Functional characterization of a -100_-102delAAG deletion-insertion polymorphism in the promoter region of the HTR3B gene

A review of the literature on the relationships between genetic polymorphisms and chemotherapy-induced nausea and vomiting

Despite current advances in antiemetic treatments, between 30% to and 60% of oncology patients experience chemotherapy-induced nausea (CIN) and 13% to 33% report chemotherapy-induced vomiting (CIV). Inter-individual differences are observed in the occurrence and severity of chemotherapy-induced nausea and vomiting (CINV). This review summarizes and critiques studies on associations between occurrence and severity of CINV and polymorphisms in serotonin receptor, drug metabolism, and drug transport pathway genes. Sixteen studies evaluated the associations between the occurrence and/or severity of CINV and single nucleotide polymorphisms (SNPs). Across these studies, three SNPs in 5-hydroxytryptamine receptor (5-HT3R) genes, two alleles of the cytochrome P450 family 2 subfamily D member 6 (CYP2D6) gene, and three SNPs in ATP binding cassette subfamily B member 1 (ABCB1) gene were associated with the occurrence and severity of CINV. Given the limited number of polymorphisms evaluated, additional research is warranted to identify new mechanisms to develop more targeted therapies.

Association of 5-HT3B Receptor Gene Polymorphisms with the Efficacy of Ondansetron for Postoperative Nausea and Vomiting

PURPOSE

Postoperative nausea and vomiting (PONV) is a common problem after general anesthesia. Although 5-hydroxytryptamine type 3 (5-HT3) receptor antagonists have significantly reduced PONV, over 35% of patients treated with ondansetron can experience PONV. In this study, we investigated whether the Y129S and -100_-102AAG deletion polymorphisms of the 5-HT3B receptor gene affect the efficacy of ondansetron in preventing PONV.

MATERIALS AND METHODS

Two hundred and forty-five adult patients who underwent laparoscopic cholecystectomy were enrolled. Ondansetron 0.1 mg/kg was intravenously administered 30 minutes before the end of surgery. Genomic DNA was prepared from blood samples using a nucleic acid isolation device. Both the Y129S variant and the -100_-102AAG deletion variant were screened for using a single base primer extension assay and a DNA direct sequencing method, respectively. The relationship between genetic polymorphisms and clinical outcomes of ondansetron treatment was investigated.

RESULTS

Among the 5-HT3B AAG deletion genotypes, the incidence of PONV was higher in patients with the homomutant than with other genotypes during the first 2 hours after surgery (p=0.02). There were no significant differences in the incidence of PONV among genotypes at 2-24 hours after surgery. In the Y129S variants of the 5-HT3B receptor gene, there were no significant differences in the incidence of PONV among genotypes during the first 2 hours and at 2-24 hours after surgery.

CONCLUSION

The response to ondansetron for PONV was significantly influenced by the -100_-102AAG deletion polymorphisms of the 5-HT3B gene. Thus, the -100_-102AAG deletion variants may be a pharmacogenetic predictor for responsiveness to ondansetron for PONV.

Pharmacogenetics of chemotherapy-induced nausea and vomiting

Chemotherapy-induced nausea and vomiting (CINV) is associated with distressing adverse effects observed in patients during cytotoxic chemotherapy. One of the potential factors explaining suboptimal response to currently used antiemetics is variability in genes encoding enzymes and proteins that play a role in the action of antiemetic drugs. Pharmacogenomics studies of CINV are sparse and focus mainly on polymorphisms associated with serotonin receptor, drug metabolism and drug transport. Currently, the role of pharmacogenetics in mechanisms of CINV has not been fully unraveled, and it is premature to implement results of pharmacogenetic association studies of antiemetic drugs in clinical practice. More uniform studies, with genetic profiles and biomarkers relevant for the proposed target and transporter mechanisms, are needed.

Does pharmacogenomics account for variability in control of acute chemotherapy-induced nausea and vomiting with 5-hydroxytryptamine type 3 receptor antagonists?

