Hypermutability of a UV-sensitive aphidicolin-resistant mutant of Chinese hamster fibroblasts

An ultraviolet light (UV)-sensitive thymidine auxotroph of Chinese hamster V79 cells that exhibits pleiotropic effects such as a high level of deoxycytidine triphosphate, slow growth, sensitivity to cytidine, and high frequencies of site-specific bromodeoxyuridine-dependent chromosomal aberrations was selected by its resistance to aphidicolin. The UV-induced mutability of this mutant and one of its revertants, which retains some of the phenotypes listed above, was studied in 3 mutation assay systems. The results showed that the mutant was hypermutable for ouabain and diphtheria-toxin-resistant mutations compared to wild-type V79 cells at the same UV dose or the same survival level. The mutant exhibits a delayed expression of maximal frequency of induced 6-thioguanine-resistant mutants. When maximal frequencies are compared at the same UV dose, the mutant also has higher mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase locus. The revertant was similar to the wild-type in UV sensitivity and mutability. Our results showed that UV sensitivity and hypermutability are correlated and controlled by a single gene. Thymidine auxotrophy or nucleotide pool imbalance apparently did not contribute to the UV sensitivity and mutability of the mutant.     

Mutation-Selection Balance under Genomic Imprinting at an Autosomal Locus

I model the effect of genomic imprinting on the equilibrium allele frequencies at an autosomal diallelic locus subject to viability selection and mutation. The population size is assumed to be very large; male and female mutation rates may be unequal. Different models examine cases of the inactivation of one gene (with both complete and partial penetrance) and of differential expression of genes according to the parent of origin. In the simplest cases the frequency of the deleterious allele is approximately twice that of a dominant nonimprinting mutant, but considerably less than that of a recessive nonimprinting mutant. Under imprinting, selection and unequal mutation rates interact: other things being equal, male-biased mutation leads to lower mutant frequencies under maternal imprinting and higher frequencies under paternal imprinting. I also model cases where just one allele is imprintable (and the other not). These models allow us to predict the frequency of a failure to imprint in a normally imprinting system, as well as the frequency of imprinting at a standard nonimprinting locus.     

Somatic mutations and the hierarchy of hematopoiesis

Clonal disease is often regarded as almost synonymous with cancer. However, it is becoming increasingly clear that our bodies harbor numerous mutant clones that are not tumors, and mostly give rise to no disease at all. Here we discuss three somatic mutations arising within the hematopoietic system: BCR‐ABL, characteristic of chronic myeloid leukemia; mutations of the PIG‐A gene, characteristic of paroxysmal nocturnal hemoglobinuria; the V617F mutation in the JAK2 gene, characteristic of myeloproliferative diseases. The population frequencies of these three blood disorders fit well with a hierarchical model of hematopoiesis. The fate of any mutant clone will depend on the target cell and on the fitness advantage, if any, that the mutation confers on the cell. In general, we can expect that only a mutation in a hematopoietic stem cell will give long‐term disease; the same mutation taking place in a cell located more downstream may produce just a ripple in the hematopoietic ocean.     

Characterization of the properties of a novel mutation in VAPB in familial amyotrophic lateral sclerosis

Following the mutation screening of genes known to cause amyotrophic lateral sclerosis (ALS) in index cases from 107 familial ALS (FALS) kindred, a point mutation was identified in vesicle-associated membrane protein-associated protein B (VAPB), or VAMP-associated protein B, causing an amino acid change from threonine to isoleucine at codon 46 (T46I) in one FALS case but not in 257 controls. This is an important finding because it is only the second mutation identified in this gene that causes ALS. In order to investigate the pathogenic effects of this mutation, we have used a motor neuron cell line and tissue-specific expression of the mutant protein in Drosophila. We provide substantial evidence for the pathogenic effects of this mutation in abolishing the effect of wild type VAPB in the unfolded protein response, promoting ubiquitin aggregate formation, and activating neuronal cell death. We also report that expression of the mutant protein in the Drosophila motor system induces aggregate deposition, endoplasmic reticulum disorganization, and chaperone up-regulation both in neurons and in muscles. Our integrated analysis of the pathogenic effect of the T46I mutation and the previously identified P56S mutation indicate extensive commonalities in the disease mechanism for these two mutations. In summary, we show that this newly identified mutation in human FALS has a pathogenic effect, supporting and reinforcing the role of VAPB as a causative gene of ALS.     

