PARP-2 depletion results in lower radiation cell survival but cell line-specific differences in poly(ADP-ribose) levels.

Poly(ADP-ribose) polymerase-2 (PARP-2) activity contributes to a cells' poly(ADP-ribosyl)ating potential and like PARP-1, has been implicated in several DNA repair pathways including base excision repair and DNA single strand break repair. Here the consequences of its stable depletion in HeLa, U20S, and AS3WT2 cells were examined. All three PARP-2 depleted models showed increased sensitivity to the cell killing effects on ionizing radiation as reported in PARP-2 depleted mouse embryonic fibroblasts providing further evidence for a role in DNA strand break repair. The PARP-2 depleted HeLa cells also showed both higher constitutive and DNA damage-induced levels of polymers of ADP-ribose (PAR) associated with unchanged PARP-1 protein levels, but higher PARP activity and a concomitant lower PARG protein levels and activity. These changes were accompanied by a reduced maximal recruitment of PARP-1, XRCC1, PCNA, and PARG to DNA damage sites. This PAR-associated phenotype could be reversed in HeLa cells on re-expression of PARP-2 and was not seen in U20S and AS3WT2 cells. These results highlight the complexity of the relationship between different members of the PARP family on PAR metabolism and suggest that cell model dependent phenotypes associated with the absence of PARP-2 exist within a common background of radiation sensitivity.

Mutation in TTI2 reveals a role for triple T complex in human brain development.

Tel2-interacting proteins 1 and 2 (TTI1 and TTI2) physically interact with telomere maintenance 2 (TEL2) to form a conserved trimeric complex called the Triple T complex. This complex is a master regulator of phosphoinositide-3-kinase-related protein kinase (PIKKs) abundance and DNA damage response signaling. Using a combination of autozygosity mapping and high-throughput sequencing in a large consanguineous multiplex family, we found that a missense c.1307T>A/p.I436N mutation in TTI2 causes a human autosomal recessive condition characterized by severe cognitive impairment, microcephaly, behavioral troubles, short stature, skeletal anomalies, and facial dysmorphic features. Immunoblotting experiment showed decreased amount of all Triple T complex components in the patient skin fibroblasts. Consistently, a drastically reduced steady-state level of all PIKKs tested was also observed in the patient cells. Combined with previous observations, these findings emphasises the role of the TTI2 gene in the etiology of intellectual disability and further support the role of PIKK signaling in brain development and functioning.

The impact of cyclin-dependent kinase 5 depletion on poly(ADP-ribose) polymerase activity and responses to radiation.

Cyclin-dependent kinase 5 (Cdk5) has been identified as a determinant of sensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors. Here, the consequences of its depletion on cell survival, PARP activity, the recruitment of base excision repair (BER) proteins to DNA damage sites, and overall DNA single-strand break (SSB) repair were investigated using isogenic HeLa stably depleted (KD) and Control cell lines. Synthetic lethality achieved by disrupting PARP activity in Cdk5-deficient cells was confirmed, and the Cdk5(KD) cells were also found to be sensitive to the killing effects of ionizing radiation (IR) but not methyl methanesulfonate or neocarzinostatin. The recruitment profiles of GFP-PARP-1 and XRCC1-YFP to sites of micro-irradiated Cdk5(KD) cells were slower and reached lower maximum values, while the profile of GFP-PCNA recruitment was faster and attained higher maximum values compared to Control cells. Higher basal, IR, and hydrogen peroxide-induced polymer levels were observed in Cdk5(KD) compared to Control cells. Recruitment of GFP-PARP-1 in which serines 782, 785, and 786, potential Cdk5 phosphorylation targets, were mutated to alanines in micro-irradiated Control cells was also reduced. We hypothesize that Cdk5-dependent PARP-1 phosphorylation on one or more of these serines results in an attenuation of its ribosylating activity facilitating persistence at DNA damage sites. Despite these deficiencies, Cdk5(KD) cells are able to effectively repair SSBs probably via the long patch BER pathway, suggesting that the enhanced radiation sensitivity of Cdk5(KD) cells is due to a role of Cdk5 in other pathways or the altered polymer levels.

