Substrate-specific modulation of CYP3A4 activity by genetic variants of cytochrome P450 oxidoreductase

Effects of major transporter and metabolizing enzyme gene polymorphisms on carbamazepine metabolism in Chinese patients with epilepsy

AIM

The present study aimed to evaluate the effects of SNPs of major transporter and metabolizing enzyme genes on carbamazepine (CBZ) metabolism in Chinese patients with epilepsy.

MATERIALS & METHODS

For 210 epileptic patients treated with CBZ as monotherapy, nine SNPs in candidate genes ABCB1, CYP3A4, CYP3A5, POR and EPHX1 were analyzed by PCR-RFLP or direct sequencing. Serum concentrations of CBZ, carbamazepine-10,11-epoxide (CBZE) and carbamazepine-10,11-trans dihydrodiol (CBZD) were determined by HPLC. Dose-adjusted concentrations of CBZ (CDRCBZ), CBZE (CDRCBZE), CBZD (CDRCBZ D) and CBZD:CBZE ratio were used as evaluation parameters for CBZ metabolism.

RESULTS

The ABCB1 c.3435C>T was significantly associated with the CDR of CBZ and its major metabolites. CYP3A4*1G and CYP3A5*3 could influence CBZ metabolism, while POR*28 had no effect on it. The EPHX1 c.416A>G and c.128G>C variants were significantly associated with CBZD:CBZE ratio.

CONCLUSION

Our data suggest that certain polymorphisms of major transporter and metabolizing enzyme genes could in part influence interindividual variability of CBZ metabolism in Chinese patients with epilepsy.

Electron transfer by human wild-type and A287P mutant P450 oxidoreductase assessed by transient kinetics: functional basis of P450 oxidoreductase deficiency

Cytochrome P450 oxidoreductase (POR) is a 2-flavin protein that transfers electrons from NADPH via its FAD and FMN moieties to all microsomal cytochrome P450 enzymes, including steroidogenic and drug-metabolizing P450s. Defects in the POR gene can cause POR deficiency (PORD), manifested clinically by disordered steroidogenesis, genital anomalies and skeletal malformations. We examined the POR mutant A287P, which is the most frequent cause of PORD in patients of European ancestry and partially disrupts most P450 activities in vitro. Flavin content analysis showed that A287P is deficient in FAD and FMN binding, although the mutation site is distant from the binding sites of both flavins. Externally added flavin partially restored the cytochrome c reductase activity of A287P, suggesting that flavin therapy may be useful for this frequent form of PORD. Transient kinetic dissection of the reaction of POR with NADPH and the reduction in cytochrome c by POR using stopped-flow techniques revealed defects in individual electron transfer steps mediated by A287P. A287P had impaired ability to accept electrons from NADPH, but was capable of a fast FMN → cytochrome c electron donation reaction. Thus the reduced rates of P450 activities with A287P may be due to deficient flavin and impaired electron transfer from NADPH.

Pharmacogenetics in Jewish populations

Spanning over 2000 years, the Jewish population has a long history of migration, population bottlenecks, expansions, and geographical isolation, which has resulted in a unique genetic architecture among the Jewish people. As such, many Mendelian disease genes and founder mutations for autosomal recessive diseases have been discovered in several Jewish groups, which have prompted recent genomic studies in the Jewish population on common disease susceptibility and other complex traits. Although few studies on the genetic determinants of drug response variability have been reported in the Jewish population, a number of unique pharmacogenetic variants have been discovered that are more common in Jewish populations than in other major racial groups. Notable examples identified in the Ashkenazi Jewish (AJ) population include the vitamin K epoxide reductase complex subunit 1 (VKORC1) c.106G>T (p.D36Y) variant associated with high warfarin dosing requirements and the recently reported cytochrome P450 2C19 (CYP2C19) allele, CYP2C19*4B, that harbors both loss-of-function [*4 (c.1A>G)] and increased-function [*17 (c.-806C>T)] variants on the same haplotype. These data are encouraging in that like other ethnicities and subpopulations, the Jewish population likely harbors numerous pharmacogenetic variants that are uncommon or absent in other larger racial groups and ethnicities. In addition to unique variants, common multi-ethnic variants in key drug metabolism genes (e.g., ABCB1, CYP2C8, CYP2C9, CYP2C19, CYP2D6, NAT2) have also been detected in the AJ and other Jewish groups. This review aims to summarize the currently available pharmacogenetics literature and discuss future directions for related research with this unique population.

Functional POR A503V is associated with the risk of bladder cancer in a Chinese population

Human cytochrome P450 oxidoreductase (POR) plays important roles in the metabolism of exogenous carcinogens and endogenous sterol hormones. However, few studies have explored the association between POR variants and the risk of bladder cancer. In this study, we first sequenced all 16 POR exons among 50 randomly selected controls, and found three variants, rs1135612, rs1057868 (A503V) and rs2228104, which were then assessed the relation to risk of bladder cancer in a case-control study of 1,050 bladder cancer cases and 1,404 cancer-free controls in a Chinese population. People with A503V TT genotype have a decreased risk of bladder cancer in a recessive model (TT vs. CC/CT, OR = 0.73, 95% CI = 0.57–0.93), which was more pronounced among elderly male, non-smoking, subjects. Especially, A503V TT genotype showed a protective effect in the invasive tumor stage. Functional analysis revealed that A503V activity decreased in cytochrome c reduction (50.5 units/mg vs. 135.4 units/mg), mitomycin C clearance (38.3% vs. 96.8%), and mitomycin C-induced colony formation (78.0 vs 34.3 colonies per dish). The results suggested that POR A503V might decrease the risk of bladder cancer by reducing its metabolic activity, and should be a potential biomarker for predicting the susceptibility to human bladder cancer.

