SOX21 and SOX2 collaboratively promote blastocyst formation downstream of TFAP2C.

In mammals, the first cell fate decisions segregate pluripotent fetal cells from the differentiating extraembryonic lineages. In mouse, pluripotent progenitors can be identified as early as the 16-cell (morula) stage, based on their restricted expression of SOX2. In spite of the unique SOX2 expression pattern, however, the first cell fate decision proceeds normally in the absence of Sox2, suggesting an undiscovered compensatory mechanism. We discovered that the closely related Sox paralogue SOX21 is expressed alongside, and independently of Sox2 prior to blastocyst formation. Embryos lacking both Sox2 and Sox21 exhibit a phenotype more severe than those lacking either gene alone, arresting during the morula-to-blastocyst transition. In fact, embryos lacking any three copies of Sox2 or Sox21 frequently phenocopy double knockouts, consistent with a dose-dependent genetic interaction. Finally, we discovered the mechanism promoting parallel expression of Sox2 and Sox21. We show that, like Sox2, expression of Sox21 depends on TFAP2C, and not NANOG or OCT4, clarifying gene regulatory mechanisms preceding blastocyst formation. Overall, our study reveals a novel Sox-dependent pathway regulating embryonic processes even earlier than previously appreciated.