Related Articles

The gene encoding the dopamine transporter (DAT) has been implicated in CNS disorders, but the responsible polymorphisms remain uncertain. To search for regulatory polymorphisms, we measured allelic DAT mRNA expression in substantia nigra of human autopsy brain tissues, using two marker SNPs (rs6347 in exon 9 and rs27072 in the 3′-UTR). Allelic mRNA expression imbalance (AEI), an indicator of cis-acting regulatory polymorphisms, was observed in all tissues heterozygous for either of the two marker SNPs. SNP scanning of the DAT locus with AEI ratios as the phenotype, followed by in vitro molecular genetics studies, demonstrated that rs27072 C>T affects mRNA expression and translation. Expression of the minor T allele was dynamically regulated in transfected cell cultures, possibly involving microRNA interactions. Both rs6347 and rs3836790 (intron8 5/6 VNTR) also seemed to affect DAT expression, but not the commonly tested 9/10 VNTR in the 3′UTR (rs28363170). All four polymorphisms (rs6347, intron8 5/6 VNTR, rs27072 and 3′UTR 9/10 VNTR) were genotyped in clinical cohorts, representing schizophrenia, bipolar disorder, depression, and controls. Only rs27072 was significantly associated with bipolar disorder (OR=2.1, p=0.03). This result was replicated in a second bipolar/control population (OR=1.65, p=0.01), supporting a critical role for DAT regulation in bipolar disorder.

Genetic variation in mRNA expression plays a critical role in human phenotypic diversity, but it has proven difficult to detect regulatory polymorphisms - mostly single nucleotide polymorphisms (rSNPs). Additionally, variants in the transcribed region, termed here ‘structural RNA SNPs’ (srSNPs), can affect mRNA processing and turnover. Both rSNPs and srSNPs cause allelic mRNA expression imbalance (AEI) in heterozygous individuals. We have applied a rapid and accurate AEI methodology for testing 42 genes implicated in human diseases and drug response, specifically cardiovascular and CNS diseases, and affecting drug metabolism and transport. Each gene was analyzed in physiologically relevant human autopsy tissues, including brain, heart, liver, intestines, and lymphocytes. Substantial AEI was observed in ∼55% of the surveyed genes. Focusing on cardiovascular candidate genes in human hearts, AEI analysis revealed frequent cis-acting regulatory factors in SOD2 and ACE mRNA expression, having potential clinical significance. SNP scanning to locate regulatory polymorphisms in a number of genes failed to support several previously proposed promoter SNPs discovered with use of reporter gene assays in heterologous tissues, while srSNPs appear more frequent than expected. Computational analysis of mRNA folding indicates that ∼90% of srSNPs affects mRNA folding, and hence potentially function. Our results indicate that both rSNPs and srSNPs represent a still largely untapped reservoir of variants that contribute to human phenotypic diversity.

Polymorphisms in the gene encoding SORL1, involved in cellular trafficking of APP, have been implicated in late-onset Alzheimer’s disease, by a mechanism thought to affect mRNA expression. To search for regulatory polymorphisms, we have measured allele-specific mRNA expression of SORL1 in human autopsy tissues from the prefrontal cortex of 26 Alzheimer’s patients, and 51 controls, using two synonymous marker SNPs (rs3824968 in exon 34 (11 heterozygous AD subjects and 16 controls), and rs12364988 in exon 6 (8 heterozygous AD subjects)). Significant allelic expression imbalance (AEI), indicative of the presence of cis-acting regulatory factors, was detected in a single control subject, while allelic ratios were near unity for all other subjects. We genotyped 7 SNPs in two haplotype blocks that had previously been implicated in Alzheimer’s disease. Since each of these SNPs was heterozygous in several subjects lacking AEI, this study fails to support a regulatory role for SORL1 polymorphisms in mRNA expression.

The MDR1 (ABCB1) gene encodes a membrane-bound transporter that actively effluxes a wide range of compounds from cells. The overexpression of MDR1 by multidrug-resistant cancer cells is a serious impediment to chemotherapy. MDR1 is expressed in various tissues to protect them from the adverse effect of toxins. The pharmacokinetics of drugs that are also MDR1 substrates also influence disease outcome and treatment efficacy. Although MDR1 is a well conserved gene, there is increasing evidence that its polymorphisms affect substrate specificity. Three single nucleotide polymorphisms (SNPs) occur frequently and have strong linkage, creating a common haplotype at positions 1236C>T (G412G), 2677G>T (A893S) and 3435C>T (I1145I). The frequency of the synonymous 3435C>T polymorphism has been shown to vary significantly according to ethnicity. Existing literature suggests that the haplotype plays a role in response to drugs and disease susceptibility. This review summarizes recent findings on the 3435C>T polymorphism of MDR1 and the haplotype to which it belongs. A possible molecular mechanism of action by ribosome stalling that can change protein structure and function by altering protein folding is discussed.

