Transcriptional Control of HP1a By the RNA Binding Proteins Rbmx/L1 Maintain Chromatin State in Myeloid Leukemia

• Published in: Blood 2020-11, Vol.136 (Supplement 1), p.15-15
• Main Authors: Nguyen, Diu T.T., Prieto, Camila, Liu, Zhaoqi, Wheat, Justin, Perez, Alexendar, Gourkanti, Saroj, Chou, Timothy, Barin, Ersilia, Chow, Arthur, Taggart, James, Hoskova, Katerina, Dhodapkar, Meera, Schurer, Alexandra, Barlowe, Trevor Stephen, Vu, Ly, Leslie, Christina, Steidl, Ulrich G., Rabadan, Raul, Kharas, Michael G.
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2020-142229

Mutations and aberrant expression of RNA binding proteins (RBPs) have recently been found to contribute to leukemia development (Prieto and Kharas, CSH, 2020). Previously we have identified the RBP RBMX (RNA binding motif protein, X-linked) in our shRNA in vivo screen using murine MLL-AF9 driven leukemia (Prieto et al, ASH abstract 2018). Here we uncover the role of RBMX and its retrogene RBMXL1 in normal hematopoiesis and leukemogenesis in mouse and human systems. To investigate RBMX function in normal hematopoiesis, we created an Rbmx Mx1-Cre conditional knockout model to specifically delete Rbmx in the hematopoietic system. We gender stratified our studies since Rbmx is sex-linked. Deletion of Rbmx in both female and male primary mice as well as in non-competitive transplant animals did not affect hematopoiesis. Additionally, Rbmx knockout (KO) leads to a mild reduction in multipotent progenitors (MPP2 and MPP4) in female competitive transplanted mice although no defects in long-term hematopoiesis was observed in male competitive transplanted mice. These data suggest that Rbmx is dispensable for normal hematopoiesis. To identify the role of RBMX in leukemogenesis we knockout Rbmx in MLL-AF9 murine leukemia cells and found it significantly reduced colony formation in vitro and delayed leukemogenesis in vivo, indicating that Rbmx is required for leukemia maintenance. We observed, however, that MLL-AF9 transformed cells from Rbmx KO donor mice showed no delay in leukemia initiation versus cells from wildtype. We then determined that while Rbmx deletion is effective with complete depletion of mRNA, the retrogene RBMXL1 expression is maintained in Rbmx deficient cells, which may compensate for Rbmx deletion in leukemia initiation. Indeed, depletion of RBMXL1 by shRNAs (KD) in Rbmx deficient leukemia cells results in a drastic reduction in colony formation, increased colony myeloid differentiation, and induced apoptosis in cells deficient for both RBMXL1 and RBMX compared to those only depleted of RBMX. Correspondingly, RBMXL1 KD in Rbmx deficient leukemia cells resulted in further delayed leukemogenesis in vivo, indicating that RBMXL1 is functionally redundant to RBMX and both genes are required for leukemia development and maintenance. We next investigated the role of RBMX/L1 in normal human hematopoietic and leukemia cells. We found that RBMX/L1 expression were higher in AML cell lines (n=10/11) and primary AML patient samples (n=2/4) compared to healthy i