Support for the association between the rare functional variant I425V of the serotonin transporter gene and susceptibility to obsessive compulsive disorder

Pharmacogenetics of selective serotonin reuptake inhibitors (SSRI): A serotonin reuptake transporter (SERT)-based approach

Neuropsychiatric disorders are considered to be the most common cause of disability worldwide. Serotonin and its transporter is a prominent paradigm in mood disorders. Response to selective serotonin reuptake inhibitors (SSRI) is altered due to heterogeneity in the serotonin transporter gene, SLC6A4 (solute carrier family 6 member 4). The reported polymorphisms are found to be in different regions of the transporter gene: promoter region (5-HTTLPR and various single nucleotide polymorphisms within it), intron (STin2), and exon 9 (I425V). The long and short alleles of the 5-HTTLPR gene, which are prevalent among variations, may mediate differential effects. In long allelic variant carriers, an increased response to SSRI and timely recovery is due to increased availability of SERT. Whereas, SERT availability is significantly decreased in short allelic carriers, necessitating a reduction in SSRI dosage due to the increased risk of adverse drug reactions. Thus, pharmacogenetic investigations are required to understand the impact of functional variations on the efficacy and tolerability of SSRI. Identifying the carrier variants may aid in clear-decision making of the treatment regimen, aiding the approach of personalized medication.

The investigation of nonsynonymous SNPs of human SLC6A4 gene associated with depression: An in silico approach

Genetic polymorphism of the SLC6A4 gene is associated with several behavioral disorders, including depression. Since studying the total nonsynonymous single nucleotide polymorphisms (nsSNPs) of the SLC6A4 gene at the population level is a difficult task, we aim to utilize in silico approach to detect the most deleterious nsSNPs of the SLC6A4 gene. In our study, 7 computational tools were used in the initial stage, including SIFT, Polyphen-2, PROVEAN, SNAP2, PhD-SNP, PANTHER, and SNPs&GO to find out the most damaging nsSNPs. In the second phase, we performed structural, functional, and stability analysis of SLC6A4 protein by popular computation tools, including I-Mutant 2.0 and MutPred2. Also, the ConSurf server was utilized to find the conserved region of the SLC6A4 protein to determine the relationship between these conserved regions with high-risk nsSNPs. Based on these analyses, 5 high-risk mutations of the SLC6A4 protein were selected. Then, we carried out comparative modeling by using the Robetta server and aligned the mutant protein model with the native protein structure. Later, we performed the post-translational modification and functional domain analysis of the SLC6A4 protein. This study concludes that Arginine → Tryptophan at position 79 and Arginine → Cysteine at position 104 are the two significant mutations in SLC6A4 protein which might play an essential role in causing diseases. Future studies should take these high-risk nsSNPs (rs1221448303, rs200953188) into consideration while exploring diseases related to the SLC6A4 gene. Besides, our research is the first-ever comprehensive in silico investigation of the SLC6A4 gene. Thus, the findings of this study could be beneficial for developing precision medicines against diseases caused by SLC6A4 malfunction. Furthermore, extensive wet-lab research and experiments on various model organisms might be helpful to investigate the precise role of these damaging nsSNPs of the SLC6A4 gene.

Functional and Biochemical Consequences of Disease Variants in Neurotransmitter Transporters: A Special Emphasis on Folding and Trafficking Deficits

Neurotransmitters, such as γ-aminobutyric acid, glutamate, acetyl choline, glycine and the monoamines, facilitate the crosstalk within the central nervous system. The designated neurotransmitter transporters (NTTs) both release and take up neurotransmitters to and from the synaptic cleft. NTT dysfunction can lead to severe pathophysiological consequences, e.g. epilepsy, intellectual disability, or Parkinson’s disease. Genetic point mutations in NTTs have recently been associated with the onset of various neurological disorders. Some of these mutations trigger folding defects in the NTT proteins. Correct folding is a prerequisite for the export of NTTs from the endoplasmic reticulum (ER) and the subsequent trafficking to their pertinent site of action, typically at the plasma membrane. Recent studies have uncovered some of the key features in the molecular machinery responsible for transporter protein folding, e.g., the role of heat shock proteins in fine-tuning the ER quality control mechanisms in cells. The therapeutic significance of understanding these events is apparent from the rising number of reports, which directly link different pathological conditions to NTT misfolding. For instance, folding-deficient variants of the human transporters for dopamine or GABA lead to infantile parkinsonism/dystonia and epilepsy, respectively. From a therapeutic point of view, some folding-deficient NTTs are amenable to functional rescue by small molecules, known as chemical and pharmacological chaperones.

