DNA Interstrand Cross-linking Agents Research Articles

FANCJ DNA helicase is recruited to the replisome by AND-1 to ensure genome stability

FANCJ, a DNA helicase linked to Fanconi anemia and frequently mutated in cancers, counteracts replication stress by dismantling unconventional DNA secondary structures (such as G-quadruplexes) that occur at the DNA replication fork in certain sequence contexts. However, how FANCJ is recruited to the replisome is unknown. Here, we report that FANCJ directly binds to AND-1 (the vertebrate ortholog of budding yeast Ctf4), a homo-trimeric protein adaptor that connects the CDC45/MCM2-7/GINS replicative DNA helicase with DNA polymerase α and several other factors at DNA replication forks. The interaction between FANCJ and AND-1 requires the integrity of an evolutionarily conserved Ctf4-interacting protein (CIP) box located between the FANCJ helicase motifs IV and V. Disruption of the CIP box significantly reduces FANCJ association with the replisome, causing enhanced DNA damage, decreased replication fork recovery and fork asymmetry in cells unchallenged or treated with Pyridostatin, a G-quadruplex-binder, or Mitomycin C, a DNA inter-strand cross-linking agent. Cancer-relevant FANCJ CIP box variants display reduced AND-1-binding and enhanced DNA damage, a finding that suggests their potential role in cancer predisposition.

Abstract 315: B7H3 is a potential therapeutic target in advanced metastatic prostate cancer

Abstract B7H3 is an immunomodulatory protein of the B7 family which inhibits T cell function. B7H3 mRNA is ubiquitously expressed in normal human tissues, but the protein is expressed at low levels, suggesting a tight post-transcriptional regulation. By contrast, B7H3 protein is overexpressed in several solid tumors and often correlates with poor survival, higher tumor grade, and metastasis independently of immune regulation. A few studies also have reported an association between B7H3 overexpression and therapy resistance. Since B7H3 protein is preferentially expressed on cancer cells and tumor vasculature, it is an attractive target for developing novel cancer therapies. Here we analyzed the expression of B7H3 in 26 mCRPC organoid models and examined whether B7H3 can serve as a novel therapeutic target for advanced mPCa patients that fail current treatment regimens. The B7H3 protein expression in PDXOs of various lineages including adenocarcinoma (n=19), double-negative (DNPC, n= 2), and neuroendocrine (NE, n =5) lineages, was analyzed using simple western™ assay, immunofluorescence, and flow cytometry. We tested the ex-vivo drug response of PDXOs and performed PDX preclinical trials using humanized B7H3-ADC armed with a pyrrolobenzodiazepine (PBD) warhead. PBD dimers are DNA interstrand cross-linking agents and are a poor substrate of P-gp drug transporters. Our results showed that B7-H3 was overexpressed in several PDX derived organoids of mPCa. Organoids with adenocarcinoma phenotype (n=19) demonstrated a range of B7H3 protein expression whereas RB1/TP53 mutated small cell neuroendocrine or double negative prostate cancer showed a relatively more uniform and low expression. Nonetheless, PDXOs of neuroendocrine, double negative or adenocarcinoma origin responded with maximum efficacy to B7H3-ADC ex vivo. Adenocarcinoma organoid models (n=19) displayed a wide range of responses, which was only partially correlated with B7H3 protein levels. Genetically-engineered loss of B7-H3 converted a responder to a non-responder phenotype. Our investigation for potential biomarkers of B7H3-PBD sensitivity suggested that RB1-deficiency induced replication stress and/or expression of SLFN11, an interferon pathway transcriptional target, to be the strong predictors of B7H3-PBD response, independent of tumor lineage. We further performed preclinical trials using four PDX models based on their RB1 and SLFN11 status, including a model of advanced adenocarcinoma lineage with low B7H3 expression. B7H3-PBD eradicated large established subcutaneous tumors and metastases and improved long-term overall survival in either RB1 deficient and/or SLFN11HIGH models, but showed no response in RB1+/SLFN11- tumors. In conclusion, B7H3-PBD treatment has potential to produce complete and durable response in appropriately-selected, heavily-pretreated metastatic CRPC and NEPC. Citation Format: Supreet Agarwal, Lei Fang, Juan Juan Yin, Elaine Hurt, Kathleen Kelly. B7H3 is a potential therapeutic target in advanced metastatic prostate cancer [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 315.

