Critical role of the POT1 OB domain in maintaining genomic stability

Oligonucleotide/oligosaccharide-binding (OB) domain-containing proteins have been identified as critical for telomere maintenance, DNA repair, transcription and other DNA metabolism processes. Protection of telomere 1 (POT1), a telomere binding protein, has an OB domain like single-strand binding protein (SSB1). In this issue of Oncogene, Gu et al. present evidence that POT1, like SSB1, is required to maintain genomic stability. This work, in conjunction with results from previous investigators, highlights the importance of POT1 in telomere metabolism. Inactivation of POT1 telomere protective functions in mouse models lacking p53 expression in the breast epithelium unleashes a torrent of DNA damage responses (DDRs) at the telomeres, culminating in karyotypic alterations with massive arrays of telomere fusions. Therefore, POT1 is not only required to promote telomere homeostasis, but also plays an essential role in maintaining a stable genome.

Telomeres, histone code, and DNA damage response

Genomic stability is maintained by telomeres, the end terminal structures that protect chromosomes from fusion or degradation. Shortening or loss of telomeric repeats or altered telomere chromatin structure is correlated with telomere dysfunction such as chromosome end-to-end associations that could lead to genomic instability and gene amplification. The structure at the end of telomeres is such that its DNA differs from DNA double strand breaks (DSBs) to avoid nonhomologous end-joining (NHEJ), which is accomplished by forming a unique higher order nucleoprotein structure. Telomeres are attached to the nuclear matrix and have a unique chromatin structure. Whether this special structure is maintained by specific chromatin changes is yet to be thoroughly investigated. Chromatin modifications implicated in transcriptional regulation are thought to be the result of a code on the histone proteins (histone code). This code, involving phosphorylation, acetylation, methylation, ubiquitylation, and sumoylation of histones, is believed to regulate chromatin accessibility either by disrupting chromatin contacts or by recruiting non-histone proteins to chromatin. The histone code in which distinct histone tail-protein interactions promote engagement may be the deciding factor for choosing specific DSB repair pathways. Recent evidence suggests that such mechanisms are involved in DNA damage detection and repair. Altered telomere chromatin structure has been linked to defective DNA damage response (DDR), and eukaryotic cells have evolved DDR mechanisms utilizing proficient DNA repair and cell cycle checkpoints in order to maintain genomic stability. Recent studies suggest that chromatin modifying factors play a critical role in the maintenance of genomic stability. This review will summarize the role of DNA damage repair proteins specifically ataxia-telangiectasia mutated (ATM) and its effectors and the telomere complex in maintaining genome stability.

Regulation of telomere movement by telomere chromatin structure

Beyond their role in replication and chromosome end capping, telomeres are also thought to function in meiotic chromosome pairing, meiotic and mitotic chromosome segregation as well as in nuclear organization. Observations in both somatic and meiotic cells suggest that the positioning of telomeres within the nucleus is highly specific and believed to be dependent mainly on telomere interactions with the nuclear envelope either directly or through chromatin interacting proteins. Although little is known about the mechanism of telomere clustering, some studies show that it is an active process. Recent data have suggested a regulatory role for telomere chromatin structure in telomere movement. This review will summarize recent studies on telomere interactions with the nuclear matrix, telomere chromatin structure and factors that modify telomere chromatin structure as related to regulation of telomere movement.

Mutations that affect meiosis in male mice influence the dynamics of the mid-preleptotene and bouquet stages

Meiosis pairs and segregates homologous chromosomes and thereby forms haploid germ cells to compensate the genome doubling at fertilization. Homologue pairing in many eukaryotic species depends on formation of DNA double strand breaks (DSBs) during early prophase I when telomeres begin to cluster at the nuclear periphery (bouquet stage). By fluorescence in situ hybridization criteria, we observe that mid-preleptotene and bouquet stage frequencies are altered in male mice deficient for proteins required for recombination, ubiquitin conjugation and telomere length control. The generally low frequencies of mid-preleptotene spermatocytes were significantly increased in male mice lacking recombination proteins SPO11, MEI1, MLH1, KU80, ubiquitin conjugating enzyme HR6B, and in mice with only one copy of the telomere length regulator Terf1. The bouquet stage was significantly enriched in Atm(-/-), Spo11(-/-), Mei1(m1Jcs/m1Jcs), Mlh1(-/-), Terf1(+/-) and Hr6b(-/-) spermatogenesis, but not in mice lacking recombination proteins DMC1 and HOP2, the non-homologous end-joining DNA repair factor KU80 and the ATM downstream effector GADD45a. Mice defective in spermiogenesis (Tnp1(-/-), Gmcl1(-/-), Asm(-/-)) showed wild-type mid-preleptotene and bouquet frequencies. A low frequency of bouquet spermatocytes in Spo11(-/-)Atm(-/-) spermatogenesis suggests that DSBs contribute to the Atm(-/-)-correlated bouquet stage exit defect. Insignificant changes of bouquet frequencies in mice with defects in early stages of DSB repair (Dmc1(-/-), Hop2(-/-)) suggest that there is an ATM-specific influence on bouquet stage duration. Altogether, it appears that several pathways influence telomere dynamics in mammalian meiosis.

Localized versus regional hyperthermia: comparison of xenotransplants treated with a small animal ultrasound system and waterbath limb immersion

The response of xenotransplants were compared with waterbath immersion vs focal ultrasound (US) hyperthermia using tumour growth delay, immunhistochemistry and histopathology assays. Waterbath hyperthermia was performed by limb immersion. Precautions were taken to minimize total body heating by surrounding the mouse with plastic insulators. Thermometry was performed with clinical-grade, 20-gauge needle thermocouples and monitored with a Labthermics unit. Significant differences in cytotoxicity between ultrasound and waterbath treatment of tumors at 43 degrees C were observed as determined by TUNNEL assay. Conversely, contralateral (non-treated) tumours in animals treated with similar temperature demonstrated no significant differences between modalities. Western blot analysis revealed increased hsp70 induction at 43 degrees C in waterbath vs focal ultrasound hyperthermia. Comparison of tumour growth delay between tumours heated with waterbath vs ultrasound at 43 degrees C but not at 41 degrees C revealed significant differences. This is the first study comparing localized vs regional hyperthermia using the small animal ultrasound system (SAHUS) delivery system. Consistent ultrasound hyperthermia can be achieved throughout a xenotransplant. At equivalent temperature of 43 degrees C for 60?min, waterbath hyperthermia demonstrated greater local response vs ultrasound hyperthermia.

The role of ATM in telomere structure and function

Ataxia telangiectasia (AT) is a rare human autosomal recessive disorder with a wide variety of phenotypic manifestations. AT patients are cancer prone and hypersensitive to ionizing radiation. Cells derived from AT patients require higher levels of serum factors, exhibit cytoskeletal defects, and undergo premature senescence in culture. The gene responsible for AT is ATM (ataxia-telangiectasia mutated), and its product has been implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination, and cell cycle control. Because of the homology of the human ATM gene to the TEL1 and rad3 genes of yeast, it has been suggested that mutations in ATM could lead to defective telomere maintenance. The ATM gene product influences chromosome end associations, telomere length, and telomere clustering. The defective telomere metabolism in AT cells could be due to altered interactions between the telomeres and the nuclear matrix. These interactions were studied in nuclear matrix halos before and after irradiation. Altered telomere-nuclear matrix interactions were observed in cells derived from individuals with AT. AT cells also had different nucleosomal periodicity in their telomeres from normal cells. Both telomere-nuclear matrix interactions and nucleosomal periodicity were altered by treatment of primary AT fibroblasts with ionizing radiation. This effect was not observed in cells derived from normal individuals. A link was also found between altered telomere-nuclear matrix interactions, aberrant telomere clustering, and gonadal atrophy. The telomere defect was not corrected by the ectopic expression of the catalytic subunit of telomerase (TERT). Since alteration of the yeast telomere chromatin structure is known to influence gene expression, we compared expressed sequence tags (ESTs) of Atm-null mouse cells and normal mouse cells. Several ESTs were found to be aberrantly expressed in Atm-null mouse cells. This paper summarizes our recent publications and presents some new data on the influence of ATM on telomere metabolism.

hSIR2(SIRT1) functions as an NAD-dependent p53 deacetylase

DNA damage-induced acetylation of p53 protein leads to its activation and either growth arrest or apoptosis. We show here that the protein product of the gene hSIR2(SIRT1), the human homolog of the S. cerevisiae Sir2 protein known to be involved in cell aging and in the response to DNA damage, binds and deacetylates the p53 protein with a specificity for its C-terminal Lys382 residue, modification of which has been implicated in the activation of p53 as a transcription factor. Expression of wild-type hSir2 in human cells reduces the transcriptional activity of p53. In contrast, expression of a catalytically inactive hSir2 protein potentiates p53-dependent apoptosis and radiosensitivity. We propose that hSir2 is involved in the regulation of p53 function via deacetylation.

