Form Of Familial Hypertrophic Cardiomyopathy Research Articles

Multimodality imaging in apical hypertrophic cardiomyopathy: clinical case

Apical hypertrophic cardiomyopathy is rare familial form of hypertrophic cardiomyopathy, with hypertrophy mainly affecting the apex of the left ventricle and characterized by a spade-like left ventricular cavity. Despite varying presentation, impaired capabilities of some standard instrumental methods and dissent on diagnostic criteria several diagnostic characteristics estimated by various noninvasive imaging modalities make it possible to establish a diagnosis with high accuracy. We hereby describe a case with electrocardiographic abnormalities and chest pain with suspected acute coronary syndrome. Acute coronary syndrome was excluded as well as performing invasive coronary angiography was avoided using advanced diagnostic tools including single photon emission computed tomography and apical hypertrophic cardiomyopathy was demonstrated by careful differential diagnosis.

Hypertrophic cardiomyopathy R403Q mutation in rabbit β-myosin reduces contractile function at the molecular and myofibrillar levels

In 1990, the Seidmans showed that a single point mutation, R403Q, in the human β-myosin heavy chain (MHC) of heart muscle caused a particularly malignant form of familial hypertrophic cardiomyopathy (HCM) [Geisterfer-Lowrance AA, et al. (1990) Cell 62:999-1006.]. Since then, more than 300 mutations in the β-MHC have been reported, and yet there remains a poor understanding of how a single missense mutation in the MYH7 gene can lead to heart disease. Previous studies with a transgenic mouse model showed that the myosin phenotype depended on whether the mutation was in an α- or β-MHC backbone. This led to the generation of a transgenic rabbit model with the R403Q mutation in a β-MHC backbone. We find that the in vitro motility of heterodimeric R403Q myosin is markedly reduced, whereas the actin-activated ATPase activity of R403Q subfragment-1 is about the same as myosin from a nontransgenic littermate. Single myofibrils isolated from the ventricles of R403Q transgenic rabbits and analyzed by atomic force microscopy showed reduced rates of force development and relaxation, and achieved a significantly lower steady-state level of isometric force compared with nontransgenic myofibrils. Myofibrils isolated from the soleus gave similar results. The force-velocity relationship determined for R403Q ventricular myofibrils showed a decrease in the velocity of shortening under load, resulting in a diminished power output. We conclude that independent of whether experiments are performed with isolated molecules or with ordered molecules in the native thick filament of a myofibril, there is a loss-of-function induced by the R403Q mutation in β-cardiac myosin.

Abstract 12739: Gap Junction Protein Beta 4 Plays an Important Role In Cardiac Hypertrophy and Cardiac Function in Human and Zebrafish

Introduction: Gap junction protein beta 4 (GJB4) encodes a transmembrane connexin protein (Cx30.3) that is a component of gap junctions. Mutations in this gene remain unknown to be associated with hypertrophic cardiomyopathy (HCM) and cardiac dysfunction. Hypothesis: Our hypothesis is that mutations in GJB4 can cause familial HCM, which can progresses into dilated-phase HCM (d-HCM). Methods: We have previously examined a patient and her elder brother who both presented with complicated severe HCM and whose parents are married cousins. The elder brother had died suddenly as a child and the autopsy demonstrated the development of HCM. The patient was diagnosed with HCM, which progressed to d-HCM during the 20-year follow-up. The patient has recently undergone cardiac transplantation. The parents are healthy. We hypothesized that an autosomal recessive mutation caused their familial HCM. The whole-exome sequencing was performed from each of the three participants. We generated knockout zebrafish of GJB4 and ELP2 by CRISPER/Cas9 system and analysis the volume and function of ventricle at five days post fertilization (dpf). Results: The gene exome analysis showed 34 single nucleotide polymorphisms (SNPs) that caused amino acid change for which the patient was homozygous and both parents were heterozygous after excluding all known common (>10%) SNP gene mutations in the dbSNP. Among them, GJB4 and ELP2 were the identified genes that are possibly associated with cardiac muscle. The PolyPhen-2 scores of mutations in GJB4 and ELP2 were 1.000 and 0.001 respectively. The identified missense mutation was a 981 A>C nucleotide transition in GJB4. The resultant E204A substitution replaces a polar carboxylic glutamate with a nonpolar alanine. The endodiastolic volume was significantly lower in GJB4-KO (94pl, p<0.01) and ELP2-KO (95pl, p<0.01) than in wild type (WT, 145pl) zebrafish at 5 dpf. Furthermore the ventricular ejection fraction was decreased in GJB4 KO (0.66, p<0.01) and unchanged in ELP2-KO (0.80) respectively compared with WT zebrafish (0.82). Conclusions: These results indicate both that mutations in GJB4 and ELP2 can cause a familial form of HCM and that GJB4 especially may be a new target for treatment of cardiac function and hypertrophy.

