Waldenstrom Macroglobulinemia Cell Lines Research Articles

Aberrant Expression of Spliced WNK2 Is an Early Event in MYD88 Mutated WM That Activates ERK1/2 and Supports Tumor Growth

Background: Waldenström's Macroglobulinemia (WM) patients carry MYD88 (MYD88 MUT) and CXCR4 (CXCR4 MUT) mutations in 95-97% and 30-40% of cases, respectively. The mechanisms of MYD88-driven lymphomagenesis remain to be clarified. Our previous transcriptome analysis highlighted WNK2 as a highly dysregulated gene in MYD88 MUT WM (Guerrera ML et al, Haematologica 2018). WNK2 is a serine/threonine protein kinase that negatively regulates ERK1/2 activation in a kinase-dependent manner; is a known tumor suppressor in certain solid cancers, where it is primarily silenced by promoter methylation. ERK1/2 is a pro-survival signal in WM; its activation is accompanied by the release of inflammatory cytokines and can mediate ibrutinib resistance. We previously demonstrated WNK2 transcriptional and protein upregulation in CXCR4 WT and silencing in CXCR4 MUT WM and identified several mechanisms of WNK2 transcriptional dysregulation including i) the preferential expression of short isoforms lacking the kinase domain; ii) the recurrence of an identical, aberrant, exon skipping event in all gene isoforms; and iii) aberrant methylation and chromatin accessibility in CXCR4 WT vs CXCR4 MUT WM. We therefore hypothesized that spliced WNK2 may contribute to MYD88 MUT WM oncogenesis. Methods: We investigated WNK2 regulation in 253 untreated WM patients by combined transcriptome and PacBio IsoSeq analysis in a subgroup of 12 WM and 3 healthy donors (HD). Lentiviral transduction was used to overexpress and functionally characterize WNK2 isoforms in MYD88 MUT BCWM1 and MWCL1 WM and control lymphoma cell lines. Results: We identified that tumors from patients with MYD88 MUTCXCR4 WT WM preferentially expressed the short Ensembl WNK2-207 transcript, whose annotation is incomplete. Via PacBio analysis, we identified 8 previously undocumented short isoforms of WNK2 mapping to the original WNK2-207 annotation, all of which lack the kinase domain and bear the same mRNA skipping of two exons near the 3' end. RNA-Seq analysis of 253 WM patents confirmed the IsoSeq predictions and showed that the novel PacBio isoform WNK2-PB4 was among the most expressed transcripts in WM. We next sought to functionally characterize WNK2-PB4 overexpression in MYD88 mutated lymphoma cells. We successfully cloned WNK2-PB4 from patient CD19+ BM cells and obtained an artificial construct of WNK2-PB4 that included the 2 skipped exons, WNK2-SKEX, through the Genewiz gene synthesis service. Stable cell lines overexpressing either WNK2-PB4 or WNK2-SKEX or the empty vector were successfully generated by lentiviral transduction. Overexpression of WNK2-PB4 showed increased phosphorylation of ERK1/2 and SRC family members in the BCWM1 and MWCL1 cells; in contrast, no activation of those targets was seen in MYD88 MUT DLBCL TMD8 and MYD88 WT Burkitt Ramos cell lines ( Figure 1). Increased activation of ERK1/2 was also observed in WM cell lines overexpressing WNK2-SKEX. Study of the downstream signaling of ERK1/2 by WB revealed increased phosphorylation of p90RSK in BCWM1 WNK2-PB4 cells. BCWM1 and MWCL1 overexpressing WNK2-PB4 showed greater proliferation compared to the empty vector control by CellTrace Violet, while no differences in apoptosis were noticed. By co-immunoprecipitation, we observed WNK2 complexed with HCK. Upon treatment with the highly potent dual HCK/BTK inhibitor KIN-8194, we observed dose-dependent decreases in phosphorylation levels of HCK, BTK, and ERK and total levels of WNK2 in WNK2-PB4 overexpressing BCWM.1 and MWCL-1 cells versus vector control transduced cells. Transcriptome analysis of WM samples based on a novel subtype and early/late evolutionary staging classification for WM (Hunter et al, ASH 2022) revealed that WNK2 was aberrantly upregulated from an early stage of WM and was either silenced or further upregulated with disease progression in the B-cell like or Plasma cell like WM subgroups, respectively. Conclusions: Taken together, our findings show a high selection pressure to aberrantly regulate WNK2 expression at an early stage of MYD88 MUT WM with different evolution in CXCR4 WT vs CXCR4 MUT WM, thus suggesting a role for aberrantly spliced WNK2 in the oncogenesis of MYD88 MUT WM. Overexpression of the aberrantly spliced WNK2-PB4 isoform in stable WM cell lines provided a proliferation advantage and led to ERK1/2 activation, which may be induced by the WNK2/HCK interaction.

Changes in Methylation and Chromatin Accessibility Underlie Subtype Classification and Disease Evolution in Waldenström's Macroglobulinemia

Background: We previously identified three subtypes of Waldenstrom's Macroglobulinemia (WM): B-cell like (BCL), Plasma cell like (PCL), and an intermediate group enriched for early/smoldering WM from which the other two subtypes evolved (Hunter et al, ASH 2022). Diffusion pseudo-time (DPT) analysis suggested all WM samples could be placed on a shared evolutionary path independent of subtype, and when samples were separated into Early and Late DPT values, it mirrored disease progression. The role of epigenetic dysregulation that underlies subtype classification and disease evolution remains poorly understood in WM. Methods: We therefore performed chromatin availability analysis using ATACSeq and bisulfite sequencing of WM patients. ATAC-Seq libraries were constructed from 37 CD19+ BM cells from treatment-naive WM patients along with 6 paired CD19+CD27- and CD19+CD27+ selected healthy donor (HD) peripheral blood (PB) controls and 3 HD CD138+ plasma cell (PC) samples. Enhanced reduced representational bisulfite sequencing (ERRBS) was also performed on 65 MYD88 mutated and 13 MYD88 wild type (WT) WM patients using CD19-selected BM lymphoplasmacytic cells. For 11 WM MYD88 mutated patients, CD138+ cells were also selected. In addition ERRBS was also performed on paired CD19+ and CD138+ selected samples from 5 IgM multiple myeloma (MM) patients, 6 paired CD19+CD27- and CD19+CD27+ selected healthy donor (HD) peripheral blood cells; 8 HD CD138+ selected PC; as well as MYD88 mutated BCWM.1, BCWM.2, MWCL-1, RPCI-WM1, OCI-Ly3, TMD-8, HBL-1 and SU-DHL2 cells; MYD88 wild-type OCI-Ly7, OCI-Ly19, Ramos B-cells. To infer 5-hydroxymethylcytosine (5HMC) levels, oxidative bisulfite sequencing was also performed for 73 samples including the HD, MM, WT and WM CD19/138 pairs. All ATAC-Seq and 37 of the WM ERRBS samples were part of our multi-omic study that included 253 treatment naïve patients who underwent whole exome and RNASeq sequencing. Results: A UMAP projection of the CpG ERRBS data clustered WM patients into two distinct groups ( Figure 1). One group clustered directly with HD Memory B-cells (MB) and was greatly enriched for CXCR4 mutated WM patients. The other WM group clustered with IgM MM patients and was largely CXCR4 WT. The findings are consistent with the BCL and PCL WM subtypes, which were confirmed by a perfect assignment correlation with our larger multi-omic data set wherein these subtypes were defined. Notably, BCWM.1, BCWM.2 and MWCL-1 WM cell lines clustered with the WM samples. The 5HMC data displayed no clear clustering of the patient samples but clearly separated all HD samples from those of WM and IgM MM samples, consistent with a role for 5HMC dysregulation in oncogenesis. Differential methylation analysis showed differences by MYD88 and CXCR4 mutation status, deletions in Chr6q, familial history for WM, WM subtype, and stratification by early and late DPT values. Notably, the only comparison that did not yield any differences was WM CD19+ to WM CD138+, suggesting that this may not be a meaningful epigenetic distinction in WM. Similar comparisons were performed on the ATAC peaks. Differential chromatin accessibility was noted near many of the corresponding differentially expressed genes. ATAC peak coverage significantly correlated with the expression of top genes. This was particularly true for WM subtype and DPT comparisons ( Table 1). DNA sequences from differentially accessible peaks between BCL and PCL subtypes were also evaluated for relative TF motif enrichment in open vs. closed chromatin revealing PAX5, FUBP1, and ZNF121 enrichment in BCL subtypes using PCL subtype peaks as a background control. TFs REL, SP1, SPIB, BC11A and JUN were enriched PCL subtype compared with BCL subtype. Correlation between DPT associated genes revealed two clear groups, one associated with stem cell and pre-B-cell genes (Group 1) and the other with inflammatory, myeloid, and T-cell genes (Group 2). Motif analysis of differential peaks correlating with genes in each group demonstrated comparative enrichment for MYC, MXI1, and E2F2 in group 1 while RUNX1, NFKB1, and ATF1 were in enriched in group 2. Conclusions: This is the first independent validation of our previously reported multi-omic driven WM subtype classification. The studies underscore that epigenetic differences underlie the biology of WM subclassification and support a strong role for epigenetic changes driving WM evolution.

