Molecular genetics of aortic valve disease

Notch signaling in cardiac valve development and disease

The Notch pathway is an intercellular signaling mechanism involved in multiple cell-to-cell communication processes that regulate cell fate specification, differentiation, and tissue patterning during embryogenesis and adulthood. Functional studies in the mouse have shown that a Hey-Bmp2 regulatory circuit restricts Bmp2 expression to presumptive valve myocardium (atrioventricular canal and outflow tract). Likewise, a Notch-Hey-Bmp2 axis represses Bmp2 in the endocardium. During cardiac valve formation, endocardial Notch signaling activates the epithelial-mesenchyme transition (EMT) that will give rise to the cardiac valve primordia. During this process, Notch integrates with myocardially derived signals (Bmp2 or Bmp4) to promote, via Snail1/2 activation a complete, invasive EMT in presumptive valve tissue. In humans, mutations in Notch signaling components are associated with several congenital disorders involving malformed valves, aortic arch, and defective chamber septation. Data suggest that the same embryonic Notch-Hey-Bmp2 regulatory axis is active in the adult valve. This review examines the experimental evidence supporting a role for Notch in heart valve development and homeostasis, and how altered Notch signaling may lead to valve disease in the newborn and adult.

Molecular and developmental mechanisms of congenital heart valve disease

Congenital heart disease occurs in approximately 1% of all live births and includes structural abnormalities of the heart valves. However, this statistic underestimates congenital valve lesions, such as bicuspic aortic valve (BAV) and mitral valve prolapse (MVP), that typically become apparent later in life as progressive valve dysfunction and disease. At present, the standard treatment for valve disease is replacement, and approximately 95,000 surgical procedures are performed each year in the United States. The most common forms of congenital valve disease include abnormal valve cusp morphogenesis, as in the case of BAV, or defects in extracellular matrix (ECM) organization and homeostasis, as occurs in MVP. The etiology of these common valve diseases is largely unknown. However, the study of murine and avian model systems, along with human genetic linkage studies, have led to the identification of genes and regulatory processes that contribute to valve structural malformations and disease. This review focuses on the current understanding and therapeutic implications of molecular regulatory pathways that control valve development and contribute to valve disease.

Insight into pathologic abnormalities in congenital semilunar valve disease based on advances in understanding normal valve microstructure and extracellular matrix

Congenitally diseased valves are relatively frequent causes of significant morbidity and mortality. Pathology descriptions of such valves have primarily focused on gross structural features including the number of leaflets or commissures (bicuspid/bicommissural valve) and alterations in the contour, thickness, and consistency of the leaflets (dysplastic valve). Functional correlates of these pathologic alterations are valvar stenosis, insufficiency, or both. Further characterization of the microstructural abnormalities seen in these malformed valves may not only provide insight into the correlation of distinct pathologies with their respective pathogenesis and clinical sequelae but also prove pivotal in uncovering new avenues for therapeutic interventions and prevention regimens. This review summarizes microstructural findings in congenital semilunar valve disease (CSVD) and discusses their relevance in light of recent advances in knowledge of normal valve microstructure, biology, and function. Specifically, the biological and mechanical roles of various matrix components and their interactions are discussed in the context of CSVD. Indeed, recent research in normal valves adds significant insight into CSVD and raises many hypotheses that will need to be addressed by future studies.

Hereditary disorders of connective tissue: a guide to the emerging differential diagnosis

PURPOSE

To create a practical desk reference for clinicians focused on the differential diagnosis of individuals presenting with features that suggest an inherited disorder of connective tissue.

METHODS

We searched the medical literature for distinct clinical entities that shared clinical features with Marfan syndrome and other classical inherited disorders of connective tissue.

RESULTS

Thirty-six distinct heritable disorders of connective tissue were identified that have overlapping features. These disorders were organized into two matrices according to clinical characteristics and according to causative genes.

CONCLUSIONS

A broad differential diagnosis is emerging for individuals presenting with features suggestive of altered connective tissue. Recent advances in molecular genetics have aided in the delineation of these disorders.

Genetic factors in non-syndromic congenital heart malformations

The genetic defect in most patients with non-syndromic congenital heart malformations (CHM) is unknown, although more than 40 different genes have already been implicated. Only a minority of CHM seems to be due to monogenetic mutations, and the majority occurs sporadically. The multifactorial inheritance hypothesis of common diseases suggesting that the cumulative effect of multiple genetic and environmental risk factors leads to disease, might also apply for CHM. We review here the monogenic disease genes with high-penetrance mutations, susceptibility genes with reduced-penetrance mutations, and somatic mutations implicated in non-syndromic CHM.

