Primary Acute Myeloid Leukemia Research Articles

Clinical Correlates of Venetoclax-Based Combination Sensitivities to Augment Acute Myeloid Leukemia Therapy.

By mapping drug sensitivity data to clinical features and tumor cell state, we identify novel venetoclax combinations targeting patient subtypes who lack sensitivity to Ven + azacytidine. This provides a framework for a taxonomy of AML informed by readily available sets of clinical and genetic features obtained as part of standard care. See related commentary by Becker, p. 437 . This article is featured in Selected Articles from This Issue, p. 419.

FUS-ERG induces late-onset azacitidine resistance in acute myeloid leukaemia cells

FUS-ERG is a chimeric gene with a poor prognosis, found in myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). It remains unclear whether DNA hypomethylating agents, including azacitidine (Aza), are effective in FUS-ERG-harbouring AML and how FUS-ERG induces chemoresistance. Stable Ba/F3 transfectants with FUS-ERG were repeatedly exposed to Aza for 7 days of treatment and at 21-day intervals to investigate Aza sensitivity. Stable FUS-ERG transfectants acquired resistance acquired resistance after three courses of Aza exposure. RNA sequencing (RNA-seq) was performed when Aza susceptibility began to change; genes with altered expression or transcript variants were identified. Molecular signatures of these genes were analysed using gene ontology. RNA-seq analyses identified 74 upregulated and 320 downregulated genes involved in cell motility, cytokine production, and kinase activity. Additionally, 1321 genes with altered transcript variants were identified, revealing their involvement in chromatin organisation. In a clinical case of AML with FUS-ERG, we compared whole-genome alterations between the initial MDS diagnosis and AML recurrence after Aza treatment. Genes with non-synonymous or near mutations in transcription regulatory areas (TRAs), additionally detected in AML recurrence, were collated with the gene list from RNA-seq to identify genes involved in acquiring Aza resistance in the presence of FUS-ERG. Whole-genome sequencing of clinical specimens identified 29 genes with non-synonymous mutations, including BCOR, and 48 genes located within 20 kb of 54 TRA mutations in AML recurrence. These genes were involved in chromatin organisation and included NCOR2 as an overlapping gene with RNA-seq data. Transcription regulators involved in mutated TRAs were skewed and included RCOR1 in AML recurrence. We tested the efficacy of BH3 mimetics, including venetoclax and S63845, in primary Aza-resistant AML cells treated with FUS-ERG. Primary FUS-ERG-harbouring AML cells acquiring Aza resistance affected the myeloid cell leukaemia-1 (MCL1) inhibitor S63845 but not while using venetoclax, despite no mutations in BCL2. FUS-ERG promoted Aza resistance after several treatments. The disturbance of chromatin organisation might induce this by co-repressors, including BCOR, NCOR2, and RCOR1. MCL1 inhibition could partially overcome Aza resistance in FUS-ERG-harbouring AML cells.

Constitutively Synergistic Multiagent Drug Formulations Targeting MERTK, FLT3, and BCL-2 for Treatment of AML.

Although high-dose, multiagent chemotherapy has improved leukemia survival rates, treatment outcomes remain poor in high-risk subsets, including acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) in infants. The development of new, more effective therapies for these patients is therefore an urgent, unmet clinical need. The dual MERTK/FLT3 inhibitor MRX-2843 and BCL-2 family protein inhibitors were screened in high-throughput against a panel of AML and MLL-rearranged precursor B-cell ALL (infant ALL) cell lines. A neural network model was built to correlate ratiometric drug synergy and target gene expression. Drugs were loaded into liposomal nanocarriers to assess primary AML cell responses. MRX-2843 synergized with venetoclax to reduce AML cell density in vitro. A neural network classifier based on drug exposure and target gene expression predicted drug synergy and growth inhibition in AML with high accuracy. Combination monovalent liposomal drug formulations delivered defined drug ratios intracellularly and recapitulated synergistic drug activity. The magnitude and frequency of synergistic responses were both maintained and improved following drug formulation in a genotypically diverse set of primary AML bone marrow specimens. We developed a nanoscale combination drug formulation that exploits ectopic expression of MERTK tyrosine kinase and dependency on BCL-2 family proteins for leukemia cell survival in pediatric AML and infant ALL cells. We demonstrate ratiometric drug delivery and synergistic cell killing in AML, a result achieved by a systematic, generalizable approach of combination drug screening and nanoscale formulation that may be extended to other drug pairs or diseases in the future.

