Evaluation of a Cys23Ser mutation within the human 5-HT2C receptor gene: no evidence for an association of the mutant allele with obesity or underweight in children, adolescents and young adults

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.

No evidence of association between the serotonin 2A receptor −1438G/A promoter polymorphism and childhood obesity in a Spanish population: A case-parent study and a matched case-control study

Serotonin has been related to feeding behaviour and body weight control through its suppressive effect on appetite. Conflicting results have been published in the literature regarding the association between the - 1438 G/A promoter polymorphism of the 5HT2A gene with obesity-related variables. The aim of this study was to assess the association between the--1438 G/A polymorphism of the 5HT2A gene with childhood obesity in a Spanish population. A total of 136 cases aged 6-16 years with BMI above the 97th percentile of the Spanish BMI reference data for age and gender were matched by gender and age (+/- 6 months) with 136 controls. Additionally, 43 obese children and their parents were selected for a family-based association study (case-parent study). Genotyping was carried out by polymerase chain reaction and restriction enzyme analysis. Conditional logistic regression and transmission/disequilibrium test were used to assess genotype-obesity association. In the matched case-control study, the crude and adjusted odds ratios for the association between 5HT2A--1438 G/A genotypes were nonsignificant. Likewise, no association is suggested by the case-parent study. In conclusion, it is unlikely that the--1438 G/A polymorphism of 5HT2A gene may influence obesity in a Spanish children population.

Further exploration of the possible influence of polymorphisms in HTR2C and 5HTT on body weight

Receptors of the 5-HT2C subtype are of importance for the influence of serotonin on food intake, and 2 single nucleotide polymorphisms in this gene (HTR2C)--Cys23Ser (rs6318) and -759C>T (rs3813929)--have been reported to be associated with weight and/or antipsychotic-induced weight gain. The present study aimed to replicate these associations; in addition, the 5-HTTLPR polymorphism in the promoter region of the serotonin transporter gene (SLC6A4) was assessed. The polymorphisms were genotyped in subjects recruited from the normal population (n = 510), and possible associations between genotype and body mass index (BMI) were assessed. The Ser23 allele was more common in underweight subjects (BMI <20) than in normal- and overweight (BMI > or =20) subjects (P = .006). The T allele of the -759C/T polymorphism was less common in the overweight group (BMI > or =25) (P = .007). Homozygosity for the short allele of 5-HTTLPR was more frequent in underweight subjects (P = .015). Our results are in agreement with previous studies, suggesting polymorphisms in HTR2C to be associated with body weight, particularly in women; and they also suggest that 5-HTTLPR may influence this phenotype. Further studies on the importance of the investigated genes for eating disorders and drug-induced weight gain are warranted.

Association between serotonin 5-HT-2C receptor gene (HTR2C) polymorphisms and obesity- and mental health-related phenotypes in a large population-based cohort

OBJECTIVE

Studies have shown that common single-nucleotide polymorphisms (SNPs) in the serotonin 5-HT-2C receptor (HTR2C) are associated with antipsychotic agent-induced weight gain and the development of behavioural and psychological symptoms. We aimed to analyse whether variation in the HTR2C is associated with obesity- and mental health-related phenotypes in a large population-based cohort.

METHOD

Six tagSNPs, which capture all common genetic variation in the HTR2C gene, were genotyped in 4978 men and women from the European Prospective Investigation into Cancer (EPIC)-Norfolk study, an ongoing prospective population-based cohort study in the United Kingdom. To confirm borderline significant associations, the -759C/T SNP (rs3813929) was genotyped in the remaining 16 003 individuals from the EPIC-Norfolk study. We assessed social and psychological circumstances using the Health and Life Experiences Questionnaire. Genmod models were used to test associations between the SNPs and the outcomes. Logistic regression was performed to test for association of SNPs with obesity- and mental health- related phenotypes.

