Excessive DNA Damage Research Articles

The spindle assembly checkpoint is a therapeutic vulnerability of CDK4/6 inhibitor-resistant ER+ breast cancer with mitotic aberrations.

Inhibitors of cyclin-dependent kinases 4 and 6 (CDK4/6i) are standard first-line treatments for metastatic ER+ breast cancer. However, acquired resistance to CDK4/6i invariably develops, and the molecular phenotypes and exploitable vulnerabilities associated with resistance are not yet fully characterized. We developed a panel of CDK4/6i-resistant breast cancer cell lines and patient-derived organoids and demonstrate that a subset of resistant models accumulates mitotic segregation errors and micronuclei, displaying increased sensitivity to inhibitors of mitotic checkpoint regulators TTK and Aurora kinase A/B. RB1 loss, a well-recognized mechanism of CDK4/6i resistance, causes such mitotic defects and confers enhanced sensitivity to TTK inhibition. In these models, inhibition of TTK with CFI-402257 induces premature chromosome segregation, leading to excessive mitotic segregation errors, DNA damage, and cell death. These findings nominate the TTK inhibitor CFI-402257 as a therapeutic strategy for a defined subset of ER+ breast cancer patients who develop resistance to CDK4/6i.

Olaparib enhances the Resveratrol-mediated apoptosis in breast cancer cells by inhibiting the homologous recombination repair pathway

Although sensitization of BRCA-mutated, homologous recombination (HR)-deficient breast cancer cells through PARP inhibitor is widely studied, not much is known about the treatment of BRCA-wild-type, HR-proficient breast cancer. Here, we aim to investigate whether a bioactive compound, Resveratrol (RES), can induce DNA double-strand breaks in HR-proficient breast cancer cells and Olaparib (OLA), a PARP inhibitor, can enhance the RES-mediated apoptosis by deregulating the HR repair pathway. The detailed mechanism of anti-cancer action of RES + OLA combination in breast cancer has been evaluated using in vitro, ex vivo, and in vivo preclinical model systems. OLA increased RES-mediated DNA damage, downregulated the HR pathway proteins, caused a late S/G2 cell cycle arrest, enhanced apoptosis and cell death in RES pre-treated breast cancer cells at much lower concentrations than their individual treatments. Direct measurement of HR pathway activity using a GFP plasmid-based assay demonstrated reduced HR efficiency in I-SceI endonuclease-transfected cells treated with OLA. Moreover, RES + OLA treatment also caused significant reduction in PARP1-mediated PARylation and efficiently trapped PARP1 at the DNA damage site. Upon RES treatment, PARylated PARP1 was found to interact with BRCA1, which then activated other HR pathway proteins. But after addition of OLA in RES pre-treated cells, PARP1 could not interact with BRCA1 due to inhibition of PARylation. This resulted in deregulation of HR pathway. To further confirm the role of BRCA1 in PARP1-mediated HR pathway activation, BRCA1 was knocked down that caused complete inhibition of HR pathway activity, and further enhanced apoptosis after RES + OLA treatment in BRCA1-silenced cells. In agreement with in vitro data, similar experimental results were obtained in ex vivo patient-derived breast cancer cells and in vivo xenograft mice. Thus, RES + OLA combination treatment enhanced breast cancer cell death by causing excessive DNA damage and also simultaneously inhibiting the HR pathway.

THE EFFECT OF SILENCING THE Tip60 GENE ON THE RESPONSE TO RADIOTHERAPY IN BREAST CANCER CELLS

One of the most important problems encountered in patients with triple-negative breast cancer (BC) treatment is the inadequate response of tumor tissue to treatment. The high expression of the Tip60, which is involved in the repair of DNA double-strand breaks, will increase the repair of DNA damage to be created in tumor cells, especially during the radiotherapy treatment process, thus reducing the treatment response and having a negative effect. In this study, the Tip60 gene was silenced using siRNA in MCF-7 and MDA-MB-231 cell lines, and their response to radiotherapy was monitored. To determine whether gene silencing was successful or not, Tip60 mRNA and protein expression values were measured. Cytotoxicity and DNA damage in UV-treated cells were analyzed by MTT and COMET methods, respectively. According to the results of the study, more DNA damage was observed in the MCF-7 in which the Tip60 gene was silenced and UV-treated compared to the non-Tip60 gene-silenced and UV-treated cells. On the other hand, more DNA damage was observed in the MDA-MB-321 in which the Tip60 gene was non-silenced and applied UV, compared to the cells in which the Tip60 gene was silenced. However, excessive DNA damage was already observed in the untreated MDA-MB-231. According to the results, silencing of the TiP60 gene in the MCF-7 may be beneficial in reducing resistance to radiotherapy, but no effect is expected in the MDA-MB-231. This can be explained by the fact that they are heterogeneous tumors. These data could use for future treatment development studies.

