Molecular genetics of aortic valve disease

Human Genetics of Semilunar Valve and Aortic Arch Anomalies

Lesions of the semilunar valve and the aortic arch can occur either in isolation or as part of well-described clinical syndromes. The polygenic cause of calcific aortic valve disease will be discussed including the key role of NOTCH1 mutations. In addition, the complex trait of bicuspid aortic valve disease will be outlined, both in sporadic/familial cases and in the context of associated syndromes, such as Alagille, Williams, and Kabuki syndromes. Aortic arch abnormalities particularly coarctation of the aorta and interrupted aortic arch, including their association with syndromes such as Turner and 22q11 deletion, respectively, are also discussed. Finally, the genetic basis of congenital pulmonary valve stenosis is summarized, with particular note to Ras-/mitogen-activated protein kinase (Ras/MAPK) pathway syndromes and other less common associations, such as Holt-Oram syndrome.

Single-cell reconstruction and mutation enrichment analysis identifies dysregulated cardiomyocyte and endothelial cells in congenital heart disease

Congenital heart disease (CHD) is one of the most prevalent neonatal congenital anomalies. To catalog the putative candidate CHD risk genes, we collected 16,349 variants [single-nucleotide variants (SNVs) and Indels] impacting 8,308 genes in 3,166 CHD cases for a comprehensive meta-analysis. Using American College of Medical Genetics (ACMG) guidelines, we excluded the 0.1% of benign/likely benign variants and the resulting dataset consisted of 83% predicted loss of function variants and 17% missense variants. Seventeen percent were de novo variants. A stepwise analysis identified 90 variant-enriched CHD genes, of which six (GPATCH1, NYNRIN, TCLD2, CEP95, MAP3K19, and TTC36) were novel candidate CHD genes. Single-cell transcriptome cluster reconstruction analysis on six CHD tissues and four controls revealed upregulation of the top 10 frequently mutated genes primarily in cardiomyocytes. NOTCH1 (highest number of variants) and MYH6 (highest number of recurrent variants) expression was elevated in endocardial cells and cardiomyocytes, respectively, and 60% of these gene variants were associated with tetralogy of Fallot and coarctation of the aorta, respectively. Pseudobulk analysis using the single-cell transcriptome revealed significant (P < 0.05) upregulation of both NOTCH1 (endocardial cells) and MYH6 (cardiomyocytes) in the control heart data. We observed nine different subpopulations of CHD heart cardiomyocytes of which only four were observed in the control heart. This is the first comprehensive meta-analysis combining genomics and CHD single-cell transcriptomics, identifying the most frequently mutated CHD genes, and demonstrating CHD gene heterogeneity, suggesting that multiple genes contribute to the phenotypic heterogeneity of CHD. Cardiomyocytes and endocardial cells are identified as major CHD-related cell types.NEW & NOTEWORTHY Congential heart disease (CHD) is one of the most prevalent neonatal congenital anomalies. We present a comprehensive analysis combining genomics and CHD single-cell transcriptome. Our study identifies 90 potential candidate CHD risk genes of which 6 are novel. The risk genes have heterogenous expression suggestive of multiple genes contributing to the phenotypic heterogeneity of CHD. Cardiomyocytes and endocardial cells are identified as major CHD-related cell types.

Role of Notch2 pathway in mature B cell malignancies

In recent decades, the Notch pathway has been characterized as a key regulatory signaling of cell-fate decisions evolutionarily conserved in many organisms and different tissues during lifespan. At the same time, many studies suggest a link between alterations of this signaling and tumor genesis or progression. In lymphopoiesis, the Notch pathway plays a fundamental role in the correct differentiation of T and B cells, but its deregulated activity leads to leukemic onset and evolution. Notch and its ligands Delta/Jagged exhibit a pivotal role in the crosstalk between leukemic cells and their environment. This review is focused in particular on Notch2 receptor activity. Members of Notch2 pathway have been reported to be mutated in Chronic Lymphocytic Leukemia (CLL), Splenic Marginal Zone Lymphoma (SMZL) and Nodal Marginal Zone Lymphoma (NMZL). CLL is a B cell malignancy in which leukemic clones establish supportive crosstalk with non-malignant cells of the tumor microenvironment to grow, survive, and resist even the new generation of drugs. SMZL and NMZL are indolent B cell neoplasms distinguished by a distinct pattern of dissemination. In SMZL leukemic cells affect mainly the spleen, bone marrow, and peripheral blood, while NMZL has a leading nodal distribution. Since Notch2 is involved in the commitment of leukemic cells to the marginal zone as a major regulator of B cell physiological differentiation, it is predominantly affected by the molecular lesions found in both SMZL and NMZL. In light of these findings, a better understanding of the Notch receptor family pathogenic role, in particular Notch2, is desirable because it is still incomplete, not only in the physiological development of B lymphocytes but also in leukemia progression and resistance. Several therapeutic strategies capable of interfering with Notch signaling, such as monoclonal antibodies, enzyme or complex inhibitors, are being analyzed. To avoid the unwanted multiple "on target" toxicity encountered during the systemic inhibition of Notch signaling, the study of an appropriate pharmaceutical formulation is a pressing need. This is why, to date, there are still no Notch-targeted therapies approved. An accurate analysis of the Notch pathway could be useful to drive the discovery of new therapeutic targets and the development of more effective therapies.

Point on the Aortic Bicuspid Valve

Background—Bicuspid aortic valve (BAV) disease is the most prevalent congenital heart disease in the world. Knowledge about its subtypes origin, development, and evolution is poor despite the frequency and the potential gravity of this condition. Its prognosis mostly depends on the risk of aortic aneurysm development with an increased risk of aortic dissection. Aims—This review aims to describe this complex pathology in way to improve the bicuspid patients’ management. Study design—We reviewed the literature with MEDLINE and EMBASE databases using MeSH terms such as “bicuspid aortic valve”, “ascending aorta”, and “bicuspid classification”. Results—There are various classifications. They depend on the criteria chosen by the authors to differentiate subtypes. Those criteria can be the number and position of the raphes, the cusps, the commissures, or their arrangements regarding coronary ostia. Sievers’ classification is the reference. The phenotypic description of embryology revealed that all subtypes of BAV are the results of different embryological pathogenesis, and therefore, should be considered as distinct conditions. Their common development towards aortic dilatation is explained by the aortic media’s pathological histology with cystic medial necrosis. At the opposite, BAV seems to display a profound genetic heterogeneity with both sporadic and familial forms. BAV can be even isolated or combined with other congenital malformations. Conclusions—All those characteristics make this pathology a highly complex condition that needs further genetic, embryological, and hemodynamic explorations to complete its well described anatomy.

