IDH1-Mutant Preleukemic Hematopoietic Stem Cells Can Be Eliminated by Inhibition of Oxidative Phosphorylation.

Rare preleukemic hematopoietic stem cells (pHSC) harboring only the initiating mutations can be detected at the time of acute myeloid leukemia (AML) diagnosis. pHSCs are the origin of leukemia and a potential reservoir for relapse. Using primary human samples and gene editing to model isocitrate dehydrogenase 1 (IDH1) mutant pHSCs, we show epigenetic, transcriptional, and metabolic differences between pHSCs and healthy hematopoietic stem cells (HSC). We confirm that IDH1-driven clonal hematopoiesis is associated with cytopenia, suggesting an inherent defect to fully reconstitute hematopoiesis. Despite giving rise to multilineage engraftment, IDH1-mutant pHSCs exhibited reduced proliferation, blocked differentiation, downregulation of MHC class II genes, and reprogramming of oxidative phosphorylation metabolism. Critically, inhibition of oxidative phosphorylation resulted in the complete eradication of IDH1-mutant pHSCs but not IDH2-mutant pHSCs or wild-type HSCs. Our results indicate that IDH1-mutant preleukemic clones can be targeted with complex I inhibitors, offering a potential strategy to prevent the development and relapse of leukemia. SIGNIFICANCE: A high burden of pHSCs is associated with worse overall survival in AML. Using single-cell sequencing, metabolic assessment, and gene-edited human models, we find human pHSCs with IDH1 mutations to be metabolically vulnerable and sensitive to eradication by complex I inhibition. See related commentary by Steensma.

IDH1-mutant preleukemic hematopoietic stem cells can be eliminated by inhibition of oxidative phosphorylation.

Rare preleukemic hematopoietic stem cells (pHSCs) harboring only the initiating mutations can be detected at the time of AML diagnosis. pHSCs are the origin of leukemia and a potential reservoir for relapse. Using primary human samples and gene-editing to model isocitrate dehydrogenase 1 (IDH1) mutant pHSCs, we show epigenetic, transcriptional, and metabolic differences between pHSCs and healthy hematopoietic stem cells (HSCs). We confirm that IDH1 driven clonal hematopoiesis is associated with cytopenia, suggesting an inherent defect to fully reconstitute hematopoiesis. Despite giving rise to multilineage engraftment, IDH1-mutant pHSCs exhibited reduced proliferation, blocked differentiation, downregulation of MHC Class II genes, and reprogramming of oxidative phosphorylation metabolism. Critically, inhibition of oxidative phosphorylation resulted in complete eradication of IDH1-mutant pHSCs but not IDH2-mutant pHSCs or wildtype HSCs. Our results indicate that IDH1-mutant preleukemic clones can be targeted with complex I inhibitors, offering a potential strategy to prevent development and relapse of leukemia.

Reprogramming Cancer into Antigen-Presenting Cells as a Novel Immunotherapy.

Therapeutic cancer vaccination seeks to elicit activation of tumor-reactive T cells capable of recognizing tumor-associated antigens (TAA) and eradicating malignant cells. Here, we present a cancer vaccination approach utilizing myeloid-lineage reprogramming to directly convert cancer cells into tumor-reprogrammed antigen-presenting cells (TR-APC). Using syngeneic murine leukemia models, we demonstrate that TR-APCs acquire both myeloid phenotype and function, process and present endogenous TAAs, and potently stimulate TAA-specific CD4+ and CD8+ T cells. In vivo TR-APC induction elicits clonal expansion of cancer-specific T cells, establishes cancer-specific immune memory, and ultimately promotes leukemia eradication. We further show that both hematologic cancers and solid tumors, including sarcomas and carcinomas, are amenable to myeloid-lineage reprogramming into TR-APCs. Finally, we demonstrate the clinical applicability of this approach by generating TR-APCs from primary clinical specimens and stimulating autologous patient-derived T cells. Thus, TR-APCs represent a cancer vaccination therapeutic strategy with broad implications for clinical immuno-oncology. SIGNIFICANCE: Despite recent advances, the clinical benefit provided by cancer vaccination remains limited. We present a cancer vaccination approach leveraging myeloid-lineage reprogramming of cancer cells into APCs, which subsequently activate anticancer immunity through presentation of self-derived cancer antigens. Both hematologic and solid malignancies derive significant therapeutic benefit from reprogramming-based immunotherapy. This article is highlighted in the In This Issue feature, p. 1027.

