PRMT5-Dependent Methylation of the TIP60 Coactivator RUVBL1 Is a Key Regulator of Homologous Recombination

Mechanistic Insights into CLNS1A-Mediated Chemoresistance and Tumor Progression in Non-small Cell Lung Cancer

CLNS1A is a chloride channel protein and an essential component of the methylosome complex, which additionally comprises PRMT5 and MEP50. In this study, we investigated its contribution to lung cancer and its potential as a therapeutic target. Analysis of transcriptomic datasets and western blotting revealed that CLNS1A, PRMT5, and MEP50 were overexpressed in lung cancer tissues, with elevated CLNS1A expression correlating with poor patient survival. CLNS1A overexpression enhanced platinum clearance from cells, increased the IC50 values for chemotherapy, and improved cell survival. Conversely, the knockdown of CLNS1A increased drug accumulation, reduced survival, and increased sensitivity to chemotherapy. The 3W mutant, a chloride channel-defective variant with steric hindrance at key bottleneck residues, impaired chloride ion transport, thereby reducing drug resistance, migration, and anchorage-independent growth. Mechanistically, CLNS1A promotes drug efflux through its chloride channel activity and activates the FAK-SRC-RAC1 pathway to enhance motility and clonogenicity. It also facilitates PRMT5-mediated RUVBL1 methylation to support anti-apoptotic DNA damage response signaling. In vivo, CLNS1A overexpression accelerated tumor growth and reduced survival, whereas CLNS1A knockdown sensitized tumors to cisplatin, enhancing therapeutic efficacy. These findings suggest that CLNS1A is a potential biomarker and therapeutic target, and its inhibition offers a strategy to overcome drug resistance and limit the metastatic progression of lung cancer.

Genome-wide CRISPR activation screen identifies ARL11 as a sensitivity determinant of PARP inhibitor therapy

Resistance to poly-(ADP)-ribose polymerase inhibitors (PARPi) remains a significant challenge in clinical practice, leading to treatment failure in many patients. It is crucial to better understand the molecular mechanisms that underlie PARPi resistance. In this study, utilizing a genome-wide CRISPR activation screen with olaparib, we identified ARL11 as a potential modulator of PARPi treatment response in BRCA-wild-type MDA-MB-231 cells. Mechanistically, ARL11 interacts with STING to enhance innate immunity and forms positive feedback with type I interferon (IFN) induction, which induces ARL11 up-regulation and contributes to resistance to PARPi therapy. Additionally, we observed that ARL11 interacts with the RUVBL1 and RUVBL2 (RUVBL1/2) complex, the key DNA double-strand repair proteins, facilitating DNA homologous recombination (HR) repair and significantly reducing PARPi-induced DNA double-strand damages. Clinical sample analysis reveals that the expression levels of ARL11 and RUVBL1/2 are significantly elevated in breast cancer patients compared to healthy controls. Collectively, our findings suggested that ARL11 and RUVBL1/2 may be promising therapeutic targets to sensitize breast cancer cells to PARPi therapy.

PRMT5-regulated splicing of DNA repair genes drives chemoresistance in breast cancer stem cells

Breast cancer stem cells (BCSCs) are a rare cell population that is responsible for tumour initiation, metastasis and chemoresistance. Despite this, the mechanism by which BCSCs withstand genotoxic stress is largely unknown. Here, we uncover a pivotal role for the arginine methyltransferase PRMT5 in mediating BCSC chemoresistance by modulating DNA repair efficiency. Mechanistically, we identify PRMT5 as a major regulator of DNA damage response (DDR) gene splicing in BCSCs, particularly those integral to the Fanconi Anaemia and homologous recombination pathways, with PRMT5 inhibition synergising with chemotherapy to promote BCSC apoptosis. A comparison of BCSCs and their bulk cell progeny identified some shared (ATM, DDX11, EXO1, FAN1, SLX4) but many unique (ATR, RAD17, RAD51D, RUVBL1) PRMT5-dependent alternative DDR splicing events. Surprisingly, these skipped exons and retained intron events rarely lead to substantial gene expression repression, suggesting that PRMT5 inhibition predominantly results in nuclear detention of intron-containing transcripts and the production of non-canonical isoforms with compromised protein function. Since many genes within the same DDR pathway undergo deregulated splicing, this study thus reveals additional points of vulnerability and alternative combination drug strategies that could improve the therapeutic efficacy of PRMT5 inhibitors to promote BCSC eradication.

In silico protein structural analysis of PRMT5 and RUVBL1 mutations arising in human cancers

DNA double strand breaks (DSBs) can be generated spontaneously during DNA replication and are repaired primarily by Homologous Recombination (HR). However, efficient repair requires chromatin remodeling to allow the recombination machinery access to the break. TIP60 is a complex conserved from yeast to humans that is required for histone acetylation and modulation of HR activity at DSBs. Two enzymatic activities within the TIP60 complex, KAT5 (a histone acetyltransferase) and RUVBL1 (an AAA+ ATPase) are required for efficient HR repair. Post-translational modification of RUVBL1 by the PRMT5 methyltransferase activates the complex acetyltransferase activity and facilitates error free HR repair. In S. pombe a direct interaction between PRMT5 and the acetyltransferase subunit of the TIP60 complex (KAT5) was also identified. The TIP60 complex has been partially solved experimentally in both humans and S. cerevisiae, but not S. pombe. Here, we used in silico protein structure analysis to investigate structural conservation between S. pombe and human PRMT5 and RUVBL1. We found that there is more similarity in structure conservation between S. pombe and human proteins than between S. cerevisiae and human. Next, we queried the COSMIC database to analyze how mutations occurring in human cancers affect the structure and function of these proteins. Artificial intelligence algorithms that predict how likely mutations are to promote cellular transformation and immortalization show that RUVBL1 mutations should have a more drastic effect than PRMT5. Indeed, in silico protein structural analysis shows that PRMT5 mutations are less likely to destabilize enzyme function. Conversely, most RUVBL1 mutations occur in a region required for interaction with its partner (RUVBL2). These data suggests that cancer mutations could destabilize the TIP60 complex. Sequence conservation analysis between S. pombe and humans shows that the residues identified in cancer cells are highly conserved, suggesting that this may be an essential process in eukaryotic DSB repair. These results shed light on mechanisms of DSB repair and also highlight how S. pombe remains a great model system for analyzing DSB repair processes that are tractable in human cells.

High PRMT5 levels, maintained by KEAP1 inhibition, drive chemoresistance in high-grade serous ovarian cancer

Protein arginine methyl transferases (PRMTs) are generally upregulated in cancers. However, the mechanisms leading to this upregulation and its biological consequences are poorly understood. Here, we identify PRMT5, the main symmetric arginine methyltransferase, as a critical driver of chemoresistance in high-grade serous ovarian cancer (HGSOC). PRMT5 levels and its enzymatic activity are induced in a platinum-resistant (Pt-resistant) state at the protein level. To reveal potential regulators of high PRMT5 protein levels, we optimized intracellular immunostaining conditions and performed unbiased CRISPR screening. We identified Kelch-like ECH-associated protein 1 (KEAP1) as a top-scoring negative regulator of PRMT5. Our mechanistic studies show that KEAP1 directly interacted with PRMT5, leading to its ubiquitin-dependent degradation under normal physiological conditions. At the genomic level, ChIP studies showed that elevated PRMT5 directly interacted with the promoters of stress response genes and positively regulated their transcription. Combined PRMT5 inhibition with Pt resulted in synergistic cellular cytotoxicity in vitro and reduced tumor growth in vivo in Pt-resistant patient-derived xenograft tumors. Overall, the findings from this study identify PRMT5 as a critical therapeutic target in Pt-resistant HGSOC cells and reveal the molecular mechanisms that lead to high PRMT5 levels in Pt-treated and chemo-resistant tumors.

PRMT5 inhibition sensitizes glioblastoma tumor models to temozolomide

Background

Despite multi-model therapy of maximal surgical resection, radiation, chemotherapy, and tumor-treating fields, glioblastoma patients show dismal prognosis. Protein Arginine Methyltransferase 5 (PRMT5) is overexpressed in glioblastoma and its inhibition imparts an anti-tumor effect. Even though Temozolomide (TMZ) is the standard chemotherapeutic agent in the treatment of glioblastoma, tumor cells invariably develop resistance to TMZ. However, the mechanistic role of PRMT5 in glioblastoma therapy resistance is unknown.

