Migraine Genetics
Migraine Genetics
Genome-wide association studies (GWAS) depend on the ability to map the patterns of inheritance for the most common form of genomic variation, the SNP. There are approximately 10 million common SNPs; those with a minor-allele frequency of at least 5% are transmitted across generations in blocks, allowing a few particular, or tag, SNPs to capture the great majority of SNP variation within each block. Tag SNPs are readily measured SNPs that are in strong linkage disequilibrium with multiple other SNPs, so that they can serve as a proxy for these SNPs on large-scale genotyping platforms. GWAS compare DNA from individuals with a disease (cases) and similar individuals without the disease (controls). The genome from each individual is read using SNP arrays containing large numbers of genetic variants. If one type of variant is more frequent in individuals with the disease, the SNP (often a tag SNP) is said to be "associated" with the disease. The associated SNPs are then considered to mark a region of the human genome that influences the risk of disease. GWAS identify SNPs and other variants in DNA that are associated with a disease, but cannot on their own specify which genes are causal. SNPs associated with disease by GWAS are enriched within non-coding functional elements, with a majority residing in or near ENCODE-defined regions that are outside of protein-coding genes (see part 1).
While GWAS appear to show great promise, it is important to understand that common DNA sequence variations, as identified in GWAS, do not directly lead to changes in disease-related phenotypes, but instead lead to changes in molecular phenotypes, which then affect molecular and cellular processes that lead to changes in physiological states. Understanding the extent to which and under what conditions this DNA is accessible to proteins that control expression will help to elucidate the functional consequences of these SNPs.
GWAS have identified many non-coding variants associated with common diseases and traits, like migraine. These variants are concentrated in regulatory DNA marked by deoxyribonuclease I hypersensitive sites (DHSs), which suggests pervasive involvement of regulatory DNA variation in common human disease and provides pathogenic insights into diverse disorders. For example, the connection of numerous DHSs harboring GWAS SNPs with promoters of distant genes amplifies the genetic basis of disease and trait associations, provides a wealth of plausible causal genes to explain associations, and unifies seemingly unconnected variants associated with related diseases by way of convergent perturbation of common transcription factor networks.
Migraine GWAS A meta-analysis of GWAS on migraine evaluated 6 population-based European migraine cohorts with a total sample size of 10,980 individuals (2446 cases and 8534 controls from 6 Dutch and Icelandic samples). For replication, 3 population-based samples (2 of Dutch and 1 of Australian origin) were tested. A total of 32 SNPs showed marginal evidence for association (P < 10). The best result was obtained for SNP rs9908234, located in the nerve growth factor receptor gene (P = 8 × 10). This SNP did not replicate in 3 cohorts from the Netherlands or Australia. In addition, none of the 18 additional leading SNPs that were tested in 2 replication cohorts could be replicated successfully. A possible explanation for the negative results could be reduced power to detect an association based on diagnostic inaccuracy, because diagnoses were rendered based on questionnaires rather than the gold-standard of direct specialist–patient interviews. Another explanation offered by the authors is the heterogeneity of the population-based sample, with a less severe phenotype and consequently a lower genetic risk compared with patients seen in clinical practice.
The authors of this study also investigated SNP rs1835740, located on 8q21 between the metadherin (MTDH) and PGCP genes, which was found to be significantly associated with MWA in the first GWA study of clinic-based populations. The SNP was not associated with migraine, but a gene-based analysis identified a modest gene-based significant association with migraine. The MTDH gene downregulates SLC1A2 (also known as EAAT2 and GLT-1), the gene encoding the major glutamate transporter in the brain, providing a possible link between this variant and glutamate regulation, a neurotransmitter long considered to play a role in migraine pathogenesis. Downregulation of EAAT2 could result in excess glutamate at the neuronal synapse, and this accumulation could account for the electrophysiological and imaging evidence of cortical excitability and provide a substrate for CSD, the presumed biological substrate of migraine aura, and central sensitization, postulated to be the underlying mechanism of allodynia during a migraine attack. These results also support the hypothesis that complementary pathways, such as the glutamate system, may link the Mendelian channelopathies with the pathogenetic mechanisms of more common forms of episodic neurologic disorders, such as migraine.
