Clinical heterogeneity in lymphoedema-distichiasis with FOXC2 truncating mutations

Meibomian gland development: Where, when and how?

The Meibomian gland (MG) is an indispensable adnexal structure of eye that produces meibum, an important defensive component for maintaining ocular homeostasis. Normal development and maintenance of the MGs is required for ocular health since atrophic MGs and disturbances in composition and/or secretion of meibum result in major ocular pathologies, collectively termed as Meibomian gland dysfunction (MGD). Currently available therapies for MGD merely provide symptomatic relief and do not treat the underlying deficiency of the MGs. Hence, a thorough understanding of the timeline of MG development, maturation and aging is required for regenerative purposes along with signaling molecules & pathways controlling proper differentiation of MG lineage in mammalian eye. Understanding the factors that contribute to the development of MGs, developmental abnormalities of MGs, and changes in the quality & quantity of meibum with developing phases of MGs are essential for developing potential treatments for MGD. In this review, we compiled a timeline of events and the factors involved in the structural and functional development of MGs and the associated developmental defects of MGs during development, maturation and aging.

Autosomal dominant inheritance with sex-limited manifestation: An unusual mode of transmission in humans and animals

Autosomal dominant, sex-limited inheritance is a distinct mode of transmission that should not be conflated with X-linked inheritance. From animal studies, we know that sex-limited inheritance implies the chance to "turn off" some genes in either males or females, in order to meliorate the phenotype, for example, by improving the fecundity. In this way, sex-limited genes play an important role in the evolution of diverse species of animals. In human genetics, however, the biological significance of sex-limited genes is unknown until today. When screening the literature, we found, thus far, three human examples of sex-limited transmission. Autosomal dominant, male-limited inheritance has meticulously been studied in a particular form of precocious puberty. Limitation to females was described in autosomal dominant lymphedema of the CESLR1 type, being underpinned by convincing molecular findings. Another example is white lentiginosis of Grosshans that shows clinical evidence of such mode of transmission although molecular findings are lacking as yet. In the animal kingdom, autosomal dominant sex-limited inheritance is a well-established phenomenon that has extensively been studied in various species such as butterflies, damselflies, fish (cichlids), and birds. Hence, at this point in time, it seems likely that other human examples of this mode of inheritance have previously been reported or will be published in the future.

Genetic landscape of FOXC2 mutations in lymphedema-distichiasis syndrome: Different mechanism of pathogenicity for mutations in different domains

Lymphedema-dissociated syndrome (LDS), of which the pathogenesis is not fully understood, afflicts many patients. In this study, we investigated the effect of FOXC2 gene loss-of-function on the development of LDS disease.Two Han Chinese families with LDS were recruited in this study, pathogenic mutations were identified by Sanger sequencing. Reverse-transcription PCR, subcellular localization, dual fluorescein enzymes, and other in vitro experiments were used to study the functional effects of eight FOXC2 mutations. Two pathogenic FOXC2 duplication mutations (c.930_936dup and c.931-937dup) were identified in the two families. Both mutations caused uneven distribution in the nucleus and a chromatin contraction phenotype, weakening the DNA binding activity and transcription activity. We then performed functional analysis on six additional mutations in different domains of FOXC2 that were reported to cause LDS. We found mutations located in the forkhead domain and central region dramatically reduced the transactivation ability, while mutations in activation domain-2 enhanced this ability. All 8 mutations down-regulated the transcription of ANGPT2 and affected the activity of the ERK-RAS pathway, which may cause abnormal formation of lymphatic vessels. Our findings also showed that all 8 mutations decreased the ability of interaction between FOXC2 and the Wnt4 promoter, suggesting mutations in FOXC2 may also affect the Wnt4-Frizzled-RYK signaling pathway, leading the abnormal differentiation of the meibomian glands into hair follicle cells during the embryonic period and causing distichiasis. This study expanded and revealed the potential pathogenesis mechanism.

Distichiasis: An update on etiology, treatment and outcomes

Distichiasis, an extra row of eyelashes emerging from meibomian gland orifices, occurs due to the metaplastic transition of sebaceous glands into the pilosebaceous unit. It can present congenitally, such as in lymphedema distichiasis syndrome, or secondary to acquired conditions, such as cicatrizing conjunctivitis, trachoma. This review summarizes the etiology of distichiasis, its presentation, the evolution of various surgical techniques, and their outcomes in human and animal eyes. The published literature has focused on the different treatment modalities and their outcomes; the etiopathogenesis of this condition remains elusive. Truncating mutations (missense, frameshift, and nonsense) in the Forkhead family gene FOXC2 are involved in the distichiasis-lymphedema syndrome. The treatment options are no different for congenital versus acquired distichiasis, with no specific available algorithms. Acquired distichiasis in cicatrizing ocular surface diseases is difficult to manage, and existing treatment options offer success rates of 50%-60%. The outcomes of electroepilation or direct cryotherapy are not as good as surgical excision of distichiatic lashes after splitting the anterior and posterior lamella under direct visualization. The marginal tarsectomy with or without free tarsoconjunctival graft has shown good results in eyes with congenital and acquired distichiasis. The details of differences between normal and distichiatic lash, depth, or course of distichiatic eyelashes remain largely unknown. Studies exploring the distichiatic eyelash depth might improve the outcomes of blind procedures such as cryotherapy or radiofrequency-assisted epilation.

