A Stapled BIM BH3 Helix Restores Apoptosis In Bim-Null Mantle Cell Lymphoma

Abstract 437

Mantle cell lymphoma (MCL) is a highly aggressive B-cell lymphoma resistant to conventional chemotherapy. Although defined by the characteristic t(11;14) translocation, a variety of genetic aberrations have been indentified in MCL and may contribute to its pathogenesis and chemoresistance. Of particular interest is the frequent biallelic deletion of the pro-apoptotic BCL-2 family protein BIM. BIM exerts its pro-death function via its alpha-helical BH3 death domain that has the dual capacity to inhibit anti-apoptotic proteins such as BCL-2 and MCL-1 and directly trigger pro-apoptotic proteins such as the mitochondrial executioner BAX. In an effort to replace the deleted BH3 functionality in Bim^−/− MCL, we generated stabilized alpha-helix of BCL-2 domains (SAHBs) modeled after the BIM BH3 alpha-helix. BIM SAHBs are helical, protease-resistant, and cell permeable peptides that recapitulate the pro-apoptotic activity of BIM. Indeed BIM SAHB, but not a negative control point mutant, dose-responsively activated caspase 3/7 and correspondingly inhibited the viability of a panel of MCL cell lines with IC50s in the single digit micromolar range. To extend these studies to an in vivo model of Bim^−/− lymphoproliferative disease, we reconstituted Rag2-deficient mice with the bone marrow from Bim^−/− mice and documented the development of splenomegaly and massive organ infiltration by B220-positive lymphocytes. Mice treated with intravenous BIM SAHB exhibited TUNEL positivity within the aberrant organ infiltrates, but not in the surrounding normal parenchyma. Importantly, vehicle and point mutant SAHB had no such effect, highlighting the specificity of BIM SAHB activity. Thus, therapeutic replacement of BIM's pro-apoptotic functionality using BIM SAHB has the potential to restore apoptosis in hematologic cancers that mount an apoptotic blockade by deleting Bim.