Multigene predictors of tacrolimus exposure in kidney transplant recipients

AIM

Determine the effect of the genetic variants beyond CYP3A5*3 on tacrolimus disposition.

PATIENTS & METHODS

We studied genetic correlates of tacrolimus trough concentrations with POR*28, CYP3A4*22 and ABCC2 haplotypes in a large, ethnically diverse kidney transplant cohort (n = 2008).

RESULTS

Subjects carrying one or more CYP3A5*1 alleles had lower tacrolimus trough concentrations (p = 9.2 × 10(-75)). The presence of one or two POR*28 alleles was associated with a 4.63% reduction in tacrolimus trough concentrations after adjusting for CYP3A5*1 and clinical factors (p = 0.037). In subset analyses, POR*28 was significant only in CYP3A5*3/*3 carriers (p = 0.03). The CYP3A4*22 variant and the ABBC2 haplotypes were not associated.

CONCLUSION

This study confirmed that CYP3A5*1 was associated with lower tacrolimus trough concentrations. POR*28 was associated with decreased tacrolimus trough concentrations although the effect was small possibly through enhanced CYP3A4 enzyme activity. CYP3A4*22 and ABCC2 haplotypes did not influence tacrolimus trough concentrations. Original submitted 19 December 2014; Revision submitted 2 April 2015.

Genetic variations in NADPH-CYP450 oxidoreductase in a Czech Slavic cohort

AIM

Estimating polymorphic allele frequencies of the NADPH-CYP450 oxidoreductase (POR) gene in a Czech Slavic population.

METHODS

The POR gene was analyzed in 322 individuals from a control cohort by sequencing and high resolution melting analysis.

RESULTS

We identified seven unreported SNP genetic variations, including two SNPs in the 5' flanking region (g.4965C>T and g.4994G>T), one intronic variant (c.1899-20C>T), one synonymous SNP (p.20Ala=) and three nonsynonymous SNPs (p.Thr29Ser, p.Pro384Leu and p.Thr529Met). The p.Pro384Leu variant exhibited reduced enzymatic activities compared with wild-type.

CONCLUSION

New POR variant identification indicates the number of uncommon variants might be specific for each subpopulation being investigated, particularly germane to the singular role that POR plays in providing reducing equivalents to all CYP450s in the endoplasmic reticulum. Original submitted 15 September 2014; Revision submitted 17 November 2014.

Regioselective hydroxylation of steroid hormones by human cytochromes P450

This article reviews in vitro metabolic activities [including Michaelis constants (Km), maximal velocities (Vmax) and Vmax/Km] and drug-steroid interactions [such as induction and cooperativity (activation)] of cytochromes P450 (P450 or CYP) in human tissues, including liver and adrenal gland, for 14 kinds of endogenous steroid compounds, including allopregnanolone, cholesterol, cortisol, cortisone, dehydroepiandrosterone, estradiol, estrone, pregnenolone, progesterone, testosterone and bile acids (cholic acid). First, we considered the drug-metabolizing P450s. 6β-Hydroxylation of many steroids, including cortisol, cortisone, progesterone and testosterone, was catalyzed primarily by CYP3A4. CYP1A2 and CYP3A4, respectively, are likely the major hepatic enzymes responsible for 2-/4-hydroxylation and 16α-hydroxylation of estradiol and estrone, steroids that can contribute to breast cancer risk. In contrast, CYP1A1 and CYP1B1 predominantly metabolized estrone and estradiol to 2- and 4-catechol estrogens, which are endogenous ultimate carcinogens if formed in the breast. Some metabolic activities of CYP3A4, including dehydroepiandrosterone 7β-/16α-hydroxylation, estrone 2-hydroxylation and testosterone 6β-hydroxylation, were higher than those for polymorphically expressed CYP3A5. Next, we considered typical steroidogenic P450s. CYP17A1, CYP19A1 and CYP27A1 catalyzed steroid synthesis, including hydroxylation at 17α, 19 and 27 positions, respectively. However, it was difficult to predict which hepatic drug-metabolizing P450 or steroidogenic P450 will be mainly responsible for metabolizing each steroid hormone in vivo based on these results. Further research is required on the metabolism of steroid hormones by various P450s and on prediction of their relative contributions to in vivo metabolism. The findings collected here provide fundamental and useful information on the metabolism of steroid compounds.

The influence of CYP3A, PPARA, and POR genetic variants on the pharmacokinetics of tacrolimus and cyclosporine in renal transplant recipients

Purpose

Tacrolimus (Tac) and cyclosporine (CsA) are mainly metabolized by CYP3A4 and CYP3A5. Several studies have demonstrated an association between the CYP3A5 genotype and Tac dose requirements. Recently, CYP3A4, PPARA, and POR gene variants have been shown to influence CYP3A metabolism. The present study investigated potential associations between CYP3A5*3, CYP3A4*22, PPARA c.209-1003G>A and c.208 + 3819A>G, and POR*28 alleles and dose-adjusted concentrations (C/D) of Tac and CsA in 177 renal transplant patients early post-transplant.