Microdeletion of 16q24.1-q24.2-A unique etiology of Lymphedema-Distichiasis syndrome and neurodevelopmental disorder

Interstitial deletions of 16q24.1–q24.2 are associated with alveolar capillary dysplasia, congenital renal malformations, neurodevelopmental disorders, and congenital abnormalities. Lymphedema–Distichiasis syndrome (LDS; OMIM # 153400) is a dominant condition caused by heterozygous pathogenic variants in FOXC2. Usually, lymphedema and distichiasis occur in puberty or later on, and affected individuals typically achieve normal developmental milestones. Here, we describe a boy with congenital lymphedema, distichiasis, bilateral hydronephrosis, and global developmental delay, with a de novo microdeletion of 894 kb at 16q24.1–q24.2. This report extends the phenotype of both 16q24.1–q24.2 microdeletion syndrome and of LDS. Interestingly, the deletion involves only the 3′‐UTR part of FOXC2.

Digenic Inheritance of a FOXC2 Mutation and Two PIEZO1 Mutations Underlies Congenital Lymphedema in a Multigeneration Family

BACKGROUND

The lymphatic system is essential for maintaining the balance of interstitial fluid in tissues and for returning protein-rich fluids (lymph) to the bloodstream. Congenital lymphatic defects lead to accumulation of lymph in peripheral tissues and body cavities, termed primary lymphedema. To date, only a limited number of individual genes have been identified in association with primary lymphedema. However, variability of age of onset and severity of lymphatic abnormalities within some families suggests that multiple mutations or genes may be responsible, thus hampering efforts to identify individual associated genes.

METHODS

Whole exome sequencing (WES) was performed in 4 members of a large multigeneration family with highly variable lymphedema and followed by Sanger sequencing for identified mutations in 34 additional family members. Genotypes were correlated with clinical and lymphangioscintigraphic phenotypes.

RESULTS

WES uncovered 2 different mechanotransducer PIEZO1 mutations and one FOXC2 transcription factor mutation in various combinations. Sanger sequencing confirmed the presence/absence of the 3 variants in affected and unaffected family members and co-segregation of one or more variants with disease. Genetic profiles did not clearly correlate with the highly variable severity of lymphatic abnormalities.

CONCLUSIONS

WES in lymphedema families can uncover unexpected combinations of several lymphedema-associated mutations. These findings provide essential information for genetic counseling and reveal complex gene interactions in lymphatic developmental pathways. These can offer insights into the complex spectrum of clinical and lymphatic lymphedema phenotypes and potential targets for treatment.

Imbalance between Expression of FOXC2 and Its lncRNA in Lymphedema-Distichiasis Caused by Frameshift Mutations

Forkhead-box C2 (FOXC2) is a transcription factor involved in lymphatic system development. FOXC2 mutations cause Lymphedema-distichiasis syndrome (LD). Recently, a natural antisense was identified, called lncRNA FOXC2-AS1, which increases FOXC2 mRNA stability. No studies have evaluated FOXC2 and FOXC2-AS1 blood expression in LD and healthy subjects. Here, we show that FOXC2 and FOXC-AS1 expression levels were similar in both controls and patients, and a significantly higher amount of both RNAs was observed in females. A positive correlation between FOXC2 and FOXC2-AS1 expression was found in both controls and patients, excluding those with frameshift mutations. In these patients, the FOXC2-AS1/FOXC2 ratio was about 1:1, while it was higher in controls and patients carrying other types of mutations. The overexpression or silencing of FOXC2-AS1 determined a significant increase or reduction in FOXC2 wild-type and frameshift mutant proteins, respectively. Moreover, confocal and bioinformatic analysis revealed that these variations caused the formation of nuclear proteins aggregates also involving DNA. In conclusion, patients with frameshift mutations presented lower values of the FOXC2-AS1/FOXC2 ratio, due to a decrease in FOXC2-AS1 expression. The imbalance between FOXC2 mRNA and its lncRNA could represent a molecular mechanism to reduce the amount of FOXC2 misfolded proteins, protecting cells from damage.