RESULTS

Of the six HTR2C SNPs, only the T allele of the -759C/T SNP showed borderline significant associations with higher body mass index (BMI) (0.23 kg m(-2); (95% confidence interval (CI): 0.01-0.44); P=0.051) and increased risk of lifetime major depressive disorder (MDD) (Odds ratio (OR): 1.13 (95% CI: 1.01-1.22), P=0.02). The associations between the -759C/T and BMI and lifetime MDD were independent. As associations only achieved borderline significance, we aimed to validate our findings on the -759C/T SNP in the full EPIC-Norfolk cohort (n=20 981). Although the association with BMI remained borderline significant (beta=0.20 kg m(-2); 95% CI: 0.04-0.44, P=0.09), that with lifetime MDD (OR: 1.01; 95% CI: 0.94-1.09, P=0.73) was not replicated.

CONCLUSIONS

Our findings suggest that common HTR2C gene variants are unlikely to have a major role in obesity- and mental health-related traits in the general population.

Focus on HTR2C: A possible suggestion for genetic studies of complex disorders

HTR2C is one of the most relevant and investigated serotonin receptors. Its role in important brain structures such as the midbrain, the lateral septal complex, the hypothalamus, the olfactory bulb, the pons, the choroid plexus, the nucleus pallidus, the striatum and the amygdala, the nucleus accumbens and the anterior cingulated gyrus candidate it as a promising target for genetic association studies. The biological relevance of these brain structures is reviewed by way of the focus on HTR2C activity, with a special attention paid to psychiatric disorders. Evidence from the genetic association studies that dealt with HTR2C is reviewed and discussed alongside the findings derived from the neuronatmic investigations. The reasons for the discrepancies between these two sets of reports are discussed. As a result, HTR2C is shown to play a pivotal role in many different psychiatric behaviors or psychiatric related disrupted molecular balances, nevertheless, genetic association studies brought inconsistent results so far. The most replicated association involve the feeding behavior and antipsychotic induced side effects, both weight gain and motor related: Cys23Ser (rs6318) and -759C/T (rs3813929) report the most consistent results. The lack of association found in other independent studies dampens the clinical impact of these reports. Here, we report a possible explanation for discrepant findings that is poorly or not at all usually considered, that is that HTR2C may exert different or even opposite activities in the brain depending on the structure analyzed and that mRNA editing activity may compensate possible genetically controlled functional effects. The incomplete coverage of the HTR2C variants is proposed as the best cost-benefit ratio bias to fix. The evidence of brain area specific HTR2C mRNA editing opens a debate about how the brain can differently modulate stress events, and process antidepressant treatments, in different brain areas. The mRNA editing activity on HTR2C may play a major role for the negative association results.

Pharmacogenetics of schizophrenia

There is substantial unexplained interindividual variability in the drug treatment of schizophrenia. A substantial proportion of patients respond inadequately to antipsychotic drugs, and many experience limiting side effects. As genetic factors are likely to contribute to this variability, the pharmacogenetics of schizophrenia has attracted substantial effort. The approaches have mainly been limited to association studies of polymorphisms in candidate genes, which have been indicated by the pharmacology of antipsychotic drugs. Although some advances have been made, particularly in understanding the pharmacogenetics of some limiting side effects, genetic prediction of symptom response remains elusive. Nevertheless, with improvements in defining the response phenotype in carefully assessed and homogeneous subject groups, the near future is likely to see the identification of genetic predictors of outcome that may inform the choice of pharmacotherapy.

Complex HTR2C linkage disequilibrium and promoter associations with body mass index and serum leptin

The occurrence of obesity, eating disorders, and related diseases has increased in many parts of the world. Given that few strong genetic factors have been found, it is clear that these are complex multi-factorial diseases. The serotonin receptor 2C, a member of the 5-HTergic system, has been implicated in the control of phagia and obesity. We report a detailed investigation of linkage disequilibrium (LD) within and between the HTR2C promoter and the flanking sequences around a commonly utilized marker in the second coding exon of HTR2C. We suggest that inconsistent associations between HTR2C and several phenotypes, including obesity, may be due to the LD pattern across the gene in which recombination and gene conversion have been influential. The nucleotide and haplotype distribution is consistent with that of the neutral mutation model. The number of haplotypes suggests demographic influences or over dominant selection that may have a function in HTR2C expression. Using the fine LD pattern, we describe a possible association with promoter haplotypes and diplotypes, including a GT microsatellite, and body mass index (BMI) > or =30 kgm(-2) (P<0.0001). SNP -995G>A heterozygotes, as well as promoter diplotypes, were found to marginally influence higher serum leptin corrected for percentage body fat (P=0.01), which might suggest that these subjects are leptin resistant. Our results complement previous studies of HTR2C in both mice and humans, and suggest the importance of genetic variation and elucidating the fine LD structure in uncovering the genetic factors of obesity.

