Resistance to methyl mercaptopurine riboside in cultured hamster cells. Preliminary characterization of resistant cells and conditions affecting their selection in quantitative mutation studies

The Chinese hamster FANCG/XRCC9 mutant NM3 fails to express the monoubiquitinated form of the FANCD2 protein, is hypersensitive to a range of DNA damaging agents and exhibits a normal level of spontaneous sister chromatid exchange

Fanconi anemia (FA) is a human autosomal disorder characterized by cancer susceptibility and cellular sensitivity to DNA crosslinking agents such as mitomycin C and diepoxybutane. Six FA genes have been cloned including a gene designated XRCC9 (for X-ray Repair Cross Complementing), isolated using a mitomycin C-hypersensitive Chinese hamster cell mutant termed UV40, and subsequently found to be identical to FANCG. A nuclear complex containing the FANCA, FANCC, FANCE, FANCF and FANCG proteins is needed for the activation of a sixth FA protein FANCD2. When monoubiquitinated, the FANCD2 protein co-localizes with the breast cancer susceptibility protein BRCA1 in DNA damage induced foci. In this study, we have assigned NM3, a nitrogen mustard-hypersensitive Chinese hamster mutant to the same genetic complementation group as UV40. NM3, like human FA cell lines (but unlike UV40) exhibits a normal spontaneous level of sister chromatid exchange. We show that both NM3 and UV40 are also hypersensitive to other DNA crosslinking agents (including diepoxybutane and chlorambucil) and to non-crosslinking DNA damaging agents (including bleomycin, streptonigrin and EMS), and that all these sensitivities are all corrected upon transfection of the human FANCG/XRCC9 cDNA. Using immunoblotting, NM3 and UV40 were found not to express the active monoubiquitinated isoform of the FANCD2 protein, although expression of the FANCD-L isoform was restored in the FANCG cDNA transformants, correlating with the correction of mutagen-sensitivity. These data indicate that cellular resistance to these DNA damaging agents requires FANCG and that the FA gene pathway, via its activation of FANCD2 and that protein's subsequent interaction with BRCA1, is involved in maintaining genomic stability in response not only to DNA interstrand crosslinks but also a range of other DNA damages including DNA strand breaks. NM3 and other "FA-like" Chinese hamster mutants should provide an important resource for the study of these processes in mammalian cells.

The genetic basis of resistance to ionising radiation damage in cultured mammalian cells

To test the genetic similarity of independently-isolated hamster cell mutants sensitive to ionising radiation, these were fused in pairs and the hybrids exposed to X-rays. Some mutants (irs1, irs3, xrs-1, XR-1, BLM2) were found to complement all others tested for radiosensitivity in hybrids, and are therefore in separate genetic groups. The mutants irs2 and V-E5, both isolated from V79 cells, did not complement and therefore belong to the same group. Another pair, EM7 and irs1SF, formed hybrids with intermediate levels of survival between mutant and wild-type. However, the parental cells fused to irs1SF also showed intermediate sensitivity, suggesting a semi-dominant mutant phenotype rather than a lack of complementation. Crosses of some of these hamster mutants to the radiosensitive mouse mutant M10 showed clear complementation (irs1 x M10, irs2 x M10) but for others the complementation did not greatly exceed the sensitivity of one (irs3 x M10) or both mutants (XR-1 x M10). Taken with our previously-published data, these results show that there are at least 8 genetic groups determining resistance to ionising radiation damage in rodent cells.

Fingerprinting cell lines: use of human hypervariable DNA probes to characterize mammalian cell cultures

Hypervariable DNA sequences may be used as probes to derive DNA "finger-prints" for individuals. To assess the use of the human 33.15 and 33.6 probes (isolated by Jeffreys and coworkers) for characterizing cell lines of nonhuman origin, DNA from different stocks of Chinese hamster (CH) cells was screened. All CHO (ovary) sublines could be readily distinguished from CH-V79 sublines by their fingerprints, but where two stocks had been derived recently from the same line, their fingerprints were nearly identical. Similarly fingerprints of HPRT-deficient mutants derived from one cell stock were identical. A V79 x CHO fusion hybrid showed equal fingerprint band-sharing with each parent line, while early-passage diploid CH cells had a fingerprint closer to CHO than to V79. Thus these data introduce a simple means of typing cell lines to avoid cross-contamination, of checking cell hybrids, and of assessing the divergence of cell stocks from one another.

Six complementation groups for ionising-radiation sensitivity in Chinese hamster cells

The ionising radiation-sensitive mutants irs 1, irs 2, irs 3, xrs-1 (or xrs-7), EM7 and XR-1 were fused to wild-type cells or to each other in pairs to create hybrid cells. These hybrids were checked chromosomally and their X-ray sensitivity tested. Each mutant was found to be recessive to wild-type and to complement the X-ray sensitivity of the other mutants. Thus there appear to be at least 6 complementation groups for ionising radiation sensitivity in Chinese hamster cells.

