Molecular Evolution of the CYP2D Subfamily in Primates: Purifying Selection on Substrate Recognition Sites without the Frequent or Long-Tract Gene Conversion

Abstract

The human cytochrome P450 (CYP) 2D6 gene is a member of the CYP2D gene subfamily, along with the CYP2D7P and CYP2D8P pseudogenes. Although the CYP2D6 enzyme has been studied extensively because of its clinical importance, the evolution of the CYP2D subfamily has not yet been fully understood. Therefore, the goal of this study was to reveal the evolutionary process of the human drug metabolic system. Here, we investigate molecular evolution of the CYP2D subfamily in primates by comparing 14 CYP2D sequences from humans to New World monkey genomes. Window analysis and statistical tests revealed that entire genomic sequences of paralogous genes were extensively homogenized by gene conversion during molecular evolution of CYP2D genes in primates. A neighbor-joining tree based on genomic sequences at the nonsubstrate recognition sites showed that CYP2D6 and CYP2D8 genes were clustered together due to gene conversion. In contrast, a phylogenetic tree using amino acid sequences at substrate recognition sites did not cluster the CYP2D6 and CYP2D8 genes, suggesting that the functional constraint on substrate specificity is one of the causes for purifying selection at the substrate recognition sites. Our results suggest that the CYP2D gene subfamily in primates has evolved to maintain the regioselectivity for a substrate hydroxylation activity between individual enzymes, even though extensive gene conversion has occurred across CYP2D coding sequences.

Introduction

Cytochrome P450 (CYP) enzymes catalyze the oxidation of exogenous and endogenous substrates, such as drugs, environmental chemicals, plant metabolites, fatty acids, and steroids. The enzymes are membrane-binding heme proteins, and they are widely expressed in prokaryotes and eukaryotes (Nelson et al. 1993). All the eukaryotic CYPs are considered to have been derived from a single common ancestor (Nelson 1999), although an ancestral enzyme has not yet been determined (Nelson et al. 2013; Sezutsu et al. 2013). A large variety of CYP proteins are necessary to interact with a large number of substrates, and the variety of substrates is considered to reflect the species-specificity for the habitat, environment, and diet (Gonzalez and Nebert 1990). In humans, Homo sapiens, the CYP gene family consists of 57 functional genes and 58 pseudogenes, whereas in mice, Mus musculus, there are 102 functional genes and 88 pseudogenes (Nelson et al. 2004). The putative number of CYP genes varies between organisms as shown in the Cytochrome P450 Homepage (Nelson 2009), including the Pacific transparent sea squirt, Ciona savignyi, and rice, Oryza sativa, with 97 and 323 functional genes, respectively, and as low as three expressed CYP genes in the budding yeast, Saccharomyces cerevisiae. Their genes are all members of the CYP superfamily and can be classified into two groups, with respect to their pattern of evolution (Thomas 2007; Gotoh 2012; Kawashima and Satta 2014). Products of the first group primarily function in metabolizing the endogenous substrates, and those genes undergo few gene duplications over long periods of time. On the other hand, products of the second group mainly metabolize exogenous substrates, and those genes have undergone frequent gene duplications or gene loss (birth-and-death evolution).

The hominin CYP2D subfamily, a member of the second group, consists of a functional CYP2D6 and two paralogs, CYP2D7 and CYP2D8 (Nelson 2009), and the latter two genes often are not functional in some species (Yasukochi and Satta 2011). The human CYP2D6 enzyme has a high affinity for alkaloids, and it detoxifies alkaloids (Fonne-Pfister and Meyer 1988). The enzyme is also important for metabolizing about 25% of commonly used drugs, such as antidepressants, β-blockers, and antiarrhythmics (Ingelman-Sundberg 2005). The three human CYP2D genes are located within a contiguous region of about 45 kb on chromosome 22 (Kimura et al. 1989). The nucleotide sequences of the two pseudogenes (CYP2D7P and CYP2D8P) are similar to that of CYP2D6. The CYP2D7P gene has a single frameshift mutation in the first exon, which causes a premature stop codon downstream. Alternative mRNAs from this pseudogene have been identified in human breast (Huang et al. 1997), lung (Huang et al. 1997), and brain (Pai et al. 2004; Gaedigk et al. 2005). Of them, a functional transcript was observed in some individuals in an Indian population (Pai et al. 2004). On the other hand, there is no observation of mRNA from CYP2D8P gene.

We previously identified the gene organization of the CYP2D subfamily in nonhuman primates, including the chimpanzee, Pan troglodytes, Sumatran orangutan, Pongo pygmaeus abelii, rhesus monkey, Macaca mulatta, and white-tufted-ear marmoset, Callithrix jacchus (Yasukochi and Satta 2011). Our findings reveal that the CYP2D7 gene has been duplicated from CYP2D6 in a stem lineage of humans and great apes, and the origin of CYP2D6 and CYP2D8P genes in the human genome can be traced back to a stem lineage of the New World monkeys and Catarrhini at the latest. Two functional CYP2D isoforms have been observed in macaques and marmosets, although the copy number of the CYP2D genes is different among individuals in the case of macaques (Hichiya et al. 2004; Uno et al. 2010, 2011; Cooke et al. 2012). Furthermore, phylogenetic analyses for CYP2D genes suggest that the origin of the CYP2D subfamily can be traced back to before the divergence between amniotes and amphibians (Yasukochi and Satta 2011).