Lymphedema distichiasis syndrome may be caused by FOXC2 promoter-enhancer dissociation and disruption of a topological associated domain

Lymphedema distichiasis syndrome (LDS) is a rare autosomal dominant condition characterized by lower limb lymphedema, distichiasis, and variable additional features. LDS is usually caused by heterozygous sequence variants in the FOXC2 gene located at 16q24, but in one previous instance LDS has resulted from a balanced reciprocal translocation with a breakpoint at 16q24, 120 kb distal to the FOXC2 gene suggesting a position effect. Here, we describe a second family with LDS caused by a translocation involving 16q24. The family were ascertained after detection of a paternally inherited balanced reciprocal translocation t(16;22)(q24;q13.1) in a pregnancy complicated by severe fetal hydrops. There was a past history of multiple miscarriages in the father's family, and a personal and family history of lymphedema and distichiasis, consistent with the diagnosis of LDS. Using whole genome amplified DNA from single sperm of the male proband, bead array analysis demonstrated that the FOXC2 gene was intact and the chromosome 16 breakpoint mapped to the same region 120Kb distal to the FOXC2 gene. This case highlights the clinical consequences that can arise from a translocation of genomic material without dosage imbalance, and that it is increasingly feasible to predict and characterize possible effects with improved access to molecular techniques.

FOXC2 Disease Mutations Identified in Lymphedema Distichiasis Patients Impair Transcriptional Activity and Cell Proliferation

FOXC2 is a member of the human forkhead-box gene family and encodes a regulatory transcription factor. Mutations in FOXC2 have been associated with lymphedema distichiasis (LD), an autosomal dominant disorder that primarily affects the limbs. Most patients also show extra eyelashes, a condition known as distichiasis. We previously reported genetic and clinical findings in six unrelated families with LD. Half the patients showed missense mutations, two carried frameshift mutations and a stop mutation was identified in a last patient. Here we analyzed the subcellular localization and transactivation activity of the mutant proteins, showing that all but one (p.Y109*) localized to the nucleus. A significant reduction of transactivation activity was observed in four mutants (p.L80F, p.H199Pfs*264, p.I213Tfs*18, p.Y109*) compared with wild type FOXC2 protein, while only a partial loss of function was associated with p.V228M. The mutant p.I213V showed a very slight increase of transactivation activity. Finally, immunofluorescence analysis revealed that some mutants were sequestered into nuclear aggregates and caused a reduction of cell viability. This study offers new insights into the effect of FOXC2 mutations on protein function and shows the involvement of aberrant aggregation of FOXC2 proteins in cell death.

Epigenetic priming by Dot1l in lymphatic endothelial progenitors ensures normal lymphatic development and function

Proper functioning of the lymphatic system is required for normal immune responses, fluid balance, and lipid reabsorption. Multiple regulatory mechanisms are employed to ensure the correct formation and function of lymphatic vessels; however, the epigenetic modulators and mechanisms involved in this process are poorly understood. Here, we assess the regulatory role of mouse Dot1l, a histone H3 lysine (K) 79 (H3K79) methyltransferase, in lymphatic formation. Genetic ablation of Dot1l in Tie2(+) endothelial cells (ECs), but not in Lyve1(+) or Prox1(+) lymphatic endothelial cells (LECs) or Vav1(+) definitive hematopoietic stem cells, leads to catastrophic lymphatic anomalies, including skin edema, blood–lymphatic mixing, and underdeveloped lymphatic valves and vessels in multiple organs. Remarkably, targeted Dot1l loss in Tie2(+) ECs leads to fully penetrant lymphatic aplasia, whereas Dot1l overexpression in the same cells results in partially hyperplastic lymphatics in the mesentery. Genetic studies reveal that Dot1l functions in c-Kit(+) hemogenic ECs during mesenteric lymphatic formation. Mechanistically, inactivation of Dot1l causes a reduction of both H3K79me2 levels and the expression of genes important for LEC development and function. Thus, our study establishes that Dot1l-mediated epigenetic priming and transcriptional regulation in LEC progenitors safeguard the proper lymphatic development and functioning of lymphatic vessels.

Foxc2 is essential for podocyte function

Foxc2 is one of the earliest podocyte markers during glomerular development. To circumvent embryonic lethal effects of global deletion of Foxc2, and to specifically investigate the role of Foxc2 in podocytes, we generated mice with a podocyte-specific Foxc2 deletion. Mice carrying the homozygous deletion developed early proteinuria which progressed rapidly into end stage kidney failure and death around postnatal day 10. Conditional loss of Foxc2 in podocytes caused typical characteristics of podocyte injury, such as podocyte foot process effacement and podocyte microvillus transformation, probably caused by disruption of the slit diaphragm. These effects were accompanied by a redistribution of several proteins known to be necessary for correct podocyte structure. One target gene that showed reduced glomerular expression was Nrp1, the gene encoding neuropilin 1, a protein that has been linked to diabetic nephropathy and proteinuria. We could show that NRP1 was regulated by Foxc2 in vitro, but podocyte-specific ablation of Nrp1 in mice did not generate any phenotype in terms of proteinuria, suggesting that the gene might have more important roles in endothelial cells than in podocytes. Taken together, this study highlights a critical role for Foxc2 as an important gene for podocyte function.

