Human AML Research Articles

Abstract 5495: PHI-501, a potent and novel inhibitor of NRAS mutated acute myeloid leukemia

Abstract Purpose: Development of the first-in-class inhibitor of NRAS mutated acute myeloid leukemia. PHI-501 has potent activity against receptor tyrosine kinases and intracellular kinases related to STAT, AKT, and ERK signaling. Description: Preclinical testing of PHI-501 for efficacy against NRAS mutations both in vitro and in vivo xenografts. Summary: Mutations in the RAS genes (including KRAS, NRAS, and HRAS) are discovered in about 30% of all tumors. KRAS is the most frequently mutated gene in cancers found in pancreatic, colon, and lung, while NRAS mutations around the hot spots at codons 12, 13, and 61 are more common in AML (10.3%). PHI-501 is a novel small molecule inhibitor intended for the treatment of AML in patients expressing the N-RAS activating mutation. PHI-501 presented higher selectivity within a panel of 355 protein kinases. PHI-501 exerts their anti-proliferative effects by inducing apoptosis and G0-G1 arrest and significantly induce apoptotic markers (cleaved PARP and caspase 3) in both Ba/F3-NRAS-G12D and OCI-AML3 cells. The results of Western blot analysis showed that PHI-501 attenuate phosphorylation of p70S6K1, AKT, and p38 in OCI-AML3 After 21 days of the treatment in subcutaneous OCI-AML3 human AML xenograft model, PHI-501 showed significant inhibition of tumor growth (TGI) in a dose-dependent manner. Compared to the vehicle group, the 40 mg/kg group demonstrated a 74.0% lower average tumor volume (P<0.0001) and a 78.5% lower average tumor weight (P=0.0041). No significant body weight changes were observed in any of the groups. In toxicology and pharmacokinetic study, PHI-501 exhibited pharmacokinetic profiles favorable for oral administration in rodent, dog, and monkey and displayed acceptable in vivo and in vitro safety profiles. These results show the significant ability of PHI-501 to inhibit the progression of NRAS activated AML tumor growth, making it a strong candidate for the treatment of AML. Conclusion: The preclinical evaluation of PHI-501, a novel N-RAS inhibitor, showed clear evidence of anticancer activity for AML and improved efficacy in both in vitro and in vivo models. Consequently, PHI-501 is a new potent multi-kinase inhibitor with characteristics that warrant entry into human trials for the treatment of AML in patients expressing the N-RAS activating mutation. Citation Format: Ky-Youb Nam, Jung Hee Park, Kyung Ah Kim, Inje Shin, June H-J Han, Kyu-Tae Kim, Jeong Hyeok Yoon, Hanna Cho, Seung-Hye Choi, Sandip Sengupta, Taebo Sim. PHI-501, a potent and novel inhibitor of NRAS mutated acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5495.

Abstract 1813: DIACC2010, a selective inhibitor of KIF20A

Abstract Mitotic kinesins are essential regulators of cancer cell replication and migration. The microtubule associated motor protein KIF20A (also called MKlp2), a member of the kinesin-6 family, plays an essential role during cytokinesis and is also involved in the fission of RAB6-positive vesicles from Golgi membrane, therefore contributing to intracellular vesicular trafficking (1). KIF20A plays a critical role in the development and progression of many cancers, and its high expression is associated with disease progression and poor survival outcome (2). More specifically, immature hematopoietic cells, exhibit high KIF20A expression, whereas mature peripheral blood cells do not (3). Herein we describe the preclinical anti-leukemic efficacy of DIACC2010, sole-in-class selective KIF20A inhibitor. DIACC2010 demonstrated potent and consistent cytotoxic activity in-vitro against a panel of 9 human AML cell lines, with median IC50 of 40 nM (range 10-77 nM). In the same experimental conditions, cytarabine (CYTA) had median IC50 of 207 nM (range 4-1580 nM). In parallel, no in-vitro toxicity of DIACC2010 was observed on human normal cells such as peripheral blood mononuclear cells (PBMC) and primary hepatocytes (IC50 >50 µM). DIACC2010 was subsequently evaluated in-vivo in xenograft models of CYTA-sensitive and CYTA-resistant AML cell lines: DIACC2010 significantly improved overall survival as compared to CYTA in all models, with dose-dependent efficacy. On a mechanistic standpoint, DIACC2010-exposed AML cells displayed characteristic Golgi scattering leading to cell death, as hallmarks of DIACC2010 mode of action and target engagement. Altogether, these results confirm the relevance of KIF20A-directed therapeutic approaches and support the development of DIACC2010 for the treatment of AML. (1) Coupling fission and exit of RAB6 vesicles at Golgi hotspots though kinesin-myosin interactions, S. Miserey-Lenkei et al, Nature com, 2017, 104:300 (2) Prognostic significance of KIF2A and KIF20A expression in human cancer. X. Li et al, Medicine, 2019, 98:46 (3) KIF20A, highly expressed in immature hematopoietic cells, supports the growth of HL60 cell line, H. Morita et al, Int J Immunol, 2018,108:607 Citation Format: Yves COLLETTE, Rémy CASTELLANO, Michel AURRAND-LIONS, Hélène SICARD, Norbert VEY, Cécile BOUGERET. DIACC2010, a selective inhibitor of KIF20A [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1813.

Blockade of p38 MAPK overcomes AML stem cell line KG1a resistance to 5-Fluorouridine and the impact on miRNA profiling.

Most of the AML patients in remission develop multidrug resistance after the first-line therapy and relapse. AML stem cells have gained attention for their chemoresistance potentials. Chemoresistance is a multifactorial process resulting from altered survival signaling pathways and apoptosis regulators such as MAPK, NF-κB activation and ROS production. We targeted the survival pathway p38 MAPK, NF-κB and ROS generation in human chemoresistant AML stem cell line KG1a, susceptible to enhance cell sensitivity to the chemotherapy drug 5-Fluorouridine, compared to the chemosensitive AML cell line HL60. After confirming the phenotypic characterization of KG1a and HL60 cells using flow cytometry and transcriptomic array analyses, cell treatment with the NF-κB inhibitor IKKVII resulted in a complete induction of apoptosis, and a few p38 MAPK inhibitor SB202190-treated cells underwent apoptosis. No change in the apoptosis status was observed in the ROS scavenger N-acetylcysteine-treated cells. The p38 MAPK pathway blockade enhanced the KG1a cell sensitivity to 5-Fluorouridine, which was associated with the upregulation of microribonucleic acid-(miR-)328-3p, as determined by the microarray-based miRNA transcriptomic analysis. The downregulation of the miR-210-5p in SB202190-treated KG1a cells exposed to FUrd was monitored using RT-qPCR. The miR-328-3p is known for the enhancement of cancer cell chemosensitivity and apoptosis induction, and the downregulation of miR-210-5p is found in AML patients in complete remission. In conclusion, we highlighted the key role of the p38 MAPK survival pathway in the chemoresistance capacity of the AML stem cells and potentially involved miRNAs, which may pave the way for the development of a new therapeutic strategy targeting survival signaling proteins and reduce the rate of AML relapse.