P1253: MAVORIXAFOR DISRUPTS THE CROSS TALK BETWEEN WALDENSTRÖM’S MACROGLOBULINEMIA CELLS AND THE BONE MARROW MICROENVIRONMENT RESULTING IN ENHANCED SENSITIVITY TO B-CELL TARGETED THERAPIES

Background: Waldenström’s macroglobulinemia (WM) is a rare B-cell malignancy characterized by infiltration of bone marrow (BM) and lymphoid tissues with lymphoplasmacytic cells along with hypersecretion of monoclonal immunoglobulin M (IgM) by progenitor B cells. Studies have shown that more than 90% of patients with WM have an L265P mutation in the myeloid differentiation primary response gene 88 (MYD88), and 30% to 40% of those patients show mutations in the carboxyl terminus of C-X-C receptor 4 (CXCR4). The CXCR4/C-X-C motif ligand 12 (CXCL12) axis plays an important role in the homing and retention of WM cells in the BM. The interaction of WM cells and the BM microenvironment, via cell-to-cell contacts or indirectly through secreted factors (e.g. interleukin 6 [IL-6], C-C motif ligand 5 [CCL5]), is thought to be essential for the pathogenesis of WM and to confer drug resistance. A clinical trial evaluating the efficacy of the CXCR4 antagonist, mavorixafor, in combination with the BTK inhibitor, ibrutinib in patients with MYD88L265P and CXCR4WHIM WM is currently ongoing (NCT04274738). However, the effects of mavorixafor with ibrutinib and other B-cell targeted therapies on WM cells harboring only the single mutation in MYD88L265P and without a CXCR4 mutation have not been evaluated. Aims: This study was designed to elucidate the crosstalk between WM cells and the BM stromal cell (BMSC)-based microenvironment and to explore the effect of mavorixafor in combination with different B-cell targeted therapies in a WM/BMSC co-culture model. Methods: WM cells (MWCL-1 cell line, MYD88L265PCXCR4WT) pretreated with mavorixafor and B-cell targeted therapies (BTK antagonists: ibrutinib, zanubrutinib, evobrutinib, pirtobrutinib, nemtabrutinib; or BCL2 inhibitor venetoclax) were co-cultured with established BMSCs (HS27a cells). Apoptosis, IL-6 production, and IgM release were measured after 48 or 72 hours. The effects of mavorixafor on calcium mobilization, cell migration, and adhesion to BMSCs were also measured. Results: Co-culture of WM cells with established BMSCs led to increased IgM secretion and enhanced surface expression of CXCR4 in WM cells by activation of the IL-6 receptor (IL-6R)/ Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) signaling pathway. Co-culture with established BMSCs protected WM cells from the effects of B-cell targeted therapies, reducing apoptosis of tumor cells and decreasing the IgM-reducing efficacy of the targeted therapies. Mavorixafor alone reduced IgM secretion, blocked CXCL12-induced cell migration and Ca2+ mobilization and disrupted the adhesion of WM cells to established BMSCs. Mavorixafor in combination with B-cell targeted therapies synergistically enhanced antitumor activity in WM cells, inhibited BMSC-induced IgM hypersecretion, and overcame the protective effects of BMSCs on tumor cells. Summary/Conclusion: Our studies provide preliminary evidence for the potential use of mavorixafor in disrupting the interaction of WM cells with the BMSC-based microenvironment, enhancing the efficacy of B-cell targeted therapies in the treatment of WM and potentially other lymphomas. Further studies using additional WM cell lines and/or primary patient cells are warranted to support these findings.

A New Role for the SRC Family Member HCK As a Driver of BCR/SYK Signaling in MYD88 Mutated Lymphomas

A New Role for the SRC Family Member HCK As a Driver of BCR/SYK Signaling in MYD88 Mutated Lymphomas

The Oral Proteasome Inhibitor Ixazomib, Alone and in Combination with Ibrutinib, Induces Lethality in Waldenstrom Macroglobulinemia Cells That Are Resistant to Ibrutinib

