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In somatic cells, checkpoint pathways trigger cell cycle arrest in response to unreplicated or damaged DNA by inhibiting the activity of cyclin-dependent kinases (Cdks). In the Xenopus laevis embryo, checkpoints are not operational until the midblastula transition (MBT). Studies in cell-free egg extracts indicate that a threshold concentration of nuclei, which approximates the MBT concentration, is required to elicit a checkpoint. The checkpoint response to unreplicated DNA in the extract prevents transition into mitosis by inhibiting Cdk1/cyclin B, causing an increase in the minimum amount of cyclin B necessary to enter mitosis, termed the cyclin threshold. Once the threshold of cyclin is maintained or exceeded, the system will proceed into mitosis after a lag time. We have investigated the relationship between nuclear concentration and cell cycle regulation in the extract. By precisely regulating the concentration of cyclin B and nuclear content in extract samples, we have found 1) the concentration of nuclei affects cyclin B thresholds and lag time of entry into mitosis, 2) elevated cyclin thresholds caused by DNA replication blocks are further increased by increasing the concentration of nuclei, and 3) double-stranded DNA breaks in the extract system do not affect cyclin thresholds or lag time of entry into mitosis within the range of nuclear concentrations that can be efficiently replicated. This data provides evidence of the importance of the nucleocytoplasmic ratio in normal cell cycle progression and its importance for checkpoint acquisition during early Xenopus laevis development. / Master of Science

published_or_final_version / abstract / Obstetrics and Gynaecology / Master / Master of Philosophy

Dilantin (diphenylhydantoin or DPH) has been given to epileptic mothers to control seizures during pregnancy. Previous research has demonstrated that exposure of human embryos to Dilantin in vivo results in an increased probability of abnormal development and early fetal loss. Preliminary results with cultured 1-cell and 2-cell mouse embryos demonstrated that Dilantin causes mouse embryonic cleavage events to slow during preimplantation development (Chatot et al., unpublished). Dilantin may be responsible for this by inhibiting the rate of DNA synthesis during cleavage or by affecting the expression of proteins that control cell cycle progression. The standard expression pattern of these cell cycle regulatory proteins (cyclins) has not previously been determined in the mouse preimplantation embryo model. In this study, immunolabellingtechniques have been used to determine the expression pattern of cyclins D, A, and B 1 in the first three cell cycles of preimplantation mouse embryo development.This study reveals a unique expression pattern of cyclins D, A, and B1 in the first three cell cycles of preimplantation embryo development. Examination of the beginning of the first cell cycle, or G1, indicated a moderate expression of cyclin B1 and A but no cyclin D expression. During DNA synthesis (S-phase) all cyclin expression was virtually nonexistent. Toward the end of the cell cycle at G2/M, cyclin D expression appeared to be at moderate levels while cyclins A and B 1 exhibited minimal degrees of expression.In G 1 of the second cell cycle, cyclins D and A were minimally to moderately expressed and cyclin B 1 expression was minimal. At S-phase, cyclin D expression dropped to minimal levels whereas cyclins A and B 1 were at minimal to moderate levels of expression. At G2/M of the second cell cycle, cyclin B1 was expressed at minimal to moderate levels and cyclins A and D were both expressed at minimal levels.The third cell cycle began at G 1 with cyclin B 1 being expressed at moderate levels followed by minimal to moderate levels of cyclin D expression and minimal expression for cyclin A. Cyclin D expression increased to moderate levels at S-phase and cyclin A exhibited minimal to moderate levels of expression. Cyclin B 1 was observed at moderate levels of expression at S-phase of the third cell cycle. G2 of the third cell cycle included a drop to minimal levels of expression of cyclin D, while cyclin A expression remained at minimal to moderate levels and cyclin B remained at moderate levels of expression.The cyclin expression pattern for the first three cell cycles in preimplantation mouse embryos is unique compared to known cyclin expression patterns in other species. Cyclin D is expressed in G1 and is known to be necessary for advancement to S-phase in human glioblastoma cell lines (Xiong et al., 1991). Cyclin A is active at S-phase through Win human fibroblasts and xenopus oocytes (Giordino et al., 1991; Minshul et al., 1990). Cyclin B is present at G2 through mitosis in human fibroblasts and xenopus oocytes (Pines and Hunter, 1990; Minshul et al, 1990). / Department of Biology

L'objectif de cette thèse est de mieux comprendre la régulation de la phase M du cycle cellulaire. Nos expériences ont été effectuées dans des extraits acellulaires d’embryons de Xenopus laevis. Tout d'abord, nous montrons que le moment de l'entrée en phase M est précisément déterminé par un équilibre entre l'activité de la protéine kinase CDK1 et l’activité d’une protéine phosphatase sensible à l'acide okadaïque, PP2A. Nous montrons également le rôle de la protéine CDC6 dans la régulation de l'entrée dans la première phase M embryonnaire. En effet, CDC6 inhibe CDK1 et à travers cette action régule la dynamique de cette kinase lors de l'entrée et de la progression en phase M. Ces résultats mettent en évidence un nouveau contrôle qui précise le moment du clivage embryonnaire. Ce contrôle joue un rôle clé dans la coordination entre les mécanismes de régulation du cycle cellulaire et le programme de développement de l'embryon. / The aim of this thesis is to understand better the regulation of the M-phase of the cell cycle. Experiments were done in cell-free extracts of Xenopus laevis one-cell embryos. Firstly, we show that the timing of the M-phase entry is precisely determined by a balance between the activity of CDK1 kinase and okadaic acid sensitive phosphatase, mainly PP2A. Secondly, we show the role of CDC6 protein in regulation of the entry into the first embryonic M-phase. CDC6 inhibits CDK1 and through this action regulates the dynamic of this kinase upon M-phase entry and during M-phase progression. This mechanism discovered during my PhD allows controlling precisely the timing of embryonic cleavage. This control plays a key role in coordinating the cell cycle regulating machinery and the development program of the embryo.

