Genetic heterogeneity of heritable ectopic mineralization disorders in a large international cohort.

PURPOSE: Heritable ectopic mineralization disorders comprise a group of conditions with a broad range of clinical manifestations in nonskeletal connective tissues. We report the genetic findings from a large international cohort of 478 patients afflicted with ectopic mineralization. METHODS: Sequence variations were identified using a next-generation sequencing panel consisting of 29 genes reported in association with ectopic mineralization. The pathogenicity of select splicing and missense variants was analyzed in experimental systems in vitro and in vivo. RESULTS: A total of 872 variants of unknown significance as well as likely pathogenic and pathogenic variants were disclosed in 25 genes. A total of 159 distinct variants were identified in 425 patients in ABCC6, the gene responsible for pseudoxanthoma elasticum, a heritable multisystem ectopic mineralization disorder. The interpretation of variant pathogenicity relying on bioinformatic predictions did not provide a consensus. Our in vitro and in vivo functional assessment of 14 ABCC6 variants highlighted this dilemma and provided unambiguous interpretations to their pathogenicity. CONCLUSION: The results expand the ABCC6 variant repertoire, shed new light on the genetic heterogeneity of heritable ectopic mineralization disorders, and provide evidence that functional characterization in appropriate experimental systems is necessary to determine the pathogenicity of genetic variants.

A novel autosomal recessive GJB2-associated disorder: Ichthyosis follicularis, bilateral severe sensorineural hearing loss, and punctate palmoplantar keratoderma.

Ichthyosis follicularis, a distinct cutaneous entity reported in combination with atrichia, and photophobia has been associated with mutations in MBTPS2. We sought the genetic cause of a novel syndrome of ichthyosis follicularis, bilateral severe sensorineural hearing loss and punctate palmoplantar keratoderma in two families. We performed whole exome sequencing on three patients from two families. The pathogenicity and consequences of mutations were studied in the Xenopus oocyte expression system and by molecular modeling analysis. Compound heterozygous mutations in the GJB2 gene were discovered: a pathogenic c.526A>G; p.Asn176Asp, and a common frameshift mutation, c.35delG; p.Gly12Valfs*2. The p.Asn176Asp missense mutation was demonstrated to significantly reduce the cell-cell gap junction channel activity and increase the nonjunctional hemichannel activity in the Xenopus oocyte expression system. Molecular modeling analyses of the mutant Cx26 protein revealed significant changes in the structural characteristics and electrostatic potential of the Cx26, either in hemichannel or gap junction conformation. Thus, association of a new syndrome of an autosomal recessive disorder of ichthyosis follicularis, bilateral severe sensorineural hearing loss and punctate palmoplantar keratoderma with mutations in GJB2, expands the phenotypic spectrum of the GJB2-associated disorders. The findings attest to the complexity of the clinical consequences of different mutations in GJB2.

Myocardin is required for maintenance of vascular and visceral smooth muscle homeostasis during postnatal development.

Myocardin is a muscle-restricted transcriptional coactivator that activates a serum response factor (SRF)-dependent gene program required for cardiogenesis and embryonic survival. To identify myocardin-dependent functions in smooth muscle cells (SMCs) during postnatal development, mice harboring a SMC-restricted conditional, inducible Myocd null mutation were generated and characterized. Tamoxifen-treated SMMHC-Cre(ERT2)/Myocd(F/F) conditional mutant mice die within 6 mo of Myocd gene deletion, exhibiting profound derangements in the structure of great arteries as well as the gastrointestinal and genitourinary tracts. Conditional mutant mice develop arterial aneurysms, dissection, and rupture, recapitulating pathology observed in heritable forms of thoracic aortic aneurysm and dissection (TAAD). SMCs populating arteries of Myocd conditional mutant mice modulate their phenotype by down-regulation of SMC contractile genes and up-regulation of extracellular matrix proteins. Surprisingly, this is accompanied by SMC autonomous activation of endoplasmic reticulum (ER) stress and autophagy, which over time progress to programmed cell death. Consistent with these observations, Myocd conditional mutant mice develop remarkable dilation of the stomach, small intestine, bladder, and ureters attributable to the loss of visceral SMCs disrupting the muscularis mucosa. Taken together, these data demonstrate that during postnatal development, myocardin plays a unique, and important, role required for maintenance and homeostasis of the vasculature, gastrointestinal, and genitourinary tracts. The loss of myocardin in SMCs triggers ER stress and autophagy, which transitions to apoptosis, revealing evolutionary conservation of myocardin function in SMCs and cardiomyocytes.

