Multiple myeloma (MM) is a malignancy of white blood cells called plasma cells that reside mainly in the bone marrow and is the second most common blood cancer. With increased understanding of biology, the current use of immunomodulatory (IM) drugs and proteasome inhibitors (PI) have taken over the therapeutic landscape for MM. The combination of bortezomib (PI) with lenalidomiede (IM) and dexamethasone is commonly the initial treatment of choice. Early intervention seems to provide a good outcome, but unfortunately, many patients eventually relapse.
A major goal in MM treatment is to increase the efficacy of proteasome inhibitors and prevent relapse. To investigate this, Huang et al. used unbiased mass spectrometry-based phosphoproteomics to identify potential vulnerabilities after treatment with the PI carfilzomib and discovered that splicing related proteins had significant changes in phosphorylation that is undetectable upon examination of RNA and protein abundance.
Treatment with carfilzomib resulted in an increase in phosphorylation of splicing related proteins in vitro but to determine if the response is specific to PI or a more general drug-induced stress response, phosphosites were evaluated upon treatment with a clinically used DNA alkylating agent and immunomodulatory agent, melphalan and lenalidomide respectively. There was some overlap in phosphosites between carfilzomib and melphalan, but melphalan treatment did not result in upregulation of SRSF (RNA splicing proteins) phosphorylation, while carfilzomib treatment did. These results suggest that treatment-induced stress may have a broad effect on the phosphorylation of splicing factors, but that the exact pattern of phosphosites is drug dependent, although treatment with lenalidomide resulted in minimal change in phosphorylation pattern. Interestingly, GO analysis of significant events across carfilzomib treated samples revealed enrichment of “proteasome-mediated ubiquitin-dependent protein catabolic process” and “protein polyubiquitination” suggesting that splicing may play a role in regulating protein machinery in PI treated cells.
The major rationale for proteasome inhibition in MM is to exploit the highly productive nature of plasma cells and their dependency on the proteasome to manage ER stress. PI treatment has been shown to selectively induce proteotoxic stress. Given that splicing may regulate proteins involved in ubiquitination, the authors hypothesized that increased disruption of splicing with the proof of concept spliceosome inhibitor E7107 may have a therapeutic effect in vivo.
Spliceosome inhibitors are currently in clinical trials as a therapeutic strategy in splicing factor mutant myeloid malignancies, for example AML. Many studies have shown that cells with mutant splicing factors are selectively sensitive to pharmacological splicing inhibition, and the authors show here that MM cell lines are either more or less sensitive to E1707 dependent on RNA expression levels of SF3B1. In vivo treatment with E1707 had significant survival benefit and ex vivo treatment of bone marrow cells from PI refractory patients showed promising sensitivity to E7107 with low nM range IC50.
Overall, the work presented suggests that the spliceosome is a targetable vulnerability in multiple myeloma. There is much anticipation in regard to the current clinical trial of the spliceosome inhibitor H3B-880 for AML, and if it has a good safety profile this work suggests MM patients may also benefit from this type of treatment.
A major goal in MM treatment is to increase the efficacy of proteasome inhibitors and prevent relapse. To investigate this, Huang et al. used unbiased mass spectrometry-based phosphoproteomics to identify potential vulnerabilities after treatment with the PI carfilzomib and discovered that splicing related proteins had significant changes in phosphorylation that is undetectable upon examination of RNA and protein abundance.
Treatment with carfilzomib resulted in an increase in phosphorylation of splicing related proteins in vitro but to determine if the response is specific to PI or a more general drug-induced stress response, phosphosites were evaluated upon treatment with a clinically used DNA alkylating agent and immunomodulatory agent, melphalan and lenalidomide respectively. There was some overlap in phosphosites between carfilzomib and melphalan, but melphalan treatment did not result in upregulation of SRSF (RNA splicing proteins) phosphorylation, while carfilzomib treatment did. These results suggest that treatment-induced stress may have a broad effect on the phosphorylation of splicing factors, but that the exact pattern of phosphosites is drug dependent, although treatment with lenalidomide resulted in minimal change in phosphorylation pattern. Interestingly, GO analysis of significant events across carfilzomib treated samples revealed enrichment of “proteasome-mediated ubiquitin-dependent protein catabolic process” and “protein polyubiquitination” suggesting that splicing may play a role in regulating protein machinery in PI treated cells.
The major rationale for proteasome inhibition in MM is to exploit the highly productive nature of plasma cells and their dependency on the proteasome to manage ER stress. PI treatment has been shown to selectively induce proteotoxic stress. Given that splicing may regulate proteins involved in ubiquitination, the authors hypothesized that increased disruption of splicing with the proof of concept spliceosome inhibitor E7107 may have a therapeutic effect in vivo.
Spliceosome inhibitors are currently in clinical trials as a therapeutic strategy in splicing factor mutant myeloid malignancies, for example AML. Many studies have shown that cells with mutant splicing factors are selectively sensitive to pharmacological splicing inhibition, and the authors show here that MM cell lines are either more or less sensitive to E1707 dependent on RNA expression levels of SF3B1. In vivo treatment with E1707 had significant survival benefit and ex vivo treatment of bone marrow cells from PI refractory patients showed promising sensitivity to E7107 with low nM range IC50.Overall, the work presented suggests that the spliceosome is a targetable vulnerability in multiple myeloma. There is much anticipation in regard to the current clinical trial of the spliceosome inhibitor H3B-880 for AML, and if it has a good safety profile this work suggests MM patients may also benefit from this type of treatment.
REFERENCES:
Kumar SK, Rajkumar V, Kyle RA, et al. Multiple myeloma. Nature Reviews Disease Primers. 2017;3(1). doi:10.1038/nrdp.2017.46
Huang HH, Ferguson ID, Thornton AM, et al. Proteasome inhibitor-induced modulation reveals the spliceosome as a specific therapeutic vulnerability in multiple myeloma. Nature Communications. 2020;11(1). doi:10.1038/s41467-020-15521-4
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