Chemotherapy-induced nausea and vomiting is one of the most concerning adverse drug effects from cytotoxic chemotherapy. Despite appropriate use of antiemetic guidelines, 20-30 % of patients experience breakthrough nausea and vomiting secondary to chemotherapy. To assess the variability of 5-hydroxytryptamine type 3 receptor antagonist efficacy caused by genetic variation, a review of the available literature was conducted. From the literature, three sources of pharmacogenomic variability were identified: polymorphisms associated with 5-hydroxytryptamine type 3 receptor subunits, drug metabolism via cytochromes P450, and drug transport in the body. Testing for receptor subunit polymorphisms is not applicable to a clinical setting at this time; however, cytochrome P450 2D6 testing is FDA-approved and widely accessible. Cytochrome P450 2D6 ultrarapid metabolizers and poor metabolizers displayed altered antiemetic efficacy when compared with intermediate metabolizers and extensive metabolizers. We postulate that testing for cytochrome P450 2D6 phenotypes may be the most accessible way to provide individualized antiemetic therapy in the future.

Hepatocyte nuclear factor 1 regulates the expression of the organic cation transporter 1 via binding to an evolutionary conserved region in intron 1 of the OCT1 gene

The organic cation transporter 1 (OCT1), also known as solute carrier family 22 member 1, is strongly and specifically expressed in the human liver. Here we show that the hepatocyte nuclear factor 1 (HNF1) regulates OCT1 transcription and contributes to the strong, liver-specific expression of OCT1. Bioinformatic analyses revealed strong conservation of HNF1 binding motifs in an evolutionary conserved region (ECR) in intron 1 of the OCT1 gene. Electrophoretic mobility shift and chromatin immunoprecipitation assays confirmed the specific binding of HNF1 to the intron 1 ECR. In reporter gene assays performed in HepG2 cells, the intron 1 ECR increased SV40 promoter activity by 22-fold and OCT1 promoter activity by 13-fold. The increase was reversed when the HNF1 binding sites in the intron 1 ECR were mutated or the endogenous HNF1α expression was downregulated with small interfering RNA. Following HNF1α overexpression in Huh7 cells, the intron 1 ECR increased SV40 promoter activity by 11-fold and OCT1 promoter activity by 6-fold. Without HNF1α overexpression, the increases were only 3- and 2-fold, respectively. Finally, in human liver samples, high HNF1 expression was significantly correlated with high OCT1 expression (r = 0.48, P = 0.002, n = 40). In conclusion, HNF1 is a strong regulator of OCT1 expression. It remains to be determined whether genetic variants, disease conditions, or drugs that affect HNF1 activity may affect the pharmacokinetics and efficacy of OCT1-transported drugs such as morphine, tropisetron, ondansetron, tramadol, and metformin. Beyond OCT1, this study demonstrates the validity and usefulness of interspecies comparisons in the discovery of functionally relevant genomic sequences.

Pharmacogenetic predictors of nausea and vomiting of pregnancy severity and response to antiemetic therapy: a pilot study

Background

Nausea and vomiting of pregnancy (NVP) is a common condition. The objective of this study was to evaluate the association between response to antiemetics in the treatment of NVP and genetic polymorphisms in the serotonin receptor subunit genes HTR3A and HTR3B.

Methods

Pregnant women ≥18 years of age with NVP starting antiemetic therapy with promethazine, prochlorperazine, metoclopramide, or ondansetron at ≤ 16 weeks gestational age were eligible. The study recruited 29 women with complete data and sampling who returned for their one week follow-up and were genotyped for HTR3A and HTR3B polymorphisms. Severity of NVP was captured (using Pregnancy Unique Quantification of Emesis (PUQE) and Quality of Life (QOL) tools) upon enrollment and after one week of antiemetic therapy. These measures were correlated with pharmacogenetic variability.

Results

Subjects with genotype associated with high serotonin affinity of the 5-HT_3B receptor (rs1176744, CC) required more antiemetic medications (p < 0.001) than other subjects. Those with genotypes associated with increased expression of the 5-HT_3A receptor subunit (rs1062613, CT or TT) had worse final PUQE scores (p = 0.01) than other subjects while rs3782025 variants carriers had significantly better initial (p = 0.02) and final (p = 0.02) PUQE scores than other subjects.

Conclusions

HTR3B and HTR3A gene variants may contribute to variability in response to antiemetic therapy for NVP.