Substrate recognition sites (SRSs) are important for the catalysis of target substrates, and SRSs in the CYP2 family have been determined based on their homology with bacterial CYPs (Gotoh 1992). The 3D structure of the human CYP2D6 molecule revealed several functional important sites (Wolff et al. 1985; Modi et al. 1996; de Groot et al. 1999; Rowland et al. 2006; Unwalla et al. 2010). Rowland et al. (2006) obtained the crystal structure of CYP2D6 with 3.0-Å resolution and revealed that the basic structure of the CYP2D6 is similar to other CYP members. Unwalla et al. (2010) predicted the sites of the reaction center for known CYP2D6 substrates, based on a model of ligand-bound CYP2C5 complexes as a template. Wolff et al. (1985) and de Groot et al. (1999) have reported that most substrates of CYP2D6 have a basic nitrogen at a distance of about 5–7 Å (or 10 Å) from the site of oxidation and can interact with Asp-301or Glu-216 (de Groot et al. 1999).

The human CYP2D6 enzyme has been studied across various disciplines due to its clinical importance, including pharmacokinetics, pharmacogenetics, structural biology, and clinical medicine. Therefore, CYP2D6 genetic variation in human populations has been also investigated extensively (Xie et al. 2001; Bradford 2002; Mizutani 2003; Raimundo et al. 2004; Sistonen et al. 2007; Fuselli et al. 2010; Contreras et al. 2011). More than 100 human CYP2D6 alleles, including intron variations, in samples from around the world are described in the Human Cytochrome P450 Allele Nomenclature Database (http://www.cypalleles.ki.se/cyp2d6.htm). Additionally, unequal crossover events often cause copy number variation (CNV) of the gene (Heim and Meyer 1992; Panserat et al. 1995; Steen et al. 1995; Løvlie et al. 1996; Gaedigk et al. 2010). CNV and deleterious mutations of functional CYP2D6 genes are categorized into four phenotypes based on their ability (rate) to metabolize drugs: A poor (PM), intermediate, efficient (EM), or ultrarapid metabolizer (UM) (Zanger et al. 2004). The PMs do not have any functional CYP2D6, and thus lack the ability to metabolize drugs. EMs correspond to a wild-type copy of CYP2D6 with normal function. On the other hand, UMs can have up to 13 copies of CYP2D6 gene and can rapidly metabolize medicines and thus are unaffected by drugs. The frequency of PMs is relatively higher in Europe than in other areas, whereas UMs are mainly found in North Africa (Sistonen et al. 2007).

Gene conversion among three CYP2D genes has occurred in human populations, and this probably also affects the genetic variation in drug metabolism (Hanioka et al. 1990; Heim and Meyer 1992). In human evolution, as people have only recently started to use drugs, drugs cannot be an evolutionary force for CYP2D evolution. Instead, the CYP2D enzyme must have been essential for the detoxification of foods containing plant toxins. Our previous study suggested that lineage-specific CYP2D gene expansion in vertebrates can reflect the requirement in an environment (Yasukochi and Satta 2011). This suggests that birth-and-death processes in CYP2D subfamily are associated with differences of feeding strategies among organisms. Nevertheless, there is little knowledge about the evolutionary pattern or mode of the CYP2D subfamily in primates. We here examine molecular evolution of the subfamily by comparing 14 CYP2D sequences from humans to New World monkey genomes.

Materials and Methods

Collection of Nucleotide Sequence Data of CYP2D Genes

We previously identified the gene organization of the human, chimpanzee, Sumatran orangutan, rhesus monkey, and white-tufted-ear marmoset CYP2D genes, based on nucleotide sequence data in the genome database and sequences determined by direct sequencing (Yasukochi and Satta 2011). In addition, the CYP2D6 sequence of the pygmy chimpanzee (Koch WH et al., unpublished data; GenBank accession number, DQ282163) was newly obtained from GenBank. They are human and orangutan CYP2D6, CYP2D7P, and CYP2D8P, pygmy chimpanzee (Pan paniscus) CYP2D6, common chimpanzee CYP2D6, CYP2D7, and CYP2D8P; and rhesus monkey and marmoset CYP2D6 and CYP2D8. Sequences of the common chimpanzee (from now on we will refer to the common chimpanzee as the chimpanzee.) CYP2D6 and CYP2D7 genes were used according to the annotation of our previous study (Yasukochi and Satta 2011). In addition, three CYP2D homologs from each of the cynomolgus monkey (Macaca fascicularis) and marmoset were retrieved from the NCBI (National Center for Biotechnology Information) database (http://www.ncbi.nlm.nih.gov/).

As mentioned above, in humans there are more than 100 CYP2D6 alleles that are categorized into four phenotypes. Thus, the elucidation of CYP2D molecular evolution is complicated by the presence of multiple CYP2D6 alleles in a species. Here, we use the CYP2D6*1 allele as the representative CYP2D6 allele because this allele has the highest allele frequency in human populations (Sistonen et al. 2007). For putative pseudogenes, the character “P” was added after their gene names. Sequence data used in this study and analyses for each of the sequences (see below) are summarized in supplementary table S1, Supplementary Material online.

Alignment of CYP2D Sequences and Test for Gene Conversion between Paralogous Genes

The alignment of nucleotide sequence data and translation into amino acids were performed using MEGA v5.10 (Tamura et al. 2011). The alignment was manually modified, and positions of deletions or insertions (indels) were determined. In the analysis, the pseudogene sequences were also translated according to the frame of CYP2D6, although they did not encode proteins, and transmembrane domains were predicted using SOSUI (Hirokawa et al. 1998).

The statistical test for detecting gene conversion was performed using the Two-Sample Runs Test (Takahata 1994) and GENECONV program version 1.81 (Sawyer 1989) which was run with default settings. We applied the Runs Test to CYP2D6 and CYP2D7(P) genomic sequences of only human and chimpanzee origin because we were specifically focusing on the human CYP2D6. Indels were also treated as informative sites and were used because they were important for detecting gene conversion. The global test for recombination or gene conversion events was used with 10,000 permutations of the sequence alignment to assess the significance. Allelic pairs with Sim-P value < 0.05 were excluded from further analyses.