Non-canonical WNT-signaling controls differentiation of lymphatics and extension lymphangiogenesis via RAC and JNK signaling

Development of lymphatics takes place during embryogenesis, wound healing, inflammation, and cancer. We previously showed that Wnt5a is an essential regulator of lymphatic development in the dermis of mice, however, the mechanisms of action remained unclear. Here, whole-mount immunostaining shows that embryonic day (ED) 18.5 Wnt5a-null mice possess non-functional, cyst-like and often blood-filled lymphatics, in contrast to slender, interconnected lymphatic networks of Wnt5a+/- and wild-type (wt) mice. We then compared lymphatic endothelial cell (LEC) proliferation during ED 12.5, 14.5, 16.5 and 18.5 between Wnt5a-/-, Wnt5a+/- and wt-mice. We did not observe any differences, clearly showing that Wnt5a acts independently of proliferation. Transmission electron microscopy revealed multiple defects of LECs in Wnt5a-null mice, such as malformed inter-endothelial junctions, ruffled cell membrane, intra-luminal bulging of nuclei and cytoplasmic processes. Application of WNT5A protein to ex vivo cultures of dorsal thoracic dermis from ED 15.5 Wnt5a-null mice induced flow-independent development of slender, elongated lymphatic networks after 2 days, in contrast to controls showing an immature lymphatic plexus. Reversely, the application of the WNT-secretion inhibitor LGK974 on ED 15.5 wt-mouse dermis significantly prevented lymphatic network elongation. Correspondingly, tube formation assays with human dermal LECs in vitro revealed increased tube length after WNT5A application. To study the intracellular signaling of WNT5A we used LEC scratch assays. Thereby, inhibition of autocrine WNTs suppressed horizontal migration, whereas application of WNT5A to inhibitor-treated LECs promoted migration. Inhibition of the RHO-GTPase RAC, or the c-Jun N-terminal kinase JNK significantly reduced migration, whereas inhibitors of the protein kinase ROCK did not. WNT5A induced transient phosphorylation of JNK in LECs, which could be inhibited by RAC- and JNK-inhibitors. Our data show that WNT5A induces formation of elongated lymphatic networks through proliferation-independent WNT-signaling via RAC and JNK. Non-canonical WNT-signaling is a major mechanism of extension lymphangiogenesis, and also controls differentiation of lymphatics.

FOXC2 disease-mutations identified in lymphedema-distichiasis patients cause both loss and gain of protein function

Dominant mutations in the FOXC2 gene cause a form of lymphedema primarily of the limbs that usually develops at or after puberty. In 90-95% of patients, lymphedema is accompanied by distichiasis. FOXC2 is a member of the forkhead/winged-helix family of transcription factors and plays essential roles in different developmental pathways and physiological processes. We previously described six unrelated families with primary lymphedema-distichiasis in which patients showed different FOXC2 mutations located outside of the forkhead domain. Of those, four were missense mutations, one a frameshift mutation, and the last a stop mutation. To assess their pathogenic potential, we have now examined the subcellular localization and the transactivation activity of the mutated FOXC2 proteins. All six FOXC2 mutant proteins were able to localize into the nucleus; however, the frameshift truncated protein appeared to be sequestered into nuclear aggregates. A reduction in the ability to activate FOXC1/FOXC2 response elements was detected in 50% of mutations, while the remaining ones caused an increase of protein transactivation activity. Our data reveal that either a complete loss or a significant gain of FOXC2 function can cause a perturbation of lymphatic vessel formation leading to lymphedema.

Meibomian Gland Disease: The Role of Gland Dysfunction in Dry Eye Disease

TOPIC

To discuss the pathology, causes, and ocular surface impact of meibomian gland disease (MGD), as well as its relationship to dry eye.

CLINICAL RELEVANCE

MGD is a common disorder with various contributing mechanisms and clinical manifestations. Understanding MGD pathophysiology and its relationship to dry eye is important in order to optimize diagnosis and treatment algorithms.

METHODS

A review of current literature was performed to discern MGD in terms of pathophysiology, risk factors, and ocular surface impact, and the relationship to dry eye.

RESULTS

Meibomian gland obstruction and meibocyte depletion are important components of MGD. Many pathologies can disrupt function of meibomian glands, ranging from congenital to acquired causes. Once gland disruption occurs, the quality and quantity of meibum is altered, with a negative impact on the ocular surface. Increased tear evaporation, tear hyperosmolarity, increased ocular surface staining, increased inflammation, symptomatic irritation of the eyelid and globes, as well as decreased visual acuity have all been observed.

CONCLUSION

MGD leads to changes in meibum quality and quantity that can cause evaporative dry eye and ocular surface disruption, leading to dry eye symptoms in some individuals.

