BRCA1 mislocalization leads to aberrant DNA damage response in heterozygous ABRAXAS1 mutation carrier cells.

Whilst heterozygous germline mutations in the ABRAXAS1 gene have been associated with a hereditary predisposition to breast cancer, their effect on promoting tumourigenesis at the cellular level has not been explored. Here, we demonstrate in patient-derived cells that the Finnish ABRAXAS1 founder mutation (c.1082G > A, Arg361Gln), even in the heterozygous state leads to decreased BRCA1 protein levels as well as reduced nuclear localization and foci formation of BRCA1 and CtIP. This causes disturbances in basal BRCA1-A complex localization, which is reflected by a restraint in error-prone DNA double-strand break repair pathway usage, attenuated DNA damage response and deregulated G2-M checkpoint control. The current study clearly demonstrates how the Finnish ABRAXAS1 founder mutation acts in a dominant-negative manner on BRCA1 to promote genome destabilization in heterozygous carrier cells.

Directed Alternative Splicing in Nijmegen Breakage Syndrome: Proof of Principle Concerning Its Therapeutical Application.

Over 90% of patients with Nijmegen breakage syndrome (NBS), a hereditary cancer disorder, are homoallelic for a 5 bp deletion in the NBN gene involved in the cellular response to DNA damage. This hypomorphic mutation leads to a carboxy-terminal protein fragment, p70-nibrin, with some residual function. Average age at malignancy, typically lymphoma, is 9.7 years. NBS patients are hypersensitive to chemotherapeutic and radiotherapeutic treatments, thus prevention of cancer development is of particular importance. Expression of an internally deleted NBN protein, p80-nibrin, has been previously shown to be associated with a milder cellular phenotype and absence of cancer in a 62-year-old NBS patient. Here we show that cells from this patient, unlike other NBS patients, have DNA replication and origin firing rates comparable to control cells. We used here antisense oligonucleotides to enforce alternative splicing in NBS patient cells and efficiently generate the same internally deleted p80-nibrin protein. Injecting the same antisense sequences as morpholino oligomers (VivoMorpholinos) into the tail vein of a humanized NBS murine mouse model also led to efficient alternative splicing in vivo. Thus, proof of principle for the use of antisense oligonucleotides as a potential cancer prophylaxis has been demonstrated.

DNA Repair.

Cellular chromosomal DNA is the principal target through which ionising radiation exerts it diverse biological effects. This chapter summarises the relevant DNA damage signalling and repair pathways used by normal and tumour cells in response to irradiation. Strategies for tumour radiosensitisation are reviewed which exploit tumour-specific DNA repair deficiencies or signalling pathway addictions, with a special focus on growth factor signalling, PARP, cancer stem cells, cell cycle checkpoints and DNA replication. This chapter concludes with a discussion of DNA repair-related candidate biomarkers of tumour response which are of crucial importance for implementing precision medicine in radiation oncology.

Dual PI3K/mTOR Inhibitor NVP-BEZ235 Leads to a Synergistic Enhancement of Cisplatin and Radiation in Both HPV-Negative and -Positive HNSCC Cell Lines.

The standard of care for advanced head and neck cancers (HNSCCs) is radiochemotherapy, including cisplatin. This treatment results in a cure rate of approximately 85% for oropharyngeal HPV-positive HNSCCs, in contrast to only 50% for HPV-negative HNSCCs, and is accompanied by severe side effects for both entities. Therefore, innovative treatment modalities are required, resulting in a better outcome for HPV-negative HNSCCs, and lowering the adverse effects for both entities. The effect of the dual PI3K/mTOR inhibitor NVP-BEZ235 on a combined treatment with cisplatin and radiation was studied in six HPV-negative and six HPV-positive HNSCC cell lines. Cisplatin alone was slightly more effective in HPV-positive cells. This could be attributed to a defect in homologous recombination, as demonstrated by depleting RAD51. Solely for HPV-positive cells, pretreatment with BEZ235 resulted in enhanced cisplatin sensitivity. For the combination of cisplatin and radiation, additive effects were observed. However, when pretreated with BEZ235, this combination changed into a synergistic interaction, with a slightly stronger enhancement for HPV-positive cells. This increase could be attributed to a diminished degree of DSB repair in G1, as visualized via the detection of γH2AX/53BP1 foci. BEZ235 can be used to enhance the effect of combined treatment with cisplatin and radiation in both HPV-negative and -positive HNSCCs.

