The roles of Dnmt1 cytosine methyltransferase proteins in genomic reprogramming duringmouse preimplantation development.

Inheritance of DNA methylation on imprinted genes depends on the Dnmt1 (cytosine-5-)methyltransferase protein. Methylation patterns on imprinted genes are maintained by oocytespecificDnmt1o isoform at the 8-cell stage of preimplantation development. Methylationpatterns in postimplantation embryos are maintained by the Dnmt1s isoform. To determine ifDnmt1s can functionally replace Dnmt1o, we expressed Dnmt1s in oocytes and discovered thatDnmt1s can maintain genomic imprints in the absence of Dnmt1o. However, the ability ofDnmt1s to maintain imprinting is dependant on the level of oocyte Dnmt1s. Though Dnmt1s andDnmt1o have equivalent maintenance methyltransferase functions in oocytes, the unstable natureof oocyte Dnmt1s, in comparison to oocyte Dnmt1o, leads to levels lower than what are requiredto maintain methylation at the 8-cell stage. We also determined that in cloned embryos, Dnmt1oundergoes none of its expected trafficking to 8-cell stage nuclei. Instead, these embryos exhibita mosaic pattern of Dnmt1s expression. Defects in intracellular trafficking of Dnmt1o andmisexpression of Dnmt1s, along with the intrinsic instability of Dnmt1s, might contribute toivaberrant DNA methylation in cloned embryos, thus raising concerns about the use of currentcloning technologies for therapeutic cloning.Molecular mechanisms involved in the formation of ovarian teratomas were also analyzed.Unfertilized oocytes arrest at the MII stage of meiotic maturation. After fertilization, oocytescontinue into cell division. Premature activation of MII oocytes without fertilization, can lead toovarian teratoma formation. To better understand mechanisms governing the prevention ofspontaneous oocyte activation, we investigated the molecular defects leading to formation ofovarian teratomas in the Tgkd mouse model. Tgkd is a transgene insertional mutation that leadsto reduced levels of the Inpp4b protein in MII oocytes of hemizygous Tgkd females. WildtypeGV oocytes have less Inpp4b protein than MII oocytes, and a significant decrease in Inpp4b isalso seen after fertilization. Also, the dependence of ovarian teratoma formation on the mousestrain, emphasizes the role of a strain-specific modifier on chromosome 6, possibly IP31. Thus,it is possible that oocyte Inpp4b normally suppresses spontaneous MII oocyte activation,possibly by reducing levels of IP3, an intermediate in the oocyte activation mechanism, thatoccurs following fertilization