A defect in DNA topoisomerase II activity in ataxia-telangiectasia cells

Ataxia telangiectasia mutated-dependent regulation of topoisomerase II alpha expression and sensitivity to topoisomerase II inhibitor

Topoisomerase II alpha (TOP2A) has a crucial role in proper chromosome condensation and segregation. Here we report the interaction of TOP2A with ataxia telangiectasia mutated (ATM) and its phosphorylation in an ATM-dependent manner after DNA damage. In vitro kinase assay and site-directed mutagenesis studies revealed that serine 1512 is the target of phosphorylation through ATM. Serine 1512 to Alanine mutation of TOP2A showed increased stability of the protein, retaining TOP2A activity at least with regard to cell survival activity. Ataxia telangiectasia-derived cell lines showed high levels of TOP2A that were associated with hypersensitivity to the TOP2 inhibitor etoposide. These findings suggest that ATM-dependent TOP2A modification is required for proper regulation of TOP2 stability and subsequently of the sensitivity to TOP2 inhibitor. In a lymphoblastoid cell line derived from a patient who developed MLL rearrangement, positive infant leukemia, defective ATM expression, and increased TOP2A expression were shown. It was intriguing that hypersensitivity to TOP2 inhibitor and susceptibility to MLL gene rearrangement were shown by low-dose etoposide exposure in this cell line. Thus, our findings have clinically important implications for the pathogenesis of infantile acute leukemia as well as treatment-associated secondary leukemia following exposure to TOP2 inhibitors.

Condensin-dependent localisation of topoisomerase II to an axial chromosomal structure is required for sister chromatid resolution during mitosis

Assembly of compact mitotic chromosomes and resolution of sister chromatids are two essential processes for the correct segregation of the genome during mitosis. Condensin, a five-subunit protein complex, is thought to be required for chromosome condensation. However, recent genetic analysis suggests that condensin is only essential to resolve sister chromatids. To study further the function of condensin we have depleted DmSMC4, a subunit of the complex, from Drosophila S2 cells by dsRNA-mediated interference. Cells lacking DmSMC4 assemble short mitotic chromosomes with unresolved sister chromatids where Barren, a non-SMC subunit of the complex is unable to localise. Topoisomerase II, however, binds mitotic chromatin after depletion of DmSMC4 but it is no longer confined to a central axial structure and becomes diffusely distributed all over the chromatin. Furthermore, cell extracts from DmSMC4 dsRNA-treated cells show significantly reduced topoisomerase II-dependent DNA decatenation activity in vitro. Nevertheless, DmSMC4-depleted chromosomes have centromeres and kinetochores that are able to segregate, although sister chromatid arms form extensive chromatin bridges during anaphase. These chromatin bridges do not result from inappropriate maintenance of sister chromatid cohesion by DRAD21, a subunit of the cohesin complex. Moreover, depletion of DmSMC4 prevents premature sister chromatid separation, caused by removal of DRAD21, allowing cells to exit mitosis with chromatin bridges. Our results suggest that condensin is required so that an axial chromatid structure can be organised where topoisomerase II can effectively promote sister chromatid resolution.

The genetic defect in ataxia-telangiectasia

The autosomal recessive human disorder ataxia-telangiectasia (A-T) was first described as a separate disease entity 40 years ago. It is a multisystem disease characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, radiosensitivity, predisposition to lymphoid malignancies and immunodeficiency, with defects in both cellular and humoral immunity. The pleiotropic nature of the clinical and cellular phenotype suggests that the gene product involved is important in maintaining stability of the genome but also plays a more general role in signal transduction. The chromosomal instability and radiosensitivity so characteristic of this disease appear to be related to defective activation of cell cycle checkpoints. Greater insight into the nature of the defect in A-T has been provided by the recent identification, by positional cloning, of the responsible gene, ATM. The ATM gene is related to a family of genes involved in cellular responses to DNA damage and/or cell cycle control. These genes encode large proteins containing a phosphatidylinositol 3-kinase domain, some of which have protein kinase activity. The mutations causing A-T completely inactivate or eliminate the ATM protein. This protein has been detected and localized to different subcellular compartments.

