Expression levels of Lyn, Syk, PLCγ2 and ERK in patients with chronic lymphocytic leukemia, and higher levels of Lyn are associated with a shorter treatment-free survival

B cell receptor signaling proteins as biomarkers for progression of CLL requiring first-line therapy

The molecular landscape of chronic lymphocytic leukemia (CLL) has been extensively characterized, and various potent prognostic biomarkers were discovered. The genetic composition of the B-cell receptor (BCR) immunoglobulin (IG) was shown to be especially powerful for discerning indolent from aggressive disease at diagnosis. Classification based on the IG heavy chain variable gene (IGHV) somatic hypermutation status is routinely applied. Additionally, BCR IGH stereotypy has been implicated to improve risk stratification, through characterization of subsets with consistent clinical profiles. Despite these advances, it remains challenging to predict when CLL progresses to requiring first-line therapy, thus emphasizing the need for further refinement of prognostic indicators. Signaling pathways downstream of the BCR are essential in CLL pathogenesis, and dysregulated components within these pathways impact disease progression. Considering not only genomics but the entirety of factors shaping BCR signaling activity, this review offers insights in the disease for better prognostic assessment of CLL.

B cell receptor signaling and associated pathways in the pathogenesis of chronic lymphocytic leukemia

B cell antigen receptor (BCR) signaling is a key driver of growth and survival in both normal and malignant B cells. Several lines of evidence support an important pathogenic role of the BCR in chronic lymphocytic leukemia (CLL). The significant improvement of CLL patients' survival with the use of various BCR pathway targeting inhibitors, supports a crucial involvement of BCR signaling in the pathogenesis of CLL. Although the treatment landscape of CLL has significantly evolved in recent years, no agent has clearly demonstrated efficacy in patients with treatment-refractory CLL in the long run. To identify new drug targets and mechanisms of drug action in neoplastic B cells, a detailed understanding of the molecular mechanisms of leukemic transformation as well as CLL cell survival is required. In the last decades, studies of genetically modified CLL mouse models in line with CLL patient studies provided a variety of exciting data about BCR and BCR-associated kinases in their role in CLL pathogenesis as well as disease progression. BCR surface expression was identified as a particularly important factor regulating CLL cell survival. Also, BCR-associated kinases were shown to provide a crosstalk of the CLL cells with their tumor microenvironment, which highlights the significance of the cells' milieu in the assessment of disease progression and treatment. In this review, we summarize the major findings of recent CLL mouse as well as patient studies in regard to the BCR signalosome and discuss its relevance in the clinics.

Constitutive activation of Lyn kinase enhances BCR responsiveness, but not the development of CLL in Eµ-TCL1 mice

The treatment of chronic lymphocytic leukemia (CLL) has been improved dramatically by inhibitors targeting B-cell receptor (BCR)-associated kinases. The tyrosine kinase Lyn is a key modulator of BCR signaling and shows increased expression and activity in CLL. To evaluate the functional relevance of Lyn for CLL, we generated a conditional knockin mouse model harboring a gain-of-function mutation of the Lyn gene (LynY508F), which was specifically expressed in the B-cell lineage (Lynup-B). Kinase activity profiling revealed an enhanced responsiveness to BCR stimulation in Lynup-B B cells. When crossing Lynup-B mice with Eµ-TCL1 mice (TCL1tg/wt), a transgenic mouse model for CLL, the resulting TCL1tg/wt Lynup-B mice showed no significant change of hepatomegaly, splenomegaly, bone marrow infiltration, or overall survival when compared with TCL1tg/wt mice. Our data also suggested that TCL1 expression has partially masked the effect of the Lynup-B mutation, because the BCR response was only slightly increased in TCL1tg/wt Lynup-B compared with TCL1tg/wt. In contrast, TCL1tg/wt Lynup-B were protected at various degrees against spontaneous apoptosis in vitro and upon treatment with kinase inhibitors targeting the BCR. Collectively, and consistent with our previous data in a Lyn-deficient CLL model, these data lend further suggest that an increased activation of Lyn kinase in B cells does not appear to be a major driver of leukemia progression and the level of increased BCR responsiveness induced by Lynup-B is insufficient to induce clear changes to CLL pathogenesis in vivo.

