CYP3A ranks among the most abundant cytochrome P450 enzymes in the liver, playing a dominant role in metabolic elimination of clinically used drugs. A main member in CYP3A family, CYP3A4 expression and activity vary considerably among individuals, attributable to genetic and non-genetic factors, affecting drug dosage and efficacy. However, the extent of genetic influence has remained unclear. This review assesses current knowledge on the genetic factors influencing CYP3A4 activity. Coding region CYP3A4 polymorphisms are rare and account for only a small portion of inter-person variability in CYP3A metabolism. Except for the promoter allele CYP3A4*1B with ambiguous effect on expression, common CYP3A4 regulatory polymorphisms were thought to be lacking. Recent studies have identified a relatively common regulatory polymorphism, designated CYP3A4*22 with robust effects on hepatic CYP3A4 expression. Combining CYP3A4*22 with CYP3A5 alleles *1, *3 and *7 has promise as a biomarker predicting overall CYP3A activity. Also contributing to variable expression, the role of polymorphisms in transcription factors and microRNAs is discussed.
cytochrome P450s; CYP3A4; polymorphism; biomarker
Cytochrome P450 2C19 is responsible for the metabolism of many drugs, including the activation of clopidogrel. The allele CYP2C19*17 is associated with ultra-rapid metabolizer phenotypes by increasing gene transcription. This study tests to what extent CYP2C19*17 enhances CYP2C19 expression in human liver and whether additional regulatory variants contribute to variation in CYP2C19 expression.
CYP2C19 mRNA was measured with quantitative real-time PCR (qRT-PCR), enzyme activity as metabolic velocity with S-mephenytoin as the substrate and allelic mRNA expression ratio with SNaPshot in human livers. CYP2C19 transcribed exons and a 4kb promoter region were sequenced using IonTorrent PGM or Sanger sequencing and screened for polymorphisms associated with total hepatic CYP2C19 mRNA, enzyme activity and allelic mRNA ratios.
Livers heterozygote and homozygous for CYP2C19*17 had mRNA levels 1.8-fold (p=0.028) and 2.9- fold (p=0.006), respectively, above homozygous reference allele livers. CYP2C19*17 heterozygotes were also associated with increased allelic mRNA expression (allelic ratio ~1.8-fold, SD±0.6, p<0.005), whereas CYP2C19 enzyme activity was elevated 2.3-fold, with borderline significance (p=0.06) in CYP2C19*17 carriers. One liver sample of African ancestry displayed a 2-fold allelic expression ratio, and another sample, a ~12-fold increase in metabolic velocity. Neither case was accounted for by *17, which indicates the presence of additional regulatory variants.
Our findings confirm *17 as a regulatory polymorphism enhancing hepatic CYP2C19 expression 2-fold with potential to compensate for the loss of function allele CYP2C19*2. Additional regulatory factors may also enhance CYP2C19 expression in African American populations.
CYP2C19; clopidogrel; allelic expression imbalance; polymorphism; biomarker; cytochrome P450
The HIV-1 restriction factor SAM domain and HD domain-containing protein 1 (SAMHD1) blocks HIV-1 infection in human myeloid cells. Mutations in the SAMHD1 gene are associated with rare genetic diseases including Aicardi–Goutieres syndrome. However, it is unknown whether polymorphisms of SAMHD1 are associated with infection and natural control of HIV-1 in humans. Our objective was to determine whether the expression of SAMHD1 mRNA is affected by common single nucleotide polymorphisms (SNPs) in SAMHD1 and whether the SNPs are associated with HIV-1 infection status. Using a tagging SNP approach, we determined the association between eight tagging SNPs in SAMHD1 and the mRNA expression in B-lymphocyte cell lines from 70 healthy white donors. We identified one SNP (rs1291142) that was significantly associated with SAMHD1 mRNA expression, with minor allele carriers having 30% less mRNA levels (p=0.015). However, after analyzing the published genome-wide association study data of 857 HIV-1 controllers and 2088 HIV-1 progressors from the European and African-American cohorts, we did not find a significant association between SNPs in SAMHD1 and HIV-1 infection status, including SNP rs1291142 (p>0.05). We also observed 2- to 6-fold variations of SAMHD1 mRNA levels in primary B-lymphocytes, CD4+ T-lymphocytes, and CD14+ monocytes from five healthy donors. Our results suggest that common regulatory polymorphism(s) exist in the SAMHD1 gene that affects its mRNA expression in B-lymphocyte cell lines from healthy whites. However, polymorphisms of SAMHD1 are unlikely to contribute to the infection and natural control of HIV-1 in European and African-American individuals.
