Mutagenicity testing in mammalian cells. I. Derivation of a Chinese hamster ovary cell line heterozygous for the adenine phosphoribosyltransferase and thymidine kinase loci

Analysis of In Vivo Mutation in the Hprt and Tk Genes of Mouse Lymphocytes

Determining mutant frequencies in endogenous reporter genes is a tool for identifying potentially genotoxic environmental agents, and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe a high-throughput method for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation at both the Hprt and Tk genes.

Detection of In Vivo Mutation in the Hprt and Pig-a Genes of Rat Lymphocytes

Assays for in vivo mutation are used to identify genotoxic hazards and phenotypes prone to genomic instability and cancer. The hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the phosphatidyl inositol glycan, class A (Pig-a) gene are endogenous X-linked genes that can be used as reporters of mutation in peripheral blood lymphocytes from most mammals. Here we describe methodology for measuring Hprt and Pig-a mutation in rat T-lymphocytes. The identification and selective expansion of mutant lymphocytes is based upon the phenotypic properties of Hprt- and Pig-a-deficient cells, that is, resistance to the purine analog, 6-thioguanine, or to the bacterial toxin, proaerolysin. Expanded mutants can be further analyzed by sequencing cDNA from the target transcripts for identification of small sequence alterations and by multiplex PCR analysis of genomic DNA for the detection of deletions.

Analysis of in vivo mutation in the Hprt and Tk genes of mouse lymphocytes

Assays measuring mutant frequencies in endogenous reporter genes are used for identifying potentially genotoxic environmental agents and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe methods for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation in both the Hprt and Tk genes.

Detection of in vivo mutation in the Hprt and Pig-a genes of rat lymphocytes

Assays for in vivo mutation are used to identify genotoxic hazards and phenotypes prone to genomic instability and cancer. The hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the phosphatidyl inositol glycan, class A (Pig-a) gene are endogenous X-linked genes that can be used as reporters of mutation in peripheral blood lymphocytes from most mammals. Here we describe methodology for measuring Hprt and Pig-a mutation in rat T-lymphocytes. The identification and selective expansion of mutant lymphocytes are based upon the phenotypic properties of Hprt- and Pig-a-deficient cells, i.e., resistance to the purine analog, 6-thioguanine, or to the bacterial toxin, proaerolysin. Expanded mutants can be further analyzed by sequencing cDNA from the target transcripts for identification of small sequence alterations and by multiplex PCR analysis of genomic DNA for the detection of deletions.

Genome-wide demethylation promotes triplet repeat instability independently of homologous recombination

Trinucleotide repeat instability is intrinsic to a family of human neurodegenerative diseases. The mechanism leading to repeat length variation is unclear. We previously showed that treatment with the demethylating agent 5-aza-2'-deoxycytidine (5-aza-CdR) dramatically increases triplet repeat instability in mammalian cells. Based on previous reports that demethylation increases homologous recombination (HR), and our own observations that HR destabilizes triplet repeats, we hypothesized that demethylation alters repeat stability by stimulating HR. Here, we test that hypothesis at the adenosine phosphoribosyl transferase (Aprt) locus in CHO cells, where CpG demethylation and HR have both been shown to increase CAG repeat instability. We find that the rate of HR at the Aprt locus is not altered by demethylation. The spectrum of recombinants, however, was shifted from the usual 6:1 ratio of conversions to crossovers to more equal proportions in 5-aza-CdR-treated cells. The subtle influences of demethylation on HR at the Aprt locus are not sufficient to account for its dramatic effects on repeat instability. We conclude that 5-aza-CdR promotes triplet repeat instability independently of HR.

Significance of CpG methylation for solar UV-induced mutagenesis and carcinogenesis in skin

Mutations detected in the p53 gene in human nonmelanoma skin cancers show a highly UV-specific mutation pattern, a dominance of C --> T base substitutions at dipyrimidine sites plus frequent CC --> TT tandem substitutions, indicating a major involvement of solar UV in the skin carcinogenesis. These mutations also have another important characteristic of frequent occurrences at CpG dinucleotide sites, some of which actually show prominent hotspots in the p53 gene. Although mammalian solar UV-induced mutation spectra were studied intensively in the aprt gene using rodent cultured cells and the UV-specific mutation pattern was confirmed, the second characteristic of the p53 mutations in human skin cancers had not been reproduced. However, studies with transgenic mouse systems developed thereafter for mutation research, which harbor methyl CpG-abundant transgenes as mutation markers, yielded complete reproductions of the situation of the human skin cancer mutations in terms of both the UV-specific pattern and the frequent occurrence at CpG sites. In this review, we evaluate the significance of the CpG methylation for solar UV mutagenesis in the mammalian genome, which would lead to skin carcinogenesis. We propose that the UV-specific mutations at methylated CpG sites, C --> T transitions at methyl CpG-associated dipyrimidine sites, are a solar UV-specific mutation signature, and have estimated the wavelength range effective for the solar-UV-specific mutation as 310-340 nm. We also recommend the use of methyl CpG-enriched sequences as mutational targets for studies on solar-UV genotoxicity for human, rather than conventional mammalian mutational marker genes such as the aprt and hprt genes.

Transcription-coupled DNA repair is genomic context-dependent

DNA damage is preferentially repaired in the transcribed strand of many active genes. Although the concept of DNA repair coupled with transcription has been widely accepted, its mechanisms remain elusive. We recently reported that in Chinese hamster ovary cells while ultraviolet light-induced cyclobutane pyrimidine dimers (CPDs) are preferentially repaired in the transcribed strand of dihydrofolate reductase gene, CPDs are efficiently repaired in both strands of adenine phosphoribosyltransferase (APRT) locus, in either a transcribed or nontranscribed APRT gene (1). These results suggested that the transcription dependence of repair may depend on genomic context. To test this hypothesis, we constructed transfectant cell lines containing a single, actively transcribed APRT gene, integrated at different genomic sites. Mapping of CPD repair in the integrated APRT genes in three transfectant cell lines revealed two distinct repair patterns, either preferential repair of CPDs in the transcribed strand or very poor repair in both strands. Similar kinetics of micrococcal nuclease digestion were seen for all three transfectant APRT gene domains and endogenous APRT locus. Our results suggest that both the efficiency and strand-specificity of repair of an actively transcribed gene are profoundly affected by genomic context but do not reflect changes in first order nucleosomal structure.

Mutagenicity of gamma-radiation, mitomycin C, and etoposide in the Hprt and Tk genes of Tk(+/-) mice

The recently developed Tk(+/-) mouse detects in vivo somatic cell mutation in the endogenous, autosomal Tk gene. To evaluate the sensitivity of this model, we have treated Tk(+/-) mice with three agents that induce DNA damage by different mechanisms, and determined spleen lymphocyte mutant frequencies (MFs) in the autosomal Tk gene and in the X-linked Hprt gene. gamma-Radiation, which produces single- and double-strand breaks by nonspecific oxidative stress, efficiently increased Hprt MF, but not Tk MF. Mitomycin C, which produces bulky DNA monoadducts and crosslinks, was mutagenic in both the Hprt and Tk genes, but the response was greater in the Tk gene. An inhibitor of the ligase function of DNA topoisomerase II, etoposide, did not increase Hprt MF, and induced a small, but nonsignificant increase in Tk MF. Combined with previous data, the results indicate that the two genes are differentially sensitive to many agents, and that the Tk gene is more sensitive than the Hprt gene to some, but not all types of DNA damage.