Restoration of NAD+ homeostasis protects C2C12 myoblasts and mouse levator ani muscle from mechanical stress-induced damage

ABSTRACT Excessive mechanical traction damages the levator ani muscle (LAM), increasing the incidence of pelvic floor dysfunction (PFD). In this study, we explored the effects of oxidized nicotinamide adenine dinucleotide (NAD+) on the damage to both muscle cells and LAM tissue induced by mechanical stress (MS) at the cellular and animal levels. The cell damage model was established using a four-point bending system. The LAM damage model was established using vaginal distention and traction. Exogenous addition of PJ34, an inhibitor of poly (ADP-ribose) polymerase-1 (PARP-1), and the nicotinamide mononucleotide (NMN) precursor of NAD+ increased NAD+ levels. ATP content and mitochondrial membrane potential were measured to assess mitochondrial function. NAD+ levels, cell viability, and PARP-1 activity were detected using commercial kits. DNA damage in cells was detected with immunofluorescence staining, and LAM damage was detected with tissue TUNEL staining. PARP-1 activity and DNA damage of LAM were detected by immunohistochemistry. A small amount of DNA damage and PARP-1 activation did not affect NAD+ levels, while excessive DNA damage and PARP-1 activation led to an imbalance of NAD+ homeostasis. Furthermore, increasing NAD+ levels in vivo and in vitro could rescue mitochondrial dysfunction and damage to both muscle cells and LAM tissue induced by MS. In conclusion, MS can induce damage to both C2C12 cells and LAM tissue. Restoring NAD+ homeostasis can rescue this damage by improving mitochondrial function.

Nicotinamide mononucleotide supplementation improves the quality of porcine oocytes under heat stress

BackgroundElevated ambient temperature-caused heat stress is a major concern for livestock production due to its negative impact on animal feed intake, growth, reproduction, and health. Particularly, the germ cells are extremely sensitive to the heat stress. However, the effective approach and strategy regarding how to protect mammalian oocytes from heat stress-induced defects have not been determined.MethodsGerminal vesicle (GV) porcine oocytes were cultured at 41.5 °C for 24 h to induce heat stress, and then cultured at 38.5 °C to the specific developmental stage for subsequent analysis. Nicotinamide mononucleotide (NMN) was dissolved in water to 1 mol/L for a stock solution and further diluted with the maturation medium to the final concentrations of 10 μmol/L, 20 μmol/L, 50 μmol/L or 100 μmol/L, respectively, during heat stress. Immunostaining and fluorescence intensity quantification were applied to assess the effects of heat stress and NMN supplementation on the key processes during the oocyte meiotic maturation.ResultsHere, we report that NMN supplementation improves the quality of porcine oocytes under heat stress. Specifically, we found that heat stress resulted in oocyte maturation failure by disturbing the dynamics of meiotic organelles, including the cytoskeleton assembly, cortical granule distribution and mitochondrial function. In addition, heat stress induced the production of excessive reactive oxygen species (ROS) and DNA damage, leading to the occurrence of apoptosis in oocytes and subsequent embryonic development arrest. More importantly, we validated that supplementation of NMN during heat stress restored the meiotic defects during porcine oocyte maturation.ConclusionsTaken together, our study documents that NMN supplementation is an effective approach to improve the quality of oocytes under heat stress by promoting both nuclear and cytoplasmic maturation.

Abstract 1792: FGFR3 mediated PARP1 tyrosine 158 phosphorylation promotes PARP inhibitor resistance