MicroRNA regulatory networks in cardiovascular development

The heart, more than any other organ, requires precise functionality on a second-to-second basis throughout the lifespan of the organism. Even subtle perturbations in cardiac structure or function have catastrophic consequences, resulting in lethal forms of congenital and adult heart disease. Such intolerance of the heart to variability necessitates especially robust regulatory mechanisms to govern cardiac gene expression. Recent studies have revealed central roles for microRNAs (miRNAs) as governors of gene expression during cardiovascular development and disease. The integration of miRNAs into the genetic circuitry of the heart provides a rich and robust array of regulatory interactions to control cardiac gene expression. miRNA regulatory networks also offer opportunities for therapeutically modulating cardiac function through the manipulation of pathogenic and protective miRNAs. We discuss the roles of miRNAs as regulators of cardiac form and function, unresolved questions in the field, and issues for the future.

Effects of varying Notch1 signal strength on embryogenesis and vasculogenesis in compound mutant heterozygotes

Background

Identifying developmental processes regulated by Notch1 can be addressed in part by characterizing mice with graded levels of Notch1 signaling strength. Here we examine development in embryos expressing various combinations of Notch1 mutant alleles. Mice homozygous for the hypomorphic Notch1^12f allele, which removes the single O-fucose glycan in epidermal growth factor-like repeat 12 (EGF12) of the Notch1 ligand binding domain (lbd), exhibit reduced growth after weaning and defective T cell development. Mice homozygous for the inactive Notch1^lbd allele express Notch1 missing an ~20 kDa internal segment including the canonical Notch1 ligand binding domain, and die at embryonic day ~E9.5. The embryonic and vascular phenotypes of compound heterozygous Notch1^12f/lbd embryos were compared with Notch1^+/12f, Notch1^12f/12f, and Notch1^lbd/lbd embryos. Embryonic stem (ES) cells derived from these embryos were also examined in Notch signaling assays. While Notch1 signaling was stronger in Notch1^12f/lbd compound heterozygotes compared to Notch1^lbd/lbd embryos and ES cells, Notch1 signaling was even stronger in embryos carrying Notch1^12f and a null Notch1 allele.

Results

Mouse embryos expressing the hypomorphic Notch1^12f allele, in combination with the inactive Notch1^lbd allele which lacks the Notch1 ligand binding domain, died at ~E11.5-12.5. Notch1^12f/lbd ES cells signaled less well than Notch1^12f/12f ES cells but more strongly than Notch1^lbd/lbd ES cells. However, vascular defects in Notch1^12f/lbd yolk sac were severe and similar to Notch1^lbd/lbd yolk sac. By contrast, vascular disorganization was milder in Notch1^12f/lbd compared to Notch1^lbd/lbd embryos. The expression of Notch1 target genes was low in Notch1^12f/lbd yolk sac and embryo head, whereas Vegf and Vegfr2 transcripts were increased. The severity of the compound heterozygous Notch1^12f/lbd yolk sac phenotype suggested that the allelic products may functionally interact. By contrast, compound heterozygotes with Notch1^12f in combination with a Notch1 null allele (Notch1^tm1Con) were capable of surviving to birth.

Conclusions

Notch1 signaling in Notch1^12f/lbd compound heterozygous embryos is more defective than in compound heterozygotes expressing a hypomorphic Notch1^12f allele and a Notch1 null allele. The data suggest that the gene products Notch1^lbd and Notch1^12f interact to reduce the activity of Notch1^12f.

Structural abnormalities of the pulmonary trunk in tetralogy of Fallot and potential clinical implications: a morphological study

OBJECTIVES

The purpose of this study was to determine whether intrinsic histological abnormalities of the pulmonary trunk (PT) are present from birth and interact with palliative surgery and/or repair.

BACKGROUND

Little is known about PT histology in patients with tetralogy of Fallot (TOF), especially in the era of surgical intervention in childhood.

METHODS

We studied 39 formalin-fixed necropsy heart specimens with TOF and compared them with 17 normal control heart specimens. Sections of the PT and aorta were studied by light microscopy using various stains; histological findings were graded according to severity.

