ETV6-RUNX1 is associated with the most common subtype of childhood leukemia. As few ETV6-RUNX1 carriers develop precursor B-cell acute lymphocytic leukemia (pB-ALL), the underlying genetic basis for development of full-blown leukemia remains to be identified, but the appearance of leukemia cases in timespace clusters keeps infection as a potential causal factor. Here, we present in vivo genetic evidence mechanistically connecting preleukemic ETV6-RUNX1 expression in hematopoetic stem cells/precursor cells (HSC/PC) and postnatal infections for human-like pB-ALL. In our model, ETV6-RUNX1 conferred a low risk of developing pB-ALL after exposure to common pathogens, corroborating the low incidence observed in humans. Murine preleukemic ETV6-RUNX1 pro/preB cells showed high Rag1/2 expression, known for human ETV6-RUNX1 pB-ALL. Murine and human ETV6-RUNX1 pB-ALL revealed recurrent genomic alterations, with a relevant proportion affecting genes of the lysine demethylase (KDM) family. KDM5C loss of function resulted in increased levels of H3K4me3, which coprecipitated with RAG2 in a human cell line model, laying the molecular basis for recombination activity. We conclude that alterations of KDM family members represent a disease-driving mechanism and an explanation for RAG off-target cleavage observed in humans. Our results explain the genetic basis for clonal evolution of an ETV6-RUNX1 preleukemic clone to pB-ALL after infection exposure and offer the possibility of novel therapeutic approaches.

Heat shock protein 90 (HSP90) stabilizes many client proteins, including the BCR-ABL1 oncoprotein. BCR-ABL1 is the hallmark of chronic myeloid leukemia (CML) in which treatment-free remission (TFR) is limited, with clinical and economic consequences. Thus, there is an urgent need for novel therapeutics that synergize with current treatment approaches. Several inhibitors targeting the N-terminal domain of HSP90 are under investigation, but side effects such as induction of the heat shock response (HSR) and toxicity have so far precluded their US Food and Drug Administration approval. We have developed a novel inhibitor (aminoxyrone [AX]) of HSP90 function by targeting HSP90 dimerization via the C-terminal domain. This was achieved by structure-based molecular design, chemical synthesis, and functional preclinical in vitro and in vivo validation using CML cell lines and patient-derived CML cells. AX is a promising potential candidate that induces apoptosis in the leukemic stem cell fraction (CD34CD38) as well as the leukemic bulk (CD34CD38) of primary CML and in tyrosine kinase inhibitor (TKI)-resistant cells. Furthermore, BCR-ABL1 oncoprotein and related pro-oncogenic cellular responses are downregulated, and targeting the HSP90 C terminus by AX does not induce the HSR in vitro and in vivo. We also probed the potential of AX in other therapy-refractory leukemias. Therefore, AX is the first peptidomimetic C-terminal HSP90 inhibitor with the potential to increase TFR in TKI-sensitive and refractory CML patients and also offers a novel therapeutic option for patients with other types of therapy-refractory leukemia because of its low toxicity profile and lack of HSR.

Dual- or multi-target drugs have emerged as a promising alternative to combination therapies. Proteasome inhibitors (PIs) possess synergistic activity with histone deacetylase (HDAC) inhibitors due to the simultaneous blockage of the ubiquitin-degradation and aggresome pathways. Here, we present the design, synthesis, binding modes and anticancer properties of RTS-V5 as the first-in-class dual HDAC-proteasome ligand. The inhibition of both targets was confirmed by biochemical and cellular assays as well as X-ray crystal structures of the 20S proteasome and HDAC6 complexed with RTS-V5. Cytotoxicity assays with leukemia and multiple myeloma cell lines as well as therapy-refractory primary patient-derived leukemia cells demonstrated that RTSV5 possesses potent and selective anticancer activity. Our results will thus guide the structure-based optimization of dual HDAC-proteasome inhibitors for the treatment of hematological malignancies.

The majority of childhood leukemias are precursor B cell-acute lymphoblastic leukemias (pB-ALL) caused by a combination of prenatal genetic predispositions and oncogenic events occurring after birth. Although genetic predispositions are frequent in children (>1-5%), fewer than 1% of genetically predisposed carriers will develop pB-ALL. While infectious stimuli are believed to play a major role in leukemogenesis, the critical determinants are not well defined. Here, employing murine models of pB-ALL, we show that microbiome disturbances incurred by antibiotic treatment early in life were sufficient to induce leukemia in genetically predisposed mice even in the absence of infectious stimuli and independent of T-cells. Using V4 and full-length 16S rRNA sequencing of a series of fecal samples, we found that genetic predisposition to pB-ALL (Pax5 heterozygosity or ETV6-RUNX1 fusion) shaped a distinct gut microbiome. Machine learning accurately (96.8%) predicted genetic predisposition using 40 of 3,983 amplicon sequence variants (ASVs) as proxies for bacterial species. Transplantation of either wild type (WT) or Pax5+/- hematopoietic bone marrow cells into WT recipient mice revealed that the microbiome is shaped and determined in a donor-genotype-specific manner. Gas chromatography-mass spectrometric (GC-MS) analyses of sera from WT and Pax5+/- mice demonstrated the presence of a genotype-specific distinct metabolomic profile. Taken together, our data indicate that it is a lack of commensal microbiota rather than the presence of specific bacteria that promotes leukemia in genetically predisposed mice. Future large-scale longitudinal studies are required to determine whether targeted microbiome modification in children predisposed to pB-ALL could become a successful prevention strategy.