Impaired Degradation of Neutrophil Extracellular Traps: A Possible Severity Factor of Elderly Male COVID-19 Patients.

Neutrophil extracellular traps (NETs) have been described as a potential trigger of severe COVID-19. NETs are known as extracellular DNA fibers released by neutrophils in response to infection. If the host is unable to balance efficient clearance of NETs by dornases (DNases), detrimental consequences occur. Elevated levels of NETs in COVID-19 patients are associated with higher risk of morbid thrombotic complications. Here, we studied the level of NET markers and DNase activity in a cohort of COVID-19 patients compared to healthy controls. Our data confirmed an increased level of NET markers in the plasma of COVID-19 patients, with a higher level in male compared to female patients. At the same time, there was an increased DNase activity detectable in COVID-19 patients compared to healthy controls. Importantly, there was a negative correlation of DNase activity with the age of male patients. The antimicrobial peptide LL-37, which is known to stabilize NETs against DNase degradation, is embedded in NETs upon severe acute respiratory syndrome coronavirus-2-infection. The LL-37 plasma level correlates with the NET-marker level in male COVID-19 patients, indicating a potential role of LL-37 in the risk of NET-associated thrombosis in male COVID-19 patients by stabilizing NETs against DNase degradation. In conclusion, our data identify two potential risk factors of elderly male patients which may lead to inefficient NET degradation and a subsequently higher risk of NET-associated thrombosis during COVID-19: reduced DNase activity and an increased LL-37 level.

Increased replication stress and R-loop accumulation in EGFRvIII-expressing glioblastoma present new therapeutic opportunities.

Background: The oncogene epidermal growth factor receptor variant III (EGFRvIII) is expressed in approximately one-third of all glioblastomas (GBMs). So far it is not clear if EGFRvIII expression induces replication stress in GBM cells, which might serve as a therapeutical target. Methods: Isogenetic EGFRvIII- and EGFRvIII+ cell lines with endogenous EGFRvIII expression were used. Markers of oncogenic and replication stress such as γH2AX, RPA, 53BP1, ATR, and CHK1 were analyzed using western blot, immunofluorescence, and flow cytometry. The DNA fiber assay was performed to analyze replication, transcription was measured by incorporation of EU, and genomic instability was investigated by micronuclei and CGH-Array analysis. Immunohistochemistry staining was used to detect replication stress markers and R-loops in human GBM samples. Results: EGFRvIII+ cells exhibit an activated replication stress response, increased spontaneous DNA damage, elevated levels of single-stranded DNA, and reduced DNA replication velocity, which are all indicative characteristics of replication stress. Furthermore, we show here that EGFRvIII expression is linked to increased genomic instability. EGFRvIII-expressing cells display elevated RNA synthesis and R-loop formation, which could also be confirmed in EGFRvIII-positive GBM patient samples. Targeting replication stress by irinotecan resulted in increased sensitivity of EGFRvIII+ cells. Conclusion: This study demonstrates that EGFRvIII expression is associated with increased replication stress, R-loop accumulation, and genomic instability. This might contribute to intratumoral heterogeneity but may also be exploited for individualized therapy approaches.

Exploiting Chromosomal Instability of PTEN-Deficient Triple-Negative Breast Cancer Cell Lines for the Sensitization against PARP1 Inhibition in a Replication-Dependent Manner.

Chromosomal instability (CIN) is an emerging hallmark of cancer and its role in therapeutic responses has been increasingly attracting the attention of the research community. To target the vulnerability of tumors with high CIN, it is important to identify the genes and mechanisms involved in the maintenance of CIN. In our work, we recognize the tumor suppressor gene Phosphatase and Tensin homolog (PTEN) as a potential gene causing CIN in triple-negative breast cancer (TNBC) and show that TNBC with low expression levels of PTEN can be sensitized for the treatment with poly-(ADP-ribose)-polymerase 1 (PARP1) inhibitors, independent of Breast Cancer (BRCA) mutations or a BRCA-like phenotype. In silico analysis of mRNA expression data from 200 TNBC patients revealed low expression of PTEN in tumors with a high CIN70 score. Western blot analysis of TNBC cell lines confirm lower protein expression of PTEN compared to non TNBC cell lines. Further, PTEN-deficient cell lines showed cellular sensitivity towards PARP1 inhibition treatment. DNA fiber assays and examination of chromatin bound protein fractions indicate a protective role of PTEN at stalled replication forks. In this study, we recognize PTEN as a potential CIN-causing gene in TNBC and identify its important role in the replication processes.

