The TAL1 Short Isoform Generated By PRMT1-Mediated Alternative RNA Splicing Promotes Erythroid Differentiation

The isoforms of key transcription factors in hematopoiesis such as TAL1, GATA1 and RUNX1 are generated through alternative RNA splicing regulated by the PRMT1-RBM15 axis (Zhang et al. 2015). The functions of short isoforms of GATA1 (GATA1s) and RUNX1 (RUNX1a) are well characterized, yet it is unknown how the short isoform of TAL1 (TAL1s) regulates hematopoiesis. In this presentation, we report that the short isoform of TAL1, i.e. TAL1s, is generated via alternative RNA splicing as detected by isoform specific real-time PCR reactions using RNA isolated from leukemia cell lines and primary human cord blood cells. RBM15, an RNA binding protein, which is involved in chromosome translocation to produce RBM15-MKL1 fusion protein in acute megakaryocytic leukemia, regulates the alternative RNA splicing of TAL1. RBM15 promotes the production of full-length TAL1 mRNA, while reduction of RBM15 protein level via PRMT1-mediated degradation pathway favors the production of TAL1s. RBM15 directly binds to intronic regions on TAL1 pre-mRNA. Binding of RBM15 is responsible for recruiting SF3B1-associated RNA splicing complex. Given that PRMT1 senses the hypoxia status of hematopoietic cells, the changing of TAL1s/TAL1fl ratio by PRMT1 activity may be an adaptive response of hematopoietic cells to hypoxia status. The short form TAL1s still contains the helix-loop-helix DNA binding domain but not the N terminal regions upstream of the DNA binding domain. Thus, the TAL1s may act as a dominant negative mutant of the full-length TAL1fl to block TAL1fl-regulated transcription. We demonstrated that overexpression of TAL1s not the full-length TAL1promotes the erythroid differentiation of K562 cells. Although TAL1 gene is required for both erythroid and megakaryocyte differentiation at early stage of hematopoiesis, TAL1s does not promote megakaryocyte differentiation. Therefore, fine-tuning the TAL1 isoforms by the PRMT1-RBM15 axis determine the cell fate of a MEP progenitor cell. Using immunoprecipitation assays and mass spectrometry analysis, we identified proteins specifically associated with the N terminal region of TAL1. How unique TAL1s-associated transcriptional regulatory complex plays in erythroid differentiation will be discussed in the presentation in comparison with the Tal1fl-asociated protein complex. In summary, our findings stratify another new layer of regulation by PRMT1, which relays extracellular signals (such as hypoxia signal) to transcriptional regulatory program. Given that PRMT1 is often constitutively highly expressed in leukemia cells, how overproduction of short form TAL1 interferes with normal hematopoiesis may help to explain the molecular mechanisms of many hematological malignancies associated with dysregulation of TAL1 expression.