ALBERT

Description: Key Points The incidence of venous thromboembolism is high in patients with a solid tumor and implanted port in the real-life practice setting. The risk factors for catheter-related thrombosis differ from those for venous thromboembolism unrelated to the catheter.

Description: Current precision medicine approaches typically target a single genetic mutation. However, adult acute myeloid leukemia (AML) is difficult to treat due to its genetic complexity. Approximately 30 somatic mutations have been found to be recurrent, with an average of 5-15 mutations present per patient (Cancer Genome Atlas 2013). Therefore, combinatorial genetic approaches have the power to uncover novel gene targets that can be used to tailor therapies to an individual's unique mutational spectrum. Mutations in NPM1 commonly occur in in AML. Approximately 25-30% of patients harbor a specific variant of the NPM1 mutation referred to as NPM1cA, which results in mislocalization of the Npm1 protein from the nucleus to the cytoplasm (Cancer Genome Atlas 2013; Falini 2005). While NPM1cA is considered to be a driver of AML development, mice with this mutation develop AML with a long latency (18 months) and with incomplete penetrance (Vassiliou 2011). In addition, those mice that do develop AML acquire additional genetic mutations, suggesting that NPM1cA cooperates with serially acquired mutations to drive AML development (Dovey 2017). It was recently discovered that mutations in the cohesin complex (consisting of the genes STAG2, SMC1A, SMC3, and RAD21) are also common in AML, with mutation in any of the four components resulting in haploinsufficiency (Cancer Genome Atlas 2013). Cohesin haploinsufficiency is enriched in patients with NPM1 mutations. Although cohesin mutations alone are insufficient to generate AML in mice (Viny 2015), they do result in increased hematopoietic stem and progenitor cell (HSPC) self-renewal (Mazumdar 2015; Fisher 2017). As the cohesin complex has known roles in chromosomal organization and the regulation of gene expression, we hypothesized that cohesin mutations would cooperate with NPM1cA to uniquely alter gene expression, resulting in AML. We crossed inducible NPM1cAflox/+ and SMC3flox/+ mouse models to examine this genetic interaction. Our current data show that the double mutant mice develop AML with increased penetrance compared with the Npm1cA/+ mice alone, with a trend toward decreased latency. The double mutant HSPCs also exhibit increased self-renewal in vitro compared to the Npm1cA/+ or SmcΔ/+ single mutants. To examine changes in gene expression, we performed RNA sequencing on lineage-depleted bone marrow in 3 mice from each genotype (WT, Npm1cA/+, Smc3Δ/+, and double) 4 weeks post excision. Consistent with our hypothesis, additive changes in gene expression were not observed. Instead, a unique set of genes were found to be deregulated in Npm1cA/+; Smc3Δ/+ marrow. In an effort to specifically target Npm1cA/+; Smc3Δ/+ mutant AML, we screened our list of uniquely-affected genes for those associated with AML. We found DOCK1 to be overexpressed in the double, but not Npm1cA/+ single, cells. High expression of this gene has been correlated with decreased overall and disease-free survival in AML patients (Lee 2017). To determine if DOCK1 contributes to the enhanced cell growth observed in vitro in our leukemic lines, we used an inhibitor that targets Dock1. This inhibitor induced apoptosis in our double leukemic cell lines but was less effective in our NPM1 single mutant or WT cells. Additionally, no effect was observed with a genetically unrelated AML line, MLL-AF9. Similarly, shRNA-mediated knockdown of Dock1 resulted in decreased cell viability in Npm1cA/+; Smc3Δ/+ leukemic lines but not in Npm1cA/+ only lines. We thus hypothesize that Dock1 represents a unique target for the treatment of patients harboring the Npm1/Cohesin mutational combination. Our results provide validity to the concept that combinatorial genetics can be used to target the unique genetic landscape of an individual patient. Future studies will focus on the impact of Dock1 inhibition in human Npm1/Cohesion mutated AML cell lines. Disclosures Vassiliou: KYMAB: Consultancy, Equity Ownership; Celgene: Research Funding. Levine:Isoplexis: Equity Ownership; Epizyme: Patents & Royalties; Imago: Equity Ownership; Loxo: Consultancy, Equity Ownership; Janssen: Consultancy, Honoraria; Gilead: Honoraria; Prelude: Research Funding; Novartis: Consultancy; Celgene: Consultancy, Research Funding; Roche: Consultancy, Research Funding; C4 