Prevention of DNA Replication Stress by CHK1 Leads to Chemoresistance Despite a DNA Repair Defect in Homologous Recombination in Breast Cancer.

Chromosomal instability not only has a negative effect on survival in triple-negative breast cancer, but also on the well treatable subgroup of luminal A tumors. This suggests a general mechanism independent of subtypes. Increased chromosomal instability (CIN) in triple-negative breast cancer (TNBC) is attributed to a defect in the DNA repair pathway homologous recombination. Homologous recombination (HR) prevents genomic instability by repair and protection of replication. It is unclear whether genetic alterations actually lead to a repair defect or whether superior signaling pathways are of greater importance. Previous studies focused exclusively on the repair function of HR. Here, we show that the regulation of HR by the intra-S-phase damage response at the replication is of overriding importance. A damage response activated by Ataxia telangiectasia and Rad3 related-checkpoint kinase 1 (ATR-CHK1) can prevent replication stress and leads to resistance formation. CHK1 thus has a preferred role over HR in preventing replication stress in TNBC. The signaling cascade ATR-CHK1 can compensate for a double-strand break repair error and lead to resistance of HR-deficient tumors. Established methods for the identification of HR-deficient tumors for Poly(ADP-Ribose)-Polymerase 1 (PARP1) inhibitor therapies should be extended to include analysis of candidates for intra-S phase damage response.

Cdc45 is limiting for replication initiation in humans.

Cdc45 is an essential protein that together with Mcm2-7 and GINS forms the eukaryotic replicative helicase CMG. Cdc45 seems to be rate limiting for the initial unwinding or firing of replication origins. In line with this view, Cdc45-overexpressing cells fired at least twice as many origins as control cells. However, these cells displayed an about 2-fold diminished fork elongation rate, a pronounced asymmetry of replication fork extension, and an early S phase arrest. This was accompanied by H2AX-phosphorylation and subsequent apoptosis. Unexpectedly, we did not observe increased ATR/Chk1 signaling but rather a mild ATM/Chk2 response. In addition, we detected accumulation of long stretches of single-stranded DNA, a hallmark of replication catastrophe. We conclude that increased origin firing by upregulated Cdc45 caused exhaustion of the single-strand binding protein RPA, which in consequence diminished the ATR/Chk1 response; the subsequently occurring fork breaks led to an ATM/Chk2 mediated phosphorylation of H2AX and eventually to apoptosis.

Constitutive gp130 activation rapidly accelerates the transformation of human hepatocytes via an impaired oxidative stress response.

Pro-inflammatory signaling pathways, especially interleukin 6 (IL-6), and reactive oxygen species (ROS) promote carcinogenesis in the liver. In order to elucidate the underlying oncogenic mechanism, we activated the IL-6 signal transducer glycoprotein 130 (gp130) via stable expression of a constitutively active gp130 construct (L-gp130) in untransformed telomerase-immortalized human fetal hepatocytes (FH-hTERT). As known from hepatocellular adenomas, forced gp130 activation alone was not sufficient to induce malignant transformation. However, additional challenge of FH-hTERT L-gp130 clones with oxidative stress resulted in 2- to 3-fold higher ROS levels and up to 6-fold more DNA-double strand breaks (DSB). Despite increased DNA damage, ROS-challenged FH-hTERT L-gp130 clones displayed an enhanced proliferation and rapidly developed colony growth capabilities in soft agar. As driving gp130-mediated oncogenic mechanism, we detected a decreased expression of antioxidant genes, in particular glutathione peroxidase 3 and apolipoprotein E, and an absence of P21 upregulation following ROS-conferred induction of DSB. In summary, an impaired oxidative stress response in hepatocytes with gp130 gain-of-function mutations, as detected in dysplastic intrahepatic nodules and hepatocellular adenomas, is one of the central oncogenic mechanisms in chronic liver inflammation.

