The establishment of a predictive mutational model of the forkhead domain through the analyses of FOXC2 missense mutations identified in patients with hereditary lymphedema with distichiasis

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.

Development and physiological functions of the lymphatic system: insights from human genetic studies of primary lymphedema

Primary lymphedema is a long-term (chronic) condition characterized by tissue lymph retention and swelling that can affect any part of the body, although it usually develops in the arms or legs. Due to the relevant contribution of the lymphatic system to human physiology, while this review mainly focuses on the clinical and physiological aspects related to the regulation of fluid homeostasis and edema, clinicians need to know that the impact of lymphatic dysfunction with a genetic origin can be wide ranging. Lymphatic dysfunction can affect immune function so leading to infection; it can influence cancer development and spread, and it can determine fat transport so impacting on nutrition and obesity. Genetic studies and the development of imaging techniques for the assessment of lymphatic function have enabled the recognition of primary lymphedema as a heterogenic condition in terms of genetic causes and disease mechanisms. In this review, the known biological functions of several genes crucial to the development and function of the lymphatic system are used as a basis for understanding normal lymphatic biology. The disease conditions originating from mutations in these genes are discussed together with a detailed clinical description of the phenotype and the up-to-date knowledge in terms of disease mechanisms acquired from in vitro and in vivo research models.

Loss of Foxc1 and Foxc2 function in chondroprogenitor cells disrupts endochondral ossification

Endochondral ossification initiates the growth of the majority of the mammalian skeleton and is tightly controlled through gene regulatory networks. The forkhead box transcription factors Foxc1 and Foxc2 regulate aspects of osteoblast function in the formation of the skeleton, but their roles in chondrocytes to control endochondral ossification are less clear. Here, we demonstrate that Foxc1 expression is directly regulated by the activity of SRY (sex-determining region Y)-box 9, one of the earliest transcription factors to specify the chondrocyte lineage. Moreover, we demonstrate that elevated expression of Foxc1 promotes chondrocyte differentiation in mouse embryonic stem cells and loss of Foxc1 function inhibits chondrogenesis in vitro. Using chondrocyte-targeted deletion of Foxc1 and Foxc2 in mice, we reveal a role for these factors in chondrocyte differentiation in vivo. Loss of both Foxc1 and Foxc2 caused a general skeletal dysplasia predominantly affecting the vertebral column. The long bones of the limbs were smaller, mineralization was reduced, and organization of the growth plate was disrupted; in particular, the stacked columnar organization of the proliferative chondrocyte layer was reduced in size and cell proliferation was decreased. Differential gene expression analysis indicated disrupted expression patterns of chondrogenesis and ossification genes throughout the entire process of endochondral ossification in chondrocyte-specific Foxc1/Foxc2 KO embryos. Our results suggest that Foxc1 and Foxc2 are required for normal chondrocyte differentiation and function, as loss of both genes results in disorganization of the growth plate, reduced chondrocyte proliferation, and delays in chondrocyte hypertrophy that prevents ossification of the skeleton.

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.

Heterozygous variants that disturb the transcriptional repressor activity of FOXP4 cause a developmental disorder with speech/language delays and multiple congenital abnormalities

PURPOSE

Heterozygous pathogenic variants in various FOXP genes cause specific developmental disorders. The phenotype associated with heterozygous variants in FOXP4 has not been previously described.

METHODS

We assembled a cohort of eight individuals with heterozygous and mostly de novo variants in FOXP4: seven individuals with six different missense variants and one individual with a frameshift variant. We collected clinical data to delineate the phenotypic spectrum, and used in silico analyses and functional cell-based assays to assess pathogenicity of the variants.

RESULTS

We collected clinical data for six individuals: five individuals with a missense variant in the forkhead box DNA-binding domain of FOXP4, and one individual with a truncating variant. Overlapping features included speech and language delays, growth abnormalities, congenital diaphragmatic hernia, cervical spine abnormalities, and ptosis. Luciferase assays showed loss-of-function effects for all these variants, and aberrant subcellular localization patterns were seen in a subset. The remaining two missense variants were located outside the functional domains of FOXP4, and showed transcriptional repressor capacities and localization patterns similar to the wild-type protein.