Methods

Patient-derived primary glioblastoma neurospheres (GBMNS), treated with PRMT5 inhibitor (LLY-283) or transfected with PRMT5 target-specific siRNA were treated with TMZ and subjected to in vitro functional and mechanistic studies. The intracranial mouse xenograft model was used to test the in vivo antitumor efficacy of combination treatment.

Results

We found that PRMT5 inhibition increased the cytotoxic effect and caspase 3/7 activity of TMZ in GBMNS suggesting that apoptosis is the potential mode of cell death in the combination treatment. PRMT5 inhibition abrogated the TMZ-induced G2/M cell cycle arrest. Unbiased transcriptomic studies indicate that PRMT5 inhibition negatively enriches DNA damage repair genes. Importantly, combination therapy increased DNA double-strand breaks (H2AX foci) and enhanced the DNA damage (comet assay), suggesting that the combination treatment increases the TMZ-induced DNA damage. Specifically, the LLY-283 treatment blocked homologous recombination repair in GBMNS. In vivo, LLY-283 and TMZ combination significantly curbed the tumor growth and prolonged the survival of tumor-bearing mice.

Conclusion

Concomitant treatment of LLY-283 and TMZ has significantly greater antitumor efficacy, suggesting that PRMT5 inhibition and TMZ combination could be a new therapeutic strategy for glioblastoma.

Spliceosomal vulnerability of MYCN-amplified neuroblastoma is contingent on PRMT5-mediated regulation of epitranscriptomic and metabolomic pathways

Approximately 50 % of poor prognosis neuroblastomas arise due to MYCN over-expression. We previously demonstrated that MYCN and PRMT5 proteins interact and PRMT5 knockdown led to apoptosis of MYCN-amplified (MNA) neuroblastoma. Here we evaluate the highly selective first-in-class PRMT5 inhibitor GSK3203591 and its in vivo analogue GSK3326593 as targeted therapeutics for MNA neuroblastoma. Cell-line analyses show MYCN-dependent growth inhibition and apoptosis, with approximately 200-fold greater sensitivity of MNA neuroblastoma lines. RNA sequencing of three MNA neuroblastoma lines treated with GSK3203591 reveal deregulated MYCN transcriptional programmes and altered mRNA splicing, converging on key regulatory pathways such as DNA damage response, epitranscriptomics and cellular metabolism. Stable isotope labelling experiments in the same cell lines demonstrate that glutamine metabolism is impeded following GSK3203591 treatment, linking with disruption of the MLX/Mondo nutrient sensors via intron retention of MLX mRNA. Interestingly, glutaminase (GLS) protein decreases after GSK3203591 treatment despite unchanged transcript levels. We demonstrate that the RNA methyltransferase METTL3 and cognate reader YTHDF3 proteins are lowered following their mRNAs undergoing GSK3203591-induced splicing alterations, indicating epitranscriptomic regulation of GLS; accordingly, we observe decreases of GLS mRNA m6A methylation following GSK3203591 treatment, and decreased GLS protein following YTHDF3 knockdown. In vivo efficacy of GSK3326593 is confirmed by increased survival of Th-MYCN mice, with drug treatment triggering splicing events and protein decreases consistent with in vitro data. Together our study demonstrates the PRMT5-dependent spliceosomal vulnerability of MNA neuroblastoma and identifies the epitranscriptome and glutamine metabolism as critical determinants of this sensitivity.

New insight into targeting the DNA damage response in the treatment of glioblastoma

Glioblastoma (GBM) is the most common invasive malignant tumor in human brain tumors, representing the most severe grade of gliomas. Despite existing therapeutic approaches, patient prognosis remains dismal, necessitating the exploration of novel strategies to enhance treatment efficacy and extend survival. Due to the restrictive nature of the blood-brain barrier (BBB), small-molecule inhibitors are prioritized in the treatment of central nervous system tumors. Among these, DNA damage response (DDR) inhibitors have garnered significant attention due to their potent therapeutic potential across various malignancies. This review provides a detailed analysis of DDR pathways as therapeutic targets in GBM, summarizes recent advancements, therapeutic strategies, and ongoing clinical trials, and offers perspectives on future directions in this rapidly evolving field. The goal is to present a comprehensive outlook on the potential of DDR inhibitors in improving GBM management and outcomes.

Evaluating the Cellular Roles of the Lysine Acetyltransferase Tip60 in Cancer: A Multi-Action Molecular Target for Precision Oncology

Precision (individualized) medicine relies on the molecular profiling of tumors' dysregulated characteristics (genomic, epigenetic, transcriptomic) to identify the reliance on key pathways (including genome stability and epigenetic gene regulation) for viability or growth, and then utilises targeted therapeutics to disrupt these survival-dependent pathways. Non-mutational epigenetic changes alter cells' transcriptional profile and are a key feature found in many tumors. In contrast to genetic mutations, epigenetic changes are reversable, and restoring a normal epigenetic profile can inhibit tumor growth and progression. Lysine acetyltransferases (KATs or HATs) protect genome stability and integrity, and Tip60 is an essential acetyltransferase due to its roles as an epigenetic and transcriptional regulator, and as master regulator of the DNA double-strand break response. Tip60 is commonly downregulated and mislocalized in many cancers, and the roles that mislocalized Tip60 plays in cancer are not well understood. Here we categorize and discuss Tip60-regulated genes, evaluate Tip60-interacting proteins based on cellular localization, and explore the therapeutic potential of Tip60-targeting compounds as epigenetic inhibitors. Understanding the multiple roles Tip60 plays in tumorigenesis will improve our understanding of tumor progression and will inform therapeutic options, including informing potential combinatorial regimes with current chemotherapeutics, leading to improvements in patient outcomes.

Walsh, Marazzi I, Kappei D, Guccione E, Jeyasekharan AD. A PRMT5-ZNF326 axis mediates innate immune activation upon replication stress

DNA replication stress (RS) is a widespread phenomenon in carcinogenesis, causing genomic instability and extensive chromatin alterations. DNA damage leads to activation of innate immune signaling, but little is known about transcriptional regulators mediating such signaling upon RS. Using a chemical screen, we identified protein arginine methyltransferase 5 (PRMT5) as a key mediator of RS-dependent induction of interferon-stimulated genes (ISGs). This response is also associated with reactivation of endogenous retroviruses (ERVs). Using quantitative mass spectrometry, we identify proteins with PRMT5-dependent symmetric dimethylarginine (SDMA) modification induced upon RS. Among these, we show that PRMT5 targets and modulates the activity of ZNF326, a zinc finger protein essential for ISG response. Our data demonstrate a role for PRMT5-mediated SDMA in the context of RS-induced transcriptional induction, affecting physiological homeostasis and cancer therapy.

Repression of PRMT activities sensitize homologous recombination-proficient ovarian and breast cancer cells to PARP inhibitor treatment

An "induced PARP inhibitor (PARPi) sensitivity by epigenetic modulation" strategy is being evaluated in the clinic to sensitize homologous recombination (HR)-proficient tumors to PARPi treatments. To expand its clinical applications and identify more efficient combinations, we performed a drug screen by combining PARPi with 74 well-characterized epigenetic modulators that target five major classes of epigenetic enzymes. Both type I PRMT inhibitor and PRMT5 inhibitor exhibit high combination and clinical priority scores in our screen. PRMT inhibition significantly enhances PARPi treatment-induced DNA damage in HR-proficient ovarian and breast cancer cells. Mechanistically, PRMTs maintain the expression of genes associated with DNA damage repair and BRCAness and regulate intrinsic innate immune pathways in cancer cells. Analyzing large-scale genomic and functional profiles from TCGA and DepMap further confirms that PRMT1, PRMT4, and PRMT5 are potential therapeutic targets in oncology. Finally, PRMT1 and PRMT5 inhibition act synergistically to enhance PARPi sensitivity. Our studies provide a strong rationale for the clinical application of a combination of PRMT and PARP inhibitors in patients with HR-proficient ovarian or breast cancer.

Chromodomain mutation in S. pombe Kat5/Mst1 affects centromere dynamics and DNA repair

KAT5 (S. pombe Mst1, human TIP60) is a MYST family histone acetyltransferase conserved from yeast to humans that is involved in multiple cellular activities. This family is characterized in part by containing a chromodomain, a motif associated with binding methylated histones. We show that a chromodomain mutation in the S. pombe Kat5, mst1-W66R, has defects in pericentromere silencing. mst1-W66R is sensitive to camptothecin (CPT) but only at an increased temperature of 36°C, although it is proficient for growth at this temperature. We also describe a de-silencing effect at the pericentromere by CPT that is independent of RNAi and methylation machinery. We also show that mst1-W66R disrupts recruitment of proteins to repair foci in response to camptothecin-induced DNA damage. Our data suggest a function of Mst1 chromodomain in centromere heterochromatin formation and a separate role in genome-wide damage repair in CPT.

