Gender and aging in a transgenic mouse model of hypertrophic cardiomyopathy

Mutations in the cardiac myosin heavy chain (MHC) can cause familial hypertrophic cardiomyopathy (FHC). A transgenic mouse model has been developed in which a missense (R403Q) allele and an actin-binding deletion in the alpha-MHC are expressed in the heart. We used an isovolumic left heart preparation to study the contractile characteristics of hearts from transgenic (TG) mice and their wild-type (WT) littermates. Both male and female TG mice developed left ventricular (LV) hypertrophy at 4 mo of age. LV hypertrophy was accompanied by LV diastolic dysfunction, but LV systolic function was normal and supranormal in the young TG females and males, respectively. At 10 mo of age, the females continued to present with LV concentric hypertrophy, whereas the males began to display LV dilation. In female TG mice at 10 mo of age, impaired LV diastolic function persisted without evidence of systolic dysfunction. In contrast, in 10-mo-old male TG mice, LV diastolic function worsened and systolic performance was impaired. Diminished coronary flow was observed in both 10-mo-old TG groups. These types of changes may contribute to the functional decompensation typically seen in hypertrophic cardiomyopathy. Collectively, these results further underscore the potential utility of this transgenic mouse model in elucidating pathogenesis of FHC.     

Alterations of tension-dependent ATP utilization in a transgenic rat model of hypertrophic cardiomyopathy

Although it is established that familial hypertrophic cardiomyopathy (FHC) is caused by mutations in several sarcomeric proteins, including cardiac troponin T (TnT), its pathogenesis is still not completely understood. Previously, we established a transgenic rat model of FHC expressing a human TnT molecule with a truncation mutation (DEL-TnT). This study investigated whether contractile dysfunction and electrical vulnerability observed in DEL-TnT rats might be due to alterations of intracellular Ca(2+) homeostasis, myofibrillar Ca(2+) sensitivity, and/or myofibrillar ATP utilization. Simultaneous measurements of the force of contraction and intracellular Ca(2+) transients were performed in right ventricular trabeculae of DEL-TnT hearts at 0.25 and 1.0 Hz. Rats expressing wild-type human TnT as well as nontransgenic rats served as controls. In addition, calcium-dependent ATPase activity and tension development were investigated in skinned cardiac muscle fibers. Force of contraction was significantly decreased in DEL-TnT compared with nontransgenic rats and TnT. Time parameters of Ca(2+) transients were unchanged at 0.25 Hz but prolonged at 1.0 Hz in DEL-TnT. The amplitude of the fura-2 transient was similar in all groups investigated, whereas diastolic and systolic fura-2 ratios were found elevated in rats expressing nontruncated human troponin T. In DEL-TnT rats, myofibrillar Ca(2+)-dependent tension development as well as Ca(2+) sensitivity of tension were significantly decreased, whereas tension-dependent ATP consumption ("tension cost") was markedly increased. Thus, a C-terminal truncation of the cardiac TnT molecule impairs the force-generating capacity of the cycling cross-bridges resulting in increased tension-dependent ATP utilization. Taken together, our data support the hypothesis of energy compromise as a contributing factor in the pathogenesis of FHC.     

Mice expressing mutant myosin heavy chains are a model for familial hypertrophic cardiomyopathy.

BACKGROUND: Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by ventricular hypertrophy, myocellular disarray, arrhythmias, and sudden death. Mutations in several contractile proteins, including cardiac myosin heavy chains, have been described in families with this disease, leading to the hypothesis that HCM is a disease of the sarcomere. MATERIALS AND METHODS: A mutation in the myosin heavy chain (Myh) predicted to interfere strongly with myosin's binding to actin was designed and used to create an animal model for HCM. Five independent lines of transgenic mice were produced with cardiac-specific expression of the mutant Myh. RESULTS: Although the mutant Myh represents a small proportion (1-12%) of the heart's myosin, the mice exhibit the cardiac histopathology seen in HCM patients. Histopathology is absent from the atria and primarily restricted to the left ventricle. The line exhibiting the highest level of mutant Myh expression demonstrates ventricular hypertrophy by 12 weeks of age, but the further course of the disease is strongly affected by the sex of the animal. Hypertrophy increases with age in female animals while the hearts of male show severe dilation by 8 months of age, in the absence of increased mass. CONCLUSIONS: The low levels of the transgene protein in the presence of the phenotypic features of HCM suggest that the mutant protein acts as a dominant negative. In addition, the distinct phenotypes developed by aging male or female transgenic mice suggest that extragenic factors strongly influence the development of the disease phenotype.     

