Genetic polymorphism of CYP2D6 in a keralite (South India) population

Cytochrome P450 2D6 polymorphism in eastern Indian population

OBJECTIVES:

Cytochrome P₄₅₀ 2D6 (CYP2D6) enzyme metabolizes a quarter of prescription drugs. Polymorphisms of CYP2D6 gene and resultant phenotypic variations in metabolic activity have been described in various populations. We assessed the prevalence of CYP2D6 activity phenotypes, employing dextromethorphan (DXM) as probe drug in subjects with at least two parental generations residing in eastern India.

MATERIALS AND METHODS:

Unrelated healthy subjects took 60 mg DXM after fasting overnight. Blood samples were collected 3 h after dosing and plasma separated. DXM and its primary metabolite dextrorphan (DXT) were measured by liquid chromatography with tandem mass spectrometry. The DXM-to-DXT metabolic ratio (MR) was obtained for each subject. Histogram of MR values suggested bimodal distribution. A polynomial regression equation derived through probit analysis was solved to identify the antimode of the MR values. Subjects with log(MR) < antimode were extensive metabolizers (EMs). Log(MR) ≥ antimode indicated poor metabolizers (PMs).

RESULTS:

We evaluated the results from 97 participants. The median MR was 0.209 (interquartile range: 0.090–0.609), while the antimode for MR was 3.055. From these, it was inferred that three subjects were PMs, while the rest were EMs. CYP2D6 polymorphism prevalence is low (3.09%; 95% confidence interval: 0.35%–6.54%) in the population of eastern India and matches the prevalence in other zones of the country.

CONCLUSIONS:

Differences in CYP2D6 activity has treatment implications and may lead to adverse events or therapeutic failure. Phenotyping of subjects receiving CYP2D6 metabolized drugs may help clinicians personalize treatment and avert adverse drug-drug interactions. However, the frequency of the PM phenotype is low in India, and routinely phenotyping for CYP2D6 activity will not be cost-effective. We cannot recommend it at this stage.

Evaluation of cytochrome P450 2D6 phenotyping in healthy adult Western Indians

Background:

Cytochrome P450 2D6 (CYP2D6) metabolizes around 25% of the drugs used in therapeutics and different polymorphisms have been identified in various populations. This study aimed at finding the prevalence of CYP2D6 polymorphisms using dextromethorphan as a probe drug.

Materials and Methods:

Healthy participants were administered 60 mg dextromethorphan after an overnight fast and 5 ml of blood was collected 3 h postdose. A validated laboratory method was used to measure both dextromethorphan and its active metabolite, dextrorphan from plasma. Metabolic ratio (MR) of dextromethorphan to dextrorphan was calculated for each of the participants. Probit analysis was done and antimode was defined. Individuals with log MR equal to or higher than the antimode were classified as poor metabolizers (PMs) and those with values less than antimode were categorized as extensive metabolizers (EMs).

Results:

Data from a total of 149 participants were evaluated and the median (range) of MR was 0.25 (0.03-3.01). The polynomial equation obtained in probit analysis gave an antimode for MR of 1.39. Five (3.36%) participants were PMs and 144 (96.64%) were found to be EMs. One participant had reported mild drowsiness 2 h postdose that subsided spontaneously without any intervention.

Conclusion:

The prevalence of CYP2D6 polymorphism in Western Indian population is low (3.36%) and is similar to other populations.

Meta-analyses of the effect of CYP1A1 and CYP2D6 polymorphisms on the risk of head and neck squamous cell carcinoma

BACKGROUND

CYP1A1 and CYP2D6 are important genes encoding enzymes involved in the metabolism of toxic chemicals and carcinogens. However, inconclusive results for the association between CYP1A1 and CYP2D6 polymorphisms and the risk of head and neck squamous cell carcinoma (HNSCC) have been reported. We conducted a meta-analysis to evaluate the association of CYP1A1 and CYP2D6 polymorphisms with the risk of HNSCC.

METHODS

A database search yielded 19 relevant studies. 3 polymorphisms were included in the meta-analysis: CYP1A1, CYP2D6*4 and CYP2D6*10. Random or fixed effect models were used in the analysis.

RESULTS

The CYP1A1 polymorphism was associated with HNSCC (for m1m1 vs. m1m2: odds ratio (OR) = 1.26, 95% confidence interval (CI) = 1.030-1.542, pheterogeneity = 0.025; for the recessive model: OR = 1.316, 95% CI = 1.065-1.625, pheterogeneity = 0.001). The analysis showed evidence for association between the CYP2D6*4 polymorphism and HNSCC in Asian populations; however, negative results were also observed in other models. A significant association was also observed between CYP2D6*10 polymorphism and HNSCC risk.

CONCLUSIONS

The current study demonstrates that the CYP1A1 and CYP2D6 polymorphisms are associated with susceptibility to both development and progression of HNSCC.

Characterization of the CYP2D6 gene locus and metabolic activity in Indo- and Afro-Trinidadians: discovery of novel allelic variants

BACKGROUND

The highly polymorphic CYP2D6 gene has extensively been studied in many populations, but there is a void of knowledge regarding CYP2D6 pharmacogenetics and activity in populations with unique ancestries and admixture, such as those residing in Trinidad and Tobago.

MATERIALS & METHODS

167 healthy Indo- and 103 Afro-Trinidadians were phenotyped with dextromethorphan and extensively genotyped. Gene resequencing was performed to resolve cases with genotype/phenotype discordance.

RESULTS

CYP2D6 activity did not differ between the Indo-Trinidadians and Afro-Trinidadians. Poor metabolizers were, however, more frequent in the Indo-Trinidadians (4.19 vs 1.94%), and unique allele frequency patterns were observed. Two novel nonfunctional allelic variants were found among the Indo-Trinidadians in two discordant cases. CYP2D6*100 is characterized by a single nucleotide deletion and CYP2D6*101 by a 19-bp deletion; both cause frameshifts.

CONCLUSION

Our study underscores the importance of thoroughly characterizing the genetic make up of unique populations when considering pharmacogenetic testing for individualized therapy.

Interethnic differences in genetic polymorphisms of CYP2D6 in the U.S. population: clinical implications

DNA polymorphisms have been identified in the genes encoding a number of the cytochrome P450 (CYP) enzymes, leading to wide interindividual variation in drug clearance. CYP2D6 metabolizes a significant number of clinically used medications, and genetic variants of the CYP2D6 isozyme that result in varying levels of metabolic activity are of clinical importance in some settings. The exact nature of the clinical effect caused by polymorphisms of the gene depends on the drug in question and the specific variant alleles expressed, as individual variants result in differing phenotypes with a range of levels of enzymatic activity. Compromised drug efficacy due to CYP2D6 variation has been documented with a variety of agents, and this review considers a number of examples, including the 5-HT(3)-receptor antagonists, which are used in oncology supportive care for the prophylaxis of nausea and vomiting. CYP2D6 is involved in the metabolism of all of the most commonly available agents, except granisetron, and their efficacy and side effects may therefore be affected by the CYP2D6 polymorphism. Significant interethnic differences in CYP2D6 allele frequencies have been demonstrated from studies across many countries. However, incidences of polymorphisms in the U.S. population have been challenging to characterize because of the country's wide ethnic diversity. The CYP2D6 polymorphism may become more important as robust clinical tests become widely available and as the use of multiple medications and the attendant risk for drug-drug interactions increases.

CYP2D6 genetic polymorphism in South Indian populations

This study aimed to determine the prevalence of genetic polymorphism in the CYP2D6 gene, which codes for the polymorphically expressed CYP2D6 drug-metabolizing enzyme. The common variants CYP2D6 *2, *3, *4, *5, *10, *14, and *17 were studied in the populations (n=447) of the four South Indian states namely Tamilnadu (TN), Kerala (Ker), Karnataka (Kar) and Andhra Pradesh (AP). Genetic polymorphisms were identified using polymerase chain reaction (PCR) and PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) based methods. Differences in frequencies of CYP2D6 polymorphism between each South Indian state were statistically compared, and also the frequency of South Indian population as a whole in relation to other major populations. The CYP2D6*2 allele was the most frequent variant (34.8%), followed by the *10 allele (10.2%). The *4 and *5 alleles occurred at 7.3% and 1.9% respectively. The *3, *14 and *17 alleles were not detected in the study. The *1/*2, *1/*1 and *2/*2 genotypes were the most common CYP2D6 genotypes, representing 32.7%, 19.4% and 11.8% of the South Indian population. Genotypes that predict poor metabolizer phenotype i.e. *4/*4 and *4/*5 were found at 0.6% in South Indian population. The genetic composition at the CYP2D6 locus in South Indians is distinct from Caucasian, African and even other Asian (Chinese and Japanese) populations.

Genetic polymorphism of CYP1A1, CYP2D6, GSTM1 and GSTT1 and susceptibility to acute lymphoblastic leukaemia in Indian children

BACKGROUND

Biotransformation plays a crucial role in carcinogen activity and many genetic polymorphisms in xenobiotic metabolising enzymes have been associated with an increased risk of cancer. Such polymorphisms can lead to considerable variation in the activities of these enzymes, which are crucial in carcinogen and drug metabolism. These variations could play a role in the risk of developing paediatric acute lymphoblastic leukaemia (ALL) by their varying action on environmental carcinogens.

PROCEDURE

The present study looked for two polymorphisms (m1 and m2) in the CYP1A1, CYP2D6*4 genes and deletions of the glutathione S-transferases (GSTM1 and GSTT1) in 118 paediatric ALL patients and 118 age matched control children. The polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) were used to study gene polymorphisms.

RESULTS

In children with ALL, CYP1A1 m1 polymorphism was evident in 42.4% of subjects and CYP1A1 m2 in 37.3%. These were significantly different from the results obtained for control children (20.3% for CYP1A1 m1 and 19.5% for m2). Subjects with CYP1A1 m1 homozygous variant had a sixfold risk and CYP1A1 m2 a fourfold risk. In contrast, CYP2D6*4 was more prevalent in the controls than in the cases. Subjects with GSTM1 deletions had increased risk of ALL (OR = 2.1, P = 0.009). The odds ratios for both CYP1A1 m1 and m2 homozygous polymorphisms being associated with childhood ALL was 5.67 (95% CI = 2.11-15.27). The odds ratios for both GSTM1 and GSTT1 deletions being associated with ALL was 2.78 (95% CI = 0.67-11.56).

CONCLUSIONS

These results suggest that genetic polymorphisms of xenobiotic metabolising enzymes appear to influence susceptibility to childhood ALL.

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.

Heterogeneity of the CYP2D6 gene among Malays in Malaysia

BACKGROUND

Although Malays shared an origin with Chinese, their evolution saw substantial divergences. Phenotyping studies suggested that they differed in CYP2D6 polymorphism, with higher PM prevalence but lesser right-shift for debrisoquine MRs.

OBJECTIVE

To study the genotype distribution of CYP2D6 among the Malays in Malaysia.

METHOD

We obtained DNA from 107 Malays and used PCR to determine common CYP2D6 alleles.

RESULT

CYP2D6*1 occurred at a frequency of 36.0%, duplicated gene, 0.93%, CYP2D6*4, 2.8%, CYP2D6*5, 5.1%, CYP2D6*9, 3.3%, CYP2D6*10, 49.5% and CYP2D6*17, 0.5%. The findings of CYP2D6*17 and CYP2D6*9 were novel for Asia. The frequency for CYP2D6*10 was lower than in other Asian races. The most frequent genotypes were CYP2D6*1/*10 at 39.3%. Two subjects had genotypes that predicted PM phenotype, 35% showed genotypes that predicted intermediate metabolizers and one subject had a genotype that predicted ultra-rapid metabolism.

CONCLUSION

The genetic polymorphism of CYP2D6 in Malays is different from Chinese and Far Eastern races. They may be intermediate between East Asians and Caucasians in CYP2D6 activity. Further study in relation to the evolution of races and disease prevalence may help to identify the contributions of the polymorphism in alleged susceptibility to diseases apart from delineating its contributions to ethnic differences in the pharmacology of CYP2D6 drugs.

CYP2D6 phenotypes among Malays in Malaysia

BACKGROUND

Although they originated from China, Malays have undergone a lot of intermarriages. A study suggested that CYP2D6 poor metabolism (PM) phenotype was more common in Malays compared to Chinese. CYP2D6 is highly polymorphic and is involved in the metabolism of many drugs and has been implicated in some environmentally-induced diseases. It is therefore useful to further study this polymorphism in Malays.

OBJECTIVE

To study debrisoquine metabolism phenotypes in healthy Malay volunteers.

METHOD

We administered debrisoquine to 51 Malays and used HPLC to measure urinary debrisoquine and 4-hydroxy debrisoquine to calculate debrisoquine metabolic ratios (MR).

RESULTS

Debrisoquine MR varied widely and with probit analysis we were able to identify population subsets. Although the frequency distribution for the MR showed a right shift, the shift was less than that reported for the Chinese population. We also found 2 poor metabolizers and one ultra rapid metaboliser in the population.

CONCLUSION

The genetic polymorphism of debrisoquine in Malays differs from that in the Chinese. Both their PM prevalence and their MR distribution suggest that they are intermediate between Europeans and Chinese in relation to this polymorphism. Studies to compare CYP2D6 genotypes between them and related races would be useful to further define these differences.