Learning gene regulatory networks from next generation sequencing data.

In recent years, next generation sequencing (NGS) has gradually replaced microarray as the major platform in measuring gene expressions. Compared to microarray, NGS has many advantages, such as less noise and higher throughput. However, the discreteness of NGS data also challenges the existing statistical methodology. In particular, there still lacks an appropriate statistical method for reconstructing gene regulatory networks using NGS data in the literature. The existing local Poisson graphical model method is not consistent and can only infer certain local structures of the network. In this article, we propose a random effect model-based transformation to continuize NGS data and then we transform the continuized data to Gaussian via a semiparametric transformation and apply an equivalent partial correlation selection method to reconstruct gene regulatory networks. The proposed method is consistent. The numerical results indicate that the proposed method can lead to much more accurate inference of gene regulatory networks than the local Poisson graphical model and other existing methods. The proposed data-continuized transformation fills the theoretical gap for how to transform discrete data to continuous data and facilitates NGS data analysis. The proposed data-continuized transformation also makes it feasible to integrate different types of data, such as microarray and RNA-seq data, in reconstruction of gene regulatory networks.

Cytochrome P450 2B6*5 Increases Relapse after Cyclophosphamide-Containing Conditioning and Autologous Transplantation for Lymphoma.

Cyclophosphamide (Cy) is a prodrug that depends on bioactivation by hepatic cytochrome P450 (CYP) enzymes for its cytotoxicity. We evaluated the influence of single nucleotide polymorphisms (SNPs) of CYP enzymes on the efficacy of autologous hematopoietic cell transplantation (HCT) for lymphoma. SNPs of 22 genes were analyzed in 93 patients with Hodgkin (n = 52) and non-Hodgkin lymphoma (n = 41) treated with high-dose Cy followed by autologous HCT between 2004 and 2012. Preparative regimens contained Cy (120 mg/kg) combined with carmustine/etoposide (n = 61) or Cy (6000 mg/m(2)) with total body irradiation (n = 32). Lack of complete remission as measured by pretransplant positron emission tomography was the sole clinical factor associated with increased risk of relapse (HR, 2.1). In genomic analysis, we identified a single SNP (rs3211371) in exon 9 (C > T) of the CYP2B6 gene (allele designation 2B6*5) that significantly impacted patient outcomes. After adjusting for disease status and conditioning regimen, patients with the CYP2B6*1/*5 genotype had a higher 2-year relapse rate (HR, 3.3; 95% CI, 1.6 to 6.5; P = .041) and decreased overall survival (HR, 13.5; 95% CI, 3.5 to 51.9; P = .008) than patients with the wild-type allele. Two-year progression-free survival for patients with 2 hypofunctional CYP2B6 variant genotypes (*5 and *6) was only 11% (95% CI, 1% to 39%) compared with 67% (95% CI, 55% to 77%) for patients with the wild-type CYP2B6*1 allele in exon 9. Our results suggest that CYP2B6 SNPs influence the efficacy of high-dose Cy and significantly reduce the success of autologous HCT for lymphoma patients with the CYP2B6*5 variant.

MicroRNA-mRNA Pairs Associated with Outcome in AML: From In Vitro Cell-Based Studies to AML Patients.

Cytarabine is the primary chemotherapeutic agent used for treatment of acute myeloid leukemia (AML). Disease relapse after initial remission remains one of the most pressing therapeutic challenges in the treatment of AML. Relapsed disease is often resistant to cytarabine and subsequent salvage therapy is ineffective. Recent studies have shown that some microRNAs (miRNAs) are associated with prognosis, but have not yet explored the role of miRNAs in cellular response to cytarabine. We identified 20 miRNAs that associate with the in vitro cytarabine chemo-sensitivity or apoptotic response of eight AML cell lines. Out of the 20 miRNAs, data on 18 miRNAs was available in AML patients from The Cancer Genome Atlas database. Our stepwise-integrated analyses (step 1 - miRNA-target mRNA that were significantly correlated in AML patients; step 2 - mRNAs from step 1 with significant association with overall survival (OS)) identified 23 unique miRNA-mRNA pairs predictive of OS in AML patients. As expected HOX genes (HOXA9, HOXB7, and HOXA10) were identified to be regulated by miRs as well as predictive of worse OS. Additionally, miR107-Myb, miR-378-granzyme B involved in granzyme signaling and miR10a-MAP4K4 were identified to be predictive of outcome through integrated analysis. Although additional functional validations to establish clinical/pharmacologic importance of miRNA-mRNA pairs are needed, our results from RNA electrophoretic mobility shift assay confirmed binding of miR-10a, miR-378, and miR-107 with their target genes GALNT1, GZMB, and MYB, respectively. Integration of pathogenic and pharmacologically significant miRNAs and miRNA-mRNA relationships identified in our study opens up opportunities for development of targeted/miRNA-directed therapies.

