Abstract C124: HIF-independent synthetic lethality between CDK4/6 inhibition and VHL loss across species

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene and its protein product, pVHL, occurs in ~90% of clear cell renal cell carcinoma (ccRCC) cases. pVHL is part of the ubiquitin ligase complex that targets the HIF2α transcription factor, an oncoprotein in ccRCC, for proteasomal degradation. Without functional pVHL, HIF2α accumulates and induces inappropriate transcription of angiogenic, invasive, and growth-promoting genes. Drugs inhibiting HIF2α or its downstream target VEGF are active against ccRCC, but both de novo and acquired resistance are common and no current therapies are curative. We hypothesized that loss of pVHL might create context-specific dependencies that could then be targeted therapeutically. To look for such synthetic lethal targets, we generated human and Drosophila cell pairs that were isogenic for pVHL (or its Drosophila ortholog) and screened using RNAi and chemical compound libraries. The overlap of the hits from these screens identified a hyperdependence on CDK4/6 activity in pVHL-defective cells compared to their pVHL-proficient counterparts. In secondary assays we confirmed that pharmacologic inhibition of CDK4/6 by either Abemaciclib or Palbociclib preferentially reduced viability of VHL-/-cells as compared to VHL+/+cells across a variety of human ccRCC cell lines. Importantly, inhibition of VHL-/-ccRCC cells by Palbociclib was abrogated by expressing a Palbociclib-resistant CDK6 cDNA or by knockout of the canonical CDK4/6 target pRB. Sensitivity to Palbociclib could be reversed by expressing exogenous pVHL, but not a pVHL mutant lacking its known substrate docking site. HIF2aknockout in VHL-/-cells did not eliminate the effect of CDK4/6 inhibition, thus HIF2ais not necessary for the synthetic lethal relationship between VHLand CDK4/6. Moreover, the combination of the HIF2ainhibitor PT2399 and the CDK4/6 inhibitor Palbociclib synergistically suppressed proliferation of VHL-/-ccRCC in vitro in HIF2a-sensitive cell lines. Both Palbociclib and Abemaciclib suppress VHL-/-ccRCC tumor growth in nude mice, including tumors that are PT2399-resistant. My future work will aim to identify the mechanism that underlies the synthetic lethality between VHLand CDK4/6 activity. Citation Format: Hilary Nicholson, Benjamin Housden, Norbert Perrimon, William G Kaelin. HIF-independent synthetic lethality between CDK4/6 inhibition and VHL loss across species [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C124. doi:10.1158/1535-7163.TARG-19-C124