The 5-hydroxytryptamine 2A receptor, encoded by HTR2A, is a major post-synaptic target for serotonin in the human brain and a therapeutic drug target. Despite hundreds of genetic associations investigating HTR2A polymorphisms in neuropsychiatric disorders and therapies, the role of genetic HTR2A variability in health and disease remains uncertain.
To discover and characterize regulatory HTR2A variants, we sequenced whole transcriptomes from ten human brain regions with massively-parallel RNA sequencing and measured allelic expression of multiple HTR2A mRNA transcript variants. Following discovery of functional variants, we further characterized their impact on genetic expression in vitro.
Three polymorphisms modulate the use of novel alternative exons and untranslated regions (UTRs), changing expression of RNA and protein. The frequent promoter variant rs6311, widely implicated in human neuropsychiatric disorders, decreases usage of an upstream transcription start site encoding a longer 5′UTR with greater translation efficiency. rs76665058, located in an extended 3′UTR and unique to individuals of African descent, modulates allelic HTR2A mRNA expression. The third SNP, unannotated and present in only a single subject, directs alternative splicing of exon 2. Targeted analysis of HTR2A in the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study reveals associations between functional variants and depression severity or citalopram response.
Regulatory polymorphisms modulate HTR2A mRNA expression in an isoform-specific manner, directing the usage of novel untranslated regions and alternative exons. These results provide a foundation for delineating the role of HTR2A and serotonin signaling in CNS disorders.
serotonin; 5-HT2A; HTR2A; schizophrenia; depression; mRNA expression
Blood-brain barrier; solute carrier transporter; drug transporter; OCT3; organic cation transporter; MATE1; multidrug and toxin extrusion protein; expression profiling
Genetic variation in C-type lectins influences infectious disease susceptibility but remains poorly understood. We employed allelic mRNA expression imbalance (AEI) technology for SP-A1, SP-A2, SP-D, DC-SIGN, MRC1, and Dectin-1, expressed in human macrophages and/or lung tissues. Frequent AEI, an indicator of regulatory polymorphisms, was observed in SP-A2, SP-D, and DC-SIGN. AEI was measured for SP-A2 in 38 lung tissues using four marker SNPs and was confirmed by next generation sequencing of one lung RNA sample. Genomic DNA at the SP-A2 DNA locus was sequenced by Ion Torrent technology in 16 samples. Correlation analysis of genotypes with AEI identified a haplotype block, and, specifically, the intronic SNP rs1650232 (30% MAF); the only variant consistently associated with an approximately two-fold change in mRNA allelic expression. Previously shown to alter a NAGNAG splice acceptor site with likely effects on SP-A2 expression, rs1650232 generates an alternative splice variant with three additional bases at the start of exon 3. Validated as a regulatory variant, rs1650232 is in partial LD with known SP-A2 marker SNPs previously associated with risk for respiratory diseases including tuberculosis. Applying functional DNA variants in clinical association studies, rather than marker SNPs, will advance our understanding of genetic susceptibility to infectious diseases.
C-type lectin; marker SNP; regulatory variant; SP-A2; allelic expression imbalance
Measuring allelic RNA expression ratios is a powerful approach for detecting cis-acting regulatory variants, RNA editing, loss of heterozygosity in cancer, copy number variation, and allele-specific epigenetic gene silencing. Whole transcriptome RNA sequencing (RNA-Seq) has emerged as a genome-wide tool for identifying allelic expression imbalance (AEI), but numerous factors bias allelic RNA ratio measurements. Here, we compare RNA-Seq allelic ratios measured in nine different human brain regions with a highly sensitive and accurate SNaPshot measure of allelic RNA ratios, identifying factors affecting reliable allelic ratio measurement. Accounting for these factors, we subsequently surveyed the variability of RNA editing across brain regions and across individuals.
We find that RNA-Seq allelic ratios from standard alignment methods correlate poorly with SNaPshot, but applying alternative alignment strategies and correcting for observed biases significantly improves correlations. Deploying these methods on a transcriptome-wide basis in nine brain regions from a single individual, we identified genes with AEI across all regions (SLC1A3, NHP2L1) and many others with region-specific AEI. In dorsolateral prefrontal cortex (DLPFC) tissues from 14 individuals, we found evidence for frequent regulatory variants affecting RNA expression in tens to hundreds of genes, depending on stringency for assigning AEI. Further, we find that the extent and variability of RNA editing is similar across brain regions and across individuals.
These results identify critical factors affecting allelic ratios measured by RNA-Seq and provide a foundation for using this technology to screen allelic RNA expression on a transcriptome-wide basis. Using this technology as a screening tool reveals tens to hundreds of genes harboring frequent functional variants affecting RNA expression in the human brain. With respect to RNA editing, the similarities within and between individuals leads us to conclude that this post-transcriptional process is under heavy regulatory influence to maintain an optimal degree of editing for normal biological function.
