Polθ is phosphorylated by PLK1 to repair double-strand breaks in mitosis

Exploring factors influencing choice of DNA double-strand break repair pathways

DNA double-strand breaks (DSBs) represent one of the most severe threats to genomic integrity, demanding intricate repair mechanisms within eukaryotic cells. A diverse array of factors orchestrates the complex choreography of DSB signaling and repair, encompassing repair pathways, such as non-homologous end-joining, homologous recombination, and polymerase-θ-mediated end-joining. This review looks into the intricate decision-making processes guiding eukaryotic cells towards a particular repair pathway, particularly emphasizing the processing of two-ended DSBs. Furthermore, we elucidate the transformative role of Cas9, a site-specific endonuclease, in revolutionizing our comprehension of DNA DSB repair dynamics. Additionally, we explore the burgeoning potential of Cas9's remarkable ability to induce sequence-specific DSBs, offering a promising avenue for precise targeting of tumor cells. Through this comprehensive exploration, we unravel the intricate molecular mechanisms of cellular responses to DSBs, shedding light on both fundamental repair processes and cutting-edge therapeutic strategies.

PARG is essential for Polθ-mediated DNA end-joining by removing repressive poly-ADP-ribose marks

DNA polymerase theta (Polθ)-mediated end-joining (TMEJ) repairs DNA double-strand breaks and confers resistance to genotoxic agents. How Polθ is regulated at the molecular level to exert TMEJ remains poorly characterized. We find that Polθ interacts with and is PARylated by PARP1 in a HPF1-independent manner. PARP1 recruits Polθ to the vicinity of DNA damage via PARylation dependent liquid demixing, however, PARylated Polθ cannot perform TMEJ due to its inability to bind DNA. PARG-mediated de-PARylation of Polθ reactivates its DNA binding and end-joining activities. Consistent with this, PARG is essential for TMEJ and the temporal recruitment of PARG to DNA damage corresponds with TMEJ activation and dissipation of PARP1 and PAR. In conclusion, we show a two-step spatiotemporal mechanism of TMEJ regulation. First, PARP1 PARylates Polθ and facilitates its recruitment to DNA damage sites in an inactivated state. PARG subsequently activates TMEJ by removing repressive PAR marks on Polθ.

Non-homologous end joining shapes the genomic rearrangement landscape of chromothripsis from mitotic errors

Mitotic errors generate micronuclei entrapping mis-segregated chromosomes, which are susceptible to catastrophic fragmentation through chromothripsis. The reassembly of fragmented chromosomes by error-prone DNA double-strand break (DSB) repair generates diverse genomic rearrangements associated with human diseases. How specific repair pathways recognize and process these lesions remains poorly understood. Here we use CRISPR/Cas9 to systematically inactivate distinct DSB repair pathways and interrogate the rearrangement landscape of fragmented chromosomes. Deletion of canonical non-homologous end joining (NHEJ) components substantially reduces complex rearrangements and shifts the rearrangement landscape toward simple alterations without the characteristic patterns of chromothripsis. Following reincorporation into the nucleus, fragmented chromosomes localize within sub-nuclear micronuclei bodies (MN bodies) and undergo ligation by NHEJ within a single cell cycle. In the absence of NHEJ, chromosome fragments are rarely engaged by alternative end-joining or recombination-based mechanisms, resulting in delayed repair kinetics, persistent 53BP1-labeled MN bodies, and cell cycle arrest. Thus, we provide evidence supporting NHEJ as the exclusive DSB repair pathway generating complex rearrangements from mitotic errors.

