Clinical features of hypertrophic cardiomyopathy caused by mutation of a "hot spot" in the alpha-tropomyosin gene

Obstructive hypertrophic cardiomyopathy: from genetic insights to a multimodal therapeutic approach with mavacamten, aficamten, and beyond

BACKGROUND

A cardiac condition marked by excessive growth of heart muscle cells, hypertrophic cardiomyopathy (HCM) is a complex genetic disorder characterized by left ventricular hypertrophy, microvascular ischemia, myocardial fibrosis, and diastolic dysfunction. Obstructive hypertrophic cardiomyopathy (oHCM), a subset of HCM, involves significant obstruction in the left ventricular outflow tract (LVOT), leading to symptoms like dyspnea, fatigue, and potentially life-threatening cardiac events. With advancements in genetic understanding and the introduction of novel pharmacologic agents, including cardiac myosin inhibitors like mavacamten and aficamten, there is a paradigm shift in the therapeutic approach to oHCM.

MAIN BODY

The underlying mechanisms of HCM are closely tied to genetic mutations affecting sarcomere proteins, particularly those encoded by the MYH7 and MYBPC3 genes. These mutations lead to disrupted sarcomere function, resulting in hypertrophic changes and LVOT obstruction. While genetic heterogeneity is a hallmark of HCM, clinical diagnosis relies heavily on imaging techniques such as Echocardiography and cardiac magnetic resonance imaging to assess the extent of hypertrophy and obstruction. Current pharmacological management of obstructive HCM (oHCM) focuses on alleviating symptoms rather than modifying disease progression. Beta-blockers and calcium channel blockers are primary treatment options, although their effectiveness varies among patients. Recent clinical trials have highlighted the potential of novel cardiac myosin inhibitors, including mavacamten and aficamten, in enhancing exercise capacity, reducing LVOT obstruction, and improving overall cardiac function. These innovative agents represent a significant breakthrough in targeting the fundamental pathophysiological mechanisms driving oHCM. A comprehensive literature review was conducted, utilizing top-tier databases such as PubMed, Scopus, and Google Scholar, to compile an authoritative and up-to-date overview of the current advancements in the field. This review sheds light on the updated 2024 American Heart Association (AHA) guidelines for HCM management, emphasizing the treatment cascade and tailored management for each stage of oHCM. By introducing a new paradigm for personalized medicine in oHCM, this research leverages advanced genomics, biomarkers, and imaging techniques to optimize treatment strategies.

CONCLUSIONS

The introduction of cardiac myosin inhibitors heralds a new era in the management of oHCM. By directly targeting the molecular mechanisms underpinning the disease, these novel therapies offer improved symptom relief and functional outcomes. Ongoing research into the genetic basis of HCM and the development of targeted treatments holds promise for further enhancing patient care. Future studies should continue to refine these therapeutic strategies and explore their long-term benefits and potential in diverse patient populations. This review makes a significant contribution to the field by synthesizing the most recent AHA guidelines, emphasizing the crucial role of tailored management strategies in optimizing outcomes for patients with oHCM, and promoting the incorporation of cutting-edge genomics and imaging modalities to enhance personalized care.

Thin filament cardiomyopathies: A review of genetics, disease mechanisms, and emerging therapeutics

All muscle contraction occurs due to the cyclical interaction between sarcomeric thin and thick filament proteins within the myocyte. The thin filament consists of the proteins actin, tropomyosin, Troponin C, Troponin I, and Troponin T. Mutations in these proteins can result in various forms of cardiomyopathy, including hypertrophic, restrictive, and dilated phenotypes and account for as many as 30% of all cases of inherited cardiomyopathy. There is significant evidence that thin filament mutations contribute to dysregulation of Ca2+ within the sarcomere and may have a distinct pathomechanism of disease from cardiomyopathy associated with thick filament mutations. A number of distinct clinical findings appear to be correlated with thin-filament mutations: greater degrees of restrictive cardiomyopathy and relatively less left ventricular (LV) hypertrophy and LV outflow tract obstruction than that seen with thick filament mutations, increased morbidity associated with heart failure, increased arrhythmia burden and potentially higher mortality. Most therapies that improve outcomes in heart failure blunt the neurohormonal pathways involved in cardiac remodeling, while most therapies for hypertrophic cardiomyopathy involve use of negative inotropes to reduce LV hypertrophy or septal reduction therapies to reduce LV outflow tract obstruction. None of these therapies directly address the underlying sarcomeric dysfunction associated with thin-filament mutations. With mounting evidence that thin filament cardiomyopathies occur through a distinct mechanism, there is need for therapies targeting the unique, underlying mechanisms tailored for each patient depending on a given mutation.

Cardiomyopathies: An Overview

Background: Cardiomyopathies are a heterogeneous group of pathologies characterized by structural and functional alterations of the heart. Aims: The purpose of this narrative review is to focus on the most important cardiomyopathies and their epidemiology, diagnosis, and management. Methods: Clinical trials were identified by Pubmed until 30 March 2021. The search keywords were “cardiomyopathies, sudden cardiac arrest, dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy, arrhythmogenic cardiomyopathy (ARCV), takotsubo syndrome”. Results: Hypertrophic cardiomyopathy (HCM) is the most common primary cardiomyopathy, with a prevalence of 1:500 persons. Dilated cardiomyopathy (DCM) has a prevalence of 1:2500 and is the leading indication for heart transplantation. Restrictive cardiomyopathy (RCM) is the least common of the major cardiomyopathies, representing 2% to 5% of cases. Arrhythmogenic cardiomyopathy (ARCV) is a pathology characterized by the substitution of the myocardium by fibrofatty tissue. Takotsubo cardiomyopathy is defined as an abrupt onset of left ventricular dysfunction in response to severe emotional or physiologic stress. Conclusion: In particular, it has been reported that HCM is the most important cause of sudden death on the athletic field in the United States. It is needless to say how important it is to know which changes in the heart due to physical activity are normal, and when they are pathological.

Genotype-phenotype correlations in hypertrophic cardiomyopathy: a multicenter study in Portugal and Spain of the TPM1 p.Arg21Leu variant

INTRODUCTION AND OBJECTIVES

TPM1 is one of the main hypertrophic cardiomyopathy (HCM) genes. Clinical information on carriers is relatively scarce, limiting the interpretation of genetic findings in individual patients. Our aim was to establish genotype-phenotype correlations of the TPM1 p.Arg21Leu variant in a serie of pedigrees.

METHODS

TPM1 was evaluated by next-generation sequencing in 10 561 unrelated probands with inherited heart diseases. Familial genetic screening was performed by the Sanger method. We analyzed TPM1 p.Arg21Leu pedigrees for cosegregation, clinical characteristics, and outcomes. We also estimated the geographical distribution of the carrier families in Portugal and Spain.

