Over the past ten years or so, we have seen a proliferation of reports of new cell lines of various vertebrate species, showing hypersensitivity to killing by DNA damaging agents. Regrettably, but predictably, there is no standard terminology to describe the mutants, and as a result the literature is liberally scattered with fragments of individualistic nomenclature. There is no way of imposing order at this stage, but it may be helpful to bring together in this chapter as much information as possible on the mutants now available. As well as being an aid for reference, this should serve as a pointer towards further investigation - either in characterizing the mutants we have, or in developing new ones to fill gaps in our knowledge.

The purpose of this chapter is twofold. For those intending to enter the field of DNA repair mutants, we provide an introduction to the methodology of mutant production and characterization, with particular emphasis on rapid and simple techniques. We discuss potential uses and problems of such mutants. We also hope that the chapter will be of use to those already engaged in this work, since it provides a reasonably complete survey of permanent human cell lines showing DNA repair defects, together with tables cataloguing the artificially induced repair mutants so far available.

This survey will be limited to higher eukaryote cell lines. The genetics of repair in prokaryotes, in yeast and in Drosophila, have been dealt with in other chapters of this book. Mutants will be described in terms of their sensitivity (and their cross sensitivities, i.e. sensitivities to agents other than that used for the original selection); and, where available, details will be given of the nature of the defect in the cells’ response to DNA damage. In several cases, sets of mutant cell lines have been placed in different genetic complementation groups. Less commonly, complementation or the lack of it has been established between mutants originating in different laboratories.

As will be seen from the size of Table 2, the majority of known DNA repair mutants originate from two Chinese hamster cell lines, and it is more than likely that effort will be concentrated to an increasing extent on this material. No doubt this approach will quickly yield a unified view of DNA repair mechanisms - but it is already clear that different species display surprisingly distinct repair phenotypes, and we should beware of fishing exclusively in one small area of the eukaryote gene pool.

First, then, we shall give an outline of methods for obtaining DNA repair mutants, from the initial mutagenesis, through enrichment and selection, to a complete characterization.

Mutagenesis

Standard procedures are used for this step, treating exponentially growing cell populations with chemical mutagens such as ethyl methane sulphonate (EMS), or radiation, and allowing several days (subculturing if necessary so that cells do not enter a plateau phase of growth) for expression of the induced mutations. At this stage, stocks of mutagenized cells may be stored frozen for use in later screening experiments. (For general information on mutagenesis, see Thompson & Baker, 1973; Basilico & Meiss, 1974; Kao & Puck, 1974.)

Enrichment and selection

A common strategy is next to carry out an enrichment step, to exclude a large proportion of cells that are wild-type in terms of DNA repair. A ‘suicide’ procedure is adopted: for instance, after treatment with a DNA damaging agent, the cells are incubated with a DNA precursor analogue that will be incorporated into the DNA of normal cells in the course of repair synthesis, and will prove lethal; mutant cells defective in repair, unless they are in S phase, will tend to survive. Such protocols have been described by Isomura et al. (1973), Stefanini et al. (1982), and Fiorio et al. (1983), with bromodeoxyuridine (BrdUrd) as an analogue of thymidine; on subsequent irradiation with ‘black light’, photolysis of incorporated base causes lethal DNA damage. Schultz et al. (1981b) employed an analogous method but with high specific activity [0061H]thymidine ([0061H]dThd) substituted for BrdUrd-black light. In this case, death results from radiation damage caused by decaying incorporated tritium.

A complication with this strategy is its dependence on a DNA damaging treatment in order to produce the DNA resynthesis response; the DNA damage may be sufficient to remove from the population any highly sensitive mutants. Thus the enrichment is likely to produce a preponderance of moderate rather than extreme repair defects.

An alternative suicide approach involves infection of cells with virus previously treated with DNA damaging agent to inactivate it (Shiomi & Sato, 1979). Wild-type cells repair the virus, restoring its activity, and are killed by it; repair-defective cells survive. It is, of course, important to avoid the possibility of secondary infection of surviving cells with virus released from cells that have successfully repaired it.

The most productive technique so far for selecting those clones in a mutagenized population (with or without a preceding enrichment step) that represent repair defects is the simple criterion of reduced growth/cell division following a low level of DNA damage. Thompson et al. (1980) examined individual colonies microscopically and carefully recovered those most affected. Busch et al. (1980) used an automated photographic recording device to identify colonies that did not increase in size during a specified interval after irradiation. An alternative, labour-intensive approach (which has the advantage that mutants do not undergo any genotoxic treatment during selection) is that referred to as ‘toothpicking’, where cells from individual colonies are transferred manually (using toothpicks) to establish sets of replicate colonies in multiwell plates or on agar; one set is then treated with the selecting agent, hypersensitive colonies are identified, and a cell line established from the corresponding well on the replicate plate (Shiomi et al. 1982a; Jeggo & Kemp, 1983; Robson et al. 1985).

True replica plating, i.e. where a pattern of colonies is copied directly from one culture dish to another, one of which is tested, has been used in several versions. Stamato & Waldren (1977) established replicas of Chinese hamster ovary (CHO) colonies on nylon cloth and then irradiated the master plate with ultraviolet light (u.v.); a colony that readily detached was located on the replica and later characterized as the mutant UV-1. In a recent modification, Stamato et al. (1983) identified poorly growing colonies by matching photographic images taken at intervals after y- irradiation. With the possibility of establishing multiple replicas on polyester mesh (Raetz et al. 1982; Collins, 1987), matching can be done between a reference, untreated stained replica and the master plate, treated with DNA damaging agent, incubated for preferential loss of hypersensitive colonies, and stained with a contrasting colour. There is the further possibility that assays specific for enzymic DNA repair activities can be carried out on one of the replicas. Stefanini et al. (1982) succeeded in identifying colonies showing low unscheduled DNA synthesis (UDS) in an autoradiographic assay on a replica.

A quite different, intriguingly elegant selection technique (Abbondandolo et al. 1982) makes use of a ‘multiwire proportional chamber’, which apparently discriminates between repairing and non-repairing colonies directly by comparison of levels of incorporation of [0061H]dThd.

Characterization

Following selection of clones that are possible DNA repair-defective mutants, the defect has to be confirmed and the phenotype characterized. The need is for quick and simple assays that can accommodate a large number of selected lines requiring testing at one time. We will describe here several such techniques that should make life easier for the collector of DNA repair mutants.

The parameter of fundamental importance in a mutant is its degree of sensitivity to genotoxic agents. The conventional plating assay, which measures single-cell colony forming ability over a range of doses of the agent, is precise and reproducible but time-consuming; 1 –2 weeks are required to allow development of colonies. As an alternative, cells can be plated at moderate density, treated with a range of doses of DNA damaging agent, and the combined effect of cell killing and growth inhibition assessed a few days later by visual inspection after fixing and staining, or by protein estimation or counting of attached cells. This procedure is similar to the ‘differential cytotoxicity’ assay developed by Hoy et al. (1984); an example is shown in Fig. 1. Arlett & Priestley (1983, 1984) introduced a useful method for quantifying the efficiency of repair of potentially lethal damage (PLD). In their procedure noncycling, pre-S-phase cells were exposed to a DNA damaging agent and either replated immediately onto feeder layers to form colonies or replated after a 24 h period during which repair of PLD could occur. ‘Phis method has proved to be extremely sensitive, maximizing discrimination between, for example, ataxiatelangiectasia (A-T), A-T heterozygote and wild-type fibroblasts. For permanent cell lines with less ability to arrest and therefore to repair PLD, this procedure may be more difficult to apply.

Fig. 1.