Control of serotonin transporter phosphorylation by conformational state

Serotonin transporter (SERT) is responsible for reuptake and recycling of 5-hydroxytryptamine (5-HT; serotonin) after its exocytotic release during neurotransmission. Mutations in human SERT are associated with psychiatric disorders and autism. Some of these mutations affect the regulation of SERT activity by cGMP-dependent phosphorylation. Here we provide direct evidence that this phosphorylation occurs at Thr276, predicted to lie near the cytoplasmic end of transmembrane helix 5 (TM5). Using membranes from HeLa cells expressing SERT and intact rat basophilic leukemia cells, we show that agents such as Na(+) and cocaine that stabilize outward-open conformations of SERT decreased phosphorylation and agents that stabilize inward-open conformations (e.g., 5-HT, ibogaine) increased phosphorylation. The opposing effects of the inhibitors cocaine and ibogaine were each reversed by an excess of the other inhibitor. Inhibition of phosphorylation by Na(+) and stimulation by ibogaine occurred at concentrations that induced outward opening and inward opening, respectively, as measured by the accessibility of cysteine residues in the extracellular and cytoplasmic permeation pathways, respectively. The results are consistent with a mechanism of SERT regulation that is activated by the transport of 5-HT, which increases the level of inward-open SERT and may lead to unwinding of the TM5 helix to allow phosphorylation.

Mechanism of Paroxetine (Paxil) Inhibition of the Serotonin Transporter

The serotonin transporter (SERT) is an integral membrane protein that exploits preexisting sodium-, chloride-, and potassium ion gradients to catalyze the thermodynamically unfavorable movement of synaptic serotonin into the presynaptic neuron. SERT has garnered significant clinical attention partly because it is the target of multiple psychoactive agents, including the antidepressant paroxetine (Paxil), the most potent selective serotonin reuptake inhibitor known. However, the binding site and orientation of paroxetine in SERT remain controversial. To provide molecular insight, we constructed SERT homology models based on the Drosophila melanogaster dopamine transporter and docked paroxetine to these models. We tested the predicted binding configurations with a combination of radioligand binding and flux assays on wild-type and mutant SERTs. Our data suggest that the orientation of paroxetine, specifically its fluorophenyl ring, in SERT’s substrate binding site directly depends on this pocket’s charge distribution, and thereby provide an avenue toward understanding and enhancing high-affinity antidepressant activity.

Insights to ligand binding to the monoamine transporters-from homology modeling to LeuBAT and dDAT

Understanding of drug binding to the human biogenic amine transporters (BATs) is essential to explain the mechanism of action of these pharmaceuticals but more importantly to be able to develop new and improved compounds to be used in the treatment of depression or drug addiction. Until recently no high resolution structure was available of the BATs and homology modeling was a necessity. Various studies have revealed experimentally validated binding modes of numerous ligands to the BATs using homology modeling. Here we examine and discuss the similarities between the binding models of substrates, antidepressants, psychostimulants, and mazindol in homology models of the human BATs and the recently published crystal structures of the Drosophila dopamine transporter and the engineered protein, LeuBAT. The comparison reveals that careful computational modeling combined with experimental data can be utilized to predict binding of molecules to proteins that agree very well with crystal structures.

Genetics of obsessive–compulsive disorder: a research update

The genetic study of obsessive-compulsive disorder (OCD) has made significant gains in the past decade. However, etiological gene findings are still elusive. Epidemiological studies, including family and twin studies, strongly support a genetic component for OCD. In addition, complex segregation analyses suggest the presence of at least one major gene. The neurobiology of OCD also lends support to the notion that programmed CNS-based biological processes underlie OCD symptom expression, with mapping of brain circuits to fronto-subcortical circuits in a consistent manner. Genetic linkage studies of OCD, using families with multiple affected relatives, have generated several suggestive linkage peaks, regions that may harbor a gene or genes for OCD. However, the presence of multiple linkage peaks has added to the complexity of OCD genetics, suggesting that the exploration of gene-gene interactions and gene-environment interactions, in addition to the exploration of alternate phenotypes based on symptom expression, age at onset or comorbid conditions, may be key in locating etiologic genes. Finally, candidate gene studies, while promising, are not yet associated with linkage regions, except in the case of the glutamate transporter gene SLC1A1 in 9p24. While OCD appears to have a genetic component, additional innovative research is needed to unravel the genetic influences in the disorder.

SLC6 transporters: structure, function, regulation, disease association and therapeutics

The SLC6 family of secondary active transporters are integral membrane solute carrier proteins characterized by the Na(+)-dependent translocation of small amino acid or amino acid-like substrates. SLC6 transporters, which include the serotonin, dopamine, norepinephrine, GABA, taurine, creatine, as well as amino acid transporters, are associated with a number of human diseases and disorders making this family a critical target for therapeutic development. In addition, several members of this family are directly involved in the action of drugs of abuse such as cocaine, amphetamines, and ecstasy. Recent advances providing structural insight into this family have vastly accelerated our ability to study these proteins and their involvement in complex biological processes.

