visualization-quantitative-analysis-genotoxin-induced-parp1parp2

Visualization and Quantitative Analysis of Genotoxin-Induced PARP1/PARP2 Activation in Cells Using a Fluorescent Fusion Protein-Based Reporter

Poly(ADP-ribose) (PAR) is a polymer of ADP-ribose synthesized by four members of the ADP-ribose polymerase family of enzymes&#8212;PARP1, PARP2, PARP5a, ...

Brown University

The epigenetic modifier histone deacetylase-2 (HDAC2) is frequently dysregulated in colon cancer cells. Microsatellite instability (MSI), an unfaithful replication of DNA at nucleotide repeats, occurs in about 15% of human colon tumors. MSI promotes a genetic frameshift and consequently a loss of HDAC2 in up to 43% of these tumors. We show that long-term and short-term cultures of colorectal cancers with MSI contain subpopulations of cells lacking HDAC2. These can be isolated as single cell-derived, proliferating populations. Xenografted patient-derived colon cancer tissues with MSI also show variable patterns of HDAC2 expression in mice. HDAC2-positive and HDAC2-negative RKO cells respond similarly to pharmacological inhibitors of the class I HDACs HDAC1/HDAC2/HDAC3. In contrast to this similarity, HDAC2-negative and HDAC2-positive RKO cells undergo differential cell cycle arrest and apoptosis induction in response to the frequently used chemotherapeutic 5-fluorouracil, which becomes incorporated into and damages RNA and DNA. 5-fluorouracil causes an enrichment of HDAC2-negative RKO cells in vitro and in a subset of primary colorectal tumors in mice. 5-fluorouracil induces the phosphorylation of KAP1, a target of the checkpoint kinase ataxia-telangiectasia mutated (ATM), stronger in HDAC2-negative cells than in their HDAC2-positive counterparts. Pharmacological inhibition of ATM sensitizes RKO cells to cytotoxic effects of 5-fluorouracil. These findings demonstrate that HDAC2 and ATM modulate the responses of colorectal cancer cells towards 5-FU.

The epigenetic modifier HDAC2 and the checkpoint kinase ATM determine the responses of microsatellite instable colorectal cancer cells to 5-fluorouracil.

Cancer drug resistance remains a major obstacle in clinical oncology. As most anticancer drugs are of natural origin, we investigated the anticancer potential of a standardized cold-water leaf extract from  L., termed Breastin. The phytochemical characterization by nuclear magnetic resonance spectroscopy (NMR) and low- and high-resolution mass spectrometry revealed several monoglycosidic cardenolides as major constituents (adynerin, neritaloside, odoroside A, odoroside H, oleandrin, and vanderoside). Breastin inhibited the growth of 14 cell lines from hematopoietic tumors and 5 of 6 carcinomas. Remarkably, the cellular responsiveness of odoroside H and neritaloside was not correlated with all other classical drug resistance mechanisms, i.e., ATP-binding cassette transporters (ABCB1, ABCB5, ABCC1, ABCG2), oncogenes (EGFR, RAS), tumor suppressors (TP53, WT1), and others (GSTP1, HSP90, proliferation rate), in 59 tumor cell lines of the National Cancer Institute (NCI, USA), indicating that Breastin may indeed bypass drug resistance. COMPARE analyses with 153 anticancer agents in 74 tumor cell lines of the Oncotest panel revealed frequent correlations of Breastin with mitosis-inhibiting drugs. Using tubulin-GFP-transfected U2OS cells and confocal microscopy, it was found that the microtubule-disturbing effect of Breastin was comparable to that of the tubulin-depolymerizing drug paclitaxel. This result was verified by a tubulin polymerization assay in vitro and molecular docking in silico. Proteome profiling of 3171 proteins in the NCI panel revealed protein subsets whose expression significantly correlated with cellular responsiveness to odoroside H and neritaloside, indicating that protein expression profiles can be identified to predict the sensitivity or resistance of tumor cells to Breastin constituents. Breastin moderately inhibited breast cancer xenograft tumors in vivo. Remarkably, in contrast to what was observed with paclitaxel monotherapy, the combination of paclitaxel and Breastin prevented tumor relapse, indicating Breastin's potential for drug combination regimens.

Molecular Modes of Action of an Aqueous  Extract in Cancer Cells In Vitro and In Vivo.

