Allelic variation in the serotonin 5-HT2C receptor gene and migraine

Genetics of Migraine without Aura, Migraine with Aura, Migrainous Disorder, Head Trauma Migraine without Aura and Tension-Type Headache

This review is a tribute to Professor Jes Olesen involvement in the genetics of migraine and tension-type headache as it is coming up to his 60th birthday.

Serotonin and Migraine: Biology and Clinical Implications

Migraine is the most frequent neurological disorder in the adult population worldwide, affecting up to 12% of the general population and more frequent in women (~25%). It has a high impact on our society due to its disabling nature and, therein, reduced quality of life and increased absenteeism from work. Headache is the primary clinical manifestation and it has been associated with ‘a hereditary or predisposed sensitivity of neurovascular reactions to certain stimuli or to cyclic changes in the central nervous system’ (1). Amongst the many neurotransmitters in the brain, the serotonergic (serotonin, 5-HT) system from the brainstem raphe nucleus has been most convincingly implicated in migraine pathophysiology. The documented changes in 5-HT metabolism and in the processing of central 5-HT-mediated responses during and in between migraine attacks have led to the suggestion that migraine is a consequence of a central neurochemical imbalance that involves a low serotonergic disposition. Although the exact cascade of events that link abnormal serotonergic neurotransmission to the manifestation of head pain and the accompanying symptoms has yet to be fully understood, recent evidence suggests that a low 5-HT state facilitates activation of the trigeminovascular nociceptive pathway, as induced by cortical spreading depression. In this short review, we present and discuss the original and most recent findings that support a role for altered serotonergic neurotransmission in the manifestation of migraine headache.

Genetic insights into migraine and glutamate: a protagonist driving the headache

Migraine is a complex polygenic disorder that continues to be a great source of morbidity in the developed world with a prevalence of 12% in the Caucasian population. Genetic and pharmacological studies have implicated the glutamate pathway in migraine pathophysiology. Glutamate profoundly impacts brain circuits that regulate core symptom domains in a range of neuropsychiatric conditions and thus remains a "hot" target for drug discovery. Glutamate has been implicated in cortical spreading depression (CSD), the phenomenon responsible for migraine with aura and in animal models carrying FHM mutations. Genotyping case-control studies have shown an association between glutamate receptor genes, namely, GRIA1 and GRIA3 with migraine with indirect supporting evidence from GWAS. New evidence localizes PRRT2 at glutamatergic synapses and shows it affects glutamate signalling and glutamate receptor activity via interactions with GRIA1. Glutamate-system defects have also been recently implicated in a novel FHM2 ATP1A2 disease-mutation mouse model. Adding to the growing evidence neurophysiological findings support a role for glutamate in cortical excitability. In addition to the existence of multiple genes to choreograph the functions of fast-signalling glutamatergic neurons, glutamate receptor diversity and regulation is further increased by the post-translational mechanisms of RNA editing and miRNAs. Ongoing genetic studies, GWAS and meta-analysis implicate neurogenic mechanisms in migraine pathology and the first genome-wide associated locus for migraine on chromosome X. Finally, in addition to glutamate modulating therapies, the kynurenine pathway has emerged as a candidate for involvement in migraine pathophysiology. In this review we discuss recent genetic evidence and glutamate modulating therapies that bear on the hypothesis that a glutamatergic mechanism may be involved in migraine susceptibility.

Investigation of gamma-aminobutyric acid (GABA) A receptors genes and migraine susceptibility

BACKGROUND

Migraine is a neurological disorder characterized by recurrent attacks of severe headache, affecting around 12% of Caucasian populations. It is well known that migraine has a strong genetic component, although the number and type of genes involved is still unclear. Prior linkage studies have reported mapping of a migraine gene to chromosome Xq 24-28, a region containing a cluster of genes for GABA A receptors (GABRE, GABRA3, GABRQ), which are potential candidate genes for migraine. The GABA neurotransmitter has been implicated in migraine pathophysiology previously; however its exact role has not yet been established, although GABA receptors agonists have been the target of therapeutic developments. The aim of the present research is to investigate the role of the potential candidate genes reported on chromosome Xq 24-28 region in migraine susceptibility. In this study, we have focused on the subunit GABA A receptors type epsilon (GABRE) and type theta (GABRQ) genes and their involvement in migraine.