Identification of suitable reference genes for hepatic microRNA quantitation.

BACKGROUND: MicroRNAs (miRNAs) are short (~22 nt) endogenous RNAs that play important roles in regulating expression of a wide variety of genes involved in different cellular processes. Alterations in microRNA expression patterns have been associated with a number of human diseases. Accurate quantitation of microRNA levels is important for their use as biomarkers and in determining their functions. Real time PCR is the gold standard and the most frequently used technique for miRNA quantitation. Real time PCR data analysis includes normalizing the amplification data to suitable endogenous control/s to ensure that microRNA quantitation is not affected by the variability that is potentially introduced at different experimental steps. U6 (RNU6A) and RNU6B are two commonly used endogenous controls in microRNA quantitation. The present study was designed to investigate inter-individual variability and gender differences in hepatic microRNA expression as well as to identify the best endogenous control/s that could be used for normalization of real-time expression data in liver samples. METHODS: We used Taqman based real time PCR to quantitate hepatic expression levels of 22 microRNAs along with U6 and RNU6B in 50 human livers samples (25 M, 25 F). To identify the best endogenous controls for use in data analysis, we evaluated the amplified candidates for their stability (least variability) in expression using two commonly used software programs: Normfinder and GeNormplus, RESULTS: Both Normfinder and GeNormplus identified U6 to be among the least stable of all the candidates analyzed, and RNU6B was also not among the top genes in stability. mir-152 and mir-23b were identified to be the two most stable candidates by both Normfinder and GeNormplus in our analysis, and were used as endogenous controls for normalization of hepatic miRNA levels. CONCLUSION: Measurements of microRNA stability indicate that U6 and RNU6B are not suitable for use as endogenous controls for normalizing microRNA relative quantitation data in hepatic tissue, and their use can led to possibly erroneous conclusions.

Genetic and clinical determinants of early, acute calcineurin inhibitor-related nephrotoxicity: results from a kidney transplant consortium.

BACKGROUND: Calcineurin inhibitor (CNI)-related acute nephrotoxicity is a common complication of transplantation. Clinical factors and elevated CNI levels are associated with nephrotoxicity; however, they do not fully explain the risk. Genetic factors may also predispose individuals to nephrotoxicity. METHODS: We enrolled 945 kidney recipients into a multicenter, prospective study. DNA was genotyped for 2724 single-nucleotide polymorphisms (SNPs) using a customized chip. Cox models, unadjusted and adjusted for clinical factors, examined the association between SNPs and time to early CNI-related acute nephrotoxicity in the first 6 months posttransplant. RESULTS: Cyclosporine was associated with a 1.49 hazard (95% confidence interval, 1.04-2.14) of acute nephrotoxicity relative to tacrolimus. Acute nephrotoxicity occurred in 22.6% of cyclosporine and 19.8% of tacrolimus recipients. The median (interquartile range) daily dose and trough concentration at time of nephrotoxicity were 400 mg (400-500 mg) and 228 ng/mL (190-272 ng/mL) in the cyclosporine group, and 6 mg (4-8 mg) and 12.6 ng/mL (10.2-15.9 ng/mL) in the tacrolimus group, respectively. In single-SNP adjusted analysis, nine SNPs in the XPC, CYP2C9, PAX4, MTRR, and GAN genes were associated with cyclosporine nephrotoxicity. In a multi-SNP analysis, SNPs from the same genes remained significant after adjusting for the clinical factors, showing that the SNPs are jointly and independently predictive of cyclosporine nephrotoxicity. No SNPs were associated with tacrolimus nephrotoxicity. CONCLUSION: We identified SNPs that were potentially associated with early, acute cyclosporine-related nephrotoxicity. Identifying risk SNPs before transplantation provides an opportunity for personalization of immunosuppression by identifying those who may benefit from CNI-avoidance or minimization, or assist in selecting CNI type. These SNPs require independent validation.

