From yeast to humans: Understanding the biology of DNA Damage Response (DDR) kinases

The DNA Damage Response (DDR) is a complex network of biological processes that protect cells from accumulating aberrant DNA structures, thereby maintaining genomic stability and, as a consequence, preventing the development of cancer and other diseases. The DDR pathway is coordinated by a signaling cascade mediated by the PI3K-like kinases (PIKK) ATM and ATR and by their downstream kinases CHK2 and CHK1, respectively. Together, these kinases regulate several aspects of the cellular program in response to genomic stress. Much of our understanding of these kinases came from studies performed in the 1990s using yeast as a model organism. The purpose of this review is to present a historical perspective on the discovery of the DDR kinases in yeast and the importance of this model for the identification and functional understanding of their mammalian orthologues.

CtIP is essential for early B cell proliferation and development in mice

B cell development requires efficient proliferation and successful assembly and modifications of the immunoglobulin gene products. CtIP is an essential gene implicated in end resection and DNA repair. Here, we show that CtIP is essential for early B cell development but dispensable in naive B cells. CtIP loss is well tolerated in G1-arrested B cells and during V(D)J recombination, but in proliferating B cells, CtIP loss leads to a progressive cell death characterized by ATM hyperactivation, G2/M arrest, genomic instability, and 53BP1 nuclear body formation, indicating that the essential role of CtIP during proliferation underscores its stage-specific requirement in B cells. B cell proliferation requires phosphorylation of CtIP at T847 presumably by CDK, but not its interaction with CtBP or Rb or its nuclease activity. CtIP phosphorylation by ATM/ATR at T859 (T855 in mice) promotes end resection in G1-arrested cells but is dispensable for B cell development and class switch recombination, suggesting distinct roles for T859 and T847 phosphorylation in B cell development.

Identification of ATM mutations in Korean siblings with ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a rare autosomal recessive neurodegenerative disorder. It is characterized by early-onset, progressive cerebellar ataxia, oculomotor apraxia, choreoathetosis, conjunctival telangiectasias, immunodeficiency, and an increased risk of malignancy. Although A-T is known to be the most common cause of progressive cerebellar ataxia in childhood, there have been no confirmed cases in Korea. We report the clinical and genetic findings of Korean siblings who presented with limb and truncal ataxia, oculomotor apraxia, choreoathetosis, and telangiectasias of the eyes. Sequence analysis of the ataxia-telangiectasia mutated (ATM) gene revealed a known missense mutation (c.8546G>C; p.Arg2849Pro) and a novel intronic variant of intron 17 (c.2639-19_2639-7del13). Reverse-transcription PCR and sequencing analysis revealed that the c.2639-19_2639-7del13 variant causes a splicing aberration that potentiates skipping exon 18. Because A-T is quite rare in Korea, the diagnosis of A-T in Korean patients can be delayed. We recommend that a diagnosis of A-T should be suspected in Korean patients exhibiting the clinical features of A-T.

Radiation-induced double-strand breaks require ATM but not Artemis for homologous recombination during S-phase

Double-strand breaks (DSBs) are repaired by two distinct pathways, non-homologous end joining (NHEJ) and homologous recombination (HR). The endonuclease Artemis and the PIK kinase Ataxia-Telangiectasia Mutated (ATM), mutated in prominent human radiosensitivity syndromes, are essential for repairing a subset of DSBs via NHEJ in G1 and HR in G2. Both proteins have been implicated in DNA end resection, a mandatory step preceding homology search and strand pairing in HR. Here, we show that during S-phase Artemis but not ATM is dispensable for HR of radiation-induced DSBs. In replicating AT cells, numerous Rad51 foci form gradually, indicating a Rad51 recruitment process that is independent of ATM-mediated end resection. Those DSBs decorated with Rad51 persisted through S- and G2-phase indicating incomplete HR resulting in unrepaired DSBs and a pronounced G2 arrest. We demonstrate that in AT cells loading of Rad51 depends on functional ATR/Chk1. The ATR-dependent checkpoint response is most likely activated when the replication fork encounters radiation-induced single-strand breaks leading to generation of long stretches of single-stranded DNA. Together, these results provide new insight into the role of ATM for initiation and completion of HR during S- and G2-phase. The DSB repair defect during S-phase significantly contributes to the radiosensitivity of AT cells.

