Lack of genetic susceptibility in takotsubo cardiomyopathy: a case-control study

Takotsubo cardiomyopathy (TCM), or “broken heart syndrome”, is a form of acute and transient cardiac failure [1] typically characterized by left ventricular dysfunction in the absence of corresponding obstructive coronary lesions or ruptured plaques [2]. Frequent symptoms of TCM include dyspnoea, chest pain and syncope [3]. Additional clinical features include electrocardiographic changes, such as ST-segment elevation and prolongation of QT interval, and elevated troponin levels [4]. Due to similar clinical presentation, distinction between TCM and acute coronary syndromes (ACS) requires coronary angiography [5]. Among patients presenting with symptoms of ACS, approximately 2% are diagnosed with TCM [5, 6].

Although the pathophysiological mechanisms underlying TCM are unknown, the condition commonly affects women with approximately 10% of reported cases being males [5] and disease presentation is often preceded by a stressful incident [3]. Acutely elevated plasma levels of catecholamines in TCM patients [7] and that intravenous administration of synthetic catecholamines in patients with suspected coronary artery disease (CAD) trigger the syndrome [8] suggest that exaggerated sympathetic stimulation might contribute to TCM. A rat model demonstrating development of transient myocardial hypokinesia typical for TCM after intravenous administration of epinephrine supports this hypothesis [9].

Multiple cases of TCM in the same family [10] and recurrent cases [11] indicate a genetic component. Current literature has focused on the possible existence of associations between TCM and single nucleotide polymorphisms (SNPs) in genes associated with sympathetic stress. While some studies have presented associations between TCM and functional polymorphisms, leading to exchange of amino acids, in the β1-adrenergic receptor (ADRB1) [12] and the G-protein-coupled receptor kinase 5 (GRK5) [13], which regulate cardiac sympathetic stimulation, others have not been able to replicate these findings [14, 15]. Recently, genetic variation affecting the regulatory function of the anti-apoptotic protein Bcl-associated athanogene 3 (BAG3), thought to contribute to stress resistance of myocytes, has been proposed to contribute to TCM [16].

There is no consensus regarding the influence of polymorphisms in the ADRB1 and GRK5 genes on TCM development and, to the best of our knowledge, only one study supporting a possible association between BAG3 polymorphisms and TCM has been published. Existing studies of the influence of these polymorphisms on TCM have had small sample sizes and un-matched controls. The aim of this study was therefore to investigate the possible associations between previously identified SNPs in the ADRB1, GRK5 and BAG3 genes, and TCM in a large cohort with matched controls.

By using a national register, covering all patients investigated by coronary angiography in Sweden with information about TCM (SCAAR) for participant enrolment, we have performed the so far largest genetic study of TCM patients with age- and sex-matched controls with and without CAD. We investigated the prevalence of polymorphisms earlier reported to be associated with TCM. When analyzing DNA from 258 TCM patients, we found no associations between candidate SNPs in the ADRB1, GRK5 and BAG3 gene and TCM using control subjects without CAD as reference.

Since experience of stress is frequently reported to trigger TCM, exaggerated sympathetic cardiac stimulation has been discussed as a possible physiological mechanism behind TCM. In a genetic study including 61 TCM patients, Vriz et al. presented an association between the same ADRB1 polymorphism as investigated in the current study and TCM [12], suggesting that genetic variation influencing cardiac sympathetic stimulation might contribute to TCM. This hypothesis is supported by studies indicating that the Arg389 variant of the ADRB1 has a higher sensitivity to catecholamine stimulation compared with the Gly389 variant [18]. In our study group, the Arg389 variant was the most common allele, validating a finding recently observed by an Australian study [14]. In line with the same Australian study, we found no association between the ADRB1 polymorphism (rs1801253) and TCM. Due to differences in SNP distribution between ethnic groups [19, 20], comparison with controls of a different ethnic group than the TCM patients could have affected the results in the study by Figtree et al. [14]. Both TCM patients and controls included in the study by Vriz et al. were Caucasians while Figtree et al. used Australian TCM patients and American controls with ancestry from Europe. Although intending to minimize the impact of ethnical diversity by including only people living in Sweden in the current study, we could not replicate the findings made by Vriz et al. One possible explanation could be failure in controlling for a diverse ethnical distribution of genotypes in the current study. However, this risk is considered small since the allele frequencies observed in our study were similar to those found in TCM patients in the Australian study [14] and that failure in controlling for a diverse ethnical distribution of genotypes would have required a large number of participants with different ethnicities. In addition, allele distributions observed in our study are in agreement with those of a previous study of a Scandinavian population [21]. Hence, we believe the most likely explanation to the discrepancy between our results and the findings made by Vriz et al. is the difference in sample size.