Statins are indicated for prevention of atherosclerotic cardiovascular disease. Metabolism of certain statins involves the cytochrome P450 3A (CYP3A) enzymes, and CYP3A4*22 significantly influences the dose needed for achieving optimal lipid control for atorva statin, simvastatin, and lovastatin. CYP3A4/5 combined genotype approaches have proved useful in some studies involving CYP3A substrates. We intend to compare a combined genotype analysis to our previously reported single gene CYP3A4 analysis.
A total of 235 patients receiving stable statin doses were genotyped and grouped by CYP3A4/5 status.
The number and demographic composition of the patients categorized into the combined genotype groups were consistent with those reported for other cohorts. Dose requirement was significantly associated with the ordered combined-genotype grouping; median daily doses were nearly 40% greater for CYP3A4/5 intermediate metabolizers compared with poor metabolizers, and median daily doses were nearly double for extensive metabolizers compared with poor metabolizers. The combined-genotype approach, however, did not improve the genotype-dosage correlation p-values when compared with the previously-reported analysis; values changed from 0.129 to 0.166, 0.036 to 0.185, and 0.014 to 0.044 for atorvastatin, simvastatin, and the combined statin analysis, respectively.
The previously-reported single-gene approach was superior for predicting statin dose requirement in this cohort.
CYP3A4/5 combined genotype; gene-gene interaction; pharmacogenomics; statin
Cytochrome P450 2C9 (CYP2C9) c.449G> A (*8) is common in African Americans and is associated with decreased warfarin clearance. We examined the effect of promoter region variants inherited with 449G > A on warfarin clearance, dose requirements, and CYP2C9 expression. In an African American cohort, 449G > A was in linkage disequilibrium with c. – 1766T >C (r2 = 0.89) and c. – 1188T>C (D′ =1). The combination of the – 1766C and 449A alleles with the – 1188CC genotype was associated with lower S-warfarin clearance (0.86±0.22 vs. 1.66±0.75 ml/min/m2; n=48; P <0.01) and dose requirements [33 (25–49) vs. 43 (35–56) mg/week; n= 243; P= 0.03] compared with other genotypes. In liver tissue, alleles with the – 1766C/ – 1188C/449A haplotype showed two-fold decreased mRNA expression compared with reference alleles. In a promoter reporter assay, the – 1766C/ – 1188C haplotype decreased CYP2C9 promoter activity. These data suggest that promoter region polymorphisms inherited with 449G >A decrease CYP2C9 expression and contribute to CYP2C9*8 effects on warfarin clearance and dose requirements.
African American; CYP2C9*8; genotype; polymorphism; warfarin
N-acetyltransferase 1 (NAT1) metabolizes drugs and environmental carcinogens. NAT1 alleles *10 and *11 have been proposed to alter protein level or enzyme activity compared to wild-type NAT1 *4 and to confer cancer risk, via uncertain pathways. This study characterizes regulatory polymorphisms and underlying mechanisms of NAT1 expression.