Cytotoxicity assay using multiwell plates. Wells were inoculated with 100061 cells each (suspended in PBS) in columns of four wells for each cell type: HD-1A, HD-2, HeLa and HD-1. They were irradiated with u.v. (0, 2, 5 or 10 J m−2 as shown), medium was added, and incubation continued for 4 days before fixing and staining. Different cell types give different staining intensities in the control, unirradiated wells, presumably as a result of differing rates of growth, cell size or morphology. But it can be seen, from the relative decrease in intensity with u.v. dose, that HD-1A suffers greater cytotoxicity than HD-1, and that HD-2 is particularly sensitive compared with HeLa. Qualitatively, this is in accord with known u.v. survival parameters (Johnson et al. 1985, 1986).

Fig. 1.

Cytotoxicity assay using multiwell plates. Wells were inoculated with 100061 cells each (suspended in PBS) in columns of four wells for each cell type: HD-1A, HD-2, HeLa and HD-1. They were irradiated with u.v. (0, 2, 5 or 10 J m−2 as shown), medium was added, and incubation continued for 4 days before fixing and staining. Different cell types give different staining intensities in the control, unirradiated wells, presumably as a result of differing rates of growth, cell size or morphology. But it can be seen, from the relative decrease in intensity with u.v. dose, that HD-1A suffers greater cytotoxicity than HD-1, and that HD-2 is particularly sensitive compared with HeLa. Qualitatively, this is in accord with known u.v. survival parameters (Johnson et al. 1985, 1986).

Repair DNA synthesis or UDS is a universally accepted parameter of excision repair capacity. Autoradiographic analysis of incorporated [0061H]dThd is sound but slow (Pawsey et al. 1978). UDS-can be measured by scintillation counting of acidinsoluble tritium, if replicative DNA synthesis is suppressed by hydroxyurea (Brandt et al. 1972); although there are theoretical objections to this procedure (Mullinger et al. 1983; Collins & Johnson, 1984), it is justified by the success of its results.

A near-universal feature of the cellular response to DNA damage is the temporary depression of replicative DNA synthesis (Painter, 1977). Generally the depression is more marked and the recovery slower in hypersensitive cell lines (Fig. 2). Although this assay is valuable for its generality, it cannot provide information on which aspect of repair is defective.

Fig. 2.

Inhibition of thymidine incorporation after u.v. irradiation in HeLa-XPD hybrids. [0061C]dThd-labelled cultures were pulse-labelled for 1 h with [0061H]dThd at various times after u.v. (4 J m−2). The points correspond to amounts of acid-insoluble radioactivity in irradiated versus mock-irradiated cells, expressed as a ratio of 0061H: 0061C. Points represent the average of two experiments. HD-1 (○) shows normal u.v. sensitivity by comparison with the hypersensitive cells HD-1A (▾), and HD-2 (▵) (Johnson et al. 1986, with permission of IRL Press Ltd).

Fig. 2.

Inhibition of thymidine incorporation after u.v. irradiation in HeLa-XPD hybrids. [0061C]dThd-labelled cultures were pulse-labelled for 1 h with [0061H]dThd at various times after u.v. (4 J m−2). The points correspond to amounts of acid-insoluble radioactivity in irradiated versus mock-irradiated cells, expressed as a ratio of 0061H: 0061C. Points represent the average of two experiments. HD-1 (○) shows normal u.v. sensitivity by comparison with the hypersensitive cells HD-1A (▾), and HD-2 (▵) (Johnson et al. 1986, with permission of IRL Press Ltd).

Fig. 3.

Incision profiles of two hamster cell lines (KI, ▵ — ▵;UV-5, ▴ – – – ▴)and two human cell lines (HeLa, ○ — ○; HD-2, • – – – •) following irradiation with a range of u.v. doses. Cells were prelabelled with [0061H]dThd, incubated before and after u.v. irradiation with hydroxyurea and cytosine arabinoside, and accumulated DNA breaks (i.e. incomplete repair events) were measured by alkaline unwinding and Si nuclease digestion of unwound DNA, in situ on chamber slides (Collins et al. 1982).

Fig. 3.

Incision profiles of two hamster cell lines (KI, ▵ — ▵;UV-5, ▴ – – – ▴)and two human cell lines (HeLa, ○ — ○; HD-2, • – – – •) following irradiation with a range of u.v. doses. Cells were prelabelled with [0061H]dThd, incubated before and after u.v. irradiation with hydroxyurea and cytosine arabinoside, and accumulated DNA breaks (i.e. incomplete repair events) were measured by alkaline unwinding and Si nuclease digestion of unwound DNA, in situ on chamber slides (Collins et al. 1982).

DNA breaks reflect the balance between production of damage and its removal, and measuring breaks at different times after DNA damage is inflicted may give clues as to the nature of a repair defect. Sucrose gradient sedimentation, nucleoid sedimentation, filter elution and alkaline unwinding are alternative methods for measuring breaks (Cook & Brazell, 1976; Friedberg & Hanawalt, 1981). The last- alkaline unwinding — is fairly sensitive and very quick. Many chemical agents, and ionizing radiation, produce frank DNA breaks that diminish with time of repair incubation. When u.v. damage is repaired, DNA breaks are only transiently present, but if the later stages of repair are blocked with DNA synthesis inhibitors incision events accumulate as readily measurable breaks, and so incision-defective lines can be easily identified. A particularly quick assay for breaks and repair is based on the use of Lab Tek ‘chamber slides’ (Collins et al. 1982); all steps are performed on these slides, and the incision profile of several cell lines can be obtained within a day (Fig- 3).

Complementation analysis

Once a mutant cell line is characterized, it is important to know whether it is a novel variant or whether it fits into a known complementation group. For this, it is necessary in some way to combine the cells being tested with cells of a known complementation group and to assay for restoration of DNA repair activity (complementation), indicating whether or not the two cell lines are in the same complementation group. Complementation analysis is carried out on heterokaryons (the immediate product of fusion of cells of two distinct stocks, with two or more nuclei in a mixed cytoplasm; useful for short term studies); or, more rarely, on hybrids (i.e. the cell line derived from heterokaryons, carrying genetic components from each parent cell type). Complementation analysis by microinjection of cell extract of one type into cells of another type is a recent innovation, described by Hoeijmakers (1987). Complementation/non-complementation can be assessed on the basis of whether the fused cells/hybrids/microinjected cells have restored the level of DNA repair or damage tolerance towards wild-type values.

In the case of heterokaryon analysis, it is crucial to be able to distinguish true heterokaryons from homokaryons where cells of the same type have fused. This can be done by growing the two cell lines with latex beads of different diameters; then only those binucleate cells with beads of both sizes need be scored (Murnane & Painter, 1982; Stefanini et al. 1985). Alternatively, one cell type may be heavily labelled with [0061H]dThd for at least one generation before the fusion, so that heterokaryons are identified as binucleate cells with one densely labelled nucleus (e.g. see Johnson et al. 1985).

The assay for UDS in xeroderma pigmentosum (XP) heterokaryons by autoradiography is described, for example, by Pawsey et al. (1979). Complementation between different ataxia-telangiectasia (A-T) strains is also assayed in heterokaryons by autoradiography but in this case recovery of the ability to inhibit replicative synthesis after X-irradiation is the index of complementation (Murnane & Painter, 1982; Jaspers & Bootsma, 1982). Conversely, restoration of the ability to replicate DNA after u.v. irradiation marks complementation between different Cockayne Syndrome (CS) strains (Tanaka et al. 1981). Lehmann (1982a) also demonstrated complementation between CS strains on the basis of the recovery of RNA synthetic ability after u.v. An approach that addresses a biological end-point of complementation is that of Day et al. (1975). They infected XP heterokaryons with u.v.-irradiated adenovirus. If complementation of repair activity occurs between the two XP partners, the damaged virus is repaired and this is readily detected as a restoration of plaque-forming ability.

