Harnessing the E3 ligase SPOP for targeted degradation of the NUP98::KDM5A fusion oncoprotein.

Daxx is a multifunctional protein that regulates a variety of cellular processes, including transcription, cell cycle, and apoptosis. SPOP is a BTB (Bric-a-brac/Tramtrack/Broad complex) protein that constitutes Cul3-based ubiquitin ligases. Here we show that SPOP serves as an adaptor of Daxx for the ubiquitination by Cul3-based ubiquitin ligase and subsequent degradation by the proteasome. Expression of SPOP with Cul3 markedly reduced Daxx level, and this degradation was blocked by SPOP-specific short hairpin RNAs. Inhibition of the proteasome by MG132 caused the prevention of Daxx degradation in parallel with the accumulation of ubiquitinated Daxx. Expression of SPOP with Cul3 reversed Daxx-mediated repression of ETS1- and p53-dependent transcription, and short hairpin RNA-mediated knock down of SPOP blocked the recovery of their transcriptional activation. Furthermore, Daxx degradation led to the cleavage of poly(ADP-ribose) polymerase and the increase in the number of terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein nick end-labeling-positive apoptotic cells. These results suggest that SPOP/Cul3-ubiquitin ligase plays an essential role in the control of Daxx level and, thus, in the regulation of Daxx-mediated cellular processes, including transcriptional regulation and apoptosis.

Decision letter: SPOP mutation leads to genomic instability in prostate cancer

Adaptor Protein Self-Assembly Drives the Control of a Cullin-RING Ubiquitin Ligase

Understanding the functional significance of the essential elements in maintaining genomic stability provides insights into the process of tumor initiation and progression, and predicts therapeutic responses. One such element that has recently attracted significant attention is the Speckle-Type Poz Protein (SPOP), an E3 ubiquitin ligase adaptor protein. SPOP is frequently mutated or has altered expression in various cancers, including prostate, renal and endometrial. SPOP is involved in the regulation of proteasome-mediated degradation of several oncoproteins. Moreover, recent data also indicate SPOP's direct involvement in the DNA damage response. SPOP mutants induce alternations in the DNA damage repair pathway by promoting the error-prone Non-homologous end joining (NHEJ) pathway. SPOP has been linked with significant functions in cellular signaling pathways and cancer suppression. This mini-review will discuss recent findings regarding SPOP's role in genomic stability in the pathological setting.

The Speckle type Poz Protein (SPOP) is a tumor suppressor that is often mutated in prostate cancer. Mutation of SPOP is linked to genomic instability and it plays a critical role in prostate cancer initiation. SPOP mutations are found to be in the heterozygous state in patients. The relevance of these mutations on its protein expression has not been systematically investigated. Here we report that mutations in the center MATH domain of SPOP lead to enhanced protein stability in prostate cancer. Among the mutations, F102C, S119N, F125V and WI31G showed increased protein stability. Further we show that these mutations in the MATH domain increase half-life of SPOP protein. Interestingly, the mutant SPOPs enhanced the endogenous wild-type SPOP level. We further explored the structural insights of these SPOP mutants. We present a computational model to predict how SPOP mutations potentially lead to conformational changes within the protein, promoting its stability. Finally, immunohistochemistry in clinical samples of prostate cancer augment our findings. Together, we highlight a subset of clinically relevant SPOP mutations that have impact on endogenous SPOP protein stability, allowing to further understand its role in prostate cancer initiation and evaluating of potentially altered therapeutic response in patients harboring these mutations. Citation Format: Joshua Fried, Vinayak Khattar, Jinlu Ma, Qinghua Zeng, Wei Zhang, Bo Xu. Mutations in the center MATH domain of Speckle Type BTB/POZ Protein lead to enhanced proteinstability in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4448.

MP41-06 SPOP MUTATIONS IN PROSTATE CANCER ACROSS DEMOGRAPHICALLY DIVERSE PATIENT COHORTS.

