Isoform-Specific Potentiation of Stem and Progenitor Cell Engraftment by AML1/RUNX1

Abstract

Background AML1/RUNX1 is the most frequently mutated gene in leukaemia and is central to the normal biology of hematopoietic stem and progenitor cells. However, the role of different AML1 isoforms within these primitive compartments is unclear. Here we investigate whether altering relative expression of AML1 isoforms impacts the balance between cell self-renewal and differentiation in vitro and in vivo.Methods and FindingsThe human AML1a isoform encodes a truncated molecule with DNA-binding but no transactivation capacity. We used a retrovirus-based approach to transduce AML1a into primitive haematopoietic cells isolated from the mouse. We observed that enforced AML1a expression increased the competitive engraftment potential of murine long-term reconstituting stem cells with the proportion of AML1a-expressing cells increasing over time in both primary and secondary recipients. Furthermore, AML1a expression dramatically increased primitive and committed progenitor activity in engrafted animals as assessed by long-term culture, cobblestone formation, and colony assays. In contrast, expression of the full-length isoform AML1b abrogated engraftment potential. In vitro, AML1b promoted differentiation while AML1a promoted proliferation of progenitors capable of short-term lymphomyeloid engraftment. Consistent with these findings, the relative abundance of AML1a was highest in the primitive stem/progenitor compartment of human cord blood, and forced expression of AML1a in these cells enhanced maintenance of primitive potential both in vitro and in vivo.ConclusionsThese data demonstrate that the “a” isoform of AML1 has the capacity to potentiate stem and progenitor cell engraftment, both of which are required for successful clinical transplantation. This activity is consistent with its expression pattern in both normal and leukaemic cells. Manipulating the balance of AML1 isoform expression may offer novel therapeutic strategies, exploitable in the contexts of leukaemia and also in cord blood transplantation in adults, in whom stem and progenitor cell numbers are often limiting.

Overexpression of RUNX1 short isoform has an important role in the development of myelodysplastic/myeloproliferative neoplasms

“…10,11 Enforced expression in murine hematopoietic cells enhances engraftment after transplantation. 12 Consistent with this, RUNX1a expands hematopoietic stem cells, 13,14 and dominant inhibition by RUNX1a results in monopoiesis. 15 Therefore, expression levels of each RUNX1 isoform are likely to be important for normal and malignant hematopoiesis.…”

Runx1 deletion or dominant inhibition reduces Cebpa transcription via conserved promoter and distal enhancer sites to favor monopoiesis over granulopoiesis

“…24 Transduction of murine marrow cells with MIG-RUNX1a, expressing RUNX1a and GFP, followed by transplantation into irradiated, syngeneic recipients led to a 2-to 3-fold in vivo expansion of GFP ϩ cells 4-8 weeks later compared with cells transduced with the empty MIG retroviral vector ( Figure 3A top left), similar to results reported previously. 25 Analysis of the vector-or RUNX1a-transduced, GFP ϩ populations found that expression of RUNX1a not only increased the total number of GFP ϩ cells but also increased the proportion of LSK HSCs 2.4-fold and lin Ϫ Sca-1 Ϫ c-kit ϩ progenitors 1.9-fold ( Figure 3A). The proportion of B220 ϩ B-lymphoid cells was reduced 2.2-fold, whereas the Ter119 ϩ erythroid population was only minimally affected.…”

Cooperating gene mutations in acute myeloid leukemia: a review of the literature

“…The expression of each isoform appears to be time dependent during embryogenesis but also in adult hematopoiesis. 113 AML1 is normally expressed in all hematopoietic lineages and acts to regulate the expression of various genes specific to hematopoiesis, including M-CSF receptor, IL-3, myeloperoxydase and TCRb genes. [114][115][116][117] Mice lacking AML1 or CBFb have no fetal liver hematopoiesis, showing that the heterodimeric complex CBF is essential for definitive hematopoiesis of all lineages.…”