Abstract 5761: An alternative isoform of the metastasis susceptibility gene Brd4 promotes EMT and metastatic progression

Recent work from our laboratory has identified the bromodomain 4 (Brd4) as a suppressor of tumor progression and metastasis. Brd4 however has an alternatively spliced isoform that truncates the protein and substitutes a different 3′-UTR. Unlike the longer isoform, expression of the short isoform (ΔC) does not impair primary tumor growth of a highly metastatic mammary tumor cell line orthotopically implanted into the mammary fat pad, although pulmonary metastasis is significantly reduced compared to control tumors. Unexpectedly, a dramatic enhancement of metastatic colonization is observed following injection of the cells directly into the circulation of mice. In vivo invasion of the cell line was not impaired by expression of the short isoform suggesting that the inability of the orthotopic tumors to metastasize was not due to the inability of the tumor cells to escape from the primary tumor. Consistent with the tail vein experiments, in vitro analyses demonstrate that ΔC-expressing cells exhibit a more malignant phenotype with an increased ability to grow in a three dimensional culture compared to control cells. Furthermore, ΔC-expressing cells are able to form tumorspheres in serum-free and non adherent conditions, suggesting a reversion to a more stem-like state. Microarray gene expression analysis demonstrates that ectopic expression of ΔC modulates the expression of a subset of EMT markers and stem cell markers. Microarray analysis further shows that ectopic expression of ΔC induces a gene expression signature that predicts poor survival in multiple human breast cancer datasets. This expression signature nearly perfectly matches a molecular classifier of high-grade tumors. Taken together, these data suggest that the short isoform may be a metastasis-promoting molecule and that metastasis susceptibility mediated by Brd4 may be the result of a complex equilibrium between the metastasis-suppressing long isoform and the metastasis-promoting shorter isoform. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5761.