If You Build It, Patients with Rare Cancers Will Come: A Successful Clinical Trial in Relapsed and Refractory JMML.

Juvenile myelomonocytic leukemia (JMML) is a rare pediatric hematologic malignancy with a high relapse rate and a poor prognosis hallmarked by RAS pathway mutations. Stieglitz and colleagues conducted a phase II clinical trial using the MEK inhibitor trametinib to treat patients with relapsed and refractory juvenile myelomonocytic leukemia and observed an objective response rate of 50% and an overall survival of 80% after 4 years. See related article by Stieglitz et al., p. 1590 (4) .

RNA shielding of P65 is required to potentiate oncogenic inflammation in TET2 mutated clonal hematopoiesis.

TET2 mutations (mTET2) are common genetic events in myeloid malignancies and clonal hematopoiesis (CH). These mutations arise in the founding clone and are implicated in many clinical sequelae associated with oncogenic feedforward inflammatory circuits. However, the direct downstream effector of mTET2 responsible for the potentiation of this inflammatory circuit is unknown. To address this, we performed scRNA and scATAC-seq in COVID-19 patients with and without TET2-mutated CH reasoning that the inflammation from COVID-19 may highlight critical downstream transcriptional targets of mTET2. Using this approach, we identified MALAT1, a therapeutically tractable lncRNA, as a central downstream effector of mTET2 that is both necessary and sufficient to induce the oncogenic pro-inflammatory features of mTET2 in vivo. We also elucidate the mechanism by which mTET2 upregulate MALAT1 and describe an interaction between MALAT1 and P65 which leads to RNA "shielding" from PP2A dephosphorylation thus preventing resolution of inflammatory signaling.

Progenitor Hierarchy of Chronic Myelomonocytic Leukemia Identifies Inflammatory Monocytic-Biased Trajectory Linked to Worse Outcomes.

Myeloblast expansion is a hallmark of disease progression and comprises CD34+ hematopoietic stem and progenitor cells (HSPC). How this compartment evolves during disease progression in chronic myeloid neoplasms is unknown. Using single-cell RNA sequencing and high-parameter flow cytometry, we show that chronic myelomonocytic leukemia (CMML) CD34+ HSPC can be classified into three differentiation trajectories: monocytic, megakaryocyte-erythroid progenitor (MEP), and normal-like. Hallmarks of monocytic-biased trajectory were enrichment of CD120b+ inflammatory granulocyte-macrophage progenitor (GMP)-like cells, activated cytokine receptor signaling, phenotypic hematopoietic stem cell (HSC) depletion, and adverse outcomes. Cytokine receptor diversity was generally an adverse feature and elevated in CD120b+ GMPs. Hypomethylating agents decreased monocytic-biased cells in CMML patients. Given the enrichment of RAS pathway mutations in monocytic-biased cells, NRAS-competitive transplants and LPS-treated xenograft models recapitulated monocytic-biased CMML, suggesting that hematopoietic stress precipitates the monocytic-biased state. Deconvolution of HSPC compartments in other myeloid neoplasms and identifying therapeutic strategies to mitigate the monocytic-biased differentiation trajectory should be explored. SIGNIFICANCE: Our findings establish that multiple differentiation states underlie CMML disease progression. These states are negatively augmented by inflammation and positively affected by hypomethylating agents. Furthermore, we identify HSC depletion and expansion of GMP-like cells with increased cytokine receptor diversity as a feature of myeloblast expansion in inflammatory chronic myeloid neoplasms. This article is highlighted in the In This Issue feature, p. 476.