Background: Prostate cancer (PC) is the single most common and second-most lethal cancer in men, with over 268,000 estimated cases and over 34,500 estimated deaths in the US in 2022. The Speckle-Type POZ protein (SPOP) mutant subclass of PC accounts for 10% to 15% of all primary PC cases. SPOP is an adaptor for Cullin3/Ring (CUL3-RING)-type E3 ubiquitin ligase complexes and provides substrate specificity. The Cancer Genome Atlas (TCGA) studies show that SPOP is the most frequently mutated gene in primary prostate cancer (PC). Interestingly, PC-associated SPOP mutations are always missense and occur in a heterozygous fashion. The current gap in knowledge is the lack of understanding of the role of wildtype SPOP in PC. Methods: By utilizing prostate specific SPOP knockout (KO) mice, we recently reported increased levels of AR and MYC protein and increased cellular turnover (both proliferation and apoptosis) in the prostate luminal epithelium compared to wildtype prostates. We now characterized these mice for the expression of Cre protein and SPOP mRNA at different age using immunohistochemistry and RNA in situ hybridization. Furthermore, we performed RNA-sequencing analysis in the SPOP knockout mice and matched control littermates. Moreover, we performed RNA-seq in LNCaP, LNCaP-Abl, and RWPE1 cells following SPOP inhibition via siRNA targeting SPOP. Finally, we compared our SPOP inhibition signature from in vitro cell lines and prostate specific SPOP knockout murine model to gain insights about the role of wildtype SPOP protein in the prostate epithelium. Result: Using our Spopfl/fl;PBCre+ model, we observed SPOP floxed cells are rapidly lost and the murine prostate epithelium was repopulated with SPOP wildtype carrying cells. Similarly, knockdown (KD) of SPOP through siRNA treatment in a panel of PC cell lines resulted in a significant reduction in cell viability. These observations suggest that SPOP is important for the normal prostate cell viability. Further transcriptomic profiling of SPOP KO (from transgenic murine model) as well as siSPOP treated in vitro prostate cell lines revealed a significant reduction in the transcriptional activity of the AR. Conclusion: Our data illustrate for the first time a critical role for SPOP in the growth and survival of the prostate epithelium and prostate cancer cell. Our findings further validates SPOP as a important therapeutic target for the treatment of prostate cancer. Citation Format: Kinza Rizwan, Darlene Skapura, Cammy Mason, Cristian Coarfa, Nicholas Mitsiades, Damian Young, Salma Kaochar. SPOP loss places the prostate luminal epithelial cells at a selective disadvantage [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B069.

The Speckle type Poz Protein (SPOP), an E3 ubiquitin ligase adaptor, has recently been identified as the gene that has the most common somatic point mutations in prostate cancer. SPOP mutations are associated with genomic alterations, indicating a role for SPOP in the maintenance of genome stability. We, and others, have recently demonstrated a critical role of SPOP in the DNA damage response (DDR), suggesting SPOP mutants may represent a subgroup of patients that have hyper sensitivity to DNA damaging therapies. However, how SPOP mutations might impact its function and their roles in the progress of prostate tumorigenesis remain to be extensively studied. Genomic studies have found that SPOP mutations are clustered within its substrate binding pocket. Of a particular interest, Serine119 is found to be frequently mutated to Asparagine (S119N). Using computational modeling, we found that Ser119 resides in the SBC-MATH binding interface and is in close contact with the non-polar residue of the SPOP-binding consensus motif. Therefore we hypothesized that phosphorylation of Serine 119 might directly affect substrate binding. First we characterized prostate cancer cells expressing this mutation and we found that they displayed a suboptimal DNA damage response with prolonged DNA repair. Using an in situ proximity ligation assay, we demonstrate that Serine 119 is essential for SPOP ionizing irradiation induced interaction with the Ataxia Telangiectasia Mutated kinase (ATM), a major hub protein in the DDR. With the evidence of SPOP being a phospho-protein, we further provide in vitro evidence that ATM phosphorylates SPOP on Serine 119 in response to DNA damage. Characterization of the functional significance of ATM-mediated SPOP phosphorylation indicates a wide range of downstream targets regulating cell cycle progression and DNA repair. Taken together, our data reveal a critical pathway linking ATM and SPOP in regulation of prostate cancer initiation and therapeutic responses to DNA damage. This also provides the first evidence of a pathophysiological relevant mutation linked to ATM phosphorylation in the DDR. Citation Format: Joshua Fried, Rebecca Boohaker, Qinghua Zeng, Wei Zhang, Bo Xu. Characterization of the ATM-SPOP pathway in prostate cancer tumorigenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2752.