Immunological studies with different classes of mutants affected at the adenosine kinase locus in CHO cells

Adenosine kinase (AK) from CHO cells has been purified to homogeneity and specific antibodies to it have been raised in rabbits. Using this antibody, the presence of a specific cross-reacting protein (CRP) in cell extracts of different classes of mutants resistant to purine nucleoside analogs which are affected in AK has been investigated by the immunoblotting technique. Results of our studies show that 31 of the 32 independently selected class A AK- mutants (obtained at high frequency in presence of adenosine analogs toyocamycin, tubercidin, 6-methylmercaptopurine riboside, or pyrazofurin and containing no measurable activity of AK in cell extracts) contained similar amounts of a specific CRP as seen in the parental AK+ cells. The CRP in the parental and different mutant cell lines has the same relative molecular mass as purified AK. Similar results were obtained with two mutants each of the class B and C type (selected in presence of C-nucleosides formycin A and formycin B), which are also affected in AK but show novel properties. The presence of equivalent amounts of the CRP in the vast majority of the class A mutants strongly indicates that the high frequency of those mutants in CHO cells is not a result of an epigenetic or deletion type of event, but that such mutants may contain missense types of mutations at a presumed "mutational hot spot" within the structural gene for adenosine kinase.

Novel mutants of CHO cells resistant to adenosine analogs and containing biochemically altered form of adenosine kinase in cell extracts

Stable mutants which are approximately five- and eightfold resistant to an inosine analog, formycin B (Fomr) have been selected in a single-step from Chinese hamster ovary cells at a frequency of approximately 10(-6). Cross-resistance studies with these mutants show that the Fomr mutants exhibit increased resistance to all adenosine analogs (N-and C-nucleosides) examined and, in accordance with their cross-resistance pattern, the mutants exhibited decreased cellular uptake and phosphorylation of formycin B and various adenosine analogs. In cell hybrids formed with sensitive cells, the drug-resistant phenotype of these mutants behaved recessively. However, unlike mutants resistant to adenosine analogs that have been obtained previously, which contain no measurable activity of adenosine kinase (AK) in cell extracts, the two Fomr mutants studied contained about 60 and 110% of the enzyme activity (compared to the parental cells) in their cell extracts. Biochemical studies with AK from the mutant cells show that in comparison to the wild-type enzyme, the mutant enzymes required much higher concentrations of the adenosine analog N7-(delta 2-isopentenyl) formycin A for similar inhibition of [3H]adenosine phosphorylation. These results indicate that AK from the Fomr mutants has lower affinity for phosphorylation of adenosine analogs in comparison to the enzyme from the parental cells. The genetic lesion in the Fomr mutants may thus be directly affecting the structural gene for AK.

6-Methylmercaptopurine riboside resistance in human lymphocytes in the in vivo somatic cell mutation test

Additional drug-resistance markers are being investigated to broaden the in vivo somatic cell mutation test in human lymphocytes (PBL). The adenosine kinase (AK) locus was chosen for study because Gupta and Singh [Gupta RS, Singh B: Mutat Res 113:441-454, 1983] have demonstrated that in Chinese hamster ovary cells, mutants affected at this locus are obtained at a very high spontaneous frequency and that the response of this locus to different types of mutagens was comparable to that of the hypoxanthine-guanine phosphoribosyl transferase locus. The adenosine analog 6-methylmercaptopurine riboside (MeMPR) was used as the selective agent for obtaining AK-deficient mutants. Cultures of mitogen-stimulated PBL were set up in the presence (test) and absence (control) of the selective agent. Resistant cells capable of synthesizing DNA in the presence of MeMPR were labeled with tritiated thymidine and enumerated autoradiographically. The variant frequency (Vf) was calculated as the ratio of the number of labeled nuclei in the test relative to that in the control. Human PBL were found to be sensitive to MeMPR inhibition of DNA synthesis and exhibited a dose-dependent decrease in Vf with increasing concentrations of MeMPR. However, no leveling off of the dose-response curve was observed. Thus the background level of Vf was probably lower than the practical detection limit of the test (4.0 X 10(-7) with a 50-ml blood sample). It was concluded that, because of the autosomal recessive nature of the AK gene, the background Vf in human PBL is too low to allow a useful baseline to be established for the in vivo somatic mutation test.

