Cellular response of X-ray sensitive hamster mutant cell lines to gemcitabine, cisplatin and 5-fluorouracil

Five mutant Chinese hamster cell lines deficient in DNA repair with the corresponding parental cell lines were used to determine their sensitivity to cisplatin, 5-fluorouracil and gemcitabine. The mutations in the cell lines led to defective single strand break repair (EM-C11), defective recombination mediated repair (irs1SF), defective double strand break repair (XR-V15B, a Ku-80 mutant and CR-C1, a DNA-PKcs mutant) and an AT-like mutation (VC-4). All mutant cell lines had an impaired doubling time during exponential growth and an increased sensitivity to X-irradiation. We may conclude that for cisplatin-induced cytotoxicity the homologous recombination-associated DNA repair plays an important role in the repair of the cisplatin induced lesions, confirming previous results. In 5-FU and gemcitabine induced toxicity to cells, repair processes involved with radiation-induced damage were not implicated. This is in striking contrast to the role of cisplatin in radiosensitization where inhibition of the NHEJ pathway is implicated, and to the role of gemcitabine in sensitization where specific interference with the HR pathway is implicated.

Cellular response to pulsed low-dose rate irradiation in X-ray sensitive hamster mutant cell lines

The role of DNA repair mechanisms in the cellular response to low dose rate (LDR) irradiation was studied with the aim to gain insight in the process of sublethal damage (SLD) repair. Chinese hamster cell lines mutated in either DNA single strand break (ssb) repair or DNA double strand break (dsb) repair by non homologous end joining (NHEJ) and homologous recombination (HR), or showing an AT-like phenotype, were irradiated in plateau-phase either at high dose rate (HDR, 3.3 Gy/min) or at pulsed low dose rate (p-LDR, average 1 Gy/h). Cell survival after irradiation was assessed using the clonogenic assay. A change in sensitivity when the dose rate was decreased was observed for all parental cell lines and the DNA ssb repair mutant. No difference in cell survival after p-LDR versus. HDR irradiation was observed for the two NHEJ mutants, the AT-like mutant and the HR mutant. Based on these results we conclude that single strand break repair does not play a role in the dose rate effect. The AT like protein, functional NHEJ and XRCC3 are required for the dose rate effect.

Induction of DNA breaks and apoptosis in crosslink-hypersensitive V79 cells by the cytostatic drug beta-D-glucosyl-ifosfamide mustard

To study molecular aspects of cytotoxicity of the anticancer drug beta-D-glucose-ifosfamide mustard we investigated the potential of the agent to induce apoptosis and DNA breakage. Since beta-D-glucose-ifosfamide mustard generates DNA interstrand crosslinks, we used as an in vitro model system a pair of isogenic Chinese hamster V79 cells differing in their sensitivity to crosslinking agents. CL-V5B cells are dramatically more sensitive (30-fold based on D(10) values) to the cytotoxic effects of beta-D-glucose-ifosfamide mustard as compared to parental V79B cells. After 48 h of pulse-treatment with the agent, sensitive cells but not the resistant parental line undergo apoptosis and necrosis, with apoptosis being the predominant form of cell death (70 and 20% of apoptosis and necrosis, respectively). Apoptosis increased as a function of dose and was accompanied by induction of DNA double-strand breaks in the hypersensitive cells. Furthermore, a strong decline in the level of Bcl-2 protein and activation of caspases-3, -8 and -9 were observed. The resistant parental cells were refractory to all these parameters. Bcl-2 decline in the sensitive cells preceded apoptosis, and transfection-mediated overexpression of Bcl-2 protected at least in part from apoptosis. From the data we hypothesize that non-repaired crosslinks induced by beta-D-glucose-ifosfamide mustard are transformed into double-strand breaks which trigger apoptosis via a Bcl-2 dependent pathway.

Restoration of nucleotide excision repair in a helicase-deficient XPD mutant from intragenic suppression by a trichothiodystrophy mutation

The UV-sensitive V-H1 cell line has a T46I substitution mutation in the Walker A box in both alleles of XPD and lacks DNA helicase activity. We characterized three partial revertants that curiously display intermediate UV cytotoxicity (2- to 2.5-fold) but normal levels of UV-induced hprt mutations. In revertant RH1-26, the efficient removal of pyrimidine (6-4) pyrimidone photoproducts from both strands of hprt suggests that global-genomic nucleotide excision repair is normal, but the pattern of cyclobutane pyrimidine dimer removal suggests that transcription-coupled repair (TCR) is impaired. To explain the intermediate UV survival and lack of RNA synthesis recovery in RH1-26 after 10 J of UV/m(2), we propose a defect in repair-transcription coupling, i.e., the inability of the cells to resume or reinitiate transcription after the first TCR event within a transcript. All three revertants carry an R658H suppressor mutation, in one allele of revertants RH1-26 and RH1-53 and in both alleles of revertant RH1-3. Remarkably, the R658H mutation produces the clinical phenotype of trichothiodystrophy (TTD) in several patients who display intermediate UV sensitivity. The XPD(R658H) TTD protein, like XPD(T46I/R658H), is codominant when overexpressed in V-H1 cells and partially complements their UV sensitivity. Thus, the suppressing R658H substitution must restore helicase activity to the inactive XPD(T46I) protein. Based on current knowledge of helicase structure, the intragenic reversion mutation may partially compensate for the T46I mutation by perturbing the XPD structure in a way that counteracts the effect of this mutation. These findings have implications for understanding the differences between xeroderma pigmentosum and TTD and illustrate the value of suppressor genetics for studying helicase structure-function relationships.

The relationship between the RBE of alpha particles and the radiosensitivity of different mutations of Chinese hamster cells

The RBE of alpha-particles in different mutations of Chinese hamster cells was determined with the aim of identifying differences in the sensitivity to x-ray and alpha-particle-induced DNA damage. Two parental lines of Chinese hamster cells and four radiosensitive mutants were irradiated with different single doses of x-rays and alpha-particles and clonogenic cell survival was determined. Radiosensitivity to x-rays varied by a factor of 5 between the cell strains whereas sensitivity to alpha-particle irradiation was almost identical among all strains. The RBE is only determined by the sensitivity of the cells towards x-rays. Since cells with different defects of repair or cell cycle control have different radiosensitivities, we conclude that the effects of x-ray irradiation and the RBE are mostly determined by the activity of repair processes.

Telomerase suppression by chromosome 6 in a human papillomavirus type 16-immortalized keratinocyte cell line and in a cervical cancer cell line

BACKGROUND

High-risk human papillomavirus (HPV) types play a major role in the development of cervical cancer in vivo and can induce immortalization of primary human keratinocytes in vitro. Activation of the telomere-lengthening enzyme telomerase constitutes a key event in both processes. Because losses of alleles from chromosome 6 and increased telomerase activity have been observed in high-grade premalignant cervical lesions, we analyzed whether human chromosome 6 harbors a putative telomerase repressor locus that may be involved in HPV-mediated immortalization.

METHODS

Microcell-mediated chromosome transfer was used to introduce chromosomes 6 and 11 to the in vitro generated HPV type 16 (HPV16)-immortalized keratinocyte cell line FK16A and to the in vivo derived HPV16-containing cervical cancer cell line SIHA: Hybrid clones were analyzed for growth characteristics, telomerase activity, human telomerase reverse transcriptase (hTERT) and HPV16 E6 expression, and telomere length. FK16A hybrid clones were also transduced with an hTERT-containing retrovirus to examine the effect of ectopic hTERT expression on growth. Statistical tests were two-sided.