Background: Prostate cancer (PC) is the single most common and second-most lethal cancer in men, with over 268,000 estimated cases and over 34,500 estimated deaths in the US in 2022. The Speckle-Type POZ protein (SPOP) mutant subclass of PC accounts for 10% to 15% of all primary PC cases. SPOP is an adaptor for Cullin3/Ring (CUL3-RING)-type E3 ubiquitin ligase complexes and provides substrate specificity. The Cancer Genome Atlas (TCGA) studies show that SPOP is the most frequently mutated gene in primary prostate cancer (PC). Interestingly, PC-associated SPOP mutations are always missense and occur in a heterozygous fashion. The current gap in knowledge is the lack of understanding of the role of wildtype SPOP in PC. Methods: By utilizing prostate specific SPOP knockout (KO) mice, we recently reported increased levels of AR and MYC protein and increased cellular turnover (both proliferation and apoptosis) in the prostate luminal epithelium compared to wildtype prostates. We now characterized these mice for the expression of Cre protein and SPOP mRNA at different age using immunohistochemistry and RNA in situ hybridization. Furthermore, we performed RNA-sequencing analysis in the SPOP knockout mice and matched control littermates. Moreover, we performed RNA-seq in LNCaP, LNCaP-Abl, and RWPE1 cells following SPOP inhibition via siRNA targeting SPOP. Finally, we compared our SPOP inhibition signature from in vitro cell lines and prostate specific SPOP knockout murine model to gain insights about the role of wildtype SPOP protein in the prostate epithelium. Result: Using our Spopfl/fl;PBCre+ model, we observed SPOP floxed cells are rapidly lost and the murine prostate epithelium was repopulated with SPOP wildtype carrying cells. Similarly, knockdown (KD) of SPOP through siRNA treatment in a panel of PC cell lines resulted in a significant reduction in cell viability. These observations suggest that SPOP is important for the normal prostate cell viability. Further transcriptomic profiling of SPOP KO (from transgenic murine model) as well as siSPOP treated in vitro prostate cell lines revealed a significant reduction in the transcriptional activity of the AR. Conclusion: Our data illustrate for the first time a critical role for SPOP in the growth and survival of the prostate epithelium and prostate cancer cell. Our findings further validates SPOP as a important therapeutic target for the treatment of prostate cancer. Citation Format: Kinza Rizwan, Darlene Skapura, Cammy Mason, Cristian Coarfa, Nicholas Mitsiades, Damian Young, Salma Kaochar. SPOP: An essential gene for normal and prostate tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1705.

Personalized treatment, which is designed to only block oncogenic pathways activated by specific cancer mutations, is rapidly rising as a superior treatment for cancer. One gene that has recently been identified as highly mutated in a variety of cancers is Speckle Type POZ protein (SPOP), a substrate binding subunit of an E3 ubiquitin ligase that has been shown to target Sonic Hedgehog (SHH) downstream effectors for degradation. Interestingly, recent studies have found that SPOP is downregulated in up to 70% of breast cancer tumors and, furthermore, a correlation has been found between high levels of SHH signaling and poor prognostic pathological breast cancer features. Previous studies from our laboratory have shown that SPOP binds directly to the SHH downstream effector GLI3 to target it for degradation in a manner that is disrupted by prostate cancer-associated SPOP mutations. Since SHH signaling has been shown to be a key player in breast cancer oncogenesis, it is likely that SPOP plays a similar role in breast cancer progression. We therefore hypothesize that downregulation of SPOP induces hyper-activated SHH signaling to promote breast cancer progression through stabilization of GLI3. In order to study this, we employed a lentivirus-mediated approach to generate SPOP knockdown breast cancer cells. Next, we carried out western blot analysis to examine GLI3 protein levels when SPOP is downregulated followed by co-immunoprecipitation analysis to examine a GLI3/SPOP interaction. Finally, we used MG132 treatment to block the proteasome pathway in order to determine whether SPOP-mediated degradation of GLI3 functions through the proteasome. Our results indicate that SPOP knockdown promotes upregulation of the SHH downstream effector GLI3 through a direct physical interaction. Furthermore, we show that SPOP likely targets GLI3 for degradation through the proteasome pathway as MG132 treatment promotes GLI3 stabilization even in the presence of SPOP. Our collective results therefore indicate that SPOP targets GLI3 for degradation through the proteasome pathway which, in turn, would cause hyper-activated SHH signaling and subsequent progression of breast cancer oncogenesis.