TFOS DEWS II pathophysiology report

The TFOS DEWS II Pathophysiology Subcommittee reviewed the mechanisms involved in the initiation and perpetuation of dry eye disease. Its central mechanism is evaporative water loss leading to hyperosmolar tissue damage. Research in human disease and in animal models has shown that this, either directly or by inducing inflammation, causes a loss of both epithelial and goblet cells. The consequent decrease in surface wettability leads to early tear film breakup and amplifies hyperosmolarity via a Vicious Circle. Pain in dry eye is caused by tear hyperosmolarity, loss of lubrication, inflammatory mediators and neurosensory factors, while visual symptoms arise from tear and ocular surface irregularity. Increased friction targets damage to the lids and ocular surface, resulting in characteristic punctate epithelial keratitis, superior limbic keratoconjunctivitis, filamentary keratitis, lid parallel conjunctival folds, and lid wiper epitheliopathy. Hybrid dry eye disease, with features of both aqueous deficiency and increased evaporation, is common and efforts should be made to determine the relative contribution of each form to the total picture. To this end, practical methods are needed to measure tear evaporation in the clinic, and similarly, methods are needed to measure osmolarity at the tissue level across the ocular surface, to better determine the severity of dry eye. Areas for future research include the role of genetic mechanisms in non-Sjögren syndrome dry eye, the targeting of the terminal duct in meibomian gland disease and the influence of gaze dynamics and the closed eye state on tear stability and ocular surface inflammation.

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.

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.

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.

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.

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.

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.

The new era of the lymphatic system: no longer secondary to the blood vascular system

The blood and lymphatic systems are the two major circulatory systems in our body. Although the blood system has been studied extensively, the lymphatic system has received much less scientific and medical attention because of its elusive morphology and mysterious pathophysiology. However, a series of landmark discoveries made in the past decade has begun to change the previous misconception of the lymphatic system to be secondary to the more essential blood vascular system. In this article, we review the current understanding of the development and pathology of the lymphatic system. We hope to convince readers that the lymphatic system is no less essential than the blood circulatory system for human health and well-being.

Overlapping biomarkers, pathways, processes and syndromes in lymphatic development, growth and neoplasia

Recent discoveries in molecular lymphology, developmental biology, and tumor biology in the context of long-standing concepts and observations on development, growth, and neoplasia implicate overlapping pathways, processes, and clinical manifestations in developmental disorders and cancer metastasis. Highlighted in this review are some of what is known (and speculated) about the genes, proteins, and signaling pathways and processes involved in lymphatic/blood vascular development in comparison to those involved in cancer progression and spread. Clues and conundra from clinical disorders that mix these processes and mute them, including embryonic rests, multicentric nests of displaced cells, uncontrolled/invasive "benign" proliferation and lymphogenous/hematogenous "spread", represent a fine line between normal development and growth, dysplasia, benign and malignant neoplasia, and "metastasis". Improved understanding of these normal and pathologic processes and their underlying pathomechanisms, e.g., stem cell origin and bidirectional epithelial-mesenchymal transition, could lead to more successful approaches in classification, treatment, and even prevention of cancer and a whole host of other diseases.

Microcephaly, intellectual impairment, bilateral vesicoureteral reflux, distichiasis, and glomuvenous malformations associated with a 16q24.3 contiguous gene deletion and a Glomulin mutation

Two hereditary syndromes, lymphedema-distichiasis (LD) syndrome and blepharo-chelio-dontic (BCD) syndrome include the aberrant growth of eyelashes from the meibomian glands, known as distichiasis. LD is an autosomal dominant syndrome primarily characterized by distichiasis and the onset of lymphedema usually during puberty. Mutations in the forkhead transcription factor FOXC2 are the only known cause of LD. BCD syndrome consists of autosomal dominant abnormalities of the eyelid, lip, and teeth, and the etiology remains unknown. In this report, we describe a proband that presented with distichiasis, microcephaly, bilateral grade IV vesicoureteral reflux requiring ureteral re-implantation, mild intellectual impairment and apparent glomuvenous malformations (GVM). Distichiasis was present in three generations of the proband's maternal side of the family. The GVMs were severe in the proband, and maternal family members exhibited lower extremity varicosities of variable degree. A GLMN (glomulin) gene mutation was identified in the proband that accounts for the observed GVMs; no other family member could be tested. TIE2 sequencing revealed no mutations. In the proband, an additional submicroscopic 265 kb contiguous gene deletion was identified in 16q24.3, located 609 kb distal to the FOXC2 locus, which was inherited from the proband's mother. The deletion includes the C16ORF95, FBXO31, MAP1LC3B, and ZCCHC14 loci and 115 kb of a gene desert distal to FOXC2 and FOXL1. Thus, it is likely that the microcephaly, distichiasis, vesicoureteral, and intellectual impairment in this family may be caused by the deletion of one or more of these genes and/or deletion of distant cis-regulatory elements of FOXC2 expression.