Another GWAS that was conducted involved a large population-based cohort of 23,230 women (5122 migraineurs and 18,108 non-migraineurs), with complete genotype and phenotype information, and verified European ancestry from the Women's Genome Health Study. This study identified 3 SNPs with genome-wide significant association for common migraine at the population level (rs2651899 [1p36.32, PRDM16], rs10166942 [2q37.1, TRPM8], and rs11172113 [12q13.3, LRP1]). These SNPs were significant in a meta-analysis among 3 replication cohorts, and met genome-wide significance in a meta-analysis combining the discovery and replication cohorts. Two of the SNPs (rs2651899 and rs10166942) were specific for migraine compared with those without migraine, although none of the SNP associations were preferential for MWA or MWoA, nor were any associations specific for individual migraine features. The SNP rs10166942 was also found to be stronger among women (this may relate to the higher prevalence of migraine in women). The LRP1 locus, a member of the lipoprotein receptor family that serves as a sensor of the extracellular environment, is involved in the proliferation of vascular smooth muscle cells (VSMCs) and modulates synaptic transmission. The LRP1 protein and glutamate (N-methyl-D-aspartate) receptors are colocalized on neurons. This finding provides a further link between genetic associations and altered glutamate homeostasis in the pathogenesis of migraine. Importantly, however, this study also identified a new locus, TRPM8, which encodes a sensor for cold and cold-induced burning pain and is primarily expressed in sensory neurons, is a target in animal models of neuropathic pain. This new locus could be a pathophysiologic link between migraine and other pain syndromes. This study also suggests a shared pathophysiology between MWA and MWoA.
The first GWA clinic-based study of MWoA recently replicated associations at the TRPM8 and LRP1 loci, and reported several additional associated loci. In this study, GWA data from 2326 clinic-based German and Dutch individuals with MWoA and 4580 population-matched controls were analyzed. SNPs at 2 of 12 loci demonstrated convincing replication at 1q22 (MEF2D) and 3p24 (TGFBR2). The MEF2D protein is a transcription factor that is highly expressed in the brain, regulates neuronal differentiation, and restricts the number of excitatory synapses when activated.MEF2D dysregulation may affect neuronal excitatory neurotransmission in individuals with MWoA. In support of this hypothesis, MEF transcriptional targets have been associated with other neurologic disorders, including epilepsy. The TGFBR2 protein is involved in the regulation of cell proliferation and differentiation, as well as in extracellular matrix production. Missense mutations in TGFBR2 has been shown to cause monogenic familial aortic dissection as well as migrainous headaches in 11/14 mutation carriers in a large multigenerational family. This locus is therefore an attractive candidate gene for migraine, especially because migraine sufferers are known to have increased risk of carotid dissection.
In this important study, SNPs at the PHACTR1 and ASTN2 loci also showed suggestive evidence of replication. The PHACTR1 locus encodes a protein (a member of the PHACTR/scapinin family) that controls synaptic activity and synaptic morphology and has been implicated in endothelial cell function and susceptibility to early-onset myocardial infarction. The link between PHACTR and migraine pathogenesis could therefore either be neuronal, through aberrant synaptic transmission, or vascular, because endothelial dysfunction, cardiovascular disease, and myocardial infarction have all been linked to migraine.
Variants within the ASTN2 locus, a member of the astroactin gene family, have a role in glial-guided migration and appear important for the development of the laminar architecture of cortical regions of the brain. It is unclear how this links to the pathophysiology of migraine.
Other Migraine Genes Migraine is comorbid with numerous other disorders, and it is likely that an underlying shared biology between 2 comorbid disorders is driven, in part, by a similar genetic substrate. Recently, 2 distinct missense mutations in 2 independent families with MWA and familial advanced sleep phase syndrome was identified in the gene encoding casein kinase I δ (CKIδ). The CKIδ protein is pivotal in the function of the biological clock that influences the circadian rhythm. The resulting alterations (T44A and H46R) occurred in the conserved catalytic domain of CKIδ, where they caused reduced enzyme activity. Mice engineered to carry the CKIδ-T44A allele demonstrated significantly lowered mechanical and thermal sensory thresholds after infusion with nitroglycerin, a prototypic migraine trigger, and a significant increase in cFOS-positive neurons was demonstrated in the trigeminal-cervical complex. Also, consistent with the premise that migraine is characterized by neuronal hyperexcitability and that CSD of neuronal and glial membranes is responsible for the migraine aura, CKIδ-T44A mice demonstrated a reduced threshold for CSD after the application of potassium chloride to exposed cerebral cortex, and increased spontaneous and evoked calcium signaling in astrocytes. These in vitro and in vivo data suggest that CKIδ activity may contribute to the pathogenesis of migraine and potentially uncover protein targets of CKIδ as new targets for drug discovery.
A role has been suggested for the 2-pore domain (K2P) potassium channel, TWIK-related spinal cord potassium channel (TRESK, encoded by KCNK18), in pain pathways and general anesthesia. K2P channels are expressed throughout the CNS and have a key role in controlling neuronal resting membrane potential and neuronal excitability. They represent a major target for many volatile anesthetics and neuroprotective agents, and have been implicated in a variety of pain pathways. The TRESK K2P channel, encoded by the KCNK18 gene, is expressed in the spinal cord, where it helps modulate cellular excitability. The channel is directly activated by calcineurin after Gqα receptor stimulation and a subsequent rise in intracellular calcium. It is, therefore, a potential target of pain mediators that exert their action via these pathways, and a role has recently been suggested for TRESK in the calcineurin inhibitor-induced pain syndrome.TRESK is also activated by volatile anesthetics, such as halothane, which have been shown to inhibit CSD.
