Mutations in FOXC2 Are Strongly Associated With Primary Valve Failure in Veins of the Lower Limb

The Transcriptional Control of Lymphatic Vascular Development

More than 100 years ago, Florence Sabin suggested that lymphatic vessels develop by sprouting from preexisting blood vessels, but it is only over the past decade that the molecular mechanisms underpinning lymphatic vascular development have begun to be elucidated. Genetic manipulations in mice have identified a transcriptional hub comprised of Prox1, CoupTFII, and Sox18 that is essential for lymphatic endothelial cell fate specification. Recent work has identified a number of additional transcription factors that regulate later stages of lymphatic vessel differentiation and maturation. This review highlights recent advances in our understanding of the transcriptional control of lymphatic vascular development and reflects on efforts to better understand the activities of transcriptional networks during this discrete developmental process. Finally, we highlight the transcription factors associated with human lymphatic vascular disorders, demonstrating the importance of understanding how the activity of these key molecules is regulated, with a view toward the development of innovative therapeutic avenues.

Dysmorphogenesis of lymph nodes in Foxc2 haploinsufficient mice

Dysmorphogenesis of lymph nodes displayed in a fork head transcription factor Foxc2 haploinsufficient mice--a model for lymphedema-distichiasis syndrome--was studied by immunohistochemistry and electron microscopy. The Foxc2 heterozygous mice manifested lymph node hyperplasia composed of conspicuous proliferation of endothelial cells forming the lymphatic sinus and α-smooth muscle actin (SMA)-immunopositive fibroblast-like cells in the lymphatic pulp, particularly around the sinus. The hyperplastic sinus endothelial cells and the SMA-positive cells demonstrated distinct immunolocalization of platelet-derived growth factor (PDGF)-B, a crucial chemoattractant for vascular mural cell recruitment, and its receptor, PDGFR-β, respectively. The observations suggest that the sinus endothelial cells elicit abnormal recruitment of the fibroblast-like cells as a type of vascular mural cells via PDGF-B/PDGFR-β signaling in lymph nodes of the Foxc2 heterozygotes. Furthermore, in Foxc2 heterozygous lymph nodes, recruited SMA-positive cells displayed an intense immunoreaction for vascular endothelial growth factor (VEGF)-C, a highly specific lymphangiogenic factor, and its receptor, VEGFR-3, was preferentially distributed in the lymphatic sinus endothelial cells. These findings suggest that an interactive cycle between lymphatic sinus endothelial cells and the fibroblast-like cells, which involves PDGF-B/PDGFR-β and VEGF-C/VEGFR-3 signaling, is essential for aberrant hyperplasia of the lymphatic sinus and the fibroblast-like cells in Foxc2 haploinsufficiency.

Effects of disturbed flow on vascular endothelium: pathophysiological basis and clinical perspectives

Vascular endothelial cells (ECs) are exposed to hemodynamic forces, which modulate EC functions and vascular biology/pathobiology in health and disease. The flow patterns and hemodynamic forces are not uniform in the vascular system. In straight parts of the arterial tree, blood flow is generally laminar and wall shear stress is high and directed; in branches and curvatures, blood flow is disturbed with nonuniform and irregular distribution of low wall shear stress. Sustained laminar flow with high shear stress upregulates expressions of EC genes and proteins that are protective against atherosclerosis, whereas disturbed flow with associated reciprocating, low shear stress generally upregulates the EC genes and proteins that promote atherogenesis. These findings have led to the concept that the disturbed flow pattern in branch points and curvatures causes the preferential localization of atherosclerotic lesions. Disturbed flow also results in postsurgical neointimal hyperplasia and contributes to pathophysiology of clinical conditions such as in-stent restenosis, vein bypass graft failure, and transplant vasculopathy, as well as aortic valve calcification. In the venous system, disturbed flow resulting from reflux, outflow obstruction, and/or stasis leads to venous inflammation and thrombosis, and hence the development of chronic venous diseases. Understanding of the effects of disturbed flow on ECs can provide mechanistic insights into the role of complex flow patterns in pathogenesis of vascular diseases and can help to elucidate the phenotypic and functional differences between quiescent (nonatherogenic/nonthrombogenic) and activated (atherogenic/thrombogenic) ECs. This review summarizes the current knowledge on the role of disturbed flow in EC physiology and pathophysiology, as well as its clinical implications. Such information can contribute to our understanding of the etiology of lesion development in vascular niches with disturbed flow and help to generate new approaches for therapeutic interventions.