Novel mutation in the FOXC2 gene in three generations of a family with lymphoedema-distichiasis syndrome

Lymphoedema-distichiasis syndrome (LDS, OMIM #153400) is a genetic disorder with an autosomal dominant pattern of inheritance caused by mutations in the FOXC2 gene. Affected individuals typically present with lower extremity lymphoedema and distichiasis. The most common types of mutations in FOXC2 gene include small deletions and insertions, but duplications, duplications-insertions, missense and nonsense mutations were also found. Herein, we describe three generations of a family diagnosed with LDS caused by a new mutation in the FOXC2 gene. This mutation is a frameshift due to a deletion of the nucleotides (CC) in C repeats between C590 [corrected] and C595 [corrected]. This mutation leads to protein truncation as a result of an earlier insertion of a stop codon. To the best of our knowledge, this is the first description of this mutation in the literature and could be coupled with an atypical lymphoscintigram.

Primary lymphedema with coarctation of the aorta: possible new syndrome or variant of Irons-Bianchi syndrome?

We present a boy with congenital lymphedema, a congenital heart defect (coarctation of the aorta), and mild dysmorphic features. Clinical impression and targeted investigations ruled out Noonan syndrome and Milroy syndrome, but it was not clear whether or not he had Irons-Bianchi syndrome. We discuss the genomic and lymphoscintigraphy evaluation of this case, and review whether the small number of current case reports represent the original Irons-Bianchi syndrome or variants. We anticipate that ongoing molecular investigations such as Next Generation Sequencing will delineate a currently clinically defined phenotypic spectrum.

Massively parallel sequencing and identification of genes for primary lymphoedema: a perfect fit

Primary lymphoedema is a clinically and genetically heterogeneous group of disorders characterized by disruption of the lymphatic system. To date, the majority of the causative genes in primary lymphoedema have been identified through linkage analysis in large families with multiple affected subjects. Studies aimed at isolating additional genes responsible for primary lymphoedema have been hampered by cohorts comprised primarily of sporadic cases and small affected kindreds. In the absence of genetic heterogeneity, recent development of massively parallel DNA sequencing technology, specifically exome sequencing, has provided novel paradigms for disease gene identification in such cohorts. In this review, we summarize the novel approaches to disease gene discovery with massively parallel sequencing also known as Next Generation Sequencing (NGS), and show how the selection of unrelated affected cases from clinically homogenous phenotypic subclassifications is proving to be a successful approach for disease gene discovery in primary lymphoedema.

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.

Spinal extradural arachnoid cysts in lymphedema-distichiasis syndrome

PURPOSE

Lymphedema-distichiasis syndrome is characterized by the presence of lower limb lymphedema and supernumerary eyelashes arising from the Meibomian glands. Spinal extradural arachnoid cysts have been observed in some families but their true frequency is unknown. The aim of this study is to determine the frequency of spinal extradural arachnoid cysts in lymphedema distichiasis syndrome.

METHODS

We collected clinical information from all 45 living members of a complete family of 48 members and performed molecular analysis of the FOXC2 gene in 30 individuals. We obtained spinal magnetic resonance imaging from all family members with a FOXC2 gene mutation.

RESULTS

Twelve family members carried a mutation in the FOXC2 gene and had clinical features of lymphedema-distichiasis syndrome. Of these, 58% (seven individuals) had extradural arachnoid cysts.

DISCUSSION

We suggest that a follow-up protocol for lymphedema-distichiasis syndrome families should include spinal magnetic resonance imaging for all affected members so that the timing of surgery for removal of these cysts can be optimized.

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.

Lymphatic development

The lymphatic system is essential for fluid homeostasis, immune responses, and fat absorption, and is involved in many pathological processes, including tumor metastasis and lymphedema. Despite its importance, progress in understanding the origins and early development of this system has been hampered by lack of defining molecular markers and difficulties in observing lymphatic cells in vivo and performing genetic and experimental manipulation of the lymphatic system. Recent identification of new molecular markers, new genes with important functional roles in lymphatic development, and new experimental models for studying lymphangiogenesis has begun to yield important insights into the emergence and assembly of this important tissue. This review focuses on the mechanisms regulating development of the lymphatic vasculature during embryogenesis.

Linkage and sequence analysis indicate that CCBE1 is mutated in recessively inherited generalised lymphatic dysplasia

Generalised lymphatic dysplasia (GLD) is characterised by extensive peripheral lymphoedema with visceral involvement. In some cases, it presents in utero with hydrops fetalis. Autosomal dominant and recessive inheritance has been reported. A large, non-consanguineous family with three affected siblings with generalised lymphatic dysplasia is presented. One child died aged 5 months, one spontaneously miscarried at 17 weeks gestation, and the third has survived with extensive lymphoedema. All three presented with hydrops fetalis. There are seven other siblings who are clinically unaffected. Linkage analysis produced two loci on chromosome 18, covering 22 Mb and containing 150 genes, one of which is CCBE1. A homozygous cysteine to serine change in CCBE1 has been identified in the proband, in a residue that is conserved across species. High density SNP analysis revealed homozygosity (a region of 900 kb) around the locus for CCBE1 in all three affected cases. This indicates a likely ancestral mutation that is common to both parents; an example of a homozygous mutation representing Identity by Descent (IBD) in this pedigree. Recent studies in zebrafish have shown this gene to be required for lymphangiogenesis and venous sprouting and are therefore supportive of our findings. In view of the conserved nature of the cysteine, the nature of the amino acid change, the occurrence of a homozygous region around the locus, the segregation within the family, and the evidence from zebrafish, we propose that this mutation is causative for the generalised lymphatic dysplasia in this family, and may be of relevance in cases of non-immune hydrops fetalis.