Spontaneously immortalized cell lines obtained from adult Atm null mice retain sensitivity to ionizing radiation and exhibit a mutational pattern suggestive of oxidative stress

The study of Ataxia-telangiectasia (A-T) has benefited significantly from mouse models with knockout mutations for the Atm (A-T mutation) locus. While these models have proven useful for in vivo studies, cell cultures from Atm null embryos have been reported to grow poorly and then senesce. In this study, we initiated primary cultures from adult ears and kidneys of Atm homozygous mice and found that these cultures immortalized readily without loss of sensitivity to ionizing radiation and other Atm related cell cycle defects. A mutational analysis for loss of expression of an autosomal locus showed that ionizing radiation had a mutagenic effect. Interestingly, some spontaneous mutants exhibited a mutational pattern that is characteristic of oxidative mutagenesis. This result is consistent with chronic oxidative stress in Atm null cells. In total, the results demonstrate that permanent cell lines can be established from the tissues of adult mice homozygous for Atm and that these cell lines will exhibit expected and novel consequences of this deficiency.

Characterization of ataxia telangiectasia fibroblasts with extended life-span through telomerase expression

Ataxia-telangiectasia (A-T) is an autosomal recessive disease characterized by progressive cerebellar degeneration, immunodeficiencies, genomic instability and gonadal atrophy. A-T patients are hypersensitive to ionizing radiation and have an elevated cancer risk. Cells derived from A-T patients require higher levels of serum factors, exhibit cytoskeletal defects and undergo premature senescence in culture. We show here that expression of the catalytic subunit of telomerase (hTERT) in primary A-T patient fibroblasts can rescue the premature senescence phenotype. Ectopic expression of hTERT does not rescue the radiosensitivity or the telomere fusions in A-T fibroblasts. The hTERT+AT cells also retain the characteristic defects in cell-cycle checkpoints, and show increased chromosome damage before and after ionizing radiation. Although A-T patients have an increased susceptibility to cancer, the expression of hTERT in A-T fibroblasts does not stimulate malignant transformation. These immortalized A-T cells provide a more stable cell system to investigate the molecular mechanisms underlying the cellular phenotypes of Ataxia-telangiectasia.

Ataxia-telangiectasia: chronic activation of damage-responsive functions is reduced by alpha-lipoic acid

Cells from patients with the genetic disorder ataxia-telangiectasia (A-T) are hypersensitive to ionizing radiation and radiomimetic agents, both of which generate reactive oxygen species capable of causing oxidative damage to DNA and other macromolecules. We describe in A-T cells constitutive activation of pathways that normally respond to genotoxic stress. Basal levels of p53 and p21(WAF1/CIP1), phosphorylation on serine 15 of p53, and the Tyr15-phosphorylated form of cdc2 are chronically elevated in these cells. Treatment of A-T cells with the antioxidant alpha-lipoic acid significantly reduced the levels of these proteins, pointing to the involvement of reactive oxygen species in their chronic activation. These findings suggest that the absence of functional ATM results in a mild but continuous state of oxidative stress, which could account for several features of the pleiotropic phenotype of A-T.

Adenovirus-mediated antisense ATM gene transfer sensitizes prostate cancer cells to radiation

Treatment failure after radiation therapy of prostate cancer (PC) could be a significant problem. Our objective is to design genetic radiosensitizing strategies for the treatment of PC. Cells from individuals with the genetic disorder ataxia telangiectasia (AT) are hypersensitive to ionizing radiation. Therefore, we examined whether attenuation of the AT gene product, AT mutated (ATM), in PC cells could result in an increased intrinsic radiosensitivity. A p53-mutant PC cell line, PC-3 was infected with adenoviral vectors, expressing antisense ATM RNA to various domains of the ATM gene. Immunoblot analyses of cellular extracts from antisense ATM-transfected PC-3 cells showed attenuated expression of the ATM protein within 2 days of viral infection. Compared with cells infected with an adeno-beta-galactosidase vector, antisense ATM-transfected PC-3 cells showed aberrant control of S-phase cell-cycle checkpoints after exposure to ionizing radiation. Under these conditions, the intrinsic radiosensitivity of the PC-3 cells was enhanced. Consequently antisense ATM gene therapy could serve as a paradigm for strategies that target the cellular survival mechanisms of an irradiated tumor cell and may provide therapeutic benefit to patients undergoing radiation therapy for PC.

Inactivation of 14-3-3sigma influences telomere behavior and ionizing radiation-induced chromosomal instability

Telomeres are complexes of repetitive DNA sequences and proteins constituting the ends of linear eukaryotic chromosomes. While these structures are thought to be associated with the nuclear matrix, they appear to be released from this matrix at the time when the cells exit from G(2) and enter M phase. Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. The 14-3-3sigma gene has been reported to be a checkpoint control gene, since it promotes G(2) arrest following DNA damage. Here we demonstrate that inactivation of this gene influences genome integrity and cell survival. Analyses of chromosomes at metaphase showed frequent losses of telomeric repeat sequences, enhanced frequencies of chromosome end-to-end associations, and terminal nonreciprocal translocations in 14-3-3sigma(-/-) cells. These phenotypes correlated with a reduction in the amount of G-strand overhangs at the telomeres and an altered nuclear matrix association of telomeres in these cells. Since the p53-mediated G(1) checkpoint is operative in these cells, the chromosomal aberrations observed occurred preferentially in G(2) after irradiation with gamma rays, corroborating the role of the 14-3-3sigma protein in G(2)/M progression. The results also indicate that even in untreated cycling cells, occasional chromosomal breaks or telomere-telomere fusions trigger a G(2) checkpoint arrest followed by repair of these aberrant chromosome structures before entering M phase. Since 14-3-3sigma(-/-) cells are defective in maintaining G(2) arrest, they enter M phase without repair of the aberrant chromosome structures and undergo cell death during mitosis. Thus, our studies provide evidence for the correlation among a dysfunctional G(2)/M checkpoint control, genomic instability, and loss of telomeres in mammalian cells.

Meiotic telomere distribution and Sertoli cell nuclear architecture are altered in Atm- and Atm-p53-deficient mice

The ataxia telangiectasia mutant (ATM) protein is an intrinsic part of the cell cycle machinery that surveys genomic integrity and responses to genotoxic insult. Individuals with ataxia telangiectasia as well as Atm(-/-) mice are predisposed to cancer and are infertile due to spermatogenesis disruption during first meiotic prophase. Atm(-/-) spermatocytes frequently display aberrant synapsis and clustered telomeres (bouquet topology). Here, we used telomere fluorescent in situ hybridization and immunofluorescence (IF) staining of SCP3 and testes-specific histone H1 (H1t) to spermatocytes of Atm- and Atm-p53-deficient mice and investigated whether gonadal atrophy in Atm-null mice is associated with stalling of telomere motility in meiotic prophase. SCP3-H1t IF revealed that most Atm(-/-) p53(-/-) spermatocytes degenerated during late zygotene, while a few progressed to pachytene and diplotene and some even beyond metaphase II, as indicated by the presence of a few round spermatids. In Atm(-/-) p53(-/-) meiosis, the frequency of spermatocytes I with bouquet topology was elevated 72-fold. Bouquet spermatocytes with clustered telomeres were generally void of H1t signals, while mid-late pachytene and diplotene Atm(-/-) p53(-/-) spermatocytes displayed expression of H1t and showed telomeres dispersed over the nuclear periphery. Thus, it appears that meiotic telomere movements occur independently of ATM signaling. Atm inactivation more likely leads to accumulation of spermatocytes I with bouquet topology by slowing progression through initial stages of first meiotic prophase and an ensuing arrest and demise of spermatocytes I. Sertoli cells (SECs), which contribute to faithful spermatogenesis, in the Atm mutants were found to frequently display numerous heterochromatin and telomere clusters-a nuclear topology which resembles that of immature SECs. However, Atm(-/-) SECs exhibited a mature vimentin and cytokeratin 8 intermediate filament expression signature. Upon IF with ATM antibodies, we observed ATM signals throughout the nuclei of human and mouse SECs, spermatocytes I, and haploid round spermatids. ATM but not H1t was absent from elongating spermatid nuclei. Thus, ATM appears to be removed from spermatid nuclei prior to the occurrence of DNA nicks which emanate as a consequence of nucleoprotamine formation.