Lafreniere et al examined the potential role of TRESK in typical MWA. They sequenced the entire coding region of the KCNK18 gene in a panel and examined whether TRESK is involved in migraine by screening the KCNK18 gene in subjects diagnosed with migraine. They reported a frameshift mutation, F139WfsX24, which segregates perfectly with typical MWA in a large pedigree. They also identified prominent TRESK expression in migraine-salient areas, such as the trigeminal ganglion. Functional characterization of this mutation demonstrates that it causes a complete loss of TRESK function and that the mutant subunit suppresses wild-type channel function through a dominant-negative effect, thus explaining the dominant penetrance of this allele. These results support a role for TRESK in the pathogenesis of typical MWA and further support the role of this channel as a potential therapeutic target.
This finding has potential therapeutic implications. Based on the fact that currently available specific acute migraine medications (triptans and ergotamine) are receptor agonists, because TRESK is an ion channel expressed at the surface of the neuron, it should be possible to develop novel small compound agonists of TRESK that would be highly selective and potent, and could be used as acute or preventive drugs. This target may be highly desirable, since the gene is expressed in a very limited set of neurons, at least in mice, and as such, modulating its activity should have minimal effect on tissues where it is not expressed. The trigeminal ganglion also lies outside the blood–brain barrier and is thus more accessible to small compounds than other structures located behind the blood–brain barrier (ie, in the brain or brainstem). The challenge will be to develop small compound TRESK agonists that can modulate the excitability of TRESK-expressing neurons without increasing unwanted anesthetic or immune system side effects.
The possibility of migraine as an X-linked disorder is suggested by the female preponderance of and evidence that a high proportion of affected males have a greater number of affected first-degree relatives. In a linkage analysis of 6 migraine pedigrees and a case–control cohort study that evaluated 11 candidate genes, 2 distinct susceptibility variants were identified at Xq27 (marked by DXS8043—DXS297) and Xq28 (DXS8061—XqTer). Because these susceptibility regions were also associated in the case–control population, the possibility was raised that these loci are not pedigree-specific and may be contributing to migraine in the general population.
Migraine, particularly MWA, is associated with several hereditary and acquired cerebrovascular disorders, including arterial dissection, ischemic stroke, and cardiovascular disease. It has been speculated that migraine and stroke may both be triggered by hypoperfusion and could therefore exist on a continuum of vascular complications in a subset of patients who have these hereditary or acquired comorbid vascular conditions. Indeed, evidence suggests that certain migraine mutations may increase the vulnerability for stroke by facilitating ischemic depolarizations. FHM type 1 (FHM1) mutant mice, with mutations in CaV2.1 voltage-gated calcium channels, develop earlier onset of anoxic depolarization and more frequent peri-infarct depolarizations, resulting in more extensive cerebral infarctions and worse neurologic outcomes. The authors proposed that enhanced susceptibility to ischemia-induced CSD predisposes migraine sufferers to infarction during mild ischemic events and that this in part may account for the higher stroke risk in migraineurs.
The mechanism underlying the frequent association between migraine and genetic small-vessel arteriopathies is unclear, but a common underlying factor may be the propensity for transient cerebral ischemia or hypoperfusion to trigger an underlying cascade of events that culminate in a migraine attack. Indeed, recent experimental data have indicated that focal, mild, and transient hypoperfusion (cerebral ischemia without infarction) can trigger CSD. MWA and MWoA have been associated as a common trait in several genetic small-vessel vasculopathies, as illustrated later.
Cerebral Autosomal Dominant Arteriopathy With Subcortical Infarcts and Leukoencephalopathy The clinical observation of an association between migraine and several genetic vasculopathies supports the concept that transient hypoperfusion or ischemic depolarization increases the susceptibility to migraine. One such genetic vasculopathy is cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). CADASIL is a systemic, non-amyloid, non-atherosclerotic vasculopathy and is the most common monogenic-inherited form of adult-onset stroke and vascular dementia, linked to mutations in the Notch3 gene (located on chromosome 19p13.2-p13.1), which encodes a cell surface receptor that, in human adult tissue, is solely expressed on VSMCs. The Notch3 protein is part of a signal transduction pathway, critical for aspects of vascular development, homeostasis, and VSMC differentiation. Most mutations in CADASIL are missense mutations and involve the loss or gain of a cysteine residue in the Notch3 protein (specifically the extracellular epidermal-growth-factor-like repeat). The Notch3 transgenic mouse model demonstrates that degeneration of VSMCs precedes the deposition of granular osmiophilic material in the basement membrane and extracellular matrix of VSMCs, and mutations in the NOTCH3 gene have a gain-of-function effect on the protein and that dysfunctional anchoring of VSMCs to the extracellular matrix and adjacent cells triggers VSMC degeneration. The variable disease course of CADASIL, ranging from relatively mild to very severe, might be an indication for the involvement of other genetic and environmental modifying factors.
