Alterations in the alpha2 isoform of Na,K-ATPase associated with familial hemiplegic migraine type 2

The Dawn and Advancement of the Knowledge of the Genetics of Migraine

Background: Migraine is a prevalent episodic brain disorder known for recurrent attacks of unilateral headaches, accompanied by complaints of photophobia, phonophobia, nausea, and vomiting. Two main categories of migraine are migraine with aura (MA) and migraine without aura (MO). Main body: Early twin and population studies have shown a genetic basis for these disorders, and efforts have been invested since to discern the genes involved. Many techniques, including candidate-gene association studies, loci linkage studies, genome-wide association, and transcription studies, have been used for this goal. As a result, several genes were pinned with concurrent and conflicting data among studies. It is important to understand the evolution of techniques and their findings. Conclusions: This review provides a chronological understanding of the different techniques used from the dawn of migraine genetic investigations and the genes linked with the migraine subtypes.

Unravelling the Genetic Landscape of Hemiplegic Migraine: Exploring Innovative Strategies and Emerging Approaches

Migraine is a severe, debilitating neurovascular disorder. Hemiplegic migraine (HM) is a rare and debilitating neurological condition with a strong genetic basis. Sequencing technologies have improved the diagnosis and our understanding of the molecular pathophysiology of HM. Linkage analysis and sequencing studies in HM families have identified pathogenic variants in ion channels and related genes, including CACNA1A, ATP1A2, and SCN1A, that cause HM. However, approximately 75% of HM patients are negative for these mutations, indicating there are other genes involved in disease causation. In this review, we explored our current understanding of the genetics of HM. The evidence presented herein summarises the current knowledge of the genetics of HM, which can be expanded further to explain the remaining heritability of this debilitating condition. Innovative bioinformatics and computational strategies to cover the entire genetic spectrum of HM are also discussed in this review.

Start Me Up: How Can Surrounding Gangliosides Affect Sodium-Potassium ATPase Activity and Steer towards Pathological Ion Imbalance in Neurons?

Gangliosides, amphiphilic glycosphingolipids, tend to associate laterally with other membrane constituents and undergo extensive interactions with membrane proteins in cis or trans configurations. Studies of human diseases resulting from mutations in the ganglioside biosynthesis pathway and research on transgenic mice with the same mutations implicate gangliosides in the pathogenesis of epilepsy. Gangliosides are reported to affect the activity of the Na⁺/K⁺-ATPase, the ubiquitously expressed plasma membrane pump responsible for the stabilization of the resting membrane potential by hyperpolarization, firing up the action potential and ion homeostasis. Impaired Na⁺/K⁺-ATPase activity has also been hypothesized to cause seizures by several mechanisms. In this review we present different epileptic phenotypes that are caused by impaired activity of Na⁺/K⁺-ATPase or changed membrane ganglioside composition. We further discuss how gangliosides may influence Na⁺/K⁺-ATPase activity by acting as lipid sorting machinery providing the optimal stage for Na⁺/K⁺-ATPase function. By establishing a distinct lipid environment, together with other membrane lipids, gangliosides possibly modulate Na⁺/K⁺-ATPase activity and aid in “starting up” and “turning off” this vital pump. Therefore, structural changes of neuronal membranes caused by altered ganglioside composition can be a contributing factor leading to aberrant Na⁺/K⁺-ATPase activity and ion imbalance priming neurons for pathological firing.

Procyanidin C1 from Viola odorata L. inhibits Na+,K+-ATPase

Members of the Viola genus play important roles in traditional Asian herbal medicine. This study investigates the ability of Viola odorata L. extracts to inhibit Na⁺,K⁺-ATPase, an essential animal enzyme responsible for membrane potential maintenance. The root extract of V. odorata strongly inhibited Na⁺,K⁺-ATPase, while leaf and seeds extracts were basically inactive. A UHPLC-QTOF-MS/MS metabolomic approach was used to identify the chemical principle of the root extract’s activity, resulting in the detection of 35,292 features. Candidate active compounds were selected by correlating feature area with inhibitory activity in 14 isolated fractions. This yielded a set of 15 candidate compounds, of which 14 were preliminarily identified as procyanidins. Commercially available procyanidins (B1, B2, B3 and C1) were therefore purchased and their ability to inhibit Na⁺,K⁺-ATPase was investigated. Dimeric procyanidins B1, B2 and B3 were found to be inactive, but the trimeric procyanidin C1 strongly inhibited Na⁺,K⁺-ATPase with an IC₅₀ of 4.5 µM. This newly discovered inhibitor was docked into crystal structures mimicking the Na₃E₁∼P·ADP and K₂E₂·P_i states to identify potential interaction sites within Na⁺,K⁺-ATPase. Possible binding mechanisms and the principle responsible for the observed root extract activity are discussed.

Advances in genetics of migraine

Background

Migraine is a complex neurovascular disorder with a strong genetic component. There are rare monogenic forms of migraine, as well as more common polygenic forms; research into the genes involved in both types has provided insights into the many contributing genetic factors. This review summarises advances that have been made in the knowledge and understanding of the genes and genetic variations implicated in migraine etiology.

Findings

Migraine is characterised into two main types, migraine without aura (MO) and migraine with aura (MA). Hemiplegic migraine is a rare monogenic MA subtype caused by mutations in three main genes - CACNA1A, ATP1A2 and SCN1A - which encode ion channel and transport proteins. Functional studies in cellular and animal models show that, in general, mutations result in impaired glutamatergic neurotransmission and cortical hyperexcitability, which make the brain more susceptible to cortical spreading depression, a phenomenon thought to coincide with aura symptoms. Variants in other genes encoding ion channels and solute carriers, or with roles in regulating neurotransmitters at neuronal synapses, or in vascular function, can also cause monogenic migraine, hemiplegic migraine and related disorders with overlapping symptoms. Next-generation sequencing will accelerate the finding of new potentially causal variants and genes, with high-throughput bioinformatics analysis methods and functional analysis pipelines important in prioritising, confirming and understanding the mechanisms of disease-causing variants.

With respect to common migraine forms, large genome-wide association studies (GWAS) have greatly expanded our knowledge of the genes involved, emphasizing the role of both neuronal and vascular pathways. Dissecting the genetic architecture of migraine leads to greater understanding of what underpins relationships between subtypes and comorbid disorders, and may have utility in diagnosis or tailoring treatments. Further work is required to identify causal polymorphisms and the mechanism of their effect, and studies of gene expression and epigenetic factors will help bridge the genetics with migraine pathophysiology.