CONCLUSION

Collectively, our findings show that heterozygous loss-of-function variants in FOXP4 are associated with an autosomal dominant neurodevelopmental disorder with speech/language delays, growth defects, and variable congenital abnormalities.

Insights into the pathogenicity of missense variants in the forkhead domain of FOX proteins underlying Mendelian disorders

Forkhead box (FOX) proteins are members of a conserved family of transcription factors. Pathogenic variants in FOX genes have been shown to be responsible for several human genetic diseases. Here, we have studied the molecular and structural features of germline pathogenic variants in seven FOX proteins involved in Mendelian disorders and compared them with those of variants present in the general population (gnomAD). Our study shows that the DNA-binding domain of FOX proteins is particularly sensitive to damaging variation, although some family members show greater mutational tolerance than others. Next, we set to demonstrate that this tolerance depends on the inheritance mode of FOX-linked disorders. Accordingly, genes whose variants underlie recessive conditions are supposed to have a greater tolerance to variation. This is what we found. As expected, variants responsible for disorders with a dominant inheritance pattern show a higher degree of pathogenicity compared to those segregating in the general population. Moreover, we show that pathogenic and likely pathogenic variants tend to affect mutually exclusive sites with respect to those reported in gnomAD. The former also tend to affect sites with lower solvent exposure and a higher degree of conservation. Our results show the value of using publicly available databases and bioinformatics to gain insights into the molecular and structural bases of disease-causing genetic variation.

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.

Role of FOXC2 and PITX2 rare variants associated with mild functional alterations as modifier factors in congenital glaucoma

Congenital glaucoma (CG) is a severe and inherited childhood optical neuropathy that leads to irreversible visual loss and blindness in children. CG pathogenesis remains largely unexplained in most patients. Herein we have extended our previous studies to evaluate the role of FOXC2 and PITX2 variants in CG. Variants of the proximal promoter and transcribed sequence of these two genes were analyzed by Sanger sequencing in a cohort of 133 CG families. To investigate possible oligogenic inheritance involving FOXC2 or PITX2 and CYP1B1, we also analyzed FOXC2 and PITX2 variants in a group of 25 CG cases who were known to carry CYP1B1 glaucoma-associated genotypes. The functional effect of three identified variants was assessed by transactivation luciferase reporter assays, protein stability and subcellular localization analyses. We found eight probands (6.0%) who carried four rare FOXC2 variants in the heterozygous state. In addition, we found an elevated frequency (8%) of heterozygous and rare PITX2 variants in the group of CG cases who were known to carry CYP1B1 glaucoma-associated genotypes, and one of these PITX2 variants arose de novo. To the best of our knowledge, two of the identified variants (FOXC2: c.1183C>A, p.(H395N); and PITX2: c.535C>A, p.(P179T)) have not been previously identified. Examination of the genotype-phenotype correlation in this group suggests that the presence of the infrequent PITX2 variants increase the severity of the phenotype. Transactivation reporter analyses showed partial functional alteration of three identified amino acid substitutions (FOXC2: p.(C498R) and p.(H395N); PITX2: p.(P179T)). In summary, the increased frequency in PCG patients of rare FOXC2 and PITX2 variants with mild functional alterations, suggests they play a role as putative modifier factors in this disease further supporting that CG is not a simple monogenic disease and provides novel insights into the complex pathological mechanisms that underlie CG.