RUVBL1 ubiquitination by DTL promotes RUVBL1/2-β-catenin-mediated transcriptional regulation of NHEJ pathway and enhances radiation resistance in breast cancer

Radiotherapy effectiveness in breast cancer is limited by radioresistance. Nevertheless, the mechanisms behind radioresistance are not yet fully understood. RUVBL1 and RUVBL2, referred to as RUVBL1/2, are crucial AAA+ ATPases that act as co-chaperones and are connected to cancer. Our research revealed that RUVBL1, also known as pontin/TIP49, is excessively expressed in MMTV-PyMT mouse models undergoing radiotherapy, which is considered a murine spontaneous breast-tumor model. Our findings suggest that RUVBL1 enhances DNA damage repair and radioresistance in breast cancer cells both in vitro and in vivo. Mechanistically, we discovered that DTL, also known as CDT2 or DCAF2, which is a substrate adapter protein of CRL4, promotes the ubiquitination of RUVBL1 and facilitates its binding to RUVBL2 and transcription cofactor β-catenin. This interaction, in turn, attenuates its binding to acetyltransferase Tat-interacting protein 60 (TIP60), a comodulator of nuclear receptors. Subsequently, ubiquitinated RUVBL1 promotes the transcriptional regulation of RUVBL1/2-β-catenin on genes associated with the non-homologous end-joining (NHEJ) repair pathway. This process also attenuates TIP60-mediated H4K16 acetylation and the homologous recombination (HR) repair process. Expanding upon the prior study's discoveries, we exhibited that the ubiquitination of RUVBL1 by DTL advances the interosculation of RUVBL1/2-β-catenin. And, it then regulates the transcription of NHEJ repair pathway protein. Resulting in an elevated resistance of breast cancer cells to radiation therapy. From the aforementioned, it is evident that targeting DTL-RUVBL1/2-β-catenin provides a potential radiosensitization approach when treating breast cancer.

Protein Arginine Methyltransferases in Pancreatic Ductal Adenocarcinoma: New Molecular Targets for Therapy

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignant disease with a low 5-year overall survival rate. It is the third-leading cause of cancer-related deaths in the United States. The lack of robust therapeutics, absence of effective biomarkers for early detection, and aggressive nature of the tumor contribute to the high mortality rate of PDAC. Notably, the outcomes of recent immunotherapy and targeted therapy against PDAC remain unsatisfactory, indicating the need for novel therapeutic strategies. One of the newly described molecular features of PDAC is the altered expression of protein arginine methyltransferases (PRMTs). PRMTs are a group of enzymes known to methylate arginine residues in both histone and non-histone proteins, thereby mediating cellular homeostasis in biological systems. Some of the PRMT enzymes are known to be overexpressed in PDAC that promotes tumor progression and chemo-resistance via regulating gene transcription, cellular metabolic processes, RNA metabolism, and epithelial mesenchymal transition (EMT). Small-molecule inhibitors of PRMTs are currently under clinical trials and can potentially become a new generation of anti-cancer drugs. This review aims to provide an overview of the current understanding of PRMTs in PDAC, focusing on their pathological roles and their potential as new therapeutic targets.

PRMT5-mediated homologous recombination repair is essential to maintain genomic integrity of neural progenitor cells

Maintaining genomic stability is a prerequisite for proliferating NPCs to ensure genetic fidelity. Though histone arginine methylation has been shown to play important roles in safeguarding genomic stability, the underlying mechanism during brain development is not fully understood. Protein arginine N-methyltransferase 5 (PRMT5) is a type II protein arginine methyltransferase that plays a role in transcriptional regulation. Here, we identify PRMT5 as a key regulator of DNA repair in response to double-strand breaks (DSBs) during NPC proliferation. Prmt5F/F; Emx1-Cre (cKO-Emx1) mice show a distinctive microcephaly phenotype, with partial loss of the dorsal medial cerebral cortex and complete loss of the corpus callosum and hippocampus. This phenotype is resulted from DSBs accumulation in the medial dorsal cortex followed by cell apoptosis. Both RNA sequencing and in vitro DNA repair analyses reveal that PRMT5 is required for DNA homologous recombination (HR) repair. PRMT5 specifically catalyzes H3R2me2s in proliferating NPCs in the developing mouse brain to enhance HR-related gene expression during DNA repair. Finally, overexpression of BRCA1 significantly rescues DSBs accumulation and cell apoptosis in PRMT5-deficient NSCs. Taken together, our results show that PRMT5 maintains genomic stability by regulating histone arginine methylation in proliferating NPCs.

Overview of the development of protein arginine methyltransferase modulators: Achievements and future directions

Protein methylation is a post-translational modification (PTM) that organisms undergo. This process is considered a part of epigenetics research. In recent years, there has been an increasing interest in protein methylation, particularly histone methylation, as research has advanced. Methylation of histones is a dynamic process that is subject to fine control by histone methyltransferases and demethylases. In addition, many non-histone proteins also undergo methylation, and these modifications collectively regulate physiological phenomena, including RNA transcription, translation, signal transduction, DNA damage response, and cell cycle. Protein arginine methylation is a crucial aspect of protein methylation, which plays a significant role in regulating the cell cycle and repairing DNA. It is also linked to various diseases. Therefore, protein arginine methyltransferases (PRMTs) that are involved in this process have gained considerable attention as a potential therapeutic target for treating diseases. Several PRMT inhibitors are in phase I/II clinical trials. This paper aims to introduce the structure, biochemical functions, and bioactivity assays of PRMTs. Additionally, we will review the structure-function of currently popular PRMT inhibitors. Through the analysis of various data on known PRMT inhibitors, we hope to provide valuable assistance for future drug design and development.

Nuclear to cytoplasmic transport is a druggable dependency in MYC-driven hepatocellular carcinoma

The MYC oncogene is often dysregulated in human cancer, including hepatocellular carcinoma (HCC). MYC is considered undruggable to date. Here, we comprehensively identify genes essential for survival of MYChigh but not MYClow cells by a CRISPR/Cas9 genome-wide screen in a MYC-conditional HCC model. Our screen uncovers novel MYC synthetic lethal (MYC-SL) interactions and identifies most MYC-SL genes described previously. In particular, the screen reveals nucleocytoplasmic transport to be a MYC-SL interaction. We show that the majority of MYC-SL nucleocytoplasmic transport genes are upregulated in MYChigh murine HCC and are associated with poor survival in HCC patients. Inhibiting Exportin-1 (XPO1) in vivo induces marked tumor regression in an autochthonous MYC-transgenic HCC model and inhibits tumor growth in HCC patient-derived xenografts. XPO1 expression is associated with poor prognosis only in HCC patients with high MYC activity. We infer that MYC may generally regulate and require altered expression of nucleocytoplasmic transport genes for tumorigenesis.

Exploiting the DNA Damage Response for Prostate Cancer Therapy

Prostate cancers that progress despite androgen deprivation develop into castration-resistant prostate cancer, a fatal disease with few treatment options. In this review, we discuss the current understanding of prostate cancer subtypes and alterations in the DNA damage response (DDR) that can predispose to the development of prostate cancer and affect its progression. We identify barriers to conventional treatments, such as radiotherapy, and discuss the development of new therapies, many of which target the DDR or take advantage of recurring genetic alterations in the DDR. We place this in the context of advances in understanding the genetic variation and immune landscape of CRPC that could help guide their use in future treatment strategies. Finally, we discuss several new and emerging agents that may advance the treatment of lethal disease, highlighting selected clinical trials.