Mapping the locus for familial hypertrophic cardiomyopathy to chromosome 11 in a family with a case of apical hypertrophic cardiomyopathy of the Japanese type

To identify the disease locus of familial hypertrophic cardiomyopathy (FHC) in a Chinese family, a genetic linkage study was performed using polymorphisms from various chromosomal regions. This family has eight affected members, including a case with typical features of apical hypertrophic cardiomyopathy of the Japanese type. The results revealed significant evidence of linkage of polymorphisms on chromosome 11p13–q13 and FHC in this family with a maximal lod score of 3.38 at θ = 0.00. Our data suggest that the locus responsible for FHC in this family maps to chromosome 11 and that the molecular basis of FHC in the case of apical hypertrophic cardiomyopathy of the Japanese type might be similar to that of other affected members in the same family. Further studies are needed to elucidate the whole spectrum of the genetic basis of apical hypertrophic cardiomyopathy of the Japanese type. Received: 15 June 1995 / Revised: 22 August 1995     

Echocardiographic-determined septal morphology in Z-disc hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) can be classified into at least four major anatomic subsets based upon the septal contour, and the location and extent of hypertrophy: reverse curvature-, sigmoidal-, apical-, and neutral contour-HCM. Here, we sought to identify genetic determinants for sigmoidal-HCM and hypothesized that Z-disc-HCM may be associated preferentially with a sigmoidal phenotype. Utilizing PCR, DHPLC, and direct DNA sequencing, we performed mutational analysis of five genes encoding cardiomyopathy-associated Z-disc proteins. The study cohort consisted of 239 unrelated patients with HCM previously determined to be negative for mutations in the eight genes associated with myofilament-HCM. Blinded to the Z-disc genotype status, the septal contour was graded qualitatively using standard transthoracic echocardiography. Thirteen of the 239 patients (5.4%) had one of 13 distinct HCM-associated Z-disc mutations involving residues highly conserved across species and absent in 600 reference alleles: LDB3 (6), ACTN2 (3), TCAP (1), CSRP3 (1), and VCL (2). For this subset with Z-disc-associated HCM, the septal contour was sigmoidal in 11 (85%) and apical in 2 (15%). While Z-disc-HCM is uncommon, it is equal in prevalence to thin filament-HCM. In contrast to myofilament-HCM, Z-disc-HCM is associated preferentially with sigmoidal morphology.     

Structural analysis of obscurin gene in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a cardiac disease characterized by left ventricular hypertrophy with diastolic dysfunction. Molecular genetic studies have revealed that HCM is caused by mutations in genes for sarcomere/Z-band components including titin/connectin and its associate proteins. However, disease-causing mutations can be found in about half of the patients, suggesting that other disease-causing genes remain to be identified. To explore a novel disease gene, we searched for obscurin gene (OBSCN) mutations in HCM patients, because obscurin interacts with titin/connectin. Two linked variants, Arg4344Gln and Ala4484Thr, were identified in a patient and functional analyses demonstrated that Arg4344Gln affected binding of obscurin to Z9-Z10 domains of titin/connectin, whereas Ala4484Thr did not. Myc-tagged obscurin showed that Arg4344Gln impaired obscurin localization to Z-band. These observations suggest that the obscurin abnormality may be involved in the pathogenesis of HCM.     