RAD51AP1 and RAD54L Can Underpin Two Distinct RAD51-Dependent Routes of DNA Damage Repair via Homologous Recombination.

Homologous recombination DNA repair (HR) is a complex DNA damage repair pathway and an attractive target of inhibition in anti-cancer therapy. To help guide the development of efficient HR inhibitors, it is critical to identify compensatory HR sub-pathways. In this study, we describe a novel synthetic interaction between RAD51AP1 and RAD54L, two structurally unrelated proteins that function downstream of the RAD51 recombinase in HR. We show that concomitant deletion of RAD51AP1 and RAD54L further sensitizes human cancer cell lines to treatment with olaparib, a Poly (adenosine 5′-diphosphate-ribose) polymerase inhibitor, to the DNA inter-strand crosslinking agent mitomycin C, and to hydroxyurea, which induces DNA replication stress. We also show that the RAD54L paralog RAD54B compensates for RAD54L deficiency, although, surprisingly, less extensively than RAD51AP1. These results, for the first time, delineate RAD51AP1- and RAD54L-dependent sub-pathways and will guide the development of inhibitors that target HR stimulators of strand invasion.

Use of the cell division assay to diagnose Fanconi anemia patients' hypersensitivity to mitomycin C.

The recently reported cell division assay (CDA) was optimized to measure the relative sensitivity of cells to cytotoxic drugs in vitro. Here, we investigated the in vitro hypersensitivity of lymphocytes from Fanconi anemia (FA) patients, to cytotoxic drugs using CDA. Peripheral blood mononuclear cells (PBMC) as well as cell lines derived from FA patients were treated with two DNA interstrand crosslinking (ICL) agents, mitomycin C and cyclophosphamide. Our data indicate that the CDA detects hypersensitivity of cells from FA patients to mitomycin C. Further, cell lines derived from FA‐patients were also hypersensitive to mitomycin C as well as cyclophosphamide, when assayed by the CDA. This study suggests that the CDA is a useful alternative for the diagnosis of FA patients' hypersensitivity to ICL agents.

Substituents Have a Large Effect on Photochemical Generation of Benzyl Cations and DNA Cross-Linking.

Photoactivated DNA interstrand cross-linking agents have a wide range of biological applications. Recently, several aryl boronates have been reported to induce DNA interstrand cross-link (ICL) formation via carbocations upon photoirradiation. Herein, we synthesized a series of new bifunctional phenyl compounds to test the generality of such a mechanism, and to understand how the chemical structure influences carbocation formation and the DNA cross-linking process. These compounds efficiently form DNA ICLs via generated benzyl cations upon 350 nm irradiation. The DNA cross-linking efficiency and the pathway for carbocation generation depend on both the aromatic substituents and the leaving groups. Bromine as a leaving group facilitates the DNA cross-linking process in comparison with trimethyl ammonium salt. Both electron-donating and -withdrawing substituents induce bathochromic shifts, which favor photoinduced DNA ICL formation. For the bromides, the benzyl cation intermediates were generated through oxidation of the corresponding benzyl radicals. However, for the ammonia salts, the benzyl cations were formed through two pathways: either through oxidation of the benzyl radicals or by direct heterolysis of the C-N bond. Photoinduced C-N homolysis to form benzyl radicals occurred with compounds having donating substituents, whereas direct heterolysis of the C-N bond occurred with those bearing withdrawing substituents. The adducts formed between 1 a and four natural nucleosides were characterized, indicating that the alkylation sites for the photogenerated benzyl cations are dG, dA, and dC.

