AML1/Runx1 is a frequent target of leukemia-associated gene aberration, and it encodes a transcription factor essential for definitive hematopoiesis. We previously reported that the AML1 molecules with trans-activation subdomains retained can rescue in vitro hematopoietic defects of AML1-deficient mouse embryonic stem (ES) cells when expressed by using a knock-in approach. Extending this notion to in vivo conditions, we found that the knock-in ES cell clones with AML1 mutants, which retain trans-activation subdomains but lack C-terminal repression subdomains including the conserved VWRPY motif, contribute to hematopoietic tissues in chimera mice. We also found that germline mice homozygous for the mutated AML1 allele, which lacks the VWRPY motif, exhibit a minimal effect on hematopoietic development, as was observed in control knock-in mice with full-length AML1. On the other hand, reduced cell numbers and deviant CD4 expression were observed during early T-lymphoid ontogeny in the VWRPY-deficient mice, whereas the contribution to the thymus by the corresponding ES cell clones was inadequate. These findings demonstrate that AML1 with its trans-activating subdomains is essential and sufficient for hematopoietic development in the context of the entire mouse. In addition, its transrepression activity, depending on the Cterminal VWRPY motif, plays a role in early thymocyte development. IntroductionVertebrate hematopoietic development is characterized by the sequential appearance of two cell populations, further defined as primitive and definitive hematopoiesis. 1 In mice, for example, primitive hematopoiesis is first seen in the form of blood islands in the yolk sac of the 7.5-day-old (E7.5) mouse embryo. It is thought that this cell population is directly differentiated from hemangioblasts, a bipotent precursor. Primitive hematopoiesis consists predominantly of a large and nucleated erythroid population containing embryonic-type hemoglobin. In contrast to the restricted and temporal development of this first wave, which diminishes at midgestation, definitive hematopoiesis originates from the aorta-gonad-mesonephros (AGM) region, where stem cells with long-term repopulating ability of multilineage hematopoiesis emerge at approximately E9.5, as a result of budding, from the ventral endothelial cells of the great vessels. 2,3 The stem cells then migrate into the fetal liver and proliferate to rapidly establish the definitive hematopoiesis of all lineages, including progenitors for T-and B-lymphoid populations. Active sites for definitive hematopoiesis are transferred to bone marrow and spleen before birth and function throughout life within these organs.These stem cells are equipped with a number of critical transcription factors that play pivotal roles in determining the fate of the cells at discrete developmental stages. Most of these molecules have been identified by isolating DNA-binding proteins to known cis-regulatory elements of lineage-specific genes or by cloning the DNA targets of leukemia-associated chrom...
AML1/Runx1 is a frequent target of human leukemia-associated gene aberration and encodes a transcription factor with nonredundant biologic functions in initial development of definitive hematopoiesis, T-cell development, and steadystate platelet production. AML1/Runx1 and 2 closely related family genes, AML2/ Runx3 and AML3/Runx2/Cbfa1, present in mammals, comprise the Runt-domain transcription factor family. Although they have similar structural and biochemical properties, gene-targeting experiments have identified distinct biologic roles. To directly determine the presence of functional overlap among runt-related transcription factor (Runx) family molecules, we replaced the C-terminal portion of acute myeloid leukemia 1 (AML1) with that derived from its family members, which are variable in contrast to conserved Runt domain, using the gene knock-in method. We found that C-terminal portions of either AML2 or AML3 could functionally replace that of AML1 for myeloid development in culture and within the entire mouse. However, while AML2 substituted for AML1 could effectively rescue lymphoid lineages, AML3 could not, resulting in a smaller thymus and lymphoid deficiency in peripheral blood. Substitution by the C-terminal portion of AML3 also led to high infantile mortality and growth retardation, suggesting that AML1 has as yet unidentified effects on these phenotypes. Thus, the C-terminal portions of Runx family members have both similar and distinct biologic functions. ( IntroductionThe runt-related transcription factor (Runx) family is a newly established gene family consisting of 3 molecules so far identified in mammalian cells: acute myeloid leukemia 1 (AML1)/Runx1, AML2/Runx3, and AML3/Runx2/core-binding factor ␣ 1 (Cbf␣1). [1][2][3][4][5] Runx gene is known to encode a DNA-binding subunit of the CBF transcription factor complex, which is a heterodimer complex consisting of one Runx molecule and one molecule of the common non-DNA-binding subunit CBF. 6,7 The association between these subunits is mediated through the signature domain of 128 amino acid residues near the N-terminus of Runx molecules, known as the Runt domain. 8,9 This domain is tightly conserved through evolution with homology to the Drosophila orthologue Runt and also functions in the sequence-specific DNA binding to the core DNA sequence, TGT/cGGT. [8][9][10] This sequence is present in a number of candidate target genes although critical target(s) responsible for their biologic functions are not yet thoroughly identified.In contrast to the Runt domains, whose DNA and amino acid sequences are highly homologous among all Runx family molecules with over 90% identity, their C-termini show substantial differences, with only about 60% of the sequences identical. [1][2][3][4][5] Nevertheless, these C-terminal portions share biochemically equivalent components, each of which is responsible for their respective properties as transcriptional factors, such as nuclear translocation, nuclear matrix targeting, trans-activation, auto-inhibition, and tran...
