LB56

Identifying cell growth control pathways repressed by the oncogene TBX2 in breast cancer cells

Zenobia D Costa, Nyree Crawford, Hannah Farmer, Keara Redmond, Niamh Buckley, Paul Mullan

Queen's University Belfast, UK

Background
TBX2 is transcription factor located on 17q23, a region that is amplified in approximately 15% of breast cancers. 17q23 amplification correlates with poor prognosis and also occurs more frequently in BRCA1- and BRCA2-linked breast cancers. TBX2 acts as a transcriptional repressor and switches off a number of key cell cycle regulatory molecules such as p21^WAF1 and p14^ARF. The aim of this study was to identify novel TBX2 target genes and to assess their role in proliferation or growth control.

Results
A breast cancer disease specific array was used to compare MCF-7 cells with siRNA mediated knockdown of TBX2 to scrambled siRNA. We identified >700 potential TBX2 regulated genes (+/- 2-fold). Twelve targets with known roles in cell growth (including characteristics such as cell proliferation, invasion, metastasis, angiogenesis and apoptosis) were chosen and among these genes, 4 were consistently shown to be targets of TBX2 by real time qPCR and Northern Blotting. All four targets showed growth inhibitory effects. In particular, CST6 (a cysteine protease inhibitor) was shown profound apoptotic activity when exogenously expressed in MCF7 cells (which have low constitutive CST6 expression) but did not induce apoptosis in the non-transformed cell line MCF10-A. We are now in a process of defining the mechanism of CST6 repression by TBX2 using various promoter analysis techniques. We are also investigating the mode of growth inhibition by CST6.

Conclusion
We have identified a number of novel TBX2 repressed target genes including a protease inhibitor (CST6). Reconstitution of CST6 into TBX2-expressing cells elicits a strong growth inhibition phenotype. We aim to use this information to develop new therapeutic approaches to target TBX2 driven breast cancers.