Mutagen treatment of single Chinese hamster ovary cells produces colonies mosaic for glucose-6-phosphate dehydrogenase activity

Mutation in G6PD gene leads to loss of cellular control of protein glutathionylation: mechanism and implication

More than 400 million people are susceptible to oxidative stress due to glucose-6-phosphate dehydrogenase (G6PD) deficiency. Protein glutathionylation is believed to be responsible for loss of protein function and/or cellular signaling during oxidative stress. To elucidate the implications of G6PD deficiency specifically in cellular control of protein glutathionylation, we used hydroxyethyldisulfide (HEDS), an oxidant which undergoes disulfide exchange with existing thiols. G6PD deficient (E89) cells treated with HEDS showed a significant increase in protein glutathionylation compared to wild-type (K1) cells. In order to determine whether increase in global protein glutathionylation by HEDS leads to loss of function of an important protein, we compared the effect of HEDS on global protein glutathionylation with that of Ku protein function, a multifunctional DNA repair protein, using a novel ELISA. E89 cells treated with HEDS showed a significant loss of Ku protein binding to DNA. Cellular protein thiol and GSH, whose disulfide is involved in protein glutathionylation, were decreased by HEDS in E89 cells with no significant effect in K1 cells. E89 cells showed lower detoxification of HEDS, that is, conversion of disulfide HEDS to free sulfhydryl mercaptoethanol (ME), compared to K1 cells. K1 cells maintained their NADH level in the presence of HEDS but that of E89 cells decreased by tenfold following a similar exposure. NADPH, a cofactor required to maintain reduced form of the thiols, was decreased more in E89 than K1 cells. The specific role of G6PD in the control of such global protein glutathionylation and Ku function was further demonstrated by reintroducing the G6PD gene into E89 (A1A) cells, which showed a normal phenotype.

Varying the nucleic acid composition of siRNA molecules dramatically varies the duration and degree of gene silencing

The utility of short interfering RNA (siRNA) as a means of gene silencing depends on several factors. These include the degree to which a gene can be silenced, the length of time for which the gene remains silenced, the degree of recovery of gene function, and the effects of the silencing process on general cell functions. We hypothesized that changing the nucleic acid composition of the siRNA constructs used for silencing would affect these parameters. With siRNA gene silencing of the glucose-6-phosphate dehydrogenase gene as a baseline, we found that siDNA molecules have an effect that is similar in duration but lesser in degree, whereas hybrid DNA:RNA molecules have an effect that is enormously greater in both duration and degree.

Time versus replication dependence of EMS-induced delayed mutation in Chinese hamster cells

We have previously observed in Chinese hamster cells that ethyl methane sulfonate (EMS) induces mutations which are distributed over at least 10-14 cell divisions following treatment. This delayed appearance of mutations could be explained by EMS-induced lesions which remain in DNA and have a probability that is significantly less than 1.0 of producing base mispairing errors during successive replication cycles (replication-dependent). Alternatively, delayed mutation may be a time-dependent process in which a slow acting or damage inducible error-prone repair process removes persistent DNA lesions and replaces them with an incorrect base during the course of 7-10 days of colony growth following EMS exposure. To address this question, the distribution of HGPRT delayed mutation events (fifth division or later) in cells plated immediately for exponential growth after EMS treatment was compared with the distribution in cells which remained under confluent growth conditions for 8 days and then were replated. Both the distribution and rate of accumulation of delayed mutations (mutations/cell division) were similar in the two culture conditions. In contrast, the frequency of early mutations (before the fifth division) in the confluent population was reduced more than 2-fold compared to dividing cells. A comparison of the frequency of EMS-induced DNA lesions in the two populations revealed that the density inhibited population contained one third the DNA lesions of the exponential population. These results argue against a time-dependent process since, if this mechanism applies, one would expect an increase in early mutant events and a decrease in delayed events in the confluent population. The results, however, are consistent with a replication model in which potential early mutant lesions are preferentially removed in the density inhibited culture during the 8 days of incubation while lesions producing late mutants are not removed.

EMS and UV-light-induced colony sectoring and delayed mutation in Chinese hamster cells

PURPOSE

To review studies of mutagen-induced colony sectoring which demonstrate that UV light and EMS produce delayed mutational events in Chinese hamster ovary cells.