Methylation of the ATM promoter in glioma cells alters ionizing radiation sensitivity

Glioblastomas are among the malignancies most resistant to radiation therapy. In contrast, cells lacking the ATM protein are highly sensitive to ionizing radiation. The relationship between ATM protein expression and radiosensitivity in 3 glioma cell lines was examined. T98G cells exhibited normal levels of ATM protein, whereas U118 and U87 cells had significantly lower levels of ATM and increased (>2-fold) sensitivity to ionizing radiation compared to T98G cells. The ATM promoter was methylated in U87 cells. Demethylation by azacytidine treatment increased ATM protein levels in the U87 cells and decreased their radiosensitivity. In contrast, the ATM promoter in U118 cells was not methylated. Further, expression of exogenous ATM did not significantly alter the radiosensitivity of U118 cells. ATM expression is therefore heterogeneous in the glioma cells examined. In conclusion, methylation of the ATM promoter may account for the variable radiosensitivity and heterogeneous ATM expression in a fraction of glioma cells.

A critical role for Pin2/TRF1 in ATM-dependent regulation. Inhibition of Pin2/TRF1 function complements telomere shortening, radiosensitivity, and the G(2)/M checkpoint defect of ataxia-telangiectasia cells

Cells derived from patients with the human genetic disorder ataxia-telangiectasia (A-T) display many abnormalities, including telomere shortening, premature senescence, and defects in the activation of S phase and G(2)/M checkpoints in response to double-strand DNA breaks induced by ionizing radiation. We have previously demonstrated that one of the ATM substrates is Pin2/TRF1, a telomeric protein that binds the potent telomerase inhibitor PinX1, negatively regulates telomere elongation, and specifically affects mitotic progression. Following DNA damage, ATM phosphorylates Pin2/TRF1 and suppresses its ability to induce abortive mitosis and apoptosis (Kishi, S., Zhou, X. Z., Nakamura, N., Ziv, Y., Khoo, C., Hill, D. E., Shiloh, Y., and Lu, K. P. (2001) J. Biol. Chem. 276, 29282-29291). However, the functional importance of Pin2/TRF1 in mediating ATM-dependent regulation remains to be established. To address this question, we directly inhibited the function of endogenous Pin2/TRF1 in A-T cells by stable expression of two different dominant-negative Pin2/TRF1 mutants and then examined their effects on telomere length and DNA damage response. Both the Pin2/TRF1 mutants increased telomere length in A-T cells, as shown in other cells. Surprisingly, both the Pin2/TRF1 mutants reduced radiosensitivity and complemented the G(2)/M checkpoint defect without inhibiting Cdc2 activity in A-T cells. In contrast, neither of the Pin2/TRF1 mutants corrected the S phase checkpoint defect in the same cells. These results indicate that inhibition of Pin2/TRF1 in A-T cells is able to bypass the requirement for ATM in specifically restoring telomere shortening, the G(2)/M checkpoint defect, and radiosensitivity and demonstrate a critical role for Pin2/TRF1 in the ATM-dependent regulation of telomeres and DNA damage response.

ATM as a target for novel radiosensitizers

DNA damage checkpoints are complex signal transduction pathways that are critical for normal cellular recovery following potentially lethal genotoxic insults. The ataxia-telangiectasia mutated (ATM) protein kinase is a critical component in these pathways and integrates the cellular response to damage by phosphorylating key proteins involved in cell cycle regulation and DNA repair. Lack of normal ATM function in the inherited ataxia-telangiectasia (A-T) syndrome results in a pleiotropic clinical syndrome characterized by a marked increased risk of cancer and profound hypersensitivity to ionizing radiation. Cells derived from patients with A-T share some of these attributes with genomic instability, loss of normal cell cycle arrest pathways, defects in DNA repair and increased radiation sensitivity. The radiosensitivity of A-T cells suggests that pharmacological inhibitors of the ATM kinase should be effective radiosensitizing agents. In fact, caffeine inhibits ATM kinase activity at concentrations that result in an A-T-like phenotype with loss of cell cycle checkpoints and hypersensitivity to ionizing radiation. Although the clinical use of caffeine as a radiosensitizer is limited by potentially lethal systemic toxicities, more potent methyl xanthines may selectively inhibit the ATM pathway at clinically achievable levels. Interestingly, caffeine and other methyl xanthines preferentially radiosensitize cells that lack normal p53 function. Because p53 is commonly inactivated in epithelial malignancies, this suggests that small molecule inhibitors of ATM might selectively sensitize the majority of tumors to the lethal effects of ionizing radiation while sparing normal tissues.

