Inability of RUNX1/AML1 to breach AML1-ETO block of embryonic stem cell definitive hematopoiesis

The t(8;21)(q22:q22) translocation associated with acute myeloid leukemia fuses the AML1/RUNX1 N-terminal portion located on chromosome 21 to most of the ETO/MTG8 gene on chromosome 8. Various investigators have shown that the fusion product AML1-ETO on its own is unable to promote leukemia. Early studies using transgenic mouse models demonstrated that the direct knock-in of the fusion protein expression is embryonic lethal, similar to the AML1 knockout, suggesting that AML1-ETO has a dominant negative role over AML1. Using the embryonic stem cells generated for such studies, we show here that the presence of the fusion product AML1-ETO blocks definitive hematopoiesis in vitro as well, in both one and two step methylcellulose methods of embryonic stem cell hematopoietic differentiation. However, there is a very low occurrence of macrophage colonies, similar to the knock-in mice that display macrophages in cell cultures of yolk sac derived cells. In addition, we show that exogenous expression of AML1 is unable to bypass this AML1-ETO induced definitive hematopoietic block in these cells. This inability is not linked to an inability to reverse gene expression inhibition by AML1-ETO of the PU.1 gene associated with stem cell maintenance and myeloid differentiation. Our results suggest that AML1-ETO functions in a complex competitive manner with AML1 involving transcriptional regulation, protein-protein interactions and post-transcriptional mechanism(s) affecting early embryonic hematopoiesis and possibly leukemogenesis.

Expression of AML/Runx and ETO/MTG family members during hematopoietic differentiation of embryonic stem cells

Runx1/AML1 plays important roles in hematopoiesis, including the commitment of cells to hematopoiesis during embryonic development, and in the maintenance of hematopoietic cell populations. It is also one of the most common genes involved in chromosomal translocations related to leukemia. One such translocation is t(8;21), which fuses the Runx1 gene to the MTG8/ETO gene and generates the Runx1-MTG8 (AML1-ETO) fusion gene. Both Runx1 and MTG8 have two additional family members that are much less studied in hematopoiesis. Here we report the expression of every member of the Runx and MTG families as well as the Runx heterodimerization partner CBFbeta during hematopoietic differentiation of murine embryonic stem cells. We observed substantially increased expression of Runx1, Runx2, and MTG16 during hematopoietic differentiation. Furthermore, the increase in Runx2 expression is delayed relative to Runx1 expression, suggesting their possible sequential contribution to hematopoiesis.

Acute myeloid leukemia with the 8q22;21q22 translocation: secondary mutational events and alternative t(8;21) transcripts

Nonrandom and somatically acquired chromosomal translocations can be identified in nearly 50% of human acute myeloid leukemias. One common chromosomal translocation in this disease is the 8q22;21q22 translocation. It involves the AML1 (RUNX1) gene on chromosome 21 and the ETO (MTG8, RUNX1T1) gene on chromosome 8 generating the AML1-ETO fusion proteins. In this review, we survey recent advances made involving secondary mutational events and alternative t(8;21) transcripts in relation to understanding AML1-ETO leukemogenesis.

Involvement of a Novel Q-SNARE, D12, in Quality Control of the Endomembrane System

The cellular endomembrane system requires the proper kinetic balance of synthesis and degradation of its individual components, which is maintained in part by a specific membrane fusion apparatus. In this study, we describe the molecular properties of D12, which was identified from a mouse expression library. This C-terminal anchored membrane protein has sequence similarity to both a yeast soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE), Use1p/Slt1p, and a recently identified human syntaxin 18-binding protein, p31. D12 formed a tight complex with syntaxin 18 as well as Sec22b and bound to alpha-SNAP, indicating that D12 is a SNARE protein. Although the majority of D12 is located in the endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments at steady state, overexpression or knockdown of D12 had no obvious effects on membrane trafficking in the early secretory pathway. However, suppression of D12 expression caused rapid appearance of lipofuscin granules, accompanied by apoptotic cell death without the apparent activation of the unfolded protein response. The typical cause of lipofuscin formation is the impaired degradation of mitochondria by lysosomal degradative enzymes, and, consistent with this, we found that proper post-Golgi maturation of cathepsin D was impaired in D12-deficient cells. This unexpected observation was supported by evidence that D12 associates with VAMP7, a SNARE in the endosomal-lysosomal pathway. Hence, we suggest that D12 participates in the degradative function of lysosomes.