Genetic variants of ACE are suspected risk factors in cardiovascular disease, but the alleles responsible for the variations remain unidentified. To search for regulatory polymorphisms, allelic angiotensin I–converting enzyme (ACE) mRNA expression was measured in 65 heart tissues, followed by genotype scanning of the ACE locus. Marked allelic expression imbalance (AEI) detected in five African-American subjects was associated with single-nucleotide polymorphisms (SNPs) (rs7213516, rs7214530, and rs4290) residing in conserved regions 2−3 kb upstream of ACE. Moreover, each of the SNPs affected transcription in reporter gene assays. SNPs rs4290 and rs7213516 were tested for associations with adverse cardiovascular outcomes in hypertensive patients with coronary disease (International Verapamil SR Trandolapril Study Genetic Substudy (INVEST-GENES), n = 1,032). Both SNPs were associated with adverse cardiovascular outcomes, largely attributable to nonfatal myocardial infarction in African Americans, showing an odds ratio of 6.16 (2.43−15.60) (P < 0.0001) for rs7213516. The high allele frequency in African Americans (16%) compared to Hispanics (4%) and Caucasians (<1%) suggests that these alleles contribute to variation between populations in cardiovascular risk and treatment outcomes.

Background

P-glycoprotein (Pgp) is a drug efflux pump that transports natural products, including taxanes and other chemotherapeutic agents, from cells. Several frequent polymorphisms in ATP binding cassette gene B1 (ABCB1) may influence Pgp levels and drug efflux. The purpose of this review was to assess the clinical significance of ABCB1 polymorphisms in oncology.

Methods

Peer-reviewed studies were identified through a search of PubMed/MEDLINE (1990-2008) and the ASCO abstracts (2003-2008) database. Included studies described clinical trials where ABCB1 genotyping was performed in patients with cancer. Search terms included ABCB1, Pgp, docetaxel, paclitaxel, irinotecan, imatinib and anticancer agent. Studies were excluded if the manuscript was not available in English.

Results

The influence of polymorphisms in ABCB1 2677G>T/A, 3435C>T, and 1236C>T and progression-free and overall survival in 309 patients from the Australian Ovarian Cancer Study treated with paclitaxel/carboplatin demonstrated that compared to homozygote GG carriers at 2677, women with the minor T/A alleles were significantly less likely to relapse following treatment. Other trials of ABCB1 genotyping in breast and prostate cancer patients receiving taxanes have shown inconsistent results. Pharmacokinetic studies where ABCB1 was genotyped and patients received irinotecan or imatinib have also shown inconsistent results.

Conclusion

A number of commercially available drugs are substrates for Pgp, and the ABCB1-variant genotypes are frequent and functionally significant, which may have future implications for drug dosing.

Introduction

The human multi-drug resistance gene (MDR1, ABCB1) codes for P-glycoprotein (P-gp), an important membrane-bound efflux transporter known to confer anti-cancer drug resistance as well as affect the pharmacokinetics of many drugs and xenobiotics. A number of single nucleotide polymorphisms (SNPs) have been identified throughout the ABCB1 gene which may have an effect on P-gp expression levels and function. Haplotype as well as genotype analysis of SNPs is becoming increasingly important in identifying genetic variants underlying susceptibility to human disease. Three SNPs, 1236C>T, 2677G>T, and 3435C>T have been repeatedly shown to predict changes in the function of P-gp. The frequencies with which these polymorphisms exist in a population have also been shown to be ethnically related.

Methods

In this study, 95 individuals representative of the entire ethnic make-up of the United States were compared to 101 individuals from an Ashkenazi Jewish population. These individuals were analyzed by genomic sequencing and PCR-RFLP to calculate their genotype frequencies.

Results

Twenty-five SNPs were located in the exons of the ABCB1 gene. All of the polymorphisms identified were in parts of the ABCB1 gene product predicted to be intracellular, and 16 appear to be novel as compared to those listed by NCBI. Frequencies of the 1236C>T and 2677G>T/A/C SNPs were similar for the American and Ashkenazi populations (64.2% and 60.4% respectively for 1236C>T – χ2 is 0.30 p≤1; 55.8% and 64.4% for 2677G>T/A/C χ2 is 1.49 p≤1), but were different for 3435C>T (24.2% for the American population and 69.3% for the Ashkenazi population χ2 is 39.927 p<0.001). The 1236T/2677T/3435T haplotype occurred in 23.6% (SE 0.013) of the Ashkenazi population.

Conclusion

The SNP at location 3435C>T plays a significant role in the ABCB1 gene. The haplotype and genotype analysis from these data may be used as a basis for studies on the relationship between ABCB1 genotypes and drug efficacy, drug toxicity, disease susceptibility or other phenotypes.

