Inhibitory Receptors and Phosphatases Enable Oncogenic Tyrosine Kinase Signaling In B Cell Lineage Leukemia

B cells are selected at multiple developmental checkpoints for an intermediate level of (pre-) B cell receptor (BCR) signaling strength: either insufficient or hyperactive signaling (e.g. from an autoreactive BCR) results in cell death. Acute lymphoblastic leukemia (ALL) is the most frequent type of cancer in children and typically arises from pre-B cells, a large fraction of which are autoreactive. In ∼25% of patients, ALL is driven by an oncogenic tyrosine kinase (e.g. BCR-ABL1 in Ph⁺ ALL) and defines the ALL subgroup with the worst clinical outcome. Ph⁺ ALL cells invariably develop resistance against tyrosine kinase inhibitors (TKI). Here we tested the hypothesis that inherent mechanisms of negative selection to eliminate autoreactive clones with hyperactive pre-BCR signaling are still active in transformed pre-B cells and identified a potential therapeutic target for ALL patients.

The BCR-ABL1 oncogene mimics a constitutively active pre-BCR and an incremental increase of pre-BCR downstream signaling (ITAM overexpression) was indeed sufficient to induce cell death in Ph⁺ ALL, but not in normal pre-B cells with low baseline signaling strength. TKI-treatment, while designed to kill leukemia cells, seemingly paradoxically rescued Ph⁺ ALL cells in this experimental setting.

Patient-derived Ph⁺ ALL cells differ from normal pre-B cells by expression of high levels of ITIM containing inhibitory receptors including PECAM1, CD300A and LAIR1. However, ITAM containing activation receptors like CD79B was absent on the cell surface, and there was point or frame-shift mutation for both CD79A and CD79B. Importantly, high expression levels of ITIM-receptors are predictive of poor outcome in two clinical trials. In the COG trial (P9906; n=207) for children high-risk ALL, mRNA levels of PECAM1, CD300A and LAIR1 at diagnosis positively correlated with early minimal residual disease (MRD) findings on day 29 (p<0.0005), and negatively correlated with overall survival (OS) rate (p<0.02) or relapse free survival (RFS) rate (p<0.05). In the ECOG trial (E2993; n=215) for adults ALL, PECAM1 mRNA level negatively correlated with OS rate (p=0.0285).

Genetic studies revealed that Pecam1, Cd300a and Lair1 receptors are critical to calibrate oncogenic signaling strength and prevent excessive tyrosine kinase activity through recruitment of the inhibitory phosphatases Ptpn6 (SHP1) and Inpp5d (SHIP1). Deletion of Pecam1, Cd300a or Lair1 in Ph+ ALL cells caused increased ROS levels, accumulation of p53, p21 and p27 protein, G0/G1cell cycle arrest and cellular senescence. P-STAT5 and p-ERK were increased after Lair1 deletion. Transplant experiments indicated that Lair1 deletion resulted in rapid leukemia regression and prolonged survival of recipient mice. Leukemia cell death caused by Lair1 deletion could be rescued by overexpression of the inhibitory phosphatase Ptpn6 (SHP1) or Inpp5d (SHIP1). Genetic deletion of Ptpn6 and Inpp5d caused cell death in BCR-ABL1 ALL cells but neither in normal pre-B cells with weak baseline signaling, nor myeloid cells (normal and BCR-ABL1-transformed), which -unlike B cells- are not subject to negative selection of auto-reactive clones. To test if these findings are also relevant to other subtypes of ALL, we used a model for NRAS^G12D-driven ALL. BCR-ABL1 (∼25%) and NRAS lesions (∼30%) account for more than half cases of ALL. Consistently, NRAS^G12D Pecam1^-/-, Cd300a^-/- leukemia cells were prone to G0/G1 cell cycle arrest and cellular senescence (p<0.01).

Using chimeric PECAM1, CD300A and LAIR1 receptor decoys and a novel small molecule inhibitor of INPP5D (SHIP1), we demonstrate that inhibitory phosphatase signaling represents a potential novel class of therapeutic targets for tyrosine kinase-driven ALL.

These results indicated that inhibitory receptors and downstream phosphatases are critical regulators of oncogenic signaling strength in tyrosine kinase-driven ALL, and identified ITIM-receptors and phosphatases as members of a potential novel class of therapeutic targets. The concept of pharmacological perturbance of oncogenic signaling equilibrium in leukemia cells by inhibition (e.g. TKI-treatment) or exaggeration of signaling strength (e.g. blockade of ITIM-receptors) may lead to the discovery of multiple additional therapeutic targets and broaden our repertoire of currently available therapeutic intervention.

No relevant conflicts of interest to declares.