The signaling pathways responsible for proper development are often the same ones implicated in cancer formation. Sonic Hedgehog (Shh) signaling is a critical developmental pathway that directs cerebellar development and ventral neural tube patterning. Shh ligand binds the receptor Patched, removing Patched's inhibition of Smoothened and allowing for activation of Gli transcription factors. When ligand is absent, Gli proteins are processed to their repressor form. In vertebrates, this signaling cascade requires the primary cilium, a microtubule-based projection found on every vertebrate cell. The ciliary GTPase Arl13b regulates Shh signaling at multiple levels and affects the production of Gli activator but not Gli repressor. Overactivated Shh signaling can cause medulloblastoma (MB), a tumor of the cerebellum, and new molecular therapies are needed. Genetic ablation of cilia prevents tumors in mice, but cilia are a poor therapeutic target. Arl13b is a strong candidate since its loss reduces Shh activity while preserving Gli repressor, which protects against MB formation.
This dissertation covers my work on the requirement of Gli repressor for ventral neural tube patterning. By deleting Arl13b--and thus disrupting Gli activator function--we uncovered a patterning defect in the neural tube that corrected over time; I showed this recovery was dependent on Gli repressor. I also describe the generation of a mouse model of MB in which I could delete Arl13b in the cerebellum before tumor formation or in an existing tumor. However, after the generations of crosses required to make the model, the tumor phenotype disappeared, leading to the conclusion that a genetic modifier was interfering with tumor formation.
The core of this dissertation demonstrates that the loss of Arl13b in a mouse model of MB inhibits tumor formation. By disrupting Arl13b function in cell culture, I demonstrate that the loss of Arl13b results in reduced Shh signaling--and related growth and proliferation phenotypes. I show that Arl13b is not an essential gene in adulthood in mice. Taken together, my results provide the proof of principle that Arl13b is a promising target for the treatment of MB as its loss counteracts tumorigenic signaling while leaving cilia function largely intact.
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