Regulation of DNA-dependent protein kinase activity by ionizing radiation-activated abl kinase is an ATM-dependent process

Ionizing radiation (IR) treatment results in activation of the nonreceptor tyrosine kinase c-Abl because of phosphorylation by ATM. In vitro evidence indicates that DNA-dependent protein kinase (DNA-PK) can also phosphorylate and thus potentially activate Abl kinase activity in response to IR exposure. To unravel the role of ATM and DNA-PK in the activation of Abl, we assayed Abl, ATM, and DNA-PK activity in ATM- and DNA-PKcs-deficient cells after irradiation. Our results show that despite the presence of higher than normal levels of DNA-PK kinase activity, c-Abl fails to become activated after IR exposure in ATM-deficient cells. Conversely, normal activation of both ATM and c-Abl occurs in DNA-PKcs-deficient cells, indicating that ATM but not DNA-PK is required for activation of Abl in response to IR treatment. Moreover, activation of Abl kinase activity by IR correlates well with activation of ATM activity in all phases of the cell cycle. These results indicate that ATM is primarily responsible for activation of Abl in response to IR exposure in a cell cycle-independent fashion. Examination of DNA-PK activity in response to IR treatment in Abl-deficient cells expressing mutant forms of Abl or in normal cells exposed to an inhibitor of Abl suggests an in vivo role for Abl in the down-regulation of DNA-PK activity. Collectively, these results suggest a convergence of the ATM and DNA-PK pathways in the cellular response to IR through c-Abl kinase.

Influence of ATM function on interactions between telomeres and nuclear matrix

The ATM (ataxia telangiectasia mutated) gene product has been implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination, and cell cycle control. The human ATM protein shows similarity to several yeast and mammalian proteins involved in meiotic recombination and cell cycle progression. Because of the homology of the human ATM gene to the TEL1 and rad3 genes of yeast, it has been suggested that mutations in ATM could lead to defective telomere maintenance. Recently, we have shown that the ATM gene product, which is defective in the cancer-prone disorder ataxia telangiectasia (AT), influences chromosome end associations and telomere length. A possible hypothesis explaining these results is that the defective telomere metabolism in AT cells is due to altered interactions between the telomeres and the nuclear matrix. These interactions were examined in nuclear matrix halos prior to and after irradiation. A difference was observed in the ratio of soluble and matrix-associated telomeric DNA between cells derived from AT and normal individuals. Treatment with ionizing radiation affected the ratio of soluble and matrix-associated telomeric DNA only in the AT cells. To test the hypothesis that the ATM gene product is involved in interactions between telomeres and the nuclear matrix, such interactions were examined in human cells expressing either a dominant-negative effect or complementation of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper motif mimics the altered interactions of telomere and nuclear matrix seen in AT cells. Fibroblasts from AT individuals transfected with a wild-type ATM gene had corrected telomere-nuclear matrix interactions. In experiments designed to determine whether there is a link between the altered telomere-nuclear matrix interactions and defective telomere movement and clustering, a significant difference was observed in the ratio of soluble compared to matrix-associated telomeric DNA sequences in meiocytes of Atm(-/-) and control mice. These results suggest that the ATM gene influences the interactions between telomeres and the nuclear matrix and that alterations in telomere chromatin could be at least partly responsible for the pleiotropic phenotypes of the ATM gene. This paper summarizes our recent publications on the influence of inactivation of ATM on the interaction of telomeres with nuclear matrix in somatic and germ cells.

High frequency of hypermethylation at the 14-3-3 sigma locus leads to gene silencing in breast cancer

Expression of 14-3-3 final sigma (final sigma) is induced in response to DNA damage, and causes cells to arrest in G(2). By SAGE (serial analysis of gene expression) analysis, we identified final sigma as a gene whose expression is 7-fold lower in breast carcinoma cells than in normal breast epithelium. We verified this finding by Northern blot analysis. Remarkably, final sigma mRNA was undetectable in 45 of 48 primary breast carcinomas. Genetic alterations at final sigma such as loss of heterozygosity were rare (1/20 informative cases), and no mutations were detected (0/34). On the other hand, hypermethylation of CpG islands in the final sigma gene was detected in 91% (75/82) of breast tumors and was associated with lack of gene expression. Hypermethylation of final sigma is functionally important, because treatment of final sigma-non-expressing breast cancer cell lines with the drug 5-aza-2'-deoxycytidine resulted in demethylation of the gene and synthesis of final sigma mRNA. Breast cancer cells lacking final sigma expression showed increased number of chromosomal breaks and gaps when exposed to gamma-irradiation. Therefore, it is possible that loss of final sigma expression contributes to malignant transformation by impairing the G(2) cell cycle checkpoint function, thus allowing an accumulation of genetic defects. Hypermethylation and loss of final sigma expression are the most consistent molecular alterations in breast cancer identified so far.

Regulation of the hTERT telomerase catalytic subunit by the c-Abl tyrosine kinase

BACKGROUND

Telomeres consist of repetitive (TTAGGG) DNA sequences that are maintained by the multisubunit telomerase ribonucleoprotein. Telomerase consists of an RNA, which serves as template for the sequence tracts, and a catalytic subunit that functions in reverse transcription of the RNA template. Cloning and characterization of the human catalytic subunit of telomerase (hTERT) has supported a role in cell transformation. How telomerase activity is regulated, however, is largely unknown.

RESULTS

We show here that hTERT associates directly with the c-Abl protein tyrosine kinase. We also found that c-Abl phosphorylates hTERT and inhibits hTERT activity. Moreover, our findings demonstrate that exposure of cells to ionizing radiation induces tyrosine phosphorylation of hTERT by a c-Abl-dependent mechanism. The functional significance of the c-Abl-hTERT interaction is supported by the demonstration that cells deficient in c-Abl show telomere lengthening.

CONCLUSIONS

The ubiquitously expressed c-Abl tyrosine kinase is activated by DNA double-strand breaks. Our finding of telomere lengthening in c-Abl-deficient cells and the functional interactions between c-Abl and hTERT support a role for c-Abl in the regulation of telomerase function.

Antisense ATM gene therapy: a strategy to increase the radiosensitivity of human tumors

Atm, the gene mutated in ataxia-telangiectasia (AT) patients, is an essential component of the signal transduction pathway that responds to DNA damage due to ionizing radiation (IR). We attenuated ATM protein expression in human glioblastoma cells by expressing antisense RNA to a functional domain of the atm gene. While ATM expression decreased, constitutive expression of p53 and p21 increased. Irradiated ATM-attenuated cells failed to induce p53, demonstrated radioresistant DNA synthesis, and increased radiosensitivity. Antisense-ATM gene therapy in conjunction with radiation therapy may provide a novel strategy for the treatment of cancer.

Ionizing radiation activates the ATM kinase throughout the cell cycle

The ATM protein kinase is a critical intermediate in a number of cellular responses to ionizing irradiation (IR) and possibly other stresses. ATM dysfunction results in abnormal checkpoint responses in multiple phases of the cell cycle, including G1, S and G2. Though downstream targets of the ATM kinase are still being elucidated, it has been demonstrated that ATM acts upstream of p53 in a signal transduction pathway initiated by IR and can phosphorylate p53 at serine 15. The cell cycle stage-specificity of ATM activation and p53Ser15 phosphorylation was investigated in normal lymphoblastoid cell line (GM536). Ionizing radiation was found to enhance the kinase activity of ATM in all phases of the cell cycle. This enhanced activity was apparent immediately after treatment of cells with IR, but was not accompanied by a change in the abundance of the ATM protein. Since IR activates the ATM kinase in all phases of the cell cycle, DNA replication-dependent strand breaks are not required for this activation. Further, since p53 protein is not directly required for IR-induced S and G2-phase checkpoints, the ATM kinase likely has different functional targets in different phases of the cell cycle. These observations indicate that the ATM kinase is necessary primarily for the immediate response to DNA damage incurred in all phases of the cell cycle.

