Genetic Testing for Myocarditis

Myocarditis clinically manifests with chest pain, electrocardiographic changes, and serum biomarker elevation indicating myocardial injury. Often attributed to acute or recent viral infections, the familial clustering of myocarditis was thought to represent inherited susceptibility to infection. Pathologically, myocarditis is characterized by inflammatory cell infiltrates, often mononuclear, but not restricted to lymphocytes. Although myocarditis most commonly resolves, there are recurrent cases. During episodes of myocarditis, myocardial edema and inflammation can also be associated with arrhythmias, especially life-threatening ventricular tachycardia (VT). Risk stratification for future adverse cardiovascular events after myocarditis can be challenging, especially in those who present in early adulthood when the risk benefit assessment must be made for a longer life span.

Arrhythmogenic cardiomyopathy (AC) refers to genetically mediated cardiomyopathies that can alter the right and/or left ventricle and commonly associate with enhanced risk for ventricular arrhythmias, especially VT. AC is linked to pathogenic variants in genes encoding desmosomal components, including plakophilin 2 and desmoplakin, PKP2 and DSP, respectively. The overlap of myocarditis and AC has now come into focus, with an appreciation that myocarditis can represent a “hot phase” of AC gene mutations.¹ The work of Ammirati et al² in this issue of JACC: Heart Failure describes a cohort of patients with myocarditis, comparing outcomes with 2 other cohorts, one without genetic testing and one with genetic testing but without desmosomal gene mutations.² The authors identify relevant differences among these groups at the time of initial presentation and for longer-term sequelae; those with desmosomal gene mutations are more likely to have recurrence of myocarditis and future risk for VT.

Features present at the time of hospitalization distinguished desmosomal-associated myocarditis (DAM) from the other cohorts.² Those with desmosomal gene mutations were more likely to have a family history of myocarditis and a family history of sudden cardiac death. There was also more non-sustained ventricular tachycardia (NSVT) seen in the DAM group compared with the groups without genetic testing or with negative genetic testing. Interestingly, the authors also noted that there were cardiac magnetic resonance features that also helped predict long-term risk for cardiovascular events. The presence of late gadolinium enhancement in the septum or a ring-like structure of delayed enhancement both portended greater risk for cardiovascular events.

Myocarditis is more commonly seen in men, but among the 3 groups in the study, there were more women in the DAM group compared with the other 2, and this is in part because of the inherited contribution to myocarditis. Cardiomyopathies caused by desmosomal gene mutations are most commonly inherited as autosomal dominant conditions, accounting for more equal distribution among male and female individuals. Why men are more likely to manifest with myocarditis features is not known, but variable expressivity and penetrance is common in genetically mediated cardiomyopathies. For AC, it has also been reported that the presence of “second hits” contributes to early onset.³ The second hit can be an additional genetic variant, or it can also include environmental features, including intense exercise or other physiological stressor. It has also been observed that the proband typically manifests with an earlier age of onset compared with other family members detected through cascade genetic testing.³ For a clinician managing a patient with acute myocarditis, it is critical to obtain accurate family history and integrate this information with clinical findings of NSVT, delayed enhancement on magnetic resonance imaging, and results from gene panel testing. Together, these data help identify the patients who are at greatest risk for myocarditis and NSVT recurrence.

Practices regarding genetic testing vary widely and depend on payer systems as well as provider knowledge and familiarity with genetic testing. Turnaround times for return of genetic testing results also vary widely, but this should also not be a barrier for inpatient testing because results return can be part of outpatient management. Genetic testing for cardiomyopathy and arrhythmias relies on gene panel testing, in which up to 170 genes can be tested at the same time. The larger gene panels have improved yield, but these larger panels also are more likely to identify variants of uncertain significance. Variants of uncertain significance are rare gene changes about which there is insufficient information for full interpretation, and these uncertain variants are considered medically inactionable.

Although more study is needed, it is noteworthy that the DSP gene appears over-represented among the desmosomal genes reported with the acute inflammatory myocarditis process. The DSP gene encodes desmoplakin, a large intracellular component of the desmosome. The DSP gene itself is comparatively intolerant to those genetic variants that prematurely truncate the protein (referred to as “loss of function” or LoF variants). In the gnomAD database of human genetic variation, DSP’s probability of LoF is 1 (indicating intolerance to truncations), whereas PKP2, DSG2, DSC2, JUP, and TMEM43 each have probabilities of LoF of zero (indicating tolerance to truncations). This genetic feature may signify unique properties of DSP and influences the ability to interpret DSP genetic testing.

Clinically, DSP mutations can target the left ventricle (LV), and other AC genes appear to affect the right ventricle. Because myocarditis is seen more as an LV disease, this may also explain the predominance of DSP mutations in DAM.¹ Beyond myocarditis, the pattern of inflammation in DSP-linked LV disease also can be initially diagnosed as sarcoidosis, seen as uptake using 18F-fluorodeoxyglucose positron emission tomography scanning. Perhaps even most striking is that VT can occur even with relatively well-preserved LV function, a feature noted in the article by Smith et al¹ and also seen in the article by Ammirati et al.² The exact mechanism by which DSP mutations lead to inflammation and VT is not known, but a variety of inflammatory mechanisms have been implicated, including transforming growth factor-beta and nuclear factor-κB cascades.⁴

In particular, the precise role of innate and adaptive immune pathways in disease progression remains poorly understood. Many patients with AC present with serum autoantibodies targeting epitopes associated with the intercalated disc as well as the desmosome. Interestingly, anti-DSG2 antibodies have been found in patients with AC with and without mutation in this gene, suggesting a shared adaptive immune response underlying advanced disease.⁴ In early stages of disease, however, susceptibility to infection was historically thought to drive the “second hits” resulting in younger age at presentation. Recent evidence from animal and cellular models suggests that reduced expression of desmosome genes is sufficient to activate a transcriptional program resulting in sterile inflammation.⁴ In patients with DAM, this background gene expression may exacerbate inflammatory triggers when combined with known clinical risk factors such as strenuous exercise.¹ These preclinical studies suggest that inhibition of innate immune pathways such as nuclear factor-κB may prevent the release of cytokines and reduce the recruitment of immune cells to cardiac tissue in cases of DAM.

DSP mutations are also associated with inflammatory skin diseases ranging from severe diseases like epidermis bullosa to milder disorders like eczema. DSP and desmosomes are highly expressed in skin, an important barrier for infection. The overlap of DSP mutations with palmoplantar hyperkeratoma and wooly hair with cardiomyopathy is well established. Dry or flaky skin, or hyperkeratosis, can be seen in DSP mutations and may be overlooked in patients with cardiomyopathy. Broad immunosuppression, including topical steroids, can be effective, but newer targeted immune therapies are available to treat these skin disorders. Ustekinumab, a monoclonal antibody that targets interleukin-12 and interleukin-23, has been used to treat DSP-related skin disease.⁵ At this point, there is only anecdotal evidence to support utility for immunosuppression in DSP-related myocarditis, with the most common agent being glucocorticoid steroids, typically before the recognition of the genetic contribution to disease. Going forward, evaluating the relevant immune dysregulation and cell type responsible should be a high priority because there are a wealth of agents that could be used.

Myocarditis has a unique presentation but a range of etiologies, and clinicians now need to include a careful consideration of the genetic contributions to myocarditis. Genetic testing for cardiomyopathy and arrhythmias should be included in the clinical workflow, alongside a family history for myocarditis and sudden cardiac death. When a genetic contribution is identified, this identifies a patient at greater risk for recurrence of myocarditis and ventricular arrhythmias.