Rare coding variants of the adenosine A3 receptor are increased in autism: on the trail of the serotonin transporter regulome

BACKGROUND

Rare genetic variation is an important class of autism spectrum disorder (ASD) risk factors and can implicate biological networks for investigation. Altered serotonin (5-HT) signaling has been implicated in ASD, and we and others have discovered multiple, rare, ASD-associated variants in the 5-HT transporter (SERT) gene leading to elevated 5-HT re-uptake and perturbed regulation. We hypothesized that loci encoding SERT regulators harbor variants that impact SERT function and/or regulation and therefore could contribute to ASD risk. The adenosine A3 receptor (A3AR) regulates SERT via protein kinase G (PKG) and other signaling pathways leading to enhanced SERT surface expression and catalytic activity.

METHODS

To test our hypothesis, we asked whether rare variants in the A3AR gene (ADORA3) were increased in ASD cases vs. controls. Discovery sequencing in a case-control sample and subsequent analysis of comparison exome sequence data were conducted. We evaluated the functional impact of two variants from the discovery sample on A3AR signaling and SERT activity.

RESULTS

Sequencing discovery showed an increase of rare coding variants in cases vs. controls (P=0.013). While comparison exome sequence data did not show a significant enrichment (P=0.071), combined analysis strengthened evidence for association (P=0.0025). Two variants discovered in ASD cases (Leu90Val and Val171Ile) lie in or near the ligand-binding pocket, and Leu90Val was enriched individually in cases (P=0.040). In vitro analysis of cells expressing Val90-A3AR revealed elevated basal cGMP levels compared with the wildtype receptor. Additionally, a specific A3AR agonist increased cGMP levels across the full time course studied in Val90-A3AR cells, compared to wildtype receptor. In Val90-A3AR/SERT co-transfections, agonist stimulation elevated SERT activity over the wildtype receptor with delayed 5-HT uptake activity recovery. In contrast, Ile171-A3AR was unable to support agonist stimulation of SERT. Although both Val90 and Ile171 were present in greater numbers in these ASD cases, segregation analysis in families showed incomplete penetrance, consistent with other rare ASD risk alleles.

CONCLUSIONS

Our results validate the hypothesis that the SERT regulatory network harbors rare, functional variants that impact SERT activity and regulation in ASD, and encourages further investigation of this network for other variation that may impact ASD risk.

Common and rare alleles of the serotonin transporter gene, SLC6A4, associated with Tourette's disorder

To evaluate the hypothesis that functionally over-expressing alleles of the serotonin transporter (SERT) gene (solute carrier family 6, member 4, SLC6A4) are present in Tourette's disorder (TD), just as we previously observed in obsessive compulsive disorder (OCD), we evaluated TD probands (N = 151) and controls (N = 858). We genotyped the refined SERT-linked polymorphic region 5-HTTLPR/rs25531 and the associated rs25532 variant in the SLC6A4 promoter plus the rare coding variant SERT isoleucine-to-valine at position 425 (I425V). The higher expressing 5-HTTLPR/rs25531 LA allele was more prevalent in TD probands than in controls (χ(2)  = 5.75; P = 0.017; odds ratio [OR], 1.35); and, in a secondary analysis, surprisingly, it was significantly more frequent in probands who had TD alone than in those who had TD plus OCD (Fisher's exact test; P = 0.0006; OR, 2.29). Likewise, the higher expressing LAC haplotype (5-HTTLPR/rs25531/rs25532) was more frequent in TD probands than in controls (P = 0.024; OR, 1.33) and also in the TD alone group versus the TD plus OCD group (P = 0.0013; OR, 2.14). Furthermore, the rare gain-of-function SERT I425V variant was observed in 3 male siblings with TD and/or OCD and in their father. Thus, the cumulative count of SERT I425V becomes 1.57% in OCD/TD spectrum conditions versus 0.15% in controls, with a recalculated, family-adjusted significance of χ(2) = 15.03 (P < 0.0001; OR, 9.0; total worldwide genotyped, 2914). This report provides a unique combination of common and rare variants in one gene in TD, all of which are associated with SERT gain of function. Thus, altered SERT activity represents a potential contributor to serotonergic abnormalities in TD. The present results call for replication in a similarly intensively evaluated sample. © 2013 Movement Disorder Society.

