Radiosensitivity and oxidative signalling in ataxia telangiectasia: an update

Proliferation in imaginal discs requires cell growth and is linked to patterning processes controlled by secreted cell-signalling molecules. To identify new genes involved in the control of cell proliferation we have screened a collection of P-lacW insertion mutants that result in lethality in the larval/pupal stages, and characterized a novel gene, patufet (ptuf). Inactivation of ptuf by a P element insertion in the 5' untranslated region leads to aberrant imaginal disc morphology characterized by a reduction in mass of discs and disorganization of disc cells where no folding or patterning can be detected. Moreover, apoptotic cells can be observed in these small and abnormal mutant discs. To examine the role of ptuf we have studied its clonal behaviour in genetic mosaics generated by mitotic recombination. The mutation causes reduced cell viability, smaller cell size and stops vein differentiation. Non-autonomous effects, such as abnormal differentiation of wild-type cells surrounding the clones, are also observed. We have cloned the ptuf gene of Drosophila melanogaster and found that it encodes a selenophosphate synthetase, which is the first identified in insects. Mutant flies transformed with the full-length cDNA show complete reversion of lethality and disc phenotype. Northern blot analysis and in situ hybridization indicate that the ptuf gene is expressed in imaginal discs as well as at different stages of development. The synthesis of selenoproteins by the selenophosphate synthetase, the role of selenoproteins in the maintenance of the oxidant/antioxidant balance of the cell and its possible implications in imaginal disc morphogenesis are discussed.     

Poly(ADP-ribose) polymerase activity is not affected in ataxia telangiectasia cells and knockout mice

Poly(ADP-ribose) polymerase (PARP) is a constitutive factor of the DNA damage surveillance network in dividing cells. Based on its capacity to bind to DNA strand breaks, PARP plays a regulatory role in their resolution in vivo. ATM belongs to a large family of proteins involved in cell cycle progression and checkpoints in response to DNA damage. Both proteins may act as sensors of DNA damage to induce multiple signalling pathways leading to activation of cell cycle checkpoints and DNA repair. To determine a possible relationship between PARP and ATM, we examined the PARP response in an ATM-null background. We demonstrated that ATM deficiency does not affect PARP activity in human cell lines or Atm-deficient mouse tissues, nor does it alter PARP activity induced by oxidative damage or gamma-irradiation. Our results support a model in which PARP and ATM could be involved in distinct pathways, both effectors transducing the damage signal to cell cycle regulators.     

Radiation-activated DNA-binding protein constitutively present in ataxia telangiectasia nuclei

We have recently described the appearance of a specific DNA-binding protein in nuclei from human cells exposed to ionizing radiation which was not detected in nuclear extracts from unperturbed cells (Singh, S. P., and Lavin, M. F. (1990) Mol. Cell. Biol. 10, 5279-5285). We report here a similar activity which is constitutively present in nuclei of both unirradiated and irradiated cells from patients with the human genetic disorder ataxia telangiectasia (A-T). Activity was present in unirradiated nuclear extracts from 3 A-T cell lines of different complementation groups, but was not detected or was present only at a low level in 4 controls. Active protein was detected in the cytoplasm of both cell types. Exposure to ionizing radiation did not change the amount of DNA binding activity in A-T nuclei but led to an increase in nuclei from 4 control cell lines. Purification of the binding activities from A-T nuclei and control cytoplasm was carried out by affinity chromatography, as described previously for control extracts (Teale, B., Singh, S. P., Khanna, K. K., Findik, D., and Lavin, M. F. (1992) J. Biol. Chem. 267, 10295-10301). Southwestern analysis and UV cross-linking confirmed the presence of a major DNA-binding species at 70 kDa in both cases with a minor binding activity at 47 kDa also evident. It was not possible to distinguish between the binding activities from A-T and control cells under different conditions, and phosphorylation was required for binding activity in both cases. Footprint analysis revealed that the same sequence was being recognized by the control and A-T proteins. The constitutive presence of a specific radiation-responsive DNA-binding protein in A-T cells may be indicative of a continuous state of stress in these cells.     