Mutations in filamin C cause a new form of familial hypertrophic cardiomyopathy.

Mutations in different genes encoding sarcomeric proteins are responsible for 50-60% of familial cases of hypertrophic cardiomyopathy (HCM); however, the genetic alterations causing the disease in one-third of patients are currently unknown. Here we describe a case with familial HCM of unknown cause. Whole-exome sequencing reveals a variant in the gene encoding the sarcomeric protein filamin C (p.A1539T) that segregates with the disease in this family. Sequencing of 92 HCM cases identifies seven additional variants segregating with the disease in eight families. Patients with FLNC mutations show marked sarcomeric abnormalities in cardiac muscle, and functional analysis reveals that expression of these FLNC variants resulted in the formation of large filamin C aggregates. Clinical studies indicate that FLNC-mutated patients have higher incidence of sudden cardiac death. On the basis of these findings, we conclude that mutations in the gene encoding the sarcomeric protein filamin C cause a new form of familial HMC.

Creatine Kinase Adenosine Triphosphate and Phosphocreatine Energy Supply in a Single Kindred of Patients With Hypertrophic Cardiomyopathy

A lethal and extensively characterized familial form of hypertrophic cardiomyopathy (HC) is due to a point mutation (Arg403Gln) in the cardiac β-myosin heavy chain gene. Although this is associated with abnormal energy metabolism and progression to heart failure in an animal model, in vivo cardiac energetics have not been characterized in patients with this mutation. Noninvasive phosphorus saturation transfer magnetic resonance spectroscopy was used to measure the adenosine triphosphate supplied by the creatine kinase (CK) reaction and phosphocreatine, the heart's primary energy reserve, in 9 of 10 patients from a single kindred with HC caused by the Arg403GIn mutation and 17 age-matched healthy controls. Systolic and diastolic function was assessed by echocardiography in all 10 patients with HC. The patients with HC had impairment of diastolic function and mild systolic dysfunction, when assessed using global systolic longitudinal strain. Myocardial phosphocreatine was significantly decreased by 24% in patients (7.1 ± 2.3 μmol/g) compared with the controls (9.4 ± 1.2 μmol/g; p = 0.003). The pseudo-first-order CK rate-constant was 26% lower (0.28 ± 0.15 vs 0.38 ± 0.07 s⁻¹, p = 0.035) and the forward CK flux was 44% lower (2.0 ± 1.4 vs 3.6 ± 0.9 μmol/g/s, p = 0.001) than in the controls. The contractile abnormalities did not correlate with the metabolic indexes. In conclusion, myocardial phosphocreatine and CK-ATP delivery are significantly reduced in patients with HC caused by the Arg403Gln mutation, akin to previous results from mice with the same mutation. A lack of a relation between energetic and contractile abnormalities suggests the former result from the sarcomeric mutation and not a late consequence of mechanical dysfunction.