Identification of predictive markers of cytarabine response in AML by integrative analysis of gene-expression profiles with multiple phenotypes.

AIM: To identify gene-expression signatures predicting cytarabine response by an integrative analysis of multiple clinical and pharmacological end points in acute myeloid leukemia (AML) patients. MATERIALS & METHODS: We performed an integrated analysis to associate the gene expression of diagnostic bone marrow blasts from acute myeloid leukemia (AML) patients treated in the discovery set (AML97; n = 42) and in the independent validation set (AML02; n = 46) with multiple clinical and pharmacological end points. Based on prior biological knowledge, we defined a gene to show a therapeutically beneficial (detrimental) pattern of association of its expression positively (negatively) correlated with favorable phenotypes such as intracellular cytarabine 5´-triphosphate levels, morphological response and event-free survival, and negatively (positively) correlated with unfavorable end points such as post-cytarabine DNA synthesis levels, minimal residual disease and cytarabine LC(50). RESULTS: We identified 240 probe sets predicting a therapeutically beneficial pattern and 97 predicting detrimental pattern (p ≤ 0.005) in the discovery set. Of these, 60 were confirmed in the independent validation set. The validated probe sets correspond to genes involved in PIK3/PTEN/AKT/mTOR signaling, G-protein-coupled receptor signaling and leukemogenesis. This suggests that targeting these pathways as potential pharmacogenomic and therapeutic candidates could be useful for improving treatment outcomes in AML. CONCLUSION: This study illustrates the power of integrated data analysis of genomic data as well as multiple clinical and pharmacologic end points in the identification of genes and pathways of biological relevance.

Novel polymorphisms associated with tacrolimus trough concentrations: results from a multicenter kidney transplant consortium.

BACKGROUND: The CYP4503A5*1 genotype is associated with lower tacrolimus concentrations. Although its effect is important, it incompletely explains the variability in tacrolimus concentrations and has a relatively low minor allele frequency in whites relative to African Americans (AA). METHODS: We studied clinical and recipient genetic correlates of dose-normalized tacrolimus troughs (n=12,277) in the first 6 months posttransplant using a customized single-nucleotide polymorphism chip with 2722 variants in a large, ethnically diverse (144 AA and 551 non-AA) adult kidney transplant population through a seven-center consortium. RESULTS: During the 6-month study, AAs had consistently lower median (interquartile range) troughs than non-AAs, 6.2 (4.4-8.4) ng/mL vs. 8.3 (6.4-10.4) ng/mL (P<0.0001), despite 60% higher daily doses, 8 (5-10) mg vs. 5 (4-7) mg (P<0.0001). The median tacrolimus trough concentration in week 1 posttransplant was particularly low in AAs (2.1 [1.2-3.5] ng/mL) compared with non-AAs (5.0 [3.1-8.2] ng/mL) (P<0.0001), despite similar initial doses. In single-variant analysis, CYP3A5*3 (rs776746) was the top variant (P=2.4×10) associated with troughs. After adjustment for CYP3A5*3, clinical factors and race, 35 additional variants were identified (P<0.01, not significant at false discovery rate 20%). In the final multivariant, regression models beginning with these variants and clinical factors, seven variants were identified in the non-AA and seven variants in the AA group towards the first trough concentrations. Rs776746 (CYP3A5), rs2239393 (COMT) and diabetes were the only factors common in both populations. CONCLUSION: We identified variants beyond CYP3A5*3, which may further explain pharmacokinetic variability of tacrolimus and demonstrated that important variants differ by race.

Genetic determinants of mycophenolate-related anemia and leukopenia after transplantation.

