The authors uncover a new role for the RNA-binding protein Msi2 in the regulation of hematopoietic stem cell homeostasis and leukemogenesis. Msi2 is required for the maintenance of the balance between progenitor renewal and differentiation, and its overexpression cooperates with oncogenic events to induce aggressive leukemia. Msi2 expression is also elevated in human myeloid leukemias and may be a new prognostic marker and therapeutic target in acute myeloid leukemia. RNA-binding proteins of the Musashi (Msi) family are expressed in stem cell compartments and in aggressive tumors, but they have not yet been widely explored in the blood. Here we demonstrate that Msi2 is the predominant form expressed in hematopoietic stem cells (HSCs), and its knockdown leads to reduced engraftment and depletion of HSCs in vivo. Overexpression of human MSI2 in a mouse model increases HSC cell cycle progression and cooperates with the chronic myeloid leukemia–associated BCR-ABL1 oncoprotein to induce an aggressive leukemia. MSI2 is overexpressed in human myeloid leukemia cell lines, and its depletion leads to decreased proliferation and increased apoptosis. Expression levels in human myeloid leukemia directly correlate with decreased survival in patients with the disease, thereby defining MSI2 expression as a new prognostic marker and as a new target for therapy in acute myeloid leukemia (AML).

Key Points: Treatment-naive and relapsed/refractory MDS patients receiving venetoclax and HMAs have an ORR of 59% with 63% of responders proceeding to transplant. Allogeneic stem cell transplantation after treatment with venetoclax in combination with HMA is associated with prolonged survival.

Large chromosomal deletions are among the most common molecular abnormalities in cancer, yet the identification of relevant genes has proven difficult. The 5q− syndrome, a subtype of myelodysplastic syndrome (MDS), is a chromosomal deletion syndrome characterized by anemia and thrombocytosis. Although we have previously shown that hemizygous loss of RPS14 recapitulates the failed erythroid differentiation seen in 5q− syndrome, it does not affect thrombocytosis. Here we show that a microRNA located in the common deletion region of 5q− syndrome, miR-145, affects megakaryocyte and erythroid differentiation. We find that miR-145 functions through repression of Fli-1, a megakaryocyte and erythroid regulatory transcription factor. Patients with del(5q) MDS have decreased expression of miR-145 and increased expression of Fli-1. Overexpression of miR-145 or inhibition of Fli-1 decreases the production of megakaryocytic cells relative to erythroid cells, whereas inhibition of miR-145 or overexpression of Fli-1 has a reciprocal effect. Moreover, combined loss of miR-145 and RPS14 cooperates to alter erythroid-megakaryocytic differentiation in a manner similar to the 5q− syndrome. Taken together, these findings demonstrate that coordinate deletion of a miRNA and a protein-coding gene contributes to the phenotype of a human malignancy, the 5q− syndrome.

The tyrosine kinase JAK3 plays a well-established role during normal lymphocyte development and is constitutively phosphorylated in several lymphoid malignancies. However, its contribution to lymphomagenesis remains elusive. In this study, we used the newly identified activating JAK3A572V mutation to elucidate the effect of constitutive JAK3 signaling on murine lymphopoiesis. In a bone marrow transplantation model, JAK3A572V induces an aggressive, fatal, and transplantable lymphoproliferative disorder characterized by the expansion of CD8+TCRαβ+CD44+CD122+Ly-6C+ T cellsthat closely resemble an effector/memory T-cell subtype. Compared with wild-type counterparts, these cells show increased proliferative capacities in response to polyclonal stimulation, enhanced survival rates with elevated expression of Bcl-2, and increased production of interferon-γ (IFNγ) and tumor necrosis factor-α (TNFα), correlating with enhanced cytotoxic abilities against allogeneic target cells. Of interest, the JAK3A572V disease is epidermotropic and produces intraepidermal microabscesses. Taken together, these clinical features are reminiscent of those observed in an uncommon but aggressive subset of CD8+ human cutaneous T-cell lymphomas (CTCLs). However, we also observed a CD4+ CTCL-like phenotype when cells are transplanted in an MHC-I–deficient background. These data demonstrate that constitutive JAK3 activation disrupts T-cell homeostasis and induces lymphoproliferative diseases in mice.

The hypomethylating agents (HMA) azacitidine and decitabine are both approved by the FDA for the treatment of myelodysplastic syndromes (MDS). Although heralded as a significant advancement, HMA lead to responses in less than half of patients and for those that respond most will relapse. As such, there is a crucial need to improve frontline therapy approaches. One promising strategy involves combining azacitidine or decitabine with investigational or existing therapies with the goal of achieving synergistic activity and better patient outcomes. The purpose of this paper is to critically review the efficacy and safety of reported HMA-based combination regimens in patients with higher-risk MDS.