozygotes for the haplotype 29-C and a 11.5 kb haplotype that has been shown to lack the entire CYP2D6 gene. In addition, two extensive metabolizer individuals among the 146 tested for were compound heterozygotes for the haplotype 29-C and a 29 kb haplotype carrying the defective CYP2D68* B allele (haplotype 29-B).(ABSTRACT TRUNCATED AT 250 WORDS)

N-acetylation phenotype and genotype and risk of bladder cancer in benzidine-exposed workers

Several studies in subjects occupationally exposed to arylamine carcinogens have shown increased risks for bladder cancer associated with the slow acetylator phenotype. To follow up these reports, a case-control study of N-acetylation and bladder cancer risk was carried out among subjects occupationally exposed to benzidine, in benzidine dye production and use facilities in China. Thirty-eight bladder cancer cases and 43 controls from these factories were included for study of acetylation phenotype, by dapsone administration, and for polymorphisms in the NAT2 gene, by a polymerase chain reaction (PCR)-based test. In contrast to previous studies, no increase in bladder cancer risk was found for the slow N-acetylation phenotype (OR = 0.3; 95% CI = 0.1-1.3) or for slow N-acetylation-associated double mutations in NAT2 (OR = 0.5; 95% CI = 0.1-1.8). Examination of specific mutations and adjustment for age, weight, city and tobacco use did not alter the results. When examined by level of benzidine exposure in the cases, the bladder cancer risks associated with low (OR = 0.3, 95% CI = 0.0-2.2), medium (OR = 0.7, 95% CI = 0.1-4.5) and high (OR = 0.6, 95% CI = 0.1-3.5) exposure showed no interaction between genotype and benzidine exposure, within the range of exposures experienced by subjects in this study. This study, which is the first to incorporate phenotypic and genotypic analyses, provides evidence that the NAT2-related slow N-acetylation polymorphism is not associated with an increased risk of bladder cancer in workers exposed to benzidine, and may have a protective effect.

Stereoselective hydroxylation of mexiletine in human liver microsomes: implication of P450IID6--a preliminary report

Mexiletine, a class Ib antiarrhythmic drug, is used clinically as a racemic mixture of two enantiomers. Aromatic and aliphatic hydroxylation are the two major metabolic steps. In the liver, this metabolism is catalyzed by the cytochrome P450IID6, an isoenzyme of cytochrome P450 due to genetic polymorphism in humans. In the present study, the metabolism of the two stereoisomers was compared in vitro in human liver microsomes. Parahydroxylation (aromatic hydroxylation) is favored for S(+)-mexiletine with a mean intrinsic clearance higher than for R(-)-mexiletine. The R(-) enantiomer exhibits a threefold higher mean Vmax value for aliphatic hydroxylation than S(+)-mexiletine. We showed that (i) the high-affinity component of dextrometorphan O-demethylation was competitively inhibited by R(-)- and S(+)-mexiletine and that (ii) hydroxylations of the two enantiomers were very strongly competitively inhibited by quinidine. Hydroxylation reactions of mexiletine exhibit stereoselectivity in vitro in human liver microsomes and are catalyzed by P450IID6.

Prediction of phenotype for acetylation and for debrisoquine hydroxylation by DNA-tests in healthy human volunteers

The debrisoquine/sparteine-type polymorphism of drug oxidation and the polymorphism for acetylation are two common inherited variations in human drug metabolism. The phenotypes for hydroxylation and acetylation can be predicted be newly developed methods based on mutation-specific amplification of DNA by the polymerase chain reaction (PCR), which also allow for identification of heterozygous carriers of one mutant allele. In the present study, the results of genotyping of 81 healthy European volunteers were compared with the phenotype obtained by the classical biochemical approach using debrisoquine and caffeine as probe drugs. Genotyping correctly predicted all 73 extensive metabolisers (EMs) and 6 out of 8 poor metabolisers (PMs) of debrisoquine. All 48 rapid acetylators and 33 of 35 slow acetylators were predicted. Overall, the DNA analysis result matched the in vivo phenotype in 97.5% of individuals.

A high performance liquid chromatographic method for the quantitation of flecainide in plasma

A routine high performance liquid chromatographic method for the rapid determination of flecaïnide (Flecaine), using a novel internal standard, N-methylflecaïnide, has been developed. After deproteinization of spiked samples, flecaïnide was totally recovered at neutral pH. Flecaïnide and the internal standard were separated on a reversed phase XL 3 microns ODS column using 10 mM phosphate buffer, pH 3.0: acetonitrile (70:30) as mobile phase, in less than 10 min. With spectrofluorometric detection, the limit of quantitation for flecaïnide was 10 ng/mL. Intra- and inter-assay precision variations were 0.24% and 1.4%.

