Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Pathology and Laboratory Medicine

Supervisor

Tuck, Alan B.

2nd Supervisor

Chambers, Ann F.

Joint Supervisor

Abstract

TBX3 is a transcriptional regulator involved in embryonic development and in tumorigenesis of several cancer types. There are two isoforms of TBX3 (TBX3iso1 and TBX3iso2) with different DNA binding domains. The large-scale functional roles of TBX3iso1 and TBX3iso2 were characterized in a breast cancer context. Both TBX3 isoforms induced invasiveness and an epithelial-to-mesenchymal (EMT) phenotype. Coupling data from genome-wide ChIP-array and RNA-Seq studies provided a novel list of genes regulated by each isoform. Both TBX3 isoforms regulate expression of several EMT-related genes, including SLUG and TWIST1. Importantly, TBX3 is a direct regulator of SLUG, and SLUG expression is required for TBX3-induced migration and invasion.

Assessing TBX3 expression in early stage breast cancers by immunohistochemistry (IHC) revealed high expression in low-grade lesions. Within a second non-high-grade cohort, there was an association between TBX3 expression in the pre-invasive ductal carcinoma in situ (DCIS) and size of the invasive focus. Additionally, there was a positive correlation between TBX3/SLUG, and TBX3/TWIST1 expression by IHC in the invasive carcinoma. Pathway analysis of transcriptomics data revealed altered expression of several proteases and their inhibitors, consistent with the ability of tumor cells to degrade basement membrane. These findings strongly suggest the involvement of TBX3 in the promotion of invasiveness and progression of early stage pre-invasive DCIS to invasive carcinoma through the low-grade molecular pathway.

Interestingly, only TBX3iso1 overexpressing cells exhibited increased tumorigenic potential in mouse xenograft experiments. Transcriptomics data and functional studies revealed that TBX3iso1 overexpression promotes angiogenesis and secretion of cancer-associated cytokines (including osteopontin) which is able to induce tubule formation by endothelial cells in vitro. Tumorigenic TBX3iso1 overexpressing cells also had elevated hyaluronan synthase 2 (HAS2) levels and high levels of hyaluronan retention. These factors may contribute to the survival of cells and promote angiogenesis, allowing the formation of primary tumors in vivo.

In conclusion, I have found evidence for a role of TBX3iso1 and TBX3iso2 in direct modulation of EMT and invasiveness, and a role for TBX3iso1 in inducing angiogenesis. Together these, along with previous work showing anti-senescence and pro-proliferative activities of TBX3, suggest multiple potential activities for promotion of malignancy of breast cancer.

Available for download on Wednesday, January 01, 2020

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