Hybrid cell lines are most easily produced when each parental variant involved in the fusion carries a different selectable genetic marker. Then, under appropriate selection, only successfully fused heterokaryons will survive to divide. Once established, hybrid cell cultures can be tested for a range of repair end-points such as UDS, incision capacity and host cell reactivation of virus; or for survival after treatment with DNA damaging agent, either by the standard plating assay for single cell colony-forming ability (Thompson et al. 1981, 1985a; Johnson et al. 1985, 1986; Duckworth-Rysiecki et al. 1985, 1986), or by a quick assay for cell survival and growth resembling the cytotoxicity assay described above (Cleaver, 1982; Thompson et al. 1985a).

In practice, provided sufficient clones are examined to remove the potential problems associated with chromosome segregation, hybrids should provide a wider view of complementation than can be achieved from heterokaryons. In one case hybrids have revealed discordance between UDS data and other repair assays. Johnson et al. (1985) backcrossed a permanent XP D-like hybrid with fibroblasts from different XP complementation groups. With XP C, E and F cells u.v. sensitivity and DNA repair ability were restored to normal; with XP D all hybrids remained u.v.-sensitive, and with the exception of somewhat elevated UDS levels, no other repair assay indicated complementation.

In this section, we list mutants that have been induced in established cell lines by techniques similar to those described above. Tables 1-4 cover cell lines representing four different species: human (Table 1), Chinese hamster (Table 2), mouse (Table 3), and frog (Table 4).

Table 1.

Human mutant cell lines

Human mutant cell lines
Human mutant cell lines
Table 2.

Hamster mutant cell lines

Hamster mutant cell lines
Hamster mutant cell lines
Table 3.

Mouse mutant cell lines

Mouse mutant cell lines
Mouse mutant cell lines
Table 4.

Frog mutant cell lines

Frog mutant cell lines
Frog mutant cell lines

As is well known, many human DNA repair mutants occur naturally, as clinical conditions. There are numerous reviews of the work done with primary cultures of cells from patients with autosomal recessive traits such as XP, A-T, etc. (see e.g., Lehmann, 1982b). In the following section, we will describe those cases in which permanent cell lines have been derived from such cultures, making them potentially useful in long-term molecular and cellular genetic analysis.

Permanent fibroblast cell lines have been derived from most of the complementation groups of XP. The line XP12ROSV is the simian virus 40 (SV40)-transformed counterpart of XP12RO, of group A, and was obtained by G. Veldhuisen and D. Bootsma. A well-characterized subclone, XP12RPO-MI, has been described (Royer-Pokora & Haseltine, 1984). Barbis et al. (1986) and Hashimoto et al. (1986) report other SV40-transformed XP A lines, derived from GM2009 and XP5NI, respectively. Immortal XP C lines have been produced by Daya-Grosjean et al. (1986), and by Canaani et al. (1986). In each case transformation was achieved by an origin-defective SV40 recombinant plasmid. One XP D line, derived from GM434 by means of the plasmid pSV7, has been described together with an XP E line from GM241S (Moses et al. 1986). An immortal SV40-transformed XP F line was derived from XP2YO by Yagi & Takebe (1983), and an XP G line from GM3021 by Barbis et al. (1986). A permanent XP variant cell line (from GM2359) has also been reported (Barbis et al. 1986).

Another important source of immortal XP cell lines, this time lymphoblastoid in origin (following infection with Epstein Barr Virus (EBV)) was described many years ago by Andrews et al. (1974). Most XP complementation groups are now represented by lymphoblastoid derivatives (NIGMS Human Genetic Mutant Repository, 1985).

Permanent cell lines of Bloom’s syndrome are available; one, SV889BL, was produced by SV40 transformation (K. Sperling, personal communication: see Squires et al. 1982), and two others, GM3498B-HV1 and -HV2, by transfection with DNA from a tumorigenic mouse cell line carrying a single copy of Harvey murine sarcoma virus (Doniger et al. 1983). The latter were found to exhibit the high spontaneous sister chromatid exchange (SCE) frequency characteristic of untransformed Bloom’s syndrome cells (Doniger et al. 1983). Moses et al. (1986) reported an additional permanent Bloom’s cell line derived from GM2548A by use of the recombinant plasmid pSV7, and Shiraishi et al. (1983) reported results with an EBV-transformed cell line.

Ataxia-telangiectasia is represented by the SV40-transformed fibroblast line AT5BIVA, by the pSVori-transformed AT25F fibroblast line (Murnane et al. 1985) and the successful immortalization of A-T fibroblasts by transfection with a plasmid pSV3gpt containing the SV40 early region (Mayne et al. 1986b). Hashimoto et al. (1986) report the production of three A-T lines (from AT10S) using, for transformation, either the whole SV40 genome alone cloned into pBR322 or a combination of SV40 plus Moloney murine sarcoma virus. Henderson & Ribecky (1980) report the production of an AT lymphoblastoid cell line, and there are now many such lines listed in NIGMS Human Genetic Mutant Repository (1985).

The first permanent fibroblast cultures from Cockayne Syndrome (CS3BE, CSIAN) and the immunodeficient condition known as 46BR have been reported by Mayne et al. (1986a,b), while Henderson & Ribecky (1980) described the production and properties of an EBV-transformed CS lymphoblastoid line. Finally, permanent cell lines from Fanconi’s anaemia have been described by Henderson & Ribecky (1980) and Duckworth-Rysiecki et al. (1985) (lymphoblastoid), and Duckworth-Rysiecki et al. (1986) (fibroblast-derived).

A fundamental problem with virally transformed cell lines such as these is the possibility that the process of transformation alters the phenotypic response to DNA damaging agents. This was illustrated some time ago for rat cells (Waters et al. 1977) and also for human cells (Heddle & Arlett, 1980). More recently, permanent AT lines were shown to be somewhat less sensitive to X-rays than their precursor fibroblasts (Murnane et al. 1985). It was known that the line XP12ROSV was hypersensitive to SCE formation after EMS treatment or u.v. irradiation (Wolff et al. 1977). Heddle & Arlett (1980) found that this line was also hypersensitive to killing by EMS; but the corresponding primary XP strain showed no hypersensitivity in either killing or SCE production by EMS. Furthermore, excision repair (indicated by u.v.-dependent incision of DNA) is drastically reduced in SV40- transformed normal human cells as well as in the SV40-transformed Bloom’s syndrome line (Squires et al. 1982). Viral transformation is often associated with defective repair of O0061-alkylated purines (Day et al. 1987) and with enhanced rates of chromosome breakage and SCE (Wolff et al. 1977; Popescu et al. 1983), and Teo et al. (1983) suggest that the sensitivity of XP12ROSV to ethylating agents, and the defective removal of O0061-ethyl groups, results from transformation rather than from the XP phenotype. The report of poor O0061-alkyl removal from an XP C lymphoblastoid line by Altamirano-Dimas et al. (1979) may also be explained in this way. By contrast, there are several reports of transformed XP lines where DNA repair capabilities are much restored. The enhanced repair of cross-links in an SV40- transformed XP A line reported by Gantt et al. (1984) is one example. In other cases the complete wild-type, repair-competent phenotype is restored, as in the case of the spontaneously transformed XP line described by Thielmann et al. (1983), or the XP G line produced by Moses et al. (1986). Moreover, high rates of reversion of XP A cell lines to u.v.-resistant phenotypes have been reported (Takano et al. 1982; Royer-Pokora & Haseltine, 1984), while many AT lymphoblastoid cell lines display wild-type chromosome stability (Cohen et al. 1979). Clearly it is necessary to exercise great caution when regarding transformed cell lines as the accurate counterparts of the primary repair-defective or hypersensitive cell strains.

In the context of viral transformation u.v.- and 4-nitroquinoline-l-oxide (4- NQO)-sensitive permanent human fibroblast lines from a normal embryo have been reported in the absence of selection, by Suzuki & Kuwata (1979) and Suzuki & Fuse (1981), following complex double infection with Rous Sarcoma virus and SV40. From two of these clones revertants have been isolated after EMS mutagenesis and u.v. selection, though frequencies of reversion are not provided (Suzuki & Kuwata, 1979).