Pseudo-mutant P53 is a unique phenotype of DNMT3A-mutated pre-leukemia.

Pre-leukemic clones carrying DNMT3A mutations have a selective advantage and an inherent chemoresistance, however the basis for this phenotype has not been fully elucidated. Mutations affecting the gene TP53 occur in pre-leukemic hematopoietic stem/progenitor cells (preL-HSPC) and lead to chemoresistance. Many of these mutations cause a conformational change and some of them were shown to enhance self-renewal capacity of preL-HSPC. Intriguingly, a misfolded P53 was described in AML blasts that do not harbor mutations in TP53, emphasizing the dynamic equilibrium between wild-type (WT) and "pseudo-mutant" conformations of P53. By combining single cell analyses and P53 conformation-specific monoclonal antibodies we studied preL-HSPC from primary human DNMT3A-mutated AML samples. We found that while leukemic blasts express mainly the WT conformation, in preL-HSPC the pseudo-mutant conformation is the dominant. HSPC from non-leukemic samples expressed both conformations to a similar extent. In a mouse model we found a small subset of HSPC with a dominant pseudo-mutant P53. This subpopulation was significantly larger among DNMT3AR882H-mutated HSPC, suggesting that while a pre-leukemic mutation can predispose for P53 misfolding, additional factors are involved as well. Treatment with a short peptide that can shift the dynamic equilibrium favoring the WT conformation of P53, specifically eliminated preL-HSPC that had dysfunctional canonical P53 pathway activity as reflected by single cell RNA sequencing. Our observations shed light upon a possible targetable P53 dysfunction in human preL-HSPC carrying DNMT3A mutations. This opens new avenues for leukemia prevention.

Metalloproteinase inhibition reduces AML growth, prevents stem cell loss, and improves chemotherapy effectiveness

AbstractAcute myeloid leukemia (AML) is a blood cancer of the myeloid lineage. Its prognosis remains poor, highlighting the need for new therapeutic and precision medicine approaches. AML symptoms often include cytopenias linked to loss of healthy hematopoietic stem and progenitor cells (HSPCs). The mechanisms behind HSPC decline are complex and still poorly understood. Here, intravital microscopy (IVM) of a well-established experimental model of AML allows direct observation of the interactions between healthy and malignant cells in the bone marrow (BM), suggesting that physical dislodgment of healthy cells by AML through damaged vasculature may play an important role. Multiple matrix metalloproteinases (MMPs), known to remodel extracellular matrix, are expressed by AML cells and the BM microenvironment. We reason MMPs could be involved in cell displacement and vascular leakiness; therefore, we evaluate the therapeutic potential of MMP pharmacological inhibition using the broad-spectrum inhibitor prinomastat. IVM analyses of prinomastat-treated mice reveal reduced vascular permeability and healthy cell clusters in circulation and lower AML infiltration, proliferation, and cell migration. Furthermore, treated mice have increased retention of healthy HSPCs in the BM and increased survival following chemotherapy. Analysis of a human AML transcriptomic database reveals widespread MMP deregulation, and human AML cells show susceptibility to MMP inhibition. Overall, our results suggest that MMP inhibition could be a promising complementary therapy to reduce AML growth and limit HSPC loss and BM vascular damage caused by MLL-AF9 and possibly other AML subtypes.

3219 – RNA DEADENYLATION SUBUNIT CNOT3 PROMOTES MYELOID LEUKEMIA BY DRIVING TRANSLATION OF ONCOGENIC TARGETS

How processes that affect post-transcriptional and translational regulation impact leukemia development is not well studied. The CCR4-NOT (CNOT) complex is one of two major multi-subunit polyA deadenylation complexes which mediate the shortening of the poly(A) tails and is recruited to m6A transcripts to mediate mRNA degradation. We surveyed several genome-wide CRISPR screens and observed several subunits of the CNOT complex ranked as genes essential for survival of AML cells. The highest ranked CNOT3 adaptor subunit demonstrated elevated expression in AML patients and associated with a poor prognosis in independent AML patient cohorts. Depletion of CNOT3 in human AML cells resulted in inhibition of cell growth, increased myeloid differentiation, cell cycle arrest and delayed leukemia development in vivo. Meanwhile, CNOT3 loss in normal cord-blood CD34+ cells promoted differentiation without inducing cell death, suggesting a potential therapeutic window to target CNOT3 in leukemia. Using a CRISPR saturation mutagenesis screen, we identified NOT box in the C-term domain of CNOT3 to be most critical for cell viability, indicating a functional dependence of CNOT3 on interaction with the deadenylation complex. Transcriptomic and proteomic profiling upon CNOT3 depletion revealed activation of p53 pathways and decrease of MYC- target genes. We further observed a marked decrease in c-MYC protein without alternation in total abundancy and half-life of c-MYC mRNA upon CNOT3 ablation. Mass spectrometry analysis of CNOT3 immunoprecipitation captured proteins involved in translation machinery. Moreover, depletion of CNOT3 significantly reduced the abundancy of c-MYC mRNA in the actively translating polysome fraction. Overall, we uncovered a critical role of the CNOT deadenylation complex in myeloid leukemia and translational control of critical oncogenic targets including c-MYC. How processes that affect post-transcriptional and translational regulation impact leukemia development is not well studied. The CCR4-NOT (CNOT) complex is one of two major multi-subunit polyA deadenylation complexes which mediate the shortening of the poly(A) tails and is recruited to m6A transcripts to mediate mRNA degradation. We surveyed several genome-wide CRISPR screens and observed several subunits of the CNOT complex ranked as genes essential for survival of AML cells. The highest ranked CNOT3 adaptor subunit demonstrated elevated expression in AML patients and associated with a poor prognosis in independent AML patient cohorts. Depletion of CNOT3 in human AML cells resulted in inhibition of cell growth, increased myeloid differentiation, cell cycle arrest and delayed leukemia development in vivo. Meanwhile, CNOT3 loss in normal cord-blood CD34+ cells promoted differentiation without inducing cell death, suggesting a potential therapeutic window to target CNOT3 in leukemia. Using a CRISPR saturation mutagenesis screen, we identified NOT box in the C-term domain of CNOT3 to be most critical for cell viability, indicating a functional dependence of CNOT3 on interaction with the deadenylation complex. Transcriptomic and proteomic profiling upon CNOT3 depletion revealed activation of p53 pathways and decrease of MYC- target genes. We further observed a marked decrease in c-MYC protein without alternation in total abundancy and half-life of c-MYC mRNA upon CNOT3 ablation. Mass spectrometry analysis of CNOT3 immunoprecipitation captured proteins involved in translation machinery. Moreover, depletion of CNOT3 significantly reduced the abundancy of c-MYC mRNA in the actively translating polysome fraction. Overall, we uncovered a critical role of the CNOT deadenylation complex in myeloid leukemia and translational control of critical oncogenic targets including c-MYC.

