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N B Varshaver

Publications and source records attributed to N B Varshaver.

At least 19 recordsLinked to original sources
PubMed1995

[Characteristics of mutants induced by the c-Ha-ras1 oncogene and the nature of the oncogene's mutagenic action].

Chinese hamster cell clones of independent origin, which were resistant to purine base analogs and induced by the activated c-Ha-ras1 oncogene, were isolated. It was shown that the isolated clones stably retained resistance after cultivation on a medium without an analog, confirming mutational nature of the resistance. Most of the clones are able to grow on the HAT medium, retaining partial activity of the hypoxanthine phosphoribosyltransferase enzyme (HPRT); i.e., they are leaky mutants. Analysis by blot-hybridization did not reveal the presence of human ras-sequences in any of the mutants studied. Evidently, the mutagenic action of the oncogene is not insertional, and resistance is not linked to the stably integrated oncogene. The mutagenic effect of c-Ha-ras1 is likely to be of the "hit-and-run" type.

Animals↗
PubMed1992

The activated human c-Ha-ras-1 oncogene as a mutagen.

The induction of gene mutations and chromosome aberrations by plasmid pEJ6.6 carrying the activated c-Ha-ras-1 oncogene from human bladder carcinoma was studied in cultured Chinese hamster cells. Both an increase in the frequency of hypoxanthine-phosphoribosyltransferase-deficient (HPRT-) mutants and chromosome aberrations was observed after pEJ6.6 transfection as compared to control series (pBR322). In order to define whether it is the oncogene which is responsible for the mutagenic effect of pEJ6.6, a derivative of c-Ha-ras-1 carrying a deletion in its coding region was constructed. As shown in all experiments, the frequency of HPRT- mutants after treatment with pEJ6.6 plasmid exceeded that in control dishes treated by pEJ6.6 plasmid with an inactivated oncogene. The effect was rather weak but statistically significant. Thus, the results of experiments carried out show that the mutagenic activity of pEJ6.6 plasmid is chiefly determined by its oncogene. The role of the mutagenic effects of activated oncogenes in malignant transformation is discussed.

Animals↗
PubMed1992

[Role of activated cellular c-Ha-ras-1 oncogene in the mutagenic effect of the plasmid pEJ6.6].

The role of the activated oncogene c-Ha-ras-1 from human bladder carcinoma integrated into the pEJ6.6 plasmid in the mutagenic effect of the plasmid was studied in Chinese hamster cells. The frequency of hypoxanthine-phosphoribosyltransferase defective (HPRT-) mutants after treatment with pEJ6.6 containing an active c-Ha-ras-1 exceeded that in control dishes treated with a derivative of pEJ6.6 plasmid with an inactivated oncogene. The inactivation was achieved by introducing a deletion into the coding region of the oncogene. The mutagenic effect was rather weak but statistically significant. Thus, the data obtained show that the mutagenic activity of pEJ6.6 plasmid is determined by its oncogene. The role of mutagenic effects of activated cellular oncogenes in malignant transformation is discussed.

Animals↗
PubMed1992

[The plasmid pEJ6.6 carrying the activated c-Ha-ras-1 oncogene increases the mutation frequency in Chinese hamster cells].

The induction of gene mutations and chromosome aberrations by the plasmid pEJ6.6 carrying the activated c-Ha-ras-1 oncogene from human bladder carcinoma was studied in cultured Chinese hamster cells. Both an increase in the frequency of gene mutations to 6-mercaptopurine resistance and of chromosome aberrations was observed after pEJ6.6 treatment as compared to control series (pBR322). Thus the results of experiments carried out show that the pEJ6.6 plasmid possesses a mutagenic activity.

Animals↗
PubMed1989

Reversion to normal phenotype induced by SV40 in a spontaneously transformed malignant Chinese hamster cell line.

By using a selection procedure that excluded the transforming effect of SV40, reversions to several properties of normal phenotype were for the first time obtained in a transformed Chinese hamster cell line after SV40 infection. The value of induction to recovery of contact inhibition was typical for SV40-induced reverse gene mutations. Thirteen of 15 isolated revertant clones were T-antigen positive, thus synthesizing the product of viral oncogene. Therefore, in the majority of clones reversion occurred in spite of the presence of viral transforming protein. Dot hybridization revealed the presence of SV40 DNA in all revertants including those expressing no T antigen. The virus rescued from one T-antigen positive and two negative clones proved to be infectious. Reversion to contact inhibition was followed by reversion as regards serum requirements and growth in soft agar. However, in all cases reversion was partial. Karyologic analysis of revertant clones showed that six clones maintained the hypodiploid karyotype of the parental clone, six revertants were near-tetraploid, and one was near triploid. The possible events underlying the SV40-induced reversions to normal phenotype and the role of virus-induced mutations in viral carcinogenesis are discussed.