Polymorphisms in MMP-9 and TIMP-2 in Chinese patients with varicose veins

BACKGROUND

Varicose veins (VVs), a common vascular disease, are functionally characterized by dilation and tortuosity and are widely prevalent in the adult population. The pathophysiology and molecular mechanism of VVs are still unclear. A genetic risk for VVs has been demonstrated, although no genetic variant pertaining to VVs has been identified. Matrix metalloproteinases (MMPs) and their endogenous tissue inhibitors (TIMPs), which can prevent excessive extracellular matrix (ECM) degradation, greatly impact vascular remodeling and may play a vital role in patients with VVs. We evaluated a potential association between polymorphisms in the promoters of MMP-9 and TIMP-2 and the risk for VVs in the Chinese population.

MATERIALS AND METHODS

Genotyping of the promoter region polymorphisms -1562C/T in MMP-9 and -418G/C in TIMP-2 was performed with PCR and restriction fragment length polymorphism (PCR-RFLP) assays with a group of 60 patients with VVs and 60 healthy controls. Purified PCR products were sequenced.

RESULTS

A significant correlation was found between patients with VVs and controls at -1562C/T in MMP-9. The TIMP-2 gene polymorphism -418G/C was also associated with VVs.

CONCLUSIONS

Our results suggest that polymorphisms in the promoter region of MMP-9 and TIMP-2 are associated with VVs in the Chinese population.

Genetics of osteoporosis

Osteoporosis is a common disease with a strong genetic component characterized by reduced bone mass, defects in the microarchitecture of bone tissue, and an increased risk of fragility fractures. Twin and family studies have shown high heritability of bone mineral density (BMD) and other determinants of fracture risk such as ultrasound properties of bone, skeletal geometry, and bone turnover. Osteoporotic fractures also have a heritable component, but this reduces with age as environmental factors such as risk of falling come into play. Susceptibility to osteoporosis is governed by many different genetic variants and their interaction with environmental factors such as diet and exercise. Notable successes in identification of genes that regulate BMD have come from the study of rare Mendelian bone diseases characterized by major abnormalities of bone mass where variants of large effect size are operative. Genome-wide association studies have also identified common genetic variants of small effect size that contribute to regulation of BMD and fracture risk in the general population. In many cases, the loci and genes identified by these studies had not previously been suspected to play a role in bone metabolism. Although there has been extensive progress in identifying the genes and loci that contribute to the regulation of BMD and fracture over the past 15 yr, most of the genetic variants that regulate these phenotypes remain to be discovered.

Lymphatic dysfunction, not aplasia, underlies Milroy disease

OBJECTIVE

Milroy disease is an inherited autosomal dominant lymphoedema caused by mutations in the gene for vascular endothelial growth factor receptor-3 (VEGFR-3, also known as FLT4). The phenotype has to date been ascribed to lymphatic aplasia. We further investigated the structural and functional defects underlying the phenotype in humans.

METHODS

The skin of the swollen foot and the non-swollen forearm was examined by (i) fluorescence microlymphangiography, to quantify functional initial lymphatic density in vivo; and (ii) podoplanin and LYVE-1 immunohistochemistry of biopsies, to quantify structural lymphatic density. Leg vein function was assessed by colour Doppler duplex ultrasound.

RESULTS

Milroy patients exhibited profound (86-91%) functional failure of the initial lymphatics in the foot; the forearm was unimpaired. Dermal lymphatics were present in biopsies but density was reduced by 51-61% (foot) and 26-33% (forearm). Saphenous venous reflux was present in 9/10 individuals with VEGFR3 mutations, including two carriers.

