B cell receptor signaling in chronic lymphocytic leukemia

Kinetics of CLL cells in tissues and blood during therapy with the BTK inhibitor ibrutinib

The Bruton tyrosine kinase (BTK) inhibitor ibrutinib has excellent clinical activity in patients with chronic lymphocytic leukemia (CLL). Characteristically, ibrutinib causes CLL cell redistribution from tissue sites into the peripheral blood during the initial weeks of therapy. To better characterize the dynamics of this redistribution phenomenon, we correlated serial lymphocyte counts with volumetric changes in lymph node and spleen sizes during ibrutinib therapy. Kinetic parameters were estimated by applying a mathematical model to the data. We found that during ibrutinib therapy, 1.7% ± 1.1% of blood CLL cells and 2.7% ± 0.99% of tissue CLL cells die per day. The fraction of the tissue CLL cells that was redistributed into the blood during therapy was estimated to be 23.3% ± 17% of the total tissue disease burden. These data indicate that the reduction of tissue disease burden by ibrutinib is due more to CLL cell death and less to egress from nodal compartments.

Breaking good: the inexorable rise of BTK inhibitors in the treatment of chronic lymphocytic leukaemia

Although expressed in several haematological lineages and involved in multiple different signalling pathways, Bruton tyrosine kinase (BTK) plays an indispensible role in B cells in signalling from the B cell receptor (BCR) for antigen. Many B cell malignancies remain dependent on constitutive BCR signalling, making BTK a functional therapeutic target. Several BTK inhibitors (BTKi) with different kinomes and modes of action are being assessed clinically. This review documents the efficacy and toxicity of BTKi in chronic lymphocytic leukaemia (CLL). Clinically, the furthest in development is ibrutinib (trade name, Imbruvica), an irreversible BTKi, which has shown spectacular preliminary efficacy, with rapid reductions in lymph nodes accompanied by peripheral blood lymphocytosis. The lymphocytosis resolves slowly and most patients do not enter a complete remission. Nevertheless, it is possible to maintain many CLL patients, even those with adverse cytogenetic features, on drug for many months with minimal toxicities, thus potentially transforming the therapeutic paradigms for CLL. The efficacy, lack of toxicity and oral administration of BTKi will ensure their adoption in a wide range of B cell malignancies. An outstanding challenge is to incorporate BTKi with other precision medicines in a mechanism-based manner in order to dispense with conventional chemotherapy.

FAS-antisense 1 lncRNA and production of soluble versus membrane Fas in B-cell lymphoma

Impaired Fas-mediated apoptosis is associated with poor clinical outcomes and cancer chemoresistance. Soluble Fas receptor (sFas), produced by skipping of exon 6, inhibits apoptosis by sequestering Fas ligand. Serum sFas is associated with poor prognosis of non-Hodgkin's lymphomas. We found that the alternative splicing of Fas in lymphomas is tightly regulated by a long-noncoding RNA corresponding to an antisense transcript of Fas (FAS-AS1). Levels of FAS-AS1 correlate inversely with production of sFas, and FAS-AS1 binding to the RBM5 inhibits RBM5-mediated exon 6 skipping. EZH2, often mutated or overexpressed in lymphomas, hyper-methylates the FAS-AS1 promoter and represses the FAS-AS1 expression. EZH2-mediated repression of FAS-AS1 promoter can be released by DZNeP (3-Deazaneplanocin A) or overcome by ectopic expression of FAS-AS1, both of which increase levels of FAS-AS1 and correspondingly decrease expression of sFas. Treatment with Bruton's tyrosine kinase inhibitor or EZH2 knockdown decreases the levels of EZH2, RBM5 and sFas, thereby enhancing Fas-mediated apoptosis. This is the first report showing functional regulation of Fas repression by its antisense RNA. Our results reveal new therapeutic targets in lymphomas and provide a rationale for the use of EZH2 inhibitors or ibrutinib in combination with chemotherapeutic agents that recruit Fas for effective cell killing.

Molecular pathways: targeting the microenvironment in chronic lymphocytic leukemia--focus on the B-cell receptor

Interactions between malignant B lymphocytes and the tissue microenvironment play a major role in the pathogenesis of chronic lymphocytic leukemia (CLL) and other B-cell malignancies. The coexistence and coevolution of CLL cells with their tissue neighbors provided the basis for discovery of critical cellular and molecular drivers of the disease and identification of new therapeutic targets. Bone marrow stromal cells (BMSC), monocyte-derived nurselike cells (NLC), and T cells are key players in the CLL microenvironment, which activate and protect CLL cells within the tissues. CLL surface molecules, such as the B-cell antigen receptor (BCR), chemokine receptors, adhesion molecules, and TNF receptor superfamily members (e.g., CD40, BCMA, and BAFF-R) engage in cross-talk with respective tissue ligands. This cross-talk results in survival and expansion of the CLL clone, and protects CLL cells from conventional cytotoxic drugs. Inhibiting these pathways represents an alternative therapeutic strategy to more conventional chemoimmunotherapy. Here, we review central components of the CLL microenvironment, with a particular emphasis on BCR signaling, and we summarize the most relevant clinical advances with inhibitors that target the BCR-associated spleen tyrosine kinase/SYK (fostamatinib), Bruton's tyrosine kinase/BTK (ibrutinib), and PI3Kδ (idelalisib).