Animals↗
PubMed1987

[Reversibility of malignant transformation as affected by the oncogenic virus SV40. II. The characteristics of virus-induced revertant clones].

Fifteen revertant clones exhibiting contact inhibition, one of the typical characteristics of normal cells, were studied after treatment of spontaneously transformed Chinese hamster fibroblasts with SV40. The clones proved to be partial revertants, as regards to other properties of the normal phenotype--loss of the ability to grow in a medium with a low serum content and anchorage-dependence. Viral DNA was detected in all revertant clones. The expression of T-antigen--the product of viral oncogene, was observed in 13 of 15 revertants analyzed. The study of SV40 "rescued" from several revertants in permissive monkey cells has shown that the virus is non-defective. In 7 clones, reversion was accompanied with polyploidization. In the cases, reversion could be due to changes in the balance between oncogenes and suppressor genes (anti-oncogenes). The possibility of induction by SV40 of mutations in anti-oncogenes suppressing the expression of both cellular and viral oncogenes is discussed. It is suggested that reversion to the normal phenotype in clones with a near-diploid karyotype could result from such virus-induced suppressor mutations.

Animals↗
PubMed1987

[Reversal of malignant transformation induced by the oncogenic virus SV40. I. Induction of reversal to normal type for the contact inhibition trait].

The possibility of induction by the oncogenic DNA-containing virus SV40 of reversions to normal phenotype as regards contact inhibition ("flat" revertants), was studied in spontaneously transformed chinese hamster fibroblasts. Negative selection was used for detection of revertants. The method adopted allowed to study the mutagenic activity of the virus, while excluding its transforming effect. In all experiments the frequency of revertants after infection exceeded that in control series. The value of induction varied from 1.2 to 28.4 X 10(-6). The tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) known to increase the frequency of mutations induced by carcinogens in vitro, displayed no enhancing effect on the frequency of revertants induced by SV40. The lack of enhancement of virus-induced reversions after TPA treatment might be explained by the lack of the transforming effect of SV40 in the system studied. Some of the normal "flat" colonies were T-antigen positive, i. e. the viral oncogene was expressed. The role of mutations induced by SV40 in cellular genes controlling malignancy is discussed.

Animals↗
PubMed1986

[Mutagenic effect of the SV40 oncogene: induction of resistance to 6-mercaptopurine and serum independence].

The mutagenic and transforming activity of SV40 DNA fragment, corresponding to its oncogene (the gene for large T antigen) was studied in Chinese hamster cells. After expression time of 3 to 4 days, the oncogene induced mutations of resistance to 6-mercaptopurine (6MP), while the DNA encoding the SV40 late genes, as well as DNA of Chinese hamster cells, were devoid of mutagenic activity. The value of induction ranged from 10(-4) to 10(-5). After the same expression time, the oncogene induced a typical character of oncogenic transformation - independence of serum growth factors (ser+). The value of induction of ser+ variants was somewhat higher than for resistance mutations. The study of 12 clones induced by the oncogene has shown the ser+ character to be hereditary, the expression of viral oncogene being not necessary for its maintenance. The data obtained support the hypothesis in favour of the participation of mutations of cellular genes in viral carcinogenesis.

Animals↗
PubMed1986

[Integration of SV40 DNA into the cell genome and viral mutagenesis].

Integration of DNA of a temperature-sensitive SV40 mutant (tsA239) into the cell genome was studied. The viral A gene (the oncogene) encodes the tumour T antigen which is ts in the mutant and is devoid of mutagenic and transforming activity under non-permissive conditions (40 degrees C). Clones of Chinese hamster cells infected by tsA239 mutant were analysed. Those infected by wild-type SV40 served as controls. As shown by dot-hybridization, SV40 DNA was detected in cells of 14 out of 18 clones infected by tsA mutant and incubated at 40.5 degrees C, and in all 20 clones infected by tsA mutant and incubated under permissive conditions (33 degrees C), the difference between the two groups being insignificant (p greater than 0.05). By means of blot-hybridization it was established that viral DNA was integrated into the cell genome of all 12 clones analysed, belonging to the three experimental series: infection by tsA mutant, incubation at 40.5 and 33 degrees C, infection by wt SV40, incubation at 40.5 degrees C. The number of integration sites ranged from one to four in different clones. Integration of SV40 DNA in tandems was observed. The data presented allow to conclude that integration per se does not play a crucial role in determining the mutagenic and transforming effect of the virus. Obviously, what matters is the activity of viral oncogene product - the T antigen.