Systems of pattern formation within developmental biology

Applications of mathematical models to developmental biology have provided helpful insight into various subfields, ranging from the patterning of animal skin to the development of complex organ systems. Systems involved in patterning within morphology present a unique path to explain self-organizing systems. Current efforts show that patterning systems, notably Reaction-Diffusion and specific signaling pathways, provide insight for explaining morphology and could provide novel applications revolving around the formation of biological systems. Furthermore, the application of pattern formation provides a new perspective on understanding developmental biology and pathology research to study molecular mechanisms. The current review is to cover and take a more in-depth overlook at current applications of patterning systems while also building on the principles of patterning of future research in predictive medicine.

Genetics of congenital heart disease: a narrative review of recent advances and clinical implications

Congenital heart disease (CHD) is the most common human birth defect and remains a leading cause of mortality in childhood. Although advances in clinical management have improved the survival of children with CHD, adult survivors commonly experience cardiac and non-cardiac comorbidities, which affect quality of life and prognosis. Therefore, the elucidation of genetic etiologies of CHD not only has important clinical implications for genetic counseling of patients and families but may also impact clinical outcomes by identifying at-risk patients. Recent advancements in genetic technologies, including massively parallel sequencing, have allowed for the discovery of new genetic etiologies for CHD. Although variant prioritization and interpretation of pathogenicity remain challenges in the field of CHD genomics, advances in single-cell genomics and functional genomics using cellular and animal models of CHD have the potential to provide novel insights into the underlying mechanisms of CHD and its associated morbidities. In this review, we provide an updated summary of the established genetic contributors to CHD and discuss recent advances in our understanding of the genetic architecture of CHD along with current challenges with the interpretation of genetic variation. Furthermore, we highlight the clinical implications of genetic findings to predict and potentially improve clinical outcomes in patients with CHD.

Ex Vivo Models to Decipher the Molecular Mechanisms of Genetic Notch Cardiovascular Disorders

Notch is an evolutionary, conserved, cell-cell signaling pathway that is central to several biological processes, from tissue morphogenesis to homeostasis. It is therefore not surprising that several genetic mutations of Notch components cause inherited human diseases, especially cardiovascular disorders. Despite numerous efforts, current in vivo models are still insufficient to unravel the underlying mechanisms of these pathologies, hindering the development of utmost needed medical therapies. In this perspective review, we discuss the limitations of current murine models and outline how the combination of microphysiological systems (MPSs) and targeted computational models can lead to breakthroughs in this field. In particular, while MPSs enable the experimentation on human cells in controlled and physiological environments, in silico models can provide a versatile tool to translate the in vitro findings to the more complex in vivo setting. As a showcase example, we focus on Notch-related cardiovascular diseases, such as Alagille syndrome, Adams-Oliver syndrome, and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Impact statement In this review, a comprehensive overview of the limitations of current in vivo models of genetic Notch cardiovascular diseases is provided, followed by a discussion over the potential of microphysiological systems and computational models in overcoming these limitations and in potentiating drug testing and modeling of these pathologies.

Causes, Diagnosis, Risk Stratification and Treatment of Bicuspid Aortic Valve Disease: An Updated Review

The most common congenital heart disease is the bicuspid aortic valve. Understanding the pathophysiology and the altered hemodynamics is a key component for the diagnosis, risk stratification and treatment. Among others, aortic valve stenosis is the most common complication. Treatment strategies vary depending on the severity of the disease, particularly the dilation of the aorta playing a major role. Together with valve replacement, transcatheter aortic valve implantation is now considered as an alternative option with good results. With this review we would like to discuss the causes, diagnostic methods, risk stratification and treatment strategies of the bicuspid aortic valve.

Mechanical stress alters the expression of calcification-related genes in vascular interstitial and endothelial cells

OBJECTIVES

Vascular wall calcification is a major pathophysiological component of atherosclerotic disease with many similarities to osteogenesis. Mechanical stress of the vascular wall may theoretically contribute to the proliferative processes by endothelial and interstitial cells. The aim of the study was to investigate the effect of mechanical stress on the expression of some calcification-related genes in primary human endothelial and interstitial cells, and how endothelial cells may stimulate the fibroblast and smooth muscle cells.

METHODS

Human umbilical vein endothelial and interstitial cells were subjected to cyclic stretch using a FlexCell® bioreactor, and interstitial cells were also subjected to tensile strain in cultures embedded in 3-dimensional collagen gels. The medium from endothelial cells was used to stimulate the gel-cultured interstitial cells, or the endothelium was sown directly on top. For comparison, human endothelial and smooth muscle cells were isolated from aortic wall fragments of patients with and without the aortic aneurysm. The expression of genes was measured using quantitative PCR.

RESULTS

Four hours of cyclic stretch applied to cultured endothelial cells upregulated the mRNA expression of bone morphogenetic protein 2 (BMP-2), a major procalcific growth factor. When applied to a 3-dimensional culture of vascular interstitial cells, the medium from prestretched endothelial cells decreased the expression of BMP-2 and periostin mRNA in the fibroblasts. The static tension in gel-cultured interstitial cells upregulated BMP-2 mRNA expression. The addition of endothelial cells on the top of this culture also reduced mRNA of anticalcific genes, periostin and osteopontin. Similar changes were observed in smooth muscle cells from human aortic aneurysms compared to cells from the healthy aorta. Aortic aneurysm endothelial cells also showed an increased expression of BMP-2 mRNA.

CONCLUSIONS

Endothelial cells respond to mechanical stress by upregulation of pro-osteogenic factor BMP-2 mRNA and modulate the expression of other osteogenic factors in vascular interstitial cells. Endothelial cells may, thus, contribute to vascular calcification when exposed to mechanical stress.

Maternal hyperglycemia and fetal cardiac development: Clinical impact and underlying mechanisms

Congenital heart disease (CHD) is the most common type of birth defect and is both a significant pediatric and adult health problem, in light of a growing population of survivors. The etiology of CHD has been considered to be multifactorial with genetic and environmental factors playing important roles. The combination of advances in cardiac developmental biology, which have resulted in the elucidation of molecular pathways regulating normal cardiac morphogenesis, and genome sequencing technology have allowed the discovery of numerous genetic contributors of CHD ranging from chromosomal abnormalities to single gene variants. Conversely, mechanistic details of the contribution of environmental factors to CHD remain unknown. Maternal diabetes mellitus (matDM) is a well-established and increasingly prevalent environmental risk factor for CHD, but the underlying etiologic mechanisms by which pregestational matDM increases the vulnerability of embryos to cardiac malformations remains largely elusive. Here, we will briefly discuss the multifactorial etiology of CHD with a focus on the epidemiologic link between matDM and CHD. We will describe the animal models used to study the underlying mechanisms between matDM and CHD and review the numerous cellular and molecular pathways affected by maternal hyperglycemia in the developing heart. Last, we discuss how this increased understanding may open the door for the development of novel prevention strategies to reduce the incidence of CHD in this high-risk population.