The methyl methanesulfonate induced S-phase delay in XRCC1-deficient cells requires ATM and ATR.

X-ray repair cross-complementing 1 (XRCC1) is required for DNA single-strand break and base excision repair (BER) in human cells. XRCC1-deficient human cells show hypersensitivity to cell killing, increased genetic instability and a significant delay in S-phase progression after exposure to the alkylating agent methyl methanesulfonate (MMS). Using RNAi modulation of XRCC1 levels, we show here that this S-phase delay is associated with significantly increased levels of recombinational repair as visualized by Rad51 focus formation. Using ATM- and ATR-defective cells and an ATM specific kinase inhibitor we demonstrate for the first time that the MMS-induced S-phase checkpoint requires both ATM and ATR. This unique dependency is associated with phosphorylation of ATM/ATR downstream targets or effectors such as SMC1 and Chk1. These results support the hypothesis that after MMS-treatment, the presence of unresolved BER intermediates gives rise to lesions that activate both ATM and ATR and that during the consequent S-phase delay, such intermediates may be repaired by a recombinational pathway which involves the Rad51 protein.

The associations of sequence variants in DNA-repair and cell-cycle genes with cancer risk: genotype-phenotype correlations.

DNA-repair systems maintain the integrity of the human genome, and cell-cycle checkpoints are a critical component of the cellular response to DNA damage. Thus the presence of sequence variants in genes involved in these pathways that modulate their activity might have an impact on cancer risk. Many molecular epidemiological studies have investigated the association between sequence variants, particularly SNPs (single nucleotide polymorphisms), and cancer risk. For instance, ATM (ataxia telangiectasia mutated) SNPs have been associated with increased risk of breast, prostate, leukaemia, colon and early-onset lung cancer, and the intron 3 16-bp repeat in TP53 (tumour protein 53) is associated with an increased risk of lung cancer. In contrast, the variant allele of the rare CHEK2 (checkpoint kinase 2 checkpoint homologue) missense variant (accession number rs17879961) was significantly associated with a lower incidence of lung and upper aerodigestive cancers. For some sequence variants, a strong gene-environment interaction has also been noted. For instance, a greater absolute risk reduction of lung and upper aerodigestive cancers in smokers than in non-smokers carrying the I157T CHEK2 variant has been observed, as has an interaction between TP53 intron 3 16-bp repeats and multiple X-ray exposures on lung cancer risk. The challenge now is to understand the molecular mechanisms underlying these associations.

Radiosensitization by the poly(ADP-ribose) polymerase inhibitor 4-amino-1,8-naphthalimide is specific of the S phase of the cell cycle and involves arrest of DNA synthesis.

Radiosensitization caused by the poly(ADP-ribose) polymerase (PARP) inhibitor 4-amino-1,8-naphthalimide (ANI) was investigated in 10 asynchronously growing rodent (V79, CHO-Xrs6, CHO-K1, PARP-1+/+ 3T3, and PARP-1-/- 3T3) or human (HeLa, MRC5VI, IMR90, M059J, and M059K) cell lines, either repair proficient or defective in DNA-PK (CHO-Xrs6 and M059J) or PARP-1 (PARP-1-/- 3T3). Pulse exposure to ANI (1-hour contact) potentiated radiation response in rodent cells except in PARP-1(-/-) 3T3 fibroblasts. In contrast, ANI did not significantly enhance radiation susceptibility in asynchronously dividing human cells; yet, single-strand break rejoining was lengthened by ca. 7-fold in all but mouse PARP-1-/- 3T3s. Circumstantial evidence suggested that radiosensitization by ANI occurs in rapidly dividing cells only. Experiments using synchronized HeLa cells consistently showed that ANI-induced radiosensitization is specific of the S phase of the cell cycle and involves stalled replication forks. Under these conditions, prolonged contact with ANI ended in the formation of de novo DNA double-strand breaks hours after irradiation, evoking collision with uncontrolled replication forks of DNA lesions whose repair was impaired by inhibition of the PARP catalytic activity. The data suggest that increased response to radiotherapy by PARP inhibitors may be achieved only in rapidly growing tumors with a high S-phase content.

