Ma, Jian

DNA replication is a vulnerable cellular process, and its deregulation leads to genomic instability. Here, we demonstrate that chromobox protein homolog 3 (CBX3) binds replication protein A 32-kDa subunit (RPA2) and regulates RPA2 retention at stalled replication forks. CBX3 is recruited to stalled replication forks by RPA2 and inhibits ring finger and WD repeat domain 3 (RFWD3)–facilitated replication restart. Phosphorylation of CBX3 at serine-95 by casein kinase 2 (CK2) kinase augments cadherin 1 (CDH1)–mediated CBX3 degradation and RPA2 dynamics at stalled replication forks, which permits replication fork restart. Increased expression of CBX3 due to gene amplification or CK2 inhibitor treatment sensitizes prostate cancer cells to poly(ADP-ribose) polymerase (PARP) inhibitors while inducing replication stress and DNA damage. Our work reveals CBX3 as a key regulator of RPA2 function and DNA replication, suggesting that CBX3 could serve as an indicator for targeted therapy of cancer using PARP inhibitors.

Abstract Retinoblastoma (RB) protein can exert tumor suppressor functions even when it becomes phosphorylated. It is thus essential to understand how phosphorylated RB (p-RB) expression and function are regulated. Here, we demonstrated that RING finger domain protein TRIM28 bound and promoted ubiquitination and degradation of CDK4/6-phosphorylated RB protein. SETDB1, a known TRIM28 binding partner, protected p-RB from degradation through the binding of methylated RB by its Tudor domain independent of its methyltransferase activity. SETDB1 was found to be frequently overexpressed due to gene amplification and positively correlated with p-RB in prostate cancer patient specimens. Inhibition of SETDB1 expression using a gene-specific antisense oligonucleotide (ASO) reduced tumor growth but accelerated RB protein degradation, limiting the therapeutic efficacy. However, coadministration of the CDK4/6 inhibitor palbociclib blocked ASO-induced RB degradation and resulted in a much greater cancer-inhibitory effect than each inhibitor alone both in vitro and in vivo. This study identified CDK4/6-dependent, TRIM28-mediated proteasomal degradation as a mechanism of RB inactivation and reveals SETDB1 as a key inhibitor of this process. Our findings suggest that combined targeting of SETDB1 and CDK4/6 represents a viable approach for the treatment of cancers with SETDB1 gene amplification or overexpression. Significance: The identification of a role for TRIM28 and SETDB1 in regulating CDK4/6-phosphorylated RB stability uncovers a combination strategy using CDK4/6 and SETDB1 inhibition to decrease RB degradation and inhibit cancer growth.

SPOP mutation disables 53BP1 ubiquitination and exclusion from DNA double-strand break sites and causes genome instability.

Abstract The tumor-suppressor protein RB acts as a transcription repressor via interaction of its pocket domain with an LXCXE motif in histone deacetylase (HDAC) proteins such as HDAC1. Here, we demonstrate that HDAC5 deficient for the LXCXE motif interacts with both RB-N (via an FXXXV motif) and RB-C segments, and such interactions are diminished by phosphorylation of RB serine-249/threonine-252 and threonine-821. HDAC5 was frequently downregulated or deleted in human cancers such as prostate cancer. Loss of HDAC5 increased histone H3 lysine 27 acetylation (H3K27-ac) and circumvented RB-mediated repression of cell-cycle–related pro-oncogenic genes. HDAC5 loss also conferred resistance to CDK4/6 inhibitors such as palbociclib in prostate and breast cancer cells in vitro and prostate tumors in vivo, but this effect was overcome by the BET-CBP/p300 dual inhibitor NEO2734. Our findings reveal an unknown role of HDAC5 in RB-mediated histone deacetylation and gene repression and define a new mechanism modulating CDK4/6 inhibitor therapeutic sensitivity in cancer cells. Significance: This study defines a previously uncharacterized role of HDAC5 in tumor suppression and provides a viable strategy to overcome CDK4/6 inhibitor resistance in HDAC5-deficent cancer.

62 Background: We have previously reported that mutations in SPOP promote bromodomain and extra-terminal motif (BET) protein upregulation and BET inhibitor resistance in prostate cancer (Zhang et al., Nature Medicine 23(9):1055-1062, 2017). Other studies indicate that histone acetyltransferases (HATs) such as P300 are also upregulated in SPOP-mutated prostate cancer (Blattner et al., Cancer Cell 31: 436-451, 2017). Methods: We examined the anti-cancer effect of NEO2734 (Epigene Therapeutics Inc), a novel dual inhibitor of both BET and CBP-P300, in prostate cancer using cell line, organoid and patient-derived xenografts (PDX) as working models. We examined the anti-cancer effect of NEO2734 (Epigene Therapeutics Inc), a novel dual inhibitor of both BET and CBP-P300, in prostate cancer using cell line, organoid and patient-derived xenografts (PDX) as working models. Results: We demonstrated that while prostate cancer cell lines expressing F133V, the most frequent SPOP mutant, are very resistant to both BET inhibitor JQ1 and CBP/P300 inhibitor CPI-637, they are very sensitive to simultaneous cotreatment with JQ1 and CPI-637. Similarly, we demonstrated that F133V mutant cells are also very sensitive to NEO2734. We established PDX and organoid models from a patient with prostate cancer expressing a novel SPOP mutation Q165P. Computer simulation analysis reveals that the Q165P mutation causes larger conformational fluctuation, indicating that the structure of Q165P is less stable than that of wild type SPOP. Accordingly, co-immunoprecipitation assay showed that Q165P mutation impaired the dimerization ability of SPOP. We further showed that the Q165P mutant organoid was very sensitive to NEO2734 in vitro and the Q165P mutant PDX was very sensitive to NEO2734 in mice. Conclusions: These PDX-based pre-clinical studies indicate that the inhibition of both BET and CBP-P300, either by co-administration of two agents, or by treatment with the dual inhibitor, NEO2734, is effective in SPOP-mutated prostate cancer. These data provide a strong rationale for the inclusion of patients with SPOP-mutated prostate cancer in clinical studies of NEO2734.