Smac mimetics, Inhibitors Apoptosis Protein (IAP) antagonists, have been reported to treat various cancers via TNF-dependent extrinsic apoptotic pathway upon rapid autoubiquitylation and proteasomal degradation of cIAPs.
MLL (mixed lineage leukemia) rearrangements are driven by dysregulated epigenetic mechanisms. The N-terminal sequence of MLL fuses with other proteins, forming proteins such as MLL-ENL, MLL-AF9 and MLL-AF4, that require few or no additional mutations to cause leukaemia. Nup98-HOXA9 is a non-MLL fusion protein. Most AMLs are also associated with increased expression of the homeobox gene HoxA9 and its binding partner Meis1. Overexpression of MLL fusion proteins (MLL-ENL and MLL-AF9), Nup98-HoxA9 or HoxA9 and Meis 1 in progenitor/stem cells is sufficient to induce AML in mice.
Previous studies suggest that combined overexpression of IAPs and downregulation of the endogenous IAP antagonist SMAC, is associated with poor overall survival in leukaemia. To determine the sensitivity of different AML to Smac-mimetic compounds, primary tumor cells obtained from murine AML in vivo models, were treated with different concentrations of Smac-mimetics. Interestingly, we found that different AMLs have different sensitivity to the compounds tested, including the Smac-mimetic in clinical trials Birinapant. MLL-ENL leukemias were the most sensitive to all smac-mimetics tested, while MLL-AF9 had an intermediate response and HOXA9 +Meis1 and Nup98-HOXA9 cells were resistant.
Cell death induced by Smac-mimetics in our models was TNF-dependent and associated with caspase-8 activation. However, there was no correlation between sensitivity to smac-mimetic and levels of proteins that participate in TNF signaling, such as IAPs (cIAP1, XIAP), necroptosis-related (RIP3/MLKL), apoptosis-related (Caspase 8) and the sensitivity of Smac-mimetics.
Together these results suggest the smac-mimetics treatment induces apoptosis and efficiently kill AML harbouring MLL translocation. Smac-mimetics may therefore be a good candidate drug to trial for the treatment of AML and ALL.