METHODS

We have performed an association analysis in a large population of case-controls (275 unrelated Caucasian migraineurs versus 275 controls) examining a set of 3 single nucleotide polymorphisms (SNPs) in the coding region (exons 3, 5 and 9) of the GABRE gene and also the I478F coding variant of the GABRQ gene.

RESULTS

Our study did not show any association between the examined SNPs in our test population (P>0.05).

CONCLUSION

Although these particular GABA receptor genes did not show positive association, further studies are necessary to consider the role of other GABA receptor genes in migraine susceptibility.

Migraine as a sex-conditioned inherited disorder: evidences from China and the world

Migraine is a complex and heterogeneous disorder. Although several genetic models has been proposed including autosomal-dominant/autosomal recessive, sex-linked, sex-limited, mitochondrial, and multi-gene, none of these models can well-explain the transmission of the disease. We hypothesied that migraine is a sex-conditioned inherited disorder (autosomal dominant in females and autosomal recessive in males). This hypothesis is supported by the evidence such as the locations of genes associated with familial hemiplegic migraine, possibly "typical" migraine as well (dominantly on chromosome 19p, 1q, and 2q), male:female ratio of prevalence (1:2-1:4), the mostly youth-onset, the provocation by the contraceptives, the induction by menstruation, and the self-limitation after menopause. Female sex-hormones appear to be the key contributor to a higher prevalence of migraine in female. Socio-environmental factors may also play an important role.

Pharmacogenetics of migraine: genetic variants and their potential role in migraine therapy

Migraine is a paroxysmal neurological disorder affecting up to 6% of males and 18% of females in the general population, and has been demonstrated to have a strong, but complex, genetic component. Genetic investigation of migraine provides hope that new targets for medications and individual specific therapy will be developed. The identification of polymorphisms or genetic biomarkers for disease susceptibility and treatment should aid in providing a better understanding of migraine pathology and, consequently, more appropriate and efficient treatment for migraineurs. In this review, we will discuss results investigating genetic biomarkers for migraine and their potential role in future therapy planning.

A pharmacogenomic evaluation of migraine therapy

Migraine is a common idiopathic primary headache disorder with significant mental, physical and social health implications. Accompanying an intense unilateral pulsating head pain other characteristic migraine symptoms include nausea, emesis, phonophobia, photophobia and in approximately 20-30% of migraine cases, neurologic disturbances associated with the aura phase. Although selective serotonin (5-HT) receptor agonists (i.e., 5-HT(1B/1D)) are successful in alleviating migrainous symptoms in < or = 70% of known sufferers, for the remaining 30%, additional migraine abortive medications remain unsuccessful, not tested or yet to be identified. Genetic characterization of the migrainous disorder is making steady progress with an increasing number of genomic susceptibility loci now identified on chromosomes 1q, 4q, 5q, 6p, 11q, 14q, 15q, 17p, 18q, 19p and Xq. The 4q, 5q, 17p and 18q loci involve endophenotypic susceptibility regions for various migrainous symptoms. In an effort to develop individualized pharmacotherapeutics, the identification of these migraine endophenotypic loci may well be the catalyst needed to aid in this goal. In this review the authors discuss the present treatment of migraine, known genomic susceptibility regions and results from migraine (genetic) association studies. The authors also discuss pharmacogenomic considerations for more individualized migraine prophylactic treatments.

Genetic association study and meta-analysis of the HTR2C Cys23Ser polymorphism and migraine

The objectives of this study were to determine if the HTR2C Cys23Ser polymorphism is associated with migraine in a case-control study, and to perform a meta-analysis with present and previous available studies. The HTR2C gene is located at the Xq24-q28 chromosomal band. This band was linked to migraine with aura (MA) in two Australian families. Using the HTR2C Cys23Ser allelic variant, this gene has been ruled out as a migraine gene in 3 out of 4 studies. Only the Japanese study reported a higher risk for MA (OR=6.11; 95% CI=1.70-21.97, p trend<0.01). We performed a case-control study with 335 migraine subjects and 335 sex- and age-matched controls, and a meta-analysis pooling the results of the available data from MA subsets of patients. In the association study we found no significant differences among migraine and MA patients for this polymorphism. In the meta-analysis, under the fixed-effect model, the Ser allele did not confer higher risk for suffering MA (pooled OR=1.1; 99% CI=0.8-1.5, p=0.499). Our study did not confirm the HTR2C Cys23Ser polymorphism as a risk factor for migraine and MA.

