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Characterization of microRNA oncogenes and tumor suppressors in a mouse B-lymphoma model.



Characterization of microRNA oncogenes and tumor suppressors in a mouse B-lymphoma model.


Margaux Joy Bennett

Doctor of Philosophy Molecular and Cell Biology

University of California, Berkeley

Professor Lin He, Chair

Cancer of any cell type, including B-cell lymphoma, arises from a series of genetic lesions that ultimately transforms normal cells into tumor cells. These genetic lesions allow the cells to accumulate traits such as uncontrolled proliferation, resistance to cell death, and invasive growth. Functional characterization of these genetic lesions will help us to understand the molecular alterations required for lymphomagenesis, and to provide better therapeutic strategies. Several major lesions that drive oncogenesis have been well-characterized, but the complex signaling network that cooperates with and regulates these oncogenic drivers has yet to be understood. In the last decade, it has become clear that non-coding RNAs, particularly microRNAs, both as oncogenes and tumor suppressors, play a large role in cancer initiation, progression, and maintenance.

In this dissertation, I discuss the roles that the oncogenic miRNA cluster mir-17-92, and the tumor suppressive mir-34 miRNA family play in B-cell lymphomagenesis, using the Eμ-myc mouse model that mimics human Burkitt's lymphoma. First, I present our work dissecting the mir-17-92 cluster. In mammals, the mir-17-92 cluster of miRNAs has an unconventional gene structure compared to protein-coding genes in that one mir-17-92 primary transcript yields six individual miRNAs. The mir-17-92 cluster is found to be amplified in several human B-cell malignancies, and overexpression of mir-17-92 is known to accelerate Eμ-myc lymphomagenesis. To determine the mechanism behind mir-17-92's acceleration of lymphomagenesis, we dissected the components of the cluster and found that mir-19 is both necessary and sufficient to accelerate Eμ-myc B-cell lymphomagenesis. While miRNAs regulate many pathways and targets, the oncogenic ability of mir-19 is due at least in part to its capacity to downregulate the tumor suppressor phosphatase and tensin homolog (Pten). mir-19 thus promotes cell survival over c-Myc induced apoptosis. We next show a new paradigm illustrating the internal oncogenic and tumor suppressive crosstalk that occurs within the unique polycistronic cluster of mir-17-92. We show that, fascinatingly, mir-92 antagonizes mir-19's oncogenic ability in this B-cell lymphoma model by downregulating Fbw7, a negative regulator of c-Myc. Consequently, mir-92 increases the levels of c-Myc imposing a strong coupling between excessive proliferation and apoptosis. Therefore, during lymphomagenesis there is a selection to disrupt this internal antagonism of mir-17-92 to favor the development of lymphoma cells that express less mir-92 relative to that of mir-19.

In the final chapter of this dissertation I switch gears from analyzing oncogenic effects to investigating the tumor suppressive roles the mir-34 miRNA family members have in Eμ-myc lymphomagenesis. The mir-34 miRNA family consists of six miRNAs that have identical seed sequences, which are located in three genomic loci with different cistronic arrangements; in humans mir-34a is transcribed from chromosome 1, mir-34b/mir-34c is transcribed as a single transcript from chromosome 11, and the mir-449 family is transcribed as a single transcript containing mir-449a/mir-449b/mir-449c from chromosome 5. p53 transcriptionally induces the expression of mir-34a, and mir-34b/mir-34c, while the mir-449 cluster is induced by E2f1. We found that the deletion of each of these three miRNA loci exhibit an acceleration of Eμ-myc lymphomagenesis. Surprisingly, however, the complete deletion of mir-34a and mir-34b/mir-34c loci abolishes the acceleration that is seen upon the deletion of the individual miRNAs. In the B-cells of the complete mir-34 knockout, we found that the homologous mir-449 family is upregulated. We demonstrate that the mir-34 knockout mice also upregulate E2f1, which is likely driving the compensatory upregulation of mir-449. The compensatory responses of these homologous mir-34 miRNAs further the crosstalk between the E2f1 and p53 pathways. These studies provide two clear examples of how miRNAs play a major role in cancer progression. The discovery of this interesting internal antagonism within the mir-17-92 cluster, and the fine-tuned compensatory responses between the mir-34 and mir-449 miRNAs helps illuminate how miRNAs have evolved to maintain their unique polycistronic structures, and seemingly redundant families, in order to cross-activate regulatory pathways and rein in potentially oncogenic cell signals.

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