Sporadic in utero generalized edema caused by mutations in the lymphangiogenic genes VEGFR3 and FOXC2

OBJECTIVES

To investigate the genetic causes of idiopathic sporadic prenatal generalized edema.

STUDY DESIGN

In a series of 12 patients, in whom in utero generalized skin edema or hydrops fetalis had been diagnosed, we screened 3 lymphangiogenic genes, VEGFR3, FOXC2, and SOX18.

RESULTS

In 3 of the patients, we identified a mutation: 2 in VEGFR3 and 1 in FOXC2. Two of the mutations were de novo and one was either de novo or nonpenetrant inherited. In these patients, the generalized edema resorbed spontaneously, either in utero or after birth. In the 2 individuals with a VEGFR3 mutation, edema remained limited to lower limbs.

CONCLUSIONS

Mutations in the VEGFR3 and FOXC2 genes account for a subset of patients with unexplained in utero generalized subcutaneous edema and hydrops fetalis without family history of lymphedema. Lymphangiogenic genes should be screened for mutations in sporadic patients diagnosed with fetal edema.

Lymphatic endothelial cells, lymphedematous lymphangiogenesis, and molecular control of edema formation

Lymphedema, defined as the abnormal accumulation of protein-rich fluid in soft tissues, results from the dysfunction of lymphatic system, an imbalance between lymph formation and its absorption into the initial lymphatics. Primary lymphedema occurs rarely on idiopathic or developmental abnormalities, especially hypoplasia or aplasia of lymphatics. Secondary lymphedema commonly develops when lymph transport is impaired due to lymphatic damage or resection of lymph nodes in surgery, infection, and radiation. Lymphatic endothelial cells (LECs) actively participate in the phenotypic consequences of a deranged lymphangiogenesis relating to tissue fluid accumulation in the pathogenesis of lymphedema. Recent insights into molecular genetic bases have shown an updated genotype-phenotype correlation between lymphangiogenesis, lymphatic function, and lymphedema. FOXC2, EphrinB2, VEGFR-3, VEGF-C, angiopoietin-2, Prox-1 and podoplanin have proved to be important factors of the genetic cascade linking to hereditary lymphedema, and embryonic and postnatal lymphatic development. FOXC2 may have a key role in regulating interactions between LECs and smooth muscle cells, and in the morphogenesis of lymphatic valves. Reduced VEGFR-3 tyrosine kinase activity and subsequent failure in transducing sufficient physiological VEGF-C/-D signals may affect LEC function and structure in the intercellular junctions and peri-lymphatic components. Identification of genetic markers in humans and animal models would facilitate the management of environmental factors influencing the expression and severity of lymphedema, and provide a basis for developing novel targeted therapies for the disease.

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.

Importance of forkhead transcription factor Fkhl18 for development of testicular vasculature

Forkhead transcription factors are characterized by a winged helix DNA binding domain, and the members of this family are classified into 20 subclasses by phylogenetic analyses. Fkhl18 is structurally unique, and is classified into FoxS subfamily. We found Fkhl18 expression in periendothelial cells of the developing mouse fetal testis. In an attempt to clarify its function, we generated mice with Fkhl18 gene disruption. Although KO mice developed normally and were fertile in both sexes, we frequently noticed unusual blood accumulation in the fetal testis. Electron microscopic analysis demonstrated frequent gaps, measuring 100-400 nm, in endothelial cells of blood vessels. These gaps probably represented ectopic apoptosis of testicular periendothelial cells, identified by caspase-3 expression, in KO fetuses. No apoptosis of endothelial cells was noted. Fkhl18 suppressed the transcriptional activity of FoxO3a and FoxO4. Considering that Fas ligand gene expression is activated by Foxs, the elevated activity of Foxs in the absence of Fkhl18 probably explains the marked apoptosis of periendothelial cells in Fkhl18 KO mice.

Animal models for the molecular and mechanistic study of lymphatic biology and disease

The development of animal model systems for the study of the lymphatic system has resulted in an explosion of information regarding the mechanisms governing lymphatic development and the diseases associated with lymphatic dysfunction. Animal studies have led to a new molecular model of embryonic lymphatic vascular development, and have provided insight into the pathophysiology of both inherited and acquired lymphatic insufficiency. It has become apparent, however, that the importance of the lymphatic system to human disease extends, beyond its role in lymphedema, to many other diverse pathologic processes, including, very notably, inflammation and tumor lymphangiogenesis. Here, we have undertaken a systematic review of the models as they relate to molecular and functional characterization of the development, maturation, genetics, heritable and acquired diseases, and neoplastic implications of the lymphatic system. The translation of these advances into therapies for human diseases associated with lymphatic dysfunction will require the continued study of the lymphatic system through robust animal disease models that simulate their human counterparts.