RESULTS

The TPM1 p.Arg21Leu variant was identified in 25/4099 (0.61%) HCM-cases, and was absent in 6462 control individuals with other inherited cardiac phenotypes (P<.0001). In total, 83 carriers (31 probands) were identified. The combined LOD score for familial cosegregation was 3.95. The cumulative probability of diagnosis in carriers was 50% at the age of 50 years for males, and was 25% in female carriers. At the age of 70 years, 17% of males and 46% of female carriers were unaffected. Mean maximal left ventricular wall thickness was 21.4 ±7.65mm. Calculated HCM sudden death risk was low in 34 carriers (77.5%), intermediated in 8 (18%), and high in only 2 (4.5%). Survival free of cardiovascular death or heart transplant was 87.5% at 50 years. Six percent of carriers were homozygous and 18% had an additional variant. Family origin was concentrated in Galicia, Extremadura, and northern Portugal, suggesting a founder effect.

CONCLUSIONS

TPM1 p.Arg21Leu is a pathogenic HCM variant associated with late-onset/incomplete penetrance and a generally favorable prognosis.

The effect of tropomyosin variants on cardiomyocyte function and structure that underlie different clinical cardiomyopathy phenotypes

Background - Variants within the alpha-tropomyosin gene (TPM1) cause dominantly inherited cardiomyopathies, including dilated (DCM), hypertrophic (HCM) and restrictive (RCM) cardiomyopathy. Here we investigated whether TPM1 variants observed in DCM and HCM patients affect cardiomyocyte physiology differently. Methods - We identified a large family with DCM carrying a recently identified TPM1 gene variant (T201M) and a child with RCM with compound heterozygote TPM1 variants (E62Q and M281T) whose family members carrying single variants show diastolic dysfunction and HCM. The effects of TPM1 variants (T201M, E62Q or M281T) and of a plasmid containing both the E62Q and M281T variants on single-cell Ca²⁺ transients (CaT) in HL-1 cardiomyocytes were studied. To define toxic threshold levels, we performed dose-dependent transfection of TPM1 variants. In addition, cardiomyocyte structure was studied in human cardiac biopsies with TPM1 variants. Results - Overexpression of TPM1 variants led to time-dependent progressive deterioration of CaT, with the smallest effect seen for E62Q and larger and similar effects seen for the T201M and M281T variants. Overexpression of E62Q/M281T did not exacerbate the effects seen with overexpression of a single TPM1 variant. T201M (DCM) replaced endogenous tropomyosin dose-dependently, while M281T (HCM) did not. Human cardiac biopsies with TPM1 variants revealed loss of sarcomeric structures. Conclusion - All TPM1 variants result in reduced cardiomyocyte CaT amplitudes and loss of sarcomeric structures. These effects may underlie pathophysiology of different cardiomyopathy phenotypes.

Divergent effects of adrenaline in human induced pluripotent stem cell-derived cardiomyocytes obtained from hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common inherited cardiac disease that affects the heart muscle with diverse clinical outcomes. HCM can cause sudden cardiac death (SCD) during or immediately after mild to rigorous physical activity in young patients. However, the mechanism causing SCD as a result of exercise remains unknown, but exercise-induced ventricular arrhythmias are thought to be responsible for this fatal consequence. To understand the disease mechanism behind HCM in a better way, we generated patient-specific induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from HCM patients carrying either the MYBPC3-Gln1061X or TPM1-Asp175Asn mutation. We extensively investigated the effects of low to high concentrations of adrenaline on action potential characteristics, and the occurrence of arrhythmias in the presence of various concentrations of adrenaline and in wash-out condition. We classified and quantified different types of arrhythmias observed in hiPSC-CMs, and found that the occurrence of arrhythmias was dependent on concentrations of adrenaline and positions of mutations in genes causing HCM. In addition, we observed ventricular tachycardia types of arrhythmias in hiPSC-CMs carrying the TPM1-Asp175Asn mutation. We additionally examined the antiarrhythmic potency of bisoprolol in HCM-specific hiPSC-CMs. However, bisoprolol could not reduce the occurrence of arrhythmias during administration or during the wash-out condition of adrenaline in HCM-specific hiPSC-CMs. Our study demonstrates hiPSC-CMs as a promising tool for studying HCM. The experimental design used in this study could be suitable and beneficial for studying other components and drugs related to cardiac disease in general.

Summary: Different concentrations of adrenaline have divergent effects during and immediately after administration in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from hypertrophic cardiomyopathy (HCM) patients. Bisoprolol could not reduce the arrhythmias in HCM-specific hiPSC-CMs.

Hypertrophic Cardiomyopathy: Genetics, Pathogenesis, Clinical Manifestations, Diagnosis, and Therapy

Hypertrophic cardiomyopathy (HCM) is a genetic disorder that is characterized by left ventricular hypertrophy unexplained by secondary causes and a nondilated left ventricle with preserved or increased ejection fraction. It is commonly asymmetrical with the most severe hypertrophy involving the basal interventricular septum. Left ventricular outflow tract obstruction is present at rest in about one third of the patients and can be provoked in another third. The histological features of HCM include myocyte hypertrophy and disarray, as well as interstitial fibrosis. The hypertrophy is also frequently associated with left ventricular diastolic dysfunction. In the majority of patients, HCM has a relatively benign course. However, HCM is also an important cause of sudden cardiac death, particularly in adolescents and young adults. Nonsustained ventricular tachycardia, syncope, a family history of sudden cardiac death, and severe cardiac hypertrophy are major risk factors for sudden cardiac death. This complication can usually be averted by implantation of a cardioverter-defibrillator in appropriate high-risk patients. Atrial fibrillation is also a common complication and is not well tolerated. Mutations in over a dozen genes encoding sarcomere-associated proteins cause HCM. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin-binding protein C, respectively, are the 2 most common genes involved, together accounting for ≈50% of the HCM families. In ≈40% of HCM patients, the causal genes remain to be identified. Mutations in genes responsible for storage diseases also cause a phenotype resembling HCM (genocopy or phenocopy). The routine applications of genetic testing and preclinical identification of family members represents an important advance. The genetic discoveries have enhanced understanding of the molecular pathogenesis of HCM and have stimulated efforts designed to identify new therapeutic agents.

Proteomic Analysis of Endothelin-1 Targets in the Regulation of Cardiomyocyte Proliferation

Hypoxia is a fetal stressor that leads to the production of endothelin-1 (ET-1). Previous work has shown that ET-1 treatment leads to the premature terminal differentiation of fetal cardiomyocytes. However, the precise mechanism is unknown. We tested the hypothesis that the fetal cardiomyocyte proteome will be greatly altered due to ET-1-treatment, which reveals a potential molecular mechanism of ET-1-induced terminal differentiation. Over a thousand proteins were detected in the fetal cardiomyocytes and among them 75 proteins were significantly altered due to ET-1 treatment. Using IPA pathway analysis, the merged network depicted several key proteins that appeared to be involved in regulating proliferation, including: EED, UBC, ERK1/2, MAPK, Akt, and EGFR. EED protein, which is associated with regulating proliferation via epigenetic mechanisms, is of particular interest. Herein we propose a model of the molecular mechanism by which ET-1 induced cardiomyocyte terminal differentiation occurs.

Phenotype and prognostic correlations of the converter region mutations affecting the β myosin heavy chain

Objectives

The prognostic value of genetic studies in cardiomyopathies is still controversial. Our objective was to evaluate the outcome of patients with cardiomyopathy with mutations in the converter domain of β myosin heavy chain (MYH7).