Animals↗
PubMed1985

The oncogene of BAV-3 as a mutagen.

We studied the mutagenic and carcinogenic effects on mammalian cells of two EcoRI DNA fragments of bovine adenovirus type3 (BAV-3) integrated into the pBR325 plasmid. Fragment D located between 3.6 and 19.7 map units, contains the viral oncogene, fragment C, located between 44.3 and 63.7 map units, has no oncogenic activity. The BAV-3 oncogene was shown to increase significantly the frequency of 6-mercaptopurine (6MP)-resistant mutants in Chinese hamster calls. Fragment C, pBR325 without viral sequences and DNA from normal Syrian hamster cells did not have any mutagenic effect. We also looked at the combined action of the viral DNA fragments and the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), which enhances the transforming effect of carcinogens. TPA was shown to increase the mutant yield on exposure to the viral oncogene but not to induce mutagenic activity in those types of DNA that are unable to transform cells. Probably TPA does not affect the initiation of the mutation process, but acts on later stages just as it affects carcinogenic activity. Thus the results obtained confirm the existence of parallelism between the mutagenic and transforming effects of viral DNA and show that both activities are mapped in the same region of viral DNA - its oncogene.

Adenoviridae↗
PubMed1985

[Spontaneous rate of occurrence of 2 characters of the transformation phenotype in a mouse fibroblast culture].

The spontaneous rate of occurrence of two characters of malignant transformation was studied in mouse embryo fibroblasts C3H10T1/2, clone 8. This cell line, though "immortal" in vitro, is characterized by a normal phenotype in respect to many other properties. The spontaneous rate of occurrence of anchorage-independence (aga+) and dense foci on cell monolayer varied in different experiments from 0.65 X 10(-6) to 1.2 X 10(-6) and from 1.2 X 10(-6) to 3.6 X 10(-6) per cell per generation, respectively. The fluctuation test has shown that both characters occur as random spontaneous events. The altered colony morphology proved to be stable in all 28 foci of independent origin tested. In most cases, the morphological transformants were anchorage-independent. It is suggested that the occurrence of the characters studied is due to mutation in a gene with pleiotropic effect.

Animals↗
PubMed1985

[Role of the oncogene in the mutagenic activity of bovine adenovirus type 3].

The mutagenic and carcinogenic effect of two EcoRI-fragments of bovine adenovirus type 3 (BAV-3) DNA inserted into pBR325 has been studied. The C fragment (located between 3,6 and 19,7 map units) contains the viral oncogene, the C fragment (between 44,3 and 63,7 map units) displays no transforming activity. It has been established that oncogene BAV-3 statistically true increases the yield of mutants resistant to 6-mercaptopurine (6MP) in Chinese hamster cells. The C fragment, pBR325 without viral sequences and DNA fragments of different molecular weights from normal Syrian hamster cells have no mutagenic effect. The control over tumor formation in syngenic mice after injection of C3H10T 1/2 and D. C fragments and pBR325 treatment exposed a parallelism between the mutagenic and transforming effect. The study of the combined effect of viral DNA fragments and the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) which increases the transforming activity of different carcinogens, shows that the promoter increases the frequency of mutants after viral oncogene treatment and does not induce mutagenic activity of those types of DNA which are unable to transform the cells.

Adenoviridae↗
PubMed1984

Mutagenic effects of DNA-containing oncogenic viruses and malignant transformation of mammalian cells.

It was discovered in the 1970s that oncogenic viruses could induce gene mutations in mammalian cells. The phenomenon seems to be widespread: it was observed with all groups of DNA-containing viruses and some retroviruses. The mutagenic effects of the tested viruses at gene level are not locus specific. The viruses induce point mutations, including base substitutions, as well as deletions and insertions. The mutagenic effect of SV40 is controlled by the activity of the early A gene, which encodes the T antigen. Presumably, the process of integration creates the possibility for occurrence of mutations early after infection. Mutagenesis seems to be induced by an integrated virus, though to a much smaller extent. Virus-induced mutagenesis may be connected with an activation of the cell error-prone repair systems. The sum total of the experimental data shows that virus-induced mutagenesis and transformation are interrelated: (A) viruses, like other carcinogenes, display mutagenic activity; (B) viruses that are far removed from each other systematically, whose only similarity lay in being oncogenic and capable of integration, simultaneously showed the ability to induce gene mutations; (C) agents changing the rate of transformation also changed the rate of gene mutations: (D) The function of mutagenicity was mapped in the oncogene of SV40 (gene A); and the DNA of (E) mouse mammary carcinoma virus (MMTV) and avian leukosis virus (ALLV) induced tumors has been found to contain nucleotide sequences that transform 3T3NIH cells but do not carry any viral genetic information. Mutagenesis induced by oncogenic viruses may play a part in the multistage process of malignant transformation, though its contribution may be different in various specific cases and for different groups of viruses. Further studies of the uncommon mutagens, which viruses seem to be, may greatly increase our knowledge of the virus-cell relationship. An understanding of the extent of genetic danger inherent in viruses and live viral vaccines is necessary for practical medicine.