RESULTS

Introduction of human chromosome 6 but not of chromosome 11 to both cell lines yielded hybrid cells that demonstrated crisis-like features (i.e., enlarged and flattened morphology, vacuolation, and multinucleation) and underwent growth arrest after a marked lag period. In the chromosome 6 hybrid clones analyzed, telomerase activity and hTERT messenger RNA (mRNA) expression were statistically significantly reduced compared with those in the chromosome 11 hybrid clones (for telomerase activity, P =.004 for the FK16A hybrids and P =.039 for the SiHa hybrids; for hTERT mRNA expression, P =.003 for the FK16A hybrids). The observed growth arrest was associated with telomeric shortening. Ectopic expression of hTERT in FK16A cells could prevent the telomeric shortening-based growth arrest induced by chromosome 6.

CONCLUSIONS

Chromosome 6 may harbor a repressor of hTERT transcription, the loss of which may be involved in HPV-mediated immortalization.

Complementation of chromosomal aberrations in AT/NBS hybrids: inadequacy of RDS as an endpoint in complementation studies with immortal NBS cells

Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT) are rare autosomal recessive hereditary disorders characterized by radiosensitivity, chromosomal instability, immunodeficiency and proneness to cancer. Although the clinical features of both syndromes are quite distinct, the cellular characteristics are very similar. Cells from both NBS and AT patients are hypersensitive to ionizing radiation (IR), show elevated levels of chromosomal aberrations and display radioresistant DNA synthesis (RDS). The proteins defective in NBS and AT, NBS1 and ATM, respectively, are involved in the same pathway, but their exact relationship is not yet fully understood. Stumm et al. (Am. J. Hum. Genet. 60 (1997) 1246) have reported that hybrids of AT and NBS lymphoblasts were not complemented for chromosomal aberrations. In contrast, we found that X-ray-induced cell killing as well as chromosomal aberrations were complemented in proliferating NBS-1LBI/AT5BIVA hybrids, comparable to that in NBS-1LBI cells after transfer of a single human chromosome 8 providing the NBS1 gene. RDS observed in AT5BIVA cells was reduced in these hybrids to the level of that seen in immortal NBS-1LBI cells. However, the level of DNA synthesis, following ionizing radiation, in SV40 transformed wild-type cell lines was the same as in NBS-1LBI cells. Only primary wild-type cells showed stronger inhibition of DNA synthesis. In summary, these results clearly indicate that RDS cannot be used as an endpoint in functional complementation studies with immortal NBS-1LBI cells, whereas the cytogenetic assay is suitable for complementation studies with immortal AT and NBS cells.

Differential responses of Chinese hamster mutagen sensitive cell lines to low and high concentrations of calicheamicin and neocarzinostatin

To shed light on the mechanism underlying the cellular response to the radiomimetic agents calicheamicin Y(1)(1) (CAL) and neocarzinostatin (NCS), several hamster cell mutants defective in different DNA repair pathways were used. Two X-ray sensitive Chinese hamster V79 mutant cell lines, XR-V9B and V-E5 were studied for their response to the induction of cell killing, micronuclei, and G2-chromosomal aberrations relative to that of parental wild-type cells. In addition, effects of CAL and NCS on bleomycin sensitive BL-V40 cells and on UV sensitive V-H1 cells were analyzed. In general, the radiosensitive cell lines showed the highest sensitivities to CAL and NCS, but also the other mutants demonstrated differences in their responses compared to wild-type cells. With respect to cell killing, expressed as D(10)-value, enhanced sensitivities of mutants with factors up to 4.4 were recorded. For the induction of micronuclei (MN) and chromosomal aberrations (CA) all cell lines, including the parental cells, show a steep increase in the frequencies at the lowest tested doses and a leveling off at higher concentrations. Probably toxic effects at the higher exposure levels are responsible for these biphasic dose effect curves. Enhanced sensitivities of the various mutants were primarily observed at the higher exposure levels. With respect to the induction of MN increased sensitivities up to a factor of 18.1 were observed for the radiosensitive mutants, whereas for CA the mutant cell lines showed a variation from resistance (0.3) of VH-1 cells up to a 3.8-fold higher sensitivity to the radiomimetic agents. However, at the lowest tested concentrations for both MN and CA, the differences between the sensitive mutants and wild-type clearly diminished, suggesting the existence of residual and/or alternative DNA repair pathways in these mutants.

A new X-ray sensitive CHO cell mutant of ionizing radiation group 7,XR-C2, that is defective in DSB repair but has only a mild defect in V(D)J recombination

The DNA-dependent protein kinase (DNA-PK) complex plays a key role in DNA double-strand break (DSB) repair and V(D)J recombination. Using a genetic approach we have isolated cell mutants sensitive to ionizing radiation (IR) in the hope of elucidating the mechanism and components required for these pathways. We describe here, an X-ray-sensitive and DSB repair defective Chinese hamster ovary (CHO) cell line, XR-C2, which was assigned to the X-Ray Cross Complementation (XRCC) group 7. This group of mutants is defective in the XRCC7/SCID/Prkdc gene, which encodes the catalytic subunit of DNA-PK (DNA-PKcs). Despite the fact that XR-C2 cells expressed normal levels of DNA-PKcs protein, no DNA-PK catalytic activity could be observed in XR-C2, confirming the genetic analyses that these cells harbor a dysfunctional gene for DNA-PKcs. In contrast to other IR group 7 mutants, which contain undetectable or low levels of DNA-PKcs protein and which show a severe defect in V(D)J recombination, XR-C2 cells manifested only a mild defect in both coding and signal junction formation. The unique phenotype of the XR-C2 mutant suggests that a normal level of kinase activity is critical for radiation resistance but not for V(D)J recombination, whereas the overall structure of the DNA-PKcs protein appears to be of great importance for this process.

Functional link between ataxia-telangiectasia and Nijmegen breakage syndrome gene products

Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are recessive genetic disorders with susceptibility to cancer and similar cellular phenotypes. The protein product of the gene responsible for A-T, designated ATM, is a member of a family of kinases characterized by a carboxy-terminal phosphatidylinositol 3-kinase-like domain. The NBS1 protein is specifically mutated in patients with Nijmegen breakage syndrome and forms a complex with the DNA repair proteins Rad50 and Mrel1. Here we show that phosphorylation of NBS1, induced by ionizing radiation, requires catalytically active ATM. Complexes containing ATM and NBS1 exist in vivo in both untreated cells and cells treated with ionizing radiation. We have identified two residues of NBS1, Ser 278 and Ser 343 that are phosphorylated in vitro by ATM and whose modification in vivo is essential for the cellular response to DNA damage. This response includes S-phase checkpoint activation, formation of the NBS1/Mrel1/Rad50 nuclear foci and rescue of hypersensitivity to ionizing radiation. Together, these results demonstrate a biochemical link between cell-cycle checkpoints activated by DNA damage and DNA repair in two genetic diseases with overlapping phenotypes.

Involvement of apoptosis in mitomycin C hypersensitivity of Chinese hamster cell mutants

To elucidate the mechanisms of the mammalian cell defense against cross-linking agents, we studied previously cellular responses to mitomycin C (MMC) treatment in two MMC-hypersensitive hamster cell mutants' V-H4 and V-C8, as well as their parental cell line V79. In the present report, we investigated whether alterations in cell cycle checkpoints and induction of apoptosis could be responsible for the MMC hypersensitivity of the V-H4 and V-C8 mutant cell lines. First, we found that parental and mutant cells exhibited similar cell cycle responses to MMC concentrations of equivalent cytotoxicity, arguing against a defective cell cycle checkpoint in hypersensitive cell lines. In contrast, we showed that mutant cells underwent greater levels of apoptosis following MMC treatment than parental cells. These findings indicate that increased induction of apoptosis contributes to the hypersensitivity of V-H4 and V-C8 cells to the growth inhibitory effect of MMC. This differential apoptotic response was observed with both equimolar and equitoxic MMC doses and was specific to the cross-linking agent MMC, suggesting that control of the apoptotic process is altered in both MMC-hypersensitive mutants. The defective genes in V-H4 and V-C8 cells would then function in the regulation of an apoptotic pathway triggered by MMC-induced damage and independent of p53-mediated transcription.

