Hedgehog (Hh) signaling is essential for embryonic development and adult homeostasis. Aberrant pathway activity can result in various developmental disorders and cancers. The Hh receptor Patched1 (Ptc1) is a negative regulator of the pathway and acts as a tumor suppressor. Our lab and others have shown that Suppressor of fused (Su(fu)) and Kinesin family member 7 (Kif7) are major negative regulators of the pathway that function downstream of Ptc1. Medulloblastoma (MB) is the most common malignant pediatric brain tumor originating from the cerebellum. Several forms of MB have been identified, with abnormal activation of the Hh pathway associated with one major subtype. These tumors commonly show inactivating mutations in PTCH1, whereas mutations in SU(FU) are more rare. Mouse models with deletion of Ptc1 or expression of a constitutively active form of Smoothened (Smo) exhibit elevated Hh pathway activity, leading to MB formation. In this study, I examined the complex roles of Su(fu) in the formation of MB alongside the inactivation of Ptc1, Kif7 and p53 in the cerebellum. Unlike Ptc1+/- mice, Su(fu)+/- mice do not develop MB, even upon exposure to DNA damaging agents (X-ray irradiation or chemical carcinogenesis), which dramatically increases MB incidence in Ptc1+/- mutants. However, Su(fu)+/-;p53-/- mice develop MB and these MB tumor cells exhibit loss of heterozygosity of Su(fu), suggesting a protective role of p53 in tumor suppression in Su(fu) deficiency. Kif7+/- mice are not prone to tumorigenesis, even with deletion of p53. To bypass the embryonic lethality of Su(fu)- and Kif7-null mice, I generated neural stem cell-specific knockout mice, GFAP-Cre;Su(fu)f/f and GFAP-Cre;Kif7f/f. GFAP-Cre;Su(fu)f/f mice exhibit a severely disorganized cerebellum, with drastic up-regulation of p53 expression, and they survive past 1 year of age and do not develop MB. In contrast, GFAP-Cre;Kif7f/f mice appear grossly normal with only subtle cerebellar defects. These observations indicate that neither Su(fu) nor Kif7 inactivation is sufficient to drive tumorigenesis in the cerebellum. To investigate whether Su(fu) and Kif7 possess any overlapping tumor suppressor functions, I generated GFAP-Cre;Kif7f/f;Su(fu)f/f mice. Indeed, simultaneous loss of these negative regulators resulted in MB formation, which is correlated with increased Hh pathway activity as well as a lower level of p53 expression. Furthermore, I discovered a novel positive role for Su(fu) in MB development, as Su(fu) activity is required for robust formation of MB in Ptc1-deficient mice. Together, these data illustrate that Su(fu) plays a dual role in the genesis of MB, and suggest that the rare human MBs with SU(FU) mutations could be caused by simultaneous deletion of TP53 or KIF7.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/42608 |
| Date | 20 November 2013 |
| Creators | Satkunendran, Thevagi |
| Contributors | Hui, Chi-chung |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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