Deciphering the Role of Pyrvinium Pamoate in the Generation of Integrated Stress Response and Modulation of Mitochondrial Function in Myeloid Leukemia Cells through Transcriptome Analysis.

Pyrvinium pamoate, a widely-used anthelmintic agent, reportedly exhibits significant anti-tumor effects in several cancers. However, the efficacy and mechanisms of pyrvinium against myeloid leukemia remain unclear. The growth inhibitory effects of pyrvinium were tested in human AML cell lines. Transcriptome analysis of Molm13 myeloid leukemia cells suggested that pyrvinium pamoate could trigger an unfolded protein response (UPR)-like pathway, including responses to extracellular stimulus [p-value = 2.78 × 10−6] and to endoplasmic reticulum stress [p-value = 8.67 × 10−7], as well as elicit metabolic reprogramming, including sulfur compound catabolic processes [p-value = 2.58 × 10−8], and responses to a redox state [p-value = 5.80 × 10−5]; on the other hand, it could elicit a pyrvinium blunted protein folding function, including protein folding [p-value = 2.10 × 10−8] and an ATP metabolic process [p-value = 3.95 × 10−4]. Subsequently, pyrvinium was verified to induce an integrated stress response (ISR), demonstrated by activation of the eIF2α-ATF4 pathway and inhibition of mTORC1 signaling, in a dose- and time-dependent manner. Additionally, pyrvinium could co-localize with mitochondria and then decrease the mitochondrial basal oxidative consumption rate, ultimately dysregulating the mitochondrial function. Similar effects were observed in cabozantinib-resistant Molm13-XR cell lines. Furthermore, pyrvinium treatment retarded Molm13 and Molm13-XR xenograft tumor growth. Thus, we concluded that pyrvinium exerts anti-tumor activity, at least, via the modulation of the mitochondrial function and by triggering ISR.

Kdm6a deficiency restricted to mouse hematopoietic cells causes an age- and sex-dependent myelodysplastic syndrome-like phenotype.

Kdm6a/Utx, a gene on the X chromosome, encodes a histone H3K27me3 demethylase that has an orthologue on the Y chromosome (Uty) (Zheng et al. 2018). We previously identified inactivating mutations of Kdm6a in approximately 50% of mouse acute promyelocytic leukemia samples; however, somatic mutations of KDM6A are more rare in human AML samples, ranging in frequency from 2-15% in different series of patients, where their role in pathogenesis is not yet clear. In this study, we show that female Kdm6aflox/flox mice (with allele inactivation initiated by Vav1-Cre in hematopoietic stem and progenitor cells (HSPCs) have a sex-specific phenotype that emerges with aging, with features resembling a myelodysplastic syndrome (MDS). Female Kdm6a-knockout (KO) mice have an age-dependent expansion of their HSPCs with aberrant self-renewal, but they did not differentiate normally into downstream progeny. These mice became mildly anemic and thrombocytopenic, but did not develop overt leukemia, or die from these cytopenias. ChIP-seq and ATAC-seq studies showed only minor changes in H3K27me3, H3K27ac, H3K4me, H3K4me3 and chromatin accessibility between Kdm6a-WT and Kdm6a-KO mice. Utilizing scRNA-seq, Kdm6a loss was linked to the transcriptional repression of genes that mediate hematopoietic cell fate determination. These data demonstrate that Kdm6a plays an important role in normal hematopoiesis, and that its inactivation may contribute to AML pathogenesis.

TIFAB accelerates MLL-AF9−Induced acute myeloid leukemia through upregulation of HOXA9

SummaryWe previously showed stabilization of NIK−induced activation of NF-κB non-canonical signaling suppresses MLL-AF9−induced AML. In the current study, we demonstrate that deletion of NF-κB non-canonical RelB prevents the inhibitory effect of NIK stabilization in MLL-AF9 AML. Mechanistically, RelB suppresses its direct target, TIFAB, which is upregulated in human AML and correlates negatively with the survival of AML patients. Forced expression of TIFAB reverses NIK−induced impaired AML development through downregulation of RelB and upregulation of HOXA9. Consistent with upregulation of HOXA9, gene set enrichment analysis shows that forced expression of TIFAB blocks myeloid cell development, upregulates leukemia stem cell signature and induces similar gene expression patterns to those of HOXA9-MEIS1 and HOXA9-NUP98, and upregulates oxidative phosphorylation. Accordingly, forced expression of HOXA9 also largely releases the inhibitory impact of NIK stabilization via downregulation of RelB and upregulation of RelA. Our data suggest that NIK/RelB suppresses MLL-AF9−induced AML mainly through downregulation of TIFAB/HOXA9.

Vesicular trafficking is a key determinant of the statin response in acute myeloid leukemia

AbstractCholesterol homeostasis has been proposed as one mechanism contributing to chemoresistance in AML and hence, inclusion of statins in therapeutic regimens as part of clinical trials in AML has shown encouraging results. Chemical screening of primary human AML specimens by our group led to the identification of lipophilic statins as potent inhibitors of AMLs from a wide range of cytogenetic groups. Genetic screening to identify modulators of the statin response uncovered the role of protein geranylgeranylation and of RAB proteins, coordinating various aspect of vesicular trafficking, in mediating the effects of statins on AML cell viability. We further show that statins can inhibit vesicle-mediated transport in primary human specimens, and that statins sensitive samples show expression signatures reminiscent of enhanced vesicular trafficking. Overall, this study sheds light into the mechanism of action of statins in AML and identifies a novel vulnerability for cytogenetically diverse AML.