The incidence of MWA, typically the first disease symptom of CADASIL, is 5 times greater compared with the general population. Visual and sensory auras are most common in CADASIL, although 50% of the patients also experience atypical attacks with basilar, hemiplegic, or prolonged aura, and even coma. The explanation for the predilection for MWA attacks in patients with CADASIL was recently demonstrated in a study that showed that CSD, the electrophysiological substrate of migraine aura, is enhanced in mice expressing a vascular Notch3 CADASIL mutation (R90C) or a Notch3 knockout mutation. The phenotype was stronger in Notch3 knockout mice, implicating both loss of function and neomorphic mutations in its pathogenesis. The mechanism underlying CSD increased susceptibility in CADASIL is unclear, but dysfunctional communication within the neurovascular unit has been raised as a possibility. Abnormal Notch3 signaling may disrupt normal astrocyte-smooth muscle communication and result in impaired neurovascular coupling, or Notch3 mutations expressed in neural progenitor cells and transiently in newly born neurons may lead to enhanced CSD susceptibility phenotype later in life.
Retinal Vasculopathy With Cerebral Leukodystrophy Retinal vasculopathy with cerebral leukodystrophy (RVCL) is a neurovascular syndrome (formerly referred to in 3 separate families and publications as cerebroretinal vasculopathy, hereditary vascular retinopathy, and hereditary endotheliopathy, retinopathy, nephropathy, and stroke) characterized by vascular retinopathy, cognitive impairment, depression, migraine (mainly without aura), focal neurologic symptoms, and, in later disease stages, characteristic contrast-enhancing intracerebral mass lesions. Several systemic symptoms can be present as well, including renal and liver dysfunction, Raynaud's phenomenon, and gastrointestinal bleeding. Although reported independently, the discovery of the gene defect, carboxyl-terminal truncations causing frameshifts in the TREX1 gene encoding the 3'-5' DNA-specific exonuclease TREX1 (3p21.1-p21.3), showed that they are different phenotypic variants of the same genetic disorder.
It is unclear how the carboxyl truncating mutations in TREX1 lead to the phenotype or pathogenesis of RVCL. Functional studies with mutant TREX1 suggest that forms lacking their native carboxyl-termini cannot localize to their usual perinuclear site within the cell and freely diffuse throughout the cytoplasm, thereby rendering the gene product unable to perform its physiological function.
Hereditary systemic angiopathy (HSA) manifests with cerebral calcifications, retinopathy, progressive nephropathy and hepatopathy, and appears to be a variant of RVCL. Small- and medium-sized subcortical white matter lesions are seen on magnetic resonance imaging (MRI) – a finding consistent with a vasculopathy of small- and medium-sized cerebral vessels. Subjects usually present in the 4th decade of life with visual impairment, migraine-like headaches, skin rash, seizures, motor weakness, and cognitive impairment. In the later stages, patients may develop liver sclerosis, with progressive hepatic impairment and renal microangiopathy leading to organ failure and chronic anemia. HSA has several similarities with RVCL; the cerebral calcification, perivascular inflammatory response, and progressive hepatic and renal failure are unique to this disorder.
COL4A1-related Disorder Autosomal dominant COL4A1 gene-related disorders are described in at least 12 white European families with 100% penetrance of disease. Thus far, 12 different missense mutations have been described in the COL4A1 gene. Mutations in the COL4A1 gene, located on chromosome 13q34, are associated with several unique phenotypes with overlapping features, including a condition exhibiting perinatal hemorrhage with porencephaly in survivors; a small-vessel disease with hemorrhage of adult onset or with infantile hemiparesis; and hereditary angiopathy, nephropathy, aneurysms, and muscle cramps. Along with hemorrhagic strokes, COL4A1 mutations may cause variable degrees of retinal arteriolar tortuosity, cataracts, glaucoma, and anterior segment dysgenesis of the eye.
Neurologic symptoms in COL4A1 mutation carriers may vary in degree of severity, even within families. Depending on the age of onset, affected individuals present with infantile hemiparesis, seizures, visual loss, dystonia, strokes, migraine, mental retardation, cognitive impairment, and dementia. Single or recurrent intracranial hemorrhages may occur in non-hypertensive adults less than 50 years of age. These hemorrhages can occur spontaneously, subsequent to trauma, or as a result of anticoagulant use. Stroke often occurs as the first presentation of the disease, with a mean age of onset of 36 years. MRI generally demonstrates typical features of other small-vessel diseases, including diffuse leukoencephalopathy with deep white matter involvement of posterior periventricular areas, subcortical infarctions, microhemorrhages, and dilated perivascular spaces.
The association with migraine in individuals with COL4A1 mutations is unclear, and may either be coincidental or mutation specific. MWA was described in one family with hereditary porencephaly secondary to a COL4A1 mutation, while in 2 other patients the migraine subtype was not specified. However, as in 6 other families with hereditary porencephaly, migraine was not described.