The use of a cloned bacterial gene to study mutation in mammalian cells

The recombinant DNA molecule pSV2-gpt, which contains the bacterial gene coding for xanthine-guanine phosphoribosyl transferase (XGPRT) activity, was introduced into a hamster cell line lacking the equivalent mammalian enzyme (HGPRT). Hamster cell sublines were found with stable expression of XGPRT activity and were used to study mutation of the integrated pSV2-gpt DNA sequence. Mutants were selected by their resistance to 6-thioguanine (TG) under optimal conditions which were found to be very similar to those for selection of HGPRT-deficient mutants of mammalian cells. The frequency of XGPRT-deficient mutants was increased by treatment with X-rays, ethyl methanesulphonate and ethyl nitrosourea. X-Ray induction of mutants increased approximately linearly with dose up to about 500 rad, but the frequency of mutants per rad was very much higher than that usually found for 'native' mammalian genes. However, still higher frequencies of mutation were found for the hamster HGPRT gene when it had been stably transferred into the same hamster cell line. It is suggested, therefore, that transferred DNA may integrate in sequences which are more 'reactive' than most of the genome. Cell-free extracts of 10 TG-resistant mutants of XGPRT-proficient sublines showed no measurable XGPRT activity. High molecular weight DNA from XGPRT-proficient sublines used in the mutation studies hybridized with nick-translated pSV2-gpt DNA, showing two distinct bands when cut with the restriction enzyme Eco R1. This suggests that a single copy of pSV2-gpt DNA was integrated in these sublines. DNA from most spontaneous and mutagen-induced TG-resistant mutants had lost these two hybridizing bands, but one spontaneous mutant was found with rearranged pSV2-gpt sequence.

Quantitative mutagenesis at the adenosine kinase locus in Chinese hamster ovary cells. Development and characteristics of the selection system

Stable mutants exhibiting high degree of resistance (100-1000-fold) to various nucleoside analogs viz, toyocamycin, tubercidin, 6-methyl mercaptopurine riboside (6-MeMPR) and pyrazofurin, are obtained at similar frequency (congruent to 1 X 10(-4] in CHO cells. The mutants resistant to any of the above analogs exhibit similar degree of cross-resistance to the other three nucleoside analogs, and all of the mutants examined contained no measurable activity of the purine salvage pathway enzyme adenosine kinase (AK) which converts these analogs to their phosphorylated derivatives. These results indicate that very similar mutants are selected using any of these analogs. The recovery of AK- mutants in CHO cells is not affected by cell density (up to at least 5 X 10(5) cells per 100-mm diameter dish) and after treatment with mutagen(s) maximum mutagenic effect is observed after 7-8 days, which then remains unchanged for the next several days. Treatment of CHO cells with a number of mutagenic agents e.g. ethyl methanesulfonate, ICR170, ultraviolet light, and benzo[a]pyrene, led to a nearly linear concentration-dependent increase in the frequency of the AK- mutants in cultures. The mutagenic response of the AK locus to these agents compared favorably with that of the HGPRT locus (6-thioguanine resistance) within the same experiments. These results show that the selection system for AK- mutants provides an additional valuable genetic marker for quantitative mutagenesis studies in CHO cells.

Comparison of the mutagenic responses of 12 anticancer drugs at the hypoxanthine-guanine phosphoribosyl transferase and adenosine kinase loci in Chinese hamster ovary cells

5he mutagenic responses of 12 anticancer drugs--lomustine, dacarbazine, mitomycin C, chlorambucil, cis-diamminedichloroplatinum (II), busulfan, ellipticine, daunomycin, adriamycin, VM-26, VP16-213, and bleomycin--in Chinese hamster ovary (CHO) cells at two independent genetic loci which affect purine salvage pathway enzymes, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenosine kinase (AK) have been compared within the same experiments. The cellular mutants which lack either HGPRT (HGPRT- mutants) or AK (AK- mutants) activity can be directly selected in CHO cells using 6-thioguanine and 6-methyl mercaptopurineriboside (6-MeMPR) respectively, and optimal conditions for their selection have been described. Results of our studies show that all of the above drugs caused a concentration-dependent increase in the frequencies of both 6-thioguanine- and 6-MeMPR-resistant mutants. However, for a number of these drugs, namely adriamycin, daunomycin, bleomycin, and dacarbazine, which are only weakly mutagenic at the HGPRT locus, a relatively strong and clear mutagenic response was observed at the AK locus. In contrast, VM-26 showed a more pronounced effect at the HGPRT locus. For the remaining compounds, the mutagenic responses of the two loci (as indicated by the slopes of the dose-response curves) were comparable. The observed increase in the frequency of AK- mutants after treatment with various drugs, many of which predominantly induce frameshift and chromosome-breakage type of genetic lesions, suggests that like the HGPRT locus the AK locus is also capable of detecting all the different types of genetic lesions. The favorable response of the AK locus as compared to that of HGPRT locus in these studies indicates that the selection system of AK- mutants should provide a very useful genetic marker for mutagenesis studies in CHO cells.