Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia

B cells are selected for an intermediate level of B cell receptor (BCR) signaling strength: Attenuation below minimum (e.g. non-functional BCR)1 or hyperactivation above maximum (e.g. self-reactive BCR)2–3 thresholds of signaling strength causes negative selection. In ~25% of cases, acute lymphoblastic leukemia (ALL) cells carry the oncogenic BCR-ABL1 tyrosine kinase (Ph+), which mimics constitutively active pre-BCR signaling4,5. Current therapy approaches are largely focused on the development of more potent tyrosine kinase inhibitors to suppress oncogenic signaling below a minimum threshold for survival6. Here, we tested the hypothesis that targeted hyperactivation above a maximum threshold will engage a deletional checkpoint for removal of self-reactive B cells and selectively kill ALL cells. Testing various components of proximal preBCR signaling, we found that an incremental increase of Syk tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive Syk was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in ALL cells. Unlike normal pre-B cells, patient-derived ALL cells express the inhibitory receptors PECAM1, CD300A and LAIR1 at high levels. Genetic studies revealed that Pecam1, Cd300a and Lair1 are critical to calibrate oncogenic signaling strength through recruitment of the inhibitory phosphatases Ptpn67 and Inpp5d8. Using a novel small molecule inhibitor of INPP5D9, we demonstrated that pharmacological hyperactivation of SYK and engagement of negative B cell selection represents a promising new strategy to overcome drug-resistance in human ALL. Acute lymphoblastic leukemia (ALL) represents the most frequent type of cancer in children and is frequent in adults as well. While outcomes for patients with ALL have greatly improved over the past four decades, ALL driven by oncogenic tyrosine kinases (BCR-ABL1 in adults and other oncogenic fusion tyrosine kinases in childhood ALL)10 remains a clinical problem. Current efforts to improve treatment options are largely focused on the development of more potent tyrosine kinase inhibitors (TKI). However, responses to TKI are often short-lived. Our group recently identified upregulation of the BCL6 protooncogene in response to TKI-treatment as a major mechanism of drug-resistance in Ph+ ALL11. Here, we propose a strategy to overcome drug-resistance in ALL based on pharmacological hyperactivation of SYK. Pre-BCR signals are initiated from immunoreceptor tyrosine-based activation motifs (ITAM) in the cytoplasmic tail of Igα (CD79A) and Igβ (CD79B) signaling chains12 and essential for survival and proliferation of normal pre-B cells. However, hyperactive Chen et al. Page 2 Nature. Author manuscript; available in PMC 2015 November 21. A uhor M anscript