Elevating PLK1 overcomes BETi resistance in prostate cancer via triggering BRD4 phosphorylation-dependent degradation in mitosis

Bromodomain-containing protein 4 (BRD4) has emerged as a promising therapeutic target in prostate cancer (PCa). Understanding the mechanisms of BRD4 stability could enhance the clinical response to BRD4-targeted therapy. In this study, we report that BRD4 protein levels are significantly decreased during mitosis in a PLK1-dependent manner. Mechanistically, we show that BRD4 is primarily phosphorylated at T1186 by the CDK1/cyclin B complex, recruiting PLK1 to phosphorylate BRD4 at S24/S1100, which are recognized by the APC/CCdh1 complex for proteasome pathway degradation. We find that PLK1 overexpression lowers SPOP mutation-stabilized BRD4, consequently rendering PCa cells re-sensitized to BRD4 inhibitors. Intriguingly, we report that sequential treatment of docetaxel and JQ1 resulted in significant inhibition of PCa. Collectively, the results support that PLK1-phosphorylated BRD4 triggers its degradation at M phase. Sequential treatment of docetaxel and JQ1 overcomes BRD4 accumulation-associated bromodomain and extra-terminal inhibitor (BETi) resistance, which may shed light on the development of strategies to treat PCa.

ATM inhibition exploits checkpoint defects and ATM-dependent double strand break repair in TP53-mutant glioblastoma

Determining the balance between DNA double strand break repair (DSBR) pathways is essential for understanding treatment response in cancer. We report a method for simultaneously measuring non-homologous end joining (NHEJ), homologous recombination (HR), and microhomology-mediated end joining (MMEJ). Using this method, we show that patient-derived glioblastoma (GBM) samples with acquired temozolomide (TMZ) resistance display elevated HR and MMEJ activity, suggesting that these pathways contribute to treatment resistance. We screen clinically relevant small molecules for DSBR inhibition with the aim of identifying improved GBM combination therapy regimens. We identify the ATM kinase inhibitor, AZD1390, as a potent dual HR/MMEJ inhibitor that suppresses radiation-induced phosphorylation of DSBR proteins, blocks DSB end resection, and enhances the cytotoxic effects of TMZ in treatment-naïve and treatment-resistant GBMs with TP53 mutation. We further show that a combination of G2/M checkpoint deficiency and reliance upon ATM-dependent DSBR renders TP53 mutant GBMs hypersensitive to TMZ/AZD1390 and radiation/AZD1390 combinations. This report identifies ATM-dependent HR and MMEJ as targetable resistance mechanisms in TP53-mutant GBM and establishes an approach for simultaneously measuring multiple DSBR pathways in treatment selection and oncology research.

Structure and repair of replication-coupled DNA breaks

Using CRISPR/Cas9 nicking enzymes, we examine the interaction between the replication machinery and single strand breaks, one of the most common forms of endogenous DNA damage. We show that replication fork collapse at leading strand nicks generates resected single-ended double-strand breaks (seDSBs) that are repaired by homologous recombination (HR). If these seDSBs are not promptly repaired, arrival of adjacent forks creates double ended DSBs (deDSBs), which could drive genomic scarring in HR-deficient cancers. deDSBs can also be generated directly when the replication fork bypasses lagging strand nicks. Unlike deDSBs produced independently of replication, end-resection at nick-induced se/deDSBs is BRCA1-independent. Nevertheless, BRCA1 antagonizes 53BP1 suppression of RAD51 filament formation. These results highlight unique mechanisms that maintain replication fork stability.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels and activates the DNA damage response

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK + HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting-dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting-dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild-type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. E2-TopBP1 interaction promotes mitotic acetylation of CHK2, promoting phosphorylation and activation of the DNA damage response (DDR). The results present a new model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis, and activates the DDR. This is a novel mechanism of HPV16 activation of the DDR, a requirement for the viral life cycle.

