Mutations in FOXC2 (MFH-1), a forkhead family transcription factor, are responsible for the hereditary lymphedema-distichiasis syndrome

Renal anomalies and lymphedema distichiasis syndrome. A rare association?

Lymphedema distichiasis syndrome (LDS) is a rare, autosomal dominant genetic condition, characterized by lower limb lymphedema and distichiasis. Other associated features that have been reported include varicose veins, cleft palate, congenital heart defects, and ptosis. We update a previously reported family with a pathogenic variant in FOXC2 (c.412-413insT) where five affected individuals from the youngest generation had congenital renal anomalies detected on prenatal ultrasound scan. These included four fetuses with hydronephrosis and one with bilateral renal agenesis. A further child with LDS had prominence of the left renal pelvis on postnatal renal ultrasound. We also describe a second family in whom the proband and his affected son had congenital renal anomalies; left ectopic kidney, right duplex kidney, and bilateral duplex collecting systems with partial duplex kidney with mild degree of malrotation, respectively. Foxc2 is expressed in the developing kidney and therefore congenital renal anomalies may well be associated, potentially as a low penetrance feature. We propose that all individuals diagnosed with LDS should have a baseline renal ultrasound scan at diagnosis. It would also be important to consider the possibility of renal anomalies during prenatal ultrasound of at risk pregnancies, and that the presence of hydronephrosis may be an indication that the baby is affected with LDS.

Vascular heterogeneity and specialization in development and disease

Blood and lymphatic vessels pervade almost all body tissues and have numerous essential roles in physiology and disease. The inner lining of these networks is formed by a single layer of endothelial cells, which is specialized according to the needs of the tissue that it supplies. Whereas the general mechanisms of blood and lymphatic vessel development are being defined with increasing molecular precision, studies of the processes of endothelial specialization remain mostly descriptive. Recent insights from genetic animal models illuminate how endothelial cells interact with each other and with their tissue environment, providing paradigms for vessel type- and organ-specific endothelial differentiation. Delineating these governing principles will be crucial for understanding how tissues develop and maintain, and how their function becomes abnormal in disease.

In silico analysis of single nucleotide polymorphisms (SNPs) in human FOXC2 gene

Introduction: Lymphedema is abnormal accumulation of interstitial fluid, due to inefficient uptake and reduced flow, leading to swelling and disability, mostly in the extremities. Hereditary lymphedema usually occurs as an autosomal dominant trait with allelic heterogeneity.  Methods: We identified single nucleotide polymorphisms (SNPs) in the FOXC2 gene using dbSNP, analyzed their effect on the resulting protein using VEP and Biomart, modelled the resulting protein using Project HOPE, identified gene - gene interactions using GeneMANIA and predicted miRNAs affected and the resulting effects of SNPs in the 5' and 3' regions using PolymiRTS.  Results: We identified 448 SNPs - 429 were nsSNPs and 44 SNPs were in the 5' and 3' UTRs. In total, 2 SNPs have deleterious effects on the resulting protein, and a 3D model confirmed those effects. The gene - gene interaction network showed the involvement of FOXC2 protein in the development of the lymphatic system. hsa-miR-6886-5p, hsa-miRS-6886-5p , hsa-miR-6720-3p, which were affected by the SNPs rs201118690, rs6413505, rs201914560, respectively, were the most important miRNAs affected, due to their high conservation score.  Conclusions: rs121909106 and rs121909107 were predicted to have the most harmful effects, while hsa-miR-6886-5p, hsa-miR-6886-5p and hsa-miR-6720-3p were predicted to be the most important miRNAs affected. Computational biology tools have advantages and disadvantages, and the results they provide are predictions that require confirmation.

In silico analysis of single nucleotide polymorphisms (SNPs) in human FOXC2 gene

Introduction: Lymphedema is an abnormal accumulation of interstitial fluid, due to inefficient uptake and reduced flow, leading to swelling and disability, mostly in the extremities. Hereditary lymphedema usually occurs as an autosomal dominant trait with allelic heterogeneity. Methods: We identified single nucleotide polymorphisms (SNPs) in the FOXC2 gene using dbSNP, analyzed their effect on the resulting protein using VEP and Biomart, modelled the resulting protein using Project HOPE, identified gene - gene interactions using GeneMANIA and predicted miRNAs affected and the resulting effects of SNPs in the 5' and 3' regions using PolymiRTS. Results: We identified 473 SNPs - 429 were nsSNPs and 44 SNPs were in the 5' and 3' UTRs. In total, 2 SNPs - rs121909106 and rs121909107 - have deleterious effects on the resulting protein, and a 3D model confirmed those effects. The gene - gene interaction network showed the involvement of FOXC2 protein in the development of the lymphatic system. hsa-miR-6886-5p, hsa-miRS-6886-5p, hsa-miR-6720-3p, which were affected by the SNPs rs201118690, rs6413505, rs201914560, respectively, were the most important miRNAs affected, due to their high conservation score. Conclusions: rs121909106 and rs121909107 were predicted to have the most harmful effects, while hsa-miR-6886-5p, hsa-miR-6886-5p and hsa-miR-6720-3p were predicted to be the most important miRNAs affected. Computational biology tools have advantages and disadvantages, and the results they provide are predictions that require confirmation using methods such as functional studies.

Formation and function of intracardiac valve cells in the Drosophila heart

Drosophila harbors a simple tubular heart that ensures hemolymph circulation within the body. The heart is built by a few different cell types, including cardiomyocytes that define the luminal heart channel and ostia cells that constitute openings in the heart wall allowing hemolymph to enter the heart chamber. Regulation of flow directionality within a tube, such as blood flow in arteries or insect hemolymph within the heart lumen, requires a dedicated gate, valve, or flap-like structure that prevents backflow of fluids. In the Drosophila heart, intracardiac valves provide this directionality of hemolymph streaming, with one valve being present in larvae and three valves in the adult fly. Each valve is built by two specialized cardiomyocytes that exhibit a unique histology. We found that the capacity to open and close the heart lumen relies on a unique myofibrillar setting as well as on the presence of large membranous vesicles. These vesicles are of endocytic origin and probably represent unique organelles of valve cells. Moreover, we characterised the working mode of the cells in real time. Valve cells exhibit a highly flexible shape and during each heartbeat, oscillating shape changes result in closing and opening of the heart channel. Finally, we identified a set of novel valve cell markers useful for future in-depth analyses of cell differentiation in wildtype and mutant animals.

