title: An Investigation of Migraine Candidate Genes and Genomic Susceptibility Regions
Creators: name: Lea, Rod A.
Publisher: name: Griffith University
Year: 2003
Subjects: subject: migraine
subject: migraines
subject: headache
subject: headaches
subject: genes
subject: genetics
subject: genomic susceptibility regions

description: Typical migraine, comprised of migraine with aura (MA) and migraine without aura (MO), is a chronic, painful and debilitating neurovascular disease which is generally characterised by recurrent attacks of severe headache usually accompanied by nausea, vomiting, photo and phonophobia. Migraine has been shown to affect a large proportion of Caucasian populations with a recent comprehensive study indicating that around 25% of women and 8% of men suffer from the disease. Strong familial aggregation of typical migraine and an increased concordance for the disease in MZ twins over DZ twins, suggests that it has a significant genetic component. Heritability estimates are calculated to be between 40% and 60%, indicating that disease variation, in part, is explained by environmental determinants. The mode of transmission of typical migraine is not clear but is most likely multifactorial. Although the MA and MO subtypes exhibit some clinical heterogeneity, segregation analysis has suggested that there may be a common genetic aetiology for MA and MO, and a major gene contributing to typical migraine pathogenesis. This idea is substantiated by the fact that both subtypes of migraine can occur within the same family and even within the same individual, with up to 33% of sufferers experiencing both types of the disease. In addition, migraine prophylactics have been shown to result in similar effects in patients treated for both types of migraine. However, whether the two subtypes are truly separate entities or not remains unclear. At present, the type and number of genes involved in typical migraine is not known. Despite this, several studies into Familial Hemiplegic Migraine (FHM), a very severe subtype of MA, have led to the discovery that mutations in a brain specific calcium channel subunit gene (CACNA1A) located on chromosome 19, cause FHM in about 50% of affected families. FHM is a rare disease and is distinguished from typical migraine by its association with hemiparesis and clear autosomal dominant mode of inheritance. However, certain clinical features are common to both FHM and typical migraine including similarities in headache characteristics and triggers. Hence, FHM genetic studies provide a valuable model for investigating the genes involved in the more prevalent types of migraine with and without aura. For this reason the Genomics Research Centre has been conducting linkage studies utilising large Australian migraine pedigrees with a focus on the known FHM (CACNA1A) gene region on chromosome 19p13. Our results to date have indicated suggestive linkage to the FHM region on 19p13 in a large multigenerational pedigree (MF1) affected with typical migraine, with a maximum parametric LOD score of 1.92 (P = 0.001) obtained for a triplet repeat polymorphism situated in exon 47 of the CACNA1A gene. Expansion of this repeat was not observed, but is possible that mutations elsewhere in the CACNA1A gene may be responsible for migraine in this pedigree. To investigate this possibility, the current research involved sequencing two patients carrying the critical susceptibility haplotype surrounding the CACNA1A gene. The results of this mutation screen revealed no disease causing mutations or polymorphisms in any of the 47 exons screened. To determine whether the CACNA1A genomic region was implicated in typical migraine susceptibility in the general Caucasian population, 82 independent pedigrees and a large case-control group were also analysed using highly polymorphic microsatellite markers. There was no linkage or association detected in these groups and thus, it was concluded that if CACNA1A plays a role in typical migraine it does not confer a major effect on the disease. However, subsequent case-control studies of SNPs in the INSR gene, which is located ~15cM telomeric from CACNA1A, provided evidence of association to typical migraine. Thus, the INSR gene may now emerge as the new migraine susceptibility gene in this genomic region on chromosome 19. Family linkage studies conducted by Gardner et al have implicated an additional FHM susceptibility region on chromsome 1q31. Furthermore, independent research carried out by Ducros et al. has indicated a second FHM locus at 1q21-23, which is ~ 30cM centromeric to the region reported by Gardner et al. At this stage it is not clear whether there is a single locus, or two distinct loci, on the chromosome 1q region. This research also involved a family-based linkage and association approach to investigating the FHM susceptibility region on chromosome 1q31 for involvement in typical migraine susceptibility in affected Australian pedigrees. Initial multipoint ALLEGRO analysis provided strong evidence for linkage of Chr1q31 markers to typical migraine in a large multigenerational pedigree. The 1-LOD* unit support interval for suggestive linkage spanned ~18cM with a maximum allele sharing LOD* score of 3.36 obtained for marker D1S2782, P = 0.00004. Subsequent analysis of an independent sample of 82 affected pedigrees added support to the initial findings