Targeting BCR Tyrosine177 site with novel SH2-DED causes selective leukemia cell death in vitro and in vivo

Emergence of resistance to imatinib mesylate complicates the treatment of chronic myeloid leukemia (CML). Second-generation tyrosine kinase inhibitors are capable to overcome resistance mediated by most mutations except T315I. As this mutation is causative for 20% of clinically observed resistances, the need for novel treatment strategies becomes obvious and urgent. The autophosphorylated BCR/ABL Tyr177 recruits Grb2 via its SH2 domain, which is required for efficient induction of the myeloproliferative disease by BCR/ABL. The death effector domain (DED) is the critical factor for activation of caspase-8 induced apoptosis signal. We thus speculated that transduction of an exogenous SH2-DED (SD) fragment into the CML cells may inhibit the binding of BCR/ABL Tyr177 and Grb2, activate caspase-8 induced apoptosis and serve as a novel CML treatment strategy. The infection of the recombinant adenovirus Ad5/F35-SD was verified to show both cell proliferation-inhibitory and apoptosis-inducing effect. Further exploration into the underlying mechanisms revealed that Ad5/F35-SD exerted its function by binding to the phospho-BCR/ABL Tyr177 site, reducing Ras, MAPK and AKT kinase activities, and activating caspase-8 induced apoptosis signal by DED protein binding to DED domain of precursor caspase-8. Moreover, high anti-proliferative activity of Ad5/F35-SD was observed in nude mice and its leukemia-protective effect was evident in chronic myeloid leukemia model mice injected with BCR/ABL+ BaF3 cells. In conclusion, Ad5/F35-SD exhibits anti-proliferative and pro-apoptotic activity on BCR/ABL positive leukemia cells in vitro and in vivo through disruption of Grb2 SH2-phospho-BCR/ABL Tyr177 complex formation and induction of caspase-8 activation.