POSTER: AML-459 Genetic Landscape in Primary Acute Myeloid Leukemia (AML) and Relation With Wilms Tumor 1 (WT-1) Gene Expression

POSTER: AML-459 Genetic Landscape in Primary Acute Myeloid Leukemia (AML) and Relation With Wilms Tumor 1 (WT-1) Gene Expression

AML-459 Genetic Landscape in Primary Acute Myeloid Leukemia (AML) and Relation With Wilms Tumor 1 (WT-1) Gene Expression

AML-459 Genetic Landscape in Primary Acute Myeloid Leukemia (AML) and Relation With Wilms Tumor 1 (WT-1) Gene Expression

AML-025 Clinical Relevance of the RUNX1 Mutation in Egyptian Patients With Primary Refractory Acute Myeloid Leukemia

AML-025 Clinical Relevance of the RUNX1 Mutation in Egyptian Patients With Primary Refractory Acute Myeloid Leukemia

POSTER: AML-025 Clinical Relevance of the RUNX1 Mutation in Egyptian Patients With Primary Refractory Acute Myeloid Leukemia

POSTER: AML-025 Clinical Relevance of the RUNX1 Mutation in Egyptian Patients With Primary Refractory Acute Myeloid Leukemia

Bcl-2 inhibition combined with PPARα activation synergistically targets leukemic stem cell-like cells in acute myeloid leukemia

Persistence of leukemic stem cells (LSCs) is one of the determining factors to acute myeloid leukemia (AML) treatment failure and responsible for the poor prognosis of the disease. Hence, novel therapeutic strategies that target LSCs are crucial for treatment success. We investigated if targeting Bcl-2 and peroxisome proliferator activated receptor α (PPARα), two distinct cell survival regulating mechanisms could eliminate LSCs. This study demonstrate that the Bcl-2 inhibitor venetoclax combined with the PPARα agonist chiglitazar resulted in synergistic killing of LSC-like cell lines and CD34+ primary AML cells while sparing their normal counterparts. Furthermore, the combination regimen significantly suppressed AML progression in patient-derived xenograft (PDX) mouse models. Mechanistically, chiglitazar-mediated PPARα activation inhibited the transcriptional activity of the PIK3AP1 gene promoter and down-regulated the PI3K/Akt signaling pathway and anti-apoptotic Bcl-2 proteins, leading to cell proliferation inhibition and apoptosis induction, which was synergized with venetoclax. These findings suggest that combinatorial Bcl-2 inhibition and PPARα activation selectively eliminates AML cells in vivo and vitro, representing an effective therapy for patients with relapsed and refractory AML.

Vacuolar ATPase Is a Possible Therapeutic Target in Acute Myeloid Leukemia: Focus on Patient Heterogeneity and Treatment Toxicity.

Vacuolar ATPase (V-ATPase) is regarded as a possible target in cancer treatment. It is expressed in primary acute myeloid leukemia cells (AML), but the expression varies between patients and is highest for patients with a favorable prognosis after intensive chemotherapy. We therefore investigated the functional effects of two V-ATPase inhibitors (bafilomycin A1, concanamycin A) for primary AML cells derived from 80 consecutive patients. The V-ATPase inhibitors showed dose-dependent antiproliferative and proapoptotic effects that varied considerably between patients. A proteomic comparison of primary AML cells showing weak versus strong antiproliferative effects of V-ATPase inhibition showed a differential expression of proteins involved in intracellular transport/cytoskeleton functions, and an equivalent phosphoproteomic comparison showed a differential expression of proteins that regulate RNA processing/function together with increased activity of casein kinase 2. Patients with secondary AML, i.e., a heterogeneous subset with generally adverse prognosis and previous cytotoxic therapy, myeloproliferative neoplasia or myelodysplastic syndrome, were characterized by a strong antiproliferative effect of V-ATPase inhibition and also by a specific mRNA expression profile of V-ATPase interactome proteins. Furthermore, the V-ATPase inhibition altered the constitutive extracellular release of several soluble mediators (e.g., chemokines, interleukins, proteases, protease inhibitors), and increased mediator levels in the presence of AML-supporting bone marrow mesenchymal stem cells was then observed, especially for patients with secondary AML. Finally, animal studies suggested that the V-ATPase inhibitor bafilomycin had limited toxicity, even when combined with cytarabine. To conclude, V-ATPase inhibition has antileukemic effects in AML, but this effect varies between patients.