Objective

To analyse the single-nucleotide polymorphisms (SNPs): ABCB11236C>T, ABCB12677G>T/A, ABCB13435C>T and haplotypes in the ABCB1/MDR1 gene, which could contribute to genetic risk of colorectal cancer (CRC). Disease association between the ABCB1/MDR1 genotype, allele, haplotype frequencies and histological features, such as TNM classification, localization of primary carcinoma, grade of malignancy, histological type of tumour, lymphoid infiltration and vessel invasion were estimated. In this study, the potential role of SNPs of the ABCB1/MDR1 gene as a prognostic marker for CRC was analysed.

Materials and methods

Tumour specimens of 95 patients with CRC were studied. Using automated sequencing or PCR-RFLP method, DNA for three common SNPs of ABCB1/MDR1 was extracted and analysed. The results of genotyping and haplotype analysis with histopathological features, grading and clinical staging of neoplasms were correlated.

Results

A statistically significant higher frequency of T1236 allele in T1/T2 (89.7%), M0 groups (81.6%) and I/II clinical staging (82.7%) in comparison with T3/T4 (68.2%), M1 groups (47.4%) and III/IV clinical staging (65.1%) was detected. Furthermore, multivariate analysis according to Cox's proportional hazard model indicated that the T1236 allele is a good, independent prognostic factor and the presence of this allele decreases the risk of death in comparison with a group without this allele (HR = 0.26; p = 0.0424). In addition, a statistically significant higher frequency of C3435 allele and significant differences in the C3435 allele distribution in N1/N2 group (91.7% and 62.5%, respectively) than N0 group (71.2% and 44.9%, respectively) was found. Each of the eight possible haplotypes was noted in M0 or I/II group and only seven in M1 or III/IV group. Haplotype T1236-G2677-C3435 only in less advanced CRC subjects (9.6% in I/II and 9.2% in M0 group) was detected. In addition, significant differences in haplotype distributions between M0 or I/II and M1 or III/IV group were found (p = 0.01 and p = 0.05, respectively).

Conclusions

These results suggest association between T1236 allele and T1236-G2677-C3435 haplotype and less advanced CRC, so these genetic markers may play a role as potentially good prognostic factors. Differences in haplotype distributions and degree of clinical staging may suggest that some other potential SNPs, especially in regulatory region of ABCB1/MDR1 gene, may influence P-glycoprotein function and CRC progression.

Background

The xenobiotic transporters, Multidrug Resistance 1 (MDR1/ABCB1) and Breast Cancer Resistance Protein (BCRP/ABCG2) may restrict intestinal absorption of various carcinogens, including heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH). Cyclooxygenase-2 (COX-2) derived prostaglandins promote gastrointestinal carcinogenesis, affecting angiogenesis, apoptosis, and invasiveness.

The aim of this study was to investigate if polymorphisms in these genes were associated with risk of colorectal cancer (CRC), and to investigate possible interactions with lifestyle factors such as smoking, meat consumption, and NSAID use.

Methods

The following polymorphisms were analyzed; a synonymous MDR1 C3435T (rs1045642) in exon26, G-rs3789243-A in intron3, the functional BCRP C421A (rs2231142), the two COX-2 A-1195G (rs689466) and G-765C (rs20417) in the promoter region, and the COX-2 T8473C (rs5275) polymorphisms in the 3'-untranslated region. The polymorphisms were assessed together with lifestyle factors in a nested case-cohort study of 359 cases and a random cohort sample of 765 participants from the Danish prospective Diet, Cancer and Health study.

Results

Carriers of the variant allele of MDR1 intron 3 polymorphism were at 1.52-fold higher risk of CRC than homozygous wild type allele carriers (Incidence rate ratio (IRR) = 1.52, 95% Confidence Interval (CI): 1.12-2.06). Carriers of the variant allele of MDR1 C3435T exon 26 had a lower risk of CRC than homozygous C-allele carriers (IRR = 0.71 (CI:0.50-1.00)). There was interaction between these MDR1 polymorphisms and intake of red and processed meat in relation to CRC risk. Homozygous MDR1 C3435T C-allele carriers were at 8% increased risk pr 25 gram meat per day (CI: 1.00-1.16) whereas variant allele carriers were not at increased risk (p for interaction = 0.02). COX-2 and BCRP polymorphisms were not associated with CRC risk. There was interaction between NSAID use and MDR1 C3435T and COX-2 T8473C (p-values for interaction 0.001 and 0.04, respectively).

Conclusion

Two polymorphisms in MDR1 were associated with CRC risk and there was interaction between these polymorphisms and meat intake in relation to CRC risk. Our results suggest that MDR1 polymorphisms affect the relationship between meat and CRC risk.

Background

Common genetic variants that regulate gene expression are widely suspected to contribute to the etiology and phenotypic variability of complex diseases. Although high-throughput, microarray-based assays have been developed to measure differences in mRNA expression among independent samples, these assays often lack the sensitivity to detect rare mRNAs and the reproducibility to quantify small changes in mRNA expression. By contrast, PCR-based allelic expression imbalance (AEI) assays, which use a "marker" single nucleotide polymorphism (mSNP) in the mRNA to distinguish expression from pairs of genetic alleles in individual samples, have high sensitivity and accuracy, allowing differences in mRNA expression greater than 1.2-fold to be quantified with high reproducibility. In this paper, we describe the use of an efficient PCR/next-generation DNA sequencing-based assay to analyze allele-specific differences in mRNA expression for candidate neuropsychiatric disorder genes in human brain.