Animals↗
PubMed1983

The mutational origin of serum independence in Chinese hamster cells in vitro.

The genetic mechanisms determining the ability of transformed cells to grow in a medium with a low serum content (ser+) were studied in a clone of Chinese hamster cells with normal serum requirements. The fluctuation test has shown that serum independence occurs as a random spontaneous event. Its rate of occurrence is about 10(-5). The concomitant study of a gene mutation (resistance to 6-mercaptopurine--6MP) revealed similar characteristics with respect to the distribution of the number of mutants in replicative cultures. N-methyl-N1-nitro-N-nitrosoguanidine (MNNG) and SV40 significantly increased the frequency of ser+ colonies. Induction was detected after an expression time of 3-4 days, which is typical of gene mutations. In 16 out of 18 ser+ clones of independent origin the ser+ character remained stable. The results suggest that the ser+ character originates in most cases from a mutation event.

Animals↗
PubMed1983

[Genetic nature of one of the traits of malignant cell transformation in vitro].

The genetic events controlling the ability of transformed cells to grow in a medium with a low serum content (ser+) were studied. A hypodiploid clone of Chinese hamster cells with normal serum requirements (49a5ser-) was used as starting material. The results of the fluctuation tests have shown that serum-independence is a random spontaneous event. Its rate of occurrence is 1-2 . 10(-5). The concomitant study of a gene mutation (resistance to 6-mercaptopurine) revealed similar characteristics with respect to the distribution of the number of mutants in replicate cultures and the mutation rate. N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and the oncogenic SV40 virus significantly increased the frequency of ser+ colonies. In the majority of clones isolated in a medium with 1% serum (11 spontaneous and 7 induced by MNNG), the ser+ character proved to be stable after different periods of cultivation without selective pressure. The degree of serum-independence varied in different clones. The results suggest that the ability to grow in a medium with a low serum content originates, in most cases, from a mutation event.

Animals↗
PubMed1982

The role of the transforming A gene of SV40 in the mutagenic activity of the virus.

The mutagenic activity of the tsA239 mutant of SV40 which synthetizes a defective T antigen at 40 degrees C was investigated in Chinese hamster cells under permissive and nonpermissive temperature. At 33 degrees C the virus increased the yield of 6-mercaptopurine-resistant colonies after 2 days expression time by a factor of 1.6-4 as compared with the control and raised the frequency of aberrant metaphases after the same time by a factor of 1.9-3.4. In the same experiments, with the same initially infected population of Chinese hamster cells, at 40 degrees C tsA SV40 did not induce either gene mutations or chromosome aberrations at the same early stage after infection. Presumably the activity of the A gene of SV40 is necessary not only for the transforming but also for the mutagenic effect of the virus.

Animals↗
PubMed1981

Oncogenic adenovirus as mutagen for chinese hamster cells in vitro.

Oncogenic bovine adenovirus (BAV3) was shown to induce chromosome aberrations and gene mutations to 6-mercaptopurine (6MP) resistance in Chinese hamster cells. BAV3 showed the highest mutagenic effect at the chromosome level 12--24 h postinfection. After 48 h the yield of aberrations dropped to the control level, where it remained after 72 and 96 h. BAV3 showed a highly significant induction of mutations to 6MP resistance 48 h postinfection. The effect of the combined treatment of cells with 5-bromodeoxyuridine (BrdU) and BAV3 on mutagenesis at chromosome and gene levels proved to be synergistic. The mutagenic activity at the gene level of BAV3, along with the earlier established mutagenic effect of SV40, indicates that this property is probably inherent in many DNA-containing oncogenic viruses. The possible mechanisms underlying malignant transformation and mutagenesis are discussed.

Adenoviridae↗