Anxiety and affective disorder comorbidity related to serotonin and other neurotransmitter systems: obsessive-compulsive disorder as an example of overlapping clinical and genetic heterogeneity

Individuals with obsessive-compulsive disorder (OCD) have also been shown to have comorbid lifetime diagnoses of major depressive disorder (MDD; rates greater than 70%), bipolar disorder (rates greater than 10%) and other anxiety disorders (e.g. panic disorder, post-traumatic stress disorder (PTSD)). In addition, overlap exists in some common genetic variants (e.g. the serotonin transporter gene (SLC6A4), the brain-derived neurotrophic factor (BDNF) gene), and rare variants in genes/chromosomal abnormalities (e.g. the 22q11 microdeletion syndrome) found across the affective/anxiety disorder spectrums. OCD has been proposed as a possible independent entity for DSM-5, but by others thought best retained as an anxiety disorder subtype (its current designation in DSM-IV), and yet by others considered best in the affective disorder spectrum. This review focuses on OCD, a well-studied but still puzzling heterogeneous disorder, regarding alterations in serotonergic, dopaminergic and glutamatergic neurotransmission in addition to other systems involved, and how related genes may be involved in the comorbidity of anxiety and affective disorders. OCD resembles disorders such as depression, in which gene × gene interactions, gene × environment interactions and stress elements coalesce to yield OC symptoms and, in some individuals, full-blown OCD with multiple comorbid disorders.

Olanzapine, but not fluoxetine, treatment increases survival in activity-based anorexia in mice

Anorexia nervosa (AN) is an eating disorder characterized by extreme hypophagia, hyperactivity, and fear of weight gain. No approved pharmacological treatments exist for AN despite high mortality rates. The activity-based anorexia (ABA) phenomenon models aspects of AN in rodents, including progressive weight loss, reduced food intake, and hyperactivity. First, we optimized the ABA paradigm for mice. We compared mouse strains (Balb/cJ, A/J) for susceptibility with ABA, and evaluated the effects of different food access durations (2, 4, 6, 8, and 10 h) on ABA parameters. Balb/cJ mice exhibited significantly shorter survival time (days until 25% bodyweight loss) in the ABA paradigm compared with A/J mice. Furthermore, 6 h of food access reduced survival in mice housed with wheels without reducing survival in mice housed without wheels. We then evaluated the effects of chronic treatment with fluoxetine (4 weeks) or subchronic treatment with olanzapine (OLZ) (1 week) on ABA in BALB/cJ mice. OLZ (12 mg/kg/day) significantly increased survival and reduced food anticipatory activity (FAA). However, OLZ did not alter food intake or running wheel activity during ad-lib feeding (baseline) or restriction conditions, or in mice housed without wheels. Fluoxetine (18 mg/kg/day) increased food intake and reduced FAA, but did not alter survival. Here, we report for the first time that OLZ, but not fluoxetine, reduces ABA in mice. Our findings indicate further need for clinical investigations into the effects of OLZ, but not selective serotonin reuptake inhibitors, on core features of AN.

Role of mother's genes and environment in postpartum depression

Most studies of human molecular genetics and social environment interactions on health have relied heavily on the classic diathesis-stress model that treats genetic variations and environments as being either "risky" or "protective." The biological susceptibility model posits that some individuals have greater genetic reactivity to stress, leading to worse outcomes in poor environments, but better outcomes in rich environments. Using a nontruncated measure of a chronic environmental stressor--socioeconomic status--measured by education, and two polymorphisms (5-HTTLPR and STin2 VNTR) of the serotonin transporter gene (5-HTT), we find strong evidence that some women are genetically more reactive to the environment, resulting in a crossover of risks of postpartum depression for the most reactive groups. We discuss how our approach and findings provide a framework for understanding some of the confusion in the gene-environment interaction literature on stress, 5-HTT, and depression.

Association of SLC6A4 variants with obsessive-compulsive disorder in a large multicenter US family study

Genetic association studies of SLC6A4 (SERT) and obsessive-compulsive disorder (OCD) have been equivocal. We genotyped 1241 individuals in 278 pedigrees from the OCD Collaborative Genetics Study for 13 single-nucleotide polymorphisms, for the linked polymorphic region (LPR) indel with molecular haplotypes at rs25531, for VNTR polymorphisms in introns 2 and 7 and for a 381-bp deletion 3' to the LPR. We analyzed using the Family-Based Association Test (FBAT) under additive, dominant, recessive and genotypic models, using both OCD and sex-stratified OCD as phenotypes. Two-point FBAT analysis detected association between Int2 (P = 0.0089) and Int7 (P = 0.0187) (genotypic model). Sex-stratified two-point analysis showed strong association in females with Int2 (P<0.0002), significant after correction for linkage disequilibrium, and multiple marker and model testing (P(Adj) = 0.0069). The SLC6A4 gene is composed of two haplotype blocks (our data and the HapMap); FBAT whole-marker analysis conducted using this structure was not significant. Several noteworthy nonsignificant results have emerged. Unlike Hu et al., we found no evidence for overtransmission of the LPR L(A) allele (genotype relative risk = 1.11, 95% confidence interval: 0.77-1.60); however, rare individual haplotypes containing L(A) with P<0.05 were observed. Similarly, three individuals (two with OCD/OCPD) carried the rare I425V SLC6A4 variant, but none of them passed it on to their six OCD-affected offspring, suggesting that it is unlikely to be solely responsible for the 'OCD plus syndrome', as reported by Ozaki et al. In conclusion, we found evidence of genetic association at the SLC6A4 locus with OCD. A noteworthy lack of association at the LPR, LPR-rs25531 and rare 425V variants suggests that hypotheses about OCD risk need revision to accommodate these new findings, including a possible gender effect.

