Abstract: One of the hallmarks of chronic myeloid leukemia (CML) is genomic instability, that fosters the acquisition of tyrosine kinase inhibitor (TKI)-resistant BCR-ABL1 mutations and/or of additional chromosomal aberrations leading to progression to blast crisis (BC). Inactivating mutations in the SETD2 tumor suppressor occur in solid tumors and acute leukemias. SETD2 trimethylates histone H3 Lysine 36 (H3K36Me3) playing a key role in maintaining DNA integrity. We have recently demonstrated that, in CML, SETD2 loss of function may occur at the post-translational level. Reduced or null SETD2 and H3K36Me3 was detected in 83/96 (86%) patients (pts) with BC CML as compared to a pool of healthy donors and to chronic phase (CP) pts at diagnosis. Proteasome inhibition in primary cells from pts with undetectable SETD2 restored H3K36Me3 and led to accumulation of hyper-ubiquitinated SETD2. In K562 cells (SETD2/H3K36Me3low), we observed that after proteasome inhibition hyper-ubiquitinated SETD2 co-immunoprecipitates with MDM2. MDM2 inhibition rescued SETD2 expression and activity, suggesting that MDM2 is implicated in SETD2 reduced stability. Co-IP also showed that SETD2 interacts with Aurora Kinase A (AKA) a S/T kinase frequently overexpressed in CML. We found that AKA phosphorylates SETD2, and its inhibition rescued SETD2 expression and activity. To investigate whether SETD2/H3K36Me3 loss may be a druggable lesion, we performed clonogenic assays in LAMA84 (SETD2/H3K36Me3high) cells before and after SETD2 silencing, in imatinib-sensitive K562 (SETD2/H3K36Me3low) cells and in IM-resistant K562 cells, that are characterized by complete SETD2 loss. The extent of reduction of clonogenic growth after proteasomal, AKA or MDM2 inhibition was found to be inversely correlated to SETD2 residual expression. These observations were confirmed in cells from both CP (n=2) and BC (n=4) CML pts showing different levels of SETD2 expression and activity. Further experiments were performed in the aforementioned cell lines to verify if reduced clonogenic potential was due to cytostatic or cytotoxic effects. Apoptotic cell death was quantified by annexin V/propidium iodide staining and flow cytometry. Proteasomal inhibition by bortezomib, carfilzomib and ixazomib and AKA de-phosphorylation by Danusertib caused a time-dependent increase of annexin-V-positive cells by activating the mitochondrial apoptotic pathway as reflected by an increase in Bax expression and induction of the cleavage of caspase-3,-9 and PARP. Moreover, all drug treatments as single agent, at nanomolar doses (Bortezomib: 10 nM, Carfilzomib: 5 nM, Ixazomib: 10 nM and Danusertib: 500 nM) induced a significant increase of the DNA double-strand break marker γH2AX, suggesting that in a SETD2 knock-down context, proteasomal and AKA inhibition propagates genomic instability by forcing the cells through successive replication cycles, ultimately resulting in apoptosis from mitotic catastrophe. Reduced SETD2/H3K36Me3 levels, in association with MDM2 and AKA hyper-activation, were also detected when the CD34+ cell fraction of 10 CML-CP pts, was compared to the total mononuclear cell fraction or to the CD34+ compartment obtained from a pool of healthy donors. We thus hypothesized that leukemia progenitor cells, showing higher MDM2 and AKA activity and consequent SETD2 loss, accumulate genetic aberrations despite inhibition of BCR-ABL1 kinase. Studies are ongoing to verify if MDM2 or AKA inhibition may restore SETD2 expression and function and induce cell death. Finally, it has already been shown that alterations of epigenetic regulators such as the KDM4 family members control tumor cell proliferation in a variety of cancers including acute myeloid leukemia. Recent findings have identified KDM4 demethylases as putative therapeutic targets in a SETD2 mutated context and illustrated the efficacy of KDM4 inhibitors in AML therapy. Starting from these evidences, we will test the same approach in BC CML models. In conclusion, phosphorylation by AKA and ubiquitination by MDM2 contribute to SETD2 non-genomic loss of function in BC CML and in CD34+ leukemic progenitors. Restoring physiological H3K36Me3 may help to improve the outcome