Author + information
- Kirk U. Knowlton, MD∗ ()
- ↵∗Address for correspondence:
Dr. Kirk U. Knowlton, Intermountain Medical Center Heart Institute, 5169 South Cottonwood Street, Suite 520, Murray, Utah 84107.
Mechanistic studies of myocarditis have focused on cell culture and rodent models of disease. These model systems have provided valuable insight into the mechanisms by which Coxsackievirus can infect the heart and how the immune system is activated and directed against the myocardium. However, it has been challenging to translate the findings in these rodent models of myocarditis to human myocarditis in a way that identifies cause-and-effect relationships. The study by Belkaya et al. (1) in this issue of the Journal demonstrates the use of human inducible pluripotent stem cell (iPSC)-derived cardiomyocytes that are obtained from individuals that have genetic mutations to study viral infections in human cardiomyocytes. Furthermore, the authors demonstrated in patients with acute myocarditis a higher incidence of homozygous or compound heterozygous mutations in genes that have been implicated in cardiomyopathy, providing additional insight into the mechanisms associated with increased susceptibility to myocarditis.
Translating findings from rodent models of myocarditis to humans has been limited by the low frequency of the disease and the challenge of making a definitive diagnosis. Endomyocardial biopsy continues to be the gold standard for diagnosis, but its sensitivity can be limited by the focal nature of the disease. Furthermore, many patients do not routinely undergo myocardial biopsy for suspected myocarditis. Over the last several years, cardiac magnetic resonance (CMR) has proven to be a reliable, noninvasive method to diagnose myocarditis. The combination of CMR findings of myocarditis and elevated troponins as described by Belkaya et al. (1) provide a strong argument for the diagnosis of acute myocarditis in their patients.
Several host, innate immune defense mechanisms have been implicated as determinants of susceptibility to viral myocarditis in the intact heart (2). These mechanisms include STAT1 dependent, interferon signaling and toll-like receptor-3 (TLR-3) pathways. However, it is not clear whether these pathways are important within infected human cardiomyocytes. While interferons can be induced in infected cells, other cells may only respond to exogenously administered interferon and not activate transcription of interferon within the infected cells.
There are multiple other nonimmune mechanisms that are involved in the pathophysiology of viral myocarditis. These include attachment and entry of the virus to the cardiac myocyte, replication of the virus in the myocyte, and release of replicated virus from a cell to infect adjacent cells. Murine models of Coxsackieviral myocarditis and infected, isolated primary rodent myocyte cells have been effectively used to study many of these mechanisms. Among other things, they have demonstrated that proteases expressed by Coxsackievirus can directly cleave myocyte proteins. Several of these proteases affect cytoskeletal proteins and proteins important for cytoskeletal repair. Some are involved in apoptotic pathways (3). When cytoskeletal proteins such as dystrophin are mutated to prevent cleavage, susceptibility to viral infection is decreased by limiting the release of virus from the cell (4). Furthermore, deficiency in cytoskeletal proteins such as dystrophin greatly increase the severity of the virus-mediated myocarditis/cardiomyopathy (4). These observations highlight the potential importance of cytoskeletal and membrane proteins and apoptotic pathways in the pathogenesis of myocarditis.
The elucidation of genetic causes of dilated cardiomyopathy has provided substantial insight into the mechanisms of dilated cardiomyopathy. However, many of the genetic abnormalities associated with dilated cardiomyopathy have variable penetrance and, in some cases, such as with truncating titin mutations, there are a larger number of unaffected individuals in the general population that carry the mutation than patients with disease, even though the percentage of mutations is higher in those that have disease (5). This suggests that a combination of genetic factors that increase susceptibility to cardiomyopathy combined with acquired causes of cardiomyopathy, such as viral infection, may be an explanation for the variable penetrance and severity of dilated cardiomyopathy.