Class I Histone Deacetylase Inhibition for the Treatment of Sustained Atrial Fibrillation.

Current therapies are less effective for treating sustained/permanent versus paroxysmal atrial fibrillation (AF). We and others have previously shown that histone deacetylase (HDAC) inhibition reverses structural and electrical atrial remodeling in mice with inducible, paroxysmal-like AF. Here, we hypothesize an important, specific role for class I HDACs in determining structural atrial alterations during sustained AF. The class I HDAC inhibitor N-acetyldinaline [4-(acetylamino)-N-(2-amino-phenyl) benzamide] (CI-994) was administered for 2 weeks (1 mg/kg/day) to Hopx transgenic mice with atrial remodeling and inducible AF and to dogs with atrial tachypacing-induced sustained AF. Class I HDAC inhibition prevented atrial fibrosis and arrhythmia inducibility in mice. Dogs were divided into three groups: 1) sinus rhythm, 2) sustained AF plus vehicle, and 3) sustained AF plus CI-994. In group 3, the time in AF over 2 weeks was reduced by 30% compared with group 2, along with attenuated atrial fibrosis and intra-atrial adipocyte infiltration. Moreover, group 2 dogs had higher atrial and serum inflammatory cytokines, adipokines, and atrial immune cells and adipocytes compared with groups 1 and 3. On the other hand, groups 2 and 3 displayed similar left atrial size, ventricular function, and mitral regurgitation. Importantly, the same histologic alterations found in dogs with sustained AF and reversed by CI-994 were also present in atrial tissue from transplanted patients with chronic AF. This is the first evidence that, in sustained AF, class I HDAC inhibition can reduce the total time of fibrillation, atrial fibrosis, intra-atrial adipocytes, and immune cell infiltration without significant effects on cardiac function.

[Relationship between gestational age and apparent diffusion coefficient values in different regions of fetal brain from middle to late trimester].

OBJECTIVE: To analyze the relationship between gestational age and apparent diffusion coefficient (ADC) values in different regions of fetal brain from middle to late trimester. METHODS: DW images performed in 70 singleton non-sedated fetuses with questionably abnormal results on sonography and normal fetal MR imaging results were retrospectively reviewed. The median gestational age was 32.4 weeks (range, 24-38).With the formula of ADC = ln (S600/S0)/(B0-B600), the mean ADC values were obtained for fetal parietal white matter (WM), frontal WM, temporal WM, occipital WM, pons, cerebellum, basal ganglia and thalamus. The relationship of mean ADC values in different regions with gestational age was analyzed with linear regression. RESULTS: The mean ADC values were 1.77 ± 0.32 mm(2)/s (SD) in fetal parietal white matter (WM), 1.71 ± 0.32 mm(2)/s in occipital WM, 1.31 ± 0.18 mm(2)/s in thalamus, 1.34 ± 0.15 mm(2)/s in basal ganglia. And the mean ADC values in cerebellum, pons, frontal WM and temporal WM were 1.17 ± 0.16, 1.41 ± 0.18, 1.87 ± 0.18 and 1.74 ± 0.19 mm(2)/s respectively. A significant negative correlation between ADC values and gestational age was found for parietal WM, occipital WM, pons, cerebellum, basal ganglia and thalamus (P < 0.05). Frontal WM ADC (P > 0.05) and temporal WM ADC (P = 0.05) did not significantly change with gestational age whereas only a downward trend was present. The correlation coefficient (r) and coefficient of regression (b) were 0.420 and -0.045 in parietal WM; 0.470 and -0.052 in occipital WM; 0.370 and -0.027 in cerebellum; 0.027 and -0.020 in pons; 0.320 and -0.027 in thalamus; 0.300 and -0.021 in basal ganglia. The mean ADC values peaked in frontal WM and lowest in pons. The mean ADC values in white matter were higher than those in deep gray nuclei, cerebellum and pons.With the development of fetal brain,ADC values declined the fastest in cerebellum and occipital WM, followed by basal ganglia and thalamus. CONCLUSION: Regional differences in non-sedated fetal brain ADC values and their evolutions with gestational age are likely to reflect variations in brain maturation.

Myocardin is required for cardiomyocyte survival and maintenance of heart function.