Therapeutics: Equity Ownership; Qiagen: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Description: Upfront resistance to glucocorticoids (GCs) confers a poor prognosis for children with T-cell acute lymphoblastic leukemia (T-ALL). Using primary diagnostic samples from the Children's Oncology Group trial AALL1231, we previously demonstrated that one-third of patient T-ALL samples are intrinsically resistant to GCs when cultured in the presence of interleukin-7 (IL7), a cytokine that is abundant in the T-ALL microenvironment. Furthermore, we demonstrated that inhibiting JAK/STAT signaling downstream of the IL7 receptor (IL7R) with the JAK1/2 inhibitor ruxolitinib (RUX) overcomes GC resistance in these samples. The objective of the present study was to determine the mechanism of IL7-induced GC resistance in T-ALL and to identify novel therapeutic targets to enhance GC sensitivity. We utilized CCRF-CEM cells, a human T-ALL cell line, as a model system in conjunction with primary patient samples. Exposing CCRF-CEM cells to IL7 induced phosphorylation of STAT5, the predominant downstream effector of IL7R signaling. When cultured in the presence of IL7 and the GC dexamethasone (DEX), CCRF-CEM cells recapitulated the IL7-induced GC resistance phenotype observed in patient samples. This resistance could be overcome with RUX, and Bliss index analysis demonstrated a synergistic relationship between DEX and RUX in the presence of IL7. Furthermore, CRISPR/Cas9 mediated knockout of STAT5 (STAT5 KO) was sufficient to overcome resistance, implicating STAT5 as the critical mediator of IL7-induced GC resistance. DEX exposure potently induced upregulation of IL7R expression in CCRF-CEM cells. Using a luciferase reporter construct containing a series of STAT5 response elements, we demonstrated that in the presence of IL7, DEX-induced upregulation of IL7R expression is associated with increased downstream signal transduction, leading to a significant increase in STAT5 transcriptional output. We then performed RNA-seq to further assess the functional consequences of this enhanced STAT5-mediated transcription. Gene set enrichment analysis (GSEA) revealed that STAT5 target genes were significantly upregulated in cells exposed to DEX and IL7 relative to IL7 alone (normalized enrichment score -2.27; p 〈 0.001; FDR 〈 0.001), suggesting that DEX exposure augments activation of the STAT5 transcriptional program. One critical component of this program that was induced by the combination of DEX and IL7 was the anti-apoptotic family member BCL2, which was not induced by DEX alone. Further analysis of its protein expression in CCRF-CEM cells confirmed this paradoxical upregulation of BCL2 specifically by the combination of DEX and IL7. Furthermore, BCL2 was not upregulated by DEX and IL7 in STAT5 KO cells, consistent with this being a STAT5-mediated effect. IL7-induced GC resistance could be overcome with shRNA-mediated knockdown of BCL2 and with pharmacologic inhibition of BCL2 by venetoclax. Similar to the effect observed with RUX, Bliss index analysis demonstrated synergy between DEX and venetoclax in the presence of IL7. Consistent with our observations in CCRF-CEM cells, an analysis of primary diagnostic T-ALL samples revealed DEX-induced upregulation of IL7R expression in samples with IL7-induced GC resistance, which was associated with increased BCL2 expression in the presence of DEX and IL7. Finally, we performed a similar analysis in healthy murine thymocytes, and found that CD4/CD8 double negative (DN) and CD4 or CD8 single positive (SP) thymocytes, but not double positive (DP) thymocytes, exhibited profound IL7-induced GC resistance that was associated with DEX-induced upregulation of IL7R expression and increased BCL2 expression in the presence of DEX and IL7. These data are consistent with the known role of IL7 specifically at the DN and SP stages of development, and suggests that IL7-induced GC resistance is a physiologic mechanism of GC resistance in normal thymocyte development that is retained during leukemogenesis in a subset of T-ALL samples. Taken together, these data demonstrate that GCs paradoxically induce their own resistance in a subset of T-ALLs and in normal developing T-cells by augmenting a STAT5-mediated pro-survival program that results in upregulation of BCL2. Furthermore, we demonstrate that inhibition of JAK/STAT signaling or of BCL2 may have considerable therapeutic benefit to enhance GC sensitivity in T-ALL patients with IL7-induced GC resistance. Disclosures Teachey: La Roche: Consultancy; Amgen: Consultancy.