A Phosphomimetic Study Implicates Ser557 in Regulation of FOXP2 DNA Binding

FOXP2 is a transcription factor expressed in multiple tissues during embryonic development. FOXP2 regulates transcription by binding to DNA at its DNA binding domain, the forkhead domain (FHD) through the recognition helix. Ser557 is a residue located within the recognition helix that has the potential to become phosphorylated posttranslationally. In this study we investigated whether phosphorylation of Ser557 can influence the structure and DNA binding of the FOXP2 FHD. We did this by constructing S557E, a phosphomimetic mutant, and comparing its behaviour to the wild type. The mutation did not affect the secondary or tertiary structure of the protein although it did decrease the propensity of the FOXP2 FHD to form dimers. Most notably, the mutation showed significantly reduced DNA binding compared to the wild type as detected using electrophoretic mobility shift assays. Molecular docking was also performed in which the wild type, phosphomimetic mutant and phosphorylated wild-type were docked to DNA and their interactions with DNA were compared. These results indicated that the wild type forms more interactions with the DNA and that the phosphomimetic mutant as well as the phosphorylated wild type did not associate as favourably with the DNA. This indicates that phosphorylation of Ser557 could disrupt DNA binding likely due to electrostatic and steric hindrance. This suggests that phosphorylation of Ser557 in the FOXP2 FHD could act as a control mechanism for FOXP2 and ultimately could be involved in regulation of transcription.

Genetic and Functional Analyses of Two Missense Mutations in the Transcription Factor FOXL2 in Two Chinese Families with Blepharophimosis-Ptosis-Epicanthus Inversus Syndrome

BACKGROUND

Blepharophimosis-ptosis-epicanthus inversus syndrome (BPES) is a rare autosomal-dominant inherited disease. This study was carried out to investigate the genetic and functional changes within the FOXL2 gene in two Chinese families with BPES.

MATERIALS AND METHODS

DNA was extracted from the peripheral blood of 26 persons from two different Chinese BPES families (13 of which were affected), as well as 200 cataract patients to act as normal controls. FOXL2 gene mutations were detected using polymerase chain reaction (PCR) and DNA sequencing techniques. Bioinformatic analyses were performed to analyze the structures and functions of the mutant proteins. Wild-type and mutant FOXL2 genes were subcloned into pEGFP-N1 and pCDB vectors and then transfected into COS7 and HEK293T cell lines. We observed protein subcellular localization, and used quantitative real-time (qRT)-PCR and western blots to assess regulation of the target OSR2 gene.

RESULTS

We detected two novel missense mutations, c.162G>T (p.Lys54Asn) and c.308G>A (p.Arg103His), in the FOXL2 gene; one in each of the study families. Bioinformatic analyses indicated no obvious differences between the wild-type and mutant protein structures. However, they did predict that the two mutations were likely damaging to protein function. We found that the two mutated proteins were both largely distributed within the nucleus and that there was little found in the cytoplasm. The OSR2 mRNA content decreased significantly when the plasmids carrying the c.162G>T and c.308G>A were transfected into COS7 and HEK293 cell lines, when compared to the empty and the wild-type FOXL2 carrier. Western blot analyses indicated, that after transfecting the c.162G>T mutation, the OSR2 protein level was relatively similar to the wild-type, but that the cells transfected with the c.308G>A mutation showed significantly decreased levels of the OSR2 protein.

CONCLUSIONS

Our study broadens the BPES gene mutation spectrum and suggests a possible mechanism of action. It also provides reference data for the further studies of BPES.

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.

Equivalent missense variant in the FOXP2 and FOXP1 transcription factors causes distinct neurodevelopmental disorders

The closely related paralogues FOXP2 and FOXP1 encode transcription factors with shared functions in the development of many tissues, including the brain. However, while mutations in FOXP2 lead to a speech/language disorder characterized by childhood apraxia of speech (CAS), the clinical profile of FOXP1 variants includes a broader neurodevelopmental phenotype with global developmental delay, intellectual disability, and speech/language impairment. Using clinical whole-exome sequencing, we report an identical de novo missense FOXP1 variant identified in three unrelated patients. The variant, p.R514H, is located in the forkhead-box DNA-binding domain and is equivalent to the well-studied p.R553H FOXP2 variant that cosegregates with CAS in a large UK family. We present here for the first time a direct comparison of the molecular and clinical consequences of the same mutation affecting the equivalent residue in FOXP1 and FOXP2. Detailed functional characterization of the two variants in cell model systems revealed very similar molecular consequences, including aberrant subcellular localization, disruption of transcription factor activity, and deleterious effects on protein interactions. Nonetheless, clinical manifestations were broader and more severe in the three cases carrying the p.R514H FOXP1 variant than in individuals with the p.R553H variant related to CAS, highlighting divergent roles of FOXP2 and FOXP1 in neurodevelopment.

