MYD88L265P Augments Proximal B-Cell Receptor Signaling in Large B-Cell Lymphomas Via an Interaction with DOCK8.

Diffuse large B-cell lymphoma (DLBCL) is a clinically and genetically heterogeneous disease comprised of at least 5 recognized molecular subtypes. Cluster 5/ MCD tumors frequently exhibit concurrent alterations of the Toll-like receptor (TLR) and B-cell receptor (BCR) pathway members, MYD88L265P and CD79B, and have a less favorable prognosis. In normal B cells, the synergy between TLR and BCR signaling pathways integrates innate and adaptive immune responses and augments downstream NF-kB activation. Additionally, physiologic TLR9 pathway engagement, via MYD88, PYK2 and DOCK8, increases proximal BCR signaling in normal murine B cells. Although MCD/ Cluster 5 DLBCLs are selectively sensitive to BTK inhibition in in vitro studies and certain clinical trials, the role of mutated MYD88 in proximal BCR signaling remains undefined. Using engineered DLBCL cell line models, we find that concurrent MYD88L265P and CD79B alterations significantly increase the magnitude and duration of proximal BCR signaling, at the level of SYK and BTK, and augment PYK2-dependent DOCK8 phosphorylation. MYD88L265P DLBCLs have significantly increased colocalization of DOCK8 with both MYD88 and the proximal BCR-associated Src kinase, LYN, in comparison to MYD88WT DLBCLs, implicating DOCK8 in MYD88L265P/ proximal BCR crosstalk. Additionally, DOCK8 depletion selectively decreases proximal BCR signaling, cellular proliferation and viability of DLBCLs with endogenous MYD88L265P/CD79BY196Falterations and increases the efficacy of BTK blockade in these lymphomas. Therefore, MYD88L265P/DOCK8-enhanced proximal BCR signaling is a likely mechanism for the increased sensitivity of MCD/ Cluster 5 DLBCLs to BTK blockade.