Allelic variation in the 5-HT2C receptor (HTR2C) and functional responses to the 5-HT2C receptor agonist, m-chlorophenylpiperazine

Variation within the serotonin (5-HT) 5-HT₂C receptor system aligns with vulnerability to cocaine cue reactivity

Cocaine dependence remains a challenging public health problem with relapse cited as a major determinant in its chronicity and severity. Environmental contexts and stimuli become reliably associated with its use leading to durable conditioned responses ('cue reactivity') that can predict relapse as well as treatment success. Individual variation in the magnitude and influence of cue reactivity over behavior in humans and animals suggest that cue-reactive individuals may be at greater risk for the progression to addiction and/or relapse. In the present translational study, we investigated the contribution of variation in the serotonin (5-HT) 5-HT2C receptor (5-HT2CR) system in individual differences in cocaine cue reactivity in humans and rodents. We found that cocaine-dependent subjects carrying a single nucleotide polymorphism (SNP) in the HTR2C gene that encodes for the conversion of cysteine to serine at codon 23 (Ser23 variant) exhibited significantly higher attentional bias to cocaine cues in the cocaine-word Stroop task than those carrying the Cys23 variant. In a model of individual differences in cocaine cue reactivity in rats, we identified that high cocaine cue reactivity measured as appetitive approach behavior (lever presses reinforced by the discrete cue complex) correlated with lower 5-HT2CR protein expression in the medial prefrontal cortex and blunted sensitivity to the suppressive effects of the selective 5-HT2CR agonist WAY163909. Our translational findings suggest that the functional status of the 5-HT2CR system is a mechanistic factor in the generation of vulnerability to cocaine-associated cues, an observation that opens new avenues for future development of biomarker and therapeutic approaches to suppress relapse in cocaine dependence.

The current state of play on the molecular genetics of depression

BACKGROUND

It has been well established that both genes and non-shared environment contribute substantially to the underlying aetiology of major depressive disorder (MDD). A comprehensive overview of genetic research in MDD is presented. Method Papers were retrieved from PubMed up to December 2011, using many keywords including: depression, major depressive disorder, genetics, rare variants, gene-environment, whole genome, epigenetics, and specific candidate genes and variants. These were combined in a variety of permutations.

RESULTS

Linkage studies have yielded some promising chromosomal regions in MDD. However, there is a continued lack of consistency in association studies, in both candidate gene and genome-wide association studies (GWAS). Numerous factors may account for variable results including the use of different diagnostic approaches, small samples in early studies, population stratification, epigenetic phenomena, copy number variation (CNV), rare variation, and phenotypic and allelic heterogeneity. The conflicting results are also probably, in part, a consequence of environmental factors not being considered or controlled for.

CONCLUSIONS

Each research group has to identify what issues their sample may best address. We suggest that, where possible, more emphasis should be placed on the environment in molecular behavioural genetics to identify individuals at environmental high risk in addition to genetic high risk. Sequencing should be used to identify rare and alternative variation that may act as a risk factor, and a systems biology approach including gene-gene interactions and pathway analyses would be advantageous. GWAS may require even larger samples with reliably defined (sub)phenotypes.

Cortisol responses to emotional stress in men: association with a functional polymorphism in the 5HTR2C gene

The serotonin 5HTR2C receptor has been shown to mediate HPA axis activation during stress. We hypothesized that a functional polymorphism (rs6318) of the 5HTR2C gene would be associated with HPA axis response to a laboratory stress protocol. The present sample consisted of 41 men (22 African Americans, 19 Caucasians). We found that at rest men with the more active rs6318 Ser23 C allele had similar cortisol values compared to those with the less active Cys23 G allele. During laboratory stress, however, men with the Ser23 C allele exhibited the predicted significantly higher cortisol levels (p<0.001), as well as larger increases in anger (p=0.08) and depressive mood (p=0.006) ratings, compared to the Cys23 G carriers. The increase in cortisol was significantly related to the increases in ratings of anger and depression assessed before and after the emotion induction, and these correlations became nonsignificant when rs6318 genotype was covaried. We conclude that genetic variation in 5HTR2C may be associated with HPA axis activation and stimulated by emotional stress, and also with both psychological and physiological endophenotypes that increase the risk of cardiovascular disease and type-2 diabetes.