IMPORTANCE

Human papillomaviruses (HPVs) are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here, we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. We also demonstrate that the E2-TopBP1 interaction activates the DDR. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

Polymerase theta is a synthetic lethal target for killing Epstein-Barr virus lymphomas

Treatment options for Epstein-Barr virus (EBV)-cancers are limited, underscoring the need for new therapeutic approaches. We have previously shown that EBV-transformed cells and cancers lack homologous recombination (HR) repair, a prominent error-free pathway that repairs double-stranded DNA breaks; instead, EBV-transformed cells demonstrate genome-wide scars of the error-prone microhomology-mediated end joining (MMEJ) repair pathway. This suggests that EBV-cancers are vulnerable to synthetic lethal therapeutic approaches that target MMEJ repair. Indeed, we have previously found that targeting PARP, an enzyme that contributes to MMEJ, results in the death of EBV-lymphoma cells. With the emergence of clinical resistance to PARP inhibitors and the recent discovery of inhibitors of Polymerase theta (POLθ), the polymerase essential for MMEJ, we investigated the role of POLθ in EBV-lymphoma cells. We report that EBV-transformed cell lines, EBV-lymphoma cell lines, and EBV-lymphomas in AIDS patients demonstrate greater abundance of POLθ, driven by the EBV protein EBNA1, compared to EBV-uninfected primary lymphocytes and EBV-negative lymphomas from AIDS patients (a group that also abundantly expresses POLθ). We also find POLθ enriched at cellular DNA replication forks and exposure to the POLθ inhibitor Novobiocin impedes replication fork progress, impairs MMEJ-mediated repair of DNA double-stranded breaks, and kills EBV-lymphoma cells. Notably, cell killing is not due to Novobiocin-induced activation of the lytic/replicative phase of EBV. These findings support a role for POLθ not just in DNA repair but also DNA replication and as a therapeutic target in EBV-lymphomas and potentially other EBV-cancers as EBNA1 is expressed in all EBV-cancers.IMPORTANCEEpstein-Barr virus (EBV) contributes to ~2% of the global cancer burden. With a recent estimate of >200,000 deaths a year, identifying molecular vulnerabilities will be key to the management of these frequently aggressive and treatment-resistant cancers. Building on our earlier work demonstrating reliance of EBV-cancers on microhomology-mediated end-joining repair, we now report that EBV lymphomas and transformed B cell lines abundantly express the MMEJ enzyme POLθ that likely protects cellular replication forks and repairs replication-related cellular DNA breaks. Importantly also, we show that a newly identified POLθ inhibitor kills EBV-cancer cells, revealing a novel strategy to block DNA replication and repair of these aggressive cancers.

DNA repair is efficient in irradiated M phase zygotes

In somatic cells, DNA repair is attenuated during mitosis to prevent the formation of anaphase bridges and facilitate the proper segregation of sister chromatids. Irradiation-induced γH2AX foci persist for hours in M phase somatic cells. However, we observed that anaphase bridges formed in a significant fraction of mouse zygotes irradiated during mitosis. Additionally, γH2AX signals in M phase zygotes peaked 30 min after irradiation and subsequently reduced with a half-life within 1-2 h. These results suggest that the DNA repair system may operate efficiently in M phase zygotes following irradiation, leading to the frequent formation of anaphase bridges. The absence of H2AX promoted the successful segregation of sister chromatids and enhanced the development of embryos to the blastocyst stage. The DNA repair system may be differentially regulated during the M phase of the first cell cycle to ensure the immediate elimination of damaged zygotes, thereby efficiently preventing transmission of mutations to subsequent generations.

CDK4/6 inhibitor abemaciclib combined with low-dose radiotherapy enhances the anti-tumor immune response to PD-1 blockade by inflaming the tumor microenvironment in Rb-deficient small cell lung cancer

Background

Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors have shown significant activity against several solid tumors by reducing the phosphorylation of the canonical CDK4/6 substrate retinoblastoma (Rb) protein, while the anti-tumor effect of CDK4/6 inhibitors on Rb-deficient tumors is not clear. Most small cell lung cancers (SCLCs) are Rb-deficient and show very modest response to immune checkpoint blockade (ICB) despite recent advances in the use of immunotherapy. Here, we aimed to investigate the direct effect of CDK4/6 inhibition on SCLC cells and determine its efficacy in combination therapy for SCLC.