Methods

Clinical characteristics and survival of 117 affected members with mutations in the converter domain of MYH7 were compared with 409 patients described in the literature with mutations in the same region.

Results

Twenty-five mutations were evaluated (9 in our families including 3 novel (Ile730Asn, Asp717Gly and Arg719Pro)). Clinical diagnoses were hypertrophic (n=407), dilated (n=15), non-compaction (n=4) and restrictive (n=5) cardiomyopathies, unspecified cardiomyopathy (n=11), sudden death (n=50) and 35 healthy carriers. One hundred eighty-four had events (cardiovascular death or transplant). Median event-free survival was 50±2 years in our patients and 53±3 years in the literature (p=0.27). There were significant differences in the outcome between mutation: Ile736Thr had fewer events than other mutations in the region (p=0.01), while Arg719Gln (p<0.01) had reduced event-free survival.

Conclusions

Mutations in the converter region are generally associated with adverse prognosis although there are differences between mutations. The identification of a mutation in this particular region provides important prognostic information that should be considered in the clinical management of affected patients.

A new common mutation in the cardiac beta-myosin heavy chain gene in Finnish patients with hypertrophic cardiomyopathy

BACKGROUND

In the nationwide FinHCM Study including 306 Finnish patients with hypertrophic cardiomyopathy (HCM), we have previously identified two founder mutations in the alpha-tropomyosin (TPM1-D175N) and myosin-binding protein C (MYBPC3-Q1061X) genes, accounting for 18% of all cases. Objective. To screen additional mutations, previously identified in eastern Finnish cohorts with HCM, in the FinHCM Study population.

PATIENTS AND METHODS

Ten mutations in the beta-myosin heavy chain gene (MYH7), TPM1, and MYBPC3 were screened.

RESULTS

MYH7-R1053Q was found in 17 of 306 patients (5.6%). No carriers of MYH7-R719W or N696S were found. A novel TPM1-D175G mutation was found in a single patient. MYBPC3 mutations were found in 14 patients: IVS5-2A-C in two, IVS14-13G-A in two, K811del in six, and A851insT in four patients. Altogether, a HCM-causing mutation was identified in 32 patients, accounting for 10.5% of all cases. In addition, two MYBPC3 variants R326Q and V896M with uncertain pathogenicity were found in eight and in 10 patients, respectively.

CONCLUSION

Combining the present findings with our previous results, a causative mutation was identified in 28% of the FinHCM cohort. MYH7-R1053Q was the third most common mutation, and should be screened in all new cases of HCM in Finland.

Familial hypertrophic cardiomyopathy: clinical features, molecular genetics and molecular genetic testing

Hypertrophic cardiomyopathy is a Mendelian disease characterized by cardiac hypertrophy. It has a prevalence of 1:500 individuals and is the most common cause of sudden death in the young. Other complications include heart failure and the need for heart transplantation. Hypertrophic cardiomyopathy is due to sarcomeric gene mutations, however, phenocopies with myocardial hypertrophy can be due to triplet-repeat syndromes (Friedreich ataxia and myotonic dystrophy), mitochondrial and metabolic diseases. In a peculiar form associated with Wolf-Parkinson-White syndrome, the disease is caused by mutations in the gamma2 regulatory subunit of the AMP-activated protein kinase gene, leading to a glycogen storage cardiomyopathy. In spite of the growing knowledge about the molecular basis of hypertrophic cardiomyopathy, very little is still known about the genotype-phenotype correlations and their clinical implications. In this review, the clinical and molecular genetics of hypertrophic cardiomyopathy are described.

A study of tropomyosin's role in cardiac function and disease using thin-filament reconstituted myocardium

Tropomyosin (Tm) is the key regulatory component of the thin-filament and plays a central role in the cardiac muscle's cooperative activation mechanism. Many mutations of cardiac Tm are related to hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and left ventricular noncompaction (LVNC). Using the thin-filament extraction/reconstitution technique, we are able to incorporate various Tm mutants and protein isoforms into a muscle fiber environment to study their roles in Ca(2+) regulation, cross-bridge kinetics, and force generation. The thin-filament reconstitution technique poses several advantages compared to other in vitro and in vivo methods: (1) Tm mutants and isoforms are placed into the real muscle fiber environment to exhibit their effect on a level much higher than simple protein complexes; (2) only the primary and immediate effects of Tm mutants are studied in the thin-filament reconstituted myocardium; (3) lethal mutants of Tm can be studied without causing a problem; and (4) inexpensive. In transgenic models, various secondary effects (myocyte disarray, ECM fibrosis, altered protein phosphorylation levels, etc.) also affect the performance of the myocardium, making it very difficult to isolate the primary effect of the mutation. Our studies on Tm have demonstrated that: (1) Tm positively enhances the hydrophobic interaction between actin and myosin in the "closed state", which in turn enhances the isometric tension; (2) Tm's seven periodical repeats carry distinct functions, with the 3rd period being essential for the tension enhancement; (3) Tm mutants lead to HCM by impairing the relaxation on one hand, and lead to DCM by over inhibition of the AM interaction on the other hand. Ca(2+) sensitivity is affected by inorganic phosphate, ionic strength, and phosphorylation of constituent proteins; hence it may not be the primary cause of the pathogenesis. Here, we review our current knowledge regarding Tm's effect on the actomyosin interaction and the early molecular pathogenesis of Tm mutation related to HCM, DCM, and LVNC.

A systematic review and meta-analysis of genotype-phenotype associations in patients with hypertrophic cardiomyopathy caused by sarcomeric protein mutations

BACKGROUND

The genetic basis of familial hypertrophic cardiomyopathy (HCM) is well described, but the relation between genotype and clinical phenotype is still poorly characterised.

OBJECTIVE

To summarise and critically review the current literature on genotype-phenotype associations in patients with HCM and to perform a meta-analysis on selected clinical features.

DATA SOURCES

PubMed/Medline was searched up to January 2013. Retrieved articles were checked for additional publications.

SELECTION CRITERIA

Observational, cross-sectional and prospectively designed English language human studies that analysed the relationship between the presence of mutations in sarcomeric protein genes and clinical parameters.

DATA EXTRACTION AND ANALYSIS

The pooled analysis was confined to studies reporting on cohorts of unrelated and consecutive patients in which at least two sarcomere genes were sequenced. A random effect meta-regression model was used to determine the overall prevalence of predefined clinical features: age at presentation, gender, family history of HCM, family history of sudden cardiac death (SCD), and maximum left ventricular wall thickness (MLVWT). The I(2) statistic was used to estimate the proportion of total variability in the prevalence data attributable to the heterogeneity between studies.

RESULTS

Eighteen publications (corresponding to a total of 2459 patients) were selected for the pooled analysis. The presence of any sarcomere gene mutation was associated with a younger age at presentation (38.4 vs 46.0 years, p<0.0005), a family history of HCM (50.6% vs 23.1%, p<0.0005), a family history of SCD (27.0% vs 14.9%, p<0.0005) and greater MLVWT (21.0 vs 19.3 mm, p=0.03). There were no differences when the two most frequently affected genes, MYBPC3 and MYH7, were compared. A total of 53 family studies were also included in the review. These were characterised by pronounced variability and the majority of studies reporting on outcomes analysed small cross-sectional cohorts and were unsuitable for pooled analyses.