肥厚型心肌病的致病分子机制研究进展

肥厚型心肌病(Hypertrophic cardiomyopathy,HCM)是以左心室及室间隔不对称肥厚为基本特征的原发性心肌病,其发病率约为0.2%,是青少年和运动员心源性猝死的最常见原因。HCM的发病年龄、发病程度和猝死风险等临床表型具有多样性,通常呈常染色体显性遗传。目前已报道的HCM相关突变超过900种,主要定位在β肌球蛋白重链基因、肌球蛋白结合蛋白C基因、心脏肌钙蛋白T基因等13个心脏肌节蛋白基因;另一方面,越来越多的研究显示线粒体基因突变与HCM发生相关。文章在简单介绍HCM形态学特征及临床表型的基础上,着重综述了HCM的致病分子机制及其最新研究进展。     

A mathematical model of the myogenic response to systolic pressure in the afferent arteriole

Elevations in systolic blood pressure are believed to be closely linked to the pathogenesis and progression of renal diseases. It has been hypothesized that the afferent arteriole (AA) protects the glomerulus from the damaging effects of hypertension by sensing increases in systolic blood pressure and responding with a compensatory vasoconstriction (Loutzenhiser R, Bidani A, Chilton L. Circ Res 90: 1316-1324, 2002). To investigate this hypothesis, we developed a mathematical model of the myogenic response of an AA wall, based on an arteriole model (Gonzalez-Fernandez JM, Ermentrout B. Math Biosci 119: 127-167, 1994). The model incorporates ionic transport, cell membrane potential, contraction of the AA smooth muscle cell, and the mechanics of a thick-walled cylinder. The model represents a myogenic response based on a pressure-induced shift in the voltage dependence of calcium channel openings: with increasing transmural pressure, model vessel diameter decreases; and with decreasing pressure, vessel diameter increases. Furthermore, the model myogenic mechanism includes a rate-sensitive component that yields constriction and dilation kinetics similar to behaviors observed in vitro. A parameter set is identified based on physical dimensions of an AA in a rat kidney. Model results suggest that the interaction of Ca(2+) and K(+) fluxes mediated by voltage-gated and voltage-calcium-gated channels, respectively, gives rise to periodicity in the transport of the two ions. This results in a time-periodic cytoplasmic calcium concentration, myosin light chain phosphorylation, and cross-bridge formation with the attending muscle stress. Furthermore, the model predicts myogenic responses that agree with experimental observations, most notably those which demonstrate that the renal AA constricts in response to increases in both steady and systolic blood pressures. The myogenic model captures these essential functions of the renal AA, and it may prove useful as a fundamental component in a multiscale model of the renal microvasculature suitable for investigations of the pathogenesis of hypertensive renal diseases.     

Image and DNA analysis of hypertrophic myocytes in hypertensive heart disease and hypertrophic cardiomyopathy

OBJECTIVE: To investigate differences in the pathophysiology of cardiac hypertrophy between patients with hypertensive heart disease (HHD) and hypertrophic cardiomyopathy (HCM). STUDY DESIGN: The study group consisted of 30 autopsied heart disease patients (10 HHD, 10 HCM and 10 noncardiac heart disease). DNA synthesis by hypertrophic cardiac myocytes was examined, and three-dimensional myocyte structure image was investigated. DNA synthesis and the cell cycle were investigated by flow cytometry using autopsy material. Three-dimensional myocyte structure image was visualized. RESULTS: The percentage of cells in G2M phase of the cell cycle was significantly decreased in the myocardium of autopsied hearts with HCM as compared with hearts with HHD (HCM:HHD = 1.2 +/- 1.1%: 7.7 +/- 2.6%, mean +/- SD). Hypertrophic myocytes of HCM characteristically possessed myocardial disarray and irregular side-to-side branch connections between myocytes. No myocyte disarray or irregular connections could be observed in HHD. CONCLUSION: These results suggest that the mechanism of cardiac hypertrophy differs between patients with HHD and HCM and also suggest dissimilar cell vitality and latent proliferative viability of hypertrophic myocytes in a hypertrophic process between HHD and HCM. That is, hypertrophic myocytes may be called "restricted" myocytes in a morphologic and biochemical sense.