Differences in 5-hydroxytryptamine-3B haplotype frequencies between Asians and Caucasians

BACKGROUND

The 5-hydroxytryptamine (5-HT3) receptor is a ligand-operated ion channel with five different receptor subunits (5-HT3A, B, C, D, and E) found in humans. Activation of 5-HT3 receptors influences various effects such as drug-induced emesis and causes behavioral problems such as anxiety, depression and cognitive disorders. To explore interethnic differences in the response to 5-HT3 antagonists, we studied haplotype frequencies in the gene encoding the 5-HT3B receptor in Asians and Caucasians.

METHODS

Three single nucleotide polymorphisms (SNPs) of the 5-HT3B receptor gene, i.e., deletion AAG at the 5'-UTR position, 18792A>G at the intron position, and 46698G>A at the 3' near gene position, were selected and genotyped in 165 Indonesian cancer patients and 188 Caucasian healthy volunteers. Haplotypes were set with gPlink, whereas the differences in haplotype frequencies between Indonesians and Caucasians were compared using multivariate analysis.

RESULTS

The haplotype profiles based on the deletion AAG, 18792A>G and 46698G>A were AAGAA, AAGAG, AAGGG, and deletion AG in both Indonesians and Caucasians. The frequency of the AAGAG haplotype was 54.8% in Indonesians and 39.9% in Caucasians (p<0.05). The frequency of the AAGGG haplotype was 14.3% in Indonesians and 29.3% in Caucasians. Moreover, there were significant differences in the frequencies of haplotype pairs between Indonesians and Caucasians (p<0.001).

CONCLUSION

Indonesian cancer patients had significantly different AAGAG and AAGGG haplotype frequencies of the gene encoding the 5-HT3B receptor compared to healthy Caucasians. This finding could be useful for understanding interethnic differences in the response to drugs targeting the 5-HT3B receptor in cancer-treatment-related emesis.

Replication of functional serotonin receptor type 3A and B variants in bipolar affective disorder: a European multicenter study

Serotonin type 3 receptors (5-HT(3)) are involved in learning, cognition and emotion, and have been implicated in various psychiatric phenotypes. However, their contribution to the pathomechanism of these disorders remains elusive. Three single nucleotide polymorphisms (SNPs) in the HTR3A and HTR3B genes (rs1062613, rs1176744 and rs3831455) have been associated with bipolar affective disorder (BPAD) in pilot studies, and all of them are of functional relevance. We performed a European multicenter study to confirm previous results and provide further evidence for the relevance of these SNPs to the etiology of neuropsychiatric disorders. This involved analysis of the distribution of the three SNPs among 1804 BPAD cases and 2407 healthy controls. A meta-analysis revealed a pooled odds ratio of 0.881 (P = 0.009, 95% confidence intervals = 0.802-0.968) for the non-synonymous functional SNP HTR3B p.Y129S (rs1176744), thereby confirming previous findings. In line with this, the three genome-wide association study samples BOMA (Bonn-Mannheim)-BPAD, WTCCC (Wellcome Trust Case Control Consortium)-BPAD and GAIN (Genetic Association Information Network)-BPAD, including >3500 patients and 5200 controls in total, showed an overrepresentation of the p.Y129 in patients. Remarkably, the meta-analysis revealed a P-value of 0.048 (OR = 0.934, fixed effect model). We also performed expression analyses to gain further insights into the distribution of HTR3A and HTR3B mRNA in the human brain. HTR3A and HTR3B were detected in all investigated brain tissues with the exception of the cerebellum, and large differences in the A:B subunit ratio were observed. Interestingly, expression of the B subunit was most prominent in the brain stem, amygdalae and frontal cortex, regions of relevance to psychiatric disorders. In conclusion, the present study provides further evidence for the presence of impaired 5-HT(3) receptor function in BPAD.

5-HT(3) receptors: potential of individual isoforms for personalised therapy

Serotonin type 3 (5-HT(3)) receptors are ligand-gated ion channels built by five subunits of diverse composition. In humans, five subunits exist (5-HT3A-E) which are encoded by the genes HTR3A-E and are expressed in various isoforms. Recently, the importance of factors influencing receptor expression became clear, such as chaperones and microRNAs. Owing to their expression profile and physiological functions, 5-HT(3) receptors have been implicated in irritable bowel syndrome (IBS) and psychiatric disorders. Interestingly, HTR3 variants have now been shown to be associated with these conditions. This underlines the potential of 5-HT(3) receptors as therapeutic targets and may enable personalised therapies in the future.