Activation of p53 transcriptional activity requires ATM's kinase domain and multiple N-terminal serine residues of p53

The ATM protein kinase regulates the cell's response to DNA damage by regulating cell cycle checkpoints and DNA repair. ATM phosphorylates several proteins involved in the DNA-damage response, including p53. We have examined the mechanism by which ATM regulates p53's transcriptional activity. Here, we demonstrate that reintroduction of ATM into AT cells restores the activation of p53 by the radio-mimetic agent bleomycin. Further, p53 activation is lost when a kinase inactive ATM is used, or if the N-terminal of ATM is deleted. In addition, AT cells stably expressing ATM showed decreased sensitivity to Ionizing Radiation-induced cell killing, whereas cells expressing kinase inactive ATM or N-terminally deleted ATM were indistinguishable from AT cells. Finally, single point-mutations of serines 15, 20, 33 or 37 did not individually block the ATM-dependent activation of p53 transcriptional activity by bleomycin. However, double mutations of either serines 15 and 20 or serines 33 and 37 blocked the ability of ATM to activate p53. Our results indicate that the N-terminal of ATM and ATM's kinase activity are required for activation of p53's transcriptional activity and restoration of normal sensitivity to DNA damage. In addition, activation of p53 by ATM requires multiple serine residues in p53's transactivation domain.

The Ataxia telangiectasia gene product is required for oxidative stress-induced G1 and G2 checkpoint function in human fibroblasts

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by neuronal degeneration accompanied by ataxia, telangiectasias, acute cancer predisposition, and sensitivity to ionizing radiation (IR). Cells from individuals with AT show unusual sensitivity to IR, severely attenuated cell cycle checkpoint functions, and poor p53 induction in response to IR compared with normal human fibroblasts (NHFs). The gene mutated in AT (ATM) has been cloned, and its product, pATM, has IR-inducible kinase activity. The AT phenotype has been suggested to be a consequence, at least in part, of an inability to respond appropriately to oxidative damage. To test this hypothesis, we examined the ability of NHFs and AT dermal fibroblasts to respond to t-butyl hydroperoxide and IR treatment. AT fibroblasts exhibit, in comparison to NHFs, increased sensitivity to the toxicity of t-butyl hydroperoxide, as measured by colony-forming efficiency assays. Unlike NHFs, AT fibroblasts fail to show G(1) and G(2) phase checkpoint functions or to induce p53 in response to t-butyl hydroperoxide. Treatment of NHFs with t-butyl hydroperoxide activates pATM-associated kinase activity. Our results indicate that pATM is involved in responding to certain aspects of oxidative damage and in signaling this information to downstream effectors of the cell cycle checkpoint functions. Our data further suggest that some of the pathologies seen in AT could arise as a consequence of an inability to respond normally to oxidative damage.

Telomeric protein Pin2/TRF1 as an important ATM target in response to double strand DNA breaks

ATM mutations are responsible for the genetic disease ataxia-telangiectasia (A-T). ATM encodes a protein kinase that is activated by ionizing radiation-induced double strand DNA breaks. Cells derived from A-T patients show many abnormalities, including accelerated telomere loss and hypersensitivity to ionizing radiation; they enter into mitosis and apoptosis after DNA damage. Pin2 was originally identified as a protein involved in G(2)/M regulation and is almost identical to TRF1, a telomeric protein that negatively regulates telomere elongation. Pin2 and TRF1, probably encoded by the same gene, PIN2/TRF1, are regulated during the cell cycle. Furthermore, up-regulation of Pin2 or TRF1 induces mitotic entry and apoptosis, a phenotype similar to that of A-T cells after DNA damage. These results suggest that ATM may regulate the function of Pin2/TRF1, but their exact relationship remains unknown. Here we show that Pin2/TRF1 coimmunoprecipitated with ATM, and its phosphorylation was increased in an ATM-dependent manner by ionizing DNA damage. Furthermore, activated ATM directly phosphorylated Pin2/TRF1 preferentially on the conserved Ser(219)-Gln site in vitro and in vivo. The biological significance of this phosphorylation is substantiated by functional analyses of the phosphorylation site mutants. Although expression of Pin2 and its mutants has no detectable effect on telomere length in transient transfection, a Pin2 mutant refractory to ATM phosphorylation on Ser(219) potently induces mitotic entry and apoptosis and increases radiation hypersensitivity of A-T cells. In contrast, Pin2 mutants mimicking ATM phosphorylation on Ser(219) completely fail to induce apoptosis and also reduce radiation hypersensitivity of A-T cells. Interestingly, the phenotype of the phosphorylation-mimicking mutants is the same as that which resulted from inhibition of endogenous Pin2/TRF1 in A-T cells by its dominant-negative mutants. These results demonstrate for the first time that ATM interacts with and phosphorylates Pin2/TRF1 and suggest that Pin2/TRF1 may be involved in the cellular response to double strand DNA breaks.