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.

Novel FOXC2 Mutation in Hereditary Distichiasis Impairs DNA-Binding Activity and Transcriptional Activation

Distichiasis presents as double rows of eyelashes arising from aberrant differentiation of the meibomian glands of the eyelids, and it may be sporadic or hereditary. FOXC2 gene mutations in hereditary distichiasis are rarely reported. Here, we examined two generations of a Chinese family with hereditary distichiasis but without lymphedema or other features of LD syndrome. The FOXC2 gene was amplified and sequenced in all family members. Subcellular localization and luciferase assays were performed to assess the activity of the mutant FOXC2 protein. Clinical examinations showed distichiasis, lower eyelid ectropion, congenital ptosis and photophobia in all affected individuals. Sequence analysis revealed a novel frameshift mutation, c.964_965insG, in the coding region of the FOXC2 gene. This mutation caused protein truncation due to the presence of a premature stop codon. A fluorescence assay showed that this mutation did not change the nuclear localization of the protein. However, it impaired DNA-binding activity and decreased transcriptional activation. This is the first report of a FOXC2 mutation in hereditary distichiasis in the Chinese population. The findings of our study expand the FOXC2 mutation spectrum and contribute to the understanding of the genotype-phenotype correlation of this disease.

Overexpression of transcription factor FOXC2 in cultured human podocytes upregulates injury markers and increases motility

Obesity and diabetes-related kidney diseases associate with renal failure and cardiovascular morbidity, and represent a major health issue worldwide. However, the molecular mechanisms leading to their development remain poorly understood. We observed increased expression of transcription factor FoxC2 in the podocytes of obese Zucker rats that are insulin resistant and albuminuric. We also found that depletion of adiponectin, an adipocyte-derived hormone whose secretion is decreased in obesity, upregulated FOXC2 in differentiated human podocytes in vitro. Overexpression of FOXC2 in cultured human podocytes led to increased nuclear expression of FOXC2 associated with a change of cellular morphology. This was accompanied by upregulation of vimentin, a key mesenchymal marker, and active beta-catenin, associated with podocyte injury. We also observed re-organization of the actin cytoskeleton, disrupted localization of the tight junction protein ZO-1, and increased motility of podocytes overexpressing FOXC2. These data indicate that the expression of FOXC2 in podocytes needs to be tightly regulated, and that its overexpression induces a chain of cellular events leading to podocyte dysfunction. These changes may lead to podocyte detachment and depletion ultimately contributing to albuminuria. We also suggest a novel molecular mechanism linking obesity-induced decrease in adiponectin to podocyte dysfunction via upregulation of FOXC2.

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.

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.

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.

Phosphorylation regulates FOXC2-mediated transcription in lymphatic endothelial cells

One of the key mechanisms linking cell signaling and control of gene expression is reversible phosphorylation of transcription factors. FOXC2 is a forkhead transcription factor that is mutated in the human vascular disease lymphedema-distichiasis and plays an essential role in lymphatic vascular development. However, the mechanisms regulating FOXC2 transcriptional activity are not well understood. We report here that FOXC2 is phosphorylated on eight evolutionarily conserved proline-directed serine/threonine residues. Loss of phosphorylation at these sites triggers substantial changes in the FOXC2 transcriptional program. Through genome-wide location analysis in lymphatic endothelial cells, we demonstrate that the changes are due to selective inhibition of FOXC2 recruitment to chromatin. The extent of the inhibition varied between individual binding sites, suggesting a novel rheostat-like mechanism by which expression of specific genes can be differentially regulated by FOXC2 phosphorylation. Furthermore, unlike the wild-type protein, the phosphorylation-deficient mutant of FOXC2 failed to induce vascular remodeling in vivo. Collectively, our results point to the pivotal role of phosphorylation in the regulation of FOXC2-mediated transcription in lymphatic endothelial cells and underscore the importance of FOXC2 phosphorylation in vascular development.

