Mutations in Bcl9 and Pygo genes cause congenital heart defects by tissue-specific perturbation of Wnt/β-catenin signaling

Genetic alterations in human BCL9 genes have repeatedly been found in congenital heart disease (CHD) with as-of-yet unclear causality. BCL9 proteins and their Pygopus (Pygo) co-factors can participate in canonical Wnt signaling via binding to {beta}-catenin. Nonetheless, their contributions to vertebrate heart development remain uncharted. Here, combining zebrafish and mouse genetics, we document tissue-specific functions in canonical Wnt signaling for BCL9 and Pygo proteins during heart development. In a CRISPR-Cas9-based genotype-phenotype association screen, we uncovered that zebrafish mutants for bcl9 and pygo genes largely retain {beta}-catenin activity, yet develop cardiac malformations. In mouse, both systemic and lineage-specific loss of the Pygo-BCL9-{beta}-catenin complex caused heart defects with outflow tract malformations, aberrant cardiac septation and valve formation, and compact myocardium hypoplasia. Mechanistically, these phenotypes coincide with transcriptional deregulation during heart patterning, and Pygo2 associates with {beta}-catenin at cis-regulatory regions of cardiac genes. Taken together, our results establish BCL9 and Pygo as tissue-specific {beta}-catenin co-factors during vertebrate heart development. Our results further implicate alterations in BCL9 and BCL9L in human CHDs as possibly causative.