Development of calcific aortic valve disease: Do we know enough for new clinical trials?

Calcific aortic valve disease (CAVD), previously thought to represent a passive degeneration of the valvular extracellular matrix (VECM), is now regarded as an intricate multistage disorder with sequential yet intertangled and interacting underlying processes. Endothelial dysfunction and injury, initiated by disturbed blood flow and metabolic disorders, lead to the deposition of low-density lipoprotein cholesterol in the VECM further provoking macrophage infiltration, oxidative stress, and release of pro-inflammatory cytokines. Such changes in the valvular homeostasis induce differentiation of normally quiescent valvular interstitial cells (VICs) into synthetically active myofibroblasts producing excessive quantities of the VECM and proteins responsible for its remodeling. As a result of constantly ongoing degradation and re-deposition, VECM becomes disorganised and rigid, additionally potentiating myofibroblastic differentiation of VICs and worsening adaptation of the valve to the blood flow. Moreover, disrupted and excessively vascularised VECM is susceptible to the dystrophic calcification caused by calcium and phosphate precipitating on damaged collagen fibers and concurrently accompanied by osteogenic differentiation of VICs. Being combined, passive calcification and biomineralisation synergistically induce ossification of the aortic valve ultimately resulting in its mechanical incompetence requiring surgical replacement. Unfortunately, multiple attempts have failed to find an efficient conservative treatment of CAVD; however, therapeutic regimens and clinical settings have also been far from the optimal. In this review, we focused on interactions and transitions between aforementioned mechanisms demarcating ascending stages of CAVD, suggesting a predisposing condition (bicuspid aortic valve) and drug combination (lipid-lowering drugs combined with angiotensin II antagonists and cytokine inhibitors) for the further testing in both preclinical and clinical trials.

Intraluminal valves: development, function and disease

The circulatory system consists of the heart, blood vessels and lymphatic vessels, which function in parallel to provide nutrients and remove waste from the body. Vascular function depends on valves, which regulate unidirectional fluid flow against gravitational and pressure gradients. Severe valve disorders can cause mortality and some are associated with severe morbidity. Although cardiac valve defects can be treated by valve replacement surgery, no treatment is currently available for valve disorders of the veins and lymphatics. Thus, a better understanding of valves, their development and the progression of valve disease is warranted. In the past decade, molecules that are important for vascular function in humans have been identified, with mouse studies also providing new insights into valve formation and function. Intriguing similarities have recently emerged between the different types of valves concerning their molecular identity, architecture and development. Shear stress generated by fluid flow has also been shown to regulate endothelial cell identity in valves. Here, we review our current understanding of valve development with an emphasis on its mechanobiology and significance to human health, and highlight unanswered questions and translational opportunities.

Krox20 defines a subpopulation of cardiac neural crest cells contributing to arterial valves and bicuspid aortic valve

Although cardiac neural crest cells are required at early stages of arterial valve development, their contribution during valvular leaflet maturation remains poorly understood. Here, we show in mouse that neural crest cells from pre-otic and post-otic regions make distinct contributions to the arterial valve leaflets. Genetic fate-mapping analysis of Krox20-expressing neural crest cells shows a large contribution to the borders and the interleaflet triangles of the arterial valves. Loss of Krox20 function results in hyperplastic aortic valve and partially penetrant bicuspid aortic valve formation. Similar defects are observed in neural crest Krox20-deficient embryos. Genetic lineage tracing in Krox20^-/- mutant mice shows that endothelial-derived cells are normal, whereas neural crest-derived cells are abnormally increased in number and misplaced in the valve leaflets. In contrast, genetic ablation of Krox20-expressing cells is not sufficient to cause an aortic valve defect, suggesting that adjacent cells can compensate this depletion. Our findings demonstrate a crucial role for Krox20 in arterial valve development and reveal that an excess of neural crest cells may be associated with bicuspid aortic valve.

Integration of Drosophila and Human Genetics to Understand Notch Signaling Related Diseases

Notch signaling research dates back to more than one hundred years, beginning with the identification of the Notch mutant in the fruit fly Drosophila melanogaster. Since then, research on Notch and related genes in flies has laid the foundation of what we now know as the Notch signaling pathway. In the 1990s, basic biological and biochemical studies of Notch signaling components in mammalian systems, as well as identification of rare mutations in Notch signaling pathway genes in human patients with rare Mendelian diseases or cancer, increased the significance of this pathway in human biology and medicine. In the 21st century, Drosophila and other genetic model organisms continue to play a leading role in understanding basic Notch biology. Furthermore, these model organisms can be used in a translational manner to study underlying mechanisms of Notch-related human diseases and to investigate the function of novel disease associated genes and variants. In this chapter, we first briefly review the major contributions of Drosophila to Notch signaling research, discussing the similarities and differences between the fly and human pathways. Next, we introduce several biological contexts in Drosophila in which Notch signaling has been extensively characterized. Finally, we discuss a number of genetic diseases caused by mutations in genes in the Notch signaling pathway in humans and we expand on how Drosophila can be used to study rare genetic variants associated with these and novel disorders. By combining modern genomics and state-of-the art technologies, Drosophila research is continuing to reveal exciting biology that sheds light onto mechanisms of disease.

Runx2-I is an Early Regulator of Epithelial-Mesenchymal Cell Transition in the Chick Embryo

BACKGROUND

Although normally linked to bone and cartilage development, the Runt-related transcription factor, RUNX2, was reported in the mouse heart during development of the valves. We examined RUNX2 expression and function in the developing avian heart as it related to the epithelial-mesenchymal transition (EMT) in the atrioventricular canal. EMT can be separated into an activation stage involving hypertrophy and cell separation and an invasion stage where cells invade the extracellular matrix. The localization and activity of RUNX2 was explored in relation to these steps in the heart. As RUNX2 was also reported in cancer tissues, we examined its expression in the progression of esophageal cancer in staged tissues.