CONCLUSION

We propose that VEGFR3 mutations in humans cause lymphoedema through a failure of tissue protein and fluid absorption. This is due to a profound functional failure of initial lymphatics and is not explained by microlymphatic hypoplasia alone. The superficial venous valve reflux indicates the dual role of VEGFR-3 in lymphatic and venous development.

Haploinsufficiencies of FOXF1 and FOXC2 genes associated with lethal alveolar capillary dysplasia and congenital heart disease

Neonatal deaths account for about 67% of all deaths during the first year of life in the USA. Genetic defects are important factors contributing to neonatal deaths and congenital anomalies. Here we report on the identification of a 1.37 Mb de novo deletion of chromosome 16q24.1-q24.2 by microarray-based comparative genomic hybridization (aCGH) technique in a newborn boy with lethal severe alveolar capillary dysplasia and multiple congenital anomalies who died of irreversible pulmonary hypertension, respiratory failure and cor pulmonale at three days of age. The phenotypic findings and causal genes (FOXF1 and FOXC2) involved in producing this unusual syndrome are detailed. Our findings independently confirm the results in a previous publication describing multiple patients with similar clinical and genetic observations, and highlight the importance of scanning human genomes at high resolution for identifications of micro-imbalances as pathogenic causes in neonates with unexplained congenital anomalies. (c) 2010 Wiley-Liss, Inc.

Lymphangiogenesis, myeloid cells and inflammation

The lymphatic system is essential for the maintenance of tissue fluid balance, immune surveillance and the absorption of fatty acids in the gastrointestinal tract. The lymphatic circulation is also a key player in disease processes such as cancer metastasis, lymphedema and various inflammatory disorders. With the identification of specific growth factors for lymphatic endothelial cells and markers that distinguish blood and lymphatic vessels, as well as the development of in vivo imaging technologies that provide new tools to examine the lymphatic drainage function in real time, many advancements have been made in lymphatic vascular research during the past few years. Despite these significant achievements, our understanding of the role of lymphatics in disease processes other than cancer metastasis is still rather limited. The current review will focus on the recent progress made in studies of lymphatics in inflammatory disorders.

A case of lymphedema-distichiasis syndrome carrying a new de novo frameshift FOXC2 mutation

PURPOSE

Lymphedema-Distichiasis (LD, OMIM 153400) is an autosomal dominant disorder with variable expression. The mutated gene implicated is FOXC2, which encodes for a forkhead transcription factor involved in the development of the lymphatic and vascular system. LD is characterized by late childhood or pubertal onset lymphedema of the limbs and distichiasis. Other associations have been reported, including congenital heart disease, ptosis, scoliosis.

CONCLUSIONS

Here we describe a case of LD carrying a de novo frameshift mutation of FOXC2 who presented a prepubertal onset of lower limbs lymphedema and mild distichiasis associated with other anomalies such as webbing neck and ptosis.

Lymphangiogenesis: Molecular mechanisms and future promise

The growth of lymphatic vessels (lymphangiogenesis) is actively involved in a number of pathological processes including tissue inflammation and tumor dissemination but is insufficient in patients suffering from lymphedema, a debilitating condition characterized by chronic tissue edema and impaired immunity. The recent explosion of knowledge on the molecular mechanisms governing lymphangiogenesis provides new possibilities to treat these diseases.

Pathogenesis of primary varicose veins

BACKGROUND

Valvular incompetence and reflux are common features of primary varicose veins, and have long been thought to be their cause. Recent evidence, however, suggests that changes in the vein wall may precede valvular dysfunction.

METHODS

A literature search was performed using PubMed and Ovid using the keywords 'varicose vein wall changes', 'pathogenesis', 'aetiology' and 'valvular dysfunction'. Articles discussing the pathophysiology of complications of varicose veins, such as ulceration, recurrence, thrombophlebitis and lipodermatosclerosis, were excluded.