Evolution of chromosomal abnormalities in sequential cytogenetic studies of ataxia telangiectasia

Sequential cytogenetic studies of four patients with ataxia telangiectasia showed the progressive development of lymphocyte clones, each marked with a rearranged chromosome 14. Initial studies had shown random chromosomal breaks and rearrangements. Later studies in all patients showed nonrandom rearrangement of chromosome 14 with a breakpoint at 14q12 and with the distal segment translocated to either chromosome 14 or 7. The proportion of circulating lymphocytes carrying the marker tended to increase with time, accounting for the majority of the lymphocytes eventually in one case. The marked lymphocyte clones evolved further, as a result of loss of the small centric portions of the rearranged chromosome 14 (14pter leads to 14q12). Perhaps the abnormal clones in ataxia telangiectasia escape immunologic surveillance and flourish in an immunologically impaired environment. Subsequent to the loss of the centric portion of the rearranged chromosome 14, the cells may acquire additional capabilities that enhance malignant transformation.     

High frequency and error-prone DNA recombination in ataxia telangiectasia cell lines

The only specific DNA repair defect found in ataxia telangiectasia (A-T) cells is mis-repair of cleaved DNA. In this report we measured DNA recombination, given its role in DNA repair and genetic instability. Using plasmids containing selectable reporter genes, we found a higher frequency of both chromosomal recombination (>100 times) and extra-chromosomal recombination (27 times) in SV40-transformed A-T cell lines compared with in an SV40-transformed normal fibroblast cell line. Southern analysis of single A-T colonies exhibiting post-integration recombination revealed that 24/27 had undergone aberrant rearrangements; recombination in normal fibroblast colonies was achieved by gene conversion in 8/11 clones and 10/11 clones showed unchanged copies of the plasmid. Using co-transfection of two integrating plasmids, each containing a separate deletion in the xgprt reporter gene, the 27 times difference in extra-chromosomal recombination was found when the plasmids were cleaved at a distance from the reporter gene. When the plasmids were cleaved within the reporter gene, the co-transfection frequency was reduced in A-T, but was increased in normal cells. We conclude that A-T cell lines have not only a high frequency chromosomal and extra-chromosomal recombination, but also exhibit error-prone recombination of cleaved DNA.     

Defective control of apoptosis, radiosensitivity, and spindle checkpoint in ataxia telangiectasia

We examined the regulation of apoptosis, radiosensitivity, and spindle checkpoint in response to DNA-damaging agents in ataxia telangiectasia (AT)-derived lymphoblastoid cell lines (AT-LCLs), which lack AT mutated (ATM) protein expression. In addition to the previous findings that AT-LCLs are defective in regulation of cell cycle at the G1, S, and G2-M checkpoints in response to X-ray irradiation (X-IR) and are highly sensitive to X-IR (J. Biol. Chem., 271: 20486-20493, 1996), we showed for the first time that AT-LCLs were defective in X-IR-associated spindle checkpoint control. The cells were also resistant to early apoptosis as much as LCLs derived from patients with Li-Fraumeni syndrome (LFS-LCLs). Terminal deoxynucleotidyl transferase-mediated nick end labeling assay of LCLs, however, demonstrated a significant increase in apoptotic cells among AT-LCLs cultured over a longer period after X-IR. These findings were in contrast to those of LFS-LCL, which showed very little increase in terminal deoxynucleotidyl transferase-mediated nick end labeling-positive population, even in cells with hyperploidy. Thus, although early apoptosis and cell cycle controls in response to DNA damage are disrupted in both ATM and p53 mutations, cells from AT patients are much more susceptible to late-onset apoptosis than those of LFS. These differences may depend on the level of accumulation of DNA damage and/or threshold that triggers late-onset cell death in ATM or p53 mutations. Our findings allow a better understanding of the role of ATM in p53-dependent and independent signal transduction pathways in response to DNA damaging agents.     

Possible control of cell death pathways in ataxia telangiectasia. A case report

Peripheral blood monocytes (PBMCs) obtained from a boy with the neuroimmunodegenerative syndrome of ataxia telangiectasia (AT) failed to aggregate or replicate efficiently when mitogenically activated under serum-depleted conditions. These cells rapidly swelled, then slowly shrank, and flattened as they excreted vesicles containing chromatin. This accelerated cell death with loss of homoadhesiveness could be prevented in vitro in most of the homozygous PBMCs by adding large amounts of autologous serum or by adding mixtures of Th1 cytokines, serum factors, and redox agents. However, even in high-serum media containing added cytokines, 20-30% of the homozygous PBMCs quickly flattened, produced minicells, and died. Since the defective functions of the human ataxia-telangiectasia nuclear kinase gene (ATM) could be bypassed in vitro in these defective AT PMBCs by addition of appropriate cytokines and redox survival factors, it may be possible to slow the progressive losses of ATM-deficient lymphoid cells seen in vivo. Since the neuronal degeneration in AT, as seen in the retrovirus-induced neuroimmunodegenerative syndromes, may also be a consequence of impairment of the central and peripheral immune system, it may become possible to prevent the neurodegeneration in AT by using signaling therapies that upregulate the ATM-induced signal deficiencies in the developing immune system.     