Summary AML1/RUNX1, which encodes a transcription factor essential for definitive haematopoiesis, is a frequent target of leukaemia‐associated chromosome translocations. Point mutations of this gene have also recently been associated with leukaemia and myelodysplastic syndrome (MDS). To further define the frequency and biological characteristics of AML1 mutations, we have examined 170 cases of such diseases. Mutations within the runt‐domain were identified in five cases: one of de novo acute myeloid leukaemia (AML) and four of MDS. Where multiple time point samples were available, mutations were detected in the earliest samples, which persisted throughout the disease course. Of the five mutations, one was a silent mutation, two were apparent loss‐of‐function mutations caused by N‐terminal truncation, and two were insertions, I150ins and K168ins, which preserved most of the AML1 DNA‐binding domain. Both AML1 molecules with insertion mutations were non‐functional in that they were unable to rescue haematological defects in AML1‐deficient mouse embryonic stem cells. In addition, activating mutations of N‐ras, deletion of chromosome 12p, or inactivation of TP53 accompanied some of the AML1 mutations. Together, these observations strongly suggest that one‐allele inactivation of AML1 serves as an initial or early event that plays an important role in the eventual development of overt diseases with additional genetic alterations.
Allogeneic stem cell transplantation is a curative treatment for severe congenital neutropenia (SCN). However, a standard conditioning regimen and donor source have not been established. We report 3 consecutive cases of SCN who were successfully treated by cord blood transplantation (CBT) with reduced-intensity conditioning consisting of fludarabine, melphalan, and low-dose total body irradiation. All cases achieved complete donor chimerism without severe infectious complications and have maintained normal neutrophil counts for between 3 and 9 years after CBT. These results suggest that CBT with reduced-intensity conditioning can be an alternative therapy for SCN when human leukocyte antigen-matched bone marrow donor is unavailable.
A 26-year-old man with idiopathic hypereosinophilic syndrome (HES) was treated with imatinib mesylate following a 5-year history of prednisolone therapy. The patient had hypereosinophilia (absolute eosinophil counts >1500/microL) occurring in cyclic oscillations as well as histologically diagnosed eosinophilic vasculitis, bursitis, and periodic soft-tissue swellings. Laboratory data revealed high levels of serum tryptase and increased numbers of mast cells in the bone marrow, but serum interleukin 5 levels were within the normal range. The disease initially responded well to 100 mg/day of imatinib mesylate but recurred 8 weeks later. Thereafter, a daily 200-mg dose was temporarily effective. Despite the response to imatinib, the FIP1L1-PDGFRA fusion gene was not detected by fluorescence in situ hybridization analysis. Additional molecular and cytogenetic studies showed neither translocations of platelet-derived growth factor receptor (PDGFR) genes nor mutations in the c-KIT or the PDGFR genes. Although imatinib mesylate is a choice of treatment for patients with HES, its precise molecular mechanism in individual cases remains to be clarified.
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