Regulation of the cyclin-dependent kinase inhibitor p21 by TBX3

Master Thesis


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University of Cape Town

TBX3, a member of the T-box family of transcription factors, is essential in development and has emerged as an important player in the oncogenic process. It is vital for the development of the heart, breasts, limbs, teeth and genitalia and TBX3 mutations lead to Ulnar-Mammary Syndrome, which is characterised by malformations of these organs and body structures. TBX3 is overexpressed in several cancers and has been implicated in a number of oncogenic processes ranging from the bypass of senescence to the inhibition of apoptosis and more recently TBX3 has been shown to contribute directly to tumour formation, migration and invasion of melanoma and breast cancer cells. However, little is known about the molecular basis for its role in development and oncogenesis because there is a paucity of information regarding its target genes. Of interest to the current study, preliminary evidence suggest that TBX3 may repress the cyclin-dependent kinase inhibitor p21WAF1, which has a role in a myriad of processes including cell cycle arrest, senescence, apoptosis, differentiation and DNA replication and repair. The current study therefore aimed to explore this possibility and to reveal a mechanism by which TBX3 regulates p21WAF1. Results show, using luciferase reporter gene assays, that TBX3 is a potent transcriptional repressor of the p21WAF1 promoter and that this repression is due to a direct interaction between TBX3 and the p21WAF1 promoter via the TBX3 DNA-binding domain. This study also provides evidence that the ability of TBX3 to repress p21WAF1 may require the TBX3 N-terminal repression domain. In addition, the site through which TBX3 acts to repress p21WAF1 was identified close to the initiator of the p21WAF1 promoter and TBX3 is shown to bind this site in vitro and in vivo. Furthermore, this study provides evidence that a serine proline motif (S190) located within the DNA-binding domain of TBX3, may play an important role in regulating the ability of TBX3 to transcriptionally repress p21WAF1 by inhibiting binding. Lastly, the repression of p21WAF1 by TBX3 is also shown to be biologically significant in TBX3 overexpression and knock down cell culture models. Results from this study provide a detailed mechanism of how TBX3 directly transcriptionally represses p21WAF1 which adds to our understanding of how TBX3 may contribute to oncogenesis.

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