Axenfeld-Rieger Malformations Research Articles

An Iris Cyst as a Presenting Feature of Axenfeld-Rieger Malformation

An Iris Cyst as a Presenting Feature of Axenfeld-Rieger Malformation

The Role of hsa-miR-548l Dysregulation as a Putative Modifier Factor for Glaucoma-Associated FOXC1 Mutations.

Mutations of the FOXC1 transcription factor are involved in a variety of autosomal dominant ocular anterior segment defects, ranging from Axenfeld-Rieger malformations to isolated glaucoma in some patients. In this study we have evaluated the possible role of the c.*734A>T FOXC1 variant as a modifier factor of the activity of two FOXC1 mutations previously identified in families primarily affected by dominant glaucoma (haplotypes p.G447_G448insDG-c.*734A>T and p.I126S-c.*734A>T). Previous bioinformatic analyses indicated that the c.*734A>T variant is located in a potential target sequence for hsa-miR-548l. Co-expression of this miRNA with a reporter cDNA construct in which the wild-type 3'UTR sequence of FOXC1 was fused to the 3'-end of the firefly luciferase coding region, led to approximately 20% decreased luciferase activity compared to the controls, confirming the presence of a target sequence for hsa-miR-548l. In contrast, this miRNA did not show any effect on the luciferase activity associated with the mutant 3'UTR FOXC1 sequence, showing that it resulted in a loss-of-function of the has-miR-548l target sequence. In addition, functional evaluation of the two glaucoma-associated haplotypes revealed increased protein levels and transactivation, compared to the corresponding individual coding mutations (approximately 1.2-fold on average). These data support the role of hsa-miR-548l as a regulator of FOXC1 translation and provide evidence for the c.*734A>T variant as a modifier factor for the activity of coding glaucoma-associated FOXC1 mutations.

BMP4 loss-of-function mutations in developmental eye disorders including SHORT syndrome

BMP4 loss-of-function mutations and deletions have been shown to be associated with ocular, digital, and brain anomalies, but due to the paucity of these reports, the full phenotypic spectrum of human BMP4 mutations is not clear. We screened 133 patients with a variety of ocular disorders for BMP4 coding region mutations or genomic deletions. BMP4 deletions were detected in two patients: a patient affected with SHORT syndrome and a patient with anterior segment anomalies along with craniofacial dysmorphism and cognitive impairment. In addition to this, three intragenic BMP4 mutations were identified. A patient with anophthalmia, microphthalmia with sclerocornea, right-sided diaphragmatic hernia, and hydrocephalus was found to have a c.592C>T (p.R198X) nonsense mutation in BMP4. A frameshift mutation, c.171dupC (p.E58RfsX17), was identified in two half-siblings with anophthalmia/microphthalmia, discordant developmental delay/postaxial polydactyly, and poor growth as well as their unaffected mother; one affected sibling carried an additional BMP4 mutation in the second allele, c.362A>G (p.H121R). This is the first report indicating a role for BMP4 in SHORT syndrome, Axenfeld–Rieger malformation, growth delay, macrocephaly, and diaphragmatic hernia. These results significantly expand the number of reported loss-of-function mutations, further support the critical role of BMP4 in ocular development, and provide additional evidence of variable expression/non-penetrance of BMP4 mutations.

Human p32 Is a Novel FOXC1-Interacting Protein That Regulates FOXC1 Transcriptional Activity in Ocular Cells

Mutations in the human forkhead box C1 gene (FOXC1) cause Axenfeld-Rieger (AR) malformations, often leading to glaucoma. Understanding the function of FOXC1 necessitates characterizing the proteins that interact with FOXC1. This study was undertaken to isolate FOXC1-interacting proteins and determine their effects on FOXC1. To identify FOXC1-interacting proteins, a human trabecular meshwork (HTM) yeast two-hybrid (Y2H) cDNA library was screened. The interaction and colocalization between FOXC1 and its putative protein partner were confirmed by Ni(2+) pull-down assays, immunoprecipitation, and immunofluorescence, respectively. The electrophoretic mobility shift assay (EMSA) was used to study the effect of the interacting protein on FOXC1 DNA-binding ability. Dual luciferase assays using FOXC1 reporter plasmids in HTM cells were performed to determine the effect of the interaction on FOXC1 transcription activity. The human p32 protein was isolated as a putative FOXC1-interacting protein from a Y2H screen. The interaction of FOXC1 with p32 was confirmed by Ni-pull-down assays and immunoprecipitation. Although p32 is predominantly cytoplasmic, the portion of p32 that is within the nucleus colocalizes with FOXC1. The FOXC1 forkhead domain (FHD) was identified as the p32 interaction domain. p32 significantly inhibited FOXC1-mediated transcription activation in a dose-dependent manner but did not affect FOXC1 DNA-binding ability. Of interest, a FOXC1 mutation F112S displayed an impaired interaction with p32. In the study, the human p32 protein as a novel regulator of FOXC1-mediated transcription activation. Failure of p32 to interact with FOXC1 containing the disease-causing F112S mutation indicates that impaired protein interaction may be a disease mechanism for AR malformations.