Ultraviolet mutagenesis of normal and xeroderma pigmentosum variant human fibroblasts

The mutabilities of normal and xeroderma pigmentosum variant (XP4BE) human fibroblasts by ultraviolet light (UV) were compared under conditions of maximum expression of the 6-thioguanine resistance (TGr) phenotype. Selection was with 20 micrograms TG/ml on populations reseeded at various times after irradiation. Approx. 6--12 days (4--8 population doublings), depending on the UV dose, were necessary for complete expression. The induced mutation frequencies were linear functions of the UV dose but the slope of the line for normal cells extrapolated to zero induced mutants at 3 J/m2. The postreplication repair-defective XP4BE cells showed a higher frequency of TGr colonies than normal fibroblasts when compared at equal UV doses or at equitoxic treatments. The induced frequency of TGr colonies was not a linear function of the logarithm of survival for either cell type. Instead, the initial slope decreased to a constant slope for survivals less than about 50%. The UV doses and induced mutation frequencies corresponding to 37% survival of cloning abilities were 6.7 J/m2 and 6.2 X 10(-5), respectively, for normal cells and 3.75 J/m2 and 17.3 X 10(-5) for the XP4BE cells. The lack of an observable increase in the mutant frequency for normal fibroblasts exposed to slightly lethal UV doses suggests that normal postreplication repair of UV-induced lesions is error-free (or nearly so) until a threshold dose is exceeded.     

A mutational assay system for L5178Y mouse lymphoma cells, using hypoxanthine-guanine-phosphoribosyl-transferase (HGPRT) -deficiency as marker. The occurrence of a long expression time for mutations induced by X-rays and EMS.

The development of a system for the detection of somatic cell mutation to hypoxanthine-guanine-phosphoribosyl-transferase (HGPRT) (EC 2.4.2.8) deficiency in L5178Y mouse lymphoma cells is described. The selection of mutant cells was not influenced by the concentration of the selective agent 6-thioguanine (6-TG). In addition, all the mutants selected, spontaneous as well as induced ones, showed a complete loss of HGPRT activity. In reconstruction experiments, in which mutant cells were mixed with wild-type cells, the recovery of mutant cells was only markedly influenced when wild-type cells were seeded in a cell density ten times higher than the one, 5-10(4) cells/ml, used in subsequent induction experiments. X-irradiation and treatment with ethyl methanesulfonate (EMS) increased in the mutation rate above the spontaneous background. A clear-cut dose-dependent mutagenic effect after exposure to X-rays was measured. The rate of induced mutations at the HGPRT locus in lymphoma cells was 1-3-10(-7) per R, as determined after exposures of 200, 300, 400, 500 and 600 R. The time the cells needed to express their mutations was much longer than 48 h. Further study of this phenomenon showed that the optimal expression time for TGr-resistant mutants in L5178Y cells was 6 to 7 days. No indication for a dose-dependent effect on the optimal expression of the mutants was found.     

一种新型的双功能体内突变检测系统的建立

将含有两个LacI靶基因的穿梭质粒pMCLacI/neo整合入小鼠的基因组中 ,建立一种能同时分析体内表达基因和沉默基因的双功能的新型体内突变检测系统 ,使在体内比较分析不同组织、器官中表达基因和沉默基因的突变谱和突变率成为可能。486个经显微注射过 pMCLacI/neo质粒的C5 7BL/ 6小鼠的受精卵 ,分别移植入 18只假孕母鼠的输卵管中 ,产下 32只存活小鼠 ,进一步检测证实 :(1)有 5只小鼠在基因组中整合有 pMCLacI/neo基因 ;(2 )该质粒中的两个LacI靶基因 ,只有一个表达 ;(3)能有效地从小鼠基因组中回收 pMCLacI/neo基因。因此 ,成功建立了pMCLacI/neo的转基因小鼠品系 ;可以将该小鼠用于整体动物水平研究表达基因和沉默基因突变机制的模型 ;该模型为一种具有双功能的新型体内突变检测系统     

Induction of mutations in males of the fish Oryzias latipes at a specific locus after gamma-irradiation