Fanconi anemia with sun-sensitivity caused by a Xeroderma pigmentosum-associated missense mutation in XPF

BackgroundFanconi anemia (FA) is an inherited genomic instability disorder with congenital and developmental abnormalities, bone marrow failure and predisposition to cancer early in life, and cellular sensitivity to DNA interstrand crosslinks.Case presentationA fifty-one-year old female patient, initially diagnosed with FA in childhood on the basis of classic features and increased chromosomal breakage, and remarkable sun-sensitivity is described. She only ever had mild haematological abnormalities and no history of malignancy. To identify and characterise the genetic defect in this lady, who is one of the oldest reported FA patients, we used whole-exome sequencing for identification of causative mutations, and functionally characterized the cellular phenotype. Detection of the novel splice site mutation c.793-2A > G and the previously described missense mutation c.1765C > T (p.Arg589Trp) in XPF/ERCC4/FANCQ assign her as the third individual of complementation group FA-Q. Ectopic expression of wildtype, but not mutant, XPF/ERCC4/FANCQ, in patient-derived fibroblasts rescued cellular resistance to DNA interstrand-crosslinking agents. Patient derived FA-Q cells showed impaired nuclear excision repair capacity. However, mutated XPF/ERCC4/FANCQ protein in our patient’s cells, as in the two other patients with FA-Q, was detectable on chromatin, in contrast to XP-F cells, where missense-mutant protein failed to properly translocate to the nucleus.ConclusionsPatients with FA characteristics and UV sensitivity should be tested for mutations in XPF/ERCC4/FANCQ. The missense mutation p.Arg589Trp was previously detected in patients diagnosed with Xeroderma pigmentosum or Cockayne syndrome. Hence, phenotypic manifestations associated with this XPF/ERCC4/ FANCQ mutation are highly variable.

Genome-wide mutagenesis and multi-drug resistance in American trypanosomes induced by the front-line drug benznidazole

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and affects 5–8 million people in Latin America. Although the nitroheterocyclic compound benznidazole has been the front-line drug for several decades, treatment failures are common. Benznidazole is a pro-drug and is bio-activated within the parasite by the mitochondrial nitroreductase TcNTR-1, leading to the generation of reactive metabolites that have trypanocidal activity. To better assess drug action and resistance, we sequenced the genomes of T. cruzi Y strain (35.5 Mb) and three benznidazole-resistant clones derived from a single drug-selected population. This revealed the genome-wide accumulation of mutations in the resistant parasites, in addition to variations in DNA copy-number. We observed mutations in DNA repair genes, linked with increased susceptibility to DNA alkylating and inter-strand cross-linking agents. Stop-codon-generating mutations in TcNTR-1 were associated with cross-resistance to other nitroheterocyclic drugs. Unexpectedly, the clones were also highly resistant to the ergosterol biosynthesis inhibitor posaconazole, a drug proposed for use against T. cruzi infections, in combination with benznidazole. Our findings therefore identify the highly mutagenic activity of benznidazole metabolites in T. cruzi, demonstrate that this can result in multi-drug resistance, and indicate that vigilance will be required if benznidazole is used in combination therapy.

Arsenite Binds to the RING Finger Domain of FANCL E3 Ubiquitin Ligase and Inhibits DNA Interstrand Crosslink Repair.

Human exposure to arsenic in drinking water is known to be associated with the development of bladder, lung, kidney, and skin cancers. The molecular mechanisms underlying the carcinogenic effects of arsenic species remain incompletely understood. DNA interstrand cross-links (ICLs) are among the most cytotoxic type of DNA lesions that block DNA replication and transcription, and these lesions can be induced by endogenous metabolism and by exposure to exogenous agents. Fanconi anemia (FA) is a congenital disorder manifested with elevated sensitivity toward DNA interstrand cross-linking agents, and monoubiquitination of FANCD2 by FANCL is a crucial step in FA-mediated DNA repair. Here, we demonstrated that As3+ could bind to the PHD/RING finger domain of FANCL in vitro and in cells. This binding led to compromised ubiquitination of FANCD2 in cells and diminished recruitment of FANCD2 to chromatin and DNA damage sites induced by 4,5',8-trimethylpsoralen plus UVA irradiation. Furthermore, clonogenic survival assay results showed that arsenite coexposure rendered cells more sensitive toward DNA interstrand cross-linking agents. Together, our study suggested that arsenite may compromise genomic stability via perturbation of the Fanconi anemia pathway, thereby conferring its carcinogenic effect.