Genes for Common Migraine Subtypes
Genome-wide Association Studies
Genome-wide association studies (GWAS) depend on the ability to map the patterns of inheritance for the most common form of genomic variation, the SNP. There are approximately 10 million common SNPs; those with a minor-allele frequency of at least 5% are transmitted across generations in blocks, allowing a few particular, or tag, SNPs to capture the great majority of SNP variation within each block. Tag SNPs are readily measured SNPs that are in strong linkage disequilibrium with multiple other SNPs, so that they can serve as a proxy for these SNPs on large-scale genotyping platforms. GWAS compare DNA from individuals with a disease (cases) and similar individuals without the disease (controls). The genome from each individual is read using SNP arrays containing large numbers of genetic variants. If one type of variant is more frequent in individuals with the disease, the SNP (often a tag SNP) is said to be "associated" with the disease. The associated SNPs are then considered to mark a region of the human genome that influences the risk of disease. GWAS identify SNPs and other variants in DNA that are associated with a disease, but cannot on their own specify which genes are causal. SNPs associated with disease by GWAS are enriched within non-coding functional elements, with a majority residing in or near ENCODE-defined regions that are outside of protein-coding genes (see part 1).
While GWAS appear to show great promise, it is important to understand that common DNA sequence variations, as identified in GWAS, do not directly lead to changes in disease-related phenotypes, but instead lead to changes in molecular phenotypes, which then affect molecular and cellular processes that lead to changes in physiological states. Understanding the extent to which and under what conditions this DNA is accessible to proteins that control expression will help to elucidate the functional consequences of these SNPs.
GWAS have identified many non-coding variants associated with common diseases and traits, like migraine. These variants are concentrated in regulatory DNA marked by deoxyribonuclease I hypersensitive sites (DHSs), which suggests pervasive involvement of regulatory DNA variation in common human disease and provides pathogenic insights into diverse disorders. For example, the connection of numerous DHSs harboring GWAS SNPs with promoters of distant genes amplifies the genetic basis of disease and trait associations, provides a wealth of plausible causal genes to explain associations, and unifies seemingly unconnected variants associated with related diseases by way of convergent perturbation of common transcription factor networks.
Migraine GWAS A meta-analysis of GWAS on migraine evaluated 6 population-based European migraine cohorts with a total sample size of 10,980 individuals (2446 cases and 8534 controls from 6 Dutch and Icelandic samples). For replication, 3 population-based samples (2 of Dutch and 1 of Australian origin) were tested. A total of 32 SNPs showed marginal evidence for association (P < 10). The best result was obtained for SNP rs9908234, located in the nerve growth factor receptor gene (P = 8 × 10). This SNP did not replicate in 3 cohorts from the Netherlands or Australia. In addition, none of the 18 additional leading SNPs that were tested in 2 replication cohorts could be replicated successfully. A possible explanation for the negative results could be reduced power to detect an association based on diagnostic inaccuracy, because diagnoses were rendered based on questionnaires rather than the gold-standard of direct specialist–patient interviews. Another explanation offered by the authors is the heterogeneity of the population-based sample, with a less severe phenotype and consequently a lower genetic risk compared with patients seen in clinical practice.
The authors of this study also investigated SNP rs1835740, located on 8q21 between the metadherin (MTDH) and PGCP genes, which was found to be significantly associated with MWA in the first GWA study of clinic-based populations. The SNP was not associated with migraine, but a gene-based analysis identified a modest gene-based significant association with migraine. The MTDH gene downregulates SLC1A2 (also known as EAAT2 and GLT-1), the gene encoding the major glutamate transporter in the brain, providing a possible link between this variant and glutamate regulation, a neurotransmitter long considered to play a role in migraine pathogenesis. Downregulation of EAAT2 could result in excess glutamate at the neuronal synapse, and this accumulation could account for the electrophysiological and imaging evidence of cortical excitability and provide a substrate for CSD, the presumed biological substrate of migraine aura, and central sensitization, postulated to be the underlying mechanism of allodynia during a migraine attack. These results also support the hypothesis that complementary pathways, such as the glutamate system, may link the Mendelian channelopathies with the pathogenetic mechanisms of more common forms of episodic neurologic disorders, such as migraine.