Methods

All patients (n = 177) were genotyped for CYP3A4*22, CYP3A5*3, POR*28, PPARA c.209-1003G>A, and PPARA c.208 + 3819A>G using real-time polymerase chain reaction (PCR) and melting curve analysis with allele-specific hybridization probes or PCR restriction fragment length polymorphisms (RFLP) methods. Drug concentrations and administered doses were retrospectively collected from patient charts at Oslo University Hospital, Rikshospitalet, Norway. One steady-state concentration was collected for each patient.

Results

We confirmed a significant impact of the CYP3A5*3 allele on Tac exposure. Patients with POR*28 and PPARA variant alleles demonstrated 15 % lower (P = 0.04) and 19 % higher (P = 0.01) Tac C₀/D respectively. CsA C₂/D was 53 % higher among CYP3A4*22 carriers (P = 0.03).

Conclusion

The results support the use of pre-transplant CYP3A5 genotyping to improve initial dosing of Tac, and suggest that Tac dosing may be further individualized by additional POR and PPARA genotyping. Furthermore, initial CsA dosing may be improved by pre-transplant CYP3A4*22 determination.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-014-1656-3) contains supplementary material, which is available to authorized users

Impact of POR*28 on the pharmacokinetics of tacrolimus and cyclosporine A in renal transplant patients

BACKGROUND

The P450 oxidoreductase (POR)*28 variant allele has been associated with altered cytochrome P450 3A enzyme activities. Both CYP3A5 and CYP3A4 are involved in the metabolism of calcineurin inhibitors and recent data show that POR*28 may explain part of the variability observed in tacrolimus (Tac) pharmacokinetics. The aim of this study was to investigate the impact of the POR*28 allele on Tac and cyclosporine A (CsA) immunosuppressive therapies.

METHODS

Kidney transplant recipients receiving either Tac (n = 184) or CsA (n = 174), participating in a prospective multicenter trial, were genotyped for POR*28, CYP3A4*22, and CYP3A5*3.

RESULTS

CYP3A5 expressers that were carriers of at least 1 POR*28 allele had a 16.9% decrease in dose-adjusted predose concentrations when compared CYP3A5 expressers that carried the POR*1/*1 genotype (P = 0.03), indicating an increased CYP3A5 activity for POR*28 carriers. In CYP3A5, nonexpressers carrying 2 POR*28 alleles, a 24.1% (confidence interval95% = -39.4% to -4.9%; P = 0.02) decrease in dose-adjusted predose concentrations was observed for Tac, suggesting higher CYP3A4 activity. For CsA, POR*28/*28 patients not expressing CYP3A5 and not carrying the CYP3A4*22 decrease-of-function allele showed 15% lower CsA dose-adjusted predose concentrations (P = 0.01), indicating also increased CYP3A4 activity. In both cohorts (ie, Tac and CsA), the POR*28 allele was neither associated with the incidence of delayed graft function nor with biopsy-proven acute rejection. These results were further confirmed in 2 independent cohorts.

CONCLUSIONS

Our results show that the POR*28 allele is associated with increased in vivo CYP3A5 activity for Tac in CYP3A5 expressers, whereas POR*28 homozygosity was associated with a significant higher CYP3A4 activity in CYP3A5 nonexpressers for both Tac and CsA.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Single-nucleotide polymorphisms in P450 oxidoreductase and peroxisome proliferator-activated receptor-α are associated with the development of new-onset diabetes after transplantation in kidney transplant recipients treated with tacrolimus

BACKGROUND

New-onset diabetes after transplantation (NODAT) is an important complication after kidney transplantation. The etiology of the malady is multifactorial and includes both environmental and genetic factors. NODAT is a polygenic disease and many single-nucleotide polymorphisms could constitute potential risk factors. Peroxisome proliferator-activated receptor α (PPARα) and P450 oxidoreductase (POR) play a central role in the control of energy metabolism in humans. Some recent data highlighted a possible functional impact of two single-nucleotide polymorphisms in PPARα (rs4253728 G>A and rs4823613 A>G) and one coding variant in POR (rs1057868; POR*28; A503V) on the activity of their respective encoded proteins. In the present study, we assessed the association between these variants and the risk of developing NODAT after kidney transplantation.

METHODS

Development of NODAT was investigated in 101 renal transplant recipients receiving tacrolimus-based immunosuppressive therapy. Patients were genotyped for PPARα and POR. The incidence of NODAT was compared between different genotypes. Kaplan-Meier and Cox's proportional-hazard analysis were used to evaluate the association of NODAT with potential risk factors. Potential nongenetic risk factors were also considered.

RESULTS

The PPARα rs4253728A>G and POR*28 variant alleles were both independently associated with an increased risk for NODAT with respective odds ratios of 8.6 [95% confidence interval (CI)=1.4-54.2; P=0.02] and 8.1 (95% CI=1.1-58.3; P=0.04). Other risk predictors included sex and body weight.

CONCLUSION

This candidate-gene study shows that polymorphisms in PPARα and POR might predispose patients being treated with tacrolimus to the development of NODAT after kidney transplantation. Patient management after organ transplantation might benefit from genotype data.

