PTEN is a negative regulator of PI3K-AKT signaling and a potent tumor suppressor in many types of cancer. To test a tumor suppressive role of PTEN in pre-B acute lymphoblastic leukemia (ALL), we induced Cre-mediated deletion of Pten in mouse models of pre-B ALL. In contrast to its role as a tumor suppressor in other cancers, loss of one or both alleles of Pten caused rapid cell death of pre-B ALL cells and was sufficient to clear transplant recipient mice of leukemia. Small molecule inhibition of PTEN in human pre-B ALL cells resulted in AKT hyperactivation, p53 checkpoint activation and cell death. Loss of PTEN function in pre-B ALL cells was functionally equivalent to acute activation of autoreactive pre-BCR signaling, which engaged a deletional checkpoint for removal of autoreactive B cells. We propose that targeted inhibition of PTEN and hyperactivation of AKT triggers a checkpoint for elimination of autoreactive B cells and represents a new strategy to overcome drug-resistance in human ALL.

Key Points• BCR variable-region mannoses in follicular lymphoma are recognized by lectins of common opportunistic bacteria.• Introduction of N-linked sugars into the BCR variable region interferes with antigen recognition.B-cell antigen receptor (BCR) expression is a key feature of most B-cell lymphomas, but the mechanisms of BCR signal induction and the involvement of autoantigen recognition remain unclear. In follicular lymphoma (FL) B cells, BCR expression is retained despite a chromosomal translocation that links the antiapoptotic gene BCL2 to the regulatory elements of immunoglobulin genes, thereby disrupting 1 heavy-chain allele. A remarkable feature of FL-BCRs is the acquisition of potential N-glycosylation sites during somatic hypermutation. The introduced glycans carry mannose termini, which create potential novel binding sites for mannose-specific lectins. Here, we investigated the effect of N-linked variable-region glycosylation for BCR interaction with cognate antigen and with lectins of different origins. N-glycans were found to severely impair BCR specificity and affinity to the initial cognate antigen. In addition, we found that lectins from Pseudomonas aeruginosa and Burkholderia cenocepacia bind and stimulate FL cells. Human exposure to these bacteria can occur by contact with soil and water. In addition, they represent opportunistic pathogens in susceptible hosts. Understanding the role of bacterial lectins might elucidate the pathogenesis of FL and establish novel therapeutic approaches. (Blood. 2015;125(21):3287-3296)

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Bone marrow transplantation in association with conventional chemotherapy has been used to obtain long-term remissions in acute leukemia and chronic myelogenous leukemia. However, recurrence of leukemia occurs in 10 % -20 % of patients. In the past there have been reports of leukemic relapse developing in the donor cells. This has been determined using cytogenetic analysis. This type of analysis is limited to cases in which there is either a sex difference, a cytogenetic polymorphism or a protein polymorphism. We have used restriction fragmentlength polymorphisms of DNA from donor and recipient cells to evaluate the origin of leukemic cells arising following bone marrow transplantation.Two cases are presented. The first was a case of leukemic relapse following bone marrow transplantation for acute myelogenous leukemia between HLAidentical brothers. There were no useful cytogenetic markers available in this case. The second was a case of leukemic relapse following bone marrow transplantation for acute lymphoblastic leukemia, between HLA-identical brother and sister. Cytogenetic analysis of bone marrow revealed only donor-type karyotypes (70 metaphases analyzed). Informative restriction fragment-length polymorphisms were identified for both patients. In both cases we found that T lymphocytes and granulocytes were of donor origin while the leukemic cells were of recipient origin.

1582 Poster Board I-608 Background Traditional AML prognostic markers are based on clinical characterization (e.g. age) or static measurements of leukemia biology present at diagnosis, such as cytogenetics and isolated molecular events (e.g. presence of FLT3 ITD mutation). No validated methods currently exist to predict the disease response to standard AML induction chemotherapy for individual patients. Objectives: Single Cell Network Profiling (SCNP) was used to measure intracellular signaling in response to extracellular modulators in order to develop a new proteomic tool to characterize and monitor AML biology in the context of therapeutic applications. Methods Modulated SCNP using a multiparametric flow cytometry platform was performed evaluating the phosphorylation of intracellular signaling molecules in their basal states and after treatment with modulators in specific cell populations (e.g. leukemic cells). Since multiple signaling pathways may be dysregulated in AML and contribute to the likelihood of response to a given therapy, pathways that affect proliferation, apoptosis, and DNA damage were analyzed. Analyses were aimed to assess assay reproducibility, identify a signaling profile associated with likelihood of response to standard induction chemotherapy (first training set, n=34), and test extrapolation of the identified profile to a fully independent set of AML samples (second training set; n=88). Results High assay reproducibility (Pearson correlation coefficients ≥ 0.8) was observed. In the first training study univariate analysis revealed multiple “nodes” (modulated read outs of proteins in signaling pathways) associated with disease response to conventional induction therapy (i.e. AUC of ROC >0.66; p<0.05). Importantly combination of some of the independently predictive nodes improved disease response stratification (AUC of ROC up to 1.0; p<0.05). Extrapolation of the assay to a second independent set of samples revealed similar findings after accounting for clinical covariates. Specifically, for patients <60 years, the presence of intact apoptotic pathways was correlated with complete response (CR) while in samples from patients ≥60 years increased p-Akt and p-Erk levels in response to FLT3L stimulation correlated with non response (NR). Importantly, the predictive values of these nodes was independent from cytogenetic and FLT3 mutational status. Conclusions The two studies reported here show that AML biology characterization in individual patients using modulated SCNP can be performed with high technical accuracy and reproducibility to quantitatively characterize the biology of AML. This approach can be used to generate highly predictive tests for therapeutic response independently of classic prognostic factors. Disclosures Kornblau: Nodality, Inc.: Consultancy. Rosen:Nodality, Inc.: Employment, Equity Ownership. Putta:Nodality, Inc.: Employment, Equity Ownership. Cohen:Nodality, Inc.: Employment, Equity Ownership. Covey:Nodality, Inc.: Employment, Equity Ownership. Fantl:Nodality, Inc.: Employment, Equity Ownership. Gayko:Nodality, Inc.: Employment, Equity Ownership. Cesano:Nodality, Inc.: Employment, Equity Ownership.