Introduction: Dysregulated B-cell receptor (BCR) signaling drives proliferation of Waldenstrom macroglobulinemia (WM) cells, which characteristically secrete excessive IgM. To maintain IgM production and preserve cellular homeostasis, WM cells rely on optimally functioning proteasomes to degrade and recycle proteins. This biological dependence has been clinically exploited using the proteasome inhibitor (PI), bortezomib (Btz), which is capable of inducing remission in 85% of WM patients (Treon et al Clin Canc Res, 2007) as a single agent. Despite, its activity, Btz use is associated with a high incidence of sensory and autonomic neuropathies as well as its cumbersome dosing schedule and non-oral route of administration. Thus, new agents that 1.) build on the activity of Btz, 2.) have a more favorable toxicity profile and 3.) can be combined with standard of care agents such as, ibrutinib (Ibr), are highly attractive and should be prioritized for investigation. Here we describe the preclinical activity of ixazomib (Ixa), a PI that is administered orally and is associated with significantly less peripheral neuropathy; in combination with Ibr, in WM cells.Methods: WM cell lines (BCWM.1 and RPCI.WM1) and their drug-resistant subclones were used. Genomic data including whole exome sequencing (WES) and copy number variations (CNV) was used in creation of digital simulation avatars of RPCI-WM1 and RPCI-WM1/IR (Ibr-resistant, Ibr-R variant), where the salient and prominently dysregulated cellular pathways were identified. Ixa +/- Ibr was tested in digital avatars and dose-dependent impact on simulated disease growth was assessed (Fig 1A). The predictive results were experimentally validated. Cell viability was measured using trypan blue or CellTiter Glo. Apoptosis was examined by annexin-V/PI staining of drug-treated cells followed by flow cytometry analysis. Mitochondrial membrane permeability (MOMP) was determined by TMRM staining of drug-treated cells followed by flow cytometry.Results: We first profiled the growth inhibitory action of Ixa in WM cell lines (n=9); all of which were sensitive to Ixa irrespective of acquired resistance to Btz, Ibr or carfilzomib with a median IC50 of 95.59nM. We then tested whether Ixa could cooperate with Ibr to enhance WM cell kill. Indeed, the Cellworks modeling tool, predicted a significant loss of viability and induction of apoptosis in both wildtype and Ibr-R cells treated with Ibr + Ixa, as compared to either single agent (Fig 1B-D). We confirmed this in vitro, where viability of wildtype WM cells treated with Ixa (20nM), Ibr (5uM) or Ibr + Ixa for 6h significantly decreased; most notable with Ixa + Ibr (25.5% viable cells). And this effect was similar in Ibr-R cell lines (30.3% viable cells) after combination treatment. Further confirming in silico predictive modeling results, we noted a significant increase in annexin-V/PI positivity in Ixa treated wildtype (24.5%) and Ibr-R WM cell lines (33.1%). Whereas Ibr alone exhibited minimal apoptosis in WM cell lines (~11 - 15%), the combination of with Ixa + Ibr enhanced programmed cell death in wildtype (40.2%) and Ibr-R cells (50.5%). As intrinsic apoptosis is triggered through mitochondrial disruption, we also examined mitochondrial transmembrane permeability (MOMP) in cells treated with Ibr, Ixa or Ixa + Ibr. In wildtype cells, mean MOMP induced by single agent Ixa was 23.5% and in Ibr-R cells 33%; increasing to 39% and 50.4%, respectively, after treatment with Ibr + Ixa. Mechanistic studies revealed changes in proteins regulated by the ubiquitin proteasome system and those associated with BTK signaling. While Ixa alone downregulated Bcl-2 and Mcl-1 levels, this effect was more marked in cells (both wildtype and Ibr-resistant) treated with Ixa + Ibr combination. Similar effects were noted in pPLCg2, albeit with some variability between the cell lines.Conclusion: Using a combination of in silico modeling and experimental approaches, we provide first preclinical evidence on the antitumor effects of Ibr combined with the oral PI, Ixa, in WM cells, including those resistant to Ibr. Whereas further investigation on the precise molecular mechanisms of how Ibr + Ixa cooperate to induce apoptosis in WM cell is ongoing, our studies provide the basis for clinical testing of the Ibr + Ixa regimen; with a phase II clinical study in development. [Display omitted] DisclosuresSingh:Cellworks Research India Pvt. Ltd: Employment. Kumar:Cellworks: Employment. Basu:Cellworks: Employment. Trivedi:Cellworks: Employment. Majumdar:Cellworks: Employment. Abbasi:Cellworks Group Inc.: Employment. Vali:Cellworks Group Inc.: Employment. Martin:Janssen: Consultancy, Honoraria, Other: travel expenses; Genentech: Consultancy; Celgene: Consultancy; Teva: Research Funding; Novartis: Consultancy; Gilead: Consultancy, Other: travel expenses. Ansell:Bristol-Myers Squibb: Research Funding; Merck: Research Funding; Celldex: Research Funding; Affimed: Research Funding; Seattle Genetics: Research Funding. Ailawadhi:Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Pharmacyclics: Research Funding; Takeda: Consultancy, Honoraria.

Targeting CD38 with Daratumumab Suppresses B-Cell Receptor Signaling and Enhances the Activity of Ibrutinib in Waldenstrom Macroglobulinemia Cells

Introduction: Waldenstrom macroglobulinemia (WM) is characterized by malignant B-lymphoplasmacytic cells that rely on B-cell receptor (BCR) signaling, which is clinically exploited with ibrutinib (Ibr). Although effective, Ibr tx. mostly results in partial remission of disease. This signifies the need to examine combination tx. strategies that can lead to superior disease eradication and counteract onset of Ibr resistance. WM cells express several surface antigens, including CD38 and Daratumumab (Dara) is a first-in-class monoclonal antibody that binds CD38; inducing an immune effector-mediated cytolytic reaction. CD38 also functions as a molecular hub for integration of signals transmitted through the B-cell receptor complex (BCR) to promote cell proliferation. We hypothesized that cotargeting BTK (with Ibr) and CD38 (with Dara) would induce both an immune-mediated and cell intrinsic mode of death to result in enhanced WM cell kill.Methods: Wildtype WM cell lines (BCWM.1, RPCI.WM1) and ibrutinib-resistant (IR) subclones were used in experiments. CD38 MFI and surface antibody binding (sAbc) was determined by flow cytometry. Ibr (1uM), Dara (0.1ug/mL) or Ibr + Dara-mediated death through antibody dependent cell cytotoxicity (ADCC) or complement dependent cytotoxicity (CDC) was measured using the Calcein AM assay at an effector to target (E:T) ratio of 50:1. Antibody-dependent cellular phagocytosis (ADCP) was assessed using human macrophages at an E:T ratio of 2:1. An IgG1 isotype Ab was used as a control in experiments.Results: CD38 sAbc in BCWM.1 and RPCI-WM1 cells was 4864 and 35892, respectively; consistent with previously noted MFI's (Paulus et al PLOS One, 2016). Specific lysis induced by single agent Dara in wildtype WM cells, showed a mean ADCC of 24.46%, CDC: 20.29% and ADCP: 20.12%. Dara can also induce apoptosis and we noted an average of 33.35% annexin-V/PI+ WM cells after 24h Dara tx. We surmised the anti-WM activity of Dara could be enhanced by Ibr as 1.) the BTK inhibitor has been shown to improve anti-tumor T-cell responses and 2.) the combined effects of each drug could synergistically attenuate BCR-associated signaling. As anticipated, mean ADCC, CDC, ADCP and apoptosis increased to 52.7%, 38.41%, 30.19% and 57.53%, respectively in Ibr + Dara-treated wildtype WM cells. While potential mechanisms of the enhanced immune-mediated specific lysis from the combination tx. are being investigated, we observed cell signaling changes that may account for the increased cell death. As expected, Dara decreased pLyn, pSyk, pBTK, pPLCG2 and pERK as well as pAKT, pmTOR and pS6K levels in WM cells. And these proteins decreased further in Ibr + Dara treated cells. Next, we asked the question whether Dara +/- Ibr would be active in IR WM cells, which had lower CD38 sAbc (mean 8400, range 1024 - 15776). Mean ADCC by Dara in IR WM cells was 19.52%, CDC: 31.85%, ADCP: 27.5% and apoptosis at 31.3%. While ADCC was expectedly lower in IR vs WT cells, CDC and ADCP were significantly higher. We then tested the combination of Ibr + Dara in IR WM cells and noted greater ADCC (63.97%), CDC (60.34%) and apoptosis (60.23%), whereas ADCP did not increase (29.45%). We also analyzed BCR-associated protein expression and noted single agent Dara decreased pBTK, pPLCG2, pERK and pS6K, whereas the effects of Ibr + Dara were more cell line specific. As IR WM subclones are less reliant on BTK/BCR-mediated signaling, cell death from Dara (+/- Ibr) may be due to other mechanisms possibly related to alterations in the mitochondria (intrinsic to IR cells, Paulus et al BCJ 2017).Conclusion: We demonstrate in vitro activity of Dara and Ibr in WM cells, showing that 1.) Dara induces cell death through various mechanisms (ADCC, CDC, ADCP and apoptosis); 2.) while higher CD38 sAbc appears to correlate with greater ADCC, this was not the case for CDC, ADCP or apoptosis; 3.) Dara significantly modulates BCR-associated signaling; an effect preserved in IR WM cells and documented for the first time and 4.) the combination tx. exerts significantly more WM cell death in nearly all the aforementioned assays than either single agent alone and in some cases more so in IR WM cells. With further preclinical results supporting these data, to be shared at the conference, our investigations provide rationale for clinical testing of Ibr + Dara in WM. DisclosuresSher:LAM Therapeutics, Inc: Research Funding. Ansell:Bristol-Myers Squibb: Research Funding; Merck: Research Funding; Celldex: Research Funding; Affimed: Research Funding; Seattle Genetics: Research Funding. Malavasi:Jaansen Pharmaceuticals: Research Funding. Ailawadhi:Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Pharmacyclics: Research Funding; Takeda: Consultancy, Honoraria.

