Hypertrophic cardiomyopathy is characterised by a histological phenotype of myocyte disarray, but heart tissue samples from patients with dilated cardiomyopathy (DCM) often look comparatively similar to those from healthy individuals apart from conspicuous regions of fibrosis and necrosis. We have previously investigated subcellular alterations in the cytoarchitecture of mouse models of dilated cardiomyopathy and found that both the organisation and composition of the intercalated disc, i.e. the specialised type of cell–cell contact in the heart, is altered. There is also is a change in the composition of the M-band of the sarcomere due to an expression shift towards the more extensible embryonic heart (EH)-myomesin isoform. Analysis of human samples from the Sydney Human Heart Tissue Bank have revealed similar structural findings and also provided evidence for a dramatic change in overall cardiomyocyte size control, which has also been seen in the mouse. Together these changes in cytoarchitecture probably contribute to the decreased functional output that is seen in DCM.
Cytoskeleton; Intercalated disc; Dilated cardiomyopathy; Formin; M-band
The expression of the myofibrillar M-band proteins myomesin and M- protein was studied in chicken pectoral muscle and heart during differentiation using monoclonal antibodies in a double-antibody sandwich enzyme-linked immunosorbent assay, immunoblotting, and immunocytochemistry. In presumptive pectoral muscle, myomesin accumulated first, increasing from 2% of the adult concentration at day 7 to 70% by day 16 in ovo. M-protein accumulation lagged 6-7 d behind that of myomesin attaining only 40% of the adult concentration in ovo. The molecular masses of myomesin (185 kD) and M-protein (165 kD) remained constant during embryogenesis. In cultured myogenic cells the accumulation and M-band localization of myomesin preceded that of M- protein by 1.5 d. Chicken heart was shown, in addition to M-protein, to contain unique isoforms of myomesin. In hearts of 6 d embryos, a 195-kD myomesin isoform was the major species; throughout development, however, a transition to a mixture of 195 and 190 kD was observed, the latter being the major species in the adult tissue. During heart differentiation the initial accumulation of myomesin again preceded that of M-protein, albeit on an earlier time scale than in pectoral muscle with M-protein reaching adult proportions first.
Myomesin is a 185-kDa protein located in the M-band of striated muscle where it interacts with myosin and titin, possibly connecting thick filaments with the third filament system. By using expression of epitope-tagged myomesin fragments in cultured cardiomyocytes and biochemical binding assays, we could demonstrate that the M-band targeting activity and the myosin-binding site are located in different domains of the molecule. An N-terminal immunoglobulin-like domain is sufficient for targeting to the M-band, but solid-phase overlay assays between individual N-terminal domains and the thick filament protein myosin revealed that the unique head domain contains the myosin-binding site. When expressed in cardiomyocytes, the head domains of rat and chicken myomesin showed species-specific differences in their incorporation pattern. The head domain of rat myomesin localized to a central area within the A-band, whereas the head domain of chicken myomesin was diffusely distributed in the cytoplasm. We therefore conclude that the head domain of myomesin binds to myosin but that this affinity is not sufficient for the restriction of the domain to the M-band in vivo. Instead, the neighboring immunoglobulin-like domain is essential for the precise incorporation of myomesin into the M-band, possibly because of interaction with a yet unknown protein of the sarcomere.
Dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM) are classic forms of systolic and diastolic heart failure, respectively. Mutations in genes encoding sarcomere and cytoskeletal proteins are major causes of HCM and DCM. MURC, encoding muscle-restricted coiled-coil, a Z line protein, regulates cardiac function in mice. We investigated potential causal role of MURC in human cardiomyopathies.
Methods and Results
We sequenced MURC in 1,199 individuals including 383 probands with DCM, 307 with HCM and 509 healthy controls. We found six heterozygous DCM-specific missense variants (p.N128K, p.R140W, p.L153P, p.S307T, p.P324L and p.S364L) in eight unrelated probands. Variants p.N128K and p.S307T segregated with inheritance of DCM in small families (χ2=8.5, p=0.003). Variants p.N128K, p.R140W, p.L153P and p.S364L were considered probably or possibly damaging. Variant p.P324L recurred in three independent probands, including one proband with a TPM1 mutation (p.M245T). A deletion variant (p.L232-R238del) was present in three unrelated HCM probands but it did not segregate with HCM in a family who also had a MYH7 mutation (p.L970V). The phenotype in mutation carriers was notable for progressive heart failure leading to heart transplantation in four patients, conduction defects and atrial arrhythmias. Expression of mutant MURC proteins in neonatal rat cardiac myocytes transduced with recombinant adenoviruses was associated with reduced RhoA activity, lower mRNA levels of hypertrophic markers and smaller myocyte size as compared to wild type MURC.