Phylogenetic Analyses for CYP2D Sequences

The neighbor-joining (NJ) trees (Saitou and Nei 1987) for nucleotide sequences of intron 2 fragment in CYP2D genomic regions and amino acid sequences of the entire coding sequences (CDSs) were reconstructed based on the Kimura two-parameter (Kimura 1980) and p-distance models (Nei and Kumar 2000), respectively. The NJ tree was also constructed based on synonymous substitutions in the CYP2D CDSs. Bootstrap analyses for their NJ trees were performed using 1,000 replications. The maximum likelihood (ML) and maximum parsimony (MP) trees were implemented in the PHYLIP 3.69 package (Felsenstein 2009), and constructed on the basis of amino acid sequences at the SRSs. Bootstrap analyses were of 100 and 1,000 replicates for the ML and MP trees, respectively. For the ML tree, global rearrangement was allowed, and the input order of OTU was randomized with three jumbles during randomization. For the MP tree, the jumble option was set ten times. The ML and MP trees were visualized using the TreeView program version 1.6.6 (Page 1996). All positions containing any indels and missing data were eliminated from the data set (complete deletion option).

Measure of Genetic Distance among CYP2D Sequences and Estimation of Natural Selection

The genetic distances of silent substitutions, which include both synonymous substitutions in the CDSs and changes in noncoding positions, were calculated by DnaSP 5.10.01 software (Librado and Rozas 2009). A sliding window analysis for the genetic distances of silent substitutions was also implemented in the DnaSP. The number of nonsynonymous substitutions per nonsynonymous site (d_N) and synonymous substitutions per synonymous site (d_S) was estimated with MEGA v5.10, using the distance of the modified Nei–Gojobori (Zhang et al. 1998) with the Jukes–Cantor correction (Jukes and Cantor 1969) for multiple substitutions. Standard errors were calculated using 1,000 bootstrapping replicates. Ratios of d_N/d_S for nine functional CYP2D6/7/8 and six CYP2D6 CDSs were separately calculated at the SRSs and non-SRSs as well as over the entire coding region. In addition, we estimated ω ratios (=d_N/d_S) for 14 CYP2D CDSs including pseudogenes with the ML method by using CODEML in the PAML program version 4.8 (Yang 2007). Several models that estimate values of ω under different scenarios of selective pressure (M0, M1a, M2a, M7, and M8) were considered in this program. A Bayesian approach (Bayes empirical Bayes) in CODEML was used to predict positively selected codons. All indels and codons including nonsense and frameshift mutations were eliminated from the data set for the estimates of ω ratios.

Amino Acid Sequences Variation for CYP2D Sequences in Primates

Amino acid variability among the 14 CYP2D CDSs was calculated using a Wu–Kabat plot (Wu and Kabat 1970). The Wu–Kabat plot estimates the variability for each amino acid position, which is measured by the number of amino acids at a site divided by the maximum frequency of amino acid at the sites. Wu and Kabat (1970) originally evaluated the variability based on the sequence data for a single species, but here we used the sequences of orthologous genes from six different species, a method we refer to as “the modified Wu–Kabat plot.” In this approach, if only a single amino acid appears at a site, the variability is the minimum value of 1, whereas if six different species have six different amino acid at a site, the value is 6/(1/6), or 36, at a maximum.

Estimation of Nonfunctionalization Time of CYP2D8 genes

To estimate nonfunctionalization times (T_P) of CYP2D8P genes in humans and orangutans, we calculated relative times of T_P to species divergence time between the Hominidae and Old World monkeys (T_S). The relative times (T_P/T_S) were obtained from the number of nonsynonymous and synonymous substitutions between a functional gene and pseudogene according to the equation T_P/T_S = 2 (d_N/d_S − f)/(1 − f), by modifying the method of Sawai et al. (2008). The parameter f, which is the neutral fraction of substitutions, is determined by f = d_Nf/d_Sf, where d_Nf and d_Sf are nonsynonymous and synonymous divergence for the functional gene, respectively. We set the parameter T_S to 35 Ma (Satta et al. 2004). The standard errors of T_P/T_S were acquired by bootstrapping codons of CYP2D8P 10,000 times under a given f. In simulations, T_P/T_S sometimes became negative, attributed to cases where d_N/d_S < f, due to insufficient time passed for accumulation of nonsynonymous substitutions since the loss of CYP2D8 function, suggesting that it is equivalent to the case of T_P/T_S = 0.

Results

Phylogenetic Relationships among CYP2D Genes in Primates

The NJ tree based on synonymous substitutions among 20 CYP2D CDSs of seven primates (fig. 1) showed that orthologous genes did not form a single clade. In the hominid, CYP2D6 and CYP2D7 were intermingled together, but CYP2D8 did not cluster with them. In macaques, rhesus and cynomolgus monkey sequences grouped in a macaque-specific cluster. In marmosets, CYP2D6 and CYP2D8 clustered distinctly away from other primates. From this phylogeny, gene diversification was presumed to have occurred at least five times (diamonds 1–5 in fig. 1). Alternatively, the phylogeny shows that in orangutans, macaques, and marmosets, the conversion between CYP2D6 and CYP2D8 occurred in each species (diamonds 2, 4, and 5 in fig. 1). As the history of CYP2D6 and CYP2D7(P) in humans and chimpanzees is not that simple, further phylogenetic analyses were performed to identify the time and location of ectopic gene exchanges.

Fig. 1.—

NJ tree of the CYP2D genes of primates, based on synonymous substitutions of the entire coding region. The modified Nei–Gojobori method was used as a distance measure with Jukes–Cantor correction. The internal node number represents bootstrap values (>50%). The diamond represents a gene divergence. The CYP2D sequences indicated in bold were obtained from the whole-genome sequence data, and nucleotide sequences of some fragments were determined in our previous study (Yasukochi and Satta 2011). The mouse Cyp2d22 was used as an outgroup sequence. Hosa, human; papa, pygmy chimpanzee; patr, chimpanzee; poab, orangutan; mamu, rhesus monkey; mafa, cynomolgus monkey; caja, white-tufted-ear marmoset; mumu, house mouse.