Description: Myeloid transformation occurs via stepwise acquisition of driver mutations that have distinct positions in the clonal hierarchy. BCOR mutations are common in myelodysplastic syndromes (MDS), but rarely occur as initiating events, instead typically arising in the context of other pre-existing mutations, such as TET2. BCOR encodes a subunit of the Polycomb Repressive Complex 1.1 (PRC1.1) which mediates gene repression by regulating H2AK119 via RING1 ubiquitin ligase activity. We hypothesized that BCOR mutations exert context dependent effects on chromatin regulation, thereby mediating conditional clonal selection during disease progression. We used CRISPR/Cas9 genome editing to establish an isogenic human leukemia cell line model harboring BCOR, TET2, or concurrent BCOR and TET2 mutations. Whereas TET2 deficiency alone caused no effect on cell growth properties, isolated loss of BCOR caused a significant decrease in cell proliferation. Concomitant inactivation of TET2 and BCOR reversed the negative impact of BCOR mutations and resulted in enhanced proliferation suggesting that TET2 and BCOR deficiencies cooperate to drive a distinct cell growth phenotype that is not evident when either gene is knocked out alone. To define the impact of BCOR deficiency on PRC1.1 complex assembly we performed PCGF1 co-immunoprecipitation followed by mass spectrometry (IP/MS) in cells with and without BCOR deficiency. BCOR mutations caused a marked decrease in PRC1.1 complex abundance without disrupting complex integrity. Even in the absence of BCOR, PCGF1 interactions with other PRC1 components (RING1A/B, SKP1, KDM2B, USP7) were maintained. By contrast, reciprocal BCOR IP/MS in cells with and without PCGF1 deletion showed that PCGF1 is required for RING1A/B and RYBP binding, but does not regulate complex abundance. These data suggest that BCOR is critical for maintaining PRC1.1 stability while PCGF1 selectively recruits core enzymatic components to the complex. To evaluate the role of PRC1.1 enzymatic function in regulating target loci, we performed RNA-seq and ChIP-seq analysis in PCGF1 deficient cells that lack PRC1.1-RING1 interactions. 931 genes were significantly upregulated (FC〉2, padj≤0.05) in PCGF1 deficient cells compared to control. In wild type cells, these genes were bound by PRC1 (RING1B) and PRC2 (SUZ12) and marked by high levels of repressive histone marks (H3K27me3, H2AK119ub). In PCGF1 deficient cells, RING1B binding and repressive marks were lost at these loci, while BCOR binding was maintained and the active H3K27ac mark significantly increased. These data suggest that selective loss of PRC1.1 enzymatic activity causes derepression of PRC1.1 target loci. To determine whether derepression of PRC1.1 targets underlies the cooperative phenotypic effect of TET2 and BCOR inactivation, we compared the transcriptional and epigenetic state of BCOR/TET2 deficient cells to that of control, TET2, and BCOR deficient cells. Indeed, a subset of genes that were derepressed in PCGF1 deficient cells also showed increased expression, loss of repressive and gain of activating marks in BCOR-TET2 deficient cells, but not in cells with sole BCOR or TET2 deficiency. Derepressed genes were specifically enriched for involvement in developmental processes and cell signaling, including the leukemia-associated HOXA gene cluster. In contrast to PCGF1 deficient cells, BCOR deficient cells also displayed significant transcriptional downregulation of genes involved in metabolism and cell cycle progression, consistent with the negative effect of BCOR mutations on cell growth properties. In wild type cells, these BCOR-dependent loci had high levels of PRC1.1 (BCOR) binding, active histone marks, and high chromatin accessibility. Remarkably, expression of nearly all (590/622) BCOR-dependent genes was restored after introduction of concurrent TET2 deletion. Our data suggest that the BCOR-PRC1.1 complex fulfills a dual epigenetic role via RING1-dependent regulation of gene repression at some target loci and RING1-independent maintenance of active transcription at other loci. Accordingly, inactivation of BCOR alone is sufficient to derepress key leukemia-associated targets, but incurs a metabolic liability that limits transformation potential. Concurrent inactivation of TET2 may therefore enable transformation by attenuating negative impacts of BCOR mutations on active transcription. Disclosures No relevant conflicts of interest to declare.