RESULTS

Among the TOF group (1 fetus, 11 infants, 14 children, and 13 adults), 11 patients had undergone palliative and 10 patients had undergone reparative surgery at a median age of 8 years (range 2.5 to 18 years). Histological changes of grade 2 or higher were present in 59% (medionecrosis), 36% (fibrosis), 56% (cystlike formation), and 56% (abnormal elastic tissue configuration) of TOF patients. Total histology grading scores were higher in TOF hearts (median 6, range 1 to 9) compared with controls (median 1, range 0 to 6; p < 0.0001). Histological abnormalities were present among infants (median score 3.5, range 1 to 9) and after palliative surgery (median score 5, range 2 to 9) or repair (median score 7.5, range 4 to 9).

CONCLUSIONS

Marked histological abnormalities in the PT of hearts with TOF exist compared with controls. These changes were present from infancy and among patients who had undergone palliative or reparative surgery, although operations in this cohort were performed late. Our data suggest that structural abnormalities of the PT, similar to these recently shown in the aorta, are intrinsic.

Heart valve development: regulatory networks in development and disease

In recent years, significant advances have been made in the definition of regulatory pathways that control normal and abnormal cardiac valve development. Here, we review the cellular and molecular mechanisms underlying the early development of valve progenitors and establishment of normal valve structure and function. Regulatory hierarchies consisting of a variety of signaling pathways, transcription factors, and downstream structural genes are conserved during vertebrate valvulogenesis. Complex intersecting regulatory pathways are required for endocardial cushion formation, valve progenitor cell proliferation, valve cell lineage development, and establishment of extracellular matrix compartments in the stratified valve leaflets. There is increasing evidence that the regulatory mechanisms governing normal valve development also contribute to human valve pathology. In addition, congenital valve malformations are predominant among diseased valves replaced late in life. The understanding of valve developmental mechanisms has important implications in the diagnosis and management of congenital and adult valve disease.

Lack of periostin leads to suppression of Notch1 signaling and calcific aortic valve disease

The Postn gene encodes protein periostin. During embryonic development, it is highly expressed in the outflow tract (OFT) endocardial cushions of the developing heart, which give rise to several structures of the mature heart including the aortic valve. Periostin was previously implicated in osteoblast differentiation, cancer metastasis, and tooth and bone development, but its role in cardiac OFT development is unclear. To elucidate the role that periostin plays in the developing heart we analyzed cardiac OFT phenotype in mice after deletion of the Postn gene. We found that lack of periostin in the embryonic OFT leads to ectopic expression of the proosteogenic growth factor pleiotrophin (Ptn) and overexpression of delta-like 1 homolog (Dlk1), a negative regulator of Notch1, in the distal (prevalvular) cushions of the OFT. This resulted in suppression of Notch1 signaling, strong induction of the central transcriptional regulator of osteoblast cell fate Runx2, upregulation of osteopontin and osteocalcin expression, and subsequent calcification of the aortic valve. Our data suggest that periostin represses a default osteogenic program in the OFT cushion mesenchyme and promotes differentiation along a fibrogenic lineage. Lack of periostin causes derepression of the osteogenic potential of OFT mesenchymal cells, calcium deposition, and calcific aortic valve disease. These results establish periostin as a key regulator of OFT endocardial cushion mesenchymal cell fate during embryonic development.

Reduced EGFR causes abnormal valvular differentiation leading to calcific aortic stenosis and left ventricular hypertrophy in C57BL/6J but not 129S1/SvImJ mice

Epidermal growth factor receptor (EGFR) signaling contributes to aortic valve development in mice. Because developmental phenotypes in Egfr-null mice are dependent on genetic background, the hypomorphic Egfr(wa2) allele was made congenic on C57BL/6J (B6) and 129S1/SvImJ (129) backgrounds and used to identify the underlying cellular cause of EGFR-related aortic valve abnormalities. Egfr(wa2/wa2) mice on both genetic backgrounds develop aortic valve hyperplasia. Many B6-Egfr(wa2/wa2) mice die before weaning, and those surviving to 3 mo of age or older develop severe left ventricular hypertrophy and heart failure. The cardiac phenotype was accompanied by significantly thicker aortic cusps and larger transvalvular gradients in B6-Egfr(wa2/wa2) mice compared with heterozygous controls and age-matched Egfr(wa2) homozygous mice on either 129 or B6129F1 backgrounds. Histological analysis revealed cellular changes in B6-Egfr(wa2/wa2) aortic valves underlying elevated pressure gradients and progression to heart failure, including increased cellular proliferation, ectopic cartilage formation, extensive calcification, and inflammatory infiltrate, mimicking changes seen in human calcific aortic stenosis. Despite having congenitally enlarged valves, 129 and B6129F1-Egfr(wa2/wa2) mice have normal lifespans, absence of left ventricular hypertrophy, and normal systolic function. These results show the requirement of EGFR activity for normal valvulogenesis and demonstrate that dominantly acting genetic modifiers curtail pathological changes in congenitally deformed valves. These studies provide a novel model of aortic sclerosis and stenosis and suggest that long-term inhibition of EGFR signaling for cancer therapy may have unexpected consequences on aortic valves in susceptible individuals.