Finally, though from another species, the SV40-transformed Indian muntjac cell, SVM, should be mentioned in this section. SVM arose spontaneously from SV40 infection of a skin fibroblast culture and was not selected for the considerable sensitivity to u.v. and 4-NQO that it expresses (Pillidge, 1984). Its high spontaneous rate of chromosome aberrations suggested, however, that it might display repair defects and it was found to have aberrant post-replication recovery (daughter strand repair), as well as impaired ability to recover RNA synthesis after u.v. (Pillidge et al. 1986a). Moreover, u.v. induces numerous chromosome aberrations and abundant SCE in this cell (Pillidge et al. 1986b).

A quite different approach, not involving viral transformation, was adopted by Johnson et al. (1985) to establish a permanent line of XP D. HeLa (permanently growing heteroploid human) cells were fused in suspension with XP D fibroblasts, using inactivated Sendai virus. The HeLa cells had been X-irradiated prior to fusion, so that unfused HeLa cells would not be expected to proliferate. But some hybrid cells survived to form clones, having apparently gained the capacity for indefinite growth from a HeLa component, and of these one clone, HD2, closely resembles XP D in its u.v. sensitivity and DNA repair characteristics (Johnson et al. 1985).

The variety of DNA repair mutants that we have already might seem to indicate that there is no need to look for more. But DNA repair is likely to involve complex genetic systems, judging by what is now known for bacteria and by the plethora of complementation groups for XP representing just one step of repair, several of which clearly appear not to be allelic (Giannelli et al. 1982). It is unlikely, for instance, that the six complementation groups for CHO u.v. excision repair are all that exist (see Friedberg, 1987; Downes, 1986).

Mutants are invaluable for investigating the biochemistry of DNA repair. They are also, of course, exploited as recipients for transfected DNA in attempts to identify DNA repair genes. Before beginning a search for further mutants, it is wise to check that the cell line to be used will yield a suitable system; some otherwise valuable mutants are very poor at taking up foreign DNA (Schultz et al. 1985; Hoeijmakers et al. personal communication).

The frequency at which DNA repair mutants arise is remarkably high, around 1 in 100061 or even higher (Thompson et al. 1980; Busch et al. 1980; Jeggo & Kemp, 1983; Robson et al. 1985). In common with other families of CHO mutants the abundance of DNA repair mutants lends support to the possibility that CHO cells might be physically or functionally hemizygous, so that mutation in only one allele is necessary to establish a mutant phenotype (Siminovitch, 1976). The hemizygous state may result from selective methylation; azacytidine, which blocks methylation, caused the reversion of xrs mutants to a normal phenotype (Jeggo & Holliday, 1986).

It is noteworthy that certain steps in the excision repair of u.v. damage (the most-studied pathway) seem to be over-represented among mutants. Almost all show defective incision; those that do not, tend to have ill-understood faults in post-replication recovery, i.e. tolerance mechanisms for replication in the presence of damage. The later stages of excision of damage, repair synthesis and ligation, seem genetically intractable (with the possible exception of a ligation anomaly in the human condition 46BR (Henderson et al. 1985) and several mouse cell lines (Pearson & Styles, 1984; Yin et al. 1973)). This may well be because the enzymes responsible for those steps are involved also in DNA replication or other essential processes, so mutations will tend to be lethal.

There is mounting evidence that defects in processes associated with DNA replication are often accompanied by elevated sensitivity to DNA damage. Thus, temperature-sensitive CHO cell lines tsl3A and tsl5C are hypersensitive to killing by alkylating agents (Srinivasan et al. 1980), as are deoxycytidine kinase-deficient CHO mutants (Meuth, 1983). Purine auxotroph CHO lines, in a purine-starved state, perform an abortive kind of DNA repair that appears to be mutagenic (Collins, 1985, and unpublished results). And an aphidicolin-resistant hamster (V79) mutant, aphr 4-2, which has an altered DNA polymerase a with increased affinity for dCTP, is hypersensitive to both killing and mutation induction by u.v. light or MNNG (Liu et al. 1984).

It is important that DNA repair should be seen, not as an isolated phenomenon, but as one aspect of the integrated nucleic acid economy of the cell. The need now is for biochemical and genetic approaches that will allow us to investigate the relationship between normal DNA replication and the various mechanisms that the cell uses in order to cope with DNA damage and genetic misinformation.

We thank Dr Istvan Rasko for helpful comments on the manuscript.

