July 2009

Document Type


Degree Name



Dept. of Molecular and Medical Genetics


Oregon Health & Science University


Human colorectal cancer (CRC) accounts for one in three diagnosed cancers and one in two cancer-related deaths. A great deal about the underlying genetics of human CRC has been learned from human and mouse studies. Yet we still have limited understanding of how isolated genetic changes in different tissue types contribute to CRC development and progression. To investigate this elusive question, I have utilized a novel Pms2[superscript cre] system for stochastic alteration of loxP-marked cancer genes in mice. With this system, Cre activation is achieved through an unrepaired mutation event, with a frequency modulated by DNA mismatch repair status. The Pms2[superscript cre] system represents a more faithful mouse model of human cancers and can be applied to understand the outcome of random, isolated mutation of target genes. Specifically described within this work, stochastic activation of the oncogenic form of k-Ras confers a stem cell advantage as observed through increased area of cells harboring the oncogenic mutation and hyperplasia within the colon. In contrast, experiments with a conditionally inactivated TgfβrII allele demonstrate that isolated reduction of TgfβrII results in a competitive disadvantage along the crypt/villi axis and is associated with an increase in crypt apoptosis. Further, I note haploinsufficiency for reduced TgfßrII along the crypt/villus axis. Finally, I apply the Pms2[superscript cre] system to investigate the effects of activating a stabilized form of c-Myc and inactivating an allele of Smad4 alone, or in combination, in gastrointestinal tumor development. My results demonstrate that c-Myc stabilization accelerates Smad4 tumorigenesis. Smad4/c-Myc gastric/duodenal adenomas are more aggressive and invasive than Smad4-only tumors. Additionally, Smad4/c-Myc adenomas have an increased number of bone-marrow derived cells, which may be the tumor-initiating cell. Not only what genetic changes underly cancer development but in what tissue type . The TgfßrII results suggest that isolated disruption of this gene is disadvantageous in tumor initiation. The Smad4/c-Myc results suggest different tumor-initiating capabilities when these genes are altered in the epithelial versus the stroma. Thus, the work described in this thesis provides insight into how genetic changes at the single-cell level affect, in a dynamic way, tumor development.




School of Medicine



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