An in vivo CRISPR screen in chick embryos reveals a role for MLLT3 in specification of neural cells from the caudal epiblast.

Tissue development relies on the coordinated differentiation of stem cells in dynamically changing environments. The formation of the vertebrate neural tube from stem cells in the caudal lateral epiblast is a well-characterized example. Despite an understanding of the signalling pathways involved, the gene regulatory mechanisms remain poorly defined. To address this, we developed a multiplexed in vivo CRISPR screening approach in chick embryos targeting genes expressed in the caudal epiblast and neural tube. This revealed a role for MLLT3, a component of the super elongation complex, in the specification of neural fate. Perturbation of MLLT3 disrupted neural tube morphology and reduced neural fate acquisition. Mutant forms of retinoic acid receptor A lacking the MLLT3 binding domain similarly reduced neural fate acquisition. Together, these findings validate an in vivo CRISPR screen strategy in chick embryos and identify a previously unreported role for MLLT3 in caudal neural tissue specification.