Soluble PD-1 Ligands Regulate T-Cell Function in Waldenstrom's Macroglobulinemia

While immune checkpoint molecules contribute to the malignant development of several cancers, including certain types of lymphomas, their potential significance in malignant progression of Waldenstrom's Macroglobulinemia (WM), a low-grade B-cell lymphoma, has not fully been described. The main characteristic of WM is infiltration of lymphoplasmacytic cells in the bone marrow (BM) and increased monoclonal IgM production by malignant cells within BM microenvironment. While next generation sequencing analysis has identified mutations in the MYD88 gene as the driver of WM development, studies by our group have also highlighted a role for the BM microenvironment in pathophysiology of WM and shown that elevated levels of certain cytokines in the BM, such as IL-6 and IL-21, promote both cell proliferation and IgM secretion in WM.In this study, we investigated the impact of cytokines on the immune checkpoint receptor Programmed Death-1 (PD-1) and its ligands PD-L1 and PD-L2 and evaluated how these molecules contribute to malignant progression in WM.Using RT-PCR analysis we have shown that treatment of WM cell lines with IL-21 and IL-6 significantly induce PD-L1 and PD-L2 gene expression, implying that elevated levels of these cytokines within the BM microenvironment could be associated with increased PD-L1 and PD-L2 expression by malignant cells in WM patients. We therefore performed RT-PCR analysis on cells isolated from patients' BM and confirmed increased PD-L1 and PD-L2 expression by both WM and BM cells when compared normal BM cells. In contrast, flow cytometry analysis showed minimal expression of these ligands on the surface of the WM cells, with no significant difference between WM and normal BM cells, indicating that there might be a regulatory mechanism controlling surface expression of these ligands. Further analysis using ELISA and Western Blotting showed that PD-L1 and PD-L2 are found as secreted forms in both BM plasma and peripheral blood serum of the WM patients and their level is significantly higher in WM as compared to normal subjects. Interestingly, in vitro treatment of the WM cell lines by IL-21 and IL-6 increased secreted PD-L1 and PD-L2 in the culture media of the cells, providing an additional line of evidence for elevated PD-L1 and PD-L2 levels in WM patients. Subsequent analysis showed that the secreted form of the PD-L1 and PD-L2 binds to T-cells and induces signals that reduce T-cell proliferation and cell cycle progression, the effects that are associated with a reduction in p-ERK and cyclin A levels.In summary, our data identifies that secreted form of PD-1 ligands is elevated in WM and play an important role in regulating T-cell function. Given the capability of secreted form to be bioactive at distant sites, soluble PD-1 ligands may have potential to promote disease progression. DisclosuresAnsell:Seattle Genetics: Research Funding; Bristol-Myers Squibb: Research Funding; Affimed: Research Funding; Merck: Research Funding; Celldex: Research Funding.

Progression from Monoclonal gammopathy of undetermined significance of the immunoglobulin M class (IgM-MGUS) to Waldenstrom Macroglobulinemia is associated with an alteration in lipid metabolism

The molecular events that modulate the progression of monoclonal gammopathy of undetermined significance of the immunoglobulin M class (IgM-MGUS) to Waldenstrom Macroglobulinemia (WM) are mostly unknown. We implemented comparative proteomics and metabolomics analyses on patient serum samples to identify differentially expressed molecules crucial to the progression from IgM-MGUS to WM. Our data identified altered lipid metabolism as a discriminating factor between MGUS, WM, and matched normal controls. Levels of many fatty acids, including polyunsaturated fatty acids and dicarboxylic acids, were significantly downregulated in WM sera when compared to MGUS. These reductions were associated with diminished 15-LOX and PPAR protein expression and increased 5-LOX and GPX4 expression in WM versus MGUS patients’ samples. Furthermore, WM serum samples showed increased lipid peroxidation compared to MGUS. Treatment with IL-6 or TNFα, upstream regulators of differentially expressed proteins between MGUS and WM, increased lipid absorption and lipid peroxidation in WM cell lines. Knock-down of 15-LOX expression increased WM cell survival, an effect accompanied by increased 5-LOX and GPX4 expression. In summary, our data show that reduced fatty acid and lipid metabolite levels in the serum of the WM patients are associated with increased lipid peroxidation and that downregulation of 15-LOX increases the survival of WM cells. These data are highly significant in identifying the biomarkers of disease progression and designing targeted therapeutic intervention.

Discovery of a Selective, Covalent IRAK1 Inhibitor with Antiproliferative Activity in MYD88 Mutated B-Cell Lymphoma.

Interleukin 1 (IL-1) receptor-associated kinases (IRAKs) are serine/threonine kinases that play critical roles in initiating the innate immune response against foreign pathogens. Additionally, dysregulation of IRAK1 signaling plays a role in neoplastic disorders. For example, IRAK1 was shown to be important for survival and proliferation in many B-cell lymphomas, including Waldenström's macroglobulinemia (WM) and ABC subtype Diffused Large B-cell Lymphoma (DLBCL) cells. Here, we report the discovery of a highly potent and selective covalent inhibitor of IRAK1, JH-X-119-01. Intact protein MS labeling studies confirmed that JH-X-119-01 irreversibly labels IRAK1 at C302. This compound exhibited cytotoxic activity at single digit micromolar concentrations in a panel of WM, DLBCL, and lymphoma cell lines expressing MYD88. Cotreatment of JH-X-119-01 with the BTK inhibitor ibrutinib resulted in synergistic killing effects in these systems. Taken together, JH-X-119-01 represents a highly selective probe of IRAK1 for further development.

The BLIMP1-EZH2 nexus in a non-Hodgkin lymphoma.

Waldenström's macroglobulinemia (WM) is a non-Hodgkin lymphoma, resulting in antibody-secreting lymphoplasmacytic cells in the bone marrow and pathologies resulting from high levels of monoclonal immunoglobulin M (IgM) in the blood. Despite the key role for BLIMP1 in plasma cell maturation and antibody secretion, its potential effect on WM cell biology has not yet been explored. Here we provide evidence of a crucial role for BLIMP1 in the survival of cells from WM cell line models and further demonstrate that BLIMP1 is necessary for the expression of the histone methyltransferase EZH2 in both WM and multiple myeloma cell lines. The effect of BLIMP1 on EZH2 levels is post-translational, at least partially through the regulation of proteasomal targeting of EZH2. Chromatin immunoprecipitation analysis and transcriptome profiling suggest that the two factors co-operate in regulating genes involved in cancer cell immune evasion. Co-cultures of natural killer cells and cells from a WM cell line further suggest that both factors participate in immune evasion by promoting escape from natural killer cell-mediated cytotoxicity. Together, the interplay of BLIMP1 and EZH2 plays a vital role in promoting the survival of WM cell lines, suggesting a role for the two factors in Waldenström's macroglobulinaemia.