Codominance associated with overexpression of certain XPD mutations

Mutations in the XPD gene are associated with three complex clinical phenotypes, namely xeroderma pigmentosum (XP), XP in combination with Cockayne syndrome (XP-CS), and trichothiodystrophy (TTD). XP is caused by a deficiency in nucleotide excision repair (NER) that results in a high risk of skin cancer. TTD is characterized by severe developmental and neurological defects, with hallmark features of brittle hair and scaly skin, and sometimes has defective NER. We used CHO cells as a system to study how specific mutations alter the dominant/recessive behavior of XPD protein. Previously we identified the T46I and R75W mutations in two highly UV-sensitive hamster cell lines that were reported to have paradoxically high levels of unscheduled DNA synthesis. Here we report that these mutants have greatly reduced XPD helicase activity and fully defective NER in a cell-extract excision assay. We conclude that the unscheduled DNA synthesis seen in these mutants is caused by abortive "repair" that does not contribute to cell survival. These mutations, as well as the K48R canonical helicase-domain mutation, each produced codominant negative phenotypes when overexpressed in wild-type CHO cells. The common XP-specific R683W mutation also behaved in a codominant manner when overexpressed, which is consistent with the idea that this mutation may affect primarily the enzymatic activity of the protein rather than impairing protein interactions, which may underlie TTD. A C-terminal mutation uniquely found in TTD (R722W) was overexpressed but not to levels sufficiently high to rigorously test for a codominant phenotype. Overexpression of mutant XPD alleles may provide a simple means of producing NER deficiency in other cell lines.

Tk+/- mouse model for detecting in vivo mutation in an endogenous, autosomal gene

Tk+/- transgenic mice were created using an embryonic stem cell line in which one allele of the endogenous thymidine kinase (Tk) gene was inactivated by targeted homologous recombination. Breeding Tk+/- parents produced viable Tk-/- knockout (KO) mice. Splenic lymphocytes from KO mice were used in reconstruction experiments for determining the conditions necessary for recovering Tk somatic cell mutants from Tk+/- mice. The cloning efficiency of KO lymphocytes was not affected by the toxic thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) or trifluorothymidine (TFT), or by BrdUrd in the presence of lymphocytes from Tk+/- animals; however, it was easier to identify clones resistant to BrdUrd than to TFT when Tk+/- cells were present. Tk+/- mice were treated with vehicle or 100 mg/kg of N-ethyl-N-nitrosourea (ENU), and after 4 months, the frequency of Tk mutant lymphocytes was measured by resistance to BrdUrd. The frequency of Tk mutants was 22+/-5.9x10-6 in control animals and 80+/-31x10-6 in treated mice. In comparison, the frequency of Hprt mutant lymphocytes, as measured by resistance to 6-thioguanine, was 2.0+/-1.2x10-6 in control animals and 84+/-28x10-6 in the ENU-treated mice. Analysis of BrdUrd-resistant lymphocyte clones derived from the ENU-treated animals revealed point mutations in the non-targeted Tk allele. These results indicate that the selection of BrdUrd-resistant lymphocytes from Tk+/- mice may be used for assessing in vivo mutation in an endogenous, autosomal gene.

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.

Construction of a functional cDNA clone of the hamster ERCC2 DNA repair and transcription gene

The complete hamster ERCC2 cDNA was constructed in a plasmid vector from clones of three overlapping reverse transcribed/polymerase chain reaction amplified fragments using unique restriction enzyme recognition sites within the regions of overlap. This complete cDNA insert was then cloned into a mammalian expression vector, pcD2E, and tested for function by the ability to confer UV resistance to the ERCC2 mutant CHO cell line UV5. Site-specific mutagenesis was used to introduce the G347-->A and G1844-->A changes resulting in the Cys116-->Tyr and Gly615-->Glu mutations previously identified in UV5 and UVL-13 (also an ERCC2 mutant CHO cell line), respectively. The 116Tyr and 615Glu plasmids each failed to confer UV resistance to UV5 or UVL-13 cells, respectively, demonstrating that the changes identified are indeed the causative mutations in UV5 and UVL-13.

Salvage pathway of pyrimidine synthesis: divergence of substrate specificity in two related species of teleostean fish

For nucleotide synthesis, cells use purine and pyrimidine nucleosides generated either through de novo synthesis or through utilization of salvage pathways. In the pyrimidine salvage pathway, thymidine is taken up by transport proteins and phosphorylated by the enzyme thymidine kinase to thymidine monophosphate. So far, all vertebrates analyzed are able to use radioactively labeled thymidine for the biosynthesis of nucleotides in brain tissue. However, when standard autoradiographic, immunohistochemical and biochemical procedures were applied for the detection of the incorporation of tritiated thymidine and the thymidine analogue 5-bromo-2'-deoxyuridine into DNA to two species of gymnotiform fish, a divergence in substrate specificity has been revealed. Although brain cells of the two species, Apteronotus leptorhynchus and Eigenmannia sp., can utilize 5-bromo-2'-deoxyuridine for pyrimidine synthesis, only Eigenmannia sp. is able to incorporate tritiated thymidine into DNA during the S phase of the cell cycle. We hypothesize that this inability to use thymidine for nucleotide synthesis is caused either by a defect in the transport system mediating the uptake of thymidine or by a deficiency in the thymidine kinase of A. leptorhynchus.

Characterization of an apparent hotspot for spontaneous mutation in exon 5 of the Chinese hamster APRT gene

We describe an apparent hotspot for spontaneous deletions and base substitution mutations at a TTC trinucleotide direct repeat/MboII restriction site in exon 5 of the Chinese hamster APRT gene, in a region with the potential to form a relatively stable, quasipalindromic, stem-loop structure. The recurrent 3 bp TTC deletions observed at this site, which account for approx. 20% of the characterized spontaneous APRT deletions in hemizygous CHO cell lines, represent the only spontaneous deletion events that have been recovered more than once at this locus. A total of 11 independently derived, spontaneous CHO cell APRT mutants with identical 3 bp TTC deletions at this exon 5 MboII site, plus another five mutants that have single base substitutions at this site have been identified among spontaneous mutant collections in several different laboratories. Intriguingly, each of the frequently deleted or mutated bases at this exon 5 deletion hotspot site would correspond to one of the unpaired bases within a single-stranded 'loop' region of a stable, quasipalindromic, stem-loop structure that can be formed by intrastrand pairing of inverted repeats in this portion of the APRT gene sequence. An identical TTC trinucleotide direct repeat sequence at the same site in exon 5 of the human APRT gene also appears to be a hotspot for spontaneous deletion.