Retroviral expression of the NBS1 gene in cultured Nijmegen breakage syndrome cells restores normal radiation sensitivity and nuclear focus formation

The majority of cases of the autosomal recessive disorder Nijmegen breakage syndrome (NBS) are associated with null mutations in the NBS1 gene, which encodes a 95 kDa protein, nibrin. Cell lines established from NBS patients fail to express nibrin and display hypersensitivity to ionizing radiation and dysregulation of the nuclear localization of two key proteins involved in DNA repair, Mre11 and Rad50. Conclusive proof that mutations in the NBS1 gene are responsible for NBS requires that re-expression of normal nibrin in NBS cells complements these phenotypes. In the current study, retroviral expression vectors containing a normal copy of the NBS1 gene or a mutated form derived from a NBS patient were introduced into a well- characterized NBS cell line. Introduction of a normal copy of the NBS1 gene, but not the mutant form, resulted in robust expression of nibrin that displayed correct nuclear localization. Expression of nibrin also restored the ability of nibrin, Mre11 and Rad50 to complex and to redistribute within the nucleus in response to ionizing radiation. Radiation sensitivity of NBS cells expressing wild-type nibrin was restored to normal levels. Hence, introduction of the NBS1 gene can correct the phenotypes observed in NBS cells.

Characterization of the mutational profile of (+)-7R,8S-dihydroxy-9S, 10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene at the hypoxanthine (guanine) phosphoribosyltransferase gene in repair-deficient Chinese hamster V-H1 cells

Earlier studies have shown that the profile of mutations induced by (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (+)-BPDE at the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene of Chinese hamster V79 cells was dependent on the concentration of (+)-BPDE. In the present study, we examined the effect of the concentration of (+)-BPDE on its mutational profile at the hprt gene in repair-deficient V-H1 cells (a derivative of V79 cells) to explore the role of DNA repair in the dose-dependent mutational profile of (+)-BPDE. Independent hprt mutant clones were isolated after exposing V-H1 cells to dimethylsulfoxide (DMSO) or to low (4-6 nM; 95% cell survival) or high (40-48 nM; 31% cell survival) concentrations of (+)-BPDE in DMSO. The mutation frequencies for the DMSO control and for the low and high concentration groups were 0.1, 2.1 and 32.9 mutant colonies/10(5) survivors, respectively. The profile of mutations at the hprt gene was characterized for 148 (+)-BPDE-induced mutant clones and the results from the present study were compared with those obtained earlier with V79 cells. The data indicated that: (i) V-H1 cells were approximately 9-fold more sensitive to the cytotoxic effects of (+)-BPDE than V79 cells; (ii) the mutation frequency in V-H1 cells was similar to that observed in V79 cells following exposure to similar concentrations of (+)-BPDE; (iii) (+)-BPDE-induced mutations at guanine on the transcribed strand of the hprt gene were common in V-H1 cells but were extraordinarily rare in V79 cells; (iv) (+)-BPDE-induced mutations at adenine on the transcribed strand of the hprt gene were common in both V-H1 and V79 cells; (v) although exposure of V79 cells to different doses of (+)-BPDE resulted in a dose-dependent mutational profile at the hprt gene, this was not observed in V-H1 cells. Our observations indicate a defect in the transcription-coupled repair of (+)-BPDE-DNA adducts in V-H1 cells and that the repair activity deficient in V-H1 cells is essential for the dose-dependent mutational profile observed with (+)-BPDE in V79 cells.

Immortalization and characterization of Nijmegen Breakage syndrome fibroblasts

Nijmegen Breakage Syndrome (NBS) is a very rare autosomal recessive chromosomal instability disorder characterized by microcephaly, growth retardation, immunodeficiency and a high incidence of malignancies. Cells from NBS patients are hypersensitive to ionizing radiation (IR) and display radioresistant DNA synthesis (RDS). NBS is caused by mutations in the NBS1 gene on chromosome 8q21 encoding a protein called nibrin. This protein is a component of the hMre11/hRad50 protein complex, suggesting a defect in DNA double-strand break (DSB) repair and/or cell cycle checkpoint function in NBS cells. We established SV40 transformed, immortal NBS fibroblasts, from primary cells derived from a Polish patient, carrying the common founder mutation 657del5. Immortalized NBS cells, like primary cells, are X-ray sensitive (2-fold) and display RDS following IR. They show an increased sensitivity to bleomycin (3.5-fold), etoposide (2.5-fold), camptothecin (3-fold) and mitomycin C (1.5-fold), but normal sensitivity towards UV-C. Despite the clear hypersensitivity towards DSB-inducing agents, the overall rates of DSB-rejoining in NBS cells as measured by pulsed field gel electrophoresis were found to be very similar to those of wild type cells. This indicates that the X-ray sensitivity of NBS cells is not directly caused by an overt defect in DSB repair.

Mammalian X-ray-sensitive mutants which are defective in non-homologous (illegitimate) DNA double-strand break repair

In all organisms multiple pathways to repair DNA double-strand breaks (DSB) have been identified. In mammalian cells DSB are repaired by two distinct pathways, homologous and non-homologous (illegitimate) recombination. X-ray-sensitive mutants have provided a tool for the identification and understanding of the illegitimate recombination pathway in mammalian cells. Two (sub-)pathways can be distinguished, the first mediated by DNA-PK-dependent protein kinase (DNA-PK), and the second directed by the hMre11/hRad50 complex. A variety of mutants impaired in DSB repair by illegitimate recombination, with mutations in Ku, DNA-PKcs, XRCC4 or nibrin, have been described. Herein, the characterization of these mutants with respect to the impaired cellular function and the molecular defect is provided. Further studies on these mutants, as well as on new mutants impaired in as-of-yet unidentified pathways, should be helpful to a better understanding of DSB repair and of the processes leading to genome instability and cancer.

Enhanced UV-induced mutagenesis in the UV61 cell line, the Chinese hamster homologue of Cockayne's syndrome B, is associated with defective transcription coupled repair of cyclobutane pyrimidine dimers

Cells from Cockayne's syndrome (CS) patients are hypersensitive to the cytotoxic effects of UV-irradiation and are defective in transcription coupled repair (TCR). We have examined the mutagenic consequences of impaired TCR in the Chinese hamster cell line UV61, the rodent homologue of CS complementation group B. Analysis of the two major UV-induced photolesions, cyclobutane pyrimidine dimers (CPD) and pyrimidine 6-4 pyrimidone photoproducts (6-4 PP), revealed that repair of CPD from the transcribed strand was strongly reduced in UV61 cells, but repair of 6-4 PP was indistinguishable from that in wild-type hamster cells. UV-induced mutation induction was enhanced in UV61 compared to that observed in repair proficient cells. The spectrum of UV-induced base substitutions in UV61 was clearly different from that observed in wild-type hamster cells and resembled the spectrum previously observed in nucleotide excision repair deficient hamster cells. In UV61 cells a strong strand bias for mutation induction was found; assuming that premutagenic lesions occur at dipyrimidine sequences, 76% of the mutations could be attributed to lesions in the transcribed strand. These data strongly favour the hypothesis that defective TCR of CPD is responsible for the enhanced UV-induced mutagenesis in UV61 cells.

Lack of correlation between repair of DNA interstrand cross-links and hypersensitivity of hamster cells towards mitomycin C and cisplatin

The ability to repair DNA interstrand cross-links may be an important factor contributing to mitomycin C (MMC) and cisplatin cytotoxicities. We have assessed the repair of interstrand cross-links induced by MMC in two MMC-hypersensitive hamster cell mutants and their resistant parental cell line. Using a gene-specific repair assay, we found no evidence for repair of MMC cross-links in either parental or mutant cells, suggesting that persistence of DNA interstrand cross-links is not responsible for the differential toxicity of MMC towards hypersensitive cells. Repair of cisplatin-induced interstrand cross-links was efficient in resistant as well as in mutant cells. Therefore we concluded that a defect in excision repair of interstrand cross-links was not responsible for the cytotoxic effects of MMC and cisplatin in these hypersensitive mutants.