MLL1 Modulates IgM and Inflammatory Cytokines in Waldenstrom's Macroglobulinemia

Waldenström macroglobulinemia (WM) is a low-grade B-cell lymphoma associated with the accumulation of lymphoplasmacytic cells in the bone marrow (BM) and a high level of monoclonal IgM protein in the serum. Genome-wide studies revealed genomic alternations in WM, with the most prevalent being a point mutation in the myeloid differentiation primary response gene 88 (MyD88) resulting in an amino acid substitution (L265P) that is present in over 90% of WM patients. MyD88 is an effector of Toll-like receptor (TLR) signaling pathway that mediates inflammatory cytokine expression and secretion upon activation. Here, we found that activation of the TLR-MyD88 pathway in WM cells carrying the MyD88 L265P somatic mutation increases the expression of IL-6 and CCL2, two cytokines known to participate in the pathobiology of several neoplasms including B cell malignancies. Furthermore, IL-6 has been shown to modulate IgM secretion and malignant B cell growth in WM. We demonstrate that an active ERK1/2 pathway, which is elevated in WM patient samples, is required to maintain IL-6 and CCL2 expression in BCWM.1 (p=0.0339) and RPCI-WM1 (p=0.0005) cells. Further characterization identified a change in the chromatin landscape in response to TLR-MyD88 activation in WM cells. We found increased levels of trimethylation of histone 3 lysine 4 (H3K4me3) at the promoters of the inflammatory cytokines IL-6 and CCL2 by chromatin immunoprecipitation (ChIP) assay followed by qPCR (ChIP-qPCR). The H3K4me3 modification is catalyzed by six related homologs of the yeast histone methyltransferase (HMT) family. Further analysis identified the Mixed-lineage leukemia 1 (MLL1) as the enzyme bound to these promoters in response to TLR-MyD88 stimulation of WM cells. Analysis of WM cell lines and primary WM patient cells showed that MLL1 and its binding partner, menin, are expressed at significantly higher levels in CD19+CD138+cells from WM patients compared to CD19+B cells from peripheral blood of healthy donors (p<0.001). We also found an increase in H3K4me3 deposition on IL-6 and CCL2 promoters during early (1-3 hr) and late (12-24 hr) kinetics following TLR-MyD88 stimulation. This also coincided with increased deposition of MLL1 on these cytokine promoters by ChIP-qPCR. Disruption of menin-MLL1 using the MI-2 or MI-503 menin-MLL1 inhibitors significantly reduced IL-6 and CCL2 expression in WM cell lines (BCWM.1 p=0.039; RPCI-WM1 p=0.0041). This also results in significantly reduced IgM expression (p<0.0001) and secretion at inhibitor dose (5 μM) that has no effect on cell proliferation. Finally, MLL1 knockdown using RNAi significantly reduces IgM expression in BCWM.1 (p<0.0001), MWCL (p=0.0016) and RPCI-WM1 (p<0.0001) and IgM secretion in BCWM.1 (p<0.0001) and MWCL-1 (p<0.0001). Taken together, these results identify a novel role for menin-MLL1 in regulating inflammatory cytokines and IgM expression and secretion in WM and provide the rationale for targeting these molecules in WM patients. Disclosures Ansell: Mayo Clinic Rochester: Employment; Mayo Clinic Rochester: Employment; Regeneron: Research Funding; Seattle Genetics: Research Funding; Trillium: Research Funding; Seattle Genetics: Research Funding; Regeneron: Research Funding; Trillium: Research Funding; Bristol-Myers Squibb: Research Funding; Mayo Clinic Rochester: Employment; Trillium: Research Funding; Affimed: Research Funding; Affimed: Research Funding; Regeneron: Research Funding; Trillium: Research Funding; Mayo Clinic Rochester: Employment; Bristol-Myers Squibb: Research Funding; Mayo Clinic Rochester: Employment; LAM Therapeutics: Research Funding; Bristol-Myers Squibb: Research Funding; Seattle Genetics: Research Funding; LAM Therapeutics: Research Funding; LAM Therapeutics: Research Funding.

Overcoming the Supportive Stroma-Induced Proliferation in Waldenstrom's Macroglobulinemia By Selective Inhibition of the FGF/FGF-Receptor Axis

The human fibroblast growth factor receptor (FGF-R) family plays an essential role in a wide range of cellular processes, such as cell growth, proliferation, differentiation, migration and survival. It has been reported that FGF-Rs are expressed in hematopoietic cells; and FGF/FGFR signaling deregulation is largely involved in hematologic malignancies, including Waldenström macroglobulinemia (WM). WM is still an incurable disease, and patients succumb due to disease progression. Therefore, novel therapeutics designed to specifically target deregulated signaling pathways in WM are required. We aimed to investigate the role of FGF/FGF-R system in FGF-dependent WM cell lines by using an anti-pan FGF trap molecule (NSC12), responsible for FGF/FGF-R blocking. We first interrogated the GSE9656 dataset in order to confirm the expression of FGFs and FGF-Rs in WM cells, demonstrating an enrichment of several FGF- and FGF-R-isoforms in primary WM patients' derived tumor cells compared to the normal cellular counterpart (P<0.05); and demonstrated the ability of NSC12 to inhibit FGF-secretion within the conditioned media of NCS12-treated WM cells, as shown by ELISA. Wide-transcriptome profiling of NSC12-treated WM cells (BCWM.1; MWCL1) revealed a significant inhibition of Myc-target related genes, coupled with silencing of genes involved in cell cycle progression, cell proliferation, PI3K-AKT-mTOR signaling, oxidative phosphorylation (Hallmark; FDR<0.25; P<0.05). This prompted us to evaluate the anti-tumor functional sequelae exerted by NSC12 in WM cells: NSC12 induced significant inhibition of WM cell growth (BCWM1 and WMCL1) in a dose-dependent fashion (0.1-10μM; IC50 ~3μM), even in the presence of bone marrow microenvironment. In addition, a significant effect was also observed in primary tumor cells from WM patients; while no effect was observed on healthy donor-derived peripheral blood mononuclear cells. The growth inhibitory effect was associated with induction of apoptotic cell death, caspase activation and PARP cleavage, as demonstrated by flow cytometry and western blot, respectively. Moreover, we also observed a NSC12 dose-dependent increase of mitochondrial reactive oxigen species (ROS), at protein level. Cell cycle analysis revealed a reduction of the S-phase and increase of G0/G1 phase. Mechanistically, NSC12 targeted WM cells by inhibiting MAPK, JAK/STAT3 and PI3K-Akt pathways known to be FGFRs-activated signaling cascades. Importantly, the same effect was maintained in WM cells even in the presence of the supporting BM microenvironment. Functional studies demonstrated the ability of NSC12 to impair the adhesion of both cell lines to the supportive primary bone marrow stromal cells, in vitro. NCS12-dependnet anti-WM activity was also tested in combination with bortezomib, carfilzomib, everolimus and ibrutinib: the combinatory treatment (48h) resulted in a more significant dose-dependent inhibition of WM cell survival and proliferation (P<0.05), thus suggesting the rational for combining FGF-blockade with proteasome-, mTOR-, or BTK-inhibitors. In vivo studies are being performed, in order to further corroborate the anti-WM activity of NSC12 using WM animal models. Disclosures Ronca: Associazione Italiana per la Ricerca sul Canctro (AIRC): Research Funding. Rossi:Astellas: Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria; Mundipharma: Honoraria; BMS: Honoraria; Sandoz: Honoraria; Amgen: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Jazz: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Consultancy; Roche: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Roccaro:AstraZeneca: Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Associazione Italiana per al Ricerca sul Cancro (AIRC): Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; European Hematology Association: Research Funding; Associazione Italiana per al Ricerca sul Cancro (AIRC): Research Funding; Transcan2-ERANET: Research Funding; AstraZeneca: Research Funding; European Hematology Association: Research Funding; Transcan2-ERANET: Research Funding.

