Transcription Activation of an Antisense Long Noncoding RNA (lnc-Mega1) of RBM15 By RUNX1 Enhances Megakaryocyte Differentiation

RBM15 encodes an RNA binding protein, which regulates self-renewal and differentiation of hematopoietic stem cells. RBM15 knock-out promotes the proliferation of Mk progenitor cells but blocks the maturation of Mk progenitor cells as measured by polyploidy. RBM15-MKL1 fusion causes pediatric acute megakaryoblastic leukemia without Down’s syndrome. Here, we cloned a long non-coding RNA (we called Lnc-Mega1), which is transcribed in the opposite direction of the human RBM15 gene. Lnc-Mega1 is not transcribed in mice in the RBM15 locus. Blast search also indicates there are no homologues available in mouse. The 5’end of Lnc-Mega1 overlaps with the 5’ end of RBM15 transcript for 180bp. We found that RUNX1 binds to the promoter regions of RBM15 by chromatin immunoprecipitation assay. RUNX1 knock-down reduces the transcription of both RBM15 and Lnc-Mega1. Therefore, RUNX1 controls the transcription of both genes. Nevertheless, the promoter is not always bidirectional. We found in hematopoietic stem cell RBM15 is transcribed more actively than lnc-Mega1. During differentiation, both lnc-Mega-1 and RBM15 are transcribed actively. In addition, ectopic expression of Lnc-Mega1 enhances megakaryocyte differentiation in both leukemic cell lines and human CD34+ cord blood cells. The tertiary structure of the lnc-Mega1 is critical as truncations cannot expedite differentiation. Many genes important for hematopoiesis such as Pu.1, p21, and p53 contain anti-sense transcription, but the mechansims for antisense RNA molecules in regulating their sense counterparts are very different. Although RBM15 does not directly interact with lnc-Mega1, Lnc-Mega1 overexpression enhances the RBM15 protein level via increasing the translation efficiency. Given that RBM15-MKL1 is the causal protein in acute megakaryoblastic leukemia, lnc-Mega1 RNA might contribute to leukemogenesis by increasing the translation efficiency of the fusion protein.