Identification of Mcl-1 as a Novel Target in Neoplastic Mast Cells and Demonstration of Cooperative Growth-Inhibitory Effects of mcl-1 Antisense Oligonucleotides, PKC412, and AMN107.

Mcl-1 is a Bcl-2 family-member that has been described to act anti-apoptotic in various myeloid neoplasms and therefore has been proposed as a potential therapeutic target. Systemic mastocytosis (SM) is a myeloid neoplasm involving myelomastocytic progenitors. We examined the expression and functional role of Mcl-1 in neoplastic mast cells (MC), to determine whether Mcl-1 could serve as a target in MC neoplasms. As assessed by RT-PCR and immunohistochemistry, primary neoplastic MC were found to express Mcl-1 mRNA and the Mcl-1 protein in a constitutive manner in all patients analyzed. Moreover, the human MC-leukemia cell line HMC-1 was found to express Mcl-1. Transfection of these cells with Mcl-1-specific antisense oligonucleotides (ASO) or an mcl-1-specific siRNA using lipofectin resulted in a reduced survival and increased percentage of apoptotic cells compared to control. The effects of mcl-1 ASO were seen with the HMC-1.1 subclone carrying the G560V c-kit mutation (mcl-1 ASO, 250 nM: 49±4% apoptotic cells compared to control: 3±2%, p<0.05; mcl-1 siRNA: 41±5% vs control: 5±3%, p<0.05) as well as with HMC-1.2 cells carrying both the G560V c-kit mutation and the D816V c-kit mutation (mcl-1 ASO, 250 nM: 36±2% apoptotic cells compared to control: 6±1%, p<0.05; mcl-1 siRNA: 30±6% vs control: 5±2%, p<0.05). Moreover, mcl-1 ASO were found to cooperate with the tyrosine kinase inhibitors (Novartis Pharma AG) imatinib, AMN107, and PKC412 in producing growth inhibition in HMC-1.2 cells. Together, these data show that Mcl-1 is a novel survival factor and attractive target in neoplastic human MC. Whether the Mcl-1-targeting concept can be developed far enough to reach clinical application remains to be elucidated.