Another GWAS that was conducted involved a large population-based cohort of 23,230 women (5122 migraineurs and 18,108 non-migraineurs), with complete genotype and phenotype information, and verified European ancestry from the Women's Genome Health Study. This study identified 3 SNPs with genome-wide significant association for common migraine at the population level (rs2651899 [1p36.32, PRDM16], rs10166942 [2q37.1, TRPM8], and rs11172113 [12q13.3, LRP1]). These SNPs were significant in a meta-analysis among 3 replication cohorts, and met genome-wide significance in a meta-analysis combining the discovery and replication cohorts. Two of the SNPs (rs2651899 and rs10166942) were specific for migraine compared with those without migraine, although none of the SNP associations were preferential for MWA or MWoA, nor were any associations specific for individual migraine features. The SNP rs10166942 was also found to be stronger among women (this may relate to the higher prevalence of migraine in women). The LRP1 locus, a member of the lipoprotein receptor family that serves as a sensor of the extracellular environment, is involved in the proliferation of vascular smooth muscle cells (VSMCs) and modulates synaptic transmission. The LRP1 protein and glutamate (N-methyl-D-aspartate) receptors are colocalized on neurons. This finding provides a further link between genetic associations and altered glutamate homeostasis in the pathogenesis of migraine. Importantly, however, this study also identified a new locus, TRPM8, which encodes a sensor for cold and cold-induced burning pain and is primarily expressed in sensory neurons, is a target in animal models of neuropathic pain. This new locus could be a pathophysiologic link between migraine and other pain syndromes. This study also suggests a shared pathophysiology between MWA and MWoA.
The first GWA clinic-based study of MWoA recently replicated associations at the TRPM8 and LRP1 loci, and reported several additional associated loci. In this study, GWA data from 2326 clinic-based German and Dutch individuals with MWoA and 4580 population-matched controls were analyzed. SNPs at 2 of 12 loci demonstrated convincing replication at 1q22 (MEF2D) and 3p24 (TGFBR2). The MEF2D protein is a transcription factor that is highly expressed in the brain, regulates neuronal differentiation, and restricts the number of excitatory synapses when activated.MEF2D dysregulation may affect neuronal excitatory neurotransmission in individuals with MWoA. In support of this hypothesis, MEF transcriptional targets have been associated with other neurologic disorders, including epilepsy. The TGFBR2 protein is involved in the regulation of cell proliferation and differentiation, as well as in extracellular matrix production. Missense mutations in TGFBR2 has been shown to cause monogenic familial aortic dissection as well as migrainous headaches in 11/14 mutation carriers in a large multigenerational family. This locus is therefore an attractive candidate gene for migraine, especially because migraine sufferers are known to have increased risk of carotid dissection.
In this important study, SNPs at the PHACTR1 and ASTN2 loci also showed suggestive evidence of replication. The PHACTR1 locus encodes a protein (a member of the PHACTR/scapinin family) that controls synaptic activity and synaptic morphology and has been implicated in endothelial cell function and susceptibility to early-onset myocardial infarction. The link between PHACTR and migraine pathogenesis could therefore either be neuronal, through aberrant synaptic transmission, or vascular, because endothelial dysfunction, cardiovascular disease, and myocardial infarction have all been linked to migraine.
Variants within the ASTN2 locus, a member of the astroactin gene family, have a role in glial-guided migration and appear important for the development of the laminar architecture of cortical regions of the brain. It is unclear how this links to the pathophysiology of migraine.
Other Migraine Genes Migraine is comorbid with numerous other disorders, and it is likely that an underlying shared biology between 2 comorbid disorders is driven, in part, by a similar genetic substrate. Recently, 2 distinct missense mutations in 2 independent families with MWA and familial advanced sleep phase syndrome was identified in the gene encoding casein kinase I δ (CKIδ). The CKIδ protein is pivotal in the function of the biological clock that influences the circadian rhythm. The resulting alterations (T44A and H46R) occurred in the conserved catalytic domain of CKIδ, where they caused reduced enzyme activity. Mice engineered to carry the CKIδ-T44A allele demonstrated significantly lowered mechanical and thermal sensory thresholds after infusion with nitroglycerin, a prototypic migraine trigger, and a significant increase in cFOS-positive neurons was demonstrated in the trigeminal-cervical complex. Also, consistent with the premise that migraine is characterized by neuronal hyperexcitability and that CSD of neuronal and glial membranes is responsible for the migraine aura, CKIδ-T44A mice demonstrated a reduced threshold for CSD after the application of potassium chloride to exposed cerebral cortex, and increased spontaneous and evoked calcium signaling in astrocytes. These in vitro and in vivo data suggest that CKIδ activity may contribute to the pathogenesis of migraine and potentially uncover protein targets of CKIδ as new targets for drug discovery.
A role has been suggested for the 2-pore domain (K2P) potassium channel, TWIK-related spinal cord potassium channel (TRESK, encoded by KCNK18), in pain pathways and general anesthesia. K2P channels are expressed throughout the CNS and have a key role in controlling neuronal resting membrane potential and neuronal excitability. They represent a major target for many volatile anesthetics and neuroprotective agents, and have been implicated in a variety of pain pathways. The TRESK K2P channel, encoded by the KCNK18 gene, is expressed in the spinal cord, where it helps modulate cellular excitability. The channel is directly activated by calcineurin after Gqα receptor stimulation and a subsequent rise in intracellular calcium. It is, therefore, a potential target of pain mediators that exert their action via these pathways, and a role has recently been suggested for TRESK in the calcineurin inhibitor-induced pain syndrome.TRESK is also activated by volatile anesthetics, such as halothane, which have been shown to inhibit CSD.