Phase 1 study of JNJ-64619178, a protein arginine methyltransferase 5 inhibitor, in patients with lower-risk myelodysplastic syndromes

Splicing factor (SF) gene mutations are frequent in myelodysplastic syndromes (MDS), and agents that modulate RNA splicing are hypothesized to provide clinical benefit. JNJ-64619178, a protein arginine methyltransferase 5 (PRMT5) inhibitor, was evaluated in patients with lower-risk (LR) MDS in a multi-part, Phase 1, multicenter study. The objectives were to determine a tolerable dose and to characterize safety, pharmacokinetics, pharmacodynamics, and preliminary clinical activity. JNJ-64619178 was administered on a 14 days on/7 days off schedule or every day on a 21-day cycle to patients with International Prognostic Scoring System (IPSS) Low or Intermediate-1 risk MDS who were red blood cell transfusion-dependent. Twenty-four patients were enrolled; 15 (62.5 %) patients had low IPSS risk score, while 18 (75.0 %) had an SF3B1 mutation. Median duration of treatment was 3.45 months (range: 0.03-6.93). No dose limiting toxicities were observed. The 0.5 mg once daily dose was considered better tolerated and chosen for dose expansion. Twenty-three (95.8 %) patients experienced treatment-emergent adverse events (TEAE). The most common TEAEs were neutropenia (15 [62.5 %]) and thrombocytopenia (14 [58.3 %]). JNJ-64619178 pharmacokinetics was dose-dependent. Target engagement as measured by plasma symmetric di-methylarginine was observed across all dose levels; however, variant allele frequency of clonal mutations in bone marrow or blood did not show sustained reductions from baseline. No patient achieved objective response or hematologic improvement per International Working Group 2006 criteria, or transfusion independence. A tolerable dose of JNJ-64619178 was identified in patients with LR MDS. However, no evidence of clinical benefit was observed.

Copy Number Variation That Influences the Ionizing Radiation Sensitivity of Oral Squamous Cell Carcinoma

Genome instability in cancer cells causes not only point mutations but also structural variations of the genome, including copy number variations (CNVs). It has recently been proposed that CNVs arise in cancer to adapt to a given microenvironment to survive. However, how CNV influences cellular resistance against ionizing radiation remains unknown. PRMT5 (protein arginine methyltransferase 5) and APE1 (apurinic/apyrimidinic endonuclease 1), which enhance repair of DNA double-strand breaks and oxidative DNA damage, are closely localized in the chromosome 14 of the human genome. In this study, the genomics data for the PRMT5 and APE1 genes, including their expression, CNVs, and clinical outcomes, were analyzed using TCGA's data set for oral squamous cell carcinoma patients. The two genes were found to share almost identical CNV values among cancer tissues from oral squamous cell carcinoma (OSCC) patients. Levels of expression of PRMT5 and APE1 in OSCC tissues are highly correlated in cancer but not in normal tissues, suggesting that regulation of PRMT5 and APE1 were overridden by the extent of CNV in the PRMT5-APE1 genome region. High expression levels of PRMT5 and APE1 were both associated with poor survival outcomes after radiation therapy. Simultaneous down-regulation of PRMT5 and APE1 synergistically hampered DNA double-strand break repair and sensitized OSCC cell lines to X-ray irradiation in vitro and in vivo. These results suggest that the extent of CNV in a particular genome region significantly influence the radiation resistance of cancer cells. Profiling CNV in the PRMT5-APE1 genome region may help us to understand the mechanism of the acquired radioresistance of tumor cells, and raises the possibility that simultaneous inhibition of PRMT5 and APE1 may increase the efficacy of radiation therapy.

RNA methylation, homologous recombination repair and therapeutic resistance

Homologous recombination (HR) repair of DNA double-strand breaks (DSBs) is critical for maintaining genomic integrity and stability. Defects in HR increase the risk of tumorigenesis. However, many human tumors exhibit enhanced HR repair capabilities, consequently endowing tumor cells with resistance to DNA-damaging chemotherapy and radiotherapy. This review summarizes the role of RNA methylation in HR repair and therapeutic resistance in human tumors. We also analyzed the interactions between RNA methylation and other HR-modulating modifications including histone acetylation, histone deacetylation, ubiquitination, deubiquitination, protein arginine methylation, and gene transcription. This review proposes that targeting RNA methylation is a promising approach to overcoming HR-mediated therapeutic resistance.

The potential and challenges of targeting MTAP-negative cancers beyond synthetic lethality

Approximately 15% of cancers exhibit loss of the chromosomal locus 9p21.3 - the genomic location of the tumour suppressor gene CDKN2A and the methionine salvage gene methylthioadenosine phosphorylase (MTAP). A loss of MTAP increases the pool of its substrate methylthioadenosine (MTA), which binds to and inhibits activity of protein arginine methyltransferase 5 (PRMT5). PRMT5 utilises the universal methyl donor S-adenosylmethionine (SAM) to methylate arginine residues of protein substrates and regulate their activity, notably histones to regulate transcription. Recently, targeting PRMT5, or MAT2A that impacts PRMT5 activity by producing SAM, has shown promise as a therapeutic strategy in oncology, generating synthetic lethality in MTAP-negative cancers. However, clinical development of PRMT5 and MAT2A inhibitors has been challenging and highlights the need for further understanding of the downstream mediators of drug effects. Here, we discuss the rationale and methods for targeting the MAT2A/PRMT5 axis for cancer therapy. We evaluate the current limitations in our understanding of the mechanism of MAT2A/PRMT5 inhibitors and identify the challenges that must be addressed to maximise the potential of these drugs. In addition, we review the current literature defining downstream effectors of PRMT5 activity that could determine sensitivity to MAT2A/PRMT5 inhibition and therefore present a rationale for novel combination therapies that may not rely on synthetic lethality with MTAP loss.

Phase 1 Study of JNJ-64619178, a Protein Arginine Methyltransferase 5 Inhibitor, in Advanced Solid Tumors

PURPOSE

In this first-in-human, Phase 1, open-label, multicenter study, we evaluated JNJ-64619178, a selective and potent PRMT5 inhibitor, in patients with advanced malignant solid tumors or non-Hodgkin lymphomas (NHL). The primary objective was to evaluate the safety and to identify a recommended Phase 2 dose (RP2D) of JNJ-64619178.

PATIENTS AND METHODS

Adult patients with treatment-refractory advanced solid tumors or NHL and measurable disease received escalating doses of JNJ-64619178 following two schedules (Schedule A: 14 days on/7 days off; Schedule B: every day on a 21-day cycle). Safety, pharmacokinetics (PK), pharmacodynamics (PD), and clinical activity were evaluated.

RESULTS

Ninety patients received JNJ-64619178. Thrombocytopenia was identified as the only dose-limiting toxicity. JNJ-64619178 showed dose-proportional PK and robust target engagement, as measured by plasma symmetric dimethylarginine, across all dose levels. The objective response rate was 5.6% (5 of 90). Patients with adenoid cystic carcinoma (ACC) had an ORR of 11.5% (3 of 26) and a median progression-free survival of 19.1 months.

CONCLUSIONS

JNJ-64619178 demonstrated manageable dose-dependent toxicity and preliminary evidence of antitumor activity in ACC and other tumor types. Plasma exposure was dose dependent, and target inhibition was maintained with intermittent and continuous dosing. On the basis of safety, clinical activity, PK, and PD findings, two provisional RP2Ds were selected: 1.5 mg intermittently and 1.0 mg once daily. Aside from ACC, clinical benefit was limited, and biomarkers to enrich for responsiveness to PRMT5 inhibition will be needed for further development.

PRMT5 supports multiple oncogenic pathways in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell malignancy with an overall poor prognosis, particularly for patients that progress on targeted therapies. Novel, more durable treatment options are needed for patients with MCL. Protein arginine methyltransferase 5 (PRMT5) is overexpressed in MCL and plays an important oncogenic role in this disease via epigenetic and posttranslational modification of cell cycle regulators, DNA repair genes, components of prosurvival pathways, and RNA splicing regulators. The mechanism of targeting PRMT5 in MCL remains incompletely characterized. Here, we report on the antitumor activity of PRMT5 inhibition in MCL using integrated transcriptomics of in vitro and in vivo models of MCL. Treatment with a selective small-molecule inhibitor of PRMT5, PRT-382, led to growth arrest and cell death and provided a therapeutic benefit in xenografts derived from patients with MCL. Transcriptional reprograming upon PRMT5 inhibition led to restored regulatory activity of the cell cycle (p-RB/E2F), apoptotic cell death (p53-dependent/p53-independent), and activation of negative regulators of B-cell receptor-PI3K/AKT signaling (PHLDA3, PTPROt, and PIK3IP1). We propose pharmacologic inhibition of PRMT5 for patients with relapsed/refractory MCL and identify MTAP/CDKN2A deletion and wild-type TP53 as biomarkers that predict a favorable response. Selective targeting of PRMT5 has significant activity in preclinical models of MCL and warrants further investigation in clinical trials.