Transgenic Mouse α- and β-Cardiac Myosins Containing the R403Q Mutation Show Isoform-dependent Transient Kinetic Differences

Familial hypertrophic cardiomyopathy (FHC) is a major cause of sudden cardiac death in young athletes. The discovery in 1990 that a point mutation at residue 403 (R403Q) in the β-myosin heavy chain (MHC) caused a severe form of FHC was the first of many demonstrations linking FHC to mutations in muscle proteins. A mouse model for FHC has been widely used to study the mechanochemical properties of mutated cardiac myosin, but mouse hearts express α-MHC, whereas the ventricles of larger mammals express predominantly β-MHC. To address the role of the isoform backbone on function, we generated a transgenic mouse in which the endogenous α-MHC was partially replaced with transgenically encoded β-MHC or α-MHC. A His6 tag was cloned at the N terminus, along with R403Q, to facilitate isolation of myosin subfragment 1 (S1). Stopped flow kinetics were used to measure the equilibrium constants and rates of nucleotide binding and release for the mouse S1 isoforms bound to actin. For the wild-type isoforms, we found that the affinity of MgADP for α-S1 (100 μM) is ~ 4-fold weaker than for β-S1 (25 μM). Correspondingly, the MgADP release rate for α-S1 (350 s(-1)) is ~3-fold greater than for β-S1 (120 s(-1)). Introducing the R403Q mutation caused only a minor reduction in kinetics for β-S1, but R403Q in α-S1 caused the ADP release rate to increase by 20% (430 s(-1)). These transient kinetic studies on mouse cardiac myosins provide strong evidence that the functional impact of an FHC mutation on myosin depends on the isoform backbone.

What is the Effect of A Familial Hypertrophic Cardiomyopathy Mutation on Cardiac Myosin Function?

Familial hypertrophic cardiomyopathy (FHC) is a clinically and genetically heterogeneous disease which is a major cause of heart failure. The landmark discovery that a point mutation at residue 403 (R403Q) in the β-myosin heavy chain (MHC) can cause a lethal form of FHC was made in 1990, but the effect of this mutation on the functional properties of human cardiac myosin remains poorly understood. One problem has been that the prevalent mouse model for FHC expresses predominantly α-MHC. The β-MHC, however, is the predominant isoform in the ventricles of all larger mammals. Even though the α- and β-MHC share > 90 % sequence identity, they differ ∼ 2-fold in enzymatic and mechanical properties, raising the possibility that the effect of a disease mutation may depend on the isoform backbone. To address this question we used a transgenic mouse model in which the endogenous α-MHC was replaced with transgenically encoded β-MHC. A His-tag was cloned at the N-terminus of α- and β-MHC, along with the R403Q mutation, to facilitate isolation of myosin or its head subfragment-1 (S1).We find that the steady-state ATPase activity and in vitro motility of mouse α-MHC is enhanced by the R403Q mutation, as reported previously, but the R403Q mutation in a β-MHC background shows a slight reduction in activity. A more in-depth analysis of the R403Q phenotype is being undertaken by stopped-flow kinetics to measure the nucleotide turnover in these mutant S1 isoforms. In order to determine the extent of species-dependent differences, we are comparing the functional properties of β-cardiac myosin in the mouse with those in the rabbit, a model system which more closely resembles humans in protein composition and disease phenotypes.

Sarcomere Shortening Destabilizes the Ca2+ Control System in Ventricular Myocytes: Implications for Understanding Arrhythmias in Familial Hypertrophic Cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) results from mutations of contractile proteins. Certain forms of FHC are linked with a surprisingly high incidence of sudden cardiac death. It is a long-standing conundrum how mutations in motor proteins give rise to electrical arrhythmias. Quantitative modeling studies using large-scale simulations reveal an intimate link between the contractile system and the Ca2+ control system. This insight may help resolve this conundrum. Our simulations show that a small decrease in sarcomere length (SL) can destabilize the Ca2+ control system and increase the probability of spontaneous Ca2+ waves. FHC mutations on cardiac troponin T (cTnT) increase myofilament Ca2+ sensitivity, which may account for the shortened SL in cardiomyocytes from mice harboring the cTnT mutations. To test the model predictions we conducted experiments using the myofilament Ca2+ sensitizer EMD 57033 (Merck) to reduce the resting SL length. EMD in the range of 1-3 μM reduced the resting SL from 1.9 to 1.5 μm without altering sarcoplasmic reticulum Ca2+ load, systolic, or diastolic Ca2+ levels. Upon cessation of pacing (1 Hz), control myocytes (0 EMD) were quiescent but EMD treated myocytes exhibited spontaneous contractions and Ca2+ release. These results are consistent with the model predictions lending support to the idea that FHC mutations destabilize the Ca2+ control system, which in turn, become a substrate for arrhythmias.