Conclusions

The complexity of migraine disorders is mirrored by their genetic complexity. A comprehensive knowledge of the genetic factors underpinning migraine will lead to improved understanding of molecular mechanisms and pathogenesis, to enable better diagnosis and treatments for migraine sufferers.

The α2β2 isoform combination dominates the astrocytic Na+ /K+ -ATPase activity and is rendered nonfunctional by the α2.G301R familial hemiplegic migraine type 2-associated mutation

Synaptic activity results in transient elevations in extracellular K+ , clearance of which is critical for sustained function of the nervous system. The K+ clearance is, in part, accomplished by the neighboring astrocytes by mechanisms involving the Na+ /K+ -ATPase. The Na+ /K+ -ATPase consists of an α and a β subunit, each with several isoforms present in the central nervous system, of which the α2β2 and α2β1 isoform combinations are kinetically geared for astrocytic K+ clearance. While transcript analysis data designate α2β2 as predominantly astrocytic, the relative quantitative protein distribution and isoform pairing remain unknown. As cultured astrocytes altered their isoform expression in vitro, we isolated a pure astrocytic fraction from rat brain by a novel immunomagnetic separation approach in order to determine the expression levels of α and β isoforms by immunoblotting. In order to compare the abundance of isoforms in astrocytic samples, semi-quantification was carried out with polyhistidine-tagged Na+ /K+ -ATPase subunit isoforms expressed in Xenopus laevis oocytes as standards to obtain an efficiency factor for each antibody. Proximity ligation assay illustrated that α2 paired efficiently with both β1 and β2 and the semi-quantification of the astrocytic fraction indicated that the astrocytic Na+ /K+ -ATPase is dominated by α2, paired with β1 or β2 (in a 1:9 ratio). We demonstrate that while the familial hemiplegic migraine-associated α2.G301R mutant was not functionally expressed at the plasma membrane in a heterologous expression system, α2+/G301R mice displayed normal protein levels of α2 and glutamate transporters and that the one functional allele suffices to manage the general K+ dynamics.

Substitutions in the cardenolide binding site and interaction of subunits affect kinetics besides cardenolide sensitivity of insect Na,K-ATPase

Substitutions within the cardenolide target site of several insects' Na,K-ATPase α-subunits may confer resistance against toxic cardenolides. However, to which extent these substitutions alter the Na,K-ATPase's kinetic properties and how they interact with different β-subunits is not clear. The cardenolide-adapted milkweed bug Oncopeltus fasciatus possesses three paralogs of the α-subunit (A, B, and C) that differ in number and identity of resistance-conferring substitutions. We introduced these substitutions into the α-subunit of Drosophila melanogaster and combined them with the β-subunits Nrv2.2 and Nrv3. The substitutions Q111T-N122H-F786N-T797A (A-copy mimic) and Q111T-N122H-F786N (B-copy mimic) mediated high insensitivity to ouabain, yet they drastically lowered ATPase activity. Remarkably, the identity of the β-subunit was decisive and all α-subunits were less active when combined with Nrv3 than when combined with Nrv2.2. Both the substitutions and the co-expressed β-subunit strongly affected the enyzme's affinity for Na⁺ and K⁺. Na⁺ affinity was considerably higher for all enzymes expressed with nrv3 while expression with nrv2.2 mostly increased K⁺ affinity. Our results provide the first evidence that resistance against cardenolides comes at the cost of significantly altered kinetic properties of the Na,K-ATPase. The β-subunit can strongly modulate these properties but cannot fully compensate for the effect of the substitutions.

Subclinical Neuromuscular Transmission Abnormality Detected by Single-Fibre EMG is More Pronounced in Cluster Headache Than in Migraine With Aura

The aim was to investigate neuromuscular transmission (NMT) by single-fibre EMG (SFEMG) in a large series of patients having migraine with aura (MA) or cluster headache (CH). Recent studies using SFEMG have shown subclinical dysfunction of NMT in MA and CH. Forty-three patients having MA, 51 with CH and 38 healthy control subjects underwent nerve conduction studies, EMG and SFEMG during voluntary contraction of the extensor digitorum communis muscle. Twenty different potential pairs were recorded and individual, mean and total abnormal individual jitter values were calculated. The results obtained from MA patients were compared with those from CH patients. In MA patients, 32 of 860 jitters were abnormally high, whereas 73 of 1020 of the jitters showed this abnormality in CH patients. None of the control subjects, five MA patients (11.6%) and 11 CH patients (21.6%) were designated as having subclinical NMT abnormality. Thus, patients having junction dysfunction were significantly more common in the CH group. The subclinical NMT abnormality shown by SFEMG is more common in CH than in MA. These two primary headache syndromes may have some shared functional abnormality of NMT constituents which is more evident in CH.

ATP1A2 Mutations in Migraine: Seeing through the Facets of an Ion Pump onto the Neurobiology of Disease

Mutations in four genes have been identified in familial hemiplegic migraine (FHM), from which CACNA1A (FHM type 1) and SCN1A (FHM type 3) code for neuronal voltage-gated calcium or sodium channels, respectively, while ATP1A2 (FHM type 2) encodes the α2 isoform of the Na(+),K(+)-ATPase's catalytic subunit, thus classifying FHM primarily as an ion channel/ion transporter pathology. FHM type 4 is attributed to mutations in the PRRT2 gene, which encodes a proline-rich transmembrane protein of as yet unknown function. The Na(+),K(+)-ATPase maintains the physiological gradients for Na(+) and K(+) ions and is, therefore, critical for the activity of ion channels and transporters involved neuronal excitability, neurotransmitter uptake or Ca(2+) signaling. Strikingly diverse functional abnormalities have been identified for disease-linked ATP1A2 mutations which frequently lead to changes in the enzyme's voltage-dependent properties, kinetics, or apparent cation affinities, but some mutations are truly deleterious for enzyme function and thus cause full haploinsufficiency. Here, we summarize structural and functional data about the Na(+),K(+)-ATPase available to date and an overview is provided about the particular properties of the α2 isoform that explain its physiological relevance in electrically excitable tissues. In addition, current concepts about the neurobiology of migraine, the correlations between primary brain dysfunction and mechanisms of headache pain generation are described, together with insights gained recently from modeling approaches in computational neuroscience. Then, a survey is given about ATP1A2 mutations implicated in migraine cases as documented in the literature with focus on mutations that were described to completely destroy enzyme function, or lead to misfolded or mistargeted protein in particular model cell lines. We also discuss whether or not there are correlations between these most severe mutational effects and clinical phenotypes. Finally, perspectives for future research on the implications of Na(+),K(+)-ATPase mutations in human pathologies are presented.