Hypersensitivity of ataxia telangiectasia fibroblasts to ionizing radiation is associated with a repair deficiency of DNA double-strand breaks

Amphiregulin (Ar) is an EGF receptor ligand that functions to modulate the growth of both normal and malignant epithelial cells. We asked whether mouse preimplantation embryos express Ar, and if so, what the function of Ar is during preimplantation development. We used RT-PCR to show expression of Ar mRNA in mouse blastocysts, and using a polyclonal anti-Ar antibody and indirect immunofluorescence, we detected the presence of Ar protein in morula- and blastocyst-stage embryos. Ar protein was present in both the cytoplasm and nucleus in both morulae- and blastocyst-stage embryos, which is similar to Ar distribution in other cell types. Embryos cultured in Ar developed into blastocysts more quickly and also exhibited increased cell numbers compared to control embryos. In addition, 4-cell stage embryos cultured in an antisense Ar phosphorothioate-modified oligodeoxynucleotide (S-oligo) for 48 hr exhibited slower rates of blastocyst formation and reduced embryo cell numbers compared to embryos exposed to a random control S-oligo. TGF-alpha significantly improved blastocyst formation, but not cell numbers, for embryos cultured in the antisense Ar S-oligo. From these observations, we propose that Ar may function as an autocrine growth factor for mouse preimplantation embryos by promoting blastocyst formation and embryo cell number. We also propose that blastocyst formation is stimulated by Ar and TGF-alpha, while Ar appears to exert a greater stimulatory effect on cell proliferation than does TGF-alpha in these embryos.     

KARP-1 is induced by DNA damage in a p53- and ataxia telangiectasia mutated-dependent fashion

The KARP-1 (Ku86 Autoantigen Related Protein-1) gene, which is expressed from the human Ku86 autoantigen locus, appears to play a role in mammalian DNA double-strand break repair as a regulator of the DNA-dependent protein kinase complex. Here we demonstrate that KARP-1 gene expression is significantly up-regulated following exposure of cells to DNA damage. KARP-1 mRNA induction was completely dependent on the ataxia telangiectasia and p53 gene products, consistent with the presence of a p53 binding site within the second intron of the KARP-1 locus. These observations link ataxia telangiectasia, p53, and KARP-1 in a common pathway.     

Heterozygosity for mutations in the ataxia telangiectasia gene is not a major cause of radiotherapy complications in breast cancer patients

Of patients being treated by radiotherapy for cancer, a small proportion develop marked long-term radiation damage. It is believed that this is due, at least in part, to intrinsic individual differences in radiosensitivity, but the underlying mechanism is unknown. Individuals affected by the recessive disease ataxia telangiectasia (AT) exhibit extreme sensitivity to ionizing radiation. Cells from such individuals are also radiosensitive in in vitro assays, and cells from AT heterozygotes are reported to show in vitro radiosensitivity at an intermediate level between homozygotes and control subjects. In order to examine the possibility that a defect in the ATM gene may account for a proportion of radiotherapy complications, 41 breast cancer patients developing marked changes in breast appearance after radiotherapy and 39 control subjects who showed no clinically detectable reaction after radiotherapy were screened for mutations in the ATM gene. One out of 41 cases showing adverse reactions was heterozygous for a mutation (insertion A at NT 898) that is predicted to generate a truncated protein of 251 amino acids. No truncating mutations were detected in the control subjects. On the basis of this result, the estimated percentage (95% confidence interval) of AT heterozygous patients in radiosensitive cases was 2.4% (0.1-12.9%) and in control subjects (0-9.0%). We conclude that ATM gene defects are not the major cause of radiotherapy complications in women with breast cancer.     