Abstract Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi), which induce DNA damage by inhibiting PARP1 enzymatic activity and trapping PARP on the damaged DNA, are used to eliminate BRCA1/2-mutated (BRCAm) cancer. However, clinical observations suggest that BRCAm tumors develop PARPi resistance. Current strategies to overcome PARPi resistance include impeding multiple DNA repair pathways to induce excessive DNA damage. Here, we propose a novel strategy targeting oncogenic receptor tyrosine kinases to enhance PARP trapping. By developing triple-negative breast cancer (TNBC) cells with acquired talazoparib resistance, we observed a high prevalence of activated fibroblast growth factor receptor 3 (FGFR3) kinase in these cells through kinase antibody array analysis. Mass spectrometry analysis and in vitro kinase assay suggested that FGFR3 phosphorylated PARP1 at tyrosine residues 158 and 176. Biochemistry studies suggested that only PARP1 tyrosine 158 phosphorylation contributes to PARPi resistance in the cells we developed. We then developed a monoclonal antibody against tyrosine 158 phosphorylated PARP1, and found that high-level PARP1 tyrosine 158 phosphorylation positively correlated with PARPi resistance in breast cancer patient-derived xenograft models. We further demonstrated that the combination of FGFR inhibitor and PARPi delayed DNA repair with prolonged PARP trapping. Moreover, synergy between PARPi and FGFR inhibition was observed in multiple TNBC cell lines with PARPi resistance in vitro. The combination of PARPi and FGFR inhibitor also showed synergism in vivo, and treatment with the combination of PARPi and FGFR inhibitor was tolerated in mouse models. These findings reveal that PARP1 tyrosine 158 phosphorylation facilitates resolving of the PARPi-induced PARP-trapping, and that the tyrosine 158 phosphorylated PARP1 may be an effective biomarker to indicate FGFR3 mediated PARPi resistance. Citation Format: MeiKuang Chen, Yuan Gao, Weiya Xia, Yu-Han Wang, Jennifer K. Litton, Yu-Yi Chu, Funda Meric-Bernstam, Helen Piwnica-Worms, Banu Arun, Jordi Rodon Ahnert, Yongkun Wei, Wei-Chao Chang, Hung-Ling Wang, Coya Tapia, Constance T. Albarracin, Shao-Chun Wang, Ying-Nai Wang, Gabriel N. Hortobagyi, Chunru Lin, Liuqing Yang, Dihua Yu, Mien-Chie Hung. FGFR3 mediated PARP1 tyrosine 158 phosphorylation promotes PARP inhibitor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1792.

Justifying integration of liquid biopsy into routine molecular diagnostics to complement tissue genomic profiling: Experience at Fox Chase Cancer Center.

e15029 Background: Plasma-based liquid biopsy has rapidly evolved from a novel research tool toward translation into clinical use for many diseases including cancer. While there are still some gaps regarding standardization and clinical utility, as tissue next-gen sequencing had previously faced and overcome, there is a growing body of evidence supporting integration of liquid biopsy into standard of care as a diagnostic tool for precision oncology. Selected from a larger cohort, several cases are presented here illustrating the use of the PGDx elio plasma resolve assay to profile circulating tumor DNA (ctDNA) from patients whose tumor and matched normal DNA had also been characterized by a next-generation sequencing panel. Methods: The PGDx elio plasma resolve assay and bioinformatics detects single and multi-nucleotide variants in 33 genes, select amplifications and translocations, as well as microsatellite instability by sequencing on the Illumina NextSeq 500/550 platform. DNA from matched tumor and normal FFPE samples were assessed using the 275 gene Molecular Diagnostics Laboratory’s Comprehensive Cancer Panel (CCP), also on the NextSeq platform. Selected cases represent the range of diagnostic utility that might be expected from ctDNA. Results: Across all cases, there were varying levels of concordance between tissue and plasma as well as novel variants in the plasma that were not present in the tissue DNA demonstrating the benefit of incorporating ctDNA profiling to capture critical genomic information. In one rectal adenocarcinoma case, DNA variants from the primary tumor were detected in ctDNA eight months later during evaluation of a recurrent tumor, suggesting that plasma profiling might have provided early biomarker detection of recurrence. An aggressive and recurrent melanoma, however, showed no concordance between ctDNA and tumor DNA variants, indicating that some tumors may not shed sufficient DNA into the bloodstream to make ctDNA clinically relevant. Another case of melanoma had concordant detection of the driver BRAF V600E mutation in tumor and ctDNA, but ctDNA also contained a pathogenic ATM truncation not observed in the tumor sample. Current clinical trials are investigating the combination of PARP- and ATR-inhibiting drugs, which causes excessive DNA damage and cell death in the context of ATM-deficient cancers. Conclusions: These cases show that combined tissue and ctDNA profiling at the time of initial diagnosis can reveal tumor heterogeneity or distinct metastases not observed in a selected tissue sample, and that profiling during and after therapy may provide early detection of recurrence, appearance of resistance mutations, or monitoring of measurable residual disease.