of this critical subset of pts. Acknowledgments: Study supported by AIRC (project code 16996), AIL (Associazione Italiana contro le Leucemia, Linfomi e Mieloma), Italian Ministry of Health, project GR-2016-02364880. Disclosures Gugliotta: Pfizer: Honoraria; Novartis: Honoraria; Incyte: Honoraria. Castagnetti:Novartis: Honoraria; Incyte: Honoraria; Pfizer: Honoraria; Bristol Myers Squiib: Consultancy, Honoraria. Rosti:BMS: Speakers Bureau; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Pfizer: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Incyte: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Iurlo:Incyte: Other: Speaker Honoraria; Novartis: Other: Speaker Honoraria; Pfizer: Other: Speaker Honoraria. Abruzzese:Incyte: Consultancy; Novartis: Consultancy; Pfizer: Consultancy; BMS: Consultancy. Pregno:Incyte: Consultancy, Honoraria; Pfizer: Honoraria; Novartis: Honoraria; Bristol Myers Squibb: Honoraria. Crugnola:Novartis: Honoraria; Incyte: Honoraria. Albano:Novartis: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees. Bonifacio:Novartis: Honoraria; Amgen: Honoraria; Pfizer: Honoraria; Incyte: Honoraria; BMS: Honoraria. Tiribelli:Pfizer: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees. Baccarani:Novartis: Consultancy, Speakers Bureau; Incyte: Consultancy, Speakers Bureau; Takeda: Consultancy. Martinelli:Roche: Consultancy; Pfizer: Consultancy; BMS: Consultancy; Novartis: Consultancy; ARIAD: Consultancy. Cavo:amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel accommodations, Speakers Bureau; novartis: Honoraria; takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; bms: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel accommodations, Speakers Bureau. Soverini:Incyte: Consultancy.

Abstract: The SETD2 protein is a histone methyltransferase that specifically catalyzes the trimethylation of Lysine 36 on histone H3 (H3K36me3). SETD2/H3K36me3 are implicated in transcript elongation and splicing, DNA repair, chromosome segregation. SETD2 gene deletions and/or mutations (mostly frameshift or nonsense) have been reported in solid tumors (clear cell renal cell carcinoma, bladder cancer, lung cancer, melanoma, endometrial cancer) and in acute leukemias. Using a Western Blotting (WB) approach to screen for SETD2 protein expression and for H3K36me3 levels in a relatively large cohort of 80 advanced-phase chronic myeloid leukemia (CML) patients (pts), we could detect reduced or null SETD2 and H3K36me3 in 86% of pts as compared to a pool of healthy donors and to chronic phase (CP) pts at diagnosis who achieved optimal responses to TKI, but neither mutations/deletions nor transcriptional down-regulation were the underlying causes. Inhibition of proteasome-mediated degradation in primary cells from pts with undetectable SETD2 restored H3K36me3 and led to accumulation of hyper-ubiquitinated SETD2, suggesting that a functional protein is produced but rapidly degraded. Moreover, proteasome inhibition was found to induce apoptosis and to reduce clonogenic growth. In K562 cells (SETD2/H3K36me3low), co-immunoprecipitation (co-IP) performed before and after proteasome inhibition showed accumulation of the hyper-ubiquitinated form of SETD2 bound to MDM2. MDM2 inhibition by SP-141 resulted in cytostatic effects and restored SETD2 expression and activity. Superimposable results were achieved by siRNA-mediated silencing of MDM2, suggesting that MDM2 is implicated in SETD2 reduced stability. Co-IP also showed that SETD2 interacts with Aurora Kinase A a Ser-Thr kinase frequently overexpressed in CML. We found that Aurora Kinase A phosphorylates SETD2, and both pharmacological inhibition by Danusertib and siRNA-mediated silencing rescued SETD2 expression and activity. Next, to investigate whether SETD2/H3K36me3 loss may contribute to genetic instability, LAMA 84 (SETD2/H3K36Me3high) and K562 (SETD2/H3K36me3low) cells were studied by WB and immunofluorescence (IF) to assess phosphorylated histone 2A.X (γH2AX) and Rad51 foci in steady state conditions and after sub-lethal DNA damage by UV exposure. The same studies were performed after SETD2 silencing for 3 months. Cells with low or silenced SETD2 had significantly higher levels of γH2AX