Methods

The immediate impact of CDK4/6 inhibitor abemaciclib on cell cycle, cell viability and apoptosis in four SCLC cell lines was initially checked. To explore the effect of abemaciclib on double-strand DNA (ds-DNA) damage induction and the combination impact of abemaciclib coupled with radiotherapy (RT), western blot, immunofluorescence (IF) and quantitative real-time polymerase chain reaction (qRT-PCR) were performed. An Rb-deficient immunocompetent murine SCLC model was established to evaluate efficacy of abemaciclib in combination therapy. Histological staining, flow cytometry analysis and RNA sequencing were performed to analyze alteration of infiltrating immune cells in tumor microenvironment (TME).

Results

Here, we demonstrated that abemaciclib induced increased ds-DNA damage in Rb-deficient SCLC cells. Combination of abemaciclib and RT induced more cytosolic ds-DNA, and activated the STING pathway synergistically. We further showed that combining low doses of abemaciclib with low-dose RT (LDRT) plus anti-programmed cell death protein-1 (anti-PD-1) antibody substantially potentiated CD8+ T cell infiltration and significantly inhibited tumor growth and prolonged survival in an Rb-deficient immunocompetent murine SCLC model.

Conclusions

Our results define previously uncertain DNA damage-inducing properties of CDK4/6 inhibitor abemaciclib in Rb-deficient SCLCs, and demonstrate that low doses of abemaciclib combined with LDRT inflame the TME and enhance the efficacy of anti-PD-1 immunotherapy in SCLC model, which represents a potential novel therapeutic strategy for SCLC.

Onvansertib in Combination with FOLFIRI and Bevacizumab in Second-Line Treatment of KRAS-Mutant Metastatic Colorectal Cancer: A Phase Ib Clinical Study

PURPOSE

Onvansertib is a highly specific inhibitor of polo-like kinase 1 (PLK1), with demonstrated safety in solid tumors. We evaluated, preclinically and clinically, the potential of onvansertib in combination with chemotherapy as a therapeutic option for KRAS-mutant colorectal cancer.

PATIENTS AND METHODS

Preclinical activity of onvansertib was assessed (i) in vitro in KRAS wild-type and -mutant isogenic colorectal cancer cells and (ii) in vivo, in combination with irinotecan, in a KRAS-mutant xenograft model. Clinically, a phase Ib trial was conducted to investigate onvansertib at doses 12, 15, and 18 mg/m2 (days 1-5 and 14-19 of a 28-day cycle) in combination with FOLFIRI/bevacizumab (days 1 and 15) in patients with KRAS-mutant metastatic colorectal cancer who had prior oxaliplatin exposure. Safety, efficacy, and changes in circulating tumor DNA (ctDNA) were assessed.

RESULTS

In preclinical models, onvansertib displayed superior activity in KRAS-mutant than wild-type isogenic colorectal cancer cells and demonstrated potent antitumor activity in combination with irinotecan in vivo. Eighteen patients enrolled in the phase Ib study. Onvansertib recommended phase II dose was established at 15 mg/m2. Grade 3 and 4 adverse events (AE) represented 15% of all treatment-related AEs, with neutropenia being the most common. Partial responses were observed in 44% of patients, with a median duration of response of 9.5 months. Early ctDNA dynamics were predictive of treatment efficacy.

CONCLUSIONS

Onvansertib combined with FOLIFRI/bevacizumab exhibited manageable safety and promising efficacy in second-line treatment of patients with KRAS-mutant metastatic colorectal cancer. Further exploration of this combination therapy is ongoing. See related commentary by Stebbing and Bullock, p. 2005.

Polo-like Kinase 1 Inhibition in KRAS-Mutated Metastatic Colorectal Cancer

Inhibition of Polo-like kinase 1 (Plk1) is a promising new target and therapeutic strategy in metastatic colorectal cancer, especially those with KRAS mutations. New data support further development of onvansertib, and highlights the role of circulating tumor DNA in phase I clinical trials. See related article by Ahn et al., p. 2039.