The role of 5-HT2C receptor polymorphisms in the pharmacogenetics of antipsychotic drug treatment

The development of novel, atypical antipsychotics has been welcomed due to their lower incidence of extrapyramidal side effects. However, as with all pharmacotherapy, patient response is often varied and these novel compounds are not without their own side effect profile, most notably weight gain. Inter-individual variations in response to drug treatment are, in part, due to polymorphisms of the genes encoding drug targets. The importance of the serotonin system in psychiatric symptomatology and several side effects of antipsychotic drugs is well established. Thus genetic polymorphisms of two central 5-HT receptors (5-HT2A and 2C), at which many of the newer antipsychotic drugs act, are prime candidates for pharmacogenetic analysis of the effects of these drugs in the treatment of schizophrenia. To date, much work has focussed on the 5-HT2A receptor subtype. However, pharmacological evidence and recent molecular genetic studies would suggest a role for the 5-HT2C receptor in the consequences of antipsychotic treatment, particularly in relation to the development of both drug-induced dyskinesias and weight gain. This review briefly examines the pharmacology and physiology of the 5HT2C receptor in the context of its genetics, with discussion of known polymorphisms of this receptor gene and its promoter region. Associations between these polymorphisms and the development of schizophrenia, the symptom responses to antipsychotic treatment and side effects of such treatment, notably tardive dyskinesia and weight gain, are assessed and related to the presumed functionality of these polymorphisms. Such studies clearly demonstrate the potential of pharmacogenetics in optimising treatment for the individual patient.

Association of antipsychotic drug-induced weight gain with a 5-HT2C receptor gene polymorphism

A side-effect of treatment with antipsychotic drugs for schizophrenia is increased body fat, which leads to further morbidity and poor adherence to treatment. The 5-hydroxytryptamine 2C receptor (5-HT2C) has been associated with this effect; we aimed to establish whether a genetic polymorphism of the promoter region of this receptor affects weight gain after drug treatment in first-episode patients with schizophrenia. We noted significantly less weight gain in patients with the -759T variant allele (p=0.0003) than in those without this allele, who were more likely to have substantial (>7%) weight gain (p=0.002). We have identified a genetic factor that is associated with antipsychotic drug-induced weight gain.

The human genetics of eating disorders lessons from the leptin/melanocortin system

Genetic analysis of eating disorders is complex phenotypically and genotypically. As seen in the leptin/melanocortin system, however, the results can lead to a deeper understanding and to new therapies. Benefits are expected for eating disorders that stem from genetic and psychologic causes. Finally, an awareness of possible genetic causes of eating disorders will help determine the causes--and thus the treatments--in children and adolescents with eating disorders, as exemplified by obese patients with mutations in the POMC, PC1, leptin, and MC4R loci.

Genome-wide scan of obesity in Finnish sibpairs reveals linkage to chromosome Xq24

Obesity is a multifactorial trait with evidence of a genetic component. Obesity is very common in all westernized countries, including Finland, where 10% of the adult population has a body mass index of 32 kg/m2 or more. Here we report results from a three-stage genome-wide scan of obesity in 188 affected subjects (body mass index, > or =32 kg/m2) from 87 Finnish families. Initially, 374 markers with an average density of 10 centimorgans were genotyped. The strongest evidence for linkage to obesity was detected on chromosome Xq24, with the marker DXS6804 providing a maximum likelihood score (MLS) 3.14 in a model-free 2-point sibpair analysis. Fine-mapping in an extended sample set of 367 affected subjects from 166 families yielded a multipoint MLS of 3.48 over this X-chromosomal region. The Xq24 region contains a plausible candidate gene, serotonin 2C receptor, variants of which have been shown to predispose to obesity and type II diabetes in mice. Another chromosomal region also provided suggestive evidence of linkage, an area on 18q21, flanking the melanocortin-4 receptor, where a 2-point MLS of 2.42 with marker D18S1155 was obtained with a set of 367 affected subjects. In conclusion, our results in this Finnish study sample suggest that a locus on chromosome Xq24 influences the risk of obesity.