Abbondandolo
,
A.
,
Bonatti
,
S.
,
Bellazzini
,
R.
,
Betti
,
G.
,
Del Guerra
,
A.
,
Massai
,
M. M.
,
Ragadini
,
M.
,
Spandre
,
G.
&
Tonelli
,
G.
(
1982
).
Direct screening of living mammalian cell colonies for the identification of DNA repair deficient mutants by a multiwire proportional chamber
.
Radiat. environ. Biophys.
21
,
109
121
.
Altamirano-Dimas
,
M.
,
Sklar
,
R.
&
Strauss
,
B.
(
1979
).
Selectivity of the excision of alkylation products in a xeroderma pigmentosum-derived lymphoblastoid line
.
Mutat. Res.
60
,
197
206
.
Andrews
,
A. D.
,
Robbins
,
J. H.
,
Kraemer
,
K. H.
&
Buell
,
D. N.
(
1974
).
Xeroderma pigmentosum long-term lymphoid lines with increased ultraviolet sensitivity
.
J. natn. Cancer Inst.
53
,
691
693
.
Arlett
,
C. F.
&
Priestley
,
A.
(
1983
).
Defective recovery from potentially lethal damage in some human fibroblast strains
.
Int.J. Radiat. Biol.
43
,
157
167
.
Arlett
,
C. F.
&
Priestley
,
A.
(
1984
).
Deficient recovery from potentially lethal damage in some gamma-irradiated human fibroblast cell strains
.
Br.J. Cancer Suppl. VI, 227-232
.
Barbis
,
D. P.
,
Schultz
,
R. A.
&
Friedberg
,
E. C.
(
1986
).
Isolation and partial characterization of virus-transformed cell lines representing the A, G and variant complementation groups of xeroderma pigmentosum
.
Mutat. Res.
165
,
175
184
.
Barenfeld
,
L. S.
(
1984
).
A study of DNA synthesis in the Chinese hamster cells of UV-sensitive and UV-resistant clones
.
Tsitologia
26
,
343
348
.
Basilico
,
C.
&
Meiss
,
H. K.
(
1974
).
Methods for selecting and studying temperature-sensitive mutants of BHK-21 cells
.
Meth. Cell Biol.
8
,
1
22
.
Brandt
,
W. N.
,
Flamm
,
W. G.
&
Bernheim
,
N. J.
(
1972
).
The value of hydroxyurea in assessing repair synthesis of DNA in HeLa cells
.
Chem.-Biol. Interact.
5
,
327
339
.
Busch
,
D. B.
,
Cleaver
,
J. E.
&
Glaser
,
D. A.
(
1980
).
Large-scale isolation of UV-sensitive clones of CHO cells
.
Somat. Cell Genet.
6
,
407
418
.
Canaani
,
D.
,
Naiman
,
T.
,
Teitz
,
T.
&
Berg
,
P.
(
1986
).
Immortalization of xeroderma pigmentosum cells by simian virus 40 DNA having a defective origin of DNA replication
.
Somat. Cell molec. Genet.
12
,
13
20
.
Chan
,
J. Y. H.
,
Thompson
,
L. H.
&
Becker
,
F. F.
(
1984
).
DNA-ligase activities appear normal in the CHO mutant EM9
.
Mutat. Res.
131
,
209
214
.
Clarkson
,
J. M.
,
Mitchell
,
D. L.
&
Adair
,
G. M.
(
1983
).
The use of an immunological probe to measure the kinetics of DNA repair in normal and UV-sensitive mammalian cell lines
.
Mutat. Res.
112
,
287
299
.
Cleaver
,
J. E.
(
1982
).
Rapid complementation method for classifying excision repair-defective xeroderma pigmentosum cell strains
.
Somat. Cell Genet.
8
,
801
810
.
Cohen
,
M. M.
,
Sagi
,
M.
,
Ben-Zur
,
Z.
,
Schaap
,
T.
,
Voss
,
R.
,
Koh
,
G.
&
Ben-Bassat
,
H.
(
1979
).
Ataxia telangiectasia: chromosomal stability in continuous lymphoblastoid cell lines
.
Cytogenet. Cell Genet.
23
,
44
52
.
Collins
,
A.
(
1985
).
Do mammalian cells control DNA repair according to the availability of DNA precursors
.
Abst. Conf. Mechanisms of DNA Damage and Repair. Gaithersburg, MD: National Bureau of Standards
.
Collins
,
A.
(
1987
).
Replica plating of cultured human cells on polyester mesh
.
J. Tiss. Culture Meth, (in press)
.
Collins
,
A. R. S.
&
Johnson
,
R. T.
(
1984
).
The inhibition of DNA repair
.
Adv. Radiat. Biol.
11
,
71
129
.
Collins
,
A.
,
Jones
,
C.
&
Waldren
,
C.
(
1982
).
A survey of DNA repair incision activities after ultraviolet irradiation of a range of human, hamster, and hamster-human hybrid cell lines
.
J. Cell Sci.
56
,
423
440
.
Collins
,
A.
&
Waldren
,
C.
(
1982
).
Cell cycle kinetics and ultraviolet light survival in UV-1, a Chinese hamster ovary cell mutant defective in post-replication recovery
.
J. Cell Sci.
57
,
261
275
.
Cook
,
P. R.
&
Brazell
,
I. A.
(
1976
).
Detection and repair of single strand breaks in nuclear DNA
.
Nature, Lond.
263
,
679
682
.
Day
,
R. S. Ill
,
Babich
,
M. A.
,
Yarosh
,
D. B.
&
Scudiero
,
D. A.
(
1987
).
The role of O6- methylguanine in human cell killing, sister chromatid exchange induction and mutagenesis: a review
.
J. Cell Sci. Suppl.
6
,
333
353
.
Day
,
R. S.
,
Kraemer
,
K. H.
&
Robbins
,
J. H.
(
1975
).
Complementing xeroderma pigmentosum fibroblasts restore biological activity to UV damaged DNA
.
Mutat. Res.
28
,
251
255
.
Daya-Grosjean
,
L.
,
James
,
M.
,
Drougard
,
C.
&
Sarasin
,
A.
(
1986
).
The establishment and characterisation of a xeroderma pigmentosum cell line transformed by an origin-defective SV40 recombinant plasmid
.
Br.J. Cancer
54
,
353
(abstr.).
Doniger
,
J.
,
DiPaulo
,
J. A.
&
Popescu
,
N. C.
(
1983
).
Transformation of Bloom’s syndrome fibroblasts by DNA transfection
.
Science
222
,
1144
1146
.
Downes
,
C. S.
(
1986
).
Molecular biology of DNA repair
.
Mutat. Res.
166
,
221
226
.
Duckworth-Rysiecki
,
G.
,
Cornish
,
K.
,
Clarke
,
C.
&
Buckwald
,
M.
(
1985
).
Identification of two complementation groups in Fanconi anemia
.
Somat. Cell molec. Genet.
11
,
35
41
.
Duckworth-Rysiecki
,
G.
,
Ton
,
L.
,
Ng
,
I.
,
Clarke
,
C.
&
Buchwald
,
M.
(
1986
).
Characterization of a Simian virus 40-transformed Fanconi anemia fibroblast cell line
.
Mutat. Res.
166
,
207
214
.
Fiorio
,
R.
,
Frosina
,
G.
&
Abbondandolo
,
A.
(
1983
).
Isolation and preliminary characterization of UV-sensitive mutants from the human cell line EUE
.
Carcinogenesis
4
,
39
44
.
Fiorio
,
R.
,
Quintavalle
,
A.
,
Abbondandolo
,
A.
,
Bonatti
,
S.
&
Mazzaccaro
,
A.
(
1981
).
Characterization of UV-sensitive clones isolated in EUE human cells
.
Mutat. Res.
85
,
256
(abstr.).
Friedberg
,
E. C.
(
1987
).
The molecular biology of nucleotide excision repair of DNA: recent progress. J’
.
Cell Sci. Suppl.
6
,
1
24
.
Friedberg
,
E. C.
&
Hanawalt
,
P. C.
(
1981
).
DNA Repair: A Laboratory Manual of Research Procedures, vol. IB. New York, Basel: Marcel Dekker, Inc
.
Gantt
,
R.
,
Taylor
,
W. G.
,
Camalier
,
R. F.
&
Stephens
,
E. V.
(
1984
).
Repair of DNA-protein cross-links in an excision repair-deficient human cell line and its Simian virus 40-transformed derivative
.
Cancer Res.
44
,
1809
1812
.
Giannelli
,
F.
,
Pawsey
,
S. A.
&
Avery
,
J. A.
(
1982