Results

Using our assay, we successfully analyzed AEI for 70 candidate neuropsychiatric disorder genes in 52 independent human brain samples. Among these genes, 62/70 (89%) showed AEI ratios greater than 1 ± 0.2 in at least one sample and 8/70 (11%) showed no AEI. Arranging log2AEI ratios in increasing order from negative-to-positive values revealed highly reproducible distributions of log2AEI ratios that are distinct for each gene/marker SNP combination. Mathematical modeling suggests that these log2AEI distributions can provide important clues concerning the number, location and contributions of cis-acting regulatory variants to mRNA expression.

Conclusions

We have developed a highly sensitive and reproducible method for quantifying AEI of mRNA expressed in human brain. Importantly, this assay allowed quantification of differential mRNA expression for many candidate disease genes entirely missed in previously published microarray-based studies of mRNA expression in human brain. Given the ability of next-generation sequencing technology to generate large numbers of independent sequencing reads, our method should be suitable for analyzing from 100- to 200-candidate genes in 100 samples in a single experiment. We believe that this is the appropriate scale for investigating variation in mRNA expression for defined sets candidate disorder genes, allowing, for example, comprehensive coverage of genes that function within biological pathways implicated in specific disorders. The combination of AEI measurements and mathematical modeling described in this study can assist in identifying SNPs that correlate with mRNA expression. Alleles of these SNPs (individually or as sets) that accurately predict high- or low-mRNA expression should be useful as markers in genetic association studies aimed at linking candidate genes to specific neuropsychiatric disorders.

The primary inflorescence stem of Arabidopsis thaliana is rich in lignified cell walls, in both vascular bundles and interfascicular fibres. Previous gene expression studies demonstrated a correlation between expression of phenylpropanoid biosynthetic genes and a subset of genes encoding ATP-binding cassette (ABC) transporters, especially in the ABCB/multi-drug resistance/P-glycoprotein (ABCB/MDR/PGP) and ABCG/pleiotropic drug resistance (ABCG/PDR) subfamilies. The objective of this study was to characterize these ABC transporters in terms of their gene expression and their function in development of lignified cells. Based on in silico analyses, four ABC transporters were selected for detailed investigation: ABCB11/MDR8, ABCB14/MDR12, ABCB15/MDR13, and ABCG33/PDR5. Promoter::glucuronidase reporter assays for each gene indicated that promoters of ABCB11, ABCB14, ABCB15, and ABCG33 transporters are active in the vascular tissues of primary stem, and in some cases in interfascicular tissues as well. Homozygous T-DNA insertion mutant lines showed no apparent irregular xylem phenotype or alterations in interfascicular fibre lignification or morphology in comparison with wild type. However, in abcb14-1 mutants, stem vascular morphology was slightly disorganized, with decreased phloem area in the vascular bundle and decreased xylem vessel lumen diameter. In addition, abcb14-1 mutants showed both decreased polar auxin transport through whole stems and altered auxin distribution in the procambium. It is proposed that both ABCB14 and ABCB15 promote auxin transport since inflorescence stems in both mutants showed a reduction in polar auxin transport, which was not observed for any of the ABCG subfamily mutants tested. In the case of ABCB14, the reduction in auxin transport is correlated with a mild disruption of vascular development in the inflorescence stem.

Cis-acting polymorphisms that affect gene expression are now known to be frequent, although the extent and mechanisms by which such variation affects the human phenotype are, as yet, only poorly understood. Key signatures of cis-acting variation are differences in gene expression that are tightly associated with regulatory SNPs or expression Quantitative Trait Loci (eQTL) and an imbalance of allelic expression (AEI) in heterozygous samples. Such cis-acting sequence differences appear often to have been under selection within and between populations and are also thought to be important in speciation. Here we describe the example of lactase persistence. In medical research, variants that affect regulation in cis have been implicated in both monogenic and polygenic disorders, and in the metabolism of drugs. In this review we suggest that by further understanding common regulatory variations and how they interact with other genetic and environmental variables it will be possible to gain insight into important mechanisms behind complex disease, with the potential to lead to new methods of diagnosis and treatments.