Lck is a relevant target in chronic lymphocytic leukaemia cells whose expression variance is unrelated to disease outcome

Pathogenesis of chronic lymphocytic leukaemia (CLL) is contingent upon antigen receptor (BCR) expressed by malignant cells of this disease. Studies on somatic hypermutation of the antigen binding region, receptor expression levels and signal capacity have all linked BCR on CLL cells to disease prognosis. Our previous work showed that the src-family kinase Lck is a targetable mediator of BCR signalling in CLL cells, and that variance in Lck expression associated with ability of BCR to induce signal upon engagement. This latter finding makes Lck similar to ZAP70, another T-cell kinase whose aberrant expression in CLL cells also associates with BCR signalling capacity, but also different because ZAP70 is not easily pharmacologically targetable. Here we describe a robust method of measuring Lck expression in CLL cells using flow cytometry. However, unlike ZAP70 whose expression in CLL cells predicts prognosis, we find Lck expression and disease outcome in CLL are unrelated despite observations that its inhibition produces effects that biologically resemble the egress phenotype taken on by CLL cells treated with idelalisib. Taken together, our findings provide insight into the pathobiology of CLL to suggest a more complex relationship between expression of molecules within the BCR signalling pathway and disease outcome.

Comparison of the mRNA expression profile of B-cell receptor components in normal CD5-high B-lymphocytes and chronic lymphocytic leukemia: a key role of ZAP70

The B‐cell receptor (BCR) signaling pathway is of great importance for B‐cell survival and proliferation. The BCR expressed on malignant B‐CLL cells contributes to the disease pathogenesis, and its signaling pathway is currently the target of several therapeutic strategies. Although various BCR alterations have been described in B‐CLL at the protein level, the mRNA expression levels of tyrosine kinases in B‐CLL compared to that in normal CD5‐high and CD5‐low B‐lymphocytes remain unknown. In the current study, we measured the mRNA expression levels of CD79A, CD79B, LYN, SYK, SHP1, and ZAP70 in purified populations of CD5‐high B‐CLL cells, CD5‐low B‐cells from the peripheral blood of healthy donors, and CD5‐high B‐cells from human tonsils. Here, we report a clear separation in the B‐CLL dataset between the ZAP70‐high and ZAP70‐low subgroups. Each subgroup has a unique expression profile of BCR signaling components that might reflect the functional status of the BCR signaling pathway. Moreover, the ZAP70‐low subgroup does not resemble either CD5‐high B‐lymphocytes from the tonsils or CD5‐low lymphocytes from PBMC (P < 0.05). We also show that ZAP70 is the only gene that is differentially expressed in CD5‐high and CD5‐low normal B‐lymphocytes, confirming the key role of Zap‐70 tyrosine kinase in BCR signaling alterations in B‐CLL.

Chronic Lymphocytic Leukemia: Exploiting Vulnerabilities with Targeted Agents

The field of oncology has been transformed over the course of the last 20 years in large part due to the enhanced understanding of cellular biology and cellular signaling. The indolent natural history of chronic lymphocytic leukemia (CLL) has permitted extensive study of cancer biology and can in some ways be thought of a model for understanding and translating concepts to other diseases. By systematically probing the biology of CLL cells and working out in stepwise fashion the transduction of signals from the surface immunoglobulin to nuclear transcription factors, investigators have paved the way for rational targeting of therapies at natural vulnerabilities that mimic oncogene addiction. These key targets include Bruton's tyrosine kinase (BTK), phosphatidylinositol 3-kinase (PI3K), Src, Bcl2, and cyclin-dependent kinases (CDKs). In this review, we will consider these proteins and describe the current and future molecules designed to target them in CLL.