The complement receptor C3AR constitutes a novel therapeutic target in NPM1-mutated AML.

Mutated nucleophosmin 1 (NPM1) is the most common genetic alteration in acute myeloid leukemia (AML), found in ∼30% of cases. Although mutations in this gene are considered favorable according to current risk stratification guidelines, a large fraction of patients will experience relapse, demonstrating the urgent need for new treatment options. Therefore, we aimed to identify cell surface proteins specifically expressed on NPM1-mutated AML cells, allowing for potential targeting with antibody-based therapies. Herein, we report on an arrayed flow cytometry-based screen directed to 362 cell surface markers. In comparing the cell surface expression on NPM1-mutated AML cells with primitive (CD34+ CD38-) normal bone marrow cells, we identified the complement receptor C3AR as being specifically expressed in NPM1-mutated AML. By flow cytometry and single-cell RNA sequencing, we further show that normal hematopoietic stem and progenitor cells lack detectable C3AR gene and protein expression, making it particularly suitable as a target for antibody therapy. We also demonstrate that C3AR in combination with GPR56 distinguishes the leukemic stem cells (LSCs) in NPM1-mutated AML from the normal hematopoietic stem cells, defining the LSC population, as shown by transplantation into immunodeficient mice. Mechanistically, the stimulation of C3AR-expressing cells with C3a, the ligand of C3AR, leads to the activation of ERK1/2 and increased survival of AML cells, suggesting that this is an important signaling axis in this subtype of AML. Finally, we show that antibodies directed against C3AR efficiently elicit natural killer cell-mediated killing of primary AML cells ex vivo, highlighting C3AR as a candidate therapeutic target in NPM1-mutated AML.

Dysregulated Lipid Synthesis by Oncogenic IDH1 Mutation Is a Targetable Synthetic Lethal Vulnerability.

Isocitrate dehydrogenase 1 and 2 (IDH) are mutated in multiple cancers and drive production of (R)-2-hydroxyglutarate (2HG). We identified a lipid synthesis enzyme [acetyl CoA carboxylase 1 (ACC1)] as a synthetic lethal target in mutant IDH1 (mIDH1), but not mIDH2, cancers. Here, we analyzed the metabolome of primary acute myeloid leukemia (AML) blasts and identified an mIDH1-specific reduction in fatty acids. mIDH1 also induced a switch to b-oxidation indicating reprogramming of metabolism toward a reliance on fatty acids. Compared with mIDH2, mIDH1 AML displayed depletion of NADPH with defective reductive carboxylation that was not rescued by the mIDH1-specific inhibitor ivosidenib. In xenograft models, a lipid-free diet markedly slowed the growth of mIDH1 AML, but not healthy CD34+ hematopoietic stem/progenitor cells or mIDH2 AML. Genetic and pharmacologic targeting of ACC1 resulted in the growth inhibition of mIDH1 cancers not reversible by ivosidenib. Critically, the pharmacologic targeting of ACC1 improved the sensitivity of mIDH1 AML to venetoclax. SIGNIFICANCE: Oncogenic mutations in both IDH1 and IDH2 produce 2-hydroxyglutarate and are generally considered equivalent in terms of pathogenesis and targeting. Using comprehensive metabolomic analysis, we demonstrate unexpected metabolic differences in fatty acid metabolism between mutant IDH1 and IDH2 in patient samples with targetable metabolic interventions. See related commentary by Robinson and Levine, p. 266. This article is highlighted in the In This Issue feature, p. 247.

Clinical and genomic characterization of patients diagnosed with the provisional entity acute myeloid leukemia with BCR-ABL1, a Swedish population-based study.

