Functional identification of a synthetic lethal interaction between Aac11 and the Rb-E2F pathway. E.J. Morris, N.J. Dyson. Lab Molecular Oncology, MGH Cancer Center, Charlestown, MA.

   Appropriate tissue homeostasis depends on the tightly controlled balance between cell division and cell death. Accordingly, loss of control of this regulation can lead to tumorigenesis and pathology. The pRb and E2F protein family members are important regulators of G1-S progression and apoptosis. Consequently, mutations in the Rb pathway, whereby Rb inactivation leads to deregulated E2F activity, are important and frequent contributors to human neoplasia. However, regulation of E2F-mediated apoptosis in tumor cells is unclear and insight into this signaling pathway might identify novel targets for antitumor therapy. Studies of Rb and E2F function in Drosophila are ideal due mainly to smaller gene families and the ability to conduct unbiased genetic screening. Accordingly, we screened a Gal4 enhancer trap collection and generated a number of dE2f1-dependent phenotypes that could be copied by overexpression of the proapoptotic gene, reaper but not cyclin E. After some further characterization, we screened a dE2f1-dependent apoptotic wing phenotype against a genomic deficiency collection, as well as 2200 recessive lethal P-element transposon insertion lines, and identified a number of suppressor and enhancer mutations. Through secondary screening, we identified mutations in the Drosophila homolog of Antiapoptosis clone-11 (Aac11) as strong specific enhancers of multiple dE2f1-dependent phenotypes. Subsequent validation studies though transient transfection analysis demonstrated that RNA interference (RNAi) of Aac11 significantly enhanced dE2f1-induced apoptosis without affecting dE2f1 transcriptional activity. Moreover, we demonstrated that co-RNAi of both Aac11 and Rbf resulted in a synthetic lethal interaction. These data suggest that Aac11 might function in the Rb pathway as a downstream apoptotic suppressor and inhibition of Aac11 activity might be useful for treatment of tumors with Rb mutations. This work was supported by NRSA fellowship CA88474-02 (EJM).