Deletion of the Proline-Rich Region of the Murine Metastasis Susceptibility Gene Brd4 Promotes Epithelial-to-Mesenchymal Transition- and Stem Cell-Like Conversion

Abstract

The bromodomain-containing chromatin-modifying factor BRD4 is an inherited susceptibility gene for breast cancer progression and metastasis, but its functionality in these settings has yet to be explored. Here we show that deletion of either of the BRD4 bromodomains had modest effects on the metastatic suppression ability of BRD4. In contrast, expression of the natural short isoform of BRD4 that truncates the protein after the SEED domain restored progression and metastatic capacity. Unexpectedly, deletion of the proline-rich region induced mesenchymal-like conversion and acquisition of cancer stem cell-like properties, which are mediated by the carboxy-terminal P-TEFb binding domain. Deletion of this proline-rich region also induced a gene expression signature that predicted poor outcome in human breast cancer data sets and that overlapped G3 grade human breast tumors. Thus our findings suggest that BRD4 may be altering the predisposition of tumors to undergo conversion to a more de-differentiated or primitive state during metastatic progression. Cancer Res; 71(8); 3121-31. Ó2011 AACR.

Integrated cross-species transcriptional network analysis of metastatic susceptibility

“…The inherited differences in immune components in the mouse that predict DMFS in ER − patients is, to the best of our knowledge, unique evidence that polymorphisms in stromal tissue play an important role in predisposing women to disseminated disease. Our previous efforts to identify metastasis susceptibility genes have focused on tumor-autonomous factors (25,(33)(34)(35) rather than nonautonomous factors, such as immune response. Interestingly, epidemiology studies have demonstrated that polymorphisms in the human orthologs of these cell-autonomous metastasis susceptibility genes are prognostic only in ER + patients (36), consistent with the finding that the cell-autonomous TPX2 mitotic spindle checkpoint network is significant only in ER + patients.…”

“…To test this hypothesis, transcriptional analysis of the metastasis susceptibility gene Bromodomain 4 (Brd4) (19) was performed. Brd4 is transcribed as two isoforms, the longer of which is antimetastatic (19) and induces a gene signature associated with longer DMFS, but the short isoform is prometastatic and induces a gene signature indicative of poorer DMFS (20). The expression of the TPX2 network was therefore examined in the highly metastatic Mvt1 mouse mammary tumor cell line (17) stably expressing either the long or short Brd4 isoforms.…”

“…The Brd4-expressing cell line has been previously described (19,20). Briefly, the highly metastatic Mvt1 cell line (17) was transfected with long isoform Brd4 expression vectors (19) or transduced with short isoform-expressing lentiviruses (20).…”

“…Individual clones were generated, total RNA isolated, and arrayed on Affymetrix MOE430 v2 chips by the K.W.H. laboratory, as previously described (19) or by the Microarray Core in the NCI Laboratory of Molecular Technology (20). Expression data were normalized using the Partek Genomics Suite then loaded into BRB ArrayTools version 4.2.0-Beta_2 (42).…”

The Complex Nature of Breast Cancer Stem-Like Cells: Heterogeneity and Plasticity

“…To date only a few groups have addressed this issue. A number of molecular mechanisms which potentially underlie this stem cell/EMT plasticity have been identified including the p130Cas/Cyclooxygenase-2 axis and Brd4 [38,39]. Meyer et al [19] have also demonstrated that single non-invasive epitheliallike CD44 -cells [19].…”