Glutathione Metabolism Contributes to the Pathobiology of Waldenstrom's Macroglobulinemia

Cancer cells have altered energy demand due to their increased proliferative capacity. In Waldenstrom Macroglobulinemia (WM), an indolent yet incurable B-cell lymphoma, the homeostasis of the bone marrow (BM) environment is disturbed due to the infiltration of the lymphoplamacytic cells that continuously produce monoclonal IgM. An alteration in energy demand could skew the balance of key proteins and metabolites towards a permissive niche for WM tumor cells growth, and unfavorably effect on the immune response against tumor cells. Therefore, the aim of our study was to identify how changes in certain metabolites and/or proteins could contribute to the pathobiology of WM and whether the cytokine and chemokine composition of the BM microenvironment play a role in such changes. WM patient's samples including BM plasma, peripheral blood serum and BM cells (n=101) as well as equivalent normal counterparts (n=86) were collected and used for metabolomics analysis. Comprehensive targeted metabolomics analysis was performed using Capillary Electrophoresis Time-of-Flight Mass spectrometry (CE-TOFMS), CE-triple quadrupole mass spectrometry (CE-qQqMS) and Liquid Chromatography (LC-TOFMS). Normal and WM peripheral blood serums samples were also used for untargeted proteomics analysis using a fully automated proteomic technology platform that includes an Agilent 1200 Series Quaternary HPLC system connected to a Q Exactive Plus mass spectrometer. Real-time PCR analysis was performed to detect the gene expression of the relevant metabolite transporters located on the cell membrane. BM cells from control and WM patients' samples were used for flow cytometry analysis. IHC was used to detect the proteins on the BM tissues. Principal Component Analysis (PCA) and Hierarchal Clustering Analysis (HCA) on both metabolomics and proteomics data identified two distinct clusters for disease and normal samples, indicating that there are differentially expressed proteins and metabolites in WM versus normal samples. Furthermore, pathway analysis showed that the majority of the altered metabolites were the members of the glutathione (GSH) metabolism pathway. This finding was further validated not only by data obtained from metabolomics analysis of BM cells and BM plasma, but also by proteomic data WM patients serum, implying that GSH metabolism is key to the biology of WM. Moreover, stimulation of WM cell lines by IL-6 and IL-21, cytokines involved in inducing WM cell proliferation and IgM secretion, resulted in increased gene expression of the transporters mediating uptake of metabolites required for GSH synthesis, including SLC7A11, 4F2HC and LAT1, indicating that cytokines in the WM BM could modulate GSH metabolism. In addition, IHC staining of the BM tissues as well as flow cytometry analysis of patients' lymphoplasmacytic cells identified glutathione peroxidase as one of the major proteins modulating GSH metabolism in WM. In summary, our data highlight a central role for GSH metabolism in WM disease biology and indicate that intervening with the metabolic processes could be a potential therapeutic strategy for patients with WM. Disclosures Ansell: AI Therapeutics: Other: research funding for clinical trials; Seattle Genetics: Other: research funding for clinical trials; Regeneron: Other: research funding for clinical trials; Affimed: Other: research funding for clinical trials; Bristol Myers Squibb: Other: research funding for clinical trials; Pfizer: Other: research funding for clinical trials; Merck: Other: research funding for clinical trials; Takeda: Other: research funding for clinical trials.

CXCR4-targeted PET imaging using 64Cu-AMD3100 for detection of Waldenström Macroglobulinemia

ABSTRACTObjective: Waldenström Macroglobulinemia (WM) is a rare B-cell malignancy characterized by secretion of immunoglobulin M and cancer infiltration in the bone marrow. Chemokine receptor such as CXCR4 and hypoxic condition in the bone marrow play crucial roles in cancer cell trafficking, homing, adhesion, proliferation, survival, and drug resistance. Herein, we aimed to use CXCR4 as a potential biomarker to detect hypoxic-metastatic WM cells in the bone marrow and in the circulation by using CXCR4-detecting radiopharmaceutical.Methods: We radiolabeled a CXCR4-inhibitor (AMD3100) with 64Cu and tested its binding to WM cells with different levels of CXCR4 expression using gamma counter in vitro. The accumulation of this radiopharmaceutical tracer was tested in vivo in subcutaneous and intratibial models using PET/CT scan. In addition, PBMCs spiked with different amounts of WM cells ex vivo were detected using gamma counting.Results: In vitro, 64Cu-AMD3100 binding to WM cell lines demonstrated a direct correlation with the level of CXCR4 expression, which was increased in cells cultured in hypoxia with elevated levels of CXCR4, and decreased in cells with CXCR4 and HIF-1α knockout. Moreover, 64Cu-AMD3100 detected localized and circulating CXCR4high WM cells with high metastatic potential.Conclusions: In conclusion, we developed a molecularly targeted system, 64Cu-AMD3100, which binds to CXCR4 and specifically detects WM cells with hypoxic phenotype and metastatic potential in the subcutaneous and intratibial models. These preliminary findings using CXCR4-detecting PET radiopharmaceutical tracer indicate a potential technology to predict high-risk patients for the progression to WM due to metastatic potential.

Activation of the Non-Canonical Estrogen Receptor Gper As a Novel Therapeutic Strategy Against Waldenström Macroglobulinemia

Although estrogens have been shown to regulate normal B cell proliferation and differentiation, the impact of estrogen receptor (ER) signaling on B cell malignancies, including Waldenström Macroglobulinemia (WM), remains unexplored. To address this issue, we have analyzed, through preclinical WM models, the druggability either of the classical estrogen receptor ERβ, an emerging target for acute leukemias and certain types of lymphomas, and of the novel, more recently deorphanized G-protein coupled ER GPER, whose therapeutic potential in solid tumors has recently emerged.Both ERβ and GPER were found expressed at mRNA and protein level in a panel of WM and IgM-secreting lymphoma cell lines. However, while treatment with ERβ agonists did not elicit any effect, selective GPER activation significantly reduced WM cell viability. On this basis, we investigated GPER expression pattern and therapeutic potential in WM.By interrogating public microarray datasets, we found GPER transcript significantly upregulated in WM as compared to normal B cells; moreover, immunohistochemical analysis showed elevated GPER expression in lymph node biopsies from newly-diagnosed WM patients as compared to healthy lymph nodes. Treatment with the selective GPER agonist G-1 ( (±) -1-[(3aR*,4S*,9bS*)-4-(6-Bromo-1,3-benzodioxol-5-yl) -3a,4,5,9b-tetrahydro-3H cyclopenta [c]quinolin-8-yl] ethanone) led to reduced viability, clonogenicity and migration of WM and IgM-secreting lymphoma cell lines (IC50 at 48h ranging between 2.5 and 5.0 mM), including MYD88 and CXCR4 mutated cell lines, and even in the presence of bone marrow-derived stromal cells, while it did not affect healthy CD19+ B cell viability; conversely, GPER antagonists G-36 and G-15 slightly enhanced cell proliferation. The growth inhibitory activity of G-1 was associated with accumulation in G2/M cell cycle phase and induction of apoptosis, the latter phenomenon assessed by Annexin V/7AAD staining, analysis of mitochondrial membrane depolarization and cleavage of caspase 3, 7 and 9; hybridization of an antibody-array also highlighted G-1-induced down-regulation of anti-apoptotic proteins, including survivin and Bcl2. A significant effect of G-1 was also observed in primary tumor cells from refractory WM patients. Moreover, G-1 synergistically enhanced bortezomib and ibrutinib cytotoxicity in vitro. Importantly, intraperitoneal injection of G-1 (2mg/kg) significantly reduced the growth of BCWM-1 xenografts in NOD/SCID mice.To gain further insight into the consequences of selective GPER activation, we performed gene expression profile and GSEA analysis of WM cell lines treated with G-1. Of note, the p53 signaling pathway was strongly induced upon GPER activation, and upregulation of p53 and of its target genes or microRNAs (p21CIP1, Bax, Bad, PUMA and miR-34a) was confirmed both in G-1 treated WM cell lines and primary WM cells; moreover, G-1 combination with bortezomib or ibrutinib led to stronger increase in p53 and p21CIP1 levels as compared to single agent treatment. Finally, p53 knock-down partially reversed G-1-dependent anti-WM effects, thus suggesting p53 as downstream mediator of GPER.In summary, we report a significant anti-tumor activity upon selective GPER activation in WM cells, with apoptosis induction and a potent synergistic anti-WM activity in combination with ibrutinib and bortezomib. Altogether, these preclinical data suggest GPER agonists as a potential therapeutic option in WM. DisclosuresMunshi:OncoPep: Other: Board of director.