Pharmacogenomics of human P450 oxidoreductase

Cytochrome P450 oxidoreductase (POR) supports reactions of microsomal cytochrome P450 which metabolize drugs and steroid hormones. Mutations in POR cause disorders of sexual development. P450 oxidoreductase deficiency (PORD) was initially identified in patients with Antley–Bixler syndrome (ABS) but now it has been established as a separate disorder of sexual development (DSD). Here we are summarizing the work on variations in POR related to metabolism of drugs and xenobiotics. We have compiled mutation data on reported cases of PORD from clinical studies. Mutations found in patients with defective steroid profiles impact metabolism of steroid hormones as well as drugs. Some trends are emerging that establish certain founder mutations in distinct populations, with Japanese (R457H), Caucasian (A287P), and Turkish (399–401) populations showing repeated findings of similar mutations. Most other mutations are found as single occurrences. A large number of different variants in POR gene with more than 130 amino acid changes are now listed in databases. Among the polymorphisms, the A503V is found in about 30% of all alleles but there are some differences across different population groups.

Cytochrome P450 modified polycrystalline indium tin oxide film as a drug metabolizing electrochemical biosensor with a simple configuration

The development of a biocatalytic electrode consisting of cytochrome P450 (CYP) proteins would be a key technology with which to establish simple drug metabolizing biosensors or screening devices for drug inhibitors. We have successfully detected the direct electron transfer (DET) from a human CYP layer or a CYP microsome adsorbed on a bare indium tin oxide (ITO) film electrode without any modification layers and applied it to drug metabolism evaluation. We compared the electrocatalytic properties of the two ITO films with different surface nanostructures (polycrystalline or amorphous). CYP on polycrystalline ITO film enhanced the electron transfer rate of oxygen reduction about fifteen times more than with amorphous film. The polycrystalline ITO film was a suitable electrode for the adsorption of CYP proteins while maintaining efficient DET and enzymatic activity, probably because of its larger surface area and negatively charged surface. The oxygen reduction current at the polycrystalline ITO film electrodes had increased 3- to 4-fold, specifically coupled with the oxidation of drugs (testosterone and quinidine) by the monooxygenase activity of CYP. In contrast, the oxygen reduction current completely disappeared in the presence of the CYP inhibitor (ketoconazole). Similar results could be obtained from the CYP microsome with sufficiently clear responses. These results indicate that the CYP modified polycrystalline ITO electrode offers the potential for electrochemically evaluating CYP activity for drug metabolism with a simple configuration.

Pharmacogenetics of P450 oxidoreductase: implications in drug metabolism and therapy

The redox reaction of cytochrome P450 enzymes (CYP) is an important physiological and biochemical reaction in the human body, as it is involved in the oxidative metabolism of both endogenous and exogenous substrates. Cytochrome P450 oxidoreductase (POR) is the only obligate electron donor for all of the hepatic microsomal CYP enzymes. It plays a crucial role in drug metabolism and treatment by not only acting as an electron donor involved in drug metabolism mediated by CYP enzymes but also by directly inducing the transformation of some antitumor precursors. Studies have found that the gene encoding human POR is highly polymorphic, which is of considerable clinical significance as it affects the metabolism and curative effects of clinically used drugs. This review aims to discuss the effect of POR and its genetic polymorphisms on drug metabolism and therapy, as well as the potential mechanisms of POR pharmacogenetics.

Pharmacogenomics of Cytochrome P450 3A4: Recent Progress Toward the "Missing Heritability" Problem

CYP3A4 is the most important drug metabolizing enzyme in adult humans because of its prominent expression in liver and gut and because of its broad substrate specificity, which includes drugs from most therapeutic categories and many endogenous substances. Expression and function of CYP3A4 vary extensively both intra- and interindividually thus contributing to unpredictable drug response and toxicity. A multitude of environmental, genetic, and physiological factors are known to influence CYP3A4 expression and activity. Among the best predictable sources of variation are drug–drug interactions, which are either caused by pregnane X-receptor (PXR), constitutive androstane receptor (CAR) mediated gene induction, or by inhibition through coadministered drugs or other chemicals, including also plant and food ingredients. Among physiological and pathophysiological factors are hormonal status, age, and gender, the latter of which was shown to result in higher levels in females compared to males, as well as inflammatory processes that downregulate CYP3A4 transcription. Despite the influence of these non-genetic factors, the genetic influence on CYP3A4 activity was estimated in previous twin studies and using information on repeated drug administration to account for 66% up to 88% of the interindividual variation. Although many single nucleotide polymorphisms (SNPs) within the CYP3A locus have been identified, genetic association studies have so far failed to explain a major part of the phenotypic variability. The term “missing heritability” has been used to denominate the gap between expected and known genetic contribution, e.g., for complex diseases, and is also used here in analogy. In this review we summarize CYP3A4 pharmacogenetics/genomics from the early inheritance estimations up to the most recent genetic and clinical studies, including new findings about SNPs in CYP3A4 (*22) and other genes (P450 oxidoreductase (POR), peroxisome proliferator-activated receptor alpha (PPARA)) with possible contribution to CYP3A4 variable expression.