Myristoylation of cGMP-dependent protein kinase dictates isoform specificity for serotonin transporter regulation

By transporting serotonin (5-HT) into neurons and other cells, serotonin transporter (SERT) modulates the action of 5-HT at cell surface receptors. SERT itself is modulated by several processes, including the cGMP signaling pathway. Activation of SERT by cGMP requires the cGMP-dependent protein kinase (PKG). Here we show that in HeLa cells lacking endogenous PKG, expression of PKGIα or PKGIβ was required for 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) to stimulate SERT phosphorylation and 5-HT influx. Catalytically inactive PKG mutants and wild-type PKGII did not support this stimulation. However, a mutant PKGII (G2A) that was not myristoylated substituted for functional PKGI, suggesting that myristoylation and subsequent membrane association blocked productive interaction with SERT. PKG also influenced SERT expression and localization. PKGI isoforms increased total and cell surface SERT levels, and PKGII decreased cell surface SERT without altering total expression. Remarkably, these changes did not require 8-Br-cGMP or functional kinase activity and were also observed with a SERT mutant resistant to activation by PKG. Both PKGIα and PKGIβ formed detergent-stable complexes with SERT, and this association did not require catalytic activity. The nonmyristoylated PKGII G2A mutant stimulated SERT expression similar to PKGI isoforms. These results suggest multiple mechanisms by which PKG can modulate SERT and demonstrate that the functional difference between PKG isoforms results from myristoylation of PKGII.

The genetics of obsessive-compulsive disorder and Tourette syndrome: an epidemiological and pathway-based approach for gene discovery

OBJECTIVE

To provide a contemporary perspective on genetic discovery methods applied to obsessive-compulsive disorder (OCD) and Tourette syndrome (TS).

METHOD

A review of research trends in genetics research in OCD and TS is conducted, with emphasis on novel approaches.

RESULTS

Genome-wide association studies (GWAS) are now in progress in OCD and TS and will provide a platform for future discovery of common gene variants. Optimally, newer next-generation genome sequencing methods can also be used to detect larger effect genes (rare gene variants), taking advantage of pedigrees. Studies of gene networks or sets rather than individual genes will be required to elucidate biological etiology, as neural systems appear to act redundantly. Newer phenotyping strategies, such as symptom-based subtypes, cross-disorder latent class types, and intermediate phenotypes (endophenotypes) will need to be developed and tested to better align clinical and physiological measures with genetic architecture.

CONCLUSION

Although genetics research has made significant advances based on computational strength and bioinformatics advances, newer approaches to phenotyping and judicious study of gene etiological networks will be needed to uncover the genetic etiology of OCD and TS.

Genetics of obsessive-compulsive disorder

Obsessive-compulsive disorder is a common debilitating condition affecting individuals from childhood through adult life. There is good evidence of genetic contribution to its etiology, but environmental risk factors also are likely to be involved. The condition probably has a complex pattern of inheritance. Molecular studies have identified several potentially relevant genes, but much additional research is needed to establish definitive causes of the condition.

A large-scale screen for coding variants in SERT/SLC6A4 in autism spectrum disorders

In the current study we explored the hypothesis that rare variants in SLC6A4 contribute to autism susceptibility and to rigid-compulsive behaviors in autism. We made use of a large number of unrelated cases with autism spectrum disorders (approximately 350) and controls (approximately 420) and screened for rare exonic variants in SLC6A4 by a high-throughput method followed by sequencing. We observed no difference in the frequency of such variants in the two groups, irrespective of how we defined the rare variants. Furthermore, we did not observe an association of rare coding variants in SLC6A4 with rigid-compulsive traits scores in the cases. These results do not support a significant role for rare coding variants in SLC6A4 in autism spectrum disorders, nor do they support a significant role for SLC6A4 in rigid-compulsive traits in these disorders.

Assessing the validity of current mouse genetic models of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.

