BCR engagement in CLL: when translation goes wrong

In this issue of Blood, Yeomans et al identify MYC as an important target for translational regulation in chronic lymphocytic leukemia (CLL) cells after B-cell receptor (BCR) stimulation and show that current therapies suppress this induction.¹ 

Although eukaryotic ribosomes have the capability to add 6 to 9 amino acids per second, the messenger RNA (mRNA) translation machinery has the laborious daily task of producing a vast catalog of 30 000 proteins from our 18 000 coding genes.^2,3  Translation is a process frequently dysregulated in tumors and constitutes a valuable target for the development of novel anticancer therapies.⁴ 

Quiescent circulating CLL cells represent the most accessible population and are consequently more studied than CLL cells located in the bone marrow and lymph nodes (LNs). However, CLL cells attracted to LNs constitute the principal pool of proliferating leukemic cells and therefore should be the main focus of therapeutic interventions. Although the exact nature of the stimulation remains unknown, BCR activation, together with other soluble factors, triggers a cascade of signaling events leading to CLL cell survival and proliferation in highly specialized tissue microenvironments known as LN proliferation centers.⁵  BCR stimulation in CLL cells specifically induces MYC oncogene overexpression in vitro,⁶  an event also observed in CLL cells found in LNs⁶  and associated with a more aggressive disease.⁷  The BCR signaling capacity strongly depends on the mutational status of the immunoglobulin heavy chain variable (IGHV) genes. CLL cells with mutated IGHV genes usually express lower levels of surface IgM (sIgM) and present a weak response to BCR engagement associated with indolent disease. In contrast, cells with unmutated IGHV genes that predominantly express the ZAP-70 kinase respond to BCR stimulation and reflect an aggressive clinical behavior.

Consistent with this, Yeomans et al speculated that BCR stimulation induces a global mRNA translation increase and that MYC could be a crucial target for mRNA translation regulation in anti–IgM-responding CLL cells. By first analyzing 2 published gene expression profiling data sets generated from blood and LN-derived CLL cells and anti–IgM-stimulated circulating CLL cells,^5,8  the authors identified using a bioinformatics approach, growth promoting pathways activated following BCR engagement. Among the 14 clusters of genes generated by the weighted gene coexpression network analysis, expression of a module comprising 344 genes strongly correlated with anti-IgM stimulation and the presence of CLL cells in the LN. Gene ontology analysis confirmed the authors’ hypothesis, as the clustered genes were associated with just a few important biological functions, among which were “regulation of cell cycle,” “cellular response to stress,” “mRNA translation,” and an MYC-centered “metabolism” network.

With the idea to study global protein synthesis by ³⁵S-based metabolic labeling, Yeomans et al further observed an increase in bulk mRNA translation following BCR engagement, independent of the IGHV gene mutation status, that could be drastically reduced by the Bruton tyrosine kinase (BTK) and spleen tyrosine kinase (SYK) inhibitors ibrutinib and tamatinib, respectively. The authors also observed that both drugs inhibited BCR engagement-driven phosphorylation of p70S6K, a mammalian target of rapamycin complex 1 (mTORC1) activation marker and extracellular signal-regulated kinases (ERK1/2), two molecules positively regulating translation downstream of the phosphatidylinositol 3-kinase/AKT axis. The authors elegantly confirmed the increase in protein synthesis at the single cell level. Based on a protocol to image protein synthesis, Yeomans et al developed a novel cytometry-based assay combining the integration of O-propargyl-puromycin (OPP) and surface immunostaining to study the production of nascent polypeptides at the single cell level. The specificity of the response of CLL cells toward B-cell–specific stimulation was demonstrated by the absence of protein synthesis in nonmalignant T cells stimulated by CpG oligodeoxynucleotides or anti-IgM. Similar to metabolic labeling, the OPP integration confirmed the inhibitory effect of ibrutinib and tamatinib in the malignant CLL clone.

The authors further studied the mechanisms regulated by BCR engagement in CLL cells. They identified an increase in key translation initiation factors (eIFs) eIF4A and eIF4GI at both mRNA⁸  and protein levels, and a downregulated expression of the eIF4A negative regulator programmed cell death protein 4 (PDCD4). Acting respectively as RNA helicase and scaffold for the translation initiation complex, eIF4A and eIF4GI are not induced in BCR-stimulated normal B cells and therefore appear to be specific regulators of the mRNA translation machinery in CLL cells following BCR engagement. Finally, in line with previous observations,^5,6,8  the authors detected higher levels of total MYC transcripts. However, translation of an mRNA can be done by a single ribosome or by multiple ribosomes simultaneously, forming a structure called polysomes. Hence, Yeomans et al performed a polysome profiling to specifically detect RNA undergoing intense translation and, indeed, quantified higher amounts of polysomal MYC mRNA in CLL cells following BCR engagement. This finding was corroborated by the marked increase in MYC protein level, which was partially or totally abrogated by ibrutinib and tamatinib, respectively.

With this study, Yeomans et al describe for the first time, the regulation of global and MYC-specific mRNA translation in CLL cells after anti-IgM stimulation (see figure). With the advances in understanding the molecular mechanisms following BCR stimulation, there is a clear rationale for inhibiting translation in CLL cells. Current therapeutic strategies interfering with translation via mTORC1 or Src kinase inhibition were recently shown to be efficient in Eµ-Myc–driven models of B-cell lymphoma⁹  and to inhibit ERK1/2-eIF4–mediated translation of Myc,¹⁰  respectively. Additional clinical trials are required to validate any beneficial effects of direct inhibition of mRNA translation on CLL outcome. However, given the complex nature of the LN microenvironment, it will be of critical importance to identify regulatory molecules promoting RNA translation in CLL cells. Whether cytokines, chemokines, or extracellular vesicles could synergize with BCR engagement to stimulate translation of specific mRNA in primary CLL cells will require further investigations. A better understanding of the disease pathogenesis is crucial in the fight against cancer and will bring additional insight to developing new therapeutic approaches.