Inhibiting PRMT5 induces DNA damage and increases anti-proliferative activity of Niraparib, a PARP inhibitor, in models of breast and ovarian cancer

BACKGROUND

Inhibitors of Poly (ADP-Ribose) Polymerases (PARP) provide clinical benefit to patients with breast and ovarian cancers, by compromising the DNA repair activity of cancer cells. Although these agents extend progression-free survival in many patients, responses can be short lived with many patients ultimately progressing. Identification of combination partners that increase dependence of cancer cells to the DNA repair activity of PARPs may represent a strategy to increase the utility of PARP inhibitors. Protein arginine methyltransferase 5 (PRMT5) regulates DNA damage response pathways through splicing and protein modification, and inhibitors of PRMT5 have recently entered clinical trials.

METHODS

The effect of PRMT5 inhibition on the levels of DNA damage and repair markers including γH2AX, RAD51, and 53BP1 was determined using high content immunofluorescent imaging. The anti-proliferative activity of the combination of PRMT5 and PARP inhibitors was evaluated using in vitro models of breast and ovarian cancers using both cell lines and ex vivo patient derived xenografts. Finally, the combinations of PRMT5 and PARP inhibitors were evaluated in cell line xenograft models in vivo.

RESULTS

Inhibition of PRMT5 by GSK3326595 led to increased levels of markers of DNA damage. The addition of GSK3326595 to the PARP inhibitor, niraparib, resulted in increased growth inhibition of breast and ovarian cancer cell lines and patient derived spheroids. In vivo, the combination improved the partial effects on tumor growth inhibition achieved by either single agent, producing complete tumor stasis and regression.

CONCLUSION

These data demonstrate that inhibition of PRMT5 induced signatures of DNA damage in models of breast and ovarian cancer. Furthermore, combination with the PARP inhibitor, Niraparib, resulted in increased anti-tumor activity in vitro and in vivo. Overall, these data suggest inhibition of PRMT5 as a mechanism to broaden and enhance the clinical application of PARP inhibitors.

GAPDH facilitates homologous recombination repair by stabilizing RAD51 in an HDAC1-dependent manner

Homologous recombination (HR), a form of error-free DNA double-strand break (DSB) repair, is important for the maintenance of genomic integrity. Here, we identify a moonlighting protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), as a regulator of HR repair, which is mediated through HDAC1-dependent regulation of RAD51 stability. Mechanistically, in response to DSBs, Src signaling is activated and mediates GAPDH nuclear translocation. Then, GAPDH directly binds with HDAC1, releasing it from its suppressor. Subsequently, activated HDAC1 deacetylates RAD51 and prevents it from undergoing proteasomal degradation. GAPDH knockdown decreases RAD51 protein levels and inhibits HR, which is re-established by overexpression of HDAC1 but not SIRT1. Notably, K40 is an important acetylation site of RAD51, which facilitates stability maintenance. Collectively, our findings provide new insights into the importance of GAPDH in HR repair, in addition to its glycolytic activity, and they show that GAPDH stabilizes RAD51 by interacting with HDAC1 and promoting HDAC1 deacetylation of RAD51.

PRMT1 and PRMT5: on the road of homologous recombination and non-homologous end joining

DNA double-strand breaks (DSBs) are widely accepted to be the most deleterious form of DNA lesions that pose a severe threat to genome integrity. Two predominant pathways are responsible for repair of DSBs, homologous recombination (HR) and non-homologous end-joining (NHEJ). HR relies on a template to faithfully repair breaks, while NHEJ is a template-independent and error-prone repair mechanism. Multiple layers of regulation have been documented to dictate the balance between HR and NHEJ, such as cell cycle and post-translational modifications (PTMs). Arginine methylation is one of the most common PTMs, which is catalyzed by protein arginine methyltransferases (PRMTs). PRMT1 and PRMT5 are the predominate PRMTs that promote asymmetric dimethylarginine and symmetric dimethylarginine, respectively. They have emerged to be crucial regulators of DNA damage repair. In this review, we summarize current understanding and unaddressed questions of PRMT1 and PRMT5 in regulation of HR and NHEJ, providing insights into their roles in DSB repair pathway choice and the potential of targeting them for cancer therapy.

Autophagy dictates sensitivity to PRMT5 inhibitor in breast cancer

Protein arginine methyltransferase 5 (PRMT5) catalyzes mono-methylation and symmetric di-methylation on arginine residues and has emerged as a potential antitumor target with inhibitors being tested in clinical trials. However, it remains unknown how the efficacy of PRMT5 inhibitors is regulated. Here we report that autophagy blockage enhances cellular sensitivity to PRMT5 inhibitor in triple negative breast cancer cells. Genetic ablation or pharmacological inhibition of PRMT5 triggers cytoprotective autophagy. Mechanistically, PRMT5 catalyzes monomethylation of ULK1 at R532 to suppress ULK1 activation, leading to attenuation of autophagy. As a result, ULK1 inhibition blocks PRMT5 deficiency-induced autophagy and sensitizes cells to PRMT5 inhibitor. Our study not only identifies autophagy as an inducible factor that dictates cellular sensitivity to PRMT5 inhibitor, but also unearths a critical molecular mechanism by which PRMT5 regulates autophagy through methylating ULK1, providing a rationale for the combination of PRMT5 and autophagy inhibitors in cancer therapy.

PRMT5 promotes ovarian cancer growth through enhancing Warburg effect by methylating ENO1

Protein arginine methyltransferase 5 (PRMT5) is a major type II enzyme responsible for symmetric dimethylation of arginine (SDMA), and plays predominantly roles in human cancers, including in ovarian cancer. However, the exactly roles and underlying mechanisms of PRMT5 contributing to the progression of ovarian cancer mediated by reprogramming cell metabolism remain largely elusive. Here, we report that PRMT5 is highly expressed and correlates with poor survival in ovarian cancer. Knockdown or pharmaceutical inhibition of PRMT5 is sufficient to decrease glycolysis flux, attenuate tumor growth, and enhance the antitumor effect of Taxol. Mechanistically, we find that PRMT5 symmetrically dimethylates alpha-enolase (ENO1) at arginine 9 to promotes active ENO1 dimer formation, which increases glycolysis flux and accelerates tumor growth. Moreover, PRMT5 signals high glucose to increase the methylation modification of ENO1. Together, our data reveal a novel role of PRMT5 in promoting ovarian cancer growth by controlling glycolysis flux mediated by methylating ENO1, and highlights that PRMT5 may represent a promising therapeutic target for treating ovarian cancer.

Protein Arginine Methyltransferase 5 (PRMT5) Mutations in Cancer Cells

Arginine methylation is a form of posttranslational modification that regulates many cellular functions such as development, DNA damage repair, inflammatory response, splicing, and signal transduction, among others. Protein arginine methyltransferase 5 (PRMT5) is one of nine identified methyltransferases, and it can methylate both histone and non-histone targets. It has pleiotropic functions, including recruitment of repair machinery to a chromosomal DNA double strand break (DSB) and coordinating the interplay between repair and checkpoint activation. Thus, PRMT5 has been actively studied as a cancer treatment target, and small molecule inhibitors of its enzymatic activity have already been developed. In this report, we analyzed all reported PRMT5 mutations appearing in cancer cells using data from the Catalogue of Somatic Mutations in Cancers (COSMIC). Our goal is to classify mutations as either drivers or passengers to understand which ones are likely to promote cellular transformation. Using gold standard artificial intelligence algorithms, we uncovered several key driver mutations in the active site of the enzyme (D306H, L315P, and N318K). In silico protein modeling shows that these mutations may affect the affinity of PRMT5 for S-adenosylmethionine (SAM), which is required as a methyl donor. Electrostatic analysis of the enzyme active site shows that one of these mutations creates a tunnel in the vicinity of the SAM binding site, which may allow interfering molecules to enter the enzyme active site and decrease its activity. We also identified several non-coding mutations that appear to affect PRMT5 splicing. Our analyses provide insights into the role of PRMT5 mutations in cancer cells. Additionally, since PRMT5 single molecule inhibitors have already been developed, this work may uncover future directions in how mutations can affect targeted inhibition.