Lafreniere et al examined the potential role of TRESK in typical MWA. They sequenced the entire coding region of the KCNK18 gene in a panel and examined whether TRESK is involved in migraine by screening the KCNK18 gene in subjects diagnosed with migraine. They reported a frameshift mutation, F139WfsX24, which segregates perfectly with typical MWA in a large pedigree. They also identified prominent TRESK expression in migraine-salient areas, such as the trigeminal ganglion. Functional characterization of this mutation demonstrates that it causes a complete loss of TRESK function and that the mutant subunit suppresses wild-type channel function through a dominant-negative effect, thus explaining the dominant penetrance of this allele. These results support a role for TRESK in the pathogenesis of typical MWA and further support the role of this channel as a potential therapeutic target.
This finding has potential therapeutic implications. Based on the fact that currently available specific acute migraine medications (triptans and ergotamine) are receptor agonists, because TRESK is an ion channel expressed at the surface of the neuron, it should be possible to develop novel small compound agonists of TRESK that would be highly selective and potent, and could be used as acute or preventive drugs. This target may be highly desirable, since the gene is expressed in a very limited set of neurons, at least in mice, and as such, modulating its activity should have minimal effect on tissues where it is not expressed. The trigeminal ganglion also lies outside the blood–brain barrier and is thus more accessible to small compounds than other structures located behind the blood–brain barrier (ie, in the brain or brainstem). The challenge will be to develop small compound TRESK agonists that can modulate the excitability of TRESK-expressing neurons without increasing unwanted anesthetic or immune system side effects.
The possibility of migraine as an X-linked disorder is suggested by the female preponderance of and evidence that a high proportion of affected males have a greater number of affected first-degree relatives. In a linkage analysis of 6 migraine pedigrees and a case–control cohort study that evaluated 11 candidate genes, 2 distinct susceptibility variants were identified at Xq27 (marked by DXS8043—DXS297) and Xq28 (DXS8061—XqTer). Because these susceptibility regions were also associated in the case–control population, the possibility was raised that these loci are not pedigree-specific and may be contributing to migraine in the general population.
Migraine and Genetic Vasculopathies
Migraine, particularly MWA, is associated with several hereditary and acquired cerebrovascular disorders, including arterial dissection, ischemic stroke, and cardiovascular disease. It has been speculated that migraine and stroke may both be triggered by hypoperfusion and could therefore exist on a continuum of vascular complications in a subset of patients who have these hereditary or acquired comorbid vascular conditions. Indeed, evidence suggests that certain migraine mutations may increase the vulnerability for stroke by facilitating ischemic depolarizations. FHM type 1 (FHM1) mutant mice, with mutations in CaV2.1 voltage-gated calcium channels, develop earlier onset of anoxic depolarization and more frequent peri-infarct depolarizations, resulting in more extensive cerebral infarctions and worse neurologic outcomes. The authors proposed that enhanced susceptibility to ischemia-induced CSD predisposes migraine sufferers to infarction during mild ischemic events and that this in part may account for the higher stroke risk in migraineurs.
The mechanism underlying the frequent association between migraine and genetic small-vessel arteriopathies is unclear, but a common underlying factor may be the propensity for transient cerebral ischemia or hypoperfusion to trigger an underlying cascade of events that culminate in a migraine attack. Indeed, recent experimental data have indicated that focal, mild, and transient hypoperfusion (cerebral ischemia without infarction) can trigger CSD. MWA and MWoA have been associated as a common trait in several genetic small-vessel vasculopathies, as illustrated later.
Cerebral Autosomal Dominant Arteriopathy With Subcortical Infarcts and Leukoencephalopathy The clinical observation of an association between migraine and several genetic vasculopathies supports the concept that transient hypoperfusion or ischemic depolarization increases the susceptibility to migraine. One such genetic vasculopathy is cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). CADASIL is a systemic, non-amyloid, non-atherosclerotic vasculopathy and is the most common monogenic-inherited form of adult-onset stroke and vascular dementia, linked to mutations in the Notch3 gene (located on chromosome 19p13.2-p13.1), which encodes a cell surface receptor that, in human adult tissue, is solely expressed on VSMCs. The Notch3 protein is part of a signal transduction pathway, critical for aspects of vascular development, homeostasis, and VSMC differentiation. Most mutations in CADASIL are missense mutations and involve the loss or gain of a cysteine residue in the Notch3 protein (specifically the extracellular epidermal-growth-factor-like repeat). The Notch3 transgenic mouse model demonstrates that degeneration of VSMCs precedes the deposition of granular osmiophilic material in the basement membrane and extracellular matrix of VSMCs, and mutations in the NOTCH3 gene have a gain-of-function effect on the protein and that dysfunctional anchoring of VSMCs to the extracellular matrix and adjacent cells triggers VSMC degeneration. The variable disease course of CADASIL, ranging from relatively mild to very severe, might be an indication for the involvement of other genetic and environmental modifying factors.
