Translational research is a crucial bridge in treating cardiomyopathy - Progress notes


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Diagnostic Virology Laboratory Newsletter

For members of the Texas Children's Hospital medical staff

Translational research is a crucial bridge
in treating cardiomyopathy

By Jeffrey A. Towbin, M.D.


Hypertropic cardiomyopathy

Cardiomyopathies in children represent a group of heart muscle disorders in which heart failure and sudden death are key clinical features that make these diseases high risk and high impact to physicians and other caregivers, as well as potentially tragic for families. The Heart Failure and Cardiomyopathy service at Texas Children’s Hospital is the largest cardiomyopathy program in pediatrics, seeing nearly 1,000 outpatient visits yearly and a high volume of inpatients. The outcomes of these children have improved over the past decade with the introduction of better therapies that are in part based on improved understanding of the molecular basis of these disorders. We use the clinical information and molecular genetic understanding gained in our laboratories (the Phoebe Willingham Muzzy Pediatric Molecular Cardiology Laboratory) to improve care in a bedside-to-bench-to-bedside approach, also known as translational research.

Cardiomyopathies are classified by function type
These disorders are classified into separate functional types including (1) dilated cardiomyopathy (DCM), (2) hypertrophic cardiomyopathy (HCM), (3) restrictive cardiomyopathy (RCM) and (4) arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C).¹ In addition, several other forms exist including left ventricular noncompaction (LVNC) and overlap disorders with mixed functional types.¹ The genetic basis of these disorders has been studied since the late 1980s and progress has been made for all forms.

Hypertrophic cardiomyopathy (HCM): This disorder, characterized by left ventricular hypertrophy, systolic hypercontractility and diastolic dysfunction associated with myofiber disarray, is the most common cause of sudden death in young healthy subjects in the United States, particularly during athletics.² Genetic linkage studies initially demonstrated genetic heterogeneity; in other words, multiple different genetic loci on multiple different chromosomes a variety of different mutant genes leading to clinically similar disorders.³ The first gene, b-myosin heavy chain, located on chromosome 14, was first identified by the Seidman laboratory in Boston and was followed by the identification of 9 other genes.³ In all cases, these genes encode sarcomeric proteins and include cardiac actin, cardiac troponin T, cardiac troponin I, a-tropomyosin, myosin binding protein C, titin, the essential and regulatory myosin light chains, and muscle LIM protein. Recently, non-sarcomere protein-encoding genes, including AMP kinase⁴ and the a-iduronidase gene causing Fabry disease,⁵ have been identified. We have recently identified mutations in the LAMP-2 gene^6,7 as well as mutations in G4.5^8,9 as causative of HCM.

Genotype-phenotype correlations were initially performed on b-myosin heavy chain, a-tropomyosin, myosin binding protein C, and cardiac troponin T and differences in age of onset, severity of hypertrophy, and survival was reported.^10,11 However, the studies were performed on a small number of genotyped individuals and may not be representative. Recently, Dr. Michael Ackerman’s laboratory has reported that many of these initial contributions are patient specific and not gene- or mutation-specific with clinical findings widely varying among mutated individuals.¹² Therefore, risk stratification on the basis of genotype is fraught with danger. In addition, no clinical test for genetic screening of the HCM-causing genes has been available to date, but a diagnostic laboratory in Boston is expected to begin to offer a fee-for-service genetics screen in the near future.

LAMP-2, the lysosome-associated membrane protein, causes Danon disease, which is characterized by cardiomyopathy and skeletal myopathy.^6,7 The hypertrophic form of disease later changes to dilated cardiomyopathy and heart failure. Ophthalmologic, neuromuscular and learning issues also occur. This disorder supports the need to consider multi-organ systemic abnormalities in HCM. Similarly, mutations in G4.5 result in multiple forms of cardiomyopathy, including HCM and is associated with skeletal myopathy as well.^8,9 Full-blown disease due to G4.5 mutations result in Barth syndrome that includes neutropenia and 3-methyl-glutaconic aciduria.^8,9 Both G4.5 and LAMP-2 genetic analysis is available in our diagnostic laboratory (John Welsh Cardiac DNA Diagnostic Laboratory); identifying these disorders would allow better care of the full set of disorders affecting these children.