Genetic Testing for Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a disease that has grown more fascinating as new diagnostic and therapeutic tools have emerged over the last 50 years. In the early 20th century, HCM was a pathological entity. Subaortic obstruction was described at the outset but was only documented in patients after the advent of cardiac catheterization; it was first coined as idiopathic hypertrophic subaortic stenosis (IHSS) until the breakthrough of echocardiography revealed the broader phenotype. The nature of the outflow tract gradient as either dynamic or true mechanical obstruction was for years the subject of hot debate. Today HCM is considered a primary disease of the myocardium characterized by cardiac hypertrophy in the absence of a discernible clinical cause, such as hypertension or valvular heart disease.1 Typically, the left ventricle is hyperdynamic and has a small cavity size. The prevalence of HCM is at least 1:500 in the younger population,2 although it is likely to be higher in older individuals because of age-dependent expression of the disease. Early descriptions recognized sudden cardiac death (SCD) as a major clinical manifestation of HCM. While it remains the primary concern of physicians and patients, the development of internal defibrillators has provided a means to deal with this tragic event. The first series of surgeries for HCM were performed and refined about 40 years ago. The Moro procedure for septal myectomy became the standard surgical therapy for obstructive HCM. Advances in percutaneous techniques led to percutaneous septa! ablation, commonly performed by injecting alcohol into the main septal branches of the left anterior descending coronary artery. Its widespread use has led to ongoing debates about the superiority of surgical myectomy versus alcohol septal ablation, yet this debate remains unsettled due to the lack of large-scale randomized studies. While a familial form of HCM was recognized in the mid-20th century, the molecular genetic basis of HCM was revealed only in the last two decades with the development and application of modern molecular genetic techniques. Today, one of the most common questions asked by both physicians and patients centers around the availability and utility of genetic testing to diagnose and predict the risk of SCD.

Genetic Testing For Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a disease that has grown more fascinating as new diagnostic and therapeutic tools have emerged over the last 50 years. In the early 20th century, HCM was a pathological entity. Subaortic obstruction was described at the outset but was only documented in patients after the advent of cardiac catheterization; it was first coined as "idiopathic hypertrophic subaortic stenosis" (IHSS) until the breakthrough of echocardiography revealed the broader phenotype. The nature of the outflow tract gradient as either dynamic or true mechanical obstruction was for years the subject of hot debate.Today HCM is considered a primary disease of the myocardium characterized by cardiac hypertrophy in the absence of a discernible clinical cause, such as hypertension or valvular heart disease.1 Typically, the left ventricle is hyperdynamic and has a small cavity size. The prevalence of HCM is at least 1:500 in the younger population,2 although it is likely to be higher in older individuals because of age-dependent expression of the disease. Early descriptions recognized sudden cardiac death (SCD) as a major clinical manifestation of HCM. While it remains the primary concern of physicians and patients, the development of internal defibrillators has provided a means to deal with this tragic event.The first series of surgeries for HCM were performed and refined about 40 years ago. The Moro procedure for septal myectomy became the standard surgical therapy for obstructive HCM. Advances in percutaneous techniques led to percutaneous septa! ablation, commonly performed by injecting alcohol into the main septal branches of the left anterior descending coronary artery. Its widespread use has led to ongoing debates about the superiority of surgical myectomy versus alcohol septal ablation, yet this debate remains unsettled due to the lack of large-scale randomized studies.While a familial form of HCM was recognized in the mid-20th century, the molecular genetic basis of HCM was revealed only in the last two decades with the development and application of modern molecular genetic techniques. Today, one of the most common questions asked by both physicians and patients centers around the availability and utility of genetic testing to diagnose and predict the risk of SCD.