BACKGROUND: Mycophenolate-related anemia and leukopenia are well-known toxicities after transplantation. Toxicity leads to dose reduction, addition of colony-stimulating factors or erythropoietin, or discontinuation of immunosuppressive therapy. The causes of and risk factors associated with toxicity are unclear. METHODS: We studied the association between mycophenolate-related anemia and leukopenia and 2724 single nucleotide polymorphisms (SNP) in 978 patients undergoing living or deceased donor kidney transplant. Patients were followed up to time of first anemia (hemoglobin<10 gm/dL or hematocrit<30%) or first leukopenia (white blood cell [WBC] count <3000 cells/mm), which required clinical intervention in the first 6 months after transplant. RESULTS: Anemia occurred in 87 (9.5%) subjects and leukopenia in 224 (22.9%). In single SNP analyses, none of the SNPs were associated with time to leukopenia at a false discovery rate (FDR) of 20%. However, SNPs from the IL12A, HUS, CYP2C8 genes were associated with time to anemia, allowing for an FDR of 20%. To assess the independence of these SNPs as predictors of anemia, we conducted a multi-SNP analysis including one SNP from each of the three genes. All three SNPs were associated with time to anemia, after adjusting for recipient age, weight, posttransplant dialysis and antiviral drug use, and stratifying by clinical center. CONCLUSION: Although these SNPs require validation in an independent population, our results suggest that genetics may play a role in risk of mycophenolate-related hematologic toxicity. This may ultimately provide for better management of maintenance immunosuppression and gives insights into potential mechanism(s) by which toxicity occurs.

Single-nucleotide polymorphisms, acute rejection, and severity of tubulitis in kidney transplantation, accounting for center-to-center variation.

BACKGROUND: Acute rejection (AR) is associated with worse renal allograft outcomes. Therefore, this study investigated single-nucleotide polymorphisms (SNPs) to identify genetic variants associated with AR, accounting for center variation, in a multicenter, prospective, observation study. METHODS: We enrolled patients from six transplant centers, five in the United States and one in Canada. A total of 2724 SNPs were genotyped. We accounted for center variation in AR rates by stratifying by transplant center and using novel knowledge discovery methods. RESULTS: There was significant center variation in AR rates across the six transplant sites (P<0.0001). Accounting for this difference and clinical factors independently associated with AR, we identified 15 novel SNPs associated with AR with stratification by transplant center (P<0.05). We also identified 15 novel SNPs associated with severity of tubulitis scores, after adjusting for transplant center and other clinical factors independently associated with severity of tubulitis (P<0.05). There was some overlap with one SNP associated with AR and also associated with severity of tubulitis, among the top 15 SNPs. CONCLUSION: Center-to-center variation is a major challenge to genomic studies focused on AR. The SNPs associated with AR and severity of tubulitis in this study will need to be validated in independent cohort of kidney transplant recipients.

Genetic predictors of interindividual variability in hepatic CYP3A4 expression.

Variability in hepatic CYP3A4 cannot be explained by common CYP3A4 coding variants. We previously identified polymorphisms in pregnane X receptor (PXR) and ATP-binding cassette subfamily B member 1 (ABCB1) associated with CYP3A4 mRNA levels in small cohorts of human livers. However, the relative contributions of these genetic variations or of polymorphisms in other CYP3A4 regulators to variable CYP3A4 expression were not known. We phenotyped livers from white donors (n = 128) by quantitative real-time polymerase chain reaction for expression of CYP3A4, CYP3A5, and CYP3A7 and nine transcriptional regulators, coactivators, and corepressors. We resequenced hepatic nuclear factor-3-beta (HNF3beta, FoxA2), HNF4alpha, HNF3gamma (FoxA3), nuclear receptor corepressor 2 (NCoR2), and regions of the CYP3A4 promoter and genotyped informative single-nucleotide polymorphisms in PXR and ABCB1 in the same livers. CYP3A4 mRNA was positively correlated with PXR and FoxA2 and negatively correlated with NCoR2 mRNA. A common silent polymorphism and a polymorphic trinucleotide (CCT) repeat in FoxA2 were associated with CYP3A4 expression. The transcriptional activity of the FoxA2 polymorphic CCT repeat alleles (wild-type, n = 14 and variant, n = 13, 15, and 19) when assayed by luciferase reporter transactivation assays was greatest for the wild-type repeat, with deviations from this number having decreased transcriptional activity. This corresponded with higher expression of FoxA2 mRNA and its targets PXR and CYP3A4 in human livers with (CCT) n = 14 genotypes. Multiple linear regression analysis was used to quantify the contributions of selected genetic polymorphisms to variable CYP3A4 expression. This approach identified sex and polymorphisms in FoxA2, HNF4alpha, FoxA3, PXR, ABCB1, and the CYP3A4 promoter that together explained as much as 24.6% of the variation in hepatic CYP3A4 expression.

