A first step towards the understanding of the 5-HT3 receptor subunit heterogeneity from a computational point of view

A randomized trial evaluating the association between related gene polymorphism and nausea and vomiting induced by cisplatin multi-day chemotherapy

PURPOSE

We aim to investigate the correlation between gene polymorphisms and cisplatin chemotherapy-induced nausea and vomiting (CINV), which was prevented by olanzapine or aprepitant triple antiemetic regimen.

METHODS

Before chemotherapy, the blood samples of 89 malignant tumor patients who received multi-day chemotherapy with cisplatin were collected for sequencing and typing. As there were duplicate patients enrolled in different chemotherapy cycles, there were a total of 190 cases. The patients were divided into two groups randomly, who received the triple antiemetic regimen of olanzapine or aprepitant combined with 5-HT3RA and dexamethasone. The main evaluation indicators were the total protection (TP) rate in the acute phase (0-24 h), the delayed phase (25-120 h) and the overall phase (0-120 h).

RESULTS

Univariate analysis was performed on genetic loci that reached H-W balance with TP. In the olanzapine group, increased TP in the acute phase was associated with HTR3A rs1176719 non-GG (P < 0.05) genotype etc. Increased TP in the delayed phase was associated with HTR3A rs1176719 non-GG (P < 0.05) genotype etc. In the aprepitant group, increased TP in the acute phase was associated with the MTHFR rs1801131 TT (P < 0.05) genotype etc. Increased TP in the delayed phase was associated with HTR3A rs1062613 CC (P < 0.05) genetype ect. Multivariate Logistic regression analysis showed that HTR3B rs7943062GG (P < 0.05) genotype etc. were correlated with increased TP in the delayed phase. MTHFR rs1801131TT genotype was associated with increased TP in the acute phase (P < 0.05) and delayed phase (P < 0.05).

CONCLUSION

This study found that gene polymorphisms, including HTR3B (rs1062613, rs1176719, rs2276303), HTR3B (rs45460698, rs7943062), HTR3C (rs6766410), ERCC1 (rs3212986), ERCC4 (rs744154) and MTHFR(rs1801131), may be independent prognostic factors for CINV.

Discriminating between 5-HT₃A and 5-HT₃AB receptors

The 5-HT3B subunit was first cloned in 1999, and co-expression with the 5-HT3A subunit results in heteromeric 5-HT₃AB receptors that are functionally distinct from homomeric 5-HT₃A receptors. The affinities of competitive ligands at the two receptor subtypes are usually similar, but those of non-competitive antagonists that bind in the pore often differ. A competitive ligand and allosteric modulator that distinguishes 5-HT₃A from 5-HT₃AB receptors has recently been described, and the number of non-competitive antagonists identified with this ability has increased in recent years. In this review, we discuss the differences between 5-HT₃A and 5-HT₃AB receptors and describe the possible sites of action of compounds that can distinguish between them.

Association between variants of 5-hydroxytryptamine receptor 3C (HTR3C) and chemotherapy-induced symptoms in women receiving adjuvant treatment for breast cancer

Administration of chemotherapy is associated with a wide array of symptoms affecting quality of life. Genetic risk factors for severity of chemotherapy-induced symptoms have not been determined. The present study aimed to explore the associations between polymorphisms in candidate genes and chemotherapy-induced symptoms. Women treated with at least two cycles of adjuvant doxorubicin and cyclophosphamide, with or without paclitaxel for early breast cancer (n = 105) completed the memorial symptom assessment scale and provided blood for genotyping. DNA was extracted from peripheral blood leukocytes and assayed for single nucleotide polymorphisms (SNPs) in GTP cyclohydrolase 1 (GCH1, rs10483639, rs3783641, and rs8007267), catecholamine-o-methyltransferase (COMT, rs4818), and 5-hydroxytryptamine (serotonin) receptor 3C (HTR3C, rs6766410, and rs6807362). Genotyping of HTR3C revealed a significant association between the presence of rs6766410 and rs6807362 SNPs (K163 and G405 variants) and increased severity of symptoms (p = 0.0001 and p = 0.007, respectively). Multiple regressions revealed that rs6766410 and rs6807362, but not age or stage at diagnosis, predicted severity of symptoms (p = 0.001 and p = 0.006, respectively) and explained 12 % of the variance in each regression model. No association was found between the genetic variants of CGH1 or COMT and symptom score. Our study indicates, for the first time, an association between variants of HTR3C and severity of chemotherapy-induced symptoms. Analyzing these genetic variants may identify patients at increased risk for the development of chemotherapy-induced symptoms and targeting the serotonin pathway may serve as a novel treatment strategy for these patients.

Exploring a potential palonosetron allosteric binding site in the 5-HT(3) receptor

Palonosetron (Aloxi) is a potent second generation 5-HT(3) receptor antagonist whose mechanism of action is not yet fully understood. Palonosetron acts at the 5-HT(3) receptor binding site but recent computational studies indicated other possible sites of action in the extracellular domain. To test this hypothesis we mutated a series of residues in the 5-HT3A receptor subunit (Tyr(73), Phe(130), Ser(163), and Asp(165)) and in the 5-HT3B receptor subunit (His(73), Phe(130), Glu(170), and Tyr(143)) that were previously predicted by in silico docking studies to interact with palonosetron. Homomeric (5-HT(3)A) and heteromeric (5-HT(3)AB) receptors were then expressed in HEK293 cells to determine the potency of palonosetron using both fluorimetric and radioligand methods to test function and ligand binding, respectively. The data show that the substitutions have little or no effect on palonosetron inhibition of 5-HT-evoked responses or binding. In contrast, substitutions in the orthosteric binding site abolish palonosetron binding. Overall, the data support a binding site for palonosetron at the classic orthosteric binding pocket between two 5-HT3A receptor subunits but not at allosteric sites previously identified by in silico modelling and docking.

Approaching the 5-HT₃ receptor heterogeneity by computational studies of the transmembrane and intracellular domains

5-hydroxytryptamine type-3 receptor (5-HT₃), an important target of many neuroactive drugs, is a cation selective transmembrane pentamer whose functional stoichiometries and subunit arrangements are still debated, due to the extreme complexity of the system. The three dimensional structure of the 5-HT₃R subunits has not been solved so far. Moreover, most of the available structural and functional data is related to the extracellular ligand-binding domain, whereas the transmembrane and the intracellular receptor domains are far less characterised, although they are crucial for receptor function. Here, for the first time, 3D homology models of the transmembrane and the intracellular receptor domains of all the known human 5-HT₃ subunits have been built and assembled into homopentameric (5-HT(3A)R, 5-HT(3B)R, 5-HT(3C)R, 5-HT(3D)R and 5-HT(3E)R) and heteropentameric receptors (5-HT(3AB), 5-HT(3AC), 5-HT(3AD) and 5-HT(3AE)), on the basis of the known three-dimensional structures of the nicotinic-acetylcholine receptor and of the ligand gated channel from Erwinia chrysanthemi. The comparative analyses of sequences, modelled structures, and computed electrostatic properties of the single subunits and of the assembled pentamers shed new light both on the stoichiometric composition and on the physicochemical requirements of the functional receptors. In particular, it emerges that a favourable environment for the crossing of the pore at the transmembrane and intracellular C terminus domain levels by Ca²⁺ ions is granted by the maximum presence of two B subunits in the 5-HT₃ pentamer.