Gene Conversion between CYP2D Genes in Humans and Chimpanzees

For simplicity, CYP2D6 and CYP2D7(P) genes in humans and chimpanzees were designated as H6, H7, C6, and C7, respectively. Informative sites of the four sequences were classified into three categories, supporting each of the three topologies. They were ((H6, C6), (H7, C7)), ((H6, H7), (C6, C7)), and ((H6, C7), (C6, H7)) (fig. 2A) and were named α, β, and γ, respectively. As CYP2D6 and CYP2D7(P) duplication occurred before the divergence of humans and great apes, the topology α represents an original phylogenetic relationship. On the other hand, the topology β suggests gene conversion within each species, whereas the topology γ might be caused by recurrent substitutions. In fact, a site showing the topology γ was found only once (at the 2213 site) in the 6,924 bp compared, whereas α and β sites showed a block distribution (α1, α2, and β1–β3) (fig. 2B and supplementary fig. S1, Supplementary Material online).

Fig. 2.—

(A) The possible topologies at parsimony informative sites. In analysis, α topology was expected when a gene duplication event occurred before speciation, whereas β topology was expected when gene conversion or recombination occurred after its duplication. H6, human CYP2D6 gene; H7, human CYP2D7P; C6, chimpanzee CYP2D6; C7, chimpanzee CYP2D7. (B) The alignment of nucleotide sequences based on parsimony informative sites in the CYP2D6 and CYP2D7(P) of the human and chimpanzee. The white column indicates α topology. The filled black column indicates β topology. The filled gray column indicates γ topology.

We examined interlocus gene conversion (or recombination) using both Takahata’s Two-Sample Runs Test and the GENECONV program. The Runs Test exhibited significant runs of both α and β informative sites (table 1) with P values of <0.001. The analysis by GENECONV was performed using all the CYP2D genes of five primates, and it suggested significant gene conversion events in three β regions (supplementary table S2, Supplementary Material online).

Table 1.

Two-Sample Runs Test for Detection of Gene Conversion

Sister Group Status	A	B	P	Z	ka
(β, γ), α	26	201	≈6.9 × 10−25	10	5
			≈7.0 × 10−26	9	4
			≈2.6 × 10−27	8	4
			≈1.9 × 10−28	7	3
			≈5.1 × 10−30	6	3
			≈2.4 × 10−31	5	2
			≈4.2 × 10−33	4	2
			≈9.5 × 10−35	3	1
			≈8.5 × 10−37	2	1
			≪0.001	≤10
(α, γ), β	202	25	≈5.9 × 10−25	10	5
			≈6.3 × 10−26	9	4
			≈2.3 × 10−27	8	4
			≈1.7 × 10−28	7	3
			≈4.7 × 10−30	6	3
			≈2.3 × 10−31	5	2
			≈4.1 × 10−33	4	2
			≈9.5 × 10−35	3	1
			≈8.5 × 10−37	2	1
			≪0.001	≤10

Note.—α, a sister group state, [(H6, C6),(H7, C7))]; β, a sister group state, [(H6, H7),(C6, C7))]; γ, a sister group state, [(H6, C7),(H7, C6))]. H6, human CYP2D6; H7, human CYP2D7P; C6, chimpanzee CYP2D6; C7, chimpanzee CYP2D7. A, site defined as one that does not support a certain sister group status; B, site defined as one that supports the specified sister group status; Z, continuum of the same sister group status (run).

^aZ = 2 k when the number of Z is even, and Z = 2 k + 1, when the number of Z is odd.

To determine the timing of the conversion, we constructed a synonymous tree for each of the three β regions of H6, H7, C6, and C7, with orangutan CYP2D6 and CYP2D7P (O6 and O7) as outgroups (fig. 3). In the trees for the three β regions, orthologous genes did not form a single cluster. For the β1 region, the distance between the orthologs, H6 and C6, was d_H6C6 = 0.013 ± 0.004 and comparable to the genome average (1.23%; Fujiyama et al. 2002). For the paralogous genes, d_C6C7 (0.006 ± 0.003) was smaller than the genomic average. Both C6 and C7 were closer to H6 (0.014 ± 0.004) than to H7 (0.029 ± 0.006), suggesting that it is likely that C6 converts C7. These observations indicate that gene conversion between CYP2D6 and CYP2D7(P) occurred at least once in the β1 region in an ancestral population of chimpanzees and humans. In the β2 and β3 regions, on the other hand, independent gene conversion seemed to have occurred in humans and chimpanzees, respectively. In addition, the average genetic divergences between H6/C6 and H7/C7 in the β2 and β3 regions were 0.031 ± 0.007 and 0.029 ± 0.007, respectively. The divergence also reflected the gene conversion between CYP2D6 and CYP2D7(P) in an ancestor population of humans and chimpanzees.

Fig. 3.—

NJ trees based on the genetic distances of three β regions in CYP2D6 and CYP2D7(P) genes of the human, chimpanzee, and orangutan. The genetic distances (three decimal places) were based on silent substitutions, which include synonymous substitutions in coding regions and changes in noncoding regions. H6, human CYP2D6 gene; H7, human CYP2D7P; C6, chimpanzee CYP2D6; C7, chimpanzee CYP2D7; O6, orangutan CYP2D6; O7, orangutan CYP2D7P.