Novel single nucleotide polymorphisms in the promoter and intron 1 of human pregnane X receptor/NR1I2 and their association with CYP3A4 expression.

The hypothesis was tested that sequence diversity in pregnane X receptor (PXR) cis-regulatory regions is a significant determinant of variation in inducible and constitutive CYP3A4 expression. A combination of comparative genomics and computational algorithms was used to select regions of the human PXR promoter and intron 1 that were resequenced in the polymorphism discovery resource 24 DNA subset. PXR single nucleotide polymorphisms (SNP) were then genotyped in donor human livers phenotyped for CYP3A4 and multidrug resistance protein 1 mRNA and primary human hepatocytes phenotyped for basal and rifampin-inducible CYP3A4 activity. The human PXR promoter [16.9 kilobase (kb)] was significantly larger than in rodents (2.9 kb). Eighty-nine SNPs were identified in the promoter and intron 1 of PXR. The SNPs most consistently associated with CYP3A4 phenotypic measures were a 44477T>C(-1359) promoter SNP (in linkage disequilibrium with SNP 463970, a 6-base pair deletion in intron 1a, and SNP 46551, a C nucleotide insertion in intron 1b); SNP 63396C>T in intron 1 (in linkage disequilibrium with SNP 63704A>G, a 63813(CAAA)(CA) variable repeat, and SNP 65104T>C); and SNP 56348C>A, SNP 69789A>G, and SNP 66034T>C. Donor livers with the variant PXR alleles had altered hepatic expression of PXR targets compared with livers with PXR wild-type alleles. These results identified PXR promoter and intron 1 SNPs associated with PXR target gene expression (CYP3A4) in donor livers and cultured hepatocytes and that a striking number of the linked intron 1 SNPs will affect putative binding sites for hepatic nuclear factor 3beta (FOXA2), a transcription factor linked with PXR expression.

MDR1 genotype is associated with hepatic cytochrome P450 3A4 basal and induction phenotype.

OBJECTIVES: Variant cytochrome P450 (CYP) 3A4 alleles cannot explain human variation in CYP3A4 expression. This study investigated whether common single-nucleotide polymorphisms (SNPs) in multidrug resistance 1 (MDR1), encoding P-glycoprotein, or the pregnane X receptor (PXR) were associated with basal or inducible CYP3A4 expression. METHODS: MDR1 G2677T and C3435T SNPs and a PXR 6-base pair (bp) deletion were genotyped in the deoxyribonucleic acid from 144 human livers in 3 cohorts each phenotyped for basal or rifampin (INN, rifampicin)-inducible hepatic CYP3A4 expression (or both) and in 57 human small bowel biopsy specimens from 3 cohorts each phenotyped for either basal or rifampin-induced CYP3A4 expression (or both). RESULTS: Hepatic CYP3A4 expression/function was significantly higher in persons homozygous for the MDR1 2677T (Ser893) allele compared with persons homozygous for 2677G (Ala893) in all 3 hepatic cohorts. For example, homozygous MDR1 2677 TT livers had higher midazolam hydroxylase activity than homozygous 2677 GG livers (1831 +/- 1336 pmol x min(-1) x mg protein(-1) versus 1060 +/- 552 pmol x min(-1) x mg protein(-1), P = .03). In 2 of the 3 groups the association was observed in men but not in women. For example, homozygous MDR1 2677 TT male hepatocytes had significantly higher testosterone 6beta-hydroxylase activity compared with homozygous 2677 GG livers (0.120 +/- 0.06 pmol x min(-1) x mg protein(-1) versus 0.069 +/- 0.04 pmol x min(-1) x mg protein(-1), P = .0002). Conversely, rifampin induction of testosterone 6beta-hydroxylation in primary human hepatocytes was significantly higher in persons homozygous for 2677G (12.0 +/- 5.7-fold) compared with MDR1 homozygous TT carriers (7.3 +/- 4.6-fold) (P = .01). Suggestive evidence for higher CYP3A4 expression in MDR1 2677T carriers was also observed in human intestines. CYP3A4 expression was also related to a 6-bp deletion in PXR in 2 of the liver cohorts. Two-factor ANOVA analysis revealed a significant interaction between the MDR1 2677 SNP and the PXR 6-bp deletion influencing CYP3A4 expression (P = .007). CONCLUSIONS: Individuals homozygous for the MDR1 2677T allele show enhanced constitutive CYP3A4 expression in the liver and intestine, as compared with those homozygous for the MDR1 2677G allele, particularly in men. Conversely, the magnitude of CYP3A4 induction by rifampin is greater in persons with the MDR1 2677G allele and is inversely related to baseline CYP3A4 expression. MDR1 likely influences basal CYP3A4 expression by limiting the intracellular concentration of an endogenous regulator. MDR1 genotype may be a useful predictor of basal CYP3A4 and the extent of some CYP3A4-mediated drug interactions in humans. Moreover, the influence of MDR1 genotype on CYP3A4 expression adds additional complexity to determining the relative contribution of the MDR1 alleles to the disposition of substrates shared by CYP3A4 and MDR1/P-glycoprotein.

