Takuro Arimura

Activation of MAPK pathways links LMNA mutations to cardiomyopathy in Emery-Dreifuss muscular dystrophy.

Mutations in LMNA, which encodes nuclear Lamins A and C cause diseases affecting various organs, including the heart. We have determined the effects of an Lmna H222P mutation on signaling pathways involved in the development of cardiomyopathy in a knockin mouse model of autosomal dominant Emery-Dreifuss muscular dystrophy. Analysis of genome-wide expression profiles in hearts using Affymetrix GeneChips showed statistically significant differences in expression of genes in the MAPK pathways at the incipience of the development of clinical disease. Using real-time PCR, we showed that activation of MAPK pathways preceded clinical signs or detectable molecular markers of cardiomyopathy. In heart tissue and isolated cardiomyocytes, there was activation of MAPK cascades and downstream targets, implicated previously in the pathogenesis of cardiomyopathy. Expression of H222P Lamin A in cultured cells activated MAPKs and downstream target genes. Activation of MAPK signaling by mutant A-type lamins could be a cornerstone in the development of heart disease in autosomal dominant Emery-Dreifuss muscular dystrophy.

Replication of the association between a chromosome 9p21 polymorphism and coronary artery disease in Japanese and Korean populations

AbstractCoronary artery disease (CAD) has become a major health problem in many countries. Recent genome-wide association studies have identified the association between rs1333049 on chromosome 9p21 and susceptibility to CAD in Caucasoid populations. In this study, we evaluated the associations of rs1333049 with CAD in Japanese (604 patients and 1,151 controls) and Koreans (679 patients and 706 controls). We found a significant association in both Japanese [odds ratio (OR) = 1.30, 95% confidence interval (CI); 1.13–1.49, p = 0.00027, allele count model] and Koreans (OR = 1.19, 95% CI; 1.02–1.38, p = 0.025, allele count model). These observations demonstrated that chromosome 9p21 was the susceptibility locus for CAD also in East Asians.

Cardiac Ankyrin Repeat Protein Gene (ANKRD1) Mutations in Hypertrophic Cardiomyopathy

OBJECTIVES The purpose of this study was to explore a novel disease gene for hypertrophic cardiomyopathy (HCM) and to evaluate functional alterations caused by mutations. BACKGROUND Mutations in genes encoding myofilaments or Z-disc proteins of the cardiac sarcomere cause HCM, but the disease-causing mutations can be found in one-half of the patients, indicating that novel HCM-susceptibility genes await discovery. We studied a candidate gene, ankyrin repeat domain 1 (ANKRD1), encoding for the cardiac ankyrin repeat protein (CARP) that is a Z-disc component interacting with N2A domain of titin/connectin and N-terminal domain of myopalladin. METHODS We analyzed 384 HCM patients for mutations in ANKRD1 and in the N2A domain of titin/connectin gene (TTN). Interaction of CARP with titin/connectin or myopalladin was investigated using coimmunoprecipitation assay to demonstrate the functional alteration caused by ANKRD1 or TTN mutations. Functional abnormalities caused by the ANKRD1 mutations were also examined at the cellular level in neonatal rat cardiomyocytes. RESULTS Three ANKRD1 missense mutations, Pro52Ala, Thr123Met, and Ile280Val, were found in 3 patients. All mutations increased binding of CARP to both titin/connectin and myopalladin. In addition, TTN mutations, Arg8500His, and Arg8604Gln in the N2A domain were found in 2 patients, and these mutations increased binding of titin/connectin to CARP. Myc-tagged CARP showed that the mutations resulted in abnormal localization of CARP in cardiomyocytes. CONCLUSIONS CARP abnormalities may be involved in the pathogenesis of HCM.

Autophagic degradation of nuclear components in mammalian cells

Autophagy is an evolutionally conserved intracellular mechanism for the degradation of organelles and proteins. Here we demonstrate the presence of perinuclear autophagosomes/autolysosomes containing nuclear components in nuclear envelopathies caused by mutations in the genes encoding A-type lamins (LMNA) and emerin (EMD). These autophagosomes/autolysosomes were sometimes bigger than nucleus. The autophagic nature is further supported by up-regulation of LC3-II in LmnaH222P/H222P fibroblasts. In addition, inhibition of autophagy led to the accumulation of nuclear abnormalities and reduced cell viability, highly suggesting a beneficial role of autophagy, at least in these cells. Similar giant autophagosomes/autolysosomes were seen even in wild-type cells, albeit rarely, implying that this “nucleophagy” is not confined to the diseased condition, but may be seen even in physiologic conditions to clean up nuclear wastes produced by nuclear damage.