Large Genetic Distances in the CYP2D Region among Primates

We examined genetic divergence in the fragment of intron 2, which appeared to be scarcely affected by gene conversion across the CYP2D sequences of four Catarrhini species examined (supplementary fig. S2, Supplementary Material online). The recombination signal was not detected in intron 2 through both Runs test and GENECONV, but genetic distances (d_{_intron2}) among sequences in the region were still large. For example, d_{_intron2} between the human and orangutan CYP2D6 was 0.08 ± 0.020 (supplementary table S3A, Supplementary Material online). A sliding window analysis helped elucidate the large genetic distances across the CYP2D genomic region (supplementary fig. S3, Supplementary Material online). To examine whether the genetic divergence was enhanced uniquely in the CYP2D region, we calculated the d_S of the TCF20 (transcription factor 20) gene that was located adjacent to the CYP2D8(P) gene. The genetic distances among primate TCF20 sequences were much lower than d_{_intron2} of CYP2D (P < 0.001). The value of d_S between human and orangutan TCF20 sequences was 0.03 ± 0.002 (supplementary table S3B, Supplementary Material online), in good agreement with the genome average (about 3%, Stewart and Disotell 1998; Satta et al. 2004; Peng et al. 2009). This result suggests that the large divergence is specific to the CYP2D nonconverted region.

As methylation of CpG sites often results in the large divergence between sequences, we compared GC content and the proportion of CpG sites in the CYP2D with those in TCF20 CDSs. The GC contents of CYP2D CDSs and partial intron 2 sequences ranged from 63% to 69%, whereas those of the TCF20 gene were approximately 52%. The average proportion of CpG sites in the entire CYP2D gene was much higher than that in the TCF20 (P < 0.001; CYP2D, 6.4%; TCF20, 1.8%). In fact, when CpG methylation-related sites (CG, TG, and CA) were removed from CYP2D CDSs, their genetic distances became lower, for example, d_S = 0.017 ± 0.012 between H6 and C6 intron 2 sequences (supplementary table S4, Supplementary Material online). These results suggest that genetic distances in the CYP2D region have been uniquely enhanced by the high mutation rate at the CpG sites.

Amino Acid Sequences Variation for CYP2D Subfamily in Primates

The putative amino acid sequences of the CYP2D genes including pseudogenes were aligned. The aligned region is approximately 500 residues long (see supplementary fig. S4, Supplementary Material online). The amino acid sequences within some regions under the functional and structural constraints, such as heme-binding regions, were relatively conserved among the 14 CYP2D genes from the five primates. On the other hand, the sequence similarities of the transmembrane region were rather low.

The amino acid sequences at SRSs between CYP2D7P and CYP2D8P genes in Hominidae appeared to be more similar to those between CYP2D6 and CYP2D7P. In contrast, at non-SRSs, amino acid sequences of CYP2D6 and CYP2D7P were more similar to each other than those of CYP2D7P and CYP2D8P (supplementary fig. S4, Supplementary Material online). In the rhesus monkey and marmoset, amino acid sequences between CYP2D6 and CYP2D8 genes were more similar at non-SRSs than at SRSs. To confirm this observation, NJ trees of SRSs and non-SRSs for amino acid sequences and that of intron regions for nucleotide sequences were compared (fig. 4). In the SRS tree, all the CYP2D6 genes from primates were monophyletic from the CYP2D7(P) and CYP2D8(P) (fig. 4A). The MP and ML trees also supported the same topology even though an outgroup sequence was changed (supplementary fig. S5, Supplementary Material online). The NJ trees of non-SRSs and intron regions showed that paralogous genes of each species formed an intermingled cluster for Hominidae (fig. 4B and C).

Fig. 4.—

Three phylogenetic trees of primate CYP2D genes. (A) NJ tree based on amino acid sequences at SRSs. (B) NJ tree based on amino acid sequences at non-SRSs. (C) NJ tree based on nucleotide sequences in intron regions. A p-distance was used as a distance measure for amino acid sequences (A and B). A genetic distance of nucleotide sequences was calculated by using the Kimura two-parameter model (C). Diamond represents a gene divergence. Only bootstrap values over 50% are shown in the NJ tree. The mouse Cyp2d22 was used as an outgroup sequence. Hosa, human; papa, pygmy chimpanzee; patr, chimpanzee; poab, orangutan; mamu, rhesus monkey; caja; white-tufted-ear marmoset; mumu, house mouse.

The variability within the CYP2D6 amino acid sequences was evaluated using a modified Wu–Kabat plot (See Materials and Methods) (fig. 5). The level of variability was not significantly different between SRSs and non-SRSs. High heterogeneities of amino acid variability were observed around αB and αJ’ helix tail ends and the β2-2 sheet, suggesting that a functional constraint was not so strong in these regions.

Fig. 5.—

The variability level at amino acid residues among the CYP2D6 genes in six primates by modified Wu–Kabat plot. Gray boxes indicate the SRSs identified by Gotoh (1992). The ordinate axis represents the variability of amino acid residues. The abscissa axis represents the position of the human CYP2D6 molecule.

Estimation of Nonfunctionalization Time of CYP2D8 Enzymes in Hominidae

A putative nonsense mutation in the CYP2D8P gene (site 344 between SRS-4 and SRS-5) was shared between human and chimpanzee lineages (fig. 6 and supplementary fig. S4, Supplementary Material online), but it was not shared with orangutans. Additional six independent nonsense and frameshift mutations were found in each of the human, chimpanzee, and orangutan CYP2D8P sequences. Based on these observations, pseudogenization occurred independently in a human/chimpanzee ancestor and in an orangutan lineage. No frameshift and nonsense mutations, however, were observed in the rhesus monkey or marmoset CYP2D8 sequences, suggesting that they are functional. The single independent frameshift mutation of CYP2D7P was detected in human and orangutan, but not in the chimpanzee sequence.