Foxc2CreERT2 knock-in mice mark stage-specific Foxc2-expressing cells during mouse organogenesis

Foxc2, a member of the winged helix transcription factor family, is essential for eye, calvarial bone, cardiovascular and kidney development in mice. Nevertheless, how Foxc2-expressing cells and their descendent cells contribute to the development of these tissues and organs has not been elucidated. Here, we generated a Foxc2 knock-in (Foxc2^CreERT2 ) mouse, in which administration of estrogen receptor antagonist tamoxifen induces nuclear translocation of Cre recombinase in Foxc2-expressing cells. By crossing with ROSA-LacZ reporter mice (Foxc2^CreERT2 ; R26R), the fate of Foxc2 positive (Foxc2⁺ ) cells was analyzed through LacZ staining at various embryonic stages. We found Foxc2⁺ cell descendants in the supraoccipital and exoccipital bone in E18.5 embryos, when tamoxifen was administered at embryonic day (E) 8.5. Furthermore, Foxc2⁺ descendant cranial neural crest cells at E8-10 were restricted to the corneal mesenchyme, while Foxc2⁺ cell derived cardiac neural crest cells at E6-12 were found in the aorta, pulmonary trunk and valves, and endocardial cushions. Foxc2⁺ cell descendant contributions to the glomerular podocytes in the kidney were also observed following E6.5 tamoxifen treatment. Our results are consistent with previous reports of Foxc2 expression during early embryogenesis and the Foxc2^CreERT2 mouse provides a tool to investigate spatiotemporal roles of Foxc2 and contributions of Foxc2⁺ expressing cells during mouse embryogenesis.

Early Mandibular Distraction to Relieve Robin Severe Airway Obstruction in Two Siblings with Lymphedema-Distichiasis Syndrome

Although micrognathia and cleft palate have been reported in patients with Lymphedema-distichiasis syndrome (LDS), the classic Robin sequence with glossoptosis and airway obstruction has not been previously described in patients with genetically confirmed LDS. Here we report on two female siblings with LDS confirmed by a FOXC2 mutation who presented at birth with severe airway obstruction related to Robin sequence. Respiratory obstruction was successfully managed by early distraction osteogenesis. Our report highlights the unusual occurrence of Robin sequence in LDS patients and advises distraction osteogenesis to resolve breathing problems in LDS patients who present with Robin related severe airway obstruction.

Endothelial Cell Responses to Biomechanical Forces in Lymphatic Vessels

SIGNIFICANCE

Lymphatic vessels are important components of the cardiovascular and immune systems. They contribute both to the maintenance of normal homeostasis and to many pathological conditions, such as cancer and inflammation. The lymphatic vasculature is subjected to a variety of biomechanical forces, including fluid shear stress and vessel circumferential stretch.

RECENT ADVANCES

This review will discuss recent advances in our understanding of biomechanical forces in lymphatic vessels and their role in mammalian lymphatic vascular development and function.

CRITICAL ISSUES

We will highlight the importance of fluid shear stress generated by lymph flow in organizing the lymphatic vascular network. We will also describe how mutations in mechanosensitive genes lead to lymphatic vascular dysfunction.

FUTURE DIRECTIONS

Better understanding of how biomechanical and biochemical stimuli are perceived and interpreted by lymphatic endothelial cells is important for targeting regulation of lymphatic function in health and disease. Important remaining critical issues and future directions in the field will be discussed in this review. Antioxid. Redox Signal. 25, 451-465.

Building the drains: the lymphatic vasculature in health and disease

The lymphatic vasculature is comprised of a network of endothelial vessels found in close proximity to but separated from the blood vasculature. An essential tissue component of all vertebrates, lymphatics are responsible for the maintenance of fluid homeostasis, dissemination of immune cells, and lipid reabsorption under healthy conditions. When lymphatic vessels are impaired due to invasive surgery, genetic disorders, or parasitic infections, severe fluid build-up accumulates in the affected tissues causing a condition known as lymphedema. Malignant tumors can also directly activate lymphangiogenesis and use these vessels to promote the spread of metastatic cells. Although their first description goes back to the times of Hippocrates, with subsequent anatomical characterization at the beginning of the 20th-century, the lack of identifying molecular markers and tools to visualize these translucent vessels meant that investigation of lymphatic vessels fell well behind research of blood vessels. However, after years under the shadow of the blood vasculature, recent advances in imaging technologies and new genetic and molecular tools have accelerated the pace of research on lymphatic vessel development. These new tools have facilitated both work in classical mammalian models and the emergence of new powerful vertebrate models like zebrafish, quickly driving the field of lymphatic development back into the spotlight. In this review, we summarize the highlights of recent research on the development and function of the lymphatic vascular network in health and disease. WIREs Dev Biol 2016, 5:689-710. doi: 10.1002/wdev.246 For further resources related to this article, please visit the WIREs website.

