Small Molecule Inhibitor of Neutrophil Elastase Normalizes Myeloid Differentiation and Improves Impaired Cell Survival Triggered by Elastase Mutations In Patients with Severe Congenital Neutropenia and Acute Myeloid Leukemia

Abstract 386

Heterozygous mutations in the neutrophil elastase gene ELANE have been identified as the primary cause of severe congenital neutropenia (SCN) associated with recurring severe infections and evolution to acute myeloid leukemia (AML). As of today, more than 50 substitution, truncation, insertion and deletion mutations have been identified. Animal studies based on knock-in or knockout of ELANE in mice failed to produce severe neutropenia phenotype. We and others previously reported that expression of various mutants but not wild type neutrophil elastase (NE) in human but not murine cells triggers accelerated apoptosis. We also reported that expression of mutant NE (del.145-152), identified in SCN patients one of whom evolved to develop MDS/AML, in human promyelocytic tet-off HL60 cells causes both accelerated apoptosis and characteristic block of myeloid differentiation similar to that seen in bone marrow of SCN patients. Examination of the tertiary structure of NE revealed that most of the mutations leave the active site of the mutant protease intact. We identified a small molecule inhibitor of neutrophil elastase, a derivative of L-malic acid (Merck, USA), that blocked the proteolytic activity of NE by approximately 80% and was capable of restoring impaired myeloid differentiation and normalizing production of myeloid cells expressing del145-152 NE mutant. It is important to note that block of proteolytic activity of NE with the NE-SMI had no adverse effect on control human myeloid progenitor cells expressing wild type NE, thus confirming the gain-of-function effect of NE mutants. More than 20% of SCN patients with NE mutations evolve to develop AML. Molecular modeling and analysis of the tertiary structures of NE available through the Protein Database revealed that 16 different mutations identified in AML patients affect predominantly the N95 or N144 glycosylation sites or the binding pocket of the protease suggesting that altered substrate specificity of the mutant enzyme is the cause of accelerated apoptosis and block of myeloid differentiation in SCN/AML. We sought to obtain bone marrow samples from 2 unrelated SCN/AML patients both on G-CSF treatment harboring either C122Y or insPQ94. Bone marrow purified CD34+ and/or CD34-/CD33+ myeloid progenitors from the patients showed basal level of apoptosis in a range of 20–25%, which gradually increased reaching 40–50% apoptosis by 3 days of culture. Importantly, treatment of primary bone marrow-derived cells with NE-SMI substantially reduced accelerated apoptosis to near initial rate with approximately up to 2-fold reduction of apoptosis by 3 days of culture as determined by flow cytometry. Thus, our findings demonstrate that 1) small molecule inhibitor of neutrophil elastase is effective in blocking accelerated apoptosis triggered by three different NE mutations identified in SCN patients evolved to develop MDS/AML and 2) the small molecule inhibitor of NE is a promising therapeutic agent that should be considered for testing in clinical trials in SCN/AML patients.