MER Signaling Axis: A Novel Therapeutic Target in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of CD19+/CD5+/CD23+ monoclonal B-cells in peripheral blood, bone marrow and lymphoid tissues with a highly variable disease course.

B-cell receptor (BCR)-signal inhibitors, specifically Ibrutinib which targets Bruton's tyrosine kinase (BTK) and Idelalisib which targets PI3Kδ, are effective in relapsed/refractory CLL patients. Despite BCR-signal-inhibitors are changing the management of CLL, these agents are not curative and resistance can develop, often leading to the transformation of CLL into more aggressive B-cell lymphomas limiting their therapeutics options. Thus, improved understanding of the nature and extent of other active receptor tyrosine kinase (RTK) signals in CLL cells is critical for development of additional therapies to treat these high-risk patients. Our recent discovery of MER, a RTK of the TAM (Tyro3, AXL, MER) family, in CLL cells may offer such an option.

In order to analyze MER expression status in CLL cells from previously untreated CLL patients, we employed western blot, flow cytometry and qRT-PCR analyses. We detected low to high level expression of MER in CLL cells from ~50% of CLL patients as compared to normal, purified B cells. We also found that MER in CLL cells remained constitutively phosphorylated (active). Furthermore, our preliminary analysis suggested that MER expression in CLL cells may be positively associated with high-risk, unmutated IGHV CLL patients.

In vitro ligation of Gas6 (growth arrest-specific gene 6), a specific ligand for all the TAM receptors, robustly activated MER as compared to AXL or Tyro3. Interestingly, activation of MER preferentially activated Erk1/2, NF-κB, and BTK of the BCR pathway in CLL cells. Partial depletion of MER in CLL cells using a specific-siRNA resulted in inhibition of P-Erk1/2, P-NF-κB and P-BTK, but not P-AKT, suggesting that MER may crosstalk with the BCR signaling pathway in CLL cells likely via interaction with CD79a, LYN or BTK. To address this hypothesis, we immunoprecipitated MER from CLL cell lysates and found a direct complex formation between MER and LYN.

We also analyzed leukemic B cells from the Eµ-TCL1 transgenic mice which recapitulate highly aggressive human CLL for the expression of MER by western blot/flow cytometry. Results showed robust expression of MER in TCL1 leukemic B cells purified from mice with advanced CLL. Finally, treatment of CLL mice with a high-affinity MER inhibitor (UNC-2025) via oral gavage significantly reduced leukemic tumor burden. In total, our findings suggest that expression/activation of MER may potentiate CLL cell survival via a crosstalk with the BCR signal thus, could be an attractive therapeutic target for the progressive CLL patients.

No relevant conflicts of interest to declare.