FLT3 Inhibition Downregulates EZH2 in AML and Promotes Myeloid Differentiation

Internal tandem duplication mutations in the Fms-like tyrosine kinase 3 (FLT3-ITD) are frequently recurring in AML and confer a poor prognosis. FLT3 inhibitors (FLT3i) such as gilteritinib are efficacious in relapsed AML. Clinical responses to FLT3i include myeloid differentiation of the FLT3-ITD clone in about 50% of patients. How FLT3i induce this response in a subset of patients is unknown.The FLT3i-induced differentiation response seen in clinical trials has not previously been demonstrated in animal models. We modeled FLT3i-induced differentiation in murine Flt3 ITD/ITDDnmt3a -/- AML model (Meyer et al., Cancer Discovery, 2016). Treatment with FLT3i in vitro accelerated differentiation of cKIT+ leukemic splenocytes as assessed by colony morphology in serial re-plating assays. To characterize the differentiation response in vivo, we transplanted CD45.2+ leukemic splenocytes from moribund mice into sub-lethally irradiated healthy congenic CD45.1+ mice. After confirmation of engraftment at 2 weeks post-irradiation, mice were treated with vehicle or gilteritinib for 4 weeks. Animals treated with gilteritinib demonstrated increased neutrophil and decreased stem/progenitor cell populations, recapitulating the clinically observed increase in granulocytic differentiation of the FLT3-ITD clone.We next sought to understand the molecular mechanism of FLT3i-induced differentiation. We used a proteomic-based screen in a human AML cell line treated with FLT3i to identify novel targets of FLT3-ITD that could be potential mediators of the differentiation response. We identified downregulation of Enhancer of Zeste Homolog 2 (EZH2), the catalytic component of the Polycomb Repressive Complex 2 (PRC2). EZH2 and PRC2 were previously shown to be required for leukemic maintenance in mouse models of MLL-AF9 AML. We treated murine Flt3 ITD/ITDDnmt3a -/- cKIT+ leukemic splenocytes with FLT3i or the EZH1/2 inhibitor (EZH1/2i). Both promoted myeloid differentiation to similar degrees as assessed by colony morphology in this model. We hypothesized that FLT3-ITD regulates EZH2 to maintain leukemia cells in a stem/progenitor cell state. We, therefore, characterized the effect of FLT3i on PRC2 in more detail. We confirmed that FLT3i decreases EZH2 protein levels in FLT3-ITD cell lines and primary human AML within 24 hours of treatment as suggested by our proteomic data (Figure 1A-B). We found that the mechanism of EZH2 downregulation is complex with both transcriptional effects and a decrease in EZH2 protein half-life. ChIP-Seq for H3K27me3 demonstrated decreased peaks at the transcription start sites of PRC2 target genes (Figure 1C). RNA-Seq gene expression profiles of FLT3i- and EZH1/2i-treated human AML cells overlapped at 253 differentially expressed genes (Figure 1D). Critically, both FLT3i and EZH1/2i expression profiles enriched in differentiated myeloid cell gene signatures.Overall, we found that EZH2 is a novel, unexpected, and clinically relevant target of FLT3-ITD. Our data suggest that reduced EZH2 activity following FLT3 inhibition promotes myeloid differentiation of FLT3-ITD leukemic cells, providing a mechanistic explanation for the FLT3i-induced differentiation response seen in patients. These data demonstrate that FLT3-ITD has at least two functions in leukemogenesis, the well described activation of signaling pathways, and second, a previously undefined, regulation of PRC2 to maintain a myeloid stem cell state. Our results may lead to improved approaches to therapy for FLT3 mutated AML. [Display omitted] DisclosuresBernt: Syndax: Research Funding; Merck: Other: Spouse is an employee of Merck.. Carroll: Incyte Pharmaceuticals: Research Funding; Janssen Pharmaceutical: Consultancy.

Thyrointegrin αVβ3 Antagonist: Implications in Acute Myeloid Leukemia

Leukemic cells are able to receive and send several signals within bone marrow niche that play an important role in their survival. One of the important crosstalk is the interaction between the bone marrow microenvironment proteins (vitronectin, fibronectin, fibrinogen, and ostepontin) and thyrointegrin αVβ3 on leukemic cells, generating ligand-specific outside-in signals that are relevant to a variety of cell functions, including gene transcription, cell division, cell attachment, and motilityOur previous experiment using in vivo AML animal models with primary AML cells and cell lines have shown significant reduction of leukemic cell burden 74% and >95% (P<0.0001), respectively, after daily subcutaneous treatment with thyrointegrin αvβ3 antagonist fb-PMT (Ki 0.23 nM) at 3 and 10 mg/kg, for 3-4 weeks. In this study we focused on evaluations of the molecular effects of fb-PMT in leukemic cells. Acute myeloid leukemia cell lines (K562-Luc and KG1a cells) were cultured in 50 cm² cell culture flasks with 10 mL phenol red free RPMI media containing 10% fetal bovine albumin. The leukemic cells were treated (at 50% confluence) with 30 µM fb-PMT for 48 hours. Total RNA was immediately isolated from harvested cells using Triazole and used for microarray analysis. Overall, there were 370 significantly down-regulated gene expression records and 273 significantly up-regulated gene expression records, expression of which were changed at least 1.5-fold in fb-PMT-treated human leukemic cells. Significant examples of the fb-PMT-induced gene expression signatures (GES) of pathway's interference include SNAI, MYC, HIF1A, TWIST1, and TFAP2C (P<0.05). Notably, inference of potential contribution to the fb-PMT anticancer activity of the interference with these pathways seems highly congruent with their known biological functions such as cell cycle control (MYC), survival and maintenance of stem cells (HIF1A, TFAP2C), and essential features of the malignant phenotype (TWIST1, SNAI) (Figure 1).Consistently, examples of the fb-PMT-induced GES of transcriptional pathway's activation include RB1, IRF9, MAML1, RAP1A, and GATA4 pathways (P<0.05), known biological functions of which appear highly consistent with the hypothesis that activation of these pathways might contribute to fb-PMT anticancer activity. Finally, we found that fb-PMT interfered with estrogen signaling in human AML cells. The fb-PMT was associated with decreased phosphorylation and nuclear enrichment of Erα (Figure 1).Collectively, our in vivo study and genomic data have shown the key role thyrointegrin αvβ3 in leukemogenesis. The thyrointegrin αvβ3 antagonist fb-PMT demonstrated potent anticancer actions on human AML through the molecular interference mechanism with multiple signaling pathways supporting growth and survival of leukemic cells [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Targeting PRMT9 Suppresses Acute Myeloid Leukemia Maintenance

