Role Of The Transcription Factor Iroquois Homeobox 2 (Irx2)In Breast Cancer Progression And Metastasis

The onset of the metastatic cascade -the main cause of cancer-related death- is the dissemination of single tumor cells into distant organs. The presence of disseminated tumor cells (DTC) in the bone marrow (BM) of cancer patients is an independent prognostic factor for the occurrence of metastatic relapse in breast cancer and other epithelial tumors. Our current results suggest that the transcription factor IRX2 might represent a novel tumor suppressor gene which might in particular contribute to early hematogenous dissemination. The purpose of this study is to gain insights into the role of the transcription factor IRX2 during breast cancer progression and metastasis. In order to identify genes that could account for early hematogenous dissemination we compared expression profiles of primary breast tumors with or without detectable DTC in the BM. We further analyzed the transcript expression by quantitative RT-PCR analysis in an independent set of primary breast tumor samples. With the purpose to provide further evidence about the clinical relevance and the biological function of the IRX2 transcription factor we also performed different computer-based analyses using a large publicly available patient data set. Potential transcriptional targets of the IRX2 protein were moreover validated in IRX2-knock-down model systems and by gene-reporter-assays. We were able to discover that low IRX2 expression in primary breast tumors is associated with BM status. In addition, we could show that low IRX2 expression was associated with decreased long time survival of breast cancer patients. Furthermore, by using stringent thresholds in a bioinformatical approach we could identify 22 potential transcriptional targets of the IRX2 transcription factor. Most interestingly, we found that LRIG1 might be a crucial target of the IRX2 gene product. We could reveal a positive correlation between IRX2 and LRIG1 expression in primary breast tumors and we could identify a IRX2 binding site in the proximal LRIG1-promotor which is effectively transactivated by the IRX2 protein in vitro. Additionally we could show that shRNA-mediated knock-down of IRX2 expression in cultured breast cancer cell lines is attended by decreased LRIG1 expression. Our present results imply that the IRX2 gene product might represent a novel tumor suppressor protein. Loss of IRX2 expression could contribute in particular to early hematogenous dissemination. In addition, our findings show a direct transcriptional regulation of LRIG1 by the IRX2 transcription factor. Since LRIG1 is an important negative regulator of receptor tyrosine kinases, we assume that decreased IRX2 expression may led to a more aggressive phenotype by sustaining the over expression of receptor tyrosine kinases, like EGFR and ERBB2. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3099. doi:10.1158/1538-7445.AM2011-3099