Variation in oral clearance of saquinavir is predicted by CYP3A5*1 genotype but not by enterocyte content of cytochrome P450 3A5.

OBJECTIVE: Saquinavir, a widely prescribed human immunodeficiency virus 1 protease inhibitor, has a low and variable oral bioavailability that has been attributed to extensive first-pass extraction mediated by hepatic or intestinal cytochrome P450 (CYP) 3A4 and intestinal P-glycoprotein (P-gp). The polymorphic CYP3A5 has also been shown to influence the saquinavir metabolite/parent urinary ratio, suggesting a role for CYP3A5. METHODS: Twenty healthy subjects received a single oral dose of saquinavir (600 mg) with water (control) and, on a separate occasion, with Seville orange juice (a selective intestinal CYP3A4/5 inhibitor). Hepatic CYP3A4 activity was evaluated by use of the erythromycin breath test. Duodenal biopsy specimens were used to assess relative intestinal CYP3A4 and CYP3A5 protein contents. Relative P-gp content was also assessed in the biopsy specimens and in lymphocytes. Genetic polymorphisms in MDR1 (in exon 21 and 26), CYP3A5 (*1 and *3), and CYP3A4*1B were identified by direct sequencing. Saquinavir plasma concentrations were measured by tandem liquid chromatography-mass spectrometry. Pharmacokinetic parameter estimates (maximum concentration, time to reach maximum concentration, area under the concentration-time curve, apparent oral clearance [CL/F]) were computed by standard noncompartmental methods. Stepwise multiple regression analysis was used to identify the hepatic or intestinal variables that predicted variation in saquinavir pharmacokinetic measures. RESULTS: Baseline saquinavir CL/F was not correlated with liver CYP3A4 activity (the erythromycin breath test result), intestinal CYP3A4 content, or intestinal P-gp content (r(2) = 0.08, 0.08, and 0.007, respectively; P > .2). MDR1 genotype and lymphocyte P-gp content were also not predictive. Among the 6 subjects expressing intestinal CYP3A5, the mean saquinavir CL/F was almost twice as high as for the 14 nonexpressors (36.7 L/h [95% confidence interval (CI), 18.7-54.6 L/h] and 19.3 L/h [95% CI, 11.2-27.4 L/h], respectively; P = .03). However, among the 6 CYP3A5 expressors, there was an unexpected negative correlation between CL/F and intestinal CYP3A5 content (r(2) = 0.58, P = .05). Seville orange juice decreased the mean CL/F in all 20 subjects from 24.5 L/h (95% CI, 16.7-32.3 L/h) to 14.7 L/h (95% CI, 8.4-20.6 L/h) (P = .05). The effect size did not appear to be influenced by CYP3A5 expression. CONCLUSIONS: The CYP3A5*1 genotype is associated with increased saquinavir CL/F. This does not appear to reflect intestinal CYP3A5 expression and presumably reflects the contribution of hepatic CYP3A5. The interaction with Seville orange juice in subjects not expressing CYP3A5 supports a role for intestinal CYP3A4. However, the modest nature of the interaction, combined with the inability to detect a correlation between CL/F and CYP3A4 enterocyte content, supports our recent in vitro work suggesting a smaller contribution of intestinal CYP3A4 than has been assumed.