Molecular and biochemical characterization of new X-ray-sensitive hamster cell mutants defective in Ku80

Ku, a heterodimer of approximately 70 and approximately 80 kDa subunits, is a nuclear protein that binds to double-stranded DNA ends and is a component of the DNA-dependent protein kinase (DNA-PK). Cell lines defective in Ku80 belong to group XRCC5 of ionizing radiation-sensitive mutants. Five new independent Chinese hamster cell mutants, XR-V10B, XR-V11B, XR-V12B, XR-V13B and XR-V16B, that belong to this group were isolated. To shed light on the nature of the defect in Ku80, the molecular and biochemical characteristics of these mutants were examined. All mutants, except XR-V12B, express Ku80 mRNA, but no Ku80 protein could clearly be detected by immunoblot analysis in any of them. DNA sequence analysis of the Ku80 cDNA from these mutants showed a deletion of 252 bp in XR-V10B; a 6 bp deletion that results in a new amino acid residue at position 107 and the loss of two amino acid residues at positions 108 and 109 in XR-V11B; a missense mutation resulting in a substitution of Cys for Tyr at position 114 in XR-V13B; and two missense mutations in XR-V16B, resulting in a substitution of Met for Val at position 331 and Arg for Gly at position 354. All these mutations cause a similar, 5-7-fold, increase in X-ray sensitivity in comparison to wild-type cells, and a complete lack of DNA-end binding and DNA-PK activities. This indicates that all these mutations lead to loss of the Ku80 function due to instability of the defective protein.

Hypersensitivity of Ku-deficient cells toward the DNA topoisomerase II inhibitor ICRF-193 suggests a novel role for Ku antigen during the G2 and M phases of the cell cycle

Ku antigen is a heterodimer, comprised of 86- and 70-kDa subunits, which binds preferentially to free DNA ends. Ku is associated with a catalytic subunit of 450 kDa in the DNA-dependent protein kinase (DNA-PK), which plays a crucial role in DNA double-strand break (DSB) repair and V(D)J recombination of immunoglobulin and T-cell receptor genes. We now demonstrate that Ku86 (86-kDa subunit)-deficient Chinese hamster cell lines are hypersensitive to ICRF-193, a DNA topoisomerase II inhibitor that does not produce DSB in DNA. Mutant cells were blocked in G2 at drug doses which had no effect on wild-type cells. Moreover, bypass of this G2 block by caffeine revealed defective chromosome condensation in Ku86-deficient cells. The hypersensitivity of Ku86-deficient cells toward ICRF-193 was not due to impaired in vitro decatenation activity or altered levels of DNA topoisomerase IIalpha or -beta. Rather, wild-type sensitivity was restored by transfection of a Ku86 expression plasmid into mutant cells. In contrast to cells deficient in the Ku86 subunit of DNA-PK, cells deficient in the catalytic subunit of the enzyme neither accumulated in G2/M nor displayed defective chromosome condensation at lower doses of ICRF-193 compared to wild-type cells. Our data suggests a novel role for Ku antigen in the G2 and M phases of the cell cycle, a role that is not related to its role in DNA-PK-dependent DNA repair.

Human chromosome 21 determines growth factor dependence in human/mouse B-cell hybridomas

Interleukin 6 (IL-6) serves as a growth factor for mouse plasmacytomas. As a model for IL-6-mediated growth of plasmacytomas, we study IL-6-dependent B-cell hybridomas, which can be generated through fusion of B lymphocytes with a plasmacytoma cell line, e.g., SP2/0. In the present report, we have investigated the peculiar behavior of B-cell hybridomas with respect to IL-6 dependence. We demonstrate that although newly generated hybridomas are IL-6 dependent, many hybridomas lose this dependency at frequencies as high as 50%, shortly after fusion. We speculated that the loss of IL-6-dependent growth is due to the well-known chromosomal instability of B-cell hybridomas. Consequently, loss of IL-6 dependence is the result of loss of a specific chromosome(s). This model implies the existence of an "IL-6 dependency" gene, the loss of which makes hybridomas capable of proliferating in the absence of IL-6. Because SP2/0 is IL-6 independent, the IL-6-dependent phenotype of B-cell hybridomas, and hence the IL-6 dependency gene, must be derived from the B lymphocyte. We have tested this model by generating human/mouse B-cell hybridomas through fusion of human B lymphocytes with SP2/0. We then analyzed the human chromosome content of 10 IL-6-dependent and 14 IL-6-independent subclones. From that analysis we concluded that the presence of human chromosome 21 correlated with IL-6 dependence. This correlation was confirmed by microcell fusion experiments in which a single copy of chromosome 21 was introduced into IL-6-independent hybridomas, resulting in reconstitution of the IL-6-dependent phenotype. We therefore conclude that chromosome 21 carries an IL-6 dependency gene.

XR-C1, a new CHO cell mutant which is defective in DNA-PKcs, is impaired in both V(D)J coding and signal joint formation

DNA-dependent protein kinase (DNA-PK) plays an important role in DNA double-strand break (DSB) repair and V(D)J recombination. We have isolated a new X-ray-sensitive CHO cell line, XR-C1, which is impaired in DSB repair and which was assigned to complementation group 7, the group that is defective in the XRCC7 / SCID ( Prkdc ) gene encoding the catalytic subunit of DNA-PK (DNA-PKcs). Consistent with this complementation analysis, XR-C1 cells lackeddetectable DNA-PKcs protein, did not display DNA-PK catalytic activity and were complemented by the introduction of a single human chromosome 8 (providing the Prkdc gene). The impact of the XR-C1 mutation on V(D)J recombination was quite different from that found in most rodent cells defective in DNA-PKcs, which are preferentially blocked in coding joint formation, whereas XR-C1 cells were defective in forming both coding and signal joints. These results suggest that DNA-PKcs is required for both coding and signal joint formation during V(D)J recombination and that the XR-C1 mutant cell line may prove to be a useful tool in understanding this pathway.

Methyl methanesulfonate-induced hprt mutation spectra in the Chinese hamster cell line CHO9 and its xrcc1-deficient derivative EM-C11

The Chinese hamster cell mutant EM-C11, which is hypersensitive to the cell killing effects of alkylating agents compared to its parental line CHO9, has been used to study the impact of base excision repair on the mutagenic effects of DNA methylation damage. This cell line has a defect in the xrcc1 gene. XRCC1 can interact with DNA polymerase-beta, thereby suppressing strand displacement, and DNA ligase III, both of which have been implicated in base excision repair. XRCC1 may, therefore, allow efficient ligation of single-strand breaks generated during base excision repair. Both EM-C11 and CHO9 cells were treated with methyl methanesulfonate (MMS), a DNA-methylating agent reacting predominantly with nitrogen atoms generating adducts which are substrates for the base excision repair pathway. EM-C11 cells are much more sensitive to the cytotoxic effects of MMS than CHO9: for EM-C11, the dose of MMS inducing 10% survival is 6-fold lower than that for CHO9. In contrast, mutation induction at the hprt locus following MMS is similar in EM-C11 and CHO9. Molecular analysis of hprt gene mutations showed that although the largest class of hprt mutations, both in EM-C11 and CHO9 cells, consisted of GC > AT transitions, most likely caused by O6-methylguanine, the size of this class was smaller in EM-C11. The fraction of deletion mutants in EM-C11, however, was twice as large as that found in CHO9 cells. These results suggest that reduced ligation efficiency of single-strand breaks generated during base excision repair, as result of a defect in XRCC1, may lead to the formation of deletions.