Sirt1 Protects Subventricular Zone-Derived Neural Stem Cells from DNA Double-Strand Breaks and Contributes to Olfactory Function Maintenance in Aging Mice.

DNA damage is assumed to accumulate in stem cells over time and their ability to withstand this damage and maintain tissue homeostasis is the key determinant of aging. Nonetheless, relatively few studies have investigated whether DNA damage does indeed accumulate in stem cells and whether this contributes to stem cell aging and functional decline. Here, we found that, compared with young mice, DNA double-strand breaks (DSBs) are reduced in the subventricular zone (SVZ)-derived neural stem cells (NSCs) of aged mice, which was achieved partly through the adaptive upregulation of Sirt1 expression and non-homologous end joining (NHEJ)-mediated DNA repair. Sirt1 deficiency abolished this effect, leading to stem cell exhaustion, olfactory memory decline, and accelerated aging. The reduced DSBs and the upregulation of Sirt1 expression in SVZ-derived NSCs with age may represent a compensatory mechanism that evolved to protect stem cells from excessive DNA damage, as well as mitigate memory loss and other stresses during aging.

Environmentally Relevant Hexavalent Chromium Disrupts Elemental Homeostasis and Induces Apoptosis in Zebrafish Liver

Although hexavalent chromium Cr [VI] is known as a toxicant in the aquatic environment, its effect in low, environmentally relevant concentration (ERC; 2mgL-1) is less characterized. Against this backdrop, the effects of Cr [VI] in ERC on zebrafish liver has been investigated in this study. Fluorescence microscopy and gel electrophoresis detected excess DNA damage and cell death via apoptosis in 2mgL-1 Cr [VI]-treated fish when compared with that of control. Besides, there were transcriptional activations of p53, Bax, Caspase 9 and Caspase 3 genes but downregulation of Bcl2 gene in the treated group, confirming the apoptotic pathway. Energy dispersive X-ray fluorescence (EDXRF) data showed significant (p < 0.05) increase in hepatic content of Cr, selenium, iron, manganese, calcium, sulfur and magnesium but depletion of zinc, copper and cobalt in the treated group. Collectively, the study shows that even a low, ERC of Cr [VI] is toxic to the zebrafish as it elicited marked apoptosis in the hepatocytes and altered the liver elemental profile.

CDK4/6 inhibitors induce replication stress to cause long‐term cell cycle withdrawal

CDK4/6 inhibitors arrest the cell cycle in G1‐phase. They are approved to treat breast cancer and are also undergoing clinical trials against a range of other tumour types. To facilitate these efforts, it is important to understand why a cytostatic arrest in G1 causes long‐lasting effects on tumour growth. Here, we demonstrate that a prolonged G1 arrest following CDK4/6 inhibition downregulates replisome components and impairs origin licencing. Upon release from that arrest, many cells fail to complete DNA replication and exit the cell cycle in a p53‐dependent manner. If cells fail to withdraw from the cell cycle following DNA replication problems, they enter mitosis and missegregate chromosomes causing excessive DNA damage, which further limits their proliferative potential. These effects are observed in a range of tumour types, including breast cancer, implying that genotoxic stress is a common outcome of CDK4/6 inhibition. This unanticipated ability of CDK4/6 inhibitors to induce DNA damage now provides a rationale to better predict responsive tumour types and effective combination therapies, as demonstrated by the fact that CDK4/6 inhibition induces sensitivity to chemotherapeutics that also cause replication stress.

Excessive DNA damage mediates ECM degradation via the RBBP8/NOTCH1 pathway in sporadic aortic dissection

Stanford type A aortic dissection (TA-AD) is a life-threatening disease. Most cases of aortic dissection (AD) are sporadic rather than inherited. Unlike that of inherited AD, the pathogenesis of sporadic AD is still unclear. In the current study, we aimed to explore the pathogenesis of sporadic AD through transcriptome sequencing data analyses. We downloaded sporadic TA-AD transcriptome profiles from Gene Expression Omnibus (GEO) and found response to DNA damage stimulus was activated in AD. Furthermore, by conducting mouse AD tissue single cell RNA sequencing and immunostaining, we found that DNA damage mainly occurred in smooth muscle cells (SMCs) and fibroblasts. Next, we examined the repair patterns in response to DNA damage and found the linker molecules RBBP8/NOTCH1 between DNA damage/repair and extracellular matrix (ECM) organization through protein–protein interaction analysis. Thus, we proposed that DNA damage could contribute to AD by regulating ECM changes. To explore the underlying mechanism, we knocked down the DNA repair-related gene RBBP8 in aortic SMCs, which could exacerbate DNA damage, and observed decreased expression level of NOTCH1. Inhibition of NOTCH1 with crenigacestat in vivo accelerated β-aminopropionitrile-induced formation of AD and increased mortality. Meanwhile, phenotype switching of SMCs was induced by Notch1 knockdown or inhibition; this switching occurred via a pathway involving downregulation of contractile marker gene expression and upregulation of MMP2 expression, which might aggravate ECM degradation. In conclusion, excessive DNA damage is a characteristic pathological change of sporadic aortic dissection, which might contribute to ECM changes and AD development via action on the NOTCH1 pathway.

