Hypertrophic Cardiomyopathy: From “Heart Tumour” to a Complex Molecular Genetic Disorder

Next Generation Sequencing and Linkage Analysis for the Molecular Diagnosis of a Novel Overlapping Syndrome Characterized by Hypertrophic Cardiomyopathy and Typical Electrical Instability of Brugada Syndrome

BACKGROUND

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant inherited disorder; mutations in at least 20 genes have been associated. Brugada syndrome (BrS) is an autosomal dominant inherited disorder caused by mutations mainly in theSCN5Agene. A new clinical entity that consists of HCM, typical electrical instability of BrS and sudden death (SD), is described.

METHODS AND RESULTS

The family was constituted by 7 members, 4 of who presented clinical features of HCM and electrical instability of BrS. The clinical presentation of proband was ventricular fibrillation. All members were clinically evaluated by physical examination, 12-lead electrocardiography, 2-dimensional echocardiography, stress test, electrocardiogram Holter, flecainide test, and electrophysiological study. An integrated linkage analysis and next generation sequencing (NGS) approach was used to identify the causative mutation. Linkage with the α-tropomyosin (TPM1) gene on chromosome 15q22 was identified. The NGS study identified a missense mutation within theTPM1gene (c.574G>A; p.E192K), exactly located in a binding domain with polycystin-2 protein. No other pathogenic mutations were identified.

CONCLUSIONS

This is the first report of an association between HCM and BrS, and the first to use a combined approach of linkage and NGS to identify a causative mutation in SD. The present study expands the clinical spectrum of disorders associated with theTPM1gene and may be useful to report novel mechanisms of electrical instability in HCM and BrS.

Mutations in alpha-actinin-2 cause hypertrophic cardiomyopathy: a genome-wide analysis

OBJECTIVES

This study describes a genome-wide linkage analysis of a large family with clinically heterogeneous hypertrophic cardiomyopathy (HCM).

BACKGROUND

Familial HCM is a disorder characterized by genetic heterogeneity. In as many as 50% of HCM cases, the genetic cause remains unknown, suggesting that other genes may be involved.

METHODS

Clinical evaluation, including clinical history, physical examination, electrocardiography, and 2-dimensional echocardiography, was performed, and blood was collected from family members (n = 23) for deoxyribonucleic acid analysis. The family was genotyped with markers from the 10-cM AB PRISM Human Linkage mapping set (Applied Biosystems, Foster City, California), and 2-point linkage analysis was performed.

RESULTS

Affected family members showed marked clinical diversity, ranging from asymptomatic individuals to those with syncope, heart failure, and premature sudden death. The disease locus for this family was mapped to chromosome 1q42.2-q43, near the marker D1S2850 (logarithm of odds ratio = 2.82, theta = 0). A missense mutation, Ala119Thr, in the alpha-actinin-2 (ACTN2) gene was identified that segregated with disease in the family. An additional 297 HCM probands were screened for mutations in the ACTN2 gene using high-resolution melt analysis. Three causative ACTN2 mutations, Thr495Met, Glu583Ala, and Glu628Gly, were identified in an additional 4 families (total 1.7%) with HCM.

CONCLUSIONS

This is the first genome-wide linkage analysis that shows mutations in ACTN2 cause HCM. Mutations in genes encoding Z-disk proteins account for a small but significant proportion of genotyped HCM families.

Combinatorial effects of double cardiomyopathy mutant alleles in rodent myocytes: a predictive cellular model of myofilament dysregulation in disease

Inherited cardiomyopathy (CM) represents a diverse group of cardiac muscle diseases that present with a broad spectrum of symptoms ranging from benign to highly malignant. Contributing to this genetic complexity and clinical heterogeneity is the emergence of a cohort of patients that are double or compound heterozygotes who have inherited two different CM mutant alleles in the same or different sarcomeric gene. These patients typically have early disease onset with worse clinical outcomes. Little experimental attention has been directed towards elucidating the physiologic basis of double CM mutations at the cellular-molecular level. Here, dual gene transfer to isolated adult rat cardiac myocytes was used to determine the primary effects of co-expressing two different CM-linked mutant proteins on intact cardiac myocyte contractile physiology. Dual expression of two CM mutants, that alone moderately increase myofilament activation, tropomyosin mutant A63V and cardiac troponin mutant R146G, were shown to additively slow myocyte relaxation beyond either mutant studied in isolation. These results were qualitatively similar to a combination of moderate and strong activating CM mutant alleles alphaTmA63V and cTnI R193H, which approached a functional threshold. Interestingly, a combination of a CM myofilament deactivating mutant, troponin C G159D, together with an activating mutant, cTnIR193H, produced a hybrid phenotype that blunted the strong activating phenotype of cTnIR193H alone. This is evidence of neutralizing effects of activating/deactivating mutant alleles in combination. Taken together, this combinatorial mutant allele functional analysis lends molecular insight into disease severity and forms the foundation for a predictive model to deconstruct the myriad of possible CM double mutations in presenting patients.