CONCLUSIONS

The presence of a mutation in any sarcomere gene is associated with a number of clinical features. The heterogeneous nature of the disease and the inconsistency of study design precludes the establishment of more precise genotype-phenotype relationships. Large scale studies examining the relation between genotype, disease severity, and prognosis are required.

Two founder mutations in the alpha-tropomyosin and the cardiac myosin-binding protein C genes are common causes of hypertrophic cardiomyopathy in the Finnish population

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is predominantly caused by a large number of various mutations in the genes encoding sarcomeric proteins. However, two prevalent founder mutations for HCM in the alpha-tropomyosin (TPM1-D175N) and myosin-binding protein C (MYBPC3-Q1061X) genes have previously been identified in eastern Finland.

OBJECTIVE

To assess the prevalence of these founder mutations in a large population of patients with HCM from all over Finland. Patients and methods. We screened for two founder mutations (TPM1-D175N and MYBPC3-Q1061X) in 306 unrelated Finnish patients with HCM from the regions covering a population of ∼4,000,000.

RESULTS

The TPM1-D175N mutation was found in 20 patients (6.5%) and the MYBPC3-Q1061X in 35 patients (11.4%). Altogether, the two mutations accounted for 17.9% of the HCM cases. In addition, 61 and 59 relatives of the probands were found to be carriers of TPM1-D175N and MYBPC3-Q1061X, respectively. The mutations showed regional clustering. TPM1-D175N was prevalent in central and western Finland, and MYBPC3-Q1061X in central and eastern Finland.

CONCLUSION

The TPM1-D175N and MYBPC3-Q1061X mutations account for a substantial part of all HCM cases in the Finnish population, indicating that routine genetic screening of these mutations is warranted in Finnish patients with HCM.

A novel alpha-tropomyosin mutation associates with dilated and non-compaction cardiomyopathy and diminishes actin binding

BACKGROUND

Dilated cardiomyopathy (DCM) is characterized by idiopathic dilatation and systolic contractile dysfunction of the ventricle(s) leading to an impaired systolic function. The origin of DCM is heterogeneous, but genetic transmission of the disease accounts for up to 50% of the cases. Mutations in alpha-tropomyosin (TPM1), a thin filament protein involved in structural and regulatory roles in muscle cells, are associated with hypertrophic cardiomyopathy (HCM) and very rarely with DCM.

METHODS AND RESULTS

Here we present a large four-generation family in which DCM is inherited as an autosomal dominant trait. Six family members have a cardiomyopathy with the age of diagnosis ranging from 5 months to 52 years. The youngest affected was diagnosed with dilated and non-compaction cardiomyopathy (NCCM) and died at the age of five. Three additional children died young of suspected heart problems. We mapped the phenotype to chromosome 15 and subsequently identified a missense mutation in TPM1, resulting in a p.D84N amino acid substitution. In addition we sequenced 23 HCM/DCM genes using next generation sequencing. The TPM1 p.D84N was the only mutation identified. The mutation co-segregates with all clinically affected family members and significantly weakens the binding of tropomyosin to actin by 25%.

CONCLUSIONS

We show that a mutation in TPM1 is associated with DCM and a lethal, early onset form of NCCM, probably as a result of diminished actin binding caused by weakened charge-charge interactions. Consequently, the screening of TPM1 in patients and families with DCM and/or (severe, early onset forms of) NCCM is warranted. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

New polymorphisms in human MEF2C gene as potential modifier of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy is caused by mutations in genes encoding sarcomeric proteins. Its variable phenotype suggests the existence of modifier genes. Myocyte enhancer factor (MEF) 2C could be important in this process given its role as transcriptional regulator of several cardiac genes. Any variant affecting MEF2C expression and/or function may impact on hypertrophic cardiomyopathy clinical manifestations. In this candidate gene approach, we screened 209 Caucasian hypertrophic cardiomyopathy patients and 313 healthy controls for genetic variants in MEF2C gene by single-strand conformation polymorphism analysis and direct sequencing. Functional analyses were performed with transient transfections of luciferase reporter constructions. Three new variants in non-coding exon 1 were found both in patients and controls with similar frequencies. One-way ANOVA analyses showed a greater left ventricular outflow tract obstruction (p = 0.011) in patients with 10C+10C genotype of the c.-450C(8_10) variant. Moreover, one patient was heterozygous for two rare variants simultaneously. This patient presented thicker left ventricular wall than her relatives carrying the same sarcomeric mutation. In vitro assays additionally showed a slightly increased transcriptional activity for both rare MEF2C alleles. In conclusion, our data suggest that 15 bp-deletion and C-insertion in the 5'UTR region of MEF2C could affect hypertrophic cardiomyopathy, potentially by affecting expression of MEF2C and therefore, the expression of their target cardiac proteins that are implicated in the hypertrophic process.

Low-grade inflammation and the phenotypic expression of myocardial fibrosis in hypertrophic cardiomyopathy

Objective

To investigate the role of inflammation in the phenotypic expression of myocardial fibrosis in hypertrophic cardiomyopathy (HCM).

Design

Clinical study.

Setting

Kuopio University Hospital and University of Eastern Finland, Kuopio, Finland.

Subjects

Twenty-four patients with a single HCM-causing mutation D175N in the α-tropomyosin gene and 17 control subjects.

Main outcome measures

Endomyocardial biopsy samples taken from the patients with HCM were compared with matched myocardial autopsy specimens. Levels of high-sensitivity C-reactive protein (hsCRP) and proinflammatory cytokines were measured in patients and controls. Myocardial late gadolinium enhancement (LGE) in cardiac MRI (CMRI) was detected.

Results

Endomyocardial samples in patients with HCM showed variable myocyte hypertrophy and size heterogeneity, myofibre disarray, fibrosis, inflammatory cell infiltration and nuclear factor kappa B (NF-κB) activation. Levels of hsCRP and interleukins (IL-1β, IL-1RA, IL-6, IL-10) were significantly higher in patients with HCM than in control subjects. In patients with HCM, there was a significant association between the degree of myocardial inflammatory cell infiltration, fibrosis in histopathological samples and myocardial LGE in CMRI. Levels of hsCRP were significantly associated with histopathological myocardial fibrosis. hsCRP, tumour necrosis factor α and IL-1RA levels had significant correlations with LGE in CMRI.

Conclusions

A variable myocardial and systemic inflammatory response was demonstrated in patients with HCM attributable to an identified sarcometric mutation. Inflammatory response was associated with myocardial fibrosis, suggesting that myocardial fibrosis in HCM is an active process modified by an inflammatory response.