Description: Application of autologous T cells genetically engineered to express CD19-specific chimeric antigen receptors (CAR-T) is highly effective in the treatment of B cell malignancies. To this date, application of CAR-T therapy beyond CD19 remains challenging due to the inability to control CAR-T reactivity in patients and the lack of tumor-associated antigens exclusively expressed by malignant cells. The interleukin-3 receptor alpha chain (CD123) is a promising immunotherapeutic target and associated with leukemia-initiating compartments in myeloid- or lymphoid derived diseases. However, in contrast to CD19, CD123 is a precarious target due to its prevalent expression on healthy hematopoietic stem and progenitor cells (HSPC) as well as endothelial cells. Thus, CAR-T lacking any fine-tuned control mechanisms are at risk to cause life threatening toxicities or can only act as bridging therapy to an allogeneic stem cell transplantation. To extend application of CAR-T therapy and safely redirect CAR-engineered T cells to challenging targets such as CD123, a switch-controllable universal CAR-T platform (UniCAR) was recently introduced. The UniCAR system consists of two components: (1) a non-reactive inducible second generation CAR with CD28/CD3ζ stimulation for an inert manipulation of T cells (UniCAR-T) and (2) soluble targeting modules (TM) enabling UniCAR-T reactivity in an antigen-specific manner. Here we provide late stage pre-clinical data for UniCAR-T in combination with a CD123-specific TM (TM123) for treatment of acute leukemia. Primary patient-derived CD123-positive leukemic blasts were efficiently eradicated by TM123-redirected clinical-grade manufactured UniCAR-T in vitro and in vivo. Activation, cytolytic responses and cytokine release were proven to be strictly switch-controlled. Moreover, anti-leukemic responses of UniCAR-T were demonstrated to be comparable to conventional CD123-specific CAR-T in vitro. In contrast to conventional CD123 CAR-T, TM123-redirected UniCAR-T discriminate between CD123high malignant cells and CD123low healthy cells with negligible toxicity towards HSPC in vivo. As 4-1BB mediated co-stimulation is known to enhance CAR-T activity in vivo, a novel CD123-specific targeting module bearing a covalently bound trimeric 4-1BB ligand (4-1BBL) was developed and characterized for co-stimulation at the leukemic site in trans. Specific binding of TM123-4-1BBL was demonstrated against native 4-1BB as well as CD123-positive leukemic blasts. In long-term tumor eradication models, TM123-4-1BBL ameliorated the killing capability of UniCAR-T in vitro. Additionally, the increased hydrodynamic radius of trimeric 4-1BBL-coupled TM123 prolonged plasma half-life and enhanced bioavailability in vivo. In conclusion, UniCAR-T maintain high anti-leukemic efficacy, while adding a sophisticated mechanism for immediate control to improve safety and versatility of CD123-directed CAR-T therapy. Moreover, switching between several TMs from short to moderate plasma half-life allows for an individualized treatment of various leukemic settings while minimizing potential adverse effects. Disclosures Loff: GEMoaB Monoclonals GmbH: Employment. Meyer:Cellex Patient Treatment GmbH: Employment. Dietrich:Cellex Patient Treatment GmbH: Employment. Spehr:Cellex Patient Treatment GmbH: Employment. Julia:Cellex Patient Treatment GmbH: Employment. Gründer:GEMoaB Monoclonals GmbH: Employment. Franke:GEMoaB Monoclonals GmbH: Employment. Bachmann:GEMoaB Monoclonals GmbH: Equity Ownership. Ehninger:Bayer: Research Funding; GEMoaB Monoclonals GmbH: Employment, Equity Ownership; Cellex Gesellschaft fuer Zellgewinnung mbH: Employment, Equity Ownership. Ehninger:GEMoaB Monoclonals GmbH: Employment, Equity Ownership; Cellex Gesellschaft fuer Zellgewinnung mbH: Equity Ownership. Cartellieri:Cellex Patient Treatment GmbH: Employment.