Managing Brain Extracellular K(+) during Neuronal Activity: The Physiological Role of the Na(+)/K(+)-ATPase Subunit Isoforms

During neuronal activity in the brain, extracellular K⁺ rises and is subsequently removed to prevent a widespread depolarization. One of the key players in regulating extracellular K⁺ is the Na⁺/K⁺-ATPase, although the relative involvement and physiological impact of the different subunit isoform compositions of the Na⁺/K⁺-ATPase remain unresolved. The various cell types in the brain serve a certain temporal contribution in the face of network activity; astrocytes respond directly to the immediate release of K⁺ from neurons, whereas the neurons themselves become the primary K⁺ absorbers as activity ends. The kinetic characteristics of the catalytic α subunit isoforms of the Na⁺/K⁺-ATPase are, partly, determined by the accessory β subunit with which they combine. The isoform combinations expressed by astrocytes and neurons, respectively, appear to be in line with the kinetic characteristics required to fulfill their distinct physiological roles in clearance of K⁺ from the extracellular space in the face of neuronal activity. Understanding the nature, impact and effects of the various Na⁺/K⁺-ATPase isoform combinations in K⁺ management in the central nervous system might reveal insights into pathological conditions such as epilepsy, migraine, and spreading depolarization following cerebral ischemia. In addition, particular neurological diseases occur as a result of mutations in the α2- (familial hemiplegic migraine type 2) and α3 isoforms (rapid-onset dystonia parkinsonism/alternating hemiplegia of childhood). This review addresses aspects of the Na⁺/K⁺-ATPase in the regulation of extracellular K⁺ in the central nervous system as well as the related pathophysiology. Understanding the physiological setting in non-pathological tissue would provide a better understanding of the pathological events occurring during disease.

Migraine pathophysiology: lessons from mouse models and human genetics

Migraine is a common, disabling, and undertreated episodic brain disorder that is more common in women than in men. Unbiased genome-wide association studies have identified 13 migraine-associated variants pointing at genes that cluster in pathways for glutamatergic neurotransmission, synaptic function, pain sensing, metalloproteinases, and the vasculature. The individual pathogenetic contribution of each gene variant is difficult to assess because of small effect sizes and complex interactions. Six genes with large effect sizes were identified in patients with rare monogenic migraine syndromes, in which hemiplegic migraine and non-hemiplegic migraine with or without aura are part of a wider clinical spectrum. Transgenic mouse models with human monogenic-migraine-syndrome gene mutations showed migraine-like features, increased glutamatergic neurotransmission, cerebral hyperexcitability, and enhanced susceptibility to cortical spreading depression, which is the electrophysiological correlate of aura and a putative trigger for migraine. Enhanced susceptibility to cortical spreading depression increased sensitivity to focal cerebral ischaemia, and blocking of cortical spreading depression improved stroke outcome in these mice. Changes in female hormone levels in these mice modulated cortical spreading depression susceptibility in much the same way that hormonal fluctuations affect migraine activity in patients. These findings confirm the multifactorial basis of migraine and might allow new prophylactic options to be developed, not only for migraine but potentially also for migraine-comorbid disorders such as epilepsy, depression, and stroke.

Familial hemiplegic migraine treated by sodium valproate and lamotrigine

Background

Familial hemiplegic migraine (FHM) is a rare monogenic subtype of migraine with aura that includes motor auras. Prophylactic treatment of FHM often has marginal effects and involves a trial-and-error strategy based on therapeutic guidelines for non-hemiplegic migraine and on case reports in FHM.

Methods

We assessed the response to prophylactic medication in an FHM family and sequenced the FHM2 ATP1A2 gene in all available relatives.

Results

A novel p.Met731Val ATP1A2 mutation was identified. Attack frequency was reduced significantly with sodium valproate monotherapy ( n = 1) and attacks ceased completely with a combination of sodium valproate and lamotrigine ( n = 2).

Conclusions

We report dramatic prophylactic effects of sodium valproate and lamotrigine in an FHM2 family, making these drugs worth considering in the treatment of other FHM patients.

Familial hemiplegic migraine mutations affect Na,K-ATPase domain interactions

Familial hemiplegic migraine (FHM) is a monogenic variant of migraine with aura. One of the three known causative genes, ATP1A2, which encodes the α2 isoform of Na,K-ATPase, causes FHM type 2 (FHM2). Over 50 FHM2 mutations have been reported, but most have not been characterized functionally. Here we study the molecular mechanism of Na,K-ATPase α2 missense mutations. Mutants E700K and P786L inactivate or strongly reduce enzyme activity. Glutamic acid 700 is located in the phosphorylation (P) domain and the mutation most likely disrupts the salt bridge with Lysine 35, thereby destabilizing the interaction with the actuator (A) domain. Mutants G900R and E902K are present in the extracellular loop at the interface of the α and β subunit. Both mutants likely hamper the interaction between these subunits and thereby decrease enzyme activity. Mutants E174K, R548C and R548H reduce the Na(+) and increase the K(+) affinity. Glutamic acid 174 is present in the A domain and might form a salt bridge with Lysine 432 in the nucleotide binding (N) domain, whereas Arginine 548, which is located in the N domain, forms a salt bridge with Glutamine 219 in the A domain. In the catalytic cycle, the interactions of the A and N domains affect the K(+) and Na(+) affinities, as observed with these mutants. Functional consequences were not observed for ATP1A2 mutations found in two sporadic hemiplegic migraine cases (Y9N and R879Q) and in migraine without aura (R51H and C702Y).

Pathophysiology of Migraine

Migraine is a serious health problem which impair quality of life. It is the second most common primary headache that affects approximately more than %10 people in general population. Migraine pathophysiology is still unclear. Increasing results of studies suggest to migraine pathophysiology is related with primary neuronal mechanisms. Migraine pain starts in which region of brain and what brain regions are activated in different stages is unenlightened. There is evidences that growing number of studies which using new imaging techniques as positron emission tomography (PET) and functional magnetic resonans imaging (fMRI) show that migraine and cluster headaches are related with neuronal structures and vasodilatation. There are four phases to a migraine. The prodrome phase, aura, the attack, and the postdrome phase. Some datas obtained from last ten years indicate that cortical excitability has increased in interictal phase too. For many years, studies in rodents show trgimenial nerve is activated and it leads to vasodilatation and neurogenic inflammation in the headache phase. Although the majority of patients encountered in clinical practice are migraine without aura or chronic migraine, experimental studies of the migraine pathophysiology are focusing on the aura model which is used cortical spreading depression.