Since breast cancer is the second leading cause of cancer deaths among American women there is a compelling need to uncover novel and superior treatment regimens for women suffering from this disease. Interestingly, several studies have indicated that up to 70% of breast cancer patients have downregulation of the E3 ubiquitin ligase Speckle Type POZ Protein (SPOP) thus indicating that SPOP is a critical tumor suppressor gene in breast cancer. Our lab has previously shown that SPOP targets GLI3 for ubiquitination to promote SHH signaling in prostate cancer patients, a finding that we hypothesized to be true for breast cancer as well since SHH signaling is often found to be hyper-activated in advanced breast cancer. We further hypothesized that SPOP downregulated breast cancer cells therefore will likely have a differential response to targeted therapy and that SHH targeted drugs could prove to be beneficial for SPOP downregulated breast cancer patients. First of all, to study the effect of downregulated SPOP, a lentivirus carrying an shRNA against SPOP was generated and infected into MCF-7 cells. Proliferation assays were utilized to confirm that downregulated SPOP plays a role in the increased proliferation of breast cancer cells. Next, quantitative PCR was performed to determine the effect of downregulated SPOP on the expression of GLI3-dependent SHH target genes. Finally, a natural compound library was utilized to find a novel therapeutic strategy for SPOP downregulated breast cancer. Our findings confirmed that SPOP knockdown increases proliferation of MCF-7 cells in vitro. Furthermore, quantitative PCR confirmed that GLI3 target genes are upregulated in MCF-7 cells when SPOP expression is low. Finally, our natural compound library screen identified novel therapeutic compounds that specifically target SPOP downregulated MCF-7 cells in a manner that involves disruption of GLI3-dependent SHH signaling. These findings therefore provide critical insight into novel treatment strategies for patients suffering from SPOP downregulated breast cancer.

The purpose of this study is to functionally annotate endometrial cancer (EC) specific mutants of SPOP (Speckle-type POZ protein). SPOP is the substrate adaptor for the E3 ligase complex CUL3-SPOP-RBX1, which is responsible for mediating the ubiquitination, and subsequent proteasomal degradation of a number of protein substrates including the SRC3 (NCOA3/AIB1) oncoprotein. Our laboratory has previously shown that SPOP is frequently mutated in serous and clear cell ECs, two clinically aggressive subtypes of EC. Importantly, the vast majority of SPOP mutants localized within the MATH domain, which is responsible for substrate binding. We hypothesize that EC associated SPOP mutations encode loss-of-function mutants that abolish or attenuate substrate degradation. To test this hypothesis, we initially focused our attention on five SPOP mutations we uncovered in ECs (E47K, S80R, P94A, M117I and R121Q). HEK293 cells were transiently co-transfected with a HA-tagged SRC3-expressing construct and a MYC-tagged construct expressing either vector alone, wildtype SPOP, or mutant forms of SPOP. Protein lysates from transfected cells were subjected to Western blotting to measure exogenous levels of HA-SRC3 and MYC-SPOP. Cells exogenously expressing wildtype SPOP or the E47K, P94A, M117I or R121Q mutants exhibited lower levels of HA-SRC3 than cells expressing the vector control. In contrast, cells expressing the S80R mutant had elevated levels of HA-SRC3 compared to cells expressing either wildtype SPOP or the vector control. Co-immumoprecipitation experiments are being performed to assess the substrate binding capacity of SPOP mutants. Our findings indicate that the SPOP-S80R mutation, which is recurrent in EC, has an impaired ability to down-regulate SRC3 protein levels compared to wildtype SPOP. Ongoing and future studies aim to elucidate the mechanism that underlies the effect of SPOP-S80R, and to functionally annotate additional SPOP mutations in EC. Citation Format: Fred Lozy, Mary Ellen Urick, Meghan Rudd, Deena Maurer, Daphne W. Bell. Functional analysis of endometrial cancer mutants of the E3 ubiquitin ligase adaptor protein, SPOP. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 62.