Soluble PD-1 ligands regulate T-cell function in Waldenstrom macroglobulinemia

AbstractAlthough immune checkpoint molecules regulate the progression of certain cancers, their significance in malignant development of Waldenstrom macroglobulinemia (WM), an incurable low-grade B-cell lymphoma, remains unknown. Recently, cytokines in the bone marrow (BM) microenvironment are shown to contribute to the pathobiology of WM. Here, we investigated the impact of cytokines, including interleukin-6 (IL-6) and IL-21, on immune regulation and particularly on the programmed death-1 (PD-1) and its ligands PD-L1 and PD-L2. We showed that IL-21, interferon γ, and IL-6 significantly induced PD-L1 and PD-L2 gene expression in WM cell lines. Increased PD-L1 and PD-L2 messenger RNA was also detected in patients' BM cells. Patients' nonmalignant BM cells, including T cells and monocytes, showed increased PD-L1, but minimal or undetectable PD-L2 surface expression. There was also very modest PD-L1 and PD-L2 surface expression by malignant WM cells, suggesting that ligands are cleaved from the cell surface. Levels of soluble ligands were higher in patients' BM plasma and blood serum than controls. Furthermore, IL-21 and IL-6 increased secreted PD-L1 in the culture media of WM cell lines, implying that elevated levels of soluble PD-1 ligands are cytokine mediated. Soluble PD-1 ligands reduced T-cell proliferation, phosphorylated extracellular signal-regulated kinase and cyclin A levels, mitochondrial adenosine triphosphate production, and spare respiratory capacity. In conclusion, we identify that soluble PD-1 ligands are elevated in WM patients and, in addition to surface-bound ligands in WM BM, could regulate T-cell function. Given the capability of secreted forms to be bioactive at distant sites, soluble PD-1 ligands have the potential to promote disease progression in WM.

PAK4 Inhibition Impacts Growth and Survival, and Increases Sensitivity to DNA-Damaging Agents in Waldestrostom Macroglobulinemia

p21-activated kinases (PAKs) are effectors for the small GTPases Cdc42 and Rac that control a wide range of cellular processes including cytoskeleton reorganization, cell proliferation, survival and motility. Based on our initial results showing significant biological and functional role of PAK4 in plasma cell malignancies, we investigated its role in Waldenström macroglobulinemia (WM). We first confirmed high level of PAK4 expression and activity in WM cells. Both genetic depletion via stable lentiviral knock down and pharmacological inhibition of PAK4 via allosteric modulator KPT-9274 decreased tumor cell growth and survival in a dose and time dependent manner in a panel of WM cell lines. The growth inhibitory effect was associated with induction of apoptotic cell death, caspases activation and PARP cleavage as well as modulation of pro-apoptotic proteins such as PUMA. In addition, related-PAK4 biological processes such as migration, invasion and adhesion to bone marrow stromal cells (BMSCs) were significantly impacted by the inhibitor. A significant effect of KPT9274 was also observed in primary tumor cells from both newly diagnosed and relapse WM patients, including primary cells from ibrutinib resistant patients.To gain further insight into the consequences of PAK4 inhibition, we performed gene expression profile in WM cell lines treated with KPT-9274. Intriguingly we detected a significant impact on expression of Fanconi Anemia (FA)/BRCA pathway related genes, a DNA-damage response pathway essential for the repair of DNA interstrand crosslinks (ICLs) and regulation of cellular responses to replication stress. Since FA/BRCA deficient cells are defective in the formation of Rad51 foci, a crucial component of the homologous recombination (HR) repair machinery, we examined the effect of PAK4/NAMPT inhibition on Rad51 as well its impact on HR. We observed that, apart from inducing DNA damage, assessed by γH2AX expression, KPT-9274 specifically inhibited Rad51 and the double strand break repair by the homologous recombination pathway. Protein pull down analysis revealed the existence of a complex between nicotinamide phosphoribosyltransferase (NAMPT) and the conserved PAK binding domain; treatment with KPT-9274 disrupted this interaction and inhibited the enzymatic activity of NAMPT. NAD is necessary for PARP function especially with increasing DNA damage; and its depletion severely hamper DNA repair. Consistent with this, we observed that nicotinic acid supplementation rescued the effect of KPT-9274 on NAMPT inhibition as well as DNA damage and repair.As FA/BRCA pathway has been shown to affects sensitivity to alkylating agents, we next combined KPT-9274 with either melphalan or bendamustine in MM and WM cells. The combination treatment resulted in a synergistic time- and dose-dependent inhibition of cell survival and growth in cell lines and primary patient tumor cells. We observed a synergistic induction of apoptosis and activation of caspase-3/7, -8 and -9 following treatment with combined versus single-agent therapy. Similarly, in PAK4 knock-down WM cells bendamustine significantly increased the rate of early and late apoptotic cells (60%) compared to scrambled knockdown cells (37%).In summary, we report a significant anti-proliferative activity of dual PAK4-NAMPT inhibition in WM cells, with induction of apoptosis, DNA-damage response and FA/BRCA pathway disruption and a strong synergistic activity in combination with DNA damaging agents such as Bendamustine. Altogether, our data suggests dual PAK4-NAMPT inhibition represents a novel therapeutic strategies in WM. DisclosuresSenapedis:Karyopharm Therapeutics Inc: Employment, Equity Ownership. Baloglu:Karyopharm Therapeutics Inc: Employment, Equity Ownership. Oliva:Takeda: Honoraria; Celgene: Honoraria. Anderson:Oncopep: Other: scientific founder; C4 Therapeutics: Other: scientific founder; Gilead Sciences: Membership on an entity's Board of Directors or advisory committees; Millenium Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees; MedImmune: Membership on an entity's Board of Directors or advisory committees. Treon:Pharmacyclics: Consultancy, Research Funding.

