AML1/Runx1 Negatively Regulates the Number of Quiescent Hematopoietic Stem Cells in Adult Hematopoiesis.

Transcription factor AML1 (also called Runx1), which was initially isolated from the t(8;21) chromosomal translocation frequently found in the acute myelogenous leukemia FAB M2 subtype, is essential for the development of multilineage hematopoiesis in mouse embryos. By analyzing conditional AML1 knockout mice, we have previously shown that AML1 negatively regulates the number of immature hematopoietic cells defined as lineage-negative, CD34⁻ Sca-1⁺ c-Kit⁺ (34KSL) cells in adult hematopoiesis, while it is required for megakaryocytic maturation and lymphocytic development. The former is a significant observation because an increase in hematopoietic stem/progenitor cells due to defective AML1 function may be closely related to the development of human leukemia. In support of this is the fact that mice in which leukemic chimeric protein AML1/ETO is expressed in hematopoietic cells are subject to myeloproliferative disease and develop leukemia after additional mutation. However, it has remained yet to be determined how AML1 contributes to homeostasis of hematopoietic stem cells (HSCs). To address this issue, we analyzed in detail HSC function in the absence of AML1. Notably, cells in the Hoechst 33342 side population (SP) fraction are increased in number in AML1-deficient bone marrow, which suggests enrichment of quiescent HSCs. We quantitatively evaluated HSCs by bone marrow transplantation assays using limiting dilution and found a significant increase in HSC number within the AML1-deficient bone marrow. These results indicate that the number of quiescent HSCs is negatively regulated by AML1, loss of which may result in accumulation of leukemic stem cell pool in AML1-related leukemia.