Genetic effects of ATP1A2 in familial hemiplegic migraine type II and animal models

Na⁺/K⁺-ATPase alpha 2 (Atp1a2) is an integral plasma membrane protein belonging to the P-type ATPase family that is responsible for maintaining the sodium (Na⁺) and potassium (K⁺) gradients across cellular membranes with hydrolysis of ATP. Atp1a2 contains two subunits, alpha and beta, with each having various isoforms and differential tissue distribution. In humans, mutations in ATP1A2 are associated with a rare form of hereditary migraines with aura known as familial hemiplegic migraine type II. Genetic studies in mice have revealed other neurological effects of Atp1a2 in mice including anxiety, fear, and learning and motor function disorders. This paper reviews the recent findings in the literature concerning Atp1a2.

Genes, molecules and patients--emerging topics to guide clinical pain research

This review selectively explores some areas of pain research that, until recently, have been poorly understood. We have chosen four topics that relate to clinical pain and we discuss the underlying mechanisms and related pathophysiologies contributing to these pain states. A key issue in pain medicine involves crucial events and mediators that contribute to normal and abnormal pain signaling, but remain unseen without genetic, biomarker or imaging analysis. Here we consider how the altered genetic make-up of familial pains reveals the human importance of channels discovered by preclinical research, followed by the contribution of receptors as stimulus transducers in cold sensing and cold pain. Finally we review recent data on the neuro-immune interactions in chronic pain and the potential targets for treatment in cancer-induced bone pain.

Expression of mutant α1 Na/K-ATPase defective in conformational transition attenuates Src-mediated signal transduction

The α1 Na/K-ATPase possesses both pumping and signaling functions. Using purified enzyme we found that the α1 Na/K-ATPase might interact with and regulate Src activity in a conformation-dependent manner. Here we further explored the importance of the conformational transition capability of α1 Na/K-ATPase in regulation of Src-related signal transduction in cell culture. We first rescued the α1-knockdown cells by wild-type rat α1 or α1 mutants (I279A and F286A) that are known to be defective in conformational transition. Stable cell lines with comparable expression of wild type α1, I279A, and F286A were characterized. As expected, the defects in conformation transition resulted in comparable degree of inhibition of pumping activity in the mutant-rescued cell lines. However, I279A was more effective in inhibiting basal Src activity than either the wild-type or the F286A. Although much higher ouabain concentration was required to stimulate Src in I279A-rescued cells, extracellular K(+) was comparably effective in regulating Src in both control and I279A cells. In contrast, ouabain and extracellular K(+) failed to produce detectable changes in Src activity in F286A-rescued cells. Furthermore, expression of either mutant inhibited integrin-induced activation of Src/FAK pathways and slowed cell spreading processes. Finally, the expression of these mutants inhibited cell growth, with I279A being more potent than that of F286A. Taken together, the new findings suggest that the α1 Na/K-ATPase may be a key player in dynamic regulation of cellular Src activity and that the capability of normal conformation transition is essential for both pumping and signaling functions of α1 Na/K-ATPase.

Migraine headache: a review of the molecular genetics of a common disorder

This tutorial summarises the state-of-the-art on migraine genetics and looks at the possible future direction of this field of research. The view of migraine as a genetic disorder, initially based on epidemiological observations of transmission of the condition within families, was subsequently confirmed by the identification of monogenic forms of "syndromic" migraine, such as familial hemiplegic migraine. We are currently witnessing a change in the way genetic analysis is used in migraine research: rather than studying modalities of inheritance in non-monogenic forms of migraine and in the persistent modalities of migraine headache, researchers are now tending to focus on the search for genetic markers of dysfunction in biological systems. One example of the evolution of migraine genetic research is provided by the recent efforts to shed light on the pharmacogenomic mechanisms of drug response in migraineurs. In addition, novel molecular approaches about to be introduced are expected to further increase knowledge on this topic and improve patient management.

Inhibition of phosphorylation of na+,k+-ATPase by mutations causing familial hemiplegic migraine

The neurological disorder familial hemiplegic migraine type II (FHM2) is caused by mutations in the α2-isoform of the Na(+),K(+)-ATPase. We have studied the partial reaction steps of the Na(+),K(+)-pump cycle in nine FHM2 mutants retaining overall activity at a level still compatible with cell growth. Although it is believed that the pathophysiology of FHM2 results from reduced extracellular K(+) clearance and/or changes in Na(+) gradient-dependent transport processes in neuroglia, a reduced affinity for K(+) or Na(+) is not a general finding with the FHM2 mutants. Six of the FHM2 mutations markedly affect the maximal rate of phosphorylation from ATP leading to inhibition by intracellular K(+), thereby likely compromising pump function under physiological conditions. In mutants R593W, V628M, and M731T, the defective phosphorylation is caused by local perturbations within the Rossmann fold, possibly interfering with the bending of the P-domain during phosphoryl transfer. In mutants V138A, T345A, and R834Q, long range effects reaching from as far away as the M2 transmembrane helix perturb the function of the catalytic site. Mutant E700K exhibits a reduced rate of E(2)P dephosphorylation without effect on phosphorylation from ATP. An extremely reduced vanadate affinity of this mutant indicates that the slow dephosphorylation reflects a destabilization of the phosphoryl transition state. This seems to be caused by insertion of the lysine between two other positively charged residues of the Rossmann fold. In mutants R202Q and T263M, effects on the A-domain structure are responsible for a reduced rate of the E(1)P to E(2)P transition.

Genetic basis of pain variability: recent advances

An estimated 15-50% of the population experiences pain at any given time, at great personal and societal cost. Pain is the most common reason patients seek medical attention, and there is a high degree of individual variability in reporting the incidence and severity of symptoms. Research suggests that pain sensitivity and risk for chronic pain are complex heritable traits of polygenic origin. Animal studies and candidate gene testing in humans have provided some progress in understanding the heritability of pain, but the application of the genome-wide association methodology offers a new tool for further elucidating the genetic contributions to normal pain responding and pain in clinical populations. Although the determination of the genetics of pain is still in its infancy, it is clear that a number of genes play a critical role in determining pain sensitivity or susceptibility to chronic pain. This review presents an update of the most recent findings that associate genetic variation with variability in pain and an overview of the candidate genes with the highest translational potential.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.

