Novel roles of a transcription elongation regulator in mammary gland development and tumorigenesis.

Abstract

Abstract Abstract #6013 Historically, transcriptional regulation in mammary gland development and breast cancer development is thought to primarily occur at the stage of transcription initiation. However, recent studies of genome-wide transcription suggest that the majority of “inactive” genes in human cells experience transcription initiation and many of them have binding of RNA polymerase at the promoter-proximal region. Thus, transcription elongation may represent a widespread, yet vastly under-appreciated step for mammalian gene regulation. Cofactor of BRCA1 (COBRA1), which was first identified as a BRCA1-binding protein, acts as an integral subunit of the negative elongation factor (NELF) complex to stall RNA polymerase II (RNAPII) during estrogen-stimulated transcription elongation. COBRA1 also shares a number of target genes for transcriptional regulation with BRCA1 in breast cancer cells. In support of its potential role in tumor suppression, COBRA1 is preferentially expressed in luminal epithelial cells in normal mammary ducts; and its reduced expression is strongly correlated with metastatic breast cancer. Knockdown of COBRA1 leads to elevated hormone-dependent transcription and accelerated cell proliferation in vitro. To further investigate the biological function of COBRA1, we compared tumor growth of control and COBRA1 knockdown human breast cancer cells in a xenograft mouse model. In contrast to tumors from the control cells whose growth was strictly estrogen-dependent, tumors from COBRA1-depleted cells achieved sustained growth even in the absence of exogenously added estrogen. This result suggests that depletion of COBRA1 either allows hormone-independent cell proliferation or sensitizes cells to grow under extremely low levels of estrogen. In a complementary approach, a Cre-lox system was used to delete mouse Cobra1 in whole body or in mammary gland. Mouse embryos deficient in Cobra1 do not develop beyond E5.5; and mammary gland-specific Cobra1 knockout mice exhibit severe defects in alveologenesis and lactogenesis. The molecular basis for the Cobra1-associated developmental defects and the possible impact of Cobra1 deletion on mammary tumorigenesis will be discussed. By linking regulation of transcription elongation with mammary gland-specific functions, our work establishes a novel paradigm for gene regulation in mammary gland biology and breast cancer research. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6013.