Mutations in hamster single-strand break repair gene XRCC1 causing defective DNA repair

The molecular basis for the DNA repair dysfunction observed in mutant Chinese hamster ovary cell lines of X-ray repair cross complementing group 1 (XRCC1) is unknown and the exact role of the XRCC1 protein remains unclear. To help clarify the role of the XRCC1 gene we analyzed four mutant cell lines of this complementation group and a revertant cell line for XRCC1 protein content and for sequence alterations in the XRCC1 coding region. Immunoblot analysis of cellular extracts indicated that each of four mutant lines was lacking XRCC1 protein, whereas the repair-proficient revertant line derived from one of these mutants contained a normal level of XRCC1. Although each of these cell lines expressed XRCC1 mRNA, we found in all cases a distinct point mutation resulting in crucial alterations in the encoded XRCC1 protein sequence of 633 amino acids. Two of the mutations cause non-conservative amino acid changes, Glu102-->Lys and Cys390-->Tyr, at positions that are invariant among hamster, mouse and human XRCC1 sequences and are located in putative functional domains. A third debilitating mutation disrupts RNA splicing, generating multiple transcripts of different length that contain deletions spanning a region of >100 amino acids in the midsection of the XRCC1 coding sequence. A fourth mutation results in a termination codon that shortens the open reading frame to 220 amino acids, however, in the revertant cell line a further mutation in the same codon, Stop221-->Leu, permits translation of a full-length functional variant protein. These mutational data indicate the importance of the putative functional regions in XRCC1, such as the BRCA1 C-terminal (BRCT) domain found in common with BRCA1 and other DNA repair and cell cycle checkpoint proteins, and also regions necessary for interaction with DNA polymerase beta and DNA ligase III.

Reversal of methylation tolerance by transfer of human chromosome 2

Human cell lines resistant to N-methyl-N-nitrosourea (MNU) were previously assigned to two complementation groups. Members of group I are defective in mismatch correction [S. Ceccotti, G Aquilina, P. Macpherson, M. Yamada, P. Karran, M. Bignami, Processing of O6-methylguanine by mismatch correction in human cell extracts. Current Biol. 6 (1996) 1528-1531]. To identify the mechanism responsible for the less pronounced phenotype of the second complementation group, we characterized the persistence of MNU-induced O6-methylguanine (O6-meGua) and mutation induction at the hypoxanthine guanine phosphoribosyl-transferase (HPRT) locus. Group II clones are unable to repair the premutagenic base O6-meGua and are as mutable by MNU as group I clones and the parental HeLaMR cells. MNU-induced SCE were undetectable in group I clones and drastically reduced in group II in comparison with the parental cells. These observations are consistent with a defective processing of DNA methylation damage by members of both groups. Group II clones exhibit a moderate spontaneous mutator phenotype at the HPRT gene but significant instability at mononucleotide repeat microsatellites. Introduction of a single human chromosome 2 (but not of chromosome 3 or 7) into group II cells partially reverts both MNU resistance and the increased spontaneous mutation rate. The properties of group II variants are consistent with methylation tolerance and a partially defective mismatch repair. We propose that members of group II are defective in the chromosome 2-based mismatch correction gene GTBP/hMSH6.

Molecular analysis of ERCC2 mutations in the repair deficient hamster mutants UVL-1 and V-H1

The cDNA sequence of the Chinese hamster ERCC2 nucleotide excision repair and transcription gene from the UVL-1 Chinese hamster ovary (CHO) mutant cell line and the V-H1 Chinese hamster V79 mutant line was analyzed. ERCC2 encodes a presumed ATP-dependent DNA helicase and is single copy in CHO lines due to the structural hemizygosity of chromosome 9. Both UVL-1 and V-H1 have intermediate levels of (6-4) photoproduct repair but are as highly UV sensitive as the group 2 mutants that have no detectable repair. Deficiency in cyclobutane dimer removal has also been shown for V-H1. In UVL-1, a single base substitution resulting in an Arg75-->Trp substitution in helicase domain Ia was identified. The equivalent amino acid position is also Arg in the human, mouse, Xiphophorus maculatus, Saccharomyces cerevisiae, and Schizosaccharomyces pombe homologs. In V-H1, a single base substitution resulting in a Thr46-->Ile substitution in helicase domain I (the ATP-binding domain) was identified in both alleles. The equivalent amino acid position is also Thr in the five homologs. Analysis of three V-H1 partial revertants revealed that they still have the original V-H1 mutation in both alleles, indicating that these are second site reversion events. Site-specific mutagenesis was used to introduce the Thr46-->Ile, Arg75-->Trp, and Lys48-->Arg (helicase domain I) mutations into a hamster ERCC2 expression plasmid. These plasmids each failed to confer UV resistance to group 2 mutant cells, further demonstrating that the changes identified are the causative mutations in V-H1 and UVL-1. Correlations between specific mutations, biochemical activities, and repair phenotype are discussed.

Molecular processes and radiosensitivity

BACKGROUND

DNA double-strand breaks (DSB) are the most genotoxic lesions induced by ionizing radiation. At least 2 different pathways for DSB repair have been identified, homologous and non-homologous recombination.

METHODS

Studies on X-ray-sensitive mutants have led to the identification of several genes involved in processing of DSB in bacteria, yeast and mammalian cells.

RESULTS AND CONCLUSION

In mammalian cells non-homologous recombination is the main pathway for DSB repair, while the role of homologous recombination in DSB repair awaits clarification. It is known that, in addition to DNA repair, other safeguards control the human cellular response to ionizing radiation, such as cell cycle regulation and mechanisms involved in scavenging of free radicals produced by ionizing radiation.

Phenotypic heterogeneity in nucleotide excision repair mutants of rodent complementation groups 1 and 4

Rodent ultraviolet light (UV)-sensitive mutant cells in complementation groups (CGs) 1 and 4 normally are known for their extraordinary (approximately 80-100 x) sensitivity to mitomycin C (MMC), although some CG1 mutants with reduced MMC sensitivity were previously reported (Stefanini et al. (1987) Cytotechnology 1, 91). We report here new CG1 and CG4 mutants with only 1.6-10 x wild-type MMC sensitivity despite low unscheduled DNA synthesis (UDS) levels. Mutant UV140, in UV CG4, has approximately 3.8 x the UV sensitivity of parental line AA8, approximately 1.6 x wild-type MMC sensitivity, wild-type X-ray and ethyl methanesulfonate (EMS) sensitivity, and is only slightly (approximately 1.4 x) hypermutable to 8-azaadenine resistance by UV light. It has moderately decreased incision of UV-damaged DNA, has moderately decreased removal of (6-4) photoproducts, and is profoundly deficient in UDS after UV. After UV, it shows abnormally decreased DNA synthesis and persistently decreased RNA synthesis. In addition a cell-free extract of this mutant displays strongly reduced nucleotide excision repair synthesis using DNA treated with N-acetoxy-acetyl-amino-fluorene (AAF). The extract selectively fails to complement extracts of group 1 and 4 mutants consistent with the notion that the affected proteins, ERCC1 and ERCC4, are part of the same complex and that mutations in one subunit also affect the other component. Mutant UV212 is a CG1 mutant with approximately 3.3 x wild-type UV and approximately 5-10 x wild-type MMC sensitivity, with profoundly deficient UDS and hypermutability (approximately 5.8 x) by UV. Mutant UV201, probably in CG1, is only slightly (approximately 1.5 x) UV-sensitive and has near wild-type (1.02X) UV mutability. These unusual group 1 and 4 mutants demonstrate that the unique UV and MMC sensitivity phenotypes displayed by these groups can be separated and support the idea that they are the result of distinct repair functions of the corresponding ERCC1 and ERCC4 genes: nucleotide excision repair for UV lesions and a separate repair pathway for removal of interstrand crosslinks.