Migraine- and dystonia-related disease-mutations of Na+/K+-ATPases: relevance of behavioral studies in mice to disease symptoms and neurological manifestations in humans

The two autosomal dominantly inherited neurological diseases: familial hemiplegic migraine type 2 (FHM2) and familial rapid-onset of dystonia-parkinsonism (Familial RDP) are caused by in vivo mutations of specific alpha subunits of the sodium-potassium pump (Na(+)/K(+)-ATPase). Intriguingly, patients with classical FHM2 and RDP symptoms additionally suffer from other manifestations, such as epilepsy/seizures and developmental disabilities. Recent studies of FHM2 and RDP mouse models provide valuable tools for dissecting the vital roles of the Na(+)/K(+)-ATPases, and we discuss their relevance to the complex patient symptoms and manifestations. Thus, it is interesting that mouse models targeting a specific α-isoform cause different, although still comparable, phenotypes consistent with classical symptoms and other manifestations observed in FHM2 and RDP patients. This review highlights that use of mouse models have broad potentials for future research concerning migraine and dystonia-related diseases, which will contribute towards understanding the, yet unknown, pathophysiologies.

New directions in migraine

Migraine is a highly prevalent neurological disorder imparting a major burden on health care around the world. The primary pathology may be a state of hyperresponsiveness of the nervous system, but the molecular mechanisms are yet to be fully elucidated. We could now be at a watershed moment in this respect, as the genetic loci associated with typical forms of migraine are being revealed. The genetic discoveries are the latest step in the evolution of our understanding of migraine, which was initially considered a cerebrovascular condition, then a neuroinflammatory process and now primarily a neurogenic disorder. Indeed, the genetic findings, which have revealed ion channels and transporter mutations as causative of migraine, are a powerful argument for the neurogenic basis of migraine. Modulations of ion channels leading to amelioration of the migraine 'hyperresponsive' brain represent attractive targets for drug discovery. There lies ahead an exciting and rapidly progressing phase of migraine translational research, and in this review we highlight recent genetic findings and consider how these may affect the future of migraine neurobiology and therapy.

Stabilization of the α2 isoform of Na,K-ATPase by mutations in a phospholipid binding pocket

The α2 isoform of Na,K-ATPase plays a crucial role in Ca(2+) handling, muscle contraction, and inotropic effects of cardiac glycosides. Thus, structural, functional, and pharmacological comparisons of α1, α2, and α3 are of great interest. In Pichia pastoris membranes expressing human α1β1, α2β1, and α3β1 isoforms, or using the purified isoform proteins, α2 is most easily inactivated by heating and detergent (α2 ≫ α3 > α1). We have examined an hypothesis that instability of α2 is caused by weak interactions with phosphatidylserine, which stabilizes the protein. Three residues, unique to α2, in trans-membrane segments M8 (Ala-920), M9 (Leu-955), and M10 (Val-981) were replaced by equivalent residues in α1, singly or together. Judged by the sensitivity of the purified proteins to heat, detergent, "affinity" for phosphatidylserine, and stabilization by FXYD1, the triple mutant (A920V/L955F/V981P, called α2VFP) has stability properties close to α1, although single mutants have only modest or insignificant effects. Functional differences between α1 and α2 are unaffected in α2VFP. A compound, 6-pentyl-2-pyrone, isolated from the marine fungus Trichoderma gamsii is a novel probe of specific phospholipid-protein interactions. 6-Pentyl-2-pyrone inactivates the isoforms in the order α2 ≫ α3 > α1, and α2VFP and FXYD1 protect the isoforms. In native rat heart sarcolemma membranes, which contain α1, α2, and α3 isoforms, a component attributable to α2 is the least stable. The data provide clear evidence for a specific phosphatidylserine binding pocket between M8, M9, and M10 and confirm that the instability of α2 is due to suboptimal interactions with phosphatidylserine. In physiological conditions, the instability of α2 may be important for its cellular regulatory functions.

Ouabain binding site in a functioning Na+/K+ ATPase

The Na(+)/K(+) ATPase is an almost ubiquitous integral membrane protein within the animal kingdom. It is also the selective target for cardiotonic derivatives, widely prescribed inhibitors for patients with heart failure. Functional studies revealed that ouabain-sensitive residues distributed widely throughout the primary sequence of the protein. Recently, structural work has brought some consensus to the functional observations. Here, we use a spectroscopic approach to estimate distances between a fluorescent ouabain and a lanthanide binding tag (LBT), which was introduced at five different positions in the Na(+)/K(+) ATPase sequence. These five normally functional LBT-Na(+)/K(+) ATPase constructs were expressed in the cell membrane of Xenopus laevis oocytes, operating under physiological internal and external ion conditions. The spectroscopic data suggest two mutually exclusive distances between the LBT and the fluorescent ouabain. From the estimated distances and using homology models of the LBT-Na(+)/K(+) ATPase constructs, approximate ouabain positions could be determined. Our results suggest that ouabain binds at two sites along the ion permeation pathway of the Na(+)/K(+) ATPase. The external site (low apparent affinity) occupies the same region as previous structural findings. The high apparent affinity site is, however, slightly deeper toward the intracellular end of the protein. Interestingly, in both cases the lactone ring faces outward. We propose a sequential ouabain binding mechanism that is consistent with all functional and structural studies.

Pain as a channelopathy

Mendelian heritable pain disorders have provided insights into human pain mechanisms and suggested new analgesic drug targets. Interestingly, many of the heritable monogenic pain disorders have been mapped to mutations in genes encoding ion channels. Studies in transgenic mice have also implicated many ion channels in damage sensing and pain modulation. It seems likely that aberrant peripheral or central ion channel activity underlies or initiates many pathological pain conditions. Understanding the mechanistic basis of ion channel malfunction in terms of trafficking, localization, biophysics, and consequences for neurotransmission is a potential route to new pain therapies.

