Tracing the Transitions from Pluripotency to Germ Cell Fate with CRISPR Screening

Early mammalian development entails a series of cell fate transitions that includes transit through naive pluripotency to post-implantation epiblast. This subsequently gives rise to primordial germ cells (PGC), the founding population of the germline lineage. To investigate the gene regulatory networks that control these critical cell fate decisions, we developed a compound-reporter system to track cellular identity in a model of PGC specification (PGC-like cells; PGCLC), and coupled it with unbiased genome-wide CRISPR screening. This enabled identification of key genes both for exit from pluripotency and for acquisition of PGC fate, with further characterisation revealing a central role for the transcription factors Nr5a2 and Zfp296 in germline ontogeny. Abrogation of these genes results in significantly impaired PGCLC development due to widespread activation (Nr5a2-/-) or inhibition (Zfp296-/-) of WNT pathway components. This leads to aberrant upregulation of the somatic programme or failure to appropriately activate germline genes in PGCLC, respectively, and consequently loss of germ cell identity. Overall our study places Zfp296 and Nr5a2 as key components of an expanded PGC gene regulatory network, and outlines a transferable strategy for identifying critical regulators of complex cell fate transitions.