The Axenfeld-Rieger syndrome gene FOXC1 contributes to left-right patterning

Normal body situs requires precise spatiotemporal expression of the Nodal-Lefty-Pitx2 cascade in the lateral plate mesoderm. The ultimate output of this patterning is establishment of the left-right axis, which provides vital cues for correct organ formation and function. Mutations, deletions and duplications in PITX2 and FOXC1 lead to the rare genetic disease Axenfeld-Rieger syndrome (ARS). While situs defects are not a recognised feature of ARS, partial penetrance of cardiac septal defects and valve incompetence is observed; both of these congenital heart defects (CHDs) also occur following disruption of left-right patterning. Here we investigated whether foxc1 genes have a critical role in specifying organ situs. We demonstrate that CRISPR/Cas9 generated mutants for the zebrafish paralogs foxc1a and foxc1brecapitulate ARS phenotypes including craniofacial dysmorphism, hydrocephalus and intracranial haemorrhage. Furthermore, foxc1a-/-; foxc1b-/- mutant animals display cardiac and gut situs defects. Modelling FOXC1 duplication by transient mRNA overexpression revealed that increased foxc1 dosage also results in organ situs defects. Analysis of known left-right patterning genes revealed a loss in expression of the NODAL antagonist lefty2 in the lateral plate mesoderm. Consistently, LEFTY2 mutations are known to cause human cardiac situs defects. Our data reveal a novel role for the forkhead-box transcription factor foxc1 in patterning of the left-right axis, and provide a plausible mechanism for the incidence of congenital heart defects in Axenfeld-Rieger syndrome patients.