Generation of Human Induced Pluripotent Stem Cells from Extraembryonic Tissues of Fetuses Affected by Monogenic Diseases

The generation of human induced pluripotent stem cells (hiPSCs) derived from an autologous extraembryonic fetal source is an innovative personalized regenerative technology that can transform own-self cells into embryonic stem-like ones. These cells are regarded as a promising candidate for cell-based therapy, as well as an ideal target for disease modeling and drug discovery. Thus, hiPSCs enable researchers to undertake studies for treating diseases or for future applications of in utero therapy. We used a polycistronic lentiviral vector (hSTEMCCA-loxP) encoding OCT4, SOX2, KLF4, and cMYC genes and containing loxP sites, excisible by Cre recombinase, to reprogram patient-specific fetal cells derived from prenatal diagnosis for several genetic disorders, such as myotonic dystrophy type 1 (DM1), β-thalassemia (β-Thal), lymphedema-distichiasis syndrome (LDS), spinal muscular atrophy (SMA), cystic fibrosis (CF), as well as from wild-type (WT) fetal cells. Because cell types tested to create hiPSCs influence both the reprogramming process efficiency and the kinetics, we used chorionic villus (CV) and amniotic fluid (AF) cells, demonstrating how they represent an ideal cell resource for a more efficient generation of hiPSCs. The successful reprogramming of both CV and AF cells into hiPSCs was confirmed by specific morphological, molecular, and immunocytochemical markers and also by their teratogenic potential when inoculated in vivo. We further demonstrated the stability of reprogrammed cells over 10 and more passages and their capability to differentiate into the three embryonic germ layers, as well as into neural cells. These data suggest that hiPSCs-CV/AF can be considered a valid cellular model to accomplish pathogenesis studies and therapeutic applications.

Foxc1 and Foxc2 deletion causes abnormal lymphangiogenesis and correlates with ERK hyperactivation

The lymphatic vasculature is essential for maintaining interstitial fluid homeostasis, and dysfunctional lymphangiogenesis contributes to various pathological processes, including inflammatory disease and tumor metastasis. Mutations in FOXC2 are dominantly associated with late-onset lymphedema; however, the precise role of FOXC2 and a closely related factor, FOXC1, in the lymphatic system remains largely unknown. Here we identified a molecular cascade by which FOXC1 and FOXC2 regulate ERK signaling in lymphatic vessel growth. In mice, lymphatic endothelial cell-specific (LEC-specific) deletion of Foxc1, Foxc2, or both resulted in increased LEC proliferation, enlarged lymphatic vessels, and abnormal lymphatic vessel morphogenesis. Compared with LECs from control animals, LECs from mice lacking both Foxc1 and Foxc2 exhibited aberrant expression of Ras regulators, and embryos with LEC-specific deletion of Foxc1 and Foxc2, alone or in combination, exhibited ERK hyperactivation. Pharmacological ERK inhibition in utero abolished the abnormally enlarged lymphatic vessels in FOXC-deficient embryos. Together, these results identify FOXC1 and FOXC2 as essential regulators of lymphangiogenesis and indicate a new potential mechanistic basis for lymphatic-associated diseases.

Cdk5 and Foxc2--a new relationship in the lymphatic vasculature

Lymphatic vessel dysfunction is associated with various pathologic conditions, including immunologic disorders, lymphedema, as well as tumor dissemination. Yet, the knowledge about the regulation of lymphatic vessel development is still limited. Our study elucidates cyclin dependent kinase 5 (Cdk5) as an essential player in the development of lymphatic vessels. Deletion of Cdk5 in the mouse endothelium results in severe lymphedema formation and embryonic lethality. On the mechanistic level, we show that Cdk5 phosphorylates the forkhead transcription factor Foxc2 which regulates Foxc2-dependent transcription. In summary, our study elucidates the Cdk5-Foxc2 interaction as a critical regulator of lymphatic vessel development.

Segregated Foxc2, NFATc1 and Connexin expression at normal developing venous valves, and Connexin-specific differences in the valve phenotypes of Cx37, Cx43, and Cx47 knockout mice

Venous valves (VVs) are critical for unidirectional blood flow from superficial and deep veins towards the heart. Congenital valve aplasia or agenesis may, in some cases, be a direct cause of vascular disease, motivating an understanding of the molecular mechanisms underlying the development and maintenance of VVs. Three gap junction proteins (Connexins), Cx37, Cx43, and Cx47, are specifically expressed at VVs in a highly polarized fashion. VVs are absent from adult mice lacking Cx37; however it is not known if Cx37 is required for the initial formation of valves. In addition, the requirement of Cx43 and Cx47 for VV development has not been studied. Here, we provide a detailed description of Cx37, Cx43, and Cx47 expression during mouse vein development and show by gene knockout that each Cx is necessary for normal valve development. The valve phenotypes in the knockout lines exhibit Cx-specific differences, however, including whether peripheral or central VVs are affected by gene inactivation. In addition, we show that a Cx47 null mutation impairs peripheral VV development but does not affect lymphatic valve formation, a finding of significance for understanding how some CX47 mutations cause inherited lymphedema in humans. Finally, we demonstrate a striking segregation of Foxc2 and NFATc1 transcription factor expression between the downstream and upstream faces, respectively, of developing VV leaflets and show that this segregation is closely associated with the highly polarized expression of Cx37, Cx43, and Cx47. The partition of Foxc2 and NFATc1 expression at VV leaflets makes it unlikely that these factors directly cooperate during the leaflet elongation stage of VV development.

FoxC2 Enhances BMP7-Mediated Anabolism in Nucleus Pulposus Cells of the Intervertebral Disc

Bone-morphogenetic protein-7 (BMP-7) is a growth factor that plays a major role in mediating anabolism and anti-catabolism of the intervertebral disc matrix and cell homeostasis. In osteoblasts, Forkhead box protein C2 (FoxC2) is a downstream target of BMPs and promotes cell proliferation and differentiation. However, the role FoxC2 may play in degenerative human intervertebral disc tissue and the relationship between FoxC2 and BMP-7 in nucleus pulposus (NP) cells remain to be elucidated. This study aims to investigate the presence and signaling mechanisms of FoxC2 in degenerative human intervertebral disc tissue and NP cells. Western blot and real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) analyses were used to measure FoxC2 expression in the NP tissue and cells. Transfections were carried out to measure the effect of FoxC2 on BMP-7-mediated extracellular matrix upregulation. Adenoviral knock-down of Smad1 was performed to investigate the mechanism of BMP-7-induced FoxC2 expression. In degenerative NP tissue, FoxC2 was markedly upregulated and positively correlated with increased disc degeneration. Induction of NP cell proliferation was confirmed by using cell counting kit-8 assay, immunocytochemistry and real-time qRT-PCR for Ki67. FoxC2 led to decreased noggin expression and increased Smad1/5/8 phosphorylation. During combined treatment with BMP-7, FoxC2 greatly potentiated anabolism through synergistic mechanisms on ECM formation. Combination therapy using BMP-7 and FoxC2 may be beneficial to the treatment of intervertebral disc degeneration.