Regulation and localization of the Bloom syndrome protein in response to DNA damage

Bloom syndrome (BS) is an autosomal recessive disorder characterized by a high incidence of cancer and genomic instability. BLM, the protein defective in BS, is a RecQ-like helicase, presumed to function in DNA replication, recombination, or repair. BLM localizes to promyelocytic leukemia protein (PML) nuclear bodies and is expressed during late S and G2. We show, in normal human cells, that the recombination/repair proteins hRAD51 and replication protein (RP)-A assembled with BLM into a fraction of PML bodies during late S/G2. Biochemical experiments suggested that BLM resides in a nuclear matrix-bound complex in which association with hRAD51 may be direct. DNA-damaging agents that cause double strand breaks and a G2 delay induced BLM by a p53- and ataxia-telangiectasia mutated independent mechanism. This induction depended on the G2 delay, because it failed to occur when G2 was prevented or bypassed. It coincided with the appearance of foci containing BLM, PML, hRAD51 and RP-A, which resembled ionizing radiation-induced foci. After radiation, foci containing BLM and PML formed at sites of single-stranded DNA and presumptive repair in normal cells, but not in cells with defective PML. Our findings suggest that BLM is part of a dynamic nuclear matrix-based complex that requires PML and functions during G2 in undamaged cells and recombinational repair after DNA damage.

Telomeric protein Pin2/TRF1 induces mitotic entry and apoptosis in cells with short telomeres and is down-regulated in human breast tumors

Telomeres are essential for cell survival and have been implicated in the mitotic control. The telomeric protein Pin2/TRF1 controls telomere elongation and its expression is tightly regulated during cell cycle. We previously reported that overexpression of Pin2/TRF1 affects mitotic progression. However, the role of Pin2/TRF1 at the interface between cell division and cell survival remains to be determined. Here we show that overexpression of Pin2 induced apoptosis in cells containing short telomeres, but not in cells with long telomeres. Furthermore, before entering apoptosis, Pin2-expressing cells first accumulated in mitosis and strongly stained with the mitosis-specific MPM2 antibody. Moreover, Pin2-induced apoptosis is potentiated by arresting cells in mitosis, but suppressed by accumulating cells in G1. In addition, overexpression of Pin2 also resulted in activation of caspase-3, and its proapoptotic activity was significantly reduced by inhibition of caspase-3. These results indicate that up-regulation of Pin2/TRF1 can specifically induce entry into mitosis and apoptosis, likely via a mechanism related to activation of caspase-3. Significantly, we also found that, out of 51 human breast cancer tissues and 10 normal controls examined, protein levels of Pin2/TRF1 in tumors were significantly lower than in normal tissues, as detected by immunoblotting analysis and immunocytochemistry. Since down-regulation of Pin2/TRF1 allows cells to maintain long telomeres, these results suggest that down-regulation of Pin2/TRF1 may be important for cancer cells to extend their proliferative potential.

ATM: a mediator of multiple responses to genotoxic stress

The ATM protein kinase is the product of the gene responsible for the pleiotropic recessive disorder ataxia-telangiectasia. ATM-deficient cells show enhanced sensitivity and greatly reduced responses to genotoxic agents that generate DNA double strand breaks (DSBs), such as ionizing radiation and radiomimetic chemicals, but exhibit normal responses to DNA adducts and base modifications induced by other agents. Therefore, DSBs are most likely the predominant signal for the activation of ATM-mediated pathways. Identification of the ATM gene triggered extensive research aimed at elucidating the numerous functions of its large multifaceted protein product. While ATM has both nuclear and cytoplasmic functions, this review will focus on its roles in the nucleus where it plays a central role in the very early stages of damage detection and serves as a master controller of cellular responses to DSBs. By activating key regulators of multiple signal transduction pathways, ATM mediates the efficient induction of a signaling network responsible for repair of the damage, and for cellular recovery and survival.