Heterozygous FOXC1 Mutation (M161K) Associated with Congenital Glaucoma and Aniridia in an Infant and a Milder Phenotype in Her Mother

Purpose: To report the genetic basis for congenital glaucoma with clinical aniridia in an infant and a milder phenotype in her mother. Methods: Prospective case series. Results: An infant girl with almost complete lack of iris tissue was referred and treated for congenital glaucoma. Although the presumed clinical diagnosis was aniridia (On-line Mendelian Inheritance in Man [OMIM] AN2, # 106210), PAX6 sequencing was normal. Examination of the infant's mother was significant for Axenfeld-Rieger malformation (ARM): prominent Schwabe line, subtle iris hypoplasia, iris stands bridging the angle, increased intraocular pressure, and glaucomatous optic nerve cupping. Both parents and the infant underwent diagnostic FOXC1 DNA sequencing. A heterozygous M161K FOXC1 mutation was found in the infant and her mother but not in the father, who had a normal ocular examination. Discussion: The spectrum of intrafamilial phenotypic variation associated with heterozygous FOXC1 mutation can be wide. FOXC1 mutation can be a cause of congenital glaucoma with clinical aniridia. Although such infants resemble the AN2 phenotype, the glaucoma of AN2 due to PAX6 mutation is typically secondary with onset several years after birth.

Genotype-Phenotype Correlations in Axenfeld-Rieger Malformation and Glaucoma Patients withFOXC1andPITX2Mutations

To improve the understanding of Axenfeld-Rieger Malformation (ARM)-associated glaucoma and to determine the best glaucoma treatment for patients with ARM who have known genetic defects in FOXC1 or PITX2. Clinical data were collected from patients with diagnosed ARM, in whom we had previously identified disease-causing mutations in either the FOXC1 or PITX2 genes, by examination of patient records and use of clinical questionnaires. One hundred twenty-six patients with ARM, representing 20 different probands, with FOXC1 and PITX2 alterations were included in the study. ARM-associated glaucoma is a bilateral anterior segment dysgenesis disease that affects males and females equally. Seventy-five percent of the patients with ARM who participated in this study had glaucoma that had developed in adolescence or early adulthood. Of note, the patients with nonocular findings were more likely to have PITX2 defects than FOXC1 defects. Glaucoma in only 18% of patients with either PITX2 or FOXC1 genetic defects responded to medical or surgical treatment (used solely or in combination). Patients with FOXC1 mutations have the mildest prognosis for glaucoma development, whereas patients with PITX2 defects and patients with FOXC1 duplication have a more severe prognosis for glaucoma development than do patients with FOXC1 mutations. In the present study, current medical therapies do not successfully lower intraocular pressure or prevent progression of glaucoma in patients with ARM who have FOXC1 or PITX2 alterations. This clinical study also provides useful diagnostic criteria to identify the gene responsible for ARM.

Reduced Human and Murine Corneal Thickness in an Axenfeld-Rieger Syndrome Subtype