Independent FHC-related cardiac troponin T mutations exhibit specific alterations in myocellular contractility and calcium kinetics

Mutations in cardiac troponin T (cTnT) are linked to a severe form of Familial Hypertrophic Cardiomyopathy. Patients carrying mutations flanking the tropomyosin-binding domain of cTnT (R92L and Δ160E) develop distinct clinical syndromes. In order to better understand the cellular pathophysiology underlying these clinically relevant differences, we studied isolated adult left ventricular myocytes from independent transgenic cTnT mouse lines carrying either a 35% (Δ160E) or 50% (R92L) replacement of the endogenous cTnT with the mutant forms. Measurement of baseline myocellular contraction revealed that the Δ160E cells had significant decreases in the peak rate of contraction and percent shortening as compared to either R92L or Non-TG myocytes. In addition, while both Δ160E and R92L myocytes demonstrated a decrease in the peak rate of relaxation as compared to Non-TG, the magnitude of the difference was significantly greater in Δ160E cells. Concurrent myocyte [Ca 2+] i transient measurements revealed that while the alterations in the peak rates and times of the rise and decline of the [Ca 2+] i transient were similar to the changes in the respective measures of sarcomeric mechanics, R92L cells also exhibited reduced rates of the rise and decline of the [Ca 2+] i transient but did not exhibit these reductions in terms of sarcomeric mechanics. Of note, only Δ160E, and not R92L myocytes, demonstrated significant reductions in SR Ca 2+ load and uptake, corresponding to the impairments seen in the [Ca 2+] i and mechanical transients. Finally, Western analysis revealed a significant Δ160E-specific reduction in the SERCA2a/PLB ratio, which may well underlie the observed alterations in Ca 2+ homeostasis. Therefore, independent cTnT mutations result in significant mutation-specific effects in Ca 2+ handling that may, in part, contribute to the observed clinical variability in cTnT-related FHC.

Changes in the chemical and dynamic properties of cardiac troponin T cause discrete cardiomyopathies in transgenic mice

Cardiac troponin T (cTnT) is a central component of the regulatory thin filament. Mutations in cTnT have been linked to severe forms of familial hypertrophic cardiomyopathy. A mutational "hotspot" that leads to distinct clinical phenotypes has been identified at codon 92. Although the basic functional and structural roles of cTnT in modulating contractility are relatively well understood, the mechanisms that link point mutations in cTnT to the development of this complex cardiomyopathy are unknown. To address this question, we have taken a highly interdisciplinary approach by first determining the effects of the residue 92 mutations on the molecular flexibility and stability of cTnT by means of molecular dynamics simulations. To test whether the predicted alterations in thin filament structure could lead to distinct cardiomyopathies in vivo, we developed transgenic mouse models expressing either the Arg-92-Trp or Arg-92-Leu cTnT proteins in the heart. Characterization of these models at the cellular and whole-heart levels has revealed mutation-specific early alterations in transcriptional activation that result in distinct pathways of ventricular remodeling and contractile performance. Thus, our computational and experimental results show that changes in thin filament structure caused by single amino acid substitutions lead to differences in the biophysical properties of cTnT and alter disease pathogenesis.

Laing early onset distal myopathy: slow myosin defect with variable abnormalities on muscle biopsy

Background: Laing early onset distal myopathy (MPD1) is an autosomal dominant myopathy caused by mutations within the slow skeletal muscle fibre myosin heavy chain gene, MYH7. It is allelic with...

Differential cross-bridge kinetics of FHC myosin mutations R403Q and R453C in heterozygous mouse myocardium

The kinetic effects of the cardiac myosin point mutations R403Q and R453C, which underlie lethal forms of familial hypertrophic cardiomyopathy (FHC), were assessed using isolated myosin and skinned strips taken from heterozygous (R403Q/+ and R453C/+) male mouse hearts. Compared with wild-type (WT) mice, actin-activated ATPase was increased by 38% in R403Q/+ and reduced by 45% in R453C/+, maximal velocity of regulated thin filament (V(RTF)) in the in vitro motility assay was increased by 8% in R403Q/+ and was not different in R453C/+, myosin concentration at half-maximal V(RTF) was reduced by 30% in R403Q/+ and not different in R453C/+, and the characteristic frequency for oscillatory work production (b frequency), determined by sinusoidal analysis in the skinned strip at maximal calcium activation, was 27% lower in R403Q/+ and 18% higher in R453C/+. The calcium sensitivity for isometric tension in the skinned strip was not different in R403Q/+ (pCa(50) 5.64 +/- 0.02) and significantly enhanced in R453C/+ (5.82 +/- 0.03) compared with WT (5.58 +/- 0.02). We conclude that isolated myosin and skinned strips of R403Q/+ and R453C/+ myocardium show marked differences in cross-bridge kinetic parameters and in calcium sensitivity of force production that indicate different functional roles associated with the location of each point mutation at the molecular level.