Mutations in isocitrate dehydrogenase isoform 1 (IDH1) are primarily found in secondary glioblastoma (GBM) and low-grade glioma but are rare in primary GBM. The standard treatment for GBM includes radiation combined with temozolomide, an alkylating agent. Fortunately, IDH1 mutant gliomas are sensitive to this treatment, resulting in a more favorable prognosis. However, it's estimated that up to 75 % of IDH1 mutant gliomas will progress to WHO grade IV over time and develop resistance to alkylating agents. Therefore, understanding the mechanism(s) by which IDH1 mutant gliomas confer sensitivity to alkylating agents is crucial for developing targeted chemotherapeutic approaches. The base excision repair (BER) pathway is responsible for repairing most base damage induced by alkylating agents. Defects in this pathway can lead to hypersensitivity to these agents due to unresolved DNA damage. The coordinated assembly and disassembly of BER protein complexes are essential for cell survival and for maintaining genomic integrity following alkylating agent exposure. These complexes rely on poly-ADP-ribose formation, an NAD-dependent post-translational modification synthesized by PARP1 and PARP2 during the BER process. At the lesion site, poly-ADP-ribose facilitates the recruitment of XRCC1. This scaffold protein helps assemble BER proteins like DNA polymerase beta (Polβ), a bifunctional DNA polymerase containing both DNA synthesis and 5'-deoxyribose-phosphate lyase (5'dRP lyase) activity. Here, we confirm that IDH1 mutant glioma cells have defective NAD metabolism, but still produce sufficient nuclear NAD for robust PARP1 activation and BER complex formation in response to DNA damage. However, the overproduction of 2-hydroxyglutarate, an oncometabolite produced by the IDH1 R132H mutant protein, suppresses BER capacity by reducing Polβ protein levels. This defines a novel mechanism by which the IDH1 mutation in gliomas confers cellular sensitivity to alkylating agents and to inhibitors of the poly-ADP-ribose glycohydrolase, PARG.

Oncometabolite 2-hydroxyglutarate suppresses basal protein levels of DNA polymerase beta that enhances alkylating agent and PARG inhibition induced cytotoxicity.

Protein poly-ADP-ribosylation (PARylation) is a post-translational modification formed by transferring successive units of ADP-ribose to target proteins to form poly-ADP-ribose (PAR) chains. PAR plays a critical role in the DNA damage response (DDR) by acting as a signaling platform to promote the recruitment of DNA repair factors to the sites of DNA damage that bind via their PAR-binding domains (PBDs). Several classes of PBD families have been identified, which recognize distinct parts of the PAR chain. Proteins encoding PBDs play an essential role in conveying the PAR-mediated signal through their interaction with PAR chains, which mediates many cellular functions, including the DDR. The WWE domain, encoded in 12 human proteins, identifies the iso-ADP-ribose moiety of the PAR chain. PARylation is a heterogeneous structure that is highly dynamic in cells. Capturing the dynamics of PARylation is essential to understanding its role in the DDR, which can be achieved by expanding the tool kit for PAR detection and tracking mediated by the unique binding capability of various sensors. We recently described the WWE domain of RNF146 as a robust genetically encoded probe, when fused to EGFP, for the detection of PAR in live cells. Expanding on this, we used structural prediction tools to evaluate all of the WWE domains encoded in human proteins, evaluating each as molecular PAR probes in live cells. We demonstrate unique PAR dynamics when tracked by WWE-encoded PAR binding domains, in addition to an engineered macrodomain, that can be exploited for modulation of the PAR-dependent DNA damage response.

Overexpression of the WWE domain of RNF146 modulates poly-(ADP)-ribose dynamics at sites of DNA damage.

Head and Neck Squamous Cell Carcinoma (HNSCC) imposes a significant health burden, necessitating innovative therapeutic strategies to enhance treatment efficacy. Current treatments, such as surgery, radiation, and chemotherapy, have limited effectiveness and yield severe side effects, emphasizing the need for targeted therapies. We have focused on DNA polymerase beta (Polβ) and its roles in replication stress, cellular responses to DNA damaging-therapies, and DNA damage response modifiers. Our investigations reveal a regulatory role for base excision repair (BER) proteins, including Polβ, in the cellular response to inhibitors of poly(ADP-ribose) glycohydrolase (PARG), an enzyme involved in poly(ADP-ribose) (PAR) degradation. The inhibition of PARG, in HNSCC cells, elicits replication stress and activates the PAR-induced S-phase/ATR checkpoint, leading to a block to replication, cell cycle arrest, and the onset of apoptosis. However, Polβ overexpression mitigates this response, reducing replication-stress-induced PAR foci formation, suggesting a modulation of replication checkpoint activation. We found that PARG inhibitor treatment is ineffective on HNSCC cells that overexpress Polβ, implying that the PARG inhibitor-induced PAR and apoptotic response is dependent on the level of Polβ. Further, our in vitro experiments demonstrate that combining PARG and ATR/CHK1 inhibitors overcomes Polβ-mediated treatment resistance in HNSCC cells, producing synergistic effects compared to the individual treatment conditions. Our findings suggest a possible treatment paradigm for HNSCC, employing ATR or CHK1 inhibitors in combination with PARG inhibitors. This strategy offers a promising path for more effective HNSCC treatments, potentially overcoming Polβ-related resistance.