Modeling rare gene variation to gain insight into the oldest biomarker in autism: construction of the serotonin transporter Gly56Ala knock-in mouse

Alterations in peripheral and central indices of serotonin (5-hydroxytryptamine, 5-HT) production, storage and signaling have long been associated with autism. The 5-HT transporter gene (HTT, SERT, SLC6A4) has received considerable attention as a potential risk locus for autism-spectrum disorders, as well as disorders with overlapping symptoms, including obsessive-compulsive disorder (OCD). Here, we review our efforts to characterize rare, nonsynonymous polymorphisms in SERT derived from multiplex pedigrees carrying diagnoses of autism and OCD and present the initial stages of our effort to model one of these variants, Gly56Ala, in vivo. We generated a targeting vector to produce the Gly56Ala substitution in the Slc6a4 locus by homologous recombination. Following removal of a neomycin resistance selection cassette, animals exhibiting germline transmission of the Ala56 variant were bred to establish a breeding colony on a 129S6 background, suitable for initial evaluation of biochemical, physiological and behavioral alterations relative to SERT Gly56 (wild-type) animals. SERT Ala56 mice were achieved and exhibit a normal pattern of transmission. The initial growth and gross morphology of these animals is comparable to wildtype littermate controls. The SERT Ala56 variant can be propagated in 129S6 mice without apparent disruption of fertility and growth. We discuss both the opportunities and challenges that await the physiological/behavioral analysis of Gly56Ala transgenic mice, with particular reference to modeling autism-associated traits.

Chronic reductions in serotonin transporter function prevent 5-HT1B-induced behavioral effects in mice

BACKGROUND

Obsessive-compulsive disorder (OCD) is characterized by intrusive thoughts, images, or impulses and/or repetitive stereotypical behavior. Obsessive-compulsive disorder patients exhibit reduced prepulse inhibition (PPI) and symptom exacerbation after challenge with 5-HT1B receptor agonists. Recently, gain-of-function alleles of the serotonin transporter (5-HTT) have been associated with OCD. We tested the hypothesis that reducing 5-HTT function chronically, either genetically or via serotonin reuptake inhibitor (SRI) treatment, attenuates PPI deficits and perseverative hyperlocomotion induced by 5-HT1B agonists in mice.

METHODS

Mice received subchronic or chronic pretreatment with the SRI fluoxetine and acute treatment with RU24969 (5-HT1A/1B agonist) or 8-OH-DPAT (5-HT1A agonist) and were assessed for PPI, locomotor activity, and spatial patterns of locomotion. The same measures were evaluated in 5-HTT wild-type (WT), heterozygous (HT), and knockout (KO) mice after RU24969 treatment. The effects of WAY100635 (5-HTA antagonist) or GR127935 (5-HT1B/D antagonist) pretreatment on RU24969-induced effects were evaluated. Finally, 5-HT1B binding and functional coupling were assessed in 5-HTT-WT, -HT, and -KO mice, and normal fluoxetine-treated mice.

RESULTS

Chronic, but not subchronic, fluoxetine treatment prevented RU24969-induced PPI deficits and perseverative hyperlocomotion. These RU24969-induced effects were mediated via 5-HT1B and not 5-HT1A receptors. 5-HTT-KO mice showed no effects of RU24969, and 5-HTT-HT mice exhibited intermediate phenotypes. 5-HT1B binding and functional coupling were reduced in the globus pallidus and substantia nigra of 5-HTT-KO mice.

CONCLUSIONS

Our results demonstrate that chronic, but not subchronic, fluoxetine treatment and 5-HTT knockout robustly attenuate 5-HT1B agonist-induced PPI deficits and perseverative hyperlocomotion. These results may have implications for the etiology and treatment of OCD.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

SERT Ileu425Val in autism, Asperger syndrome and obsessive-compulsive disorder

BACKGROUND

SERT I425V, an uncommon missense single nucleotide polymorphism producing a gain-of-function of the serotonin transporter (SERT), was originally found to segregate with a primarily obsessive-compulsive disorder (OCD) but complexly comorbid phenotype in two unrelated families.

OBJECTIVE

As two individuals with SERT I425V and OCD also had Asperger syndrome (AS), an autism spectrum disorder, and as other rare SERT variants have recently shown significant associations with autism, we set out to extend our original OCD study by genotyping additional autism/AS and OCD samples.

METHODS

Case-control association study of SERT I425V in 210 AS/autism probands and 215 controls, plus 335 OCD probands and their family members.

RESULTS

SERT I425V was not found in any of the individuals with AS/autism, OCD alone or OCD comorbid with AS and other disorders, or in controls. This results in new estimates of SERT I425V having a 1.5% prevalence in 530 individuals with OCD from five unrelated families genotyped by us and by one other group and a 0.23% frequency in four control populations totaling 1300 individuals, yielding a continuing significant OCD-control difference (Fisher's exact test corrected for family coefficient of identity P=0.004, odds ratio=6.54).

