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Given the plasticity of hematopoietic stem and progenitor cells, multiple routes of differentiation must be blocked in the the pathogenesis of acute myeloid leukemia, the molecular basis of which is incompletely understood. We report that posttranscriptional repression of the transcription factor ARID3A by miR-125b is a key event in the pathogenesis of acute megakaryoblastic leukemia (AMKL). AMKL is frequently associated with trisomy 21 and GATA1 mutations (GATA1s), and children with Down syndrome are at a high risk of developing the disease. The results of our study showed that chromosome 21–encoded miR-125b synergizes with Gata1s to drive leukemogenesis in this context. Leveraging forward and reverse genetics, we uncovered Arid3a as the main miR-125b target behind this synergy. We demonstrated that, during normal hematopoiesis, this transcription factor promotes megakaryocytic differentiation in concert with GATA1 and mediates TGFβ-induced apoptosis and cell cycle arrest in complex with SMAD2/3. Although Gata1s mutations perturb erythroid differentiation and induce hyperproliferation of megakaryocytic progenitors, intact ARID3A expression assures their megakaryocytic differentiation and growth restriction. Upon knockdown, these tumor suppressive functions are revoked, causing a blockade of dual megakaryocytic/erythroid differentiation and subsequently of AMKL. Inversely, restoring ARID3A expression relieves the arrest of megakaryocytic differentiation in AMKL patient-derived xenografts. This work illustrates how mutations in lineage-determining transcription factors and perturbation of posttranscriptional gene regulation can interact to block multiple routes of hematopoietic differentiation and cause leukemia. In AMKL, surmounting this differentiation blockade through restoration of the tumor suppressor ARID3A represents a promising strategy for treating this lethal pediatric disease.

Gain of chromosome 21 (Hsa21) is among the most frequent aneuploidies in leukemia. However, it remains unclear how partial or complete amplifications of Hsa21 promote leukemogenesis and why children with Down syndrome (i.e. trisomy 21) are particularly at risk of leukemia development. Here, we propose that RUNX1 isoform disequilibrium with RUNX1A bias is key to Down syndrome-associated myeloid leukemia (ML-DS). Starting with Hsa21-focused CRISPR-Cas9 screens, we uncovered a strong and specific RUNX1 dependency in ML-DS cells. Expression of the RUNX1A isoform is elevated in ML-DS patients, and mechanistic studies using murine ML-DS models and patient-derived xenografts (PDXs) revealed that excess RUNX1A synergizes with the pathognomonic Gata1s mutation during leukemogenesis by displacing RUNX1C from its endogenous binding sites and inducing oncogenic programs in complex with the MYC cofactor MAX. These effects were reversed by restoring the RUNX1A:RUNX1C equilibrium in PDXs in vitro and in vivo. Moreover, pharmacological interference with MYC:MAX dimerization using MYCi361 exerted strong anti-leukemic effects. Thus, our study highlights the importance of alternative splicing in leukemogenesis, even on a background of aneuploidy, and paves the way for the development of specific and targeted therapies for ML-DS, as well as for other leukemias with Hsa21 aneuploidy or RUNX1 isoform disequilibrium.

The noncoding genome presents a largely untapped source of biological insights, including thousands of long noncoding RNA (lncRNA) loci. While some produce bona fide lncRNAs, others exert transcript-independent cis-regulatory effects, and a lack of predictive features renders mechanistic dissection challenging. Here, we describe MYNRL15, a CTCF-enriched lncRNA locus and pan-myeloid leukemia dependency initially identified by expression-guided CRISPR interference screens. We show that accessibility and integrity of the MYNRL15 locus is required for myeloid leukemia maintenance; its perturbation selectively impairs acute myeloid leukemia (AML) cells compared to hematopoietic stem and progenitor cells in vitro, and depletes AML xenografts in vivo. While the MYNRL15 transcript and neighboring protein-coding genes appear dispensable, dense CRISPR tiling of the locus revealed two crucial candidate cis-regulatory DNA elements which drive the perturbation phenotype. Disruption of these elements triggers the formation of a tumor-suppressive, long-range chromatin interaction. By integrating transcriptome profiling with a CRISPR-Cas9 knockout screen of genes from the gained interaction region, we pinpointed two downregulated, potent cancer dependency genes as effectors of MYNRL15 disruption: WDR61 and IMP3. Finally, guided by distinctive features of the MYNRL15 locus, we find that elevated CTCF density characterizes a set of lncRNA loci enriched in leukemia vulnerabilities (22.6-24.2% essentiality rate). A catalog of CTCF-enriched lncRNA loci (C-LNCs) in 18 cell types representing different cancer entities and tissues is provided with this study, towards refining the search for noncoding oncogenic vulnerabilities in leukemia and other malignancies.