MURC mutations impart loss-of-function effects on MURC functions and are likely causal variants in human DCM. The causal role of a deletion mutation in HCM is uncertain.
heart failure; genetics; cardiomyopathy; mutation; RhoA
Dilated cardiomyopathy (DCM), a genetically heterogeneous disorder, causes heart failure and rhythm disturbances. The majority of identified DCM genes encode structural proteins of the contractile apparatus and cytoskeleton. Recently, genetic defects in calcium and potassium regulation have been discovered in patients with DCM, implicating an alternative disease mechanism. The full spectrum of genetic defects in DCM, however, has not been established.
To identify a novel gene for DCM at a previously mapped locus, define the spectrum of mutations in this gene within a DCM cohort, and determine the frequency of DCM among relatives inheriting a mutation in this gene.
Design, Setting, and Participants
Refined mapping of a DCM locus on chromosome 3p in a multigenerational family and mutation scanning in 156 unrelated pro-bands with DCM, prospectively identified at the Mayo Clinic between 1987 and 2004. Relatives underwent screening echocardiography and electrocardiography and DNA sample procurement.
Main Outcome Measure
Correlation of identified mutations with cardiac phenotype.
Refined locus mapping revealed SCN5A, encoding the cardiac sodium channel, as a candidate gene. Mutation scans identified a missense mutation (D1275N) that cosegregated with an age-dependent, variably expressed phenotype of DCM, atrial fibrillation, impaired automaticity, and conduction delay. In the DCM cohort, additional missense (T220I, R814W, D1595H) and truncation (2550-2551insTG) SCN5A mutations, segregating with cardiac disease or arising de novo, were discovered in unrelated probands. Among individuals with an SCN5A mutation 27% had early features of DCM (mean age at diagnosis, 20.3 years), 38% had DCM (mean age at diagnosis, 47.9 years), and 43% had atrial fibrillation (mean age at diagnosis, 27.8 years).
Heritable SCN5A defects are associated with susceptibility to early-onset DCM and atrial fibrillation. Similar or even identical mutations may lead to heart failure, arrhythmia, or both.
Dilated Cardiomyopathy (DCM) is characterized by systolic dysfunction, followed by heart failure necessitating cardiac transplantation. The genetic basis is well established by the identification of mutations in sarcomere and cytoskeleton gene/s. Modifier genes and environmental factors are also considered to play a significant role in the variable expression of the disease, hence various mechanisms are implicated and one such mechanism is oxidative stress. Nitric Oxide (NO), a primary physiological transmitter derived from endothelium seems to play a composite role with diverse anti-atherogenic effects as vasodilator. Three functional polymorphisms of endothelial nitric oxide synthase (NOS3) gene viz., T-786C of the 5′ flanking region, 27bp VNTR in intron4 and G894T of exon 7 were genotyped to identify their role in DCM. A total of 115 DCM samples and 454 controls were included. Genotyping was carried out by PCR -RFLP method. Allelic and genotypic frequencies were computed in both control & patient groups and appropriate statistical tests were employed. A significant association of TC genotype (T-786C) with an odds ratio of 1.74, (95% CI 1.14 - 2.67, p = 0.01) was observed in DCM. Likewise the GT genotypic frequency of G894T polymorphism was found to be statistically significant (OR 2.10, 95% CI 1.34–3.27, p = 0.0011), with the recessive allele T being significantly associated with DCM (OR 1.64, 95% CI 1.18 - 2.30, p = 0.003). The haplotype carrying the recessive alleles of G894T and T-786C, C4bT was found to exhibit 7 folds increased risk for DCM compared to the controls. Hence C4bT haplotype could be the risk haplotype for DCM. Our findings suggest the possible implication of NOS3 gene in the disease phenotype, wherein NOS3 may be synergistically functioning in DCM associated heart failure via the excessive production of NO in cardiomyocytes resulting in decreased myocardial contractility and systolic dysfunction, a common feature of DCM phenotype.
Myofibrillogenesis, the process of sarcomere formation, requires close interactions of sarcomeric proteins and various components of sarcomere structures. The myosin thick filaments and M-lines are two key components of the sarcomere. It has been suggested that myomesin proteins of M-lines interact with myosin and titin proteins and keep the thick and titin filaments in order. However, the function of myomesin in myofibrillogenesis and sarcomere organization remained largely enigmatic. No knockout or knockdown animal models have been reported to elucidate the role of myomesin in sarcomere organization in vivo. In this study, by using the gene-specific knockdown approach in zebrafish embryos, we carried out a loss-of-function analysis of myomesin-3 and slow myosin heavy chain 1 (smyhc1) expressed specifically in slow muscles. We demonstrated that knockdown of smyhc1 abolished the sarcomeric localization of myomesin-3 in slow muscles. In contrast, loss of myomesin-3 had no effect on the sarcomeric organization of thick and thin filaments as well as M- and Z-line structures. Together, these studies indicate that myosin thick filaments are required for M-line organization and M-line localization of myomesin-3. In contrast, myomesin-3 is dispensable for sarcomere organization in slow muscles.