Human iPSC-derived cardiomyocytes show promise as a model to study many of these steps of viral infection that occur in the cardiac myocyte. This can include assessment of innate immune responses within the cardiac myocyte and the effect of exogenous antiviral defense mechanisms, such as interferon stimulation in the human cardiac myocyte. However, they are not effective for the study of circulating cellular immune responses. Innate immune mechanisms are present in the cardiomyocyte, but there are also innate and adaptive immune mechanisms in other cell types that may be important in determining susceptibility to viral infection in vivo.
Belkaya et al. (1) present data from 2 groups of experiments. In the first case, they evaluated the effect of Coxsackievirus B3 (CVB3) infection on normal and mutated human iPSC-derived cardiomyocytes. They extended previous work demonstrating that human iPSC-derived cardiac myocytes can be infected by Coxsackievirus and that the virus can replicate and induce a cytopathic effect (6). Furthermore, their data assessed the effects of disruption of TLR3 and STAT1 activity in infected myocytes. The alteration in TLR3 that was studied has been implicated in a patient with viral myocarditis. STAT1 signaling is required for interferon signaling, a known antiviral mechanism. The data demonstrate that alteration of neither of these innate immune signaling molecules influences viral replication or cytopathic effects. However, the data do not exclude the possibility that alterations in TLR3 or STAT1 can change susceptibility to viral myocarditis in vivo where an intact circulating cellular immune response may be important. Furthermore, even though there is no evidence of induction of interferon signaling in infected human iPSC-derived cardiac myocytes, it appears that the myocytes can respond to exogenous interferon, as has been demonstrated previously (6).
The second series of experiments addressed the potential association between genetic causes of cardiomyopathy and myocarditis. Data from mouse models of myocarditis have demonstrated that genetic alteration of cardiac cytoskeletal proteins can alter susceptibility to viral heart disease by altering the ability of the virus to exit from the infected cell to infect adjacent cells. Because many of the genetic causes of cardiomyopathy are due to abnormalities in cardiac cytoskeletal or contractile proteins, it is logical to determine whether there is an increase in genetic alterations in the same genes that cause genetic cardiomyopathy in patients in whom viral myocarditis is diagnosed initially. This study demonstrates that, in patients with acute myocarditis, there is an increase in the percentage who have homozygous or compound heterozygous variants in genes that have been associated with dilated cardiomyopathy. Interestingly, they found that potentially pathogenic variants occurred in the genes DSP, PKP2, and TNNI3 that code for the cytoskeletal and contractile proteins desmoplakin, plakophilin-2, and troponin I type 3. In addition, there were alterations in BAG3, which encodes BCL2-associated athanogene 3, which is an important mediator of apoptosis. Other genes that were abnormal in acute myocarditis included SCN5A, which encodes the sarcolemmal sodium channel, voltage gated type V alpha subunit, which has been associated with the dystrophin-glycoprotein complex (7). Finally, RYR2, which encodes the ryanodine receptor 2, was mutated. It is localized to the membrane of the sarcoplasmic reticulum. Because each of the homozygous or compound heterozygous variants was only identified in a single patient, the current studies do not prove in each case that the variant identified is definitively a cause for increased susceptibility to myocarditis. However, overall, the data provide compelling evidence that genes involved in hereditary dilated cardiomyopathy can contribute to susceptibility to myocarditis. This is a significant insight that overcomes some of the challenges that exist for studying the overlap between genetic and acquired forms of cardiomyopathy.
As important as the observations are in the current article by Belkaya et al. (1), one must bear in mind that human iPS-derived cardiomyocytes are not fully matured cardiomyocytes. Therefore, observations of the human iPS-derived cardiomyocytes might not be identical to those in intact adult cardiomyocytes. Furthermore, the genetic studies in this paper focused primarily on variants in genes that are known to cause cardiomyopathy; it is possible that variants that are important in altering the circulating cellular immune response or other aspects of the innate immune response may also affect susceptibility to acute myocarditis. Last, it is has not been proven that the patients analyzed had a viral cause for their acute myocarditis, but given the young age of the patients, viral infections are common causes of acute myocarditis.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Knowlton has reported that he has no relationships relevant to the contents of this paper to disclose.
- 2017 American College of Cardiology Foundation
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