P-glycoprotein attenuates DNA repair activity in multidrug-resistant cells by acting through the Cbp-Csk-Src cascade.

Recent studies have demonstrated that P-glycoprotein (P-gp) expression impairs DNA interstrand cross-linking agent-induced DNA repair efficiency in multidrug-resistant (MDR) cells. To date, the detailed molecular mechanisms underlying how P-gp interferes with Src activation and subsequent DNA repair activity remain unclear. In this study, we determined that the C-terminal Src kinase-binding protein (Cbp) signaling pathway involved in the negative control of Src activation is enhanced in MDR cells. We also demonstrated that cells that ectopically express P-gp exhibit reduced activation of DNA damage response regulators, such as ATM, Chk2, Braca1 and Nbs1 and hence attenuated DNA double-strand break repair capacity and become more susceptible than vector control cells to DNA interstrand cross-linking (ICL) agents. Moreover, we demonstrated that P-gp can not only interact with Cbp and Src but also enhance the formation of inhibitory C-terminal Src kinase (Csk)-Cbp complexes that reduce phosphorylation of the Src activation residue Y416 and increase phosphorylation of the Src negative regulatory residue Y527. Notably, suppression of Cbp expression in MDR cells restores cisplatin-induced Src activation, improves DNA repair capacity, and increases resistance to ICL agents. Ectopic expression of Cbp attenuates cisplatin-induced Src activation and increases the susceptibility of cells to ICL agents. Together, the current results indicate that P-gp inhibits DNA repair activity by modulating Src activation via Cbp-Csk-Src cascade. These results suggest that DNA ICL agents are likely to have therapeutic potential against MDR cells with P-gp-overexpression.

DNA Interstrand Crosslinks by H-pin Polyamide (S)-seco-CBI Conjugates.

Although DNA interstrand crosslinking (ICL) agents are widely used as antitumor drugs, DNA sequence-specific ICL agents are quite rare. In this study, H-pin imidazole-pyrrole polyamide 1-(chloromethyl)-2,3-dihydro-1H-benzo[e]indol-5-ol (seco-CBI) conjugates that produce sequence-specific DNA ICLs were designed and synthesized. Conjugates with H-pin polyamide and seco-CBI moieties were constructed to recognize a 7 bp DNA sequence, and their reactivity and selectivity in DNA alkylation were evaluated by using high-resolution denaturing gel electrophoresis and sequence-specific plasmid cleavage. One conjugate (6), which contained a chiral (S)-seco-CBI, exhibited greater sequence-specific ICL activity toward the target DNA sequence and was cytotoxic to a cancer cell line. Molecular modeling studies indicated that the greater activity of 6 resulted from the relative orientation of the cyclopropane group in the (S)-CBI unit.

Inactivation of BRCA2 in human cancer cells identifies a subset of tumors with enhanced sensitivity towards death receptor-mediated apoptosis.

PurposeDNA repair defects due to detrimental BRCA2-mutations confer increased susceptibility towards DNA interstrand-crosslinking (ICL) agents and define patient subpopulations for individualized genotype-based cancer therapy. However, due to the side effects of these drugs, there is a need to identify additional agents, which could be used alone or in combination with ICL-agents. Therefore, we investigated whether BRCA2-mutations might also increase the sensitivity towards TRAIL-receptors (TRAIL-R)-targeting compounds.Experimental designTwo independent model systems were applied: a BRCA2 gene knockout and a BRCA2 gene complementation model. The effects of TRAIL-R-targeting compounds and ICL-agents on cell viability, apoptosis and cell cycle distribution were compared in BRCA2-proficient versus-deficient cancer cells in vitro. In addition, the effects of the TRAIL-R2-targeting antibody LBY135 were assessed in vivo using a murine tumor xenograft model.ResultsBRCA2-deficient cancer cells displayed an increased sensitivity towards TRAIL-R-targeting agents. These effects exceeded and were mechanistically distinguishable from the well-established effects of ICL-agents. In vitro, ICL-agents expectedly induced an early cell cycle arrest followed by delayed apoptosis, whereas TRAIL-R-targeting compounds caused early apoptosis without prior cell cycle arrest. In vivo, treatment with LBY135 significantly reduced the tumor growth of BRCA2-deficient cancer cells in a xenograft model.ConclusionsBRCA2 mutations strongly increase the in vitro- and in vivo-sensitivity of cancer cells towards TRAIL-R-mediated apoptosis. This effect is mechanistically distinguishable from the well-established ICL-hypersensitivity of BRCA2-deficient cells. Our study thus defines a new genetic subpopulation of cancers susceptible towards TRAIL-R-targeting compounds, which could facilitate novel therapeutic approaches for patients with BRCA2-deficient tumors.