Decreased energetics in murine hearts bearing the R92Q mutation in cardiac troponin T

The thin filament protein cardiac troponin T (cTnT) is an important regulator of myofilament activation. Here we report a significant change in cardiac energetics in transgenic mice bearing the missense mutation R92Q within the tropomyosin-binding domain of cTnT, a mutation associated with a clinically severe form of familial hypertrophic cardiomyopathy. This functional domain of cTnT has recently been shown to be a crucial modulator of contractile function despite the fact that it does not directly interact with the ATP hydrolysis site in the myosin head. Simultaneous measurements of cardiac energetics using 31P NMR spectroscopy and contractile performance of the intact beating heart revealed both a decrease in the free energy of ATP hydrolysis available to support contractile work and a marked inability to increase contractile performance upon acute inotropic challenge in hearts from R92Q mice. These results show that alterations in thin filament protein structure and function can lead to significant defects in myocardial energetics and contractile reserve.

Decreased energetics in murine hearts bearing the R92Q mutation in cardiac troponin T.

The thin filament protein cardiac troponin T (cTnT) is an important regulator of myofilament activation. Here we report a significant change in cardiac energetics in transgenic mice bearing the missense mutation R92Q within the tropomyosin-binding domain of cTnT, a mutation associated with a clinically severe form of familial hypertrophic cardiomyopathy. This functional domain of cTnT has recently been shown to be a crucial modulator of contractile function despite the fact that it does not directly interact with the ATP hydrolysis site in the myosin head. Simultaneous measurements of cardiac energetics using 31P NMR spectroscopy and contractile performance of the intact beating heart revealed both a decrease in the free energy of ATP hydrolysis available to support contractile work and a marked inability to increase contractile performance upon acute inotropic challenge in hearts from R92Q mice. These results show that alterations in thin filament protein structure and function can lead to significant defects in myocardial energetics and contractile reserve.

Mechanism of regulation of Acanthamoeba myosin-IC by heavy-chain phosphorylation.

The ATPase activity of myosin-Is from lower eukaryotes is activated by phosphorylation by the p21-activated kinase family at the TEDS site on an actin-binding surface-loop. This actin-binding loop is the site of a cardiac myosin-II mutation responsible for some forms of familial hypertrophic cardiomyopathy. To determine the mechanism of myosin-I regulation by heavy-chain phosphorylation (HCP) and to better understand the importance of this loop in the function of all myosin isoforms, we performed a kinetic investigation of the regulatory mechanism of the Acanthamoeba myosin-IC motor domain (MIC(IQ)). Phosphorylated and dephosphorylated MIC(IQ) show actin-activated ATPase activity; however, HCP increases the ATPase activity >20-fold. HCP does not greatly affect the rate of phosphate release from MIC in the absence of actin, as determined by single turnover experiments. Additionally, HCP does not significantly affect the affinity of myosin for actin in the absence or presence of ATP, the rate of ATP-induced dissociation of actoMIC(IQ), the affinity of ADP, or the rate of ADP release. Sequential-mix single-turnover experiments show that HCP regulates the rate of phosphate release from actin-bound MIC(IQ). We propose that the TEDS-containing actin-binding loop plays a direct role in regulating phosphate release and the force-generating (A-to-R) transition of myosin-IC.

