| The retinoblastoma tumor suppressor, pRB, in mouse and humans, functions to suppress neoplasia and coordinate proper growth and development. One of the most critical, and best characterized, interactors of pRB is the E2F/DP family of transcription factors. pRB represses E2F-dependent transcription, and inactivation of pRB allows E2F/DP mediated target gene expression. Activation of E2F target genes drives the cell through G1 phase and into S phase by coordinate expression of cell cycle regulators and replication factors. Tumor studies and tissue culture analysis show that inactivation of pRB or expression of E2F1-3 is sufficient to deregulate the cell cycle and drive S-phase entry. We analyzed the role of E2F/DP in the mouse in vivo by inactivating DP1, the major dimerization partner of E2F1-6, through gene targeting. The DP proteins are important for all functions of the E2F heterodimer and DP1-deficiency was expected to compromise most E2F/DP activity, including DNA binding, and transcriptional activation and repression through pRB family members. We studied DP1-deficient embryos using gross dissections and in situ analyses and found that DP1 is indeed required for proper development in the mouse, specifically in the extra-embryonic tissue. Loss of DP1 led to decreased numbers of trophoblast-derived cells and reduced DNA replication within these tissues. Bypassing the extra-embryonic requirement for DP1 through use of DP1-deficient chimeras, we found high contribution of DP1-deficient cells to many tissues between E14.5 and E17.5. This suggests DP1, and hence, E2F activity, are not critical for development of most of the embryo proper. |