Minocycline Attenuates Excessive DNA Damage Response and Reduces Ectopic Calcification in Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum (PXE) is a hereditary ectopic calcification disorder affecting the skin, eyes, and blood vessels. Recently, the DNA damage response (DDR), in particular PARP1, was shown to be involved in aberrant mineralization, raising the hypothesis that excessive DDR/PARP1 signaling also contributes to PXE pathogenesis. Using dermal fibroblasts of patients with PXE and healthy controls, (lesional) skin tissue, and abcc6a‒/‒ zebrafish, we performed expression analysis of DDR/PARP1 targets with QRT-PCR, western blot, immunohistochemistry, and enzyme activity assays before and after treatment with the PARP1 inhibitor minocycline. PARP1 and the ATM‒p21‒p53 axis was found to be significantly increased in PXE. In addition, PARP1 downstream targets IL-6, signal transducer and activator of transcription 1/3, TET1, and RUNX2 were upregulated, whereas the RUNX2 antagonist microRNA-204 was decreased. In PXE fibroblasts, DDR/PARP1 signaling increased with advancing ectopic calcification. Minocycline treatment attenuated DDR/PARP1 overexpression and reduced aberrant mineralization in PXE fibroblasts and abcc6a‒/‒ zebrafish. In summary, we showed the involvement of excessive DDR/PARP1 signaling in PXE pathophysiology, identifying a signal transducer and activator of transcription‒driven cascade resulting in increased expression of the epigenetic modifier TET1 and procalcifying transcription factor RUNX2. Minocycline attenuated this deleterious molecular mechanism and reduced ectopic calcification both invitro and invivo, fueling the exciting prospect of a therapeutic compound for PXE.

Altered H3 histone acetylation impairs high-fidelity DNA repair to promote cerebellar degeneration in spinocerebellar ataxia type 7.

SUMMARYA common mechanism in inherited ataxia is a vulnerability of DNA damage. Spinocerebellar ataxia type 7 (SCA7) is a CAG-polyglutamine-repeat disorder characterized by cerebellar and retinal degeneration. Polyglutamine-expanded ataxin-7 protein incorporates into STAGA co-activator complex and interferes with transcription by altering histone acetylation. We performed chromatic immunoprecipitation sequencing ChIP-seq on cerebellum from SCA7 mice and observed increased H3K9-promoter acetylation in DNA repair genes, resulting in increased expression. After detecting increased DNA damage in SCA7 cells, mouse primary cerebellar neurons, and patient stem-cell-derived neurons, we documented reduced homology-directed repair (HDR) and single-strand annealing (SSA). To evaluate repair at endogenous DNA in native chromosome context, we modified linear amplification-mediated high-throughput genome-wide translocation sequencing and found that DNA translocations are less frequent in SCA7 models, consistent with decreased HDR and SSA. Altered DNA repair function in SCA7 may predispose the subject to excessive DNA damage, leading to neuron demise and highlights DNA repair as a therapy target.

CRISPR screens guide the way for PARP and ATR inhibitor biomarker discovery.

DNA repair pathways are heavily studied for their role in cancer initiation and progression. Due to the large amount of inherent DNA damage in cancer cells, tumor cells profoundly rely on proper DNA repair for efficient cell cycle progression. Several current chemotherapeutics promote excessive DNA damage in cancer cells, thus leading to cell death during cell cycle progression. However, if the tumor has efficient DNA repair mechanisms, DNA-damaging therapeutics may not be as effective. Therefore, directly inhibiting DNA repair pathways alone and in combination with chemotherapeutics that cause DNA damage may result in improved clinical outcomes. Nevertheless, tumors can acquire resistance to DNA repair inhibitors. It is essential to understand the genetic mechanisms underlying this resistance. Genome-wide CRISPR screening has emerged as a powerful tool to identify biomarkers of resistance or sensitivity to DNA repair inhibitors. CRISPR knockout and CRISPR activation screens can be designed to investigate how the loss or overexpression of any human gene impacts resistance or sensitivity to specific inhibitors. This review will address the role of CRISPR screening in identifying biomarkers of resistance and sensitivity to DNA repair pathway inhibitors. We will focus on inhibitors targeting the PARP1 and ATR enzymes, and how the biomarkers identified from CRISPR screens can help inform the treatment plan for cancer patients.