).
Differences in patterns of complementation of the more common groups of xeroderma pigmentosum: possible implications
.
Cell
29
,
451
458
.
Hama-Inaba
,
H.
,
Hieda-Shiomi
,
N.
&
Sato
,
K.
(
1983
).
Inhibition and recovery of DNA synthesis after X-irradiation in radiosensitive mouse-cell mutants
.
Mutat. Res.
120
,
161
165
.
Hashimoto
,
T.
,
Nakano
,
Y.
,
Koji Owada
,
M.
,
Kakunaga
,
T.
&
Furnyama
,
J.
(
1986
).
Establishment of cell lines derived from ataxia telangiectasia and xeroderma pigmentosum patients with high radiation sensitivity
.
Mutat. Res.
166
,
215
220
.
Heddle
,
J. A.
&
Arlett
,
C. F.
(
1980
).
Untransformed xeroderma pigmentosum cells are not hypersensitive to sister-chromatid exchange production by ethyl methanesulphonate - implications for the use of transformed cell lines and for the mechanism by which SCE arise
.
Mutat. Res.
72
,
119
125
.
Henderson
,
E. E.
&
Ribecky
,
R.
(
1980
).
DNA repair in lymphoblastoid cell lines established from human genetic disorders
.
Chem.-Biol. Interact.
33
,
63
81
.
Henderson
,
L. M.
,
Arlett
,
C. F.
,
Harcourt
,
S. A.
,
Lehmann
,
A. R.
&
Broughton
,
B. C.
(
1985
).
Cells from an immunodeficient patient (46BR) with a defect in DNA ligation are hypomutable but hypersensitive to the induction of sister chromatid exchanges
.
Proc. natn. Acad. Sci. U.SA.
82
,
2044
2048
.
HoEIJMAKERS
,
J. H. J.
(
1987
).
Characterization of genes and proteins involved in excision repair of human cells. J’
.
Cell Sci. Suppl.
6
,
111
125
.
Hori
,
T.-A.
,
Shiomi
,
T.
&
Sato
,
K.
(
1983
).
Human chromosome 13 compensates a DNA repair defect in UV-sensitive mouse cells by mouse-human cell hybridization
.
Proc. natn. Acad. Sci. U.SA.
80
,
5655
5659
.
Hoy
,
C. A.
,
Salazar
,
E. P.
&
Thompson
,
L. H.
(
1984
).
Rapid detection of DNA-damaging agents using repair-deficient CHO cells
.
Mutat. Res.
130
,
321
332
.
Hoy
,
C. A.
,
Thompson
,
L. H.
, Mooney, C.-L. &
Salazar
,
E. P.
(
1985a
).
Defective DNA crosslink removal in Chinese hamster cell mutants hypersensitive to bifunctional alkylating agents
.
Cancer Res.
45
,
1737
1743
.
Hoy
,
C. A.
,
Thompson
,
L. H.
,
Salazar
,
E. P.
&
Stewart
,
S. A.
(
1985b
).
Different genetic alterations underlie dual hypersensitivity of CHO mutant UV-1 to DNA methylating and crosslinking agents
.
Somat. Cell molec. Genet.
11
,
523
—532.
Isomura
,
K.
,
Nikaido
,
O.
,
Horikawa
,
M.
&
Sugahara
,
T.
(
1973
).
Repair of DNA damage in ultraviolet-sensitive cells isolated from HeLa S3 cells
.
Radiat. Res.
53
,
143
—152.
Jaspers
,
N. J. G.
&
Bootsma
,
D.
(
1982
).
Genetic heterogeneity in ataxia-telangiectasia studied by cell fusion
.
Proc. natn. Acad. Sci. U.SA.
79
,
2641
2644
.
Jeggo
,
P. A.
&
Holliday
,
R.
(
1986
).
Reversion of a defect in DNA repair induced at high frequency by azacytidine
.
Br.J. Cancer
54
,
350
(abstr.).
Jeggo
,
P. A.
&
Kemp
,
L. M.
(
1983
).
X-ray sensitive mutants of Chinese hamster ovary cell line. Isolation and cross-sensitivity to other DNA damaging agents
.
Mutat. Res.
112
,
313
327
.
Jeggo
,
P. A.
&
Kemp
,
L. M.
(
1985
).
Isolation and characterisation of X-ray sensitive mutants of the CHO cell line
.
Br.J. Cancer
51
,
609
(abstr.).
Johnson
,
R. T.
,
Squires
,
S.
,
Elliott
,
G. C.
,
Koch
,
G. L. E.
&
Rainbow
,
A. J.
(
1985
).
Xeroderma pigmentosum D-Hela hybrids with low and high ultraviolet sensitivity associated with normal and diminished DNA repair ability, respectively.^
.
Cell Sci.
76
,
115
—133.
Johnson
,
R. T.
,
Squires
,
S.
,
Elliott
,
G. C.
,
Rainbow
,
A. J.
,
Koch
,
G. L. E.
&
Smith
,
M.
(
1986
).
Analysis of repair in XP-HeLa hybrids; lack of correlation between excision repair of UV damage and adenovirus reactivation in an XP(D)-like cell line
.
Carcinogenesis
7
,
1733
1738
.
Jones
,
N. J.
,
Debenham
,
P. G.
&
Thacker
,
J.
(
1986
).
New X-ray-sensitive mutants of cultured hamster cells
.
Br.J. Cancer
54
,
349
(abstr.).
Kao, F.-T. &
Puck
,
T. T.
(
1974
).
Induction and isolation of auxotrophic mutants in mammalian cells
.
Meth. Cell Biol.
8
,
23
39
.
Kemp
,
L. M.
,
Sedgwick
,
S. G.
&
Jeggo
,
P. A.
(
1984
).
X-ray sensitive mutants of Chinese hamster ovary cells defective in double-strand break rejoining
.
Mutat. Res.
132
,
189
196
.
Kuroki
,
T.
&
Miyashita
,
S. Y.
(
1977
).
Isolation of UV-sensitive clones from mouse cell lines by Lederberg style replica plating
.
J. cell. Physiol.
90
,
79
90
.
Lehmann
,
A. R.
(
1982a
).
Three complementation groups in Cockayne’s Syndrome
.
Mutat. Res.
106
,
345
356
.
Lehmann
,
A. R.
(
1982b
).
Xeroderma pigmentosum, Cockayne Syndrome and ataxia telangiectasia: Disorders relating DNA repair to carcinogenesis
.
Cancer Surveys
1
,
93
118
.
Liu
,
P. K.
, Chang, C.-C. &
Trosko
,
J. E.
(
1984
).
Evidence for mutagenic repair in V79 cell mutant with aphidicolin-resistant DNA polymerase-α
.
Somat. Cell molec. Genet.
10
,
235
245
.
Mayne
,
L. V.
,
Priestley
,
A.
,
James
,
M. R.
&
Burke
,
J. F.
(
1986a
).
Efficient immortalization and morphological transformation of human fibroblasts by transfection with SV40 DNA linked to a dominant marker
.
Expl Cell Res.
162
,
530
538
.
Mayne
,
L. V.
,
Priestley
,
A.
,
Jones
,
T.
&
Arlett
,
C. F.
(
1986b
).
Cloning of human DNA repair genes: 1. Immortalisation of primary strains. 2. Gene transfer and selection for resistance to DNA damaging agents
.
Br.J. Cancer 5A, 350 (abstr
.).
Meuth
,
M.
(
1983
).
Deoxycytidine kinase-deficient mutants of Chinese hamster ovary cells are hypersensitive to DNA alkylating agents
.
Mutat. Res.
110
,
383
391
.
Meyn
,
R. E.
,
Jenkins
,
S. F.
&
Thompson
,
L. H.
(
1982
).
Defective removal of DNA cross-links in a repair-deficient mutant of Chinese hamster cells
.
Cancer Res.
42
,
3106
3110
.
Mitchell
,
D. L.
,
Clarkson
,
J. M.
&
Adair
,
G. M.
(
1986
).
The DNA of UV-irradiated normal and excision-deficient mammalian cells undergoes relaxation in an initial stage of DNA repair
.
Mutat. Res.
165
,
123
128
.
Moses
,
R. E.
,
Timme
,
T. L.
&
Wood
,
C. M.
(
1986
).
Immortalization of human DNA repair deficient fibroblasts
.
Abst. 7th Int. Cong. Human Genetics, West Berlin, vol. II, p. 690
.
Mullinger
,
A. M.
,
Collins
,
A. R. S.
&
Johnson
,
R. T.
(
1983
).
