<p>Progression through the G1, S, G2 and M phases of the cell cycle is controlled by cyclin-dependent kinases (Cdks) and cyclins. These proteins form active Cdk:cyclin complexes that phosphorylate specific substrates. The Cdk:cyclin complexes of the G1/S transition regulate the progression of cells into the S phase by phosphorylating the retinoblastoma protein (Rb). This prevents Rb from sequestering E2F, a transcription factor that induces expression of genes required for DNA synthesis. This process is in part regulated by a family of Cdk inhibitors (CKIs) called the Ink4 family (<u>In</u>hibitors of Cd<u>k4</u>). The Ink4 family of CKIs consists of four members; p16<sup>Ink4a</sup>, p15<sup>Ink4b</sup>, p18<sup>Ink4c</sup> and p19<sup>Ink4d</sup>, and they bind specifically to Cdk4 and Cdk6, thereby negatively regulating their kinase activities and cell cycle progression. Because of its cell cycle inhibitory role, p16<sup>Ink4a</sup> is frequently mutated or deleted in human cancer, whereas the other <i>Ink4</i> genes are only occasionally altered in cancer. The overall aim of this thesis was to study the roles of p18<sup>Ink4c</sup> and p19<sup>Ink4d</sup> using <i>in vivo</i> models of cancer and embryonic development. In paper I, we analyzed the tumor spectrum in mice lacking <i>p53</i>, <i>Ink4c</i> and <i>Ink4d</i>. p53 is a tumor suppressor and one of the most frequently mutated genes in human cancer. Mice carrying mutated <i>p53</i> alleles are highly tumor-prone but develop predominantly lymphomas. However, the combined loss of <i>p53</i> and <i>Ink4c</i> (but not <i>Ink4d</i>) caused a shift in the tumor spectrum to increased incidences of hemangiomas and hemangiosarcomas, as well as appearance of medulloblastomas, a tumor of the cerebellum. These data, revealed in the absence of p53, suggest a cell-type specific tumor suppressing role for p18<sup>Ink4c</sup>. In paper II, loss of <i>Ink4c</i> was evaluated in another tumor-prone mouse model; the Eµ-<i>Myc</i> mouse. This is a transgenic mouse overexpressing c-Myc in B cells causing clonal B cell lymphomas. Surprisingly, precancerous B cells and lymphomas from Eµ-<i>Myc</i> mice exhibited elevated levels of p18<sup>Ink4c</sup> mRNA and protein despite high rates of proliferation. Moreover, loss of <i>Ink4c</i> in this model did not affect the rate of cell proliferation or the onset of tumor development. We conclude from these studies that <i>Ink4c</i> is not an important tumor suppressor of Myc-induced lymphomas. To gain insight into the role of <i>Ink4</i> genes in early vertebrate development, the African clawed frog, <i>Xenopus laevis</i>, was analyzed for the presence of <i>Ink4</i> homologs. Paper III describes the cloning and characterization of a gene homologous to <i>Ink4d</i>, <i>Xl-Ink4d</i>. This CKI is expressed throughout frog embryo development, making <i>Xl</i>-Ink4d the only CKI present during the cleavage stages of <i>X. laevis</i>. Antisense morpholino oligonucleotides directed against <i>Xl-Ink4d</i> were used to knock down the protein level of <i>Xl</i>-Ink4d during development. This resulted in defects in head tissues and reduced expression of <i>Twist</i>, a gene important for neural crest cell migration. We therefore propose that <i>Xl</i>-Ink4d is important for proper neural crest differentiation in the frog.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:umu-1357 |
| Date | January 2007 |
| Creators | Nilsson, Lisa |
| Publisher | Umeå University, Molecular Biology (Faculty of Science and Technology), Umeå : Molekylärbiologi (Teknisk-naturvetenskaplig fakultet) |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
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