RESULTS

A specific isoform, RUNX2-I, is present and required for EMT by endothelia of the atrioventricular canal. Knockdown of RUNX2-I inhibits the cell-cell separation that is characteristic of initial activation of EMT. Loss of RUNX2-I altered expression of EMT markers to a greater extent during activation than during subsequent cell invasion. Transforming growth factor beta 2 (TGFβ2) mediates activation during cardiac endothelial EMT. Consistent with a role in activation, RUNX2-I is regulated by TGFβ2 and its activity is independent of similarly expressed Snai2 in regulation of EMT. Examination of RUNX2 expression in esophageal cancer showed its upregulation concomitant with the development of dysplasia and continued expression in adenocarcinoma.

CONCLUSIONS

These data introduce the RUNX2-I isoform as a critical early transcription factor mediating EMT in the developing heart after induction by TGFβ2. Its expression in tumor tissue suggests a similar role for RUNX2 in the EMT of metastasis. Developmental Dynamics 247:542-554, 2018. © 2017 Wiley Periodicals, Inc.

Protein-altering and regulatory genetic variants near GATA4 implicated in bicuspid aortic valve

Bicuspid aortic valve (BAV) is a heritable congenital heart defect and an important risk factor for valvulopathy and aortopathy. Here we report a genome-wide association scan of 466 BAV cases and 4,660 age, sex and ethnicity-matched controls with replication in up to 1,326 cases and 8,103 controls. We identify association with a noncoding variant 151 kb from the gene encoding the cardiac-specific transcription factor, GATA4, and near-significance for p.Ser377Gly in GATA4. GATA4 was interrupted by CRISPR-Cas9 in induced pluripotent stem cells from healthy donors. The disruption of GATA4 significantly impaired the transition from endothelial cells into mesenchymal cells, a critical step in heart valve development.

The genetic component of bicuspid aortic valve and aortic dilation. An exome-wide association study

BACKGROUND

Bicuspid aortic valve is the most common cardiovascular congenital malformation affecting 2% of the general population. The incidence of life-threatening complications, the high heritability, and familial clustering rates support the interest in identifying risk or protective genetic factors. The main objective of the present study was to identify population-based genetic variation associated with bicuspid aortic valve and concomitant ascending aortic dilation.

MATERIALS AND METHODS

A cross-sectional exome-wide association study was conducted in 565 Spanish cases and 484 controls. Single-marker and gene-based association analyses enriched for low frequency and rare genetic variants were performed on this discovery stage cohort and for the subsets of cases with and without ascending aortic dilation. Discovery-stage association signals and additional markers indirectly associated with bicuspid aortic valve, were genotyped in a replication cohort that comprised 895 Caucasian cases and 1483 controls.

RESULTS

Although none of the association signals were consistent across series, the involvement of HMCN2 in calcium metabolism and valve degeneration caused by calcium deposit, and a nominal but not genome-wide significant association, supported it as an interesting gene for follow-up studies on the genetic susceptibility to bicuspid aortic valve.

CONCLUSIONS

The absence of a genome-wide significant association signal shows this valvular malformation may be more genetically complex than previously believed. Exhaustive phenotypic characterization, even larger datasets, and collaborative efforts are needed to detect the combination of rare variants conferring risk which, along with specific environmental factors, could be causing the development of this disease.

Hemodynamics driven cardiac valve morphogenesis

Mechanical forces are instrumental to cardiovascular development and physiology. The heart beats approximately 2.6 billion times in a human lifetime and heart valves ensure that these contractions result in an efficient, unidirectional flow of the blood. Composed of endocardial cells (EdCs) and extracellular matrix (ECM), cardiac valves are among the most mechanically challenged structures of the body both during and after their development. Understanding how hemodynamic forces modulate cardiovascular function and morphogenesis is key to unraveling the relationship between normal and pathological cardiovascular development and physiology. Most valve diseases have their origins in embryogenesis, either as signs of abnormal developmental processes or the aberrant re-expression of fetal gene programs normally quiescent in adulthood. Here we review recent discoveries in the mechanobiology of cardiac valve development and introduce the latest technologies being developed in the zebrafish, including live cell imaging and optical technologies, as well as modeling approaches that are currently transforming this field. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Notch1 Mutation Leads to Valvular Calcification Through Enhanced Myofibroblast Mechanotransduction

OBJECTIVE

Calcific aortic valve disease (CAVD) is a significant cardiovascular disorder, and controversy exists as to whether it is primarily a dystrophic or osteogenic process in vivo. In this study, we sought to clarify the mechanism of CAVD by assessing a genetic mutation, Notch1 heterozygosity, which leads to CAVD with 100% penetrance in humans.

APPROACH AND RESULTS

Murine immortalized Notch1(+/-) aortic valve interstitial cells (AVICs) were isolated and expanded in vitro. Molecular signaling of wild-type and Notch1(+/-) AVICs were compared to identify changes in pathways that have been linked to CAVD-transforming growth factor-β1/bone morphogenetic protein, mitogen-activated protein kinase, and phosphoinositide 3-kinase/protein kinase B-and assessed for calcification potential. Additionally, AVIC mechanobiology was studied in a physiologically relevant, dynamic mechanical environment (10% cyclic strain) to investigate differences in responses between the cell types. We found that Notch1(+/-) AVICs resembled a myofibroblast-like phenotype expressing higher amounts of cadherin-11, a known mediator of dystrophic calcification, and decreased Runx2, a known osteogenic marker. We determined that cadherin-11 expression is regulated by Akt activity, and inhibition of Akt phosphorylation significantly reduced cadherin-11 expression. Moreover, in the presence of cyclic strain, Notch1(+/-) AVICs exhibited significantly upregulated phosphorylation of Akt at Ser473 and smooth muscle α-actin expression, indicative of a fully activated myofibroblast. Finally, these Notch1-mediated alterations led to enhanced dystrophic calcific nodule formation.

CONCLUSIONS

This study presents novel insights in our understanding of Notch1-mediated CAVD by demonstrating that the mutation leads to AVICs that are fully activated myofibroblasts, resulting in dystrophic, but not osteogenic, calcification.

