Abstract LB-245: Systems-based pharmacogenomics reveals that the ubiquitin ligase SCF-SKP2 is a molecular determinant of clinical response to bortezomib-based therapy
Authors: Malek Ehsan , Abdel Malek Mohamed , Driscoll James J. 
Published: August 2015

The molecular events that dictate the initiation, progression and therapeutic response in multiple myeloma (MM) remain incompletely understood. The ubiquitin (Ub) ligase Skp1-Cullin-1-Skp2 (SCFSkp2) complex promotes proliferation by inducing proteasomal degradation of the cyclin-dependent kinase inhibitor p27Kip1. Skp2 overexpression and reduced p27 levels are frequent events in human cancers and are associated with poor prognosis. Methods: Prospective pharmacogenomics was performed using next generation exome sequencing gene expression profiles (GEPs) compiled from the APEX/SUMMIT phase III clinical trial deposited in dataset GSE9782. 255 relapsed MM patients were randomized to bortezomib (BTZ) vs. dexamethasone (DEX) alone and progression-free (PFS) and overall survival (OS) then correlated with the pre-treatment GEPs. RPMI8226 cells were exposed for 8 months to successively escalated doses of the proteasome inhibitors BTZ, carfilzomib or ixazomib to generate chemoresistant cell lines. Side population (SP) cells were isolated from MMCLs and patients’ samples by flow cytometry sorting based upon Hoechst 33342 staining. Results: Pharmacogenomic correlation of GEPs with clinical response revealed hyperexpression of CUL1 and SKP2 in tumors from MM patients that did not respond to BTZ. CUL1 and SKP2 hyperexpression was correlated with significantly reduced PFS and OS after treatment with BTZ but not with DEX. Cullin-1 and Skp2 were elevated in cells generated with acquired PI-resistance. CUL1 or SKP2 genetic ablation significantly enhanced the sensitivity of chemoresistant cells to PIs. A high-throughput assay was then employed using GFP-tagged p27 to screen large databases and chemical libraries in order to identify novel lead compounds that inhibited the Skp2-dependent ubiquitination of p27. Treatment with DT204, a novel lead compound identified here, reduced p27 ubiquitination, its accumulation in myeloma cells and impaired Skp2 association with Cullin-1 and Cks1, a critical rate-limiting effector of Skp2 activity. The anti-myeloma effect of DT204 was diminished using a phosphorylation-defective p27 mutant. Co-treatment with DT204 enhanced the anti-myeloma effect of BTZ against MM cell lines, patient tumor cells, stem cells and the tumor-initiating clonogenic side population (SP). The in-vivo efficacy and survival benefit of DT204 as monotherapy or in combination with BTZ will be reported. Conclusion: Taken together, the results demonstrate that pharmacologics to selectively disrupt the SCFSkp2 complex-p27 axis within the UPS holds promise to overcome chemoresistance with therapeutic benefit for myeloma patients. Skp2, Cullin1 can be utilized as biomarkers to select MM patients who will benefit from early experimental therapeutics of Skp2-inhibitors.

Note: This abstract was not presented at the meeting.

Citation Format: Ehsan Malek, Mohamed Abdel Malek, James J. Driscoll. Systems-based pharmacogenomics reveals that the ubiquitin ligase SCF-SKP2 is a molecular determinant of clinical response to bortezomib-based therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-245. doi:10.1158/1538-7445.AM2015-LB-245