The homeobox gene HLXB9 encodes for the transcription factor HB9, which is essential for pancreatic as well as motor neuronal development. Beside its physiologic expression pattern, aberrant HB9 expression has been observed in several neoplasias. Especially in infant translocation t(7;12) acute myeloid leukemia aberrant HB9 expression is the only known molecular hallmark and assumed to be a key factor in leukemic transformation. However, up to now only poor functional data exist addressing the oncogenic potential of HB9 or its influence on hematopoiesis. We investigated the influence of HB9 on cell proliferation and cell cycle in vitro, as well as on hematopoietic stem cell differentiation in vivo using murine and human model systems. In vitro, HB9 expression led to premature senescence in human HT1080 and murine NIH3T3 cells, providing for the first time evidence for an oncogenic potential of HB9. Onset of senescence was characterized by induction of the p53-p21 tumor suppressor network, resulting in growth arrest, accompanied by morphological transformation and expression of senescence-associated β-galactosidase. In vivo, HB9-transduced primary murine hematopoietic stem and progenitor cells underwent a profound differentiation arrest and accumulated at the megakaryocyte/erythrocyte progenitor stage. In line, gene expression analyses revealed de novo expression of erythropoiesis-related genes in human CD34+ hematopoietic stem and progenitor cells upon HB9 expression. In summary, the novel findings of HB9 dependent premature senescence and myeloid-biased perturbed hematopoietic differentiation shed light on the oncogenic properties of HB9 in translocation t(7;12) acute myeloid leukemia for the first time.

Inhibition of more than one cancer‐related pathway by multi‐target agents is an emerging approach in modern anticancer drug discovery. Here, based on the well‐established synergy between histone deacetylase inhibitors (HDACi) and alkylating agents, we present the discovery of a series of alkylating HDACi using a pharmacophore‐linking strategy. For the parallel synthesis of the target compounds, we developed an efficient solid‐phase‐supported protocol using hydroxamic acids immobilized on resins (HAIRs) as stable and versatile building blocks for the preparation of functionalized HDACi. The most promising compound, 3 n, was significantly more active in apoptosis induction, activation of caspase 3/7, and formation of DNA damage (γ‐H2AX) than the sum of the activities of either active principle alone. Furthermore, to demonstrate the utility of our preloaded resins, the HAIR approach was successfully extended to the synthesis of a proof‐of‐concept proteolysis‐targeting chimera (PROTAC), which efficiently degrades histone deacetylases.

The impact of LMO2 expression on cell lineage decisions during T‐cell leukemogenesis remains largely elusive. Using genetic lineage tracing, we have explored the potential of LMO2 in dictating a T‐cell malignant phenotype. We first initiated LMO2 expression in hematopoietic stem/progenitor cells and maintained its expression in all hematopoietic cells. These mice develop exclusively aggressive human‐like T‐ALL. In order to uncover a potential exclusive reprogramming effect of LMO2 in murine hematopoietic stem/progenitor cells, we next showed that transient LMO2 expression is sufficient for oncogenic function and induction of T‐ALL. The resulting T‐ALLs lacked LMO2 and its target‐gene expression, and histologically, transcriptionally, and genetically similar to human LMO2‐driven T‐ALL. We next found that during T‐ALL development, secondary genomic alterations take place within the thymus. However, the permissiveness for development of T‐ALL seems to be associated with wider windows of differentiation than previously appreciated. Restricted Cre‐mediated activation of Lmo2 at different stages of B‐cell development induces systematically and unexpectedly T‐ALL that closely resembled those of their natural counterparts. Together, these results provide a novel paradigm for the generation of tumor T cells through reprogramming in vivo and could be relevant to improve the response of T‐ALL to current therapies.

AC133 is a prominent surface marker of CD34+ and CD34- hematopoietic stem/progenitor cell (HSPC) subsets. AC133+ HSPCs contain high progenitor cell activity and are capable of hematopoietic reconstitution. Furthermore, AC133 is used for prospective isolation of tumor-initiating cells in several hematological malignancies. Nucleolin is a multifunctional factor of growing and cancer cells, which is aberrantly active in certain hematological neoplasms, and serves as a candidate molecular target for cancer therapy. Nucleolin is involved in gene transcription and RNA metabolism and is prevalently expressed in HSPCs, as opposed to differentiated hematopoietic tissue. The present study dissects nucleolin-mediated activation of surface AC133 and its cognate gene CD133, via specific interaction of nucleolin with the tissue-dependent CD133 promoter P1, as a mechanism that crucially contributes to AC133 expression in CD34+ HSPCs. In mobilized peripheral blood (MPB)-derived HSPCs, nucleolin elevates colony-forming unit (CFU) frequencies and enriches granulocyte-macrophage CFUs. Furthermore, nucleolin amplifies long-term culture-initiating cells and also promotes long-term, cytokine-dependent maintenance of hematopoietic progenitor cells. Active β-catenin, active Akt and Bcl-2 levels in MPB-derived HSPCs are nucleolin-dependent, and effects of nucleolin on these cells partially rely on β-catenin activity. The study provides new insights into molecular network relevant to stem/progenitor cells in normal and malignant hematopoiesis.