Inter-ethnic genetic variations and novel variant identification in the partial sequences of CYP2B6 gene in Pakistani population

Introduction

The cytochrome P450 2 (CYP2) is a major family of the cytochrome enzymes, comprising of several subfamilies that aggregated together in the form of clusters in the genome. The CYP2 subfamilies include the CYP2A, CYP2B, CYP2F, CYP2G, CYP2S and CYP2T genes (Simonsson et al., 2003). Among these, the CYP2B located at chromosome 19 of the human genome, holding nine exons encodes for 491 amino acids containing protein (Yamano et al., 1989). The human CYP2B6 gene has two known loci: the functional CYP2B6 and its non-functional pseudogene CYP2B7, located in the center of the CYP2A18P locus inside a 112 kb block (Yamano et al., 1989). The CYP2B6 enzyme is involved in the metabolic activation and inactivation of several drugs including anticancer such as cyclophosphamide and ifosfamide (Chang et al., 1993; Granvil et al., 1999; Roy et al., 1999), antimalarial such as artemesinin (Simonsson et al., 2003), antiviral such as efavirenz, and antidepressant such as bupropion (Faucette et al., 2000). Although the total fraction of the CYP2B6 enzyme is small as compared to the total hepatic P450 family, however, it metabolizes a vast majority of pharmaceutical drugs. In addition to 7–8% of the marketed pharmaceutical drugs, CYP2B6 metabolizes certain exogenous and endogenous substances such as nicotine and testosterone (Rosenbrock et al., 1999), in conjunction with other cytochrome enzymes.

The single nucleotide polymorphisms (SNPs) in the CYP2B6 gene may affect the expression and enzyme activity of the translated protein, resulting in significant differences in the pharmacokinetics of CYP2B6-metabolized drugs among individuals and ethnicities, in turn, leading to variations in efficacy and toxicity (Desta et al., 2007; Nyakutira et al., 2008; Aurpibul et al., 2012). Several important variants of CYP2B6 gene, i.e. CYP2B6*2 (C64T), *3 (C777A), *4 (A785G), *5 (C1459T), *6 (G516T and A785G), *7 (G516T, A785G and C1459T) (Lang et al., 2001), *8 (A415G) and *9 (G516T) have been discovered (Lamba et al., 2003), besides the wildtype CYP2B6*1 allele. About 38 protein variants of the highly polymorphic CYP2B6 gene have been identified so far (http://www.cypalleles.ki.se/CYP2B6.htm), (Zanger & Klein, 2013). Genetic variants of the enzyme result in substrate, and expression dependent functional changes.

Numerous other studies have reported that genetic variations that alter the CYP2B6 enzyme expression result in altered drug responses (Hesse et al., 2000; Coller et al., 2002; Lerman et al., 2002). Studies have also shown that there are major differences in the amount of enzyme and its activity amongst different individuals. Ethnicity and sex-based differences in CYP2B6 gene expression and splice variants have been reported previously (Lamba et al., 2003). For example, CYP2B6 *4 variant has been shown to result in enhanced expression and altered activity of the enzymes (Gadel, Friedel & Kharasch, 2015). A SNP rs3745274 (c.516G>T) leading to CYP2B6*6 allele was alone responsible for aberrant splicing, resulting in a low-CYP2B6 expression phenotype. In recent years, studies have identified CYP2B6*6 in association with enhanced plasma concentrations of certain drugs (Aurpibul et al., 2012), including efavirenz.