Fig. 6.—

The chronological order of evolutionary events in the primate CYP2D gene cluster, based on the results of this study. The number in a box represents the CYP2D isoforms. A cross mark signifies a loss of function (pseudogene). The number of the site on a box represents a putative deleterious mutation site. A filled arrow and stop mark represent frameshift and nonsense mutations, respectively. The number of the site on the bottom represents the total number of their mutations.

To confirm the relaxation of functional constraint in the CYP2D8P pseudogenes, we estimated ω ratios under different selective scenarios for the specific lineages by the ML methods, based on the expected topology of CYP2D8(P) gene tree, (marmoset, (rhesus monkey, (orangutan, (human, chimpanzee)))). The analysis was performed in the similar manner of Zhao et al. (2010). The average ω value across all branches was 0.34 (table 2A), and the ω values on branches of human/chimpanzee/orangutan (ω = 1.00, table 2B) CYP2D8P genes were higher than those across other branches (ω = 0.02, table 2B and C).

Table 2.

Estimates of ω Ratios for the Five Primate CYP2D8(P) Genes under Various Models

	Model	np	ω	Likelihood	Model Comparison	P Value
A	All branches same ω	10	0.34	−3,220
B	Hominidae branches ω2, other branches ω1	13	ω1 = 0.02, ω2 = 1.00	−3,205	B versus A	3.8 × 10−8
C	Hominidae branches ω2 = 1, other branches ω1	12	ω1 = 0.02, ω2 = 1.00	−3,205	C versus B	1.0

Note.—np, number of parameters.

It is interesting to know when the human and orangutan CYP2D8P genes have lost their function. The relative time (T_P/T_S) of pseudogenization of the gene was calculated using the method described in Materials and Methods (and see supplementary fig. S6, Supplementary Material online). We did not consider gene conversion in CYP2D8P from the CYP2D6 and CYP2D7(P) genes, because CYP2D8P genes in Hominidae consistently formed clusters distinct from the other CYP2D genes in α and β regions (supplementary fig. S1, Supplementary Material online). The relative times (T_P/T_S) were 0.50 and 0.16 in human and orangutan CYP2D8Ps, respectively. Bootstrap analyses showed that mean and standard errors of T_P/T_S values were 0.54 ± 0.29 and 0.21 ± 0.20 in human and orangutan genes, respectively. The calibration time corresponding to T_S was set to be the divergence time of 35 Ma between Hominidae and Old World monkeys (Satta et al. 2004). The pseudogenization times of the human and orangutan CYP2D8Ps were estimated to be about 17.5 Ma (19 ± 10 Ma) and 5.6 Ma (7.4 ± 7 Ma), respectively. Although both estimates have large standard errors, the orangutan CYP2D8 enzyme has lost its function more recently than its counterpart in humans and chimpanzees.

Selection Pressure Operating on CYP2D Genes

We calculated the mean d_N and d_S values for about 1,500-bp CDSs of functional CYP2D6/D7/D8 genes for six primates (table 3). We then examined the ratio of d_N/d_S in SRSs and non-SRSs separately. In the SRSs, the ratios were 0.79 among the three CYP2D genes and 0.48 among only CYP2D6 genes, whereas those in non-SRSs were 0.32 and 0.24, respectively. The mean d_N value of SRSs among the three CYP2D genes was the highest (0.10 ± 0.017), and that of non-SRSs among CYP2D6 genes was the lowest (0.03 ± 0.004). A chi-square test showed no statistically significant differences in d_N and d_S values between SRSs and non-SRSs (supplementary table S5, Supplementary Material online). However, d_N values at SRSs among functional CYP2D genes were significantly higher than those at non-SRSs (P = 5.8 × 10⁻¹⁰ by Wilcoxon signed-rank test), although the length of SRSs was short.

Table 3.

Mean d_S and d_N Distances among the Functional CYP2D6/D7/D8 Genes and among CYP2D6 Genes

	No. of OTU	SRSs		Non-SRSs		Entire Coding Region
		$\overline{d_{\text{N}}}$	$\overline{d_{\text{N}}/d_{\text{S}}}$	$\overline{d_{\text{N}}}$	$\overline{d_{\text{N}}/d_{\text{S}}}$	$\overline{d_{\text{N}}}$	$\overline{d_{\text{S}}}$	$\overline{d_{\text{N}}/d_{\text{S}}}$
Primate functional CYP2D6/D7/D8 genesa	9	0.10 ± 0.017	0.79*	0.04 ± 0.004	0.32*	0.05 ± 0.005	0.12 ± 0.010	0.39*
Primate CYP2D6 genes	6	0.05 ± 0.012	0.48*	0.03 ± 0.004	0.24*	0.03 ± 0.004	0.11 ± 0.010	0.28*

Note.—The d_S and d_N values are calculated by the modified Nei–Gojobori method with Jukes–Cantor (R = 2.25; Zhang et al. 1998).

^aExcluding CYP2D7P genes of the human and orangutan and CYP2D8P genes of Hominidae.

*P < 0.05 (d_N > d_S; Wilcoxon signed-rank test).

By using the PAML program with different selective scenarios (models M0–M8), we estimated values of ω for functional CYP2D genes in five primates examined (supplementary table S6, Supplementary Material online). In the program, proportions P₀ of sites with ω < 1 (0 < ω < 1) were more than 75% whereas proportions P₂ of site with ω > 1 were less than 2%.

