Date of Award

8-2009

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

Supervisor

Juliet M. Daniel

Language

English

Abstract

Kaiso is a member of the BTB/POZ-ZF (hereafter POZ-ZF) family of transcription factors that was initially identified as a p120 interaction partner. Like typical POZ-ZF transcription factors, Kaiso has an N-terminal POZ domain that facilitates protein-protein interactions and a DNA-binding zinc finger region at the C-terminus. Kaiso is implicated in the Wnt signaling pathway and has important roles in both normal development and in disease pathogenesis. In the Xenopus model system, Kaiso depletion results in gastrulation defects during embryogenesis. However, Kaiso expression rescues the axis-duplication phenotype resulting from β-catenin overexpression in Xenopus tadpoles. Additionally, Kaiso has been shown to bind the xWnt11 promoter directly and downregulate expression of this non-canonical Wnt ligand involved in directing cell movement, adhesion, and polarity. Taken together these two findings confirm Kaiso's role as a negative regulator of Wnt signaling in Xenopus. In the mouse model system, however, it was found that Kaiso was dispensable throughout embryogenesis and adulthood. In the Apc^Min/+ model of intestinal cancer, constitutive Wnt signaling induces polyp formation in the small intestines of affected mice and this closely mimics the human disease familial adenomatous polyposis (FAP). Kaiso null mice actually display a resistance to intestinal tumorigenesis; they exhibit a delay in tumor formation and a reduction in tumor size and number. Hence, in contract to the Xenopus system, data from the mouse model hints at Kaiso's role as a positive regulator of Wnt signaling. To resolve this discrepancy, we engineered an intestinal-specific Kaiso overexpression mouse model and observed the phenotype in a wildtype and Apc^Min/+ background.

Kaiso overexpression mice were created by pronuclear injection of Kaiso cDNA under control of the intestinal-specific villin promoter. Four founder animals were initially identified but only three of these founders, A, B, and, C, produced viable and fertile Kaiso transgenic offspring. Western blot analysis of A, B, and C mouse intestinal tissue showed that Kaiso is efficiently overexpressed in the intestines at high, moderate, and low levels, respectively. RT-PCR analysis showed that Kaiso overexpression is limited to villin positive tissues, including the kidney, small intestine, and large intestine. Immunohistochemical analysis showed that Kaiso overexpression had no overt effect on normal intestinal development. When crossed with Apc^Min/+ mice, high levels of Kaiso overexpression correlated with increased intestinal tumorigenesis. This implicated Kaiso as a positive regulator of Wnt signaling in the mammalian mouse model and supports previous independent studies. Future experiments should include the analysis of Kaiso's effect on cell proliferation along the crypt-villus axis in the intestines of Kaiso overexpression mice. Further investigation of Kaiso's role in intestinal development and tumorigenesis could be achieved through crosses with p120 intestinal-specific misexpression mice in wildtype and Apc^Min/+ backgrounds.

McMaster University Library

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