Abstract MP233: Investigating Genotoxic-induced Innate Immune Pathways Leading To Heart Failure With Preserved Ejection Fraction For Developing Effective Treatments

Background: A hallmark of heart failure pathologies is excessive and cumulative DNA damage, leading to an increased and accelerated cardiac cellular senescence. We aimed to investigate the role of DNA breaks in inducing inflammation leading to adverse remodeling, premature senescence, and cardiac dysfunction leading to onset of Heart Failure with preserved Ejection Fraction (HFpEF), a specific type of disease prevalent in the aging population. Results: We use heart-focused ionizing radiation, a novel in vivo technique to induce aging related DNA damage and onset of diastolic dysfunction and HFpEF (n= 4-5 per group &amp; per species; animals used: Rattus norvegicus and Mus musculus). We performed functional analysis, histological tissue assessment and molecular investigations to evaluate and validate this novel animal model technique. DNA damage response is upregulated in aging-related Heart Failure, and here demonstrated in our novel heart-focused radiation technique. Using in vitro ionizing radiation exposure alters cardiomyocyte morphology and activates inflammation, leading to upregulation of fibrosis and senescence markers. Finally using experimental inhibitor compounds targeting the cGAS-STING axis of innate immunity, we show effective attenuation of this relevant pathway, leading to decreased systemic inflammation, cardiomyocyte remodeling, and senescence associated with DNA damage injury. Conclusions: We present a novel technique to induce genotoxic outcomes and inflammation of the heart, leading to HFpEF pathology. We elucidate novel connections from the onset and accumulation of unresolved DNA damage, to activation of specialized innate immune cellular responses, and ultimately upregulation of cardiac tissue specific inflammation, fibrosis and senescence. Finally we demonstrate that antagonizing the cGAS-STING pathway could allow a precision medicine approach to treating genotoxic and inflammatory HFpEF.

Abstract 1965: Discovery of ZN-c3, a potent Wee-1 inhibitor with a differentiated pharmacologic and kinase selectivity profile

Abstract Genome instability initiated by DNA damage or DNA replication stress is a driver of tumor development. To ensure the accuracy of DNA replication, DNA damage response (DDR) mediated by various cell cycle checkpoints activate the DNA repair system. One such checkpoint system is the G2-M checkpoint which is involved in the repair and prevention of excessive DNA damage. A key regulator of the G2-M checkpoint is the Wee1 kinase which causes cell cycle arrest to prevent damaged cells from entering mitosis. Thus, inhibition of Wee1 should prevent G2-M checkpoint activation which would lead to unscheduled mitotic entry and induction of apoptosis. Small-molecule inhibitors of Wee1 are currently being tested in several tumor types. Preliminary results show promising clinical activity of AZD-1775, a potent inhibitor of Wee1, however, achieving an optimal dose and a schedule that is well-tolerated has been a challenge. Here, we describe the characterization of ZN-c3, a potent, selective and orally bioavailable small-molecule inhibitor of Wee1. ZN-c3 exhibits potent anti-proliferative activity across multiple tumor cell lines. Compared to AZD-1775, ZN-c3 has superior selectivity profile against a panel of kinases. In vivo, ZN-c3 induced tumor regression as a single agent in several tumor models including non-small cell lung cancer, ovarian cancer, and uterine sarcoma. In combination studies, ZN-c3 potentiates the antitumor efficacies of carboplatin and gemcitabine. The pharmacokinetic and efficacy profiles of ZN-c3 and AZD-1775 were compared in tumor xenograft models. ZN-c3 showed higher plasma and tumor exposure than AZD-1775, which correlated with more potent tumor growth inhibition than AZD-1775. In summary, ZN-c3, with its key differentiated pharmacologic and kinase selectivity profile, is a promising new generation of DDR inhibitor. ZN-c3 is currently in clinical development for the treatment of advanced solid cancers. Citation Format: Jiali Li, Brant Boren, Peter Q. Huang, Kevin D. Bunker, Fernando Doñate, Ahmed A. Samatar. Discovery of ZN-c3, a potent Wee-1 inhibitor with a differentiated pharmacologic and kinase selectivity profile [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1965.

