The BCL2 inhibitor venetoclax has recently emerged as an important component of acute myeloid leukemia (AML) therapy. Notably, use of this agent has revealed a previously unrecognized form of pathogenesis characterized by monocytic disease progression. We demonstrate that this form of disease arises from a fundamentally different type of leukemia stem cell (LSC), which we designate as monocytic LSC (m-LSC), that is developmentally and clinically distinct from the more well-described primitive LSC (p-LSC). The m-LSC is distinguished by a unique immunophenotype (CD34−, CD4+, CD11b−, CD14−, CD36−), unique transcriptional state, reliance on purine metabolism, and selective sensitivity to cladribine. Critically, in some instances, m-LSC and p-LSC subtypes can co-reside in the same patient with AML and simultaneously contribute to overall tumor biology. Thus, our findings demonstrate that LSC heterogeneity has direct clinical significance and highlight the need to distinguish and target m-LSCs as a means to improve clinical outcomes with venetoclax-based regimens. Significance: These studies identify and characterize a new type of human acute myeloid LSC that is responsible for monocytic disease progression in patients with AML treated with venetoclax-based regimens. Our studies describe the phenotype, molecular properties, and drug sensitivities of this unique LSC subclass. This article is featured in Selected Articles from This Issue, p. 1949

Chemo-resistance in acute myeloid leukemia (AML) patients is driven by leukemic stem cells (LSCs) resulting in high rates of relapse and low overall survival. Here, we demonstrate that upregulation of the splicing factor, RBM17 preferentially marks and sustains LSCs and directly correlates with shorten patient survival. RBM17 knockdown in primary AML cells leads to myeloid differentiation and impaired colony formation and in vivo engraftment. Integrative multi-omics analyses show that RBM17 repression leads to inclusion of poison exons and production of nonsense-mediated decay (NMD)-sensitive transcripts for pro-leukemic factors and the translation initiation factor, EIF4A2. We show that EIF4A2 is enriched in LSCs and its inhibition impairs primary AML progenitor activity. Proteomic analysis of EIF4A2-depleted AML cells shows recapitulation of the RBM17 knockdown biological effects, including pronounced suppression of proteins involved in ribosome biogenesis. Overall, these results provide a rationale to target RBM17 and/or its downstream NMD-sensitive splicing substrates for AML treatment.

Acute myeloid leukemia (AML) is fueled by leukemic stem cells (LSC) whose determinants are challenging to discern from hematopoietic stem cells (HSC) or uncover by approaches focused on general cell properties. We have identified a set of RNA-binding proteins (RBP) selectively enriched in human AML LSCs. Using an in vivo two-step CRISPR-Cas9 screen to assay stem cell functionality, we found 32 RBPs essential for LSCs in MLL-AF9;NrasG12D AML. Loss-of-function approaches targeting key hit RBP ELAVL1 compromised LSC-driven in vivo leukemic reconstitution, and selectively depleted primitive malignant versus healthy cells. Integrative multiomics revealed differentiation, splicing, and mitochondrial metabolism as key features defining the leukemic ELAVL1–mRNA interactome with mitochondrial import protein, TOMM34, being a direct ELAVL1-stabilized target whose repression impairs AML propagation. Altogether, using a stem cell–adapted in vivo CRISPR screen, this work demonstrates pervasive reliance on RBPs as regulators of LSCs and highlights their potential as therapeutic targets in AML. Significance: LSC-targeted therapies remain a significant unmet need in AML. We developed a stem-cell–adapted in vivo CRISPR screen to identify key LSC drivers. We uncover widespread RNA-binding protein dependencies in LSCs, including ELAVL1, which we identify as a novel therapeutic vulnerability through its regulation of mitochondrial metabolism. This article is highlighted in the In This Issue feature, p. 171

Developing strategies to enhance the response to bromodomain and extraterminal domain (BET) inhibitors and effectively eradicate cancer stem cells would represent a major cancer treatment advance against leukemia. Through a functional CRISPR screen, we identified the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, as a critical regulator of MYC expression and BET inhibitor sensitivity in human acute myeloid leukemia (AML). Constitutive or pharmacological activation of AHR repressed MYC and synergized with BET inhibitors to inhibit MYC transcription and suppress leukemia growth across diverse AML models. Mechanistically, AHR directly up-regulated a noncanonical target, ELMSAN1, a component of the MiDAC histone deacetylase complex, which promotes histone deacetylation at MYC regulatory elements. ELMSAN1 depletion led to up-regulation of MYC and impaired AHR signaling–induced BET inhibitor sensitization. In vivo, AHR agonists enhanced BET inhibitor efficacy in patient-derived xenografts and murine leukemia models, enabling the use of lower BET inhibitor doses while preserving therapeutic benefit and reducing toxicity. This combination suppressed leukemia stem cell (LSC) gene signatures and reduced LSC frequency, with minimal impact on normal hematopoietic stem and progenitor cells in both human cord blood xenografts and immunocompetent mouse models. Together, these findings uncover a MYC-repressive, nongenetic AHR-ELMSAN1 axis that enhances BET-targeting therapies and selectively impairs LSCs, providing a compelling rationale for clinical translation in AML and potentially other MYC-driven cancers.

Dysregulation of RNA N ⁶ -methyladenosine (m ⁶ A) readers has been linked to various diseases, but the therapeutic potential of small-molecule inhibitors targeting them is of interest. Here, we reported the identification and characterization of a potent and selective first-in-class inhibitor (YL-5092) of YTHDC1, a nuclear RNA m ⁶ A reader. We provided a high-resolution cocrystal structure of the YTHDC1–YL-5092 complex. In acute myeloid leukemia (AML) models, YL-5092 blocked the binding of YTHDC1 to its m ⁶ A substrates and reduced mRNA stability, resulting in apoptosis of AML cells and myeloid differentiation. In multiple xenograft models of AML representing disease heterogeneity, YL-5092 alone or in combination with standard AML therapy eliminated leukemia and extended survival. Moreover, YL-5092 functionally impaired leukemia stem cells yet spared normal hematopoietic counterparts. Collectively, our work demonstrates the efficacy of a selective YTHDC1 inhibitor and suggests that targeting of m ⁶ A readers is a potential strategy in the treatment of hematologic cancers.