Myosin; Myomesin 3; M-line; Sarcomere
Cardiomyopathies are a heterogeneous group of heart muscle disorders and are classified as 1) Hypertrophic Cardiomyopathy (HCM) 2) Dilated cardiomyopathy (DCM) 3) Restrictive cardiomyopathy (RCM) and 4) Arrhythmogenic right ventricular dysplasia (ARVD) as per WHO classification, of which HCM and DCM are common. HCM is a complex but relatively common form of inherited heart muscle disease with prevalence of 1 in 500 individuals and is commonly associated with sarcomeric gene mutations. Cardiac muscle troponin I (TNNI-3) is one such sarcomeric protein and is a subunit of the thin filament-associated troponin-tropomyosin complex involved in calcium regulation of skeletal and cardiac muscle contraction. Mutations in this gene were found to be associated with a history of sudden cardiac death in HCM patients.
Therefore the present study aims to identify for mutations associated with troponin I gene in a set of HCM patients from Indian population.
MATERIALS AND METHODS:
Mutational analyses of 92 HCM cases were carried out following PCR based SSCP analysis.
The study revealed band pattern variation in 3 cases from a group of 92 HCM patients. This band pattern variation, on sequencing revealed base changes, one at nt 2560 with G>T transversion in exon-5 region with a wobble and others at nt 2479 and nt 2478 with G>C and C>G transversions in the intronic region upstream of the exon 5 on sequencing. Further analysis showed that one of the probands showed apical form of hypertrophy, two others showing asymmetric septal hypertrophy. Two of these probands showed family history of the condition.
Hence, the study supports earlier reports of involvement of TNNI-3 in the causation of apical and asymmetrical forms of hypertrophy.
Genetic variation; hypertrophic cardiomyopathy; sudden cardiac death; troponin-I
Human myofibrillogenesis regulator 1, a novel 17-kDa protein, is closely involved in cardiac hypertrophy. We studied the molecular mechanism that links MR-1 to hypertrophic response. Hypertrophic hallmarks such as cell size and [3H]-leucine incorporation were significantly increased when MR-1 was transfected into cardiomyocytes for 48 h. However, sarcomere organization was promoted when MR-1 was transfected for 8 h. The finding that cardiac hypertrophy was induced long after increase of sarcomere organization indicates that the promoted sarcomere organization may be one of the crucial factors causing hypertrophy. Furthermore, when MR-1 was transfected into cardiomyocytes, the nuclear localization of myomesin-1 was shifted to the cytoplasm. Transfection with small ubiquitin-like modifier-1 (SUMO-1) mimicked the effect of MR-1 inducing translocation of myomesin-1. However, transfection with SUMO-1 in MR-1-silenced cardiomyocytes failed to induce translocation and sarcomere organization, even though SUMO-1 expression was at the same level. Overexpression of MR-1 may induce cardiomyocyte hypertrophy via myomesin-1-mediated sarcomere organization.
cardiomyocyte; hypertrophy; myofibrillogenesis regulator; sarcomere organization
Mutations in a variety of myofibrillar genes cause dilated cardiomyopathy (DCM) in humans, usually with dominant inheritance and incomplete penetrance. Here, we sought to clarify the functional effects of the previously identified DCM-causing TTN 2-bp insertion mutation (c.43628insAT) and generated a titin knock-in mouse model mimicking the c.43628insAT allele.
Mutant embryos homozygous for the Ttn knock-in mutation developed defects in sarcomere formation and consequently died before E9.5. Heterozygous mice were viable and demonstrated normal cardiac morphology, function and muscle mechanics. mRNA and protein expression studies on heterozygous hearts demonstrated elevated wild-type titin mRNA under resting conditions, suggesting that up-regulation of the wild-type titin allele compensates for the unstable mutated titin under these conditions.
When chronically exposed to angiotensin II or isoproterenol, heterozygous mice developed marked left ventricular dilatation (p<0.05) with impaired fractional shortening (p<0.001) and diffuse myocardial fibrosis (11.95 ± 2.8% versus 3.7 ±1.1%). Thus, this model mimics typical features of human dilated cardiomyopathy and may further our understanding of how titin mutations perturb cardiac function and remodel the heart.
Cardiomyopathy; Development; Sarcomere formation; Genetics; Mouse model; Heart failure; Pathogenesis; Titin
Myocardial hibernation (MH) is a well-known feature of human ischaemic cardiomyopathy (ICM), whereas its presence in human idiopathic dilated cardiomyopathy (DCM) is still controversial. We investigated the histological and molecular features of MH in left ventricle (LV) regions of failing DCM or ICM hearts. We examined failing hearts from DCM (n = 11; 41.9 ± 5.45 years; left ventricle-ejection fraction (LV-EF), 18 ± 3.16%) and ICM patients (n = 12; 58.08 ± 1.7 years; LVEF, 21.5 ± 6.08%) undergoing cardiac transplantation, and normal donor hearts (N, n = 8). LV inter-ventricular septum (IVS) and antero-lateral free wall (FW) were transmurally (i.e. sub-epicardial, mesocardial and sub-endocardial layers) analysed. LV glycogen content was shown to be increased in both DCM and ICM as compared with N hearts (P < 0.001), with a U-shaped transmural distribution (lower values in mesocardium). Capillary density was homogenously reduced in both DCM and ICM as compared with N (P < 0.05 versus N), with a lower decrease independent of the extent of fibrosis in sub-endocardial and sub-epicardial layers of DCM as compared with ICM. HIF1-α and nestin, recognized ischaemic molecular hallmarks, were similarly expressed in DCM-LV and ICM-LV myocardium. The proteomic profile was overlapping by ˜50% in DCM and ICM groups. Morphological and molecular features of MH were detected in end-stage ICM as well as in end-stage DCM LV, despite epicardial coronary artery patency and lower fibrosis in DCM hearts. Unravelling the presence of MH in the absence of coronary stenosis may be helpful to design a novel approach in the clinical management of DCM.