Speckle Type POZ Protein (SPOP), despite its tumor type-dependent role in tumorigenesis, primarily as a tumor suppressor gene is associated with a variety of different cancers. However, its function in pancreatic cancer remains uncertain. SPOP expression and the association between its expression and patient prognosis and immune function were evaluated using The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), The Tumor Immune Estimation Resource 2.0 (TIMER2.0) database, cBioportal, and various bioinformatic databases. Enrichment analysis of SPOP and the association between SPOP expression with clinical stage and grade were analyzed using the R software package. Then immunohistochemistry (IHC) was used to estimate the correlation between SPOP and tumor-infiltrating lymphocytes (TILs) in patients with pancreatic cancer. As part of our study, we assessed that SPOP was anomalously expressed in kinds of cancers, associated with clinical stage and outcomes. Meanwhile, SPOP also played a crucial role in the tumor microenvironment (TME). The expression level of SPOP was significantly correlated to tumor-infiltrating immune cells (TICs) in pancreatic cancer. Our study uncovered the potential corrections in SPOP with TICs, suggesting that SPOP may act as a biomarker for immunotherapy in pancreatic cancer.

BackgroundTumor suppressor epigenetic silencing plays an important role in non-small cell lung cancer (NSCLC) development and progression. Previously, the expression of speckle-type POZ protein (SPOP) has been found to be significantly inhibited in NSCLC. Our research aimed to investigate the molecular mechanisms, clinical significance and epigenetic alteration of SPOP in NSCLC.Materials and methodsBisulfite sequencing PCR and methylation-specific PCR were performed to test gene methylation. Chromatin immunoprecipitation (ChIP) was performed to detect transcription factor C/EBPα combinations and the promoter of the SPOP gene. Furthermore, we evaluated the effects of C/EBPα siRNA on SPOP expression, tumor cell migration and proliferation via MTT and Transwell assays in vitro and tumor growth in vivo. The relationship between the methylation status of the SPOP gene and clinicopathologic characteristics was investigated.ResultsHypermethylation was found in the CpG island of the SPOP gene promoter in NSCLC tissues, and this methylation was found to be correlated with SPOP expression. SPOP promoter methylation was associated with the pathology grade. The transcriptional activities were significantly inhibited by the hypermethylation of specific CpG sites within the SPOP gene promoter, while 5-aza-2′-deoxycytidine significantly increased SPOP gene expression. C/EBPα also played a key role in SPOP regulation. Five C/EBPα binding sites in the CpG island of the SPOP gene promoter were identified by ChIP. Inhibition of C/EBPα significantly reduced SPOP expression. SPOP mediated the C/EBPα-regulated suppression of invasion, migration and proliferation in vitro and tumor growth in vivo.ConclusionsSPOP function and expression in NSCLS were regulated by DNA methylation and C/EBPα transcriptional regulation combination effects, indicating that the SPOP promoter methylation status could be utilized as an epigenetic biomarker and that the C/EBPα-SPOP signaling pathway could be a potential therapeutic target in NSCLC.

Membrane‐less organelles in cells are large, dynamic protein/protein or protein/RNA assemblies that have been reported in some cases to have liquid droplet properties. However, the molecular interactions underlying the recruitment of components are not well understood. Herein, we study how the ability to form higher‐order assemblies influences the recruitment of the speckle‐type POZ protein (SPOP) to nuclear speckles. SPOP, a cullin‐3‐RING ubiquitin ligase (CRL3) substrate adaptor, self‐associates into higher‐order oligomers; that is, the number of monomers in an oligomer is broadly distributed and can be large. While wild‐type SPOP localizes to liquid nuclear speckles, self‐association‐deficient SPOP mutants have a diffuse distribution in the nucleus. SPOP oligomerizes through its BTB and BACK domains. We show that BTB‐mediated SPOP dimers form linear oligomers via BACK domain dimerization, and we determine the concentration‐dependent populations of the resulting oligomeric species. Higher‐order oligomerization of SPOP stimulates CRL3SPOP ubiquitination efficiency for its physiological substrate Gli3, suggesting that nuclear speckles are hotspots of ubiquitination. Dynamic, higher‐order protein self‐association may be a general mechanism to concentrate functional components in membrane‐less cellular bodies.