Overexpression of Rb and E2F-1 in ataxia-telangiectasia lymphocytes

AT cells exhibit defective cell cycle regulation following DNA damage. Previous studies have shown that induction of p53 and p21 proteins are delayed in response to ionizing radiation, resulting in the failure of G1/S checkpoint in AT cells. In this study, further investigation of the molecular mechanisms underlying G1/S phase progression in AT cells was conducted. Exponentially growing normal and AT cells were exposed to 2 Gy of ionizing radiation and the expression levels and functional activities of Rb and E2F-1 proteins were determined. We observed overexpression of hyperphosphorylated Rb and E2F-1 proteins in AT cells, which was unaffected post-irradiation. Furthermore, gel shift assays showed that E2F-1-DNA binding was constitutive in AT cells, whereas it was inhibited in control cells following exposure to ionizing radiation. The data suggests that abnormalities in the function of Rb and E2F-1 proteins may also be responsible for the failure of AT cells to arrest in the G1/S checkpoint in response to DNA damage.

The radioprotective effect of BBI is associated with the activation of DNA repair-relevant genes

PURPOSE

To investigate the molecular mechanisms of the radioprotective effect of the Bowman-Birk proteinase inhibitor (BBI) in normal human skin fibroblasts (HSF).

MATERIAL AND METHODS

The effect of BBI pre-treatment on p53 protein level and on mRNA levels of downstream genes (ERCC3, Gadd45 and p53) was investigated.

RESULTS

As indicated by time-course experiments based on clonogenic assays, a 6 h pre-incubation with BBI before irradiation of HSF with a single dose of 6 Gy resulted in maximum radioprotection. In non-irradiated cells, pre-incubation with BBI resulted in an increased level of p53 protein. Concomitantly, enhanced mRNA levels of the ERCC3 and the Gadd45 genes were observed. As a consequence, BBI-treated cells showed accelerated DNA repair compared with untreated cells when irradiated.

CONCLUSIONS

The radioprotective effect of the Bowman-Birk proteinase inhibitor was accompanied by elevated mRNA expression of repair-relevant genes prior to irradiation. Activation of the DNA-repair machinery induced by pre-treatment with BBI is one possible mechanism of the radioprotective effect of BBI.

The Schizosaccharomyces pombe S-phase checkpoint differentiates between different types of DNA damage

We have identified an S-phase DNA damage checkpoint in Schizosaccharomyces pombe. This checkpoint is dependent on Rad3, the S. pombe homolog of the mammalian ATM/ATR checkpoint proteins, and Cds1. Cds1 had previously been believed to be involved only in the replication checkpoint. The requirement of Cds1 in the DNA damage checkpoint suggests that Cds1 may be a general target of S-phase checkpoints. Unlike other checkpoints, the S. pombe S-phase DNA damage checkpoint discriminates between different types of damage. UV-irradiation, which causes base modification that can be repaired during G1 and S-phase, invokes the checkpoint, while gamma-irradiation, which causes double-stranded breaks that cannot be repaired by a haploid cell if induced before replication, does not invoke the checkpoint. Because the same genes are required to respond to UV- and gamma-irradiation during G2, this discrimination may represent an active suppression of the gamma response during S-phase.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Characterization and cell cycle regulation of the related human telomeric proteins Pin2 and TRF1 suggest a role in mitosis

Telomeres are essential for preserving chromosome integrity during the cell cycle and have been specifically implicated in mitotic progression, but little is known about the signaling molecule(s) involved. The human telomeric repeat binding factor protein (TRF1) is shown to be important in regulating telomere length. However, nothing is known about its function and regulation during the cell cycle. The sequence of PIN2, one of three human genes (PIN1-3) we previously cloned whose products interact with the Aspergillus NIMA cell cycle regulatory protein kinase, reveals that it encodes a protein that is identical in sequence to TRF1 apart from an internal deletion of 20 amino acids; Pin2 and TRF1 may be derived from the same gene, PIN2/TRF1. However, in the cell Pin2 was found to be the major expressed product and to form homo- and heterodimers with TRF1; both dimers were localized at telomeres. Pin2 directly bound the human telomeric repeat DNA in vitro, and was localized to all telomeres uniformly in telomerase-positive cells. In contrast, in several cell lines that contain barely detectable telomerase activity, Pin2 was highly concentrated at only a few telomeres. Interestingly, the protein level of Pin2 was highly regulated during the cell cycle, being strikingly increased in G2+M and decreased in G1 cells. Moreover, overexpression of Pin2 resulted in an accumulation of HeLa cells in G2+M. These results indicate that Pin2 is the major human telomeric protein and is highly regulated during the cell cycle, with a possible role in mitosis. The results also suggest that Pin2/TRF1 may connect mitotic control to the telomere regulatory machinery whose deregulation has been implicated in cancer and aging.