Despite intense investigation over the past century, the molecular mechanisms that regulate maintenance and adaptation of the heart during postnatal development are poorly understood. Myocardin is a remarkably potent transcriptional coactivator expressed exclusively in cardiac myocytes and smooth muscle cells during postnatal development. Here we show that myocardin is required for maintenance of cardiomyocyte structure and sarcomeric organization and that cell-autonomous loss of myocardin in cardiac myocytes triggers programmed cell death. Mice harboring a cardiomyocyte-restricted null mutation in the myocardin gene (Myocd) develop dilated cardiomyopathy and succumb from heart failure within a year. Remarkably, ablation of the Myocd gene in the adult heart leads to the rapid-onset of heart failure, dilated cardiomyopathy, and death within a week. Myocd gene ablation is accompanied by dissolution of sarcomeric organization, disruption of the intercalated disc, and cell-autonomous loss of cardiomyocytes via apoptosis. Expression of myocardin/serum response factor-regulated myofibrillar genes is extinguished, or profoundly attenuated, in myocardin-deficient hearts. Conversely, proapoptotic factors are induced and activated in myocardin-deficient hearts. We conclude that the transcriptional coactivator myocardin is required for maintenance of heart function and ultimately cardiomyocyte survival.

Inhibition of the DNA Damage Response Attenuates Ectopic Calcification in Pseudoxanthoma Elasticum.

Pseudoxanthoma elasticum (PXE) is a heritable ectopic calcification disorder with multiorgan clinical manifestations. The gene at default, ABCC6, encodes an efflux transporter, ABCC6, which is a critical player regulating the homeostasis of inorganic pyrophosphate, a potent endogenous anticalcification factor. Previous studies suggested that systemic inorganic pyrophosphate deficiency is the major but not the exclusive cause of ectopic calcification in PXE. In this study, we show that the DNA damage response (DDR) and poly(ADP-ribose) (PAR) pathways are involved locally in PXE at sites of ectopic calcification. Genetic inhibition of PAR polymerase 1 gene PARP1, the predominant PAR-producing enzyme, showed a 54% reduction of calcification in the muzzle skin in Abcc6‒/‒Parp1‒/‒ mice, compared with that of age-matched Abcc6‒/‒Parp1+/+ littermates. Subsequently, oral administration of minocycline, an inhibitor of DDR/PAR signaling, resulted in an 86% reduction of calcification in the muzzle skin of Abcc6‒/‒ mice. Minocycline treatment also attenuated the DDR/PAR signaling and reduced the calcification of dermal fibroblasts derived from patients with PXE. The anticalcification effect of DDR/PAR inhibition was not accompanied by alterations in plasma inorganic pyrophosphate concentrations. These results suggest that local DDR/PAR signaling in calcification-prone tissues contributes to PXE pathogenesis and that its inhibition might provide a promising treatment strategy for ectopic calcification in PXE, a currently intractable disease.

Functional Assessment of Missense Variants in the ABCC6 Gene Implicated in Pseudoxanthoma Elasticum, a Heritable Ectopic Mineralization Disorder.

Pseudoxanthoma elasticum, a heritable multisystem ectopic mineralization disorder, is caused by inactivating mutations in the ABCC6 gene. The encoded protein, ABCC6, a transmembrane transporter, has a specialized efflux function in hepatocytes by contributing to plasma levels of inorganic pyrophosphate, a potent inhibitor of mineralization in soft connective tissues. Reduced plasma inorganic pyrophosphate levels underlie the ectopic mineralization in pseudoxanthoma elasticum. In this study, we characterized the pathogenicity of three human ABCC6 missense variants using an adenovirus-mediated liver-specific ABCC6 transgene expression system in an Abcc6-/- mouse model of pseudoxanthoma elasticum. Variants p.L420V and p.R1064W were found benign because they had abundance and plasma membrane localization in hepatocytes similar to the wild-type human ABCC6 transgene, normalized plasma inorganic pyrophosphate levels, and prevented mineralization in the dermal sheath of vibrissae in muzzle skin, a phenotypic hallmark in the Abcc6-/- mice. In contrast, p.S400F was shown to be pathogenic because it failed to normalize plasma inorganic pyrophosphate levels and had no effect on ectopic mineralization despite its normal expression and proper localization in hepatocytes. These results showed that adenovirus-mediated hepatic ABCC6 expression in Abcc6-/- mice can provide a model system to effectively elucidate the multifaceted functional consequences of human ABCC6 missense variants identified in patients with pseudoxanthoma elasticum.

Myocardin regulates expression of contractile genes in smooth muscle cells and is required for closure of the ductus arteriosus in mice.