RESULTS AND CONCLUSION

Positive family history, age, sex and pregnancy are important risk factors for varicose vein formation. Areas of intimal hyperplasia and smooth muscle cell proliferation are often noted in varicose veins, although regions of atrophy are also present. The total elastin content in varicose as opposed to non-varicose veins is reduced; changes in overall collagen content are uncertain. Matrix metalloproteinases (MMPs), including MMP-1, MMP-2, MMP-3, MMP-7 and MMP-9, and tissue inhibitor of metalloproteinase (TIMP) 1 and TIMP-3 are upregulated in varicose veins. Activation of the endothelium stimulates the recruitment of leucocytes and the release of growth factors, leading to smooth muscle cell proliferation and migration. Dysregulated apoptosis has also been demonstrated in varicose veins. An understanding of the pathophysiology of varicose veins is important in the identification of potential therapeutic targets and treatment strategies.

Polymorphism of matrix metalloproteinase genes (MMP1 and MMP3) in patients with varicose veins

BACKGROUND

Several risk factors for varicose veins have been identified: female gender, combined with obesity and pregnancy, occupations requiring standing for long periods, sedentary lifestyle, history of deep-vein thrombosis and family history. However, no specific gene variants related to a wide prevalence of varicosities in general population have been identified. Extracellular matrix composition, predominantly maintained by matrix metalloproteinases (MMPs), may affect the vein-wall structure, which may lead to dilation of vessels and cause varicosities.

AIMS

MMP-1 (tissue collagenase I) and MMP-3 (stromelysin I) expression was found to be raised in varicose veins compared with normal vessels. Therefore, a study was conducted to evaluate a potential association between MMP1 and MMP3 promoter polymorphisms and a risk of varicose veins.

METHODS

Genotyping for the presence of the polymorphisms -1607dupG (rs1799750) in MMP1 and -1171dupA (rs3025058) in the MMP3 promoter region was performed using PCR and restriction-fragment length polymorphism assays in a group of 109 patients diagnosed with varicose veins and 112 healthy controls.

RESULTS

The frequencies of the MMP1 and MMP3 alleles (minor allele frequency 0.440 in patients vs. 0.451 in the controls for MMP1-1607*G and 0.514 vs. 0.469 for MMP3-1171*dupA, respectively) and of genotypes did not differ significantly between patients and controls.

CONCLUSIONS

The MMP1-1607dupG and MMP3-1171dupA promoter polymorphisms are not valuable markers of susceptibility for varicose veins.

FOXC2 controls formation and maturation of lymphatic collecting vessels through cooperation with NFATc1

The mechanisms of blood vessel maturation into distinct parts of the blood vasculature such as arteries, veins, and capillaries have been the subject of intense investigation over recent years. In contrast, our knowledge of lymphatic vessel maturation is still fragmentary. In this study, we provide a molecular and morphological characterization of the major steps in the maturation of the primary lymphatic capillary plexus into collecting lymphatic vessels during development and show that forkhead transcription factor Foxc2 controls this process. We further identify transcription factor NFATc1 as a novel regulator of lymphatic development and describe a previously unsuspected link between NFATc1 and Foxc2 in the regulation of lymphatic maturation. We also provide a genome-wide map of FOXC2-binding sites in lymphatic endothelial cells, identify a novel consensus FOXC2 sequence, and show that NFATc1 physically interacts with FOXC2-binding enhancers. As damage to collecting vessels is a major cause of lymphatic dysfunction in humans, our results suggest that FOXC2 and NFATc1 are potential targets for therapeutic intervention.

Foxc2 transcription factor: a newly described regulator of angiogenesis

Angiogenesis is a critical process to form new blood vessels from preexisting vessels under physiologic and pathologic conditions and involves cellular and morphologic changes such as endothelial cell proliferation, migration, and vascular tube formation. Despite evidence that angiogenic factors, including vascular endothelial growth factor and Notch, control various aspects of angiogenesis, the molecular mechanisms underlying gene regulation in blood vessels and surrounding tissues are not fully understood. Importantly, recent studies demonstrate that Forkhead transcription factor Foxc2 directly regulates expression of various genes involved in angiogenesis, CXCR4, integrin beta3, Delta-like 4 (Dll4), and angiopoietin 2, thereby controlling angiogenic processes. Thus, Foxc2 is now recognized as a novel regulator of vascular formation and remodeling. This review summarizes current knowledge about the function of Foxc2 in angiogenesis and discusses prospects for future research in Foxc2-mediated pathologic angiogenesis in cardiovascular disease.