Molecular mechanisms of inherited thoracic aortic disease - from gene variant to surgical aneurysm

Aortic dissection is a catastrophic event that has a high mortality rate. Thoracic aortic aneurysms are the clinically silent precursor that confers an increased risk of acute aortic dissection. There are several gene mutations that have been identified in key structural and regulatory proteins within the aortic wall that predispose to thoracic aneurysm formation. The most common and well characterised of these is the FBN1 gene mutation that is known to cause Marfan syndrome. Others less well-known mutations include TGF-β1 and TGF-β2 receptor mutations that cause Loeys-Dietz syndrome, Col3A1 mutations causing Ehlers-Danlos Type 4 syndrome and Smad3 and-4, ACTA2 and MYHII mutations that cause familial thoracic aortic aneurysm and dissection. Despite the variation in the proteins affected by these genetic mutations, there is a unifying pathological end point of medial degeneration within the wall of the aorta characterised by vascular smooth muscle cell loss, fragmentation and loss of elastic fibers, and accumulation of proteoglycans and glycosaminoglycans within vascular smooth muscle cell-depleted areas of the aortic media. Our understanding of these mutations and their post-translational effects has led to a greater understanding of the pathophysiology that underlies thoracic aortic aneurysm formation. Despite this, there are still many unanswered questions regarding the molecular mechanisms. Further elucidation of the signalling pathways will help us identify targets that may be suitable modifiers to enhance treatment of this often fatal condition.

RANKL-OPG and RAGE modulation in vascular calcification and diabetes: novel targets for therapy

Type 2 diabetes is associated with increased cardiovascular morbidity and mortality and early vascular ageing. This takes the form of atherosclerosis, with progressive vascular calcification being a major complication in the pathogenesis of this disease. Current research and drug targets in diabetes have hitherto focused on atherosclerosis, but vascular calcification is now recognised as an independent predictor of cardiovascular morbidity and mortality. An emerging regulatory pathway for vascular calcification in diabetes involves the receptor activator for nuclear factor κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG). Important novel biomarkers of calcification are related to levels of glycation and inflammation in diabetes. Several therapeutic strategies could have advantageous effects on the vasculature in patients with diabetes, including targeting the RANKL and receptor for AGE (RAGE) signalling pathways, since there has been little success-at least in macrovascular outcomes-with conventional glucose-lowering therapy. There is substantial and relevant clinical and basic science evidence to suggest that modulating RANKL-RANK-OPG signalling, RAGE signalling and the associated proinflammatory milieu alters the natural course of cardiovascular complications and outcomes in people with diabetes. However, further research is critically needed to understand the precise mechanisms underpinning these pathways, in order to translate the anti-calcification strategies into patient benefit.

Normal and abnormal development of the aortic wall and valve: correlation with clinical entities

Dilation of the wall of the thoracic aorta can be found in patients with a tricuspid (TAV) as well as a bicuspid aortic valve (BAV) with and without a syndromic component. BAV is the most common congenital cardiovascular malformation, with a population prevalence of 0.5–2 %. The clinical course is often characterised by aneurysm formation and in some cases dissection. The non-dilated aortic wall is less well differentiated in all BAV as compared with TAV, thereby conferring inherent developmental susceptibility. Furthermore, a turbulent flow, caused by the inappropriate opening of the bicuspid valve, could accelerate the degenerative process in the aortic wall. However, not all patients with bicuspidy develop clinical complications during their life. We postulate that the increased vulnerability for aortic complications in a subset of patients with BAV is caused by a defect in the early development of the aorta and aortic valve. This review discusses histological and molecular genetic aspects of the normal and abnormal development of the aortic wall and semilunar valves. Aortopathy associated with BAV could be the result of a shared developmental defect during embryogenesis.

Multi-detector CT angiography of the aortic valve-Part 2: disease specific findings

The aortic valve and adjacent structures should be routinely evaluated on all thoracic cross-sectional imaging studies. Echocardiography and magnetic resonance imaging (MRI) are the main imaging techniques used for assessment of the aortic valve and related pathology but multi-detector computed tomography (MDCT) can offer valuable complimentary information in some clinical scenarios. MDCT is the definite means of assessing aortic valvular calcification, acute aortic syndrome and for non-invasive assessment of the coronary arteries. MDCT also has an emerging role in the planning and follow-up of trans-catheter aortic valve replacement. This article reviews the spectrum of aortic valve disease highlighting the key MDCT imaging features.

Cardiovascular genomics: implications for acute and critical care nurses

As genomic health care becomes commonplace, nurses will be asked to provide genomic care in all health care settings including acute care and critical care. Three common cardiac conditions are reviewed, Marfan syndrome, bicuspid aortic valve, and hypertrophic cardiomyopathy, to provide acute care and critical care nurses with an overview of these pathologies through the lens of genomics and relevant case studies. This information will help critical care nursing leaders become familiar with genetics related to common cardiac conditions and prepare acute care and critical care nurses for a new phase in patient diagnostics, with greater emphasis on early diagnosis and recognition of conditions before sudden cardiac death.

Etiology of valvular heart disease-genetic and developmental origins

Valvular heart disease occurs as either a congenital or acquired condition and advances in medical care have resulted in valve disease becoming increasingly prevalent. Unfortunately, treatments remain inadequate because of our limited understanding of the genetic and molecular etiology of diseases affecting the heart valves. Therefore, surgical repair or replacement remains the most effective option, which comes with additional complications and no guarantee of life-long success. Over the past decade, there have been significant advances in our understanding of cardiac valve development and, not surprisingly, mutations in these developmental genes have been identified in humans with congenital valve malformations. Concurrently, there has been a greater realization that acquired valve disease is not simply a degenerative process. Molecular investigation of acquired valve disease has identified that numerous signaling pathways critical for normal valve development are re-expressed in diseased valves. This review will discuss recent advances in our understanding of the development of the heart valves, as well as the implications of these findings on the genetics of congenital and acquired valvular heart disease.

Architectural trends in the human normal and bicuspid aortic valve leaflet and its relevance to valve disease

The bicuspid aortic valve (AV) is the most common cardiac congenital anomaly and has been found to be a significant risk factor for developing calcific AV disease. However, the mechanisms of disease development remain unclear. In this study we quantified the structure of human normal and bicuspid leaflets in the early disease stage. From these individual leaflet maps average fiber structure maps were generated using a novel spline based technique. Interestingly, we found statistically different and consistent regional structures between the normal and bicuspid valves. The regularity in the observed microstructure was a surprising finding, especially for the pathological BAV leaflets and is an essential cornerstone of any predictive mathematical models of valve disease. In contrast, we determined that isolated valve interstitial cells from BAV leaflets show the same in vitro calcification pathways as those from the normal AV leaflets. This result suggests the VICs are not intrinsically different when isolated, and that external features, such as abnormal microstructure and altered flow may be the primary contributors in the accelerated calcification experienced by BAV patients.