HSP90 Inhibitor PU-H71 in Combination with BH3-Mimetics in the Treatment of Acute Myeloid Leukemia.

Targeting the molecular chaperone HSP90 and the anti-apoptotic proteins MCL1 and BCL2 may be a promising novel approach in the treatment of acute myeloid leukemia (AML). The HSP90 inhibitor PU-H71, MCL1 inhibitor S63845, and BCL2 inhibitor venetoclax were assessed as single agents and in combination for their ability to induce apoptosis and cell death in leukemic cells. AML cells represented all major morphologic and molecular subtypes including FLT3-ITD and TP53 mutant AML cell lines and a variety of patient-derived AML cells. Results: PU-H71 and combination treatments with MCL1 inhibitor S63845 or BCL2 inhibitor venetoclax induced cell cycle arrest and apoptosis in susceptible AML cell lines and primary AML. The majority of the primary AML samples were responsive to PU-H71 in combination with BH3 mimetics. Elevated susceptibility to PU-H71 and S63845 was associated with FLT3 mutated AML with CD34 < 20%. Elevated susceptibility to PU-H71 and venetoclax was associated with primary AML with CD117 > 80% and CD11b < 45%. The combination of HSP90 inhibitor PU-H71 and MCL1 inhibitor S63845 may be a candidate treatment for FLT3-mutated AML with moderate CD34 positivity while the combination of HSP90 inhibitor PU-H71 and BCL2 inhibitor venetoclax may be more effective in the treatment of primitive AML with high CD117 and low CD11b positivity.

Transcriptional Response to Standard AML Drugs Identifies Synergistic Combinations.

Unlike genomic alterations, gene expression profiles have not been widely used to refine cancer therapies. We analyzed transcriptional changes in acute myeloid leukemia (AML) cell lines in response to standard first-line AML drugs cytarabine and daunorubicin by means of RNA sequencing. Those changes were highly cell- and treatment-specific. By comparing the changes unique to treatment-sensitive and treatment-resistant AML cells, we enriched for treatment-relevant genes. Those genes were associated with drug response-specific pathways, including calcium ion-dependent exocytosis and chromatin remodeling. Pharmacological mimicking of those changes using EGFR and MEK inhibitors enhanced the response to daunorubicin with minimum standalone cytotoxicity. The synergistic response was observed even in the cell lines beyond those used for the discovery, including a primary AML sample. Additionally, publicly available cytotoxicity data confirmed the synergistic effect of EGFR inhibitors in combination with daunorubicin in all 60 investigated cancer cell lines. In conclusion, we demonstrate the utility of treatment-evoked gene expression changes to formulate rational drug combinations. This approach could improve the standard AML therapy, especially in older patients.

The MLL-Menin Interaction is a Therapeutic Vulnerability in NUP98-rearranged AML.

Chromosomal translocations involving the NUP98 locus are among the most prevalent rearrangements in pediatric acute myeloid leukemia (AML). AML with NUP98 fusions is characterized by high expression of HOXA and MEIS1 genes and is associated with poor clinical outcome. NUP98 fusion proteins are recruited to their target genes by the mixed lineage leukemia (MLL) complex, which involves a direct interaction between MLL and Menin. Here, we show that therapeutic targeting of the Menin-MLL interaction inhibits the propagation of NUP98-rearrranged AML both ex vivo and in vivo. Treatment of primary AML cells with the Menin inhibitor revumenib (SNDX-5613) impairs proliferation and clonogenicity ex vivo in long-term coculture and drives myeloid differentiation. These phenotypic effects are associated with global gene expression changes in primary AML samples that involve the downregulation of many critical NUP98 fusion protein-target genes, such as MEIS1 and CDK6. In addition, Menin inhibition reduces the expression of both wild-type FLT3 and mutated FLT3-ITD, and in combination with FLT3 inhibitor, suppresses patient-derived NUP98-r AML cells in a synergistic manner. Revumenib treatment blocks leukemic engraftment and prevents leukemia-associated death of immunodeficient mice transplanted with NUP98::NSD1 FLT3-ITD-positive patient-derived AML cells. These results demonstrate that NUP98-rearranged AMLs are highly susceptible to inhibition of the MLL-Menin interaction and suggest the inclusion of AML patients harboring NUP98 fusions into the clinical evaluation of Menin inhibitors.

