Migraine with aura is not linked to the FHM gene CACNA1A or the chromosomal region, 19p13

Pathophysiological Bases of Comorbidity in Migraine

Despite that it is commonly accepted that migraine is a disorder of the nervous system with a prominent genetic basis, it is comorbid with a plethora of medical conditions. Several studies have found bidirectional comorbidity between migraine and different disorders including neurological, psychiatric, cardio- and cerebrovascular, gastrointestinal, metaboloendocrine, and immunological conditions. Each of these has its own genetic load and shares some common characteristics with migraine. The bidirectional mechanisms that are likely to underlie this extensive comorbidity between migraine and other diseases are manifold. Comorbid pathologies can induce and promote thalamocortical network dysexcitability, multi-organ transient or persistent pro-inflammatory state, and disproportionate energetic needs in a variable combination, which in turn may be causative mechanisms of the activation of an ample defensive system with includes the trigeminovascular system in conjunction with the neuroendocrine hypothalamic system. This strategy is designed to maintain brain homeostasis by regulating homeostatic needs, such as normal subcortico-cortical excitability, energy balance, osmoregulation, and emotional response. In this light, the treatment of migraine should always involves a multidisciplinary approach, aimed at identifying and, if necessary, eliminating possible risk and comorbidity factors.

Genetics of migraine: an update

An important genetic component of migraine was systematically established by epidemiological studies in the 1990s. Over the past 15 years, significant progress has been made in unraveling the genetic basis and pathophysiological mechanisms of familial hemiplegic migraine, a rare and severe autosomal-dominant subtype of migraine with aura. Three different causative genes (CACNA1A, ATP1A2 and SCN1A), all of which are involved in cerebral ion translocation, have been identified. Functional studies and mouse models have shown that mutations in these genes, by different mechanisms, cause a disturbed cerebral glutamate homeostasis and, thus, increase susceptibility to cortical spreading depression, the likely correlate of migraine aura. More recently, genome-wide association studies have, for the first time, detected robust risk variants associated with the more common, genetically complex types of migraine, which has generated new perspectives for genetic research in migraine. This review summarizes the current knowledge about migraine genetics, with a focus on both familial hemiplegic migraine and recent results of genome-wide association studies.

Chromosome 1p36 in migraine with aura: association study of the 5HT(1D) locus

Migraine with aura (MA) may share some but not all risk factors with other forms of migraine. As common migraine without aura (MO) has been associated with the chromosome 1p36 locus, we tested its involvement in MA by using two-point parametric linkage analysis to analyze 64 multiplex MA families. A logarithm of the odds score of 1.9 was suggestive of chromosome 1p36 linkage to MA. The transmission disequilibrium test analysis was then performed in 79 nuclear families with one MA parent and one MA offspring. We identified the presence of genetic association at chromosome 1p36 with MA (P=0.045, Bonferroni corrected): the locus encoding the 5HT(1D) receptor gene. Although these data suggest that the 1p36 locus may protect against MA, consistent with the role of the 5HT(1D) receptor in migraine treatment with triptan drugs, the study is subject to the limitations associated with studying a small number of affected families. As a result, we contrast evidence suggesting that the chromosome 1p36 locus is strongly MO associated with our finding that 1p36 has a more limited contribution to MA in the families we analyzed. Further work using a genome-wide association study approach in familial typical migraine, consisting of those affected by MO or MA, will serve to further distinguish how and why MA differs from MO.

Migraine with and without headache

Transient visual and neurological episodes are relatively common and can occur for the first time in middle and old age. In many cases these transient events are migraine auras. An aura is a transient, stereotypical, visual or neurological episode usually lasting 4 to 60 minutes in duration. Migraine is usually, but not always, associated with headache and can be accompanied by systemic and autonomic symptoms. Diagnosis is dependent on International Headache Society criteria. The pathophysiology is believed to involve neurovascular mechanisms. There is a hereditary component to migraine. When migraine auras occur in the absence of headache they are termed acephalgic migraines. Late onset migraine accompaniment is an acephalgic migraine that presents in middle-aged and older adults. It is usually benign. Migraines can be mimicked by other more serious conditions. Most patients with a stable migraine pattern and normal neurological evaluation do not require further testing. Some patients with atypical presentation, older age, or suspected secondary causes need further investigation.

Migraine genetics

Migraine with aura (MA) and migraine without aura (MO) are primary headaches prevalent in the general population that carry a substantial familial liability. Based on the model of migraine as a complex disease, a multifactorial type of inheritance has been suggested, but familial hemiplegic migraine (FHM), classified as a subtype of MA, shows an autosomal dominant transmission pattern and is due to mutations in three genes encoding for neural channel subunits. These FHM mutations, however, account for a minority of the FHM families and are not usually found in sporadic HM or in the typical migraines MA/MO. This implies that the genetic predisposition to the typical migraines may be different and that FHM could be better classified as a type of syndromic migraine rather than a MA subtype. Linkage and genome-wide scans have disclosed several chromosomal liability loci in selected families with MA/MO. It is likely that typical migraine genes will be discovered in the future. Epigenetic mechanisms, especially those acting in the early stages of neural development, are here proposed to be involved in the genetics of the typical migraines, especially if the typical migraines are modeled as evolutionarily conserved behaviors (sickness behavior) enacted out of a genetic repertoire.