A role for PKCzeta in potentiation of the topoisomerase II activity and etoposide cytotoxicity by wortmannin.

Enhanced cytotoxicity of etoposide by wortmannin, an inhibitor of enzymes holding a phosphatidylinositol 3-kinase domain, was investigated in eight cell lines proficient or deficient for DNA double-strand break repair. Wortmannin stimulated the decatenating activity of topoisomerase II, promoted etoposide-induced accumulation of DNA double-strand breaks, shifted the specificity for cell killing by etoposide from the S to G1 phase of the cell cycle, and potentiated the cytotoxicity of etoposide through two mechanisms. (a) Sensitization to high, micromolar amounts of etoposide required integrity of the nonhomologous end-joining repair pathway. (b) Wortmannin dramatically increased the susceptibility to low, submicromolar amounts of etoposide in a large fraction of the cell population irrespective of the status of ATM, Ku86, and DNA-PKCS. It is shown that this process correlates depression of phosphatidylinositol 3-kinase-dependent phosphorylation of the atypical, zeta isoform of protein kinase C (PKCzeta). Stable expression of a dominant-negative, kinase-dead mutant of PKCzeta in a tumor cell line reproduced the hypersensitivity pattern induced by wortmannin. The results are consistent with up-regulation of the topoisomerase II activity in relation to inactivation of PKCzeta and indicate that PKCzeta may be a useful target to improve the efficiency of topoisomerase II poisons at low concentration.

Afatinib, an Irreversible EGFR Family Inhibitor, Shows Activity Toward Pancreatic Cancer Cells, Alone and in Combination with Radiotherapy, Independent of KRAS Status.

BACKGROUND: Pancreatic adenocarcinoma is characterized by a high frequency of KRAS mutations and frequent deregulation of the epidermal growth factor receptor (EGFR) and other EGFR family members such as HER2/ErbB2. The EGFR inhibitor erlotinib is approved for treatment of pancreatic cancer, but has shown modest activity in most patients. OBJECTIVE: Here we investigated the activity of afatinib, a second-generation irreversible pan-EGFR family kinase inhibitor, alone or in combination with ionizing radiation, toward pancreatic cancer cells. METHODS: The influence of afatinib on cell proliferation, cell cycle distribution, clonogenic survival, nuclear fragmentation, ploidy, and centrosome amplification following irradiation was determined. Expression and phosphorylation of HER receptors, Akt, DNA-PKcs, and ERK1/2 was characterized by Western blot analysis. RESULTS: Afatinib was growth-inhibitory for all three cell lines but cytotoxic only toward BxPC3 (KRAS (wt)) and Capan-2 (KRAS (mut)) cells, both of which express high levels of EGFR, HER2, and HER3 receptors. Afatinib increased the radiosensitivity of BxPC3 and Capan-2 cells, prevented the radio-induced phosphorylation of Akt, and induced mitotic catastrophe following irradiation. In comparison, Panc-1 cells (KRAS (mut)) expressing low levels of EGFR family receptors were resistant to afatinib-induced radiosensitization. LIMITATIONS: These results must be confirmed in vivo. CONCLUSIONS: Afatinib showed cytotoxic and radiosensitizing effects toward a subset of pancreatic cancer cells which was closely correlated with expression of EGFR, HER2, and HER3 receptors, but not with KRAS status.

Genetic biomarkers of therapeutic radiation sensitivity.