Biology of the eyelash hair follicle: an enigma in plain sight

Because of their crucial impact on our perception of beauty, eyelashes constitute a prime target for the cosmetic industry. However, when compared with other hair shafts and the mini-organs that produce them [eyelash hair follicles (ELHFs)], knowledge on the biology underlying growth and pigmentation of eyelashes is still rudimentary. This is due in part to the extremely restricted availability of human ELHFs for experimental study, underappreciation of their important sensory and protective functions and insufficient interest in understanding why they are distinct from scalp hair follicles (HFs) (e.g. ELHFs produce shorter hair shafts, do not possess an arrector pili muscle, have a shorter hair cycle and undergo greying significantly later than scalp HFs). Here we synthesize the limited current knowledge on the biology of ELHFs, in humans and other species, their role in health and disease, the known similarities with and differences from other HF populations, and their intrinsic interethnic variations. We define major open questions in the biology of these intriguing mini-organs and conclude by proposing future research directions. These include dissecting the molecular and cellular mechanisms that underlie trichomegaly and the development of in vitro models in order to interrogate the distinct molecular controls of ELHF growth, cycling and pigmentation and to probe novel strategies for the therapeutic and cosmetic manipulation of ELHFs beyond prostaglandin receptor stimulation.

Understanding lymphangiogenesis in knockout models, the cornea, and ocular diseases for the development of therapeutic interventions

A major focus of cancer research for several decades has been understand the ability of tumors to induce new blood vessel formation, a process known as angiogenesis. Unfortunately, only limited success has been achieved in the clinical application of angiogenesis inhibitors. We now know that lymphangiogenesis, the growth of lymphatic vessels, likely also plays a major role in tumor progression. Thus, therapeutic strategies targeting lymphangiogenesis or both lymphangiogenesis and angiogenesis may represent promising approaches for treating cancer and other diseases. Importantly, research progress toward understanding lymphangiogenesis is significantly behind that related to angiogenesis. A PubMed search of "angiogenesis" returns nearly 80,000 articles, whereas a search of "lymphangiogenesis" returns 2,635 articles. This stark contrast can be explained by the lack of molecular markers for identifying the invisible lymphatic vasculature that persisted until less than 2 decades ago, combined with the intensity of research interest in angiogenesis during the past half century. Still, significant strides have been made in developing strategies to modulate lymphangiogenesis, largely using ocular disease models. Here we review the current knowledge of lymphangiogenesis in the context of knockout models, ocular diseases, the biology of activators and inhibitors, and the potential for therapeutic interventions targeting this process.

Identification of HOXD4 Mutations in Spinal Extradural Arachnoid Cyst

Spinal extradural arachnoid cyst (SEDAC) is a cyst in the spinal canal that protrudes into the epidural space from a defect in the dura mater and leads to neurological disturbances. We previously showed that familial SEDAC is caused by FOXC2 mutation; however, the causal gene of sporadic SEDAC has not been identified. To identify the causal gene of sporadic SEDAC, we performed whole exome sequencing for 12 subjects with sporadic SEDAC and identified heterozygous HOXD4 loss-of-function mutations in three subjects. HOXD4 haplo-insufficiency causes SEDAC and a transcriptional network containing HOXD4 and FOXC2 is involved in the development of the dura mater and the etiology of SEDAC.

mafba is a downstream transcriptional effector of Vegfc signaling essential for embryonic lymphangiogenesis in zebrafish

Koltowska et al. used a forward genetic screen in zebrafish to identify the transcription factor mafba as essential for lymphatic vessel development. Vegfc signaling increases mafba expression to control downstream transcription, and this relationship is SoxF transcription factor-dependent.

The lymphatic vasculature plays roles in tissue fluid balance, immune cell trafficking, fatty acid absorption, cancer metastasis, and cardiovascular disease. Lymphatic vessels form by lymphangiogenesis, the sprouting of new lymphatics from pre-existing vessels, in both development and disease contexts. The apical signaling pathway in lymphangiogenesis is the VEGFC/VEGFR3 pathway, yet how signaling controls cellular transcriptional output remains unknown. We used a forward genetic screen in zebrafish to identify the transcription factor mafba as essential for lymphatic vessel development. We found that mafba is required for the migration of lymphatic precursors after their initial sprouting from the posterior cardinal vein. mafba expression is enriched in sprouts emerging from veins, and we show that mafba functions cell-autonomously during lymphatic vessel development. Mechanistically, Vegfc signaling increases mafba expression to control downstream transcription, and this regulatory relationship is dependent on the activity of SoxF transcription factors, which are essential for mafba expression in venous endothelium. Here we identify an indispensable Vegfc–SoxF–Mafba pathway in lymphatic development.

Association of polymorphisms near the FOXC2 gene with the risk of varicose veins in ethnic Russians

OBJECTIVE

To investigate the association of polymorphisms located near the FOXC2 gene with the risk of varicose veins in ethnic Russians.

METHODS

Allele, genotype, and haplotype frequencies were determined in the sample of 474 patients with primary varicose veins and in the control group of 478 individuals without a history of chronic venous disease.

RESULTS

Polymorphisms rs7189489, rs4633732, and rs1035550 showed the association with the increased risk of varicose veins, but none of the observed associations remained significant after correction for multiple testing. Haplotype analysis revealed the association of haplotype rs7189489 C-rs4633732 T-rs34221221 C-rs1035550 C-rs34152738 T-rs12711457 G with the increased risk of varicose veins (OR = 2.67, P = 0.01).

CONCLUSIONS

Our results provide evidence that the studied polymorphisms do not play a major role in susceptibility to varicose veins development in the Russian population.