and were unable to induce homologous recombination (HR) repair after DNA damage. Clonogenic assays performed in LAMA 84 cells before and after SETD2 silencing, in K562 (SETD2/H3K36me3low) and in imatinib-resistant (IM-R) K562 cells which have lost SETD2 expression and activity, suggested that reduction of clonogenic growth after proteasomal or MDM2 inhibition is strictly dependent on SETD2 expression and functional status (Figure 1A). First and second generation proteasome inhibitors (bortezomib, carfilzomib and ixazomib) inhibited the clonogenic potential of the mononuclear cell fraction from both CP (n=2) and blast crisis (BC) (n=4) CML pts at subnanomolar concentrations, with the extent of anti-tumor activity clearly anti-correlated with SETD2 expression and H3K36me3 levels: pts with lower SETD2 expression showed lower LD50 when compared with pts with higher SETD2 expression and H3K36me3 levels (Figure 1B). Similarly, clonogenic assays performed by administrating increasing doses of SP-141 (from 0.25 to 1.25 µM) suggested that MDM2 specific inhibition had more significant effects in BC-CML pts showing low SETD2 levels and activity as compared to BC-CML pts showing intermediate SETD2 levels and activity and to CP CML pts. In conclusion, phosphorylation by Aurora Kinase A and ubiquitination by MDM2 contribute to SETD2 non-genomic loss of function in advanced-phase CML. Loss of SETD2/H3K36me3 is associated with increased DNA damage and impaired HR repair. Restoring physiological H3K36me3 levels may help improve the outcome of this critical subset of pts. Acknowledgments: study supported by AIRC (project code 16996) and AIL (Associazione Italiana contro le Leucemia, Linfomi e Mieloma). Figure 1. Figure 1. Disclosures Castagnetti: Incyte: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; Bristol Meyers Squibb: Consultancy, Honoraria; Novartis: Consultancy, Honoraria. Gugliotta:Novartis: Honoraria; Pfizer: Honoraria; Bristol-Myers Squibb: Honoraria; Incyte: Honoraria. Abruzzese:Pfizer: Consultancy; Novartis: Consultancy; BMS: Consultancy; Ariad: Consultancy. Bonifacio:Incyte: Consultancy; Pfizer: Consultancy; Amgen: Consultancy; Novartis: Research Funding; Bristol Myers Squibb: Consultancy. Martinelli:Ariad/incyte: Consultancy; Pfizer: Consultancy; Celgene: Consultancy; Amgen: Consultancy; Janssen: Consultancy; Roche: Consultancy. Cavo:Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Soverini:Bristol Myers Squibb: Consultancy; Incyte Biosciences: Consultancy; Novartis: Consultancy.

Abstract: The SETD2 gene encodes the only methyltransferase responsible for histone H3 lysine 36 trimethylation (H3K36Me3) in humans. H3K36me3 play a key role in preserving the fidelity of transcription elongation and splicing. In addition, SETD2/H3K36me3 have more recently been implicated in the maintenance of genomic integrity by regulating homologous recombination (HR) repair, Mismatch Repair (MMR) mitotic spindle assembly and chromosome segregation. SETD2 deletions and/or inactivating mutations occur in many solid tumors and have recently been found also in acute leukemias. We have reported that the HMC-1.1 and -1.2 mast cell leukemia (MCL) cell lines and many advanced systemic mastocytosis (SM) patients (pts) display H3K36Me3 deficiency as a result of non-genomic loss of function of SETD2. Proteasome inhibition restored SETD2 protein expression and H3K36me3, suggesting that a functional protein is produced but rapidly degraded. In an attempt to uncover the mechanisms underlying this phenomenon, we used an in silico approach to identify candidate SETD2-interacting proteins, followed by experimental confirmation by co-immunoprecipitation (co-IP). We found that, after proteasomal inhibition, SETD2 co-immunoprecipitates with the ubiquitin E3 ligase MDM2. Treatment with the MDM2 inhibitor SP-141 rescued SETD2 expression and H3K36Me3, suggesting that MDM2 may play a role in SETD2 degradation in ASM and MCL. Moreover, SP-141 treatment of HMC-1 cells at micromolar doses induced cytostatic but not cytotoxic effects as shown by cell growth curves. Clonogenic assays supported the cytostatic effects of SP141 in HMC-1.1 and -1.2 cells. siRNA-mediated knock-down of MDM2 also rescued