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Effect of CYP3A4*18B polymorphisms and interactions with OPRM1 A118G on postoperative fentanyl requirements in patients undergoing radical gastrectomy

The present study aimed to investigate the effect of cytochrome P450 3A4 (CYP3A4)*18B polymorphisms and the interaction of the µ opioid receptor gene (OPRM1) A118G and CYP3A4*18B polymorphisms on postoperative fentanyl analgesia in Chinese Han patients undergoing radical gastrectomy. In total, 97 patients scheduled to undergo radical gastrectomy under general anesthesia were enrolled in this study. Post‑operative patient‑controlled intravenous analgesia of fentanyl was administered as analgesia up to 48 h following surgery. Venous blood (2 ml) was obtained from each patient to measure the OPRM1 A118G and CYP3A4*18B genotypes. The differences in fentanyl consumption and adverse effects were compared among the genotypes at 24 and 48 h following surgery. In the first 48 h following surgery, patients in the CYP3A4*18B/*18B group consumed significantly less fentanyl compared with patients in the *1/*1 group (P=0.032). With regards to the joint genetic effect, during the 48‑h period, patients with AA and *1*18B polymorphisms received fewer fentanyl doses compared with those with AG and *1*1 (P=0.049), while patients with AG and *1*18B polymorphisms received significantly fewer fentanyl doses compared with those with AG and *1*1 (P=0.010), and patients with *18B*18B polymorphisms received significantly fewer fentanyl doses compared with those with AA and *1*1 (P=0.024) or those with AG and *1*1 polymorphisms (P=0.006). No correlation between OPRM1 A118G and CYP3A4*18B and postoperative nausea, vomiting and dizziness was found. Results demonstrated that 48 h following surgery, patients with the CYP3A4*18B/*18B genotype required less fentanyl than patients with the CYP3A4*1/*1 genotype to control pain. Additionally, the combined genotype of CYP3A4*18B and OPRM1 A118G may affect fentanyl doses administered for pain control, but not postoperative nausea, vomiting and dizziness.

P450 oxidoreductase: genotyping, expression, purification of recombinant protein, and activity assessments of wild-type and mutant protein

P450 oxidoreductase (POR) is the flavoprotein that transfers electrons from NADPH to microsomal cytochrome P450 enzymes and to some other proteins. Protocols for genotyping human POR for common polymorphisms are described. Expression in E. coli of recombinant human POR, its purification, and different methods of assessing the effect of amino-acid sequence variants of POR on the activity of various cytochromes P450 are also described.

Identification of six novel P450 oxidoreductase missense variants in Ashkenazi and Moroccan Jewish populations

BACKGROUND

The enzyme NADPH-P450 oxidoreductase (POR) is the main electron donor to all microsomal CYPs. The possible contribution of common POR variants to inter- and intra-individual variability in drug metabolism is of great pharmacogenetic interest.

AIM

To search for POR polymorphic alleles and estimate their frequencies in a Jewish population.

MATERIALS & METHODS

We analyzed the POR gene in 301 Ashkenazi and Moroccan Jews.

RESULTS

A total of 30 POR SNPs were identified, nine in the noncoding regions and 21 in the protein-coding regions (ten synonymous, 11 missense). Six of these missense variants are previously undescribed (S102P, V164M, V191M, D344N, E398A and D648N).

CONCLUSION

The data collected in this study on missense POR SNPs, interpreted in light of the crystallographic structure of human POR, indicate that some POR missense variants may be potential biomarkers for future POR pharmacogenetic screening.

Effect of the P450 oxidoreductase 28 polymorphism on the pharmacokinetics of tacrolimus in Chinese healthy male volunteers

PURPOSE

To assess the influence of the P450 oxidoreductase 28 SNP (POR 28) on tacrolimus pharmacokinetics in the Chinese population.

METHODS

Seventy-one healthy Chinese volunteers enrolled in the study received an oral dose of 2 mg of tacrolimus after providing written informed consent. CYP3A5 3 was genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and POR*28 was genotyped by PCR-direct sequencing. Tacrolimus whole blood concentrations were determined by ultra performance liquid chromatography-tandem mass spectrometry and the pharmacokinetics analyses was evaluated by nonparametric methods.

RESULTS

The frequencies of CYP3A5 3 and POR 28 allele were 73.3 % and 29.6 %, respectively. No significant differences existed in tacrolimus pharmacokinetics between the POR 28 CC homozygotes (n = 32) and the POR 28 T allele (n = 39) in all subjects. The mean tacrolimus AUC0-24, AUC0-∞ and Cmax for the POR 28 CC (n = 14) homozygotes in CYP3A5 expressers (CYP3A5 1/ 1 or 1/ 3 genotype) were 71.5 ± 38.9 h ng/mL, 94.3 ± 58.3 h ng/mL and 17.6 ± 9.8 ng/mL, which were much higher than the POR 28 CT heterozygotes (n = 17) of 46.7 ± 24.9 h ng/mL, 57.4 ± 33.9 h ng/mL and 11.2 ± 6.4 ng/mL (P < 0.05, respectively). We did not observe any significant differences in tacrolimus pharmacokinetics between the POR 28 CC homozygotes (n = 18) and POR 28 T carriers (n = 22) in CYP3A5 nonexpressers (CYP3A5 3/ 3 carriers).

CONCLUSIONS

The POR 28 CT genotype presented a significantly lower level of tacrolimus exposure (AUC, Cmax) compared with the POR 28 CC genotype in CYP3A5-expressing subjects. It suggested that the POR 28 genetic polymorphism might also be responsible for the marked interindividual variability of tacrolimus besides the CYP3A5 3 genetic polymorphism.