High-frequency illegitimate integration of transfected DNA at preintegrated target sites in a mammalian genome

To examine the mechanisms of recombination governing the illegitimate integration of transfected DNA into a mammalian genome, we developed a cell system that selects for integration events in defined genomic regions. Cell lines with chromosomal copies of the 3' portion of the adenine phosphoribosyltransferase (APRT) gene (targets) were established. The 5' portion of the APRT gene, which has no homology to the integrated 3' portion, was then electroporated into the target cell lines, and selection for APRT gene function was applied. The reconstruction of the APRT gene was detected at frequencies ranging from less than 10(-7) to 10(-6) per electroporated cell. Twenty-seven junction sequences between the integrated 5' APRT and its chromosomal target were analyzed. They were found to be randomly distributed in a 2-kb region immediately in front of the 3' portion of the APRT gene. The junctions fell into two main classes: those with short homologies (microhomologies) and those with inserted DNA of uncertain origin. Three long inserts were shown to preexist elsewhere in the genome. Reconstructed cell lines were analyzed for rearrangements at the target site by Southern blotting; a variety of simple and complex rearrangements were detected. Similar analysis of individual clones of the parental cell lines revealed analogous types of rearrangement, indicating that the target sites are unstable. Given the high frequency of integration events at these sites, we speculate that transfected DNA may preferentially integrate at unstable mammalian loci. The results are discussed in relation to possible mechanisms of DNA integration.

Development of a novel mouse tk+/- embryonic stem cell line for use in mutagenicity studies

A tk+/- mouse embryonic stem (ES) cell line, designated 1G2, has been created in which one allele of the thymidine kinase (tk) gene was inactivated by targeted homologous recombination. This line is an analog of the mouse lymphoma tk+/- L5178Y cell line, which is used widely to assess the mutagenicity of chemical agents. Treatment of 1G2 cells with the alkylating agent N-ethyl-N-nitrosourea (ENU) resulted in a dose-related increase in trifluorothymidine-resistant colonies. Mutant frequencies of 152 and 296 per 10(6) cells were determined for 0.1 and 0.3 mg/ml doses of ENU, compared with a spontaneous mutant frequency of 15 per 10(6) cells. The data indicate that tk+/- 1G2 ES cells may be useful for the creation of a transgenic mouse model for assessing in vivo mutation using an endogenous autosomal gene.

Growth rates of hprt and tk mutant CHO cell lines

The growth rates of 31 X-ray-induced hypoxanthine phosphoribosyl transferase (hprt) deficient mutants of CHO-K1 cells were measured. Mutants had been classified as (1) full-deletion, (2) partial deletion or rearrangement, or (3) unchanged by Southern blot analyses. No relationship between growth rate and deletion type was observed. Even where all hprt-specific bands were missing, proliferation rates in culture were normal. Additionally, in CHO-AT3-2 cells, which are heterozygous at the tk locus, no difference in growth rates between a spontaneous hprt mutant and its parent was observed, although double hprt-tk-/- mutants grew more slowly.

Illegitimate recombination induced by DNA double-strand breaks in a mammalian chromosome

We examined DNA double-strand-break-induced mutations in the endogenous adenine phosphoribosyl-transferase (APRT) gene in cultured Chinese hamster ovary cells after exposure to restriction endonucleases. PvuII, EcoRV, and StuI, all of which produce blunt-end DNA double-strand breaks, were electroporated into CHO-AT3-2 cells hemizygous at the APRT locus. Colonies of viable cells containing mutations at APRT were expanded, and the mutations that occurred during break repair were analyzed at the DNA sequence level. Restriction enzyme-induced mutations consisted of small deletions of 1 to 36 bp, insertions, and combinations of insertions and deletions at the cleavage sites. Most of the small deletions involved overlaps of one to four complementary bases at the recombination junctions. Southern blot analysis revealed more complex mutations, suggesting translocation, inversion, or insertion of larger chromosomal fragments. These results indicate that blunt-end DNA double-strand breaks can induce illegitimate (nonhomologous) recombination in mammalian chromosomes and that they play an important role in mutagenesis.

Illegitimate recombination induced by DNA double-strand breaks in a mammalian chromosome

We examined DNA double-strand-break-induced mutations in the endogenous adenine phosphoribosyl-transferase (APRT) gene in cultured Chinese hamster ovary cells after exposure to restriction endonucleases. PvuII, EcoRV, and StuI, all of which produce blunt-end DNA double-strand breaks, were electroporated into CHO-AT3-2 cells hemizygous at the APRT locus. Colonies of viable cells containing mutations at APRT were expanded, and the mutations that occurred during break repair were analyzed at the DNA sequence level. Restriction enzyme-induced mutations consisted of small deletions of 1 to 36 bp, insertions, and combinations of insertions and deletions at the cleavage sites. Most of the small deletions involved overlaps of one to four complementary bases at the recombination junctions. Southern blot analysis revealed more complex mutations, suggesting translocation, inversion, or insertion of larger chromosomal fragments. These results indicate that blunt-end DNA double-strand breaks can induce illegitimate (nonhomologous) recombination in mammalian chromosomes and that they play an important role in mutagenesis.

Molecular analysis of CXPD mutations in the repair-deficient hamster mutants UV5 and UVL-13

The cDNA sequence of the Chinese hamster xeroderma pigmentosum group D (CXPD) nucleotide excision repair gene was analyzed from three Chinese hamster ovary (CHO) cell lines: repair proficient strain AA8 and repair deficient, UV complementation group 2 strains UV5 and UVL-13. CXPD encodes a presumed ATP-dependent DNA helicase and is single copy in CHO lines due to the hemizygosity of chromosome 9. Comparison of the deduced wild-type AA8 CXPD protein sequence with that of the Chinese hamster V79 lung-derived cell line revealed two amino acid polymorphisms. Position 285 is glutamine in AA8 and arginine in V79, and position 298 is alanine in AA8 and threonine in V79. Comparison with the human XPD, Saccharomyces cerevisiae RAD3, and Schizosaccharomyces pombe rad15 homologs shows variability at these positions. Analysis of the CXPD sequence in the repair deficient CHO lines UV5 and UVL-13 revealed, in each case, a single base substitution resulting in an amino acid substitution. Position 116 is tyrosine in UV5 and cysteine in AA8, and the corresponding positions of XPD, RAD3, and rad15 are cysteine. Position 615 is glutamic acid in UVL-13 and glycine in AA8, and the corresponding positions of XPD, RAD3, and rad15 are glycine. In both UV5 and UVL-13, positions 285 and 298 are glutamine and alanine, respectively, as seen in AA8. These results suggest that cysteine 116 and glycine 615 are critical to the repair function of CXPD.

