Migraine is a common neurovascular disorder with a strong genetic component. Symptoms include headache of moderate to severe intensity and recurring attacks. Diagnosis is made according to the criteria of the International Classification of Headache Disorders (ICHD-II) from the International Headache Society (IHS) [
1]. There are two main subgroups of the disorder: migraine with aura (MA) and migraine without aura (MO), with the latter being the most common subtype. Aura is defined as a period with variable focal neurological symptoms, most commonly affecting vision and occasionally also the sensory system, that precede and sometimes accompany the headache phase. Many patients having migraine with aura sometimes also experience attacks without aura. In concordance with the global lifetime prevalence of migraine, which has been reported to be 14% [
2], the lifetime prevalence of migraine in the Swedish population is 13,8% [
3], which corresponds to more than one million people. Thus, migraine is a major neurological disease of considerable public health relevance. There is a female to male predominance, with a peak in middle aged women [
2,
4]. There also seems to be some regional differences in prevalence with more migraine patients in Europe and North America compared to the rest of the world [
2]. Having relatives with migraine is a strong risk factor and heritability has been estimated to be as high as 50% [
5].
An alternative hypothesis for migraine headaches is increased brain excitability. This is supported by genetic findings in familial hemiplegic migraine (FHM), a rare monogenic subtype of MA, where mutations are found in ion channels, potentially influencing cellular excitability [
9,
10]. This is in agreement with the finding that topiramate, a calcium channel blocker generally used to treat epilepsy, has been shown to be effective in migraine as well [
11]. Genetic studies in the common forms of migraine have had limited success in the past decade. However, in three recent genome-wide association studies (GWAS) on large European migraine cohorts, a first set of genetic risk factors for migraine has been identified [
12‐
14]. In the first study by Anttila et al., the single nucleotide polymorphism (SNP) rs1835740 was identified as associated with migraine in a two stage GWAS in seven North European migraine populations [
12]. A strong association with migraine was found for the minor allele of rs1835740 on 8q22.1 (p = 5.38 × 10
−9). This genetic risk factor was stronger for MA, than MO. The SNP rs1835740 is located in a 27 kb haplotype block between two genes involved in glutamate homeostasis,
MTDH (metadherin, astrocyte elevated gene 1) and
PGCP (plasma glutamate carboxypeptidase). Anttila et al. further performed an expression quantitative trait locus (eQTL) analysis which revealed that the rs1835740 risk allele was associated with higher
MTDH expression. In addition,
MTDH was also recently found to be associated with migraine in a genome-wide meta-analysis including six population-based European cohorts [
15]. The second GWAS was performed on a cohort consisting of more than 20,000 American women of European descent. Results were replicated in two smaller independent samples of European migraine patients [
13]. Three associations were found; rs2651899, rs10166942 and rs11172113, none of which were preferential for MA or MO. Rs2651899 is located in the gene
PRDM16 (PR domain containing 16), a zinc finger transcription factor whose potential role in migraine is unclear. Rs10166942 is located close to the transcription start site for
TRPM8 (Transient receptor potential cation channel, subfamily M, member 8).
TRPM8 is a sensor for cold mainly expressed in the peripheral nervous system, and is likely to be involved in neuropathic pain [
16]. Rs11172113 maps to
LRP1 (Low density lipoprotein receptor-related protein 1), an endocytotic receptor protein involved in multiple cellular functions, such as modulating neuronal glutamate signaling [
17] and lipid homeostasis [
18], as well as being involved in the clearance of apoptotic cells [
19] and amyloid precursor protein [
20]. Findings of association with both rs10166942 and rs11172113 were recently replicated in a Scandinavian cohort consisting of more than 2,500 migraine patients [
21]. The third GWAS was performed on German and Dutch migraine patients without aura, and the material partially overlapped with the Anttila study [
14]. The study found associations with four new loci:
MEF2D (myocyte enhancer factor 2D),
TGFBR2 (transforming growth factor β receptor 2),
PHACTR1 (phosphatase and actin regulator 1) and
ASTN2 (astrotactin 2), and also replicated the previously reported associations with
TRMP8 and
LRP1. The SNPs with the strongest association at the respective locus were: rs3790455 (
MEF2D), rs7640543 (
TGFBR2), rs9349379 (
PHACTR1), and rs6478241 (
ASTN2).
MEF2D is a transcription factor that promotes the survival of newly formed neurons in the brain [
22].
MEF2D also influences the number of excitatory synapses in an activity dependent manner, and might thus influence neuronal excitability, which is an appealing role for a migraine candidate gene.
PHACTR1 regulates the activity of PP1 (protein phosphatase 1), which is known to influence synaptic activity and morphology [
23,
24].
TGFBR2 is a serine-threonine kinase involved in proliferation, and
ASTN2 codes for a protein expected to influence neuronal migration [
25,
26].
Due to previous findings of mutations in ion channels causing different forms of FHM, genetic studies of migraine have up until recently mainly been limited to these areas of the genome. However, recent advancements in high-throughput genotyping technology have enabled more hypothesis free searches of the whole genome, revealing new susceptibility loci for migraine. Findings from these GWAS should be replicated in independent populations, since genetic risk- or protective factors can vary between different populations. Therefore the aim of the present study was to perform a replication study in a Swedish cohort on eight SNPs identified as genetic risk factors for migraine in large European and American GWAS.