The catalytic subunit of telomerase is expressed in developing brain neurons and serves a cell survival-promoting function

Telomerase, a specialized reverse transcriptase (RT) linked to cell immortalization and cancer, has been thought not to be expressed in postmitotic cells. We now report that telomerase activity and its essential catalytic subunit, telomerase reverse transcriptase (TERT), are expressed in neurons in the brains of rodents during embryonic and early postnatal development, and are subsequently downregulated. Suppression of TERT expression in cultured embryonic hippocampal neurons increases their vulnerability to apoptosis and excitotoxicity. Overexpression of TERT in PC12 cells suppresses apoptosis induced by trophic factor withdrawal. TERT exerts its anti-apoptotic action at an early stage of the cell death process prior to mitochondrial dysfunction and caspase activation. TERT may serve a neuron survival-promoting function in the developing brain, and downregulation of TERT in the adult brain may contribute to increased neuronal vulnerability in various age-related neurodegenerative disorders.

Altered telomere nuclear matrix interactions and nucleosomal periodicity in ataxia telangiectasia cells before and after ionizing radiation treatment

Cells derived from ataxia telangiectasia (A-T) patients show a prominent defect at chromosome ends in the form of chromosome end-to-end associations, also known as telomeric associations, seen at G(1), G(2), and metaphase. Recently, we have shown that the ATM gene product, which is defective in the cancer-prone disorder A-T, influences chromosome end associations and telomere length. A possible hypothesis explaining these results is that the defective telomere metabolism in A-T cells are due to altered interactions between the telomeres and the nuclear matrix. We examined these interactions in nuclear matrix halos before and after radiation treatment. A difference was observed in the ratio of soluble versus matrix-associated telomeric DNA between cells derived from A-T and normal individuals. Ionizing radiation treatment affected the ratio of soluble versus matrix-associated telomeric DNA only in the A-T cells. To test the hypothesis that the ATM gene product is involved in interactions between telomeres and the nuclear matrix, we examined such interactions in human cells expressing either a dominant-negative effect or complementation of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper motif mimics the altered interactions of telomere and nuclear matrix similar to that of A-T cells. A-T fibroblasts transfected with wild-type ATM gene had corrected telomere-nuclear matrix interactions. Further, we found that A-T cells had different micrococcal nuclease digestion patterns compared to normal cells before and after irradiation, indicating differences in nucleosomal periodicity in telomeres. These results suggest that the ATM gene influences the interactions between telomeres and the nuclear matrix, and alterations in telomere chromatin could be at least partly responsible for the pleiotropic phenotypes of the ATM gene.

Paclitaxel sensitivity correlates with p53 status and DNA fragmentation, but not G2/M accumulation

PURPOSE

The antitumor agent paclitaxel (Taxol) has been shown to arrest cells in mitosis through microtubule stabilization and to induce apoptosis. The tumor suppressor gene p53 is implicated in the regulation of cell cycle checkpoints and can mediate apoptotic cell death. Although initial studies demonstrated that various DNA-damaging agents can induce p53, more recent studies have also shown p53 induction following nonDNA-damaging agents, including paclitaxel. We investigated the influence of p53 abrogation on paclitaxel-induced cell kill and correlated the extent of mitotic arrest and DNA fragmentation by paclitaxel with the drug's cytotoxic effect.

MATERIALS AND METHODS

The parental human colorectal carcinoma cell line (RKO) with wild-type p53 alleles, and two transfected RKO cell lines with inactivated p53 (RKO.p53.13 with transfected mutant p53 and RC 10.3 with HPV-16-derived E6 gene) were exposed to graded doses of paclitaxel (1-100 nM) for 24-h intervals. The functional status of p53 in cells was assessed by thymidine and BrdU incorporation following exposure to ionizing radiation (4 Gy). Reproductive integrity following paclitaxel treatment was assessed by clonogenic assay. Immunolabeling and microscopic evaluation were used to assess mitotic accumulation and micronucleation. Apoptosis was assayed using DNA fragmentation analyses.

RESULTS

A 4-fold increase in paclitaxel sensitivity was observed among RKO cells deficient in p53 function compared with wild-type RKO cells (IC 50: 4 nM, 1 nM, 1nM for RKO, RKO.p53.13, RC 10.3, respectively). The increased cytotoxic effect in RKO cells with inactive p53 correlated with an increased propensity towards micronucleation and DNA fragmentation following paclitaxel treatment. However, no significant difference in peak mitotic accumulation was observed among RKO cells with functional or abrogated p53.

CONCLUSIONS

RKO cells lacking functional p53 demonstrate significantly enhanced sensitivity to paclitaxel compared with that of wild-type RKO cells. This response corresponded with increased micronucleation and DNA fragmentation in cells deficient in p53 function. Although previous published reports of enhanced paclitaxel sensitivity in p53-deficient cells correlated this finding with increased G2/M arrest, we did not observe any significant correlation between paclitaxel-induced cell kill and the degree of mitotic arrest. Our data suggest that apoptosis is the predominant mechanism of paclitaxel cytotoxicity in RKO cells and is likely mediated by a p53-independent process.

Atm inactivation results in aberrant telomere clustering during meiotic prophase

A-T (ataxia telangiectasia) individuals frequently display gonadal atrophy, and Atm-/- mice show spermatogenic failure due to arrest at prophase of meiosis I. Chromosomal movements take place during meiotic prophase, with telomeres congregating on the nuclear envelope to transiently form a cluster during the leptotene/zygotene transition (bouquet arrangement). Since the ATM protein has been implicated in telomere metabolism of somatic cells, we have set out to investigate the effects of Atm inactivation on meiotic telomere behavior. Fluorescent in situ hybridization and synaptonemal complex (SC) immunostaining of structurally preserved spermatocytes I revealed that telomere clustering occurs aberrantly in Atm-/- mice. Numerous spermatocytes of Atm-/- mice displayed locally accumulated telomeres with stretches of SC near the clustered chromosome ends. This contrasted with spermatogenesis of normal mice, where only a few leptotene/zygotene spermatocytes I with clustered telomeres were detected. Pachytene nuclei, which were much more abundant in normal mice, displayed telomeres scattered over the nuclear periphery. It appears that the timing and occurrence of chromosome polarization is altered in Atm-/- mice. When we examined telomere-nuclear matrix interactions in spermatocytes I, a significant difference was observed in the ratio of soluble versus matrix-associated telomeric DNA sequences between meiocytes of Atm-/- and control mice. We propose that the severe disruption of spermatogenesis during early prophase I in the absence of functional Atm may be partly due to altered interactions of telomeres with the nuclear matrix and distorted meiotic telomere clustering.

Telomerase activity as a measure for monitoring radiocurability of tumor cells

Radiotherapy plays a key role in the treatment of many tumors. It is difficult to determine what fraction of tumor cells survives after treatment with ionizing radiation. A convenient and sensitive biochemical assay could be efficacious in determining the potential success of radiotherapy. Since telomerase activity is frequently associated with the malignant phenotype, we sought to determine whether a correlation existed between ionizing radiation-induced cell killing and telomerase activity. We evaluated telomerase activity in two telomerase-positive and one telomerase-negative human cell line exposed to ionizing radiation. Telomerase activity was determined using a PCR-based telomeric repeat amplification protocol coupled with ELISA. We found ionizing radiation treatment to decrease the telomerase activity (in plateau phase cells of RKO, HeLa; and growing cells of RKO) in a dose-dependent manner, which correlated with cell death in in vitro tests as well as during tumor regression in nude mice. In contrast, growing HeLa cells after 24 h postradiation treatment showed an increase in telomerase activity, but there was no increase in the levels of mRNA of hTERT. To assess the sensitivity of the telomerase activity assay, we performed mixing experiments of HeLa and AG1522 cell extracts. These studies showed that telomerase activity could be detected in lysate equal to a single HeLa cell when mixed with 10,000 AG1522 cells. Our results indicate that even a few surviving neoplastic cells can be detected by telomerase activity assay. Therefore, detection of telomerase activity may be a useful monitor of radiotherapeutic efficacy and an early predictor of outcome.

