Expression of a Knocked-In AML1-ETO Leukemia Gene Inhibits the Establishment of Normal Definitive Hematopoiesis and Directly Generates Dysplastic Hematopoietic Progenitors

Abstract

The t(8;21)-encoded AML1-ETO chimeric product is believed to be causally involved in up to 15% of acute myelogenous leukemias through an as yet unknown mechanism. To directly investigate the role of AML1-ETO in leukemogenesis, we used gene targeting to create anAML1-ETO “knock-in” allele that mimics the t(8;21). Unexpectedly, embryos heterozygous for AML1-ETO(AML1-ETO/+) died around E13.5 from a complete absence of normal fetal liver–derived definitive hematopoiesis and lethal hemorrhages. This phenotype was similar to that seen following homozygous disruption of either AML1 orCBFβ. However, in contrast to AML1- or CBFβ-deficient embryos, fetal livers from AML1-ETO/+ embryos contained dysplastic multilineage hematopoietic progenitors that had an abnormally high self-renewal capacity in vitro. To further document the role of AML1-ETO in these growth abnormalities, we used retroviral transduction to express AML1-ETO in murine adult bone marrow–derived hematopoietic progenitors. AML1-ETO–expressing cells were again found to have an increased self-renewal capacity and could be readily established into immortalized cell lines in vitro. Taken together, these studies suggest that AML1-ETO not only neutralizes the normal biologic activity of AML1 but also directly induces aberrant hematopoietic cell proliferation.

Decreased expression of transcription factor GATA‐2 in haematopoietic stem cells in patients with aplastic anaemia

“…However, there was no significant difference in expression levels of AML1 and SCL mRNA in CD34-positive cells between aplastic anaemia patients and normal subjects in contrast to GATA-2. AML1 was the most common gene involved in acute myelogenous leukaemia, in which myeloid differentiation was blocked by a disturbance of the proper function of AML1 by the fusion protein AML1-ETO (Okuda et al, 1998). It has also been shown that the expression of SCL was upregulated along with erythroid differentiation (Elwood et al, 1998;Valtieri et al, 1998), consistent with the result in this study that SCL mRNA expression was lower in the CD34-positive fraction than in the CD34-negative fraction.…”

The Aml1‐eto Chimaeric Transcription Factor in Acute Myeloid Leukaemia: Biology and Clinical Significance

“…This strategy resulted in the expression of the chimaeric gene from the endogenous AML1 regulatory sequences, thus accurately mimicking the t(8;21). Under these conditions, expression of AML1-ETO produced an embryonic lethal phenotype that was almost identical to the one observed after the loss of AML1 or CBFb (Yergeau et al, 1997;Okuda et al, 1998). This result formally demonstrates that AML1-ETO can function as a dominant inhibitor of normal AML1/CBFb activity.…”

“…Direct proof of the role of AML1-ETO in haemopoietic cell transformation has come from experiments in which an AML1-ETO chimaeric gene was created by knocking ETO into the AML1 genomic locus (Yergeau et al, 1997;Okuda et al, 1998). This strategy resulted in the expression of the chimaeric gene from the endogenous AML1 regulatory sequences, thus accurately mimicking the t(8;21).…”

“…This result formally demonstrates that AML1-ETO can function as a dominant inhibitor of normal AML1/CBFb activity. However, although AML1-or CBFb-de®cient embryos lacked detectable haemopoietic progenitors, fetal livers from AML1-ETO expressing embryos contained dysplastic multilineage haemopoietic progenitors (Okuda et al, 1998). Moreover, AML1-ETO expressing cells differentiated abnormally and had a high self-renewal capacity, readily establishing immortal cell lines in culture.…”

Novel loss‐of‐function mutations of the haematopoiesis‐related transcription factor, acute myeloid leukaemia 1/runt‐related transcription factor 1, detected in acute myeloblastic leukaemia and myelodysplastic syndrome

“…One possibility is that the AML1 dosage influences the fate of the haematopoietic progenitor cells, determining whether they self-renew or commit to differentiation. To support this possibility, haematopoietic progenitors that harbour engineered AML1-MTG8 allele(s) have been shown to have an increased potential for self-renewal (Okuda et al, 1998;Rhoades et al, 2000;de Guzman et al, 2002;Mulloy et al, 2002;Schwieger et al, 2002). In addition, multipotent haematopoietic stem cells appeared at earlier stages of development in AML1-haplo-insufficient mice, compared with the time they appeared in wild-type mice (Cai et al, 2000).…”