Preclinical development of ARD103, a CLL-1 CAR-T produced by a 3-day DashCAR technology, for acute myeloid leukemia.

119 Background: Acute myeloid leukemia (AML) is the most common acute leukemia in adults. More than half of the AML patients experienced primary refractory or relapse after current treatments, such as chemotherapy, hematopoietic stem cell transplantation, and targeted therapy. AML patients are in urgent need of novel therapeutic strategies. Leukemic stem cells (LSCs), which cause the treatment failure and leukemia relapsed. Human C-type lectin-like molecule-1 (CLL-1) is a surface antigen expressed on both AML malignant blast cells and LSCs, but not on normal hematopoietic stem cells, suggesting that CLL-1 is a compelling target with low off-tumor toxicity for novel AML treatment. Methods: ARD103 is a second-generation chimeric antigen receptor (CAR)-T cell, expressing a novel extracellular anti-CLL-1 scFv fused to a CD28 transmembrane domain, a CD28 co-stimulatory domain, and a CD3ζ activation domain. ARD103 was generated using lentiviral vector in an optimized 3-day DashCAR manufacturing process and demonstrated high percentage of CAR population (&gt;80%), high percentages of memory T cells (Tscm and Tcm are &gt;70%) and profound effector cytokine production (IFN-γ, granzyme A, perforin and granulysin). Results: ARD103, CLL-1 CAR-T exhibited specific killing on various CLL-1-expressing AML cell lines, such as U937, THP-1, and HL60, correlated to the levels of CLL-1 expression. In the AML U937-Luc xenograft mouse model, the CLL-1 CAR-T cells prolonged the survival time significantly (p &lt; 0.0001), with superior CAR-T cell persistence, to the end point of the study at 48 days compared to the median survival time at 23 days in the untreated or control groups. Furthermore, the xenograft mice receiving as minimum as 3 x105 CAR-T cells per mouse were able to elicit effective recall responses after tumor rechallenge and survived for more than 60 days. The off-target binding interactions of ARD103 CAR-T cells using a human cell microarray showed a specific and strong interaction with CLL-1 but not with other human membrane/extracellular proteins. In the aspect of on-target off-tumor toxicity, the hematotoxicity assessment on ARD103 exerted modest (5%-23%) or no effect on colony formation inhibition of CD34+ cells derived from human normal bone marrow or peripheral blood, respectively. Furthermore, autologous ARD103 cells were successfully prepared from relapsed/refractoryAML patients and demonstrated to mediate specific cytotoxicity to primary AML blasts and LSCs derived from the same patients. Conclusions: Taken together, the ARD103 CLL-1 CAR-T candidate has demonstrated excellent anti-AML potency and efficacy with minimal safety concerns, and warrant to be further developed preclinically and clinically for the treatment of relapsed/refractory AML patients. Keywords: Acute myeloid leukemia; CLL-1; Chimeric antigen receptor-T cell; Immunotherapy.

Casein Kinase 2 (CK2): A Possible Therapeutic Target in Acute Myeloid Leukemia.

The protein kinase CK2 (also known as casein kinase 2) is one of the main contributors to the human phosphoproteome. It is regarded as a possible therapeutic strategy in several malignant diseases, including acute myeloid leukemia (AML), which is an aggressive bone marrow malignancy. CK2 is an important regulator of intracellular signaling in AML cells, especially PI3K-Akt, Jak-Stat, NFκB, Wnt, and DNA repair signaling. High CK2 levels in AML cells at the first time of diagnosis are associated with decreased survival (i.e., increased risk of chemoresistant leukemia relapse) for patients receiving intensive and potentially curative antileukemic therapy. However, it is not known whether these high CK2 levels can be used as an independent prognostic biomarker because this has not been investigated in multivariate analyses. Several CK2 inhibitors have been developed, but CX-4945/silmitasertib is best characterized. This drug has antiproliferative and proapoptotic effects in primary human AML cells. The preliminary results from studies of silmitasertib in the treatment of other malignancies suggest that gastrointestinal and bone marrow toxicities are relatively common. However, clinical AML studies are not available. Taken together, the available experimental and clinical evidence suggests that the possible use of CK2 inhibition in the treatment of AML should be further investigated.

FLT3-directed UniCAR T-cell therapy of acute myeloid leukaemia.