We measured allelic NAT1 mRNA expression and translation, as a function of multiple transcription start sites, alternative splicing, and three 3′-polyadenylation sites in human livers (one of which discovered in this study), B lymphocytes, and transfected cells. In a clinical study of 469 HIV/AIDS patients treated with the NAT1/NAT2 substrate sulfamethoxazole (SMX), associations were tested between SMX induced hypersensitivity and NAT1 *10 and *11 genotypes, together with known NAT2 polymorphisms.
NAT1*10 and *11 were determined to act as common regulatory alleles accounting for most NAT1 expression variability, both leading to increased translation into active protein. NAT1*11 (2.4% minor allele frequency) affected 3′polyadenylation site usage, thereby increasing formation of NAT1 mRNA with intermediate length 3′UTR (major isoform) at the expense of the short isoform, resulting in more efficient protein translation. NAT1 *10 (19% minor allele frequency) increased translation efficiency without affecting 3′-UTR polyadenylation site usage. Livers and B-lymphocytes with *11/*4 and *10/*10 genotypes displayed higher NAT1 immunoreactivity and NAT1 enzyme activity than the reference genotype *4/*4. Patients who carry *10/*10 and *11/*4 (‘fast NAT1 acetylators’) were less likely to develop hypersensitivity to SMX, but this was observed only in subjects also carrying a slow NAT2 acetylator genotype.
NAT1 *10 and *11 significantly increase NAT1 protein level/enzyme activity, enabling the classification of carriers into reference and rapid acetylators. Rapid NAT1 acetylator status appears to protect against SMX toxicity by compensating for slow NAT2 acetylator status.
N-acetyltransferase; NAT1; polyadenylation; allelic expression imbalance; sulfamethoxazole; cotrimoxazole; protein translation; acetylator phenotype; idiosyncratic drug reactions
Sulfamethoxazole (SMX) is a commonly used antibiotic for prevention of infectious diseases associated with HIV/AIDS and immune-compromised states. SMX-induced hypersensitivity is an idiosyncratic cutaneous drug reaction with genetic components. Here, we tested association of candidate genes involved in SMX bioactivation and antioxidant defense with SMX-induced hypersensitivity.
Seventy seven single nucleotide polymorphisms (SNPs) from 14 candidate genes were genotyped and assessed for association with SMX-induced hypersensitivity, in a cohort of 171 HIV/AIDS patients. SNP rs761142 T > G, in glutamate cysteine ligase catalytic subunit (GCLC), was significantly associated with SMX-induced hypersensitivity, with an adjusted p value of 0.045. This result was replicated in a second cohort of 249 patients (p = 0.025). In the combined cohort, heterozygous and homozygous carriers of the minor G allele were at increased risk of developing hypersensitivity (GT vs TT, odds ratio = 2.2, 95% CL 1.4-3.7, p = 0.0014; GG vs TT, odds ratio = 3.3, 95% CL 1.6 – 6.8, p = 0.0010). Each minor allele copy increased risk of developing hypersensitivity 1.9 fold (95% CL 1.4 – 2.6, p = 0.00012). Moreover, in 91 human livers and 84 B-lymphocytes samples, SNP rs761142 homozygous G allele carriers expressed significantly less GCLC mRNA than homozygous TT carriers (p < 0.05).
rs761142 in GCLC was found to be associated with reduced GCLC mRNA expression and with SMX-induced hypersensitivity in HIV/AIDS patients. Catalyzing a critical step in glutathione biosynthesis, GCLC may play a broad role in idiosyncratic drug reactions.