Acute myeloid leukemia (AML) with t(9;22)(q34;q11), also known as AML with BCR-ABL1, is a rare, provisional entity in the WHO 2016 classification and is considered a high-risk disease according to the European LeukemiaNet 2017 risk stratification. We here present a retrospective, population-based study of this disease entity from the Swedish Acute Leukemia Registry. By strict clinical inclusion criteria we aimed to identify genetic markers further distinguishing AML with t(9;22) as a separate entity. Twenty-five patients were identified and next-generation sequencing using a 54-gene panel was performed in 21 cases. Interestingly, no mutations were found in NPM1, FLT3, or DNMT3A, three frequently mutated genes in AML. Instead, RUNX1 was the most commonly mutated gene, with aberrations present in 38% of the cases compared to around 10% in de novo AML. Additional mutations were identified in genes involved in RNA splicing (SRSF2, SF3B1) and chromatin regulation (ASXL1, STAG2, BCOR, BCORL1). Less frequently, mutations were found in IDH2, NRAS, TET2, and TP53. The mutational landscape exhibited a similar pattern as recently described in patients with chronic myeloid leukemia (CML) in myeloid blast crisis (BC). Despite the concomitant presence of BCR-ABL1 and RUNX1 mutations in our cohort, both features of high-risk AML, the RUNX1-mutated cases showed a superior overall survival compared to RUNX1 wildtype cases. Our results suggest that the molecular characteristics of AML with t(9;22)/BCR-ABL1 and CML in myeloid BC are similar and do not support a distinction of the two disease entities based on their underlying molecular alterations.

Clonal competition within complex evolutionary hierarchies shapes AML over time.

Clonal heterogeneity and evolution has major implications for disease progression and relapse in acute myeloid leukemia (AML). To model clonal dynamics in vivo, we serially transplanted 23 AML cases to immunodeficient mice and followed clonal composition for up to 15 months by whole-exome sequencing of 84 xenografts across two generations. We demonstrate vast changes in clonality that both progress and reverse over time, and define five patterns of clonal dynamics: Monoclonal, Stable, Loss, Expansion and Burst. We also show that subclonal expansion in vivo correlates with a more adverse prognosis. Furthermore, clonal expansion enabled detection of very rare clones with AML driver mutations that were undetectable by sequencing at diagnosis, demonstrating that the vast majority of AML cases harbor multiple clones already at diagnosis. Finally, the rise and fall of related clones enabled deconstruction of the complex evolutionary hierarchies of the clones that compete to shape AML over time.

A high-content cytokine screen identifies myostatin propeptide as a positive regulator of primitive chronic myeloid leukemia cells.

Aberrantly expressed cytokines in the bone marrow (BM) niche are increasingly recognized as critical mediators of survival and expansion of leukemic stem cells. To identify regulators of primitive chronic myeloid leukemia (CML) cells, we performed a high-content cytokine screen using primary CD34+ CD38low chronic phase CML cells. Out of the 313 unique human cytokines evaluated, 11 were found to expand cell numbers ≥2-fold in a 7-day culture. Focusing on novel positive regulators of primitive CML cells, the myostatin antagonist myostatin propeptide gave the largest increase in cell expansion and was chosen for further studies. Herein, we demonstrate that myostatin propeptide expands primitive CML and normal BM cells, as shown by increased colony-forming capacity. For primary CML samples, retention of CD34-expression was also seen after culture. Furthermore, we show expression of MSTN by CML mesenchymal stromal cells, and that myostatin propeptide has a direct and instant effect on CML cells, independent of myostatin, by demonstrating binding of myostatin propeptide to the cell surface and increased phosphorylation of STAT5 and SMAD2/3. In summary, we identify myostatin propeptide as a novel positive regulator of primitive CML cells and corresponding normal hematopoietic cells.

CD36 defines primitive chronic myeloid leukemia cells less responsive to imatinib but vulnerable to antibody-based therapeutic targeting.

Tyrosine kinase inhibitors (TKIs) are highly effective for the treatment of chronic myeloid leukemia (CML), but very few patients are cured. The major drawbacks regarding TKIs are their low efficacy in eradicating the leukemic stem cells responsible for disease maintenance and relapse upon drug cessation. Herein, we performed ribonucleic acid sequencing of flow-sorted primitive (CD34+CD38low) and progenitor (CD34+ CD38+) chronic phase CML cells, and identified transcriptional upregulation of 32 cell surface molecules relative to corresponding normal bone marrow cells. Focusing on novel markers with increased expression on primitive CML cells, we confirmed upregulation of the scavenger receptor CD36 and the leptin receptor by flow cytometry. We also delineate a subpopulation of primitive CML cells expressing CD36 that is less sensitive to imatinib treatment. Using CD36 targeting antibodies, we show that the CD36 positive cells can be targeted and killed by antibody-dependent cellular cytotoxicity. In summary, CD36 defines a subpopulation of primitive CML cells with decreased imatinib sensitivity that can be effectively targeted and killed using an anti-CD36 antibody.