A novel FOXC2 mutation in spinal extradural arachnoid cyst

Spinal extradural arachnoid cyst (SEDAC) is a cyst in the spinal canal, which causes spinal cord compression and subsequent neurological damage. We previously identified two FOXC2 mutations in two SEDAC families. The FOXC2 mutations have been shown to be responsible for lymphedema-distichiasis syndrome (LDS), which includes SEDAC as an occasionally associated phenotype. We encountered a non-familial patient with SEDAC associated with LDS, and identified a novel nonsense mutation in FOXC2, c.349C>T (p.Q117*).

Combining Foxc2 and Connexin37 deletions in mice leads to severe defects in lymphatic vascular growth and remodeling

Connexins (Cxs), proteins that are vital for intercellular communication in vertebrates, have recently been shown to play a critical role in lymphatic development. However, our knowledge is currently limited regarding the functional relationships of Cxs with other proteins and signaling pathways. Cell culture studies have shown that Cx37 is necessary for coordinated activation of the transcription factor NFATc1, which cooperates with Foxc2 (another transcription factor) during lymphatic endothelial development. These data suggest that Cxs, Foxc2, and NFATc1 are part of a common developmental pathway. Here, we present a detailed characterization of Foxc2(+/-)Cx37(-/-) mice, demonstrating that lymphatic network architecture and valve formation rely on the concurrent embryonic expression and function of Foxc2 and Cx37. Foxc2(+/-)Cx37(-/-) mice have lymphedema in utero, exhibit craniofacial abnormalities, show severe dilation of intestinal lymphatics, display abnormal lacteal development, lack lymphatic valves, and typically die perinatally (outcomes not seen in Foxc2(+/-) or Cx37(-/-) mice separately). We provide a rigorous, quantitative documentation of lymphatic vascular network changes that highlight the specific structural alterations that occur in Foxc2(+/-)Cx37(-/-) mice. These data provide further evidence suggesting that Foxc2 and Cx37 are elements in a common molecular pathway directing lymphangiogenesis.

A Novel Missense Mutation in FLT4 Causes Autosomal Recessive Hereditary Lymphedema

BACKGROUND

Primary lymphedema covers around 10% of all lymphedema cases. Most cases segregate as an autosomal dominant trait and rarely manifest autosomal recessive inheritance. Our research aimed to map and ultimately to hunt the mutation that causes hereditary lymphedema in an extended consanguineous Muslim family consisting of several affected individuals.

METHODS AND RESULTS

We attempted molecular diagnosis by applying homozygosity mapping and whole genome linkage analysis. A candidate locus of 2.3 Mb located on chromosome 5q35.3 was identified, yielding an overall LOD score of 3.18. This locus has been previously linked to congenital lymphedema, namely by the FLT4 gene. Mutations in FLT4 that were previously described in Muslim-Israeli families were discarded as culprit using sequence analysis. Sanger sequencing the gene revealed a novel missense variant in exon 28 (NM_182925.4: c.3704C>G; p.Ser1235Cys). This variant has perfect segregation within the extended family and was not previously reported in either common or pathogenic variants databases.

CONCLUSIONS

Our mutation is the first reported pathogenic variant located outside the tyrosine kinase domains of the VEGFR3 receptor, and the second to portray autosomal recessive inheritance. The homozygous substitution of serine by cysteine at position 1235 affects protein tyrosine kinase activity, possibly through a null effect mechanism rather than a negative dominant effect. Our variant is associated with a mild phenotype, possibly reflecting some residual receptor activity, most probably attributed to the variant's location beyond the TK domains.

Downregulation of FoxC2 Increased Susceptibility to Experimental Colitis: Influence of Lymphatic Drainage Function?

BACKGROUND

Although inflammation-induced expansion of the intestinal lymphatic vasculature (lymphangiogenesis) is known to be a crucial event in limiting inflammatory processes, through clearance of interstitial fluid and immune cells, considerably less is known about the impact of an impaired lymphatic clearance function (as seen in inflammatory bowel diseases) on this cascade. We aimed to investigate whether the impaired intestinal lymphatic drainage function observed in FoxC2 mice would influence the course of disease in a model of experimental colitis.

METHODS

Acute dextran sodium sulfate colitis was induced in wild-type and haploinsufficient FoxC2 mice, and survival, disease activity, colonic histopathological injury, neutrophil, T-cell, and macrophage infiltration were evaluated. Functional and structural changes in the intestinal lymphatic vessel network were analyzed, including submucosal edema, vessel morphology, and lymphatic vessel density.

RESULTS

We found that FoxC2 downregulation in FoxC2 mice significantly increased the severity and susceptibility to experimental colitis, as displayed by lower survival rates, increased disease activity, greater histopathological injury, and elevated colonic neutrophil, T-cell, and macrophage infiltration. These findings were accompanied by structural (dilated torturous lymphatic vessels) and functional (greater submucosal edema, higher immune cell burden) changes in the intestinal lymphatic vasculature.

CONCLUSIONS

These results indicate that sufficient lymphatic clearance plays a crucial role in limiting the initiation and perpetuation of experimental colitis and those disturbances in the integrity of the intestinal lymphatic vessel network could intensify intestinal inflammation. Future therapies might be able to exploit these processes to restore and maintain adequate lymphatic clearance function in inflammatory bowel disease.

Cdk5 controls lymphatic vessel development and function by phosphorylation of Foxc2

The lymphatic system maintains tissue fluid balance, and dysfunction of lymphatic vessels and valves causes human lymphedema syndromes. Yet, our knowledge of the molecular mechanisms underlying lymphatic vessel development is still limited. Here, we show that cyclin-dependent kinase 5 (Cdk5) is an essential regulator of lymphatic vessel development. Endothelial-specific Cdk5 knockdown causes congenital lymphatic dysfunction and lymphedema due to defective lymphatic vessel patterning and valve formation. We identify the transcription factor Foxc2 as a key substrate of Cdk5 in the lymphatic vasculature, mechanistically linking Cdk5 to lymphatic development and valve morphogenesis. Collectively, our findings show that Cdk5-Foxc2 interaction represents a critical regulator of lymphatic vessel development and the transcriptional network underlying lymphatic vascular remodeling.

