Genetics of Dilated Cardiomyopathy

Abstract

Dilated cardiomyopathy (DCM) is defined as dilation and/or reduced function of one or both ventricles and remains a common disease worldwide. An estimated 40% of cases of familial DCM have an identifiable genetic cause. Accordingly, there is a fast-growing interest in the field of molecular genetics as it pertains to DCM. Many gene mutations have been identified that contribute to phenotypically significant cardiomyopathy. DCM genes can affect a variety of cardiomyocyte functions, and particular genes whose function affects the cell–cell junction and cytoskeleton are associated with increased risk of arrhythmias and sudden cardiac death. Through advancements in next-generation sequencing and cardiac imaging, identification of genetic DCM has improved over the past couple decades, and precision medicine is now at the forefront of treatment for these patients and their families. In addition to standard treatment of heart failure and prevention of arrhythmias and sudden cardiac death, patients with genetic cardiomyopathy stand to benefit from gene mechanism–specific therapies.

EPIDEMIOLOGY AND PATHOGENESIS OF DILATED CARDIOMYOPATHY

The cardiomyopathies are defined as myocardial disorders with abnormal structure or function of the heart. Broadly, these can be subdivided into dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), arrhythmogenic cardiomyopathy (ACM), and left ventricular noncompaction cardiomyopathy. DCM is characterized by dilation and loss of function of one or both ventricles and can be due to secondary causes such as infiltrative disease, metabolic derangements, valvular disease, toxins, medication, and many more. Although ischemic cardiomyopathies are more common in the United States (59% versus 41%) (1), patients with nonischemic DCM are more likely to be women, nonwhite, and younger than those with ischemic cardiomyopathies. The true prevalence of nonischemic DCM is not fully identified but is likely underestimated. An epidemiological study performed in Olmstead county, MN, from 1975 to 1984 using autopsy data, echocardiography, and angiography found a DCM prevalence of 36.5 in 100,000 patients and a man-to-woman ratio of 3:4 (2). This differs from several other studies performed in various regions, a difference that may reflect geographical and ethnic contributions to the frequency of DCM (3–6). Up to 50% of nonischemic DCM is genetic or idiopathic (7). Familial DCM is defined as (a) the presence of two or more relatives with DCM or (b) the presence of one relative with DCM and sudden cardiac death (SCD) prior to the age of 35 years. An estimated 40% of cases of familial DCM have an identifiable genetic cause (8). As such, there is a fast-growing interest in the field of molecular genetics as it pertains to DCM. Many genes have been identified that may contribute to phenotypically significant cardiomyopathy (9).

HCM:

hypertrophic cardiomyopathy

ACM:

arrhythmogenic cardiomyopathy

A significant portion (20–38%) of DCM may have an oligogenic basis; multiple rare variants from different unlinked loci and inconstant penetrance may cause a similar phenotype of DCM. The effects of environmental insults such as myocarditis, chemotherapy, and alcohol on the phenotypic expression of DCM in patients with genetic mutations are termed gene–environment interactions (GxE) and have also been studied (10, 11). In contrast, HCM and ACM fit more classically into a Mendelian model due to few highly penetrant rare variants affecting the sarcomere or desmosome, respectively. In fact, for HCM and ACM, a single mutation may explain most genetic causes in individual families (12–14). As with HCM, variants in the sarcomere genes are a frequent cause of DCM. For example, truncating mutations in the giant sarcomeric protein Titin (TTN) are the most common cause of DCM in adults but may also cause HCM (15–18) in rare cases. Mechanistically, the distinction from HCM is that DCM variants of the sarcomere gene are generally a loss-of-function mutation resulting in impaired force generation and decreased systolic function of the ventricle. DCM-causing mutations are not limited to the sarcomere. They may affect force transmission, mechanical stress, signaling, desmosomal proteins, nuclear structure and function, ion channel activity, protein turnover, and calcium hemostasis (19).

As with adults, genetic testing has become integral to the diagnosis, prognostication, and treatment of the pediatric population. Although DCM variants that directly disturb aspects of cardiomyocyte function are also present in younger patients, many genes associated with inborn errors of metabolism cause DCM (20, 21). Interestingly, a genetic cause in a pediatric patient is identified more commonly than in an adult patient (54% versus 27%) (22, 23). The 5-year survival in pediatric DCM that is familial is 94%, but this same cohort of patients has a relatively high 5-year transplantation rate of 38% (24, 25). These trends differ in idiopathic DCM and myocarditis, emphasizing the importance of genetic testing in all pediatric patients with DCM (26).

