In the present study, we have demonstrated a critical role for YY1 in mouse embryonic development. Mouse embryos lacking YY1 develop to the blastocyst stage and are implanted but die shortly thereafter. These embryos show a severe defect in the development of the embryonic and extraembryonic tissues at a developmental stage that coincides with rapid cell proliferation and differentiation. Our findings provide the first demonstration of a pivotal role for YY1 in vertebrate development.
During mouse development, cleavage of the fertilized embryo to the blastocyst-stage embryo (E3.5) marks the first differentiation event. This event is the first morphological asymmetry observed in the embryo highlighted by the formation of the blastocyst inner cell mass and the trophectoderm which give rise to the embryo proper and contribute to the placental tissue, respectively (reviewed in references 13
). The blastocyst-stage embryo prepares for uterine implantation by generating two specialized tissues, the trophoblast and primitive endoderm, which later contribute to the formation of the placenta. At the time of implantation, proteolytic degradation of the uterine epithelium and attachment of the hatched blastocyst occur. Following blastocyst implantation and prior to gastrulation, mouse embryos must achieve a rapid proliferative expansion of the inner cell mass and its associated trophectoderm, resulting in the formation of the egg cylinder (E6.5), which consists of the inner epiblast and the outer visceral endoderm. This event marks the differentiation of the three earliest cell types, the endoderm, the mesoderm, and the ectoderm. Embryos that lack a threshold number of epiblast cells due to either proliferative defects or cell losses fail to gastrulate and are arrested prior to the formation of the primitive streak (reviewed in reference 31
). The YY1 mutants fail prior to the formation of the primitive streak, suggesting that YY1 is likely to be necessary for epiblast proliferation or differentiation events prior to gastrulation. Taken together, these findings suggest that YY1 is a key molecule that is involved in regulating genes whose products are pivotal for differentiation and/or proliferation during early embryogenesis. The fact that YY1 is detected in both the inner cell mass and the trophectoderm of the preimplantation blastocyst (Fig. M) is consistent with such a role for YY1 in regulating genes for both embryonic and extraembryonic tissues.
What might be the downstream target genes for YY1 during early embryogenesis whose misregulation may account for the observed defects in the YY1−/−
embryos? The phenotypes of the YY1 mutant embryos are reminiscent of those described for the evx-1 (29
), Fgf-4 (7
), fug-1 (5
), rad51 (19
), and β1 integrin (6
) mutant mice. These mice share a feature with the YY1−/−
embryos in which proliferative expansion of the embryo does not allow the morphogenesis of the mutant embryos to the pregastrulation stage. All of these genes are crucial for the implanting embryo. Therefore, it is possible that YY1 may be essential for regulating the expression of these or other genes important for the formation of the pregastrulation embryo (reviewed in reference 4
Recently, a putative Drosophila
YY1 homolog, pleiohomeotic (Pho), has been described (1
). Mutations in Pho result in Drosophila
embryonic lethality (10
). Pho and YY1 have extensive amino acid identity in the zinc finger region (95% in the entire zinc finger region and 100% in zinc fingers 2 and 3), suggesting that they are likely to recognize similar (if not identical) DNA sequences. Outside the zinc finger region, there is very little sequence conservation except for a 22-amino-acid region located in the central portion of the proteins (1
). The transcription activation domain situated at the N terminus of YY1 (2
) appears to be absent in Pho. Therefore, it is not clear whether the same molecular mechanism underlies the biological functions of both proteins. Regardless, our results indicate that murine YY1 may have a crucial role similar to that of Drosophila
Pho in early embryonic development, suggesting an evolutionarily conserved function for YY1 prior to the separation of arthropods and vertebrates.
In summary, we have shown that mice lacking YY1 exhibit early embryonic lethality at the time around implantation, revealing a crucial role for YY1 in early mouse development. We postulate that YY1 may regulate genes whose products are essential for the rapid proliferation and differentiation of mouse embryos around the time of implantation. The expression pattern of YY1 and the phenotypes displayed by a subset of the YY1 heterozygotes raise the additional possibility that YY1 may also be required for later-stage embryogenesis. The observation that a subset of YY1 heterozygotes is growth retarded and has neurulation defects suggests that both alleles of YY1 are necessary for normal embryonic growth and development. Future experiments will focus on delineating YY1 function and the mechanism of action in later embryonic development by selective inhibition of YY1 expression with conditional knockout technology. These results being taken together, YY1 appears indispensable for mouse embryonic development and is one of the few transcription factors characterized to date that has such an early role in mouse embryogenesis. Given the fact that YY1 is highly conserved among human (22
), mouse (8
), avian (5a
), and Xenopus
) species, it is likely that YY1 plays a crucial developmental role in these organisms as well.