The occurrence of acute or late normal tissue reactions after therapeutic radiotherapy and cellular responses in in vitro radiosensitivity assays do not correlate well suggesting that to date no one test system is suitable for predicting the risk or severity of such reactions. New insights into the underlying molecular mechanisms of this sensitivity are coming from studies that assess associations between common polymorphisms in DNA damage detection and repair genes and the development of adverse reactions to radiotherapy. The presence of such variants may alter protein function and an individual's capacity to repair damaged DNA modifying the response of the normal tissue. Polymorphisms in the XRCC1, ATM, hHR21 and TGFbeta1 genes have been shown to be associated with an increased risk of developing an adverse normal tissue reaction to radiotherapy, whilst one variant in the ATM gene has been reported to be radioprotective. Functional studies, taking into account either the haplotypes or the combined genotypes when multiple polymorphisms in a gene are present, will be necessary to establish the mechanistic basis of these associations. In the future association studies can only benefit from the analysis of multiple genes in large, well-characterized cohorts in particular to identify genetic factors that might specifically influence the temporal occurrence of these adverse reactions.

Cdk5 promotes DNA replication stress checkpoint activation through RPA-32 phosphorylation, and impacts on metastasis free survival in breast cancer patients.

Cyclin dependent kinase 5 (Cdk5) is a determinant of PARP inhibitor and ionizing radiation (IR) sensitivity. Here we show that Cdk5-depleted (Cdk5-shRNA) HeLa cells show higher sensitivity to S-phase irradiation, chronic hydroxyurea exposure, and 5-fluorouracil and 6-thioguanine treatment, with hydroxyurea and IR sensitivity also seen in Cdk5-depleted U2OS cells. As Cdk5 is not directly implicated in DNA strand break repair we investigated in detail its proposed role in the intra-S checkpoint activation. While Cdk5-shRNA HeLa cells showed altered basal S-phase dynamics with slower replication velocity and fewer active origins per DNA megabase, checkpoint activation was impaired after a hydroxyurea block. Cdk5 depletion was associated with reduced priming phosphorylations of RPA32 serines 29 and 33 and SMC1-Serine 966 phosphorylation, lower levels of RPA serine 4 and 8 phosphorylation and DNA damage measured using the alkaline Comet assay, gamma-H2AX signal intensity, RPA and Rad51 foci, and sister chromatid exchanges resulting in impaired intra-S checkpoint activation and subsequently higher numbers of chromatin bridges. In vitro kinase assays coupled with mass spectrometry demonstrated that Cdk5 can carry out the RPA32 priming phosphorylations on serines 23, 29, and 33 necessary for this checkpoint activation. In addition we found an association between lower Cdk5 levels and longer metastasis free survival in breast cancer patients and survival in Cdk5-depleted breast tumor cells after treatment with IR and a PARP inhibitor. Taken together, these results show that Cdk5 is necessary for basal replication and replication stress checkpoint activation and highlight clinical opportunities to enhance tumor cell killing.

EPI-CT: in vitro assessment of the applicability of the γ-H2AX-foci assay as cellular biomarker for exposure in a multicentre study of children in diagnostic radiology.

PURPOSE: To conduct a feasibility study on the application of the γ-H2AX foci assay as an exposure biomarker in a prospective multicentre paediatric radiology setting. MATERIALS AND METHODS: A set of in vitro experiments was performed to evaluate technical hurdles related to biological sample collection in a paediatric radiology setting (small blood sample volume), processing and storing of blood samples (effect of storing blood at 4°C), the reliability of foci scoring for low-doses (merge γ-H2AX/53BP1 scoring), as well as the impact of contrast agent administration as potential confounding factor. Given the exploratory nature of this study and the ethical constraints related to paediatric blood sampling, blood samples from adult volunteers were used for these experiments. In order to test the feasibility of pooling the γ-H2AX data when different centres are involved in an international multicentre study, two intercomparison studies in the low-dose range (10-500 mGy) were performed. RESULTS: Determination of the number of X-ray induced γ-H2AX foci is feasible with one 2 ml blood sample pre- and post-computed tomography (CT) scan. Lymphocyte isolation and fixation on slides is necessary within 5 h of blood sampling to guarantee reliable results. The possible enhancement effect of contrast medium on the induction of DNA DSB in a patient study can be ruled out if radiation doses and the contrast agent concentration are within diagnostic ranges. The intercomparison studies using in vitro irradiated blood samples showed that the participating laboratories, executing successfully the γ-H2AX foci assay in lymphocytes, were able to rank blind samples in order of lowest to highest radiation dose based on mean foci/cell counts. The dose response of all intercomparison data shows that a dose point of 10 mGy could be distinguished from the sham-irradiated control (p = 0.006). CONCLUSIONS: The results demonstrate that it is feasible to apply the γ-H2AX foci assay as a cellular biomarker of exposure in a multicentre prospective study in paediatric CT imaging after validating it in an in vivo international pilot study on paediatric patients.

