A Protease-Resistant PML-RARα Has Increased Leukemogenic Potential in a Murine Model of Acute Promyelocytic Leukemia (APL).

Previous work from our group has demonstrated that the azurophil granule protease, neutrophil elastase (NE) can cleave PML-RARα (PR), the fusion protein that initiates acute promyelocytic leukemia; we identified valines 420 and 432 within the PML domain as the initial sites of NE cleavage. Furthermore, neutrophil elastase deficiency reduces the penetrance of leukemia in a knock-in model of APL in which the human PR cDNA is knocked into the 5′ untranslated region of the murine cathepsin G gene (Lane and Ley, Cell 2003) In this study, we demonstrate that mutation of residues V420 and V432 to arginine (2VR) confers resistance to proteolytic cleavage by NE. V420 and V432 are also cleaved using marrow extracts derived from NE deficient mice, indicating that other marrow-derived proteases can cleave PR at these two valines; the 2VR mutation confers resistance to these proteases as well. To determine the role that cleavage of PR plays in leukemogenesis, we generated protease-resistant PR mice by inserting the PR(2VR) allele into the murine cathepsin G locus via homologous recombination. Analysis of marrow from 6–8 week old preleukemic mice indicates that myeloid progenitors from PR(WT) and PR(2VR) mice both have increased replating efficiency in methocellulose cultures with a trend towards increased efficiency with PR(2VR)mice (WT=0%, PR(WT)=0.45%, PR(2VR)=1.65% at 4^th replating, p=NS). Both PR(WT) and PR(2VR) mice develop a phenotypically identical, fatal myeloid leukemia characterized by splenic infiltration with blasts that co-express CD117 (c-kit) and Gr-1, and that are morphologically promyelocytes. A tumor watch revealed that PR(2VR) mice (n=89) demonstrated a shortened latency of leukemia development (274 days) compared to PR(WT) mice (n=41, 473 days, p=0.0003). These results indicate that resistance to proteolysis confers an increased leukemogenic potential to the PR fusion protein, which may be due to increased levels of full length PR in the myeloid progenitors of these mice. Based on these observations, we suggest NE-mediated cleavage of PR is not required for its activity. Therefore, the mechanism by which NE deficiency affects APL penetrance may be indirect.