Dilated cardiomyopathy (DCM): This disorder is characterized by a dilated left ventricle with systolic dysfunction.¹ Mitral regurgitation and ventricular arrhythmias may be associated. Studies of the genetic basis of DCM were relatively slow to get underway, in part due to the late recognition that this disease was genetically based. To date, nearly 20 genetic loci have been identified and 13 genes are now known.¹ We identified the first of these genes, dystrophin,¹³ the cause of X-linked cardiomyopathy and speculated that the final common pathway for DCM is the cytoskeleton and the link between the sarcolemma and sarcomere.¹⁴ Over the past decade, other mutations in a variety of genes have been identified including sarcolemmal/cytoskeletal genes (d-sarcoglycan, metavinculin, desmin), Z-disk protein-encoding genes (ZASP, a-actinin-2, MLP, titin), and sarcomeric protein-encoding genes (b-myosin heavy chain, a-tropomyosin, myosin binding protein-C, troponin T).^1,14,17 In addition, mutations in lamin A/C, a nuclear envelope protein and G4.5/tafazzin (with uncertain function), have been identified.^8,15 Our laboratory identified many of these genes (delta-sarcoglycan, ZASP, a-actinin-2, MLP). Unfortunately, to date little has been done regarding genotype:phenotype correlations.

Additional causes of DCM have been identified as well. In babies, abnormalities of mitochondrial function due to mitochondrial DNA (mtDNA) mutations, import protein-encoding genes of the genome and mutations in metabolic protein-encoding genes, particularly those of the fatty acid oxidation pathway such as the acyldehydrogenese genes (MCAD, LCAD, VLCAD and the trifunctional protein) are important causes, along with Barth syndrome, caused by mutations in G4.5 (in boys).^9,17 As noted for HCM, skeletal muscle abnormalities are common in these patients as well.

Non-genetic causes of disease are also relatively common. In young children, myocarditis is particularly prevalent although this certainly occurs throughout childhood and adolescence. The common viral causes of myocarditis and DCM include adenovirus, parvovirus and the enteroviruses, particularly coxsackieviruses B3 and B4.²⁰ Recently, we demonstrated that over 35 percent of cases of myocarditis can be identified etiologically using polymerase chain reaction (PCR) analysis of myocardial specimens²⁰ and previously showed the utility of PCR evaluation of tracheal aspirates in intubated children “too sick to biopsy”.²¹ Currently, we offer fee-for-service screening of viral genome in our diagnostic laboratory (CLIA-approved John Welsh Cardiac DNA Diagnostic Laboratory) with rapid report turn-around, as well as DNA screening for neonatal gene causes of DCM including G4.5 and mitochondrial proteins SCO2 and SURF1 (See website http://www.bcm.tmc.edu/pedi/cardio/research/welsh.html).

Left ventricular noncompaction (LVNC): This disorder, characterized by deep trabeculations in the LV endocardium, particularly in the apex and free wall, apical hypertrophy, with intermittent systolic dysfunction is an under-recognized disorder.²² The disorder has an unpredictable course, with some patients developing progressive heart failure necessitating transplantation, others developing an “undulating phenotype” in which the echocardiographic features alternate between a DCM-like disorder and an HCM-like disorder, and still others having primarily diastolic dysfunction. In some cases, LVNC is associated with systemic disorders such as Barth syndrome, mitochondrial or metabolic disorders.^9-22

The inheritance pattern in LVNC varies. The majority of cases are transmitted as autosomal dominant traits, but X-linked and mitochondrial transmission is also relatively common. We have identified the two autosomal dominant genes thus far reported
(a-dystrobrevin, ZASP)^9,17 and an X-linked gene, G4.5^9,23 (the gene responsible for Barth syndrome) has also been identified. As previously noted, mutation screening for G4.5 in young males with LVNC is available in our diagnostic laboratory as a fee-for-service test (John Welsh Pediatric Cardiac Diagnostic Laboratory; see http://www.bcm.tmc.edu/pedi/cardio/research/welsh.html). These children also commonly have skeletal myopathy as well as other systemic abnormalities. Most of these children do well; however, very young children (<1 year of age) with metabolic instability tend to have the worst clinical course.

