Analysis of the phenotypic abnormalities in lymphoedema-distichiasis syndrome in 74 patients with FOXC2 mutations or linkage to 16q24

Tbx1 is regulated by forkhead proteins in the secondary heart field

Transcriptional regulation in a tissue-specific and quantitative manner is essential for developmental events, including those involved in cardiovascular morphogenesis. Tbx1 is a T-box-containing transcription factor that is responsible for many of the defects observed in 22q11 deletion syndrome in humans. Tbx1 is expressed in the secondary heart field (SHF) and is essential for cardiac outflow tract (OFT) development. We previously reported that Tbx1 is regulated by sonic hedgehog by means of forkhead (Fox) transcription factors in the head mesenchyme and pharyngeal endoderm, but how it is regulated in the SHF is unknown. Here, we show that Tbx1 expression in the SHF is regulated by Fox proteins through a combination of two evolutionarily conserved Fox binding sites in a dose-dependent manner. Cell fate analysis using the Tbx1 enhancer suggests that SHF-derived Tbx1-expressing cells contribute extensively to the right ventricular myocardium as well as the OFT during early development and ultimately give rise to the right ventricular infundibulum, pulmonary trunk, and pulmonary valves. These results suggest that Fox proteins are involved in most, if not all, Tbx1 expression domains and that Tbx1 marks a subset of SHF-derived cells, particularly those that uniquely contribute to the right-sided outflow tract and proximal pulmonary artery.

Milroy disease and the VEGFR-3 mutation phenotype

Primary congenital lymphoedema (Milroy disease) is a rare autosomal dominant condition for which a major causative gene defect has recently been determined. Mutations in the vascular endothelial growth factor receptor 3 (VEGFR-3) gene have now been described in 13 families world-wide. This is a review of the condition based on the clinical findings in 71 subjects from 10 families. All 71 individuals have a mutation in VEGFR-3. Ninety per cent of the 71 individuals carrying a VEGFR-3 mutation showed signs of oedema, which was confined in all cases to the lower limbs. In all but two cases onset of swelling was from birth. Other symptoms and signs included cellulitis (20%), large calibre leg veins (23%), papillomatosis (10%), and upslanting toenails (10%). In males, hydrocoele was the next most common finding after oedema (37%). Thorough clinical examination of these patients indicates that there are few clinical signs in addition to lower limb oedema. Rigorous phenotyping of patients produces a high yield of VEGFR-3 mutations.

Identification of cis-regulatory elements for MECP2 expression

Rett syndrome (RTT) is an X-linked dominant disabling neurodevelopmental disorder caused by loss of function mutations in the MECP2 gene, located at Xq28, which encodes a multifunctional protein. MECP2 expression is regulated in a developmental stage and cell-type-specific manner. The need for tightly controlled MeCP2 levels in brain is strongly suggested by neurologically abnormal phenotypes of mouse models with mild overexpression and by mental retardation in human males with MECP2 duplication. We set out to identify long-range cis-regulatory sequences that differentially regulate MECP2 transcription and, when mutated, may contribute to the pathogenesis of RTT, autism or X-linked mental retardation. By inter-species sequence comparisons, we detected 27 highly conserved non-coding DNA sequences within a 210 kb region covering MECP2. We functionally confirmed four enhancer and two silencer elements by performing luciferase reporter assays in four different human cell lines. The transcription factor binding capability of the identified regulatory elements was tested by gel shift assays. To locate the human MECP2 core promoter, we dissected the promoter region by reporter assays with deletion constructs. We then used chromosome conformation capture methods to document long-range interactions of three enhancers and two silencers with the MECP2 promoter. Acting over distances of up to 130 kb, these elements may influence chromatin configurations and regulate MECP2 transcription. Our study has defined the "MECP2 functional expression module" and identified enhancer and silencer elements that are likely to be responsible for the tissue-specific, developmental stage-specific or splice-variant-specific control of MeCP2 protein expression.

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.

Lymphangiogenesis in development and human disease

The lymphatic vasculature forms a vessel network that drains interstitial fluid from tissues and returns it to the blood. Lymphatic vessels are also an essential part of the body's immune defence. They have an important role in the pathogenesis of several diseases, such as cancer, lymphoedema and various inflammatory conditions. Recent biological and technological developments in lymphatic vascular biology will lead to a better understanding and treatment of these diseases.