Description: Background Pediatric classical Hodgkin lymphoma (cHL) is a clonal disorder in an inflammatory background, also known as the microenvironment. This microenvironment is of major importance for growth and survival of the malignant Hodgkin/Reed Sternberg (HRS) cells. HRS cells and the microenvironment communicate through chemo- and cytokines. Blood biomarkers result from this active crosstalk, and may be a surrogate for lymphoma viability (Steidl et al, JCC 2011). Blood biomarkers are important because they hold the promise to be easily available and cost-effective. One promising biomarker in adult patients with cHL is the "Thymus and Activation-Regulated Chemokine, TARC (Plattell et al, Haematologica 2012). Elevated TARC levels are also described in patients with atopic dermatitis (Hijnen et al, J All Clin Immunol 2004). In adult cHL patients about 85% of patients have significantly elevated levels of TARC in pre-treatment serum or plasma compared to healthy controls (Plattell et al). So far nothing is known about TARC in pediatric cHL patients. To define its value as a diagnostic marker in pediatric cHL patients, we compared TARC levels of pediatric cHL patients with control patients. This study was IRB-approved and registered under Dutch Trial registry number 6876. Methods After providing informed consent, plasma and serum samples were collected of newly diagnosed cHL patients before start of treatment. To define normal values of TARC in children, samples were collected from non-cHL randomly selected patients from the hematology, endocrinology and oncology outpatient clinic. Baseline characteristics including history of atopic dermatitis were collected. These control patients were divided in three age groups (0-9,10-14 and 15-18 years). TARC levels were measured by enzyme-linked immunosorbent assay (R&D systems, Human CCL17/TARC Quantikine ELISA Kit). TARC levels of the cHL patients were compared to the control group to obtain ROC curves and calculate the AUC, cross-validated sensitivity and specificity and accuracy of TARC as a diagnostic marker. We hypothesized that pediatric cHL patients had elevated pretreatment TARC levels in both serum and plasma. Analyses were done using SAS V9.4. Results Fourteen cHL patients were included with a median age of 14 (range 11-17) years. Ten (71.4%) were female. Eighty patients were included in the control group with a median age of 12 (range 10 months-18) years. Twenty-nine patients (36.3%) were included in age group 0-9, 25 (31.2%) in age group 10-14 and 26 (32.5%) in age group 15-18. Thirty-nine (48.8%) were female. Patients of the control group had a median TARC value of 71 (range 18-762) pg/ml for plasma and 318 (range 27-1300) pg/ml for serum. TARC plasma and serum levels decreased with age (Spearman correlation -0.26, 2-tailed p=0.0204), but there were no statistically significant pairwise comparisons found between the pre-specified age groups. In the eight control patients (10%) with atopic dermatitis no significantly higher plasma and serum levels were found (plasma median with eczema 97 versus 70 pg/ml without eczema (p=0.71) and serum median with eczema 643 versus 317 pg/ml (p=0.71)). Plasma was collected in 14 cHL patients, and all had elevated TARC levels, with a median plasma level of 18449 (range 1635-55821) pg/mL. Serum samples were collected in 8/14 cHL patients and all had elevated serum TARC levels. Median serum level: 46703 (range 12817-149739) pg/ml. The plasma TARC levels of cHL patients were significantly higher than those of the control group patients (p

Description: MYC amplification (amp) is a marker of poor prognosis in many non-hematologic malignancies. While MYC translocations in B cell lymphoma (BCL) have been extensively studied, little is known about the significance of MYC amp. Recent studies describe increased MYC copy numbers (3-10 copies/cell) to be associated with more aggressive BCL. The WHO 2017 does not include MYC amp in the definition of high-grade BCL (HGBCL) with MYC and BCL2 and/or BCL6 rearrangement ("double-hit lymphoma", DHL). However, it also states that high-level MYC amp occurring together with a MYC rearrangement, and concurrent with BCL2 rearrangement likely has a similar clinical impact as classic DHL. Although increased MYC copy number is commonly identified by routine fluorescence in situ hybridization (FISH) testing in BCL, the experience of our large cytogenetics reference laboratory suggests that high-level MYC amp, defined as uncountable MYC signals, is far less common. Therefore, we sought to characterize