Idiosyncratic drug reaction; Sulfamethoxazole; Hypersensitivity; Glutamate cysteine ligase catalytic subunit (GCLC); Association; HIV/AIDS
Polymorphisms in and around the Cholesteryl Ester Transfer Protein (CETP) gene have been associated with HDL levels, risk for coronary artery disease (CAD), and response to therapy. The mechanism of action of these polymorphisms has yet to be defined. We used mRNA allelic expression and splice isoform measurements in human liver tissues to identify the genetic variants affecting CETP levels. Allelic CETP mRNA expression ratios in 56 human livers were strongly associated with several variants 2.5–7 kb upstream of the transcription start site (e.g., rs247616 p = 6.4×10−5, allele frequency 33%). In addition, a common alternatively spliced CETP isoform lacking exon 9 (Δ9), has been shown to prevent CETP secretion in a dominant-negative manner. The Δ 9 expression ranged from 10 to 48% of total CETP mRNA in 94 livers. Increased formation of this isoform was exclusively associated with an exon 9 polymorphism rs5883-C>T (p = 6.8×10−10) and intron 8 polymorphism rs9930761-T>C (5.6×10−8) (in high linkage disequilibrium with allele frequencies 6–7%). rs9930761 changes a key splicing branch point nucleotide in intron 8, while rs5883 alters an exonic splicing enhancer sequence in exon 9.
The effect of these polymorphisms was evaluated in two clinical studies. In the Whitehall II study of 4745 subjects, both rs247616 and rs5883T/rs9930761C were independently associated with increased HDL-C levels in males with similar effect size (rs247616 p = 9.6×10−28 and rs5883 p = 8.6×10−10, adjusted for rs247616). In an independent multiethnic US cohort of hypertensive subjects with CAD (INVEST-GENE), rs5883T/rs9930761C alone were significantly associated with increased incidence of MI, stroke, and all-cause mortality in males (rs5883: OR 2.36 (CI 1.29–4.30), p = 0.005, n = 866). These variants did not reach significance in females in either study. Similar to earlier results linking low CETP activity with poor outcomes in males, our results suggest genetic, sex-dependent CETP splicing effects on cardiovascular risk by a mechanism independent of circulating HDL-C levels.
The dopamine receptor D2 (encoded by DRD2) is implicated in susceptibility to mental disorders and cocaine abuse, but mechanisms responsible for this relationship remain uncertain. DRD2 mRNA exists in two main splice isoforms with distinct functions: D2 long (D2L) and D2 short (D2S, lacking exon 6), expressed mainly postsynaptically and presynaptically, respectively. Two intronic single-nucleotide polymorphisms (SNPs rs2283265 (intron 5) and rs1076560 (intron 6)) in high linkage disequilibrium (LD) with each other have been reported to alter D2S/D2L splicing and several behavioral traits in human subjects, such as memory processing. To assess the role of DRD2 variants in cocaine abuse, we measured levels of D2S and D2L mRNA in human brain autopsy tissues (prefrontal cortex and putamen) obtained from cocaine abusers and controls, and genotyped a panel of DRD2 SNPs (119 abusers and 95 controls). Robust effects of rs2283265 and rs1076560 on reducing formation of D2S relative to D2L were confirmed. The minor alleles of rs2283265/rs1076560 were considerably more frequent in Caucasians (18%) compared with African Americans (7%). Also, in Caucasians, rs2283265/rs1076560 minor alleles were significantly overrepresented in cocaine abusers compared with controls (rs2283265: 25 to 9%, respectively; p=0.001; OR=3.4 (1.7–7.1)). Several SNPs previously implicated in diverse clinical association studies are in high LD with rs2283265/rs1076560 and could have served as surrogate markers. Our results confirm the role of rs2283265/rs1076560 in D2 alternative splicing and support a strong role in susceptibility to cocaine abuse.
