The Oligosaccharyltransferase Complex: A Novel Vulnerability in Multiple Myeloma

Abstract

Multiple myeloma (MM) is described as a clonal expansion of malignant plasma cells in the bone marrow characterized by several clinical symptoms: bone lesions, hypercalcemia, anemia, serum monoclonal gammopathy, immune suppression, and multiple organ failure. Despite the success of advanced treatments, disease relapse is common and incurable. Thus, there is an unmet clinical need to identify novel targets for relapsed/refractory MM treatment.

The oligosaccharyltransferase (OST) complex catalyzes the transfer of N-glycan to a nascent translated protein which is a critical step in N-glycosylation, the most abundant post-translational modification of secretory and membrane-bound proteins. Aberrant expression of the OST complex or N-glycosylation is a diagnostic marker for solid tumors. However, the roles of the OST complex and N-glycosylation in MM development are virtually unknown. The mammal OST complex has two isoforms: OST-A and OST-B, with distinct catalytic subunits. Both isoforms share six subunits: OST4, RPN1, RPN2, DDOST, DAD1, and TMEM258. Analyzing the publicly available genomic data, we found that the shared subunits of the OST complex are abundantly expressed in MM cell lines and MM patient-derived cells. The expression levels of the OST subunits in relapsed MM patients are significantly higher than those in newly diagnosed MM patients. Intriguingly, patients expressing elevated levels of the OST complex are highly correlated with poor prognosis. These findings lead us to hypothesize that the OST complex might play a crucial role in MM.

To address this hypothesis, we used the CRISPR/Cas9 system to knockout the OST's shared subunits and found that MM cells are dependent on the OST complex. Deletion of DDOST and DAD1 (two central structural subunits of the OST complex) impaired MM cell growth, arrested the cell cycle, and induced apoptosis. Using the subcutaneous xenograft model, we found that knockout of DDOST or DAD1 suppressed MM growth in vivo and extended the survival of tumor-bearing mice. Intriguingly, suppression of the OST complex's enzymatic activity by a specific inhibitor, NGI-1, recapitulated the phenotypes of the OST complex deletion. Of note, NGI-1 sensitizes the MM cells and bortezomib-resistant MM cells (both MM cell lines and primary patient cells) to bortezomib treatment. Mechanistically, we found that disruption of the OST complex in MM cells significantly suppressed transcriptomic signatures of MM pathology (NF-κB signaling, glycolysis, MYC targets, and cell cycle) and induced apoptosis pathway as well as inflammatory pathway (interferon alpha and gamma). Intriguingly, we found that all known genes responsible for the bortezomib-resistant phenotype were significantly downregulated upon the suppression of the OST complex.

In conclusion, we identified the OST complex as a novel vulnerability in MM cells. Targeting the OST complex incorporation with Bortezomib might provide a novel and effective combined treatment for relapsed/refractory MM patients. We're currently investigating the underlying mechanisms of how the OST complex regulates MM pathology and developing patient-derived xenograft models to validate the therapeutic efficacy of combined treatment (NGI-1 and Bortezomib).