Discussion

Gene Conversion in the CYP2D Cluster in Primates, but Not at the SRSs

Although some previous studies have reported that gene conversion affects evolution of CYP2D genes in humans (Kimura et al. 1989; Gonzalez and Nebert 1990; Masimirembwa et al. 1996), the phylogenetic tree based on amino acids at SRSs in this study indicates little effect of gene conversion between SRS sequences of CYP2D6 and CYP2D8. The SRSs identified by Gotoh (1992) were used in the present analysis, but de Graaf et al. (2007) have also proposed six SRSs. We reconstructed the NJ tree by using amino acids of the recognition sites identified by de Graaf et al. (2007), and confirmed the topological differences between SRSs and non-SRSs (data not shown). Although the definition of SRSs was slightly different between Gotoh (1992) and de Graaf et al. (2007), it is clear that the pattern of nucleotide substitution accumulation is different between the SRSs and non-SRSs based on both definitions.

In the comparison of amino acid sequences among CYP2D genes of five primates examined, the sequences of CYP2D8 in the rhesus monkey and marmoset showed more similarities with those of human CYP2D6 (H6) and CYP2D7P (H7) than the human CYP2D8P (H8), for example, 92% between H6 and rhesus monkey CYP2D8 (R8), 91% between H7 and R8, and 89% between H8 and R8. However, CYP2D8(P) genes in all the five primates share two Alu elements, AluSX and AluSg4, in intron 1, whereas there is no Alu element in intron 1 of all CYP2D6 and CYP2D7P genes (Yasukochi and Satta 2011). In addition, the phylogenetic tree of intron 2 fragment, which is scarcely affected by gene conversion, shows that CYP2D8(P) sequences form a distinct clade from CYP2D6 and CYP2D7(P) sequences. Therefore, the CYP2D7 gene is likely to have been duplicated from the CYP2D6 after the divergence of hominid species and Old World monkeys at the latest.

Difference in Metabolic Activity Level of CYP2D Enzymes among Primates

Our findings suggest that SRSs in CYP2D6 and CYP2D8 are scarcely affected by gene conversion to retain substrate specificity for each of their isoforms. CYP2D6 and CYP2D8 in the cynomolgus monkey metabolize the same compounds as humans, namely bufuralol and dextromethorphan, although their drug-metabolizing activity levels are different from those in humans (Uno et al. 2010). In addition, the CYP2D enzyme activity in macaques is more efficient than that in humans. In the marmoset, CYP2D19, a putative CYP2D6 ortholog, exhibits no activity for debrisoquine 4-hydroxylase but has high activity for debrisoquine 5-, 6-, 7-, and 8-hydroxylase, whereas CYP2D30, a putative CYP2D8 ortholog, exhibits high activity for the former and low activity for the latter (Cooke et al. 2012). These results suggest that the functional difference between CYP2D6 and CYP2D8 molecules is likely regioselectivity (and may be stereoselectivity) rather than the metabolism of different substrates. Amino acid sequences at SRSs have been likely maintained by purifying selection to retain this regioselectivity even though gene conversion affects the other regions of the gene.

Our results show that macaques and marmosets have at least two functional CYP2D isoforms, whereas Hominidae species appear to have one functional isoform only. Nishimuta et al. (2011) measured the relative activity of cynomolgus monkey CYP2D6 and CYP2D8 to human CYP2D6 by using the intrinsic clearance (CL_int) value, which is commonly used to measure hepatic drug metabolic activity, and revealed that the CL_int in monkeys was higher than in the human. This result indicates more effective metabolism in monkeys. Although the actual activity in nonhuman Hominidae is unknown, human CYP2D enzyme had less activity than that of monkeys. It is interesting to know when this decrease in activity occurred.

Enhanced Genetic Distances Caused by the Methylation of CpG Sites

Genetic distances at intron 2 that appeared unaffected by gene conversion between CYP2D orthologous genes in primates were higher than expected, although all informative sites support the ordinary gene-cluster tree. On the other hand, the distances of TCF20 gene located adjacent to the CYP2D region were in good agreement with the expectation of species divergence. In this study, we found that the substitution rate was specifically enhanced in CYP2D genes only (but not adjacent genes). This increased the mutation rate in the CYP2D genes could be due to CpG site methylation as GC content is higher in these genes relative to the TCF20 gene. However, such CpG sites have been normally conserved by purifying selection if the gene is important (Uno and Osada 2011). The degeneration of CpG sites was possibly caused by a loss of functional constraint in the coding region of a gene. The one possible cause for relaxation of functional constraint on CYP2D in hominoids is discussed below.

Pseudogenization of the CYP2D Gene in Hominidae

A nonsense mutation in CYP2D8Ps was shared in the human and chimpanzee, but two frameshift mutations were independently found in the orangutan CYP2D8P, showing that CYP2D8Ps have independently lost the function in common ancestors of human/chimpanzee and orangutan lineages. The pseudogenization times of human/chimpanzee and orangutan CYP2D8Ps were estimated to be about 17.5 and 5.6 Ma, respectively. Previous studies reported that the divergence times of humans from orangutans were estimated at about 18 Ma (Satta et al. 2004; Steiper and Young 2006). Therefore, our estimation of the pseudogenization time suggests that the CYP2D8 in human/chimpanzee may have immediately lost its function after the divergence of Hominidae species.

The average ω value across all branches in a tree of 14 CYP2D genes including both functional genes and pseudogenes estimated by CODEML was significantly lower than unity (P < 1.4 × 10⁻⁷ by likelihood-ratio test), indicating that purifying selection has generally acted on CYP2D genes. In addition, a two-ratio model exhibited that the ω value of Hominidae CYP2D8P clade was unity. Therefore, the evolutionary mode of the CYP2D8P genes may have switched to neutrality after the recent pseudogenization.

In general, animals are thought to perceive bitter or astringent taste as a method to avoid toxins in foods. The Old World monkeys tend to take in more antifeedants, such as condensed tannins and monoterpenes, compared with chimpanzees (Wrangham et al. 1998). Monkeys with smaller body size are thought to have an advantage for detoxifying plant metabolites, compared with humans and great apes, because the expression rates of microsomal enzyme activity negatively correlates with mammalian body size (Lambert 2002).