Genetic variants of PXR (NR1I2) and CAR (NR1I3) and their implications in drug metabolism and pharmacogenetics.

The defense mechanisms responsible for protecting the body from endogenous toxins are also involved in the metabolism of drugs and are composed of phase I and phase II drug metabolizing enzymes, as well as drug transporters. Numerous drugs and chemicals have been shown to modulate the expression of the genes involved in these three drug-detoxifying processes. Induction of these genes contributes to both auto-induction of drug clearance and to drug-drug interactions in combination therapies. The orphan nuclear receptors PXR (pregnane X receptor) and CAR (Constitutive androstane receptor) are xenosensors that mediate drug-induced changes by increasing transcription of genes that are involved in drug clearance and disposition. Co-administration of drugs, one of which is a nuclear receptor agonist or antagonist, can either lead to altered clearance of the second drug and severe toxicity, or a loss of therapeutic efficacy or an imbalance in physiological substrate concentrations, providing a novel molecular mechanism for drug-drug interactions. Thus, genetic variability in these nuclear receptors will contribute to human variation in the magnitude of clinically significant drug-drug interactions. This review describes common PXR and CAR genetic variants that have been identified to date in the human population and the functional consequence of these variant alleles. In addition, alternatively spliced variants of PXR and CAR that may also contribute to individual variability as well as tissue specific expression of these receptors are also described. Identification of PXR and CAR genetic variants and alternatively spliced mRNAs may ultimately allow predictions of interindividual differences in target gene induction and drug-drug interactions.

PXR (NR1I2): splice variants in human tissues, including brain, and identification of neurosteroids and nicotine as PXR activators.

To gain insight on the expression of pregnane X receptor (PXR), we analyzed PXR.1 and PXR alternatively spliced transcripts in a panel of 36 human tissues. PXR.1 was expressed in many more tissues than previously determined, including human bone marrow and select regions of the human brain. In each of these tissues, we observed alternative splicing of various exons of PXR that generated multiple distinct PXR isoforms. The most abundant PXR alternative mRNA transcripts lacked 111 nucleotides, deleting 37 amino acids from the PXR LBD (PXR.2), or lacked 123 nt, deleting 41 amino acids from the PXR LBD (PXR.3). CYP3A4, a gene transcriptionally regulated by PXR, showed incomplete overlap with PXR in its tissue distribution. Quantitation of PXR mRNAs in human liver demonstrated that PXR.2 and PXR.3 represented 6.7% and 0.32% of total PXR mRNA transcripts. Brain expression of PXR prompted analysis of whether some brain acting chemicals were PXR ligands. The neurosteroids allopregnanolone and pregnanolone activated PXR and induced transcription of a CYP3A4-luciferase reporter. Nicotine, the psychoactive and addictive chemical in cigarettes, and a known inducer of brain CYP2B6, was an efficacious activator of PXR and inducer of CYP3A4 transcription. Because nicotine activation of PXR will enhance metabolism of nicotine to the non-psychoactive cotinine, these results provide one molecular mechanism for the development of tolerance to nicotine. Moreover, the identification of PXR in many human tissues, such as brain, and activation by tissue specific ligands (such as neurosteroids) suggests additional biological roles for this receptor in these tissues.

Expression of constitutive androstane receptor splice variants in human tissues and their functional consequences.