AML is a heterogenous disease in which prognosis and treatment is determined by recurrent genetic mutations and chromosomal abnormalities. Some genetic alterations result in aberrant RNA translation, which is exploited by leukemia stem cells (LSCs) to produce short-lived oncoproteins (c-Myc, Mcl-1) to promote cell survival. Since LSCs are considered as the source of treatment failure and relapse, it is critical to understand how LSCs hijack the translational machinery in order to develop effective therapeutics in AML.Arginine methylation catalyzed by protein arginine methyltransferases 1-9 (PRMT1-9) regulates various activities, including RNA translation. We revealed that PRMT1 over activation contributes to leukemia maintenance (Blood, 2019; Blood, 2019). Unlike PRMT1, the recently described PRMT9, is not defined in leukemia. Follow up of AML patients from existing datasets (GSE12417, TARGET) confirms that patients with higher levels of PRMT9 correlates with decreased overall survival. In our AML cohort, we observed significantly increased PRMT9 mRNA levels in AML CD34 + cells relative to normal counterparts (Fig. 1A), spurring our interest in evaluating its function in AML. We thus developed a Prmt9 conditional KO mouse (Mx1-Cre/Prmt9 f/f) and crossed it with an MLL-AF9 (MA9) knock-in mouse to generate Prmt9-KO/MA9 mice for further transplant assay. As a result, Prmt9-KO significantly delayed leukemogenesis in recipient mice carrying MA9 transplants evidenced by prolonged survival (Fig. 1B). Prmt9-KO impaired leukemia-initiating activity evidenced by secondary transplantation that significant less residual CD45.2 + MA9 leukemia cells were found in BM of secondary recipients receiving Prmt9-KO cells relative to those of Prmt9-WT controls. We next validated PRMT9 function in human AML. PRMT9-KD potentially impaired survival of primary AML CD34 + cells, whereas large sparing normal counterparts (Fig. 1C). Moreover, we designed a WT PRMT9 construct and corresponding catalytically dead mutant, both resistant to shPRMT9 (WT-R and MUT-R). Unlike MUT-R, WT-R expression rescued survival effects seen following shPRMT9 treatment, indicating that PRMT9 catalytic activity is required for AML survival (Fig. 1D). To define PRMT9 downstream effectors, we searched for methylated substrates in Molm13 with inducible PRMT9-KD or corresponding controls by performing SILAC analysis coupled with quantitative Mass-Spec (MS) (Fig. 1E). Complete analyses of normalized methyl peptides SILAC ratios revealed more prominent down-regulation of R-methylation in PRMT9-KD cells, with 49 methyl sites downregulated. Notably, methylated PAPB1 C-terminus peptide containing R493 was most significantly depleted upon PRMT9-KD. PABP1 potentiates translation initiation by binding to the poly(A) mRNA tail and interacting with factors like eIF4G. We thus generated an anti-R493 methylation antibody. Indeed, PRMT9 was confirmed to catalyze R493 methylation through in-vitro and cellular methylation assays (Fig. 1F, G). To define the function of R493 methylation, we ectopically expressed either WT PABP1 or the R493K mutant in Molm13 cells and then engineered those cells to express shPABP1 to KD endogenous PABP1. Notably, unlike WT PABP1, expression of R493K suppressed protein synthesis and increased apoptosis after endogenous PABP1-KD, suggesting that PRMT9 mediated PABP1-R493 methylation promotes AML viability (Fig. 1H, I).To discover PRMT9 inhibitors, we conducted a structure-based virtual screen of 960,000 compounds from NCI and Zinc compound libraries (Fig. 1J). We requested the top 300 candidates to assess their anti-leukemia activity in Molm13 cells (Fig. 1K). The top 20 most effective compounds were further analyzed via PRMT9 methylation assay (R493 antibody detection). NSC641396 exhibited the most potent PRMT9 inhibitory effects, as evidenced by decreased PABP1 R493 methylation levels at a dose <1 µΜ (Fig. 1L). NSC641396 treatment on AML CD34 + cells altered polysome profiling, decreased protein biosynthesis and reduced levels of short-lived proteins, suggesting translation inhibition (Fig. 1M).Taken together, our results suggest that PRMT9 plays an oncogenic role in AML or LSC maintenance, by promoting PABP1 methylation mediated translation activity. Further studies are needed to explore if targeting PRMT9 with newly identified lead compound ablates LSCs activity. [Display omitted] DisclosuresMarcucci: Novartis: Other: Speaker and advisory scientific board meetings; Agios: Other: Speaker and advisory scientific board meetings; Abbvie: Other: Speaker and advisory scientific board meetings.

Multi-Modal Targeting of FLT3 with Chimeric Antigen Receptor T Cell Immunotherapy and Tyrosine Kinase Inhibition in High-Risk Pediatric Leukemias

Background:Clinical outcomes for children with FLT3-mutant AML and infants with KMT2A-rearranged (KMT2A-R) B-ALL remain dismal. These leukemias share a common feature of aberrant activation of FLT3 kinase signaling, which occurs by activating FLT3 mutations in AML and by overexpression of wild-type FLT3 in KMT2A-R ALL. Several FLT3 tyrosine kinase inhibitors (FLT3i) are approved for adults with FLT3-mutant AML, but potential efficacy against KMT2A-R ALL remains incompletely characterized and may differ from responses in AML. We previously developed and preclinically validated chimeric antigen receptor (CAR) T cells directed against FLT3 (FLT3CART), which importantly showed potent anti-leukemia activity in preclinical models of both childhood FLT3-mutant AML and infant KMT2A-R ALL (Chien CD et al. ASH 2016). In the current studies, we hypothesized that combinatorial targeting of these two high-risk leukemia subtypes with FLT3CART and the selective next-generation FLT3i gilteritinib would have superior activity and potentially mitigate therapeutic resistance now known to occur with kinase inhibitors or CAR T cell immunotherapy.Methods and Results:We first assessed in vitro sensitivity of human FLT3-mutant AML and KMT2A-R ALL cell lines to gilteritinib, a second-generation selective FLT3i with established clinical activity in FLT3-mutant AML and unknown activity in KMT2A-R ALL. As detrimental effects of kinase inhibitors (e.g., dasatinib, ruxolitinib) upon CAR T cells have been reported, we evaluated for similar effects with gilteritinib co-incubated in vitro with CD3/CD28-bead activated healthy human donor T cells. However, we observed minimal deleterious effects of gilteritinib on normal T cell viability, immunophenotype, and IL-2 and interferon-gamma (IFNg) production. We validated combinatorial effects of gilteritinib and FLT3CART-induced cytotoxicity against FLT3-mutant AML and KMT2A-R ALL cell lines in vitro without impairment of IL-2/IFNg production. We then assessed this dual therapy approach in luciferase+ FLT3-mutant AML (MOLM14) and KMT2A-R ALL (SEM) cell line murine xenograft models. As predicted, both FLT3CART and gilteritinib monotherapies transiently inhibited in vivo leukemia proliferation, although leukemia progression eventually occurred. Conversely, FLT3CART and gilteritinib combination therapy strikingly induced enhanced and sustained leukemia clearance in all assessed AML and ALL cell line xenograft models (Figure 1). Confirmatory studies in our established childhood FLT3-mutant AML and KMT2A-R ALL patient-derived xenograft (PDX) models have also demonstrated potent anti-leukemia efficacy of combined FLT3CART and gilteritinib therapy.Earlier-generation FLT3i have been reported to increase cell surface FLT3 expression on FLT3-mutant AML cells. Given the known importance of target antigen site density for CAR T cell efficacy, we reasoned that a sequential approach to dual therapy with FLT3i ‘priming’ followed by FLT3CART may be superior to a simultaneous treatment strategy. In vitro studies with leukemia cell lines and in vivo studies with PDX models indeed confirmed gilteritinib-induced increases in FLT3 surface antigen density in FLT3-mutant AML cells. Intriguingly, we observed contrasting effects in KMT2A-R ALL cell lines and PDX with decreased surface FLT3 expression upon gilteritinib exposure. Ongoing studies are currently validating gilteritinib priming for FLT3CART given these initial data suggesting potentially divergent sequencing approaches in FLT3-mutant AML versus KMT2A-R ALL.Conclusions:Taken together, our preclinical studies demonstrate that dual targeting with FLT3CART immunotherapy and gilteritinib is a promising therapeutic strategy in FLT3-mutant AML and, importantly, also in KMT2A-R ALL. Notably, we also report minimal negative effects of gilteritinib on FLT3CART, suggesting that FLT3i may be used to enhance CAR T cell immunotherapy without inhibiting T cell function. Phase 1 clinical trials of FLT3CART will open soon for adults and children with FLT3-mutant AML and/or KMT2A-R ALL. [Display omitted] DisclosuresFry: Sana Biotechnology: Current Employment, Current equity holder in publicly-traded company; ElevateBio: Research Funding. Tasian: Kura Oncology: Consultancy; Aleta Biotherapeutics: Consultancy; Gilead Sciences: Research Funding; Incyte Corporation: Research Funding.