A homolog of FHM2 is involved in modulation of excitatory neurotransmission by serotonin in C. elegans

The C. elegans eat-6 gene encodes a Na(+), K(+)-ATPase alpha subunit and is a homolog of the familial hemiplegic migraine candidate gene FHM2. Migraine is the most common neurological disorder linked to serotonergic dysfunction. We sought to study the pathophysiological mechanisms of migraine and their relation to serotonin (5-HT) signaling using C. elegans as a genetic model. In C. elegans, exogenous 5-HT inhibits paralysis induced by the acetylcholinesterase inhibitor aldicarb. We found that the eat-6(ad467) mutation or RNAi of eat-6 increases aldicarb sensitivity and causes complete resistance to 5-HT treatment, indicating that EAT-6 is a component of the pathway that couples 5-HT signaling and ACh neurotransmission. While a postsynaptic role of EAT-6 at the bodywall NMJs has been well established, we found that EAT-6 may in addition regulate presynaptic ACh neurotransmission. We show that eat-6 is expressed in ventral cord ACh motor neurons, and that cell-specific RNAi of eat-6 in the ACh neurons leads to hypersensitivity to aldicarb. Electron microscopy showed an increased number of synaptic vesicles in the ACh neurons in the eat-6(ad467) mutant. Genetic analyses suggest that EAT-6 interacts with EGL-30 Galphaq, EGL-8 phospholipase C and SLO-1 BK channel signaling to modulate ACh neurotransmission and that either reduced or excessive EAT-6 function may lead to increased ACh neurotransmission. Study of the interaction between eat-6 and 5-HT receptors revealed both stimulatory and inhibitory 5-HT inputs to the NMJs. We show that the inhibitory and stimulatory 5-HT signals arise from distinct 5-HT neurons. The role of eat-6 in modulation of excitatory neurotransmission by 5-HT may provide a genetic explanation for the therapeutic effects of the drugs targeting 5-HT receptors in the treatment of migraine patients.

The neurobiology of migraine

The understanding of migraine has moved well beyond its traditional characterization as a "vascular headache." In considering the basic neurobiology of migraine, it is important to begin with the concept of migraine as not merely a headache, but rather a heterogeneous array of episodic symptoms. Among the array of phenomena experienced by migraine patients are visual disturbances, nausea, cognitive dysfunction, fatigue, and sensitivity to light, sound, smell, and touch. These symptoms may occur independently or in any combination, and in some patients occur even in the absence of headache. The diversity and variability of symptoms experienced by migraine patients belies a complex neurobiology, involving multiple cellular, neurochemical, and neurophysiological processes occurring at multiple neuroanatomical sites. Migraine is a multifaceted neurobiological phenomenon that involves activation of diverse neurochemical and cellular signaling pathways in multiple regions of the brain. Propagated waves of cellular activity in the cortex, possibly involving distinct glial and vascular signaling mechanisms, can occur along with activation of brainstem centers and nociceptive pathways. Whether different brain regions become involved in a linear sequence, or as parallel processes, is uncertain. The modulation of brain signaling by genetic factors, and by sex and sex hormones, provides important clues regarding the fundamental mechanisms by which migraine is initiated and sustained. Each of these mechanisms may represent distinct therapeutic targets for this complex and commonly disabling disorder.

Biological science of headache channels

Several episodic neurological diseases, including familial hemiplegic migraine (FHM) and different types of epilepsy, are caused by mutations in ion channels, and hence classified as channelopathies. The classification of FHM as a channelopathy has introduced a new perspective in headache research and has strengthened the idea of migraine as a disorder of neural excitability. Here we review recent studies of the functional consequences of mutations in the CACNA1A and SCNA1A genes (encoding the pore-forming subunit of Ca(V)2.1 and Na(V)1.1 channels) and the ATPA1A2 gene (encoding the alpha(2) subunit of the Na(+)/K(+) pump), responsible for FHM1, FHM3, and FHM2, respectively. These studies show that: (1) FHM1 mutations produce gain-of-function of the Ca(V)2.1 channel and, as a consequence, increased glutamate release at cortical synapses and facilitation of induction and propagation of cortical spreading depression (CSD); (2) FHM2 mutations produce loss-of-function of the alpha(2) Na(+)/K(+)-ATPase; and (3) the FHM3 mutation accelerates recovery from fast inactivation of Na(V)1.5 channels. These findings are consistent with the hypothesis that FHM mutations share the ability to render the brain more susceptible to CSD, by causing excessive synaptic glutamate release (FHM1) or decreased removal of K(+) and glutamate from the synaptic cleft (FHM2) or excessive extracellular K(+) (FHM3).

Genetics of headaches

Insight into the molecular mechanisms involved in primary headaches is important to identify drug targets for improving treatment of patients, but essentially lacking. Genetic research is increasingly successful in pinpointing these mechanisms. Most progress has been made for Familial Hemiplegic Migraine, a rare subtype of migraine with aura. Three genes (CACNA1A, ATP1A2 and SCN1A) have been identified that all encode ion transporters. Cellular and transgenic mouse studies suggest that neuronal hyperexcitability and increased susceptibility to cortical spreading depression, the correlate of migraine aura, are important molecular mechanisms in migraine. Investigating monogenic diseases in which migraine is a prominent feature such as CADASIL, which is caused by mutations in the NOTCH3 gene, can help understanding the pathology of migraine. Candidate gene association studies and linkage studies in the common forms of migraine were less successful. Except for the MTHFR gene no gene variant has been identified yet. Convincingly demonstrated genetic findings in other primary headaches such as cluster headache and tension-type headache are even rarer. However, with current technical possibilities of massive genotyping and international efforts to collect large well-phenotyped patient cohorts, the first gene variants for various primary headache types are likely to be discovered in the coming decade.

Molecular genetics of migraine

Migraine is an episodic neurovascular disorder that is clinically divided into two main subtypes that are based on the absence or presence of an aura: migraine without aura (MO) and migraine with aura (MA). Current molecular genetic insight into the pathophysiology of migraine predominantly comes from studies of a rare monogenic subtype of migraine with aura called familial hemiplegic migraine (FHM). Three FHM genes have been identified, which all encode ion transporters, suggesting that disturbances in ion and neurotransmitter balances in the brain are responsible for this migraine type, and possibly the common forms of migraine. Cellular and animal models expressing FHM mutations hint toward neuronal hyperexcitability as the likely underlying disease mechanism. Additional molecular insight into the pathophysiology of migraine may come from other monogenic syndromes (for instance cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, which is caused by NOTCH3 mutations), in which migraine is prominent. Investigating patients with common forms of migraine has had limited successes. Except for 5',10'-methylenetetrahydrolate reductase, an enzyme in folate metabolism, the large majority of reported genetic associations with candidate migraine genes have not been convincingly replicated. Genetic linkage studies using migraine subtypes as an end diagnosis did not yield gene variants thus far. Clinical heterogeneity in migraine diagnosis may have hampered the identification of such variants. Therefore, the recent introduction of more refined methods of phenotyping, such as latent-class analysis and trait component analysis, may be certainly helpful. Combining the new phenotyping methods with genome-wide association studies may be a successful strategy toward identification of migraine susceptibility genes. Likely the identification of reliable biomarkers for migraine diagnosing will make these efforts even more successful.

