Glioblastoma (GBM) is the most common and aggressive brain tumor in adults. Yet, there are currently no highly effective therapies to treat patients with GBM and ~90% patients die within 2 years of diagnosis. The current standard of care treatment for GBM is surgery, radiation and chemotherapy (temozolomide), which significantly reduces tumor burden and volume in patients but does not prevent tumor regrowth and recurrence, resulting in only modest survival benefits. It is thought that GBM tumors contain slow-dividing GBM stem-like cells (GSCs) that are missed by surgery and survive radiation and chemotherapy to cause tumor regrowth. Developing drugs that specifically kill GSCs, therefore, will likely improve patient outcomes and survival.
In a recent paper published in Nucleic Acids Research Cancer, Dr. Patrick Paddison and his team sought to identify genes that are required for GSC survival but are dispensable for normal neural stem cells. Drugs that inhibit the function of these genes will likely kill GSCs while sparing normal healthy brain tissue. To identify GBM-specific survival genes, they performed CRISPR-Cas9 lethality screens in GSCs and NSCs. A top hit from their screen was FBXO42, a little studied gene that encodes an F-box protein that identifies proteins for targeted degradation by E3 ubiquitin ligase complexes. Only a few FBXO42 substrates have been identified so far (p53, ING4, and RBPJ) but they all play important roles in tumorigenesis. While the team showed that FBXO42 requires its substrate recognition and E3 ubiquitin ligase interaction domains for GBM viability, deletion of any of the previously identified FBXO42 substrates failed to rescue GBM viability following FBXO42 inactivation. Furthermore, FBOX42 loss did not lead to p53 accumulation as previously reported. Overall, their results indicate that FBOX42 loss leads to ubiquitination of an unidentified FBXO42 substrate that triggers GBM cell death.
While not yet identifying the critical FBXO42 substrates that cause GSC death, Dr. Paddison’s team discovered that FBXO42 loss caused either arrest during or a significant delay in mitosis. Time-lapse microscopy revealed that the FBXO42 deleted GSCs were arrested specifically in metaphase, during which the newly replicated chromosomes are connected to microtubule spindles and align in the center of the nucleus in preparation for their separation and cellular division. FBXO42 deleted GSCs suffered from a distorted spindle and dramatic increases in misaligned chromosomes compared to control GSCs. Thus, the GSC require FBXO42 to maintain proper spindle assembly and prevent mitotic arrest and cell death.
To further validate FBXO42 as a target for GBM therapy, Dr. Paddison’s team modeled FBXO42 loss in a mouse model of GBM. As expected, loss of FBXO42 caused tumor regression, although subsequent tumor outgrowth was observed. While FBXO42 was only required in two of five patient-derived GSC isolates tested, Dr. Paddison and his team discovered that ~15% of cancer cell lines from all tissue types appear to depend on FBXO42 activity using publicly available datasets. They subsequently validated their predictions using breast, bone, and gastric cancer cell lines. Overall, their results indicate that drugs that block the function of FBXO42 could be used to treat a wide variety of cancers.
Looking to the future, Dr. Paddison and his team’s paper highlights FBXO42 as an exciting new target for cancer therapy. However, several key questions remain: what are the FBXO42 substrates that mediate spindle assembly defects and cell in GSCs? How does FBXO42 ubiquitinate its substrate? Does FBXO42 K48-polyubiquitinate or K63-polyubiquinate its substrates leading to substrate degradation or changes in substrate activity, respectively? Which biomarkers predict sensitivity to FBOX42-targeted therapy? Can drugs be developed to selectively bind and inhibit FBXO42? Answering these questions will continue to drive work completed in the Paddison lab.
The spotlighted research was funded by the National Institutes of Health.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Patrick Paddison and James Olson contributed to this work.
Hoellerbauer P, Kufeld M, Arora S, Mitchell K, Girard EJ, Herman JA, Olson JM, Paddison PJ. 2024. FBXO42 activity is required to prevent mitotic arrest, spindle assembly checkpoint activation and lethality in glioblastoma and other cancers. NAR Cancer. 6, zcae021.
Nick Salisbury is a postdoctoral fellow in the Galloway lab at Fred Hutch. Nick's research focuses on understanding how DNA viruses, such as Merkel cell polyomavirus, cause cancer and developing new targeted therapies to treat malignancies caused by viruses. Originally from UK, he completed his BA and PhD at University of Cambridge before moving to US in 2016.