RNA-Seq; Whole transcriptome; Allele expression; mRNA expression; Functional genetics; Regulatory polymorphism; eQTL; Read alignment; Next generation sequencing; Bioinformatics
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
Flavopiridol is a cyclin-dependent kinase inhibitor in phase II clinical development for treatment of various forms of cancer. When administered with a pharmacokinetically (PK)-directed dosing schedule, flavopiridol exhibited striking activity in patients with refractory chronic lymphocytic leukemia. This study aimed to evaluate pharmacogenetic factors associated with inter-individual variability in pharmacokinetics and outcomes associated with flavopiridol therapy.
Thirty-five patients who received single-agent flavopiridol via the PK-directed schedule were genotyped for 189 polymorphisms in genes encoding 56 drug metabolizing enzymes and transporters. Genotypes were evaluated in univariate and multivariate analyses as covariates in a population PK model. Transport of flavopiridol and its glucuronide metabolite was evaluated in uptake assays in HEK-293 and MDCK-II cells transiently transfected with SLCO1B1. Polymorphisms in ABCC2, ABCG2, UGT1A1, UGT1A9, and SLCO1B1 were found to significantly correlate with flavopiridol PK in univariate analysis. Transport assay results indicated both flavopiridol and flavopiridol-glucuronide are substrates of the SLCO1B1/OATP1B1 transporter. Covariates incorporated into the final population PK model included bilirubin, SLCO1B1 rs11045819 and ABCC2 rs8187710. Associations were also observed between genotype and response. To validate these findings, a second set of data with 51 patients was evaluated, and overall trends for associations between PK and PGx were found to be consistent.
Polymorphisms in transport genes were found to be associated with flavopiridol disposition and outcomes. Observed clinical associations with SLCO1B1 were functionally validated indicating for the first time its relevance as a transporter of flavopiridol and its glucuronide metabolite. A second 51-patient dataset indicated similar trends between genotype in the SLCO1B1 and other candidate genes, thus providing support for these findings. Further study in larger patient populations will be necessary to fully characterize and validate the clinical impact of polymorphisms in SLCO1B1 and other transporter and metabolizing enzyme genes on outcomes from flavopiridol therapy.
Hepatocellular carcinoma (HCC) is on the rise worldwide. HCC responds poorly to chemotherapy. Lapatinib is an inhibitor of EGFR and HER2/NEU both implicated in hepatocarcinogenesis. This trial was designed to determine the safety and efficacy of lapatinib in HCC.
A Fleming phase II design with a single stage of 25 patients with a 90% power to exclude a true response rate of < 10% and detect a true response rate of ≥30% was utilized. The dose of lapatinib was 1,500 mg/d administered orally in 28-day cycles. Tumor and blood specimens were analyzed for expression of HER2/NEU/CEP17 and status of downstream signal pathway proteins.
Twenty-six patients with HCC enrolled on this study. 19% had one prior therapy. Most common toxicities were diarrhea (73%), nausea (54%) and rash (42%). No objective responses were observed. Ten (40%) patients had stable disease (SD) as their best response including 6 (23%) with SD lasting > 120 days. Median progression-free-survival was 1.9 months and median overall survival 12.6 months. Patients who developed a rash had a borderline statistically significant longer survival. Tissue and blood specimens were available on >90% of patients. No somatic mutations in EGFR (exons 18–21) were found. In contrast to our previous findings, we did not find evidence of HER2/NEU somatic mutations. PTEN, P-AKT and P70S6K expression did not correlate with survival.
Lapatinib is well-tolerated but appears to benefit only a subgroup of patients for whom predictive molecular or clinical characteristics are not yet fully defined.
Personality traits related to emotion processing are, at least in part, heritable and genetically determined. Dopamine D2 receptor signaling is involved in modulation of emotional behavior and activity of associated brain regions such as the amygdala and the prefrontal cortex. An intronic single nucleotide polymorphism within the D2 receptor gene (DRD2, rs1076560, guanine>thymine - G>T) shifts splicing of the two protein isoforms (D2 short, D2S, mainly presynaptic, and D2 long, D2L) and has been associated with modulation of memory performance and brain activity. Here, our aim was to investigate the association of DRD2 rs1076560 genotype with personality traits of emotional stability and with brain physiology during processing of emotionally relevant stimuli. DRD2 genotype and Big Five Questionnaire scores were evaluated in 134 healthy subjects demonstrating that GG subjects have reduced ‘emotion control’ compared with GT subjects. fMRI in a sample of 24 individuals indicated greater amygdala activity during implicit processing and greater dorsolateral prefrontal cortex (DLPFC) response during explicit processing of facial emotional stimuli in GG subjects compared with GT. Other results also demonstrate an interaction between DRD2 genotype and facial emotional expression on functional connectivity of both amygdala and dorsolateral prefrontal regions with overlapping medial prefrontal areas. Moreover, rs1076560 genotype is associated with differential relationships between amygdala/DLPFC functional connectivity and emotion control scores. These results suggest that genetically determined D2 signaling may explain part of personality traits related to emotion processing and individual variability in specific brain responses to emotionally relevant inputs.
amygdala; DRD2; dopamine; emotion; fMRI; prefrontal cortex
Variation of the gene coding for D2 receptors (DRD2) has been associated with risk for schizophrenia and with working memory deficits. A functional intronic SNP (rs1076560) predicts relative expression of the two D2 receptors isoforms, D2S (mainly pre-synaptic) and D2L (mainly post-synaptic). However, the effect of functional genetic variation of DRD2 on striatal dopamine D2 signaling and on its correlation with prefrontal activity during working memory in humans is not known.