High levels of RAD51 perturb DNA replication elongation and cause unscheduled origin firing due to impaired CHK1 activation.

In response to replication stress ATR signaling through CHK1 controls the intra-S checkpoint and is required for the maintenance of genomic integrity. Homologous recombination (HR) comprises a series of interrelated pathways that function in the repair of DNA double strand breaks and interstrand crosslinks. In addition, HR, with its key player RAD51, provides critical support for the recovery of stalled forks during replication. High levels of RAD51 are regularly found in various cancers, yet little is known about the effect of the increased RAD51 expression on intra-S checkpoint signaling. Here, we describe a role for RAD51 in driving genomic instability caused by impaired replication and intra-S mediated CHK1 signaling by studying an inducible RAD51 overexpression model as well as 10 breast cancer cell lines. We demonstrate that an excess of RAD51 decreases I-Sce-I mediated HR despite formation of more RAD51 foci. Cells with high RAD51 levels display reduced elongation rates and excessive dormant origin firing during undisturbed growth and after damage, likely caused by impaired CHK1 activation. In consequence, the inability of cells with a surplus of RAD51 to properly repair complex DNA damage and to resolve replication stress leads to higher genomic instability and thus drives tumorigenesis.

Heterozygous mutations in PALB2 cause DNA replication and damage response defects.

Besides mutations in BRCA1/BRCA2, heterozygous defects in PALB2 are important in breast cancer predisposition. PALB2 heterozygosity increases the risk of malignancy about sixfold. PALB2 interacts with BRCA1 and BRCA2 to regulate homologous recombination and mediate DNA damage response. Here we show, by analysing lymphoblastoid cell lines from heterozygous female PALB2 mutation carriers, that PALB2 haploinsufficiency causes aberrant DNA replication/damage response. Mutation carrier cells show increased origin firing and shorter distance between consecutive replication forks. Carrier cell lines also show elevated ATR protein, but not phosphorylation levels, and a majority of them display aberrant Chk1-/Chk2-mediated DNA damage response. Elevated chromosome instability is observed in primary blood lymphocytes of PALB2 mutation carriers, indicating that the described mechanisms of genome destabilization operate also at the organism level. These findings provide a new mechanism for early stages of breast cancer development that may also apply to other heterozygous homologous recombination signalling pathway gene mutations in hereditary cancer predisposition.

DNA damage by X-rays and their impact on replication processes.

BACKGROUND: Replication-dependent radiosensitization of tumors ranks among the most promising tools for future improvements in tumor therapy. However, cell cycle checkpoint signaling during S phase is a key for maintaining genomic stability after ionizing irradiation allowing DNA damage repair by stabilizing replication forks, inhibiting new origin firing and recruiting DNA repair proteins. As the impact of the different types of DNA damage induced by ionizing radiation on replication fork functionality has not been investigated, this study was performed in tumor cells treated with various agents that induce specific DNA lesions. METHODS: U2OS cells were exposed to methyl methanesulfonate (MMS) to induce base damage, low or high concentrations of hydrogen peroxide for the induction of SSBs, Topotecan to induce DSBs at replication, Mitomycin C (MMC) to induce interstrand cross-links or ionizing irradiation to analyze all damages. Chk1 phosphorylation, origin firing and replication fork progression, and cell cycle distribution were analyzed. RESULTS: In our system, the extent of Chk1 phosphorylation was dependent on the type of damage induced and prolonged Chk1 phosphorylation correlated with the inhibition of replication initiation. Ionizing radiation, high concentrations of hydrogen peroxide, and Topotecan affected replication elongation much more strongly that the other agents. Almost all agents induced a slight increase in the S phase population but subsequent G2 arrest was only observed in response to those agents that strongly inhibited replication elongation and caused prolonged Chk1 phosphorylation. CONCLUSIONS: Our data suggest that to improve radiotherapy, radiosensitivity in S phase could be increased by combining irradiation with agents that induce secondary DSB or inhibit checkpoint signaling, such as inhibitors of PARP or Chk1.