Genetic biomarkers for migraine

Biomarkers are physical signs or laboratory measurements that occur in association with a pathological process and have putative diagnostic and/or prognostic utility. In migraine, clinical, radiological, and biochemical biomarkers might be helpful to improve diagnosis, get insight in pathophysiology, and facilitate treatment choices. Genetic biomarkers are defined as genetic variations (mutations or polymorphisms) that can predict disease susceptibility, disease outcome, or treatment response. As yet, only a few genetic biomarkers for migraine are available. Mutations in 3 different genes responsible for familial hemiplegic migraine, a monogenetic subtype of migraine with aura, and the MTHFR C677T polymorphism in common forms of migraine are clear examples. Many positive findings from linkage studies and association studies in common forms of migraine have not been replicated, and are therefore of less clinical use. In this review, we will discuss genetic biomarkers in migraine.

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 5ht2c receptors in affective disorders

5-HT(2C) receptors are predominantly localised in the brain and their dysregulation may contribute to particular symptoms of anxiety and depression. The marked affinity of several clinically established psychotropic agents sites (e.g., tricyclic antidepressants, clozapine, fluoxetine) for 5-HT(2C) receptor has generated interest in the therapeutic potential of selective, high affinity 5-HT(2C) receptor ligands. Like the selective serotonin re-uptake inhibitor (SSRI) fluoxetine, high affinity selective agonists such as Ro 60-0175 and Ro 60-0332 have potent in vivo activity in animal models suggestive of therapeutic action against depression, obsessive-compulsive disorder (OCD) and panic disorders. In contrast, 5-HT(2C) receptor antagonists such as SB-200646A or SB-221284 show signs of anxiolytic-like activity in tests for conditioned and phobic-like anxiety in rodents whereas they are inactive in tests indicative of antidepressant, antiOCD and antipanic activity. These results are consistent with an important hypothesis proposing that 5-HT has a complex, dual action on the neural mechanism of anxiety by either facilitating or inhibiting different kinds of anxiety in different brain regions. They also suggest that 5-HT(2C) receptor subtypes play an important role in the therapeutic properties of SSRIs. Certain 5-HT(2C) receptor antagonists may possess negative efficacy at 5-HT(2C) receptors and, as inverse agonists, may control constitutive receptor activity possibly characterising some psychopathological states. Receptor variants exist in the human population and indicate possible associations between somatic mutations in the 5-HT(2C) receptor and psychopathology or response to drug treatment. Selective 5-HT(2C) receptor ligands may offer innovative and improved therapeutic opportunities for the biological treatment of specific aspects of psychiatric syndromes.

Serotonin 2C receptor gene Cys23Ser polymorphism: a candidate genetic risk factor of migraine with aura in Japanese population

OBJECTIVES

The goal of this study is to clarify the association between migraine and Serotonin 2C receptor Cys23Ser polymorphism in Japanese population.

MATERIALS AND METHOD

This study included 37 individuals with migraine with aura (MWA), 80 with migraine without aura, 43 with tension type headache (TH) and 360 with controls. The genotypes of Cys23Ser polymorphism were confirmed by polymerase chain reaction-restriction fragment length polymorphism techniques.

RESULTS

The Ser allele frequency in control subjects is much less than that in Caucasian population. The Ser allele frequency in patients with MWA was higher than that in control subjects.

CONCLUSION

The present study provides that 5HTR2c Cys23Ser polymorphism may be associated with MWA in Japanese population.

Common genetic risk factors for psychiatric and simatic disorders

There is increasing knowledge about considerable comorbidity between psychiatric and somatic diseases, questioning whether variations in genes could be predisposing factors for both conditions. With respect to the multiple interactions between brain and body investigations have centered on variants in several candidate genes for proteins that mediate these interactions and therefore also have implications in psychiatric disorders. The available data, although still preliminary and rare, indicate the importance of polymorphic variants in genes coding for the serotonin (5-hydroxytryptamine, 5-HT) transporter (5-HTT), the 5-HT_2A receptor, proinflammatory cytokines, and the angiotensin-converting enzyme (ACE) in migraine, fibromyalgia, cardiovascular disorders, and psychiatric conditions. The role played by these various polymorphisms remains to be determined, as does whether they are indicative of common pathophysiological mechanisms or identify a subgroup of patients with somatic disorders that are more closely related to psychiatric symptoms. Nevertheless, they do at least illustrate the potential influence of genetic differences on illness course and treatment outcome, and might be a rational approach to drug development and treatment paradigms.