Presenilin: RIP and beyond

Over the years the presenilins (PSENs), a family of multi-transmembrane domain proteins, have been ascribed a number of diverse potential functions. Recent in vivo evidence has supported the existence of PSEN functions beyond its well-established role in regulated intramembrane proteolysis. In this review, we will briefly discuss the ability of PSEN to modulate cellular signaling pathways through gamma-secretase cleavage of transmembrane proteins. Additionally, we will critically examine the proposed roles of PSEN in the regulation of beta-catenin function, protein trafficking, calcium regulation, and apoptosis.

Review: Does lowering cholesterol have an impact on the progression of aortic stenosis?

Several studies suggest that atherosclerotic disease is not a focal disease restricted to culprit lesions in the intima of the arterial wall, but seems to act as a general disease affecting the entire cardiovascular system. Evolving research has lately focused on the atherosclerotic component in calcific aortic stenosis (AS) as it seems that the valve is affected in a pattern similar to that of the vasculature. The hope is therefore, that we someday in the management of patients with calcific AS can apply some of the same treatment strategies as in atherosclerotic vascular disease. This article reviews the pathophysiological mechanisms of calcific AS, reviews current clinical trials of statin use in aortic stenosis and reports on on-going trials, evaluating whether cholesterol lowering therapy can slow disease progression in different populations. Finally, we review if computerized tomography, biomarkers, and clinical characteristics such as left ventricular ejection fraction, can be useful in stratifying patients to potential benefit of statin therapy.

Mouse models of congenital cardiovascular disease

Congenital heart defects occur in nearly 1% of human live births and many are lethal if not surgically repaired. In addition, the genetic contribution to congenital or acquired cardiovascular diseases that are silent at birth, but progress to cause significant disease in later life is being increasingly appreciated. Heart development and structure are highly conserved between mouse and human. The discoveries that are being made in this model system are highly relevant to understanding the pathogenesis of human heart defects whether they occus in isolation, or in the context of a syndrome. Many of the genes required for cardiovascular development were discovered fortuitously when early lethality or structural defects were observed in mouse mutants generated for other purposes, and relevant genes continue to be defined in this manner. Candidate genes for this process are being identified by their roles other species, or by their expression in pertinent tissues in mice. In this review, I will briefly summarize heart development as currently understood in the mouse, and then discuss how complementary studies in mouse and human have identified genes and pathways that are critical for normal cardiovascular development, and for maintaining the structure and function of this organ system throughout life.

Mutational and energetic studies of Notch 1 transcription complexes

Notch proteins constitute the receptors of a highly conserved signaling pathway that influences cell fate decisions both during development and in adulthood. A proteolytic cascade induced by ligand stimulation results in release of the intracellular Notch domain from the cell membrane, allowing it to enter the nucleus and form a complex with a DNA-bound transcription factor called CSL (CBF-1/RBP-J kappa, Suppressor of Hairless, and Lag-1) and a coactivator of the Mastermind family. Assembly of this Notch nuclear complex is the key step in the transcriptional response to a Notch signal. In the studies reported here, we mapped residues important for the stabilization of this multiprotein-DNA complex using site-directed mutagenesis, determined the affinity of the three-domain form of CSL for its various partners, and investigated sources of cooperativity in complex formation by monitoring the influence of various components of the complex on the interactions of CSL with its other partners. Our findings are consistent with a model for complex assembly in which the RBP-J kappa-associated molecule domain of Notch increases the effective concentration of the ankyrin domain for its binding site on the Rel-homology region of CSL, enabling docking of the ankyrin domain and subsequent recruitment of the Mastermind-like coactivator.

Novel NOTCH1 mutations in patients with bicuspid aortic valve disease and thoracic aortic aneurysms

OBJECTIVES

Bicuspid aortic valve is a common condition and is associated with a significantly increased risk of developing thoracic aortic aneurysms and acute aortic dissection. Patient-specific prediction of the risk of developing thoracic aortic aneurysm, however, is imprecise. We hypothesize that genotypic variations in patients with bicuspid aortic valves contribute to this observed variability in aortic phenotype. We, therefore, investigated the potential relationship between mutations in regions of NOTCH1 recently reported to be associated with bicuspid aortic valve and the phenotype of bicuspid aortic valve and thoracic aortic aneurysms in unrelated patients undergoing surgical repair.