Migraine: a genetic disease?

Migraines carry a substantial genetic liability, and in families affected with the typical migraines (migraine with, MA, and without aura, MO) linkage to some chromosomal loci has been reported. As yet however, no genes are known for MA/MO, while the three genes discovered as responsible for familial hemiplegic migraine (FHM) are not involved in the typical migraines. Accordingly, we propose to consider FHM as a syndromic migraine and not as a variety of MA. Moreover, we suggest that epigenetic mechanisms play a role in the determination of the typical migraines, and that the primary headaches represent behavioural responses (sickness behaviour, fight-or-flight responses), having adaptive advantage and having been evolutionary conserved, in which pain represents a signal of homeostatic imbalance. Epigenetic mechanisms and this proposed genetic behavioural model could be usefully incorporated into headache genetic research.

A novel ATP1A2 gene mutation in an Irish familial hemiplegic migraine kindred

OBJECTIVE

We studied a large Irish Caucasian pedigree with familial hemiplegic migraine (FHM) with the aim of finding the causative gene mutation.

BACKGROUND

FHM is a rare autosomal-dominant subtype of migraine with aura, which is linked to 4 loci on chromosomes 19p13, 1q23, 2q24, and 1q31. The mutations responsible for hemiplegic migraine have been described in the CACNA1A gene (chromosome 19p13), ATP1A2 gene (chromosome 1q23), and SCN1A gene (chromosome 2q24).

METHODS

We performed linkage analyses in this family for chromosome 1q23 and performed mutation analysis of the ATP1A2 gene.

RESULTS

Linkage to the FHM2 locus on chromosome 1 was demonstrated. Mutation screening of the ATP1A2 gene revealed a G to C substitution in exon 22 resulting in a novel protein variant, D999H, which co-segregates with FHM within this pedigree and is absent in 50 unaffected individuals. This residue is also highly conserved across species.

CONCLUSIONS

We propose that D999H is a novel FHM ATP1A2 mutation.

The primary headaches: genetics, epigenetics and a behavioural genetic model

The primary headaches, migraine with (MA) and without aura (MO) and cluster headache, all carry a substantial genetic liability. Familial hemiplegic migraine (FHM), an autosomal dominant mendelian disorder classified as a subtype of MA, is due to mutations in genes encoding neural channel subunits. MA/MO are considered multifactorial genetic disorders, and FHM has been proposed as a model for migraine aetiology. However, a review of the genetic studies suggests that the FHM genes are not involved in the typical migraines and that FHM should be considered as a syndromic migraine rather than a subtype of MA. Adopting the concept of syndromic migraine could be useful in understanding migraine pathogenesis. We hypothesise that epigenetic mechanisms play an important role in headache pathogenesis. A behavioural model is proposed, whereby the primary headaches are construed as behaviours, not symptoms, evolutionarily conserved for their adaptive value and engendered out of a genetic repertoire by a network of pattern generators present in the brain and signalling homeostatic imbalance. This behavioural model could be incorporated into migraine genetic research.

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.

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.

Genetic biomarkers for migraine

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

Migraine genetics: an update

A growing interest in genetic research in migraine has resulted in the identification of several chromosomal regions that are involved in migraine. However, the identification of mutations in the genes for familial hemiplegic migraine (FHM) forms the only true molecular genetic knowledge of migraine thus far. The increased number of mutations in the FHM1 (CACNA1A) and the FHM2 (ATP1A2) genes allow studying the relationship between genetic findings in both genes and the clinical features in patients. A wide spectrum of symptoms is seen in patients. Additional cerebellar ataxia and (childhood) epilepsy can occur in FHM1 and FHM2. Functional studies show a dysfunction in ion transport as the key factor in the pathophysiology of (familial hemiplegic) migraine that predict an increased susceptibility to cortical spreading depression--the underlying mechanism of migraine aura.

Genetik der Migräne

Twin and family studies provide evidence of a genetic component in migraine, in particular migraine with aura (MA). Familial hemiplegic migraine (FHM) is a rare monogenic subtype of MA for which three causative genes have been identified: CACNA1A (FHM1), ATP1A2 (FHM2), and SCN1A (FHM3). Mutations in these genes are also found in some patients with sporadic hemiplegic migraine. Linkage studies have identified several gene loci for the more common forms of migraine; however, identification of the respective causative genes is still pending. This review summarizes recent developments in the genetics of migraine and their implications for molecular genetic testing. We further discuss the roles of CACNA1A, ATP1A2, and SCN1A in the pathophysiology of cortical spreading depression, which is the likely correlate of migraine aura.