Family-based studies to identify genetic variants that cause congenital heart defects

Congenital heart defects (CHDs) are the most common congenital abnormalities. Analysis of large multigenerational families has led to the identification of a number of genes for CHDs. However, identifiable variations in these genes are the cause of a small proportion of cases of CHDs, suggesting significant genetic heterogeneity. In addition, large families with CHDs are rare, making the identification of additional genes difficult. Next-generation sequencing technologies will provide an opportunity to identify more genes in the future. However, the significant genetic variation between individuals will present a challenge to distinguish between 'pathogenic' and 'benign' variants. We have demonstrated that the analysis of multiple individuals in small families using combinations of algorithms can reduce the number of candidate variants to a small, manageable number. Thus, the analysis of small nuclear families or even distantly related 'sporadic' cases may begin to uncover the 'dark matter' of CHD genetics.

Endothelial nitric oxide signaling regulates Notch1 in aortic valve disease

The mature aortic valve is composed of a structured trilaminar extracellular matrix that is interspersed with aortic valve interstitial cells (AVICs) and covered by endothelium. Dysfunction of the valvular endothelium initiates calcification of neighboring AVICs leading to calcific aortic valve disease (CAVD). The molecular mechanism by which endothelial cells communicate with AVICs and cause disease is not well understood. Using a co-culture assay, we show that endothelial cells secrete a signal to inhibit calcification of AVICs. Gain or loss of nitric oxide (NO) prevents or accelerates calcification of AVICs, respectively, suggesting that the endothelial cell-derived signal is NO. Overexpression of Notch1, which is genetically linked to human CAVD, retards the calcification of AVICs that occurs with NO inhibition. In AVICs, NO regulates the expression of Hey1, a downstream target of Notch1, and alters nuclear localization of Notch1 intracellular domain. Finally, Notch1 and NOS3 (endothelial NO synthase) display an in vivo genetic interaction critical for proper valve morphogenesis and the development of aortic valve disease. Our data suggests that endothelial cell-derived NO is a regulator of Notch1 signaling in AVICs in the development of the aortic valve and adult aortic valve disease.

Loss of function of parathyroid hormone receptor 1 induces Notch-dependent aortic defects during zebrafish vascular development

OBJECTIVE

Coarctation of the aorta is rarely associated with known gene defects. Blomstrand chondrodysplasia, caused by mutations in the parathyroid hormone receptor 1 (PTHR1) is associated with coarctation of the aorta in some cases, although it is unclear whether PTHR1 deficiency causes coarctation of the aorta directly. The zebrafish allows the study of vascular development using approaches not possible in other models. We therefore examined the effect of loss of function of PTHR1 or its ligand parathyroid hormone-related peptide (PTHrP) on aortic formation in zebrafish.

APPROACH AND RESULTS

Morpholino antisense oligonucleotide knockdown of either PTHR1 or PTHrP led to a localized occlusion of the mid-aorta in developing zebrafish. Confocal imaging of transgenic embryos showed that these defects were caused by loss of endothelium, rather than failure to lumenize. Using a Notch reporter transgenic ([CSL:Venus]qmc61), we found both PTHR1 and PTHrP knockdown-induced defective Notch signaling in the hypochord at the site of the aortic defect before onset of circulation, and the aortic occlusion was rescued by inducible Notch upregulation.

CONCLUSIONS

Loss of function of either PTHR1 or PTHrP leads to a localized aortic defect that is Notch dependent. These findings may underlie the aortic defect seen in Blomstrand chondrodysplasia, and reveal a link between parathyroid hormone and Notch signaling during aortic development.

Impact of notch signaling on inflammatory responses in cardiovascular disorders

Notch signaling is a major pathway in cell fate decisions. Since the first reports showing the major role of Notch in embryonic development, a considerable and still growing literature further highlights its key contributions in various pathological processes during adult life. In particular, Notch is now considered as a major player in vascular homeostasis through the control of key cellular functions. In parallel, confounding evidence emerged that inflammatory responses regulate Notch signaling in vitro in endothelial cells, smooth muscle cells or vascular infiltrating cells and in vivo in vascular and inflammatory disorders and in cardiovascular diseases. This review presents how inflammation influences Notch in vascular cells and, reciprocally, emphasizes the functional role of Notch on inflammatory processes, notably by regulating key cell functions (differentiation, proliferation, apoptosis/survival, activation). Understanding how the disparity of Notch receptors and ligands impacts on vasculature biology remains critical for the design of relevant and adequate therapeutic strategies targeting Notch in this major pathological context.

Cardiac outflow tract anomalies

The mature outflow tract (OFT) is, in basic terms, a short conduit. It is a simple, although vital, connection situated between contracting muscular heart chambers and a vast embryonic vascular network. Unfortunately, it is also a focal point underlying many multifactorial congenital heart defects (CHDs). Through the use of various animal models combined with human genetic investigations, we are beginning to comprehend the molecular and cellular framework that controls OFT morphogenesis. Clear roles of neural crest cells (NCC) and second heart field (SHF) derivatives have been established during OFT formation and remodeling. The challenge now is to determine how the SHF and cardiac NCC interact, the complex reciprocal signaling that appears to be occurring at various stages of OFT morphogenesis, and finally how endocardial progenitors and primary heart field (PHF) communicate with both these colonizing extra-cardiac lineages. Although we are beginning to understand that this dance of progenitor populations is wonderfully intricate, the underlying pathogenesis and the spatiotemporal cell lineage interactions remain to be fully elucidated. What is now clear is that OFT alignment and septation are independent processes, invested via separate SHF and cardiac neural crest (CNC) lineages. This review will focus on our current understanding of the respective contributions of the SHF and CNC lineage during OFT development and pathogenesis.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

The congenital bicuspid aortic valve can experience high-frequency unsteady shear stresses on its leaflet surface

The bicuspid aortic valve (BAV) is a common congenital malformation of the aortic valve (AV) affecting 1% to 2% of the population. The BAV is predisposed to early degenerative calcification of valve leaflets, and BAV patients constitute 50% of AV stenosis patients. Although evidence shows that genetic defects can play a role in calcification of the BAV leaflets, we hypothesize that drastic changes in the mechanical environment of the BAV elicit pathological responses from the valve and might be concurrently responsible for early calcification. An in vitro model of the BAV was constructed by surgically manipulating a native trileaflet porcine AV. The BAV valve model and a trileaflet AV (TAV) model were tested in an in vitro pulsatile flow loop mimicking physiological hemodynamics. Laser Doppler velocimetry was used to make measurements of fluid shear stresses on the leaflet of the valve models using previously established methodologies. Furthermore, particle image velocimetry was used to visualize the flow fields downstream of the valves and in the sinuses. In the BAV model, flow near the leaflets and fluid shear stresses on the leaflets were much more unsteady than for the TAV model, most likely due to the moderate stenosis in the BAV and the skewed forward flow jet that collided with the aorta wall. This additional unsteadiness occurred during mid- to late-systole and was composed of cycle-to-cycle magnitude variability as well as high-frequency fluctuations about the mean shear stress. It has been demonstrated that the BAV geometry can lead to unsteady shear stresses under physiological flow and pressure conditions. Such altered shear stresses could play a role in accelerated calcification in BAVs.