Genome-wide CRISPR screens identify PKMYT1 as a therapeutic target in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with an overall 5-year survival rate of <12% due to the lack of effective treatments. Novel treatment strategies are urgently needed. Here, PKMYT1 is identified through genome-wide CRISPR screens as a non-mutant, genetic vulnerability of PDAC. Higher PKMYT1 expression levels indicate poor prognosis in PDAC patients. PKMYT1 ablation inhibits tumor growth and proliferation in vitro and in vivo by regulating cell cycle progression and inducing apoptosis. Moreover, pharmacological inhibition of PKMYT1 shows efficacy in multiple PDAC cell models and effectively induces tumor regression without overt toxicity in PDAC cell line-derived xenograft and in more clinically relevant patient-derived xenograft models. Mechanistically, in addition to its canonical function of phosphorylating CDK1, PKMYT1 functions as an oncogene to promote PDAC tumorigenesis by regulating PLK1 expression and phosphorylation. Finally, TP53 function and PRKDC activation are shown to modulate the sensitivity to PKMYT1 inhibition. These results define PKMYT1 dependency in PDAC and identify potential therapeutic strategies for clinical translation.

Pan-cancer analysis of polo-like kinase family genes reveals polo-like kinase 1 as a novel oncogene in kidney renal papillary cell carcinoma

Background

Polo-like kinases (PLKs) are a kinase class of serine/threonine with five members that play crucial roles in cell cycle regulation. However, their biological functions, regulation, and expression remain unclear. This study revealed the molecular properties, oncogenic role, and clinical significance of PLK genes in pan-cancers, particularly in kidney renal papillary cell carcinoma (KIRP).

Methods

We evaluated the mutation landscape, expression level, and prognostic values of PLK genes using bioinformatics analyses and explored the association between the expression level of PLK genes and tumor microenvironment (TME), immune subtype, cancer immunotherapy, tumor stemness, and drug sensitivity. Finally, we verified the prognostic value in patients with KIRP through univariate and multivariate analyses and nomogram construction.

Results

PLK genes are extensively altered in pan-cancer, which may contribute to tumorigenesis. These genes are aberrantly expressed in some types of cancer, with PLK1 being overexpressed in 31 cancers. PLK expression is closely associated with the prognosis of various cancers. The expression level of PLK genes is related with sensitivity to diverse drugs and cancer immunity as well as cancer immunotherapy. Importantly, we verified that PLK1 was overexpressed in KIRP tissues and could be an unfavorable prognostic biomarker in patients with KIRP. Hence, PLK1 may serve as an oncogenic gene in KIRP and should be explored in future studies.

Conclusions

Our study comprehensively reports the molecular characteristics and biological functions of PLK family gens across human cancers and recommends further investigation of these genes as potential biomarkers and therapeutic targets, especially in KIRP.

RAD52-dependent mitotic DNA synthesis is required for genome stability in Cyclin E1-overexpressing cells

Overexpression of Cyclin E1 perturbs DNA replication, resulting in DNA lesions and genomic instability. Consequently, Cyclin E1-overexpressing cancer cells increasingly rely on DNA repair, including RAD52-mediated break-induced replication during interphase. We show that not all DNA lesions induced by Cyclin E1 overexpression are resolved during interphase. While DNA lesions upon Cyclin E1 overexpression are induced in S phase, a significant fraction of these lesions is transmitted into mitosis. Cyclin E1 overexpression triggers mitotic DNA synthesis (MiDAS) in a RAD52-dependent fashion. Chemical or genetic inactivation of MiDAS enhances mitotic aberrations and persistent DNA damage. Mitosis-specific degradation of RAD52 prevents Cyclin E1-induced MiDAS and reduces the viability of Cyclin E1-overexpressing cells, underscoring the relevance of RAD52 during mitosis to maintain genomic integrity. Finally, analysis of breast cancer samples reveals a positive correlation between Cyclin E1 amplification and RAD52 expression. These findings demonstrate the importance of suppressing mitotic defects in Cyclin E1-overexpressing cells through RAD52.