Another way of influencing the cardiac stress response to catecholamines is GRK5 mediated desensitisation of β-adrenergic receptors. The Gln41Leu GRK5 polymorphism has been shown to affect βAR desensitisation during exposure to catecholamine stimulation [22] and has therefore been suggested to contribute to TCM [13]. Although genetic variation in GRK5 is a reasonable pathophysiological hypothesis, we found no association between this polymorphism and TCM. However, since the minor allele frequency is very low, finding of an association requires a larger sample size unless the impact of the polymorphism is very large.

Recently, the anti-apoptotic BAG3, shown to increase susceptibility to damage during catecholamine stress, has been presented as another genetic candidate [16]. In 2015 d’Avenia et al. [16] analyzed 6 BAG3 SNPs in 70 TCM patients and 81 healthy blood donors with no signs of cardiovascular disease. The most frequent SNP (rs8946), coding for a regulating microRNA [16], was found to be more common among TCM patients than controls with an absolute difference in allele frequency of more than 10%. The same SNP was proposed to potentially impair a signaling pathway leading to up-regulation of BAG3 in cardiomyocytes in response to epinephrine. In the current study the BAG3 SNP was genotyped and we were unable to validate the finding of an association between this SNP (rs8946) and TCM. Despite the larger sample size and greater power to detect an existing association between the polymorphism and TCM, than in the study of d’Avenia et al. [16], we were unable to corroborate the existence of such an association, which may suggest a spurious finding in their study.

Clinical characteristics associated with TCM in the current study were medication for psychiatric disease and experience of stress at disease manifestation. These findings validate other studies proposing psychiatric [23, 24] and stress [4] comorbidity in TCM. To our knowledge, no other studies have compared the prevalence of preceding stress among TCM patients with the prevalence in controls. The only cardiovascular risk factor observed in a higher degree among TCM patients than controls without CAD was smoking. Unexpectedly, controls with and without CAD were more likely to report heritability of TCM than TCM patients. This is probably due to misunderstanding of the question. In contrast to TCM patients, controls may not be familiar with the diagnosis “broken heart” and might have assumed we were referring to CAD.

Limitations of this study included an unknown representativity of the study groups for the general population. Since patients were recruited by mail there is a possibility of selection bias. Another limitation is that the controls without CAD might not have been healthy, the only examinations performed served to exclude CAD and previous myocardial infarction. A potential heterogeneity of this control group might have influenced the results. The use of self-reported questionnaires for data acquisition entails a risk for self-report bias, which might decrease the internal validity of the current study. Furthermore, the call rates of the studied SNPs were low, possibly due to the origin of DNA. Although DNA purified from saliva has been reported to be of similar quality as blood DNA it cannot be excluded that some of the samples were of poor quality. Although we have performed the largest genetic study of TCM patients so far, the number of subjects included is still low. The power of the present study allowed us to detect an association between the two common polymorphisms and TCM only if the minor allele frequencies differ more than ten absolute percent when the TCM group is compared to any of the control groups. In order to provide strong evidence for genetic associations, a larger sample size is required. Future genome-wide association studies should be done to search for unknown genetic variants associated with TCM. Preferably, all included participants should belong to the same ethnic group. This arrangement would provide the best possible conditions for discovering genetic predisposition to TCM.

By performing the so far largest genetic study of TCM, we were unable to demonstrate an association between the proposed candidate SNPs in the ADRB1, GRK5 and BAG3 genes, and TCM. This strongly indicates a need for expanding the search in order to clarify the potential genetic predisposition to TCM, preferably using a genome-wide approach. Discovering genetic variation explaining disease susceptibility might entail both the understanding of intracellular signalling dysfunction among TCM patients and give future treatment suggestions.