SETD2 expression and activity, further supporting the hypothesis that SETD2 hyper-ubiquitination by MDM2 plays a role in SETD2 reduced stability and proteasomal degradation. Co-IP also showed that SETD2 interacts with Aurora Kinase A, as it was suggested in silico. We found that Aurora A is overexpressed in advanced SM and may target SETD2 for phosphorylation. Both pharmacological inhibition by Danusertib and siRNA-mediated silencing of Aurora A rescued SETD2 expression and activity, raising the hypothesis that phosphorylation by Aurora A might be the trigger for MDM-2 mediated degradation of SETD2. To evaluate whether increased DNA damage and reduced HR proficiency can be observed in SETD2/H3K36Me3-deficient SM, we used western blotting (WB) and immunofluorescence (IF) to assess phosphorylated histone 2A.X (γH2AX) and Rad51 foci. Compared to cells from healthy controls, SETD2- and H3K36Me3-deficient cell lines and pts had significantly higher levels of γH2AX and lower levels of Rad51. RNA-seq in SETD2-deficient pts showed evidence of transcription and splicing defects like transcription-induced chimeras, intron retention and non-canonical splicing patterns not observed in healthy donors. Next, the ROSAD816V cell line, which displays SETD2 and H3K36me3 levels superimposable to healthy donors, was studied by WB and IF to assess γH2AX and Rad51 in steady state and after sub-lethal DNA damage by UV exposure. The same experiments were carried out after SETD2 silencing for 2 months. Cells with silenced SETD2 had significantly higher levels of γH2AX and were unable to activate the HR repair. Interestingly, clonogenic assays in ROSAD816V cells before and after SETD2 silencing showed that reduction of clonogenic potential after proteasomal or MDM2 inhibition is indeed SETD2-dependent (Figure 1A). Finally, we performed clonogenic assays to evaluate the therapeutic potential of bortezomib, carfilzomib and ixazomib in neoplastic mast cells from 3 patients with advanced SM and we observed in all cases that both first and second generation inhibitors induced a significant reduction of clonogenic activity at nanomolar doses (Figure 1B). Taken together, our results suggest that AKA and MDM2-mediated post-translational modifications contribute to SETD2 non-genomic loss of function in advanced SM. Loss of SETD2 and H3K36me3 is associated with increased DNA damage and transcription and splicing defects in patients. Inhibiting AKA or MDM2 activity or proteasome-mediated degradation are promising therapeutic strategies in patients with low SETD2 expression levels. Acknowledgments: study supported by AIRC (project code 16996) and AIL (Associazione Italiana contro le Leucemia, Linfomi e Mieloma). Figure 1. Figure 1. Disclosures Bonifacio: Incyte: Consultancy; Pfizer: Consultancy; Amgen: Consultancy; Novartis: Research Funding; Bristol Myers Squibb: Consultancy. Pagano:Janssen: Speakers Bureau; Merck: Speakers Bureau; Gilead: Speakers Bureau; Basilea: Speakers Bureau; Pfizer: Speakers Bureau. Valent:Novartis: Honoraria; Pfizer: Honoraria; Incyte: Honoraria. Cavo:Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Soverini:Novartis: Consultancy; Incyte Biosciences: Consultancy; Bristol Myers Squibb: Consultancy.

Abstract: FMS-like tyrosine kinase 3 (FLT3) is among the most common driver genes recurrently mutated in acute myeloid leukemia (AML), accounting for approximately 30% of cases. Activating mutations of the FLT3 receptor include internal tandem duplications (ITD) that map to the auto-inhibitory juxtamembrane (JM) domain or point mutations within the tyrosine kinase domain (TKD). Several FLT3 tyrosine kinase inhibitors have been developed in the last few years, but midostaurin is currently the only one approved for the treatment of newly diagnosed patients harboring FLT3 mutations. Here we describe for the first time a novel in-frame deletion in exon 14 (JM domain) of the FLT3 gene, that we identified in a young woman with CBFb-MYH11-positive AML. We demonstrated that this novel FLT3 variant is pathogenic, since it is responsible for constitutive activation of FLT3 receptor. Finally, ex-vivo studies demonstrated that this novel mutation is sensitive to midostaurin.