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.

Generation of mouse ES cell lines engineered for the forced induction of transcription factors

Here we report the generation and characterization of 84 mouse ES cell lines with doxycycline-controllable transcription factors (TFs) which, together with the previous 53 lines, cover 7–10% of all TFs encoded in the mouse genome. Global gene expression profiles of all 137 lines after the induction of TFs for 48 hrs can associate each TF with the direction of ES cell differentiation, regulatory pathways, and mouse phenotypes. These cell lines and microarray data provide building blocks for a variety of future biomedical research applications as a community resource.

A complex regulatory network of transcription factors critical for ocular development and disease

The PITX2 'homeobox' and FOXC1 and FOXC2 'forkhead box' transcription factors are critical for eye development and cause human ocular diseases when mutated. We have identified biochemical and genetic links between these transcription factors and a transcriptional regulator protein PRKC apoptosis Wilms' tumor 1 regulator (PAWR) that we propose to functionally connect all these proteins in a common pathway critically involved in eye development. We discovered all binary physical interactions between FOXC1, PITX2, FOXC2 and PAWR. Importantly, PAWR modulates the abilities of PITX2, FOXC1 and FOXC2 to activate their genetic targets. Together with either FOXC1 or FOXC2, PAWR increases PITX2 activity. PAWR reduces PITX2 activity in the absence of FOXC1 or FOXC2. At the same time, PAWR also exerts different regulatory effects on different FOXC target sites. Furthermore, morpholino knockdown of pitx2, foxc1 and pawr in zebrafish indicate that PAWR, FOXC1 and PITX2 genetically interact, and are in the same developmental pathway. These data for the first time tie PITX2, FOXC1, FOXC2 and PAWR into a common regulatory pathway. We have therefore identified a functional link between three transcription factors, modulated by PAWR, which we propose underlies the similar ocular phenotypes and glaucoma pathology caused by mutations of these genes.

Foxd3 is an essential Nodal-dependent regulator of zebrafish dorsal mesoderm development

Establishment of the embryonic mesoderm is dependent on integration of multiple signaling and transcriptional inputs. We report that the transcriptional regulator Foxd3 is essential for dorsal mesoderm formation in zebrafish, and that this function is dependent on the Nodal pathway. Foxd3 gain-of-function results in expanded dorsal mesodermal gene expression, including the Nodal-related gene cyclops, and body axis dorsalization. Foxd3 knockdown embryos displayed reduced expression of cyclops and mesodermal genes, axial defects similar to Nodal pathway loss-of-function, and Nodal pathway activation rescued these phenotypes. In MZoep mutants inactive for Nodal signaling, Foxd3 did not rescue mesoderm formation or axial development, indicating that the mesodermal function of Foxd3 is dependent on an active downstream Nodal pathway. A previously identified foxd3 mutant, sym1, was described as a predicted null mutation with neural crest defects, but no mesodermal or axial phenotypes. We find that Sym1 protein retains activity and can induce strong mesodermal expansion and axial dorsalization. A subset of sym1 homozygotes displays axial defects and reduced cyclops and mesodermal gene expression, and penetrance of the mesodermal phenotypes is enhanced by Foxd3 knockdown. Therefore, sym1 is a hypomorphic allele, and reduced Foxd3 function results in a reduction of cyclops expression, and subsequent mesodermal and axial defects. These results demonstrate that Foxd3 is an essential upstream regulator of the Nodal pathway in zebrafish dorsal mesoderm development and establish a broadly conserved role for Foxd3 in vertebrate mesodermal development.