Directed Evolution of a Highly Specific FN3 Monobody to the SH3 Domain of Human Lyn Tyrosine Kinase

Affinity reagents of high affinity and specificity are very useful for studying the subcellular locations and quantities of individual proteins. To generate high-quality affinity reagents for human Lyn tyrosine kinase, a phage display library of fibronectin type III (FN3) monobodies was affinity selected with a recombinant form of the Lyn SH3 domain. While a highly specific monobody, TA8, was initially isolated, we chose to improve its affinity through directed evolution. A secondary library of 1.2 × 10⁹ variants was constructed and screened by affinity selection, yielding three variants, two of which have affinities of ~ 40 nM, a 130-fold increase over the original TA8 monobody. One of the variants, 2H7, displayed high specificity to the Lyn SH3 domain, as shown by ELISA and probing arrays of 150 SH3 domains. Furthermore, the 2H7 monobody was able to pull down endogenous Lyn from a lysate of Burkitt's lymphoma cells, thereby demonstrating its utility as an affinity reagent for detecting Lyn in a complex biological mixture.

Concomitant high expression of Toll-like receptor (TLR) and B-cell receptor (BCR) signalling molecules has clinical implications in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive disease with frequent relapse. Targeted therapies against B-cell receptor (BCR) molecules have demonstrated improved outcomes in relapsed cases. However, clinical responses are slow and selective, with failure to attain complete remission in a significant subset of patients. Complex interaction of BCR signal transduction with toll-like receptor (TLR) and other pathways in MCL remains unknown, thus averting progress in development of targeted therapies. We have performed detailed digital quantification of BCR/TLR signalling molecules and their effector pathways in a cohort (n = 81) of MCL patients and correlated these data with overall survival. Hierarchical clustering model based on BCR/TLR genes revealed two distinct (BCR^high and BCR^low ) subsets of patients (n = 32; 40%) with significant differences in expression (>1.5-fold change; p < 0.05). Higher levels of BTK/SYK/BLNK/CARD11/PLCG signalosome and lower expression of MALT1/BCL10 genes suggested tonic pattern of BCR activation. Amplified expression of TLR6/TLR7/TLR9 was noted in concert with hyper-responsiveness of BCR machinery. MYD88, a key TLR adaptor molecule, was not upregulated in any of these clusters, which may suggest a 'cross-talk' between BCR and TLR pathways. In sync with BCR/TLR signalling, we recorded significantly enhanced expression of genes associated with NF-kB pathway in BCR^high subset of MCL patients. On univariate analysis, the BCR^high patients showed a trend towards inferior clinical response to a standardized treatment protocol, compared with the BCR^low group (log rank, p = 0.043). In conclusion, we have identified hyperactive BCR/TLR signalling pathways and their effector downstream targets in a subset of MCL patients and associated it with poor clinical outcomes. Our study provides quantitative evidence at RNA expression level of possible concomitant collaboration between TLR and BCR signalling molecules in MCL. These data will provide further insights for future functional studies and, hence, development of targeted therapies for MCL patients. Copyright © 2015 John Wiley & Sons, Ltd.

Ibrutinib: a paradigm shift in management of CLL

B-cell receptor (BCR) signaling plays a vital role in B-cell malignancies; Bruton tyrosine kinase is a critical mediator of this signaling. BCR signaling, either constitutively or following antigen binding, leads to activation of several downstream pathways involved in cell survival, proliferation and migration. The efficacy observed in studies of the Bruton tyrosine kinase inhibitor, ibrutinib, confirms that BCR signaling is critical for the growth of B-cell malignancies. Ibrutinib characteristically induces redistribution of malignant B cells from tissues into the peripheral blood and rapid resolution of adenopathy. Furthermore, ibrutinib therapy results in normalization of lymphocyte counts and improvement in cytopenias. Ibrutinib has been shown to have an excellent safety profile and does not cause myelosuppression. Early data from combination studies of ibrutinib with anti-CD20 monoclonal antibodies have shown more rapid responses compared to those seen with ibrutinib monotherapy. Current data strongly support continued clinical evaluation of ibrutinib in B-cell malignancies.

