Chronic Active B-Cell Receptor Signaling in Lymphoma

Abstract SCI-26

We have developed loss-of-function, RNA interference-based screens to reveal genes essential for cancer cell proliferation and survival. In parallel, we are using high-throughput RNA resequencing (RNA-seq) to identify somatic mutations and other structural abnormalities in cancer. The intersection of these two data sets has helped us to discover novel pathogenetic pathways in lymphoma that are amenable to therapeutic attack. The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) has constitutive activation of the NF-κB pathway, which we traced to the signaling adapter CARD11. In some ABC DLBCL biopsies (∼10%), somatic mutations produce CARD11 isoforms that spontaneously activate NF-κB signaling. In ABC DLBCL tumors with wild-type CARD11, we defined a “chronic active” form of B-cell receptor (BCR) signaling that activates NF-κB. Such ABC DLBCLs are killed by knockdown of BCR signaling components, such as Bruton's tyrosine kinase (BTK), or components of the BCR itself. Over one-fifth of ABC DLBCLs have mutations in the CD79B or CD79A subunits of the BCR. In 18 percent of cases, mutations occur in a single tyrosine residue in the critical “ITAM” signaling motif, generating BCRs that avoid negative autoregulation by the LYN tyrosine kinase. Based on these findings, we are conducting clinical trials of ibrutinib in relapsed/refractory DLBCL. Ibrutinib is an irreversible and highly selective small-molecule inhibitor of BTK. Thus far, ibrutinib monotherapy has induced many complete and partial responses in patients with ABC DLBCL, including those with “primary refractory” tumors that had never responded to any prior therapy. One patient has been in a sustained complete response for over 19 months, taking ibrutinib daily with no discernible side effects. Of note, ABC DLBCL tumors with and without CD79B mutations have responded, suggesting that BCR pathway addiction may be a prevalent feature in this lymphoma subtype. More recently, we have uncovered a “tonic” form of BCR signaling in Burkitt lymphoma that engages the prosurvival PI(3) kinase pathway. Two-thirds of Burkitt lymphoma cell lines die upon knockdown of BCR subunits or the proximal kinase SYK, due to loss of PI(3) kinase signaling. Moreover, a gene expression signature of PI(3) kinase activity is more highly expressed in Burkitt lymphoma biopsies than in biopsies of other aggressive lymphomas. Tonic BCR signaling in Burkitt lymphoma is mechanistically distinct from chronic active BCR signaling in ABC DLBCL, since it does not engage BTK, CARD11, or NF-κB. RNA-seq revealed that 70 percent of sporadic Burkitt lymphoma cases harbor somatic mutations that potentiate the action of the transcription factor TCF3 by preventing its inhibitory heterodimerization with the DNA-binding inhibitor ID3. TCF3 promotes tonic BCR signaling and PI(3) kinase activity in Burkitt lymphoma by transactivating the immunoglobulin heavy- and light-chain genes, thereby increasing surface BCR expression, and by repressing the phosphatase SHP-1, a potent negative regulator of BCR signaling. Hence, inhibitors of proximal BCR signaling and the PI(3) kinase pathway should be evaluated in Burkitt lymphoma, especially in patients for whom high-dose chemotherapy is infeasible, such as older individuals and those with the endemic form of this lymphoma.