Mouse RUNX1C regulates Pre-Megakaryocytic/Erythroid output and maintains survival of Megakaryocyte Progenitors

RUNX1 is crucial for the regulation of megakaryocyte specification, maturation and thrombopoiesis. Runx1 possesses two promoters: the distal P1 and proximal P2 promoters. The major protein isoforms generated by P1 and P2 are RUNX1C and RUNX1B respectively, which differ solely in their N-terminal amino acid sequences. RUNX1C is the most abundantly expressed isoform in adult hematopoiesis, present in all RUNX1-expressing populations, including the cKit+ hematopoietic stem and progenitor cells (HSPCs). RUNX1B expression is more restricted, being highly expressed in the megakaryocyte lineage but downregulated during erythropoiesis. We generated a Runx1 P1 knock-in of RUNX1B, termed P1-MRIPV. This mouse line lacks RUNX1C expression but has normal total RUNX1 levels, solely comprising RUNX1B. Utilizing this mouse line we establish a specific requirement for the P1-RUNX1C isoform in megakaryopoiesis, which cannot be entirely compensated for by RUNX1B overexpression. P1 knock-in Megakaryocyte Progenitors (MkP) have reduced proliferative capacity and undergo increased cell death, resulting in thrombocytopenia. P1 knock-in Pre-Megakaryocyte/Erythroid Progenitors (PreMegEs) demonstrate an erythroid-specification bias, evident from increased erythroid colony-forming ability and decreased megakaryocyte output. At a transcriptional level, multiple erythroid-specific genes are upregulated and megakaryocyte-specific transcripts are downregulated. In addition, pro-apoptotic pathways are activated in P1 knock-in PreMegEs, presumably accounting for the increased cell death in the MkP compartment. Unlike in the conditional adult Runx1 null models, megakaryocytic maturation is not affected in the P1 knock-in mice, suggesting that RUNX1B can regulate endomitosis and thrombopoiesis. Therefore, despite the high degree of structural similarity, RUNX1B and RUNX1C isoforms have distinct and specific roles in adult megakaryopoiesis.