Adaptor chimeric antigen receptor (CAR) T-cell therapy offers solutions for improved safety and antigen escape, which represent main obstacles for the clinical translation of CAR T-cell therapy in myeloid malignancies. The adaptor CAR T-cell platform 'UniCAR' is currently under early clinical investigation. Recently, the first proof of concept of a well-tolerated, rapidly switchable, CD123-directed UniCAR T-cell product treating patients with acute myeloid leukaemia (AML) was reported. Relapsed and refractory AML is prone to high plasticity under therapy pressure targeting one single tumour antigen. Thus, targeting of multiple tumour antigens seems to be required to achieve durable anti-tumour responses, underlining the need to further design alternative AML-specific target modules (TM) for the UniCAR platform. We here present the preclinical development of a novel FMS-like tyrosine kinase 3 (FLT3)-directed UniCAR T-cell therapy, which is highly effective for in vitro killing of both AML cell lines and primary AML samples. Furthermore, we show in vivo functionality in a murine xenograft model. PET analyses further demonstrate a short serum half-life of FLT3 TMs, which will enable a rapid on/off switch of UniCAR T cells. Overall, the presented preclinical data encourage the further development and clinical translation of FLT3-specific UniCAR T cells for the therapy of AML.

Pathogen and human NDPK-proteins promote AML cell survival via monocyte NLRP3-inflammasome activation.

A history of infection has been linked with increased risk of acute myeloid leukaemia (AML) and related myelodysplastic syndromes (MDS). Furthermore, AML and MDS patients suffer frequent infections because of disease-related impaired immunity. However, the role of infections in the development and progression of AML and MDS remains poorly understood. We and others previously demonstrated that the human nucleoside diphosphate kinase (NDPK) NM23-H1 protein promotes AML blast cell survival by inducing secretion of IL-1β from accessory cells. NDPKs are an evolutionary highly conserved protein family and pathogenic bacteria secrete NDPKs that regulate virulence and host-pathogen interactions. Here, we demonstrate the presence of IgM antibodies against a broad range of pathogen NDPKs and more selective IgG antibody activity against pathogen NDPKs in the blood of AML patients and normal donors, demonstrating that in vivo exposure to NDPKs likely occurs. We also show that pathogen derived NDPK-proteins faithfully mimic the catalytically independent pro-survival activity of NM23-H1 against primary AML cells. Flow cytometry identified that pathogen and human NDPKs selectively bind to monocytes in peripheral blood. We therefore used vitamin D3 differentiated monocytes from wild type and genetically modified THP1 cells as a model to demonstrate that NDPK-mediated IL-1β secretion by monocytes is NLRP3-inflammasome and caspase 1 dependent, but independent of TLR4 signaling. Monocyte stimulation by NDPKs also resulted in activation of NF-κB and IRF pathways but did not include the formation of pyroptosomes or result in pyroptotic cell death which are pivotal features of canonical NLRP3 inflammasome activation. In the context of the growing importance of the NLRP3 inflammasome and IL-1β in AML and MDS, our findings now implicate pathogen NDPKs in the pathogenesis of these diseases.

TAK-981, a SUMOylation inhibitor, suppresses AML growth immune-independently.

Acute myeloid leukemia (AML) generally has an unsatisfactory prognosis despite the recent introduction of new regimens, including targeted agents and antibodies. To find a new druggable pathway, we performed integrated bioinformatic pathway screening on large OHSU and MILE AML databases, discovered the SUMOylation pathway, and validated it independently with an external data set (totaling 2959 AML and 642 normal sample data). The clinical relevance of SUMOylation in AML was supported by its core gene expression which is correlated with patient survival, European LeukemiaNet 2017 risk classification, and AML-relevant mutations. TAK-981, a first-in-class SUMOylation inhibitor currently under clinical trials for solid tumors, showed antileukemic effects with apoptosis induction, cell-cycle arrest, and induction of differentiation marker expression in leukemic cells. It exhibited potent nanomolar activity, often stronger than that of cytarabine, which is part of the standard of care. TAK-981's utility was further demonstrated in invivo mouse and human leukemia models as well as patient-derived primary AML cells. Our results also indicate direct and cancer cell-inherent anti-AML effects by TAK-981, different from the type 1 interferon and immune-dependent mechanism in a previous solid tumor study. Overall, we provide a proof-of-concept for SUMOylation as a new targetable pathway in AML and propose TAK-981 as a promising direct anti-AML agent. Our data should prompt studies on optimal combination strategies and transitions to clinical trials in AML.

Association of CDSS score and 60-day mortality in Chinese patients with non-APL acute myeloid leukemia: a retrospective cohort study.

