ABSTRACT: TET1, TET2 and TET3 are DNA demethylases with critical roles during early embryonic development and cell differentiation. To assess the contributions of TET proteins to cell function during early development, single and compound knockouts of Tet genes in mouse pluripotent embryonic stem cells (ESCs) were generated. ESCs lacking all alleles of Tet1 and Tet2, or all alleles of Tet1, Tet2 and Tet3 form epiblast-like cells and epiblast stem cells in culture, indicating that TET proteins are not required to transit between naïve, formative and primed pluripotency. Moreover, in differentiation protocols, ESCs with double-knockouts of Tet1 and Tet2 or triple-knockouts of Tet1, Tet2 and Tet3 do not differentiate normally , fail to activate somatic gene expression and retain expression of pluripotency transcription factors. Therefore, TET1 and TET2, but not TET3 act redundantly to facilitate somatic differentiation. Importantly, ESCs with double knockouts of Tet1 and Tet2, or triple knockouts of Tet1, Tet2 and Tet3 cells do differentiate into primordial germ cell-like cells (PGCLCs). Furthermore, PGCLC differentiation of Tet1, Tet2 double knockouts, or Tet1, Tet2, Tet3 triple knockouts occurs with high efficiency in the presence or absence of PGC-promoting cytokines. Moreover, acquisition of a PGCLC transcriptional programme occurs more rapidly in the absence of TET proteins. These results establish TET proteins as key regulators enabling epiblast cells to choose between somatic and germline fates. The functions of Tet1 and Tet2, but not Tet3 are required to enable epiblast cells to undergo somatic differentiation and in the absence of TET protein function, epiblast cell differentiation defaults to the germline.