Functional Genomic Landscape of Epigenetic Dependencies in Myeloma

The recent development of comprehensive functional genomic approaches based on the CRISPR/Cas9 gene editing technology and its variants has created new opportunities to assess in a quantitative manner the significance of a given gene for survival and proliferation across different tumor cell lines; or in comparison to those of other genes within the same tumor cell. These comparisons may be particularly informative in the context of genes regulating the epigenome of tumor cells, since selective chemical probes for many epigenetic regulators are not currently available. We examined the results from our analysis of genome-wide CRISPR/Cas9-based gene editing studies in a panel of n=10 MM cell lines (with AVANA pooled library of ~70,000 sgRNAs; 3-4 sgRNAs per gene) and publicly available CRISPR data on cell lines from other hematologic malignancies and solid tumors. We ranked all genes according to the depletion of their sgRNAs; and as in prior studies of our group and others, we designated as essential genes those with sgRNA depletion rank <3000 (based on the MAGECK algorithm). We focused for this study on genes with known or presumed roles in epigenetic regulation and identified several genes (including BRD4, CHD4, PRMT5, PRMT1, SETD1A, JMJD6, HDAC3, et.c.) which appear to be essential for practically all MM cell lines tested, and also broadly essential for >80% of non-MM cell lines. We also identified genes which are essential for most MM cell lines tested and for sizeable subsets of non-MM tumors, including BRD2, SETDB1, EP300, DOT1L CREBBP, and EZH2. It is notable that MM cell lines exhibited higher dependency ranks for EP300, CREBBP and DOT1L compared to a collection of 300+ non-MM cell lines, suggesting that pharmacological inhibitors that can phenocopy the genetic dependency on these genes merit further consideration for therapeutic applications in MM. We also detected genes which are essential for smaller numbers of MM cell lines tested, including ATAD2, and various histone lysine demethylases (KDMs) or methyltransferases (KMTs); while several other epigenetic genes appear to be redundant for in vitro cell survival and proliferation of MM cells. When we examined all genes with median sgRNA depletion ranks <3000 in MM cell lines and selected those sgRNAs showing the most variable effect across the 10 MM cell lines, we identified epigenetic genes such as HDAC3, HDAC8, JMJD6 and KDM2A, implying a differential role of these epigenetic dependencies across different types of MM cell lines. Of interest, the large majority of epigenetic genes identified as dependencies in these CRISPR studies are not recurrently mutated in MM; and, given their copy number status, their essentiality could not be attributed to a confounding effect of excess DNA damage from CRISPR editing of amplified genes. Furthermore, these epigenetic dependencies were not over-represented among the most overexpressed in MM cells vs. normal plasma cells. Overall, these observations indicate that the functional dependency of MM cells on these epigenetic genes cannot be readily detected based on conventional molecular profiling platforms utilized so far to identify promising therapeutic targets. Our results provide novel insights into the epigenetic landscape of MM and establish EP300, CREBBP and DOT1L as epigenetic dependencies with preferential role in MM and thus potential new therapeutic targets in this disease. DisclosuresMitsiades:Takeda: Other: Employment of family member; Novartis: Research Funding; Janssen/Johnson & Johnson: Research Funding; TEVA: Research Funding; Ono: Research Funding; Abbvie: Research Funding.

Synthetic Lethality Targeting the Aid/RAD51 Axis for Treatment of AICDA-Expressing Lymphoma