DIAGNOSIS AND IMAGING

When evaluating for genetic DCM, it is essential to perform a detailed medical history. In addition, clinicians should obtain baseline hematologic and metabolic data, as well as a baseline electrocardiogram, which may abnormal in HCM, ACM, or DCM. Furthermore, patients should undergo ambulatory cardiac monitoring to evaluate arrythmia burden for further risk stratification. Building a detailed three-generation family history is crucial for determining risk factors for disease progression and SCD as well as classifying cases as familial or sporadic DCM (27). Echocardiography should be included in the initial work-up and screening for patients with suspected DCM. Cardiac magnetic resonance imaging (CMRI) can also be useful in characterization of cardiomyopathy and is frequently utilized in the diagnosis of genetic DCM.

The imaging diagnosis of DCM on echocardiogram is defined as the presence of fractional shortening of <25%, left ventricular ejection fraction (LVEF) of <45%, and left ventricular end diastolic diameter of >2.7 cm/m² or >117% predicted when corrected for age and body surface area (>2 standard deviations from the upper limits of normal values) (28). Any known secondary cause of myocardial disease must be excluded prior to making a diagnosis of genetic or idiopathic DCM (29). Phenotype-negative individuals who have confirmed variant mutations for DCM may undergo screening by echocardiography every 1–5 years.

Echocardiography

On 2D echocardiography, the LVEF is assessed using the biplane Simpson method with the use of contrast agents to opacify the left ventricular endocardium in the case of poor image quality (30). Linear measurements for estimation of LVEF (Teichholz method) should be avoided due to inaccuracy. Also useful in the estimation of LVEF is 3D echocardiography, which has shown greater reproducibility, accuracy, and inter- and intra-reader consistency when compared to conventional 2D echocardiography techniques (31, 32). This improvement is likely due to several factors, including geometric assumptions with 2D echocardiography in which the heart is modeled as an ellipsoid shape, multiple acquisitions with 2D that can introduce more error, and foreshortening that can affect 2D calculation. Compared to CMRI, a 3D echocardiogram underestimates left ventricular volumes due to overall lower spatial resolution.

Aside from the imaging definition of DCM, other associated findings may be present on a DCM patient’s echocardiogram. As DCM progresses, the left ventricle continues to remodel, becoming more spherical. This change is directly measured by an increased ratio of the short axis to the long axis (33). Functional mitral regurgitation results from dilation of the mitral valve annulus and consequent tethering of the mitral valve leaflets. Careful attention should be given to evaluate for left ventricular thrombus, pulmonary hypertension, and right ventricular dysfunction or dilation.

Cardiac Magnetic Resonance Imaging

CMRI is currently the gold standard for assessment of biventricular volumes and function (34). In addition, the main strength of CMRI is the enhanced ability for tissue characterization and scar pattern analysis (35–38). It is a useful additional test in conjunction with echocardiography to further characterize the phenotype of DCM, as well as to rule out secondary causes of DCM (39). For instance, postischemic DCM with severe myocardial hypoperfusion or scarring may have a similar phenotype of ventricular dilation with reduced function but have distinctive findings on CMRI compared with nonischemic DCM, associated with different prognosis and treatment (40).

Late gadolinium enhancement (LGE) imaging comprises a standard sequence with CMRI to evaluate for focal myocardial fibrosis. A majority (58%) of DCM patients have no LGE, while up to 28% may have a characteristic midmyocardial pattern of enhancement (41, 42). The presence or absence of LGE has large prognostic implications. A study following 472 patients with DCM for 5.3 years noted that those with midmyocardial LGE had a threefold increase in all-cause mortality and fivefold increase in a composite end point of SCD and aborted SCD. A smaller study with 65 patients also showed worse prognosis for DCM patients with midmyocardial LGE, reporting an eightfold increase in heart failure, appropriate internal defibrillator (ICD) firing, and cardiac death (43).

ICD:

internal defibrillator

There is overlap between ACM and DCM. Specific genes such as LMNA, SCN5A, FLNC, RBM20, PLN, DSP, DES, and TMEM43 can cause an arrhythmogenic DCM and may require ICD outside of the traditional primary prevention criteria (44–49). CMRI is also used clinically in DCM patients for additional SCD risk stratification. For example DSP and FLNC mutations may have a characteristic extensive ring-shaped LGE, which is a midmyocardial or subepicardial LGE pattern in three contiguous walls on the short axis view (50). Other forms of DCM do not typically present with this ring-shaped enhancement and can have heterogenous scar patterns (Figure 1).

Figure 1.

Phase-sensitive inversion recovery MRI sequences showing examples of nonischemic patterns of LGE in DCM (arrows).

(a) Characteristic ring-like subepicardial LGE in patient with DCM and DSP mutation. (b) Anteroseptal midmyocardial LGE in a patient with idiopathic DCM. (c) Patient with Duchenne’s muscular dystrophy demonstrating subepicardial LGE of the anterolateral and inferolateral walls. Abbreviations: DCM, dilated cardiomyopathy; LGE, late gadolinium enhancement; MRI, magnetic resonance imaging.