The ABCB1 gene encodes the P-glycoprotein (Pgp) protein, which is thought to transport various antiepileptic drugs. The single nucleotide polymorphism (SNP) (C3435T) in exon 26 of this gene correlates with the altered expression levels of P-glycoprotein, range of drug response and clinical conditions. In order to investigate the influence of this polymorphism on the susceptibility to and efficacy of carbamazepine therapy, we evaluated the allelic frequency and genotype distribution of this variant in 162 epilepsy patients from the Republic of Macedonia. Statistically significant differences were detected neither in the allelic frequency and genotype distribution between carbamazepine-resistant and carbamazepine-responsive epilepsy patients nor between the subgroups of carbamazepine (CBZ)-responsive patients treated with different CBZ doses. However, the T-allele was enriched in CBZ-responsive patients who required higher maintenance CBZ doses, This observation was substantiated by the findings that the median total plasma levels were the lowest in patients with CC (20 μmol/L) followed by CT (23 μmol/L) and TT (29 μmol/L) genotypes. Patients with a CC genotype also had a higher likelihood of response compared to patients with CT or TT genotypes over a wide range (400–1000 mg/day) of initial doses of CBZ. The T allele showed a reduced expression of ~5% compared to the C allele in peripheral blood mononuclear cells in heterozygotes for the variant. This difference might be translated into ~10% difference in homozygotes for the variant, which would explain the trend towards a dose-dependent efficacy of the CBZ treatment in patients with different genotypes. A larger prospective study is warranted to clarify the clinical utility of a genotypespecific individualized CBZ therapy.

Background:

Genetic variability in ABCB1, encoding the P-glycoprotein efflux transporter, has been linked to altered methadone maintenance treatment dose requirements. However, subsequent studies have indicated that additional environmental or genetic factors may confound ABCB1 pharmacogenetics in different methadone maintenance treatment settings. There is evidence that genetic variability in OPRM1, encoding the mu opioid receptor, and ABCB1 may interact to affect morphine response in opposite ways. This study aimed to examine whether a similar gene-gene interaction occurs for methadone in methadone maintenance treatment.

Methods:

Opioid-dependent subjects (n = 119) maintained on methadone (15–300 mg/day) were genotyped for five single nucleotide polymorphisms of ABCB1 (61A > G; 1199G > A; 1236C > T; 2677G > T; 3435C > T), as well as for the OPRM1 118A > G single nucleotide polymorphism. Subjects’ methadone doses and trough plasma (R)-methadone concentrations (Ctrough) were compared between ABCB1 haplotypes (with and without controlling for OPRM1 genotype), and between OPRM1 genotypes (with and without controlling for ABCB1 haplotype).

Results:

Among wild-type OPRM1 subjects, an ABCB1 variant haplotype group (subjects with a wild-type and 61A:1199G:1236C:2677T:3435T haplotype combination, or homozygous for the 61A:1199G:1236C:2677T:3435T haplotype) had significantly lower doses (median ± standard deviation 35 ± 5 versus 180 ± 65 mg/day, P < 0.01) and Ctrough (78 ± 22 versus 177 ± 97 ng/mL, P < 0.05) than ABCB1 wild-type subjects. Among subjects with the most common ABCB1 haplotype combination (wild-type with 61A:1199G:1236T:2677T:3435T), the OPRM1 118 A/G genotype was associated with a significantly higher Ctrough than 118 A/A (250 ± 126 versus 108 ± 36 ng/mL, P = 0.016). No ABCB1 haplotype group or OPRM1 genotype was associated with dose or Ctrough without taking into account confounding genetic variability at the other locus. Therefore, two interacting pharmacogenetic determinants of methadone maintenance treatment response were identified, ie, ABCB1, where variants are associated with lower methadone requirements, and OPRM1, where the variant is associated with higher methadone requirements.

Conclusion:

These opposing pharmacogenetic effects therefore need to be considered in combination when assessing methadone maintenance treatment pharmacogenetics.

Background

It has been reported that the histone deacetylase inhibitor (iHDAc) trichostatin A (TSA) induces an increase in MDR1 gene transcription (ABCB1). This result would compromise the use of iHDACs in combination with other cytotoxic agents that are substrates of P-glycoprotein (Pgp). It has also been reported the use of alternative promoters by the ABCB1 gene and the existence of a translational control of Pgp protein. Finally, the ABCB1 gene is located in a genetic locus with the nested gene RUNDC3B in the complementary DNA strand, raising the possibility that RUNDC3B expression could interfere with ABCB1 alternative promoter regulation.

Methods

A combination of RT-PCR, real time RT-PCR, Western blot and drug accumulation assays by flow cytometry has been used in this study.

Results

The iHDACs-induced increase in MDR1 mRNA levels is not followed by a subsequent increase in Pgp protein levels or activity in several pancreatic and colon carcinoma cell lines, suggesting a translational control of Pgp in these cell lines. In addition, the MDR1 mRNA produced in these cell lines is shorter in its 5′ end that the Pgp mRNA produced in cell lines expressing Pgp protein. The different size of the Pgp mRNA is due to the use of alternative promoters. We also demonstrate that these promoters are differentially regulated by TSA. The translational blockade of Pgp mRNA in the pancreatic carcinoma cell lines could be related to alterations in the 5′ end of the MDR1 mRNA in the Pgp protein expressing cell lines. In addition, we demonstrate that the ABCB1 nested gene RUNDC3B expression although upregulated by TSA is independent of the ABCB1 alternative promoter used.

