Human 5-HT5A receptor gene: systematic screening for DNA sequence variation and linkage mapping on chromosome 7q34-q36 using a polymorphism in the 5' untranslated region

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

GABRB3 gene expression increases upon ethanol exposure in human embryonic stem cells

Ionotropic receptors are the target for most mood-defining compounds. Chronic exposure to ethanol (EtOH) alters receptor-mediated responses and the numbers of these channels and specific subunits; as well as induces anxiolytic, sedative, and anesthetic activity in the human brain. However, very little is known regarding the effects of EtOH on ionotropic receptor transcription during early human development (preimplantation). Using two separate human embryonic stem cell lines the study shows that low amounts of EtOH (20 mM) alters transcription of the ionotropic subunit GABRB3. Changes in ionotrophic receptor expression influence the central nervous system development and have been shown to produce brain abnormalities in animal models. These results suggest that low concentrations of EtOH can alter ionotropic receptor transcription during early human development (preimplantation), which may be a contributing factor to the neurological phenotypes seen in fetal alcohol spectrum disorder (FASD).

Case-control association study of 65 candidate genes revealed a possible association of a SNP of HTR5A to be a factor susceptible to bipolar disease in Bulgarian population

BACKGROUND

Bipolar affective disorder (BAD) is a psychiatric illness characterized by episodes of mania and depression. Although the etiology is not clear, epidemiological studies suggest it is a result of an interaction of genetic and environmental factors. Despite of enormous efforts and abundant studies conducted, none has yet been identified definitively a gene susceptible to bipolar disorder.

METHODS

Ninety-four Bulgarian patients diagnosed with bipolar disorder and 184 Bulgarian healthy individuals, were used for genotyping of 191 single nucleotide polymorphisms (SNPs) by TaqMan and/or Invader assays. Seventeen SNPs that revealed P value less than 0.05 in the first screening were genotyped using an additional independent set of samples, consisting of 78 BAD cases and 372 controls.

RESULTS

After applying the Bonferonni correction on genotyping results of 172 cases and 556 controls, only one SNP, rs1800883, in the HTR5A gene revealed a significant level of P value (P=0.000097; odds ratio=1.80 (95%CI, 1.27-2.54); corrected P=0.017).

CONCLUSIONS

Our findings suggest that HTR5A gene could play an important role in the pathogenesis of bipolar disorder in our population. However these findings should be viewed with caution and replication studies in other populations are necessary in support of these findings.

No association of attention-deficit/hyperactivity disorder with genes of the serotonergic pathway in Han Chinese subjects

Attention-deficit/hyperactivity disorder (ADHD) is a complex psychiatric syndrome with cardinal symptoms of inattention, hyperactivity and impulsivity, and is a significant risk factor for poor health outcomes in both adolescence and adulthood. Etiology is clearly multifactoral, with probable contributions from both genetic and environmental factors. The genetic contribution is prominent, with estimated heritability at about 0.80. Although effects in dopamine metabolism have long been implicated in the etiology of ADHD, the role for serotonin has gained more attention in recent years. The current study examined five variants in three serotonin genes [those that code for serotonin receptors 2A (HTR2A), 5A (HTR5A) and 6 (HTR6)] in a relatively large sample of ADHD nuclear families. The transmission disequilibrium test (TDT) and the extended transmission disequilibrium test (ETDT) were performed to test for evidence of distorted transmission of alleles or haplotypes. No significant biased transmission was observed. These results do not support a substantial role of these serotonin gene in ADHD, however, additional work may be warranted before this association is definitively discounted.

Investigation of the human serotonin receptor gene HTR3B in bipolar affective and schizophrenic patients

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) mediates a multitude of central nervous functions by activating 5-HT receptor subtypes. A dysfunction of serotonergic neurotransmission is considered to play a major role in the pathophysiology of complex neuropsychiatric disorders. In our study, a mutation screen of the serotonin receptor gene HTR3B was carried out to explore a putative contribution to the etiology of bipolar affective disorder (BPAD) and schizophrenia (SZ). Screening of 49 patients suffering from BPAD, 78 patients with SZ and 62 control individuals revealed eleven sequence variations including a 3 bp deletion within the 5'UTR (5' untranslated region), four exonic and five intronic SNPs as well as a point mutation in the 3'UTR of HTR3B. Four of these sequence variations have not been described previously. Statistical computation rated most variants as probably non-disease related polymorphisms. However, IVS6 + 31C > T, IVS6 + 40C > A, and 1386T > C were solely detected in bipolar affective patients and in none of the controls. Interestingly, we observed a significant underrepresentation of the 3 bp deletion -100_-102delAAG in an extended sample of 162 bipolar affected patients compared to controls (allele-wise: 8% vs. 15%, P = 0.006, OR = 0.49, 95% CI: 0.3-0.82; genotype-wise: 15,5% vs. 29,0%, P = 0.005, OR = 0.45, 95% CI: 0.26-0.77). We suggest that this deletion may influence translational efficiency, thereby possibly affecting the development of bipolar affective disease.