Transgenic expression of human cytokines in immunodeficient mice does not facilitate myeloid expansion of BCR-ABL1 transduced human cord blood cells.

Several attempts have been made to model chronic myeloid leukemia (CML) in a xenograft setting but expansion of human myeloid cells in immunodeficient mice has proven difficult to achieve. Lack of cross-reacting cytokines in the microenvironment of the mice has been proposed as a potential reason. In this study we have used NOD/SCID IL2-receptor gamma deficient mice expressing human SCF, IL-3 and GM-CSF (NSGS mice), that should be superior in supporting human, and particularly, myeloid cell engraftment, to expand BCR-ABL1 expressing human cells in order to model CML. NSGS mice transplanted with BCR-ABL1 expressing cells became anemic and had to be sacrificed due to illness, however, this was not accompanied by an expansion of human myeloid cells but rather we observed a massive expansion of human T-cells and macrophages/histiocytes. Importantly, control human cells without BCR-ABL1 expression elicited a similar reaction, although with a slight delay of disease induction, suggesting that while BCR-ABL1 contributes to the inflammatory reaction, the presence of normal human hematopoietic cells is detrimental for NSGS mice.

Primary cells in BCR/FGFR1-positive 8p11 myeloproliferative syndrome are sensitive to dovitinib, ponatinib, and dasatinib.

OBJECTIVES: Translocations involving the fibroblast growth factor receptor 1 (FGFR1) gene are associated with the 8p11 myeloproliferative syndrome (EMS), a rare neoplasm that following a usually short chronic phase progresses into acute myeloid or lymphoid leukemia. The treatment commonly involves chemotherapy and, if possible, allogeneic stem cell transplantation which is the only therapeutic option for long-term survival. Given the aggressive course of EMS, we here evaluated tyrosine kinase inhibitors as treatment options to delay disease progression. METHODS: We described a new case of EMS and used chromosome analyses, PCR, and sequencing to investigate the underlying genetic aberrations. The sensitivity to several tyrosine kinase inhibitors was tested in vitro on the EMS cell line KG1 and on primary cells from the newly diagnosed EMS patient. RESULTS: A translocation involving chromosomes 8 and 22 was detected, and a BCR/FGFR1 fusion gene was confirmed and characterized by sequencing. KG1 cells and primary EMS cells displayed distinct sensitivity to dovitinib, ponatinib, and dasatinib as compared to normal bone marrow control cells. CONCLUSIONS: These results suggest that treatment with tyrosine kinase inhibitors may be beneficial for patients with EMS during the search for a suitable stem cell donor and for those not eligible for transplantation.

IL1RAP antibodies block IL-1-induced expansion of candidate CML stem cells and mediate cell killing in xenograft models.

Chronic myeloid leukemia (CML) is currently treated with tyrosine kinase inhibitors, but these do not effectively eliminate the CML stem cells. As a consequence, CML stem cells persist and cause relapse in most patients upon drug discontinuation. Furthermore, no effective therapy exists for the advanced stages of the disease. Interleukin-1 receptor accessory protein (IL1RAP; IL1R3) is a coreceptor of interleukin-1 receptor type 1 and has been found upregulated on CML stem cells. Here, we show that primitive (CD34+CD38-) CML cells, in contrast to corresponding normal cells, express a functional interleukin-1 (IL-1) receptor complex and respond with NF-κB activation and marked proliferation in response to IL-1. IL1RAP antibodies that inhibit IL-1 signaling could block these effects. In vivo administration of IL1RAP antibodies in mice transplanted with chronic and blast phase CML cells resulted in therapeutic effects mediated by murine effector cells. These results provide novel insights into the role of IL1RAP in CML and a strong rationale for the development of an IL1RAP antibody therapy to target residual CML stem cells.