Conclusions

The results show that the increase in MDR1 mRNA expression after iHDACs treatment is clinically irrelevant since this mRNA does not render an active Pgp protein, at least in colon and pancreatic cancer cell lines. Furthermore, we demonstrate that TSA in fact, regulates differentially both ABCB1 promoters, downregulating the upstream promoter that is responsible for active P-glycoprotein expression. These results suggest that iHDACs such as TSA may in fact potentiate the effects of antitumour drugs that are substrates of Pgp. Finally, we also demonstrate that TSA upregulates RUNDC3B mRNA independently of the ABCB1 promoter in use.

Here, we evaluated relationships between ABCB1 (P-glycoprotein, MDR1) polymorphisms and paclitaxel (Taxol)-induced toxicity and pharmacokinetics. Twenty-six patients were assessable for pharmacogenetics and pharmacokinetics, 22 for neurotoxicity and 18 for myelotoxicity. Patients carrying two reference alleles for the ABCB1 3435C>T polymorphism trended toward a reduced risk to develop neuropathy as compared to patients carrying at least one variant allele (P=0.09). Additionally, patients that were homozygous variant at the 2677 and 3435 loci had a significantly greater percent decrease in absolute neutrophil count at nadir (P=0.02). Neither polymorphism correlated with paclitaxel pharmacokinetics. This pilot study suggests that paclitaxel-induced neuropathy and neutropenia might be linked to inherited variants of ABCB1 through a mechanism that is unrelated to altered plasma pharmacokinetics.

Several investigations demonstrated that the polymorphisms of multidrug resistance gene (MDR1) gene contribute to interindividual variability in bioavailability and tissue distribution of its substrates. Genotyping of closely spaced single-nucleotide polymorphism (SNP) markers frequently yields highly correlated data, owing to extensive linkage disequilibrium (LD) between markers. The product of multidrug resistance gene (P-gp) is an important molecule, which regulating the bioavailability of many drugs, including calcineurin inhibitors. It also reported that some MDR1 gene polymorphisms (such as 3435C>T) was associated with significantly reduced intestinal P-gp expression in T/T homozygotes. The aim of this study is to develop genotyping assays for polymorphisms of the MDR1 gene, which are believed to have functional properties and to assess the distribution of variant alleles in renal patients (UK Caucasoid). A total of ten polymorphisms in the MDR-1 gene were selected for analysis. Haplotype assays were performed by using EH programme in 172 individuals. The following possible haplotype was apparent (G-41, C-145, C-129, C+139, C+1236, G+2677, G+2956, C+3435, C+4030 and A+4036). This finding suggests the importance of haplotype assignment for the MDR1 gene.

Background

Multidrug resistance (MDR) phenotype of malignant cells is the major problem in the chemotherapy of neoplasia. The treatment of leukemia with retinoids is aimed on the induction of leukemic cells differentiation. However the interconnections between retinoid regulated differentiation of leukemic cells and regulation of MDR remains unclear.

Methods

Four lines of cultured leukemic cells of diverse types of differentiation were infected with RARα gene and stable transfectants were isolated. We investigated the differentiation of these cells as well as the expression of RARα and MDR1 genes and P-glycoprotein (Pgp, MDR protein) functional activity in these cells.

Results

All RARα transfected sublines demonstrated the increase in the quantity of RARα mRNA. All these sublines became more differentiated. Intrinsic activity of MDR1 gene (but not Pgp functional activity) was increased in one of the transfectants. All-trans-retinoic acid (ATRA) induced Pgp activity in two of three infectants to a larger extent than in parental cells.

Conclusion

The data show that RARα regulates MDR1/ Pgp activity in human leukemic cells, in the first place, Pgp activity induced by ATRA. These results show that RARα overexpression in leukemic cells could result in MDR.

Background:

Given that HIV-protease inhibitors (HIV-PIs) are substrates/inhibitors of the multidrug transporter ABCB1, can induce ABCB1 expression, and are used in combination with doxorubicin for AIDS-Kaposi's Sarcoma (KS) treatment, the role that ABCB1 plays in mediating multidrug resistance (MDR) in a fully transformed KS cell line (SLK) was explored.

Methods:

The KS cells were exposed to both acute and chronic treatments of physiological concentrations of different HIV-PIs (indinavir, nelfinavir, atazanavir, ritonavir, or lopinavir), alone or together with doxorubicin. The ABCB1 mRNA and protein expression levels were then assessed by qRT–PCR and western blotting, flow cytometry, and immunofluorescence.

Results:

Chronic treatment of SLK cells with one of the five HIV-PIs alone or together resulted in increased resistance to doxorubicin. Co-treatment with one of the HIV-PIs in combination with doxorubicin resulted in a synergistic increase in resistance to doxorubicin, and the degree of resistance was found to correlate with the expression of ABCB1. The SLK cells were also revealed to be cross-resistant to the structurally unrelated drug paclitaxel.