We have studied frequencies of mutations induced at the b locus of the fish, Medaka Oryzias latipes, after gamma-irradiation. Homozygotes for the b locus have colorless melanophores whose phenotypic expression can be distinguished from that of the wild type. An advantage of the use of oviparous fish for detection of skin color mutations is that the mutant phenotype can be confirmed as early as 1.5 days after fertilization because of the transparent egg membrane of the embryo. Wild-type (B/B) male fish were exposed to 4.75 or 9.5 Gy of 137Cs gamma-rays at a dose rate of 0.95 Gy/min and then mated with the female testers (b/b). A total of 77,761 F1 offspring were examined for mutation and other abnormalities. In the control, we had 1 mutant among 22,068 offspring, resulting in a mutation rate of 4.53 X 10(-5)/locus/gamete. However, this mutant embryo died before hatching. Therefore, in an attempt to present specific-locus mutation frequencies in the fish, the frequencies of color mutants that survived more than 4 days after hatching were used as frequencies of viable mutants; (number of viable color mutants)/(number of hatched fry that survived more than 4 days after hatching). In the 4.75 Gy-irradiated group the viable mutant frequencies were 45.0 X 10(-5), 69.7 X 10(-5) and 0/locus/gamete, while exposure to 9.5 Gy resulted in mutation rates of 217 X 10(-5), 130 X 10(-5) and 8.06 X 10(-5), respectively, for sperm, spermatids and spermatogonia. In comparison with viable color mutant frequencies those of the total color mutants, which include such mutants as ones that died before hatching (defined as number of total color mutants/number of fertilized eggs minus number of early deaths), were considerably higher. For sperm, spermatids, and spermatogonia after exposure to 4.75 Gy, the frequencies were 1180 X 10(-5), 629 X 10(-5) and 9.90 X 10(-5)/locus/gamete, respectively, and in 9.5-Gy-irradiated fish, the frequencies were 1940 X 10(-5), 953 X 10(-5) and 55.5 X 10(-5). Although our data are incomplete, the present results were compared with mutation induction in mice. We concluded that the frequencies of viable color mutants in the fish can be compared with those in mice.     

Effects of a poly(ADP-ribose) polymerase inhibitor on mutation frequencies in dividing and quiescent C3H10T1/2 cells

The effect of 3-methoxybenzamide, an inhibitor of poly(ADP-ribose) polymerase, on N-methyl-N′-nitro-N-nitrosoguanidine-induced mutations has been examined in exponentially dividing cells where this inhibitor is a strong potentiator of cytotoxicity and in quiescent cells where the inhibitor has little or no effect on cell survival. The yield of mutants decreased in dividing cells by approximately 70% while mutation frequencies showed small but statistically significant increases in quiescent cells. These results suggest that apparent decreased mutation frequencies observed in the presence of ADP-ribosylation inhibitors are due to selective inhibition of expression of mutations in dividing cells caused by an irreversible G₂ cell cycle block.     

The role of RamA on the development of ciprofloxacin resistance in Salmonella enterica serovar Typhimurium

Active efflux pump is a primary fluoroquinolone resistant mechanism of clinical isolates of Salmonella enterica serovar Typhimurium. RamA is an essential element in producing multidrug resistant (MDR) S. enterica serovar Typhimurium. The aim of the present study was to elucidate the roles of RamA on the development of ciprofloxacin, the first choice for the treatment of salmonellosis, resistance in S. enterica serovar Typhimurium. Spontaneous mutants were selected via several passages of S. enterica serovar Typhimurium CVCC541 susceptible strain (ST) on M-H agar with increasing concentrations of ciprofloxacin (CIP). Accumulation of ciprofloxacin was tested by the modified fluorometric method. The expression levels of MDR efflux pumps were determined by real time RT-PCR. In ST and its spontaneous mutants, the ramA gene was inactivated by insertion of the kan gene and compensated on a recombinant plasmid pGEXΦ(gst-ramA). The mutant prevention concentration (MPC) and mutant frequencies of ciprofloxacin against ST and a spontaneous mutant in the presence, absence and overexpression of RamA were tested. Four spontaneous mutants (SI1-SI4) were obtained. The SI1 (CIP MICs, 0.1 mg/L) without any target site mutation in its quinolone resistant determining regions (QRDRs) and SI3 (CIP MICs, 16 mg/L) harboring the Ser83→Phe mutation in its QRDR of GyrA strains exhibited reduced susceptibility and resistance to multidrugs, respectively. In SI1, RamA was the main factor that controlled the susceptibility to ciprofloxacin by activating MdtK as well as increasing the expression level of acrAB. In SI3, RamA played predominant role in ciprofloxacin resistance via increasing the expression level of acrAB. Likewise, the deficiency of RamA decreased the MPCs and mutant frequencies of ST and SI2 to ciprofloxacin. In conclusion, the expression of RamA promoted the development of ciprofloxacin resistant mutants of S. enterica serovar Typhimurium. The inhibition of RamA could decrease the appearance of the ciprofloxacin resistant mutants.     