Broken chromosomes heading into mitosis: More than one way to patch a flat tire

A cell dealing with a broken chromosome in mitosis is like a driver dealing with a flat tire on the highway: damage repair must occur under non-ideal circumstances. Mitotic chromosome breaks encounter problems related to structures called micronuclei. These aberrant nuclei are linked to cell death, mutagenesis, and cancer. In the last few years, a flurry of studies illuminated two mechanisms that prevent mitotic problems related to micronuclei. One mechanism prevents micronuclei from forming during mitosis and involves DNA Polymerase Theta, a DNA repair regulator that patches up broken mitotic chromosomes. A second mechanism is activated after micronuclei form and then rupture, and involves CIP2A and TOPBP1 proteins, which patch micronuclear fragments to promote their subsequent mitotic segregation. Here, we review recent progress in this field of mitotic DNA damage and discuss why multiple mechanisms exist. Future studies in this exciting area will reveal new DNA break responses and inform therapeutic strategies.

DNA strand breaks at centromeres: Friend or foe?

Centromeres are large structural regions in the genomic DNA, which are essential for accurately transmitting a complete set of chromosomes to daughter cells during cell division. In humans, centromeres consist of highly repetitive α-satellite DNA sequences and unique epigenetic components, forming large proteinaceous structures required for chromosome segregation. Despite their biological importance, there is a growing body of evidence for centromere breakage across the cell cycle, including periods of quiescence. In this review, we provide an up-to-date examination of the distinct centromere environments at different stages of the cell cycle, highlighting their plausible contribution to centromere breakage. Additionally, we explore the implications of these breaks on centromere function, both in terms of negative consequences and potential positive effects.

EXO1 protects BRCA1-deficient cells against toxic DNA lesions

Inactivating mutations in the BRCA1 and BRCA2 genes impair DNA double-strand break (DSB) repair by homologous recombination (HR), leading to chromosomal instability and cancer. Importantly, BRCA1/2 deficiency also causes therapeutically targetable vulnerabilities. Here, we identify the dependency on the end resection factor EXO1 as a key vulnerability of BRCA1-deficient cells. EXO1 deficiency generates poly(ADP-ribose)-decorated DNA lesions during S phase that associate with unresolved DSBs and genomic instability in BRCA1-deficient but not in wild-type or BRCA2-deficient cells. Our data indicate that BRCA1/EXO1 double-deficient cells accumulate DSBs due to impaired repair by single-strand annealing (SSA) on top of their HR defect. In contrast, BRCA2-deficient cells retain SSA activity in the absence of EXO1 and hence tolerate EXO1 loss. Consistent with a dependency on EXO1-mediated SSA, we find that BRCA1-mutated tumors show elevated EXO1 expression and increased SSA-associated genomic scars compared with BRCA1-proficient tumors. Overall, our findings uncover EXO1 as a promising therapeutic target for BRCA1-deficient tumors.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK+HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We also demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. The results present a model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis and E2 acetylation on K111 by p300 increases, promoting interaction with Top1 that protects K112 from ubiquitination and therefore E2 proteasomal degradation.

Importance

Human papillomaviruses are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

Boveri and beyond: Chromothripsis and genomic instability from mitotic errors

Mitotic cell division is tightly monitored by checkpoints that safeguard the genome from instability. Failures in accurate chromosome segregation during mitosis can cause numerical aneuploidy, which was hypothesized by Theodor Boveri over a century ago to promote tumorigenesis. Recent interrogation of pan-cancer genomes has identified unexpected classes of chromosomal abnormalities, including complex rearrangements arising through chromothripsis. This process is driven by mitotic errors that generate abnormal nuclear structures that provoke extensive yet localized shattering of mis-segregated chromosomes. Here, we discuss emerging mechanisms underlying chromothripsis from micronuclei and chromatin bridges, as well as highlight how this mutational cascade converges on the DNA damage response. A fundamental understanding of these catastrophic processes will provide insight into how initial errors in mitosis can precipitate rapid cancer genome evolution.