ANKRD1, the gene encoding cardiac ankyrin repeat protein, is a novel dilated cardiomyopathy gene.

OBJECTIVES 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. BACKGROUND 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

Functional analysis of titin/connectin N2-B mutations found in cardiomyopathy

Hypertrophic cardiomyopathy and dilated cardiomyopathy are two major clinical phenotypes of “idiopathic” cardiomyopathy. Recent molecular genetic analyses have now revealed that “idiopathic” cardiomyopathy is caused by mutations in genes for sarcomere components. We have recently reported several mutations in titin/connectin gene found in patients with hypertrophic cardiomyopathy or dilated cardiomyopathy. A hypertrophic cardiomyopathy-associated titin/connectin mutation (Arg740Leu) was found to increase the binding to actinin, while other dilated cardiomyopathy-associated titin/connectin mutations (Ala743Val and Val54Met) decreased the binding to actinin and Tcap/telethonin, respectively. We also reported several other mutations in the N2-B region of titin/connectin found in hypertrophic cardiomyopathy and dilated cardiomyopathy. Since the N2-B region expresses only in the heart, it was speculated that functional alterations due to the mutations cause cardiomyopathies. In this study, we investigated the functional changes caused by the N2-B region mutations by using yeast-two-hybrid assays. It was revealed that a hypertrophic cardiomyopathy-associated mutation (Ser3799Tyr) increased the binding to FHL2 protein, whereas a dilated cardiomyopathy-associated mutation (Gln4053ter) decreased the binding. In addition, another TTN mutation (Arg25618Gln) at the is2 region was found in familial DCM. Because FHL2 protein is known to tether metabolic enzymes to N2-B and is2 regions of titin/connectin, these observations suggest that altered recruitment of metabolic enzymes to the sarcomere may play a role in the pathogenesis of cardiomyopathies.

Dilated cardiomyopathy‐associated BAG3 mutations impair Z‐disc assembly and enhance sensitivity to apoptosis in cardiomyocytes

Dilated cardiomyopathy (DCM) is characterized by dilation of left ventricular cavity with systolic dysfunction. Clinical symptom of DCM is heart failure, often associated with cardiac sudden death. About 20–35% of DCM patients have apparent family histories and it has been revealed that mutations in genes for sarcomere proteins cause DCM. However, the disease‐causing mutations can be found only in about 17% of Japanese patients with familial DCM. Bcl‐2‐associated athanogene 3 (BAG3) is a co‐chaperone protein with antiapoptotic function, which localizes at Z‐disc in the striated muscles. Recently, BAG3 gene mutations in DCM patients were reported, but the functional abnormalities caused by the mutations are not fully unraveled. In this study, we analyzed 72 Japanese familial DCM patients for mutations in BAG3 and found two mutations, p.Arg218Trp and p.Leu462Pro, in two cases of adult‐onset DCM without skeletal myopathy, which were absent from 400 control subjects. Functional studies at the cellular level revealed that the DCM‐associated BAG3 mutations impaired the Z‐disc assembly and increased the sensitivities to stress‐induced apoptosis. These observations suggested that BAG3 mutations present in 2.8% of Japanese familial DCM patients caused DCM possibly by interfering with Z‐disc assembly and inducing apoptotic cell death under the metabolic stress. 32:1481–1491, 2011. ©2011 Wiley Periodicals, Inc.

Nebulette Mutations are Associated with Dilated Cardiomyopathy and Endocardial Fibroelastosis

OBJECTIVES Four variants (K60N, Q128R, G202R, and A592E) in the nebulette gene were identified in patients with dilated cardiomyopathy (DCM) and endocardial fibroelastosis. We sought to determine if these mutations are cardiomyopathy causing. BACKGROUND Nebulette aligns thin filaments and connects them with the myocardial Z-disk, playing a role in mechanosensation. METHODS We generated transgenic mice with cardiac-restricted overexpression of human wild-type or mutant nebulette. Chimera and transgenic mice were examined at 4, 6, and 12 months of age by echocardiography and cardiac magnetic resonance imaging. 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. RESULTS We identified lethal cardiac structural abnormalities in mutant embryonic hearts (K60N and Q128R). Founders of the mutant mouse lines developed DCM with severe heart failure. An irregular localization pattern for nebulette and impaired desmin expression were noted in the proband and chimeric Q128R mice. Mutant G202R and A592E mice exhibited left ventricular dilation and impaired function with specific changes in I-band and Z-disk proteins by 6 months of age. The mutations modulated distribution of nebulette in the sarcomere and Z-disk during stretch of H9C2 cells. CONCLUSIONS Nebulette is a new susceptibility gene for endocardial fibroelastosis and DCM. Different mutations in nebulette trigger specific mechanisms, converging to a common pathological cascade leading to endocardial fibroelastosis and DCM.