Control of gene expression by translational recoding

Like all rules, even the genetic code has exceptions: these are generically classified as “translational recoding.” Almost every conceivable mode of recoding has been documented, including signals that redefine translational reading frame and codon assignation. While first described in viruses, it is becoming clear that sequences that program elongating ribosomes to shift translational reading frame are widely used by organisms in all domains of life, thus expanding both the coding capacity of genomes and the modes through which gene expression can be regulated at the posttranscriptional level. Instances of programmed ribosomal frameshifting and stop codon reassignment are opening up new avenues for treatment of numerous inborn errors of metabolism. The implications of these findings on human health are only beginning to emerge.

Rare non-synonymous variations in the transcriptional activation domains of GATA5 in bicuspid aortic valve disease

Bicuspid aortic valve (BAV) is the commonest congenital heart disease and a highly heritable trait; however, only the NOTCH1 gene has been linked to limited cases of BAV in humans. Recently, the transcription factor GATA5 has been shown to have an essential role in aortic valve development, and targeted deletion of Gata5 in mice is associated with partially penetrant BAV formation. Here, we investigated the relationship between GATA5 gene variants and BAV with its associated aortopathy. One hundred unrelated individuals with confirmed BAV were prospectively recruited. Following collection of clinical information and DNA extraction, the coding regions and splice signal sequences of the GATA5 gene were screened for sequence variations. The clinical characteristics of the cohort included a male predominance (77%), mean age of diagnosis 29 ± 22 years, associated aortopathy in 59% and positive family history for BAV in 13%. Genetic analysis identified the presence of 4 rare non-synonymous variations within the GATA5 transcriptional activation domains, namely Gln3Arg, Ser19Trp, Tyr142His and Gly166Ser, occurring in one patient each. Gln3Arg and Tyr142His substitutions affect highly conserved and functionally relevant residues, and are likely to impact on the transcriptional activation of GATA5 target regions. A novel Ser19Trp variation was identified at a highly conserved amino acid residue in one patient, while the Gly166Ser variant was found in a familial case of BAV and associated aortopathy. Rare non-synonymous variations in the functionally important GATA5 transcriptional activation domains may be important in the pathogenesis of BAV disease in humans.

Factors other than genotype account largely for the phenotypic variation of the pulmonary valve in Syrian hamsters

Understanding of the aetiology of congenitally anomalous pulmonary valves remains incomplete. The aim of our study, therefore, was to elucidate the degree to which the phenotypic variation known to exist for the pulmonary valve relies on genotypic variation. Initially, we tested the hypothesis that genetically alike individuals would display similar valvar phenotypes if the phenotypic arrangement depended entirely, or almost entirely, on the genotype. Thus, we examined pulmonary valves from 982 Syrian hamsters belonging to two families subject to systematic inbreeding by crossing siblings. Their coefficient of inbreeding was 0.999 or higher, so they could be considered genetically alike. External environmental factors were standardized as much as possible. A further 97 Syrian hamsters from an outbred colony were used for comparative purposes. In both the inbred and outbred hamsters, we found valves with a purely trifoliate, or tricuspid, design, trifoliate valves with a more or less extensive fusion of the right and left leaflets, bifoliate, or bicuspid, valves with fused right and left leaflets, with or without a raphe located in the conjoined arterial sinus, and quadrifoliate, or quadricuspid, valves. The incidence of the different valvar morphological variants was similar in the outbred and inbred colonies, except for the bifoliate pulmonary valves, which were significantly more frequent in the hamsters from one of the two inbred families. Results of crosses between genetically alike hamsters revealed no significant association between the pulmonary valvar phenotypes as seen in the parents and their offspring. The incidence of bifoliate pulmonary valves, nonetheless, was higher than statistically expected in the offspring of crosses where at least one of the parents possessed a pulmonary valve with two leaflets. Our observations are consistent with the notion that the basic design of the pulmonary valve, in terms of whether it possesses three or two leaflets, relies on genotypic determinants. They also denote that the bifoliate condition of the valve is the consequence of complex inheritance, with reduced penetrance and variable expressivity. Moreover, in showing that the incidence of the bifoliate pulmonary valve significantly differs in two different isogenetic backgrounds, our data suggest that genetic modifiers might be implicated in directing the manifestation of such specific pulmonary valvar malformations. Finally, our findings indicate that factors other than the genotype, operating during embryonic life and creating developmental noise, or random variation, play a crucial role in the overall phenotypic variation involving the pulmonary valve.

Coordinating tissue interactions: Notch signaling in cardiac development and disease

The Notch pathway is a crucial cell-fate regulator in the developing heart. Attention in the past centered on Notch function in cardiomyocytes. However, recent advances demonstrate that region-specific endocardial Notch activity orchestrates the patterning and morphogenesis of cardiac chambers and valves through regulatory interaction with multiple myocardial and neural crest signals. Notch also regulates cardiomyocyte proliferation and differentiation during ventricular chamber development and is required for coronary vessel specification. Here, we review these data and highlight disease connections, including evidence that Notch-Hey-Bmp2 interplay impacts adult heart valve disease and that Notch contributes to cardiac arrhythmia and pre-excitation syndromes.

Human myxomatous mitral valve prolapse: role of bone morphogenetic protein 4 in valvular interstitial cell activation

Myxomatous mitral valve prolapse (MVP) is the most common cardiac valvular abnormality in industrialized countries and a leading cause of mitral valve surgery for isolated mitral regurgitation. The key role of valvular interstitial cells (VICs) during mitral valve development and homeostasis has been recently suggested, however little is known about the molecular pathways leading to MVP. We aim to characterize bone morphogenetic protein 4 (BMP4) as a cellular regulator of mitral VIC activation towards a pathologic synthetic phenotype and to analyze the cellular phenotypic changes and extracellular matrix (ECM) reorganization associated with the development of myxomatous MVP. Microarray analysis showed significant up regulation of BMP4-mediated signaling molecules in myxomatous MVP when compared to controls. Histological analysis and cellular characterization suggest that during myxomatous MVP development, healthy quiescent mitral VICs undergo a phenotypic activation via up regulation of BMP4-mediated pathway. In vitro hBMP4 treatment of isolated human mitral VICs mimics the cellular activation and ECM remodeling as seen in MVP tissues. The present study characterizes the cell biology of mitral VICs in physiological and pathological conditions and provides insights into the molecular and cellular mechanisms mediated by BMP4 during MVP. The ability to test and control the plasticity of VICs using different molecules may help in developing new diagnostic and therapeutic strategies for myxomatous MVP.