Contractile physiology of lymphatics

The lymphatic system has important roles in body fluid regulation, macromolecular homeostasis, lipid absorption, and immune function. To accomplish these roles, lymphatics must move fluid and its other contents (macromolecules, lipids/chylomicra, immune cells) from the interstitium through the lymphatics, across the nodes, and into the great veins. Thus, the principal task of the lymphatic vascular system is transport. The body must impart energy to the lymph via pumping mechanisms to propel it along the lymphatic network and use pumps and valves to generate lymph flow and prevent its backflow. The lymphatic system utilizes both extrinsic pumps, which rely on the cyclical compression and expansion of lymphatics by surrounding tissue forces, and intrinsic pumps, which rely on the intrinsic rapid/phasic contractions of lymphatic muscle. The intrinsic lymph pump function can be modulated by neural, humoral, and physical factors. Generally, increased lymph pressure/stretch of the muscular lymphatics activates the intrinsic lymph pump, while increased lymph flow/shear in the muscular lymphatics can either activate or inhibit the intrinsic lymph pump depending on the pattern and magnitude of the flow. To regulate lymph transport, lymphatic pumping and resistance must be controlled. A better understanding of these mechanisms could provide the basis for the development of better diagnostic and treatment modalities for lymphatic dysfunction.

Diagnosis and therapy in children with lymphoedema

Introduction

Lymphoedema (LE) is a disorder characterized by persistent swelling caused by impaired lymphatic drainage because of various aetiologies, including lymphatic injury and congenital functional or anatomical defects.

Objective

Literature review and expert opinion about diagnosis and treatment of LE in children.

Results

LE is rare in children, with a prevalence of about 1.15/100,000 persons, 20 years old. The management of LE in children differs considerably from adults in terms of origin, co-morbidity and therapeutic approach. The objective of this presentation is to discuss practical issues related to clinically relevant information on the diagnosis, aetiology, work-up and treatment of LE in children. In contrast to adults, who usually experience secondary LE because of acquired lymphatic failure, most cases in children have a primary origin. The diagnosis can be made mainly on the basis of careful personal and family history, and physical examination. LE in children can be part of a syndrome if there are other concomitant phenotypic abnormalities and if a genetic defect is recognizable. Treatment of LE is mostly conservative utilizing decongestive LE therapy including compression therapy, directed exercises, massage and skincare. In the neonate, initial observation alone may be sufficient, as delayed lymphatic development and maturation can result in spontaneous improvement. The role of parents is crucial in providing the necessary input.

Conclusion

We present a review emphasizing a practical approach to treating a child with LE according to current publications and our own experience.

Developmental and pathological lymphangiogenesis: from models to human disease

The lymphatic vascular system, the body's second vascular system present in vertebrates, has emerged in recent years as a crucial player in normal and pathological processes. It participates in the maintenance of normal tissue fluid balance, the immune functions of cellular and antigen trafficking and absorption of fatty acids and lipid-soluble vitamins in the gut. Recent scientific discoveries have highlighted the role of lymphatic system in a number of pathologic conditions, including lymphedema, inflammatory diseases, and tumor metastasis. Development of genetically modified animal models, identification of lymphatic endothelial specific markers and regulators coupled with technological advances such as high-resolution imaging and genome-wide approaches have been instrumental in understanding the major steps controlling growth and remodeling of lymphatic vessels. This review highlights the recent insights and developments in the field of lymphatic vascular biology.