Family-based association studies between 5-HT5A receptor gene and schizophrenia

BACKGROUND

Pharmacological and neurodevelopmental data support the idea that the gene, which codes for the 5-HT(5A) receptor is an important candidate gene for schizophrenia susceptibility. However, previous genetic studies focusing on this gene yielded conflicting results, potentially because of: (i) stratification biases of case-control association studies, (ii) genetic and phenotypic heterogeneity of schizophrenia, and (iii) variability in the loci analyzed (the 5-HT(5A) gene having many polymorphic sites).

METHODS

A transmission disequilibrium test was used in the present study aimed at investigating two polymorphisms in exon 1 of the 5-HT(5A) gene, the A12T silent substitution and the C43T transversion leading to a 15Pro --> Ser substitution, in 103 patients with DSM-IV diagnosis of schizophrenia, and their 206 parents.

RESULTS

We found an excess of transmission of the 12T allele from the parents to their affected children (P = 0.02), with evidence for linkage disequilibrium between the 12T-43C haplotype and schizophrenia (P = 0.002). Furthermore, patients with the 12T allele had a significantly later age at onset (P = 0.003), and the Q-TDT approach confirmed that this allele was transmitted with an older age at onset (P = 0.01).

CONCLUSIONS

These data provided convergent evidence for a significant role of the 5-HT(5A) gene in schizophrenia and more specifically in patients with later age at onset.

Multiple signal transduction pathways mediated by 5-HT receptors

Among human serotonin (5-HT) receptor subtypes, each G protein-coupled receptor subtype is reported to have one G protein-signaling cascade. However, the signaling may not be as simple as previously thought to be. 5-HT5A receptors are probably the least well understood among the 5-HT receptors, but the authors found that 5-HT5A receptors couple to multiple signaling cascades. When the 5-HT5A receptors were expressed in undifferentiated C6 glioma cells, they modulated the level of second messengers. For example, activation of 5-HT5A receptors inhibited the adenylyl cyclase activity and subsequently reduced the cAMP level, as previously reported. In addition to this known signaling via Gi/Go, 5-HT5A receptors are coupled to the inhibition of ADP-ribosyl cyclase and cyclic ADP ribose formation. On the other hand, activation of 5-HT5A receptors transiently opened the K+ channels, presumably due to the increase in intracellular Ca2+ after formation of inositol (1,4,5) trisphosphate. The K+ currents were inhibited by both heparin and pretreatment with pertussis toxin, suggesting the cross-talk between Gi/Go protein and phopholipase C cascade. Thus, the authors results indicate that 5-HT5A receptors couple to multiple second messenger systems and may contribute to the complicated physiological and pathophysiological states. Although this multiple signaling has been reported only for 5-HT5A/5-HT1 receptors so far, it is possible that other 5-HT receptor subtypes bear similar complexity. As a result, in addition to the wide variety of expression patterns of each 5-HT receptor subtype, it is possible that multiple signal transduction systems may add complexity to the serotonergic system in brain function. The investigation of these serotonergic signaling and its impairment at cellular level may help to understand the symptoms of brain diseases.

Genetic variations and polymorphisms of G protein-coupled receptors: functional and therapeutic implications

G protein-coupled receptors (GPCRs) represent a major class of proteins in the genome of many species, including humans. In addition to the mapping of a number of human disorders to regions of the genome containing GPCRs, a growing body of literature has documented frequently occurring variations (i.e. polymorphisms) in GPCR loci. In this article, we use a domain-based approach to systematically examine examples of genetic variation in the coding and noncoding regions of GPCR loci. Data to date indicate that residues in GPCRs are involved in ligand binding and coupling to G proteins and that regulation can be altered by polymorphisms. Studies of GPCR polymorphisms have also uncovered the functional importance of residues not previously implicated from other approaches that are involved in the function of GPCRs. We predict that studies of GPCR polymorphisms will have a significant impact on medicine and pharmacology, in particular, by providing new means to subclassify patients in terms of both diagnosis and treatment.

Genetic variation in the 5-HT5A receptor gene in patients with bipolar disorder and major depression

In the present study, genetic variation of the 5-HT5A receptor was analyzed in patients affected by affective disorders and healthy controls. The sample consisted of 181 patients with major depression, 88 patients with bipolar affective disorder (BP) and 157 unrelated controls (C), all of Spanish origin. Two polymorphisms (-19G/C and 12A/T) in the 5-HT5A receptor gene were analyzed by polymerase chain reaction amplification and subsequent enzyme digestion. No genotype, allele or haplotype differences were found when we compared patients and controls. When clinical variables were considered as possible tools for detecting genetic heterogeneity, no differences were found. Our results suggest that the polymorphisms analyzed in the 5-HT5A receptor gene do not play a major role in the pathogenesis of affective disorders.