pathologic features; hibernating myocardium; chronic heart failure; idiopathic dilated cardiomyopathy; ischaemic microenvironment; nestin
Four variants (K60N, Q128R, G202R and A592E) in the nebulette gene (NEBL) were identified in patients with dilated cardiomyopathy (DCM) and endocardial fibroelastosis (EFE). We sought to determine if these mutations cause cardiomyopathy.
Nebulette aligns thin filaments and connects them with the myocardial Z-disk.
We produced transgenic mice with cardiac-restricted over-expression of human wild-type (WT) or mutant nebulette. Chimera and transgenic mice were examined at 4, 6 and 12 months of age by echocardiography and cardiac MRI. The hearts from embryos and adult mice were assessed by histopathologic, immunohistochemical, ultrastructural and protein analyses. Rat H9C2 cardiomyoblasts with transient expression of nebulette underwent cyclic mechanical strain.
We identified lethal cardiac structural abnormalities in mutant embryonic hearts (K60N and Q128R). Founders of the mutant mice lines developed DCM with severe heart failure. An irregular localization pattern for nebulette and impaired desmin expression was noted in the proband and chimera Q128R mice. Mutant G202R and A592E mice exhibited left ventricular dilation and impaired cardiac function accompanied with the specific changes in I-band or Z-disk proteins by 6 months of age, respectively. The mutations modulated distribution of nebulette in the sarcomere and the Z-disks during stretch of H9C2 cells.
NEBL is a new susceptibility gene for EFE and DCM. Different mutations in nebulette trigger specific mechanisms converging to a common pathological cascade leading to EFE and DCM.
nebulette; dilated cardiomyopathy; endocardial fibroelastosis; Z-disk
Cardiomyocyte contraction is regulated by phosphorylation of sarcomeric proteins. Throughout the heart regional and transmural differences may exist in protein phosphorylation. In addition, phosphorylation of sarcomeric proteins is altered in cardiac disease. Heterogeneity in protein phosphorylation may be larger in hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) as it may be caused by multiple mutations in genes encoding different sarcomeric proteins. Moreover, HCM is characterized by asymmetric remodelling of the heart. In the present study we assessed if local differences in sarcomeric protein phosphorylation are more evident in primary HCM or DCM than in non-failing donors. Thereto, phosphorylation of the two main target proteins of the beta-adrenergic receptor pathway, troponin I (cTnI) and myosin binding protein C (cMyBP-C) was analysed in different parts in the free left ventricular wall of end–stage failing HCM and DCM patients and donors obtained during transplant surgery. Intra-patient variability in protein phosphorylation within tissue samples of approximately 2 g wet weight was comparable between donor, HCM and DCM samples and could partly be attributed to the precision of the technique. Thus, our data indicate that within the precision of the measurements small, biopsy-sized cardiac tissue samples are representative for the region of the free left ventricular wall from which they were obtained.
Cardiomyopathy; Phosphorylation; Physiology
The muscle M-band protein myomesin comprises a 36 nm long filament made of repetitive immunoglobulin–helix modules that can stretch to 2.5-fold this length, demonstrating substantial molecular elasticity.
Active muscles generate substantial mechanical forces by the contraction/relaxation cycle, and, to maintain an ordered state, they require molecular structures of extraordinary stability. These forces are sensed and buffered by unusually long and elastic filament proteins with highly repetitive domain arrays. Members of the myomesin protein family function as molecular bridges that connect major filament systems in the central M-band of muscle sarcomeres, which is a central locus of passive stress sensing. To unravel the mechanism of molecular elasticity in such filament-connecting proteins, we have determined the overall architecture of the complete C-terminal immunoglobulin domain array of myomesin by X-ray crystallography, electron microscopy, solution X-ray scattering, and atomic force microscopy. Our data reveal a dimeric tail-to-tail filament structure of about 360 Å in length, which is folded into an irregular superhelical coil arrangement of almost identical α-helix/domain modules. The myomesin filament can be stretched to about 2.5-fold its original length by reversible unfolding of these linkers, a mechanism that to our knowledge has not been observed previously. Our data explain how myomesin could act as a highly elastic ribbon to maintain the overall structural organization of the sarcomeric M-band. In general terms, our data demonstrate how repetitive domain modules such as those found in myomesin could generate highly elastic protein structures in highly organized cell systems such as muscle sarcomeres.