Experiments on the effect of the medium in mutation tests with the HGPRT system in cultured mammalian cells

Higher mutation frequencies were observed on 8AG than on 8AG medium when HGPRT-deficient mutants were being selected in V79 Chinese hamster cells.2 alternative explanations for the effect of the medium were considered, namely (1), that mechanisms are present that cause resistance to 8AG only, in addition to that (or those) causing resistance to both drugs, and (2), that mutants with low HGPRT content survive on 8AG but not on 6TG medium, owing to lower affinity of 8AG for the enzyme. The second explanation was favoured as a result of various experimental approaches, including kinetics of expression on the 2 media, cross-resistance at different expression times and serial selection on the 2 media.     

Hamster and rat fetal cells have low spontaneous mutation frequencies and rates

Somatic cells of whole Syrian hamster fetuses (gestation day 13) were isolated and tested by an in vivo/in vitro mutation assay for spontaneous mutation frequencies using independent 6-thioguanine (6-TG), diphtheria toxin (DT), and ouabain mutation selection systems. Optimum conditions were ascertained. For 6-TG mutants, a total of 21 mutants were found in cells from 24 litters on 1993 plates, for an overall mutant frequency of 1.8 x 10(-7) per viable cell with 12 positive litters. In all, 26 litters were tested using DT; 77 mutants were found in 840 plates, yielding an overall mutant frequency of 2.6 x 10(-7), with 20 positive litters. No correlations or familial effects were found among 23 litters tested for both DT and 6-TG. Of 14 litters which were tested for ouabain mutants, 4 were positive, with a total of 5 mutants found on 988 plates, for an overall mutant frequency of 7.6 x 10(-8). For 14 F344 rat fetuses, the overall 6-TG spontaneous mutation frequency was determined to be 1.6 x 10(-7). From the data, estimates of mutation rates were calculated. For mutation to 6-TG resistance the rate was 8.3 x 10(-8), for mutation to DT resistance the rate was 8.1 x 10(-8) and for ouabain, the spontaneous mutation rate was 5.7 x 10(-8). For F344 rat, the spontaneous mutation rate was 1.1 x 10(-7). Induced mutant frequencies after in utero exposure to 1 mmol/kg N-ethyl-N-nitrosourea (ENU) were 311, 135 and 200 times the spontaneous value for 6-TG, DT and ouabain, respectively, for Syrian hamster fetal cells and 125 times the spontaneous 6-TG value for fetal F344 rat cells. Both spontaneous mutation frequencies and underlying spontaneous mutation rates are low, consistent with the view that fetal cells exercise extremely tight control over DNA fidelity.     

Deficiency of double mutant recombinants in crosses of phage T4

Summary Only 1.4% of the double mutant recombinants expected on the basis of wild-type recombination frequencies were observed in the combined data from two-factor crosses between a gene 37 amber mutant, amB280, and eighteen different temperature sensitive mutants which were also defective in gene 37. Similar, though less extreme, deficiencies of double mutant recombinants were observed by Doermann and Parma (1968) for mutants in several other genes. In our amB280xts crosses, frequencies of wild-type recombinants were in reasonably good agreement with those expected from the map positions of the mutants determined in crosses not involving amB280. Wild-type and double mutant recombinants were found at comparable frequencies when each of three other gene 37 amber mutants was crossed to a gene 37 temperature sensitive mutant.Experiments were performed to test whether the deficiency of double mutant recombinants in the amB280xts crosses could be explained by assuming that they occurred primarily in heterozygous particles, where their expression was masked. However, no evidence in support of this explanation was found. Other possible explanations, that the deficiency of double mutants was due to their inviability or the inability of double mutant chromosomes to replicate, were also inconsistent with our observations. The hypothesis considered to most plausibly explain our evidence is that the process by which double mutant recombinant chromosomes are formed is inhibited in the vicinity of a poorly suppressed am mutation.     