Founder mutations in hypertrophic cardiomyopathy patients in the Netherlands

In this part of a series on cardiogenetic founder mutations in the Netherlands, we review the Dutch founder mutations in hypertrophic cardiomyopathy (HCM) patients. HCM is a common autosomal dominant genetic disease affecting at least one in 500 persons in the general population. Worldwide, most mutations in HCM patients are identified in genes encoding sarcomeric proteins, mainly in the myosin-binding protein C gene (MYBPC3, OMIM #600958) and the beta myosin heavy chain gene (MYH7, OMIM #160760). In the Netherlands, the great majority of mutations occur in the MYBPC3, involving mainly three Dutch founder mutations in the MYBPC3 gene, the c.2373_2374insG, the c.2864_2865delCT and the c.2827C>T mutation. In this review, we describe the genetics of HCM, the genotype-phenotype relation of Dutch founder MYBPC3 gene mutations, the prevalence and the geographic distribution of the Dutch founder mutations, and the consequences for genetic counselling and testing. (Neth Heart J 2010;18:248-54.).

Thin filament mutations: developing an integrative approach to a complex disorder

Sixteen years ago, mutations in cardiac troponin (Tn)T and α-tropomyosin were linked to familial hypertrophic cardiomyopathy, thus transforming the disorder from a disease of the β-myosin heavy chain to a disease of the cardiac sarcomere. From the outset, studies suggested that mutations in the regulatory thin filament caused a complex, heterogeneous pattern of ventricular remodeling with wide variations in clinical expression. To date, the clinical heterogeneity is well matched by an extensive array of nearly 100 independent mutations in all components of the cardiac thin filament. Significant advances in our understanding of the biophysics of myofilament activation, coupled to the emerging evidence that thin filament linked cardiomyopathies are progressive, suggests that a renewed focus on the most proximal events in both the molecular and clinical pathogenesis of the disease will be necessary to achieve the central goal of using genotype information to manage affected patients. In this review, we examine the existing biophysical and clinical evidence in support of a more proximal definition of thin filament cardiomyopathies. In addition, new high-resolution, integrated approaches are presented to help define the way forward as the field works toward developing a more robust link between genotype and phenotype in this complex disorder.

Enhanced active cross-bridges during diastole: molecular pathogenesis of tropomyosin's HCM mutations

Three HCM-causing tropomyosin (Tm) mutants (V95A, D175N, and E180G) were examined using the thin-filament extraction and reconstitution technique. The effects of Ca(2+), ATP, phosphate, and ADP concentrations on cross-bridge kinetics in myocardium reconstituted with each of these mutants were studied at 25°C, and compared to wild-type (WT) Tm at physiological ionic strength (200 mM). All three mutants showed significantly higher (2-3.5 fold) low Ca(2+) tension (T(LC)) and stiffness than WT at pCa 8.0. High Ca(2+) tension (T(HC)) was significantly higher for E180G than that for WT, whereas T(HC) of V95A and D175N was similar to WT; high Ca(2+) stiffness (Y(HC)) had the same trend. The Ca(2+) sensitivity of isometric force was significantly greater for V95A and E180G than for WT, whereas that of D175N remained the same as for WT; for all mutants, cooperativity was lower than for WT. Nine kinetic constants and the cross-bridge distribution were deduced using sinusoidal analysis. The number of force-generating cross bridges was similar among the D175N, E180G, and WT Tm forms, but it was significantly larger in the case of V95A than WT. We conclude that the increased number of actively cycling cross bridges at pCa 8 is the major cause of Tm mutation-related HCM pathogenesis, which may result in diastolic dysfunction. Decreased contractility (T(act)) in V95A and D175N may further contribute to the severity of myocyte hypertrophy and related prognosis of the disease.

Risk stratification for sudden cardiac death in hypertrophic cardiomyopathy: systematic review of clinical risk markers

We performed a systematic literature review of recommended 'major' and 'possible' clinical risk markers for sudden cardiac death (SCD) in hypertrophic cardiomyopathy (HCM). We searched the Medline, Embase and Cochrane databases for articles published between 1971 and 2007. We included English language reports on HCM patients containing follow-up data on the endpoint (sudden) cardiac death using survival analysis. Analysis was undertaken using the quality of reporting of meta-analyses (QUORUM) statement checklist. The quality was checked using a quality assessment form from the Cochrane Collaboration. Thirty studies met inclusion criteria and passed quality assessment. The use of the six major risk factors (previous cardiac arrest or sustained ventricular tachycardia, non-sustained ventricular tachycardia, extreme left ventricular hypertrophy, unexplained syncope, abnormal blood pressure response, and family history of sudden death) in risk stratification for SCD as recommended by international guidelines was supported by the literature. In addition, left ventricular outflow tract obstruction seems associated with a higher risk of SCD. Our systematic review provides sound evidence for the use of the six major risk factors for SCD in the risk stratification of HCM patients. Left ventricular outflow tract obstruction could be included in the overall risk profile of patients with a marked left ventricular outflow gradient under basal conditions.

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.

Diagnostic, prognostic, and therapeutic implications of genetic testing for hypertrophic cardiomyopathy

Over the last 2 decades, the pathogenic basis for the most common heritable cardiovascular disease, hypertrophic cardiomyopathy (HCM), has been investigated extensively. Affecting approximately 1 in 500 individuals, HCM is the most common cause of sudden death in young athletes. In recent years, genomic medicine has been moving from the bench to the bedside throughout all medical disciplines including cardiology. Now, genomic medicine has entered clinical practice as it pertains to the evaluation and management of patients with HCM. The continuous research and discoveries of new HCM susceptibility genes, the growing amount of data from genotype-phenotype correlation studies, and the introduction of commercially available genetic tests for HCM make it essential that the modern-day cardiologist understand the diagnostic, prognostic, and therapeutic implications of HCM genetic testing.

Molecular genetics of sudden cardiac death

Sudden cardiac death (SCD) is one of the most common causes of death. An important number of sudden deaths, especially in the young, are due to genetic heart disorders, both with structural and arrhythmogenic abnormalities. In recent years, significant advances have been made in understanding the genetic basis of SCD. Identification of the genetic causes of sudden death is important because close relatives are also at potential risk of having a fatal cardiac condition. A comprehensive post-mortem investigation is vital to determine the cause and manner of death and provides the opportunity to assess the potential risk to the family after appropriate genetic counselling. In this paper, we present an update of the different genetic causes of sudden death, emphasizing their importance for the forensic pathologist due to his relevant role in the diagnosis and prevention of SCD.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Sarcomeric proteins and inherited cardiomyopathies

Over the last two decades, a large number of mutations have been identified in sarcomeric proteins as a cause of hypertrophic, dilated or restrictive cardiomyopathy. Functional analyses of mutant proteins in vitro have revealed several important functional changes in sarcomeric proteins that might be primarily involved in the pathogenesis of each cardiomyopathy. Creation of transgenic or knock-in animals expressing mutant proteins in their hearts confirmed that these mutations in genes for sarcomeric proteins induced distinct types of cardiomyopathies and provided useful animal models to explore the molecular pathogenic mechanisms and potential therapeutics of cardiomyopathy in vivo. In this review, I discuss the functional consequences of mutations in different sarcomeric proteins found in hypertrophic, dilated, and restrictive cardiomyopathies in conjunction with their effects on cardiac structure and function in vivo and their possible molecular and cellular mechanisms, which underlie the pathogenesis of these inherited cardiomyopathies.

Genetics of hypertrophic cardiomyopathy: one, two, or more diseases?