The contraction and relaxation cycles of active muscles generate substantial mechanical forces, both axially and radially, that place extraordinary stress on the molecular structures within the muscle fibers. These forces are sensed and buffered by unusually long and elastic filament proteins with highly repetitive domain structures. Myomesin is one such repetitive filament protein that is thought to form bridges between the main contractile filaments of the muscle, providing the muscle structure with resistance in the radial dimension. To investigate how the repetitive structure of myomesin contributes to muscle elasticity, we determined the overall architecture of its complete repetitive domain array using a combination of four complementary structural biology methods. Our study reveals a long, dimeric tail-to-tail filament structure folded into an irregular superhelical coil arrangement of almost identical domain modules separated by short linkers. When we applied tension to these myomesin filaments, we found they could stretch to about 2.5 times their original length by unfolding these linkers, and then return to their original state when the tension was removed. Our findings explain how myomesin might adapt its overall length in response to the changing dimensions of the contracting and relaxing muscle, so acting as a highly elastic ribbon that maintains the overall structural organization of the muscle fibers. More generally, these findings demonstrate how repetitive domain modules, such as those in myomesin, can provide elasticity to highly organized biological structures.
Myosin binding protein C (MYBPC) is a crucial component of the sarcomere and an important regulator of muscle function. While mutations in different myosin binding protein C (MYBPC) genes are well known causes of various human diseases, such as hypertrophic (HCM) and dilated (DCM) forms of cardiomyopathy as well as skeletal muscular disorders, the underlying molecular mechanisms remain not well understood. A variety of MYBPC3 (cardiac isoform) mutations have been studied in great detail and several corresponding genetically altered mouse models have been generated. Most MYBPC3 mutations may cause haploinsufficiency and with it they may cause a primary increase in calcium sensitivity which is potentially able to explain major features observed in HCM patients such as the hypercontractile phenotype and the well known secondary effects such as myofibrillar disarray, fibrosis, myocardial hypertrophy and remodelling including arrhythmogenesis. However the presence of poison peptides in some cases cannot be fully excluded and most probably other mechanisms are also at play. Here we shall discuss MYBPC interacting proteins and possible pathways linked to cardiomyopathy and heart failure.
Myosin binding protein C; Myocardial function
TTN-encoded titin, CSRP3-encoded muscle LIM protein, and TCAP-encoded telethonin are Z-disc proteins essential for the structural organization of the cardiac sarcomere and the cardiomyocyte’s stretch sensor. All three genes have been established as cardiomyopathy-associated genes for both dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM). Here, we sought to characterize the frequency, spectrum, and phenotype associated with HCM-associated mutations in these three genes in a large cohort of unrelated patients evaluated at a single tertiary outpatient center.
DNA was obtained from 389 patients with HCM (215 male, left ventricular wall thickness of 21.6 ± 6 mm) and analyzed for mutations involving all translated exons of CSRP3 and TCAP and targeted HCM-associated exons (2, 3, 4, and 14) of TTN using polymerase chain reaction (PCR), denaturing high performance liquid chromatography (DHPLC), and direct DNA sequencing. Clinical data were extracted from patient records and maintained independent of the genotype.
Overall, 16 patients (4.1%) harbored a Z-disc mutation: 12 had a MLP mutation and 4 patients a TCAP mutation. No TTN mutations were detected. Seven patients were also found to have a concomitant myofilament mutation. Seven patients with a MLP-mutation were found to harbor the DCM-associated, functionally characterized W4R mutation. W4R-MLP was also noted in a single white control subject. Patients with MLP/TCAP-associated HCM clinically mimicked myofilament-HCM.
Approximately 4.1% of unrelated patients had HCM-associated MLP or TCAP mutations. MLP/TCAP-HCM phenotypically mirrors myofilament-HCM and is more severe than the subset of patients who still remain without a disease-causing mutation. The precise role of W4R-MLP in the pathogenesis of either DCM or HCM warrants further investigation.
Genetics; Genes; Hypertrophy; Cardiomyopathy; Z-disc; Muscle LIM protein; Telethonin; TCAP; Titin
Patients with inherited dilated cardiomyopathy (DCM) frequently die with severe heart failure (HF) or die suddenly with arrhythmias, although these symptoms are not always observed at birth. It remains unclear how and when HF and arrhythmogenic changes develop in these DCM mutation carriers. In order to address this issue, properties of the myocardium and underlying gene expressions were studied using a knock-in mouse model of human inherited DCM caused by a deletion mutation ΔK210 in cardiac troponinT.
