The Axenfeld-Rieger Syndrome Gene Contributes to Left-Right Patterning.

Precise spatiotemporal expression of the -- cascade in the lateral plate mesoderm establishes the left-right axis, which provides vital cues for correct organ formation and function. Mutations of one cascade constituent and, separately, the Forkhead transcription factor independently cause a multi-system disorder known as Axenfeld-Rieger syndrome (ARS). Since cardiac involvement is an established ARS phenotype and because disrupted left-right patterning can cause congenital heart defects, we investigated in zebrafish whether contributes to organ laterality or situs. We demonstrate that CRISPR/Cas9-generated and mutants exhibit abnormal cardiac looping and that the prevalence of cardiac situs defects is increased in ; homozygotes. Similarly, double homozygotes exhibit isomerism of the liver and pancreas, which are key features of abnormal gut situs. Placement of the asymmetric visceral organs relative to the midline was also perturbed by mRNA overexpression of and . In addition, an analysis of the left-right patterning components, identified in the lateral plate mesoderm of mutants, reduced or abolished the expression of the antagonist . Together, these data reveal a novel contribution from to left-right patterning, demonstrating that this role is sensitive to gene dosage, and provide a plausible mechanism for the incidence of congenital heart defects in Axenfeld-Rieger syndrome patients.