CONCLUSION

As several other uncommon, less well quantitated genetic variations occur with an OCD phenotype, including chromosomal anomalies and some other rare gene variants (SGCE, GCH1 and SLITRK1), a tentative conclusion is that OCD resembles other complex disorders in being etiologically heterogeneous and in having both highly penetrant familial subtypes associated with rare alleles or chromosomal anomalies, as well as having a more common, polygenetic form that may involve polymorphisms in such genes as BDNF, COMT, GRIN2beta, TPH2, HTR2A and SLC1A1.

Association study between obsessive-compulsive disorder and serotonergic candidate genes

BACKGROUND

To date, research examining the relationship between serotonergic genes and obsessive-compulsive disorder (OCD) has yielded conflicting results. The purpose of this study is to investigate the association between four serotonergic polymorphisms (STin2 VNTR and 5-HTTLPR of the SLC6A4 gene, and A-1438G (rs6311) and T102C (rs6313) of the HTR2A gene) and OCD.

METHODS

99 OCD patients, 456 non-OCD psychiatric patients, and 420 healthy controls from a homogeneous Spanish Caucasian population were genotyped using standard methods.

RESULTS

All groups showed Hardy-Weinberg equilibrium for the analyzed genetic variability. A-1438G and T102C polymorphisms were in complete linkage disequilibrium. OCD patients showed an excess of STin2.12 carriers (12/12, 12/10, and 12/9 genotypes) compared with healthy controls (chi(2) (1)=7.21, corrected p=0.021; OR=3.38, 95% CI=1.32-8.62) and non-OCD psychiatric patients (chi(2) (1)=6.70, corrected p=0.030; OR=3.24, 95% CI=1.27-8.26). However, no differences were found between non-OCD patients and healthy controls (chi(2) (1)=0.05, corrected p>1; OR=1.04, 95% CI=0.72-1.51). No significant differences were found with respect to A-1438G and 5-HTTLPR polymorphisms.

CONCLUSIONS

Our data provide supporting evidence of an association between the STin2 VNTR polymorphism of the SLC6A4 gene and OCD.

Molecular genetics of the platelet serotonin system in first-degree relatives of patients with autism

Elevated platelet serotonin (5-hydroxytryptamine, 5-HT) is found in a subset of children with autism and in some of their first-degree relatives. Indices of the platelet serotonin system, including whole blood 5-HT, 5-HT binding affinity for the serotonin transporter (K(m)), 5-HT uptake (V(max)), and lysergic acid diethylamide (LSD) receptor binding, were previously studied in 24 first-degree relatives of probands with autism, half of whom were selected for elevated whole blood 5-HT levels. All subjects were then genotyped for selected polymorphisms at the SLC6A4, HTR7, HTR2A, ITGB3, and TPH1 loci. Previous studies allowed an a priori prediction of SLC6A4 haplotypes that separated the subjects into three groups that showed significantly different 5-HT binding affinity (K(m), p=0.005) and 5-HT uptake rate (V(max), p=0.046). Genotypes at four individual polymorphisms in SLC6A4 were not associated with platelet 5-HT indices. Haplotypes at SLC6A4 and individual genotypes of polymorphisms at SLC6A4, HTR7, HTR2A, ITGB3, and TPH1 showed no significant association with whole blood 5-HT. Haplotype analysis of two polymorphisms in TPH1 revealed a nominally significant association with whole blood 5-HT (p=0.046). These initial studies of indices of the 5-HT system with several single-nucleotide polymorphisms at loci in this system generate hypotheses for testing in other samples.

A large case-control study of common functional SLC6A4 and BDNF variants in obsessive-compulsive disorder

Both serotonin transporter (SLC6A4) and brain-derived neurotrophic factor (BDNF) genes have shown positive associations with obsessive-compulsive disorder (OCD) and some other psychiatric disorders, but these results have not been consistently replicated. To explore the hypothesis that this variability might result from the effects of differing combinations of overlooked variants within SLC6A4 together with small OCD and control sample sizes, we studied three common functional polymorphisms (5-HTTLPR, STin2, and the newly discovered SNP, rs25531) in the largest sample size of OCD patients (N=347) and controls (N=749) ever investigated. During methods development, we found evidence for potential SLC6A4 genotyping problems with earlier methodology, a third possible contributor to variability in earlier studies. A fourth possible explanation might be SLC6A4 x BDNF interactions, which prompted us to investigate combined genotypes of BDNF V66M with the three SLC6A4 loci. Except for a nominal association with rs25531 alone, which did not survive correction for multiple comparisons, we found no evidence for any of these other variants being associated alone or together with OCD, and we therefore also examined clinical OCD subtypes within the sample to evaluate clinical heterogeneity. Subgroups based on the age of OCD onset, gender, familiality, factor analysis-derived symptom dimensions, or comorbidity with other psychiatric disorders failed to identify SLC6A4- or BDNF-associated phenotypes, with one exception of overall number of comorbid anxiety disorders being significantly associated with 5-HTTLPR/rs25531. We conclude that despite their attractiveness as candidate genes in OCD, our data provide no support for association in this large OCD patient sample and point toward the need to examine other genes as candidates for risk determinants in OCD.

