Myelodysplastic/myeloproliferative diseases

Myeloproliferative neoplasms: Morphology and clinical practice

In myeloproliferative neoplasms (MPNs), controversy persists regarding the usefulness and reproducibility of bone marrow (BM) features. Disagreements concerning the WHO classification are mainly focused on the discrimination between essential thrombocythemia (ET) and prefibrotic/early primary myelofibrosis (prePMF) and prodromal polycythemia vera (PV). Criticism mostly refers to lack of standardization of distinctive BM features precluding correct morphological pattern recognition. The distinction between WHO-defined ET and prePMF is not trivial because outcome is significantly worse in prePMF. Morphology was generally considered to be non-specific for the diagnosis of PV. Recent studies have revealed under-diagnosis of morphologically and biologically consistent PV.

Endogenous oncogenic Nras mutation initiates hematopoietic malignancies in a dose- and cell type-dependent manner

Both monoallelic and biallelic oncogenic NRAS mutations are identified in human leukemias, suggesting a dose-dependent role of oncogenic NRAS in leukemogenesis. Here, we use a hypomorphic oncogenic Nras allele and a normal oncogenic Nras allele (Nras G12D(hypo) and Nras G12D, respectively) to create a gene dose gradient ranging from 25% to 200% of endogenous Nras G12D/+. Mice expressing Nras G12D(hypo)/G12D(hypo) develop normally and are tumor-free, whereas early embryonic expression of Nras G12D/+ is lethal. Somatic expression of Nras G12D/G12D but not Nras G12D/+ leads to hyperactivation of ERK, excessive proliferation of myeloid progenitors, and consequently an acute myeloproliferative disease. Using a bone marrow transplant model, we previously showed that ∼ 95% of animals receiving Nras G12D/+ bone marrow cells develop chronic myelomonocytic leukemia (CMML), while ∼ 8% of recipients develop acute T-cell lymphoblastic leukemia/lymphoma [TALL] (TALL-het). Here we demonstrate that 100% of recipients transplanted with Nras G12D/G12D bone marrow cells develop TALL (TALL-homo). Although both TALL-het and -homo tumors acquire Notch1 mutations and are sensitive to a γ-secretase inhibitor, endogenous Nras G12D/+ signaling promotes TALL through distinct genetic mechanism(s) from Nras G12D/G12D. Our data indicate that the tumor transformation potential of endogenous oncogenic Nras is both dose- and cell type-dependent.

Endogenous oncogenic Nras mutation promotes aberrant GM-CSF signaling in granulocytic/monocytic precursors in a murine model of chronic myelomonocytic leukemia

Oncogenic NRAS mutations are frequently identified in myeloid diseases involving monocyte lineage. However, its role in the genesis of these diseases remains elusive. We report a mouse bone marrow transplantation model harboring an oncogenic G12D mutation in the Nras locus. Approximately 95% of recipient mice develop a myeloproliferative disease resembling the myeloproliferative variant of chronic myelomonocytic leukemia (CMML), with a prolonged latency and acquisition of multiple genetic alterations, including uniparental disomy of oncogenic Nras allele. Based on single-cell profiling of phospho-proteins, a novel population of CMML cells is identified to display aberrant granulocyte-macrophage colony stimulating factor (GM-CSF) signaling in both the extracellular signal-regulated kinase (ERK) 1/2 and signal transducer and activator of transcription 5 (Stat5) pathways. This abnormal signaling is acquired during CMML development. Further study suggests that aberrant Ras/ERK signaling leads to expansion of granulocytic/monocytic precursors, which are highly responsive to GM-CSF. Hyperactivation of Stat5 in CMML cells is mainly through expansion of these precursors rather than up-regulation of surface expression of GM-CSF receptors. Our results provide insights into the aberrant cytokine signaling in oncogenic NRAS-associated myeloid diseases.

Radionuclide imaging of bone marrow disorders

Noninvasive imaging techniques have been used in the past for visualization the functional activity of the bone marrow compartment. Imaging with radiolabelled compounds may allow different bone marrow disorders to be distinguished. These imaging techniques, almost all of which use radionuclide-labelled tracers, such as ^99mTc-nanocolloid, ^99mTc-sulphur colloid, ¹¹¹In-chloride, and radiolabelled white blood cells, have been used in nuclear medicine for several decades. With these techniques three separate compartments can be recognized including the reticuloendothelial system, the erythroid compartment and the myeloid compartment. Recent developments in research and the clinical use of PET tracers have made possible the analysis of additional properties such as cellular metabolism and proliferative activity, using ¹⁸F-FDG and ¹⁸F-FLT. These tracers may lead to better quantification and targeting of different cell systems in the bone marrow. In this review the imaging of different bone marrow targets with radionuclides including PET tracers in various bone marrow diseases are discussed.