Impaired binding of ZASP/Cypher with phosphoglucomutase 1 is associated with dilated cardiomyopathy

AIMS Z-band alternatively spliced PDZ-motif protein (ZASP)/Cypher is a Z-disc component of which several dilated cardiomyopathy (DCM)-associated mutations have been reported. Most of the mutations were found in exons 4 and 10 of ZASP/Cypher gene LDB3 and both exons were expressed preferentially in the heart. The aim of this study was to investigate the functional alteration of ZASP/Cypher caused by the DCM-associated mutations. METHODS AND RESULTS The yeast-two-hybrid method was used to identify the protein bound to a domain encoded by exon 4 of LDB3. Interaction of ZASP/Cypher with the binding protein was investigated in relation to the functional alterations caused by LDB3 mutations. Localization of the ZASP/Cypher-binding protein was examined at the cellular level in rat cardiomyocytes. Phosphoglucomutase 1 (PGM1), a metabolic enzyme involved in glycolysis and gluconeogenesis, was identified as a protein interacting with ZASP/Cypher. PGM1 bound to ZASP/Cypher at the domains encoded by exons 4 and 10. Two LDB3 mutations in exon 4 (Ser189Leu and Thr206Ile) and another mutation in exon 10 (Ile345Met) reduced the binding to PGM1. PGM1 showed diffuse localization in the cytoplasm of rat cardiomyocytes under standard culture conditions, and distribution at the Z-discs was observed under stressed culture conditions. Binding of endogenous PGM1 and ZASP/Cypher was found to be enhanced by stress in rat cardiomyocytes. CONCLUSION ZASP/Cypher anchors PGM1 to Z-disc under conditions of stress. The impaired binding of PGM1 to ZASP/Cypher might be involved in the pathogenesis of DCM.

Endothelin‐1 Induces Myofibrillar Disarray and Contractile Vector Variability in Hypertrophic Cardiomyopathy–Induced Pluripotent Stem Cell–Derived Cardiomyocytes

Background Despite the accumulating genetic and molecular investigations into hypertrophic cardiomyopathy (HCM), it remains unclear how this condition develops and worsens pathologically and clinically in terms of the genetic–environmental interactions. Establishing a human disease model for HCM would help to elucidate these disease mechanisms; however, cardiomyocytes from patients are not easily obtained for basic research. Patient‐specific induced pluripotent stem cells (iPSCs) potentially hold much promise for deciphering the pathogenesis of HCM. The purpose of this study is to elucidate the interactions between genetic backgrounds and environmental factors involved in the disease progression of HCM. Methods and Results We generated iPSCs from 3 patients with HCM and 3 healthy control subjects, and cardiomyocytes were differentiated. The HCM pathological phenotypes were characterized based on morphological properties and high‐speed video imaging. The differences between control and HCM iPSC‐derived cardiomyocytes were mild under baseline conditions in pathological features. To identify candidate disease‐promoting environmental factors, the cardiomyocytes were stimulated by several cardiomyocyte hypertrophy‐promoting factors. Interestingly, endothelin‐1 strongly induced pathological phenotypes such as cardiomyocyte hypertrophy and intracellular myofibrillar disarray in the HCM iPSC‐derived cardiomyocytes. We then reproduced these phenotypes in neonatal cardiomyocytes from the heterozygous Mybpc3‐targeted knock in mice. High‐speed video imaging with motion vector prediction depicted physiological contractile dynamics in the iPSC‐derived cardiomyocytes, which revealed that self‐beating HCM iPSC‐derived single cardiomyocytes stimulated by endothelin‐1 showed variable contractile directions. Conclusions Interactions between the patients genetic backgrounds and the environmental factor endothelin‐1 promote the HCM pathological phenotype and contractile variability in the HCM iPSC‐derived cardiomyocytes.