DNA polymerase beta expression in head & neck cancer modulates the poly(ADP-ribose)-mediated replication checkpoint.

The multitude of DNA lesion types, and the nuclear dynamic context in which they occur, presents a challenge for genome integrity maintenance as this requires the engagement of different DNA repair pathways. Specific "repair controllers" that facilitate DNA repair pathway crosstalk between double-strand break (DSB) repair and base excision repair (BER) and that regulate BER protein engagement at lesion sites have yet to be identified. Here, we find that DNA polymerase β (Polβ), crucial for BER, is ubiquitylated in a BER complex-dependent manner by TRIP12, an E3 ligase that partners with UBR5 to restrain DSB repair signaling. Furthermore, we find that TRIP12, but not UBR5, controls cellular levels and chromatin loading of Polβ. Required for Polβ foci formation, TRIP12 influences Polβ involvement after radiation-induced DNA damage, a process regulated by TRIP12-mediated ubiquitylation of Polβ. Notably, excessive TRIP12-mediated engagement of Polβ affects DSB formation and radiation sensitivity, underscoring its role in promoting precedence for BER over DSB repair. The herein discovered function of TRIP12, in the governance of Polβ-directed BER, supports a role for TRIP12 in assuring BER lesion removal at complex DSB sites to optimize DSB repair at the nexus of DNA repair pathways.

TRIP12's role in the governance of DNA polymerase β involvement in DNA damage response and repair.

Non-small-cell lung cancer NSCLC is the major diagnosed type of lung cancers in the USA and Europe. It is generally related to poor prognosis and low rates of survival. Oleandrin is a cardiac glycoside occurring naturally in Nerium oleander (Apocynaceae).

Oleandrin-mediated suppression of MELK induces apoptosis, autophagy, and ferroptosis in human non-small cell lung cancer cells.

Recombinant adeno-associated virus (rAAV) is a replication-defective viral vector used in hundreds of human gene therapy trials, resulting in five FDA-approved therapies. Despite this success, rAAV-based gene therapies suffer from dose-limiting toxicities, resulting in several severe adverse reactions, including death. Previously, we discovered that rAAV rapidly kills mouse NPCs in vitro and in vivo. This vector contains a minimal genome comprised of 145-base pair inverted terminal repeats (ITRs) with a T-shaped hairpin structure that appears to be necessary and sufficient for this toxicity. However, the mechanism for AAV ITR toxicity is not known, and there have been few attempts to engineer ITRs to attenuate rAAV toxicity. In the current study, we explore the molecular mechanisms that drive dose-dependent rAAV toxicity in dividing human NPCs (hNPCs) and test whether disrupting these mechanisms mitigates this toxicity. Recombinant AAV infection induces aberrant cell cycle progression with activation of the ATM /CHK1/CHK2 pathway and expression of the DNA damage markers γH2AX and 53BP1. Affinity-based proteomics indicate that AAV ITRs bind to Poly-(ADP-Ribose)polymerase 1 (PARP1) and other DNA damage response (DDR) proteins involved in single-strand break repair (SSBR). Recombinant AAV infection attenuates poly-(ADP-ribose) (PAR) formation and mimics the antiproliferative effects of pharmacological PARP inhibitors used in cancer therapy. Moreover, treatment of hNPCs with PARP inhibitors is sufficient to reproduce many features of rAAV-induced toxicity. Finally, we demonstrate that eliminating the T-shaped hairpin within the AAV ITR reduces binding to SSBR proteins and the resulting rAAV toxicity. These findings suggest that rAAV infection induces replication stress and cell death in dividing hNPCs by functionally depleting PARP1 and other DDR proteins that are essential for DNA replication. This work fills substantial gaps in the understanding of the mechanisms of rAAV toxicity and has important implications for the development of safer rAAV-based human gene therapies.

AAV Kills Dividing Cells by Depleting PARP1 and Other DNA Damage Response Proteins.