Phenotypic cellular characterization of an ataxia telangiectasia patient carrying a causal homozygous missense mutation.

Most disease-causing mutations in Ataxia telangiectasia (AT) patients correspond to truncating mutations in the ATM gene with very few cases of AT patients carrying two missense sequence alterations being reported. The cellular phenotype of a lymphoblastoid cell line established from an AT patient (AT173) who showed classical clinical AT features, and carried two homozygous missense alterations, the 378T>A variant and 9022C>T located within the ATM kinase domain, has been characterized. ATM mRNA was detectable and the ATM protein level was approximately 50% of that seen in normal cell lines. Functional analysis of this protein revealed a total absence of ATM kinase activity measured either in vitro or in vivo, before and after exposure to ionizing radiation. The AT173 cell line was hypersensitive to ionizing radiation and exhibited a G1 cell cycle arrest defect and an accumulation of cells in G2 phase of the cell cycle after irradiation, a response that is identical to that seen in AT cell lines carrying truncating mutations. These phenotypic features strongly suggest that the 9022C>T (R3008C) missense mutation is the disease-causing mutation and that the presence of ATM protein is not always predictive of a normal cellular phenotype.

Poly(ADP-ribose) polymerase (PARP-1) is not involved in DNA double-strand break recovery.

BACKGROUND: The cytotoxicity and the rejoining of DNA double-strand breaks induced by gamma-rays, H2O2 and neocarzinostatin, were investigated in normal and PARP-1 knockout mouse 3T3 fibroblasts to determine the role of poly(ADP-ribose) polymerase (PARP-1) in DNA double-strand break repair. RESULTS: PARP-1-/- were considerably more sensitive than PARP-1+/+ 3T3s to induced cell kill by gamma-rays and H2O2. However, the two cell lines did not show any significant difference in the susceptibility to neocarzinostatin below 1.5 nM drug. Restoration of PARP-1 expression in PARP-1-/- 3T3s by retroviral transfection of the full PARP-1 cDNA did not induce any change in neocarzinostatin response. Moreover the incidence and the rejoining kinetics of neocarzinostatin-induced DNA double-strand breaks were identical in PARP-1+/+ and PARP-1-/- 3T3s. Poly(ADP-ribose) synthesis following gamma-rays and H2O2 was observed in PARP-1-proficient cells only. In contrast neocarzinostatin, even at supra-lethal concentration, was unable to initiate PARP-1 activation yet it induced H2AX histone phosphorylation in both PARP1+/+ and PARP-1-/- 3T3s as efficiently as gamma-rays and H2O2. CONCLUSIONS: The results show that PARP-1 is not a major determinant of DNA double-strand break recovery with either strand break rejoining or cell survival as an endpoint. Even though both PARP-1 and ATM activation are major determinants of the cell response to gamma-rays and H2O2, data suggest that PARP-1-dependent poly(ADP-ribose) synthesis and ATM-dependent H2AX phosphorylation, are not inter-related in the repair pathway of neocarzinostatin-induced DNA double-strand breaks.

Control of the G2/M checkpoints after exposure to low doses of ionising radiation: implications for hyper-radiosensitivity.