The defect in the AT-like hamster cell mutants is complemented by mouse chromosome 9 but not by any of the human chromosomes

X-ray sensitive Chinese hamster V79 cells mutants, V-C4, V-E5 and V-G8, show an abnormal response to X-ray-induced DNA damage. Like ataxia telangiectasia (AT) cells, they display increased cell killing, chromosomal instability and a diminished inhibition of DNA synthesis following ionizing radiation. To localize the defective hamster gene (XRCC8) on the human genome, human chromosomes were introduced into the AT-like hamster mutants, by microcell mediated chromosome transfer. Although, none of the human chromosomes corrected the defect in these mutants, the defect was corrected by a single mouse chromosome, derived from the A9 microcell donor cell line. In four independent X-ray-resistant microcell hybrid clones of V-E5, the presence of the mouse chromosome was determined by fluorescent in situ hybridization, using a mouse cot-1 probe. By PCR analysis with primers specific for different mouse chromosomes and Southern blot analysis with the mouse Ldlr probe, the mouse chromosome 9, was identified in all four X-ray-resistant hybrid clones. Segregation of the mouse chromosome 9 from these hamster-mouse microcell hybrids led to the loss of the regained X-ray-resistance, confirming that mouse chromosome 9 is responsible for complementation of the defect in V-E5 cells. The assignment of the mouse homolog of the ATM gene to mouse chromosome 9, and the presence of this mouse chromosome only in the radioresistant hamster cell hybrids suggest that the hamster AT-like mutant are homologous to AT, although they are not complemented by hamster chromosome 11.

A CHO mutant, UV40, that is sensitive to diverse mutagens and represents a new complementation group of mitomycin C sensitivity

A new mitomycin C (MMC)-sensitive rodent line, UV40, has been identified in the collection of ultraviolet light- (UV-) sensitive mutants of Chinese hamster ovary (CHO) cells isolated at the previous Facility for Automated Experiments in Cell Biology (FAECB). It was isolated from an UV mutant hunt using mutagenesis of AA8 cells with the DNA intercalating frameshift mutagen ICR170. It is complemented by CHO-UV-1, irsl, irs3, irslSF, MC5, V-C8 and V-H4 with respect to its MMC sensitivity based on cell survival. Despite having approx. 4 X normal UV sensitivity and increased sensitivity to UV inhibition of DNA replication, it has near-normal incision kinetics of UV irradiated DNA, and normal (6-4) photoproducts removal. It also is not hypermutable by UV, and shows near normal levels of UV inhibition of RNA synthesis. UV40 also has approx. 11 x .10 x .5 x and 2 x AA8 sensitivity to MMC, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), and X-rays, respectively. Thus, its defect apparently does not involve nucleotide excision repair but rather another process, possibly in replicating past lesions. The spontaneous chromosomal aberration frequency is elevated to 20% in UV40, and the baseline frequency of sister chromatid exchange is also approximately 4-fold increased. The phenotype of UV40 appears to differ from all other rodent mutants that have so far been described.

Ku86 defines the genetic defect and restores X-ray resistance and V(D)J recombination to complementation group 5 hamster cell mutants

X-ray-sensitive hamster cells in complementation groups 4, 5, 6, and 7 are impaired for both double-strand break repair and V(D)J recombination. Here we show that in two mutant cell lines (XR-V15B and XR-V9B) from group 5, the genetic defects are in the gene encoding the 86-kDa subunit of the Ku autoantigen, a nuclear protein that binds to the double-stranded DNA ends. These mutants express Ku86 mRNA containing deletions of 138 and 252 bp, respectively, and the encoded proteins contain internal, in-frame deletions of 46 and 84 amino acids. Two X-ray-resistant revertants of XR-V15B expressed two Ku86 transcripts, one with and one without the deletion, suggesting that reversion occurred by activation of a silent wild-type allele. Transfection of full-length cDNA encoding hamster Ku86 into XR-V15B cells resulted in a complete rescue of DNA-end-binding (DEB) activity and Ku70 levels, suggesting that Ku86 stabilizes the Ku70 polypeptide. In addition, cells expressing wild-type levels of DEB activity were fully rescued for X-ray resistance and V(D)J recombination, whereas cells expressing lower levels of DEB activity were only partially rescued. Thus, Ku is an essential component of the pathway(s) utilized for the resolution of DNA double-strand breaks induced by either X rays or V(D)J recombination, and mutations in the Ku86 gene are responsible for the phenotype of group 5 cells.

Spectrum of spontaneously occurring mutations in the HPRT gene of the Chinese hamster V79 cell mutant V-H4, which is homologous to Fanconi anemia group A

The mitomycin C (MMC)-hypersensitive Chinese hamster V79 cell mutant V-H4 has a cellular phenotype similar to Fanconi anemia (FA), and has been shown to be homologous to FA group A. To examine consequences of the defect in V-H4 cells on spontaneous mutagenesis, we studied the frequency and nature of spontaneous mutations at the hypoxanthine phosphoribosyltransferase (HPRT) locus in this mutant and the parental V79 cells. The mutation rates expressed as the number of mutations per cell per generation were 8.7 x 10(-7) and 3.7 x 10(-7) for V-H4 and V79 cells respectively. The molecular spectrum of 42 spontaneous hprt mutants of V-H4 cells was determined and compared with the previously described spectrum of spontaneous mutations at the HPRT locus of Chinese hamster V79 cells. The spectra of spontaneous mutations in the hprt gene of both cell lines are predominated by base pair substitutions and splice mutations. Among the base changes, V-H4 shows a larger frequency of transitions (13/42; 31%) than transversions (3/42; 7%), whereas in V79 transversions are observed more often than transitions (P < 0.001; Wilcoxon test). The frequency of splice mutations in V-H4 (17/42; 40%), which affects exon 4 almost exclusively, is not significantly different from V79. The fraction of deletions in V-H4 is low (6/42; 14%), and comparable to the level in V79. This is in contrast with the published molecular spectrum of spontaneous hprt mutants in FA (group D) cells, which consists predominantly of deletions.

A gene that regulates DNA replication in response to DNA damage is located on human chromosome 4q

Inhibition of replicative DNA synthesis following gamma-irradiation is observed in eukaryotic cells but is defective in cells derived from patients with the cancer-prone inherited disorder ataxia-telangiectasia (A-T) and in A-T-like Chinese hamster cell mutants. Chinese hamster cells show a less pronounced inhibition of DNA synthesis after gamma-irradiation when compared to irradiated human HeLa or mouse A9 cells. Therefore, to identify new human genes involved in the regulation of DNA replication in response to ionizing radiation in mammalian cells, single human chromosomes were introduced into Chinese hamster cells by microcell-mediated chromosome transfer. It is found that a new gene on human chromosome 4q inhibits DNA synthesis following gamma- and UV irradiation in hamster cells. However, this delay of DNA replication did not improve cell survival or the level of chromosomal aberrations induced by X-rays, indicating that the lack of the inhibition of DNA synthesis after X-irradiation is not a prerequisite for the X-ray sensitivity and chromosomal instability, which is observed in A-T and A-T-like hamster cells.

Complementation analysis of the murine scid cell line

It has been shown that several X-ray-sensitive Chinese hamster cell mutants defective in repair of DNA double-strand breaks (DSBs) are also impaired in the process of V(D)J recombination. The hamster mutants with this phenotype represent three distinct complementation groups, represented by the xrs series, XR-1 and V-3. The murine scid cell line also shows the same phenotype, and therefore we examined whether the scid mutant represents a new complementation group or belongs to one of the existing groups. Scid cells were fused with hamster cell mutants representing the three complementation groups. Hybrids between V-3 and scid cells were only partially complemented for X-ray sensitivity, whereas hybrids derived from fusions with the other mutants were resistant to X rays. These results suggest that V-3 and scid cells are defective in the same gene. To confirm this finding, a single human chromosome 8, which is known to carry the scid gene, was introduced into V-3 cells by microcell-mediated chromosome transfer. Nine hybrid clones derived from V-3 and carrying human chromosome 8 were obtained, and seven were found to be partially complemented for X-ray sensitivity. When human chromosome 8 was introduced into scid cells, seven of eight hybrid clones became resistant to X rays. The results indicate that the defective genes in V-3 and scid are both localized on human chromosome 8. This supports the results from the fusion analysis that V-3 and scid cells are defective in the same gene.

