In vivo RNAi screening identifies as a target for combination therapy with TKIs in BCP-ALL.

Despite the addition of tyrosine kinase inhibitors (TKIs) to the treatment of patients with B-cell precursor acute lymphoblastic leukemia ( BCP-ALL), relapse both with and without mutations is a persistent clinical problem. To identify -independent genetic mediators of response to the TKI dasatinib, we performed in vivo and in vitro RNA interference (RNAi) screens in a transplantable syngeneic mouse model of BCP-ALL. By using a novel combination of a longitudinal screen design and independent component analysis of screening data, we identified hairpins that have distinct behavior in different therapeutic contexts as well as in the in vivo vs in vitro settings. In the set of genes whose loss sensitized BCP-ALL cells to dasatinib, we identified , which regulates intracellular levels of platelet-activating factor (PAF), as an in vivo-specific mediator of therapeutic response. loss significantly sensitized leukemia cells to the multiple TKIs, indicating that inhibition of PAFAH1B3 in combination with TKI treatment may be an effective therapeutic strategy for BCP-ALL patients. PAF-induced cell death as well as surface levels of PAF receptor (PAFR) in our model are altered upon dasatinib treatment and depend on the local leukemia microenvironment; the response of KO vs overexpressing cells to dasatinib is also dependent on microenvironmental context. Antagonism of the PAFR partially reverses the observed sensitization to TKI treatment upon loss in vivo, suggesting that signaling via the PAF/PAFR pathway is at least partially responsible for this effect.