1. Selective Killing Of Cancer Cells By Suppression Of Geminin Activity Eukaryotic cells normally restrict genome duplication to once per cell division. In metazoa, re-replication of DNA during a single S phase seems to be prevented solely by suppressing CDT1 activity, a protein required for loading the replicative MCM DNA helicase. However, siRNA suppression of geminin (a specific inhibitor of CDT1) arrested proliferation only of cells derived from cancers by inducing DNA re-replication and DNA damage that spontaneously triggered apoptosis. None of these effects were detected either in cells derived from normal human tissues or in cells immortalized by a viral oncogene. To induce these effects in noncancer cells required suppression of both geminin and cyclin A, another cell cycle regulator. Therefore, initiating DNA replication in some cancer cells is limited solely by regulating the level of CDT1 activity with geminin, whereas noncancer cells contain additional safeguards that prevent DNA re-replication. These results show that inhibition of geminin activity could be used to selectively kill cancer cells without harming other cells. 2. A patent is now pending based on inhibition of geminin activity as a novel cancer therapy. 3. Differentiation Of Trophoblast Stem Cells Into Giant Cells Is Triggered By p57/Kip2 Inhibition Of CDK1 Activity Genome endoreduplication during mammalian development is a rare event for which the mechanism is unknown. It first appears when fibroblast growth factor 4 (FGF4) deprivation induces differentiation of trophoblast stem (TS) cells into the nonproliferating trophoblast giant (TG) cells required for embryo implantation. Here we show that RO3306 inhibition of cyclin-dependent protein kinase 1 (CDK1), the enzyme required to enter mitosis, induced differentiation of TS cells into TG cells. In contrast, RO3306 induced abortive endoreduplication and apoptosis in embryonic stem cells, revealing that inactivation of CDK1 triggers endoreduplication only in cells programmed to differentiate into polyploid cells. Similarly, FGF4 deprivation resulted in CDK1 inhibition by overexpressing two CDK-specific inhibitors, p57/KIP2 and p21/CIP1. TS cell mutants revealed that p57 was required to trigger endoreduplication by inhibiting CDK1, while p21 suppressed expression of the checkpoint protein kinase CHK1, thereby preventing induction of apoptosis. Furthermore, Cdk2(-/-) TS cells revealed that CDK2 is required for endoreduplication when CDK1 is inhibited. Expression of p57 in TG cells was restricted to G-phase nuclei to allow CDK activation of S phase. Thus, endoreduplication in TS cells is triggered by p57 inhibition of CDK1 with concomitant suppression of the DNA damage response by p21. 4. Developmentally Programmed Endoreduplication in Animals Development of a fertilized egg into an adult human requires trillions of cell divisions, the vast majority of which duplicate their genome once and only once. Nevertheless, trophoblast giant cells and megakaryocytes in mammals circumvent this rule by duplicating their genome multiple times without undergoing cell division, a process generally referred to as 'endoreduplication'. In contrast, arthropods such as Drosophila endoreduplicate their genome in most larval tissues, as well as in many adult tissues. Endoreduplication requires that cells prevent entrance into or completion of mitosis and cytokinesis under conditions that permit assembly of prereplication complexes. In addition, cells must prevent induction of apoptosis in response to incomplete DNA replication or DNA damage that may occur during the ensuing sequence of 'endocycles'. Thus, developmentally regulated endoreduplication results in terminal cell differentiation. Recent progress has revealed both differences and similarities in the mechanisms employed by flies and mammals to change from mitotic cell cycles to 'endocycles'. The critical step, however, appears to be switching from a CDK-dependent form of the anaphase promoting complex (APC) to one that functions only in the absence of CDK activity. 5. Cip/Kip cyclin-dependent protein kinase inhibitors and the road to polyploidy Cyclin-dependent kinases (CDKs) play a central role in the orderly transition from one phase of the eukaryotic mitotic cell division cycle to the next. In this context, p27Kip1 (one of the CIP/KIP family of CDK specific inhibitors in mammals) or its functional analogue in other eukarya prevents a premature transition from G1 to S-phase. Recent studies have revealed that expression of a second member of this family, p57Kip2, is induced as trophoblast stem (TS) cells differentiate into trophoblast giant (TG) cells. p57 then inhibits CDK1 activity, an enzyme essential for initiating mitosis, thereby triggering genome endoreduplication (multiple S-phases without an intervening mitosis). Expression of p21Cip1, the third member of this family, is also induced in during differentiation of TS cells into TG cells where it appears to play a role in suppressing the DNA damage response pathway. Given the fact that p21 and p57 are unique to mammals, the question arises as to whether one or both of these proteins are responsible for the induction and maintenance of polyploidy during mammalian development. 6. The Human RNase H2 Enzyme Eukaryotic RNase H2 is a heterotrimeric enzyme that selectively degrades the RNA primers used to initiate DNA synthesis at replication forks. The catalytic subunit of human RNase H2, RNASEH2A, is well conserved and similar to the monomeric prokaryotic RNase HII. In contrast, the RNASEH2B and RNASEH2C subunits from human and Saccharomyces cerevisiae share very little homology, although they both form soluble B/C complexes that may serve as a nucleation site for the addition of RNASEH2A to form an active RNase H2, or for interactions with other proteins to support different functions. The RNASEH2B subunit has a PIP-box and confers PCNA binding to human RNase H2. Unlike Escherichia coli RNase HII, eukaryotic RNase H2 acts processively and hydrolyzes a variety of RNA/DNA hybrids with similar efficiencies, suggesting multiple cellular substrates. Moreover, of five analyzed mutations in human RNASEH2B and RNASEH2C linked to Aicardi-Goutires Syndrome (AGS), only one, R69W in the RNASEH2C protein, exhibits a significant reduction in specific activity, revealing a role for the C subunit in enzymatic activity. Near-normal activity of four AGS-related mutant enzymes was unexpected in light of their predicted impairment causing the AGS phenotype.
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