A novel type of X-ray-sensitive Chinese hamster cell mutant with radioresistant DNA synthesis and hampered DNA double-strand break repair

It has been shown that the Chinese hamster cell mutant V-C8 is sensitive to different DNA damaging agents, such as mitomycin C (MMC), alkylating agents, UV light, and X-rays. We found that V-C8 is also sensitive to the following radiomimetic agents: bleomycin (approximately 2-fold, based on D10 values), H2O2 (approximately 2-fold), streptonigrin (approximately 11-fold), and etoposide (approximately 8-fold). Two independent spontaneous MMC-resistant revertants isolated from V-C8 cells show a level of cell killing by X-rays, EMS, and UV light which is similar to that of wild-type cells, suggesting that the observed pattern of cross-sensitivity of V-C8 cells to a wide spectrum of DNA damaging agents results from a single mutation. V-C8 cells also display radioresistant DNA synthesis following gamma-irradiation which, however, remained almost unchanged in the V-C8 revertants. The measurement of the level and rate of repair of DNA single- and double-strand breaks (SSBs and DSBs, respectively) by the DNA elution technique showed that the V-C8 mutant has a slower repair of DSBs induced by gamma-rays. The described unique phenotype of V-C8 cells suggested that V-C8 represents a novel type of mutant amongst X-ray-sensitive hamster cell mutants. To confirm this, complementation analysis with other X-ray-sensitive mutants was performed. V-C8 cells were fused with EM9, XR-1, xrs5, sxi-1, V-3, V-E5, irs3, and BLM2 mutant cells, representing different complementation groups. All the obtained hybrids regained X-ray resistance (or bleomycin resistance in the case of V-C8/BLM2 hybrids) similar to that of wild-type cells, indicating that V-C8 represents a new complementation group. The results presented indicate that V-C8 is defective in a gene involved in a pathway operating in the responses to different DNA damaging agents in mammalian cells.

The effect of defective DNA double-strand break repair on mutations and chromosome aberrations in the Chinese hamster cell mutant XR-V15B

The radiosensitive Chinese hamster cell line XR-V15B was used to study the effect of decreased rejoining of DNA double-strand breaks (DSBs) on gene mutations and chromosome aberrations. XR-V15B cells are hypersensitive to the cytotoxic effects of neocarzinostatin (NCS) and methyl methanesulfonate (MMS). Both mutagens induced more chromosome aberrations in XR-V15B cells than in the parental cell strain. The clastogenic action of NCS was characterized by the induction of predominantly chromosome-type aberrations in cells of both strains, whereas MMS induced mainly chromatid aberrations. The frequency of induced gene mutations at the hprt locus was not increased compared to the parental V79 cells when considering the same survival level. Molecular analysis by multiplex polymerase chain reaction (PCR) of mutants induced by NCS revealed a high frequency of deletions in cells of both cell lines. Methyl methane-sulfonate induced mainly mutations without visible changes in the PCR pattern, which probably represent point mutations. Our findings suggest a link between a defect in DNA DSB repair and increased cytotoxic and clastogenic effects. However, a decreased ability to rejoin DNA DSBs does not seem to influence the incidence and types of gene mutations at the hprt locus induced by NCS and MMS.

A new complementation group of mitomycin C-hypersensitive Chinese hamster cell mutants that closely resembles the phenotype of fanconi anemia cells

Three Mitomycin C (MMC)-hypersensitive mutants (CL-V1B, CL-V5B, and CL-V101B) were isolated from Chinese hamster V79B cells by the replica plating technique. In comparison to the parental cell line, CL-V1B, CL-V5B, and CL-V101B show about a 22-, 32-, and 13-fold increased sensitivity to MMC, respectively (judged by the D10). These mutants are also sensitive to other DNA cross-linking agents, such as 1,2,3,4-diepoxybutane (9-, 19-, and 12-fold, respectively) and cis-diamminedichloroplatinum(II) (17-, 12-, and 6-fold, respectively). CL-V5B and CL-V101B display an exclusive sensitivity to DNA cross-linking agents, whereas CL-V1B also shows an increased sensitivity to monofunctional alkylating agents, such as methyl methanesulfonate (3-fold) and ethyl methanesulfonate (2-fold), and UV254mm (2-fold). Approximately 2-3-fold higher levels of spontaneous chromosomal aberrations are found in these three mutants in comparison to wild-type V79B cells. At a MMC survival level of 80%, CL-V5B demonstrates a 16-fold higher level of MMC-induced chromosomal damage than V79B. Despite phenotypical heterogeneity within this group of mutants, hybrid clones derived after fusion remained MMC sensitive, indicating that these mutants belong to the same complementation group. To determine whether the mutants represent a new complementation group among other Chinese hamster cell mutants that also display hypersensitivity to MMC, CL-V1B cells were fused with mutants representing different complementation groups i.e., irs1, irs3, irs1SF, UV20, UV41, V-H4, and V-C8 cells. In all cases, the derived hybrids regained MMC sensitivity similar to wild-type cells, indicating that the CL-V1B mutant represents a new complementation group. The phenotype of CL-V1B, CL-V5B, and CL-V101B cells closely resembles the phenotype of Fanconi anemia cells, suggesting that these hamster mutants could be defective in a gene that is involved in this disorder.

The pattern of mutations induced by neocarzinostatin and methyl methanesulfonate in the ataxia telangiectasia-like Chinese hamster cell line V-E5

The Chinese hamster cell line V-E5 is a mutant cell line isolated from V79 cells. The phenotypic characteristics of V-E5 strongly resemble those of cells from patients suffering from the genomic instability syndrome ataxia telangiectasia. In order to further characterize the mutant cell line and to get insight into the underlying genetic defect we compared the clastogenic and mutagenic effects of neocarzinostatin (NCS) and methyl methanesulfonate (MMS) in V-E5 and V79 wild-type cells (V79-LE). V-E5 cells were 2-3 times more sensitive to the cytotoxic effect of NCS or MMS. The clastogenic action of NCS was characterized by the predominant induction of chromosome breaks and dicentrics in both cell lines, whereas MMS mainly induced chromatid-type aberrations. The frequency of mutations induced by NCS as well as MMS was slightly enhanced in V-E5 cells compared to V79 cells treated with the same dose. However, the mutant cell line was found to be hypomutable when considering the same survival level as in the parental cell line. Molecular analysis of mutants induced by NCS revealed a high frequency of total deletions of the hprt gene in both cell lines. In contrast, among MMS-induced mutations only 11% deletion mutations were found in V79-LE, whereas in V-E5 MMS-induced deletions were seen in 52% of the hprt-deficient mutants. These results are discussed with respect to a possible relation between genomic instability, cell cycle control and mutational spectra.

Mechanisms and biomarkers of genotoxicity. Molecular dosimetry of chemical mutagens

The relevance of the use of DNA adduct frequencies as a parameter for the extent of mutation induction by monofunctional alkylating agents was investigated in cultured Chinese hamster cells and in rat skin fibroblasts treated in vivo with the test chemicals. The nature of the biologically significant DNA adducts was investigated by DNA sequence analysis of mutations induced at the hypoxanthine-guanine phophoribosyltransferase (hprt) gene. The results show that under conditions where O6-alkylguanine is a persistent DNA lesion more than 50% of the mutations are GC to AT transitions indicating that the frequency of O6-alkylguanine is a good parameter for mutation induction. However, in target cells which are able to remove alkyl groups from the O6 position of guanine, alkylating agents with a low nucleophilic selectivity (e.g. N-ethyl-N-nitrosourea (ENU) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG)) exert most of their mutagenic activity most likely via the induction of O2-ethylthymine.