Activation Induced Cytidine Deaminase (AICDA or AID) is a DNA directed cytidine deaminase that induces somatic hypermutation and immunoglobulin class switching in stimulated B-lymphocytes. AICDA is a DNA damaging enzyme, producing DNA base pair mismatches that can subsequently be converted into mutations, DNA single strand breaks (SSB), or DNA double strand breaks (DSB). Although AICDA is normally expressed at high levels only in activated B-cells, a wide range of cancer types show constitutive overexpression of AICDA. In malignant cells, as in B-cells, AICDA promotes hypermutation and genomic instability. We previously demonstrated that cells expressing AICDA are critically dependent on the DNA repair factor RAD51 for their survival. We have developed novel small molecule inhibitors of RAD51 that selectively kill AICDA overexpressing cells. One of these, CYT01A, depletes the nucleus of RAD51, thus reducing DNA double strand break repair activity. We have shown CYT01A to be a potent (EC50~150nM) RAD51 inhibitor that reduces the efficiency of RAD51 focus formation, leading to sensitization to excess DNA damage while preserving baseline recombination/repair activity. Here we present the in vitro and in vivo pharmacologic characterization of CYT01A. We first analyzed potential off-target liabilities by carrying out kinome inhibition and safety panel (Panlabs) screens. CYT01A showed negligible inhibitory activity against 429 kinases, receptors, or other pharmacologically important non-RAD51 targets. These data are consistent with high specificity for AICDA and predict in vivo tolerability. We next measured metabolic stability of CYT01A in mouse liver microsome incubation assays, demonstrating CYT01A stability in both the absence and presence of NADPH. Measurement of Caco-2 cell permeability, as a predictor of oral bioavailability, revealed low to moderate efflux of CYT01A. Consistent with these permeability data, CYT01A showed modest oral availability, but relatively long clearance times. Daily oral dosing at 80 mg/kg for 7-14 days resulted in significant reduction in AICDA expressing lymphomas, in a human-to-mouse xenograft model. This demonstrates efficacious iin vivo exposure despite relatively low oral availability. By contrast, AICDA null lymphomas showed no significant tumor response versus vehicle controls, consistent with the synthetic lethal relationship between AICDA and RAD51. Finally, we demonstrated that repeat daily administration of CYT01A up to the maximum dose tested (80mg/kg) was well tolerated and resulted in no observable changes in complete blood counts (CBC), bone marrow function or output, or body weights. Taken together these data represent a definitive proof-of-principle demonstration that in vivo targeting of RAD51 is an effective strategy to treat AICDA expressing cancers. These data form the basis for continued preclinical development of an AICDA/RAD51 synthetic lethal therapeutic paradigm that may be applicable to a wide range of both solid tumors and hematologic malignancies. DisclosuresMills:Cyteir Therapeutics: Employment, Equity Ownership. Cyr:Cyteir Therapeutics: Employment, Equity Ownership. Maclay:Cyteir Therapeutics: Employment, Equity Ownership. Day:Cyteir Therapeutics: Employment, Equity Ownership. Patrick:Cyteir Therapeutics: Consultancy, Equity Ownership. Clas:Cyteir Therapeutics: Consultancy.

SIRT7: a sentinel of genome stability.

SIRT7 is a class III histone deacetylase that belongs to the sirtuin family. The past two decades have seen numerous breakthroughs in terms of understanding SIRT7 biological function. We now know that this enzyme is involved in diverse cellular processes, ranging from gene regulation to genome stability, ageing and tumorigenesis. Genomic instability is one hallmark of cancer and ageing; it occurs as a result of excessive DNA damage. To counteract such instability, cells have evolved a sophisticated regulated DNA damage response mechanism that restores normal gene function. SIRT7 seems to have a critical role in this response, and it is recruited to sites of DNA damage where it recruits downstream repair factors and directs chromatin regulation. In this review, we provide an overview of the role of SIRT7 in DNA repair and maintaining genome stability. We pay particular attention to the implications of SIRT7 function in cancer and ageing.

Acceleration or Brakes: Which Is Rational for Cell Cycle-Targeting Neuroblastoma Therapy?

Unrestrained proliferation is a common feature of malignant neoplasms. Targeting the cell cycle is a therapeutic strategy to prevent unlimited cell division. Recently developed rationales for these selective inhibitors can be subdivided into two categories with antithetical functionality. One applies a “brake” to the cell cycle to halt cell proliferation, such as with inhibitors of cell cycle kinases. The other “accelerates” the cell cycle to initiate replication/mitotic catastrophe, such as with inhibitors of cell cycle checkpoint kinases. The fate of cell cycle progression or arrest is tightly regulated by the presence of tolerable or excessive DNA damage, respectively. This suggests that there is compatibility between inhibitors of DNA repair kinases, such as PARP inhibitors, and inhibitors of cell cycle checkpoint kinases. In the present review, we explore alterations to the cell cycle that are concomitant with altered DNA damage repair machinery in unfavorable neuroblastomas, with respect to their unique genomic and molecular features. We highlight the vulnerabilities of these alterations that are attributable to the features of each. Based on the assessment, we offer possible therapeutic approaches for personalized medicine, which are seemingly antithetical, but both are promising strategies for targeting the altered cell cycle in unfavorable neuroblastomas.