Association of a 5-HT(5A) receptor polymorphism, Pro15Ser, to schizophrenia

Several different lines of evidence suggest that genes involved in serotonergic neurotransmission are factors in the pathogenesis of schizophrenia. For example, 5-HT(5A) knockout mice revealed decreased locomotor response to lysergic diethylamide (LSD), which produces a psychotic-like state in healthy people. Recently, we reported a naturally occurring conservative Pro15Ser substitution in the 5-HT(5A) receptor. Here, we evaluate whether this substitution is associated with schizophrenia in a sample including 249 unrelated Japanese schizophrenia patients and 253 unrelated controls. Patients and controls were genotyped for the Pro15Ser polymorphism by a PCR-RFLP assay. Ser15 allele frequencies were 0.07 in patients with schizophrenia and 0.02 in controls (chi(2) = 17.42, df =1, P < 0.0001). thus, we detected a highly significant association of pro15ser to schizophrenia in a large population of japanese schizophrenia patients and controls. since case-control studies have an inherent potential for false-positive results due to population stratification, this finding is preliminary pending further studies, including studies using the transmission/disequilibrium test to eliminate stratification bias or control loci to assess ethnic matching of cases and controls.

Pharmacogenomics and schizophrenia

Although antipsychotic drugs are effective in alleviating schizophrenic symptoms, individual differences in patient response suggest that genetic components play a major role, and pharmacogenetic studies have indicated the possibility for a more individually based pharmacotherapy. The new field of pharmacogenomics, which focuses on genetic determinants of drug response at the level of the entire human genome, is important for development and prescription of safer and more effective individually tailored drugs. DNA microarray (DNA chip) analysis enables genome-wide scanning, using the high-density single nucleotide polymorphisms map. Pharmacogenomics will aid in understanding how genetics influence disease development and drug response, and contribute to discovery of new treatments. The rate of discovery of those polymorphisms will depend on the quality of the drug response phenotype. Prospective genotyping of schizophrenic patients for the many genes at the level of the drug target, drug metabolism, and disease pathways will contribute to individualized therapy matching the patient's unique genetic make-up with an optimally effective drug.

Unified approach to the analysis of genetic variation in serotonergic pathways

Serotonin is a key neurotransmitter in the central nervous system, and dysregulation of serotonergic pathways has been implicated in the pathogenesis of many complex psychiatric diseases. Polymorphisms of many of the genes involved in serotonin biosynthesis, catabolism, and response have been reported, suggesting that genetic variability may underlie the development of diseases such as schizophrenia, obsessive compulsive disorder, and suicide. A number of single-gene polymorphisms in serotonergic pathways have been examined in these and other diseases, but to date results from this candidate gene approach have been disappointing. Although this may be because the detection of a small effect may require the analysis of large numbers of patients and controls, an alternative explanation is that the clinical importance of a single subtle genetic variant may be overlooked unless other functionally related genes are studied in tandem. To facilitate an integrated analysis, we have developed a PCR-SSP-based assay that permits the simultaneous genotyping of 13 single nucleotide polymorphisms in 9 serotonergic genes under identical conditions. These genes include tryptophan hydroxylase, tryptophan dioxygenase, monoamine oxidase A, and the serotonin receptors 5HT1A, 5HT1D-alpha, 5HT1D-beta, 5HT2A, 5HT2C, and 5HT5A. Using this technology, we have genotyped 100 Caucasoid control individuals and demonstrate that this approach is reliable, quick, cheap, and easy to interpret. We anticipate that this will facilitate the analysis of the genetic basis of susceptibility and phenotypic variability of a number of complex psychiatric diseases. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:621-627, 1999.

Identification of a naturally occurring Pro15-Ser15 substitution in the serotonin5A receptor gene in alcoholics and healthy volunteers

We screened the serotonin5A receptor gene coding region in 186 unrelated alcoholic patients and 187 controls. A relatively abundant amino acid substitution and two synonymous DNA substitutions were detected. Two synonymous variants, A12T and C789T, had rarer-allele frequencies of 23% and 1%, respectively. The Pro15Ser substitution is located in the amino terminal, extracellular domain of the receptor adjacent to a putative phosphorylation site. Pro15Ser had rarer-allele frequencies of 8.1% and 5.9% in Finnish alcoholic patients and controls, respectively (p=n.s.).

Assignment of the human serotonin 4 receptor gene (HTR4) to the long arm of chromosome 5 (5q31-q33)

In the present study, we report the chromosomal localization of the human 5-HT4 receptor gene (HTR4) by the analysis of somatic cell hybrids. Based upon genomic sequences of the HTR4 gene, a primer set was selected that reacted with human genomic DNA but not mouse or hamster genomic DNA. Using monochromosomal hybrid cell lines of the NIGMS Mapping Panel 2 we localized the HTR4 gene to human chromosome 5. To confirm the localization on chromosome 5 and to further sublocalize the HTR4 gene, the radiation hybrid panel RH3 was similarly analysed. Significant linkage was obtained to genetic marker D5S2654. This localizes the HTR4 gene to human chromosome 5q31-q33.