Trans-dichlorooxovandium (IV) complex as a novel photoinducible DNA interstrand crosslinker for cancer therapy

Although DNA interstrand crosslinking (ICL) agents such as mitomycin C, cisplatin and psoralen serve as potent anticancer drugs, these agents are known to have dose-limiting toxic effects on normal cells. Moreover, tumor resistance to these agents has been reported. Here, we show that trans-dichlorooxovanadium (IV) complex of pyrenyl terpyridine (VDC) is a novel photoinducible DNA crosslinking agent. By a combination of in vitro and ex vivo experiments including plasmid-based assays, we find that VDC forms monoadducts on the DNA and can be activated by UV-A and visible light to generate DNA interstrand crosslinks. VDC efficiently activates Fanconi anemia (FA) pathway of DNA interstrand crosslink repair. Strikingly, photoinduction of VDC induces prolonged activation of cell cycle checkpoint and a high degree of cell death in homologous recombination (HR)/ICL repair defective cells. Moreover, VDC specifically targets cells that express pathological RAD51C mutants. These data imply that VDC can be potentially used for cancer therapy and suggest that tumors arising in patients with gene mutations in FA and HR repair pathway can be specifically targeted by a photoactivatable VDC.

Diagnosis of Fanconi Anemia By Chromosome Breakage Tests Using 3 Different Scoring Systems and Whole Genome Sequencing Among Patients with Aplastic Anemia in Korean

BACKGROUD: Fanconi anemia (FA), an inherited bone marrow failure syndrome with impaired DNA repair system, is characterized by cytopenias, congenital abnormalities, and predisposition to malignancy as a consequence of chromosomal instability and hypersensitivity to DNA interstrand cross-linking agents. Differential diagnosis of FA and aplastic anemia requires integrated work-up including physical findings, bone marrow histologic findings and chromosome breakage test. Yet, there have been no consensus criteria for chromosome breakage test, which depend on each laboratory's own decision. The aim of our study was 1) to investigate the incidence of FA showing positive results for chromosome breakage test among patients diagnosed with aplastic anemia, and 2) to investigate the frequency of the gene mutations related to inherited bone marrow failure syndrome in patients with aplastic anemia. In addition to chromosome breakage test, we performed whole genome sequencing with bone marrow mononuclear cells in 18 pediatric patients with aplastic anemia whose bone marrow specimen was available.METHOD: We reviewed total 79 chromosome breakage tests from 67 patients who had been on suspicion of aplastic anemia between May 2005 and April 2015. MMC and DEB stress test were performed at concentration of 50ng/mL and 100ng/mL both on peripheral blood of suspicious patients and normal controls, respectively. The scoring of chromosome breakages test was performed, based on widely used 3 different scoring systems: those proposed by Jean Soulier, Barch MJ, and Arleen D. Auerbach. In each cases, we applied 3 different scoring systems and compared the concordance rate. In 16 among 67 patients, we performed whole genome sequencing.RESULTS: The median age of the pediatric patients was 11years (range, 7months - 19 years) and the male-to-female ratio was 1.39:1. Of 67 enrolled patients, 8 had been tested twice or 3 times because of ambiguous results. In these cases, we chose the last results. Five of 67 patients satisfied the all three criteria mentioned above, which shows 7.5% (5/67) of positive rates. Other 3 of 67 patients met only one or more of Soulier's prerequisites. Among those, one fulfilled both Barch MJ's system and Aeurbach's, and remain 2 patients were positive in each system, respectively. Mutation variants of BMF syndrome related genes were detected in 25% (4/16 patients); RPS19 (1 patient), PAX5 (1 patient), and FANC (3 patient). Inherited predisposition to myeloid leukemia related genes were detected in 56.3% (9/16 patients) and gene variants were MSH6 (5 patients), ATM (2 patients), PMS2 (1 patients), and MLH1 (1 patients). Coexisting somatic mutations of oncogene (ERB2) was detected in 6.3% (1/16 patients). Among 3 patients with fanconi anemia gene mutations, 2 patients showed positive results for chromosome breakage test and the other1patient showed negative results for chromosome breakage test.CONCLUSION: The frequency of FA based on chromosome breakage test among patients with pancytopenia suspicious of aplastic anemia was 7.5% by Soulier's prerequisites, but 9.0% when based on either of 3 different criteria. Molecular testing can additionally detect FA in 4 (25.0%) among 16 patients showing negative result by chromosome breakage test. Our study shows it is necessary to standardize a diagnostic scoring system as well as to develop complementary molecular test for accurate diagnosis of FA. DisclosuresNo relevant conflicts of interest to declare.

