Abstract 2331: Transcriptional repressor PRDM1/Blimp-1 directly regulates transcriptional elongation factor ELL3 during terminal B-cell differentiation

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA As the master regulator of B-cell differentiation, PRDM1 is well known for its ability to extinguish a network of transcription factors essential in maintaining the B-cell phenotype. In addition PRDM1 is a tumor suppressor in Diffuse Large B-Cell Lymphoma (DLBCL). However its molecular targets remain to be clearly elucidated. For that purpose we have identified novel targets of PRDM1 by chromatin immunoprecipitation (ChIP) and deep sequencing. One of the strongest associations with PRDM1 was observed at the Eleven-Nineteen Lysine-rich Leukemia (ELL) 3 promoter. The ELL family of transcriptional elongation factors (ELL, ELL2, and ELL3) is reported to increase the catalytic rate of RNA polymerase (pol) II transcription through assembly of the Super Elongation Complex (SEC), although their role in activated B-cells is unknown. Two potential consensus PRDM1 binding sites were mapped within the ELL3 proximal promoter and endogenous PRDM1 binding was confirmed by direct ChIP-qPCR analysis in multiple cell lines. The cloned ELL3 promoter was highly active in B-cell lines and co-transfection with PRDM1 significantly repressed activity. Mutation of both PRDM1 binding sites eliminated repression, while mutation of either individual site resulted in a partial loss of repression. To establish the expression pattern of ELL family members in normal human B-cells, primary naive B-cells were isolated by negative selection and either mildly activated with IL-2 and IL-4 or differentiated in the presence of IL-2, IL-21, anti-IgM, and CD40 cross-linking antibody. Naive B-cells did not express detectable protein levels of any ELL family members. Mild activation induced the expression of ELL and ELL3 while in contrast differentiation selectively suppressed ELL3 while inducing ELL2 and PRDM1. Expression of ELL, ELL2, and ELL3 was further profiled in a panel of B-cell lymphomas and multiple myeloma cell lines. Cell lines representing mature activated B-cells (Burkitts, DLBCL) displayed robust expression of ELL3 while lines of a pre B-cell, naive B-cell or differentiated plasma cell did not express ELL3. ELL2 expression was observed only in the plasma cell lines, consistent with previous reports. ELL was ubiquitously expressed at low levels. shRNA knockdown of ELL3 in ELL3 expressing B-cell lymphomas reveals that ELL3 is required for cell proliferation. Together these results establish ELL3 as an activated B-cell restricted elongation factor and suggests that a switch from ELL3 to ELL2 occurs during terminal differentiation. Dependence on ELL3 for proliferation of lymphoma lines suggests it may be a valid therapeutic target in ELL3 high expressing tumors and is consistent with suppression by the tumor suppressor PRDM1. Citation Format: Lou-Ella M.M. Alexander, January M. Watters, Michelle Maurin, Kenneth L. Wright. Transcriptional repressor PRDM1/Blimp-1 directly regulates transcriptional elongation factor ELL3 during terminal B-cell differentiation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2331. doi:10.1158/1538-7445.AM2014-2331