The incidence of MWA, typically the first disease symptom of CADASIL, is 5 times greater compared with the general population. Visual and sensory auras are most common in CADASIL, although 50% of the patients also experience atypical attacks with basilar, hemiplegic, or prolonged aura, and even coma. The explanation for the predilection for MWA attacks in patients with CADASIL was recently demonstrated in a study that showed that CSD, the electrophysiological substrate of migraine aura, is enhanced in mice expressing a vascular Notch3 CADASIL mutation (R90C) or a Notch3 knockout mutation. The phenotype was stronger in Notch3 knockout mice, implicating both loss of function and neomorphic mutations in its pathogenesis. The mechanism underlying CSD increased susceptibility in CADASIL is unclear, but dysfunctional communication within the neurovascular unit has been raised as a possibility. Abnormal Notch3 signaling may disrupt normal astrocyte-smooth muscle communication and result in impaired neurovascular coupling, or Notch3 mutations expressed in neural progenitor cells and transiently in newly born neurons may lead to enhanced CSD susceptibility phenotype later in life.
Retinal Vasculopathy With Cerebral Leukodystrophy Retinal vasculopathy with cerebral leukodystrophy (RVCL) is a neurovascular syndrome (formerly referred to in 3 separate families and publications as cerebroretinal vasculopathy, hereditary vascular retinopathy, and hereditary endotheliopathy, retinopathy, nephropathy, and stroke) characterized by vascular retinopathy, cognitive impairment, depression, migraine (mainly without aura), focal neurologic symptoms, and, in later disease stages, characteristic contrast-enhancing intracerebral mass lesions. Several systemic symptoms can be present as well, including renal and liver dysfunction, Raynaud's phenomenon, and gastrointestinal bleeding. Although reported independently, the discovery of the gene defect, carboxyl-terminal truncations causing frameshifts in the TREX1 gene encoding the 3'-5' DNA-specific exonuclease TREX1 (3p21.1-p21.3), showed that they are different phenotypic variants of the same genetic disorder.
It is unclear how the carboxyl truncating mutations in TREX1 lead to the phenotype or pathogenesis of RVCL. Functional studies with mutant TREX1 suggest that forms lacking their native carboxyl-termini cannot localize to their usual perinuclear site within the cell and freely diffuse throughout the cytoplasm, thereby rendering the gene product unable to perform its physiological function.
Hereditary Systemic Angiopathy
Hereditary systemic angiopathy (HSA) manifests with cerebral calcifications, retinopathy, progressive nephropathy and hepatopathy, and appears to be a variant of RVCL. Small- and medium-sized subcortical white matter lesions are seen on magnetic resonance imaging (MRI) – a finding consistent with a vasculopathy of small- and medium-sized cerebral vessels. Subjects usually present in the 4th decade of life with visual impairment, migraine-like headaches, skin rash, seizures, motor weakness, and cognitive impairment. In the later stages, patients may develop liver sclerosis, with progressive hepatic impairment and renal microangiopathy leading to organ failure and chronic anemia. HSA has several similarities with RVCL; the cerebral calcification, perivascular inflammatory response, and progressive hepatic and renal failure are unique to this disorder.
COL4A1-related Disorder Autosomal dominant COL4A1 gene-related disorders are described in at least 12 white European families with 100% penetrance of disease. Thus far, 12 different missense mutations have been described in the COL4A1 gene. Mutations in the COL4A1 gene, located on chromosome 13q34, are associated with several unique phenotypes with overlapping features, including a condition exhibiting perinatal hemorrhage with porencephaly in survivors; a small-vessel disease with hemorrhage of adult onset or with infantile hemiparesis; and hereditary angiopathy, nephropathy, aneurysms, and muscle cramps. Along with hemorrhagic strokes, COL4A1 mutations may cause variable degrees of retinal arteriolar tortuosity, cataracts, glaucoma, and anterior segment dysgenesis of the eye.
Neurologic symptoms in COL4A1 mutation carriers may vary in degree of severity, even within families. Depending on the age of onset, affected individuals present with infantile hemiparesis, seizures, visual loss, dystonia, strokes, migraine, mental retardation, cognitive impairment, and dementia. Single or recurrent intracranial hemorrhages may occur in non-hypertensive adults less than 50 years of age. These hemorrhages can occur spontaneously, subsequent to trauma, or as a result of anticoagulant use. Stroke often occurs as the first presentation of the disease, with a mean age of onset of 36 years. MRI generally demonstrates typical features of other small-vessel diseases, including diffuse leukoencephalopathy with deep white matter involvement of posterior periventricular areas, subcortical infarctions, microhemorrhages, and dilated perivascular spaces.
The association with migraine in individuals with COL4A1 mutations is unclear, and may either be coincidental or mutation specific. MWA was described in one family with hereditary porencephaly secondary to a COL4A1 mutation, while in 2 other patients the migraine subtype was not specified. However, as in 6 other families with hereditary porencephaly, migraine was not described.