Poly(ADP-ribose) polymerases 1 and 2 (PARP1/PARP2), and poly(ADP-ribose) glycohydrolase (PARG), modulate the level of poly(ADP-ribose) (PAR), a post-translational protein modification, in response to DNA damage or replication stress. Here, we find that replication-dependent and PARP1/PARP2-mediated PARylation recruits the base excision repair (BER)/single-strand break repair (SSBR) scaffold protein XRCC1 and the associated factors DNA polymerase β (POLB), aprataxin (APTX), and DNA ligase isoform 3 (LIG3). Further, these BER/SSBR proteins promote resistance to inhibitors of PARP1/PARP2 and PARG, as loss of these proteins sensitizes glioblastoma and ovarian cancer cells to each. In addition, depletion of these replication-associated BER/SSBR factors leads to enhanced PAR levels and PARG inhibitor-induced activation of the ATR/CHK1 S-phase checkpoint kinases. Both PARG inhibition and ATR inhibition lead to elevated ATM- and DNA-PK-dependent KAP1 phosphorylation. In turn, inhibition of either ATR or CHK1 enhances the cellular response to PARG inhibitors. Finally, inhibition of the ATR regulators PRMT1 or PRMT5 synergizes with PARG inhibition, implicating replication-associated BER/SSBR and PARylation in the activation of the PRMT1/PRMT5/ATR axis. This study highlights the role of BER/SSBR in protecting the cell during S-phase to suppress PARylation-induced checkpoint activation, which may suggest a potential intervention strategy for PARG inhibitor-resistant tumors.

Replication-associated base excision repair/single-strand break repair regulates PARG inhibitor response via the PRMT1/PRMT5/ATR axis.

Ubiquitin-specific protease 1 (USP1) is an emerging target for poly(ADP-ribose) polymerase 1 (PARP1) inhibitor-resistant and BRCA1/BRCA2 mutant tumors. USP1 is a deubiquitylating enzyme responsible for the removal of the mono-ubiquitin mark on FANCD2, PARP1, and the replication factor proliferating cell nuclear antigen (PCNA), among other proteins. USP1 facilitates proper PCNA-mediated polymerase switching from error-prone trans-lesion synthesis DNA polymerases to replicative DNA polymerases. Due to the critical role of USP1 in DNA synthesis and DNA repair, and the discovery that USP1 deubiquitylates PARP1, USP1 inhibitors (USP1i) were found to have a synthetic lethal relationship with PARP1 inhibitors (PARPi), suggesting a mechanistic link between poly(ADP-ribose) (PAR) dynamics and USP1-mediated ubiquitin hydrolysis. However, the relationship between USP1 inhibition and inhibitors of poly(ADP-ribose) glycohydrolase (PARGi), the primary enzyme responsible for PAR hydrolysis, has not been resolved. Using cell cytotoxicity, synergy, PCNA-ubiquitin, and PAR analyses, it is demonstrated herein that PARG inhibition, combined with USP1 inhibition, leads to increased levels of mono-ubiquitinated PCNA, decreased PAR accumulation, and synergistic cytotoxicity between ML323, a potent USP1i, and PDD00017273, a model PARGi. Future studies will focus on the mechanism that contributes to USP1/PARG synthetic lethality, the mechanism of cell death, and the impact of USP1 on PAR/ubiquitin dynamics and replication stress signaling.

Synergistic Cellular Toxicity from Inhibition of Poly(ADP-ribose) Glycohydrolase (PARG) and Ubiquitin-Specific Protease 1 (USP1).

DNA polymerase beta (Polβ) is a 39 kDa, single polypeptide enzyme that possesses both gap tailoring and nucleotidyl transferase activity and is the key polymerase involved in base excision repair (BER) and the final steps of active gene demethylation. We demonstrated that residues in the mouse Polβ protein, L301 and V303, are critical for Polβ's interaction with the BER scaffolding protein X-ray repair cross-complementing 1 (XRCC1), and mutation of these residues impairs Polβ's ability to bind to XRCC1, negatively impacting BER complex assembly. We developed Polβ knock-in mice to explore how defects with this essential protein complex impact genome stability in the mouse. We found these mice to be viable and fertile yet exhibited a modest reduction in body weight. Here, we examined the protein and mRNA levels in tissues from wild-type (WT), heterozygous (HET), and homozygous (HOM) Polβ mice and the derived fibroblast cell lines. We show that HOM mice have significantly diminished Polβ protein levels, as compared to WT mice, in several tissues, yet Polβ mRNA levels were not significantly different, suggesting the decreased levels of Polβ protein could not be attributed to lower gene expression. Upon examination of Polβ stability in mouse ear fibroblasts derived from WT and HOM mice, results are consistent with human cell studies that the Polβ protein is unstable and undergoes proteasome-mediated degradation. Finally, we evaluated WT, and HOM, liver and brain genomic DNA samples for 5-methylcytosine/5-hydroxymethylcytosine (5mC/5hmC) levels by nanopore sequencing to investigate the impact of suppressed Polβ protein levels on active gene demethylation. As expected, we found tissue-specific trends in methylation, when comparing the brain and liver. However, we were unable to discern substantial differences in methylation levels between WT and HOM mice, suggesting that in the absence of external stressors, low Polβ levels do not impact methylation patterns.