Axenfeld-Rieger malformations of the anterior segment are clinically heterogeneous, and up to 50% of cases are attributable to PITX2 or FOXC1 mutation. In view of PITX2's contribution to corneal development and the altered CCT in some FOXC1-related cases, this study was undertaken to investigate whether a related phenotype is associated with the PITX2/Pitx2 mutation. Central corneal thickness (CCT) was measured in patients and mice with PITX2/Pitx2 mutations. CCT in affected individuals and unaffected first-degree relatives from a large PITX2 mutation pedigree was measured with ultrasonic pachymetry. For murine measurements, the optical coherence tomogram (OCT) was calibrated against plastic films whose thickness had been determined with scanning electron microscopy (SEM). Subsequently, CCT was measured in ex vivo eyes from Pitx2(+/-) and wild-type murine littermates by using OCT. CCT in individuals with the PITX2 mutation (mean 484 microm; range, 425-519; n = 8) was significantly lower than in their unaffected first-degree relatives (mean 582 microm; range, 550-590; n = 5; P = 0.0002, t-test). Scanning electron microscopy (SEM) and OCT measurements of reference films correlated closely (r = 0.9995) and subsequent OCT analysis of murine eyes revealed a significant reduction in CCT in Pitx2(+/-) compared with wild-type littermates (Pitx2(+/-): mean, 72 microm; range, 57-87, n = 6; wt: mean, 88 microm; range, 63-100; n = 6, P = 0.035, t-test). The results show that PITX2/Pitx2 mutation results in reduced corneal thickness and provides the first example of reduced CCT in a genetic subtype of glaucoma. These data will facilitate management of developmental glaucoma and offer potential for guiding molecular genetic testing in patients with Axenfeld-Rieger. The similar CCT reduction observed in patients and mice with comparable mutations emphasizes the utility of this murine model. The technical advance of optical murine CCT measurement also provides scope for serial in vivo imaging of the developing anterior segment and determining the effects of altered CCT on measured IOP.

Novel Mutations ofFOXC1andPITX2in Patients with Axenfeld-Rieger Malformations

To determine the prevalence of FOXC1 and PITX2 mutations and to assess clinical phenotypes in a cohort of German patients with Axenfeld-Rieger malformations. All coding exons of the FOXC1 and PITX2 genes were amplified by PCR from genomic DNA and subjected to direct DNA sequencing. Analysis of mutations in control subjects was performed by restriction fragment length polymorphism (RFLP) analysis. Sequence variants were identified by DNA sequencing in 15 of 19 cases. Mutation screening identified four potentially pathogenic FOXC1 mutations causing amino acid substitutions (P79R, Y115S, G149D, and M161V) that were not present in 100 control subjects. In addition, two different 1-bp deletions causing a frameshift and subsequent premature stop codon were identified in two subjects. One patient harbored a FOXC1 nonsense mutation (S48X). Mutation screening also identified two potentially pathogenic PITX2 mutations (P64L and P64R) in two index patients that were excluded in 100 healthy control subjects. The findings in the present study clearly demonstrate that FOXC1 and PITX2 mutations are responsible for a significant proportion of Axenfeld-Rieger malformations in Germany.

Regulation of FOXC1 Stability and Transcriptional Activity by an Epidermal Growth Factor-activated Mitogen-activated Protein Kinase Signaling Cascade

Mutations in the FOXC1 transcription factor gene result in Axenfeld Rieger malformations, a disorder that affects the anterior segment of the eye, the teeth, and craniofacial structures. Individuals with this disorder possess an elevated risk for developing glaucoma. Previous work in our laboratory has indicated that FOXC1 transcriptional activity may be regulated by phosphorylation. We report here that FOXC1 is a short-lived protein (t 1/2< 30 min), and serine 272 is a critical residue in maintaining proper stability of FOXC1. Furthermore, we have demonstrated that activation of the ERK1/2 mitogen-activated protein kinase through epidermal growth factor stimulation is required for maximal FOXC1 transcriptional activation and stability. Finally, we have demonstrated that FOXC1 is targeted to the ubiquitin 26 S proteasomal degradation pathway and that amino acid residues 367-553, which include the C-terminal transactivation domain of FOXC1, are essential for ubiquitin incorporation and proteolysis. These results indicate that FOXC1 protein levels and activity are tightly regulated by post-translational modifications.

Axenfeld-Rieger malformation and distinctive facial features: Clues to a recognizable 6p25 microdeletion syndrome

Deletion of distal 6p is associated with a distinctive clinical phenotype including Axenfeld-Rieger malformation, hearing loss, congenital heart disease, dental anomalies, developmental delay, and a characteristic facial appearance. We report the case of a child where recognition of the specific ocular and facial phenotype, led to identification of a 6p microdeletion arising from a de novo 6:18 translocation. Detailed analysis confirmed deletion of the FOXC1 forkhead gene cluster at 6p25. CNS anomalies included hydrocephalus and hypoplasia of the cerebellum, brainstem, and corpus callosum with mild to moderate developmental delay. Unlike previous reports, hearing was normal.