PURPOSE OF REVIEW

Hypertrophic cardiomyopathy is the most common identifiable cause of sudden death in the young. This review details the history of hypertrophic cardiomyopathy, recent discoveries in its genetic underpinnings and important genotype-phenotype relationships described in recent studies.

RECENT FINDINGS

Since the discovery of the genetic underpinnings of hypertrophic cardiomyopathy in 1989 hundreds of mutations scattered among at least 10 sarcomeric genes confer the pathogenetic substrate for this 'disease of the sarcomere/myofilament'. More recently, the genetic spectrum of hypertrophic cardiomyopathy has expanded to encompass mutations in Z-disc-associated genes (Z-disc hypertrophic cardiomyopathy) and glycogen storage diseases mimicking hypertrophic cardiomyopathy (metabolic hypertrophic cardiomyopathy). Recent genotype-phenotype studies have discovered an important relationship between the morphology of the left ventricle, its underlying genetic substrate and the long-term outcome of this disease.

SUMMARY

Genomic medicine has entered clinical practice and the diagnostic utility of genetic testing for hypertrophic cardiomyopathy is clearly evident, but with the growing number of hypertrophic cardiomyopathy-associated genes strategic choices have to be made. With recent discoveries in genotype-phenotype relationships, especially pertaining to the echocardiographic septal shape and the underlying pathogenetic mutation, time has come to subdivide the one disease we call hypertrophic cardiomyopathy.

Prevalence and spectrum of mutations in the sarcomeric troponin T and I genes in a cohort of Spanish cardiac hypertrophy patients

We sequenced the coding exons of the cardiac troponins T (TNNT2) and I (TNNI3) genes in 115 Spanish HCM-patients (32% with a family history of the disease). Only two (2%) had mutations in the TNNT2 (Arg278>Cys and Arg92>Lys). These mutations were associated with variable clinical outcomes. No patient had TNNI3-mutation. We also genotyped these patients and 320 healthy controls for a 5 bp insertion/deletion (I/D) polymorphism in intron 3 of TNNT2. DD-homozygotes for the 5 bp I/D polymorphism were significantly more frequent among the patients (OR=1.83, 95% CI=2.10-5.16).

Rescue of tropomyosin-induced familial hypertrophic cardiomyopathy mice by transgenesis

Familial hypertrophic cardiomyopathy (FHC) is a disease caused by mutations in contractile proteins of the sarcomere. Our laboratory developed a mouse model of FHC with a mutation in the thin filament protein alpha-tropomyosin (TM) at amino acid 180 (Glu180Gly). The hearts of these mice exhibit dramatic systolic and diastolic dysfunction, and their myofilaments demonstrate increased calcium sensitivity. The mice also develop severe cardiac hypertrophy, with death ensuing by 6 mo. In an attempt to normalize calcium sensitivity in the cardiomyofilaments of the hypertrophic mice, we generated a chimeric alpha-/beta-TM protein that decreases calcium sensitivity in transgenic mouse cardiac myofilaments. By mating mice from these two models together, we tested the hypothesis that an attenuation of myofilament calcium sensitivity would modulate the severe physiological and pathological consequences of the FHC mutation. These double-transgenic mice "rescue" the hypertrophic phenotype by exhibiting a normal morphology with no pathological abnormalities. Physiological analyses of these rescued mice show improved cardiac function and normal myofilament calcium sensitivity. These results demonstrate that alterations in calcium response by modification of contractile proteins can prevent the pathological and physiological effects of this disease.

Autonomic cardiac control in animal models of cardiovascular diseases II. Variability analysis in transgenic rats with α-tropomyosin mutations Asp175Asn and Glu180Gly

Animal models of cardiovascular diseases allow to investigate relevant pathogenetic mechanisms in detail. In the present study, the mutations Asp175Asn and Glu180Gly in alpha-tropomyosin (TPM1), known cause familiar hypertrophic cardiomyopathy (FHC) were studied for changes in hemodynamic parameters and spontaneous baroreflex regulation in transgenic rats in comparison to transgenic and non-transgenic controls by telemetry. Heart rate variability (HRV) and blood pressure variability (BPV) were analyzed using time- and frequency domain, as well as non-linear measures. The dual sequence method was used for the estimation of the baroreflex regulation. In transgenic rats harboring mutated TPM1, changes in HRV were detected during exercise, but not at rest. Both mutations, Asp175Asn and Glu180Gly, caused increased low frequency power. In addition, in animals with mutation Asp175Asn a reduced total HRV was observed. BPV did not show any differences between all transgenic animal lines. During exercise, a strong increase in the number of bradycardic and tachycardic fluctuations accompanied with decreased baroreflex sensitivity (BRS) was detected in animals with either TPM1 mutation, Asp175Asn or Glu180Gly. These data suggest, that the analysis of cardiac autonomic control, particularly of baroreflex regulation, represents a powerful non-invasive approach to investigate the effects of subtle changes in sarcomeric architecture on cardiac physiology in vivo. In case of mutations Asp175Asn or Glu180Gly in TPM1, early detection of alterations in autonomic cardiac control could help to prevent sudden cardiac death in affected persons.

Microarray analysis of gene expression during early stages of mild and severe cardiac hypertrophy

Familial hypertrophic cardiomyopathy (FHC) is a disease characterized by ventricular hypertrophy, fibrosis, and aberrant systolic and/or diastolic function. We previously developed two transgenic mouse models that carry FHC-associated mutations in α-tropomyosin (TM): FHC α-TM175 mice show patchy areas of mild ventricular disorganization and limited hypertrophy, whereas FHC α-TM180 mice exhibit severe hypertrophy and fibrosis and die within 6 mo. To obtain a better understanding of the molecular mechanisms associated with the early onset of cardiac hypertrophy, we conducted a detailed comparative analysis of gene expression in 2.5-mo-old control, FHC α-TM175, and α-TM180 ventricular tissue. Results show that 754 genes (from a total of 22,600) were differentially expressed between the nontransgenic (NTG) and the FHC hearts. There are 178 differentially regulated genes between NTG and the FHC α-TM175 hearts, 388 genes are differentially expressed between NTG and FHC α-TM180 hearts, and 266 genes are differentially expressed between FHC α-TM175 and FHC α-TM180 hearts. Genes that exhibit the largest increase in expression belong to the “secreted/extracellular matrix” category, and those with the most significant decrease in expression are associated with “metabolic enzymes.” Confirmation of the microarray analysis was conducted by quantitative real-time PCR on gene transcripts commonly associated with cardiac hypertrophy.

Cine MR Imaging of Myocardial Contractile Impairment in Patients with Hypertrophic Cardiomyopathy Attributable to Asp175Asn Mutation in the α-Tropomyosin Gene

PURPOSE

To prospectively investigate the relationship between myocardial contractile impairment and left ventricular (LV) hypertrophy measured at cardiac magnetic resonance (MR) imaging in patients with hypertrophic cardiomyopathy (HCM) caused by the substitution of aspartic acid 175 with asparagine (ie, Asp175Asn mutation) in the alpha-tropomyosin gene (TPM1).