By 1 month, DCM mice had already enlarged hearts, but showed no symptoms of HF and a much lower mortality than at 2 months or later. At around 2 months, some would die suddenly with no clear symptoms of HF, whereas at 3 months, many of the survivors showed evident symptoms of HF. In isolated left ventricular myocardium (LV) from 2 month-mice, spontaneous activity frequently occurred and action potential duration (APD) was prolonged. Transient outward (Ito) and ultrarapid delayed rectifier K+ (IKur) currents were significantly reduced in DCM myocytes. Correspondingly, down-regulation of Kv4.2, Kv1.5 and KChIP2 was evident in mRNA and protein levels. In LVs at 3-months, more frequent spontaneous activity, greater prolongation of APD and further down-regulation in above K+ channels were observed. At 1 month, in contrast, infrequent spontaneous activity and down-regulation of Kv4.2, but not Kv1.5 or KChIP2, were observed.
Our results suggest that at least three steps of electrical remodeling occur in the hearts of DCM model mice, and that the combined down-regulation of Kv4.2, Kv1.5 and KChIP2 prior to the onset of HF may play an important role in the premature sudden death in this DCM model. DCM mice at 1 month or before, on the contrary, are associated with low risk of death in spite of inborn disorder and enlarged heart.
Recognizing that inhibitors of phosphodiesterase type 5 (PDE5) are increasingly employed in patients with pulmonary hypertension and right ventricular failure, we examined PDE5 expression in the human right ventricle (RV) and its impact on myocardial contractility.
Methods and Results
Tissue extracts from the RV of 20 patients were assayed for PDE5 expression using immunoblot and immunohistochemical (IHC) staining. Tissues were selected from groups of non-failing (NF) organ donors and transplant recipients with end-stage ischemic cardiomyopathy (ICM) or idiopathic dilated cardiomyopathy (DCM). Among DCM patients, subgroups with mild or severe RV dysfunction (RVD) and prior LV assist devices (LVAD) were analyzed separately. Our results showed that PDE5 abundance increased more than four-fold in the RVs of the ICM compared to NF group. In DCM, PDE5 up-regulation was more moderate and varied with the severity of RV dysfunction. IHC confirmed that cardiac myocytes contributed to the up-regulation in the failing hearts. In functional studies, PDE5 inhibition produced little change in developed force (DF) in RV trabeculae from NF hearts, but produced a moderate increase in RV trabeculae from failing hearts.
Our results showed the etiology- and severity-dependent up-regulation of myocyte PDE5 expression in the RV and the impact of this up-regulation on myocardial contractility. These findings suggest that RV PDE5 expression could contribute to the pathogenesis of RV failure and direct myocardial responses to PDE5 inhibition may modulate the indirect responses mediated by RV afterload reduction.
PDE5; cGMP; heart failure; myocardium; contractility
Dilated cardiomyopathy (DCM) is the most common cardiomyopathy, characterized by ventricular dilatation, systolic dysfunction, and progressive heart failure. DCM is the most common diagnosis leading to heart transplantation and places a significant burden on healthcare worldwide. The advent of induced pluripotent stem cells (iPSCs) offers an exceptional opportunity for creating disease-specific models, investigating underlying mechanisms, and optimizing therapy. Here we generated cardiomyocytes (CMs) from iPSCs derived from patients of a DCM family carrying a point mutation (R173W) in the gene encoding sarcomeric protein cardiac troponin T. Compared to the control healthy individuals in the same family cohort, DCM iPSC-CMs exhibited altered Ca2+ handling, decreased contractility, and abnormal sarcomeric α-actinin distribution. When stimulated with β-adrenergic agonist, DCM iPSC-CMs showed characteristics of failure such as reduced beating rates, compromised contraction, and significantly more cells with abnormal sarcomeric α-actinin distribution. β-adrenergic blocker treatment and over-expression of sarcoplasmic reticulum Ca2+ ATPase (Serca2a) improved DCM iPSC-CMs function. Our study demonstrated that human DCM iPSC-CMs recapitulated to some extent the disease phenotypes morphologically and functionally, and thus can serve as a useful platform for exploring molecular and cellular mechanisms and optimizing treatment of this particular disease.
Heart failure (HF) is associated with excessive extracellular matrix (ECM) deposition and abnormal ECM degradation leading to cardiac fibrosis. Connective Tissue Growth Factor (CTGF) modulates ECM production during inflammatory tissue injury, but available data on CTGF gene expression in failing human heart and its response to mechanical unloading are limited.
Methods and Results
LV tissue from patients undergoing cardiac transplantation for ischemic (ICM; n=20) and dilated (DCM; n=20) cardiomyopathies, and from nonfailing (NF; n=20) donor hearts were examined. Paired samples (n=15) from patients undergoing LV assist device (LVAD) implantation as “bridge to transplant” (34-1145 days) were also analyzed. There was more interstitial fibrosis in both ICM and DCM compared to NF hearts. Hydroxyproline concentration was also significantly increased in DCM relative to NF samples. The expression of CTGF,TGFB1, COL1-A1, COL3-A1, MMP2 and MMP9 mRNAs in ICM and DCM were also significantly elevated as compared to NF controls. Although TGFB1, CTGF, COL1-A1, and COL3-A1 mRNA levels were reduced by unloading, there was only a modest reduction in tissue fibrosis and no difference in protein-bound hydroxyproline concentration between pre- and post-LVAD tissue samples. The persistent fibrosis may be related to a concomitant reduction in MMP9 mRNA and protein levels following unloading.