Essential structural and functional determinants within the forkhead domain of FOXC1.

The forkhead domain (FHD)-containing developmental transcription factor FOXC1 is mutated in patients presenting with Axenfeld-Rieger malformations. In this paper, we report the introduction of positive, negative or neutral charged amino acids into critical positions within the forkhead domain of FOXC1 in an effort to better understand the essential structural and functional determinants within the FHD. We found that FOXC1 is intolerant of mutations at I87. Additionally, alterations of amino acids within alpha-helix 1 of the FOXC1 FHD affected both nuclear localization and transactivation. Amino acids within alpha-helix 3 were also found to be necessary for transactivation and can have roles in correct localization. Interestingly, changing amino acids within alpha-helix 3, particularly R127, resulted in altered DNA-binding specificity and granted FOXC1 the ability to bind to a novel DNA sequence. Given the limited topological variation of FHDs, due to the high conservation of residues, we anticipate that models of forkhead domain function derived from these data will be relevant to other members of the FOX family of transcription factors.

The wing 2 region of the FOXC1 forkhead domain is necessary for normal DNA-binding and transactivation functions.

To determine the biochemical defects that underlie Axenfeld-Rieger malformations, to determine a functional role for wing 2 in FOXC1, and to understand how mutations in this region disrupt FOXC1 function. Sequencing DNA from patients with Axenfeld-Rieger malformation resulted in the identification of two novel missense mutations (G165R and R169P) in wing 2 of FOXC1. Site-directed mutagenesis was used to introduce these mutations, as well as previously reported mutation (M161K), into the FOXC1 cDNA. These FOXC1 mutants were evaluated to determine their ability to localize to the nucleus, bind DNA and activate gene expression. Two novel missense mutations were identified in unrelated patients, in wing 2 of the FOXC1 forkhead domain. Because there had been no previous biochemical analysis, the mutation M161K was also investigated. All three mutant proteins localized correctly to the nucleus. The G165R mutation maintained wild-type levels of DNA binding; however, both the M161K and R169P mutations displayed reduced DNA binding ability. Biochemical analysis showed that all three mutations disrupt FOXC1's transactivation ability. Biochemical analysis of mutations G165R and R169P and of a previously reported mutation, M161K, demonstrate the functional significance of wing 2. M161K and R169P disrupt DNA binding of FOXC1, consistent with the hypothesis that wing 2 is necessary for DNA binding. The results also suggest that wing 2 plays a role in gene activation. These results provide the first insights into how mutations in wing 2 disrupt FOXC1 function.

Identification and analysis of a novel mutation in the FOXC1 forkhead domain.

To determine the genetic and biochemical defects that underlie Axenfeld-Rieger malformations, identify the pathogenic mutation causing these malformations, and understand how these mutations alter protein function. FOXC1 was amplified from a proband with Axenfeld-Rieger malformations and the proband's mother. PCR products were sequenced to identify the pathogenic mutation. Site-directed mutagenesis was used to introduce this mutation into the FOXC1 cDNA. A synthetic mutation at the same position was also introduced, and both natural and synthetic proteins were tested for their ability to localize to the nucleus, bind DNA, and transactivate gene expression. A novel missense mutation (L86F) was identified in FOXC1 in this family. The mutation is located in alpha-helix 1 of the forkhead domain. Biochemical assays showed that the L86F mutation does not affect nuclear localization of FOXC1, but reduces DNA binding and significantly reduces transactivation. The severity of the disruption to FOXC1 protein activity does not appear to correspond well with the severity of the phenotype in the patient. Analogous studies using a L86P, a known alpha-helix breaker, severely disrupts FOXC1 function, revealing the importance of helix 1 in FOXC1 structure and function. A novel mutation in helix 1 of the FOXC1 forkhead domain has been identified and the importance of position 86 in FOXC1 activity demonstrated. These studies also identified the role of helix 1 in FOXC1 function and provide further evidence for the lack of strong genotype-phenotype correlation in FOXC1 pathogenesis. Normal development appears to be dependent on tight upper and lower thresholds of FOXC1 activity.