Two molecularly distinct G2/M cell cycle arrests are induced after exposure to ionising radiation (IR) depending on the cell cycle compartment in which the cells are irradiated. The aims of this study were to determine whether there are threshold doses for their activation and investigate the molecular pathways and possible links between the G2 to M transition and hyper-radiosensitivity (HRS). Two human glioblastoma cell lines (T98G-HRS(+) and U373-HRS(-)) unsynchronized or enriched in G2 were irradiated and flow cytometry with BrdU or histone H3 phosphorylation analysis used to assess cell cycle progression and a clonogenic assay to measure radiation survival. The involvement of ATM, Wee1 and PARP was studied using chemical inhibitors. We found that cells irradiated in either the G1 or S phase of the cell cycle transiently accumulate in G2 in a dose-dependent manner after exposure to doses as low as 0.2Gy. Only Wee1 inhibition reduced this G2 accumulation. A block of the G2 to M transition was found after irradiation in G2 but occurs only above a threshold dose, which is cell line dependent, and requires ATM activity after exposure to doses above 0.5Gy. A failure to activate this early G2/M checkpoint correlates with low dose radiosensitization. These results provide evidence that after exposure to low doses of IR two distinct G2/M checkpoints are activated, each in a dose-dependent manner, with distinct threshold doses and involving different damage signalling pathways and confirm links between the early G2/M checkpoint and hyper-radiosensitivity.

Identification and functional consequences of a novel MRE11 mutation affecting 10 Saudi Arabian patients with the ataxia telangiectasia-like disorder.

Ten new patients with ataxia telangiectasia-like disorder (ATLD) from three unrelated Saudi Arabian families have been identified aged 5-37 representing the largest cohort of ATLD patients ever identified. They presented with an early-onset, slowly progressive, ataxia plus ocular apraxia phenotype with an absence of tumor development, even in the oldest patient. Extra-neurological features such as telangiectasia, raised alpha-fetoprotein and reduced immunoglobulin levels were absent. No translocations were found in the two investigated patients, and the presence of microcephaly was noted in four out of eight ascertained patients. All patients are homozygous for a novel missense mutation (630G-->C, W210C) of the MRE11 gene. The cellular consequences of this amino acid change, localized in the nuclease domain of the Mre11 protein, have been determined in fibroblast cultures established from two individuals. They showed high constitutive levels of Mre11 and Rad50 proteins compared with cells from normal individuals but a very low level of the Nbs1 protein. After exposure to ionizing radiation, a dose-dependent defect in ataxia telangiectasia mutated (ATM)-serine 1981, p53-serine 15 and Chk2 phosphorylation, and p53 stabilization were noted, together with a failure to form Mre11 foci and enhanced radiation sensitivity. Formation of gammaH2AX foci was similar to that seen in normal fibroblasts under the experimental conditions examined. These results emphasize the importance of functional interactions among the three proteins of the Mre11-Rad50-Nbs1 complex and lend support to a role of this complex as a sensor of DNA double-strand breaks, acting upstream of ATM.

Functional consequences of ATM sequence variants for chromosomal radiosensitivity.

The ATM [for ataxia-telangiectasia (A-T) mutated] protein plays a key role in the detection and cellular response to DNA double-strand breaks. Several single-nucleotide polymorphisms (SNPs) have been described in the ATM gene; however, their association with cancer risk or radiosensitivity remains to be fully established. In this study, the functional consequences of specific ATM SNPs on in vitro radiosensitivity, as assessed by micronuclei (MN) formation, were measured in lymphoblastoid cell lines established from 10 breast cancer (BC) patients carrying different ATM missense SNPs, six A-T patients, six A-T heterozygotes (A-T het), and six normal individuals. The BC, A-T het, and A-T cell line groups showed significantly higher mean levels of MN formation after exposure to ionizing radiation (IR) than did the group containing normal cell lines, with similar levels in the BC and A-T het groups. Within the BC lines studied, the group composed of the six carrying the linked 2572T>C (858F>L) and 3161C>G (1054P>R) variants had a higher level of MN after IR exposure compared to that observed in the remaining four BC or in the normal cell lines. This increase was not related to the constitutive ATM mRNA level, which was similar in these BC and the normal cell lines. Our results indicate that alterations in the ATM gene, including the presence of heterozygous mutations and the 2572C and 3161G variant alleles, are associated with increased in vitro chromosomal radiosensitivity, perhaps by interfering with ATM function in a dominant-negative manner.