Allosteric modulation of the 5-HT(3) receptor

5-Hydroxytryptamine type 3 (5-HT(3)) receptors are ligand-gated ion channels that play important roles in depression, anxiety, substance abuse, emesis, inflammatory pain, spinal nociception, gastrointestinal function, and cardiovascular reflexes. Probably the most studied modulators of 5-HT(3) receptors are the high affinity competitive 'setron' antagonists typified by ondansetron. However, there exists a broad range of compounds that modulate the 5-HT(3) receptor, not through the orthosteric site but by binding to allosteric sites. Most notable are therapeutic compounds ascribed to certain targets but that allosterically modulate 5-HT(3) receptors at clinically relevant concentrations.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Polymorphism in HTR3D shows different risks for acute chemotherapy-induced vomiting after anthracycline chemotherapy

Aims: Serotonin (5-hydroxytryptamine 3; 5-HT3) receptors are involved in chemotherapy-induced nausea and vomiting (CINV), and 5-HT3 antagonists are part of the ‘gold standard’ antiemetic treatment during chemotherapy. We investigated the correlation of common variants in 5-HT3 receptor subunit genes with the occurrence of CINV. Materials & methods: A total of 110 previously characterized chemotherapy-naive women with primary breast cancer treated with anthracycline-containing chemotherapy served as a study group for mutational analysis by direct sequencing. Eight common SNPs in the 5-HT3 receptor genes, HTR3A, HTR3B, HTR3D and HTR3E, were selected for association analysis. Results: A nonsynonymous variant in HTR3D, p.G36A (rs6443930), was found to be over-represented in nonresponders, assuming a log-additive inheritance model (p = 0.048). Cox proportional regression analysis resulted in a hazards ratio of 0.36 for homozygous carriers of the C allele to vomit within 24 h after first chemotherapy administration (p = 0.049). Conclusion: Our data supports the hypothesis that 5-HT3 receptors play an important role in the pathogenesis of CINV. Along with previously identified HTR3 polymorphisms, the HTR3D p.G36A variant could also contribute to facilitating individual risk predictions.

3B but which 3B and that's just one of the questions: the heterogeneity of human 5-HT3 receptors

The 5-hydroxytryptamine 3 (5-HT3) receptor is expressed widely in the central and peripheral nervous systems, where it mediates or modulates a wide range of physiological processes. The receptor is targeted by drugs administered for nausea and/or emesis and irritable bowel syndrome and has been proposed as a potential drug target in various psychiatric disorders. The 5-HT3 receptor is a pentameric ligand-gated ion channel and belongs to the Cys-loop receptor family. In contrast to the immense heterogeneity characterizing other Cysloop receptors, native 5-HT3 receptors historically have been considered a much more homogenous receptor population. However, the recent discovery of additional 5-HT3 subunits and the dawning realization that central and peripheral 5-HT3 receptor populations might comprise several subtypes characterized by distinct functional properties has emphasized the complexity of human 5-HT3 receptor signaling. In this review potential implications of these findings and of the entirely new layer of interindividual diversity introduced to the 5-HT3 receptor system by genetic variations will be outlined.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Serotonin type 3 receptor genes: HTR3A, B, C, D, E

The 5-HT3 receptor is a ligand-gated ion channel composed of five subunits. To date, five different human subunits are known, 5-HT3A–E, which are encoded by the serotonin receptor genes HTR3A, HTR3B, HTR3C, HTR3D and HTR3E, respectively. Functional receptors are pentameric complexes of diverse composition. Different receptor subtypes seem to be involved in chemotherapy-induced nausea and vomiting (CINV), irritable bowel syndrome and psychiatric disorders. 5-HT3 receptor antagonists are established in the therapy of CINV and irritable bowel syndrome. HTR3A and HTR3B polymorphisms may also contribute to the etiology of psychiatric disorders and serve as predictors in CINV and in the medical treatment of psychiatric patients.