Role of animal models in HCM research

Hypertrophic cardiomyopathy (HCM) is a complex cardiovascular genetic disorder characterized by marked clinical and genetic heterogeneity. Major advances have been made in the clinical characterization of patients with HCM and in identifying causative gene mutations. However, many questions remain regarding the underlying disease mechanisms. Furthermore, in a disease where no pharmacological treatments currently exists which can either prevent or cause regression of disease, processes to identify novel therapies are the crucial next steps. Animal models of HCM have already proved to be universally useful in confirming gene causation and dissecting out key molecular pathways involved in the development of HCM and its sequelae, including heart failure and sudden death. These findings have led to studies in animal models investigating novel therapeutic approaches in HCM, specifically targeting the development and progression of cardiac hypertrophy, fibrosis, and heart failure. This review will provide a brief summary of some of the key animal models of HCM and how these models have been utilized to understand disease mechanisms and to investigate new potential therapies. Ongoing studies using animal models of HCM will lead to a greater understanding of disease pathogenesis and will facilitate the translation of these findings to improved clinical outcomes in HCM patients.

Severe heart failure and early mortality in a double-mutation mouse model of familial hypertrophic cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy (FHC) is characterized by genetic and clinical heterogeneity. Five percent of FHC families have 2 FHC-causing mutations, which results in earlier disease onset, increased cardiac dysfunction, and a higher incidence of sudden death events. These observations suggest a relationship between the number of gene mutations and phenotype severity in FHC.

METHODS AND RESULTS

We sought to develop, characterize, and investigate the pathogenic mechanisms in a double-mutant murine model of FHC. This model (designated TnI-203/MHC-403) was generated by crossbreeding mice with the Gly203Ser cardiac troponin I (TnI-203) and Arg403Gln alpha-myosin heavy chain (MHC-403) FHC-causing mutations. The mortality rate in TnI-203/MHC-403 mice was 100% by age 21 days. At age 14 days, TnI-203/MHC-403 mice developed a significantly increased ratio of heart weight to body weight, marked interstitial myocardial fibrosis, and increased expression of atrial natriuretic factor and brain natriuretic peptide compared with nontransgenic, TnI-203, and MHC-403 littermates. By age 16 to 18 days, TnI-203/MHC-403 mice rapidly developed a severe dilated cardiomyopathy and heart failure, with inducibility of ventricular arrhythmias, which led to death by 21 days. Downregulation of mRNA levels of key regulators of Ca(2+) homeostasis in TnI-203/MHC-403 mice was observed. Increased levels of phosphorylated STAT3 were observed in TnI-203/MHC-403 mice and corresponded with the onset of disease, which suggests a possible cardioprotective response.

CONCLUSIONS

TnI-203/MHC-403 double-mutant mice develop a severe cardiac phenotype characterized by heart failure and early death. The presence of 2 disease-causing mutations may predispose individuals to a greater risk of developing severe heart failure than human FHC caused by a single gene mutation.

Mutation analysis of the natriuretic peptide precursor B (NPPB) gene in patients with hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a genetically heterogenous disease caused by mutations in genes that primarily encode sarcomeric proteins. No mutation is identified in up to 40% of HCM patients, suggesting other causative genes exist. Natriuretic peptide precursor B (NPPB; also known as "BNP") is a cardiac hormone involved in body fluid homeostasis and cardiac myocyte growth. NPPB concentrations are markedly increased in patients with ventricular hypertrophy, and it is therefore possible mutations in the NPPB gene could cause HCM.

METHODS

Genomic DNA was extracted from peripheral blood in 238 consecutive probands with HCM. The coding regions and intron/exon boundaries in the NPPB gene were amplified by PCR, and products were screened for sequence variants using high-performance liquid chromatography, followed by direct DNA sequencing.

RESULTS

Four sequence variants in the NPPB gene were identified in 9 of the 238 probands screened. Two of the variants were intronic, one was a synonymous variant at codon 79, and the final variant resulted in an amino acid substitution from arginine to histidine at codon 47 (Arg47His). The Arg47His variant was identified in a control population consisting of 204 chromosomes at an allelic frequency of 0.5%, and is therefore unlikely to cause disease.

CONCLUSION

No disease causing mutations were identified in the NPPB gene in this cohort, indicating that mutations in this gene are unlikely to be responsible for HCM.

GENES, CALCIUM AND MODIFYING FACTORS IN HYPERTROPHIC CARDIOMYOPATHY

1. Familial hypertrophic cardiomyopathy (FHC) is a primary disorder of the myocardium characterized by remarkable diversity in clinical presentations, ranging from no symptoms to severe heart failure and sudden cardiac death. 2. Over the past 15 years, at least 11 genes have been identified, defects of which cause FHC. Most of these genes encode proteins that comprise the basic contractile unit of the heart (i.e. the sarcomere). 3. Genetic studies are now beginning to have a major impact on the diagnosis in FHC, as well as in guiding treatment and preventative strategies. Although much is known about which genes cause disease, relatively little is known about the molecular steps leading from the gene defect to the clinical phenotype and what factors modify the expression of the mutant genes. 4. Concurrent studies in cell culture and animal models of FHC are now beginning to shed light on the signalling pathways involved in FHC and the role of both environmental and genetic modifying factors. Calcium dysregulation appears to be important in the pathogenesis of FHC. 5. Understanding these basic molecular mechanisms will ultimately improve our knowledge of the basic biology of heart muscle function and will therefore provide new avenues for diagnosis and treatment not only for FHC, but also for a range of human cardiovascular diseases.

Long-term follow-up of patients with obstructive hypertrophic cardiomyopathy treated with dual-chamber pacing

In this study, patients with obstructive hypertrophic cardiomyopathy (HC) were treated with dual-chamber pacemaker therapy. Long-term follow-up analysis showed that dual-chamber pacemaker therapy in selected patients resulted in a significant reduction in symptoms and in the left ventricular outflow tract gradient, which was maintained up to 10 years after implantation. Dual-chamber pacing is of potential long-term benefit in selected groups of patients with obstructive HC.