The constitutive androstane receptor (CAR) NR1I3 is a transcription factor that upon activation by xenobiotics induces transcription of drug-metabolizing and drug transporter genes. Our goal was to identify whether alternative splicing of CAR makes an important contribution to the generation of novel CAR proteins. The wild-type CAR mRNA (CAR.1) and splice variants (SVs) were amplified from human liver cDNAs and in a panel of cDNAs from 36 human tissues, using exon 1- and 3'-untranslated region primers, cloned and sequenced. Twenty-two unique hCAR splice variants (CAR-SVs) containing different combinations of splicing (deletion of exons 2, 4, 5, 7, partial deletion of exon 9, or insertion of 12 or 15 base pairs from introns 6 or 7) were identified. CAR mRNAs were expressed in small intestine, kidney, testis, adrenal, and brain caudate nucleus. Intestine expressed only CAR.1 mRNA, whereas spleen, heart, and prostate expressed only CAR-SVs. In vitro transcription and translation of CAR-SVs lacking exon 2 (missing ATG start site) generated CAR proteins that differed in M(r) from CAR.1. These CAR-SVs used a translation initiation site in exon 1, generating CAR with a unique amino-terminal sequence. Transcripts lacking part of exon 9 altered the CAR reading frame generating CAR proteins with a unique carboxy-terminal end. CAR-SVs demonstrated compromised binding to CYP2B6 NR elements and transcriptional activation of a CYP2B6 luciferase reporter. The expression of CAR in additional human cell types increases the range of tissue specific transcriptional responses regulated by this receptor, suggesting additional biological roles for CAR and CAR-SV proteins in these tissues.

Hepatic CYP2B6 expression: gender and ethnic differences and relationship to CYP2B6 genotype and CAR (constitutive androstane receptor) expression.

CYP2B6 metabolizes many drugs, and its expression varies greatly. CYP2B6 genotype-phenotype associations were determined using human livers that were biochemically phenotyped for CYP2B6 (mRNA, protein, and CYP2B6 activity), and genotyped for CYP2B6 coding and 5'-flanking regions. CYP2B6 expression differed significantly between sexes. Females had higher amounts of CYP2B6 mRNA (3.9-fold, P < 0.001), protein (1.7-fold, P < 0.009), and activity (1.6-fold, P < 0.05) than did male subjects. Furthermore, 7.1% of females and 20% of males were poor CYP2B6 metabolizers. Striking differences among different ethnic groups were observed: CYP2B6 activity was 3.6- and 5.0-fold higher in Hispanic females than in Caucasian (P < 0.022) or African-American females (P < 0.038). Ten single nucleotide polymorphisms (SNPs) in the CYP2B6 promoter and seven in the coding region were found, including a newly identified 13072A>G substitution that resulted in an Lys139Glu change. Many CYP2B6 splice variants (SV) were observed, and the most common variant lacked exons 4 to 6. A nonsynonymous SNP in exon 4 (15631G>T), which disrupted an exonic splicing enhancer, and a SNP 15582C>T in an intron-3 branch site were correlated with this SV. The extent to which CYP2B6 variation was a predictor of CYP2B6 activity varied according to sex and ethnicity. The 1459C>T SNP, which resulted in the Arg487Cys substitution, was associated with the lowest level of CYP2B6 activity in livers of females. The intron-3 15582C>T SNP (in significant linkage disequilibrium with a SNP in a putative hepatic nuclear factor 4 (HNF4) binding site) was correlated with lower CYP2B6 expression in females. In conclusion, we found several common SNPs that are associated with polymorphic CYP2B6 expression.

The orphan nuclear receptor HNF4alpha determines PXR- and CAR-mediated xenobiotic induction of CYP3A4.

The drug metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is thought to be involved in the metabolism of nearly 50% of all the drugs currently prescribed. Alteration in the activity or expression of this enzyme seems to be a key predictor of drug responsiveness and toxicity. Currently available studies indicate that the ligand-activated nuclear receptors pregnane X receptor (PXR; NR1I2) and constitutive androstane receptor (CAR; NR1I3) regulate CYP3A4 expression. However, in cell-based reporter assays, CYP3A4 promoter activity was most pronounced in liver-derived cells and minimal or modest in non-hepatic cells, indicating that a liver-specific factor is required for physiological transcriptional response. Here we show that the orphan nuclear receptor hepatocyte nuclear factor-4alpha (HNF4alpha; HNF4A) is critically involved in the PXR- and CAR-mediated transcriptional activation of CYP3A4. We identified a specific cis-acting element in the CYP3A4 gene enhancer that confers HNF4alpha binding and thereby permits PXR- and CAR-mediated gene activation. Fetal mice with conditional deletion of Hnf4alpha had reduced or absent expression of CYP3A. Furthermore, adult mice with conditional hepatic deletion of Hnf4alpha had reduced basal and inducible expression of CYP3A. These data identify HNF4alpha as an important regulator of coordinate nuclear-receptor-mediated response to xenobiotics.