The role of ubiquitin networks in the optimization of BCMA-targeting therapies in Multiple Myeloma

• Applicant: Professor Dr. Florian Bassermann
• Subject Area: Hematology, Oncology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537477296

Multiple myeloma (MM) is an incurable mature B-cell malignancy with a median survival of approximately 5 years. The poor prognosis results from early relapse and the acquisition of resistance towards current treatment modalities which typically include proteasome inhibitors (PI), immunomodulatory drugs (IMiDs), antibodies as well as chemotherapy combined with autologous stem cell transplantation. Recently, immune cell based therapies including chimeric antigen receptor T-cell (CAR-T) and bi-specific T cell engaging antibody (BiTE) therapies against plasma cell/MM cell specific surface proteins such as B cell maturating antigen (BCMA) and G protein-coupled receptor, class C, group 5, member D (GPRC5D) have demonstrated very compelling response- and PFS-rates in patients with relapsed/refractory MM. Eventually however, all patients relapse and loss or reduction of BCMA expression in terms of antigen escape is considered to be an important underlying mechanism. We recently investigated whether different MM-relevant target surface-proteins, including BCMA, GPRC5D and SLAMF7, may be regulated via the ubiquitin-proteasome-system (UPS). BCMA was found to be the only candidate undergoing strong poly-ubiquitylation in multiple myeloma cells that resulted in a very short half-life of the protein, suggesting that much of its regulation occurs on the post-translational level. Proteasomal inhibition (PI) enhanced BCMA surface expression and enhanced the efficacy of BCMA-directed CAR-T cells in co-culture experiments with MM cell lines and primary PI resistant MM-patient cells. Moreover, our proteome wide screens for BCMA-specific ubiquitin E3 ligases and deubiquitylases (DUBs) revealed distinct promising candidates for further validation. Finally, we obtained first evidence on the presence of a regulatory transcriptional feedback loop between BCMA and downstream NF-κB-dependent signaling. Based on these studies, we here propose to (I) elucidate the biochemical and cell biological nature of BCMA ubiquitylation and validate E3 ligase and DUB candidates derived from our proteome-wide BCMA interaction screen, (II) determine the role of NF-κB in BCMA transcription and the impact of BCMA stabilization on oncogenic NF-κB signaling and (III) investigate mechanistic insights from Aims 1 and 2 in co-culture systems and murine MM models with regard to BCMA CAR-T efficacy and BCMA signaling and perform validation in primary patient samples from defined MM patient cohorts. We anticipate, that our multi-disciplinary approach will yield a thorough understanding of how the ubiquitin machinery regulates BCMA abundance and how these insights can be exploited for the optimization of BCMA-directed immunotherapies in MM.

• DFG Programme: Research Grants