Identification of a new variant CYP2D6 allele lacking the codon encoding Lys-281: possible association with the poor metabolizer phenotype

A variant CYP2D6(C) P450 protein was found in a liver characterized by deficient microsomal metabolism of bufuralol and sparteine, prototypical substrates for the debrisoquine-sparteine drug oxidation polymorphism. This protein was present at decreased levels in liver and had a slightly different relative mobility on SDS-polyacrylamide gels. The cDNA cloning and sequencing of the variant, designated CYP2D6(C), revealed that its mRNA lacked a single codon resulting in deletion of Lys281. This was the result of a three base pair deletion at the 3' end of CYP2D6 exon 5. The CYP2D6(C) P450, produced in HepG2 cells using vaccinia virus mediated cDNA expression displayed Km values toward bufuralol, debrisoquine and sparteine that were not significantly different from wild type CYP2D6. These data suggest that the poor metabolizer phenotype in livers expressing CYP2D6(C) is not due to a catalytically defective enzyme but perhaps due to decreased levels of the P450 protein in microsomal membranes. Low microsomal CYP2D6(C) contents could result from deficient membrane insertion or decreased stability of the P450 protein. A polymerase chain reaction-based procedure, developed to detect CYP2D6(C) alleles, indicates that this variant probably represents less than 1.5% of all CYP2D6 alleles.

The metabolism of mexiletine in relation to the debrisoquine/sparteine-type polymorphism of drug oxidation

1. The relationship between the metabolism of the antiarrhythmic drug mexiletine and the debrisoquine/sparteine-type polymorphism was studied in vitro, using microsomes from six human livers, and in vivo, in nine healthy drug-free volunteers with wide variation in their ability to hydroxylate debrisoquine. 2. There was a strong and similar correlation between the formation rate of both major mexiletine metabolites, p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM), and the high affinity component of dextromethorphan O-demethylase activity in human liver microsomes (rs = 0.94; P less than 0.01). 3. There were marked interindividual differences in the amounts of PHM and HMM excreted in the urine over 48 h after a single 200 mg oral dose of mexiletine hydrochloride. Recoveries of both metabolites were correlated inversely with the debrisoquine/4-hydroxydebrisoquine (D/HD) urinary metabolic ratio (rs = -0.83; P = 0.006 and rs = -0.85; P = 0.004, respectively) and were lower in poor metabolisers of debrisoquine (PM) than in extensive metabolisers (EM). Moreover, PM had the highest values of mexiletine/PHM and mexiletine/HMM urinary ratios. In addition, there was a strong correlation between these two indices of mexiletine hydroxylation and the D/HD metabolic ratios (rs = 0.92; P = 0.001 and rs = 0.90; P = 0.001, respectively). 4. After mexiletine pretreatment, the values for D/HD ratio were significantly increased in EM while corresponding values in PM were similar. 5. These findings are in accordance with previous in vitro data suggesting that PHM and HMM formation is predominantly catalyzed by the genetically variable human liver cytochrome P450IID6 isoenzyme responsible for the debrisoquine/sparteine-type polymorphism of drug oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Single-dose quinidine treatment inhibits mexiletine oxidation in extensive metabolizers of debrisoquine

Urinary elimination of unchanged mexiletine, p-hydroxymexiletine (PHM), hydroxymethylmexiletine (HMM) and mexiletine N-glucuronide conjugate (MGC) was investigated before and after treatment with quinidine. All subjects were phenotyped as extensive metabolizers for debrisoquine oxidation. The total recovery of mexiletine and metabolites was significantly reduced after quinidine pretreatment. It is concluded that pretreatment with a very low dose of quinidine inhibits markedly the elimination of both major mexiletine metabolites (PHM and HMM) and likely decreases the overall elimination of mexiletine. That should lead to changes in mexiletine disposition and have clinical consequences during combination therapy with both drugs.