METHODS

We performed a targeted mutational analysis of NOTCH1 using genomic DNA from 48 unrelated subjects with concomitant bicuspid aortic valve and thoracic aortic aneurysm using denaturing high-performance liquid chromatography and DNA sequencing. We focused on exons in which mutations associated with bicuspid aortic valve have been reported previously. Results were compared with control subjects with trileaflet aortic valves (n = 94), bicuspid aortic valves, and normal aortas (n = 22) and in subjects with tricuspid aortic valves and thoracic aortic aneurysms (n = 28).

RESULTS

Four unique, nonsynonymous (3 novel) variants were identified in 5 (10.4%) of 48 patients with concomitant bicuspid aortic valves and thoracic aortic aneurysms compared with only 3 (2.1%) of 144 control subjects (P = .02). Of these, 2 novel missense mutations, A1343V and P1390T, were observed only in patients with bicuspid aortic valves and tricuspid aortic aneurysms.

CONCLUSIONS

This targeted analysis involving NOTCH1 exons previously implicated in familial and sporadic bicuspid aortic valve demonstrates overrepresentation of NOTCH1 missense variants among patients with bicuspid aortic valves and thoracic aortic aneurysms. Identification of aneurysm-predisposing susceptibility genes may lead to gene-directed surgical therapy of the ascending aorta for patients with bicuspid aortic valves.

Notch signaling in vascular development and physiology

Notch signaling is an ancient intercellular signaling mechanism that plays myriad roles during vascular development and physiology in vertebrates. These roles include regulation of artery/vein differentiation in endothelial and vascular smooth muscle cells, regulation of blood vessel sprouting and branching during both normal development and tumor angiogenesis, and the differentiation and physiological responses of vascular smooth muscle cells. Defects in Notch signaling also cause inherited vascular and cardiovascular diseases. In this review, I summarize recent findings and discuss the growing relevance of Notch pathway modulation for therapeutic applications in disease.

Cooperative assembly of higher-order Notch complexes functions as a switch to induce transcription

Notch receptors control differentiation and contribute to pathologic states such as cancer by interacting directly with a transcription factor called CSL (for CBF-1/Suppressor of Hairless/Lag-1) to induce expression of target genes. A number of Notch-regulated targets, including genes of the hairy/enhancer-of-split family in organisms ranging from Drosophila to humans, are characterized by paired CSL-binding sites in a characteristic head-to-head arrangement. Using a combination of structural and molecular approaches, we establish here that cooperative formation of dimeric Notch transcription complexes on promoters with paired sites is required to activate transcription. Our findings identify a mechanistic step that can account for the exquisite sensitivity of Notch target genes to variation in signal strength and developmental context, enable new strategies for sensitive and reliable identification of Notch target genes, and lay the groundwork for the development of Notch pathway inhibitors that are active on target genes containing paired sites.

The genetics of cardiac birth defects

Congenital heart disease likely results from a complex mixture of environmental and genetic factors. Recent work has elucidated rare single gene mutations that cause a variety of cardiac defects, but the etiologies of more common disease remains unknown. Here, we review the known genetic causes of cardiac malformations and discuss future approaches for addressing sporadic congenital heart disease as a complex trait.

Structures of CSL, Notch and Mastermind proteins: piecing together an active transcription complex

Notch signaling mediates communication between cells, and is essential for proper cell fate decisions in the developing embryo and the adult organism. Signaling initiates proteolytic release of the receptor Notch from the membrane, whereupon the intracellular portion of Notch (NotchIC) localizes to the nucleus and engages the DNA-binding transcription factor CSL. CSL is required for both repression and activation of Notch target genes, and is the focal point of a transcriptional switch, mediating interactions with transcriptional coregulators. Activation of transcription requires corepressor displacement from CSL by NotchIC and the recruitment of the transcriptional coactivator Mastermind to the complex. Several recently determined and exciting structures of CSL, NotchIC, and an active transcription complex composed of CSL, NotchIC and Mastermind have revealed new insights into transcriptional regulation in the Notch pathway.

Cardiomyopathy with a unique finding of bicuspid aortic valve in Becker's muscular dystrophy

We describe a patient with Becker's muscular dystrophy and cardiac failure caused by a combination of dilated cardiomyopathy and congenital bicuspid aortic valve with aortic stenosis. There is no documented association between congenital valve disease and human dystrophinopathies, and to our knowledge, this is the first reported case.