Pakistan is ethnically diverse; however, little is known about the distribution of CYP2B6 genetic variations in a country of over 210 million population. Various parts of the country possess a unique lifestyle, diverse genetic background, dietary habits, culture, and geographical environment. The five ethnicities focused on in the current study are the largest genetically and linguistically distinct ethnicities of Pakistan. Among these, the Punjabi is the largest one residing in the Punjab province. Geographically the area of Punjab is comprised of eastern Pakistan and northern India. The Sindhi ethnicity primarily resides in Sindh province located in Southeast of Pakistan. Pathan ethnic population is about 15–18% of total Pakistani population. Pathan are predominantly residing in Khyber Pakhtunkhwa province situated in northwestern region of Pakistan. Balochi people are residing predominantly in Balochistan province situated in the South and Western region of Pakistan. The Saraiki people live in Southern Punjab and Southern Khyber Pakhtunkhwa. The Saraiki people are distinguished in culture and norms from the closely residing Punjabi and Pathan ethnic groups. Previously published studies about these Pakistani population ethnic groups inferred considerable genetic variation (Qamar et al., 2002; Quintana-Murci et al., 2004; Bergström et al., 2020). Furthermore, distinct genetic ancestry of these groups has also been reported in worldwide population genetics project (Li et al., 2008). The CYP2B6 gene contains many SNPs in addition to some copy number variations. However, only a few might alter enzyme activity or be associated with certain diseases. Keep in view this scenario, we investigated a part of CYP2B6 from the individuals belonging from five major ethnic groups of Pakistan to infer their genetic composition across this important drug-metabolizing enzyme.

Methods & materials

Results

Population structure analysis

The Fixation index (Fst) statistic was employed to investigate the genetic composition across CYP2B6 gene among ethnic groups’ samples. The high pairwise Fst genetic distinction was observed between Seraiki and Sindhi ethnic groups (Fst = 0.13392, P-value = 0.026) as well as between Seraiki and Balochi groups (Fst = 0.04303, P-value = −0.0030). However, the degree of genetic distinction was low between Pathan and Punjabi ethnic groups (Fig. 1). Moreover, when these ethnic groups were compared with the 1000 Genomes samples, the Seraiki population showed high genetic distinction (P-value =< 0.01) in comparison with the East Asian: Han Chinese (CHB), Japanese (JPT); South Asian: Bengali (BEB), Sri Lankan Tamil (STU), Indian Telugu (ITU), Gujrati Indian (GIH); and European populations: CEU. (Utah Residents CEPH with Northern and Western European Ancestry) (Fig. 2).

AMOVA was used to estimates the genetic deviation at a molecular level among and within various ethnic groups samples. The significance of AMOVA was assessed by 99,000 permutations (Table 2). The AMOVA model revealed that genetic diversity due to variations among-ethnic groups is 1.08%, and within-ethnic groups is 98.92% (P-value = 0.28615). Similarly, a variance factor (Est. Var) of 0.50731 was observed within population; however, among-ethnic groups variance factor was found low 0.00554. The among-ethnic group based genetic variance in this model was although low but still significant, suggesting that ethnic groups are genetically distinct. High pairwise nucleotide differences (D_A) were observed between Seraiki and Sindhi, Seraiki and Balochi ethnic groups.

Table 2:

Analysis of genetic distinction across CYP2B6.

Source of variation	Df	SS.	Est. Var	%	P-value
Among populations	4	2.488	0.00554	1.08	<0.28615
Within population	99	50.244	0.50731	98.92	<0.28615

DOI: 10.7717/peerj.11149/table-2

Notes:

df: degrees of freedom; SS: the sum of squares deviation; Est.Var: estimates of variance components; %: percentage of total variance contributed by each component.

Similarly, a high mean pairwise difference (πxy) was found in the Seraiki and Sindhi, Seraiki, and Balochi ethnic groups. The highest within-population genetic distinction (i.e., π) was observed for Balochi ethnic group samples (Fig. S2). Moreover, significant differentiation between Seraiki and other Asian/European populations was found as evident by the Nei’s distance (D_A) during comparison of Pakistani ethnicities CYP2B6 SNPs with additional Asian and European populations data acquired from the 1000 Genomes Project (Fig. 3).