Analysis of genomic integrity and p53-dependent G1 checkpoint in telomerase-induced extended-life-span human fibroblasts

Life span determination in normal human cells may be regulated by nucleoprotein structures called telomeres, the physical ends of eukaryotic chromosomes. Telomeres have been shown to be essential for chromosome stability and function and to shorten with each cell division in normal human cells in culture and with age in vivo. Reversal of telomere shortening by the forced expression of telomerase in normal cells has been shown to elongate telomeres and extend the replicative life span (H. Vaziri and S. Benchimol, Curr. Biol. 8:279-282, 1998; A. G. Bodnar et al., Science 279:349-352, 1998). Extension of the life span as a consequence of the functional inactivation of p53 is frequently associated with loss of genomic stability. Analysis of telomerase-induced extended-life-span fibroblast (TIELF) cells by G banding and spectral karyotyping indicated that forced extension of the life span by telomerase led to the transient formation of aberrant structures, which were subsequently resolved in higher passages. However, the p53-dependent G1 checkpoint was intact as assessed by functional activation of p53 protein in response to ionizing radiation and subsequent p53-mediated induction of p21(Waf1/Cip1/Sdi1). TIELF cells were not tumorigenic and had a normal DNA strand break rejoining activity and normal radiosensitivity in response to ionizing radiation.

Penclomedine-induced DNA fragmentation and p53 accumulation correlate with reproductive cell death in colorectal carcinoma cells with altered p53 status

Penclomedine, a synthetic pyridine derivative, has documented antitumor activity and is being investigated in clinical trials. Its mechanism of action is unknown although it may be metabolized to a free radical, DNA-reactive species. We previously reported that telomerase positive colorectal carcinoma (RKO) cells with abrogated p53 function were more sensitive to penclomedine than were telomerase positive cells with wild-type p53. The present study demonstrates that significant differences in DNA fragmentation in response to penclomedine were observed in RKO cells lacking functional p53 compared with RKO cells with normal p53 function. No differences in DNA fragmentation in response to ionizing radiation were seen in RKO cells with normal or abrogated p53 function. RKO cells with functional p53 respond to penclomedine treatment with a dose-dependent increase in p53 protein levels. However, RKO cells with abrogated p53 function did not show any such change in p53 protein levels. Further, p53-independent increase of p21 was observed, although the significance of this response remains uncertain. These studies suggest that penclomedine may have a therapeutic advantage in killing cells that have abrogated p53 function.

A preliminary report: frequency of A-T heterozygotes among prostate cancer patients with severe late responses to radiation therapy

PURPOSE

To investigate whether a significant proportion of prostate cancer patients who have late sequelae after high-dose external-beam conformal radiation therapy are radio-sensitive because they are carriers of ataxia-telangiectasia, that is, are heterozygous for mutations in the ATM gene.

PATIENTS AND METHODS

A group of prostate cancer patients were selected who experienced severe late sequelae, specifically proctitis or cystitis, after high-dose external-beam conformal radiation therapy, together with a control group of patients treated in the same way but who did not have severe late effects. Blood samples were taken from these patients, genomic DNA extracted, and mutations sought in the ATM gene.

RESULTS

Of 17 late-effect patients in whom most or all of the ATM gene has been examined, significant mutations (17.6%) were identified in three. No significant mutations were found in the control group. The incidence of ataxia- telangiectasia heterozygotes in the United States population is 1% to 2%.

DISCUSSION

These preliminary data suggest that a disproportionate number, but by no means all, of prostate cancer radiotherapy patients who experience severe late effects are ataxia-telangiectasia heterozygotes. If this conclusion is confirmed, these individuals could be identified prospectively and, with dose de-escalation, spared a great deal of discomfort and suffering. As a corollary, if most of the small late-effects population were prospectively identifiable, the dose to the remaining population could potentially be escalated. Present methods of identifying mutations in a large gene, such as ATM, are cumbersome and expensive, but the technology is evolving rapidly, so that rapid screening of the ATM gene is imminent.

Molecular cloning and chromosomal localization of Chinese hamster telomeric protein chTRF1. Its potential role in chromosomal instability

Chinese hamster cells frequently have altered karyotypes. To investigate the basis of recent observations that karyotypic alterations are related to telomeric fusions, we asked whether these alterations are due to lack of telomere repeat binding factor/s. Further, Chinese hamster chromosomes contain large blocks of interstitial telomeric repeats, which are preferentially involved in chromosome breakage and exchange, rendering it an interesting model for such studies. Here, we report on the cloning and the chromosomal localization of the Chinese hamster telomere repeat binding factor, chTRF1. The sequence analysis revealed, similar to human TRF1 (hTRF1), an N-terminal acidic domain, a TRF1 specific DNA binding motif and a C-terminal Myb type domain. Unlike mouse TRF1 (mTRF1), chTRF1 shows 97.5% identity to hTRF1. chTRF1 gene was localized on the long arm of chromosome 5. In vitro translation of chTRF1 resulted in protein product similar in molecular weight to hTRF1. Immunostaining of Chinese hamster ovary cells (CHO) with anti-TRF1 antibody revealed punctate nuclear staining. At metaphase, antibodies failed to detect TRF1 on most of the chromosome ends and the interstitial telomeric repeat bands. These studies suggest that chTRF1 does not bind the interstitial telomeric repeats, and its presence at the metaphase chromosome ends is limited. The later could be a factor contributing to frequent karyotypic alterations observed in Chinese hamster cells.

Functional role for the c-Abl tyrosine kinase in meiosis I

The c-Abl tyrosine kinase is activated by ionizing radiation and certain other DNA-damaging agents. The DNA-dependent protein kinase (DNA-PK) and the ataxia telangiectasia mutated (ATM) gene product, effectors in the DNA damage response, contribute to the induction of c-Abl activity. The present study demonstrates that c-Abl is expressed in mouse and rat testes, and predominantly in pachytene spermatocytes of meiosis I. The results also demonstrate that c-Abl interacts directly with meiotic chromosomes. In concert with a requirement for c-Abl at the pachytene stage, we show that, in contrast to wild-type mice, testes from Abl-/- mice exhibit defects in spermatogenesis. These findings provide the first demonstration that c-Abl plays a functional role in meiosis.

Influence of ATM function on telomere metabolism

The ATM gene product, which is defective in the cancer-prone disorder ataxia telangiectasia, has been implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination and cell cycle control. The ATM gene has homology with the TEL1 gene of yeast, mutations of which lead to shortened telomeres. To test the hypothesis that the ATM gene product is involved in telomere metabolism, we examined telomeric associations (TA), telomere length, and telomerase activity in human cells expressing either dominant-negative or complementing fragments of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper (LZ) motif mimics that of ataxia telangiectasia (A-T) cells. These transfected RKO cells relative to transfected controls had a higher frequency of cells with TA and shortened telomeres, but no detectable change in telomerase activity. In addition, the percentage of cells with TA after gamma irradiation was higher in the transfected RKO cells with dominant negative activity of the ATM gene, compared to control cells. SV40 transformed fibroblasts derived from an A-T patient and transfected with a complementing carboxyl terminal kinase region of the ATM gene had a reduced frequency of cells with TA, with no effect on the telomere length or telomerase activity. The present studies using isogenic cells with manipulated ATM function demonstrate a role for the ATM gene product in telomere metabolism.

Fragments of ATM which have dominant-negative or complementing activity

The ATM protein has been implicated in pathways controlling cell cycle checkpoints, radiosensitivity, genetic instability, and aging. Expression of ATM fragments containing a leucine zipper motif in a human tumor cell line abrogated the S-phase checkpoint after ionizing irradiation and enhanced radiosensitivity and chromosomal breakage. These fragments did not abrogate irradiation-induced G1 or G2 checkpoints, suggesting that cell cycle checkpoint defects alone cannot account for chromosomal instability in ataxia telangiectasia (AT) cells. Expression of the carboxy-terminal portion of ATM, which contains the PI-3 kinase domain, complemented radiosensitivity and the S-phase checkpoint and reduced chromosomal breakage after irradiation in AT cells. These observations suggest that ATM function is dependent on interactions with itself or other proteins through the leucine zipper region and that the PI-3 kinase domain contains much of the significant activity of ATM.

