Prostate cancer–associated SPOP mutations confer resistance to BET inhibitors through stabilization of BRD4

Abstract

The bromodomain and extra-terminal (BET) family of proteins, comprised of four members including BRD2, BRD3, BRD4 and the testis-specific isoform BRDT, largely function as transcriptional co-activators 1–3 and play critical roles in various cellular processes, including cell cycle, apoptosis, migration and invasion 4,5. As such, BET proteins enhance the oncogenic functions of major cancer drivers by either elevating their expression such as c-Myc in leukemia 6,7 or by promoting transcriptional activities of oncogenic factors such as AR and ERG in the prostate cancer setting 8. Pathologically, BET proteins are frequently overexpressed and clinically linked to various types of human cancers 5,9,10, therefore pursued as attractive therapeutic targets for selective inhibition in patients. To this end, a number of bromodomain inhibitors, including JQ1 and I-BET, have been developed 11,12 and shown promising outcomes in early clinical trials. Despite resistance to BET inhibitor has been documented in pre-clinical models 13–15 the molecular mechanisms underlying acquired resistance are largely unknown. Here, we report that Cullin 3SPOP earmarks BET proteins including BRD2, BRD3 and BRD4 for ubiquitination-mediated degradation. Pathologically, prostate cancer-associated SPOP mutants fail to interact with and promote the destruction of BET proteins, leading to their elevated abundance in SPOP-deficient prostate cancer. As a result, prostate cancer cells and prostate cancer patient-derived organoids harboring SPOP mutations are more resistant to BET inhibitor-induced cell growth arrest and apoptosis. Therefore, our results elucidate the tumor suppressor role of SPOP in prostate cancer by negatively controlling BET protein stability, and also provide a molecular mechanism for BET inhibitor resistance in prostate cancer patients bearing SPOP mutations.

Targeting Bromodomain and Extra-Terminal (BET) Family Proteins in Castration-Resistant Prostate Cancer (CRPC)

“…This is likely to reflect the demonstration that patients with lower BRD4 expression are more likely to present with local disease (lymph node negative, no distant disease) and receive radical treatment; an observation that warrants further investigation. SPOP mutant cancers have been identified as a subgroup of prostate cancer with increased BRD4 expression (31,32). Although we were unable to confirm these findings, this is likely explained by the small numbers (4 cases) of SPOP mutant cases and different patient population.…”

“…Nuclear BRD4 expression (continuous variable; per 100 HS) in CRPC biopsies was not significantly associated with OS (HR 0.63, 95% CI 0.32-1.24; p=0.18). Although SPOP mutant PC have been associated with increased BRD4 protein expression, we found no clear association between SPOP mutations (4 of 39 cases with SPOP status) and BRD4 protein expression (Supplementary Figure S3) (31,32). Taken together, these data suggest that higher nuclear BRD4 expression at diagnosis, but not CRPC, is associated with poorer patient outcome.…”

“…BRD4 is a critical coregulator of AR (29,30). Furthermore, increased BRD4 protein stability in substrate-binding adaptor speckle-type POZ protein (SPOP) mutant prostate cancers is associated with AR signaling (31,32). BET inhibitors (BETi) and proteolysis targeting chimera (PROTAC) induced BET protein degraders have anti-tumor activity in CRPC models (29,30,(33)(34)(35)(36), attenuating AR signaling and potently reducing C-MYC expression (29,30,33,34,36).…”

AZD5153 Inhibits Prostate Cancer Cell Growth in Vitro and in Vivo

“…Cancer-associated SPOP mutations are observed in the meprin and TRAF (Tumor necrosis factor receptor-associated factor) homology (MATH) domain, inhibiting substrate binding [40]. Recent studies have demonstrated that BET proteins, including BRD2, BRD3 and BRD4, are the targets of SPOP [18][19][20]. Wild-type SPOP binds to the a degron motif in the BET proteins, causing their ubiquitination and proteasomal degradation [18][19][20].…”

“…Recent studies have demonstrated that BET proteins, including BRD2, BRD3 and BRD4, are the targets of SPOP [18][19][20]. Wild-type SPOP binds to the a degron motif in the BET proteins, causing their ubiquitination and proteasomal degradation [18][19][20]. Conversely, prostate cancer-associated SPOP mutants are not able to bind to BET proteins, which will lead to impaired BET proteins proteasomal degradation [18][19][20].…”