Identification of novel craniofacial regulatory domains located far upstream of SOX9 and disrupted in Pierre Robin sequence

Mutations in the coding sequence of SOX9 cause campomelic dysplasia (CD), a disorder of skeletal development associated with 46,XY disorders of sex development (DSDs). Translocations, deletions, and duplications within a ∼2 Mb region upstream of SOX9 can recapitulate the CD-DSD phenotype fully or partially, suggesting the existence of an unusually large cis-regulatory control region. Pierre Robin sequence (PRS) is a craniofacial disorder that is frequently an endophenotype of CD and a locus for isolated PRS at ∼1.2-1.5 Mb upstream of SOX9 has been previously reported. The craniofacial regulatory potential within this locus, and within the greater genomic domain surrounding SOX9, remains poorly defined. We report two novel deletions upstream of SOX9 in families with PRS, allowing refinement of the regions harboring candidate craniofacial regulatory elements. In parallel, ChIP-Seq for p300 binding sites in mouse craniofacial tissue led to the identification of several novel craniofacial enhancers at the SOX9 locus, which were validated in transgenic reporter mice and zebrafish. Notably, some of the functionally validated elements fall within the PRS deletions. These studies suggest that multiple noncoding elements contribute to the craniofacial regulation of SOX9 expression, and that their disruption results in PRS.

Casein kinase 2 prevents mesenchymal transformation by maintaining Foxc2 in the cytoplasm

Nuclear Foxc2 is a transcriptional regulator of mesenchymal transformation during developmental epithelial-mesenchymal transition (EMT) and has been associated with EMT in malignant epithelia. Our laboratory has shown that in normal epithelial cells Foxc2 is maintained in the cytoplasm where it promotes an epithelial phenotype. The Foxc2 amino terminus has a consensus casein kinase 2 (CK2) phosphorylation site at serine 124, and we now show that CK2 associates with Foxc2 and phosphorylates this site in vitro. Knockdown or inhibition of the CK2α/α' kinase subunit in epithelial cells causes de novo accumulation of Foxc2 in the nucleus. Mutation of serine 124 to leucine promotes constitutive nuclear localization of Foxc2 and expression of mesenchymal genes, whereas an S124D phosphomimetic leads to constitutive cytoplasmic localization and epithelial maintenance. In malignant breast cancer cells, the CK2β regulatory subunit is downregulated and FOXC2 is found in the nucleus, correlating with an increase in α-smooth muscle actin (SMA) expression. Restoration of CK2β expression in these cells results in cytoplasmic localization of Foxc2, decreased α-SMA expression and reduced cell migration and invasion. In contrast, knockdown of CK2β in normal breast epithelial cells leads to FOXC2 nuclear localization, decreased E-cadherin expression, increased α-SMA and vimentin expression, and enhanced cell migration and invasion. Based on these findings, we propose that Foxc2 is functionally maintained in the cytoplasm of normal epithelial cells by CK2α/α'-mediated phosphorylation at serine 124, which is dependent on proper targeting of the holoenzyme via the CK2β regulatory subunit.

Evidence for SH2 domain-containing 5'-inositol phosphatase-2 (SHIP2) contributing to a lymphatic dysfunction

The lymphatic vasculature plays a critical role in a number of disease conditions of increasing prevalence, such as autoimmune disorders, obesity, blood vascular diseases, and cancer metastases. Yet, unlike the blood vasculature, the tools available to interrogate the molecular basis of lymphatic dysfunction/disease have been lacking. More recently, investigators have reported that dysregulation of the PI3K pathway is involved in syndromic human diseases that involve abnormal lymphatic vasculatures, but there have been few compelling results that show the direct association of this molecular pathway with lymphatic dysfunction in humans. Using near-infrared fluorescence lymphatic imaging (NIRFLI) to phenotype and next generation sequencing (NGS) for unbiased genetic discovery in a family with non-syndromic lymphatic disease, we discovered a rare, novel mutation in INPPL1 that encodes the protein SHIP2, which is a negative regulator of the PI3K pathway, to be associated with lymphatic dysfunction in the family. In vitro interrogation shows that SHIP2 is directly associated with impairment of normal lymphatic endothelial cell (LEC) behavior and that SHIP2 associates with receptors that are associated in lymphedema, implicating its direct involvement in the lymphatic vasculature.

Biomolecular mechanisms in varicose veins development

Varicose veins (VVs) can be described as tortuous and dilated palpable veins, which are more than 3 mm in diameter. They are one of the clinical presentations of chronic venous disorders, which are a significant cause of morbidity. The prevalence of VVs has been estimated at 25-33% in women and 10-20% in men and is still increasing at an alarming rate. Family history, older age, female, pregnancy, obesity, standing occupations, and a history of deep venous thrombosis are the predominant risk factors. A great amount of factors are implicated in the pathogenesis of VVs, including changes in hydrostatic pressure, valvular incompetence, deep venous obstruction, ineffective function of calf muscle pump, biochemical and structural alterations of the vessel wall, extracellular matrix abnormalities, impaired balance between growth factors or cytokines, genetic alterations, and several other mechanisms. Nevertheless, the issue of pathogenesis in VVs is still not completely known, even if a great progress has been made in understanding their molecular basis. This kind of studies appears promising and should be encouraged, and perhaps the new insight in this matter may result in targeted therapy or possibly prevention.

A five generation family with a novel mutation in FOXC2 and lymphedema worsening to hydrops in the youngest generation

We describe a five generation family with dominantly inherited lymphedema, but no distichiasis, in which 3/3 affected offspring in the fifth generation have died of fetal hydrops and related birth defects. Mutational analysis disclosed a novel mutation in FOXC2 (R121C) in affected members. We searched for possible genetic influences on the greater severity of lymphedema (hydrops) in the fifth generation. Karyotypes disclosed an extra band in Xp in one affected fetus, but this was also found in the mother. Copy number variation (CNV) studies on four members of the pedigree (mother of the three severely affected fetuses/infants; one severely affected; a full, and a half, unaffected sibs) did not detect the source of the Xp band or a possible influence on the severe phenotype. However, use of SNP arrays did allow identification of the portion of the maternal proximal Xp shared by a hydrops-affected daughter and son which was not shared by an unaffected daughter from the same sibship.