Novel mutations affecting the Na, K ATPase alpha model complex neurological diseases and implicate the sodium pump in increased longevity

Mutations affecting the Na⁺, K⁺ ATPase alpha subunit have been implicated in at least two distinct human diseases, rapid-onset dystonia Parkinsonism (RDP), and familial hemiplegic migraine (FHM). Over 40 mutations have been mapped to the human ATP1A2 and ATP1A3 genes and are known to result in RDP, FHM or a variant of FHM with neurological complications. To develop a genetically tractable model system for investigating the role of the Na⁺, K⁺ ATPase in neural pathologies we performed genetic screens in Drosophila melanogaster to isolate loss-of-function alleles affecting the Na⁺, K⁺ ATPase alpha subunit. Flies heterozygous for these mutations all exhibit reduced respiration, consistent with a loss-of-function in the major ATPase. However, these mutations do not affect all functions of the Na⁺, K⁺ ATPase alpha protein since embryos homozygous for these mutations have normal septate junction paracellular barrier function and tracheal morphology. Importantly, all of these mutations cause neurological phenotypes and, akin to the mutations that cause RDP and FHM, these new alleles are missense mutations. All of these alleles exhibit progressive stress-induced locomotor impairment suggesting neuromuscular dysfunction, yet neurodegeneration is observed in an allele-specific manner. Surprisingly, studies of longevity demonstrate that mild hypomorphic mutations in the sodium pump significantly improve longevity, which was verified using the Na⁺, K⁺ ATPase antagonist ouabain. The isolation and characterization of a series of new missense alleles of ATPalpha in Drosophila provides the foundation for further studies of these neurological diseases and the role of sodium pump impairment in animal longevity.

The structure of the Na+,K+-ATPase and mapping of isoform differences and disease-related mutations

The Na+,K+-ATPase transforms the energy of ATP to the maintenance of steep electrochemical gradients for sodium and potassium across the plasma membrane. This activity is tissue specific, in particular due to variations in the expressions of the alpha subunit isoforms one through four. Several mutations in alpha2 and 3 have been identified that link the specific function of the Na+,K+-ATPase to the pathophysiology of neurological diseases such as rapid-onset dystonia parkinsonism and familial hemiplegic migraine type 2. We show a mapping of the isoform differences and the disease-related mutations on the recently determined crystal structure of the pig renal Na+,K+-ATPase and a structural comparison to Ca2+-ATPase. Furthermore, we present new experimental data that address the role of a stretch of three conserved arginines near the C-terminus of the alpha subunit (Arg1003-Arg1005).

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.

Diverse functional consequences of mutations in the Na+/K+-ATPase alpha2-subunit causing familial hemiplegic migraine type 2

Mutations in ATP1A2, the gene coding for the Na(+)/K(+)-ATPase alpha(2)-subunit, are associated with both familial hemiplegic migraine and sporadic cases of hemiplegic migraine. In this study, we examined the functional properties of 11 ATP1A2 mutations associated with familial or sporadic hemiplegic migraine, including missense mutations (T263M, T376M, R383H, A606T, R763H, M829R, R834Q, R937P, and X1021R), a deletion mutant (del(K935-S940)ins(I)), and a frameshift mutation (S966fs). According to the Na(+)/K(+)-ATPase crystal structure, a subset of the mutated residues (Ala(606), Arg(763), Met(829), and Arg(834)) is involved in important interdomain H-bond networks, and the C terminus of the enzyme, which is elongated by the X1021R mutation, has been implicated in voltage dependence and formation of a third Na(+)-binding site. Upon heterologous expression in Xenopus oocytes, the analysis of electrogenic transport properties, Rb(+) uptake, and protein expression revealed pronounced and markedly diverse functional alterations in all ATP1A2 mutants. Abnormalities included a complete loss of function (T376M), impaired plasma membrane expression (del(K935-S940)ins(I) and S966fs), and altered apparent affinities for extracellular cations or reduced enzyme turnover (R383H, A606T, R763H, R834Q, and X1021R). In addition, changes in the voltage dependence of pump currents and the increased rate constants of the voltage jump-induced redistribution between E(1)P and E(2)P states were observed. Thus, mutations that disrupt distinct interdomain H-bond patterns can cause abnormal conformational flexibility and exert long range consequences on apparent cation affinities or voltage dependence. Of interest, the X1021R mutation severely impaired voltage dependence and kinetics of Na(+)-translocating partial reactions, corroborating the critical role of the C terminus of Na(+)/K(+)-ATPase in these processes.

A novel de novo nonsense mutation in ATP1A2 associated with sporadic hemiplegic migraine and epileptic seizures

Familial hemiplegic migraine (FHM) is a severe dominant form of migraine with aura associated with transient hemiparesis. Several other neurological signs and symptoms can be associated with FHM such as cerebellar abnormalities, cerebral edema and coma after minor head trauma, epileptic seizures and mental retardation. The sporadic form of hemiplegic migraine named SHM, presents with identical clinical symptoms. Here we report a case of a young hemiplegic migraine patient, 11 years old, who had the first hemiplegic attack at the age of 10 years. This patient has a clinical history of epileptic seizures in the childhood successfully controlled with drug therapy. No familiarity for any type of migraine or seizures can be observed within the paternal or maternal line. The patient who can therefore be considered a sporadic case, carries a novel de novo nonsense mutation p.Tyr1009X in the ATP1A2 gene (FHM2), leading to a truncated alpha-2 subunit of the Na+/K+-ATPase pump thus lacking the last 11 amino acids. The novel mutation identified confirms the role of FHM2 gene in forms of hemiplegic migraine associated with epilepsy with both familial and sporadic occurrence, and expands the spectrum of mutations related to these forms of the disease.

Novel mutation confirms seizure locus SCN1A is also familial hemiplegic migraine locus FHM3

Although SCN1A, the gene encoding the neuronal voltage-gated sodium channel, type 1A, is a well-recognized target of mutations underlying a spectrum of epilepsy syndromes, and lies within an extended 12-Mb disease-associated haplotype at the familial hemiplegic migraine-3 locus, it remains to be confirmed that mutations within this gene itself cause syndromes that include migraine phenotypes. The novel T1174S missense mutation of this gene was detected segregating in a family with a heterozygous female child who presented with myoclonus and an abnormal electroencephalogram, and in her heterozygous mother, who had an ataxic migraine syndrome similar to that of her own mother. This three-generation family exhibits the broad phenotypic spectrum of the dominant neuronal hyperexcitability syndromes produced by even a given allele of this sodium channel gene. It also exhibits the second allele of this sodium channel gene associated with a migraine syndrome similar to those caused at the two other familial hemiplegic migraine loci, confirming that this gene itself, not some linked gene, is the familial hemiplegic migraine-3 locus.