Diadenosine 5', 5'''-P(1),P(4)-tetraphosphate (Ap4A) is synthesized in response to DNA damage and inhibits the initiation of DNA replication.

The level of intracellular diadenosine 5', 5'''-P(1),P(4)-tetraphosphate (Ap4A) increases several fold in mammalian cells treated with non-cytotoxic doses of interstrand DNA-crosslinking agents such as mitomycin C. It is also increased in cells lacking DNA repair proteins including XRCC1, PARP1, APTX and FANCG, while >50-fold increases (up to around 25 μM) are achieved in repair mutants exposed to mitomycin C. Part of this induced Ap4A is converted into novel derivatives, identified as mono- and di-ADP-ribosylated Ap4A. Gene knockout experiments suggest that DNA ligase III is primarily responsible for the synthesis of damage-induced Ap4A and that PARP1 and PARP2 can both catalyze its ADP-ribosylation. Degradative proteins such as aprataxin may also contribute to the increase. Using a cell-free replication system, Ap4A was found to cause a marked inhibition of the initiation of DNA replicons, while elongation was unaffected. Maximum inhibition of 70-80% was achieved with 20 μM Ap4A. Ap3A, Ap5A, Gp4G and ADP-ribosylated Ap4A were without effect. It is proposed that Ap4A acts as an important inducible ligand in the DNA damage response to prevent the replication of damaged DNA.

186P - Palb2 Mrna Expression As a Predictive and Prognostic Marker in Advanced Non-Small-Cell Lung Cancer (Nsclc) Patients (P) Treated with Cisplatin- Docetaxel Chemotherapy

ABSTRACT Aim: The Spanish Lung Cancer Group (SLCG) undertook an industry-independent, two biomarker-directed, randomized trial in advanced NSCLC p. The study (NCT00617656/GECP-BREC) compared non-selected cisplatin-based chemotherapy with therapy customized according to BRCA1/RAP80 mRNA levels. The trial was closed prematurely due to a detrimental effect in the biomarker-directed arm. Further genetic analysis defined PALB2, the bridging molecule that connects BRCA1 and BRCA2, as a promising biomarker to elucidate DNA repair mechanisms. Methods: We assessed mRNA levels of PALB2, RIF1, 53BP1, RNF8 and ATM (as biomarkers with critical roles in homologous recombination [HR]) in tissue from 177 cisplatin plus docetaxel-treated NSCLC p in the BREC trial. We correlated our findings with PFS, OS and response. Results: Median age 62; 79.1% male; 51.4% adenocarcinoma. Results of PALB2 mRNA expression were intriguing. PFS was 5.6 months (m) for p with high/intermediate (H-I) PALB2 and 4.1 m for p with low (L) PALB2 (p = 0.0018). OS was 13.2 m for p with H-I PALB2 compared to 9.9 for p with L PALB2 (p = 0.0377). In the univariate analysis, H-I PALB2 was a marker of longer PFS (HR = 0.56, 95% CI, 0.38, 0.80; p = 0.002) and OS (HR = 0.64, 95% CI, 0.41, 0.98; p = 0.0394). In the multivariate analysis, only H-I PALB2 was associated with longer PFS (here HR = 0.56 and p = 0.0022) and OS (HR = 0.61 and p = 0.0343). Among 143 p with data for response and PALB2 expression, 49.5% of p with H-I PALB2 were responders, compared to only 28% with L PALB2 (p = 0.0131). No significant differences were found for PFS, OS or response according to expression status of the other 4 biomarkers. Conclusions: Our findings reveal PALB2 to be a predictive and prognostic biomarker in advanced NSCLC p treated with docetaxel plus cisplatin. The BRCA1-PALB2-BRCA2 network is a critical determinant of responsiveness to DNA interstrand cross-linking agents and antimicrotubule agents. In our study, BRCA1 had no effect on treatment outcome of this population. PALB2 was found to be a strong marker to predict sensitivity to antimicrotubule agents, without affecting sensitivity to DNA damage-based chemotherapy. Disclosure: All authors have declared no conflicts of interest.