Hypersensitivity of lymphoblastoid lines derived from ataxia telangiectasia patients to the induction of chromosomal aberrations by etoposide (VP-16)

Mammalian DNA topoisomerase II represents the cellular target of many antitumor drugs, such as epipodophyllotoxin VP-16 (etoposide). The mechanism by which VP-16 exerts its cytotoxic and antineoplastic actions has not yet been firmly established, although the unique correlation between sensitivity to ionizing radiation and to topoisomerase II inhibitors suggest the involvement of DNA double-strand breaks. In the present study we analyzed the chromosomal sensitivity of lymphoblastoid cell lines derived from ataxia telangiectasia (AT) patients to low concentrations of the drug. Our results indicate that AT derived cells are hypersensitive to the clastogenic activity of VP-16 either when the drug is present for the whole duration of the cell cycle or specifically in the G2 phase, confirming that the induction of DNA double strand breaks, to which AT cells seem typically sensitive, could have an important role in the biological activity of VP-16.

Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.

Human cells (normal and ataxia telangiectasia) transfected with pR plasmid are hypersensitive to DNA strand-breaking agents

Ataxia telangiectasia (AT) cells are known to be hypersensitive to ionizing radiations and to drugs such as bleomycin and epipodophyllotoxin VP16, a topoisomerase II poison. Both of these produce DNA double-strand breaks even if through different mechanisms. In this work we analyzed the sensitivity to bleomycin and to epipodophyllotoxin of AT cells after transfection with pR plasmid. This plasmid, interacting with bacterial SOS repair pathways, expresses itself in mammalian cells conferring cell resistance to the SOS inducers UV and 4NQO and cell sensitivity to different drugs such as bleomycin. This effect is presumably due to the interaction of pR products with double-strand breaks. Our findings indicate that pR plasmid, in both AT lines tested (AT5BIVA fibroblasts and ATL6 lymphoblasts), expresses itself (increasing UV protection) and amplifies the already enhanced AT cell sensitivity to both bleomycin and VP16.

Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.

Aberrant DNA topoisomerase II activity, radioresistance and inherited susceptibility to cancer

Inherited susceptibility to a wide variety of neoplasias (Li-Fraumeni syndrome), has been shown in studies of one cancer-prone family, to have an intriguing association with an aberrant c-raf-1 gene and inheritance of a radioresistant phenotype in their non-cancerous skin fibroblasts. This association together with observations that DNA topoisomerases, when defective, can introduce errors into DNA and that these enzymes are perturbed in vitro by serine/threonine kinases similar to raf encoded proteins, prompted investigation of DNA topoisomerase activity of the family's fibroblasts. Since radioresistance was transferred to murine cells (NIH-3T3) when the aberrant c-raf-1 gene from this family was transfected, we also examined transformants containing this and other oncogenes. V-raf/c-myc and EJ-ras transformants were examined, the former because the family's skin fibroblasts also have 3-8-fold elevated myc expression (not apparently relevant to radioresistance) and the latter because ras, like raf, conveys radioresistance. The family members' fibroblasts and the three transfected murine lines, showed a similar perturbation of a spermidine and ATP-dependent DNA catenation activity (typical of DNA topoisomerase II). There was a significant positive correlation (r = 0.93; P = 0.0026) between the degree of activation of topoisomerase II and one measure of radioresistance (the Dq value). Relaxation of DNA supercoiling (topoisomerase I activity and other DNA nicking enzymes) was not abnormal. Cytotoxicity assays and evaluation of the influence of topoisomerase II inhibitors on DNA/protein complex formation, corroborated the existence of a qualitative topoisomerase II defect in the family's cells and transfectants. Although the contention that the qualitative topoisomerase II abnormalities observed here may be associated with malfunction is highly speculative, these findings may be relevant to the mechanism of oncogenesis, not only in this family, but with raf and ras type oncogenes.

Induction of DNA strand breaks and DNA-protein cross-links in normal human skin fibroblasts following exposure to 254 nm UV radiation

Levels of DNA strand breaks and DNA-protein cross-links (DPCs) were measured using the alkaline elution assay in normal human skin fibroblasts irradiated with 0-200 J m-2 of 254 nm UV radiation and incubated for 0-24 h. On incubation, the yields of both single-strand breaks (SSBs) and DPCs increased with similar kinetics and remained elevated. In addition, when SSBs were measured under conditions in which DPCs were not eliminated by treatment with proteinase K, a measurable yield of SSBs could not be detected. Hence, the SSBs that form in the UV-irradiated cells following incubation appear to be associated with the DPCs.