CTGF may be a key regulator of fibrosis during maladaptive remodeling and progression to HF. Although mechanical unloading normalizes most genotypic and functional abnormalities, its effect on ECM remodeling during HF is incomplete.
Remodeling; Heart-assist device; Gene expression; collagens
We sought to identify a novel gene for dilated cardiomyopathy (DCM).
DCM is a heritable, genetically heterogeneous disorder that remains idiopathic in a majority of patients. Familial cases provide an opportunity to discover unsuspected molecular bases of DCM, enabling preclinical risk detection.
Two large families with autosomal dominant DCM were studied. Genome-wide linkage analysis was used to identify a disease locus, followed by fine mapping and positional candidate gene sequencing. Mutation scanning was then performed in 278 unrelated subjects with idiopathic DCM, prospectively identified at the Mayo Clinic.
Overlapping loci for DCM were independently mapped to chromosome 10q25-q26. DNA sequencing of affected individuals in each family revealed distinct heterozygous missense mutations in exon 9 of RBM20, encoding RNA binding motif protein 20. Comprehensive coding sequence analyses identified missense mutations clustered within this same exon in six additional DCM families. Mutations segregated with DCM (composite logarithm of the odds score >11.49), were absent in 480 control samples, and altered residues within a highly conserved arginine/serine (RS)-rich region. Expression of RBM20 messenger RNA was confirmed in human heart tissue.
Our findings establish RBM20 as a DCM gene and reveal a mutation hotspot in the RS domain. RBM20 is preferentially expressed in the heart and encodes motifs prototypical of spliceosome proteins that regulate alternative pre-mRNA splicing, thus implicating a functionally distinct gene in human cardiomyopathy. RBM20 mutations are associated with young age at diagnosis, end-stage heart failure, and high mortality.
dilated cardiomyopathy; genetics; linkage analysis; mutation; RBM20
We evaluated ankyrin repeat domain 1 (ANKRD1), the gene encoding cardiac ankyrin repeat protein (CARP), as a novel candidate gene for dilated cardiomyopathy (DCM) through mutation analysis of a cohort of familial or idiopathic DCM patients, based on the hypothesis that inherited dysfunction of mechanical stretch-based signaling is present in a subset of DCM patients.
CARP, a transcription coinhibitor, is a member of the titin-N2A mechanosensory complex and translocates to the nucleus in response to stretch. It is up-regulated in cardiac failure and hypertrophy and represses expression of sarcomeric proteins. Its overexpression results in contractile dysfunction.
In all, 208 DCM patients were screened for mutations/variants in the coding region of ANKRD1 using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct deoxyribonucleic acid sequencing. In vitro functional analyses of the mutation were performed using yeast 2-hybrid assays and investigating the effect on stretch-mediated gene expression in myoblastoid cell lines using quantitative real-time reverse transcription–polymerase chain reaction.
Three missense heterozygous ANKRD1 mutations (P105S, V107L, and M184I) were identified in 4 DCM patients. The M184I mutation results in loss of CARP binding with Talin 1 and FHL2, and the P105S mutation in loss of Talin 1 binding. Intracellular localization of mutant CARP proteins is not altered. The mutations result in differential stretch-induced gene expression compared with wild-type CARP.
ANKRD1 is a novel DCM gene, with mutations present in 1.9% of DCM patients. The ANKRD1 mutations may cause DCM as a result of disruption of the normal cardiac stretch-based signaling.
DCM; CARP; ANKRD1; mutations
Mutations in a sarcomeric protein can cause hypertrophic cardiomyopathy (HCM) or dilated cardiomyopathy (DCM), the opposite ends of a spectrum of phenotypic responses of the heart to mutations. We posit the contracting phenotypes could result from differential effects of the mutant proteins on interactions among the sarcomeric proteins. To test the hypothesis, we generated transgenic mice expressing either cardiac troponin T (cTnT)-Q92 or cTnT-W141, known to cause HCM and DCM, respectively, in the heart.
Methods and results
We phenotyped the mice by echocardiography, histology and immunoblotting, and real-time polymerase chain reaction. We detected interactions between the sarcomeric proteins by co-immunoprecipitation and determined Ca2+ sensitivity of myofibrillar protein ATPase activity by Carter assay. The cTnT-W141 mice exhibited dilated hearts and decreased systolic function. In contrast, the cTnT-Q92 mice showed smaller ventricles and enhanced systolic function. Levels of cardiac troponin I, cardiac α-actin, α-tropomyosin, and cardiac troponin C co-immunoprecipitated with anti-cTnT antibodies were higher in the cTnT-W141 than in the cTnT-Q92 mice, as were levels of α-tropomyosin co-immunoprecipitated with an anti-cardiac α-actin antibody. In contrast, levels of cardiac troponin I co-immunoprecipitated with an anti-cardiac α-actin antibody were higher in the cTnT-Q92 mice. Ca2+ sensitivity of myofibrillar ATPase activity was increased in HCM but decreased in DCM mice compared with non-transgenic mice.