Mutation induction in Escherichia coli incubated in the reaction mixture of NADPH-dependent lipid peroxidation of rat-liver microsomes

Experiments were carried out to examine mutation induction in E. coli cells incubated in the reaction mixture of NADPH-dependent lipid peroxidation of microsomes isolated from rat liver. The results obtained were as follows: (1) Lipid peroxidation of microsomes occurred extensively on incubation with NADPH and Fe2+. In the E. coli WP2uvrA(pKM101) system, the mutation frequency to streptomycin resistance increased markedly when the cells were incubated in the reaction mixture of microsomal lipid peroxidation. The induced mutation frequencies were dependent on the extent of the lipid peroxidation. (2) It was also found that the mutations were induced at the same rate as in the case of (1) when the cells were added to the microsomal suspensions after the reactions due to the short-lived free radicals had terminated. (3) The cytotoxicity of the lipid peroxidation products was larger in the DNA repair-defective mutant, E. coli SR18 (uvrArecA) than the wild-type strain, SR749. From these results it is concluded that some DNA-damaging and mutagenic substances are indeed produced in the degradation process of peroxidized polyunsaturated fatty acids in liver microsomal lipids.     

Enhanced mutability at the tk locus in the radiosensitive double-strand break repair mutant xrs5

The thymidine kinase locus (tk) has been utilised as the target locus to measure the induced mutation frequency following X-irradiation in the X-ray-sensitive xrs5 mutant and its parent CHO K1 line of Chinese hamster cells. Mutations to tk- cells were measured by plating cells in selective medium containing trifluorothymidine after a post-irradiation expression time of 4 days. Our results show that the mutation frequency was 3-4 times higher in the xrs5 mutant than in the CHO K1 cell line. This enhanced mutation frequency in xrs5 is though to result from the deficiency in DNA double-strand break repair in this cell line which also results in the enhanced cell killing and higher frequencies of chromosomal aberrations in response to X-irradiation. The findings of the present study suggest that DNA double-strand break is a critical lesion leading to mutations in irradiated cells.     

Characterization of a UV-resistant mutant of CHO-K1 with normal repair activity

The biological and repair responses of Mut 8–16, an ultraviolet radiation (UV)-resistant derivative of CHO-K1, were characterized with respect to UV and to the active chemical carcinogen, benzo[a]pyrene-4,5-oxide. In comparison to the parent, the UV-survival response curve of this mutant showed a significantly larger shoulder but little or no difference in the slope of the exponential survival region. In addition, the mutant cell line demonstrated significantly larger mutation frequencies at high survival UV fluences, but smaller mutation frequencies at high survival equitoxic concentrations of the carcinogen benzo[a]pyrene-4,5-epoxide relative to the parent cell. However, these relative differences in mutation frequencies between parent and mutant appeared to decrease as survival decreased. Despite these observations there were no measurable differences in excision-repair, or in post-replication repair although the mutant appeared to show a nominal reduction (not an enhancement) of replication-repair activity following the UV exposure. These data imply there is another lesion recognition system in CHO cells whose effects on survival and mutation are best observed at low doses of carcinogen and/or radiation but which are masked at higher doses where major repair processes dominate. The dissimilar relationship of cytotoxicity to mutation induction frequency observed in UV and carcinogen treated mutant vs. parent cell lines, imply that the probabilities for lethality and mutation are independent of one another in the presence of otherwise unrepaired (residual) damage.     

The use of correction factors in the determination of mutant frequencies in populations of human diploid skin fibroblasts.

The need and validity for using a number of correction factors in the determination of mutant frequencies in human diploid skin fibroblasts was investigated. Resistance to the purine analogue 8-azaguanine was used as selective system. It appears that, under the conditions used, there were no effect of the cell density on the cloning efficiency. Therefore observed mutant frequencies need to be corrected for cloning efficiency. The reliability of using Lesch-Nyhan cells as a prototype mutant in the estimation of the efficiency of mutant recovery was investigated and found to hold true. The relation between the efficiency of mutant recovery and cell density, which reflects the degree of inter-clone metabolic cooperation, was measured. The use of this relationship enhances the accuracy of the estimated mutant frequencies. With these correction factors the mutation rate was estimated to be 5.7 - 10(-6) per cell per generation. The influence of intra-clone metabolic cooperation was determined and found to be small if present.     

Streptomycin-resistant mutant production in a continuous-flow UV mutation device

Summary A continuous-flow UV-induced mutation device which incorporates starting strain cultivation, UV irradiation and mutant reproduction was conceptualized and tested in this study using streptomycin resistance as an indicator of mutant production. For the experimental conditions employed and populations used, the mutation frequency for streptomycin resistance ranged from 10^–4 to 10^–5 cfu/ml. These mutation frequencies are comparable with conventional batch UV mutation methods and represent a gain of 3 orders of magnitude over the spontaneous mutation frequency.