Metabolic gatekeepers in B-cell malignancies

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ABSTRACT The introduction of small molecule inhibitors of kinases downstream of the B-cell receptor (BCR) or its oncogenic mimics substantially improved outcomes for patients with B-cell malignancies. However, treatment with kinase- inhibitors alone invariably selects for drug-resistant clones. The development of subsequent relapse remains a central problem in the treatment of patients with B-ALL, CLL and mantle cell lymphoma (MCL), which continue to have poor outcomes or remain incurable diseases. Over the past five years, our R35-supported research led to a new paradigm for the treatment of B-cell tumors: Unlike other cancer-types, B-cell malignancies are highly susceptible to targeted engagement of negative selection, a B-cell-intrinsic mechanism to protect against harmful autoantibodies and autoimmune disease. Contrary to established dogma, mechanisms of negative selection are not only active in preventing autoimmune disease but also represent an entirely novel class of therapeutic targets in B-cell malignancies. The main impetus of this renewal application is now to leverage these new concepts for the development of treatment strategies that will overcome conventional mechanisms of drug-resistance in B-ALL, CLL and MCL. One of these approaches is based on pharmacological hyperactivation of BCR-downstream kinases to mimic excessive signaling-strength from an autoreactive BCR. Targeted cancer-therapy is traditionally focused on kinase-inhibitors to suppress oncogenic signaling. Our concept of kinase-hyperactivation effectively represents the opposite. We pursue the new paradigm that negative selection is predicated on a B-lymphoid transcriptional program to restrict energy-abundance, a mechanism we termed `metabolic gatekeeper'. Given that transformed B-cells have higher energy-demands than their normal counterparts, we hypothesize that metabolic gatekeeper mechanisms to limit energy-supply will prevent B-cell transformation by setting low thresholds for elimination of pre-malignant clones based on energy-stress. Our laboratory recently developed genetic systems to identify factors that dictate the unique morphology of B- cells, i.e. that B-cells have a smaller cell size and fewer mitochondria than any other somatic cell type. We examine how maintenance of a small cell-size affects B-cell selection and how B-cells engineered to inducibly increase cell size and mitochondrial mass become more prone to malignant transformation. Long-term biomass-measurements of single B-cell lymphoma cells revealed periodic phases of shrinking (quiescence, BCL6+) and expanding (proliferation, MYC+) cell mass. We will examine the new paradigm that BCL6 and MYC mark iterative cycles of dormancy and activation. In analogy to sleep-wake phases, we hypothesize that the quiescent BCL6-phase is essential for recovery and regeneration. These and other observations will lead to new concepts for the understanding of how cell-size and the length of recovery-periods regulate energy-supply and survival of transformed B-cells and create new opportunities for therapeutic intervention in B-cell malignancies with a focus on B-ALL, CLL and MCL.