the clinical, pathologic and cytogenetic features of patients with BCL showing high-level MYC amp. The Mayo Clinic cytogenetic database was retrospectively reviewed for all cases of BCL with high-level MYC amp seen by FISH from January 2010 - February 2018. All FISH studies reported as MYC amp were re-reviewed by a cytogeneticist to verify the level of amp and the MYC probes involved. Pathology was reviewed by two independent hematopathologists. Clinical information was collected through chart review. Survival analysis was performed using Kaplan-Meier curves and the Wilcoxon rank-sum test. FISH analysis for MYC aberrations identified 44/9715 (0.45%) cases with high-level MYC amp. Of cases with available H&E, the most common morphology was diffuse large BCL (DLBCL) (82%; 28/34), followed by HGBCL (15%; 5/34) and plasmablastic BCL (3%; 1/34). Hans cell of origin (COO) algorithm immunohistochemistry (IHC) identified 21/25 (84%) germinal center B-cell-like (GCB), and 4/25 (16%) non-GCB cases. 21/27 (78%) cases were BCL2+ by IHC. MYC+ by IHC was ≥40% in 21/28 (75%) and

Description: A 15-year-old African-American male with a history of thrombosis was examined for the Factor V HR2 sequence variation (c.3980A〉G, p.H1327R) using PCR followed by RsaI digestion. An anomalous restriction pattern was observed: one strand was uncut (wild type) and the other was cleaved once. However, the fragment sizes were 516bp and 312bp instead of the 447bp and 381bp expected from HR2. Sanger sequencing of the PCR product detected a sequence variant (c.3845A〉G) in this patient that creates a RsaI site explaining the novel band pattern. The c.3845A〉G variant is found within one of four 27bp repeats found in exon 13 of the F5 gene. The HR2 variant is also present in one of the repeats. In this patient's case, the cleavage occurred in an abnormal location (c.3845A〉G instead of c.3980A〉G), creating the novel banding pattern observed. Six additional patients being tested for hypercoagulable states were evaluated with the FV HR2 assay and were found to have the same abnormal restriction pattern. The ethnicity of these other patients was not available to our laboratory. Sanger sequencing confirmed the presence of c.3845A〉G sequence variant in each of these patients. Unlike the HR2 and FV Leiden variants, the c.3845A〉G, p.H1282R variant (rs143333036) is most common in African populations (Minor Allele Frequency 0.012) but the clinical consequences of this variant are unclear. The variant lies within a 27bp repeat similar to the HR2 site and might cause it to be associated with a thrombotic risk similar to the HR2 variant. Further investigation will be required to determine whether the c.3845A〉G variant is a population variant with no clinical consequences or whether it represents a genetic prothrombotic risk factor in certain patient populations. Nevertheless, in the laboratory the c.3845A〉G, p.H1282R sequence variant may confound results when testing patients for the HR2 (rs1800595) variation. Disclosures No relevant conflicts of interest to declare.

Description: Background ADAMTS13 circulates in a folded conformation, which is mediated by interactions between the C-terminal CUB domains and its central Spacer domain. Binding of ADAMTS13 to the VWF D4-CK domains disrupts the CUB-Spacer interaction, inducing a structural change that extends ADAMTS13 into an open conformation that enhances catalytic efficiency ~2-fold. This mechanism supports a model in which ADAMTS13 unfolding induces exposure of an exosite in the Spacer domain that interacts with the VWF A2 domain, increasing the affinity between the two molecules, and, therefore, the rate of proteolysis. The D4-CK-mediated conformational activation of ADAMTS13 can be mimicked in vitro with the use of antibodies that disrupt the CUB-Spacer interaction, such as the previously published anti-CUB antibody, Ab17G2. We recently generated a novel, activating antibody against the Spacer domain (Ab3E4). Aim To characterize the mechanism by which the Ab17G2 and Ab3E4 enhance the catalytic efficiency of ADAMTS13. Methods The effects of the Ab17G2 and Ab3E4 on the activity of ADAMTS13 were studied using FRETS-VWF73. The effects of the Ab17G2 and Ab3E4 on the kinetics of VWF96 (VWF G1573-R1668) proteolysis were characterized using an in-house assay. ELISA was used to investigate conformational changes in ADAMTS13 induced by the Ab17G2 and Ab3E4. Results Both Ab17G2 and Ab3E4 enhanced FRETS-VWF73 proteolysis by ~1.7-fold. This result was reproduced using the VWF96 substrate; the Ab17G2 and Ab3E4 enhanced