Management of chronic lymphocytic leukemia (CLL) in the era of B-cell receptor signal transduction inhibitors

The treatment of patients with chronic lymphocytic leukemia (CLL), an indolent B-cell lymphoma is in the midst of a transformation. There are a large number of promising new therapeutic agents and cellular therapies being studied which exhibit remarkable activity, favorable toxicity profiles, convenient administration schedules, and treatment options are rapidly expanding. The recent advances in the management of CLL exemplify the value of translational medicine. This review highlights key aspects of B-cell receptor (BCR) signaling in relation to novel inhibitors of the BCR signaling pathway, currently at various stages of preclinical and clinical development.

Does B cell receptor signaling in chronic lymphocytic leukaemia cells differ from that in other B cell types?

Chronic lymphocytic leukaemia (CLL) is an incurable malignancy of mature B cells. CLL is important clinically in Western countries because of its commonality and because of the significant morbidity and mortality associated with the progressive form of this incurable disease. The B cell receptor (BCR) expressed on the malignant cells in CLL contributes to disease pathogenesis by providing signals for survival and proliferation, and the signal transduction pathway initiated by engagement of this receptor is now the target of several therapeutic strategies. The purpose of this review is to outline current understanding of the BCR signal cascade in normal B cells and then question whether this understanding applies to CLL cells. In particular, this review studies the phenomenon of anergy in CLL cells, and whether certain adaptations allow the cells to overcome anergy and allow full BCR signaling to take place. Finally, this review analyzes how BCR signals can be therapeutically targeted for the treatment of CLL.

Role of spleen tyrosine kinase in the pathogenesis of chronic lymphocytic leukemia

The antigen-dependent B-cell receptor (BCR) is triggered by binding to external antigens and transmits signals in normal B lymphocytes. Tonic signaling through the BCR plays a crucial role in the pathogenesis and progression of chronic lymphocytic leukemia (CLL). Spleen tyrosine kinase (Syk) is a key component of both BCR signals, and regulates multiple physiological functions of B lymphocytes. Studies have defined enhanced gene expression and protein expression of Syk in CLL cells which are closely related to the status of the immunoglobulin heavy chain variable region genes (IgVH). Recently, abrogating the BCR-induced signaling pathway by Syk inhibitors has represented a novel and active therapeutic approach for CLL. Studies of the correlation between Syk and ZAP-70 expression in CLL cells have brought a new perspective to determining the value of Syk in evaluating the effect of therapy and the prognosis of CLL. Therefore, we here review the role of Syk in the pathogenesis of CLL and provide an update of progress in the clinical development of Syk inhibitors.

Signaling pathways in lymphoma: pathogenesis and therapeutic targets

Lymphoma is the fifth most common cancer in the USA. Most lymphomas are classified as non-Hodgkin’s lymphoma, and nearly 95% of these cancers are of B-cell origin. B-cell receptor (BCR) surface expression and BCR functional signaling are critical for survival and proliferation of both healthy B cells, as well as most B-lymphoma cells. Agents that inhibit various components of the BCR signaling pathway, as well as parallel signaling pathways, are currently in clinical trials for the treatment of various lymphoma subtypes, including those targeting isoforms of PI3K, mTOR and BTK. In this review, we describe the signaling pathways in healthy mature B cells, the aberrant signaling in lymphomatous B cells and the rationale for clinical trials of agents targeting these pathways as well as the results of clinical trials to date. We propose that the entry into a kinase inhibitor era of lymphoma therapy will be as transformative for our patients as the advent of the antibody or chemotherapy era before it.