Targeting precursor BCR signaling in ALL

In this issue of Blood, Kim et al¹  investigated the preclinical therapeutic potential of targeting precursor B-cell receptor (pre-BCR) signaling and its mechanism of action in acute lymphoblastic leukemia (ALL). Ibrutinib, a US Food and Drug Administration (FDA)–approved inhibitor of Bruton tyrosine kinase (BTK), was demonstrated to interfere with pre-BCR signaling and specifically suppress in vitro and in vivo cell proliferation of B-ALL cells that express a functional pre-BCR. The synergistic activity with conventional chemotherapeutic agents corroborates ibrutinib as a new therapeutic opportunity for pre-BCR⁺ ALL.

Pre-B-cell ALL is a hematological malignancy that arises from an oncogenic transformation of an early B-lymphocyte progenitor. It comprises multiple subtypes with distinct constellations of somatic genetic alterations. Despite cure rates exceeding 80% in children, disease-specific and treatment-related toxicity remain important causes of morbidity and mortality in children and adults.²  Therefore, development of targeted therapies that focus on specific oncogenic mechanisms of ALL cells are of great interest to improve outcomes. In this issue of Blood, the investigators explored the activity and mechanisms of action of the BTK inhibitor ibrutinib in preclinical models of pre-B-ALL.

The basic premises for this study were that: (1) targeting BCR signaling has been proved a successful therapeutic strategy in mature B-cell malignancies^3-5 ; (2) signaling of the mature BCR is largely similar to that of pre-BCR, which plays a critical role during early B-cell development, promoting survival and expansion of progenitor cells with productively rearranged pre-BCRs⁶ ; and (3) a distinct subset of pre-B-ALL (∼15%) is selected for expression and activity of a functional pre-BCR (pre-BCR⁺).⁷  This group includes TCF3-PBX1⁺ ALL that is selectively sensitive to BCR signaling inhibitors.⁸  In the remaining ALL cases (eg, BCR-ABL1⁺ ALL),⁹  the dominant leukemic clones lack expression of a functional pre-BCR because of nonproductive immunoglobulin heavy chain (IGH) gene rearrangement and/or deregulation of other pathway components.

During early B-cell development, functional pre-BCRs are expressed for a short period after productive IGH rearrangement, allowing progenitor cells to transition into the pool of mature peripheral B cells. Survival signals from the pre-BCRs are transduced by the immunoglobulin-α (Igα) and Igβ signaling chains and include activation of downstream kinases (eg, LYN, SYK, BTK, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit δ)¹⁰  and transcriptional changes (eg, expression of B-cell CLL/lymphoma 6 [BCL6]) (see figure). Given the critical role of BTK for B-cell development and lymphomagenesis, BTK inhibitors have been extensively investigated. To date, ibrutinib is the only BTK inhibitor that is FDA-approved for treatment of mature B-cell lymphoproliferative malignancies, specifically mantle cell lymphoma, chronic lymphocytic leukemia, small lymphocytic lymphoma, and Waldenstrom macroglobulinemia.

To explore the functional role of BTK in ALL, Kim et al began their study by screening BTK expression and activity (autophosphorylation at residue Y223) in a panel of B-ALL cell lines. The results indicated activated BTK in BCR-ABL1⁺ and pre-BCR⁺ ALL cells; however, only pre-BCR⁺ cells were sensitive to ibrutinib at clinically relevant concentrations. Ibrutinib acted through the inhibition of cell proliferation rather than through the induction of apoptosis. The exclusive sensitivity of pre-BCR⁺ ALL led the investigators to further explore the effects of ibrutinib on the activity of the pre-BCR signaling cascade. These studies revealed that ibrutinib abrogates induced and autonomous pre-BCR signaling, resulting in reduced calcium release and suppression of AKT and its downstream targets, including FOXO transcription factors, ERK, and mTOR signaling pathways, in pre-BCR⁺ ALL cells.

Kim et al, using gene expression profiling and western blot, observed that prolonged treatment with ibrutinib induces a feedback compensatory mechanism characterized by suppression of negative regulators (CD22, CD72, and PTPN6) and upregulation of downstream targets of BTK such as protein kinase C β. Similar results were previously reported for pre-BCR⁺/TCF3-PBX1⁺ ALL cells treated with the dual SRC/ABL kinase inhibitor dasatinib. A key defining feature of pre-BCR⁺ B-ALL is the constitutive expression of the transcription factor BCL6, which is induced following induction of pre-BCR signaling to sustain cell survival⁷ ; therefore, it is reasonable to expect reduced BCL6 expression upon treatment with ibrutinib, which was clearly demonstrated by the authors.

Because signals transduced by BTK regulate tissue homing, Kim et al explored the potential of ibrutinib to disrupt the interactions of leukemia cells with their microenvironment. Similar to its effects in mature B-cell malignancies, ibrutinib compromised the interactions of leukemic cells with their supportive microenvironments, as shown by the inhibited chemotaxis of ALL cells toward the chemokine C-X-C motif chemokine ligand 12 (CXCL12), independent of pre-BCR status (see figure).

Having characterized the mechanisms of action of ibrutinib in leukemic cells, Kim et al further investigated the importance of alternative targets for the activity of ibrutinib in pre-BCR⁺ ALL cells. Using CRISPR-Cas9–mediated genome editing, the authors nicely showed that both BTK and B-lymphocyte kinase are relevant targets of ibrutinib in pre-BCR⁺ ALL cells.

As a compelling follow-up to the FDA approval of ibrutinib for mature B-cell malignancies, these discoveries suggest that inhibition of pre-BCR signaling with BTK inhibitors may represent an effective therapeutic option for pre-BCR⁺ ALL patients. Kim et al provided another important step toward clinical translation by demonstrating that single-agent ibrutinib significantly prolonged survival in mouse xenografts models of pre-BCR⁺ ALL. Moreover, its combinations with conventional ALL chemotherapeutic agents, dexamethasone or vincristine, synergistically reduced viability and proliferation of pre-BCR⁺ ALL cells.

Overall, the work by Kim et al exploring new insights in the activity and mechanisms of actions of ibrutinib represents a significant therapeutic advance in ALL and provides a strong rationale for the clinical development in the ALL subset with functional pre-BCR expression. Establishment of diagnostic approaches to accurately detect pre-BCR⁺ B-ALL will help identify patients that may benefit from inhibition of pre-BCR signaling.