Conclusion:

These studies suggest that ABCB1 is primarily responsible for mediating MDR in SLK cells selected with either HIV-PIs alone or in combination with doxorubicin. Therefore, the roles that ABCB1 and drug cocktails play in mediating MDR in KS in vivo should be evaluated.

Neuronal expression of ATP-binding cassette, sub-family B (MDR/TAP), member 1 (ABCB1) has been demonstrated after brain ischemia. To investigate whether ABCB1 polymorphisms are associated with the development, risk factors (hypertension, dyslipidemia, and diabetes mellitus), severity (National Institutes of Health Stroke Scale, NIHSS), and sequelae (Modified Barthel Index, MBI) of ischemic stroke (IS), four single nucleotide polymorphisms (SNPs) of the ABCB1 gene [rs4148727, promoter, -154T>C; rs3213619, 5'-untranslation region (5'UTR), -129T>C); rs1128503, synonymous, Gly412 (C>T); rs3842, 3'UTR, A>G] were analyzed in 121 IS patients and 291 control subjects. SNPStats and SPSS 18.0 were used to obtain odds ratios (OR), 95% confidence intervals (CI), and p values. Multiple logistic regression models (codominant1, codominant2, dominant, recessive, and log-additive models) were applied to analyze the genetic data. The rs3842 SNP was weakly associated with the development of IS (p=0.020 in codominant1 model and p=0.028 in dominant model). In the analysis of clinical phenotypes, ABCB1 polymorphisms were nominally associated with hypertension (rs3213619 and rs3842, p<0.05), dyslipidemia (rs1128503, p<0.05), diabetes (rs3842, p<0.05), and NIHSS (rs4148727, p<0.05). Interestingly, rs3842 showed statistically strong association between IS with hypertension and IS without hypertension (Fisher's exact p=0.003, OR=0.11, 95% CI=0.03-0.51 in recessive model). These results suggest that the ABCB1 gene may be associated with the development and clinical phenotypes of IS in Korean population.

In order to comprehensively screen genetic variants leading to differential expression of the important human ABCB1 gene in the primary drug-metabolizing organ, ABCB1 mRNA expression levels were measured in 73 normal liver tissue samples from Chinese subjects. A set of Tag SNPs. were genotyped. In addition, imputation was performed within a 500 kb region around the ABCB1 gene using the reference panels of 1,000 Genome project and HapMap III. Bayesian regression was used to assess the strength of associations by compute Bayes Factors for imputed SNPs. Through imputation and linkage disequilibrium analysis, the imputed loci rs28373093, rs1002205, rs1029421, rs2285647, and rs10235835, may represent independent and strong association signals. rs28373093, a polymorphism 1.5 kb upstream from the ABCB1 transcription start site, has the strongest association. 2677 G>A/T and 3435C>T confer a clear gene-dosage effect on ABCB1 mRNA expression. The systematic characterization of gene-wide common quantitative trait loci associated with ABCB1 mRNA expression in normal liver tissues would provide the candidate markers to ABCB1-relevant clinical phenotypes in Chinese population.

Background

Smoking, dietary factors, and alcohol consumption are known life style factors contributing to gastrointestinal carcinogenesis. Genetic variations in carcinogen handling may affect cancer risk. The multidrug resistance 1(MDR1/ABCB1) gene encodes the transport protein P-glycoprotein (a phase III xenobiotic transporter). P-glycoprotein is present in the intestinal mucosal lining and restricts absorption of certain carcinogens, among these polycyclic aromatic hydrocarbons. Moreover, P-glycoprotein transports various endogenous substrates such as cytokines and chemokines involved in inflammation, and may thereby affect the risk of malignity. Hence, genetic variations that modify the function of P-glycoprotein may be associated with the risk of colorectal cancer (CRC). We have previously found an association between the MDR1 intron 3 G-rs3789243-A polymorphism and the risk of CRC in a Danish study population. The aim of this study was to investigate if this MDR1 polymorphism was associated with risk of colorectal adenoma (CA) and CRC in the Norwegian population.

Methods

Using a case-control design, the association between the MDR1 intron 3 G-rs3789243-A polymorphism and the risk of colorectal carcinomas and adenomas in the Norwegian population was assessed in 167 carcinomas, 990 adenomas, and 400 controls. Genotypes were determined by allelic discrimination. Odds ratio (OR) and 95 confidence interval (95% CI) were estimated by binary logistic regression.

Results

No association was found between the MDR1 polymorphism (G-rs3789243-A) and colorectal adenomas or cancer. Carriers of the variant allele of MDR1 intron 3 had odds ratios (95% CI) of 0.97 (0.72–1.29) for developing adenomas, and 0.70 (0.41–1.21) for colorectal cancer, respectively, compared to homozygous wild type carriers.

Conclusion

The MDR1 intron 3 (G-rs3789243-A) polymorphism was not associated with a risk of colorectal adenomas or carcinomas in the present Norwegian study group. Thus, this MDR1 polymorphism does not seem to play an important role in colorectal carcinogenesis in this population.