Hypomorphic Protein Expression of DNA Polymerase Beta in Polβ Knock-In Transgenic Mice Does Not Impact Global DNA Methylation Levels in the Midbrain.

DNA polymerase β (Polβ) stability is tightly regulated by specific binding partners that prevent its degradation by the proteasome. Ubiquitin (Ub) chains, differing in their structural arrangements (topology), contribute to distinct cellular functions. To investigate Polβ ubiquitylation, we utilized CRISPR/Cas9-modified cell lines expressing endogenous, mClover-tagged Polβ. Our results reveal that Polβ is primarily modified with ubiquitin chains linked through lysine residues 27 [Ub(K27)] and 29 [Ub(K29)]. We further observed that Ub(K27) and Ub(K29) chains impact the utilization of ubiquitin chains linked through lysine residues 6 [Ub(K6)] and 48 [Ub(K48)] upon Polβ ubiquitylation. The ubiquitin ligase TRIP12 preferentially attaches Ub(K27) and Ub(K29) chains to Polβ. While Polβ ubiquitylation with Ub(K27) appears to have a non-degradative role, mixed ubiquitin chains and Ub(K48) chains trigger Polβ degradation. Following DNA damage, Polβ and XRCC1 translocate from the cytosol to the nucleus, while ubiquitylated Polβ relocates from the chromatin to the cytosol. This translocation of Polβ, modified with Ub(K27) chains, promotes the dissociation of the Polβ/XRCC1 complex within chromatin, and oxidative stress enhances their association in the cytosol. These findings demonstrate that atypical ubiquitin chain modifications play crucial roles in DNA repair and the DNA damage response, underscoring the unexpected importance of these chain topologies in maintaining genome integrity.

Article	Year
The epigenetic modifier HDAC2 and the checkpoint kinase ATM determine the responses of microsatellite instable colorectal cancer cells to 5-fluorouracil. Cell biology and toxicology\| PubMed ID: 35608750	2023
Molecular Modes of Action of an Aqueous Extract in Cancer Cells In Vitro and In Vivo. Molecules (Basel, Switzerland)\| PubMed ID: 36838857	2023
Oncometabolite 2-hydroxyglutarate suppresses basal protein levels of DNA polymerase beta that enhances alkylating agent and PARG inhibition induced cytotoxicity. DNA repair\| PubMed ID: 38897003	2024
Overexpression of the WWE domain of RNF146 modulates poly-(ADP)-ribose dynamics at sites of DNA damage. DNA repair\| PubMed ID: 40403420	2025
DNA polymerase beta expression in head & neck cancer modulates the poly(ADP-ribose)-mediated replication checkpoint. DNA repair\| PubMed ID: 40472742	2025
TRIP12's role in the governance of DNA polymerase β involvement in DNA damage response and repair. Nucleic acids research\| PubMed ID: 40613707	2025
Oleandrin-mediated suppression of MELK induces apoptosis, autophagy, and ferroptosis in human non-small cell lung cancer cells. Phytomedicine : international journal of phytotherapy and phytopharmacology\| PubMed ID: 40916237	2025
AAV Kills Dividing Cells by Depleting PARP1 and Other DNA Damage Response Proteins. bioRxiv : the preprint server for biology\| PubMed ID: 41279006	2025
Replication-associated base excision repair/single-strand break repair regulates PARG inhibitor response via the PRMT1/PRMT5/ATR axis. NAR cancer\| PubMed ID: 41480640	2025
Synergistic Cellular Toxicity from Inhibition of Poly(ADP-ribose) Glycohydrolase (PARG) and Ubiquitin-Specific Protease 1 (USP1). Toxics\| PubMed ID: 41745836	2026
Hypomorphic Protein Expression of DNA Polymerase Beta in Polβ Knock-In Transgenic Mice Does Not Impact Global DNA Methylation Levels in the Midbrain. Biomolecules\| PubMed ID: 41897348	2026
Ubiquitylation of DNA polymerase β via atypical K27 chains signals a DNA damage response. Nucleic acids research\| PubMed ID: 41978267	2026

Wynand P. Roos

Biography

JoVE Journal Publications

Other Publications