Waldenstrom macroglobulinemia cells devoid of BTKC481S or CXCR4WHIM-like mutations acquire resistance to ibrutinib through upregulation of Bcl-2 and AKT resulting in vulnerability towards venetoclax or MK2206 treatment

Although ibrutinib is highly effective in Waldenstrom macroglobulinemia (WM), no complete remissions in WM patients treated with ibrutinib have been reported to date. Moreover, ibrutinib-resistant disease is being steadily reported and is associated with dismal clinical outcome (overall survival of 2.9–3.1 months). To understand mechanisms of ibrutinib resistance in WM, we established ibrutinib-resistant in vitro models using validated WM cell lines. Characterization of these models revealed the absence of BTKC481S and CXCR4WHIM-like mutations. BTK-mediated signaling was found to be highly attenuated accompanied by a shift in PI3K/AKT and apoptosis regulation-associated genes/proteins. Cytotoxicity studies using the AKT inhibitor, MK2206±ibrutinib, and the Bcl-2-specific inhibitor, venetoclax±ibrutinib, demonstrated synergistic loss of cell viability when either MK22016 or venetoclax were used in combination with ibrutinib. Our findings demonstrate that induction of ibrutinib resistance in WM cells can arise independent of BTKC481S and CXCR4WHIM-like mutations and sustained pressure from ibrutinib appears to activate compensatory AKT signaling as well as reshuffling of Bcl-2 family proteins for maintenance of cell survival. Combination treatment demonstrated greater (and synergistic) antitumor effect and provides rationale for development of therapeutic strategies encompassing venetoclax+ibrutinib or PI3K/AKT inhibitors+ibrutinib in ibrutinib-resistant WM.

Isogenic Loss of TNFAIP3 in Waldenstrom Macroglobulinemia Enhances MYD88L265P-Driven Signaling

Recent next generation sequencing of paired normal and tumor lymphoma tissue highlights a significant role for the MYD88L265P mutations in Waldenstrom Macroglobulinemia (WM) and diffuse large B-cell lymphoma (DLBCL). In recent work we have found that MYD88L265P signaling is constitutively active in both WM and DLBCL cells leading to heightened MYD88L265P, IRAK and TRAF6 oligomerization and TAK1 and NF-kB activation. However, little else is know about the full impact of the mutation, especially in combination with other genetic events. Recent mouse models suggest that MYD88L265P alone may not be sufficient to induce tumor formation and that the pathway requires additional genetic hits. Interplay between MYD88L265P and other oncogenic events are further supported by the fact that TNFAIP3 (A20) inactivation often accompanies MYD88L265P. These data, combined with the finding that loss of TNFAIP3 promotes accumulation of MYD88L265PB-cells in a mouse model, suggest a potential relationship between these genetic events, and more importantly, a possible clinical impact. The aim of this study was to generate a human cell line model which mimics the effect of MYD88L265P and TNFAIP3 loss to further understand the cellular consequence of these genomic alterations in lymphoma.To mimic the TNFAIP3 loss in combination with the MYD88L265P mutation,we used the WM cell line MWCL1, which harbors a heterozygous mutation in MYD88. For the TNFAIP3 specific knock out, we used the transcription activator-like effector nucleases (TALENs) genome editing approach to induce a double stranded break in exon 5 of the TNFAIP3 gene. TALENs were expressed in MWCL1 cells by transient transfection, and single-cell clones were isolated. Sanger sequencing of the TNFAIP3 gene was performed to identify clones that carried the deletion and all clones were screened by Western blot to confirm knock down of A20.We successfully generated a panel of MWCL1 cell lines that harbor a heterozygous deletion in the TNFAIP3 gene. Western blot analysis confirm a 2.4 fold reduction in A20 expression in the heterozygous cell line (MWCL1A20-/A20+), Because A20 has been shown the be a central negative regulator of NF-κB activation, we first measured the impact of A20 loss on phosphorylation of NF-κB p65 and found that MWCL1A20-/A20+ cells had higher base line levels of pNF-κB compared to the wild type (WT) cell line (1.77 fold). Furthermore, time course stimulation experiments with the TLR ligand lipopolysaccharide (LPS) showed that MWCL1A20-/A20+ cells had a higher amount of pNF-κB after 30 (3 fold) and 60 (1.5 fold) minutes of stimulation. We next looked at expression of NF-κB target genes by quantitative real time PCR and found that IL-6 (2.5 fold; p≤ 0.05) and CXCL10 (IP10) (4 fold; p≤ 0.05) were upregulated in MWCL1A20-/A20+cells compared to WT.In summary, we have established a new WM cell line model which mimics the effect of the MYD88L265P mutation in combination with a heterozygous loss of the TNFAIP3 gene. We show that loss of TNFAIP3 results in a higher baseline and TLR-stimulated phosphorylation of NF-κB as well as in higher expression IL-6 and IP10. Overall, this cell line will be a useful tool to study biological consequences of TNFAIP3loss in WM patients as well as novel therapeutic strategies. DisclosuresAnsell:BMS, Seattle Genetics, Merck, Celldex and Affimed: Research Funding.

Altered Expression of Immune Checkpoint Molecules Including Programmed Cell Death-1 (PD-1) and Its Ligands PD-L1/PD-L2 in Waldenstrom's Macroglobulinemia

Waldenstrom's Macroglobulinemia (WM) is an indolent B-cell malignancy that is characterized by bone marrow infiltration of lymphoplasmacytic cells and excessive synthesis of IgM. Despite improved treatment strategies and increased understanding of the underlying molecular mechanisms contributing to malignant disease progression, WM remains incurable. While whole exome sequencing analysis has revealed the genomic events associated with malignant B cell transformation in WM, the bone marrow (BM) microenvironment is also emerging as a major player in the pathogenesis of WM. Previously, our lab has reported that an array of cytokines, including IL-21 and IL-6 are increased in the WM BM and such increases are associated with high proliferation rates and IgM secretion by WM cells. In this study, we aimed to identify how immune checkpoint molecules in the BM microenvironment, specifically programmed cell death-1 (PD-1) and its ligands PD-L1 and PD-L2, contribute to malignant cell expansion in WM.Using Real-Time PCR analysis, we show that the expression of PD-1, PD-L1 and PD-L2 are all significantly elevated in the cells of WM BM as compared to their normal BM counterparts. Interestingly, PD-L1 expression is also upregulated in the malignant WM cells. These findings were further confirmed by flow cytometry analysis. Subsequent in vitro analysis showed that PD-L1 and PD-L2 are cleaved from the WM cells and these soluble ligands (sPD-L1 and sPD-L2) are able to bind to the cells in the BM microenvironment, including T-cells. This results in reduced activation and proliferation of T-cells, and is accompanied by a reduction in cyclin A and p-ERK levels. In validation of the in vitro data, we also found sPD-L1 and sPD-L2 in the bone marrow plasma obtained from WM patients. While sPD-L2 was found to be present in both WM and normal BM plasma (though in a reduced level in WM), sPD-L1 was exclusive to the WM samples, implying that the function of these molecules are differentially regulated in WM. Furthermore, overexpression of PD-L1 in a WM cell line, MWCL.1, resulted in an increase in IgM secretion. Interestingly, treatment with IL-21 increased the expression of PD-L1 in WM cells, indicating that elevated IL-21 in WM BM could contribute to increased expression of PD-L1 by malignant WM cells.In summary, our data support a role for PD-1 ligands as immune regulatory molecules in WM. Further studies are ongoing to explore the molecular mechanisms by which PD-1/PD-L1/PD-L2 signaling affects the pathophysiology of WM cells. DisclosuresNo relevant conflicts of interest to declare.