CIP2A coordinates phosphosignaling, mitosis, and the DNA damage response

Human cancers share requirements for phosphorylation-dependent signaling, mitotic hyperactivity, and survival after DNA damage. The oncoprotein CIP2A (cancerous inhibitor of PP2A) can coordinate all these cancer cell characteristics. In addition to controlling cancer cell phosphoproteomes via inhibition of protein phosphatase PP2A, CIP2A directly interacts with the DNA damage protein TopBP1 (topoisomerase II-binding protein 1). Consequently, CIP2A allows DNA-damaged cells to enter mitosis and is essential for mitotic cells that are defective in homologous recombination (HR)-mediated DNA repair (e.g., BRCA mutants). The CIP2A-TopBP1 complex is also important for clustering fragmented chromosomes at mitosis. Clinically, CIP2A is a disease driver for basal-like triple-negative breast cancer (BL-TNBC) and a promising cancer therapy target across many cancer types.

The impact of chromatin on double-strand break repair: Imaging tools and discoveries

Eukaryotic nuclei are constantly being exposed to factors that break or chemically modify the DNA. Accurate repair of this DNA damage is crucial to prevent DNA mutations and maintain optimal cell function. To overcome the detrimental effects of DNA damage, a multitude of repair pathways has evolved. These pathways need to function properly within the different chromatin domains present in the nucleus. Each of these domains exhibit distinct molecular- and bio-physical characteristics that can influence the response to DNA damage. In particular, chromatin domains highly enriched for repetitive DNA sequences, such as nucleoli, centromeres and pericentromeric heterochromatin require tailored repair mechanisms to safeguard genome stability. Work from the past decades has led to the development of innovative imaging tools as well as inducible DNA damage techniques to gain new insights into the impact of these repetitive chromatin domains on the DNA repair process. Here we summarize these tools with a particular focus on Double-Strand Break (DSB) repair, and discuss the insights gained into our understanding of the influence of chromatin domains on DSB -dynamics and -repair pathway choice.

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.

Emerging roles of the CIP2A-TopBP1 complex in genome integrity

CIP2A is an inhibitor of the tumour suppressor protein phosphatase 2A. Recently, CIP2A was identified as a synthetic lethal interactor of BRCA1 and BRCA2 and a driver of basal-like breast cancers. In addition, a joint role of TopBP1 (topoisomerase IIβ-binding protein 1) and CIP2A for maintaining genome integrity during mitosis was discovered. TopBP1 has multiple functions as it is a scaffold for proteins involved in DNA replication, transcriptional regulation, cell cycle regulation and DNA repair. Here, we briefly review details of the CIP2A-TopBP1 interaction, its role in maintaining genome integrity, its involvement in cancer and its potential as a therapeutic target.

Mechanisms of synthetic lethality between BRCA1/2 and 53BP1 deficiencies and DNA polymerase theta targeting

A synthetic lethal relationship exists between disruption of polymerase theta (Polθ), and loss of either 53BP1 or homologous recombination (HR) proteins, including BRCA1; however, the mechanistic basis of these observations are unclear. Here we reveal two distinct mechanisms of Polθ synthetic lethality, identifying dual influences of 1) whether Polθ is lost or inhibited, and 2) the underlying susceptible genotype. Firstly, we find that the sensitivity of BRCA1/2- and 53BP1-deficient cells to Polθ loss, and 53BP1-deficient cells to Polθ inhibition (ART558) requires RAD52, and appropriate reduction of RAD52 can ameliorate these phenotypes. We show that in the absence of Polθ, RAD52 accumulations suppress ssDNA gap-filling in G2/M and encourage MRE11 nuclease accumulation. In contrast, the survival of BRCA1-deficient cells treated with Polθ inhibitor are not restored by RAD52 suppression, and ssDNA gap-filling is prevented by the chemically inhibited polymerase itself. These data define an additional role for Polθ, reveal the mechanism underlying synthetic lethality between 53BP1, BRCA1/2 and Polθ loss, and indicate genotype-dependent Polθ inhibitor mechanisms.