Heritable alterations at the adenine phosphoribosyltransferase (APRT) locus in human lymphoblastoid cell lines

Human lymphoblastoid cell lines derived from WI-L2 exhibit unexpected frequencies of diaminopurine (DAP) resistant mutants. The background mutant fractions of 10(-7) to 10(-8) in untreated cultures are much lower than the frequencies expected for loss of a heterozygous autosomal locus (10(-5) to 10(-6), yet much higher than expected for a homozygous locus (10(-10) to 10(-12). We used aminopterin, adenine and thymidine (AAT) to select DAP-sensitive (DAPS) revertants from one resistant line. The background frequency of DAPR in these revertant cell lines ranged from 3.5 to 6.5 x 10(-4), approximately the square root of 10(-7). Thus these data suggest that both alleles of aprt are inactivated at similarly high frequencies. They also indicate that the DAPS revertants were heterozygotes (aprt +/-) or hemizygotes (aprt +/0) and that WI-L2 was homozygous (aprt+/+). Mutational dose-response studies with X-rays, ethyl methanesulfonate (EMS), and ICR-191 were conducted in 4 of these revertant cell lines. EMS and ICR-191, which induce mainly point mutations, did not induce an increase in mutant fraction. A dose of 200 cGy X-rays, however, induced a frequency of 10(-3). Treatment of DAPR cells with 5-azacytidine induced a significant increase in reversion to DAPS. Southern blot analysis of the aprt gene after digestion with MspI or HpaII also suggests that differential methylation changes may play a major role in the generation of DAP sensitivity and resistance.

Protein turnover, growth and proliferation in CHO cells. Variation within and between mutant classes for salvage pathway enzymes

We have examined the clonal variation in rates of amino acid transport, protein synthesis, protein degradation, growth and proliferation for CHO cells with mutations in the purine and pyrimidine salvage pathways. First we compared three clonal cell lines, each with a different mutation, with the heterozygous parental line AT3-2. Overall, the correlation between rates of protein turnover, growth and proliferation was excellent. The slower growth and proliferation of one mutant, AB3 (TK-, APRT-), is explained by a low intrinsic rate of protein synthesis coupled with a smaller response in rates of amino acid transport, protein synthesis and protein degradation to insulin, serum and dexamethasone. Secondly, we compared seven aza-adenine-resistant and 14 thioguanine-resistant mutants of AT3-2 and found significant differences in control and insulin-stimulated rates of protein turnover both within and between mutant populations. A significant difference between the populations was unexpected because each individual cell line was cloned from a spontaneous pre-existing mutation in AT3-2, and each population should have the same average rate. Remarkably, all 24 mutants had lower rates of protein synthesis than AT3-2. We cannot explain the data solely in terms of mutations in the salvage pathways. Rather, we propose that the mutant survivors have randomly down-regulated the intrinsically fixed growth factor-regulated pathways of protein turnover, resulting in a broad spectrum of lower metabolic rates.

Characterization of Chinese hamster ovary cells stably transformed by a plasmid with an inducible APRT gene

A plasmid was constructed by fusion of a selectable mammalian gene, hamster adenine phosphoribosyltransferase (APRT), to the Zn(2+)-inducible sheep metallothionein I (MT I) promoter. This plasmid was used to produce stable Chinese hamster ovary (CHO) cell transformants by electroporation to study the effects of induced gene expression on DNA-mediated transformation. The sheep MT Ia promoter was chosen for these experiments because it regulates gene expression differently than murine MT promoters, exhibiting low basal levels of gene expression in uninduced conditions. We have shown that in the absence of Zn2+, there is very low expression of a sheep MT I-APRT fusion gene in stable CHO cells transformants; induction of APRT mRNA and enzyme activity by Zn2+ produced a "threshold" response, from low basal levels to high induced levels, in Zn2+ responsive stable transformant clones. In electroporation experiments, transformation frequencies were unaffected by Zn2+ treatments during a preselection period, but the presence of Zn2+ during selection increased the recovery of stable transformant clones 8- to 10-fold. All stable transformants analyzed displayed Zn(2+)-inducible APRT enzyme activity. Our results indicate that stable mammalian cell transformants with inducible genes under regulation of the sheep MT I promoter should be useful, because of low basal and high induced expression, for studies in which modulation of transcriptional activity is required.

Ultraviolet stimulation of intermolecular homologous recombination in Chinese hamster ovary cells

We previously showed that ultraviolet (UV) irradiation of cotransfected plasmid DNA molecules stimulated genetic transformation that depended on intermolecular homologous recombination in Chinese hamster ovary (CHO) cells. Repair-proficient cells and an excision repair complementation class 1 (ERCC1) UV-sensitive DNA repair-deficient mutant responded similarly to UV stimulation in cotransfections with plasmids containing linker insertion-disrupted copies of the herpes simplex virus thymidine kinase (HSV-TK) gene. In this study, we cotransfected homologous DNA molecules containing nonoverlapping deletions of the hamster adenine phosphoribosyltransferase (APRT) gene into APRT-deficient CHO ERCC1 (UVL-10) and ERCC2 (UVL-1) excision-repair mutants and parental repair-proficient CHO cells. UV damage in cotransfected circular plasmid molecules stimulated transformation in repair-proficient cells and an ERCC1 mutant, but not in an ERCC2 mutant. Linearization of plasmids prior to cotransfection greatly enhanced transformation frequencies in all three cell lines, but UV stimulation using linear recombination substrates was no longer evident. Our results suggest (i) that the ERCC1 gene defect in CHO UVL-10 cells does not affect UV stimulation of homology-dependent extra-chromosomal recombination, and (ii) that a CHO cell ERCC2 excision-repair mutant, although recombination proficient, may exhibit altered recombination in response to UV damage.

Mouse transgenes in human cells detect specific base substitutions

We describe a system of transgenic human cell lines that detects and identifies specific point mutations at defined positions within a gene. The target transgenome is a mouse adenine phosphoribosyltransferase (APRT) gene rendered nonfunctional by introduction of a substitution at either of two bases that comprise a splice acceptor site. Reversion at a mutated site results in the expression of wild-type mouse APRT and consequent growth of APRT+ transgenic cell colonies. Site-specific reversion to wild-type sequence is confirmed by regeneration of a previously destroyed diagnostic Pst I site. Two independent cell clones, each with mutant transgenomes bearing an A----G transition, exhibited an up to 7500-fold, dose-dependent induction of reversion following treatment with ethyl methanesulfonate. Treatment of these clones with 2-aminopurine resulted in no induction of revertants. In contrast, another transgenic cell clone, bearing a G----A transition, reverted as a consequence of 2-aminopurine, but not ethyl methanesulfonate, treatment. These data confirm for human cells the proposed mechanisms of action of these mutagens and provide evidence for the utility of our site-specific reversion method for mutagenesis studies.