Cell growth state determines susceptibility of repair DNA synthesis to inhibition by hydroxyurea and 1-β-D- arabinofuranosylcytosine
.
Carcinogenesis
4
,
1039
1043
.
Murnane
,
J. P.
,
Fuller
,
L. F.
&
Painter
,
R. B.
(
1985
).
Establishment and characterization of a permanent pSVori transformed ataxia-telangiectasia cell line
.
Expl Cell Res.
158
,
119
126
.
Murnane
,
J. P.
&
Painter
,
R. B.
(
1982
).
Complementation of the defects in DNA synthesis in irradiated and unirradiated ataxia telangiectasia cells
.
Proc. natn. Acad. Sci. U.SA.
79
,
1960
1963
.
NIGMS Human Genetic Mutant Cell
Repository
(
1985
).
U.S. Department of Health and Human Services
.
Washington
:
N.I.H. Publication, no. 85-2011
.
Painter
,
R. B.
(
1977
).
Rapid test to detect agents that damage human DNA
.
Nature, Lond.
265
,
650
651
.
Pawsey
,
S. A.
,
Magnus
,
I. A.
,
Ramsay
,
C. A.
,
Benson
,
P. F.
&
Giannelli
,
F.
(
1978
).
Clinical, genetic and DNA repair studies on a consecutive series of patients with xeroderma pigmentosum
.
Quart. J. Med.
48
,
179
210
.
Pearson
,
C.
&
Styles
,
J. A.
(
1984
).
Resistance of mouse lymphoma L5178YAII cells to alkylation with methylmethane sulphonate resides in a late step of excision repair
.
J. Cell Sci.
68
,
35
48
.
PiLLiDGE
,
L.
(
1984
).
Analysis of UV sensitivity and DNA repair in two Indian muntjac cell lines. PhD thesis, University of Cambridge
.
PiLLiDGE
,
L.
,
Downes
,
C. S.
&
Johnson
,
R. T.
(
1986a
).
Defective postreplication recovery and UV sensitivity in a simian virus 40-transformed Indian muntjac cell line
.
Int. J. Radiat. Biol.
50
,
119
136
.
PiLLiDGE
,
L.
,
Musk
,
S. R. R.
,
Johnson
,
R. T.
&
Waldren
,
C. A.
(
1986b
)
… Excessive chromosome fragility and abundance of sister chromatid exchanges induced by UV in an Indian muntjac cell line defective in postreplication (daughter strand) repair
.
Mutat. Res.
166
,
265
273
.
Pinto
,
R. L
,
Manukyan
,
K. L.
,
Vikhanskaya
,
F. L.
,
Manuilova
,
E. S.
,
Shapiro
,
N. I.
&
Zhestyanikov
,
V. D.
(
1980
).
Post-replication repair of DNA in UV-sensitive clones of the Chinese hamster
.
Tsitologia
22
,
1085
1095
.
Popescu
,
N. C.
,
Amsbaugh
,
S. C.
&
DiPaulo
,
J. A.
(
1983
).
Human and rodent transformed cells are more sensitive to in vitro induction of SCE by N-methyl-N’-nitrosoguanidine (MNNG) than normal cells
.
Hum. Genet.
63
,
53
57
.
Raetz
,
C. R. H.
,
Wermuth
,
M. N.
,
McIntyre
,
T. M.
,
Esko
,
J.
&
Wing
,
B. C.
(
1982
).
Somatic cell cloning in polyester stacks
.
Proc. natn. Acad. Sci. U.SA.
79
,
3223
3227
.
Robson
,
C. N.
, Harris, A. Lr. &
Hickson
,
I. D.
(
1985
).
Isolation and characterization of Chinese hamster ovary cell lines sensitive to mitomycin C and bleomycin
.
Cancer Res.
45
,
5305
5309
.
Robson
,
C. N.
,
Harris
,
A. L.
&
Hickson
,
I. D.
(
1986
).
Characterisation of mitomycin C sensitive mutants of CHO-K1 cells and their use as hosts for the cloning of human DNA repair genes
.
Br.J. Cancer
54
,
350
(abstr.).
Rosenstein
,
B. S.
&
Chao
,
C. C.-K.
(
1985a
).
Isolation of a mutant cell line derived from ICR 2A frog cells hypersensitive to the induction of non-dimer DNA damages by solar ultraviolet radiation
.
Somat. Cell molec. Genet.
11
,
339
344
.
Rosenstein
,
B. S.
&
Chao
,
C. C.-K.
(
1985b
).
Characterization of DNA repair in a mutant cell line derived from ICR 2A frog cells that is hypersensitive to non-dimer DNA damages induced by solar ultraviolet radiation
.
Mutat. Res.
146
,
191
196
.
Rosenstein
,
B.
&
Ohlsson-Wilhelm
,
B. M.
(
1979
).
Isolation of UV-sensitive clones from a haploid frog cell line
.
Somat. Cell Genet.
5
,
117
128
.
Royer-Pokora
,
B.
&
Haseltine
,
W. A.
(
1984
).
Isolation of UV-resistant revenants from a xeroderma pigmentosum complementation group A cell line
.
Nature, Lond.
311
,
390
392
.
Sato
,
K.
&
Hieda
,
N.
(
1979a
).
Isolation of a mammalian cell mutant sensitive to 4- nitroquinoline-1-oxide
.
Int.J. Radiat. Biol.
35
,
83
87
.
Sato
,
K.
&
Hieda
,
N.
(
1979b
).
Isolation and characterization of a mutant mouse lymphoma cell sensitive to methyl methanesulfonate and X rays
.
Radiat. Res.
78
,
167
171
.
Sato
,
K.
&
Setlow
,
R. B.
(
1981
).
DNA repair in a UV-sensitive mutant of a mouse cell line
.
Mutat. Res.
84
,
443
455
.
Schultz
,
R. A.
,
Barbis
,
D. P.
&
Friedberg
,
E. C.
(
1985
).
Studies on gene transfer and reversion to UV resistance in xeroderma pigmentosum cells
.
Somat. Cell molec. Genet.
11
,
617
624
.
Schultz
,
R. A.
, Chang, C.-C. &
Trosko
,
J. E.
(
1981a
).
The mutation studies of mutagensensitive and DNA repair mutants of Chinese hamster fibroblasts
.
Environ. Mutagen.
3
,
141
150
.
Schultz
,
R. A.
,
Trosko
,
J. E.
&
Chang
,
C.-C.
(
1981b
).
Isolation and partial characterization of mutagen sensitive and DNA repair mutants of Chinese hamster fibroblasts
.
Environ. Mutagen.
3
,
53
64
.
Shiomi
,
T.
,
Hieda-Shiomi
,
N.
&
Sato
,
K.
(
1982a
).
Isolation of UV-sensitive mutants of mouse L5178Y cells by a cell suspension spotting method
.
Somat. Cell Genet.
8
,
329
345
.
Shiomi
,
T.
,
Hieda-Shiomi
,
N.
&
Sato
,
K.
(
1982b
).
A novel mutant of mouse lymphoma cells sensitive to alkylating agents and caffeine
.
Mutat. Res.
103
,
61
69
.
Shiomi
,
T.
&
Sato
,
K.
(
1979
).
Isolation of UV-sensitive variants of human FL cells by a viral suicide method
.
Somat. Cell Genet.
5
,
193
201
.
Shiraishi
,
Y.
,
Yosida
,
T. H.
&
Sandberg
,
A. A.
(
1983
).
Analyses of bromodeoxyuridine- associated sister chromatid exchanges (SCEs) in Bloom syndrome based on cell fusion: single and twin SCEs in endoreduplication
.
Proc. natn. Acad. Sci. U.S.A.
80
,
4369
4373
.
Siciliano
,
M. J.
,
Carrano
,
A. V.
&
Thompson
,
L. H.
(
1985
).
Chromosome 19 corrects the complementing DNA repair mutations present in CHO cells
.
Cytogenet. Cell Genet.
40
,
744
745
.
Siminovitch
,
L.
(
1976
).
On the nature of hereditable variation in cultured somatic cells
.
Cell
1
,
1
-
11
.
Squires
,
S.
,
Johnson
,
R. T.
&
Collins
,
A. R. S.
(
1982
).
Initial rates of DNA incision in UV- irradiated human cells. Differences between normal, xeroderma pigmentosum and tumour cells
.
Mutat. Res.
95
,
389
404
.
Srinivasan
,
P. R.
,
Gupta
,
R. S.
&
Siminovitch
,
L.
(
1980
).