Molecular lymphangiogenesis: new players

The function of the lymphatic vascular system as a conduit for immune cells and excess tissue fluid has been known for over a century, but the molecular players involved in lymphangiogenesis--the formation of lymphatic vessels--have remained unknown until recently. Signals mediated by vascular endothelial growth factor-C, the homeodomain transcription factor PROX1 and the forkhead transcription factor FOXC2 have been implicated in the growth, morphogenesis and hierarchic organization of the lymphatic vascular network. Recent results have also shown the importance of the angiopoietin-Tie and ephrin-Eph signaling systems in lymphangiogenesis, suggesting that these pathways regulate the lymphatic vascular system in a manner similar to, yet distinct from, their regulation of angiogenesis. This review provides an overview of the molecular players involved in lymphangiogenesis, with special emphasis on recently discovered molecular mechanisms.

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

The FOX family of transcription factor genes is an evolutionary conserved, yet functionally diverse class of transcription factors that are important for regulation of energy homeostasis, development and oncogenesis. The proteins encoded by FOX genes are characterized by a conserved DNA-binding domain known as the forkhead domain (FHD). To date, disease-causing mutations have been identified in eight human FOX genes. Many of these mutations result in single amino acid substitutions in the FHD. We analyzed the molecular consequences of two disease-causing missense mutations (R121H and S125L) occurring in the FHD of the FOXC2 gene that were identified in patients with hereditary lymphedema with distichiasis (LD) to test the predictive capacity of a FHD structure/function model. On the basis of the FOXC2 solution structure, both FOXC2 missense mutations are located on the DNA-recognition helix of the FHD. A mutation model based on the parologous FOXC1 protein predicts that these FOXC2 missense mutations will impair the DNA-binding and transcriptional activation ability of the FOXC2 protein. When these mutations were analyzed biochemically, we found that both mutations did indeed reduce the DNA binding and transcriptional capacity. In addition, the R121H mutation affected nuclear localization of FOXC2. Together, these data indicate that these FOXC2 missense mutations are functional nulls and that FOXC2 haploinsufficiency underlies hereditary LD and validates the predictive ability of the FOXC1-based FHD mutational model.

Autosomal dominant inheritance of left ventricular outflow tract obstruction

Most nonsyndromic congenital heart malformations (CHMs) in humans are multifactorial in origin, although an increasing number of monogenic cases have been reported recently. We describe here four new families with presumed autosomal dominant inheritance of left ventricular outflow tract obstruction (LVOTO), consisting of hypoplastic left heart (HLHS) or left ventricle (HLV), aortic valve stenosis (AS) and bicuspid aortic valve (BAV), hypoplastic aortic arch (HAA), and coarctation of the aorta (CoA). LVOTO in these families shows a wide clinical spectrum with some family members having severe anomalies such as hypoplastic left heart, and others only minor anomalies such as mild aortic valve stenosis. This supports the suggestion that all anomalies of the LVOTO spectrum are developmentally related and can be caused by a single gene defect.

Lymphoedema-distichiasis and FOXC2: unreported mutations, de novo mutation estimate, families without coding mutations

Lymphoedema-distichiasis (LD) is a syndromic form of primary lymphoedema, where mutations in the gene for the developmental transcription factor FOXC2 have been shown to be causative. The disorder has been considered very rare, but our group has now ascertained 34 families and 11 sporadic cases in the UK. Two families with LD have no mutation in the coding region of FOXC2, although both are consistent with linkage to the FOXC2 locus. A deletion has been ruled out as a possible cause of LD in these families, leaving promoter mutations as the most likely cause. Sixteen previously unpublished mutations are reported, plus an estimate of the frequency of new mutations in this disorder.

A novel frameshift mutation of FOXC2 gene in a family with hereditary lymphedema-distichiasis syndrome associated with renal disease and diabetes mellitus

Lymphedema-distichiasis (LD) syndrome is a clinically variable autosomal dominant disorder. The disorder is caused by mutations in the forkhead transcription factor FOXC2 gene on chromosome band 16q24.3. Here, we report the sequence of the FOXC2 gene in a German-Irish family with LD in six affected relatives over three generations and identify a single adenine base pair insertion at nt 1006--1007. This insertion creates a frameshift mutation that predicts a premature stop at codon 462. In addition to LD, four of the affected family members have renal disease and three have diabetes mellitus (DM), not usually seen in the LD syndrome. Polymorphisms of FOXC2 in diabetics have been studied in different populations. Our sequence analysis of the 5' untranslated region (UTR) C-512T shows the homozygous T allele in all family members tested. The sequencing data in this family suggests the possibility of a novel phenotype-haplotype. This novel phenotype, LD/renal disease/type 2 diabetes, might be the result of a combination of the nt 1006--1007 insA and the upstream UTR homozygous T polymorphism.