Influence of various polymorphic variants of cytochrome P450 oxidoreductase (POR) on drug metabolic activity of CYP3A4 and CYP2B6

Cytochrome P450 oxidoreductase (POR) is known as the sole electron donor in the metabolism of drugs by cytochrome P450 (CYP) enzymes in human. However, little is known about the effect of polymorphic variants of POR on drug metabolic activities of CYP3A4 and CYP2B6. In order to better understand the mechanism of the activity of CYPs affected by polymorphic variants of POR, six full-length mutants of POR (e.g., Y181D, A287P, K49N, A115V, S244C and G413S) were designed and then co-expressed with CYP3A4 and CYP2B6 in the baculovirus-Sf9 insect cells to determine their kinetic parameters. Surprisingly, both mutants, Y181D and A287P in POR completely inhibited the CYP3A4 activity with testosterone, while the catalytic activity of CYP2B6 with bupropion was reduced to approximately ∼70% of wild-type activity by Y181D and A287P mutations. In addition, the mutant K49N of POR increased the CLint (Vmax/Km) of CYP3A4 up to more than 31% of wild-type, while it reduced the catalytic efficiency of CYP2B6 to 74% of wild-type. Moreover, CLint values of CYP3A4-POR (A115V, G413S) were increased up to 36% and 65% of wild-type respectively. However, there were no appreciable effects observed by the remaining two mutants of POR (i.e., A115V and G413S) on activities of CYP2B6. In conclusion, the extent to which the catalytic activities of CYP were altered did not only depend on the specific POR mutations but also on the isoforms of different CYP redox partners. Thereby, we proposed that the POR-mutant patients should be carefully monitored for the activity of CYP3A4 and CYP2B6 on the prescribed medication.

Altered human CYP3A4 activity caused by Antley-Bixler syndrome-related variants of NADPH-cytochrome P450 oxidoreductase measured in a robust in vitro system

NADPH-cytochrome P450 oxidoreductase (CYPOR) variants have been described in patients with perturbed steroidogenesis and sexual differentiation, related to Antley-Bixler syndrome (ABS). It is important to determine the effect of these variants on CYP3A4, the major drug-metabolizing cytochrome P450 (P450) in humans. In this study, 12 CYPOR_ABS variants were separately coexpressed with CYP3A4 in a robust in vitro system to evaluate the effects of these variants on CYP3A4 activity in a milieu that recapitulates the stoichiometry of the mammalian systems. Full-length CYPOR variants were coexpressed with CYP3A4, resulting in relative expression levels comparable to those found in hepatic tissue. Dibenzylfluorescein (DBF), a CYP3A-specific reporter substrate (Biopharm Drug Dispos 24:375-384, 2003), was used to compare the variants and wild-type (WT) CYPOR activities with that of human liver microsomes. CYP3A4, combined with WT CYPOR, demonstrated kinetic parameters (k(cat) and K(m)) equal to those for pooled human liver microsomes. CYPOR variants Y181D, Y459H, V492E, L565P, and R616X all demonstrated maximal loss of CYP3A4 catalytic efficiency, whereas R457H and G539R retained ∼10 and 30% activities, respectively. Conversely, variants P228L, M263V, A287P, and G413S each showed WT-like capacity (k(cat)/K(m)), with the A287P variant being formerly reported to exhibit substantially lower catalytic efficiency. In addition, Q153R exhibited 60% of WT CYPOR capacity to support the DBF O-debenzylation reaction, contradicting increased catalytic efficiency (k(cat)/K(m)) relative to that for the WT, reported previously. Our data indicate the importance of use of simulated, validated in vitro systems, employing full-length proteins with appropriate stoichiometric incorporation of protein partners, when pharmacogenetic predictions are to be made for P450-mediated biotransformation.

Identification of cytochrome P450 oxidoreductase gene variants that are significantly associated with the interindividual variations in warfarin maintenance dose

Cytochrome P450 oxidoreductase (POR) is required for drug metabolism by all microsomal cytochrome P450 enzymes. The aim of this study was to investigate whether any of the common single nucleotide polymorphisms (SNPs) in the POR gene and its flanking intergenic sequences correlate with interindividual variations in the warfarin maintenance dose (which is determined partly by rates of warfarin metabolism) in patients undergoing anticoagulation therapy. Warfarin dose and patients' demographic and clinical information were collected from 124 patients, who had attained a stable warfarin dose while receiving treatment at the Stratton VA Medical Center. Genomic DNAs were isolated from blood samples and were genotyped for 15 SNPs (including 10 SNPs on the POR gene). Association analysis was performed on 122 male patients by linear regression. Simple regression analysis revealed that vitamin K epoxide reductase complex subunit 1 (VKORC1) -1639A>G, CYP2C9*2, CYP2C9*3, age, and chronic aspirin therapy were significantly associated with warfarin dose. In contrast, multiple regression analysis revealed that, in addition to several known factors contributing to the variations in warfarin maintenance dose (VKORC1 -1639A>G, CYP2C9*2, CYP2C9*3, CYP4F2 rs2108622, and chronic aspirin therapy), three common POR SNPs (-173C>A, -208C>T, and rs2868177) were also significantly associated with variations in warfarin maintenance dose. These results indicate, for the first time, that three common SNPs in the POR gene may contribute to the interindividual variability in warfarin maintenance dose. Further studies on functional characterization of the POR SNPs identified, including their impact on the in vivo metabolism of additional drugs, are needed.