Haplotypes composition

A total of five haplotypes with haplotype diversity (Hd) of 0.7276 were identified for all the 104 sample sequences. The haplotype-1 was found with predominant frequency in Sindhi, Pathan, Punjabi, and Balochi samples. The haplotype-2 was also found in the same ethnic groups including Seraiki, whereas, haplotype-3 was found at high frequency only in the Seraiki group (Fig. S3). The AMOVA analysis of the pairwise distance between haplotypes revealed that Seraiki and Sindhi predominant haplotype-2 showing substantial genetic differentiation against haplotype-5 (i.e., predominant in Balochi samples) and haplotype-1 (i.e., predominant in Sindhi and Balochi). Moreover, the haplotype-3 (i.e. predominant in Seraiki samples) showing significant genetic distinction against haplotype-5 and haplotype-4 (predominant in Balochi and Sindhi samples (Fig. 4)). During medium-joining haplotype networking analysis, the main haplotypes of Punjabi, Pathan, Sindhi and Seraiki ethnic groups persisted on common nodes, whereas the Balochi samples predominant haplotype-5 constituted a separate node, showing the least genetic connectivity with the rest of the ethnic groups (Fig. 4).

Discussion

The DNA sequencing approach can be implemented effectively to assess the allelic distribution of the human DNA sequences and to estimate interethnic genetic variation among populations. This approach has been frequently implemented across the world to identify specific individual polymorphisms. The Pakistani population is a heterogeneous admixture and exhibits high ethnic diversity due to complex demography and various invasions, including Central Asian, Arabs and British colonial invasions (Bhatti et al., 2017). Therefore, allelic distribution-based assessment of the drug-metabolizing enzymes coding genes, including CYP2B6 in different ethnicities of the Pakistani population is expected to be a complex but an interesting project to study.

Current study provides the genetic structure of the CYP2B6 gene in major ethnic groups across the Pakistani population. The enzyme encoding from CYP2B6 is responsible for the metabolism of various pharmacological drugs including efavirenz (Ward et al., 1991). The polymorphisms present in the CYP2B6 gene showed a distinction of allelic frequencies and genetic variability among Pakistani ethnic groups. These differences may lead to variations in the drug-metabolizing potential of the CYP2B6 enzyme. The former global population genomics projects like Human Genome Diversity Project—HGDP (Auton et al., 2015; Bergström et al., 2020) identified genetic distinction among various Pakistani ethnic groups. The various Pakistani population groups (Indus-valley populations) revealing genetic patterns akin to the European, Caucasian, and Indian populations (Metspalu et al., 2011).

The determination of the various single nucleotide polymorphisms and allelic distribution of human genes at a population level and their association with disease phenotypes have been reported previously (Hamblin, Thompson & Di Rienzo, 2002; Nyakutira et al., 2008). Several genetic variants of CYP2B6 are reported to have a significant association with different drug pharmacokinetics in multiple populations across the world, including African, Asian, and South American (Klein et al., 2005; Nyakutira et al., 2008; Radloff et al., 2013). In the current study, we observed significant genetic variation across CYP2B6 locus in Pakistani ethnicities in comparison to worldwide, including South Asian, East Asian, and European populations. The pairwise population genetics analyses of the data inferred less genetic differentiation among Punjabi, Pathan, Balochi samples, while the Seraiki, Sindhi, and Balochi samples displayed high genetic differentiation. The highest value of pairwise genetic differences was observed between the Seraiki and Sindhi groups (Fig. 1).

Interestingly the Seraiki group showed uniform genetic composition and the lowest intra-population genetic variance (Fig. S2). Therefore, we can expect a uniform phenotype in the Seraiki population within the context of drugs metabolized by the CYP2B6 enzyme. The Seraiki group seems to be distinct from the Punjabi ethnicity, despite these groups have close demography as known from Bronze age Harappan civilization (the Pre-Aryan people) (Ahmed & Khan, 2017; Shackle, 1977; Hashmi & Majeed, 2014).