Intracranial dural venous anomalies in familial cervical cystic hygroma

This report describes a 3-year-old male with a familial cervical cystic hygroma and intracranial dural venous abnormalities consisting of a falcine sinus between the superior sagittal and the straight sinus with a rudimentary posterior part of the superior sagittal sinus; a prominent occipital sinus with narrowed transverse sinuses; and no intraparenchymal vascular abnormality. We hypothesize that a genetic factor that resulted in familial cervical cystic hygromas may also have caused the intracranial dural venous anomalies.

Long-range control of gene expression: emerging mechanisms and disruption in disease

Transcriptional control is a major mechanism for regulating gene expression. The complex machinery required to effect this control is still emerging from functional and evolutionary analysis of genomic architecture. In addition to the promoter, many other regulatory elements are required for spatiotemporally and quantitatively correct gene expression. Enhancer and repressor elements may reside in introns or up- and downstream of the transcription unit. For some genes with highly complex expression patterns--often those that function as key developmental control genes--the cis-regulatory domain can extend long distances outside the transcription unit. Some of the earliest hints of this came from disease-associated chromosomal breaks positioned well outside the relevant gene. With the availability of wide-ranging genome sequence comparisons, strong conservation of many noncoding regions became obvious. Functional studies have shown many of these conserved sites to be transcriptional regulatory elements that sometimes reside inside unrelated neighboring genes. Such sequence-conserved elements generally harbor sites for tissue-specific DNA-binding proteins. Developmentally variable chromatin conformation can control protein access to these sites and can regulate transcription. Disruption of these finely tuned mechanisms can cause disease. Some regulatory element mutations will be associated with phenotypes distinct from any identified for coding-region mutations.

Foxc2 is expressed in developing lymphatic vessels and other tissues associated with lymphedema–distichiasis syndrome

The molecular events involved in lymphatic development are poorly understood. Hence, the genes responsible for hereditary lymphedema are of great interest due to the potential for providing insights into the mechanisms of lymphatic development, the diagnosis, prevention and treatment of lymphedema, and lymphangiogenesis during tumor growth. Mutations in the FOXC2 transcription factor cause a major form of hereditary lymphedema, the lymphedema-distichiasis syndrome. We have conducted a study of Foxc2 expression during mouse development using immunohistochemistry, and examined its expression in lymphatics compared to its paralog Foxc1 and to Vegfr-3, Prox1 and other lymphatic and blood vascular proteins. We have found that Foxc2 is expressed in lymphatic primordia, jugular lymph sacs, lymphatic collectors and capillaries, as well as in podocytes, developing eyelids and other tissues associated with abnormalities in lymphedema-distichiasis syndrome.

Defective valves and abnormal mural cell recruitment underlie lymphatic vascular failure in lymphedema distichiasis

Lymphatic vessels are essential for the removal of interstitial fluid and prevention of tissue edema. Lymphatic capillaries lack associated mural cells, and collecting lymphatic vessels have valves, which prevent lymph backflow. In lymphedema-distichiasis (LD), lymphatic vessel function fails because of mutations affecting the forkhead transcription factor FOXC2. We report that Foxc2(-/-) mice show abnormal lymphatic vascular patterning, increased pericyte investment of lymphatic vessels, agenesis of valves and lymphatic dysfunction. In addition, an abnormally large proportion of skin lymphatic vessels was covered with smooth muscle cells in individuals with LD and in mice heterozygous for Foxc2 and for the gene encoding lymphatic endothelial receptor, Vegfr3 (also known as Flt4). Our data show that Foxc2 is essential for the morphogenesis of lymphatic valves and the establishment of a pericyte-free lymphatic capillary network and that it cooperates with Vegfr3 in the latter process. Our results indicate that an abnormal interaction between the lymphatic endothelial cells and pericytes, as well as valve defects, underlie the pathogenesis of LD.

Ocular pathology in congenital heart disease

PURPOSE

To describe the ocular findings in subjects with congenital heart disease (CHD).

METHODS

In a prospective study, the same observer examined 240 consecutive patients with CHD admitted to the medical centre. Two independent geneticists performed identification of syndromes.