Myocardin (Myocd) is a potent transcriptional coactivator that has been implicated in cardiovascular development and adaptation of the cardiovascular system to hemodynamic stress. To determine the function of myocardin in the developing cardiovascular system, Myocd(F/F)/Wnt1-Cre(+) and Myocd(F/F)/Pax3-Cre(+) mice were generated in which the myocardin gene was selectively ablated in neural crest-derived SMCs populating the cardiac outflow tract and great arteries. Both Myocd(F/F)/Wnt1-Cre(+) and Myocd(F/F)/Pax3-Cre(+) mutant mice survived to birth, but died prior to postnatal day 3 from patent ductus arteriosus (PDA). Neural crest-derived SMCs populating the ductus arteriosus (DA) and great arteries exhibited a cell autonomous block in expression of myocardin-regulated genes encoding SMC-restricted contractile proteins. Moreover, Myocd-deficient vascular SMCs populating the DA exhibited ultrastructural features generally associated with the SMC synthetic, rather than contractile, phenotype. Consistent with these findings, ablation of the Myocd gene in primary aortic SMCs harvested from Myocd conditional mutant mice caused a dramatic decrease in SMC contractile protein expression. Taken together, these data demonstrate that myocardin regulates expression of genes required for the contractile phenotype in neural crest-derived SMCs and provide new insights into the molecular and genetic programs that may underlie PDA.

Correction to: Myocardin regulates mitochondrial calcium homeostasis and prevents permeability transition.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Adenovirus-Mediated ABCC6 Gene Therapy for Heritable Ectopic Mineralization Disorders.

Loss-of-function mutations in the ABCC6 gene cause pseudoxanthoma elasticum and type 2 generalized arterial calcification of infancy, heritable ectopic mineralization disorders without effective treatment. ABCC6 encodes the putative efflux transporter ABCC6, which is predominantly expressed in the liver. Although the substrate of ABCC6 remains unknown, recent studies showed that pseudoxanthoma elasticum is a metabolic disorder caused by reduced circulating levels of pyrophosphate, a potent mineralization inhibitor. We hypothesized that reconstitution of ABCC6 might counteract ectopic mineralization in an Abcc6-/- mouse model of pseudoxanthoma elasticum. Intravenous administration of a recombinant adenovirus expressing wild-type human ABCC6 in Abcc6-/- mice showed sustained high-level expression of human ABCC6 in the liver for up to 4 weeks, increasing pyrophosphate levels in plasma. In addition, adenovirus injection every 4 weeks restored plasma pyrophosphate levels and, consequently, significantly reduced ectopic mineralization in the skin of young mice. By contrast, the same treatment in old mice with already established mineral deposits failed to reduce mineralization. These results suggest that adenovirus-mediated ABCC6 gene delivery, when initiated early, is a promising prevention therapy for pseudoxanthoma elasticum and generalized arterial calcification of infancy, diseases that currently lack preventive or therapeutic options.

Rapid AAV-Neutralizing Antibody Determination with a Cell-Binding Assay.

Recombinant adeno-associated virus (rAAV) has been developed as a successful vector for both basic research and human gene therapy. However, neutralizing antibodies (NAbs) against AAV capsids can abolish AAV infectivity on target cells, reducing the transduction efficacy. Absence of AAV NAb has become a prerequisite qualification for patients enrolled in gene therapy trials. Nevertheless, accurate assessment of AAV NAb has remained a challenging task. Here we developed a rapid assay based on the observations that AAV NAb inhibits rAAV binding to the host cell surface and NAb titers are negatively related to the amount of AAV genomes binding to the target cells. By quantifying the AAV genome on the target cells in the presence of anti-sera, AAV NAb titers can be accurately determined. The titer determined by this assay correlates well with the classical transduction-based assays. A major advantage of this method is that it can be carried out with a 30-min binding assay without the lengthy wait for a transduction outcome. This assay is independent of transduction performance of AAV serotype in the target cells. Therefore, the AAV cell-binding assay for NAb determination offers an alternative method for in vivo NAb assay.

Inhibition of Tissue-Nonspecific Alkaline Phosphatase Attenuates Ectopic Mineralization in the Abcc6-/- Mouse Model of PXE but Not in the Enpp1 Mutant Mouse Models of GACI.