Ueno (2001) proposed the idea that primates have developed two different strategies to avoid unpredictable toxic risk. The first is the evolution of a detoxification system (physical adaptation), and the second is an enlargement of daily food diversity to dilute the harmful effect of toxin in foods (behavioral adaptation). Although both strategies are not mutually exclusive, this study supports that monkeys have utilized the former strategy primarily, whereas humans and great apes have utilized the latter. In fact, great apes probably ingest a variety of foods in a day to reduce the toxic effects of plants, although it has been reported that alkaloid-containing plants account for 14% of the diets of the chimpanzees examined (Simmen et al. 1999). Hominids are more likely to show insensitivity to the toxic effects than monkeys because of larger body size. It is possible that some CYP2D genes are pseudogenized in humans and great apes due to the relaxation of functional constraints in the detoxification system, whereas functional CYP2D genes have been retained in macaques and marmosets to efficiently detoxify substrates.

Comparison of Amino Acid Residues at SRSs among CYP2D Molecules in Primates

To evaluate differences in CYP2D function, we compared amino acid residues at the SRSs of each CYP2D gene among primate species. Phe-120 at SRS-1 and Phe-483 at SRS-6 are important for CYP2D6 ligand binding (de Graaf et al. 2007), and in fact, Phe-120 was conserved in the primates examined, with the exception of Ile-120 in the chimpanzee CYP2D7 (Patr-CYP2D7) (supplementary fig. S7A, Supplementary Material online). As Phe-120 is responsible for direct π–π interactions with aromatic rings of substrates (de Graaf et al. 2007), the Patr-CYP2D7 with Ile-120 may lack metabolic activity or have altered regioselectivity in metabolism. The comparison of amino acids at SRSs also shows that Phe-483 is conserved in all primate CYP2D molecules. The side chains of Phe-481 and Phe-483 are involved in interacting with an aromatic ring of a substrate although the importance for direct substrate recognition by these sites is ambiguous (de Groot et al. 1999; Rowland et al. 2006). Although all CYP2D6 and Patr-CYP2D7 molecules have Phe-481, all CYP2D8 molecules have Val-481 (supplementary fig. S7B, Supplementary Material online). In addition, Ala-482 is conserved in all CYP2D6s and Patr-CYP2D7, whereas Gly-482 is present in all CYP2D8s (supplementary fig. S7C, Supplementary Material online). As amino acids at 481 and 482 are unique for CYP2D6s/CYP2D7 and CYP2D8s, there is a possibility that those specificities confer differences in ligand regioselectivity in metabolism.

Purifying Selection Has Acted on SRSs within the CYP2D Subfamily

All the values of the d_N/d_S ratios for functional CYP2D genes in primates were significantly less than unity at SRSs (P = 9.1 ×10⁻⁴ by Wilcoxon signed-rank test), non-SRSs (P = 2.9 ×10⁻¹¹), and over the entire coding region (P = 2.9 × 10⁻¹¹). These results support purifying selection on the SRSs and non-SRSs within the CYP2D subfamily among different species. However, positive selection has been reported to act on SRSs in four CYP2 subfamilies (CYP2A, 2B, 2C, and 2D) (Gotoh 1992), suggesting that different evolutionary forces have acted on SRSs between CYP2D orthologs and paralogs. Based on the higher d_N/d_S ratios and d_N values at SRSs among functional CYP2D genes relative to non-SRSs, we assume that the functional constraint on SRSs of the CYP2D subfamily is weaker than that on non-SRSs.

The inference of ω ratios under different scenarios of selective pressure in the PAML program showed a remarkably larger proportions of sites with ω < 1 (>75%) than those with ω > 1 (<2%). This result supports that amino acid sequences on the SRSs are totally conserved by purifying selection; however, some codons in the region may have been affected by diversifying selection. The Bayesian approach in the PAML analysis identified five positively selected sites (sites 167, 304, 373, 478, and 479; posterior probability, P > 0.95) under models M2a and M8 that allow for positive selection (in this case probably diversifying selection). Of the five codons, four are located within the SRSs, suggesting that the functional constraint of SRSs has been relaxed or diversifying selection has acted on their codons.

Although humans normally have only one functional CYP2D gene, mice have nine functional Cyp2d genes (Nelson et al. 2004), and there are 12 rodent Cyp2d genes that show evidence of positive selection (Thomas 2007). In contrast, we have found evidence of purifying selection in primate CYP2D genes. As a hypothesis, drug-metabolizing enzymes may have evolved by “plant–animal warfare” (Gonzalez and Nebert 1990; Heim and Meyer 1992), and the human CYP2D6 enzyme also has a very high affinity for plant toxins such as alkaloids (Fonne-Pfister and Meyer 1988). Ingelman-Sundberg (2005) has assumed that the difference in selective pressures for dietary detoxification potential resulted in the variance of the number of functional CYP2D genes between the human and mouse. The results of our study suggest that each CYP2D member in primates has evolved to retain different regioselectivity for a substrate in order to keep this function, and the moderate purifying selection may operate on each CYP2D gene, even though gene conversion widely affects the CYP2D genomic region.

In this study, we found that gene conversion and purifying selection have affected homogeneities between CYP2D paralogous genes in primates. In contrast, amino acids at SRSs have been moderately diversified between their paralogous genes. According to the number of functional CYP2D genes, macaques and marmosets appear to metabolize toxins in food more efficiently than Hominidae. Our findings, moreover, suggest that the methylation of CpG sites enhanced the divergence of CYP2D genes in the primates examined. In the near future, the development of a genome database for a variety of species will enable the analysis of intraspecies or interspecies genome-wide sequences.

Supplementary Material

Supplementary tables S1–S6 and figures S1–S7 are available at Genome Biology and Evolution online (http://www.gbe.oxfordjournals.org/).