DNA Repair and Therapeutic Strategies in Cancer Stem Cells

First-line cancer treatments successfully eradicate the differentiated tumour mass but are comparatively ineffective against cancer stem cells (CSCs), a self-renewing subpopulation thought to be responsible for tumour initiation, metastasis, heterogeneity, and recurrence. CSCs are thus presented as the principal target for elimination during cancer treatment. However, CSCs are challenging to drug target because of numerous intrinsic and extrinsic mechanisms of drug resistance. One such mechanism that remains relatively understudied is the DNA damage response (DDR). CSCs are presumed to possess properties that enable enhanced DNA repair efficiency relative to their highly proliferative bulk progeny, facilitating improved repair of double-strand breaks induced by radiotherapy and most chemotherapeutics. This can occur through multiple mechanisms, including increased expression and splicing fidelity of DNA repair genes, robust activation of cell cycle checkpoints, and elevated homologous recombination-mediated DNA repair. Herein, we summarise the current knowledge concerning improved genome integrity in non-transformed stem cells and CSCs, discuss therapeutic opportunities within the DDR for re-sensitising CSCs to genotoxic stressors, and consider the challenges posed regarding unbiased identification of novel DDR-directed strategies in CSCs. A better understanding of the DDR mediating chemo/radioresistance mechanisms in CSCs could lead to novel therapeutic approaches, thereby enhancing treatment efficacy in cancer patients.

STC2 activates PRMT5 to induce radioresistance through DNA damage repair and ferroptosis pathways in esophageal squamous cell carcinoma

Radioresistance is the major reason for the failure of radiotherapy in esophageal squamous cell carcinoma (ESCC). Previous evidence indicated that stanniocalcin 2 (STC2) participates in various biological processes of malignant tumors. However, researches on its effect on radioresistance in cancers are limited. In this study, STC2 was screened out by RNA-sequencing and bioinformatics analyses as a potential prognosis predictor of ESCC radiosensitivity and then was determined to facilitate radioresistance. We found that STC2 expression is increased in ESCC tissues compared to adjacent normal tissues, and a higher level of STC2 is associated with poor prognosis. Also, STC2 mRNA and protein expression levels were higher in radioresistant cells than in their parental cells. Further investigation revealed that STC2 could interact with protein methyltransferase 5 (PRMT5) and activate PRMT5, thus leading to the increased expression of symmetric dimethylation of histone H4 on Arg 3 (H4R3me2s). Mechanistically, STC2 can promote DDR through the homologous recombination and non-homologous end joining pathways by activating PRMT5. Meanwhile, STC2 can participate in SLC7A11-mediated ferroptosis in a PRMT5-dependent manner. Finally, these results were validated through in vivo experiments. These findings uncovered that STC2 might be an attractive therapeutic target to overcome ESCC radioresistance.

SET7-mediated TIP60 methylation is essential for DNA double-strand break repair

The repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) is crucial for maintaining genomic integrity and is involved in numerous fundamental biological processes. Post-translational modifications by proteins play an important role in regulating DNA repair. Here, we report that the methyltransferase SET7 regulates HR-mediated DSB repair by methylating TIP60, a histone acetyltransferase and tumor suppressor involved in gene expression and protein stability. We show that SET7 targets TIP60 for methylation at K137, which facilitates DSB repair by promoting HR and determines cell viability against DNA damage. Interestingly, TIP60 demethylation is catalyzed by LSD1, which affects HR efficiency. Taken together, our findings reveal the importance of TIP60 methylation status by SET7 and LSD1 in the DSB repair pathway. [BMB Reports 2022; 55(11): 541-546].

SET7-mediated TIP60 methylation is essential for DNA double-strand break repair

The repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) is crucial for maintaining genomic integrity and is involved in numerous fundamental biological processes. Post-translational modifications by proteins play an important role in regulating DNA repair. Here, we report that the methyltransferase SET7 regulates HR-mediated DSB repair by methylating TIP60, a histone acetyltransferase and tumor suppressor involved in gene expression and protein stability. We show that SET7 targets TIP60 for methylation at K137, which facilitates DSB repair by promoting HR and determines cell viability against DNA damage. Interestingly, TIP60 demethylation is catalyzed by LSD1, which affects HR efficiency. Taken together, our findings reveal the importance of TIP60 methylation status by SET7 and LSD1 in the DSB repair pathway. [BMB Reports 2022; 55(11): 541-546].

PRMT7 Inhibitor SGC8158 Enhances Doxorubicin-Induced DNA Damage and Its Cytotoxicity

Protein arginine methyltransferase 7 (PRMT7) regulates various cellular responses, including gene expression, cell migration, stress responses, and stemness. In this study, we investigated the biological role of PRMT7 in cell cycle progression and DNA damage response (DDR) by inhibiting PRMT7 activity with either SGC8158 treatment or its specific siRNA transfection. Suppression of PRMT7 caused cell cycle arrest at the G₁ phase, resulting from the stabilization and subsequent accumulation of p21 protein. In addition, PRMT7 activity is closely associated with DNA repair pathways, including both homologous recombination and non-homologous end-joining. Interestingly, SGC8158, in combination with doxorubicin, led to a synergistic increase in both DNA damage and cytotoxicity in MCF7 cells. Taken together, our data demonstrate that PRMT7 is a critical modulator of cell growth and DDR, indicating that it is a promising target for cancer treatment.

The Role of Protein Arginine Methyltransferases in DNA Damage Response

In response to DNA damage, cells have developed a sophisticated signaling pathway, consisting of DNA damage sensors, transducers, and effectors, to ensure efficient and proper repair of damaged DNA. During this process, posttranslational modifications (PTMs) are central events that modulate the recruitment, dissociation, and activation of DNA repair proteins at damage sites. Emerging evidence reveals that protein arginine methylation is one of the common PTMs and plays critical roles in DNA damage response. Protein arginine methyltransferases (PRMTs) either directly methylate DNA repair proteins or deposit methylation marks on histones to regulate their transcription, RNA splicing, protein stability, interaction with partners, enzymatic activities, and localization. In this review, we summarize the substrates and roles of each PRMTs in DNA damage response and discuss the synergistic anticancer effects of PRMTs and DNA damage pathway inhibitors, providing insight into the significance of arginine methylation in the maintenance of genome integrity and cancer therapies.

FAM135B sustains the reservoir of Tip60-ATM assembly to promote DNA damage response

BACKGROUND

Recently, the mechanism by which cells adapt to intrinsic and extrinsic stresses has received considerable attention. Tat-interactive protein 60-kDa/ataxia-telangiectasia-mutated (TIP60/ATM) axis-mediated DNA damage response (DDR) is vital for maintaining genomic integrity.

METHODS

Protein levels were detected by western blot, protein colocalisation was examined by immunofluorescence (IF) and protein interactions were measured by co-immunoprecipitation, proximity ligation assay and GST pull-down assays. Flow cytometry, comet assay and IF assays were used to explore the biological functions of sequence similarity 135 family member B (FAM135B) in DDR. Xenograft tumour, FAM135B transgenic mouse models and immunohistochemistry were utilised to confirm in vitro observations.

RESULTS

We identified a novel DDR regulator FAM135B which could protect cancer cells from genotoxic stress in vitro and in vivo. The overexpression of FAM135B promoted the removal of γH2AX and 53BP1 foci, whereas the elimination of FAM135B attenuated these effects. Consistently, our findings revealed that FAM135B could promote homologous recombination and non-homologous end-joining repairs. Further study demonstrated that FAM135B physically bound to the chromodomain of TIP60 and improved its histone acetyltransferase activity. Moreover, FAM135B enhanced the interactions between TIP60 and ATM under resting conditions. Intriguingly, the protein levels of FAM135B dramatically decreased following DNA damage stress but gradually increased during the DNA repair period. Thus, we proposed a potential DDR mechanism where FAM135B sustains a reservoir of pre-existing TIP60-ATM assemblies under resting conditions. Once cancer cells suffer DNA damage, FAM135B is released from TIP60, and the functioning pre-assembled TIP60-ATM complex participates in DDR.

CONCLUSIONS

We characterised FAM135B as a novel DDR regulator and further elucidated the role of the TIP60-ATM axis in response to DNA damage, which suggests that targeting FAM135B in combination with radiation therapy or chemotherapy could be a potentially effective approach for cancer treatment.