Inhibition of autophagy enhances DNA damage-induced apoptosis by disrupting CHK1-dependent S phase arrest

DNA damage has been shown to induce autophagy, but the role of autophagy in the DNA damage response and cell fate is not fully understood. BO-1012, a bifunctional alkylating derivative of 3a-aza-cyclopenta[a]indene, is a potent DNA interstrand cross-linking agent with anticancer activity. In this study, BO-1012 was found to reduce DNA synthesis, inhibit S phase progression, and induce phosphorylation of histone H2AX on serine 139 (γH2AX) exclusively in S phase cells. Both CHK1 and CHK2 were phosphorylated in response to BO-1012 treatment, but only depletion of CHK1, but not CHK2, impaired BO-1012-induced S phase arrest and facilitated the entry of γH2AX-positive cells into G2 phase. CHK1 depletion also significantly enhanced BO-1012-induced cell death and apoptosis. These results indicate that BO-1012-induced S phase arrest is a CHK1-dependent pro-survival response. BO-1012 also resulted in marked induction of acidic vesicular organelle (AVO) formation and microtubule-associated protein 1 light chain 3 (LC3) processing and redistribution, features characteristic of autophagy. Depletion of ATG7 or co-treatment of cells with BO-1012 and either 3-methyladenine or bafilomycin A1, two inhibitors of autophagy, not only reduced CHK1 phosphorylation and disrupted S phase arrest, but also increased cleavage of caspase-9 and PARP, and cell death. These results suggest that cells initiate S phase arrest and autophagy as pro-survival responses to BO-1012-induced DNA damage, and that suppression of autophagy enhances BO-1012-induced apoptosis via disruption of CHK1-dependent S phase arrest.

范可尼贫血症与DNA交联损伤修复

Fanconi anemia (FA) is a multigenic recessive disorder caused by mutations in 15 FA or FA-like genes and deficiency in any of these genes renders cells hypersensitive to DNA interstrand cross-linking (ICL) agents. In this review, we summarise our current knowledge about how these FA proteins participate in ICL repair.

PI3K Inhibition Augments the Therapeutic Efficacy of a 3a-aza-Cyclopenta[α]indene Derivative in Lung Cancer Cells

The synergistic targeting of DNA damage and DNA repair is a promising strategy for the development of new chemotherapeutic agents for human lung cancer. The DNA interstrand cross-linking agent BO-1509, a derivative of 3a-aza-cyclopenta[α]indene, was synthesized and combined with the phosphoinositide 3-kinase (PI3K) inhibitor LY294002 to treat human lung cancer cells. Our results showed that the BO-1509 and LY294002 combination synergistically killed lung cancer cells in culture and also suppressed the growth of lung cancer xenografts in mice, including those derived from gefitinib-resistant cells. We also found that LY294002 suppressed the induction of several DNA repair proteins by BO-1509 and inhibited the nuclear translocation of Rad51. On the basis of the results of the γH2AX foci formation assays, LY294002 apparently inhibited the repair of DNA damage that was induced by BO-1509. According to the complete blood profile, biochemical enzyme analysis, and histopathologic analysis of major organs, no apparent toxicity was observed in mice treated with BO-1509 alone or in combination with LY294002. Our results suggest that the combination of a DNA cross-linking agent with a PI3K inhibitor is a feasible strategy for the treatment of patients with lung cancer.