Mesenchymal-epithelial transition in epithelial response to injury: the role of Foxc2

Overexpression of the forkhead family transcription factor Foxc2 has been shown to activate epithelial-mesenchymal transition (EMT) and correlate with tumor metastasis. In this study, we show that both mRNA and protein levels of Foxc2 increase 1 day after kidney ischemia/reperfusion in sublethally injured tubular cells and that the protein is located in the cytoplasm rather than the nucleus of these cells. in vitro studies of cultured tubular cells confirm the cytoplasmic location of Foxc2 and show that increased cytoplasmic expression of Foxc2 correlates with epithelial differentiation rather than dedifferentiation. Silencing of Foxc2 by RNAi in these cells led to EMT and increased cell migration. In contrast, Foxc2 is found in both the nucleus and cytoplasm of cultured fibroblasts, with RNAi leading to increased expression of epithelial markers and impaired cell migration. Consistent with a subcellular localization dependence of Foxc2 function, overexpression of Foxc2 in renal epithelial cells resulted in de novo nuclear expression of the protein and promotion of a mesenchymal/fibroblast phenotype. These results suggest that Foxc2 may have regulatory functions independent of its nuclear transcriptional activity and that upregulation of endogenous Foxc2 in the cytoplasm of injured tubular cells activates epithelial cell redifferentiation rather than dedifferentiation during organ repair.

FOXL2 mutations and genomic rearrangements in BPES

The FOXL2 gene is one of 10 forkhead genes, the mutations of which lead to human developmental disorders, often with ocular manifestations. Mutations in FOXL2 are known to cause blepharophimosis syndrome (BPES), an autosomal dominant eyelid malformation associated (type I) or not (type II) with ovarian dysfunction, leading to premature ovarian failure (POF). In addition, a few mutations have been described in patients with isolated POF. Here, we review all currently described FOXL2 sequence variations and genomic rearrangements in BPES and POF. Using a combined mutation detection approach, it is possible to identify the underlying genetic defect in a major proportion (88%) of typical BPES patients. Of all genetic defects found in our BPES cohort, intragenic mutations represent 81%. They include missense changes, frameshift and nonsense mutations, in-frame deletions, and duplications, that are distributed along the single-exon gene. Genomic rearrangements comprising both deletions encompassing FOXL2 and deletions located outside its transcription unit, represent 12% and 5% of all genetic defects in our BPES cohort, respectively. One of the challenges of genetic testing in BPES is the establishment of genotype-phenotype correlations, mainly with respect to the ovarian phenotype. Genetic testing should be performed in the context of genetic counseling, however, and should be systematically complemented by a multidisciplinary clinical follow-up. Another challenge for health care professionals involved in BPES is the treatment of the eyelid phenotype and the prevention or treatment of POF.

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.

Missense mutations in the forkhead domain of FOXL2 lead to subcellular mislocalization, protein aggregation and impaired transactivation

Mutations of the FOXL2 gene have been shown to cause blepharophimosis syndrome (BPES), characterized by an eyelid malformation associated with premature ovarian failure or not. Recently, polyalanine expansions and truncating FOXL2 mutations have been shown to lead to protein mislocalization, aggregation and altered transactivation. Here, we study the molecular consequences of 17 naturally occurring FOXL2 missense mutations. Most of them map to the conserved DNA-binding forkhead domain (FHD). The subcellular localization and aggregation pattern of the mutant FOXL2 proteins in COS-7 cells was variable and ranged from a diffuse nuclear distribution like the wild-type to extensive nuclear aggregation often in combination with cytoplasmic mislocalization and aggregation. We also studied the transactivation capacity of the mutants in FOXL2 expressing granulosa-like cells (KGN). Several mutants led to a loss-of-function, while others are suspected to induce a dominant negative effect. Interestingly, one mutant that is located outside the FHD (S217F), appeared to be hypermorphic and had no effect on intracellular protein distribution. This mutation gives rise to a mild BPES phenotype. In general, missense mutations located in the FHD lead to classical BPES and cannot be correlated with expression of the ovarian phenotype. However, a potential predictive value of localization and transactivation assays in the making of genotype-phenotype correlations is proposed. This is the first study to demonstrate that a significant number of missense mutations in the FHD of FOXL2 lead to mislocalization, protein aggregation and altered transactivation, and to provide insights into the pathogenesis associated with missense mutations of FOXL2 in human disease.