STAT5 Signaling Promotes Resistance to Ivosidenib in IDH1-Mutated AML

Mutations in isocitrate dehydrogenase 1 (IDH1) promote leukemic transformation through the production of an oncometabolite, 2-hydroxyglutarate (2-HG). Ivosidenib is an inhibitor of mutant IDH1 approved for the treatment of IDH1-mutated AML. Treatment with ivosidenib can induce terminal differentiation of leukemic blasts via suppression of 2-HG. However, ivosidenib as a single agent has limited efficacy, highlighting the need to better understand the mechanisms of drug resistance.The study of drug resistance mechanisms has been hindered by the lack of IDH1/2-mutated AML cell lines models. To address this issue, we derived an Idh1-mutated AML cell line from bone marrow cells of a murine AML model generated by crossing mice expressing mutant Idh1R132H with mice expressing mutant Npm1 (Npm1c). This cell line (henceforth termed “OCI-mIDH1/N”) undergoes partial myeloid differentiation in response to ivosidenib treatment in vitro.The establishment of OCI-mIDH1/N enabled us to perform a genome-wide CRISPR knockout screen to identify genes that upon inactivation, increased the differentiation response to ivosidenib. Through this screen, we identified C-type lectin member 5a (Clec5a) as one of the top hits. Clec5a encodes a cell surface receptor that signals through the intracellular spleen tyrosine kinase (SYK). To confirm this hit, we generated Clec5a knockout clones of OCI-mIDH1/N cells. Consistent with results of the screen, ivosidenib treatment induced higher levels of Gr-1, a myeloid differentiation marker, on Clec5a-/- cells than on parental Clec5a+/+ cells. Next, we investigated the role in SYK. Clec5a-/- cells had lower levels of pSYK compared with Clec5a+/+ cells, and overexpression of Syk in Clec5a-/- cells reversed their sensitization to ivosidenib. Furthermore, direct inhibition of SYK with fostamatinib was sufficient to sensitize Clec5a+/+ cells to ivosidenib. Our findings show that CLEC5A-SYK signaling promotes resistance to ivosidenib-induced differentiation.Mechanistically, we found that CLEC5A-SYK signaling drives ivosidenib resistance through STAT5 dependent expression of self-renewal genes in the HOX family. Clec5a-/- and Syk-/- OCI-mIDH1/N cells exhibited lower levels of STAT5 activation compared with wildtype cells. Furthermore, SYK inhibitor treatment downregulated pSTAT5 and decreased STAT5 occupancy at HOX gene clusters. Functionally, overexpression of constitutively active STAT5 in Clec5a-/- and Syk-/- cells reversed their heightened sensitivity to ivosidenib and increased the expression of key HOX self-renewal genes. To determine the clinical relevance of these findings, we analyzed two independent RNA-seq datasets from patients treated with IDH inhibitors (Quek et al., 2018; Wang et al., 2021). Gene set enrichment analysis revealed that poor responders expressed significantly higher levels of STAT5 target genes compared with responders in both datasets.Mutations in the receptor tyrosine kinase (RTK) pathway have previously been shown to be associated with resistance to IDH inhibitors. Given that these pathways signal through STAT5, we hypothesized that the mechanism by the RTK mutations confer ivosidenib resistance is through STAT5 activation. To test this hypothesis, we ectopically expressed KRASG12D or PTPN11E76G in OCI-mIDH1/N cells. Expression of the oncogenes increased pSTAT5 and conferred resistance to ivosidenib-induced differentiation. Importantly, knockdown of Stat5 completely restored their sensitivity to ivosidenib, in support for our hypothesis.Next, we tested if the STAT5 inhibitor pimozide synergizes with ivosidenib to treat patient-derived xenograft (PDX) models of human IDH1-mutated AML. In two PDX models where ivosidenib alone induced moderate differentiation, the addition of pimozide to ivosidenib significantly increased the expression of myeloid differentiation markers on AML cells. In two PDX models where ivosidenib alone induced no differentiation, pimozide alone was sufficient to induce profound differentiation of AML cells. Collectively, these results suggest that ivosidenib resistant cells shift their dependence from 2-HG to STAT5 to maintain their undifferentiated state.In summary, our findings demonstrate that STAT5 is a critical mediator of resistance to ivosidenib, and combination therapy with pimozide and ivosidenib is a promising therapeutic approach for IDH1-mutated AML. DisclosuresChan: BMS: Research Funding; AbbVie: Research Funding.

CCS1477, a Novel p300/CBP Bromodomain Inhibitor, Enhances Efficacy of Azacitidine and Venetoclax in Pre-Clinical Models of Acute Myeloid Leukaemia and Lymphoma

E1A binding protein (p300) and CREB binding protein (CBP) are two closely related histone acetyl transferases that co-activate key oncogenes such as MYC and IRF4 which are relevant in a number of haematological malignancies. Here we describe the effects of CCS1477, a new orally available inhibitor of the p300/CBP bromodomains, given alone or in combination with azactidine or venetoclax in pre-clinical models of AML and B-cell lymphoma.As a monotherapy, daily oral dosing with CCS1477 (5, 10 and 20mg/kg) caused a significant dose-dependent reduction in tumour growth in a MOLM-16 xenograft model of AML, with regressions observed at the highest and well tolerated dose of 20mg/kg/qd. The inhibition of tumour growth during drug treatment was accompanied by a significant reduction in tumour expression of MYC by qPCR. In murine retroviral transduction and transplantation models of human AML initiated by either MLL-AF10 or MLL-AF9, daily monotherapy by oral gavage with CCS1477 at 30mg/kg for 42 days, caused a highly significant prolongation of survival. All seven control treated mice died of AML within 160 days whereas only one of seven CCS1477 treated mice had died in each model by 300 days.In the MOLM-16 xenograft model, a sub-maximal dose of CCS1477 (10mg/kg) demonstrated superior tumour growth inhibition by comparison with azacitidine (0.5mg/kg i.p) and with significant combination benefit when these two agents were combined. In this same model venetoclax (50 or 100 mg/kg/qd oral) had no effect on tumour growth, indicating that MOLM-16 tumours are intrinsically resistant. CCS1477 dosed daily at 10mg/kg restored sensitivity to venetoclax as evidenced by significantly greater tumour growth inhibition when compared with CCS1477 given alone. In a venetoclax-sensitive xenograft model of AML (MV-4-11), CCS1477 (10mg/kg) and venetoclax (100mg/kg) resulted in a similar and partial reduction in tumour growth over a 21 day dosing period. When venetoclax and CCS1477 were combined there was a significant combination benefit compared to either drug given alone. The DOHH-2 xenograft model of B-cell lymphoma was also sensitive to venetoclax (50mg/kg) and with a similar impact on tumour growth compared with CCS1477 (10mg/kg). In this model the combination of CCS1477 and venetoclax was more profound and caused complete tumour stasis during the dosing period.These data support the clinical testing of CCS1477 in combination with azacitidine and/or venetoclax in AML and B-cell lymphomas. CCS1477 is currently in Phase I/II clinical trials in AML, Non-Hodgkin lymphoma (including B-cell lymphoma) and multiple myeloma. (NCT04068597). DisclosuresBrooks: CellCentric Ltd: Current Employment, Current holder of individual stocks in a privately-held company. Knurowski: CellCentric Ltd: Current Employment, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Hughes: CellCentric Ltd: Consultancy, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Clegg: CellCentric Ltd: Current Employment, Current holder of individual stocks in a privately-held company. West: CellCentric Ltd: Current Employment, Current holder of individual stocks in a privately-held company. Pegg: CellCentric Ltd: Current Employment, Current holder of individual stocks in a privately-held company. Somervaille: Novartis: Consultancy, Honoraria.