METHODS AND RESULTS

Since the late 1940s, it has been known that the treatment of a single bacterial or yeast cell with mutagenic agents produces complete mutant colonies (pures) and colonies composed of both mutant and non-mutant cell types (mosaics) with various sectored patterns. A similar sectoring phenomenon has been observed in Chinese hamster ovary cells (CHO) using the DNA alkylating agent ethyl methane sulphonate (EMS) or ultraviolet light. However, unlike bacteria and yeast, a significant fraction of CHO mutant colonies contained sectors of less than 1/2; i.e. 1/4, 1/8 and 1/16 sectors, suggesting a delayed production of mutations. Using various colony-replating approaches, it was found that these mutagenic agents produced the ratio of mutant to wild-type cells expected for a delayed mutational process which produces mutant events for at least 12-14 cell divisions following treatment. This delayed mutation phenomenon was observed at both the glucose-6-phosphate dehydrogenase (G6PD) and hypoxanthine guanine phosphoribosyltransferase (HGPRT) loci. Various mutational mechanisms for the production of delayed mutations are discussed.

CONCLUSIONS

These studies suggest that mutagens such as UV light and EMS induce long-term alterations in mammalian cells that act to increase the 'apparent' spontaneous mutation frequency. This delayed mutational decrease in stability of the genome may explain the accumulation over time of the multiple genetic changes observed in malignant tumours.

Sensitivity to chemical oxidants and radiation in CHO cell lines deficient in oxidative pentose cycle activity

In this paper we examine the susceptibility of a series of G6PD- CHO cell lines to a variety of chemical oxidants. Addition of these drugs to K1D, the parental cell line, results in as much as a 20-fold increase in pentose cycle (PC) activity over control values. In two of our mutant lines, E16 and E48, little or no stimulation of PC activity is seen. These lines are shown to be much more susceptible to the toxic effects of the chemical oxidants t-butyl hydroperoxide and diamide. PC activity is also stimulated by ionizing radiation in K1D cells. One of the G6PD- cell lines has an increased aerobic radiation response compared to the parental line. However, since this is not the case with the other G6PD- cell lines, it is unclear whether this represents a difference in the absolute value of PC activity or some additional variable that may be influencing the results.

Increased sensitivity to killing by restriction enzymes in the XR-1 DNA double-strand break repair-deficient mutant

Repair or misrepair of DNA double-strand breaks (DSBs) is critical in determining cellular survival after gamma-irradiation. In this report, we focus on the cellular and biochemical consequences of restriction enzyme induced DSBs in wild-type Chinese hamster ovary (CHO) cells and the DNA DSB repair-defective mutant XR-1. We find that XR-1 possesses reduced cellular survival after the introduction of restriction enzymes that produce either cohesive or blunt ends. XR-1's sensitivity to killing by restriction enzymes strongly mimics its response to gamma-rays. Using pulsed field electrophoresis, we find that for each enzyme, similar numbers of DNA DSBs are being introduced in both cell lines. The simplest explanation for the increased sensitivity to restriction enzymes in the mutant is that the biochemical defect in XR-1 is not confined to the repair of ionizing radiation induced ends, but extends to DSBs that possess ligatable 3'-hydroxyl and 5'-phosphate ends as well.

Intragenic interspecific complementation of glucose 6-phosphate dehydrogenase in human-hamster cell hybrids

A new variant of human glucose 6-phosphate dehydrogenase (G6PD), provisionally designated G6PD Harilaou, was observed in a Greek boy affected by severe hemolytic anemia. G6PD Harilaou was associated with very severe deficiency of enzyme activity, which measured about 1% of normal in the patient's fibroblasts. By fusion of Harilaou fibroblasts with a similarly enzyme-deficient mutant CHO cell line, we have isolated a hybrid cell line that has a G6PD activity 5-10 times higher than either of the parental cells. By electrophoretic analysis we show that most of this activity is associated with a hybrid dimeric G6PD molecule consisting of one hamster and one human subunit. We suggest that this heterologous quasi-interallelic complementation is effected by a catalytically abnormal hamster subunit stabilizing a catalytically abnormal and unstable Harilaou subunit. This approach may be useful for the study of dimer formation and stability in human G6PD.

Genetic analysis of XR-1 mutation in hamster and human hybrids

We investigated the dominant/recessive nature of the XR-1 mutant locus in intraspecies Chinese hamster ovary (CHO) hybrids and interspecies hybrids with human cell lines that possess different radioresistances. The XR-1 cell is abnormally sensitive to killing by gamma rays in the G1 phase of the cell cycle, while late-S-phase cells have wild-type resistance. [3H]Thymidine selection was used to eliminate the resistant S-phase population. In both intraspecies and interspecies hybrids, the XR-1 mutation is recessive to the wild-type cell and is not influenced by differences in chromosome ploidy. Analysis of hybrids between human ataxia telangiectasia fibroblasts AT5BI and XR-1 cells revealed that they possess different genetic defects as they complemented each other in three of four hybrids tested. These data suggest that the XR-1 locus is evolutionarily conserved between hamster and human cells.