A novel selection system for recombinational and mutational events within an intron of a eucaryotic gene

In order to identify a poison sequence that might be useful in studying illegitimate recombination of mammalian cell chromosomes, several DNA segments were tested for their ability to interfere with gene expression when placed in an intron. A tRNA gene and its flanking sequences (267 bp) were shown to inhibit SV40 plaque formation 100-fold, when inserted into the intron in the T-antigen gene. Similarly, when the same DNA segment was placed in the second intron of the adenosine phosphoribosyl transferase (APRT) gene from CHO cells, it inhibited transformation of APRT-CHO cells 500-fold. These two tests indicated that the 267-bp DNA segment contained a poison sequence. The poison sequence did not affect replication since the replication of poisoned SV40 genomes was complemented by viable SV40 genomes and poisoned APRT genes were stably integrated into cell chromosomes. Cleavage of the poison sequence in the SV40 T-antigen intron by restriction enzymes indicated that the tRNA structural sequences and the 5' flanking sequences were not required for inhibition of SV40 plaque formation. Sequence analysis of viable mutant SV40, which arose after transfection of poisoned genomes, localized the poison sequence to a 35 bp segment immediately 3' of the tRNA structural sequences.

Spontaneous CHO APRT heterozygotes reflect high-frequency, allele-specific deletion of the chromosome Z4 APRT gene

In Chinese hamster ovary (CHO) cells, heterozygotes for the adenine phosphoribosyltransferase (APRT) locus arise spontaneously at high frequencies. Paradoxically, such heterozygotes yield APRT mutants only at much lower spontaneous rates, suggesting that the high-frequency event may occur at only one of the two APRT genes. In an attempt to understand the genetic basis for the apparent refractivity of one of the APRT alleles to the high-frequency genetic event and to determine whether differences in the genomic environments of the two CHO APRT alleles specifically render one gene more susceptible to high-frequency spontaneous deletion or inactivation, we have mapped the wild-type APRT allele in 16 independently derived spontaneous APRT heterozygotes. In 15 of these 16 heterozygotes, the functional, wild-type APRT gene was found to reside on the Z7 chromosome, indicating that the high-frequency event is indeed highly specific for the Z4 APRT allele. All but one of these heterozygotes were hemizygous for the APRT locus, suggesting that the high-frequency event generally involves deletion rather than spontaneous inactivation or mutation of the Z4 APRT allele.

Inhibition of transient gene expression in Chinese hamster ovary cells by triplet-sensitized UV-B irradiation of transfected DNA

The biological effectiveness of thymine-thymine cyclobutane dimers specifically induced by photosensitized ultraviolet-B irradiation was analyzed by host-cell reactivation of triplet-sensitized, UV-B irradiated plasmid pRSV beta gal DNA transfected into normal and repair-deficient Chinese hamster ovary cells. For comparison, pRSV beta gal DNA was also UV-C irradiated and transfected into the same cell lines. Ultraviolet endonuclease-sensitive site induction was determined after UV-C irradiation or acetophenone-sensitized UV-B irradiation of plasmid pRSV beta gal DNA. These data were used to calculate the number of cyclobutane pyrimidine dimers required to inactivate expression of the lacZ reporter gene in each irradiation condition. Transfection with UV-C-irradiated plasmid DNA resulted in a significantly greater reduction of reporter gene expression than did transfection with acetophenone-sensitized UV-B-irradiated pRSV beta gal DNA at equivalent induction of enzyme-sensitive sites. Since only a fraction of the inhibition could be accounted for by noncyclobutane dimer photoproducts, these results suggest that cytosine-containing pyrimidine cyclobutane dimers may be more effective than thymine-thymine dimers in inhibiting transient gene expression as measured in such host-cell reactivation experiments in mammalian cells.

Preferential loss or inactivation of chromosome Z4 APRT allele in CHO cells

In CHO cells, heterozygotes for the adenine phosphoribosyltransferase (APRT) locus arise spontaneously at high frequencies. However, such heterozygotes always yield APRT- mutants at low spontaneous rates. In an attempt to determine whether differences in the genomic environments of the two CHO APRT alleles might render one gene more susceptible to high-frequency spontaneous inactivation or deletion, we have mapped the functional APRT allele in four different spontaneous APRT heterozygotes. In each case, the functional APRT gene was found to reside on the Z7 chromosome; it was always the Z4 APRT allele that had been lost or inactivated. Two of these heterozygotes were shown to be physically hemizygous while the other two retained two copies of the APRT gene, indicating that the high-frequency event can involve either spontaneous deletion or inactivation.

Targeted homologous recombination at the endogenous adenine phosphoribosyltransferase locus in Chinese hamster cells

We have developed a system that permits analysis of targeted homologous recombination at an endogenous, chromosomal gene locus in cultured mammalian cells. Using a hemizygous, adenine phosphoribosyltransferase (APRT)-deficient, Chinese hamster ovary (CHO) cell mutant as a transfection recipient, we have demonstrated correction of a nonrevertible deletion mutation by targeted homologous recombination. Transfection with a plasmid carrying a fragment of the APRT gene yielded APRT+ recombinants at a frequency of approximately 4.1 x 10(-7). The ratio of targeted recombination to nontargeted integrations of plasmid sequences was approximately 1:4000. Analysis of 31 independent APRT+ recombinants revealed conversions of the endogenous APRT gene, targeted integration at the APRT locus, and a third class of events in which the plasmid donor APRT fragment was converted to a full-length, functional gene.

High-frequency deletion event at aprt locus of CHO cells: detection and characterization of endpoints

Two mechanisms are implicated in generating recessive drug resistance mutants at the adenine phosphoribosyltransferase (aprt) locus of Chinese hamster ovary (CHO) cells, one of which is a spontaneous high-frequency deletion of the entire gene. We have isolated and mapped a 19-kb fragment carrying aprt and its flanking sequences. A Southern blot study of 198 independent deletion mutants revealed that two different mutants have one of their breakpoints within the 19-kb region analyzed. One of these has an upstream breakpoint which could be narrowed down to a 4-kb fragment containing repetitive sequences. The other mutant has a breakpoint within a 410-bp sequence located 8.5 kb downstream of the aprt gene and which carries several elements similar to those signaling V-(D)-J joining in immunoglobulin and T-cell receptor gene rearrangements. In each case the other breakpoint lay outside of the analyzed region. These results support the previous indications that the deletions created by this spontaneous event are large.

Gene dosage mutants at adenine phosphoribosyltransferase locus induced by colcemid in Chinese hamster V79-AP4 cells

Pseudodiploid Chinese hamster V79-AP4 cells, functionally diploid at the adenine phosphoribosyltransferase (aprt) locus, were treated with colcemid, a well-known aneuploidizing agent, under various experimental conditions. Aneuploid and tetraploid cells and variants resistant to 10 micrograms/ml of 2,6-diaminopurine (DAP), which selects for presumptive aprt+/- heterozygotes in the untreated cells, were induced. Many of the induced variants were hypotetraploid with three (rather than four) chromosomes carrying the aprt gene. Dot-blot and Southern analysis of the DNA of these clones confirmed that they had three copies of the aprt gene. Their APRT specific enzymatic activity was 60-80% of that of wild-type V79-AP4. The results of these and other experiments suggest that in these variants resistance to DAP is due to an altered aprt gene dosage and point to a possible genetic effect of colcemid and other aneuploidizing agents in somatic mammalian cells.