Treatment with DC/AML Fusion Vaccine and CD3xCD123 Bi-Specific T-Cell Engager (CD123-CODV-TCE) for Treatment of Acute Myeloid Leukemia

Treatment with DC/AML Fusion Vaccine and CD3xCD123 Bi-Specific T-Cell Engager (CD123-CODV-TCE) for Treatment of Acute Myeloid Leukemia

RNA Polymerase II Is a Therapeutic Target to Overcome FLT3 Inhibitor Resistance Mediated By the Bone Marrow Microenvironment

Background: Gilteritinib is a clinically active FLT3 tyrosine kinase inhibitor (TKI) approved for relapsed/refractory FLT3-mutant AML, but nearly all patients treated with gilteritinib and other FLT3 TKIs eventually develop clinical resistance. Activating RAS/MAPK pathway mutations are a predominant non-FLT3 dependent resistance mechanism in patients treated with gilteritinib. AML blasts can also develop FLT3 TKI resistance secondary to paracrine MAPK activation stimulated by FLT3 Ligand, FGF2, or other protective cytokines within the bone marrow microenvironment (BME). To identify potential targets that sensitize AML cells to gilteritinib-induced apoptosis in a model of the BME, we performed a genome-wide CRISPR/Cas9 death screen in MOLM-14 FLT3-ITD+ human AML cells cultured in bone marrow stromal cell conditioned media. We hypothesize that identified genes represent promising combinatorial therapeutic targets that can enhance clinical efficacy of FLT3 TKIs in AML.Methods: To model stroma-mediated TKI resistance, we used the HS5 human bone marrow stromal cell line that secretes multiple cytokines (G-CSF, GM-CSF, FGF2) and supports myeloid progenitor proliferation in co-culture. MOLM-14 CRISPRi cells transduced with CRISPRi-v2 genome-wide sgRNA library were cultured in HS5 conditioned media for 24 hours and then treated with gilteritinib 250 nM. Cells were stained with a fluorogenic caspase 3/7 reagent and then fixed after 24 hours of drug treatment. Caspase-3 positive cells were sorted from the entire drug-treated cell population by FACS and guide RNAs enriched or depleted in this sample as compared to an untreated T0 sample were determined by NGS.Results: We identified several gene-level hits that were enriched in the apoptotic population (FDR <0.2). Among these, we identified multiple transcription factors or regulators of transcriptional activation. The latter included multiple components of RNA pol II machinery (POLR2G, RTF1) and multiple subunits of Mediator (MED12, MED30, MED21, MED11), a complex that regulates RNA pol II activity and has been shown to modulate super-enhancer-associated genes in AML cells. To validate select hits, we transduced MOLM-14 and MV411 CRISPRi cells with a tetracycline-inducible sgRNA expression vector. Using this system, we found that conditional knockdown of MED12, the top scoring Mediator subunit in our screen, significantly sensitized MOLM-14 (FgH1) cells to gilteritinib while modestly augmenting the cytotoxicity of gilteritinib in MV411 (FgH1) cells when compared to a non-targeting sgRNA. Functionally, MED12 associates with MED13, Cyclin C, and CDK8 to form the CDK8 kinase module of Mediator. MED12 knockdown, as expected, led to suppression of STAT1 S7272 and STAT5 S726 phosphorylation, known targets of CDK8. Based on these results, we hypothesized that CDK8 inhibition would augment apoptosis induced by gilteritinib in HS5 conditioned media. Using SEL120, a novel CDK8 inhibitor already in early phase AML clinical trials, we performed an 8 x 8 dose matrix drug synergy analysis of gilteritinib and SEL120 in multiple FLT3-mutant AML cell lines using Bliss independence modeling. We found that SEL120 was synergistic with gilteritinib in inducing apoptosis in MOLM-14 and MV411 cells in HS5 conditioned media (Bliss synergy scores of 2.44 and 11.45 with most synergistic area scores of 10.91 and 26.85, respectively). We also found combinatorial activity against MOLM-14 cells harboring secondary NRAS activating mutations (G12C and Q61K), suggesting the therapeutic combination could potentially overcome cell intrinsic and extrinsic MAPK-activating resistance mechanisms. Lastly, we found that gilteritinib and SEL120 combined to impart greater cytotoxicity than either drug alone in a primary sample (AML #1) from a patient with newly diagnosed AML possessing a FLT3-ITD mutation at high mutant allele ratio.Conclusions: The results and validation of our CRISPRi screen suggest that combined CDK8 and FLT3 inhibition is a novel strategy for augmenting gilteritinib cytotoxicity. Assessment of the activity of the combination in additional primary AML samples and in vivo murine models of AML is planned. Additional candidate targets already described and other Mediator and RNA pol II subunits from our screen are also being further evaluated to precisely define the transcriptional programs that influence FLT3 inhibitor resistance. DisclosuresLogan: Pharmacyclics, Astellas, Jazz, Kite, Kadmon, Autolus, Amphivena: Research Funding; Amgen, Pfizer, AbbVie: Consultancy. Gilbert: Denali Therapeutics: Ended employment in the past 24 months, Other: Spouse/Significant Other's Employment; GSK: Consultancy, Research Funding; AstraZeneca: Research Funding; Chroma Medicine: Consultancy, Other: Co-founder. Smith: Revolutions Medicine: Research Funding; AbbVie: Research Funding; Daiichi Sankyo: Consultancy; Amgen: Honoraria; FUJIFILM: Research Funding; Astellas Pharma: Consultancy, Research Funding.