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C): This disorder is characterized by a thin-walled dilated RV with fibrofatty infiltration of the RV. These patients suffer from VT with a left bundle branch block morphology, right heart failure, syncope and sudden death.^24-26 ARVD/C is considered the most common cause of sudden death in young healthy adults and athletes in Italy but has not been considered to be as common in the United States.

ARVD/C is inherited as an autosomal dominant disorder. Multiple genetic loci have been identified but only two genes have been discovered, the ryanodine receptor (RYR2, the same gene responsible for catecholaminergic polymorphic VT)²⁷ and desmoplakin,²⁸ an important protein of the adherens junction and desmosomes. An autosomal recessive disorder with complex phenotype, Naxos syndrome, characterized by palmoplantar keratoderma, wooly hair, and ARVD/C, initially described in an inbred population on the Greek Island of Naxos,²⁹ results from homozygous mutations in another adherens junction protein called plakoglobin²⁹ A similar autosomal recessive disorder in which the RV and LV are affected, called Carvajal syndrome, is due to homozygous mutations in desmoplakin.³⁰ The search for the remaining genes, as well as for improved diagnostic and therapeutic approaches, is under way, funded by a grant from NIH/NHLBI funding for the ARVD Registry. This program includes the Clinical Coordinating Center in Tucson, Arizona (Dr. Frank Marcus), Data Coordinating Center in Rochester, New York (Dr. Wojciech Zareba) and Genetic Center in Houston, Texas (Dr. Jeffrey Towbin). We have recently identified mutations in demosomal protein-encoding genes (desmoplakin, plakophilin-2) as disease-causing.

Therapies based on molecular genetics
The genetic studies of cardiomyopathies have identified several novel new concepts. Firstly, it appears that the disease-causing genes interrupt molecular and protein pathways that, when disrupted, cause the clinical phenotype.¹⁴ Hence, HCM is a disease of the sarcomere, DCM is a disease of the sarcomere-sarcolemma link, and ARVD is a disease of the desmosome. Understanding these pathways could enable targeted therapies to be developed.

In DCM, disruption of the sarcolemma-sarcomere link plus the addition of mechanical stress, appears to lead to the development of clinical disease over time. In order to test this hypothesis, reduction of mechanical should help to improve (i.e., reverse remodel) ventricular size and function. Reduction of mechanical stress is most effectively attained by use of ventricular assist device (VAD) therapy. Vatta et al reported work performed by our group in which VAD therapy was used in DCM.^31,32 Pre-VAD therapy, dystrophin protein was abnormal in most patients with loss of the amino (N)-terminus noted. Since the N-terminus of dystrophin binds to the sarcomere via the actin cytoskeleton, this loss of linkage to the sarcomere appears to be important. VAD therapy was shown to normalize N-terminal dystrophin and in some patients normalizes (i.e., reverse remodels) the LV.^33,34 Since dystrophin cleavage also causes heart dysfunction in coxsackie myocarditis, understanding the role of dystrophin-targeted therapy could improve outcomes in DCM and myocarditis patients. Similarly, it is possible that ACE inhibitor/b-blocker therapy is efficacious in these patients due to reduction in mechanical stress and dystrophin cleavage, similar to that seen in LVAD therapy.

Therefore, the information gained at the bench has made a significant impact in clinical care in cardiomyopathies. In the future, improved translation of research and clinical experience/expertise in cardiomyopathy is likely to reduce morbidity, mortality and the need for transplantation.

Jeffrey A. Towbin, M.D. is professor & chief, Pediatric Cardiology; director, Phoebe Willingham Muzzy Pediatric Molecular Cardiology Laboratory; Texas Children's Hospital Foundation chair of Pediatric Cardiac Research; director, Heart Failure Program; and co-director, Cardiovascular Genetics Clinic, Baylor College of Medicine, Texas Children's Hospital

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