Two novel functional mutations in the Na+,K+-ATPase alpha2-subunit ATP1A2 gene in patients with familial hemiplegic migraine and associated neurological phenotypes

Mutations in the ATP1A2 gene, encoding the alpha2-subunit of the Na+,K+-ATPase, are associated with familial hemiplegic migraine type 2. The majority of ATP1A2 mutations were reported in patients with hemiplegic migraine without any additional neurological findings. Here, we report on two novel ATP1A2 mutations that were identified in two Portuguese probands with hemiplegic migraine and interesting additional clinical features. The proband's of family 1 (with a V362E mutation) had mood alterations, classified as a borderline personality. The proband in family 2 (with a P796S mutation) had mild mental impairment, in addition to hemiplegic migraine; more severe mental retardation was observed in his brother, who also had hemiplegic migraine and carried the same mutation. Cell-survival assays clearly showed abnormal functioning of mutant Na+,K+-ATPase, indicating that both ATP1A2 mutants are disease causing. Additionally, our results suggest a possible causal relationship of the ATP1A2 mutations with the complex clinical phenotypes observed in the probands.

Molecular mechanisms of migraine?

Migraine is a common debilitating neurological disease characterised by attacks of severe headache with or without preceding aura. Its aetiology remains elusive; however it is clear that an interplay of genetic and environmental components play an important role. Familial hemiplegic migraine (FHM) is a rare and severe variant of migraine with aura and follows an autosomal dominant pattern of inheritance. This disease is genetically heterogeneous,with three causative genes having been identified. This review uses insights garnered from FHM to try and shed light on possible migraine disease pathogenesis.

Recurrent ATP1A2 mutations in Portuguese families with familial hemiplegic migraine

Familial hemiplegic migraine is a rare autosomal dominant subtype of migraine with aura. Three genes have been identified, all involved in ion transport. There is considerable clinical variation associated with FHM mutations. Genotype-phenotype correlation studies are needed, but are challenging mainly because the number of carriers of individual mutations is low. One exception is the recurrent T666M mutation in the FHM1 CACNA1A gene that was identified in almost one-third of FHM families and showed variable associated clinical features and severity, both within and among FHM families. Similar studies in the FHM2 ATP1A2 gene have not been performed because of the low number of carriers with individual mutations. Here we report on the recurrence of ATP1A2 mutations M731T and T376M that affect sodium-potassium pump functioning in two Portuguese FHM families. Considerably increasing the number of mutation carriers with these mutations indicated a clear genotype-phenotype correlation: both mutations are associated with pure FHM. In addition, we show that recurrent mutations for ATP1A2 are more frequent than previously thought, which has implications for genotype-phenotype correlations and genetic testing.

Migraine: a complex genetic disorder

Although family and twin studies show that there is a genetic component to migraine, no genes predisposing to common forms of the disorder have been identified. The most encouraging findings have emerged from the identification of genes causing rare mendelian traits that phenotypically resemble migraine. These studies have pointed migraine research towards ion-transport genes; however, there is no direct evidence of the involvement of these genes in common forms of migraine. Family-based linkage studies have identified several chromosomal regions linked to common forms of migraine, but there is little consistency between studies. The modest success in the identification of contributing gene variants has stimulated research into more effective strategies. These include new phenotyping methods for genetic studies and new study designs-such as case-control and whole-genome association studies-to identify common variants contributing to the trait.

Migraine: gene mutations and functional consequences

PURPOSE OF REVIEW

Genetic and functional studies of mutations in familial hemiplegic migraine reveal a major role for disturbed ion transport. Gene identification in common, multifactorial migraine remains challenging.

RECENT FINDINGS

Several new mutations have been identified in FHM1, FHM2 and FHM3 genes. Functional consequences of familial hemiplegic migraine mutations point to an important role for cortical spreading depression in migraine pathophysiology. New genetic approaches have been tested in common migraine - novel chromosomal loci - but no gene variants have been identified.

SUMMARY

Identification and analysis of gene mutations in familial hemiplegic migraine revealed a major role for disturbed ion transport in this disorder. Cellular and transgenic mouse models of familial hemiplegic migraine genes suggest that increased potassium and glutamate play a role in the pathophysiology of the disorder. Despite progress, no genes have been discovered for common migraine.

Familial hemiplegic migraine

Familial hemiplegic migraine (FHM) is a rare and genetically heterogeneous autosomal dominant subtype of migraine with aura. Mutations in the genes CACNA1A and SCNA1A, encoding the pore-forming alpha(1) subunits of the neuronal voltage-gated Ca2+ channels Ca(V)2.1 and Na+ channels Na(V)1.1, are responsible for FHM1 and FHM3, respectively, whereas mutations in ATP1A2, encoding the alpha2 subunit of the Na+, K+ adenosinetriphosphatase (ATPase), are responsible for FHM2. This review discusses the functional studies of two FHM1 knockin mice and of several FHM mutants in heterologous expression systems (12 FHM1, 8 FHM2, and 1 FHM3). These studies show the following: (1) FHM1 mutations produce gain-of-function of the Ca(V)2.1 channel and, as a consequence, increased Ca(V)2.1-dependent neurotransmitter release from cortical neurons and facilitation of in vivo induction and propagation of cortical spreading depression (CSD: the phenomenon underlying migraine aura); (2) FHM2 mutations produce loss-of-function of the alpha2 Na+,K+-ATPase; and (3) the FHM3 mutation accelerates recovery from fast inactivation of Na(V)1.5 (and presumably Na(V)1.1) channels. These findings are consistent with the hypothesis that FHM mutations share the ability of rendering the brain more susceptible to CSD by causing either excessive synaptic glutamate release (FHM1) or decreased removal of K+ and glutamate from the synaptic cleft (FHM2) or excessive extracellular K+ (FHM3). The FHM data support a key role of CSD in migraine pathogenesis and point to cortical hyperexcitability as the basis for vulnerability to CSD and to migraine attacks. Hence, they support novel therapeutic strategies that consider CSD and cortical hyperexcitability as key targets for preventive migraine treatment.