Serotonin-2C and serotonin-1A receptor genes are not associated with psychotic symptomatology of mood disorders

The serotonergic system is involved in both pathophysiology and treatment of mood disorders. In the present study we investigated the possible influence of the polymorphisms of the serotonin-1A and 2C receptor genes on the symptomatology of mood disorders. Eighty-four inpatients affected by mood disorders (72 bipolar and 12 major depressive disorder) were assessed by the Operational Criteria Checklist for Psychotic Illness to score their lifetime psychotic symptomatology. The subjects were also typed for 5HT1A and 5HT2C variants using polymerase chain reaction techniques. No association was found between 5HT2C and psychopathology as defined by the four symptomatologic factors used as phenotype definition (mania, depression, delusion, and disorganization) even when bipolar subjects were analyzed separately. Only one subject with the 5HT1A variant was observed. Genetic variation at the 5HT1A and 5HT2C receptor genes does not, therefore, play a major role in the pathogenesis of mood disorders symptomatology. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:161-166, 2000.

Separate systems for serotonin and leptin in appetite control

Appetite control involves an integration of the drive signals arising form energy stores in the body with the satiety signals generated by periodic episodes of food consumption. Serotonin (5-hydroxytryptamine, 5-HT) has been implicated in the processes of within-meal satiation and postmeal satiety (5-HT1B and 5-HT2C postsynaptic receptors) which are concerned with the signals arising form the pattern of food intake. Central nervous system (CNS) 5-HT is sensitive to circulating levels of the precursor tryptophan, certain macronutrients and peripheral satiety factors such as cholecystokinin (CCK) and enterostatin. Hypothalamic 5-HT receptor systems inhibit neuropeptide Y (NPY), a potent stimulator of hunger and food intake. In contrast to the linking of 5-HT with the consequences of food ingestion, the hormone leptin (OB protein) is regarded as a signal linking adipose tissue status with a number of key CNS circuits. Leptin itself stimulates CNS leptin receptors (OB-r receptor) which link with pro-opiomelanocortin (POMC)/ MC-4 receptors. The effects of leptin may also be modulated by factors such as the corticotrophin-releasing factor (CRF), cocaine and amphetamine-regulated transcript (CART), orexins and galanin. Very little evidence exists to support any direct link between the actions of 5-HT and leptin, suggesting that they are separate systems. 5-HT is a part of an integrated network for short-acting satiety signals (episodic in nature), and leptin is a hormonal indicator of long-term (tonic) energy reserves. At a conceptual level, these may represent the distinction between 'satiety' and 'drive'. Interestingly, both 5-HT and leptin modulate the action of NPY, which may form a part of a common output pathway for the expression of appetite.

Serotonin receptor 2A, 2C, 1A genes and response to lithium prophylaxis in mood disorders

The aim of this study was to investigate the influence of serotonin receptors 2A, 2C and 1A gene variants on lithium prophylactic efficacy in mood disorders. One hundred and twenty-four subjects affected by bipolar (n=102) and major depressive (n=22) disorder were followed prospectively for an average of 52 months and were typed for 5-HT2A (T102C: n=111, HTP: n=104), 5-HT2C (n=110) and 5-HT1A (n=61) variants. Both 5-HT2A and 5-HT2C variants were not associated with lithium outcome. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not influence results. No 5-HT1A gene variant was identified. 5-HT2A and 2C variants are not, therefore, associated with lithium prophylactic efficacy in mood disorders.

Identification of a new polymorphism in the 3'-untranslated region of the human serotonin receptor 2C (5-HT2C) gene

We identified a polymorphism (2831T > G) in the 3'-untranslated region of 5-HT2C receptor gene, approximately 100 kb from a previously reported coding sequence polymorphism, 796G > C (C23S). Allele frequencies were 0.90 (T) and 0.10 (G) and cosegregation analysis of the alleles at the two loci demonstrated frequencies of 0.82 (GT), 0.08 (CT), 0.10 (GG), and 0 (CC). The increased informativity gained by analysis of both polymorphisms will prove useful for future studies of this gene in X-linked neuropsychiatric disorders.