Aneuploid nucleomegaly of bronchial cells in ataxia-telangiectasia: cytologic recognition in bronchial brushings

Ataxia-telangiectasia (AT) is an autosomal recessive disorder of childhood onset characterized by cerebellar ataxia and cutaneous and conjunctival telangiectasias, which affects many systems and organs. One histologic feature of AT is the presence of enlarged dystrophic nuclei, predominantly in satellite cells of sympathetic ganglia and dorsal roots. This paper describes the recognition of nucleomegaly of respiratory cells in bronchial brushings of a 9-year-old patient with AT. The enlarged nuclei displayed smooth nuclear contour, coarse and clumped chromatin granules, and one or two conspicuous nucleoli. The average size was 0.1015 mm in the AT case and 0.0573 mm in control cells. Ploidy analysis demonstrated an aneuploid population of cells with a DNA index of 1.31 and a S-G2M fase of 4.48% in the AT, while the control nuclei showed normal diploid values. To our knowledge, this is the first report of a description of aneuploid nucleomegaly of bronchial cells detected in bronchial smears from a patient with AT. Given that malignant transformations are usually preceded by ploidy alterations, it seems likely that the presence of an aneuploid cell population probably correlates with the increased cancer risk observed in AT patients. Cytopathologists must bear in mind these morphologic features of aneuploid nucleomegaly exhibited by certain cell populations when examining a smear from AT patients. Moreover, this finding may even represent a clue for diagnosis of AT in cases in which the disease has gone unrecognized.

A phase I and pharmacokinetic study of high dose tamoxifen and weekly cisplatin in patients with metastatic melanoma

BACKGROUND

The authors have previously demonstrated that tamoxifen (TAM) is synergistic with cisplatin (DDP) in patients with metastatic melanoma. In vitro studies have demonstrated that TAM/DDP synergy is dependent on a TAM effect that is currently under investigation. In an attempt to improve the complete response rate of this regimen, the authors initiated a Phase I trial to determine the maximum tolerated dose (MTD) of TAM that could be safely administered with weekly DDP.

METHODS

TAM was started on Day 1 at a dose of 80 mg/day and was increased by 40 mg to the MTD in groups of 3 patients. DDP (80 mg/m2) was begun on Day 2 and repeated weekly for a total of 3 weeks. During Week 4, the patients were not treated with DDP but instead evaluated for response. If disease stabilization or regression was documented, the patients received a second 3-week cycle of DDP and were then reevaluated for response. Patients with progressive disease were removed from the study.

RESULTS

In 25 consecutive patients, the overall response rate was 20%. No responses were observed in patients treated with TAM at a dose of <240 mg/day. Among 13 patients treated at or above this dose, there were 2 complete responses, 3 partial responses, 2 mixed responses, and 6 patients with progressive disease. The overall response rate for patients treated with 240 mg of TAM or higher was 38.5%. Dose-limiting toxicity, which occurred at a TAM dose of 280 mg/day, was primarily hematologic and gastrointestinal in nature. There was one toxic death (due to septic neutropenia) at this dose. There were no episodes of thrombosis.

CONCLUSIONS

A TAM dose of 240 mg/day is the recommended Phase II dose. Based on the 38.5% overall response rate at this dose, the authors have initiated a Phase II study.

p53 and ATM: cell cycle, cell death, and cancer

The development of a normal cell into a tumor cell appears to depend in part on mutations in genes that normally control cell cycle and cell death, thereby resulting in inappropriate cellular survival and tumorigenesis. ATM ("mutated in ataxia-telangiectasia") and p53 are two gene products that are believed to play a major role in maintaining the integrity of the genome such that alterations in these gene products may contribute to increased incidence of genomic changes such as deletions, translocations, and amplifications, which are common during oncogenesis. p53 is a critical participant in a signal transduction pathway that mediates either a G1 arrest or apoptosis in response to DNA damage. In addition, p53 is believed to be involved in the mitotic spindle checkpoint and in the regulation of centrosome function. Following certain cytotoxic stresses, normal ATM function is required for p53-mediated G1 arrest. ATM is also involved in other cellular processes such as S phase and G2-M phase arrest and in radiosensitivity. The understanding of the roles that both p53 and ATM play in cell cycle progression and cell death in response to DNA damage may provide new insights into the molecular mechanisms of cellular transformation and may help identify potential targets for improved cancer therapies.