Partial rescue of the prophase I defects of Atm-deficient mice by p53 and p21 null alleles

Patients with the human disorder ataxia-telangiectasia (A-T; refs 1,2) and Atm-deficient mice have a pleiotropic phenotype that includes infertility. Here we demonstrate that male gametogenesis is severely disrupted in Atm-deficient mice in the earliest stages of meiotic prophase I, resulting in apoptotic degeneration. Atm is required for proper assembly of Rad51 onto the chromosomal axial elements during meiosis. In addition, p53, p21 and Bax are elevated in testes from Atm-deficient mice. To determine whether these elevated protein levels are important factors in the meiotic disruption of Atm-deficient mice, we analysed the meiotic phenotype of Atm/p53 or Atm/p21 double mutants. In these double mutants, meiosis progressed to later stages but was only partly rescued. Assembly of Rad51 foci on axial elements remained defective, and gametogenesis proceeded only to pachytene of prophase I. Previous results demonstrated that mice homozygous for a null mutation in Rad51 (ref. 6) display an early embryonic lethal phenotype that can be partly rescued by removing p53 and/or p21. Because Atm-deficient mice are viable but completely infertile, our studies suggest that the Rad51 assembly defects and elevated levels of p53, p21 and Bax represent tissue-specific responses to the absence of Atm.     

Chromosomes and causation of human cancer and leukemia. XLVIII. T-cell acute leukemia in ataxia telangiectasia

Cytogenetic and immunologic studies were performed on the cells of an 18-year-old female with ataxia telangiectasia (AT) associated with acute lymphocytic leukemia (ALL). At the onset of the leukemia 15.4% of peripheral blood cells stimulated with phytohemagglutinin (PHA) contained a tandem translocation of the long arm of chromosome #14, i.e., t(14;14). To ascertain if these karyotypically abnormal cells and the leukemic cells had a common lineage, chromosome analyses were performed on bone marrow cells. Examination of the marrow cells on the seven occasions when leukemic cells were present in the marrow, including times when they were predominant, showed only a normal karyotype without the presence of t(14;14). However, an abnormal clone, which had the karyotype 45,XX,-9,t(9;6)(q12;q13), was identified in the marrow cells on the last examination during the terminal phase of the leukemia. Immunologically, the ALL was classified as an atypical type which had characteristics in common with certain T-cell subsets. We suggest that the malignant cells did not originate from the preexisting cells with a tandem duplication of the 14q.     

Induction of p53 protein by gamma radiation in lymphocyte lines from breast cancer and ataxia telangiectasia patients

Exposure of human cells to gamma-radiation causes levels of the tumour-suppressor nuclear protein p53 to increase in temporal association with the decrease in replicative DNA synthesis. Cells from patients with the radiosensitive and cancer-prone disease ataxia telangiectasia (AT) exhibit radioresistant DNA synthesis and show a reduced or delayed gamma-radiation-induced increase in p53 protein levels. We have used Western immunoblotting with semiquantitative densitometry to examine the gamma-radiation-induced levels of p53 protein in 57 lymphoblastoid cell lines (LCLs) derived from patients with AT, carriers of the AT gene, breast cancer patients and normal donors. We confirm the previously reported reduced induction in AT homozygote LCLs (n = 8) compared with normal donor LCLs (n = 17, P = 0.01). We report that AT heterozygote LCLs (n = 5) also have a significantly reduced p53 induction when compared with LCLs from normal donors (n = 17, P = 0.02). The response of breast cancer patient cells was not significantly different from normal donor cells but 18% (5/27) had a p53 response in the AT heterozygote range (95% confidence interval) compared with only 6% (1/17) of the normal donor cells. We found no significant correlation between p53 induction and cellular radiosensitivity in LCLs from breast cancer patients. These methods may be useful in identifying individuals at greater risk of the DNA-damaging effects of ionising radiation.     

Educated insane: a nineteenth-century psychiatric paradigm

In the 1830s the time-honored notion that excess study could lead to madness underwent a significant change in America. Under the influence of Enlightenment pedagogy and phrenology, influential superintendents like Amariah Brigham and Isaac Ray feared that the "unnatural" overstimulation of children in schools would ruin their development. In the second half of the nineteenth century, as belief in environmental determinism waned and assumptions about what is "natural" changed, this psychiatric etiology was debated; then, overthrown. By the turn of the century, education was thought to aid, not harm, the mentally ill.