The P450 oxidoreductase *28 SNP is associated with low initial tacrolimus exposure and increased dose requirements in CYP3A5-expressing renal recipients

AIM

Recently a SNP of the gene encoding P450 oxidoreductase (POR*28; rs1057868C>T) has been associated with increased in vivo CYP3A activity using midazolam as a drug probe. Because tacrolimus is metabolized by CYP3A isoenzymes, this SNP might affect tacrolimus pharmacokinetics.

MATERIALS & METHODS

To test this hypothesis we performed a study in a cohort of 298 de novo renal allograft recipients stratified according to CYP3A5 genotype, which has a known impact on tacrolimus pharmacokinetics.

RESULTS

We found that in CYP3A5 expressers (CYP3A5*1 allele carriers) receiving a standard loading dose of 0.2 mg/kg, POR*28T allele carriers had lower tacrolimus C₀ levels in the first days post-transplantation and reached target C₀ levels significantly later as compared with POR*28CC homozygous patients. The POR*28T allele carriers had significantly higher tacrolimus dose requirements throughout the first year. In CYP3A5 nonexpressers (CYP3A5*3/*3) the POR*28 SNP did not affect tacrolimus pharmacokinetics.

CONCLUSION

These data indicate that the POR*28 SNP is associated with additional increases in early tacrolimus dose-requirements in patients carrying a CYP3A5*1 allele.

High-yield expression of a catalytically active membrane-bound protein: human P450 oxidoreductase

P450 oxidoreductase (POR) is a two-flavin protein that reduces microsomal P450 enzymes and some other proteins. Preparation of active bacterially expressed human POR for biochemical studies has been difficult because membrane-bound proteins tend to interact with column matrices. To reduce column-protein interactions and permit more vigorous washing, human POR lacking 27 N-terminal residues (N-27 POR) was modified to carry a C-terminal Gly3His6-tag (N-27 POR-G3H6). When expressed in Escherichia coli, N-27 POR-G3H6 could be purified to apparent homogeneity by a modified, single-step nickel-nitrilotriacetic acid affinity chromatography, yielding 31 mg POR per liter of culture, whereas standard purification of native N-27 POR required multiple steps, yielding 5 mg POR per liter. Both POR proteins had absorption maxima at 375 and 453 nm and both reduced cytochrome c with indistinguishable specific activities. Using progesterone as substrate for bacterially expressed purified human P450c17, the Michaelis constant for 17α-hydroxylase activity supported by N-27 POR or N-27 POR-G3H6 were 1.73 or 1.49 μm, and the maximal velocity was 0.029 or 0.026 pmol steroids per picomole P450 per minute, respectively. Using 17-hydroxypregnenolone as the P450c17 substrate, the Michaelis constant for 17,20 lyase activity using N-27 POR or N-27 POR-G3H6 was 1.92 or 1.89 μm and the maximal velocity was 0.041 or 0.042 pmol steroid per picomole P450 per minute, respectively. Thus, N-27 POR-G3H6 is equally active as native N-27 POR. This expression and purification system permits the rapid preparation of large amounts of highly pure, biologically active POR and may be generally applicable for the preparation of membrane-bound proteins.

Consequences of POR mutations and polymorphisms

P450 oxidoreductase (POR) transports electrons from NADPH to all microsomal cytochrome P450 enzymes, including steroidogenic P450c17, P450c21 and P450aro. Severe POR mutations A287P (in Europeans) and R457H (in Japanese) cause the Antley-Bixler skeletal malformation syndrome (ABS) plus impaired steroidogenesis (causing genital anomalies), but the basis of ABS is unclear. We have characterized the activities of ∼40 POR variants, showing that assays based on P450c17 activities, but not cytochrome c assays, correlate with the clinical phenotype. The human POR gene is highly polymorphic: the A503V sequence variant, which decreases P450c17 activities to ∼60%, is found on ∼28% of human alleles. A promoter polymorphism (∼8% of Asians and ∼13% of Caucasians) at -152 reduces transcriptional activity by half. Screening of 35 POR variants showed that most mutants lacking activity with P450c17 or cytochrome c also lacked activity to support CYP1A2 and CYP2C19 metabolism of EOMCC (a fluorogenic non-drug substrate), although there were some remarkable differences: Q153R causes ABS and has ∼30% of wild-type activity with P450c17 but had 144% of WT activity with CYP1A2 and 284% with CYP2C19. The effects of POR variants on CYP3A4, which metabolizes nearly 50% of clinically used drugs, was examined with multiple, clinically relevant drug substrates, showing that A287P and R457H dramatically reduce drug metabolism, and that A503V variably impairs drug metabolism. The degree of activity can vary with the drug substrate assayed, as the drugs can influence the conformation of the P450. POR is probably an important contributor to genetic variation in both steroidogenesis and drug metabolism.