Effect of penclomedine (NSC-338720) on telomere fusions, chromatin blebbing, and cell viability with and without telomerase activity and abrogated p53 function

Telomeres, or chromosome ends, are essential in maintaining chromosomal integrity. Telomeres consist of a short hexameric sequence, 3'-TTAGGG-5', repeated in tandem arrays added to chromosomes by the ribonucleoprotein enzyme telomerase. In this study, we assessed whether penclomedine, a novel synthetic pyridine compound presently being evaluated in clinical trials for its anticancer activity, influences telomere fusions (chromosome end-to-end associations) and telomerase activity in cells in culture. We found that penclomedine reduced the mitotic index, induced chromosome end associations in all phases of the cell cycle, and rapidly induced chromatin blebbing in a concentration-dependent manner in both cervical carcinoma (HeLa) cells and in normal human fibroblasts (AG1522). However, the effectiveness of the drug was much more pronounced in HeLa cells. In addition, there was a drug-mediated, concentration-dependent decline in telomerase activity noted in the HeLa cells that correlated with a decrease in mitotic index and an increase in telomere fusions. Interestingly, when the mitotic index, chromatin blebbing, and telomere fusions were compared between the telomerase positive (HeLa) and negative (AG1522) cell types, penclomedine affected chromatin stability to a greater extent in those cells with detectable telomerase activity. In addition, telomerase positive colorectal carcinoma cells with abrogated p53 (RC-10.3 cells) were more sensitive to penclomedine than were telomerase positive cells with wild-type p53 (RKO cells). These studies suggest that penclomedine may have a therapeutic advantage in killing tumor cells that are positive for telomerase activity and defective in p53 function.

The involvement of telomeric sequences in chromosomal aberrations

Three functional elements are required for the stable transmission of eukaryotic chromosomes: replication origins, centromeres and telomeres. In the yeast Saccharomyces cerivisiae the DNA sequences defining each of these elements are known. The simplest and most widely conserved of these sequences is that of the telomere. As the name implies, the telomere is the end of a linear eukaryotic chromosome. Two of the main functions of the telomere are to prevent DNA loss as a consequence of replication and to prevent interactions with other chromosomal ends. Thus, telomeres play a major role in maintaining chromosome stability and consequently they have been considered as likely to be involved in some aspects of chromosomal aberration formation. The involvement of telomeric DNA sequences in stabilizing normal and broken chromosome ends, in "hot spots' for aberration formation and in delayed chromosomal instability will be reviewed here drawing on material presented at the Workshop and the published literature.

Neoplastic transformation of mouse C3H10T1/2 cells following exposure to neutrons does not involve mutation of ras gene as analyzed by SSCP and cycle sequencing

About 25% of human tumors contain a mutated member of the ras gene family. Neutron exposure is an occupational risk in several work places and while we know that cells exposed to neutrons can become transformed, the molecular basis of this process is not understood. To determine whether neutron-induced cellular transformation involves ras mutation, C3H10T1/2 cells were exposed to a single dose of 5.9 MeV neutrons. Type II and type III foci were isolated and established as cell lines. A total of 34 foci were selected and expanded for analysis of tumorigenicity, chromosomal aberrations and mutations in members of the ras gene family. The presence of mutations in genomic DNA in N-ras or K-ras of each focus was examined by either single-strand conformational polymorphism (SSCP) analysis or by asymmetric PCR coupled cell cycle sequence analysis. Although chromosomal aberrations were detected at metaphase, no alterations in either ras gene were detected. We conclude that in vitro neutron-induced transformation must occur through a mechanism other than ras mutation.

Chromosome end-to-end associations and telomerase activity during cancer progression in human cells after treatment with alpha-particles simulating radon progeny

Chromosome end-to-end associations seen at metaphase involve telomeres and are commonly observed in cells derived from individuals with ataxia telangiectasia and most types of human tumors. The associations may arise because of short telomeres and/or alterations of chromatin structure. There is a growing consensus that telomere length is stabilized by the activity of telomerase in immortal cells; however, it is not clear why some immortal cells display chromosome end-to-end associations. In the present study we evaluated chromosome end-to-end associations, telomere length and telomerase activity with the tumorigenic status of human bronchial epithelial cells immortalized with human papillomavirus. Oncogenic transformation was initiated using radon simulated alpha-particles and cells evaluated as primary, secondary and metastatic transformants. The fewest chromosome end associations and lowest telomerase activity were observed in the parental immortalized cells. However, increased levels of telomerase activity were detected in alpha-particle survivors while robust telomerase activity was seen in the tumorigenic cell lines. The tumorigenic cells that were telomerase positive and had the highest frequency of cells with chromosome end-to-end associations were also metastatic. No correlation was found between telomere length and the different stages of carcinogenicity.

Chromosome aberrations in human fibroblasts induced by monoenergetic neutrons. I. Relative biological effectiveness

The relative biological effectiveness (RBE) of neutrons for many biological end points varies with neutron energy. To test the hypothesis that the RBE of neutrons varies with respect to their energy for chromosome aberrations in a cell system that does not face interphase death, we studied the yield of chromosome aberrations induced by monoenergetic neutrons in normal human fibroblasts at the first mitosis postirradiation. Monoenergetic neutrons at 0.22, 0.34, 0.43, 1, 5.9 and 13.6 MeV were generated at the Accelerator Facility of the Center for Radiological Research, Columbia University, and were used to irradiate plateau-phase fibroblasts at low absorbed doses from 0.3 to 1.2 Gy at a low dose rate. The reference low-LET, low-dose-rate radiation was 137Cs-gamma rays (0.66 MeV). A linear dose response (Y = alphaD) for chromosome aberrations was obtained for all monoenergetic neutrons and for the gamma rays. The yield of chromosome aberrations per unit dose was high at low neutron energies (0.22, 0.34 and 0.43 MeV) with a gradual decline with the increase in neutron energy. Maximum RBE (RBEm) values varied for the different types of chromosome aberrations. The highest RBE (24.3) for 0.22 and 0.43 MeV neutrons was observed for intrachromosomal deletions, a category of chromosomal change common in solid tumors. Even for the 13.6 MeV neutrons the RBEm (11.1) exceeded 10. These results show that the RBE of neutrons varies with neutron energy and that RBEs are dissimilar between different types of asymmetric chromosome aberrations and suggest that the radiation weighting factors applicable to low-energy neutrons need firmer delineation. This latter may best be attained with neutrons of well-defined energies. This would enable integrations of appropriate quality factors with measured radiation fields, such as those in high-altitude Earth atmosphere. The introduction of commercial flights at high altitude could result in many more individuals being exposed to neutrons than occurs in terrestrial workers, emphasizing the necessity for better-defined estimates of risk.

Biochemical pharmacology of penclomedine (NSC-338720)

Penclomedine (PEN) is a synthetic pyridine derivative that has been selected for clinical development based on its activity against human and mouse breast tumors implanted in mice. Its mechanism of action was unclear, and we were interested in determining its mechanism of cytotoxicity in vitro and in vivo. We found chromosome breaks, gaps, and exchanges in P388 ascites cells from BD2F1 mice treated with 200 mg/kg PEN. Maximal observed damage occurred 24 hr after drug administration. Alkaline elution indicated only limited DNA strand breaks and interstrand cross-linking. In vitro, PEN (75 micrograms/mL) inhibited RNA and DNA syntheses almost completely. In addition, incubation of [14C]PEN with rat liver S-9 fraction in the presence of calf thymus DNA resulted in the stable transfer of radioactivity to DNA. Addition of butylated hydroxytoluene, a free radical scavenger, to the incubation mixture inhibited the binding of drug to DNA, implicating free radicals as the ultimate reactive species. These data suggest that PEN can be metabolized to free radical, DNA-reactive products, and that its cytotoxicity is due to chromosomal damage produced by monofunctional alkylation. As an alternate mechanism, the ability of PEN to inhibit cellular dihydroorotate dehydrogenase was explored. Although PEN is an inhibitor of this enzyme in cells in vivo, in vitro, and in isolated cell sonicates, HPLC analyses of ribonucleotide triphosphate pools in P388 cells showed that all triphosphates had increased, especially UTP. Addition of uridine to the cell culture failed to prevent PEN-mediated cytotoxicity, suggesting that inhibition of de novo pyrimidine biosynthesis was not likely to be an important mechanism of action of this drug. These data suggest that PEN is activated in cells to a free radical that binds DNA.