Genetics

There have been remarkable achievements in the field of genetics due to advancements in next-generation sequencing, and now a person’s entire genome can be evaluated with a single test. Many gene variants have been identified in the pathogenesis of DCM (Table 1). Most of the DCM genes are inherited in an autosomal dominant pattern and have variable penetrance (51). Autosomal recessive, X-linked, mitochondrial inheritance patterns and de novo mutations also occur (52). As discussed, DCM genes encode a wide variety of cellular functions.

Table 1.

Genes causing dilated cardiomyopathy

Gene	Gene symbol	Mode of inheritance	Classificationa
ATP Binding Cassette Subfamily C Member 9	ABCC9	AD	Limited
Actin Alpha Cardiac Muscle 1	ACTCI	AD	Moderate
Ankyrin Repeat Domain 1	ANKRDI	AD	Limited
BAG Cochaperone 3	BAG3	AD	Definitive
Cysteine And Glycine Rich Protein 3	CSRP3	AD	Limited
Cardiotrophin 1	CTFI	AD	Limited
Desmin	DES	AD	Definitive
Desmoglein 2	DSG2	AD	Limited
Desmoplakin	DSP	AD	Definitiveb
Dystrobrevin Alpha	DTNA	AD	Limited
EYA Transcriptional Coactivator And Phosphatase 4	EYA4	AD	Limited
Filamin C	FLNC	AD	Definitive
GATA Zinc Finger Domain Containing 1	GATADI	AR	Limited
Integrin Linked Kinase	ILK	AD	Limited
Junctophilin 2	JPH2	AR	Moderate
Laminin Subunit Alpha 4	LAMA4	AD	Limited
LIM Domain Binding 3	LDB3	AD	Limited
Lamin A/C	LMNA	AD	Definitive
Leucine Rich Repeat Containing 10	LRRC10	AR	NKDR/AMO
MIB E3 Ubiquitin Protein Ligase 1	MIB1	AD	NKDR/AMO
Myosin Binding Protein C 3	MYBPC3	AD	Limited
Myosin Heavy Chain 6	MYH6	AD	Limited
Myosin Heavy Chain 7	MYH7	AD	Definitive
Myosin Light Chain 2	MYL2	AD	Limited
Myosin Light Chain 3	MYL3	AD	Disputed
Myopalladin	MYPN	AD	Limited
Nebulette	NEBL	AD	Limited
Nexilin F-Actin Binding Protein	NEXN	AD	Moderate
NK2 Homeobox 5	NKX2–5	AD	Limited
Natriuretic Peptide A	NPPA	AR	NKDR
Obscurin	OBSCN	AD	Limited
PDZ And LIM Domain 3	PDLIM3	AD	Disputed
Phospholamban	PLN	AD	Moderateb
Plakophillin 2	PKP2	AD	Disputed
Pleckstrin Homology And RUN Domain Containing M2	PLEKHM2	AR	Limited
PR/SET Domain 16	PRDM16	AD	Limited
Presenilin 1	PSEN1	AD	Disputed
Presenilin 2	PSEN2	AD	Limited
RNA Binding Motif Protein 20	RBM20	AD	Definitive
Sodium Voltage-Gated Channel Alpha Subunit 5	SCN5A	AD	Definitive
Sarcoglycan Delta	SGCD	AD	Limited
T-Box Transcription Factor 20	TBX20	AD	Limited
Titin-Cap	TCAP	AD	Limited
Transmembrane Protein 43	TMEM43	AD	Definitiveb
Troponin C1, Slow Skeletal And Cardiac Type	TNNC1	AD	Definitive
Troponin I3, Cardiac Type	TNNI3	AD	Moderate
TNNI3 Interacting Kinase	TNNI3K	AD	Limited
Troponin T2, Cardiac Type	TNNT2	AD	Definitive
Tropomyosin 1	TPM1	AD	Moderate
Titin	TTN	AD	Definitive
Vinculin	VCL	AD	Moderate

^ahttps://www.clinicalgenome.org/.

^bIn arrhythmogenic right ventricular cardiomyopathy.

Abbreviations: AD, autosomal dominant; AMO, animal model only; AR, autosomal recessive; NKDR, no known disease relationship.

