RUNX1-EVI1 induces dysplastic hematopoiesis and acute leukemia of the megakaryocytic lineage in mice

EVI1 drives leukemogenesis through aberrant ERG activation

Chromosomal rearrangements involving the MDS1 and EVI1 complex locus (MECOM) on chromosome 3q26 define an aggressive subtype of acute myeloid leukemia (AML) that is associated with chemotherapy resistance and dismal prognosis. Established treatment regimens commonly fail in these patients, therefore, there is an urgent need for new therapeutic concepts that will require a better understanding of the molecular and cellular functions of the ecotropic viral integration site 1 (EVI1) oncogene. To characterize gene regulatory functions of EVI1 and associated dependencies in AML, we developed experimentally tractable human and murine disease models, investigated the transcriptional consequences of EVI1 withdrawal in vitro and in vivo, and performed the first genome-wide CRISPR screens in EVI1-dependent AML. By integrating conserved transcriptional targets with genetic dependency data, we identified and characterized the ETS transcription factor ERG as a direct transcriptional target of EVI1 that is aberrantly expressed and selectively required in both human and murine EVI1-driven AML. EVI1 controls the expression of ERG and occupies a conserved intragenic enhancer region in AML cell lines and samples from patients with primary AML. Suppression of ERG induces terminal differentiation of EVI1-driven AML cells, whereas ectopic expression of ERG abrogates their dependence on EVI1, indicating that the major oncogenic functions of EVI1 are mediated through aberrant transcriptional activation of ERG. Interfering with this regulatory axis may provide entry points for the development of rational targeted therapies.

Secondary fusion proteins as a mechanism of BCR::ABL1 kinase-independent resistance in chronic myeloid leukaemia

Drug resistance in chronic myeloid leukaemia (CML) may occur via mutations in the causative BCR::ABL1 fusion or BCR::ABL1-independent mechanisms. We analysed 48 patients with BCR::ABL1-independent resistance for the presence of secondary fusion genes by RNA sequencing. We identified 10 of the most frequently detected secondary fusions in 21 patients. Validation studies, cell line models, gene expression analysis and drug screening revealed differences with respect to proliferation rate, differentiation and drug sensitivity. Notably, expression of RUNX1::MECOM led to resistance to ABL1 tyrosine kinase inhibitors in vitro. These results suggest secondary fusions contribute to BCR::ABL1-independent resistance and may be amenable to combined therapies.

Reduced-intensity conditioning is effective for allogeneic hematopoietic stem cell transplantation in infants with MECOM-associated syndrome

Mutations in the MECOM encoding EVI1 are observed in infants who have radioulnar synostosis with amegakaryocytic thrombocytopenia. MECOM-associated syndrome was proposed based on clinical heterogeneity. Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for progressive bone marrow failure. However, data regarding allogeneic HSCT for this rare disease are limited. We retrospectively assessed overall survival, conditioning regimen, regimen-related toxicities and long-term sequelae in six patients treated with allogeneic HSCT. All patients received a reduced-intensity conditioning (RIC) regimen consisting of fludarabine, cyclophosphamide or melphalan, and rabbit anti-thymocyte globulin and/or low-dose total body/thoracic-abdominal/total lymphoid irradiation, followed by allogeneic bone marrow or cord blood transplantation from unrelated donors between 4 and 18 months of age. All patients survived and achieved stable engraftment and complete chimerization with the donor type. Moreover, no patient experienced severe regimen-related toxicities, and only lower grades of acute graft-versus-host disease were observed. Three patients treated with low-dose irradiation had relatively short stature compared to three patients not treated with irradiation. Therefore, allogeneic HSCT with RIC is an effective and feasible treatment for infants with MECOM-associated syndrome. Future studies are needed to evaluate the use of low-dose irradiation to avoid risks of other long-term sequelae.

Development of Hematopoietic Stem Cells in the Early Mammalian Embryo

Hematopoietic stem cells (HSCs) were the first stem cells discovered in humans. A. A. Maximov proposed an idea of blood stem cells that was confirmed later by McCulloch and Till experimentally. HSCs were the first type of stem cells to be used in clinics and ever since are being continually used. Indeed, a single HSC transplanted intravenously is capable of giving rise to all types of blood cells. In recent decades, human and animal HSC origin, development, hierarchy, and gene signature have been extensively investigated. Due to the constant need for donor blood and HSCs suitable for therapeutic transplants, the experimental possibility of obtaining HSCs in vitro by directed differentiation of pluripotent stem cells (PSCs) has been considered in recent years. However, despite all efforts, it is not yet possible to reproduce in vitro the ontogenesis of HSCs and obtain cells capable of long-term maintenance of hematopoiesis. The study of hematopoiesis in embryonic development facilitates the establishment and improvement of protocols for deriving blood cells from PCSs and allows a better understanding of the pathogenesis of various types of proliferative blood diseases, anemia, and immunodeficiency. This review focuses on the development of hematopoiesis in mammalian ontogenesis.