Evidence of a chromatin basis for increased mutagen sensitivity associated with multiple primary malignancies of the head and neck

Individuals at increased risk for cancer development have been reported to exhibit increased mutagen sensitivity as detected by the G2 phase chromosome breakage assay. To better understand the basis for such chromosome sensitivity, we examined 4 lymphoblastoid cell lines derived from 4 head and neck cancer patients, 2 of whom were previously shown to have normal bleomycin sensitivity and 2 high bleomycin sensitivity (and multiple primary tumors). These cell lines were studied for cell survival and initial damage and repair at both the DNA and chromosome levels, and the results compared to a normal control cell line and 3 ataxia telangiectasia homozygote cell lines. While all cell lines exhibited nearly equal levels of initial DNA damage and repair throughout the cell cycle, both the ataxia telangiectasia homozygote cell lines and 2 cell lines derived from individuals with multiple head and neck primary tumors showed increased levels of initial chromosome damage (detected by premature chromosome condensation), a reduced fast repair component and higher residual chromosome damage. Our results suggest that one component of the enhanced mutagen sensitivity phenotype observed in cancer-prone individuals may involve an inherent chromatin alteration that allows a more efficient translation of DNA damage into chromosome damage following mutagen exposure. The degree of such an alteration might then be associated with an increased risk for second primary tumors in other carcinogen-exposed sites.

Spontaneous amplification of interstitial telomeric bands in Chinese hamster ovary cells

Chinese hamster cell lines have been widely used to explore the mechanism of gene amplification under stress conditions. Investigations of gene amplification in Chinese hamster cells under nonstress conditions, however, have been limited due to technical reasons. We initiated studies on the amplification mechanism by examining the frequency of interstitial telomeric bands (ITBs) in continuously growing cultures by fluorescence in situ hybridization (FISH), using a (TTAGGG)n probe. We found that one of the chromosomes uniquely exhibit amplification of ITBs in Chinese hamster ovary (CHO) cells but not in drug-resistant mutant CHO cells or in Chinese hamster V79 cells. Amplification of ITBs could not be induced by DNA damaging agents or DNA synthesis inhibitors. To investigate the early events that mediate ITB amplification, we developed clones and determined that the frequency of acquisition of additional ITBs on the marker chromosome was in the range of 10(-5) to 10(-3) per cell doubling. Using DNA strand-specific FISH, we found that one of the initial events of amplification is isochromosome formation. Our observations support the involvement of sister chromatid fusion in the initiation of spontaneous ITB amplification.

Chromosome end associations, telomeres and telomerase activity in ataxia telangiectasia cells

Cells derived from individuals with ataxia telangiectasia (AT) show enhanced spontaneous levels of chromosomal abnormalities and are sensitive to ionizing radiations and radiomimetic drugs, as evidenced by decreased survival and increased chromosome aberration frequencies at mitosis when compared with normal cell lines. The higher base line frequencies of chromosome aberrations in part involve chromosome end-to-end associations as seen at metaphase. Since telomeres of tumor cells and aging tissues are often reduced in length, chromosome end associations may be due to loss of telomeric repeats. We studied the chromosome behavior and telomeres of two ataxia telangiectasia lymphoblastoid cell lines compared to two normal control cell lines. The ataxia telangiectasia cell lines showed higher frequencies of chromosome end associations both at metaphase and in interphase, determined in prematurely condensed chromosomes of G1 and G2 cells. They also showed higher frequencies of chromosomal breaks at metaphase and fewer telomeric signals determined using fluorescent in situ hybridization with a (TTAGGG)n probe. The frequency of telomeric repeats was variable in the ataxia telangiectasia cell lines (4.3 and 8.2 kb) compared to the normal cell lines (9.6 and 12 kb) and an inverse correlation between telomere length and chromosome end associations was observed. Both ataxia telangiectasia cell lines showed more robust telomerase activity than the normal cell lines, precluding defective enzymatic capacity as the basis for the chromosome end associations. It is possible that chromatin structure in the form of telomere-nuclear matrix interactions are variant in ataxia telangiectasia cells negatively influencing telomerase function and contributing to telomere associations.

Possible role of chromatin alteration in the radiosensitivity of ataxia-telangiectasia

Cells derived from individuals with ataxia-telangiectasia (A-T) are known to exhibit increased sensitivity to ionizing radiation and certain radiomimetic chemical agents. Here we summarize our findings regarding the role of chromosome damage and repair in this radiosensitivity. Lymphoblastoid cells derived from A-T homozygotes were characterized for initial chromosome (premature chromosome condensation) and DNA (neutral filter elution) damage and repair kinetics in cells from G1 and G2 cell cycle phases. Despite initial levels of DNA damage being similar to normal controls, A-T cells exhibited nearly a two-fold higher initial amount of chromosome damage. Different A-T cell lines exhibited differing chromosome repair capacities compared with control lymphoblastoid cell lines. These results suggest that A-T cells have an altered chromatin structure whereby DNA double-strand breaks are apparently more efficiently converted into chromosome breaks. Four A-T heterozygote cell lines were examined for chromosome damage and repair in the same fashion and all exhibited increased levels of chromosome damage, although the degree of sensitivity was more prominent in G2 phase cells (two-fold higher) than in G1 phase cells (1.5-fold higher than normal controls). These results suggest that A-T heterozygotes also exhibit an altered chromatin structure which impacts on chromosome damage expression. Of interest, A-T cells also exhibited increased chromosome stickiness after irradiation, and telomere regions appeared to be frequently involved. While the molecular basis for preferential telomere involvement is not understood, these results again suggest that structural alterations in the chromatin of A-T cells may play an important role in A-T radiosensitivity.

Increased initial levels of chromosome damage and heterogeneous chromosome repair in ataxia telangiectasia heterozygote cells

Individuals heterozygous for ataxia telangiectasia (AT) appear clinically normal but have a 2-3-fold overall excess risk of cancer. Various approaches have been used to identify AT heterozygotes, however, the results are ambiguous. We recently reported that AT homozygotes exhibit more initial chromosome damage after irradiation than normal cells despite identical levels of DNA double strand breaks (DSBs) as well as a reduced fast repair component at both the DNA and chromosome levels. To determine whether AT heterozygotes exhibit the AT or normal cellular phenotype, we compared four AT heterozygote lymphoblastoid cell lines with normal control and AT homozygote lymphoblastoid cells with regard to cell survival, initial levels of damage, and repair at the DNA and chromosome levels after gamma-irradiation in G1, S, and G2 phase (estimated by neutral DNA filter elution and premature chromosome condensation). There was no significant difference in survival, induction and repair of DNA DSBs, or chromosome repair between AT heterozygote and normal cells. In contrast, all four AT heterozygote cell lines showed increased levels of chromosome damage; G1 phase cells showed intermediate levels and G2 phase cells showed levels equivalent to the AT homozygote phenotype. These results suggest that premature chromosome condensation may be useful for detecting AT heterozygotes.

Quantitative analysis of chromosome in situ hybridization signal in paraffin-embedded tissue sections

Interphase cytogenetic analysis using chromosome-specific probes is increasingly being used to detect chromosomal aberrations on paraffin-embedded tissue sections. However, quantitative analysis of the hybridization signal is confounded by the nuclear slicing that occurs during sectioning. To determine the sensitivity and accuracy of chromosome in situ hybridization for detecting numerical chromosomal aberrations on paraffin-embedded sections, in situ hybridization was performed on sections derived from mixtures of cell populations with known frequencies of numerical chromosomal aberrations and the Chromosome Index (CI) was calculated (i.e., total number of signal spots/number of nuclei counted) as a quantitative measure of chromosome copy number. The presence of 25% or more monosomic or tetrasomic cells in a given population was easily detected as a change in CI (P < 0.05). Lower degrees of polysomy could be detected as a small percentage of nuclear fragments with > 2 signal spots. The CI was not significantly influenced by a change in section thickness from 4 to 8 microM, by an increase in cell size from 478 to 986 microM3, or by the choice of detection method (fluorescence vs. conventional bright-field microscopy). Comparative analysis of touch preparations and tissue sections from the corresponding breast tumors showed that CI accurately reflects the average copy number of chromosomes in intact nuclei and may actually be superior to in situ hybridization on whole nuclei for the detection of numerical chromosomal changes in defined histologic areas. This method is thus a sensitive and accurate means of studying genetic changes in premalignant and malignant tissue, and of assessing the genetic changes associated with specific phenotypes.

