TRPM8 genetic variant is associated with chronic migraine and allodynia

Migraine, which is characterised by recurrent pulsatile headaches associated with nausea, vomiting, photophobia, and phonophobia, is a common yet disabling disease [1] that can be clinically diagnosed using the proposed criteria in the International Classification of Headache Disorders (ICHD) [2]. Migraine is considered as a complex genetic disorder. Studies of twins and familial aggregation analyses indicate a strong genetic component in migraine, showing a heritability of 0.34–0.81 [3‐8] that can be attributed to polygenes with a modest effect [9, 10]. Several single nucleotide polymorphisms (SNPs) associated with migraine susceptibility were recently identified by genome-wide association studies (GWAS) [11]. In our study among the Han Chinese population in Taiwan, several novel variants were identified to be associated with migraine in a two-stage GWAS [12], including rs655484 in disks large homolog 2 (DLG2) and rs3781545 in GDNF family receptor alpha-1 (GFRA1), rs10803531 in G protein-coupled receptor 39 (GPR39), and rs7565931 in uridine phosphorylase 2 (UPP2). Furthermore, the association between migraine and rs10166942 in transient receptor potential melastatin 8 (TRPM8) as well as rs1172113 in low density lipoprotein receptor-related protein 1 (LRP1), the two most replicated SNPs in Caucasians, were also reproduced in our study cohort. The risk allele frequencies are listed in Additional file 1: Table S1.

Endophenotypes are clinical symptoms that differentiate patients into subdivisions with underlying genetic pathogenesis. Several endophenotypes have been proposed in migraine, including prodromal symptoms such as yawning, aura, or accompanying symptoms like nausea, vomiting and pulsating. Unilateral autonomic symptoms during attacks are also raised as endophenotypes of migraine [13]. The association of some endophenotypes and migraine implicit genotypes have been studied and reported [14, 15]. Among all migraine endophenotypes or subtypes, chronic migraine is the one with critical clinical significance because it locates at the end of more severe disease-related disability with a poorer quality of life [16]. The prevalence of chronic migraine ranges from 1.0% to 1.7% in Asian populations [17, 18]. However, no susceptible gene has been identified to be associated with chronic migraine to date. On the other hand, allodynia is another widely-studied endophenotype of migraine. Defined as the pain triggered by a normally innocuous stimulation, cutaneous allodynia is very common in migraine patients [19‐21], especially in patients with chronic migraine [22]. The presence of allodynia often represents the peripheral, central, and disinhibitory sensitisation of pain pathways in affected patients [23].

In the present study, we aimed to investigate the association between migraine endophenotypes, especially for chronic migraine and allodynia, and known susceptible genes of migraine in Taiwan. Other investigated endophenotypes included aura and migrainous features. The candidate genes were chosen based on the findings of our previous migraine GWAS, the only published study in Asians [12]. The current study adopted a two-stage design, including discovery and replication cohorts of patients with migraine. In the replication cohort, we also evaluated the cutaneous allodynia profile in patients with migraine using a 17-item questionnaire, aiming to obtain the evidence for the association between genetic variants and cutaneous allodynia, a clinical marker signaling the sensitisation of trigeminovascular system and a potential predictor of migraine chronification [19].

To the best of our knowledge, this study first demonstrates that the TRPM8 rs10166942 T allele–also known as the risk allele of migraine–makes migraine patients more susceptible to migraine chronification. This finding was confirmed by a two-stage validation with two independent cohorts. In addition, our study also demonstrates the association between the TRPM8 rs10166942 T allele and allodynia severity. However, our study did not find a difference in allodynia severity between patients with episodic and chronic migraine.

The pathophysiology of chronic migraine is an urgent issue to be solved. Prior attempts to search for the relationship between genetic polymorphism, including another TRPM8 variant rs17862920, and chronic migraine have failed [25]. However, our study demonstrated an association between the rs10166942 T allele and chronic migraine with an adjusted odds ratio of 1.62, even after controlling for other known confounding factors. This finding deserves verification by following up with patients with episodic migraine in a longitudinal design. Several factors are considered as being involved in the process of migraine chronification, including older age, female gender, lower educational status, obesity, depression, stress and poor response to acute treatment [26]. In line with prior studies, our data showed that older age, lower education and higher depressive levels were risk factors for chronic migraine.

In animal models of craniofacial pain, two studies have demonstrated the association between TRPM8 and allodynia [27, 28]. However, the association between the TRPM8 genetic variant and allodynia has never been reported in human studies with migraine. Our study can be considered as a translational research, i.e. from animal models to human patients. Our data showed that severity of allodynia in migraineurs was positively associated with being female, longer disease duration of migraine, and psychiatric comorbidities, including depression and anxiety. These findings were also consistent with previous studies [29, 30], indicating that our cohort is a representative sample for migraine.