Structural and functional analyses of disease-causing missense mutations in the forkhead domain of FOXC1

Five missense mutations (P79L, P79T, I91S, I91T and R127H) within the forkhead DNA-binding domain of the FOXC1 transcription factor, identified in patients with Axenfeld-Rieger (AR) malformations, were studied to identify the effects of these mutations on FOXC1 structure and function. Molecular modeling and threading analyses predict that the I91S and T mutations may generate local disruptions to the structure of the forkhead domain while the R127H mutation alters the electrostatic charge of the DNA binding surface of the forkhead domain. The P79L and T mutations are not predicted to grossly perturb the structure of the forkhead domain. Biological analyses indicate that all of these missense mutations cause a range of FOXC1 perturbations, including nuclear localization defects, reduced or abolished DNA binding capacity, and a reduction in the transactivation capacity of FOXC1. These experiments extend our previous hypothesis that reduced transactivation of appropriate target genes by FOXC1, underlie AR malformations mapping to human chromosome 6p25. Importantly, these results can also be applied to predict the consequences of the molecular effects of mutations of other FOX genes that have analogous missense mutations, including FOXP2, FOXE3 and FOXC2.

Molecular genetics of Axenfeld-Rieger malformations.

Axenfeld-Rieger (AR) malformations are autosomal dominant developmental defects of the anterior segment of the eye, and often result in glaucomatous blindness. AR malformations are associated with mutations in two transcription factor genes (PITX2 and FOXC1) expressed throughout eye ontogeny. Studies of disease-associated mutant proteins have provided insights into the aetiology of AR malformations, while delineating residues and domains important to DNA binding, transactivation and nuclear localization. The availability of mouse models for both PITX2 and FOXC1 has allowed detailed study of their expression and mutant phenotypes. Dissection of the normal functions and domain structures of these factors will aid in future elucidation of how alterations of the developmental program produce the dysgenic phenotypes seen in AR. There are at least two AR loci still awaiting molecular cloning on chromosomes 13q14 and 16q24. Identification of further genes implicated in aberrations of human ocular development will advance our understanding of the mechanisms whereby pattern is established in the eye, and may be of clinical value in treating the glaucoma that is the most serious consequence of AR malformations.

FOXC1 Transcriptional Regulation Is Mediated by N- and C-terminal Activation Domains and Contains a Phosphorylated Transcriptional Inhibitory Domain

Mutations in the FOXC1 gene result in Axenfeld-Rieger malformations of the anterior segment of the eye and lead to an increased susceptibility of glaucoma. To understand how the FOXC1 protein may function in contributing to these malformations, we identified functional regions in FOXC1 required for nuclear localization and transcriptional regulation. Two regions in the FOXC1 forkhead domain, one rich in basic amino acid residues, and a second, highly conserved among all FOX proteins, were necessary for nuclear localization of the FOXC1 protein. However, only the basic region was sufficient for nuclear localization. Two transcriptional activation domains were identified in the extreme N- and C-terminal regions of FOXC1. A transcription inhibitory domain was located at the central region of the protein. This region was able to reduce the trans-activation potential of the C-terminal activation domain, as well as the GAL4 activation domain. Lastly, we demonstrate that FOXC1 is a phosphoprotein, and a number of residues predicted to be phosphorylated were localized to the FOXC1 inhibitory domain. Removal of residues 215-366 resulted in a transcriptionally hyperactive FOXC1 protein, which displayed a reduced level of phosphorylation. These results indicate that FOXC1 is under complex regulatory control with multiple functional domains modulating FOXC1 transcriptional regulation.

Analyses of the Effects That Disease-Causing Missense Mutations Have on the Structure and Function of the Winged-Helix Protein FOXC1

Five missense mutations of the winged-helix FOXC1 transcription factor, found in patients with Axenfeld-Rieger (AR) malformations, were investigated for their effects on FOXC1 structure and function. Molecular modeling of the FOXC1 forkhead domain predicted that the missense mutations did not alter FOXC1 structure. Biochemical analyses indicated that, whereas all mutant proteins correctly localize to the cell nucleus, the I87M mutation reduced FOXC1-protein levels. DNA-binding experiments revealed that, although the S82T and S131L mutations decreased DNA binding, the F112S and I126M mutations did not. However, the F112S and I126M mutations decrease the transactivation ability of FOXC1. All the FOXC1 mutations had the net effect of reducing FOXC1 transactivation ability. These results indicate that the FOXC1 forkhead domain contains separable DNA-binding and transactivation functions. In addition, these findings demonstrate that reduced stability, DNA binding, or transactivation, all causing a decrease in the ability of FOXC1 to transactivate genes, can underlie AR malformations.