The Balochi ethnicity showed higher intra and interethnic genetic differentiation, although this group holds a small population size compared to the rest of Pakistani ethnicities. This heterogeneous genetic ancestry, as observed in the Balochi samples, may be due to their Aryan, Arab, Persian, Turkish, Khurdish, Dravidian, Sewais (Hindu) and black African mixed ancestries (Ahmed & Khan, 2017). Therefore, due to high intraethnic genetic differentiation, the Balochi ethnic group might exhibit distinct CYP2B6 mediated drug-metabolized and treatment response phenotypes compare to other Pakistani ethnicities.

AMOVA, which was performed based on haplotypes significance analysis to determine the degree of regional differentiation and homogeneity within and among Pakistani ethnic groups, found that the majority of the genetic variance is attributed due to within-population factors. Genetic variance among populations was only 1.08%, as inferred during AMOVA analysis (Table 2). This variability was due to the intra-ethnic group heterogeneity. The genetic variance between ethnic groups in our study was low but is still considered significant as per previous studies (Adeyemo et al., 2005). Nonetheless, the small genetic difference between ethnic groups variance does not imply that they cannot be distinguished from each other. Instead, the pairwise analysis reveals significance differences between the ethnic groups (Fig. 1). Likewise, the results based on the haplotypes composition (Fig. S4) and haplotypes network plot (Fig. 4) among ethnic groups is congruent to the pairwise Fst analysis and further confirmed the genetic distinction among ethnic groups.

The prevalence of CYP2B6*6 allele has been found variable throughout the world. Globally, G allele frequency is 0.73 and that of T allele is 0.26 (Auton et al., 2015). In the present study, the prevalence of T allele was found at 0.338 in the Pakistani population which is slightly lower than its frequency found in the South Asian populations. Its prevalence in East Asia and Europe was found lower than Pakistan at 0.215 and 0.236, respectively while its frequency was higher in American and African populations, i.e. 0.373 and 0.374 respectively (Auton et al., 2015).

If clinicians have information about the patients’ CYP2B6 gene, they may help enhance the efficacy and reduce the adverse effect by prescribing the most suitable and safest drug to the patients based on their genetic structure. Over 2.6 billion unit doses of drugs are dispensed each year in Pakistan (Ahmed et al., 2020) and approximately 7.2% marketed drugs are metabolized by CYP2B6 enzyme (Zanger & Schwab, 2013). This means that over 187 million doses of those drugs are dispensed annually in Pakistan which are metabolized by CYP2B6 enzyme. Our study shows that about one third of Pakistan’s population has a CYP2B6*6 allele. This implies that over 62 million doses of drugs dispensed annually in Pakistan may not have desired effects as patients receiving these medications possess a low activity CYP2B6 allele. For example, patients taking efavirenz may suffer from enhanced frequency and severity of adverse effects if they possess one or two CYP2B6*6 alleles. However, longitudinal studies with proper follow up are needed in the Pakistani population to confirm these findings.

Conclusion

Overall, our results demonstrate distinct genetic patterns in different ethnic groups across Pakistan. The novel heterogeneous SNP, as identified in the present study, might lead to altered drug-metabolizing potential of CYP2B6 and, therefore, additional studies are required to decipher the pharmacological impact of this SNP. The rs4803419, which has been previously shown to be associated with increased plasma efavirenz level in different populations e.g., USA, Caucasian, Black, and Hispanic (Holzinger et al., 2012) and Black South African populations (Sinxadi et al., 2015), was found in high frequency in Pakistani population. Likewise, the rs3745274 has also been previously shown in strong association with increased plasma Efavirenz level in African and Serbian populations (Olagunju et al., 2014; Sinxadi et al., 2015). This variant represents the CYP2B6*6 allele, which decreases its enzymatic activity (Klein et al., 2005). This SNP, like rs4803419, is observed in the current study for Seraiki, Sindhi, and Balochi samples with higher allelic frequencies compared to other ethnic groups. These findings provide a foundation for future studies on the mechanism and effects of CYP2B6 polymorphisms in the Pakistani population and are a step towards personalized medicine.

Supplemental Information