DNA repair as a shared hallmark in cancer and ageing

Increasing evidence demonstrates that DNA damage and genome instability play a crucial role in ageing. Mammalian cells have developed a wide range of complex and well‐orchestrated DNA repair pathways to respond to and resolve many different types of DNA lesions that occur from exogenous and endogenous sources. Defects in these repair pathways lead to accelerated or premature ageing syndromes and increase the likelihood of cancer development. Understanding the fundamental mechanisms of DNA repair will help develop novel strategies to treat ageing‐related diseases. Here, we revisit the processes involved in DNA damage repair and how these can contribute to diseases, including ageing and cancer. We also review recent mechanistic insights into DNA repair and discuss how these insights are being used to develop novel therapeutic strategies for treating human disease. We discuss the use of PARP inhibitors in the clinic for the treatment of breast and ovarian cancer and the challenges associated with acquired drug resistance. Finally, we discuss how DNA repair pathway‐targeted therapeutics are moving beyond PARP inhibition in the search for ever more innovative and efficacious cancer therapies.

Synthesis and Activity of Triazole-Adenosine Analogs as Protein Arginine Methyltransferase 5 Inhibitors

Protein arginine methyltransferase 5 (PRMT5) is an attractive molecular target in anticancer drug discovery due to its extensive involvement in transcriptional control, RNA processing, and other cellular pathways that are causally related to tumor initiation and progression. In recent years, various compounds have been screened or designed to target either the substrate- or cofactor-binding site of PRMT5. To expand the diversity of chemotypes for inhibitory binding to PRMT5 and other AdoMet-dependent methyltransferases, in this work, we designed a series of triazole-containing adenosine analogs aimed at targeting the cofactor-binding site of PRMT5. Triazole rings have commonly been utilized in drug discovery due to their ease of synthesis and functionalization as bioisosteres of amide bonds. Herein, we utilized the electronic properties of the triazole ring as a novel way to specifically target the cofactor-binding site of PRMT5. A total of about 30 compounds were synthesized using the modular alkyne-azide cycloaddition reaction. Biochemical tests showed that these compounds exhibited inhibitory activity of PRMT5 at varying degrees and several showed single micromolar potency, with clear selectivity for PRMT5 over PRMT1. Docking-based structural analysis showed that the triazole ring plays a key role in binding to the characteristic residue Phe327 in the active pocket of PRMT5, explaining the compounds' selectivity for this type-II enzyme. Overall, this work provides new structure-activity relationship information on the design of AdoMet analogs for selective inhibition of PRMT5. Further structural optimization work will further improve the potency of the top leads.

Multifaceted regulation and functions of 53BP1 in NHEJ‑mediated DSB repair (Review)

The repair of DNA double-strand breaks (DSBs) is crucial for the preservation of genomic integrity and the maintenance of cellular homeostasis. Non-homologous DNA end joining (NHEJ) is the predominant repair mechanism for any type of DNA DSB during the majority of the cell cycle. NHEJ defects regulate tumor sensitivity to ionizing radiation and anti-neoplastic agents, resulting in immunodeficiencies and developmental abnormalities in malignant cells. p53-binding protein 1 (53BP1) is a key mediator involved in DSB repair, which functions to maintain a balance in the repair pathway choices and in preserving genomic stability. 53BP1 promotes DSB repair via NHEJ and antagonizes DNA end overhang resection. At present, novel lines of evidence have revealed the molecular mechanisms underlying the recruitment of 53BP1 and DNA break-responsive effectors to DSB sites, and the promotion of NHEJ-mediated DSB repair via 53BP1, while preventing homologous recombination. In the present review article, recent advances made in the elucidation of the structural and functional characteristics of 53BP1, the mechanisms of 53BP1 recruitment and interaction with the reshaping of the chromatin architecture around DSB sites, the post-transcriptional modifications of 53BP1, and the up- and downstream pathways of 53BP1 are discussed. The present review article also focuses on the application perspectives, current challenges and future directions of 53BP1 research.

RUVBL1 promotes enzalutamide resistance of prostate tumors through the PLXNA1-CRAF-MAPK pathway

Although enzalutamide improves the overall survival of patients with metastatic prostate cancers, enzalutamide resistance (ENZR) will be inevitably developed. Emerging evidence support that alternative oncogenic pathways may bypass the androgen receptor (AR) signaling to promote ENZR progression, however, the underpinning mechanisms remain poorly defined. Here, we report that the expression of RuvB like AAA ATPase 1 (RUVBL1) is upregulated in ENZR cells and xenograft models and prostate tumors in patients. Enzalutamide increases RUVBL1 accumulation in the cytoplasm, which in turn enhances the recruitment of CRAF proto-oncogene serine/threonine kinase protein to plexin A1 (PLXNA1) and the subsequent activation of the downstream MAPK pathway. Co-overexpression of RUVBL1 and PLXNA1 defines a subgroup of prostate cancer (PCa) patients with a poor prognosis. Furthermore, pharmacological inhibition of RUVBL1 by CB-6644 suppresses ENZR cell proliferation and xenograft growth and allows re-sensitization of ENZR cells and xenografts to enzalutamide, indicating that RUVBL1 may act to substitute the AR signaling to promote cancer cell survival and ENZR development. Together, these findings may lead to the identification of RUVBL1 as a potential therapeutic target for ENZR tumors.

PRMT5 regulates ATF4 transcript splicing and oxidative stress response

Protein methyltransferase 5 (PRMT5) symmetrically dimethylates arginine residues leading to regulation of transcription and splicing programs. Although PRMT5 has emerged as an attractive oncology target, the molecular determinants of PRMT5 dependency in cancer remain incompletely understood. Our transcriptomic analysis identified PRMT5 regulation of the activating transcription factor 4 (ATF4) pathway in acute myelogenous leukemia (AML). PRMT5 inhibition resulted in the expression of unstable, intron-retaining ATF4 mRNA that is detained in the nucleus. Concurrently, the decrease in the spliced cytoplasmic transcript of ATF4 led to lower levels of ATF4 protein and downregulation of ATF4 target genes. Upon loss of functional PRMT5, cells with low ATF4 displayed increased oxidative stress, growth arrest, and cellular senescence. Interestingly, leukemia cells with EVI1 oncogene overexpression demonstrated dependence on PRMT5 function. EVI1 and ATF4 regulated gene signatures were inversely correlated. We show that EVI1-high AML cells have reduced ATF4 levels, elevated baseline reactive oxygen species and increased sensitivity to PRMT5 inhibition. Thus, EVI1-high cells demonstrate dependence on PRMT5 function and regulation of oxidative stress response. Overall, our findings identify the PRMT5-ATF4 axis to be safeguarding the cellular redox balance that is especially important in high oxidative stress states, such as those that occur with EVI1 overexpression.

Protein post-translational modifications in the regulation of cancer hallmarks

Posttranslational modifications (PTMs) of proteins, the major mechanism of protein function regulation, play important roles in regulating a variety of cellular physiological and pathological processes. Although the classical PTMs, such as phosphorylation, acetylation, ubiquitination and methylation, have been well studied, the emergence of many new modifications, such as succinylation, hydroxybutyrylation, and lactylation, introduces a new layer to protein regulation, leaving much more to be explored and wide application prospects. In this review, we will provide a broad overview of the significant roles of PTMs in regulating human cancer hallmarks through selecting a diverse set of examples, and update the current advances in the therapeutic implications of these PTMs in human cancer.

How Protein Methylation Regulates Steroid Receptor Function

Steroid receptors (SRs) are members of the nuclear hormonal receptor family, many of which are transcription factors regulated by ligand binding. SRs regulate various human physiological functions essential for maintenance of vital biological pathways, including development, reproduction, and metabolic homeostasis. In addition, aberrant expression of SRs or dysregulation of their signaling has been observed in a wide variety of pathologies. SR activity is tightly and finely controlled by post-translational modifications (PTMs) targeting the receptors and/or their coregulators. Whereas major attention has been focused on phosphorylation, growing evidence shows that methylation is also an important regulator of SRs. Interestingly, the protein methyltransferases depositing methyl marks are involved in many functions, from development to adult life. They have also been associated with pathologies such as inflammation, as well as cardiovascular and neuronal disorders, and cancer. This article provides an overview of SR methylation/demethylation events, along with their functional effects and biological consequences. An in-depth understanding of the landscape of these methylation events could provide new information on SR regulation in physiology, as well as promising perspectives for the development of new therapeutic strategies, illustrated by the specific inhibitors of protein methyltransferases that are currently available.