Homozygotes for a R869G Mutation in the β -myosin Heavy Chain Gene have a Severe Form of Familial Hypertrophic Cardiomyopathy

Familial Hypertrophic Cardiomyopathy (FHC) is an autosomal dominant disease characterised by ventricular hypertrophy, with predominant involvement of the interventricular septum. It is a monogenic disease with a high level of genetic heterogeneity (nine genes and more than 110 mutations reported so far). We describe a family with a new R869G mutation in theβ -myosin heavy chain gene (MYH7). This mutation was found in the heterozygous status in both parents and in the homozygous status in the two children. A haplotype analysis on the MYH7 locus with microsatellite markers showed that the same haplotype is transmitted within the family, suggesting a founder effect. Clinically, the father was asymptomatic with mild left ventricular hypertrophy on echocardiography. The mother had a mild form of hypertrophic cardiomyopathy and remained asymptomatic until 60 years old when an atrial fibrillation occurred. For the two children, clinical diagnosis was performed at 12 and 8 years and atrial fibrillation occurred at 17 years. For both children, the evolution was characterized by left ventricle (LV) systolic dysfunction and a severe dilatation of the left atrium before 40 years of age. Conclusions: In this family, a new R869G mutation in the MYH7 gene was found. Interestingly, a mutation was found at the homozygous status for the first time in FHC. This finding suggests that this particular mutation is compatible with life, but for homozygous subjects, age at onset of symptoms was earlier and the disease much more severe than in the heterozygous subjects, suggesting a gene-dose effect.

Animal models of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is an autosomal-dominant disease that is both clinically and genetically heterogeneous. Disease-causing mutations have been found in eight genes encoding structural components of the thick and thin filament systems of the cardiac myocyte; it has therefore been coined a disease of the sarcomere. How each mutation leads to the diverse clinical phenotypes is still obscure, and research in this area is very active. Many approaches have been used to characterize the pathogenesis of the disease. Biochemical characterization of mutant alleles expressed in vitro has shed some insight into the functional deficits of several mutant alleles of myosin heavy chain, troponin-T, and alpha-tropomyosin. Transgenic animal models for FHC have been created to gain further insight into the pathogenesis of this disease. Most of these models have been made in mice; however, recently a transgenic rabbit model has been created. In addition, there are several natural-occurring forms of FHC in animals that will be interesting to explore. The discovery of additional responsible genes and the elucidation of the molecular mechanisms of pathogenesis through the use of animal models promise improved and early diagnosis and the potential for developing specific, mutation-, or mechanism-based therapeutics.

Altered crossbridge kinetics in the alphaMHC403/+ mouse model of familial hypertrophic cardiomyopathy.

A mutation in the cardiac beta-myosin heavy chain, Arg403Gln (R403Q), causes a severe form of familial hypertrophic cardiomyopathy (FHC) in humans. We used small-amplitude (0.25%) length-perturbation analysis to examine the mechanical properties of skinned left ventricular papillary muscle strips from mouse hearts bearing the R403Q mutation in the alpha-myosin heavy chain (alphaMHC403/+). Myofibrillar disarray with variable penetrance occurred in the left ventricular free wall of the alphaMHC403/+ hearts. In resting strips (pCa 8), dynamic stiffness was approximately 40% greater than in wild-type strips, consistent with elevated diastolic stiffness reported for murine hearts with FHC. At pCa 6 (submaximal activation), strip isometric tension was approximately 3 times higher than for wild-type strips, whereas at pCa 5 (maximal activation), tension was marginally lower. At submaximal calcium activation the characteristic frequencies of the work-producing (b) and work-absorbing (c) steps of the crossbridge were less in alphaMHC403/+ strips than in wild-type strips (b=11+/-1 versus 15+/-1 Hz; c= 58+/-3 versus 66+/-3 Hz; 27 degrees C). At maximal calcium activation, strip oscillatory power was reduced (0. 53+/-0.25 versus 1.03+/-0.18 mW/mm3; 27 degrees C), which is partly attributable to the reduced frequency b, at which crossbridge work is maximum. The results are consistent with the hypothesis that the R403Q mutation reduces the strong binding affinity of myosin for actin. Myosin heads may accumulate in a preforce state that promotes cooperative activation of the thin filament at submaximal calcium but blunts maximal tension and oscillatory power output at maximal calcium. The calcium-dependent effect of the mutation (whether facilitating or debilitating), together with a variable degree of fibrosis and myofibrillar disorder, may contribute to the diversity of clinical symptoms observed in murine FHC.