AML-1 mutations outside the RUNT domain: description of two cases in myeloid malignancies

Identification of an N-terminal Transactivation Domain of Runx1 That Separates Molecular Function from Global Differentiation Function

RUNX1, or AML1, is a transcription factor that is the most frequent target for chromosomal gene translocations in acute leukemias. RUNX1 is essential for definitive hematopoiesis in embryos and profoundly influences adult steady-state hematopoiesis both positively and negatively. To investigate this wide range of normal activities and the pathological role of RUNX1, it is important to define the functions of different domains of the protein. RUNX1, RUNX2, and RUNX3 are highly conserved in their DNA binding runt homology domain and contain divergent sequences of unknown function N-terminal to this domain. Here we analyzed the role of the N-terminal sequence and the alpha-helix of the runt homology domain of Runx1 in DNA binding, transactivation, and megakaryocytopoiesis. Both the N terminus and the alpha-helix were found to reduce DNA binding of Runx1 and be essential for transactivation of the granulocyte-macrophage colony-stimulating factor and Ialpha1 promoters by Runx1. The N terminus of Runx1, including the alpha-helix, was also required for transactivation of a Gal4 reporter when expressed as fusion proteins with a Gal4 DNA binding domain, and the N terminus alone was capable of stimulating transcription when fused to the Gal4 DNA binding domain. The N terminus and the alpha-helix, however, were not required for megakaryocyte development from embryonic stem cells differentiated in vitro. Thus, our findings define a second transactivation domain of Runx1 that is differentially required for activation of transcription of some Runx1-dependent promoters and megakaryocytopoiesis.

Normal and transforming functions of RUNX1: a perspective

Converging studies from many investigators indicate that RUNX1 has a critical role in the correct maintenance of essential cellular functions during embryonic development and after birth. The discovery that this gene is also frequently mutated in human leukemia has increased the interest in the role that RUNX1 plays in both normal and transforming pathways. Here, we provide an overview of the many roles of RUNX1 in hematopoietic self-renewal and differentiation and summarize the information that is currently available on the many mechanisms of RUNX1 deregulation in human leukemia.

The RUNX genes: gain or loss of function in cancer

The RUNX genes have come to prominence recently because of their roles as essential regulators of cell fate in development and their paradoxical effects in cancer, in which they can function either as tumour-suppressor genes or dominant oncogenes according to context. How can this family of transcription factors have such an ambiguous role in cancer? How and where do these genes impinge on the pathways that regulate growth control and differentiation? And what is the evidence for a wider role for the RUNX genes in non-haematopoietic cancers?

Point mutations in the RUNX1/AML1 gene: another actor in RUNX leukemia

The RUNX1/AML1 gene is the most frequent target for chromosomal translocation in leukemia. In addition, recent studies have demonstrated point mutations in the RUNX1 gene as another mode of genetic alteration in development of leukemia. Monoallelic germline mutations in RUNX1 result in familial platelet disorder predisposed to acute myelogenous leukemia (FPD/AML). Sporadic point mutations are frequently found in three leukemia entities: AML M0 subtype, MDS-AML, and secondary (therapy-related) MDS/AML. Therapy-related leukemias resulting from anticancer treatments are not uncommon, and the incidence of RUNX1 point mutations appears comparable to the incidence of the t(8;21) AML M2 subtype and the inv(16) AML M4Eo subtype. Half of the point mutations in M0 cases are biallelic, although the frequency varies with ethnicity. Most of the RUNX1 mutations are clustered in the Runt domain and result in defective DNA binding but active beta-subunit binding, which is consistent with three-dimensional structural findings and may explain the dominant inhibitory effects. Unlike the classical tumor suppressor genes requiring biallelic inactivation, haploinsufficient RUNX1 is apparently leukemogenic. However, RUNX1 abnormalities per se are insufficient to cause full-blown leukemia. Intensive investigation of cooperating genetic alterations should elucidate leukemic mechanisms.

High incidence of somatic mutations in the AML1/RUNX1 gene in myelodysplastic syndrome and low blast percentage myeloid leukemia with myelodysplasia

A high incidence of somatically acquired point mutations in the AML1/RUNX1 gene has been reported in poorly differentiated acute myeloid leukemia (AML, M0) and in radiation-associated and therapy-related myelodysplastic syndrome (MDS) or AML. The vast majority of AML1 mutations identified in these diseases were localized in the amino (N)-terminal region, especially in the DNA-binding Runt homology domain. In this report, we show that AML1 point mutations were found in 26 (23.6%) of 110 patients with refractory anemia with excess blasts (RAEB), RAEB in transformation (RAEBt), and AML following MDS (defined these 3 disease categories as MDS/AML). Among them, 9 (8.2%) mutations occurred in the carboxy (C)-terminal region, which were exclusively found in MDS/AML and were strongly correlated with sporadic MDS/AML. All patients with MDS/AML with an AML1 mutation expressed wild-type AML1 protein and had a significantly worse prognosis than those without AML1 mutations. Most AML1 mutants lost trans-activation potential, regardless of their DNA binding potential. These data suggested that AML1 point mutation is one of the major driving forces of MDS/AML, and these mutations may represent a distinct clinicopathologic-genetic entity.