Drug resistance caused by overexpression of P-glycoprotein (P-gp), the MDR1 (ABCB1) gene product, limits the therapeutic outcome. Expression of MDR1 can be induced by divergent stimuli, and involves a number of transcriptional factors. We found that the expression of CtBP1 (C-terminal-binding protein 1), a transcriptional co-regulator, was increased (~4 – fold) in human multidrug resistant (MDR) cancer cell lines, NCI/ADR-RES and A2780/DX, as compared to their sensitive counterparts. Silencing of CtBP1 expression by RNAi decreased the MDR1 mRNA and P-gp. Knockdown of CtBP1 also enhanced the sensitivity of MDR cells to chemotherapeutic drugs that are transported by P-gp and increased intracellular drug accumulation. In a reporter gene assay, co-transfection of MDR1 promoter constructs with a CtBP1 expression vector resulted in a ~2–4-fold induction of MDR1 promoter activity. CtBP1 appeared to contribute to the activation of MDR1 transcription through directly interacting with the MDR1 promoter, as evidenced by its physical binding to the promoter region of the MDR1 gene in chromatin immunoprecipitation and electromobility shift assays. Histone modifications at the MDR1 promoter, such as mono-methylation, di-methylation, and acetylation of histone H3, were not found to be affected by silencing of CtBP1 expression. Our results reveal a novel role for CtBP1 as an activator of MDR1 gene transcription, and suggest that CtBP1 might be one of the key transcription factors involved in the induction of MDR1 gene. Therefore, CtBP1 may represent a potentially new target for inhibiting drug resistance mediated by overexpression of the MDR1 gene.

Clinical resistance to chemotherapy in acute myeloid leukemia (AML) is associated with the expression of the multidrug resistance (MDR) proteins P-glycoprotein, encoded by the MDR1/ABCB1 gene, multidrug resistant-related protein (MRP/ABCC1), the lung resistance-related protein (LRP), or major vault protein (MVP), and the breast cancer resistance protein (BCRP/ABCG2). The clinical value of MDR1, MRP1, LRP/MVP, and BCRP messenger RNA (mRNA) expression was prospectively studied in 154 newly diagnosed AML patients ≥60 years who were treated in a multicenter, randomized phase 3 trial. Expression of MDR1 and BCRP showed a negative whereas MRP1 and LRP showed a positive correlation with high white blood cell count (respectively, p < 0.05, p < 0.001, p < 0.001 and p < 0.001). Higher BCRP mRNA was associated with secondary AML (p < 0.05). MDR1 and BCRP mRNA were highly significantly associated (p < 0.001), as were MRP1 and LRP mRNA (p < 0.001) expression. Univariate regression analyses revealed that CD34 expression, increasing MDR1 mRNA as well as MDR1/BCRP coexpression, were associated with a lower complete response (CR) rate and with worse event-free survival and overall survival. When adjusted for other prognostic actors, only CD34-related MDR1/BCRP coexpression remained significantly associated with a lower CR rate (p = 0.03), thereby identifying a clinically resistant subgroup of elderly AML patients.

Abstract

Objective

Guanfacine has been shown to reduce hyperactive behaviors in children with attention-deficit/hyperactivity disorder (ADHD) and possibly in children with pervasive developmental disorder (PDD) and hyperactivity. The aim of this exploratory study was to examine whether gene variants encoding the multidrug resistance protein (MDR1 or ABCB1) , a drug transporter at the blood–brain barrier, are associated with variability in the efficacy of guanfacine in children with PDD and hyperactivity.

Methods

Children with PDD who participated in an 8-week open-label trial of guanfacine were genotyped for the C3435T single-nucleotide polymorphism (SNP) variant of the MDR1 gene, a variant reported to alter function of the transporter. The decrease from baseline to 8 weeks in parent-rated Aberrant Behavior Checklist (ABC) hyperactivity and Swanson, Nolan, and Pelham (SNAP) scores were analyzed by MDR1 genotype. Response was compared between subjects homozygous for the minor allele T of the C34535T MDR1 variant (T/T) versus other genotypes (C/T and C/C).

Results

Disruptive behavior decreased during guanfacine treatment as assessed by several end points in the 25 enrolled children (23 boys and 2 girls). Genotype data were available from 22 children. Subjects with either C/T or C/C (n = 16) genotypes showed a three-fold greater improvement than T/T MDR1 C3435T genotype (n = 6) (mean decrease of 15.1 ± 12.6, or 50.7% from baseline, versus 4.5 ± 5.1, or 15.6% from baseline) in parent-rated ABC Hyperactivity scores over 8 weeks (p = 0.03). Parent-rated ADHD SNAP scores also differed by genotype (p = 0.05).

Conclusions

Gene variants in MDR1 may influence guanfacine response on hyperactive-impulsive behaviors via altered membrane transport. If replicated in larger samples, additional studies would be important to clarify the mechanisms underlying this effect and to determine its clinical significance.