BackgroundAxenfeld-Rieger syndrome (ARS) is characterised by typical anterior segment anomalies, with or without systemic features. The discovery of causative genes identified ARS subtypes with distinct phenotypes, but our understanding is incomplete, complicated by the rarity of the condition.MethodsGenetic and phenotypic characterisation of the largest reported ARS cohort through comprehensive genetic and clinical data analyses.Results128 individuals with causative variants in PITX2 or FOXC1, including 81 new cases, were investigated. Ocular anomalies showed significant overlap but with broader variability and earlier onset of glaucoma for FOXC1-related ARS. Systemic anomalies were seen in all individuals with PITX2-related ARS and the majority of those with FOXC1-related ARS. PITX2-related ARS demonstrated typical umbilical anomalies and dental microdontia/hypodontia/oligodontia, along with a novel high rate of Meckel diverticulum. FOXC1-related ARS exhibited characteristic hearing loss and congenital heart defects as well as previously unrecognised phenotypes of dental enamel hypoplasia and/or crowding, a range of skeletal and joint anomalies, hypotonia/early delay and feeding disorders with structural oesophageal anomalies in some. Brain imaging revealed highly penetrant white matter hyperintensities, colpocephaly/ventriculomegaly and frequent arachnoid cysts. The expanded phenotype of FOXC1-related ARS identified here was found to fully overlap features of De Hauwere syndrome. The results were used to generate gene-specific management plans for the two types of ARS.ConclusionSince clinical features of ARS vary significantly based on the affected gene, it is critical that families are provided with a gene-specific diagnosis, PITX2-related ARS or FOXC1-related ARS. De Hauwere syndrome is proposed to be a FOXC1opathy.

While genetic causes are known for many syndromes involving developmental anomalies, a large number of individuals with overlapping phenotypes remain undiagnosed. Using exome-sequencing analysis and review of matchmaker databases, we have discovered four de novo missense variants predicted to affect the N-terminal region of WDR37-p.Ser119Phe, p.Thr125Ile, p.Ser129Cys, and p.Thr130Ile-in unrelated individuals with a previously unrecognized syndrome. Features of WDR37 syndrome include the following: ocular anomalies such as corneal opacity/Peters anomaly, coloboma, and microcornea; dysmorphic facial features; significant neurological impairment with structural brain defects and seizures; poor feeding; poor post-natal growth; variable skeletal, cardiac, and genitourinary defects; and death in infancy in one individual. WDR37 encodes a protein of unknown function with seven predicted WD40 domains and no previously reported human pathogenic variants. Immunocytochemistry and western blot studies showed that wildtype WDR37 is localized predominantly in the cytoplasm and mutant proteins demonstrate similar protein levels and localization. CRISPR-Cas9-mediated genome editing generated zebrafish mutants with novel missense and frameshift alleles: p.Ser129Phe, p.Ser129Cys (which replicates one of the human variants), p.Ser129Tyr, p.Lys127Cysfs, and p.Gln95Argfs. Zebrafish carrying heterozygous missense variants demonstrated poor growth and larval lethality, while heterozygotes with frameshift alleles survived to adulthood, suggesting a potential dominant-negative mechanism for the missense variants. RNA-seq analysis of zebrafish embryos carrying a missense variant detected significant upregulation of cholesterol biosynthesis pathways. This study identifies variants in WDR37 associated with human disease and provides insight into its essential role in vertebrate development and possible molecular functions.

The Forkhead Box C1 (FOXC1) gene encodes a forkhead/winged helix transcription factor involved in embryonic development. Mutations in this gene cause dysgenesis of the anterior segment of the eye, most commonly Axenfeld-Rieger syndrome (ARS), often with other systemic features. The developmental mechanisms and pathways regulated by FOXC1 remain largely unknown. There are two conserved orthologs of FOXC1 in zebrafish, foxc1a and foxc1b. To further examine the role of FOXC1 in vertebrates, we generated foxc1a and foxc1b single knockout zebrafish lines and bred them to obtain various allelic combinations. Three genotypes demonstrated visible phenotypes: foxc1a−/− single homozygous and foxc1−/− double knockout homozygous embryos presented with similar characteristics comprised of severe global vascular defects and early lethality, as well as microphthalmia, periocular edema and absence of the anterior chamber of the eye; additionally, fish with heterozygous loss of foxc1a combined with homozygosity for foxc1b (foxc1a+/−;foxc1b−/−) demonstrated craniofacial defects, heart anomalies and scoliosis. All other single and combined genotypes appeared normal. Analysis of foxc1 expression detected a significant increase in foxc1a levels in homozygous and heterozygous mutant eyes, suggesting a mechanism for foxc1a upregulation when its function is compromised; interestingly, the expression of another ARS-associated gene, pitx2, was responsive to the estimated level of wild-type Foxc1a, indicating a possible role for this protein in the regulation of pitx2 expression. Altogether, our results support a conserved role for foxc1 in the formation of many organs, consistent with the features observed in human patients, and highlight the importance of correct FOXC1/foxc1 dosage for vertebrate development.