Migraine genetics

The genetics of migraine is a fascinating and moving research area. Familial hemiplegic migraine, a rare subtype of migraine with a Mendelian pattern of inheritance, is caused by mutations in the chromosome 19 CACNA1A gene in approximately 75% of the families. The finding of mutations in an ionchannel subunit defines migraine as a channelopathy (eg, epilepsy). The genetics of the more frequent variants, migraine with and without aura, is more complex. Several loci have been studied in families and case-control studies, but need to be confirmed.

An investigation of the 5-HT2C receptor gene as a migraine candidate gene

Migraine is a common complex disorder, currently classified into two main subtypes, migraine with aura (MA) and migraine without aura (MO). The strong preponderance of females to males suggests an X-linked genetic component. Recent studies have identified an X chromosomal susceptibility region (Xq24-q28) in two typical migraine pedigrees. This region harbours a potential candidate gene for the disorder, the serotonin receptor 2C (5-HT(2C)) gene. This study involved a linkage and association approach to investigate two single nucleotide variants in the 5-HT(2C) gene. In addition, exonic coding regions of the 5-HT(2C) gene were also sequenced for mutations in X-linked migraine pedigrees. Results of this study did not detect any linkage or association, and no disease causing mutations were identified. Hence, results for this study do not support a significant role of the 5-HT(2C) gene in migraine predisposition.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

Platelet serotonin measures in migraine

OBJECTIVE AND BACKGROUND

Serotonergic mechanisms play an important role in the pathogenesis of headache. To search for potential indicators of altered serotonin homeostasis in migraine, we have investigated three parameters of the platelet serotonin (5HT) system, platelet serotonin level (PSL), platelet serotonin uptake (PSU), and monoamine oxidase (MAO-B) activity, in a group of 55 patients with migraine and in 81 healthy controls.

METHODS

After platelet separation, PSL was determined fluorimetrically; PSU was measured by incubating aliquots of platelet-rich plasma with six concentrations of 14C-5-HT for 60 seconds at 37 degrees C, followed by vacuum filtration; platelet MAO-B activity (toward kynuramine as a substrate) was determined fluorimetrically.

RESULTS

Values of the investigated measures, in patients versus controls, amounted to (mean +/- SD) 608 +/- 166 vs. 591 +/- 184 ng/10(9) platelets for PSL, 139 +/- 25 vs. 142 +/- 25 pmol 5HT/10(8) platelets/minute for Vmax of PSU, 376 +/- 62 vs. 404 +/- 72 nM for Km of PSU, and 15.8 +/- 5.1 vs. 14.3 +/- 5.7 nmol product/10(8) platelets/60 minutes for velocity of MAO-B. Mentioned parameters did not show statistical differences between patients and controls, with exception of a small difference in Km of PSU, reaching significance (P<0.01). After subgrouping of patients according to diagnosis (migraine with aura, migraine without aura, and migraine attack) and gender, no differences retained significance.

CONCLUSIONS

Our results indicate the absence of a measurable disturbance in 5HT homeostasis in migraine, as shown by platelet 5HT parameters, and they question the suitability of the use of mentioned blood elements in this regard.

Molecular genetics of migraine headaches: a review

Following the recent discovery of neural calcium channel mutations in familial hemiplegic migraine, genetic linkage and association studies have been performed world-wide in an effort to unveil the genetic basis of the more common types of migraine too. Mutations in neural calcium channels, mitochondrial DNA, serotonin receptors and transporter, dopamine receptors and genetic prothrombotic risk factors have been especially investigated and are discussed here. No unambiguous conclusions have, however, been reached. FHM remains an isolated success story in the quest for the genetic basis of migraine.

Current status of genetic discoveries in migraine: familial hemiplegic migraine and beyond

Familial hemiplegic migraine (FHM) has been related to mutations in a brain calcium channel gene among Chr19p linked FHM families. Subsequent genetic Studies in different FHM families showed that additional causative genes must reside in other regions of the genome, including the long arm of Chromosome 1. Parallel discoveries in mouse mutants involving ion channel genes have also accelerated our understanding of the spectrum and functional significance of the CNS-related ion channel disorders. These studies have clear implications for migraine, epilepsy, and ataxia. An association study was suggested that other 'susceptibility' genes like the dopamine DRD2 receptor will be important in characterizing the genetic components of the larger, heterogeneous group of migraine disorders.