Date Published: May 15, 2007
Publisher: Public Library of Science
Author(s): Shinobu Tsuzuki, Dengli Hong, Rajeev Gupta, Keitaro Matsuo, Masao Seto, Tariq Enver, Elaine Dzierzak
Abstract: BackgroundAML1/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.
Partial Text: AML1 (also termed RUNX1 or CBFA2), which encodes a transcription factor, was originally identified as a component of the fusion gene generated as a consequence of the t(8; 21) chromosomal translocation characteristic of a subset of acute myelogenous leukaemia . Subsequently, mutations in the AML1 gene were shown to be associated with a number of malignant and premalignant conditions including acute myelogenous leukaemia , childhood acute lymphocytic leukaemia, familial platelet disorder, and myelodysplastic syndromes. AML1 is thus the most frequently mutated gene in human leukaemia [2,3]. Aml1 expression marks long-term repopulating hematopoietic stem cells (HSCs) in the midgestation mouse embryo , and gene targeting experiments have unequivocally shown Aml1 to be essential for the development of the hematopoietic system; Aml1-null mice die in utero of haemorrhage and lack adult-type hematopoiesis because of a failure in the generation of definitive HSCs [5,6]. In contrast, conditional gene-targeting experiments have revealed that Aml1 is dispensable for maintenance of adult HSCs. Aml1 deficiency in adult stages is accompanied by increases in myeloid progenitors and phenotypically defined HSCs, as well as impaired development of lymphoid and megakaryocytic cells [7–9].
Here, we show that AML1a increases the competitive engraftment potential and in vitro proliferation capacity of primitive BM compartments, while AML1b exhibits an essentially opposing activity, promoting differentiation and abrogating engraftment. Expression of AML1a in human cord blood-derived hematopoietic cells gave results broadly in line with those obtained in mouse cells.