Serotonin transporter phosphorylation by cGMP-dependent protein kinase is altered by a mutation associated with obsessive compulsive disorder

Human serotonin transporter (hSERT) activity expressed in HeLa cells was stimulated by agents that release nitric oxide, stimulate soluble guanylyl cyclase, or activate cGMP-dependent protein kinase (PKG). This stimulation was blocked by a PKG inhibitor. A naturally occurring mutation, I425V, associated with obsessive-compulsive disorder and other neuropsychiatric disorders, activated hSERT and eliminated stimulation via the PKG pathway. Inhibitors of soluble guanylyl cyclase or PKG decreased activity of the I425V mutant, but not wild type, indicating that both wild-type and mutant transporters could exist in both high and low activity forms. Mutation of Thr-276 in the fifth transmembrane domain (TM5) to alanine or aspartate prevented activation of wild-type hSERT through the PKG pathway and also blocked the inhibition of I425V activity by inhibitors of the pathway. The accessibility of positions in TM5 near Thr-276 was modified in T276D, but not in I425V. These results are consistent with the hypothesis that PKG phosphorylates hSERT at Thr-276 and increases its activity by modifying the substrate permeation pathway formed, in part, by TM5. The effect of the I425V mutation may shift the balance of hSERT toward the phosphorylated form, possibly by interfering with the action of a phosphatase. However, association of hSERT with protein phosphatase 2A was not decreased in the I425V mutant.

Association studies of serotonin system candidate genes in early-onset obsessive-compulsive disorder

BACKGROUND

Family-based evidence for association at serotonin system genes SLC6A4, HTR1B, HTR2A, and brain-derived neurotrophic factor (BDNF) has been previously reported in obsessive-compulsive disorder (OCD). Early-onset OCD is a more familial form of the disorder.

METHODS

We used the transmission-disequilibrium test of association at common polymorphisms in each of these genes in 54 parent-child trios ascertained through probands with early-onset OCD.

RESULTS

No evidence for association was detected at any of the polymorphisms in the entire set of subjects. Nominally significant association was found at the HTR2A rs6311 polymorphism in subjects with tic disorder and OCD (p = .05), replicating a previous finding in Tourette syndrome and OCD. Nominally significant association was also found for the SLC6A4 HT transporter gene-linked polymorphic region (5-HTTLPR) polymorphism for female subjects (p = .03). Neither association would remain significant after statistical correction for multiple testing. Despite no individual study reporting replication, a pooled analysis of five replication studies of the SLC6A4 5-HTTLPR polymorphism supports association (p = .02).

CONCLUSIONS

Low power across individual association studies in OCD may lead to a false acceptance of the null hypothesis. Accumulation of evidence from multiple studies will be necessary to evaluate the potential role for these genes in contributing to susceptibility to OCD.

Fragile X syndrome and autism at the intersection of genetic and neural networks

Autism, an entirely behavioral diagnosis with no largely understood etiologies and no population-wide biomarkers, contrasts with fragile X syndrome (FXS), a single-gene disorder with definite alterations of gene expression and neuronal morphology. Nevertheless, the behavioral overlap between autism and FXS suggests some overlapping mechanisms. Understanding how the single-gene alteration in FXS plays out within complex genetic and neural network processes may suggest targets for autism research and illustrate strategies for relating autism to more singular genetic syndromes.

Factor analysis of obsessive-compulsive disorder YBOCS-SC symptoms and association with 5-HTTLPR SERT polymorphism

Symptoms defining obsessive-compulsive disorder (OCD) are diverse. Factor analyses of OCD symptoms have been used to evaluate obsessive-compulsive phenotypes that are thought to be more homogenous than the macro phenotype. There is evidence that three factors (symmetry/ordering, obsessions/checking, and hoarding) are familial and preliminary evidence that repetitive rituals are associated with a functional polymorphism in the promotor region of the serotonin transporter gene (5-HTTLPR). The goal of this study was to examine relationships between OCD symptom dimensions and 5-HTTLPR. We genotyped 153 subjects who met DSM-IV criteria for OCD. We used the Yale-Brown obsessive-compulsive scale symptom checklist (YBOCS-SC) to assess OCD symptoms. Using principle components analysis, we derived four factors from the 13 symptom categories of the YBOCS-SC in this patient sample, which replicated previous factor analyses of this scale. The frequencies of the S allele and the SS genotype were associated with the second factor including obsessions regarding symmetry and compulsions involving repeating, counting, and ordering/arranging. This study may contribute to understanding of molecular genetic features underlying the appearance of symptom clusters in OCD.