Mexiletine metabolism in vitro by human liver

Human livers were used in investigations of mexiletine biotransformation in vitro. The major metabolic pathways of mexiletine oxidation, to form hydroxymethylmexiletine (HMM) and p-hydroxymexiletine (PHM), were characterized in liver cell preparations. The localization of reactions in the microsomal fraction, their heat lability, NADPH requirement and inhibition by prototype cytochrome P-450 (P-450) inhibitors (CO, SKF 525-A, metyrapone and quinidine) implied that they were catalyzed by P-450. Kinetic studies of reactions were performed in microsomes from five different livers. Eadie-Hofstee plots of data gave no indication of systematic deviation from linearity, suggesting that over the range of mexiletine concentrations examined (3.3-133.3 microM), HMM and PHM were formed by a single enzymatic site. Within a liver preparation, Km and Vmax values for HMM and PHM formation were similar. Between livers, Km values of reactions were similar with only a 1.8-fold range for each reaction, whereas Vmax values showed 7.2- and 7.8-fold ranges for HMM and PHM production, respectively. There was a very strong correlation between Vmax values for both reactions. These results, coupled with a parallel effect of inhibitors (SKF-525A, metyrapone, alpha-naphtoflavone and quinidine) on HMM and PHM formation, argue that both reactions are mediated by a common P-450 or closely related isozymes. In addition, the present in vitro results support the hypothesis that the genetically variable P-450 db 1 isozyme catalyzes the oxidation of mexiletine.

Inhibitory studies of mexiletine and dextromethorphan oxidation in human liver microsomes

The cytochrome P-450dbl isozyme (P-450bdl) is responsible for the genetic sparteine-debrisoquine type polymorphism of drug oxidation in humans. To investigate the relationship between mexiletine oxidation and the activity of this isozyme, cross-inhibition studies were performed in human liver microsomes with mexiletine and dextromethorphan, a prototype substrate for P-450dbl. The formation of hydroxymethylmexiletine and p-hydroxymexiletine, two major mexiletine metabolites, was competitively inhibited by dextromethorphan. Mexiletine competitively inhibited the high affinity component of dextromethorphan O-demethylation. In addition, there was a good agreement between the apparent Km values for the formation of both mexiletine metabolites and the high affinity component of dextromethorphan O-demethylation and their respective apparent Ki values. Several drugs were tested for their ability to inhibit mexiletine oxidation. Quinidine, quinine, propafenone, oxprenolol, propranolol, ajmaline, desipramine, imipramine, chlorpromazine and amitryptiline were competitive inhibitors for the formation of hydroxymethylmexiletine and p-hydroxymexiletine as for prototype reactions of the sparteine-debrisoquine type polymorphism. Amobarbital, valproic acid, ethosuximide, caffeine, theophylline, disopyramide and phenytoin, known to be non-inhibitors of P-450dbl activity, were found not to inhibit the formation of these mexiletine metabolites. Moreover, the formation of both metabolites was strongly inhibited by an antiserum containing anti-liver/kidney microsomes antibodies type I (anti-LKMI) directed against P-450dbl. These data suggest that the formation of two major metabolites of mexiletine is predominantly catalysed by the genetically variable human liver P-450dbl.

Comparative electrophysiological effects of propafenone, 5-hydroxy-propafenone, and N-depropylpropafenone on guinea pig ventricular muscle fibers

Propafenone (Pf) is a class I antiarrhythmic drug that can be given both orally and intravenously. In order to examine whether its two major metabolites [5-hydroxypropafenone (5-OH-Pf) and N-depropylpropafenone (N-DP-Pf)] possess pharmacodynamical properties, we compared their electrophysiological effects to those of the parent drug on papillary muscle fibers from guinea pig ventricular myocardium. After baseline action potential and refractory period characteristics were measured at different pacing rates, the tissue preparations were superfused with either Pf, 5-OH-Pf, or N-DP-Pf at five different concentrations and electrophysiological characteristics were studied again. The maximal rate of depolarization (Vmax) was depressed by the three compounds only at the highest concentration, although the effect of N-DP-Pf was slightly less than the other two. Refractory periods were altered only by the highest concentration of 5-OH-Pf. Propafenone and N-DP-Pf exhibited equally slow on-set/off-set kinetics of the sodium channel block, whereas those of 5-OH-Pf were twice as long, which seems to suggest a slower rate of dissociation of the latter from the inactivated sodium channels. Thus, 5-OH-Pf and N-DP-Pf comply with the definition of the class IC antiarrhythmic drugs. The cumulative in vivo effects of the two metabolites and of the parent drug could have far reaching clinical implications, especially in the genetically predisposed extensive metabolizing subject.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.

High-performance liquid chromatographic assay for mexiletine hydroxylation in microsomes of human liver

A simple high-performance liquid chromatographic assay, using fluorescence detection, is described for determining simultaneously the production of the two major hydroxylated metabolites of mexiletine in human liver microsomes. The detection limits of hydroxymethylmexiletine and p-hydroxymexiletine are 0.35 and 0.08 nmol/ml, respectively. The assay is specific, reproducible and allows the simultaneous kinetic characterization of the reactions in small amounts of liver tissue. The assay may be used to acquire a better knowledge of the kinetic behaviour of mexiletine and of its metabolites, and to investigate if the large inter-individual variations of the mexiletine pharmacokinetics are of metabolic origin, due to variations of its hydroxylation processes.