Targeted treatment for lymphedema and lymphatic metastasis

The presence of lymphatic vessels has been known for centuries, but the key players regulating the lymphatic vessel growth and function have only been discovered during the recent decade. The lymphatic vasculature is essential for maintenance of normal fluid balance and for the immune response. Hypoplasia or dysfunction of the lymphatic vessels can lead to lymphedema. Currently, lymphedema is treated primarily by physiotherapy, compression garments, and occasionally by surgery, but the means to reconstitute the collecting lymphatic vessels and cure the condition are limited. Specific growth factor therapy has been used in experimental models to regenerate lymphatic capillaries and collecting vessels after surgical damage. Recent results provide a new concept of combining growth factor therapy with lymph node transplantation as a rationale for treating secondary lymphedema. Lymphatic vessels are also involved in lymph node and systemic metastasis of cancer cells; our understanding of mechanisms of lymphatic metastasis has increased remarkably.

New horizons for imaging lymphatic function

In this review, we provide a comprehensive summary of noninvasive imaging modalities used clinically for the diagnosis of lymphatic diseases, new imaging agents for assessing lymphatic architecture and cancer status of lymph nodes, and emerging near-infrared (NIR) fluorescent optical imaging technologies and agents for functional lymphatic imaging. Given the promise of NIR optical imaging, we provide example results of functional lymphatic imaging in mice, swine, and humans, showing the ability of this technology to quantify lymph velocity and frequencies of propulsion resulting from the contractility of lymphatic structures.

The Foxc2 transcription factor regulates angiogenesis via induction of integrin beta3 expression

Forkhead transcription factor Foxc2 is an essential regulator of the cardiovascular system in development and disease. However, the cellular and molecular functions of Foxc2 in vascular endothelial cells are still not fully understood. Here, through gene expression profiling in endothelial cells, we identified molecules associated with cell-extracellular matrix interactions, integrin beta3 (Itgb3), integrin beta5 (Itgb5), and fibronectin, as downstream targets of Foxc2. We found that Itgb3 expression is directly regulated by Foxc2 through multiple Forkhead-binding elements within two high homology regions in the Itgb3 promoter. Because Itgb3 is known to regulate angiogenesis, we further tested whether Foxc2 is directly involved in angiogenesis by regulating Itgb3 expression by in vitro experiments. Overexpression of Foxc2 significantly enhanced endothelial cell migration and adhesion, whereas this effect was strongly inhibited by Itgb3 neutralization antibody. In accordance with these results, pulmonary microvascular endothelial cells isolated from Foxc2 heterozygous mutant mice showed a marked reduction in Itgb3 expression and cell migration. Finally, ex vivo aortic ring assay to test the sprouting and microvessel formation revealed enhanced microvessel outgrowth by Foxc2 overexpression. Conversely, microvessel outgrowth from aortas of Foxc2 heterozygous mutant mice was reduced. Taken together, these results suggest that Foxc2 directly induces Itgb3 expression and regulates angiogenesis by Itgb3-mediated endothelial cell adhesion and migration.

Forkhead transcription factors and cardiovascular biology

The Forkhead family of transcription factors modulates a wide variety of cellular functions in cardiovascular tissues. In this review article, we discuss recent advances in our understanding of regulation provided by the forkhead factors in cardiac myocytes and vascular cells.

Lymphedema of the lower extremity: is it genetic or nongenetic?

Lymphedema of the lower extremities is a diagnostic challenge. Exclusion of secondary causes of limb swelling and secondary lymphedema is the initial step. Primary lymphedema is classified into idiopathic and familial (hereditary) subgroups. Hereditary lymphedema can be nonsyndromic or associated with congenital anomalies or with abnormal physical findings. A 13-year-old girl presented with unilateral lower extremity lymphedema. Her medical and family history was unremarkable. The physical examination was negative for dysmorphic features and congenital anomalies. Lymphoscintigraphy showed no evidence of lymph flow in the left lower extremity, which persisted at the delayed 2-hour image. A comprehensive clinical and family history that includes a thorough physical examination are the mainstays of the medical assessment of lymphedema in children. Isotopic lymphoscintigraphy is generally considered the gold standard for confirmation of the diagnosis. This article discusses the differential diagnosis, reviews the literature, and suggests a simplified and an updated flowchart for the classification of unilateral limb lymphedema in children.