Effects of genetic variants of human P450 oxidoreductase on catalysis by CYP2D6 in vitro

OBJECTIVES

Cytochrome P450 (P450) oxidoreductase (POR) donates electrons to all microsomal cytochrome P450s, including drug-metabolizing and steroidogenic enzymes. Severe POR mutations cause skeletal malformations and disordered steroidogenesis. The POR polymorphism A503V is found on approximately 28% of human alleles and decreases activities of CYP3A4 and steroidogenic CYP17, but not the activities of steroidogenic CYP21 or drug-metabolizing CYP1A2 and CYP2C19. CYP2D6 metabolizes about 25% of clinically used drugs; we assessed the capacity of POR variants to support the activities of human CYP2D6.

METHODS

N-27 forms of wildtype (WT), Q153R, A287P, R457H and A503V POR, and WT CYP2D6 were expressed in Escherichia coli. POR proteins in bacterial membranes were reconstituted with purified CYP2D6. Support of CYP2D6 was measured by metabolism of EOMCC (2H-1-benzopyran-3-carbonitrile,7-(ethoxy-methoxy)-2-oxo-(9Cl)), dextromethorphan and bufuralol. Michaelis constant (K(m)) and maximum velocity (V(max)) were determined in three triplicate experiments for each reaction; catalytic efficiency is expressed as V(max)/K(m).

RESULTS

Compared with WT POR, disease-causing POR mutants A287P and R457H supported no detectable CYP2D6 activity with EOMCC, but A287P supported approximately 25% activity with dextromethorphan and bufuralol. Q153R had increased function with CYP2D6 (128% with EOMCC, 198% with dextromethorphan, 153% with bufuralol). A503V supported decreased CYP2D6 activity: 85% with EOMCC, 62% with dextromethorphan and 53% with bufuralol.

CONCLUSION

POR variants have different effects depending on the substrate metabolized. Disease-causing POR mutations R457H and A287P had poor activities, suggesting that diminished drug metabolism should be considered in affected patients. The common A503V polymorphism impaired CYP2D6 activities with two commonly used drugs by 40-50%, potentially explaining some genetic variation in drug metabolism.

The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders

Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. Whereas most endocrine texts discuss adrenal, ovarian, testicular, placental, and other steroidogenic processes in a gland-specific fashion, steroidogenesis is better understood as a single process that is repeated in each gland with cell-type-specific variations on a single theme. Thus, understanding steroidogenesis is rooted in an understanding of the biochemistry of the various steroidogenic enzymes and cofactors and the genes that encode them. The first and rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a single enzyme, P450scc (CYP11A1), but this enzymatically complex step is subject to multiple regulatory mechanisms, yielding finely tuned quantitative regulation. Qualitative regulation determining the type of steroid to be produced is mediated by many enzymes and cofactors. Steroidogenic enzymes fall into two groups: cytochrome P450 enzymes and hydroxysteroid dehydrogenases. A cytochrome P450 may be either type 1 (in mitochondria) or type 2 (in endoplasmic reticulum), and a hydroxysteroid dehydrogenase may belong to either the aldo-keto reductase or short-chain dehydrogenase/reductase families. The activities of these enzymes are modulated by posttranslational modifications and by cofactors, especially electron-donating redox partners. The elucidation of the precise roles of these various enzymes and cofactors has been greatly facilitated by identifying the genetic bases of rare disorders of steroidogenesis. Some enzymes not principally involved in steroidogenesis may also catalyze extraglandular steroidogenesis, modulating the phenotype expected to result from some mutations. Understanding steroidogenesis is of fundamental importance to understanding disorders of sexual differentiation, reproduction, fertility, hypertension, obesity, and physiological homeostasis.

Impact of CYP3A4*1G polymorphism on metabolism of fentanyl in Chinese patients undergoing lower abdominal surgery

PURPOSE

This study aimed to investigate the impact of CYP3A4*1G genetic polymorphism on metabolism of fentanyl in Chinese patients undergoing lower abdominal surgery.

METHODS

176 patients receiving elective lower abdominal surgery under general anesthesia were recruited into this study. Genotyping of CYP3A4*1G was carried out by direct sequencing. The plasma fentanyl concentration was detected 30 min after anesthesia induction by high performance liquid chromatography-ultraviolet ray (HPLC-UV). The visual analog scale (VAS) was used for pain evaluation at rest during patient-controlled analgesia (PCA) treatment 0 h, 12 h and 24 h after operation. PCA fentanyl consumption and adverse effects were recorded during the first 24 h after surgery.

RESULTS

The frequency of CYP3A4*1G variant allele was 0.227 (80/352, 95% CI 0.165, 0.289) in these patients. After grouping according to the genotype of CYP3A4*1G, plasma fentanyl concentration in the *1/*1 variant (wild-type) group (12.8±6.5 ng/ml) was significantly lower than that in the *1/*1G group (16.8±9.0 ng/ml, P<0.01) and the *1G/*1G group (28.1±9.5 ng/ml, P<0.01). Patients in the *1G/*1G group (247.1±73.2 μg) consumed significantly less fentanyl than that in either the wild-type group (395.0±138.5 μg) or the *1/*1G group (359.8±120.2 μg) (P<0.01). There was a significant correlation between plasma fentanyl concentration and PCA fentanyl consumption (r=-0.552, P<0.001).

CONCLUSIONS

CYP3A4*1G polymorphism is related to the pharmacokinetics of fentanyl, and patients with CYP3A4*1G variant A allele have a lower metabolic rate of fentanyl. The specific CYP3A4*1G polymorphism may predict the individual requirement of fentanyl.