alternative splicing; cocaine; dopamine; DRD2; D2S; human; addiction and substance abuse; dopamine; neurogenetics; psychostimulants; drd2; d2s; human; alternative splicing; cocaine
CHRNA5, encoding the nicotinic α5 subunit, is implicated in multiple disorders, including nicotine addiction and lung cancer. Previous studies demonstrate significant associations between promoter polymorphisms and CHRNA5 mRNA expression, but the responsible sequence variants remain uncertain. To search for cis-regulatory variants, we measured allele-specific mRNA expression of CHRNA5 in human prefrontal cortex autopsy tissues and scanned the CHRNA5 locus for regulatory variants. A cluster of six frequent single nucleotide polymorphisms (rs1979905, rs1979906, rs1979907, rs880395, rs905740, and rs7164030), in complete linkage disequilibrium, fully account for a >2.5-fold allelic expression difference and a fourfold increase in overall CHRNA5 mRNA expression. This proposed enhancer region resides more than 13 kilobases upstream of the CHRNA5 transcription start site. The same upstream variants failed to affect CHRNA5 mRNA expression in peripheral blood lymphocytes, indicating tissue-specific gene regulation. Other promoter polymorphisms were also correlated with overall CHRNA5 mRNA expression in the brain, but were inconsistent with allelic mRNA expression ratios, a robust and proximate measure of cis-regulatory variants. The enhancer region and the nonsynonymous polymorphism rs16969968 generate three main haplotypes that alter the risk of developing nicotine dependence. Ethnic differences in linkage disequilibrium across the CHRNA5 locus require consideration of the upstream enhancer variants when testing clinical associations.
Nicotinic receptor; alpha5 subunit; gene expression; nicotine dependence; lung cancer; enhancer
CHRNA5, encoding the nicotinic α5 subunit, is implicated in multiple disorders, including nicotine addiction and lung cancer. Previous studies demonstrate significant associations between promoter polymorphisms and CHRNA5 mRNA expression, but the responsible sequence variants remain uncertain. To search for cis-regulatory variants, we measured allele-specific mRNA expression of CHRNA5 in human prefrontal cortex autopsy tissues and scanned the CHRNA5 locus for regulatory variants. A cluster of six frequent single-nucleotide polymorphisms (rs1979905, rs1979906, rs1979907, rs880395, rs905740, and rs7164030), in complete linkage disequilibrium (LD), fully account for a >2.5-fold allelic expression difference and a fourfold increase in overall CHRNA5 mRNA expression. This proposed enhancer region resides more than 13 kilobases upstream of the CHRNA5 transcription start site. The same upstream variants failed to affect CHRNA5 mRNA expression in peripheral blood lymphocytes, indicating tissue-specific gene regulation. Other promoter polymorphisms were also correlated with overall CHRNA5 mRNA expression in the brain, but were inconsistent with allelic mRNA expression ratios, a robust and proximate measure of cis-regulatory variants. The enhancer region and the nonsynonymous polymorphism rs16969968 generate three main haplotypes that alter the risk of developing nicotine dependence. Ethnic differences in LD across the CHRNA5 locus require consideration of upstream enhancer variants when testing clinical associations.
nicotinic receptor; α5 subunit; gene expression; nicotine dependence; lung cancer; enhancer
The gene encoding the target of calcium channel blockers, the α1c-subunit of the L-type calcium channel (CACNA1C) has not been well characterized and only small pharmacogenetic studies testing this gene have been published to date.
Methods and Results
Resequencing of CACNA1C was performed followed by a nested case-control study of the INternational VErapamil SR/trandolapril STudy (INVEST) GENEtic Substudy (INVEST-GENES). Of 46 polymorphisms identified, eight were assessed in the INVEST-GENES. Rs1051375 was found to have a significant interaction with treatment strategy (p=0.0001). Rs1051375 A/A genotype was associated with a 46% reduction in the primary outcome among those randomized to verapamil SR treatment compared to atenolol treatment (OR 0.54 95% CI 0.32-0.92). In heterozygous A/G individuals, there was no difference in the occurrence of the primary outcome when randomized to verapamil SR versus atenolol treatment (OR 1.47 95% CI 0.86-2.53), while homozygous G/G individuals had a greater than 4-fold increased risk of the primary outcome with verapamil treatment compared to those randomized to atenolol treatment (OR 4.59 95% CI 1.67-12.67). We did not identify allelic expression imbalance or differences in mRNA expression in heart tissue by rs1051375 genotype.