MATERIALS AND METHODS

The study protocol was approved by the hospital ethics committee, and all subjects gave written informed consent. LV mass, maximal LV wall thickness, and myocardial fractional thickening during systole were measured at cine MR imaging in 24 subjects (11 male, 13 female; mean age, 42 years; age range, 17-68 years) with the Asp175Asn mutation in TPM1 and in 17 healthy volunteers (eight men, nine women; mean age, 38 years; age range, 23-60 years). The proportion of hypokinetic LV segments was calculated as the number of LV segments with fractional thickening of less than 30% divided by the total number of segments measured. Anthropometric and biochemical correlates of LV hypertrophy were determined. Univariate and multiple linear regression analyses were used to investigate the association of the proportion of hypokinetic segments and other correlates of LV hypertrophy with LV mass and maximal wall thickness.

RESULTS

The proportion of hypokinetic segments was higher in patients with HCM than in control subjects (37% +/- 20 [standard deviation] vs 12% +/- 12, P < .001). In stepwise multiple regression analysis, the proportion of hypokinetic segments accounted for 42% (P < .001); the LV end-diastolic volume, for 24% (P = .003); and male sex, for 10% (P = .014) of the variability in LV mass in patients with HCM. The proportion of hypokinetic LV segments, which accounted for 48% of the variability in LV maximal wall thickness (P < .001), was the only variable significantly associated with maximal wall thickness.

CONCLUSION

The extent of myocardial contractile impairment is strongly and independently related to LV mass and maximal wall thickness in patients with HCM attributable to the Asp175Asn mutation in TPM1.

Gene-specific modifying effects of pro-LVH polymorphisms involving the renin-angiotensin-aldosterone system among 389 unrelated patients with hypertrophic cardiomyopathy

AIMS

The purpose of this study was to determine whether the deletion/insertion (D/I) polymorphism in the ACE-encoded angiotensin-converting enzyme or the pooled gene effect of five renin-angiotensin-aldosterone system (RAAS) polymorphisms were disease modifiers in a large cohort of unrelated patients with genotyped hypertrophic cardiomyopathy (HCM).

METHODS AND RESULTS

Five different RAAS polymorphism genotypes were established by PCR amplification of the surrounding polymorphic regions of genomic DNA in a cohort of 389 unrelated patients comprehensively genotyped for HCM-causing mutations in eight sarcomeric/myofilament genes. Patient clinical data were archived in a database blinded both to the primary myofilament defect and the polymorphism genotype. Each patient was assessed with respect to ACE genotype as well as composite pro-left ventricular hypertrophy (LVH) RAAS polymorphism score (0-5). Overall, no clinical parameter correlated independently with ACE genotype. Subset analysis of the two most common genetic subtypes of HCM, MYBPC3 (myosin binding protein C) and MYH7 (beta myosin heavy chain), demonstrated a significant pro-LVH effect of DD-ACE only in patients with MYBPC3-HCM. In MYBPC3-HCM, left ventricular wall thickness was greater in patients with DD genotype (25.8+/-5 mm) compared with DI (21.8+/-4) or II genotype (20.8+/-5, P=0.01). Moreover, extreme hypertrophy (>30 mm) was only seen in MYBPC3-HCM patients who also hosted DD-ACE. An effect of RAAS pro-LVH score was evident only in the subgroup of patients with no previously identified myofilament mutation.

CONCLUSION

This study demonstrates that RAAS genotypes may modify the clinical phenotype of HCM in a disease gene-specific fashion rather than indiscriminately.

Genetic testing for risk stratification in hypertrophic cardiomyopathy and long QT syndrome: fact or fiction?

PURPOSE OF REVIEW

Hypertrophic cardiomyopathy, affecting 1 in 500 persons, is the most common identifiable cause of sudden cardiac death in the young, whereas congenital long QT syndrome, affecting 1 in 5000 persons, is perhaps one of the most common causes of autopsy negative sudden unexplained death. Since May 2004, genetic testing has been available as a clinical diagnostic test for both hypertrophic cardiomyopathy and long QT syndrome. It is now critical to carefully scrutinize the relationships between genotype and phenotype as they pertain to clinical practice.

RECENT FINDINGS

In 1990, the molecular underpinnings of hypertrophic cardiomyopathy were exposed with the identification of a mutation in the MYH7-encoded beta myosin heavy chain. Since then, hundreds of mutations scattered among at least 14 genes confer the pathogenetic substrate for this 'disease of the sarcomere'. In 1995, the discipline of cardiac channelopathies was born with the revelation that mutations in critical cardiac channel genes cause long QT syndrome. Today, hundreds of mutations involving several cardiac channel genes account for approximately 75% of long QT syndrome. Over the past decade, scores of genotype-phenotype correlation studies in both hypertrophic cardiomyopathy and long QT syndrome have been conducted.

SUMMARY

Genomic medicine has now entered the clinical practice as it pertains to the evaluation and management of both hypertrophic cardiomyopathy and long QT syndrome. The diagnostic utility of genetic testing for both diseases is clearly evident, as well as current limitations. While treatment decisions are certainly influenced by knowing the underlying genotype in long QT syndrome, there seems to be negligible prognostic value associated with particular hypertrophic cardiomyopathy-causing mutations at this time.

The pathology of hypertrophic cardiomyopathy

Sudden cardiac death (SCD) is devastating at any age, but even more so when the individual affected is young and asymptomatic, and the death is entirely unexpected. SCD is a catastrophic complication of hypertrophic cardiomyopathy (HCM) and may be the first manifestation of this disease. HCM is an inherited intrinsic disease of the myocardium characterized by left ventricular hypertrophy without chamber dilatation, in the absence of either a systemic or other cardiac disease, which may cause a similar magnitude of hypertrophy. HCM may be a clinically silent disease. Indeed, the pathologist may be the first to encounter a case of HCM at autopsy. HCM has wide-ranging implications for affected families, who will require cardiac screening and genetic counselling even if mutations are not known. Therefore, prompt and accurate diagnosis of HCM is vital. This review article will focus on the pathological diagnosis of HCM, recent advances in the genetics of this disease, and common pitfalls which may arise, leading to diagnostic uncertainty.

Proposal for contemporary screening strategies in families with hypertrophic cardiomyopathy

Screening families with hypertrophic cardiomyopathy (HCM) presents a common clinical problem to practicing cardiologists, internists, and pediatricians. The traditional recommended strategy for screening relatives in most HCM families calls for such evaluations with echocardiography (and electrocardiogram [ECG]) on a 12- to 18-month basis, usually beginning at about age 12 years. If such tests show no evidence of left ventricular hypertrophy, i.e., without one or more segments of abnormally increased wall thickness by the time full growth and maturation is achieved (at the age of about 18 to 21 years), it has been customary practice to conclude that HCM is probably absent and reassure family members accordingly that further echocardiographic testing is unnecessary. However, novel developments in the definition of the genetic causes of HCM have defined both substantial molecular diversity and heterogeneity of the disease expression including (in some relatives) incomplete phenotypic penetrance and delayed, late-onset left ventricular hypertrophy well into adulthood. These observations have unavoidably reshaped the customary practice of genetic counseling and established a new proposed paradigm for clinical family screening of HCM families. Therefore, in the absence of genetic testing, strong consideration should be given to extending diagnostic serial echocardiography past adolescence and into mid-life for those family members with a normal echocardiogram and ECG. Of note, recent developments in laboratory DNA-based diagnosis for HCM could potentially avoid the necessity for serial echocardiography in many such relatives.