Acute myeloid leukemia (AML) is a heterogeneous hematological malignancy, and is one of the triggers of DIC, the latter is an essential factor in the early death of patients with AML. However, the timely identification of DIC remains a challenge. The Chinese DIC Scoring System (CDSS) is a common consensus widely used in China; but, there are few reports on its application in patients with AML. We undertake this retrospective cohort study to investigate the association between CDSS score and 60-day mortality. CDSS scores were evaluated after admission. The outcome was all-cause 60-day mortality. Multivariate Cox regression analyses were performed to calculate the adjusted hazard ratio (HR) and the corresponding 95% confidence interval (CI). Survival curves were plotted by Kaplan-Meier and log-rank analyses. Subgroup analyses were stratified by relevant effect covariates. A total of 570 consecutive patients with primary AML were included. We found an association between a 39% increase in 60-day mortality and a 1 point increase in CDSS score (HR = 1.39, 95% CI 1.25-1.54), which was associated with a 189% increase in 60-day mortality in CDSS scores ≥ 6 compared with that in the CDSS scores < 6 (HR = 2.89, 95% CI 1.91-4.38). After adjusting for all potential con-founders, a 27% and a 198% increase were observed (HR = 1.27, 95% CI 1.01-1.61; HR = 2.98, 95% CI 1.24-7.19), respectively. There is association between 60-day mortality and CDSS score in patients with AML. These findings may help hematologists in making informed treatment decisions.

MCL1 regulates AML cells metabolism via direct interaction with HK2. Metabolic signature at onset predicts overall survival in AMLs' patients.

We characterize the metabolic background in distinct Acute Myeloid Leukemias (AMLs), by comparing the metabolism of primary AML blasts isolated at diagnosis with that of normal hematopoietic maturing progenitors, using the Seahorse XF Agilent. Leukemic cells feature lower spare respiratory (SRC) and glycolytic capacities as compared to hematopoietic precursors (i.e. day 7, promyelocytes). According with Proton Leak (PL) values, AML blasts can be grouped in two well defined populations. The AML group with blasts presenting high PL or high basal OXPHOS plus high SRC levels had shorter overall survival time and significantly overexpressed myeloid cell leukemia 1 (MCL1) protein. We demonstrate that MCL1 directly binds to Hexokinase 2 (HK2) on the outer mitochondrial membrane (OMM). Overall, these results suggest that high PL and high SRC plus high basal OXPHOS levels at disease onset, arguably with the concourse of MCL1/HK2 action, are significantly linked with shorter overall survival time in AML. Our data describe a new function for MCL1 protein in AMLs' cells: by forming a complex with HK2, MCL1 co-localizes to VDAC on the OMM, thus inducing glycolysis and OXPHOS, ultimately conferring metabolic plasticity and promoting resistance to therapy.

RAB27B controls palmitoylation-dependent NRAS trafficking and signaling in myeloid leukemia.

RAS mutations are among the most prevalent oncogenic drivers in cancers. RAS proteins propagate signals only when associated with cellular membranes as a consequence of lipid modifications that impact their trafficking. Here, we discovered that RAB27B, a RAB family small GTPase, controlled NRAS palmitoylation and trafficking to the plasma membrane, a localization required for activation. Our proteomic studies revealed RAB27B upregulation in CBL- or JAK2-mutated myeloid malignancies, and its expression correlated with poor prognosis in acute myeloid leukemias (AMLs). RAB27B depletion inhibited the growth of CBL-deficient or NRAS-mutant cell lines. Strikingly, Rab27b deficiency in mice abrogated mutant but not WT NRAS-mediated progenitor cell growth, ERK signaling, and NRAS palmitoylation. Further, Rab27b deficiency significantly reduced myelomonocytic leukemia development in vivo. Mechanistically, RAB27B interacted with ZDHHC9, a palmitoyl acyltransferase that modifies NRAS. By regulating palmitoylation, RAB27B controlled c-RAF/MEK/ERK signaling and affected leukemia development. Importantly, RAB27B depletion in primary human AMLs inhibited oncogenic NRAS signaling and leukemic growth. We further revealed a significant correlation between RAB27B expression and sensitivity to MEK inhibitors in AMLs. Thus, our studies presented a link between RAB proteins and fundamental aspects of RAS posttranslational modification and trafficking, highlighting future therapeutic strategies for RAS-driven cancers.