Streptozotocin-induced chromosomal aberrations, SCEs and mutations in CHO-9 parental cells and in EM-C11 mutant cell line

The genotoxic effects induced by the monofunctional nitrosourea derivative streptozotocin (STZ) were investigated in Chinese hamster ovary cells, parental (CHO-9) and its mutant hypersensitive to alkylating agents, designated EM-C11. The ability of this compound to induce chromosomal aberrations, cell killing, sister-chromatid exchanges (SCEs) and mutations was evaluated on these two cell lines. The mutant cells were found to be slightly more sensitive to the killing effects of STZ than the parental cell line. EM-C11 cells also showed higher levels of STZ-induced chromosomal aberrations than CHO-9 cells, but appeared to be equally sensitive to induction of SCEs. The frequencies of STZ-induced mutations, measured as resistant Na+/K(+)-ATPase and HPRT mutants, revealed a higher sensitivity of EM-C11 to the mutagenic effects of this compound.

Studies on phenotypic complementation of ataxia-telangiectasia cells by chromosome transfer

Cells derived from patients with the cancer-prone inherited disorder ataxia-telangiectasia (A-T) show an abnormal response to ionizing radiation-induced DNA damage, such as an increased cell killing and a diminished inhibition of DNA synthesis. The enhanced killing of A-T (group D) cells by X-rays can be corrected by multiple cDNAs, mapping to different chromosomes (6, 11, 17, and 18). In order to examine whether genes located on these chromosomes complement AT-D cells, normal neo-tagged chromosomes 6, 11, 17, and 18 were introduced into AT-D cells by microcell-mediated chromosome transfer. However, correction of the enhanced killing of AT-D cells by X-rays could only be achieved by chromosome 11 and by none of the other chromosomes tested. The enhanced killing of A-T (complementation group C) cells was also corrected by chromosome 11. Usually, but not in all microcell hybrid clones, chromosome 11 also corrected the radioresistant DNA synthesis (RDS) phenotype of AT-D and AT-C cells. These results (i) confirm findings by others suggesting assignment of the ATD and ATC genes to chromosome 11, (ii) demonstrate that several genes can modify the cellular radiation response when they are taken out of their normal genomic context and/or control, and (iii) indicate that the RDS phenotype and the enhanced cell killing in A-T are independent pleiotropic features resulting from the primary mutations in A-T. Also, our findings underscore that, in establishing cDNAs as candidate genes for A-T, microcell-mediated chromosome transfer studies are needed to exclude nonspecific correcting effects of these candidate cDNA genes.

Functional complementation studies with X-ray-sensitive mutants of Chinese hamster cells closely resembling ataxia-telangiectasia cells

In order to isolate a human gene complementing the defect in A-T-like hamster cell mutants, the mutants were used as recipients for genomic DNA transfection, using either HeLa chromosomal DNA or DNA from a human cosmid library. Three primary transformants with an intermediate X-ray sensitivity and almost normal sensitivity to MMS, but retaining radioresistant DNA synthesis (RDS), were obtained. To identify the human chromosome that complements the defect in the A-T-like mutants, and to assess the degree of complementation for survival and RDS, microcell-mediated chromosome transfer was used. At least 20 independent hybrid clones between the mutant and each one of the human chromosomes 1, 2, 4, 5, 15, 17 or 18 were isolated. All hybrid clones remained X-ray sensitive, except one with chromosome 4, and another with chromosome 15, both showing an intermediate X-ray sensitivity. By using in situ hybridization we found that this partial correction was due to the presence of a mouse chromosome. In these two hybrids containing the mouse chromosome together with human chromosome 4 or 15, RDS was fully complemented only in the hybrid with chromosome 4 but not in the one containing chromosome 15, suggesting that RDS and X-ray sensitivity may be complemented independently.

Molecular analysis of ethyl methanesulfonate-induced mutations at the hprt gene in the ethyl methanesulfonate-sensitive Chinese hamster cell line EM-C11 and its parental line CHO9

The Chinese hamster cell line EM-C11 has been shown to be 5 times more sensitive than its parental line CHO9, but not hypermutable, after treatment with ethyl methanesulfonate. Ethyl methanesulfonate-induced mutational spectra were determined at the hprt locus to investigate whether the same adducts are responsible for mutation induction in both cell lines. The mutational spectra for EM-C11 and CHO9 show an important difference. GC-->AT transitions were found in both cell lines at similar frequencies; however, the spectrum of CHO9 contains a class of AT-->GC transitions, which seems to be replaced by a group of deletions in EM-C11. Since the ethyl methanesulfonate-induced mutation frequency for both lines is the same at equal exposure, it is hypothesized that the lesions leading to AT-->GC transitions in CHO9 are responsible for the deletions in EM-C11. This phenomenon might be explained if the responsible adduct(s) in CHO9 is bypassed resulting in replication errors, while blocking DNA synthesis in EM-C11 causing the observed increase in cell death. In surviving EM-C11 cells, DNA strand exchanges might have occurred at the position of stalled replication forks, leading to gross molecular changes. The adduct probably responsible for the AT-->GC transitions in CHO9 and the deletions in EM-C11 is 3-ethyladenine.

Mutagenic response and repair of cis-DDP-induced DNA cross-links in the Chinese hamster V79 cell mutant V-H4 which is homologous to Fanconi anemia (group A)

Previously, it has been shown that the V-H4 mutant of Chinese hamster V79 cells is homologous to Fanconi anemia (FA) group A cells. This hamster cell mutant shows a specific sensitivity to DNA cross-linking agents; therefore, the induction and repair of DNA cross-links were studied in V-H4 and wild-type V79 cells after cis-DDP treatment by the DNA alkaline elution technique. A significant difference in repair of these lesions in V-H4 and wild-type cells was observed. After the cis-DDP treatment (24 h) about 3 times more cross-links remained in V-H4 cells in comparison to the parental V79 cells. These results indicate that the process of cross-link repair in V-H4 cells is hampered when compared to that of wild-type cells. To assess the effect of slower removal of DNA cross-links on the mutability of V-H4, the induction of mutants at the hypoxanthine-guanine phosphoribosyltransferase locus (HPRT) by cis-DDP was studied in V-H4 and V79 cells. Despite the increased cytotoxicity of cis-DDP to V-H4 cells, the mutation induction at the HPRT locus was not significantly different in both cell lines, but when the frequency of the hprt mutants was plotted against survival, hypomutability was observed in V-H4 cells after the cis-DDP treatment.

Human chromosome 11 complements ataxia-telangiectasia cells but does not complement the defect in AT-like Chinese hamster cell mutants

It has been shown that the X-ray-sensitive Chinese hamster V79 mutants (V-E5, V-C4 and V-G8) are similar to ataxia-telangiectasia (A-T) cells. To determine whether the AT-like rodent cell mutants are defective in the gene homologous to A-T (group A, C or D), human chromosome 11 was introduced to the V-E5 and V-G8 mutant cells by microcell-mediated chromosome transfer. Forty independent hybrid clones were obtained in which the presence of chromosome 11 was determined by in situ hybridization. The presence of the region of chromosome 11q22-23 was shown by molecular analysis using polymorphic DNA markers specific for the ATA, ATC and ATD loci. Seventeen of the obtained monochromosomal Chinese hamster hybrids contained a cytogenetically normal human chromosome 11, but only twelve hybrid cell lines were shown to contain an intact 11q22-23 region. Despite the complementation of the X-ray sensitivity by a normal chromosome 11 introduced to A-T cells (complementation group D), these twelve Chinese hamster hybrid clones showed lack of complementation of X-ray and streptonigrin hypersensitivity. The observed lack of complementation does not seem to be attributable to hypermethylation of the human chromosome 11 in the rodent cell background, since 5-azacytidine treatment had no effect on the streptonigrin hypersensitivity of the hybrid cell lines. These results indicate that the gene defective in the AT-like rodent cell mutants is not homologous to the ATA, ATC or ATD genes and that the human gene complementing the defect in the AT-like mutants seems not to be located on human chromosome 11.