Mouse RUNX1C regulates premegakaryocytic/erythroid output and maintains survival of megakaryocyte progenitors.

RUNX1 is crucial for the regulation of megakaryocyte specification, maturation, and thrombopoiesis. possesses 2 promoters: the distal and proximal promoters. The major protein isoforms generated by and are RUNX1C and RUNX1B, respectively, which differ solely in their -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. RUNX1B expression is more restricted, being highly expressed in the megakaryocyte lineage but downregulated during erythropoiesis. We generated a knock-in of RUNX1B, termed This mouse line lacks RUNX1C expression but has normal total RUNX1 levels, solely comprising RUNX1B. Using this mouse line, we establish a specific requirement for the RUNX1C isoform in megakaryopoiesis, which cannot be entirely compensated for by RUNX1B overexpression. knock-in megakaryocyte progenitors have reduced proliferative capacity and undergo increased cell death, resulting in thrombocytopenia. knock-in premegakaryocyte/erythroid progenitors 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, proapoptotic pathways are activated in knock-in premegakaryocyte/erythroid progenitors, presumably accounting for the increased cell death in the megakaryocyte progenitor compartment. Unlike in the conditional adult null models, megakaryocytic maturation is not affected in the 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.