Differential Activation of V γ4V δ1 and V γ9V δ2 Cord Blood Derived Gamma Delta T Cells upon Exposure to EBV-Lcl and K562 Cells

Gamma-delta (γδ) T cells have emerged as a promising candidate for adoptive cellular immunotherapy. To harness and maximize the anti-leukemia properties of these cells, we sort to comprehensively profile the transcriptomic signatures and immune repertoire of in vitro expanded γδ T cell products. Given the reported diverse TCR γδ repertoire and naïve nature of γδ T cells found in human cord blood (CB γδ), we serially track the molecular and cellular changes in these cells upon activation in expansion cultures. Based on the established viral reactivities of γδ T cell as well as prior studies showing their cross reactivities against leukemia and cancer cells, we had previously shown that stimulating CB γδ with an irradiated EBV-LCL feeder cell-based rapid expansion protocol (REP) is capable of generating cell products with potent and specific cytotoxicity against human AML cells. In the present study, using single cell RNA sequencing (scRNA-seq) coupled with single cell TCR γδ repertoire analysis, we compared the transcription signatures between our REP expanded γδ T cell (REP γδ) and non-manipulated γδ T cells reported in literatures, showing the progressive acquisition of an adult PB derived γδ T cell (PB γδ)-like cell states. Time course analysis demonstrated complex T cell activation and maturation trajectories correlating with variable level of clonal induction throughout the course of in vitro expansion. At the end of expansion, the harvested REP γδ are predominantly of the V γ4V δ1 subtype. Nevertheless, upon exposing REP γδ to target leukemia cell line K562, outgrowth of other non-V γ4V δ1 as well as the semi-invariant V γ9V δ2 cells were observed. Taken together, our data shows that as CB γδ expand and differentiate in culture, they adopt an adult PB γδ-like program. More importantly, our data highlights the rich clonal composition of in vitro expanded CB γδ, with different clonotypes being variably activated upon exposure to different stimuli. Such characteristics can potentially overcome the challenges of cancer heterogeneity and cell persistence, with the potential of improving outcomes in cell immunotherapy. DisclosuresNo relevant conflicts of interest to declare.

Impaired RAS Proteolysis Drives Clonal Hematopoietic Transformation

Recently, the protein LZTR1 (leucine zipper-like transcriptional regulator 1) was discovered as an adaptor for a cullin 3 complex responsible for ubiquitin-mediated degradation of RAS proteins. While these data provided a novel mechanism for RAS protein regulation, there is considerable controversy as to which RAS paralogs are physiologic substrates of LZTR1. In parallel, dysregulated LZTR1 expression via aberrant splicing and mutations in both LZTR1 as well as the RAS GTPase and LZTR1 substrate RIT1 were identified in patients with clonal hematopoietic disorders. However, the effects of these alterations on normal and maliganant hematopoiesis have not been evaluated. Here we utilized a series of genetically engineered murine models for germline and conditional deletion of LZTR1, RIT1, and expression of oncogenic RIT1 mutant which revealed a key role for LZTR1 in the regulation of hematopoietic stem cell (HSC) self-renewal and delineated a series of LZTR1-regulated substrates in hematopoietic cells.Consistent with a role for LZTR1 alterations in the Noonan Syndrome, germline homozygous deletion of Lztr1 was associated with lethality between embryonic day 17.5 and birth. Lztr1-/- fetuses had massive dyserythropoiesis and apoptosis of fetal liver hematopoietic cells. Competitive transplantation of E14.5 Lztr1 null fetal liver or bone marrow from 6-week-old Mx1-cre Lztr1 conditional knockout (cKO) mice resulted in striking increased self-renewal in primary and secondary competitive transplantation assays in vivo (Fig.A-B). Interestingly, recipient animals reconstituted with Lztr1-/- cells developed fatal myeloid and lymphoid leukemias characterized by anemia, thrombocytopenia, and increased myeloid and B-lymphoid cells (Fig.C-D).In order to identify the LZTR1 substrates responsible for effects on HSCs, we evaluated levels of all RAS GTPases in Lztr1 null HSCs. This revealed elevated KRas, NRas, MRas, and Rit1 protein in LZTR1 KO cells (Fig.E), with RIT1 being most prominently elevated. Evaluation of a cohort of 4,113 patients with hematologic malignancies identified 41 patients with somatic RIT1 mutations, the majority of which cluster in the switch II region and escape LZTR1-mediated ubiquitination, resulting in RIT1 protein accumulation (Fig.F-H). Given that the impact of RIT1 mutations on hematopoiesis is unknown, we next compared Lztr1 cKO with conditional expression of one of the most common leukemia-associated RIT1 mutants that escapes LZTR1-mediated ubiquitin (Rit1 M90I). Both Lztr1 cKO and Rit1 M90I conditional expression conferred GM-CSF hypersensitivity to HSCs in vitro, cytokine independent growth to human AML cell lines in vitro, and strong competitive self-renewal in vivo (Fig. I-J). Consistent with RIT1 mutations being found primarily in myeloid neoplasm patients, aged Mx1-cre Rit1M90I/WT mice developed fatal MPN, MDS, and mixed MDS/MPN disorders (Fig.K), which were serially transplantable into sublethally irradiated recipients.Despite convergent effects of LZTR1 and RIT1 on clonal HSC advantage, LZTR1 null cell lines did not solely require RIT1 for HSC advantage as revealed by Lztr1/Rit1 double KO mice. We therefore next carried out a series of experiments in RAS-less cells and whole genome CRISPR screens to delineate factors required for LZTR1 mediated hematopoietic transformation. This revealed that KRAS as well as MRAS and its RAF phosphatase partner SHOC2 were selective dependencies for LZTR1-mediated transformation. These data indicate that multiple RAS GTPases as well as RAF activation are required for LZTR1-mediated transformation (Fig.L).While considerable prior research has evaluated oncogenic alleles of RAS which alter RAS-GTP hydrolysis on hematopoiesis, the role of modulating RAS protein abundance on hematopoiesis is unknown. Here we identify RAS proteolysis as a novel regulator of HSC function, define the spectrum of RIT1 mutations in leukemia, and identify LZTR1 and RIT1 mutations as drivers of leukemogenesis. The discovery of RAS proteolysis as a novel driver of leukemogenesis has important therapeutic implications given efforts to therapeutically degrade RAS family members. Finally, the clinical importance of K/NRAS mutations on resistance to therapies in AML motivates future studies on the potential clinical impact of LZTR1 and RIT1 alterations in myeloid neoplasm patients. [Display omitted] DisclosuresAbdel-Wahab: H3B Biomedicine: Consultancy, Research Funding; Merck: Consultancy; Foundation Medicine Inc: Consultancy; Prelude Therapeutics: Consultancy; LOXO Oncology: Consultancy, Research Funding; Lilly: Consultancy; AIChemy: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees; Envisagenics Inc.: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees.