Distribution of drug-metabolizing enzymes coding genes CYP2D6, CYP3A4, CYP3A5 alleles in a group of healthy Turkish population

Introduction

Cytochrome P450 (CYP) enzymes metabolize drugs and xenobiotics and are also involved in the carcinogen and mutagen production [1], [2]. Different population and ethnic groups show variation in the genotype and allele frequencies of CYP enzymes [3]. The activities of polymorphic CYP enzymes range from the absolute absence to high metabolizing capacities [1]. Differences in the activities of CYP enzymes, due to genetic variations, are responsible for the individual response variability to numerous drugs and carcinogens [3].

Over 80 allelic variants have been described for CYP2D6 which is also known as debrisoquine/sparteine oxidation polymorphism [4]. Clinical significance of CYP2D6 enzyme polymorphism has been well documented and an estimated 20–25% of all drugs in clinical use are metabolized at least in part by this enzyme [4]. Approximately 2–3% of total liver CYP enzymes consist of CYP2D6 and its content substantially varies among people mainly due to its genetic polymorphisms [5], [6]. It has been reported that in addition to liver, CYP2D6 also expressed in the gut and brain neurons, where endogenous substrate turnover found to be high [4].

Exploring phenotypic effects and clinical relevance of CYP3A genetic polymorphisms, is an intriguing complementary strategy for pharmacogenetic and toxicogenetic studies. Four functional CYP3A enzymes such as CYP3A4, CYP3A5, CYP3A7, and CYP3A43 have been identified in humans [7]. CYP3A4 is most abundant enzyme in adult liver and intestine and is the major enzyme involved in xenobiotic and drug metabolism [8]. CYP3A5 is the predominant form in the kidney [9]. The CYP3A4 and CYP3A5 genes have a strong haplotype structure at varying frequencies across ethnic groups [10]. In addition to liver, CYP3A5 was reported to be expressed in intestine and as the prevailing CYP3A isoform in kidney [9]. CYP3A5 converts cortisol to 6-β-hydroxycortisol in the kidney also has been implicated to salt-sensitive hypertension in humans [10].

Thus, in this study, we aimed to determine the aforementioned drug metabolizing enzymes CYP2D6, CYP3A4, CYP3A5 polymorphism in a Turkish population, to help to predict inter-individual variabilities in drug response in the Turkish population.

Materials and methods

Results

CYP2D6 allele frequencies in 297 subjects were 100% for CYP2D6*1 (WT/MU). CYP3A4 allele frequencies in 283 and 306 subjects were 100% for CYP3A4*5 (C/C) and CYP3A4*18 (T/T) respectively. CYP3A5 allele were in Hardy-Weinberg equilibrium for CYP3A5*2 (p=0.142) and frequencies for C and A allele were 91% and 9% respectively in 289 subjects. CYP3A5 allele frequencies in 267 subjects were 100% for CYP3A5*4 (WT/WT) (Table 2). Due to technical reasons, it was unable to detect the genotypes of some subjects, CYP2D6*1 (n=9), CYP3A4*5 (n=23), CYP3A5*2 (n=17) and CYP3A5*4 (n=39).

Discussion

Inter-individual variabilities in genetic factors affect the pharmacokinetic and change the efficacy and toxicity properties of drugs. Also genetic variations in CYP enzymes are the important predictors of difference in drug response such as adverse drug reactions and variability in drug efficiency. The submitted pharmacogenetic CYP enzyme polymorphism makes possible to optimize pharmacotherapy and adjust dose to individual needs [1], [2]. Therefore in the present study some of the important polymorphisms for CYP2D6, CYP3A4, and CYP3A5 in a group of Turkish population were demonstrated.

Clinically important drugs such as antipsychotics, antidepressants, anti-cancer drugs and antiarrhythmics are metabolized by CYP2D6 which enzymatic activity is highly correlated with its genetic polymorphisms [1], [2]. Multiallelic polymorphisms, which strongly related to ethnicity, determines the function of CYPs such as CYP2D6, CYP3A5, CYP2C19, CYP2C9, and as a consequence lead to distinct phenotypes as ultra-rapid, extensive, intermediate and poor metabolizers [15]. CYP2D6 alleles and their functional activities which determines abovementioned metabolizing state have been classified as normal, increased, reduced and none [16]. CYP2D6*1 is classified as alleles with normal function [4]. In the present study CYP2D6 allele frequencies in subjects were 1.0 for CYP2D6*1 (WT/WT). There is limited studies for CYP2D6*1 polymorphism in Turkish population. Aynacioglu et al. reported CYP2D6*1 with an allelic frequency of 0.37 in Turkish population [17]. In another study CYP2D6 genotypes were determined among 92 Turkish patients with breast cancer treated with tamoxifen, the most common CYP2D6 gene polymorphism was *1/*2 with a percentage of 20.6% (n=19) [18]. In addition a study from Turkey reported allelic frequency for CYP2D6*1 as 50.7% in 68 psychiatric patients [19].

CYP3A enzymes metabolizes approximately 37% of the drugs from all therapeutic categories, such as macrolide antibiotics like erythromycin, immunosuppressants cyclosporin and tacrolimus, anticancer drugs like taxol, benzodiazepines, HMG-CoA reductase inhibitors like simvastatin and atorvastatin and anesthetics [20]. Besides drugs, CYP3A4 also have a role in the metabolism of bile acids and sex steroids, including testosterone, progesterone, androstenedione [20].

Although some single nucleotide polymorphisms for CYP3A4 have been identified, they failed to explain major part of the phenotypic variability. However recently, increasing studies has shown that genetic variants in CYP3A4 contribute to inter-individual variabilities of metabolic activity [21]. In addition to there is no studies regarding the CYP3A4*5 and CYP3A4*18 polymorphisms and only limited studies on CYP3A4 and CYP3A5 polymorphism in Turkish population [22], [23], [24]. It has been reported that CYP3A4*5 and CYP3A4*18 polymorphisms are associated with the degree of the enzymatic activity [21]. In the present study CYP3A4 allele frequencies were 1.0 for both CYP3A4*5 (C/C) and CYP3A4*18 (T/T) in Turkish population.

CYP3A4/5 share significant sequence homology and have almost identical substrate specificity with somewhat differing metabolic rates [25]. Although it has been reported that CYP3A5 is an overlooked polymorphic enzyme, its allelic frequency in specific ethnic groups is important to optimize pharmacotherapy [12], [26]. Literature is limited regarding to CYP3A5 polymorphism also there is no study about CYP3A5 polymorphism in Turkish population except a few genotyping for CYP3A5*3 [23], [27]. In the present study CYP3A5 alleles were in Hardy-Weinberg equilibrium for CYP3A5*2 and CYP3A5 alleles frequencies were 1.0 for CYP3A5*4 (WT/WT) in healthy Turkish blood donors. Allelic frequencies of CYP3A5*2 and *4 in the Dutch Caucasian population were report as 0.010 and 0.000 respectively [12]. In conclusion they suggested genotyping for the CYP3A5*2 allele in CYP3A5*3 heterozygotes and CYP3A5*2 less relevant for screening purposes [12].

Cytochrome P450 enzyme system have a critical role in the metabolism and elimination of drugs and xenobiotics as well as their activity could produce carcinogens and mutagens [1], [2], [28], [29]. Present study is important for elucidating the CYP2D6, CYP3A4 and CYP3A5 drug metabolizing enzyme polymorphisms that especially have not been shown before in Turkish population. These results may help optimization of personalized pharmacological therapies and prediction of xenobiotics metabolism.