Effect of chromosome size on aberration levels caused by gamma radiation as detected by fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) is a powerful technique for detecting genomic alterations at the chromosome level. To study the effect of chromosome size on aberration formation, we used FISH to detect initial damage in individual prematurely condensed chromosomes (PCC) of gamma-irradiated G0 human cells. A linear dose response for breaks and a nonlinear dose response for exchanges was obtained using a chromosome 1-specific probe. FISH detected more chromosome 1 breaks than expected from DNA based extrapolation of Giemsa stained PCC preparations. The discrepancy in the number of breaks detected by the two techniques raised questions as to whether Giemsa staining and FISH differ in their sensitivities for detecting breaks, or is chromosome 1 uniquely sensitive to gamma-radiation. To address the question of technique sensitivity, we determined total chromosome damage by FISH using a total genomic painting probe; the results obtained from Giemsa-staining and FISH were nearly identical. To determine if chromosome 1 was uniquely sensitive, we selected four different sized chromosomes for paint probes and scored them for gamma-ray induced aberrations. In these studies the number of chromosome breaks per unit DNA increased linearly with an increase in the DNA content of the chromosomes. However, the number of exchanges per unit of DNA did not increase with an increase in chromosome size. This suggests that chromosome size may influence the levels of aberrations observed. Extrapolation from measurements of a single chromosome's damage to the whole genome requires that the relative DNA content of the measured chromosome be considered.

The contribution of DNA and chromosome repair deficiencies to the radiosensitivity of ataxia-telangiectasia

Cells derived from individuals with ataxia-telangiectasia (AT) are more sensitive to ionizing radiation and radiomimetic drugs, as evidenced by decreased survival and increased chromosome aberrations at mitosis when compared with normal cell lines. Our previous studies showed that, despite similar initial levels of DNA double-strand breaks (DSBs), AT cells express higher initial chromosome damage than do normal cells as demonstrated by the technique of premature chromosome condensation. However, this finding accounted for only a portion of the increased sensitivity (T. K. Pandita and W. N. Hittelman, Radiat. Res. 130, 94-103, 1992). The purpose of the study reported here was to examine the contribution of DNA and chromosome repair to the radiosensitivity of AT cells. Exponentially growing AT and normal lymphoblastoid cells were fractionated into cell cycle phase-enriched populations by centrifugal elutriation, and their DNA and chromosome repair characteristics were evaluated by DNA neutral filter elution (for DNA DSBs) and by premature chromosome condensation, respectively. AT cells exhibited a reduced fast-repair component in both G1- and G2-phase cells, as observed at the level of both DNA DSBs and the chromosome; however, S-phase cells showed nearly normal DNA DSB repair. The findings that AT cells exhibit an increased level of chromosome damage and a deficiency in the fast component (but not the slow component) of repair suggest that chromatin organization might play a major role in the observed sensitivity of AT cells. When survival was plotted as a function of the residual amount of chromosome damage in G1- and G2- phase cells after 90 min of repair, the curves for normal and AT cells approached each other but did not overlap. These results suggest that, although higher initial levels of chromosome damage and reduced chromosome repair capability can explain much of the radiosensitivity of AT cells, other differences in AT cells must also contribute to their sensitivity phenotype.

Resistance of human cervical carcinoma cells to tumor necrosis factor correlates with their increased sensitivity to cisplatin: evidence of a role for DNA repair and epidermal growth factor receptor

Alterations in cellular biochemistry which are associated with the development of resistance to cytotoxic peptides, such as tumor necrosis factor (TNF), may also be responsible for changes in the response of cells to cytotoxic agents. Culturing ME-180 cervical carcinoma cells in the presence of escalating concentrations of TNF resulted in the development of an ME-180 cell variant (ME-180R) resistant to TNF but expressing a 3-5-fold increased sensitivity to cisplatin (CDDP) when measured following continuous exposure (low doses) or short-term incubation with CDDP (high doses) and clonogenic analysis. Cellular platinum uptake, efflux, and nuclear platinum content as well as the extent of DNA platination were examined and found to be identical in both ME-180 parental and ME-180R cell lines. Although ME-180R cells showed a relatively higher glutathione content than ME-180 parental cells, the effect of buthionine sulfoximine on the cellular sensitivity to CDDP and glutathione S-transferase activities of both cell lines were almost identical, suggesting that glutathione content or its metabolism did not appear to play a major role in differential CDDP cytotoxicity. Unscheduled DNA synthesis following exposure to CDDP was more inducible in ME-180 parental cells than in CDDP-sensitive ME-180R cells. Alkaline elution studies of cross-linked DNA in CDDP-treated ME-180 cells suggested that accumulation of DNA adducts reached maximal levels 10-15 h after CDDP treatment and was similar in both TNF-resistant and parental cells. Within 24 h after CDDP exposure, the extent of DNA cross-linking was markedly reduced in parental cells but remained elevated in the CDDP-sensitive ME-180R cell line. To examine the proposed regulatory role of phosphorylation in CDDP and TNF-mediated cytotoxicity, epidermal growth factor (EGF) receptor tyrosine kinase activity was measured in both TNF-resistant and parental ME-180 cells. Analysis of cell lysates demonstrated a 3-4-fold higher EGF receptor tyrosine kinase activity in ME-180R cells when compared to the parental population which correlated with increased expression of EGF receptor protein by immunoblot analysis. Based upon colony-forming assays, EGF treatment of ME-180 parental cells resulted in an increased sensitivity to CDDP (similar to ME-180R cells) and 3-fold stimulation of EGF receptor tyrosine kinase activity. Taken together, these results suggest that TNF resistance in ME-180 cervical carcinoma cells correlates with both increased EGF receptor expression and enhanced CDDP cytotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Initial chromosome damage but not DNA damage is greater in ataxia telangiectasia cells

Cells derived from individuals with ataxia telangiectasia (AT) exhibit increased sensitivity to ionizing radiation and certain drugs (e.g., bleomycin, neocarzinostatin, and etoposide) as evidenced by decreased survival and increased chromosome aberrations at mitosis when compared with normal cell lines. To understand better the basis of this sensitivity, three AT and two normal lymphoblastoid cell lines were fractionated into cell cycle phase-enriched populations by centrifugal elutriation and then examined for their survival and their relative initial levels of DNA damage (neutral DNA filter elution) and chromosome damage (premature chromosome condensation). AT cells exhibited decreased levels of survival in all phases of the cell cycle; however, AT cells in early G1 phase were especially sensitive compared with normal cells in G1 phase. While AT and normal cells exhibited similar levels of initial DNA double-strand breaks in exponential populations as well as throughout the cell cycle, AT cells showed nearly twofold higher initial levels of chromosome damage than normal control cells in G1 and G2 phase. These results suggest that there is a higher rate of conversion of DNA double-strand breaks into chromosome breaks in AT cells, perhaps due to a difference in chromatin organization or stability. Thus one determining component of cellular radiosensitivity might include chromatin structure.

Assessment of the mutagenic potential of a fungicide Bavistin using multiple assays

The fungicide Bavistin was assessed for mutagenic potential by various assays. Bavistin was found to be unable to induce gene mutation in Salmonella typhimurium, but it was able to induce transfection inhibition in Mycobacterium smegmatis. Bavistin was able to induce immediate genotoxic effects in plants but these were not carried through in development as in the long term no genotoxic effects were observed by the progeny test. Bavistin did induce micronuclei formation and did cause an increase in the ratio of normochromatic to polychromatic erythrocytes in mice. It was able to induce a very low frequency of sister-chromatid exchange in human lymphocytes and in addition, it was observed that the chemical affected the mitotic index but did not affect the cell cycle duration. Present studies indicate that the pesticide shows a positive response in 4 out of 5 different test systems (Table 8) and most of the observations support that Bavistin is genotoxic.

Evaluation of Thimet 10-G for mutagenicity by 4 different genetic systems

The insecticide Thimet 10-G was tested for mutagenic activity by 4 different genetic systems. It was unable to induce gene mutation in Salmonella, transfection inhibition in Mycobacterium, micronuclei formation in mice, and sister-chromatid exchange (SCE) in human lymphocytes were evaluated. It caused in mice an increase in the ratio of normochromatic to polychromatic erythrocytes and in human lymphocytes a decrease in mitotic index and delay in cell cycle. The results indicate that the insecticide is not mutagenic in the 4 test systems used at present.