Differential interactions among the sarcomeric proteins containing cTnT-Q92 or cTnT-W141 are responsible for the contrasting phenotypes of HCM or DCM, respectively.
Cardiomyopathy; Genetics; Mutation; Mouse model; Pathogenesis; Fibrosis; Heart failure
Hypertrophic cardiomyopathy (HCM) is known to be manifested by mutations in 12 sarcomeric genes and dilated cardiomyopathy (DCM) is known to manifest due to cytoskeletal mutations. Studies have revealed that sarcomeric mutations can also lead to DCM. Therefore, in the present study, we have made an attempt to compare and analyze the genetic variations of beta-myosin heavy chain gene (β-MYH7), which are interestingly found to be common in both HCM and DCM. The underlying pathophysiological mechanism leading to two different phenotypes has been discussed in this study. Till date, about 186 and 73 different mutations have been reported in HCM and DCM, respectively, with respect to this gene.
The screening of β-MYH7 gene in both HCM and DCM has revealed some common genetic variations. The aim of the present study is to understand the pathophysiological mechanism underlying the manifestation of two different phenotypes.
MATERIALS AND METHODS:
100 controls, 95 HCM and 97 DCM samples were collected. Genomic DNA was extracted following rapid nonenzymatic method as described by Lahiri and Nurnberger (1991), and the extracted DNA was later subjected to polymerase chain reaction (PCR) based single stranded conformation polymorphism (SSCP) analysis to identify single nucleotide polymorphism (SNP)s/mutations associated with the diseased phenotypes.
RESULTS AND CONCLUSION:
Similar variations were observed in β-MYH7 exons 7, 12, 19 and 20 in both HCM and DCM. This could be attributed to impaired energy compromise, or to dose effect of the mutant protein, or to even environmental factors/modifier gene effects wherein an HCM could progress to a DCM phenotype affecting both right and left ventricles, leading to heart failure.
Diastolic dysfunction; dose effect; dilated cardiomyopathy; hypertrophic cardiomyopathy; single nucleotide polymorphism; systolic dysfunction
Dilated cardiomyopathy (DCM) is a structural heart disease with strong genetic background. Monogenic forms of DCM are observed in families with mutations located mostly in genes encoding structural and sarcomeric proteins. However, strong evidence suggests that genetic factors also affect the susceptibility to idiopathic DCM. To identify risk alleles for non-familial forms of DCM, we carried out a case-control association study, genotyping 664 DCM cases and 1,874 population-based healthy controls from Germany using a 50K human cardiovascular disease bead chip covering more than 2,000 genes pre-selected for cardiovascular relevance. After quality control, 30,920 single nucleotide polymorphisms (SNP) were tested for association with the disease by logistic regression adjusted for gender, and results were genomic-control corrected. The analysis revealed a significant association between a SNP in HSPB7 gene (rs1739843, minor allele frequency 39%) and idiopathic DCM (p = 1.06×10−6, OR = 0.67 [95% CI 0.57–0.79] for the minor allele T). Three more SNPs showed p < 2.21×10−5. De novo genotyping of these four SNPs was done in three independent case-control studies of idiopathic DCM. Association between SNP rs1739843 and DCM was significant in all replication samples: Germany (n = 564, n = 981 controls, p = 2.07×10−3, OR = 0.79 [95% CI 0.67–0.92]), France 1 (n = 433 cases, n = 395 controls, p = 3.73×10−3, OR = 0.74 [95% CI 0.60–0.91]), and France 2 (n = 249 cases, n = 380 controls, p = 2.26×10−4, OR = 0.63 [95% CI 0.50–0.81]). The combined analysis of all four studies including a total of n = 1,910 cases and n = 3,630 controls showed highly significant evidence for association between rs1739843 and idiopathic DCM (p = 5.28×10−13, OR = 0.72 [95% CI 0.65–0.78]). None of the other three SNPs showed significant results in the replication stage.
This finding of the HSPB7 gene from a genetic search for idiopathic DCM using a large SNP panel underscores the influence of common polymorphisms on DCM susceptibility.
Dilated cardiomyopathy is a severe disease of the heart muscle and often leads to chronic heart failure, eventually with the consequence of cardiac transplantation. Identification of genetic disease markers in at-risk persons could play an important role in preventive health care. Several mutations in familial forms of the disease are described. Here, we examine the role of common genetic variants on the sporadic form of dilated cardiomyopathy. By screening about 2,000 candidate genes previously related to cardiovascular disease in more than 1,900 cases and 3,600 controls, we show that a polymorphism in the HSPB7 gene (rs1739843) is strongly associated with susceptibility to dilated cardiomyopathy. We also show that the effect on disease risk is present in both German and French cohorts. Therefore, this study is an important step towards revealing insight in the genetic background of the sporadic form of dilated cardiomyopathy.