Molecular targeting of lymphoma through bromodomain inhibition and induction of apoptosis

Abstract

Diffuse large B-cell lymphoma (DLBCL) is a common cancer, with an annual incidence in the UK of approximately 5000. About 40% of patients fail first-line therapy, these refractory or relapsed (R/R) patients have a poor prognosis. A closely related cancer, Burkitt lymphoma (BL) exhibits higher cure rates but requires regimens with high toxicity to achieve this. Both frequently exhibit oncogenic MYC dysregulation, which is closely linked to apoptosis activation in healthy cells. This connection must be broken for malignant cells to develop and thrive. Targeting this relationship was an approach we employed to explore novel therapies for lymphomas. We employed novel bromodomain and extraterminal (BET) inhibitors, PLX51107 and PLX2853, in Eμ-myc lymphoma cells and human lymphoma cells to induce apoptosis. We determined that this was largely through BIM upregulation, a key pro-apoptotic protein. We concluded that this was largely mediated through downregulation of a micro RNA cluster miR-17/92, which is a target of MYC implicated in oncogenesis. High expression of pro-survival BCL-2 protein is associated with worse outcomes in DLBCL and is known to inhibit BIM. We were able to show that by inhibiting BCL-2 with a BH3-mimetic (ABT- 199) we could resensitise cells with high expression of BCL-2 to BET inhibition. This was seen in murine lymphomas manipulated to upregulate BCL-2, and human lymphomas with high BCL-2 expression both in vitro and in vivo. The combination effect was demonstrated to be synergistic in DLBCL cell lines. It co-incided with increased binding of BIM to BCL-2, priming the cells for death. We further confirmed that alternative BH3-mimetics targeting alternative BCL-2 prosurvival family members (BCL-XL and MCL-1) could combine with BET inhibitors to increase apoptosis in lymphoma. Finally, we examined a large dataset of gene expression from a phase III clinical trial in DLBCL, REMoDL-B. This identified that high expression of BCL-2 mRNA, as well as BCL-XL mRNA were associated with worse progression free survival in DLBCL cell of origin subtypes. It is clear that BCL-2 family members contribute to treatment resistance in DLBCL. We demonstrated a novel approach to overcome this by using small molecule inhibitors to upregulate a pro-apoptotic protein (BIM) with concurrent anti-apoptotic protein inhibition (BCL-2, BCL-XL, MCL-1) to result in lymphoma cell demise. Future clinical trials can be guided to improve patient survival by identifying such treatment-resistance mechanisms and over-coming them through combination evidence-based therapy design based on pre-clinical studies.

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Identifiers

ORCID for Mark Cragg: orcid.org/0000-0003-2077-089X

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