Genetics of Migraine: An Update

Observations including the long-recognized tendency of migraine to run in families, the high concordance rates for migraine in twins reared together or apart, and the association of specific mutations with a rare migraine form are consistent with a genetic contribution to the disorder. This paper summarizes major findings to date on the genetics of migraine. Study of the heritability of migraine, particularly the common forms of migraine, is beset by several challenges including the absence of easily measurable biological markers, uncertainty about the etiologic and clinical overlap among migraine types, and the apparently complex interplay of environmental and genetic factors in determining migraine phenotype. Nevertheless, significant progress has been realized in recent years. Familial hemiplegic migraine, a rare migraine variant, appears to be transmitted by a Mendelian, autosomal dominant mode of inheritance involving mutations in at least 2 genes. These genes do not seem to be critically involved in the other forms of migraine; however, several other susceptibility loci for more common forms of migraine have been identified in recent genome-wide screens and candidate-locus studies. These and other data suggest that the genetic contribution to migraine is complex, multifactorial, and subject to significant modification by environmental factors.

Migraine: new treatment options from molecular biology

Migraine is a common, disabling, multifactorial, episodic neurovascular disorder of largely unknown etiology. The disease is typically characterized by recurrent attacks of headaches and associated autonomic and neurologic symptoms. Current acute and prophylactic treatment options are far from optimal and in many cases, empirically chosen. Clearly, improved treatment is desperately needed. New drug targets may emerge from molecular research as the unravelling of the molecular basis of migraine should improve our understanding of the disease, notably why patients experience attacks so frequently. The first two migraine genes discovered in families with hemiplegic migraine encode ion transporters, emphasising that dysfunction of ion transport may be an important factor in migraine. Therefore, ion transporters can be considered as novel targets for the development of future antimigraine drugs. Molecular biologic research will increasingly become important in understanding the pathophysiology of migraine and in identifying potential molecular targets for novel treatments.

The CACNA1A and ATP1A2 genes are not involved in dominantly inherited migraine with aura

Epidemiological studies indicate that migraine with typical aura (MA) has a major genetic component but the genes for MA have not been identified. However, the autosomal dominantly inherited familial hemiplegic migraine (FHM) is often caused by mutations in the CACNA1A or ATP1A2 genes. The aim of the study was to investigate if the CACNA1A or ATP1A2 genes are involved in MA with an apparently autosomal dominant mode of inheritance. From a clinic population diagnosed by a trained physician we recruited 34 extended families (comprising 174 MA patients) with an apparently autosomal dominant mode of inheritance of MA. We performed a linkage analysis of 161 of 174 MA patients and 79 unaffected relatives using a framework marker set of 44 markers for chromosome 1 and 22 markers for chromosome 19. Linkage analysis was made with a non-parametric or autosomal dominant parametric model, either allowing for heterogeneity or not, using an affected only analysis. We identified no linkage to CACNA1A and ATP1A2 loci on chromosome 19 or 1, respectively. Additionally, at least two patients from each family and 92 healthy, unrelated controls were selected for a sequence analysis. We sequenced the 48 exons of CACNA1A and the 23 exons of ATP1A2, including promoter and flanking intron sequences. No polymorphism was identified in the CACNA1A or ATP1A2 genes with a strong correlation to MA. Our study shows that the CACNA1A or ATP1A2 genes are probably not involved in MA. To identify the genes involved in the common forms of migraine, future genetic studies should focus on MA and migraine without aura (MO) and not FHM.

Chromosome 19p13 loci in Finnish migraine with aura families

Chromosomal area 19p13 contains two migraine associated genes: a Ca(v)2.1 (P/Q-type) calcium channel alpha(1) subunit gene, CACNA1A, and an insulin receptor gene, INSR. Missense mutations in CACNA1A cause a rare Mendelian form of migraine, familial hemiplegic migraine type 1 (FHM1). Contribution of CACNA1A locus has also been studied in the common forms of migraine, migraine with (MA) and without aura (MO), but the results have been contradictory. The role of INSR is less well established: A region on 19p13 separate from CACNA1A was recently reported to be a major locus for migraine and subsequently, the INSR gene was associated with MA and MO. Our aim was to clarify the role of these loci in MA families by analyzing 72 multigenerational Finnish MA families, the largest family sample so far. We hypothesized that the potential major contribution of the 19p13 loci should be detected in a family sample of this size, and this was confirmed by simulations. We genotyped eight polymorphic microsatellite markers surrounding the INSR and CACNA1A genes on 757 individuals. Using parametric and non-parametric linkage analysis, none of the studied markers showed any evidence of linkage to MA either under locus homogeneity or heterogeneity. However, marginally positive lod scores were observed in three families, and thus for these families the results remain inconclusive. The overall conclusion is that our study did not provide evidence of a major MA susceptibility region on 19p13 and thus we were not able to replicate the INSR locus finding.

Migraine genetics

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