Human papillomavirus E6 and E7 oncoproteins alter cell cycle progression but not radiosensitivity of carcinoma cells treated with low-dose-rate radiation

PURPOSE

Low-dose-rate radiation therapy has been widely used in the treatment of urogenital malignancies. When continuously exposed to low-dose-rate ionizing radiation, target cancer cells typically exhibit abnormalities in replicative cell-cycle progression. Cancer cells that arrest in the G2 phase of the cell cycle when irradiated may become exquisitely sensitive to killing by further low-dose-rate radiation treatment. Oncogenic human papillomaviruses (HPVs), which play a major role in the pathogenesis of uterine cervix cancers and other urogenital cancers, encode E6 and E7 transforming proteins known to abrogate a p53-dependent G1 cell-cycle checkpoint activated by conventional acute-dose radiation exposure. This study examined whether expression of HPV E6 and E7 oncoproteins by cancer cells alters the cell-cycle redistribution patterns accompanying low-dose-rate radiation treatment, and whether such alterations in cell-cycle redistribution affect cancer cell killing.

METHODS AND MATERIALS

RKO carcinoma cells, which contain wild-type P53 alleles, and RKO cell sublines genetically engineered to express HPV E6 and E7 oncoproteins, were treated with low-dose-rate (0.25-Gy/h) radiation and then assessed for p53 and p21WAF1/CIP1 polypeptide induction by immunoblot analysis, for cell-cycle redistribution by flow cytometry, and for cytotoxicity by clonogenic survival assay.

RESULTS

Low-dose-rate radiation of RKO carcinoma cells triggered p53 polypeptide elevations, p21WAF1/CIP1 induction, and arrest in the G1 and G2 phases of the cell cycle. In contrast, RKO cells expressing E6 and E7 transforming proteins from high-risk oncogenic HPVs (HPV 16) arrested in G2, but failed to arrest in G1, when treated with low-dose-rate ionizing radiation. Abrogation of the G1 cell-cycle checkpoint activated by low-dose-rate radiation exposure appeared to be a characteristic feature of transforming proteins from high-risk oncogenic HPVs: RKO cells expressing E6 from a low-risk nononcogenic HPV (HPV 11) exposed to low-dose-rate radiation arrested in both G1 and G2. Surprisingly, despite differences in cell-cycle redistribution accompanying low-dose-rate radiation treatment associated with high-risk HPV transforming protein expression, no consistent differences in clonogenic survival following low-dose-rate radiation treatment were found for RKO cell sublines expressing high-risk HPV oncoproteins and arresting only in G2 during low-dose-rate radiation exposure vs. RKO cell sublines exhibiting both G1 and G2 cell-cycle arrest when irradiated.

CONCLUSION

The results of this study demonstrate that neither HPV oncoprotein expression nor loss of the radiation-activated G1 cell-cycle checkpoint alter the sensitivity of RKO carcinoma cell lines to low-dose-rate radiation exposure in vitro. Perhaps for urogenital malignancies associated with oncogenic HPVs in vivo, HPV oncoprotein-mediated abrogation of the G1 cell-cycle checkpoint may not limit the potential efficacy of low-dose-rate radiation therapy.

Ataxia-telangiectasia and the ATM gene: linking neurodegeneration, immunodeficiency, and cancer to cell cycle checkpoints

Defects in regulation of the cellular life cycle may lead to premature cellular death or malignant transformation. Most of the proteins known to be involved in these processes are mediators of mitogenic signals or components of the cell cycle machinery. It has recently become evident, however, that systems responsible for ensuring genome stability and integrity are no less important in maintaining the normal life cycle of the cell. These systems include DNA repair enzymes and a recently emerging group of proteins that alert growth regulating mechanisms to the presence of DNA damage. These signals slow down the cell cycle while DNA repair ensues. Ataxia telangiectasia (A-T) is a genetic disorder whose clinical and cellular phenotype points to a defect in such a signaling system. A-T is characterized by neurodegeneration, immunodeficiency, radiosensitivity, cancer predisposition, and defective cell cycle checkpoints. The responsible gene, ATM, was recently cloned and sequenced. ATM encodes a large protein with a region highly similar to the catalytic domain of PI 3-kinases. The ATM protein is similar to a group of proteins in various organisms which are directly involved in the cell cycle response to DNA damage. It is expected to be part of a protein complex that responds to a specific type of DNA strand break by conveying a regulatory signal to other proteins. Interestingly, the immune and nervous systems, which differ markedly in their proliferation rates, are particularly sensitive to the absence of ATM function. The identification of the ATM gene highlights the growing importance of signal transduction initiated in the nucleus rather than in the external environment, for normal cellular growth.