Differential efficiency of mutagenesis at three genetic loci in CHO cells by a benzo[a]pyrene diol epoxide

The formation of DNA adducts by the ultimate carcinogen 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene (BPDE-I) has been implicated in the process of carcinogenesis. In a line of Chinese hamster ovary (CHO) cells designated AT3-2 and in two derivative mutant lines, UVL-1 and UVL-10, originally selected for hypersensitivity to UV-irradiation, we have measured the formation of BPDE-I: DNA adducts and the production of biological damage. The quantity and quality of BPDE-I: DNA adducts formed initially in the 3 cell lines are identical over a wide range of BPDE-I doses. However, the UVL lines are unable to remove adducts from their DNA, while the AT3-2 cells remove about 50% of the BPDE-I: DNA adducts in a 24-h incubation. Correlated with this, the UVL lines are more sensitive to the lethal effects of BPDE-I than are the AT3-2 cells. Mutant frequencies were measured at the aprt, hprt and oua loci and were found to increase linearly with BPDE-I: DNA adduct formation at doses which gave greater than 50% survival. At the hprt and oua loci, the efficiency of mutation induction was similar for AT3-2 and UVL-10 cells. UVL-1 cells showed slightly higher (within a factor of 2-3) mutant frequencies in response to BPDE-I compared to AT3-2 at these two loci. However, at the aprt locus the repair-deficient cells were much more highly mutable (9-15-fold) than the repair-proficient AT3-2 cells. Based on the measured average level of adduct formation, it is calculated that 15% of the BPDE-I: DNA adducts in the aprt gene are converted into mutations. However, the possibility exists that the aprt locus is subject to higher levels of modification by BPDE-I than is the bulk DNA, which would lead to an artifactually high apparent conversion frequency.

A comparative analysis of data on the clastogenicity of 951 chemical substances tested in mammalian cell cultures

A literature review was conducted using original papers published during 1964-1985 on the in vitro clastogenicity of chemical substances. Results of tests on 951 chemical substances were abstracted from over 240 reports to form the database. The evaluation of these data relied on each author's original conclusion on a positive or negative outcome. Of these 951 substances, 447 (47%) were consistently positive either with or without activation; 417 (44%) were negative in the direct test but not tested with metabolic activation systems; 4 were negative but tested only with activation; and 30 (3%) were clearly negative both with and without activation. The remaining 53 substances gave variable results when tested under different experimental protocols or in different cell types, but were positive in at least one test. Although discrepant results were found associated with some cell types, the addition of metabolic activation systems tended to eliminate such variability. No one cell appeared to be superior in response to all clastogens. For screening purposes, the choice of cell may thus depend more on the general usefulness and reliability of a cell type than on a strong response to a particular chemical. However, the use of a suitable metabolic activation system does appear to be of critical importance. The concentration at which clastogenic effects were detected varied extensively for different test substances, ranging from a minimum of 4.3 X 10(-8) to 6.9 X 10(2) mM. Possible mechanisms of action for substances active at only high levels are discussed, but no satisfactory explanation is available at this time. The relevance of tests conducted at concentrations high enough to alter significantly the osmolarity and other culture conditions is considered, and caution urged in the interpretation of test results obtained under physiologically stressful conditions. The clastogenic potential was compared quantitatively using an index of effective concentration (D20) and one which estimates the number of cells with exchange aberrations expected per mg/ml (TR) for data obtained by using a uniform protocol and cultures of Chinese hamster lung (CHL) cells. Both values were distributed over a wide range, demonstrating the variety of genotoxic potential in chemicals. In general, a substance which was active at only high concentrations produced fewer exchange-type aberrations. In vivo activity, as measured by tumourigenic effect and formation of micronuclei in bone marrow, tended to be greater for substances with a D20 below 10(-2) mg/ml and a TR value over 10(3).(ABSTRACT TRUNCATED AT 400 WORDS)

Transformation of UV-hypersensitive Chinese hamster ovary cell mutants with UV-irradiated plasmids

Transfection of UV-hypersensitive, DNA repair-deficient Chinese hamster ovary (CHO) cell lines and parental, repair-proficient CHO cells with UV-irradiated pHaprt-1 or pSV2gpt plasmids resulted in different responses by recipient cell lines to UV damage in transfected DNA. Unlike results that have been reported for human cells, UV irradiation of transfecting DNA did not stimulate the genetic transformation of CHO recipient cells. In repair-deficient CHO cells, proportionally fewer transformants were produced with increasing UV damage than in repair-proficient cells in transfections with the UV-irradiated hamster adenine phosphoribosyltransferase (APRT) gene contained in plasmid pHaprt-1. However, transfection of CHO cells with UV-irradiated pSV2gpt resulted in neither decline in transformation frequencies in repair-deficient cell lines relative to repair-proficient cells nor stimulation of genetic transformation by UV damage in the plasmid. Blot hybridization analysis of DNA samples isolated from transformed cells showed no dramatic changes in copy number or arrangement of transfected plasmid DNA with increasing UV dose. We conclude that the responses of recipient cells to UV-damaged transfecting plasmids depend both on the type of recipient cell and the characteristics of the genetic sequence used for transfection.

Repair of (6-4)photoproducts correlates with split-dose recovery in UV-irradiated normal and hypersensitive rodent cells

Chinese hamster ovary cells and two UV-hypersensitive derivatives were used to determine the importance of DNA excision repair for split-dose recovery. In the wild-type cells 75% of the maximum theoretical recovery was observed when the fractions were delivered at 2-h intervals. Very little recovery was evident in the two hypersensitive cell lines. Using radioimmunoassays specific for (6-4)photoproducts and cyclobutane dimers, the ability of UV-irradiated repair-deficient cells representing 5 complementation groups to repair these 2 photoproducts was determined. Removal of antibody-binding sites specific for (6-4)photoproducts was 80% complete in 6 h and was defective in the UV-sensitive cells. In contrast, only 20-60% of antibody-binding sites specific for cyclobutane dimers were removed 18 h post-irradiation, and the extent of removal was the same in normal and defective cell lines. We conclude that repair of (6-4)photoproducts accounts for split-dose recovery. In addition, we conclude that a consequence of DNA repair in CHO cells is modification rather than removal of cyclobutane dimers.

Internal standards for survival: increasing the accuracy for human cell mutation assays

In mutation and cell transformation assays, it has long been recognized that the common practice of using different numbers of cells on dishes with or without selective conditions creates a source of bias in mutant fraction determination. This is simply because colony formation may be enhanced or suppressed at higher initial cell densities, depending on the assay and agent tested. We propose a solution that consists of the inclusion of an experimentally distinguishable population of cells as an internal standard for colony-forming ability at the high cell density required for detection of rare variants. This method is found to be highly satisfactory for use in measuring mutation to 6-thioguanine resistance in a diploid human B lymphoblast line. For treatment with anti-2,3-dihydroxy-1,10b-epoxy-1,2,3-trihydrofluoranthene (FDE), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and 4-nitroquinoline-oxide (4NQO), the calculated induced mutant fractions using the internal-standard method were significantly lower than those calculated using the conventional low-density-plating efficiency method. The results of these experiments and our analysis lead us to conclude that this approach is applicable to all single cell mutation or transformation assays and is a necessary feature of assays in which an accurate knowledge of the fraction of rare variants is required.