RESULTS

The commonest anatomic cardiac anomalies were ventricular or atrial septal defects (62), tetralogy of Fallot (39), pulmonary stenosis (25), and transposition of the great arteries (24). The heart lesions were divided physiologically into volume overload (90), cyanotic (87), and obstructive (63). In all, 105 syndromic subjects included the velocardiofacial syndrome (18), Down's syndrome (17), CHARGE association (6), DiGeorge syndrome (5), Williams syndrome (3), Edwards syndrome (3), Noonan syndrome (3), VACTERL association (2), and Patau syndrome (trisomy 13) (2). The paediatric team recognized 51 patients as syndromic. Two independent geneticists recognized additional 54 patients as syndromic. Positive eye findings were present in 55% (132) and included retinal vascular tortuosity (46), optic disc hypoplasia (30), trichomegaly (15), congenital ptosis (12), strabismus (11), retinal haemorrhages (8), prominent eyes (7), and congenital cataract (6). There was a strong correlation between the retinal vascular tortuosity and both a low haematocrit (P=0.000) and a low arterial oxygen saturation (P=0.002).

CONCLUSIONS

Patients with CHD are at a high risk for ocular pathology and need screening for various ocular abnormalities.

The Contribution of Meibomian Disease to Dry Eye

The tear film lipid layer is the major barrier to evaporation from the ocular surface. A decrease in its thickness or functional integrity may cause evaporative dry eye (EDE). Obstructive meibomian gland dysfunction (MGD) is the most common cause of EDE and occurs as a primary disorder or secondary to acne rosacea, seborrheic or atopic dermatitis, and with cicatrizing conjunctival disorders, such as trachoma, erythema multiforme, and cicatricial pemphigoid. MGD may be an incidental finding in asymptomatic eyes, or it may be responsible for irritative lid symptoms in the absence of dry eye. MGD-dependent EDE is diagnosed on the basis of a defined degree of MGD in a symptomatic patient showing typical ocular surface damage in the absence of an aqueous tear deficiency. When MGD occurs in a background of aqueous tear deficiency (ATD), then an additional evaporative component may assumed, depending on the extent of meibomian obstruction. However, definitive criteria are not yet established. The clinical severity of dry eye is greatest when ATD and EDE occur together, particularly in Sjogren syndrome. A hypothesis is proposed to explain the steps leading to primary, simple MGD and subsequent EDE.

Mutation of the FOXC2 gene in familial distichiasis

OBJECTIVE

To examine the FOXC2 gene in a family with hereditary distichiasis.

BACKGROUND

Distichiasis, ie, a second row of eyelashes arising from the meibomian glands of the eyelids, can be inherited either alone (Online Mendelian Inheritance in Man [OMIM] no. 126300) or, more commonly, as part of the lymphedema-distichiasis (LD) syndrome (OMIM no. 153400). More than 45 families with mutations in the FOXC2 gene and LD have been described. Both lymphedema and distichiasis are highly penetrant. Distichiasis without lymphedema is not commonly seen.

METHODS

We examined three generations of a family (N = nine members) with hereditary distichiasis but without lymphedema or other features of LD syndrome. The FOXC2 gene was polymerase chain reaction--amplified from genomic DNA from all family members and examined for mutations.

RESULTS

Clinical examination showed distichiasis of all four lids in two affected family members across two generations. There were no other consistent ophthalmologic abnormalities in the family. A cytosine-to-adenine transversion was identified in DNA from affected study participants at nucleotide position 1076, which would be predicted to cause truncation of the protein at codon 359. This change was not observed in any of the nine unaffected family members participating.

CONCLUSIONS

This finding suggests that hereditary distichiasis and LD may not be separate genetic disorders but different phenotypic expressions of the same underlying disorder. Ophthalmologists should be aware that LD may present as distichiasis alone and counsel and refer their patients appropriately.

FOXC2 truncating mutation in distichiasis, lymphedema, and cleft palate

We report a family showing autosomal-dominant segregation of upper- and lower-eyelid distichiasis (double row of eyelashes) in seven affected relatives over three generations, in addition to below-knee lymphedema of pubertal onset (lymphoedema proecox) in three. Two children had cleft palate in addition to distichiasis, but without the previously reported association with the Pierre-Robin sequence. Other ophthalmologic anomalies included divergent strabismus and early-onset myopia. This family was found to be completely linked to markers mapped to 16q24.3 and thereby proposed to be allelic to the distichiasis-lymphedema syndrome (DL, MIM 153400), although pterygium colli, congenital heart disease, or facial dysmorphism were not features found here. As FOXC2/FKLH14 mutations were found to underlie DL and diverse hereditary lymphedema conditions, this gene was examined by sequence analysis. An out-of-frame deletion (914-921del) was identified and found to segregate with the disease, further highlighting the phenotypic heterogeneity of lymphedema conditions linked to FOXC2 truncating mutations. Whether such heterogeneity is related to genotype-phenotype correlation, a hypothesis not primarily supported by the apparent loss-of-function mechanism of the mutations, or governed by modifying genes, remains to be determined.