Thirty-seven healthy subjects were genotyped for rs1076560 (G>T) and underwent SPECT with [123I]IBZM (which binds primarily to post-synaptic D2 receptors) and with [123I]FP-CIT (which binds to pre-synaptic dopamine transporters, whose activity and density is also regulated by pre-synaptic D2 receptors), as well as BOLD fMRI during N-Back working memory.
Subjects carrying the T allele (previously associated with reduced D2S expression) had striatal reductions of [123I]IBZM and of [123I]FP-CIT binding. DRD2 genotype also differentially predicted the correlation between striatal dopamine D2 signaling (as identified with factor analysis of the two radiotracers) and activity of the prefrontal cortex during working memory as measured with BOLD fMRI, which was positive in GG subjects and negative in GT.
Our results demonstrate that this functional SNP within DRD2 predicts striatal binding of the two radiotracers to dopamine transporters and D2 receptors as well as the correlation between striatal D2 signaling with prefrontal cortex activity during performance of a working memory task. These data are consistent with the possibility that the balance of excitatory/inhibitory modulation of striatal neurons may also affect striatal outputs in relationship with prefrontal activity during working memory performance within the cortico-striatal-thalamic-cortical pathway.
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
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.
Alzheimer’s disease; SORL1; Allelic expression imbalance
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.
With recent advances in understanding of the neuroscience of risk taking, attention is now turning to genetic factors that may contribute to individual heterogeneity in risk attitudes. In this paper we test for genetic associations with risk attitude measures derived from both the psychology and economics literature. To develop a long-term prospective study, we first evaluate both types of risk attitudes and find that the economic and psychological measures are poorly correlated, suggesting that different genetic factors may underlie human response to risk faced in different behavioral domains. We then examine polymorphisms in a spectrum of candidate genes that affect neurotransmitter systems influencing dopamine regulation or are thought to be associated with risk attitudes or impulsive disorders. Analysis of the genotyping data identified two single nucleotide polymorphisms (SNPs) in the gene encoding the alpha 4 nicotine receptor (CHRNA4, rs4603829 and rs4522666) that are significantly associated with harm avoidance, a risk attitude measurement drawn from the psychology literature. Novelty seeking, another risk attitude measure from the psychology literature, is associated with several COMT (catechol-O-methyl transferase) SNPs while economic risk attitude measures are associated with several VMAT2 (vesicular monoamine transporter) SNPs, but the significance of these associations did not withstand statistical adjustment for multiple testing and requires larger cohorts. These exploratory results provide a starting point for understanding the genetic basis of risk attitudes by considering the range of methods available for measuring risk attitudes and by searching beyond the traditional direct focus on dopamine and serotonin receptor and transporter genes.
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
Dopamine modulation of neuronal activity during memory tasks identifies a non-linear inverted-U shaped function. Both the dopamine transporter (DAT) and dopamine D2 receptors (encoded by DRD2) critically regulate dopamine signaling in the striatum and in prefrontal cortex during memory. Moreover, in vitro studies have demonstrated that DAT and D2 proteins reciprocally regulate each other presynaptically. Therefore, we have evaluated the genetic interaction between a DRD2 polymorphism (rs1076560) causing reduced presynaptic D2 receptor expression and the DAT 3’-VNTR variant (affecting DAT expression) in a large sample of healthy subjects undergoing BOLD - fMRI during memory tasks and structural MRI. Results indicated a significant DRD2/DAT interaction in prefrontal cortex and striatum BOLD activity during both working memory and encoding of recognition memory. The differential effect on BOLD activity of the DAT variant was mostly manifest in the context of the DRD2 allele associated with lower presynaptic expression. Similar results were also evident for gray matter volume in caudate. These interactions describe a non-linear relationship between compound genotypes and brain activity or gray matter volume. Complementary data from striatal protein extracts from wild-type and D2 knock-out animals (D2R−/−) indicate that DAT and D2 proteins interact in vivo. Taken together, our results demonstrate that the interaction between genetic variants in DRD2 and DAT critically modulates the non-linear relationship between dopamine and neuronal activity during memory processing.
working memory; Recognition Memory; FMRI; Dopamine; Transport; D2; Receptor
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.