Synthetic lethality of p97 inhibition and PSMC2-overexpressionovercomes bortezomib resistance in multiple myeloma
The aim of the present study was to analyze genomic profiles obtained from multiple myeloma (MM) patient tumors to identify prognostic classifiers of clinical response and survival following bortezomib monotherapy and then target key actionable classifiers that underly bortezomib resistance. We discovered higher levels of proteasome PSMC2 messenger RNA in myeloma cells from patients that did not respond to bortezomib and that higher PSMC2 expression correlated with inferior progression-free and overall survival. Pharmacologic screening revealed that CB-5339, an ATP-competitive inhibitor of the molecular chaperone p97/VCP that shuttles ubiquitin-conjugated substrates to proteasomes, was synthetic lethal in MM cells engineered to overexpress PSMC2. p97 co-immunoprecipitated with 26S proteasomes affinity-purified from PSMC2-overexpressing cells which exhibited greater activity and reduced bortezomib sensitivity relative to PSMC2-KO cells. Bortezomib treatment followed by in-situ cross-linking and western blotting revealed higher levels of K48- and K63-linkage-specific ubiquitin chains covalently bound to 26S proteasomes isolated from PSMC2-KO cells compared to proteasomes affinity-purified from PSMC2-overexpressing cells. Western blotting demonstrated higher levels of 19S RPs, 26S proteasomes, and a PSMC2-PSMC1-PSMD2 subcomplex in PSMC2-overexpressing cells relative to PSMC2-KO cells. CB-5339 and bortezomib co-treatment synergistically targeted tumor cells obtained from MM patients previously exposed to bortezomib therapy and reduced the in vivo growth of PSMC2-overexpressing myeloma xenografts in mice. Taken together, we have discovered a genetic vulnerability that provokes proteasome-intrinsic bortezomib resistance and validated the potential of synthetic lethality to guide therapeutic interventions. Our study reaffirms that transcriptomic heterogeneity in MM is a primary driver of prognostic variability.