A mechanism for gene-environment interaction in the etiology of congenital scoliosis

Congenital scoliosis, a lateral curvature of the spine caused by vertebral defects, occurs in approximately 1 in 1,000 live births. Here we demonstrate that haploinsufficiency of Notch signaling pathway genes in humans can cause this congenital abnormality. We also show that in a mouse model, the combination of this genetic risk factor with an environmental condition (short-term gestational hypoxia) significantly increases the penetrance and severity of vertebral defects. We demonstrate that hypoxia disrupts FGF signaling, leading to a temporary failure of embryonic somitogenesis. Our results potentially provide a mechanism for the genesis of a host of common sporadic congenital abnormalities through gene-environment interaction.

Notch signaling and cardiac repair

Notch signaling is critical for proper heart development and recently has been reported to participate in adult cardiac repair. Notch resides at the cell surface as a single pass transmembrane receptor, transits through the cytoplasm following activation, and acts as a transcription factor upon entering the nucleus. This dynamic and widespread cellular distribution allows for potential interactions with many signaling and binding partners. Notch displays temporal as well as spatial versatility, acting as a strong developmental signal, controlling cell fate determination and lineage commitment, and playing a pivotal role in embryonic and adult stem cell proliferation and differentiation. This review serves as an update of recent literature addressing Notch signaling in the heart, with attention to findings from noncardiac research that provide clues for further interpretation of how the Notch pathway influences cardiac biology. Specific areas of focus include Notch signaling in adult myocardium following pathologic injury, the role of Notch in cardiac progenitor cells with respect to differentiation and cardiac repair, crosstalk between Notch and other cardiac signaling pathways, and emerging aspects of noncanonical Notch signaling in heart.

Inhibitory role of Notch1 in calcific aortic valve disease

Aortic valve calcification is the most common form of valvular heart disease, but the mechanisms of calcific aortic valve disease (CAVD) are unknown. NOTCH1 mutations are associated with aortic valve malformations and adult-onset calcification in families with inherited disease. The Notch signaling pathway is critical for multiple cell differentiation processes, but its role in the development of CAVD is not well understood. The aim of this study was to investigate the molecular changes that occur with inhibition of Notch signaling in the aortic valve. Notch signaling pathway members are expressed in adult aortic valve cusps, and examination of diseased human aortic valves revealed decreased expression of NOTCH1 in areas of calcium deposition. To identify downstream mediators of Notch1, we examined gene expression changes that occur with chemical inhibition of Notch signaling in rat aortic valve interstitial cells (AVICs). We found significant downregulation of Sox9 along with several cartilage-specific genes that were direct targets of the transcription factor, Sox9. Loss of Sox9 expression has been published to be associated with aortic valve calcification. Utilizing an in vitro porcine aortic valve calcification model system, inhibition of Notch activity resulted in accelerated calcification while stimulation of Notch signaling attenuated the calcific process. Finally, the addition of Sox9 was able to prevent the calcification of porcine AVICs that occurs with Notch inhibition. In conclusion, loss of Notch signaling contributes to aortic valve calcification via a Sox9-dependent mechanism.

Genetically alike Syrian hamsters display both bifoliate and trifoliate aortic valves

The bifoliate, or bicuspid, aortic valve (BAV) is the most frequent congenital cardiac anomaly in man. It is a heritable defect, but its mode of inheritance remains unclear. Previous studies in Syrian hamsters showed that BAVs with fusion of the right and left coronary leaflets are expressions of a trait, the variation of which takes the form of a phenotypic continuum. It ranges from a trifoliate valve with no fusion of the coronary leaflets to a bifoliate root devoid of any raphe. The intermediate stages are represented by trifoliate valves with fusion of the coronary aortic leaflets, and bifoliate valves with raphes. The aim of this study was to elucidate whether the distinct morphological variants rely on a common genotype, or on different genotypes. We examined the aortic valves from 1 849 Syrian hamsters belonging to a family subjected to systematic inbreeding by full-sib mating. The incidence of the different trifoliate aortic valve (TAV) and bifoliate aortic valve (BAV) morphological variants widely varied in the successive inbred generations. TAVs with extensive fusion of the leaflets, and BAVs, accounted for five-sixths of the patterns found in Syrian hamsters considered to be genetically alike or virtually isogenic, with the probability of homozygosity being 0.999 or higher. The remaining one-sixth hamsters had aortic valves with a tricuspid design, but in most cases the right and left coronary leaflets were slightly fused. Results of crosses between genetically alike hamsters, with the probability of homozygosity being 0.989 or higher, revealed no significant association between the valvar phenotypes in the parents and their offspring. Our findings are consistent with the notion that the BAVs of the Syrian hamster are expressions of a quantitative trait subject to polygenic inheritance. They suggest that the genotype of the virtually isogenic animals produced by systematic inbreeding greatly predisposes to the development of anomalous valves, be they bifoliate, or trifoliate with extensive fusion of the leaflets. We infer that the same underlying genotype may account for the whole range of valvar morphological variants, suggesting that factors other than genetic ones are acting during embryonic life, creating the so-called intangible variation or developmental noise, and playing an important role in the definitive anatomic configuration of the valve. The clinical implication from our study is that congenital aortic valves with a trifoliate design, but with fusion of coronary aortic leaflets, may harbour the same inherent risks as those already recognised for BAVs with fusion of right and left coronary leaflets.

Notch signaling: simplicity in design, versatility in function

Notch signaling is evolutionarily conserved and operates in many cell types and at various stages during development. Notch signaling must therefore be able to generate appropriate signaling outputs in a variety of cellular contexts. This need for versatility in Notch signaling is in apparent contrast to the simple molecular design of the core pathway. Here, we review recent studies in nematodes, Drosophila and vertebrate systems that begin to shed light on how versatility in Notch signaling output is generated, how signal strength is modulated, and how cross-talk between the Notch pathway and other intracellular signaling systems, such as the Wnt, hypoxia and BMP pathways, contributes to signaling diversity.