Cohesin composition and dosage independently affect early development in zebrafish

Cohesin, a chromatin-associated protein complex with four core subunits (Smc1a, Smc3, Rad21 and either Stag1 or 2), has a central role in cell proliferation and gene expression in metazoans. Human developmental disorders termed “cohesinopathies” are characterised by germline mutations in cohesin or its regulators that do not entirely eliminate cohesin function. However, it is not clear if mutations in individual cohesin subunits have independent developmental consequences. Using zebrafish rad21 or stag2 mutants to change cohesin complex quantity or composition, we show that these parameters independently influence embryonic tailbud development. Both mutants have altered mesoderm induction, but only homozygous or heterozygous rad21 mutation affects cell cycle gene expression. stag2 mutants have slimmer notochords and reduced Wnt signaling in neuromesodermal progenitors as revealed by single cell RNA-sequencing. Stimulation of Wnt signaling rescues transcription and morphology in stag2, but not rad21 mutants, implying that individual cohesin mutations respond independently to cell signaling. Our results have implications for the understanding and management of cohesinopathies. Overall design: RNA-seq analysis was performed on tailbud samples from wild type zebrafish embryos, as well as those with homozygous and heterozygous rad21 mutations, and homozygous stag2b mutations. The dataset includes both, baseline samples as well as, samples treated with Wnt agonist (2.5 µM 6-bromoindirubin-3'-oxime).