α-Tropomyosin mutations Asp175Asn and Glu180Gly affect cardiac function in transgenic rats in different ways

To study the mechanisms by which missense mutations in α-tropomyosin cause familial hypertrophic cardiomyopathy, we generated transgenic rats overexpressing α-tropomyosin with one of two disease-causing mutations, Asp175Asn or Glu180Gly, and analyzed phenotypic changes at molecular, morphological, and physiological levels. The transgenic proteins were stably integrated into the sarcomere, as shown by immunohistochemistry using a human-specific anti-α-tropomyosin antibody, ARG1. In transgenic rats with either α-tropomyosin mutation, molecular markers of cardiac hypertrophy were induced. Ca2+sensitivity of cardiac skinned-fiber preparations from animals with mutation Asp175Asn, but not Glu180Gly, was decreased. Furthermore, elevated frequency and amplitude of spontaneous Ca2+waves were detected only in cardiomyocytes from animals with mutation Asp175Asn, suggesting an increase in intracellular Ca2+concentration compensating for the reduced Ca2+sensitivity of isometric force generation. Accordingly, in Langendorff-perfused heart preparations, myocardial contraction and relaxation were accelerated in animals with mutation Asp175Asn. The results allow us to propose a hypothesis of the pathogenetic changes caused by α-tropomyosin mutation Asp175Asn in familial hypertrophic cardiomyopathy on the basis of changes in Ca2+handling as a sensitive mechanism to compensate for alterations in sarcomeric structure.

Genetics of hypertrophic cardiomyopathy in eastern Finland: few founder mutations with benign or intermediary phenotypes

Hypertrophic cardiomyopathy (HCM) is a genetically and clinically heterogeneous myocardial disease caused by mutations in genes encoding sarcomeric proteins. To assess the genetic background and phenotypic expression of HCM in eastern Finland, we screened 35 unrelated patients with HCM from the Kuopio University Hospital area for variants in 9 genes encoding sarcomeric proteins with the PCR-SSCP method. We herewith describe our previous findings in five sarcomeric genes and also report hitherto unpublished data on four additional sarcomeric genes. Mutations in the cardiac myosin-binding protein C gene (MYBPC3) were most frequent, accounting for 26% of cases. A novel mutation (Gln1061X) in this gene was the most common mutation, found in 6 of 35 families and accounting for 17% of all cases. Other novel mutations in MYBPC3 (IVS5-2A --> C, IVS14-13G --> A, and Ex25deltaLys) were found in one family each. A previously described alpha-tropomyosin (TPM1) mutation (Asp175Asn) was found in 11% of cases. Haplotype analysis suggested that the two most common variants (MYBPC3-Gln1061X and TPM1-Asp175Asn) were founder mutations. Only one mutation (Arg719Trp) in the beta-myosin heavy chain gene (MYH7) was found in one family, and no disease-causing mutations were found in the genes encoding alpha-actin, cardiac troponin I, T, C, or myosin essential and regulatory light chains. Altogether, the aforementioned 6 mutations found in MYBPC3, TPM1, and MYH7 accounted for 61% of familial and 40% of all HCM cases. The mutations were associated mostly with benign or intermediary phenotypes with only few HCM-related deaths. We conclude that the genetic profile of HCM in eastern Finland is unique, characterized by few founder mutations with benign or intermediary phenotypes.

On predictors of sudden cardiac death in hypertrophic cardiomyopathy

The clinical diagnostic hallmark of hypertrophic cardiomyopathy (HCM) is unexplained cardiac hypertrophy, commonly found on an echocardiogram and in unfortunate occasions, in an autopsy. The latter is most tragic as HCM, a relatively common disease (1 ) often presenting with sudden cardiac death (SCD) in apparently healthy young individuals (2 ,3 ). Indeed, HCM is considered the most common cause of SCD in young competitive athletes (2 ). The unexpected SCD of young athletic individuals in conjunction with the results of earlier studies from major referral centers, reporting an annual mortality rate of approximately 2% to 6% (3 –5 ), led to the impression that HCM is a relatively malignant disease. Population-based studies, however, suggested a more benign course with an annual mortality rate of approximately 1% (6 –9 ). In the largest series comprised of 744 patients, the annual mortality rate was 1.2% of which approximately half were sudden unexpected deaths (9 ).

Variable clinical manifestation of a novel missense mutation in the alpha-tropomyosin (TPM1) gene in familial hypertrophic cardiomyopathy

OBJECTIVES

This study was initiated to identify the disease-causing genetic defect in a family with hypertrophic cardiomyopathy (HCM) and high incidence of sudden death.

BACKGROUND

Familial hypertropic cardiomyopathy (FHC) is an autosomal dominant transmitted disorder that is genetically and clinically heterogeneous. Mutations in 11 genes have been associated with the pathogenesis of the disease.

METHODS

We studied a large FHC family, first by linkage analysis, to identify the gene involved, and subsequently screened the gene, encoding alpha-tropomyosin (TPM1), for mutations by using single-strand conformation polymorphism and sequencing analysis.

RESULTS

Twelve family members presented clinical features of HCM, five of whom died at young age, while others had only mild clinical features. Marker analysis showed linkage for the TPM1 gene on chromosome 15q22 (maximal logarithm of the odds score is 5.16, theta = 0); subsequently, a novel missense mutation (Glu62Gln) was identified.

CONCLUSIONS

The novel mutation identified in TPM1 is associated with the clinical features of cardiac hypertrophy in all but one genetically affected member of this large family. The clinical data suggest a malignant phenotype at young age with a variable clinical manifestation and penetrance at older age. The Glu62Gln mutation is the sixth TPM1 mutation identified as the cause of FHC, indicating that mutations in this gene are very rare. This is the first reported amino acid substitution at the f-position within the coiled-coil structure of the tropomyosin protein.

Hypertrophic cardiomyopathy: from gene defect to clinical disease

Major advances have been made over the last decade in our understanding of the molecular basis of several cardiac conditions. Hypertrophic cardiomyopathy (HCM) was the first cardiac disorder in which a genetic basis was identified and as such, has acted as a paradigm for the study of an inherited cardiac disorder. HCM can result in clinical symptoms ranging from no symptoms to severe heart failure and premature sudden death. HCM is the commonest cause of sudden death in those aged less than 35 years, including competitive athletes. At least ten genes have now been identified, defects in which cause HCM. All of these genes encode proteins which comprise the basic contractile unit of the heart, i.e. the sarcomere. While much is now known about which genes cause disease and the various clinical presentations, very little is known about how these gene defects cause disease, and what factors modify the expression of the mutant genes. Studies in both cell culture and animal models of HCM are now beginning to shed light on the signalling pathways involved in HCM, and the role of both environmental and genetic modifying factors. Understanding these mechanisms will ultimately improve our knowledge of the basic biology of heart muscle function, and will therefore provide new avenues for treating cardiovascular disease in man.