Ouabain, a Cardiac Glycoside, Inhibits the Fanconi Anemia/BRCA Pathway Activated by DNA Interstrand Cross-Linking Agents

Modulation of the DNA repair pathway is an emerging target for the development of anticancer drugs. DNA interstrand cross-links (ICLs), one of the most severe forms of DNA damage caused by anticancer drugs such as cisplatin and mitomycin C (MMC), activates the Fanconi anemia (FA)/BRCA DNA repair pathway. Inhibition of the FA/BRCA pathway can enhance the cytotoxic effects of ICL-inducing anticancer drugs and can reduce anticancer drug resistance. To find FA/BRCA pathway inhibitory small molecules, we established a cell-based high-content screening method for quantitating the activation of the FA/BRCA pathway by measuring FANCD2 foci on DNA lesions and then applied our method to chemical screening. Using commercial LOPAC1280 chemical library screening, ouabain was identified as a competent FA/BRCA pathway inhibitory compound. Ouabain, a member of the cardiac glycoside family, binds to and inhibits Na+/K+-ATPase and has been used to treat heart disease for many years. We observed that ouabain, as well as other cardiac glycoside family members―digitoxin and digoxin―down-regulated FANCD2 and FANCI mRNA levels, reduced monoubiquitination of FANCD2, inhibited FANCD2 foci formation on DNA lesions, and abrogated cell cycle arrest induced by MMC treatment. These inhibitory activities of ouabain required p38 MAPK and were independent of cellular Ca2+ ion increase or the drug uptake-inhibition effect of ouabain. Furthermore, we found that ouabain potentiated the cytotoxic effects of MMC in tumor cells. Taken together, we identified an additional effect of ouabain as a FA/BRCA pathway-inhibiting chemosensitization compound. The results of this study suggest that ouabain may serve as a chemosensitizer to ICL-inducing anticancer drugs.

Malignant melanoma cells acquire resistance to DNA interstrand cross-linking chemotherapeutics by p53-triggered upregulation of DDB2/XPC-mediated DNA repair

Malignant melanoma is a cancer characterized by high chemoresistance although p53 is rarely mutated. Here, we show that p53 wild-type melanoma cells acquire resistance to cell death induced by fotemustine (FM), which is a representative of alkylating DNA interstrand cross-linking agents used in melanoma therapy. We show that drug-induced resistance is a result of p53-dependent upregulation of the nucleotide excision repair (NER) genes xeroderma pigmentosum complementation group C (XPC) and damaged DNA-binding protein 2 (DDB2), which stimulate the repair of DNA interstrand cross-links (ICLs) arising from O(6)-chloroethylguanine. Consequently, TP53 mutated cells are unable to repair ICLs, leading to prolonged ATM, ATR and checkpoint kinase 1 (CHK1) activation, and finally apoptosis. The roles of p53 and NER in ICL-triggered cell death were confirmed by knockdown of p53 and XPC. Upregulation of XPC and DDB2 in p53wt cells following a single drug treatment is a robust and sustained response that lasts for up to 1 week. Pretreatment with an inducing dose followed by a high and toxic dose of FM provoked an adaptive response as the killing outcome of the challenge dose was reduced. Upregulation of XPC and DDB2 was also observed in a melanoma mouse xenograft model following systemic administration of FM. Additionally, XPC and DDB2 induction occurred upon treatment with other cross-linking anticancer drugs, such as cisplatin and mafosfamide, indicating it is a general response of cancer cells to this group of chemotherapeutics. Collectively, the data indicate that p53-dependent upregulation of XPC and DDB2 is a key mechanism upon genotoxic stress, whereby melanoma cells acquire resistance towards DNA cross-linking agents. To our knowledge, this is the first demonstration of upregulation of NER following a single dose of a DNA interstrand cross-linker, which is a robust and long-lasting effect that impacts the killing response of cancer cells to subsequent treatments.