Semaphorins in cardiovascular medicine

During organogenesis, patterning is primarily achieved by the combined actions of morphogens. Among these, semaphorins represent a general system for establishing the appropriate wiring architecture of biological nets. Originally discovered as evolutionarily conserved steering molecules for developing axons, subsequent studies on semaphorins expanded their functions to the cardiovascular and immune systems. Semaphorins participate in cardiac organogenesis and control physiological vasculogenesis and angiogenesis, which result from a balance between pro- and anti-angiogenic signals. These signals are altered in several diseases. In this review, we discuss the role of semaphorins in vascular biology, emphasizing the mechanisms by which these molecules control vascular patterning and lymphangiogenesis, as well as in genetically inherited and degenerative vascular diseases.

Genetics of vascular malformations

Vascular anomalies are developmental defects of the vasculature and encompass a variety of disorders. The majority of these occur sporadically, yet a few are reported to be familial. The identification of genes mutated in the different malformations provides insight into their etiopathogenic mechanisms and the specific roles the associated proteins play in vascular development and maintenance. It is becoming evident that somatic mosaicism plays a major role in the formation of vascular lesions. The importance of utilizing Next-Generating Sequencing (NGS) for high-throughput and "deep" screening of both blood and lesional DNA and RNA is thus emphasized, as the somatic changes are present in low quantities. There are several examples where NGS has already accomplished discovering these changes. The identification of all the causative genes and unraveling of a holistic overview of the pathogenic mechanisms should enable generation of in vitro and in vivo models and lead to development of more effective treatments, not only targeted on symptoms.

Lymphatic collecting vessel maturation and valve morphogenesis

The lymphatic vasculature plays an essential role in the maintenance of tissue interstitial fluid balance and in the immune response. After capture of fluids, proteins and antigens by lymphatic capillaries, lymphatic collecting vessels ensure lymph transport. An important component to avoid lymph backflow and to allow a unidirectional flow is the presence of intraluminal valves. Defects in the function of collecting vessels lead to lymphedema. Several important factors and signaling pathways involved in lymphatic collecting vessel maturation and valve morphogenesis have now been discovered. The present review summarizes the current knowledge about the key steps of lymphatic collecting vessel development and maturation and focuses on the regulatory mechanisms involved in lymphatic valve formation.

A Chinese pedigree of lymphoedema-distichiasis syndrome with a novel mutation in the FOXC2 gene

Lymphoedema-distichiasis syndrome (LDS) is a syndromic form of primary lymphoedema associated with double rows of eyelashes (distichiasis). Mutations in the FOXC2 gene were reported to be associated with this syndrome. In this study, we identified in a Chinese LDS pedigree a novel FOXC2 gene mutation, C.370C>T, leading to p.Leu124Phe. The novel mutation is not a common polymorphism, but is co-inherited with the disease.

Lymphatics in nanophysiology

Lymphatic vessels are essential for fluid transport and tissue homeostasis. Recent discoveries identified several genes, including Prox1 and VEGF-C, which are required for the lymphatic vessel development in physiological conditions as well as under pathological conditions such as chronic inflammation and tumor progression. Lymphatic vessels show morphological structures that are distinct between the initial lymphatic vessels and collectors, reflecting their respective functions of fluid absorption and transport. These differential structures are crucial for the physiological function of lymphatic vasculature. VEGF-A-mediated chronic inflammation impairs the fundamental structure of the initial lymphatic vessels, leading to delayed transport of nano-scaled fluorescence tracers. This article discusses recent findings that have clarified the biological function of lymphatic vessels in physiological and pathological settings. Assessments of the lymphatic function at nano-scale levels address the major contribution of lymphatic vessels to the kinetics of drug delivery and excretion.

Genetics of lymphatic anomalies

Lymphatic anomalies include a variety of developmental and/or functional defects affecting the lymphatic vessels: sporadic and familial forms of primary lymphedema, secondary lymphedema, chylothorax and chylous ascites, lymphatic malformations, and overgrowth syndromes with a lymphatic component. Germline mutations have been identified in at least 20 genes that encode proteins acting around VEGFR-3 signaling but also downstream of other tyrosine kinase receptors. These mutations exert their effects via the RAS/MAPK and the PI3K/AKT pathways and explain more than a quarter of the incidence of primary lymphedema, mostly of inherited forms. More common forms may also result from multigenic effects or post-zygotic mutations. Most of the corresponding murine knockouts are homozygous lethal, while heterozygotes are healthy, which suggests differences in human and murine physiology and the influence of other factors.

Development of the mammalian lymphatic vasculature

The two vascular systems of our body are the blood and lymphatic vasculature. Our understanding of the cellular and molecular processes controlling the development of the lymphatic vasculature has progressed significantly in the last decade. In mammals, this is a stepwise process that starts in the embryonic veins, where lymphatic EC (LEC) progenitors are initially specified. The differentiation and maturation of these progenitors continues as they bud from the veins to produce scattered primitive lymph sacs, from which most of the lymphatic vasculature is derived. Here, we summarize our current understanding of the key steps leading to the formation of a functional lymphatic vasculature.

New developments in clinical aspects of lymphatic disease

The lymphatic system is fundamentally important to cardiovascular disease, infection and immunity, cancer, and probably obesity--the four major challenges in healthcare in the 21st century. This Review will consider the manner in which new knowledge of lymphatic genes and molecular mechanisms has demonstrated that lymphatic dysfunction should no longer be considered a passive bystander in disease but rather an active player in many pathological processes and, therefore, a genuine target for future therapeutic developments. The specific roles of the lymphatic system in edema, genetic aspects of primary lymphedema, infection (cellulitis/erysipelas), Crohn's disease, obesity, cancer, and cancer-related lymphedema are highlighted.

