Page 35 - Haematologica Vol. 110 - January 2025

REVIEW ARTICLE - Molecular pathogenesis and novel treatments for CMML L. Marando et al.
generation of reactive oxygen species, which in turn, promote the activation of the NLRP3 inflammasome.46 The latter has a key role in the development of several clinical manifestations associated with KRAS-mutant myeloproliferative neoplasms.46 In response to various signals from pathogens and internal damage, activation of NLRP3 results in release of proinflam- matory cytokines (e.g., IL-1β and IL-18)47 through gasdermin D-mediated permeabilization of the plasma membrane. If gasdermin D pores cannot be repaired, cells undergo pyro- ptosis48 with the potential to amplify inflammatory responses (Figure 3).49,50 In addition to the classical mitogen-activated protein kinase (MAPK) pathway, GTP-bound RAS also binds to p110 to activate the PI3K-AKT-mTOR signaling cascade51 (Figure 4). Thus, inhibition of the phosphoinositol-3 kinase (PI3K) pathway has been explored in multiple RAS-mutated tumor types, both as an initial strategy and in subsequent efforts to overcome resistance to RAS inhibition.52 RAS mu- tations might also have roles in modulating adaptive immune responses to tumor cells. Using an Asxl1–/– NrasG12D/+ Vav-Cre mouse model, we have shown that Nras and Asxl1 cooperate to accelerate progression of CMML to AML, with AML cells overexpressing all the inhibitory immune checkpoint ligands (PD-L1/PD-L2, CD155, and CD80/CD86), highlighting that a suppressive microenvironment could play an important role in transformation to secondary AML.53 We have also shown accumulation of clonal RAS mutant CD123/CD303+ plasma- cytoid dendritic cells in CMML patients (Figures 1C, D and 3), and have documented their association with transfor- mation to AML.54 Further work by our group has shown that these plasmacytoid dendritic cell clusters are positive for indoleamine 2,3-dioxygenase 1/2 (IDO1/2),55 with IDO being an immune-checkpoint enzyme that induces systemic immune tolerance through multiple mechanisms, including regulatory T-cell expansion and tryptophan catabolism56,57 (Figure 3).
JAK2V617F mutations in chronic myelomonocytic leukemia JAK2V617F mutations are encountered in ~10% CMML patients. By studying a large cohort of CMML patients, we found that JAK2-mutated CMML is associated with higher hemoglobin/ hematocrit levels and platelet counts, and frequently co-oc- curs with TET2 mutations. However, we did not identify an increased risk of thrombosis, acute leukemia transformation, or impact on overall survival.58 JAK2V617F mutant CMML can at times be difficult to distinguish from JAK2-mutant my- eloproliferative neoplasms with monocytosis, although the use of monocyte partitioning flow cytometry (M01/classical monocytes >94% in CMML) and focus on megakaryocytic mor- phology can help to resolve this dilemma in several cases.59 Irrespective of signaling pathway mutations of signaling pathway mutations, CMML patients demonstrate granulo- cyte-macrophage colony-stimulating factor (GM-CSF)-de- pendent hypersensitivity in colony formation assays and by phospho-STAT5 (pSTAT5) flow cytometry, as compared to healthy controls, an effect more pronounced in patients with RAS mutations.60 Lenzilumab (KB003) is a novel engineered
human immunoglobulin G1k monoclonal antibody with high affinity for human GM-CSF (Figure 5) and has activity in pre- clinical models of CMML.60 In a phase I study of lenzilumab in CMML, we documented that the drug was well tolerated and had a durable clinical benefit in 33% of the patients.61 An in- terim analysis of the PREACH-M trial (ACTRN12621000223831) evaluating the combination of lenzilumab and 5-azacitidine in CMML patients with RAS pathway mutations has shown encouraging results, with complete remissions achieved within three cycles of treatment in 55% of subjects.62 JAK2 is a primary kinase regulating all the known activities of GM- CSF.63 This, along with its constitutive activation in CMML, provides the rationale for the use of JAK inhibitors, with ruxolitinib having completed early phase testing in CMML (Table 1, Figure 5).64
The role of inflammatory monocytes in chronic myelomonocytic leukemia
In CMML, classical monocytes (CD14+/CD16-) represent the predominant monocyte subset (>94% of total) (Figure 1B).65 These have highly inflammatory transcriptional signatures66 and, as a result, CMML patients have substantially different cytokine expression levels compared to healthy controls.6 Accordingly, CMML patients have a >2-fold increased risk of cardiovascular events,67 and ~20% have an associated sys- temic inflammatory and autoimmune disease (Figure 3).68 The effects of clonal monocytes go beyond the role of these cells in inflammation and organ infiltration. CMML commonly co-occurs with histiocytic malignancies that share ancestral mutations with CMML.69-71 While this suggests a common cell of origin, it also raises the possibility that neoplastic monocyte-derived macrophages contribute to the phenotypic origins of histiocytic neoplasms.
Beyond clinical manifestations, inflammation is also im- plicated in disease progression (Figure 3). Chronic inflam- mation can fuel clonal expansion of mutated HSC while inhibiting the function of wild-type HSC.72 In a murine model of Tet2-/-, increased IL-6 levels, due to blood dissemination of gut bacteria from a dysfunctional small-intestinal barrier, were associated with a CMML-like disease.73 In addition, when inflammation is active, TET2-mediated regulation of active chromatin facilitates histone deacetylation and suppresses IL6 and IL-1β expression, leading to resolution of inflamma- tion in innate myeloid cells74 and macrophages.75 As a result, in the presence of TET2 loss-of-function variants, myeloid cells show a reduced capacity to resolve inflammation. In addition, Tet2-deficient murine and TET2-mutant human HSC, when exposed to high levels of pro-inflammatory tumor necrosis factor-a in vitro, have a strong proliferative advan- tage compared with wild-type cells.76 Equally, in vivo, under inflammatory “stress”, murine Tet2-deficient HSC expand rapidly, which results in enhanced production of inflamma- tory cytokines, including IL-6, and resistance to apoptosis.77
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