Variation in CACNA1C is associated with treatment response among hypertensive patients with stable coronary artery disease. Our data suggest a genetically-defined group of patients that benefit most from calcium channel blocker therapy, a group that benefits most from β-blocker therapy, and a third group in which calcium channel blocker and β-blocker therapy are equivalent.
genetics; pharmacology; ion channels; calcium; pharmacogenetics
Dopamine D2 receptor signalling is strongly implicated in the aetiology of schizophrenia. We have recently characterized the function of three DRD2 SNPs: rs12364283 in the promoter affecting total D2 mRNA expression; rs2283265 and rs1076560, respectively in introns 5 and 6, shifting mRNA splicing to two functionally distinct isoforms, the short form of D2 (D2S) and the long form (D2L). These two isoforms differentially contribute to dopamine signalling in prefrontal cortex and in striatum. We performed a case–control study to determine association of these variants and of their main haplotypes with several schizophrenia-related phenotypes. We demonstrate that the minor allele in the intronic variants is associated with reduced expression of %D2S of total mRNA in post-mortem prefrontal cortex, and with impaired working memory behavioural performance, both in patients and controls. However, the fMRI results show opposite effects in patients compared with controls: enhanced engagement of prefronto-striatal pathways in controls and reduced activity in patients. Moreover, the promoter variant is also associated with working memory activity in prefrontal cortex and striatum of patients, and less robustly with negative symptoms scores. Main haplotypes formed by the three DRD2 variants showed significant associations with these phenotypes consistent with those of the individual SNPs. Our results indicate that the three functional DRD2 variants modulate schizophrenia phenotypes possibly by modifying D2S/D2L ratios in the context of different total D2 density.
dopamine; D2 receptor; working memory; prefrontal cortex; striatum
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.
We sought to determine whether polymorphisms in the large-conductance calcium and voltage-dependent potassium (BK) channel β1 subunit gene, KCNMB1, are associated with blood pressure response to verapamil SR or adverse outcomes in the GENEtic substudy of the INternational VErapamil SR/trandolapril STudy (INVEST-GENES).
KCNMB1 is involved in calcium sensitivity and hypertension. The association between variability in KCNMB1 and calcium antagonist response, however, has not been assessed.
Genetic samples were collected from 5979 patients in INVEST. Blood pressure response to verapamil SR and time to achieve blood pressure control was assessed in relation to Glu65Lys and Val110Leu genotypes. The primary outcome (all cause mortality, nonfatal myocardial infarction or nonfatal stroke) was compared between genotype groups, and interaction with verapamil SR therapy was assessed.
Systolic blood pressure response to verapamil SR did not differ by KCNMB1 genotype. Lys65 variant carriers, however, achieved blood pressure control earlier than Glu65Glu individuals [1.47 (interquartile ratio 2.77) versus 2.83 (interquartile ratio 4.17) months, P = 0.01] and were less likely to require multiple drugs at the time of blood pressure control (adjusted odds ratio 0.43, 95% confidence interval 0.19–0.95). Leu110 variant carriers had a reduced risk of primary outcome (hazard ratio 0.68, 95% confidence interval 0.47–0.998). Subgroup analysis revealed this finding to be more pronounced in verapamil SR-assigned patients (hazard ratio 0.587, 95% confidence interval 0.33–1.04) compared with atenolol-assigned patients (hazard ratio 0.946, 95% confidence interval 0.56–1.59). No difference was seen in the occurrence of the primary outcome compared by codon 65 genotype.
Our findings suggest that KCNMB1 genotype influences responsiveness to verapamil SR and risk of adverse cardiovascular outcomes.
KCNMB1; polymorphism; verapamil SR
The voltage-dependent L-type calcium channel α-subunit 1c (Cav1.2, CACNA1C) undergoes extensive mRNA splicing, leading to numerous isoforms with different functions. L-type calcium channel blockers are used in the treatment of hypertension and arrhythmias, but response varies between individuals. We have studied the interindividual variability in mRNA expression and splicing of CACNA1C, in 65 heart tissue samples, taken from heart transplant recipients.