with a maximum LOD* of 1.24 (P = 0.008). Utilising the independent sample of 82 pedigrees we also performed a family-based association test. Results of this analysis indicated distortion of allele transmission at marker D1S249 (global c2(5) of 15.00, P = 0.010) in these pedigrees. These positive linkage and association results will need further confirmation by independent researchers, but overall they provide good evidence for the existence of a typical migraine locus near these markers on Chr1q31, and reinforce the idea that an FHM gene in this genomic region may also contribute to susceptibility to the more common forms of migraine. The serotonergic system has long been implicated in the pathophysiology of migraine. Researchers have therefore focused on the serotonin receptors and the genes that code for them when investigating this disease. Although serotonin receptor agonists have proven to be effective in the treatment of migraine, there has been little evidence of a serotonin receptor gene being associated with the disorder. However, in 1998, Ogilvie et al reported that a VNTR in the serotonin transporter gene (SERT) showed altered allelic distributions in a Danish migraine population. In addition to serotonin, there has been renewed interest in the involvement of the dopaminergic pathways in migraine. This interest has gained impetus since the study of Peroutka et al who reported an allelic association between the dopamine receptor gene DRD2 and migraine with aura. Another dopamine related gene, the dopamine beta-hydroxylase gene (DBH), has been localised to Chr 9q34 and codes for the enzyme that catalyses the conversion of dopamine to norepinephrine. It therefore plays an important role in dopaminergic and noradrenergic neurotransmission. Serum levels of DbH enzyme have been reported to be elevated in migrainous patients during the headache phase of an attack. Also, significantly increased DbH enzyme activity has been observed in migraine patients during the headache-free interval. Thus, the DBH gene is another good candidate for involvement in migraine pathophysiology and, to our knowledge, has not been previously implicated in this disease. Candidate gene studies may be useful strategies for identifying genes involved in complex diseases such as migraine, especially if the gene being examined contributes only a minor effect to the overall phenotype. This research also involved a linkage and association approach to investigating neurotransmitter related migraine candidate genes. Specifically, polymorphisms within the serotonin transporter gene (SERT), the dopamine receptor gene (DRD2) and the dopamine beta-hydroxylase (DBH) gene were tested in unrelated Caucasian migraineurs and non-migraine control individuals. In addition, an independent sample of 82 families affected with migraine were examined. Unrelated case-control association analysis of a DBH intragenic dinucleotide polymorphism indicated altered allelic distribution between migraine and control groups (c2 = 16.53, P = 0.019). Furthermore, the transmission/disequilibrium test (TDT) which was implemented on the family data also indicated distortion of allele transmission for the same DBH marker (c2 = 4.44, P = 0.035). Together, these results provide evidence for allelic association of the DBH gene with typical migraine susceptibility (Fisher's Combined P-value = 0.006) and indicate that further research into the role of the DBH gene in migraine aetiology is warranted. Nitric oxide (NO) is emerging as a key molecule affecting the pain associated with migraine. Since nitric oxide synthase (NOS) enzymes catalyse the synthesis of NO, the genes that code for these enzymes are good candidates for migraine molecular genetic analysis. This research involved investigating the role of a functionally relevant bi-allelic tetranucleotide polymorphism located in the promoter region of the human inducible nitric oxide synthase (iNOS) gene in migraine aetiology. A large group of migraine affected individuals were genotyped and compared to an age and sex matched group of unaffected controls. Results of a chi-squared analysis indicated that allele distributions for both migraine cases and controls were not significantly different (c2 = 1.93, P = 0.16). These findings offer no evidence for an allelic association of the tested iNOS polymorphism with the common forms of the disease and therefore do not support a role for this gene in migraine pathogenesis. In summary, this research involved linkage and association analysis of migraine candidate genes and genomic susceptibility regions. Whilst, the known FHM gene (CACNA1A) was excluded for significant involvement in typical migraine the adjacent INSR gene has been associated. Migraine is genetically heterogeneous and the results of this research also provide good evidence that the DBH gene is involved in disease predisposition, whilst the DRD2, SERT and INOS gene were not shown to be implicated. An additional susceptibility region for typical migraine is also likely to localise to chromosome 1q31. Overall, the results presented in this thesis have contributed valuable data to the understanding of the molecular genetics of migraine with and without aura. Future research into the molecular pathophysiological mechanisms of migraine will greatly facilitate the development of more effective diagnosis and treatment strategies.