Hypoxic toxicity of misonidazole in a glucose-6-phosphate dehydrogenase deficient mutant Chinese hamster ovary cell line

The metabolic activation of misonidazole (MISO) and its effects on the hexose monophosphate pathway (HMP) and on cell viability were studied in hypoxic mutant Chinese hamster ovary (CHO) cells deficient in glucose-6-phosphate dehydrogenase and their parent wildtype cells. The metabolic activation of MISO was similar in both cell lines as indicated by the binding of 14C-MISO to the acid-insoluble fraction of these cells; it was decreased by the absence of glucose. In the wildtype CHO cells, MISO caused a significant stimulation of the activity of the HMP while in the mutant CHO cells no HMP activity was measurable, even in the presence of MISO. In both cell lines clonogenicity began to decline after 2 hr and trypan blue exclusion after 4 hr of hypoxic incubation. The effect of MISO on both parameters of cell viability was somewhat more pronounced in the wildtype CHO cells. This difference became especially significant at the longer incubation times. The results indicate that reducing equivalents for the metabolic activation of MISO are provided not only by the HMP but that pathways other than the HMP, such as glycolysis or pathways starting from mitochondrial tricarboxylates, are of similar or even greater importance in this respect.

Lethal sectoring is not the basis for EMS-induced pure mutant clones in Chinese hamster cells

Treatment of G1-synchronized mammalian cells with mutagenic agents which act on one strand of the DNA at a given site would be expected to produce colonies containing both mutant and wild-type cells (mosaic). We have observed that in addition to mosaic colonies, G1-synchronized Chinese hamster ovary cells which had been treated with the single-strand mutagen ethyl methanesulfonate (EMS), produced colonies in which all the cells lacked glucose-6-phosphate dehydrogenase activity. These completely mutant (pure) colonies could be derived from a potentially mosaic colony by the "death" of the wild-type cell after the first cell division, leaving only the glucose-6-phosphate dehydrogenase (G6PD)-deficient cell to grow into a colony (lethal sectoring). Using time-lapse photography to analyze cell lineages after EMS treatment, we find that cell lysis, cell release, cell migration, or proliferative failure of one lineage at the 2-cell stage accounts for only 20-25% of the pure mutant colonies observed. This result suggests that in the Chinese hamster cell there exists a mutational mechanism which fixes the mutation in both strands of the DNA before the next replication cycle following EMS treatment.

Delayed mutation in Chinese hamster cells

The possibility was examined that mutational events can be delayed for more than one or two cell divisions following treatment of Chinese hamster cells with the DNA alkylating agent ethyl methane sulfonate. If mutations in mammalian cells are delayed, the proportion of mutant cells in colonies grown from single mutagen-treated cells will reflect the cell division at which the mutation is genetically fixed, i.e., a first division mutation yields a 1/2 mutant colony, a fifth division mutation produces a 1/32 mutant colony, etc. In the present study, replating of cells from single colonies grown for six to seven days after mutagen treatment resulted in the discrete ratios of glucose-6-phosphate dehydrogenase (G6PD)-deficient mutant to wild-type colonies expected for a delayed mutational process which produces mutations over at least 8-10 cell generations. Further, when cells from 7- to 10-day colonies, grown from ethyl methane sulfonate (EMS)-treated cells were replated into selective medium containing 6-thioguanine (6TG), the number of 6TG-resistant colonies obtained per flask was distributed over a very wide range, consistent with a mutational delay process. These results could not be explained by differences in the number of cells per colony or plating efficiency in selective medium. Assuming that the relative number of 6TG-resistant colonies per flask reflects the time of mutation, EMS treatment produced two groups of mutational events: one which occurred within the first five cell generations and another uniformly distributed over at least the next eight to nine divisions. These results support the conclusion that EMS induces mutants for at least 10-14 cell generations after treatment and raise the possibility that current methods to assess the mutagenic potential of an agent might lead to significant underestimation. The role of delayed mutation in the phenomenon of "mutation expression time" is also discussed.

Timing of mutation-fixation events in ethyl methane sulfonate-treated Chinese hamster cells

Exposure of single Chinese hamster ovary (CHO) cells to the mutagen, ethyl methane sulfonate, produces two types of mutant colonies lacking glucose-6-phosphate dehydrogenase activity: colonies uniformly deficient in enzyme activity, and mosaic colonies containing both mutant and nonmutant cell phenotypes in various relative proportions and sectored patterns (1/8, 1/4, 1/2). We find that the relative size of the mutant sector in these mosaic colonies primarily reflects the cell division at which the mutation was genetically fixed. Thus, the mutation-fixation event occurs before the first cell division in 1/2 sector and pure mutant colonies, between the first and second divisions for 1/4 sectors, and between the second and third divisions for 1/8 sectors. Delay in the formation of mutations could also explain the phenomenon of "mutation expression time" which is observed when drug resistance is used to select for mutants. Colony sectoring offers for the first time in mammalian cells the opportunity to observe an agent's effect on the timing of the mutational process.