Studies on temperature-sensitive mutants of Chinese hamster ovary cells affected in DNA synthesis
.
Somat. Cell Genet
.
6
,
567
-
582
.
Stamato
,
T. D.
,
Hinkle
,
L.
,
Collins
,
A. R. S.
&
Waldren
,
C. A.
(
1981
).
Chinese hamster ovary mutant UV-1 is hypomutable and defective in a postreplication recovery process
.
Somat. Cell Genet.
7
,
307
320
.
Stamato
,
T. D.
&
Waldren
,
C. A.
(
1977
).
Isolation of UV-sensitive variants of CHO-K1 by nylon cloth replica plating
.
Somat. Cell Genet.
3
,
431
440
.
Stamato
,
T. D.
,
Weinstein
,
R.
,
Giaccia
,
A.
&
Mackenzie
,
L.
(
1983
).
Isolation of cell-cycle- dependent gamma ray-sensitive Chinese hamster ovary cell
.
Somat. Cell Genet.
9
,
165
173
.
Stefanini
,
M.
,
Keijzer
,
W.
,
Westerveld
,
A.
&
Bootsma
,
D.
(
1985
).
Interspecies complementation analysis of xeroderma pigmentosum and UV-sensitive Chinese hamster cells
.
Expl Cell Res.
161
,
373
380
.
Stefanini
,
M.
,
Mondello
,
C.
,
Tessera
,
L.
,
Capuano
,
V.
,
Guerra
,
B. R.
&
Nuzzo
,
F.
(
1986
).
Sensitivity to DNA-damaging agents and mutation induction by UV light in UV sensitive CHO cells
.
Mutat. Res.
174
,
155
159
.
Stefanini
,
M.
,
Reuser
,
A.
&
Bootsma
,
D.
(
1982
).
Isolation of Chinese hamster ovary cells with reduced unscheduled DNA synthesis after UV irradiation
.
Somat. Cell Genet.
8
,
635
642
.
Suzuki
,
N.
&
Fuse
,
A.
(
1981
).
A UV-sensitive human clonal cell line RSa, which has low repair activity
.
Mutat. Res.
84
,
133
145
.
Suzuki
,
N.
&
Kuwata
,
T.
(
1979
).
Establishment of ultraviolet-resistant cells from the highly sensitive human clonal cell line, RSb
.
Mutat. Res.
60
,
215
219
.
Takano
,
T.
,
Noda
,
M.
&
Tamura
,
T.
(
1982
).
Transfection of cells from a xeroderma pigmentosum patient with normal human DNA confers UV resistance
.
Nature, Land.
296
,
269
270
.
Tanaka
,
K.
,
Kawai
,
K.
,
Kumahara
,
Y.
,
Ikenaga
,
M.
&
Okada
,
Y.
(
1981
).
Genetic complementation groups in Cockayne Syndrome
.
Somat. Cell Genet.
7
,
445
455
.
Teo
,
LA.
,
Lehmann
,
A. R.
,
Muller
,
R.
&
Rajewsky
,
M. F.
(
1983
).
Similar rate of O6- ethylguanine elimination from DNA in normal human fibroblast and xeroderma pigmentosum cell strains not transformed by SV40
.
Carcinogenesis
4
,
1075
1077
.
Thielmann
,
H. W.
,
Fischer
,
E.
,
Dzarlieva
,
R. T.
,
Komitowski
,
D.
,
Popanda
,
O.
&
Edler
,
L.
(
1983
).
Spontaneous in vitro malignant transformation in a xeroderma pigmentosum fibroblast line
.
Int.J. Cancer
31
,
687
700
.
Thompson
,
L. H.
&
Baker
,
R. M.
(
1973
).
Isolation of mutants of cultured mammalian cells
.
Meth. Cell Biol.
6
,
209
281
.
Thompson
,
L. H.
,
Brookman
,
K. W.
,
Dillehay
,
L. E.
,
Carrano
,
A. V.
,
Mazrimas
,
J. A.
,
Mooney
,
C. L.
&
Minkler
,
J. L.
(
1982a
).
A CHO-cell strain having hypersensitivity to mutagens, a defect in DNA strand-break repair, and an extraordinary baseline frequency of sister-chromatid exchange
.
Mutat. Res.
95
,
427
—440.
Thompson
,
L. H.
,
Brookman
,
K. W.
,
Carrano
,
A. V.
&
Dillehay
,
L. E.
(
1982b
).
Role of DNA repair in mutagenesis of Chinese hamster ovary cells by 7-bromomethylbenz(a)anthracene
.
Proc, natn. Acad. Sci. U.SA.
79
,
534
538
.
Thompson
,
L. H.
,
Brookman
,
K. W.
,
Dillehay
,
L. E.
,
Mooney
,
C. L.
&
Carrano
,
A. V.
(
1982c
).
Hypersensitivity to mutation and sister-chromatid exchange induction in CHO cell mutants defective in incising DNA containing UV lesions
.
Somat. Cell Genet.
8
,
759
773
.
Thompson
,
L. H.
,
Busch
,
D. B.
,
Brookman
,
K.
,
Mooney
,
C. L.
&
Glaser
,
D. A.
(
1981
).
Genetic diversity of UV-sensitive DNA repair mutants of Chinese hamster ovary cells
.
Proc, natn. Acad. Sci. U.SA.
78
,
3734
3737
.
Thompson
,
L. H.
,
Mooney
,
C. L.
&
Brookman
,
K. W.
(
1985a
).
Genetic complementation between UV-sensitive CHO mutants and xeroderma pigmentosum fibroblasts
.
Mutat. Res.
150
,
423
429
.
Thompson
,
L. H.
,
Mooney
,
C. L.
,
Burkhart-Schultz
,
K.
,
Carrano
,
A. V.
&
Siciliano
,
M. J.
(
1985b
).
Correction of a nucleotide-excision-repair mutation by human chromosome 19 in hamster-human hybrid cells
.
Somat. Cell molec. Genet.
11
,
87
92
.
Thompson
,
L. H.
,
Rubin
,
J. S.
,
Cleaver
,
J. E.
,
Whitmore
,
G. F.
&
Brookman
,
K.
(
1980
).
A screening method for isolating DNA repair-deficient mutants of CHO cells
.
Somat. Cell Genet.
6
,
391
405
.
Thompson
,
L. H.
,
Salazar
,
E. P.
,
Brookman
,
K. W.
,
Collins
,
C. C.
,
Stewart
,
S. A.
,
Busch
,
D. B.
&
Weber
,
C. A.
(
1987
).
Recent progress with the DNA repair mutants of Chinese hamster ovary cells
.
J. Cell Sci. Suppl.
6
,
000
000
.
Thompson
,
L. H.
,
Salazar
,
E. P.
,
Brookman
,
K. W.
&
Hoy
,
C. A.
(
1983
).
Hypersensitivity to cell killing and mutation induction by chemical carcinogens in an excision repair-deficient mutant of CHO cells
.
Mutat. Res.
112
,
329
344
.
Waldren
,
C.
,
Snead
,
D.
&
Stamato
,
T.
(
1983
).
In Cellular Responses to DNA Damage
(ed. E. C. Friedberg &
B. R.
Bridges
), pp.
637
646
.
New York
:
Alan R. Liss
.
\Nkvess
,
R.
,
Mishra
,
N.
,
Bouck
,
N.
,
DiMayorica
,
G.
&
Regan
,
J.
(
1977
).
Partial inhibition of postreplication repair and enhanced frequency of chemical transformation in rat cells infected with leukemia virus
.
Proc. natn. Acad. Sci. U.SA.
74
,
238
242
.
Wolff
,
S.
,
Rodin
,
B.
&
Cleaver
,
J. E.
(
1977
).
Sister chromatid exchanges induced by mutagenic carcinogens in normal and xeroderma pigmentosum cells
.
Nature, Land.
265
,
347
349
.
Wood
,
R. D.
&
Burki
,
H. J.
(
1982
).
Repair capability and the cellular age response for killing and mutation induction after UV
.
Mutat. Res.
95
,
505
514
.
Yagi
,
T.
&
Takebe
,
H.
(
1983
).
Establishment by SV40 transformation and characteristics of a cell line of xeroderma pigmentosum belonging to complementation group F
.
Mutat. Res.
112
,
59
66
.
Yin
,
L.
,
Chun
,
E. H. L.
&
Rutman
,
R. J.
(
1973
).
A comparison of the effects of alkylation on the DNA of sensitive and resistant Lettre-Ehrlich cells following in vivo exposure to nitrogen mustard
.
Biochim. biophys. Acta
324
,
472
481
.
Zdzienicka
,
M. Z.
&
Simons
,
J. W. I. M.
(
1986
).
Analysis of repair processes by the determination of the induction of cell killing and mutation in 2 repair deficient Chinese hamster ovary cell lines
.
Mutat. Res.
166
,
59
69
.