Multiple myeloma (MM) is a blood cancer characterized by the accumulation of abnormal plasma cells. Normally, plasma cells secrete infection-fighting antibodies; when plasma cells turn cancerous, however, the cancer cells eventually outgrow the normal plasma cells, compromising the immune system. Multiple myeloma can be treated with CAR T-cells which recognize a surface protein called BCMA to specifically eliminate cancerous MM cells. This therapy works remarkably well when all of the myeloma cells express BCMA. However, some MM cells become resistant to CAR T therapy by shedding their BCMA surface proteins, which presents two problems. For one, cancer cells that shed BCMA become effectively invisible to BCMA-targeting CAR T-cells. For another, the shed BCMA competes with BCMA that is attached to myeloma cells, so the CAR T-cells get distracted by the shed protein and become less effective at killing the cancer cells. The shed BCMA proteins are cleaved by an enzyme called gamma secretase. Based on previous studies using mice, inhibiting gamma secretase can improve the efficacy of CAR T-cells by leaving the BCMA targets in place on the cancer cells. Follow-up mouse studies led to a clinical trial that showed that combining gamma secretase inhibitors (GSIs) with CAR T-cell therapy dramatically improved patient outcomes. Despite promising results, questions remain about how gamma secretase inhibition affects the other cells in the blood system. Many other clinical trials use GSIs in combination with other drugs, as well. “I feel that there are a lot of people out there who want to know what GSI may be doing to the hematopoietic system…without considering the other therapies that we’re giving,” says Dr. David Coffey, lead author of the study published in Blood.
To investigate the broad impacts of GSIs on blood cells, Dr. David Coffey from the University of Miami and Dr. Damian Green (in collaboration with the Geoff Hill and Riddell labs of the Fred Hutch Translational Science and Therapeutics division) collected samples from MM patient bone marrow before and after treatment with GSIs. They used single-cell RNA sequencing to determine how gamma secretase inhibition changes gene expression in the different bone marrow cell types. In parallel, they used single-cell ATAC sequencing to rigorously determine how changes in DNA accessibility impacted the changes in gene expression. Surprisingly, they found that the expression of 567 genes across 20 cell types was significantly different after GSI therapy. This indicates that gamma secretase inhibition likely has effects beyond increased tumor killing in patient bone marrow.
Because gamma secretase targets BCMA, its inhibition increases the amount of BCMA on the surface of cells. This increase makes the BCMA-targeted CAR T-cells more effective, but Coffey and Green suspected that it may also increase the expression of signaling genes downstream of BCMA. Indeed, GSI treatment increased the expression of genes known to be activated by BCMA in the MM cells. Many of these genes were related to cell survival and proliferation, which could be detrimental to patient outcomes if patients are only treated with GSIs. “There should be some caution in [using GSIs as a monotherapy] because there could be some proliferative effects on the tumor cells,” says Coffey.
Alongside this potential limitation, the team found some potential unexpected benefits, as well. Inhibiting gamma secretase decreased the number of non-classical monocytes—a type of immune cell—in the bone marrow of patients. Non-classical monocytes promote tumor growth by releasing molecules that confer growth and survival potential to MM cells. When the group tested the ability of monocytes treated with GSIs to support tumor growth, they found that the treated monocytes produced significantly fewer cytokines that support cancer cells. These results are exciting and suggest that gamma secretase inhibition could provide an additional therapeutic benefit to patients by regulating the ability of monocytes to support cancer cells.
In addition to the transcriptional changes the group observed, they also discovered previously unreported effects of anti-BCMA therapy. Initial clinical trials testing GSIs excluded patients that had previously received anti-BCMA therapies. Coffey and Green took a different approach by including these patients to characterize how gamma secretase inhibitors would affect this group. Surprisingly, they found that the BCMA coding region in the genome was completely deleted in two patients who received prior anti-BCMA therapy. “While GSIs have great potential, if the patient has a deleted BCMA gene, it’s going to have no [BCMA-specific] effect,” says Coffey. This suggests that clinicians may want to genetically screen patients to make sure their BCMA gene is intact. Overall, the team revealed nuanced and unexpected roles for GSIs in the hematopoietic system of patients with multiple myeloma. “We have to be careful…there are a lot of off-target effects that we need to be aware of, and that’s really what this study brings to light,” says Coffey. In the future, the group plans on interrogating more precise molecular effects of GSI treatment on myeloma cells.
This work was supported by a donation from Richard Lane and research funding from Juno Therapeutics, the National Institute of Health, the Leukemia and Lymphoma Society, Defeat Myeloma, the Brotherton Family, and the Quest for Truth Foundation.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Stanley Riddell, Geoffrey Hill, and Andrew Cowan contributed to this work.
Coffey DG, Atilla PA, Atilla E, Landgren O, Cowen AJ, Simon S, Pont MJ, Comstock ML, Hill GR, Riddell SR, Green DJ. 2024. Single-cell analysis of the multiple myeloma microenvironment after 𝛾-secretase inhibition and CAR T-cell therapy. Blood. 10.1182/blood.2024025231.
Kelsey Woodruff is a PhD candidate in the Termini Lab at Fred Hutch Cancer Center. She studies how acute myeloid leukemia cells remodel the sugars on their membranes to reprogram cancer cell signaling. Originally from Indiana, she holds a bachelor's degree in Biochemistry from Ball State University. Outside of lab, you can find her crocheting and enjoying the Seattle summers.