Stabilization of a reactive, electrophilic carcinogen, benzo[a]pyrene diol epoxide, by mammalian cells

We have studied several features of the interactions of 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-I) with a DNA repair-proficient derivative of Chinese hamster ovary cells (CHO), AT3-2, and with a UV-light sensitive mutant, UVL-10, derived from AT3-2. Methods were developed for quantitating the amount of unhydrolysed BPDE-I associated with cells and for purifying DNA from cells under conditions where artificial labeling during preparation is minimized. In both cell types, about 30% of the BPDE-I added to a cell culture is rapidly taken up by the cells and is maintained in a cellular compartment in which the half-life of BPDE-I is about 10-fold longer than in aqueous medium. The kinetics of covalent binding to DNA were measured in both cell types and found to be described well by a single exponential process with a half-life of about 60 min. This is virtually identical to the half-life for intracellular hydrolysis of BPDE-I (57 min), consistent with the suggestion that this intracellular, relatively stable BPDE-I is responsible for binding.

Functional hemizygosity for the MDH2 locus in Chinese hamster ovary cells

Isolation of electrophoretic mobility shift mutants for a large number of enzyme loci in CHO cells has allowed the identification of many genes which are functionally hemizygous. To gain further insight into the nature of hemizygosity in CHO cells and the mechanisms by which it has arisen, we are attempting to determine whether hemizygous gene loci are clustered in a few localized chromosomal regions in CHO or are more generally distributed throughout the genome. Isozyme analysis of a series of CHO electrophoretic mobility shift mutants for MDH2 (malate dehydrogenase 2, EC 1.1.1.37) revealed that this locus is functionally hemizygous in CHO cells, but the locus could not be mapped by conventional approaches because of the similar electrophoretic mobilities of Chinese hamster and mouse MDH2 isozymes. Construction of intraspecific CHO X CHO hybrids using electrophoretic mobility shift mutants with secondary, selectable drug-resistance markers allowed us to determine that MDH2 is not closely linked to any previously mapped hemizygous marker loci in CHO, but is linked to alleles for two dizygous gene loci, PGM3 and APRT, on CHO chromosome Z7. A possible genetic basis for hemizygosity of the MDH2 locus in CHO cells is discussed.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

Cloning of a functional human adenine phosphoribosyltransferase (APRT) gene: identification of a restriction fragment length polymorphism and preliminary analysis of DNAs from APRT-deficient families and cell mutants

A complete human APRT gene has been isolated from a lambda phage genomic library using cloned mouse APRT DNA as a probe. The human gene, contained in a recombinant lambda phage designated lambda Huap15, is functional by virtue of its capacity to transfer human APRT activity to Aprt- mouse recipient cells after phage-mediated transfection. Digestion of lambda Huap15 DNA with BamH1 generated a 2.2-kb fragment that is the only fragment of eight produced to hybridize with the mouse APRT gene. This 2.2-kb BamH1 fragment is a unique, single copy sequence, and has been used to identify a restriction fragment length polymorphism (RFLP) associated with the APRT locus. Taq1 digestion and Southern blot analysis of DNAs from 49 unrelated individuals produced three different patterns. DNAs of 30 individuals produced a restriction pattern of three labeled fragments about 500 bp, 600 bp, and 2.1 kb in size, which is characteristic for individuals homozygous for the more common allele. Two individuals homozygous for the less frequent allele displayed labeled fragments of 500 bp and 2.7 kb. The remaining 17 DNA samples produced all four labeled bands as expected for heterozygous individuals. The frequency of heterozygotes in the population is about 35%, while the frequency of the less common allele is about 0.21. Restriction enzyme analysis of DNAs from two APRT-deficient brothers and from an unrelated heterozygote revealed no gross deletions or rearrangements, nor the Taq1 polymorphism.

Chromosomal rearrangements and gene expression in CHO cells: mapping of alleles for eight enzyme loci on CHO chromosomes Z3, Z4, Z5, and Z7

Analysis of CHO electrophoretic mobility shift mutants for six enzyme loci ( LDHA , GAA, IDH2 , ME1, PGM3, and MPI) that have been previously mapped to Chinese hamsters chromosomes 3 and 4 indicated that each of these loci, with the exception of IDH2 , are functionally dizygous in CHO. Segregation analysis of CHO X mouse somatic cell hybrids allowed regional gene mapping assignments for a total of eight Chinese hamster chromosome 3- or 4-derived marker loci (the above six, plus APRT and PKM2) to CHO chromosomes Z3 , Z4 , Z5 , and Z7 . For seven of these enzyme loci (all but IDH2 ), two alleles are expressed in CHO cells, each segregating with a different Z-group chromosome. These gene mapping assignments confirm genetically that CHO chromosomes Z3 , Z4 , Z5 , and Z7 are, in fact, derived from Chinese hamster chromosomes 3 and 4, and provide insight into the effects of chromosomal rearrangements on gene expression and hemizygosity in CHO cells.

Genetic effects of chromosomal rearrangements in Chinese hamster ovary cells: expression and chromosomal assignment of TK, GALK, ACP1, ADA, and ITPA loci

Polyethylene glycol-mediated fusion of Chinese hamster ovary (CHO) cells with mouse Cl1D cells produced interspecific somatic cell hybrids which slowly segregated CHO chromosomes. Cytogenetic and isozyme analysis of HAT- and bromodeoxyuridine-selected hybrid subclones and of members of a hybrid clone panel retaining different combinations of CHO chromosomes enabled provisional assignments of the following enzyme loci to CHO chromosomes: TK, GALK, and ACP1 to chromosome 7; TK and GALK to chromosome Z13; ACP1, ADA, and ITPA to chromosome Z8; and ADA and ITPA to chromosome Z9. These genetic markers reflect the origin of each of these Z group chromosomes and indicate the functional activity of alleles located on rearranged chromosomes. Identification of diploid electrophoretic shift mutations for ADA and ITPA was consistent with those observations. Assignment of the functional TK locus in TK+/- CHO-AT3-2 cells indicated that gene deletion may be responsible for TK hemizygosity in this subline.