Of note, unlike a previous study [30], our data did not show associations between chronic migraine and the severity of allodynia. In fact, the results of the associations between allodynia and chronic migraine remained undetermined. One paediatric study also did not demonstrate an association between allodynia and chronic migraine [31]. In addition, one study showed that the threshold of pressure pain as measured by quantitative sensory tests did not differ between women with episodic and chronic migraine [32]. Recent results from MAST (Migraine in America Symptoms and Treatment) study showed that increased headache frequency was associated with allodynia; however, the effect was attenuated after controlling for sociodemographic profiles [33]. Regardless of the discrepancy, our results showed that allodynia was positively associated with the disease duration of migraine, which is in line with a previous study [21].

Encoded by gene TRPM8, the TRPM8 receptor is a non-selective cation channel that serves as the primary sensor of cold and cold-induced pain in mammals. The receptor can be activated by cold ranging from 8 to 28 °C and cooling agents, including methanol and icilin [34]. The T allele in the TRPM8 variant rs10166942 is a risk allele for migraine, and our results further indicate that it is also a risk allele to allodynia in migraineurs. The association between the TRPM8 variant rs10166942 and migraine was initially found in Western populations and replicated later in Asians [12]; here, the T allele was the risk allele for migraine across all studies [11]. To date, evidence in level of molecular mechanism that determines the functional effect of rs10166942 is lacking. Based on computational predictions [35], rs10166942 is located at the regulatory region of TRPM8, the functional effect of it might be alterations in the transcriptional regulation of TRPM8 and thus the affecting the phenotypes of patients. Another possibility is that rs10166942 has a strong linkage disequilibrium with the true causal variant, which remains to be identified. A recent study [36] demonstrated that the frequency of T allele in rs10166942 is positively corelated to the latitude and climate changes, suggesting that T allele-carrying genetic variant TRPM8 played a role in adaption to cold temperatures.

We speculate that T allele of rs10166942 increases the expression level of TRPM8, sensitizes humans’ cold sensation, helping them survive from the extreme changes in temperature in high latitude regions. On the other hand, T allele in rs10166942 may be associated with the functional changes of TRPM8 that contribute to the hypersensitivity to cold sensation and temperature changes. Pathways that transmit signals of cold sensation, including trigeminothalamic tracts, are repeatedly stimulated by cold and temperature changes, and eventually become sensitized in such individuals. The central sensitization of trigeminothalamic system may lead to worsening of cutaneous allodynia, especially to temperature-related stimulations like our investigated cohort reported, and to migraine progression and chronification. Of course, to explore the hypothesis, the exact impact of genotype rs10166942 on TRPM8 expression needs to be elucidated.

The association between TRPM8 and allodynia has been investigated in animal models, but the results are controversial. Kayama et al. revealed that the activation of TRPM8 reversed the heat allodynia in a mouse meningeal inflammation model, indicating that the activation of TRPM8 is protective against allodynia in migraine [28]. The same study also introduced a cell culture model with coexpression of TRPM8 and transient receptor potential cation channel subfamily V member 1 (TRPV1), showing that the activation of TRPV1, which would lead to heat and mechanical allodynia [37], was attenuated by the activation of TRPM8. The work of Kayama et al. suggests that the pathogenesis of allodynia involves multiple nociceptors, which could partially explain how the genetic variant of the cold receptor TRPM8 could be associated with both thermal and mechanical allodynia. Another study showed that the activation of meningeal TRPM8 receptors by the TRPM8 agonist icilin was associated with facial and hind paw mechanical allodynia in rats [27]. The discrepancy in results might be derived from different model systems (topical ilicin in rats vs. inflammatory soup in mice) with different readouts (mechanical vs. thermal allodynia).

Our study has limitations. First, this was a cross-sectional study. Therefore, the causal relationship of migraine chronification and TRPM8 variant could not be ascertained. Based on our findings, a longitudinal study is warranted to elucidate the association between the TRPM8 genetic variant rs10166942 and the evolution of migraine. Second, recall bias of allodynia could have been introduced because not all participants were interviewed during headache attacks. Third, the current study was a single hospital-based study despite validation with another independent cohort; therefore, the results might not be generalisable to other migraine populations. Last, the sample size of the present study was limited, and only 6 migraine susceptible SNPs were tested considering a priori evidences (as indicated in the Methods) and the limitation of resources. However, the 6 tested SNPs were proven to be significant in the studied population, which makes our finding indicative, especially in Asian population. Indeed, further investigations of the association between TRPM8 variant and chronic migraine in different populations are warranted.

Our study shows that the TRPM8 variant rs10166942 is associated with chronic migraine and allodynia in patients with migraine. Further investigation regarding the role of TRPM8 in allodynia pathogenesis and migraine chronification may provide a novel treatment strategy.