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Bone marrow (BM) is a highly complex tissue that provides important regulatory signals to orchestrate hematopoiesis. Resident and transient cells occupy and interact with some well characterized niches to produce molecular and cellular mechanisms that interfere with differentiation, migration, survival, and proliferation in this microenvironment. The acute myeloid leukemia (AML), the most common and severe hematological neoplasm in adults, arises and develop in the BM. The osteoblastic, vascular, and reticular niches provide surface co-receptors, soluble factors, cytokines, and chemokines that mediate important functions on hematopoietic cells and leukemic blasts. There are some evidences of how AML modify the architecture and function of these three BM niches, but it has been still unclear how essential those modifications are to maintain AML development. Basic studies and clinical trials have been suggesting that disturbing specific cells and molecules into the BM niches might be able to impair leukemia competencies. Either through niche-specific molecule inhibition alone or in combination with more traditional drugs, the bone marrow microenvironment is currently considered the potential target for new strategies to treat AML patients. This review describes the cellular and molecular constitution of the BM niches under healthy and AML conditions, presenting this anatomical compartment by a new perspective: as a prospective target for current and next generation therapies.

The constitutively active BCR-ABL1 tyrosine kinase, found in t(9;22)(q34;q11) chromosomal translocation-derived leukemia, initiates an extremely complex signaling transduction cascade that induces a strong state of resistance to chemotherapy. Targeted therapies based on tyrosine kinase inhibitors (TKIs), such as imatinib, dasatinib, nilotinib, bosutinib, and ponatinib, have revolutionized the treatment of BCR-ABL1-driven leukemia, particularly chronic myeloid leukemia (CML). However, TKIs do not cure CML patients, as some develop TKI resistance and the majority relapse upon withdrawal from treatment. Importantly, although BCR-ABL1 tyrosine kinase is necessary to initiate and establish the malignant phenotype of Ph-related leukemia, in the later advanced phase of the disease, BCR-ABL1-independent mechanisms are also in place. Here, we present an overview of the signaling pathways initiated by BCR-ABL1 and discuss the major challenges regarding immunologic/pharmacologic combined therapies.

Prognostic stratification in acute myeloid leukemia (AML) relies, mostly, on cytogenetics and molecular features of leukemic blasts. The LeukemiaNet prognostic scoring system has been proposed as a standardized way of evaluating prognosis in AML. We have analysed outcomes in 65 AML cases (median age of 54 years, range 18-82) treated at five hematology centers in Brazil stritified according to the European Leukemia Net (ELN) recommendations for cytogenetic and molecular analysis. We classified patients as favorable (N = 13), intermediate-1 (N = 25), intermediate-2 (N = 15), or adverse risk (N = 9). Bone marrow transplantation (BMT) was performed in 13 patients (21%). Median follow-up was 12 months. The median overall survival (OS) for all patients was 12.4 months. Median OS was 19.8, 12.4, 10.1, and 10.4 months (p = 0.24) for patients in the favorable, intermediate-1, intermediate-2, and adverse groups, respectively. Among patients receiving BMT, median OS was 26.8 months. The ELN is a valuable tool for prognostic stratification of AML patients treated in Brazil. Nevertheless, its usefulness is limited when compared to data from developed countries.

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