Structural organization and mutational analysis of the human uncoupling protein-2 (hUCP2) gene

Uncoupling proteins (UCPs) are mitochondrial membrane transporters which are involved in dissipating the proton electrochemical gradient thereby releasing stored energy as heat. This implies a major role of UCPs in energy metabolism and thermogenesis which when deregulated are key risk factors for the development of obesity and other eating disorders. From the three different human UCPs identified so far by gene cloning both UCP2 and UCP3 were mapped in close proximity (75-150 kb) to regions of human chromosome 11 (11q13) that have been linked to obesity and hyperinsulinaemia. At the amino acid level hUCP2 has about 55% identity to hUCP1 while hUCP3 is 71% identical to hUCP2. In this study we have deduced the genomic structure of the human UCP2 gene by PCR and direct sequence analysis. The hUCP2 gene spans over 8.7 kb distributed on 8 exons. The localization of the exon/intron boundaries within the coding region matches precisely that of the hUCP1 gene and is almost conserved in the recently discovered hUCP3 gene as well. The high degree of homology at the nucleotide level and the conservation of the exon /intron boundaries among the three UCP genes suggests that they may have evolved from a common ancestor or are the result from gene duplication events. Mutational analysis of the hUCP2 gene in a cohort of 172 children (aged 7 - 13) of Caucasian origin revealed a polymorphism in exon 4 (C to T transition at position 164 of the cDNA resulting in the substitution of an alanine by a valine at codon 55) and an insertion polymorphism in exon 8. The insertion polymorphism consists of a 45 bp repeat located 150 bp downstream of the stop codon in the 3'-UTR. The allele frequencies were 0.63 and 0.37 for the alanine and valine encoded alleles, respectively, and 0.71 versus 0.29 for the insertion polymorphism. The allele frequencies of both polymorphisms were not significantly elevated in a subgroup of 25 children characterized by low Resting Metabolic Rates (RMR). So far a direct correlation of the observed genotype with (RMR) and Body Mass Index (BMI) was not evident. Expression studies of the wild type and mutant forms of UCP2 should clarify the functional consequences these polymorphisms may have on energy metabolism and body weight regulation.

The human obesity gene map: the 1997 update

An update of the human obesity gene map incorporating published results up to October 1997 is presented. Evidence from Mendelian disorders exhibiting obesity as a clinical feature; single-gene mutation rodent models; quantitative trait loci uncovered in human genome-wide scans and in crossbreeding experiments with mouse, rat, and pig models; association and case-control studies with candidate genes; and linkage studies with genes and other markers is reviewed. All chromosomal locations of the animal loci are converted into human genome locations based on syntenic relationships between the genomes. A complete listing of all of these loci reveals that all but chromosome Y of the 24 human chromosomes are represented. Some chromosomes show at least three putative loci related to obesity on both arms (1, 2, 6, 8, 11, and 20) and several on one chromosome arm only (3p, 4q, 5q, 7q, 12q, 13q, 15q, 15p, 22q, and Xq). Studies reporting negative association and linkage results are also listed, with the exception of the unlinked markers from genome-wide scans.

Serotonin transporter gene-linked polymorphic region: allele distributions in relationship to body weight and in anorexia nervosa

Several lines of evidence implicate a role for the serotonergic system in body weight regulation and eating disorders. The magnitude and duration of postsynaptic responses to serotonin (5-HT) is directed by the transport into and release from the presynaptic neuron. Recently, a common polymorphism of a repetitive element in the region of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) was identified that results in a system of two common alleles. The activity of the 5-HTT, as measured in in vitro assays and in human lymphoblastoid cell lines, is dependent on the respective genotype. We thus hypothesized that this polymorphism is relevant for weight regulation in general and is possibly involved in the etiology of anorexia nervosa (AN). Allele frequencies and genotypes were determined in a total of 385 unrelated obese children, adolescents and adults, 112 underweight subjects and 96 patients with AN. Furthermore, both parents of 98 obese children and adolescents and of 55 patients with AN, respectively, were genotyped, thus allowing to test for both association and linkage. The comparison of allele frequencies between obese and underweight probands provided no evidence for a major role of the 5-HTTLPR in weight regulation. Patients with AN had allele frequencies not significantly different to those observed for obese and underweight individuals.