Wnt and Nodal asymmetries stratify mouse laterality phenotypes in the absence of node flow

Abstract

Left-right symmetry breaking in mice is considered to occur via node leftward fluid flow. The molecular cascade within the node has been reconstructed at high resolution. However, its importance for organogenesis remains poorly understood. Here we show in mutants for the motile cilium component CCDC40 that 70% of mice develop normal situs at birth despite abrogation of node flow. The discrete morphospace output, including situs inversus totalis and heterotaxy with left isomerism, supports a novel model of symmetry-breaking, in which node flow only biases asymmetry orientation, while another mechanism, potentially self-amplifying, generates it. Longitudinal, quantitative and paired transcriptomic analyses uncover the molecular signature of laterality clusters, highlighting WNT in addition to NODAL pathway asymmetry. We identify asymmetry of cardiopulmonary progenitors, the disruption of which is associated with combined heart and lung defects in mutants. Our work refines the role of the node in the establishment of asymmetry.