the catalytic efficiency (kcat/Km) of ADAMTS13 by ~1.8- and ~2.0-fold, respectively. The activation was dependent on the conformational extension of ADAMTS13, since the antibodies could not enhance the activity of an ADAMTS13 variant that lacks the TSP2-CUB2 domains (MDTCS). Surprisingly, ADAMTS13 activation was not mediated through exposure of the Spacer or Cys-rich domain exosites as previously proposed, as the Ab17G2 and Ab3E4 efficiently enhanced proteolysis of VWF96 variants in which the Spacer/Cys-rich exosite binding sites were disrupted. Kinetic analysis of VWF96 proteolysis showed that the Ab17G2- and Ab3E4-induced activation of ADAMTS13 is primarily manifest through a ~1.5- to ~2-fold increase in enzyme turnover (kcat). Thus, contrary to the current model, this suggests that the conformational extension of ADAMTS13 influences the functionality of the active site, and not substrate binding affinity (Km). Incubating ADAMTS13 with either Ab17G2 or Ab3E4 exposed a cryptic epitope in the metalloprotease domain that was specifically detected by ELISA, further corroborating that the antibodies induce a conformational change in ADAMTS13 affecting the M domain. Conclusion Antibodies can be used as tools for understanding the structure/function of enzymes. Using activating antibodies against the Spacer and CUB1 domains of ADAMTS13, we show for the first time that the activation of ADAMTS13 following its unfolding is not a result of exposure of a functional exosite in Spacer/Cys-rich domain that increases affinity to VWF. Rather, our data are consistent with an allosteric activation mechanism upon the metalloprotease domain. We propose that ADAMTS13 unfolding causes a conformational change in the active site that further activates the enzyme. We are currently investigating whether the D4-CK-induced enhancement of ADAMTS13 proteolytic activity is also mediated by conformational changes in the active site. Disclosures Vanhoorelbeke: Ablynx: Consultancy; Shire: Consultancy.

Description: Chromosomal rearrangements of the MLL gene are responsible for 5-10% of all acute leukemias, biphenotypic leukemias and myelodysplastic syndromes. The large number of known MLL fusions (〉80) renders a precise diagnosis a demanding task. Even though all MLL rearrangements are associated with high-risk acute leukemia, the outcome (poor or very poor) is influenced by the partner gene. The applied diagnostic methods (LDI-PCR and multiplex PCR) allows the identification of MLL fusion genes at the nucleotide level, providing important information on the genetics of leukemia patients, and patient-specific biomarkers. These biomarkers are used for monitoring of minimal residual disease in acute leukemia patients during and after therapy. Thus, the identification of MLL gene fusions is necessary for rapid clinical decisions to determine the best therapy regimen. We have developed a customized NGS panel for MLL diagnostics to utilize state of the art technology at DCAL. With this new tool, the whole MLL gene is analyzed in contrast to the LDI-PCR where only the main MLL breakpoint cluster region (BCR-1) is covered. The first results of the NGS analysis of 84 patients identified MLL breakpoints located outside the main BCR-1 of MLL. Furthermore, a novel MLL partner gene USP2 was identified in 16 patients. All MLL-USP2 positive patients had a breakpoint located outside BCR-1 and within a newly defined breakpoint cluster region BCR-2. The BCR-2 site was also used in 2 other patients with MLL-AFF1 and one patient with MLL-MLLT3. These findings reveal USP2 as a new entity for MLL rearrangements affecting indifferently children aged 3 months to 10 years old (mean 30 months) with no gender bias (M/F=1.3). Interestingly, only 5/16 affected children were below 1 year of age at diagnosis and thus treated according to the Interfant trial. Clinical presentation as well as outcome associated with this new entity deserves further investigation to define whether those patients should be allocated, as other MLL-rearranged ones, in high-risk treatment groups. More MLL patients should also be analyzed to get a better idea of the frequency of breakpoints within BCR-2, especially the frequency of MLL-USP2 fusions. Indeed, standard FISH analysis and CGH array do not permit reliable detection of this fusion, explaining why they remained undetected so far. The biology of this novel MLL rearrangement also deserves further investigation, considering that USP2 is the only MLL partner fused exclusively to BCR-2. Disclosures No relevant conflicts of interest to declare.