Splice variants were measured quantitatively by polymerase chain reaction in 12 splicing loci of CACNA1C mRNA. To search for functional cis-acting polymorphisms, we determined allelic expression ratios for total CACNA1C mRNA and several splice variants using marker single nucleotide polymorphisms in exon 4 and exon 30.
Total CACNA1C mRNA levels varied ∼50-fold. Substantial splicing occurred in six loci generating two or more splice variants, some with known functional differences. Splice patterns varied broadly between individuals. Two heart tissues expressed predominantly the dihydropyridine-sensitive smooth muscle isoform of CACNA1C (containing exon 8), rather than the cardiac isoform (containing exon 8a). Lack of significant allelic expression imbalance, observed with total mRNA and several splice variants, argued against CACNA1C polymorphisms as a cause of variability. Taken together, highly variable splicing can cause profound phenotypic variations of CACNA1C function, potentially associated with disease susceptibility and response to L-type calcium channel blockers.
cis-acting polymorphism; L-type calcium channel α-subunit 1c; mRNA splicing
Variants in numerous genes are thought to affect the success or failure of cancer chemotherapy. Interindividual variability can result from genes involved in drug metabolism and transport, drug targets (receptors, enzymes, etc), and proteins relevant to cell survival (e.g., cell cycle, DNA repair, and apoptosis). The purpose of the current study is to establish a flexible, cost-effective, high-throughput genotyping platform for candidate genes involved in chemoresistance and -sensitivity, and treatment outcomes.
We have adopted SNPlex for genotyping 432 single nucleotide polymorphisms (SNPs) in 160 candidate genes implicated in response to anticancer chemotherapy.
The genotyping panels were applied to 39 patients with chronic lymphocytic leukemia undergoing flavopiridol chemotherapy, and 90 patients with colorectal cancer. 408 SNPs (94%) produced successful genotyping results. Additional genotyping methods were established for polymorphisms undetectable by SNPlex, including multiplexed SNaPshot for CYP2D6 SNPs, and PCR amplification with fluorescently labeled primers for the UGT1A1 promoter (TA)nTAA repeat polymorphism.
This genotyping panel is useful for supporting clinical anticancer drug trials to identify polymorphisms that contribute to interindividual variability in drug response. Availability of population genetic data across multiple studies has the potential to yield genetic biomarkers for optimizing anticancer therapy.
Cis-acting genetic variations can affect the amount and structure of mRNA/protein. Genomic surveys indicate that polymorphisms affecting transcription and mRNA processing, including splicing and turnover, may account for main share of genetic factors in human phenotypic variability; however, most of these polymorphisms remain yet to be discovered. We use allelic expression imbalance (AEI) as a quantitative phenotype in the search for functionalcis-acting polymorphisms in many genes includingABCB1 (multidrug resistance 1 gene, MDR1, Pgp). Previous studies have shown that ABCB1 activity correlates with a synonymous polymorphism. C3435T; however, the functional polymorphism and molecular mechanisms underlying this clinical association remained unknown. Analysis of allele-specific expression in liver autopsy samples and in vitro expression experiments showed that C3435T represents a main functional polymorphism, accounting for 1.5-to 2-fold changes in mRNA levels. The mechanism appears to involve increased mRNA turnover, probably as a result of different folding structures calculated for mRNA with the Mfold program. Other examples of the successful application of AEI analysis for studying functional polymorphism include5-HTT (serotonin transporter, SLC6A4) andOPRM1 (μ opioid receptor). AEI is therefore a powerful approach for detectingcis-acting polymorphisms affecting gene expression and mRNA processing.
ABCB1; allele-specific expression; mRNA stability; cis-acting polymorphism
Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.
G Protein-Coupled; Receptors; Drug Therapy; Pharmacogenomics; Pharmacogenetics