Medulloblastomas are the most frequent malignant childhood brain tumors, arising in the posterior fossa of children. The overall 5-year survival is 70%, although children often suffer severe long-term side effects from standard medical care. To improve progression-free survival and quality of life for these children, finding new therapeutic targets in medulloblastoma is imperative. Medulloblastoma is divided in to four molecular subgroups (WNT, SHH, Group 3 and Group 4) based on key developmental pathways essential for the initiation and maintenance of tumor development. The MYC family of proto-oncogenes regulates cell proliferation and differentiation in normal brain. Aberrant expression of MYC proteins occurs commonly in medulloblastoma. Our studies on Group 3 medulloblastoma identify the transcription factor SOX9 as a novel target for the E3 ubiquitin ligase FBW7, and show that increased stability of SOX9 confers an increased metastatic potential in medulloblastoma. Moreover, SOX9-positive cells drive distant recurrences in medulloblastoma when combining two regulatable TetON/OFF systems. MYCN depletion leads to increased SOX9 expression in Group 3 medulloblastoma cells, and the recurring tumor cells are more migratory in vitro and in vivo. Segueing to treatment of medulloblastoma, we show that BET bromodomain inhibition specifically targets MYC-amplified medulloblastoma cells by downregulating MYC and MYC-transcriptional targets, and that combining BET bromodomain- and cyclin-dependent kinase- inhibition improves survival in mice compared to single therapy. Combination treatment results in decreased MYC levels and increased apoptosis, and RNA-seq confirms upregulation of apoptotic markers along with downregulated MYC target genes in medulloblastoma cells. This thesis addresses novel findings in transcription factor biology, recurrence and treatment in Group 3 medulloblastoma, the most malignant subgroup of the disease.

Hepatocellular carcmoma (HCC) is the third most common cause of cancer-related deaths worldwide, with a gender prevalence observed in men. Recent studies have suggested that elevated activity of the androgen axis is one major host factor underlying this disparity between genders. The androgen receptor (AR) mediates function of androgen in vital developmental and oncogenic pathways by binding to genomic androgen response elements, which influence the transcription of downstream target genes. AR is overexpressed in 60-80% of human HCCs. Genetic studies further established the pivotal role ofAR in hepatocarcinogenesis, where liver-specific knockout of AR significantly reduced tumorigenicity in carcinogen- and HBV-induced HCC mouse models. However, AR-inducedhepatocarcinogenesis is far from fully understood, in part because little is known about the identity and role of direct AR-dependent targeted genes in hepatocytes. / In this study, we used genome-wide location and functional analyses to identify a critical mediator of AR signaling, cell cycle-related kinase (CCRK), in driving beta-cateninl T-cell factor (TCF)-dependent hepatocarcinogenesis. Using chromatin immunoprecipitation followed by promoter array analysis of AR-overexpressing HCC cell lines, we found a number of cell cycle-related genes that are likely under the direct modulation of AR. Cell cycle-related kinase (CCRK), previously shown to promote glioblastoma tumorigenesis, was found to be the most significantly-bound AR target ( p<0.0001). CCRK was directly up-regulated by ligand-activated AR through promoter binding and required for AR-induced G1-S cell cycle progression because (1) CCRK overexpression attenuated cell cycle blockage by AR knockdown and (2) CCRK inhibition counteracted AR-mediated cell cycle progression. Ectopic CCRK expression induced immortalized liver cell proliferation, malignant transformation and tumor formation in immunodeficient mice, whereas CCRK inhibition decreased HCC cell growth in vitro and in vivo. These functional assays demonstrated that CCRK is a potential oncogene in HCC. Mechanistically, CCRK activated beta-catenin/TCF-dependent transcription through phosphorylation of glycogen synthase kinase-3beta and induced the expressions of beta-catenin target genes, cyclin D1 (CCND1) and epidermal growth factor receptor (EGFR). Inhibition of beta-catenin/TCF signaling attenuated CCRK-induced cell cycle progression, colony formation and tumorigenicity. Conversely, HCC cell growth inhibition by CCRK knockdown was rescued by constitutively active beta-catenin or TCF. In agreement with these findings, activation of the AR/CCRK/beta-catenin axis was frequently observed in primary HCCs. More importantly, CCRK over-expression was correlated with tumor staging and poor overall survival in a cohort ofhuman HCC tissues. / Together, our data reveal a new cascade for AR function in hepatocarcinogenesis via the activation of beta-catenin/TCF signaling. This study also reveals that CCRK is a novel focal link between two prominent signaling pathways vital for HCC growth and thus represents a new therapeutic target for HCC treatment. / Feng, Hai. / Adviser: Sung Jao Yiu. / Source: Dissertation Abstracts International, Volume: 73-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 161-177). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Errata inserted inside back cover. / "August 2002" / Includes bibliographical references (leaves 146-174) / 177 leaves, [91 leaves of plates] : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Molecular Biosciences, 2002

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 103-119).