Spinal extradural arachnoid cysts associated with distichiasis and lymphedema

Spinal extradural arachnoid cysts (SEDAC) are lesions communicating to the subarachnoid space of the spinal canal via a dural defect. SEDAC occupies intraspinal space and sometimes causes neurological disturbances. Although most reported cases are sporadic, several familial cases have been described, suggesting a genetic etiology. Here we report on a family with SEDAC inherited in an autosomal dominant mode. Detailed study showed that the family has the lymphedema-distichiasis syndrome. Among family members examined, a total of ten in two generations manifested all or some of the following features: SEDAC, distichiasis and lymphedema. Seven had spinal cysts, four had both SEDAC and distichiasis, and one had SEDAC distichiasis and lymphedema; three did not have SEDAC. These findings, together with rarity of both distichiasis and lymphedema in the general population, support that all of the ten members were affected with one clinical entity, the lymphedema-distichiasis syndrome. The distribution of features illustrates the variable expressivity of clinical manifestations. Although FOXC2 mutation analysis was not performed in our family, it is likely that SEDAC is a component manifestation of lymphedema-distichiasis syndrome and more consistent in our family than those reported.

Lymphedema, cardiac septal defects, and characteristic facies: Possible new case of Irons–Bianchi syndrome

We describe a Dutch girl with fetal hydrops, congenital lymphedema of the lower legs, complex congenital cardiac malformation, and a typical face with epicanthal folds. This particular combination of symptoms has been previously described by Irons and Bianchi in 1996. Our report confirms their observation and suggests that this particular constellation of symptoms may constitute a new syndrome. Molecular analysis confirms this statement by demonstrating absence of mutations in several genes known to be involved in syndromes with lymphedema.

Linkage to the FOXC2 region of chromosome 16 for varicose veins in otherwise healthy, unselected sibling pairs

BACKGROUND

The FOXC2 gene on 16q24 is mutated in lymphoedema distichiasis (LD), in which varicose veins (VV) are a common feature. We hypothesised that this gene might be implicated in the development of VV in the normal population, therefore, after performing a classical twin study, we tested for linkage and association in white women. We also tested for linkage with haemorrhoids (H), as a separate venous anomaly at the same locus.

METHODS

A total of 2060 complete female twin pairs aged 18-80 years from the St Thomas' Adult UK Twin registry replied to questions on VV and H as part of a broader postal survey of 6600 twins (62% response rate). Dizygotic female twin pairs were tested for linkage and association to the candidate marker D16S520 (1903 individuals genotyped), which is located about 80 kb from FOXC2.

RESULTS

Casewise concordance rates were significantly higher for monozygotic than dizygotic twins for both phenotypes (VV 67% v 45%; p = 2.2x10(-6); H 68% v 59%; p = 0.01; H including during pregnancy 73% v 64%; p = 2.1x10(-4)), corresponding to additive genetic heritabilities in liability of 86% (95% confidence interval (CI) 73% to 99%) for VV and 56-61% for H (95% CI 43% to 73%). The presence of VV and H were significantly correlated. We found significant evidence of linkage to the marker for VV (MLS(ASP) = 1.37, p = 0.01; GLM(ASP/DSP) Z = 3.17 p = 0.002), but no association. Both linkage and association tests were negative for H. The combined phenotype of having VV and H did not show any evidence of linkage or association.

CONCLUSION

These results demonstrate VV and H to be heritable, related conditions, and the data strongly suggest FOXC2 to be implicated in the development of VV in the general population.

Type I congenital multiple intraspinal extradural cysts associated with distichiasis and lymphedema syndrome

OBJECTIVE

The hereditary syndrome of multiple congenital intraspinal cysts associated with distichiasis, lymphedema and other congenital deformities is extremely rare. Modern imaging techniques have promoted the non-invasive diagnosis of spinal pathology and paved the way for better surgical planning. We reviewed the clinical data, imaging studies and treatment outcomes of a 12-year-old boy with this syndrome.

CLINICAL PRESENTATION

Progressive spastic paraparesis with signs of spinal cord compression leading to frequent falls. This was associated with bilateral double row of eyelashes and pretibial edema. The MRI of thoracic spine depicted two large elongated extradural lesions extending from D5-D10 with signal intensity compatible with cerebrospinal fluid leading to severe compression of the spinal cord dorsally.

TREATMENT

Laminotomy and complete microsurgical excision of the cysts resulted in a fast and full clinical recovery of his neurological deficit.

CONCLUSION

Type I congenital intraspinal cysts is a rare etiology of cord compression syndrome and may be associated with distichiasis, lymphedema and other congenital deformaties. It has several characteristics, which include the higher incidence in thoracic spine and younger age group, disproportional sever motor deficit as compared with sensory disturbances and the excellent clinical recovery following successful surgical treatment.