Abnormality in DNA-protein binding in ataxia-telangiectasia nuclear extracts

Anomalies in DNA replication, repair and recombination in ataxia-telangiectasia (A-T) point to a defect in structure or function of chromatin. In this study we have compared DNA-protein binding in nuclear extracts from control and A-T cells using two assay systems, filter-binding and DNA-accessibility. Interestingly, the extent of DNA protein binding over a range of protein concentration was significantly lower in A-T extracts. In addition the accessibility of the restriction enzyme Eco R1 to protein-bound plasmid was greater when A-T extracts were used. This is in keeping with the reduced binding observed in the filter-binding assay.

Enhanced sensitivity to camptothecin in ataxia-telangiectasia cells and its relationship with the expression of DNA topoisomerase I

The antitumour drug camptothecin (CPT) can trap covalently bound topoisomerase I-DNA intermediates as complexes which conceal single-strand scissions. In an attempt to evaluate the cytotoxic potential of these lesions in human cells we have measured: (1) cell cycle delay and cell killing by CPT in primary and transformed fibroblasts, and in lymphoblastoid lines derived from normal, X-ray sensitive ataxia-telangiectasia (A-T) and xeroderma pigmentosum (XP) donors; (2) the properties of sublines obtained by high-dose selection in CPT: (3) levels of drug-induced DNA strand scission in intact cells; (4) the cellular availability of extractable topoisomerase I. The drug induced a marked cell cycle block in G2 phase, the magnitude of the block being closely related to cell kill. XP group A cells showed normal sensitivity to CPT, whereas A-T derived cells were consistently hypersensitive (3-5 fold) in a manner which could not be related to a primary deficiency in topoisomerase I activity, abnormal capacity for complex formation or anomalies in the intracellular generation of DNA strand breaks. A CPT-resistant A-T subline had reduced topoisomerase I activity but retained the characteristic of hypersensitivity to X-radiation. The subline lost resistance upon in vitro passage with evidence that resistance was initially an unstable feature of a subpopulation of cells. The findings have implications for the role of topoisomerase I in the in vitro phenotype of A-T cells, and the contribution made by topoisomerase I-dependent damage to the cytotoxic action of CPT.

Reduced DNA topoisomerase II activity in ataxia-telangiectasia cells

Considerable evidence supports a defect at the level of chromatin structure or recognition of that structure in cells from patients with the human genetic disorder ataxia-telangiectasia. Accordingly, we have investigated the activities of enzymes that alter the topology of DNA in Epstein Barr Virus-transformed lymphoblastoid cells from patients with this syndrome. Reduced activity of DNA topoisomerase II, determined by unknotting of P4 phage DNA, was observed in partially purified extracts from 5 ataxia-telangiectasia cell lines. The levels of enzyme activity was reduced substantially in 4 of these cell lines and to a lesser extent in the other cell line compared to controls. DNA topoisomerase I, assayed by relaxation of supercoiled DNA, was found to be present at comparable levels in both cell types. Reduced activity of topoisomerase II in ataxia-telangiectasia is compatible with the molecular, cellular and clinical changes described in this syndrome.

Damage-resistant DNA synthesis in eukaryotes

The molecular basis of sensitivity of ionizing radiation and other damaging agents is not clearly defined in eukaryotes. While a large number of mutants have been described only a few have been demonstrated to have a defect in the repair of damage to DNA. An interesting characteristic of a sub-group of these mutants, in different species extending throughout the phylogenetic scale, is the presence of damage-resistant DNA synthesis. This phenomenon is observed in cells from individuals with the genetic disorder ataxia telangiectasia, in HeLa cells treated with fluorodeoxyuridine prior to UV irradiation, in mutants of the fungus Neurospora crassa, the slime mould Dictyostelium discoideum, the fruit fly Drosophila melanogaster and possibly in the "wasted" mouse mutant. In the case of ataxia telangiectasia sensitivity is only observed to ionizing radiation or radiomimetic chemicals whereas sensitivity to a wider spectrum of mutagens is reported for the lower eukaryotic mutants. In all cases a reduced inhibition of DNA synthesis is obtained after exposure to an agent to which the cell type is hypersensitive. It is unclear how damage-resistant DNA synthesis contributes to increased sensitivity in these cells, but is unlikely to be the major mechanism predisposing to radiation-induced cell death. The description of a derivative of an ataxia telangiectasia cell line with normal sensitivity to radiation but still maintaining resistant DNA synthesis partially uncouples radioresistant DNA synthesis and radiosensitivity. This paper is designed to review the phenomenon of damage-resistant DNA synthesis in a number of mutants.