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, is caused by mutations in the ABCC6 gene encoding a putative efflux transporter ABCC6. It was recently shown that the absence of ABCC6-mediated adenosine triphosphate release from the liver and, consequently, reduced inorganic pyrophosphate levels underlie the pathogenesis of PXE. Given that tissue-nonspecific alkaline phosphatase (TNAP), encoded by ALPL, is the enzyme responsible for degrading inorganic pyrophosphate, we hypothesized that reducing TNAP levels either by genetic or pharmacological means would lead to amelioration of the ectopic mineralization phenotype in the Abcc6-/- mouse model of PXE. Thus, we bred Abcc6-/- mice to heterozygous Alpl+/- mice that display approximately 50% plasma TNAP activity. The Abcc6-/-Alpl+/- double-mutant mice showed 52% reduction of mineralization in the muzzle skin compared with the Abcc6-/-Alpl+/+ mice. Subsequently, oral administration of SBI-425, a small molecule inhibitor of TNAP, resulted in 61% reduction of plasma TNAP activity and 58% reduction of mineralization in the muzzle skin of Abcc6-/- mice. By contrast, SBI-425 treatment of Enpp1 mutant mice, another model of ectopic mineralization associated with reduced inorganic pyrophosphate, failed to reduce muzzle skin mineralization. These results suggest that inhibition of TNAP might provide a promising treatment strategy for PXE, a currently intractable disease.

IOP1, a novel hydrogenase-like protein that modulates hypoxia-inducible factor-1alpha activity.

A central means by which mammalian cells respond to low oxygen tension is through the activation of the transcription factor HIF-1 (hypoxia-inducible factor-1). Under normoxic conditions, HIF-1alpha (the alpha subunit of HIF-1) is targeted for rapid degradation by the ubiquitin-proteasome pathway. Under hypoxic conditions, this degradation is inhibited, thereby leading to the stabilization and activation of HIF-1alpha. Here, we report the identification of IOP1 (iron-only hydrogenase-like protein 1), a protein homologous with enzymes present in anaerobic organisms that contain a distinctive iron-sulfur cluster. IOP1 is present in a broad range of cell types. Knockdown of IOP1 using siRNA (small interfering RNA) in mammalian cells increases protein levels of HIF-1alpha under both normoxic and hypoxic conditions, and augments hypoxia-induced HRE (hypoxia response element) reporter gene and endogenous HIF-1alpha target gene expressions. We find that IOP1 knockdown up-regulates HIF-1alpha mRNA levels, thereby providing a mechanism by which knockdown induces the observed effects. The results collectively provide evidence that IOP1 is a component of the protein network that regulates HIF-1alpha in mammalian cells.

Myocardin regulates mitochondrial calcium homeostasis and prevents permeability transition.

Myocardin is a transcriptional co-activator required for cardiovascular development, but also promotes cardiomyocyte survival through an unclear molecular mechanism. Mitochondrial permeability transition is implicated in necrosis, while pore closure is required for mitochondrial maturation during cardiac development. We show that loss of myocardin function leads to subendocardial necrosis at E9.5, concurrent with elevated expression of the death gene Nix. Mechanistically, we demonstrate that myocardin knockdown reduces microRNA-133a levels to allow Nix accumulation, leading to mitochondrial permeability transition, reduced mitochondrial respiration, and necrosis. Myocardin knockdown elicits calcium release from the endo/sarcoplasmic reticulum with mitochondrial calcium accumulation, while restoration of microRNA-133a function, or knockdown of Nix rescues calcium perturbations. We observed reduced myocardin and elevated Nix expression within the infarct border-zone following coronary ligation. These findings identify a myocardin-regulated pathway that maintains calcium homeostasis and mitochondrial function during development, and is attenuated during ischemic heart disease. Given the diverse role of Nix and microRNA-133a, these findings may have broader implications to metabolic disease and cancer.

MCUR1 Is a Scaffold Factor for the MCU Complex Function and Promotes Mitochondrial Bioenergetics.

Mitochondrial Ca(2+) Uniporter (MCU)-dependent mitochondrial Ca(2+) uptake is the primary mechanism for increasing matrix Ca(2+) in most cell types. However, a limited understanding of the MCU complex assembly impedes the comprehension of the precise mechanisms underlying MCU activity. Here, we report that mouse cardiomyocytes and endothelial cells lacking MCU regulator 1 (MCUR1) have severely impaired [Ca(2+)]m uptake and IMCU current. MCUR1 binds to MCU and EMRE and function as a scaffold factor. Our protein binding analyses identified the minimal, highly conserved regions of coiled-coil domain of both MCU and MCUR1 that are necessary for heterooligomeric complex formation. Loss of MCUR1 perturbed MCU heterooligomeric complex and functions as a scaffold factor for the assembly of MCU complex. Vascular endothelial deletion of MCU and MCUR1 impaired mitochondrial bioenergetics, cell proliferation, and migration but elicited autophagy. These studies establish the existence of a MCU complex that assembles at the mitochondrial integral membrane and regulates Ca(2+)-dependent mitochondrial metabolism.