Regulation of the cell cycle following DNA damage in normal and Ataxia telangiectasia cells

A proportion of the population is exposed to acute doses of ionizing radiation through medical treatment or occupational accidents, with little knowledge of the immediate effects. At the cellular level, ionizing radiation leads to the activation of a genetic program which enables the cell to increase its chances of survival and to minimize detrimental manifestations of radiation damage. Cytotoxic stress due to ionizing radiation causes genetic instability, alterations in the cell cycle, apoptosis, or necrosis. Alterations in the G1, S and G2 phases of the cell cycle coincide with improved survival and genome stability. The main cellular factors which are activated by DNA damage and interfere with the cell cycle controls are: p53, delaying the transition through the G1-S boundary; p21WAF1/CIP1, preventing the entrance into S-phase; proliferating cell nuclear antigen (PCNA) and replication protein A (RPA), blocking DNA replication; and the p53 variant protein p53 as together with the retinoblastoma protein (Rb), with less defined functions during the G2 phase of the cell cycle. By comparing a variety of radioresistant cell lines derived from radiosensitive ataxia telangiectasia cells with the parental cells, some essential mechanisms that allow cells to gain radioresistance have been identified. The results so far emphasise the importance of an adequate delay in the transition from G2 to M and the inhibition of DNA replication in the regulation of the cell cycle after exposure to ionizing radiation.

rad-dependent response of the chk1-encoded protein kinase at the DNA damage checkpoint

Exposure of eukaryotic cells to agents that generate DNA damage results in transient arrest of progression through the cell cycle. In fission yeast, the DNA damage checkpoint associated with cell cycle arrest before mitosis requires the protein kinase p56chk1. DNA damage induced by ultraviolet light, gamma radiation, or a DNA-alkylating agent has now been shown to result in phosphorylation of p56chk1. This phosphorylation decreased the mobility of p56chk1 on SDS-polyacrylamide gel electrophoresis and was abolished by a mutation in the p56chk1 catalytic domain, suggesting that it might represent autophosphorylation. Phosphorylation of p56chk1 did not occur when other checkpoint genes were inactive. Thus, p56chk1 appears to function downstream of several of the known Schizosaccharomyces pombe checkpoint gene products, including that encoded by rad3+, a gene with sequence similarity to the ATM gene mutated in patients with ataxia telangiectasia. The phosphorylation of p56chk1 provides an assayable biochemical response to activation of the DNA damage checkpoint in the G2 phase of the cell cycle.

A single ataxia telangiectasia gene with a product similar to PI-3 kinase

A gene, ATM, that is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) was identified by positional cloning on chromosome 11q22-23. AT is characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. The disease is genetically heterogeneous, with four complementation groups that have been suspected to represent different genes. ATM, which has a transcript of 12 kilobases, was found to be mutated in AT patients from all complementation groups, indicating that it is probably the sole gene responsible for this disorder. A partial ATM complementary DNA clone of 5.9 kilobases encoded a putative protein that is similar to several yeast and mammalian phosphatidylinositol-3' kinases that are involved in mitogenic signal transduction, meiotic recombination, and cell cycle control. The discovery of ATM should enhance understanding of AT and related syndromes and may allow the identification of AT heterozygotes, who are at increased risk of cancer.

Tumour suppressors, kinases and clamps: how p53 regulates the cell cycle in response to DNA damage

The human tumour suppressor protein p53 is critical for regulation of the cell cycle on genotoxic insult. When DNA is damaged by radiation, chemicals or viral infection, cells respond rapidly by arresting the cell cycle. A G1 arrest requires the activity of wild-type p53, as it is not observed in cells lacking functionally wild-type protein, and at least some component of S phase and G2/M arrests is also thought to be p53-dependent. p53 functions as a transcription factor which binds specific DNA sequences, and recently major downstream targets have been identified, including p21Cip1, an inhibitor of the cell cycle kinases that also blocks the replicative but not the repair function of DNA polymerase delta auxiliary factor, PCNA. Current interest focuses on developing novel cancer therapies based on our knowledge of the activity of p53 and p21Cip1 in the cell cycle.