TTN

The giant protein TTN forms the “elastic” filament of the sarcomere, essential for the mechanical compliance of the heart muscle (53). TTN is the largest macromolecule in the human body, composed of 27,000–33,000 amino acids, and is encoded by the gene TTN, which contains 363 exons. It is heavily expressed in striated muscle tissue including cardiac myocytes (54). Modifications at the genetic, transcriptional, and post-translational levels can lead to loss or gain of function and present as HCM, ACM, or DCM. Truncating TTN variants (TTNtv) are the most common causes of genetic DCM, accounting for 20–25% of all cases. Interestingly, at least a subset of peripartum cardiomyopathy cases are attributable to TTNtv (55, 56). Greater than 60,000 missense variants of TTN have been identified, but the clinical significance of missense variations in TTN remains unknown (9).

LMNA

LMNA missense and truncating mutations account for up to 8% of genetic DCM cases. Proteins Lamin A and Lamin C are encoded by the LMNA gene via differential splicing. Mutations in LMNA lead to phenotypic expressions including premature aging, myopathies, and DCM (57). LMNA mutations also lead to conduction abnormalities, atrial and ventricular arrythmias, and SCD, which usually precede DCM and have nearly complete penetrance by the seventh decade of life (58, 59).

PLN

The gene PLN encodes phospholamban, a small, 52–amino acid transmembrane protein that inhibits sarcoplasmic reticulum Ca²⁺ATPase in its unphosphorylated form. The R14del mutation of PLN is a founder mutation in the Netherlands and Germany, associated with ACM and DCM. Although, as with other DCM genes, mutations in PLN have variable penetrance, lethal arrythmias have been described (60, 61).

RBM20

The gene RBM20 encodes the RNA binding motif 20 protein, a 1,227–amino acid protein that is expressed in both the atria and the ventricles. Mutations in RBM20 are responsible for 1–5% of genetic DCM (62). RBM20 function regulates cardiac splicing including the splicing of TTN. Thus, given the downstream consequences of RBM20 mutations, their presentation may be similar to those of TTNtv.

SCN5A

The gene SCN5A encodes the alpha unit of the main cardiac sodium channel, Na_v1.5 (63). Mutations in this gene have been associated with ACM syndromes such as Brugada and long QT syndrome. Missense mutations in SCN5A have also been identified in DCM and carry a higher risk for arrythmias (46, 47).

Cytoskeletal Genes

Various genes encode cardiac cytoskeletal proteins and are associated with DCM. Mutations in the dystrophin gene can lead to DCM in Duchenne’s muscular dystrophy, which has an X-linked inheritance pattern (64). Mutations in the sarcoglycan genes can also produce cardiomyopathy associated with muscular dystrophy from sarcolemmal instability. The gene FLNC encodes filamin C, and mutations have been described in both ACM and DCM. Filamin C has a critical function in cardiomyocytes, interacting with actin, Z-disk, the desmosome, and the dystrophin complex (65). Truncation mutations in FLNC cause DCM that is associated with a high rate of arrythmias and SCD (66).

MANAGEMENT AND EMERGING THERAPIES

The current standard treatment of DCM with reduced LVEF (<40%) is directed toward heart failure, aiming to promote reverse remodeling, improve left ventricular dilation, and improve cardiac function. The current guidelines include a combination of four evidence-based therapies (67): (a) angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, or angiotensin receptor/neprilysin inhibitor; (b) evidence-based beta-blockers; (c) aldosterone antagonists; and (d) SGLT2 (sodium-glucose transport protein 2) inhibitors. Reassessment of LVEF is typically performed after 3 months of uninterrupted medical therapy. Patients with persistently low LVEF (<35%) are at high risk for SCD and benefit from ICD (67), while those with a wide QRS (>150 ms), left bundle branch block, and NYHA (New York Heart Association) class II benefit from cardiac resynchronization therapy. Finally, patients who have refractory heart failure should be referred for advanced therapies including left ventricular assist device and transplant.

However, the heart failure guidelines concept of “one size fits all” does not fully apply to DCM. Patients who are carriers of ACM gene mutations, such as mutations of FLNC, DSP, LMNA, or PLN, may require ICD based on arrhythmic risk factors (68) rather than based on severe left ventricular dysfunction. Also, the understanding of the genetic cause provides novel treatment opportunities. Emerging treatments for DCM include gene therapy for gene replacement (as in regenerative medicine advanced therapy trials) or direct genome editing by CRISPR/Cas-9 technology (currently being tested in vitro and in vivo), signaling pathway modifiers [REALM-DCM trial (NCT03439514)], and modifiers of myofilament function 9 (65, 69, 70).

CONCLUSIONS

DCM is defined as dilation or loss of one or both ventricles and remains a common disease process worldwide. Through advancements in next-generation sequencing and cardiac imaging, identification of genetic DCM has improved over the past couple decades, and precision medicine is now at the forefront of treatment for these patients and their families. In addition to standard treatment of heart failure and prevention of SCD, patients with genetic cardiomyopathy stand to benefit from gene mechanism–specific therapies.