Cardiac melanocytes influence atrial reactive oxygen species involved with electrical and structural remodeling in mice.

Cardiac melanocyte-like cells (CMLCs) contribute to atrial arrhythmias when missing the melanin synthesis enzyme dopachrome tautomerase (Dct). While scavenging reactive oxygen species (ROS) in Dct-null mice partially suppressed atrial arrhythmias, it remains unclear if CMLCs influence atrial ROS and structure or if the electrical response of CMLCs to ROS differs from that of atrial myocytes. This study is designed to determine if CMLCs contribute to overall atrial oxidative stress or structural remodeling, and if ROS affects the electrophysiology of CMLCs differently than atrial myocytes. Immunohistochemical analysis showed higher expression of the oxidative marker 8-hydroxy-2'-deoxyguanosine in Dct-null atria versus Dct-heterozygous (Dct-het) atria. Exposing isolated CMLCs from Dct-het and Dct-null mice to hydrogen peroxide increased superoxide anion more in Dct-null CMLCs. Trichrome staining showed increased fibrosis in Dct-null atria, and treating Dct-null mice with the ROS scavenger Tempol reduced atrial fibrosis. Action potential recordings from atrial myocytes and isolated Dct-het and Dct-null CMLCs in response to hydrogen peroxide showed that the EC50 for action potential duration (APD) prolongation of Dct-null CMLCs was 8.2 ± 1.7 µmol/L versus 16.8 ± 2.0 µmol/L for Dct-het CMLCs, 19.9 ± 2.1 µmol/L for Dct-null atrial myocytes, and 20.5 ± 1.9 µmol/L for Dct-het atrial myocytes. However, APD90 was longer in CMLCs versus atrial myocytes in response to hydrogen peroxide. Hydrogen peroxide also induced more afterdepolarizations in CMLCs compared to atrial myocytes. These studies suggest that Dct within CMLCs contributes to atrial ROS balance and remodeling. ROS prolongs APD to a greater extent and induces afterdepolarizations more frequently in CMLCs than in atrial myocytes.

Desmopressin-induced posterior reversible encephalopathy syndrome.

We herein report the case of a patient who presented with an acute decrease of visual acuity, hypertension, focal seizures and transient mental dysfunction while undergoing desmopressin treatment. Neuroimaging revealed bilateral occipital-parietal lesions that presented with vasogenic edema. After controlling the hypertension and discontinuing the desmopressin treatment, the patient's condition improved. A follow-up imaging examination performed six months later showed complete resolution of the lesions. It is important to recognize posterior reversible encephalopathy syndrome (PRES) as a rare and serious complication of desmopressin administration. Both the blood pressure and water electrolyte balance should be carefully monitored in patients receiving desmopressin therapy.

Myocardin regulates BMP10 expression and is required for heart development.

Myocardin is a muscle lineage-restricted transcriptional coactivator that has been shown to transduce extracellular signals to the nucleus required for SMC differentiation. We now report the discovery of a myocardin/BMP10 (where BMP10 indicates bone morphogenetic protein 10) signaling pathway required for cardiac growth, chamber maturation, and embryonic survival. Myocardin-null (Myocd) embryos and embryos harboring a cardiomyocyte-restricted mutation in the Myocd gene exhibited myocardial hypoplasia, defective atrial and ventricular chamber maturation, heart failure, and embryonic lethality. Cardiac hypoplasia was caused by decreased cardiomyocyte proliferation accompanied by a dramatic increase in programmed cell death. Defective chamber maturation and the block in cardiomyocyte proliferation were caused in part by a block in BMP10 signaling. Myocardin transactivated the Bmp10 gene via binding of a serum response factor-myocardin protein complex to a nonconsensus CArG element in the Bmp10 promoter. Expression of p57kip2, a BMP10-regulated cyclin-dependent kinase inhibitor, was induced in Myocd-/- hearts, while BMP10-activated cardiogenic transcription factors, including NKX2.5 and MEF2c, were repressed. Remarkably, when embryonic Myocd-/- hearts were cultured ex vivo in BMP10-conditioned medium, the defects in cardiomyocyte proliferation and p57kip2 expression were rescued. Taken together, these data identify a heretofore undescribed myocardin/BMP10 signaling pathway that regulates cardiomyocyte proliferation and apoptosis in the embryonic heart.