DRD4 48 bp VNTR but not 5-HT 2C Cys23Ser receptor polymorphism is related to antipsychotic-induced weight gain

Weight gain is a major side effect of antipsychotic treatment. The antidopaminergic and antiserotonergic effect of antipsychotics may contribute to antipsychotic-induced weight gain. We, therefore conducted a prospective clinical study, to investigate whether the D4 receptor (DRD4) 48 bp (base pair) variable number of tandem repeat (VNTR) polymorphism and the 5-hydroxytryptamine 2C receptor (HT(2C)) cysteine for serine substitution at position 23 (Cys23Ser) polymorphism may influence weight gain during antipsychotic treatment in a naturalistic setting of 102 Caucasian psychiatric in-patients. Patients suffering from psychotic disorders and treated according to local clinical practice were classified as either homozygous for the shorter alleles of the DRD4 48 bp VNTR polymorphism (<7-fold repeat, group 1) or heterozygous/homozygous for the long allele (7-fold repeat or higher, group 2). HT(2C) Cys23Ser polymorphism male patients were grouped hemizygous G (Cys) or C (Ser), while female patients were GG, GC or CC and both sexes were evaluated separately. Concerning the DRD4 48 bp VNTR polymorphism the increase in body mass index was significantly less in group 1 (0.38 kg m(-2); s.d.=1.04) than in group 2 (0.89 kg m(-2); s.d.=1.23; P=0.003). The difference between the genotype groups remained significant in male patients but not in female patients. In contrast, no influence on antipsychotic-induced increase in body weight was observed for the HT(2C) Cys23Ser polymorphism. These results support the hypothesis that the DRD4 48 bp VNTR polymorphism influences antipsychotic-induced weight gain. Male patients may be more affected than female patients. Due to the limitations of the study (heterogeneity of treatment, pretreatment and concomitant therapy) further studies are required before diagnostic genotyping of the DRD4 48 bp VNTR polymorphism may be useful for individualizing therapy.

Genotype-specific weight loss treatment advice: how close are we?

Obesity, whose prevalence is continually rising, is one of the world’s greatest health care burdens. This multifactorial condition is associated with many obesity-related conditions, such as type 2 diabetes, dyslipidemia, and cardiovascular disease. Weight loss is a significant challenge facing those wishing to reduce their disease risk. Of course, like obesity itself, weight loss is a complex phenomenon dependent on many environmental and genetic influences, and thus individual responses to weight loss interventions are incredibly variable. Currently, there are 3 major interventions used to reduce weight: diet, exercise, and pharmacotherapy. The findings from studies examining gene–diet (nutrigenetic), gene–exercise (actigenetic), and gene–pharmaceutical (pharmacogenetic) interactions, although not clinically applicable at this time, are gaining awareness. This review article summarizes the current evidence to support the contribution of DNA sequence variation in genes related to energy balance (expenditure and intake) in the response to weight loss intervention. There is no doubt that replication using more rigorous study designs that include the study of interactions between multiple genes and interventions is required to move towards the development of genotype-specific weight loss treatment strategies.

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.

Variability of 5-HT2C receptor cys23ser polymorphism among European populations and vulnerability to affective disorder

Substantial evidence supports a role for dysfunction of brain serotonergic (5-HT) systems in the pathogenesis of major affective disorder, both unipolar (recurrent major depression) and bipolar.(1) Modification of serotonergic neurotransmission is pivotally implicated in the mechanism of action of antidepressant drugs(2) and also in the action of mood stabilizing agents, particularly lithium carbonate.(3) Accordingly, genes that code for the multiple subtypes of serotonin receptors that have been cloned and are expressed in brain,(4) are strong candidates for a role in the genetic etiology of affective illness. We examined a structural variant of the serotonin 2C (5-HT2C) receptor gene (HTR2C) that gives rise to a cysteine to serine substitution in the N terminal extracellular domain of the receptor protein (cys23ser),(5) in 513 patients with recurrent major depression (MDD-R), 649 patients with bipolar (BP) affective disorder and 901 normal controls. The subjects were drawn from nine European countries participating in the European Collaborative Project on Affective Disorders. There was significant variation in the frequency of the HT2CR ser23 allele among the 10 population groups included in the sample (from 24.6% in Greek control subjects to 9.2% in Scots, chi(2) = 20.9, df 9, P = 0.01). Logistic regression analysis demonstrated that over and above this inter-population variability, there was a significant excess of HT2CR ser23 allele carriers in patients compared to normal controls that was demonstrable for both the MDD (chi(2) = 7.34, df 1, P = 0.006) and BP (chi(2) = 5.45, df 1, P = 0.02) patients. These findings support a possible role for genetically based structural variation in 5-HT2C receptors in the pathogenesis of major affective disorder.