Transcriptional control of lymphatic endothelial cell type specification

The lymphatic vasculature is the "sewer system" of our body as it plays an important role in transporting tissue fluids and extravasated plasma proteins back to the blood circulation and absorbs lipids from the intestinal tract. Malfunction of the lymphatic vasculature can result in lymphedema and obesity. The lymphatic system is also important for the immune response and is one of the main routes for the spreading of metastatic tumor cells. The development of the mammalian lymphatic vasculature is a stepwise process that requires the specification of lymphatic endothelial cell (LEC) progenitors in the embryonic veins, and the subsequent budding of those LEC progenitors from the embryonic veins to give rise to the primitive lymph sacs from which the entire lymphatic vasculature will eventually be derived. This process was first proposed by Florence Sabin over a century ago and was recently confirmed by several studies using lineage tracing and gene manipulation. Over the last decade, significant advances have been made in understanding the transcriptional control of lymphatic endothelial cell type differentiation. Here we summarize our current knowledge about the key transcription factors that are necessary to regulate several aspects of lymphatic endothelial specification and differentiation.

Clinical disorders of primary malfunctioning of the lymphatic system

Primary lymphedema is defined as lymphedema caused by dysplasia of the lymph vessels. This complex group of diseases is discussed in detail from a clinical perspective. A review of the epidemiology and classification of lymphedema on the backdrop of its clinical presentation reveals weaknesses of the present classification system, which, however, is the basis for the choice of optimal patient care. Non-syndrome and syndrome types of primary lymphedema are presented in detail and related molecular findings are summarized.

Interplay of mechanotransduction, FOXC2, connexins, and calcineurin signaling in lymphatic valve formation

The directional flow of lymph is maintained by hundreds of intraluminal lymphatic valves. Lymphatic valves are crucial to prevent lymphedema, accumulation of fluid in the tissues, and to ensure immune surveillance; yet, the mechanisms of valve formation are only beginning to be elucidated. In this chapter, we will discuss the main steps of lymphatic valve morphogenesis, the important role of mechanotransduction in this process, and the genetic program regulated by the transcription factor Foxc2, which is indispensable for all steps of valve development. Failure to form mature collecting lymphatic vessels and valves causes the majority of postsurgical lymphedema, e.g., in breast cancer patients. Therefore, this knowledge will be useful for diagnostics and development of better treatments of secondary lymphedema.

To grow or not to grow: hair morphogenesis and human genetic hair disorders

Mouse models have greatly helped in elucidating the molecular mechanisms involved in hair formation and regeneration. Recent publications have reviewed the genes involved in mouse hair development based on the phenotype of transgenic, knockout and mutant animal models. While much of this information has been instrumental in determining molecular aspects of human hair development and cycling, mice exhibit a specific pattern of hair morphogenesis and hair distribution throughout the body that cannot be directly correlated to human hair. In this mini-review, we discuss specific aspects of human hair follicle development and present an up-to-date summary of human genetic disorders associated with abnormalities in hair follicle morphogenesis, structure or regeneration.

FOXC2 mutations in familial and sporadic spinal extradural arachnoid cyst

Spinal extradural arachnoid cyst (SEDAC) is a cyst in the spinal canal that protrudes into the epidural space from a defect in the dura mater. Most cases are sporadic; however, three familial SEDAC cases have been reported, suggesting genetic etiological factors. All familial cases are associated with lymphedema-distichiasis syndrome (LDS), whose causal gene is FOXC2. However, FOXC2 mutation analysis has been performed in only 1 family, and no mutation analysis has been performed on sporadic (non-familial) SEDACs. We recruited 17 SEDAC subjects consisting of 2 familial and 7 sporadic cases and examined FOXC2 mutations by Sanger sequencing and structural abnormalities by TaqMan copy number assay. We identified 2 novel FOXC2 mutations in 2 familial cases. Incomplete LDS penetrance was noted in both families. Four subjects presented with SEDACs only. Thus, SEDAC caused by the heterozygous FOXC2 loss-of-function mutation should be considered a feature of LDS, although it often manifests as the sole symptom. Seven sporadic SEDAC subjects had no FOXC2 mutations, no symptoms of LDS, and showed differing clinical characteristics from those who had FOXC2 mutations, suggesting that other gene(s) besides FOXC2 are likely to be involved in SEDAC.

Vascular endothelial growth factor receptor-2 promotes the development of the lymphatic vasculature

Vascular endothelial growth factor receptor 2 (VEGFR2) is highly expressed by lymphatic endothelial cells and has been shown to stimulate lymphangiogenesis in adult mice. However, the role VEGFR2 serves in the development of the lymphatic vascular system has not been defined. Here we use the Cre-lox system to show that the proper development of the lymphatic vasculature requires VEGFR2 expression by lymphatic endothelium. We show that Lyve-1^wt/Cre;Vegfr2^flox/flox mice possess significantly fewer dermal lymphatic vessels than Vegfr2^flox/flox mice. Although Lyve-1^wt/Cre;Vegfr2^flox/flox mice exhibit lymphatic hypoplasia, the lymphatic network is functional and contains all of the key features of a normal lymphatic network (initial lymphatic vessels and valved collecting vessels surrounded by smooth muscle cells (SMCs)). We also show that Lyve-1^Cre mice display robust Cre activity in macrophages and in blood vessels in the yolk sac, liver and lung. This activity dramatically impairs the development of blood vessels in these tissues in Lyve-1^wt/Cre;Vegfr2^flox/flox embryos, most of which die after embryonic day14.5. Lastly, we show that inactivation of Vegfr2 in the myeloid lineage does not affect the development of the lymphatic vasculature. Therefore, the abnormal lymphatic phenotype of Lyve-1^wt/Cre;Vegfr2^flox/flox mice is due to the deletion of Vegfr2 in the lymphatic vasculature not macrophages. Together, this work demonstrates that VEGFR2 directly promotes the expansion of the lymphatic network and further defines the molecular mechanisms controlling the development of the lymphatic vascular system.