Subgroup-Specific Diagnostic, Prognostic, and Predictive Markers Influencing Pediatric Medulloblastoma Treatment

Medulloblastoma (MB) is the most common malignant central nervous system tumor in pediatric patients. Mainstay of therapy remains surgical resection followed by craniospinal radiation and chemotherapy, although limitations to this therapy are applied in the youngest patients. Clinically, tumors are divided into average and high-risk status on the basis of age, metastasis at diagnosis, and extent of surgical resection. However, technological advances in high-throughput screening have facilitated the analysis of large transcriptomic datasets that have been used to generate the current classification system, dividing patients into four primary subgroups, i.e., WNT (wingless), SHH (sonic hedgehog), and the non-SHH/WNT subgroups 3 and 4. Each subgroup can further be subdivided on the basis of a combination of cytogenetic and epigenetic events, some in distinct signaling pathways, that activate specific phenotypes impacting patient prognosis. Here, we delve deeper into the genetic basis for each subgroup by reviewing the extent of cytogenetic events in key genes that trigger neoplastic transformation or that exhibit oncogenic properties. Each of these discussions is further centered on how these genetic aberrations can be exploited to generate novel targeted therapeutics for each subgroup along with a discussion on challenges that are currently faced in generating said therapies. Our future hope is that through better understanding of subgroup-specific cytogenetic events, the field may improve diagnosis, prognosis, and treatment to improve overall quality of life for these patients.

Discovery and Pharmacological Characterization of JNJ-64619178, a Novel Small-Molecule Inhibitor of PRMT5 with Potent Antitumor Activity

The protein arginine methyltransferase 5 (PRMT5) methylates a variety of proteins involved in splicing, multiple signal transduction pathways, epigenetic control of gene expression, and mechanisms leading to protein expression required for cellular proliferation. Dysregulation of PRMT5 is associated with clinical features of several cancers, including lymphomas, lung cancer, and breast cancer. Here, we describe the characterization of JNJ-64619178, a novel, selective, and potent PRMT5 inhibitor, currently in clinical trials for patients with advanced solid tumors, non-Hodgkin's lymphoma, and lower-risk myelodysplastic syndrome. JNJ-64619178 demonstrated a prolonged inhibition of PRMT5 and potent antiproliferative activity in subsets of cancer cell lines derived from various histologies, including lung, breast, pancreatic, and hematological malignancies. In primary acute myelogenous leukemia samples, the presence of splicing factor mutations correlated with a higher ex vivo sensitivity to JNJ-64619178. Furthermore, the potent and unique mechanism of inhibition of JNJ-64619178, combined with highly optimized pharmacological properties, led to efficient tumor growth inhibition and regression in several xenograft models in vivo, with once-daily or intermittent oral-dosing schedules. An increase in splicing burden was observed upon JNJ-64619178 treatment. Overall, these observations support the continued clinical evaluation of JNJ-64619178 in patients with aberrant PRMT5 activity-driven tumors.

Cellular pathways influenced by protein arginine methylation: Implications for cancer

Arginine methylation is an influential post-translational modification occurring on histones, RNA binding proteins, and many other cellular proteins, affecting their function by altering their protein-protein and protein-nucleic acid interactions. Recently, a wealth of information has been gathered, implicating protein arginine methyltransferases (PRMTs), enzymes that deposit arginine methylation, in transcription, pre-mRNA splicing, DNA damage signaling, and immune signaling with major implications for cancer therapy, especially immunotherapy. This review summarizes this recent progress and the current state of PRMT inhibitors, some in clinical trials, as promising drug targets for cancer.

SAM Competitive PRMT5 Inhibitor PF-06939999 Demonstrates Antitumor Activity in Splicing Dysregulated NSCLC with Decreased Liability of Drug Resistance

Protein arginine methyltransferase 5 (PRMT5) over-expression in hematological and solid tumors methylates arginine residues on cellular proteins involved in important cancer functions including cell cycle regulation, mRNA splicing, cell differentiation, cell signaling, and apoptosis. PRMT5 methyltransferase function has been linked with high rates of tumor cell proliferation and decreased overall survival, and PRMT5 inhibitors are currently being explored as an approach for targeting cancer-specific dependencies due to PRMT5 catalytic function. Here we describe the discovery of potent and selective S-adenosylmethionine (SAM) competitive PRMT5 inhibitors, with in vitro and in vivo characterization of clinical candidate PF-06939999. Acquired resistance mechanisms were explored through the development of drug resistant cell lines. Our data highlight compound-specific resistance mutations in the PRMT5 enzyme that demonstrate structural constraints in the co-factor binding site that prevent emergence of complete resistance to SAM site inhibitors. PRMT5 inhibition by PF-06939999 treatment reduced proliferation of NSCLC cancer cells, with dose-dependent decreases in symmetric dimethyl arginine (SDMA) levels and changes in alternative splicing of numerous pre-mRNAs. Drug sensitivity to PF-06939999 in NSCLC cells associates with cancer pathways including MYC, cell cycle and spliceosome, and with mutations in splicing factors such as RBM10. Translation of efficacy in mouse tumor xenograft models with splicing mutations provides rationale for therapeutic use of PF-06939999 in the treatment of splicing dysregulated NSCLC.

PRMT5: An Emerging Target for Pancreatic Adenocarcinoma

Simple Summary

The burden of pancreatic ductal adenocarcinoma (PDAC) increases with rising incidence, yet 5-year overall survival remains poor at 17%. Routine comprehensive genomic profiling of PDAC only finds 2.5% of patients who may benefit and receive matched targeted therapy. Protein arginine methyltransferase 5 (PRMT5) as an anti-cancer target has gained significant interest in recent years and high levels of PRMT5 protein are associated with worse survival outcomes across multiple cancer types. Inhibition of PRMT5 in pre-clinical models can lead to cancer growth inhibition. However, PRMT5 is involved in multiple cellular processes, thus determining its mechanism of action is challenging. While past reviews on PRMT5 have focused on its role in diverse cellular processes and past research studies have focused mainly on haematological malignancies and glioblastoma, this review provides an overview of the possible biological mechanisms of action of PRMT5 inhibition and its potential as a treatment in pancreatic cancer.

Abstract

The overall survival of pancreatic ductal adenocarcinoma (PDAC) remains poor and its incidence is rising. Targetable mutations in PDAC are rare, thus novel therapeutic approaches are needed. Protein arginine methyltransferase 5 (PRMT5) overexpression is associated with worse survival and inhibition of PRMT5 results in decreased cancer growth across multiple cancers, including PDAC. Emerging evidence also suggests that altered RNA processing is a driver in PDAC tumorigenesis and creates a partial dependency on this process. PRMT5 inhibition induces altered splicing and this vulnerability can be exploited as a novel therapeutic approach. Three possible biological pathways underpinning the action of PRMT5 inhibitors are discussed; c-Myc regulation appears central to its action in the PDAC setting. Whilst homozygous MTAP deletion and symmetrical dimethylation levels are associated with increased sensitivity to PRMT5 inhibition, neither measure robustly predicts its growth inhibitory response. The immunomodulatory effect of PRMT5 inhibitors on the tumour microenvironment will also be discussed, based on emerging evidence that PDAC stroma has a significant bearing on disease behaviour and response to therapy. Lastly, with the above caveats in mind, current knowledge gaps and the implications and rationales for PRMT5 inhibitor development in PDAC will be explored.

STING protects breast cancer cells from intrinsic and genotoxic-induced DNA instability via a non-canonical, cell-autonomous pathway

STING (Stimulator of Interferon Genes) is an endoplasmic reticulum-anchored adaptor of the innate immunity best known to trigger pro-inflammatory cytokine expression in response to pathogen infection. In cancer, this canonical pathway can be activated by intrinsic or drug-induced genomic instability, potentiating antitumor immune responses. Here we report that STING downregulation decreases cell survival and increases sensitivity to genotoxic treatment in a panel of breast cancer cell lines in a cell-autonomous manner. STING silencing impaired DNA Damage Response (53BP1) foci formation and increased DNA break accumulation. These newly identified properties were found to be independent of STING partner cGAS and of its canonical pro-inflammatory pathway. STING was shown to partially localize at the inner nuclear membrane in a variety of breast cancer cell models and clinical tumor samples. Interactomics analysis of nuclear STING identified several proteins of the DNA Damage Response, including the three proteins of the DNA-PK complex, further supporting a role of STING in the regulation of genomic stability. In breast and ovarian cancer patients that received adjuvant chemotherapy, high STING expression is associated with increased risk of relapse. In summary, this study highlights an alternative, non-canonical tumor-promoting role of STING that opposes its well-documented function in tumor immunosurveillance.