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

Background— Mutations in the FOXC2 gene cause lymphedema distichiasis, an inherited primary lymphedema in which a significant number of patients have varicose veins. Because lymphedema distichiasis is believed to be caused by lymphatic valve failure (reflux), and FOXC2 is highly expressed on venous valves in mouse embryos, we tested the hypothesis that FOXC2 mutations may be linked to venous valve failure and reflux.

Methods and Results— The venous system of the leg was investigated with Duplex ultrasound. Pathological reflux was recorded by color Duplex ultrasound in all 18 participants with a FOXC2 mutation, including 3 without lymphedema. Every participant with a mutation in FOXC2 showed reflux in the great saphenous vein (n=18), compared with only 1 of 12 referents (including 10 family members; P <0.0001, Fisher exact test). Deep vein reflux was recorded in 14 of 18 participants.

Conclusions— FOXC2 is the first gene in which mutations have been strongly associated with primary venous valve failure in both the superficial and deep veins in the lower limb. This gene appears to be important for the normal development and maintenance of venous and lymphatic valves.

Functional genetic analysis of mutations implicated in a human speech and language disorder

Mutations in the FOXP2 gene cause a severe communication disorder involving speech deficits (developmental verbal dyspraxia), accompanied by wide-ranging impairments in expressive and receptive language. The protein encoded by FOXP2 belongs to a divergent subgroup of forkhead-box transcription factors, with a distinctive DNA-binding domain and motifs that mediate hetero- and homodimerization. Here we report the first direct functional genetic investigation of missense and nonsense mutations in FOXP2 using human cell-lines, including a well-established neuronal model system. We focused on three unusual FOXP2 coding variants, uniquely identified in cases of verbal dyspraxia, assessing expression, subcellular localization, DNA-binding and transactivation properties. Analysis of the R553H forkhead-box substitution, found in all affected members of a large three-generation family, indicated that it severely affects FOXP2 function, chiefly by disrupting nuclear localization and DNA-binding properties. The R328X truncation mutation, segregating with speech/language disorder in a second family, yields an unstable, predominantly cytoplasmic product that lacks transactivation capacity. A third coding variant (Q17L) observed in a single affected child did not have any detectable functional effect in the present study. In addition, we used the same systems to explore the properties of different isoforms of FOXP2, resulting from alternative splicing in human brain. Notably, one such isoform, FOXP2.10+, contains dimerization domains, but no DNA-binding domain, and displayed increased cytoplasmic localization, coupled with aggresome formation. We hypothesize that expression of alternative isoforms of FOXP2 may provide mechanisms for post-translational regulation of transcription factor function.

Functional interactions between FOXC1 and PITX2 underlie the sensitivity to FOXC1 gene dose in Axenfeld-Rieger syndrome and anterior segment dysgenesis

Axenfeld-Rieger ocular dysgenesis is associated with mutations of the human PITX2 and FOXC1 genes, which encode transcription factors of the homeodomain and forkhead types, respectively. We have identified a functional link between FOXC1 and PITX2 which we propose underpins the similar Axenfeld-Rieger phenotype caused by mutations of these genes. FOXC1 and PITX2A physically interact, and this interaction requires crucial functional domains on both proteins: the C-terminal activation domain of FOXC1 and the homeodomain of PITX2. Immunofluorescence further shows PITX2A and FOXC1 to be colocalized within a common nuclear subcompartment. Furthermore, PITX2A can function as a negative regulator of FOXC1 transactivity. This work ties both proteins into a common pathway and offers an explanation of why increased FOXC1 gene dosage produces a phenotype resembling that of PITX2 deletions and mutations. Ocular phenotypes arise despite the deregulated expression of FOXC1-target genes through mutations in FOXC1 or PITX2. Ultimately, PITX2 loss of function mutations have a compound effect: the reduced expression of PITX2-target genes coupled with the extensive activation of FOXC1-regulated targets. Our findings indicate that the functional interaction between FOXC1 and PITX2A underlies the sensitivity to FOXC1 gene dosage in Axenfeld-Rieger syndrome and related anterior segment dysgeneses.