High frequency "switching" at the adenine phosphoribosyltransferase locus in multipotent mouse teratocarcinoma stem cells

Clones of multipotent mouse tetratocarcinoma stem cells, presumptively heterozygous at the adenine phosphoribosyltransferase (APRT) locus (EC 2.4.2.7), were selected for partial resistance to the purine analog 2',6'-diaminopurine (DAP). All had approximately 50% APRT activity as compared to the parental line and were found to segregate homozygous deficient cells at a high frequency (approximately 10(-2]. Homozygous deficient cells were isolated from one of the heterozygotes and were found to fall into a single class characterized by residual activity and the segregation of revertants at an equally high frequency. The revertants in turn gave rise to full mutants at comparably high frequencies. Chromosomal changes detectable with the light microscope were not associated with these transitions. Physical characterization of the APRT enzymes derived from mutant, revertant, and wild-type cells did not reveal any differences. We conclude that the reversible "switching" between heterozygosity and homozygosity is attributable to some form of gene inactivation and reactivation rather than to classical mutational events.

Expression of recessive Aprt- mutations in mouse CAK cells resulting from chromosome loss and duplication

Karyotypes of recessive mutants at the autosomal adenine phosphoribosyltransferase (Aprt) locus in a clone of the near-diploid mouse CAK cell line have been analyzed. The Aprt located on chromosome 8. One copy of chromosome 8 was morphologically abnormal in the parental clone (CAK-B3-Toyr13) from which Aprt- mutants were isolated. Among 22 mutants, there were ten in which one copy of chromosome 8 had been lost. Four of these were monosomic, and in the others duplication of the remaining homolog had occurred. These findings indicate that newly induced recessive mutations in cultured mammalian cells can be expressed as the result of loss of one chromosome carrying a wild-type allele with or without duplication of the homolog carrying the mutant allele. Loss and duplication would not be detected in cell lines lacking morphologically marked chromosomes.

Genetic effects of chromosomal rearrangements in Chinese hamster ovary cells: expression and chromosomal assignment of TK, GALK, ACP1, ADA, and ITPA loci

Polyethylene glycol-mediated fusion of Chinese hamster ovary (CHO) cells with mouse Cl1D cells produced interspecific somatic cell hybrids which slowly segregated CHO chromosomes. Cytogenetic and isozyme analysis of HAT- and bromodeoxyuridine-selected hybrid subclones and of members of a hybrid clone panel retaining different combinations of CHO chromosomes enabled provisional assignments of the following enzyme loci to CHO chromosomes: TK, GALK, and ACP1 to chromosome 7; TK and GALK to chromosome Z13; ACP1, ADA, and ITPA to chromosome Z8; and ADA and ITPA to chromosome Z9. These genetic markers reflect the origin of each of these Z group chromosomes and indicate the functional activity of alleles located on rearranged chromosomes. Identification of diploid electrophoretic shift mutations for ADA and ITPA was consistent with those observations. Assignment of the functional TK locus in TK+/- CHO-AT3-2 cells indicated that gene deletion may be responsible for TK hemizygosity in this subline.

Mutation at autosomal loci of Chinese hamster ovary cells: involvement of a high-frequency event silencing two linked alleles

Two classes of cell lines heterozygous at the galactokinase (glk) locus have been isolated from Chinese hamster ovary cells. Class I, selected by plating nonmutagenized wild-type cells at low density in medium containing 2-deoxygalactose at a partially selective concentration, underwent subsequent mutation to the glk-/- genotype at a low frequency (approximately 10(-6) per cell), which was increased by mutagenesis. Class II heterozygotes, isolated by sib selection from mutagenized wild-type cells, had a higher spontaneous frequency of mutation to the homozygous state (approximately 10(-4) per cell), which was not affected by mutagenesis. About half of the glk-/- mutants derived from a class II heterozygote, but not the heterozygote itself, were functionally hemizygous at the syntenic thymidine kinase (tk) locus. Similarly, a tk+/- heterozygote with characteristics analogous to the class II glk+/- cell lines underwent high-frequency mutation to tk-/-, and most of these mutants, but not the tk+/- heterozygote, were functionally hemizygous at the glk locus. A model is proposed, similar to that for the mutational events at the adenine phosphoribosyl transferase locus (W. E. C. Bradley and D. Letovanec, Somatic Cell Genet. 8:51-66, 1982), of two different events, high and low frequency, being responsible for mutation at either of the linked loci tk and glk. The low-frequency event may be a point mutation, but the high-frequency event, in many instances, involves coordinated inactivation of a portion of a chromosome carrying the two linked alleles. Class II heterozygotes would be generated as a result of a low-frequency event at one allele, and class I heterozygotes would be generated by a high-frequency event. Supporting this model was the demonstration that all class I glk+/- lines examined were functionally hemizygous at tk.

The use of an immunological probe to measure the kinetics of DNA repair in normal and UV-sensitive mammalian cell lines

Chinese hamster ovary cells and human fibroblasts were used to study UV-light-induced repair replication and removal of antibody-binding sites. Whereas repair replication still continued 8 h post irradiation, removal of antibody-binding sites was 80% complete within 2 h and reached a plateau by 4 h. This was found to be independent of the method of DNA isolation. UV-hypersensitive CHO cells exhibited reduced levels of repair synthesis that closely correlated with the extent of removal of antibody-binding sites. XP group A, C and D cells, each of which had less than 15% of the level of repair synthesis found in the control fibroblasts, removed less than 30% of the antibody-binding sites. Group E cells demonstrated intermediate levels of DNA-repair capacity in both assays.

High-frequency structural gene deletion as the basis for functional hemizygosity of the adenine phosphoribosyltransferase locus in Chinese hamster ovary cells

The CHO-AT3-2 Chinese hamster ovary cell line is functionally hemizygous for the adenine phosphoribosyltransferase (APRT; EC 2.4.2.7) locus. Class 1 APRT +/- heterozygotes, such as CHO-AT3-2, can be isolated at high spontaneous frequencies from wild-type CHO cell populations. Simon et al. [Simon, A. E., Taylor, M. W., Bradley, W. E. C. & Thompson, L. (1982) Mol. Cell. Biol. 2, 1126-1133] have proposed that a high-frequency event that inactivates one APRT allele might be responsible for both the spontaneous generation of class 1 APRT +/- heterozygotes and the high-frequency occurrence of APRT- mutants in class 2 APRT +/- heterozygote populations. This event appears to occur at only one of the two APRT alleles. To investigate the nature of this high-frequency event, and to determine the genetic basis for functional hemizygosity of the APRT locus in CHO-AT3-2 cells, we have mapped the APRT locus by using CHO-AT3-2-mouse somatic cell hybrids. Our data confirm that CHO-AT3-2 cells have a single functional APRT allele, which is located on the Z7 chromosome. Karyotypic analysis of CHO-AT3-2 revealed an interstitial deletion on the long arm of the Z4 chromosome, in the very region where the other APRT allele should be located. To determine whether the Z4q interstitial deletion had resulted in physical loss of the APRT gene, DNA from CHO-AT3-2-mouse cell hybrids that had either lost or retained the Z4q- chromosome was analyzed for the presence of CHO APRT coding sequences. Our data suggest that allele-specific high-frequency structural gene deletion events involving the long arm of chromosome Z4 are responsible for the spontaneous generation of functional hemizygosity at the APRT locus in CHO cells.