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T R Skopek

Publications and source records attributed to T R Skopek.

At least 19 recordsLinked to original sources
PubMed1992

Mutational spectrum at the Hprt locus in splenic T cells of B6C3F1 mice exposed to N-ethyl-N-nitrosourea.

We have determined the mutational spectrum of N-ethyl-N-nitrosourea (ENU) in exon 3 of the hypoxanthine (guanine) phosphoribosyltransferase gene (Hprt) in splenic T cells following in vivo exposure of male B6C3F1 mice (5-7 weeks old) to ENU. Hprt- mutants were isolated by culturing splenic T cells in microtiter dishes containing medium supplemented with interleukin 2, concanavalin A, and 6-thioguanine. DNA was extracted from 6-thioguanine-resistant colonies and amplified by the polymerase chain reaction (PCR) using primers flanking Hprt exon 3. Identification of mutant sequences and purification of mutant DNA from contaminating wild-type Hprt DNA was accomplished by denaturing-gradient gel electrophoresis. Purified mutant DNA was then sequenced. Treatment of mice with ENU at 40 mg/kg of body weight produced a Hprt- mutant frequency of 7.3 x 10(-5) in splenic T cells, approximately 35-fold above background levels. Sixty-nine of the 521 Hprt- mutants analyzed contained mutations in exon 3 (13%). Transversions and transitions at A.T base pairs dominated the spectrum; 62 of the 69 exon 3 mutations were at A.T base pairs (14 different sites). Thirteen of 14 thymine bases undergoing mutation (61 of 62 mutations at A.T bases) were located on the nontranscribed strand of exon 3. The majority of the remaining mutations (6 of 69) were transitions at a single G.C base pair. These results suggest the importance of thymidine alkylation in ENU-induced mutagenesis in vivo. The mouse Hprt- T-cell cloning/sequencing assay described here may represent a useful system for studying the molecular mechanism of chemically induced mutation occurring in vivo in an endogenous gene.

Animals↗
PubMed1992

Molecular dosimetry of ethylene oxide: formation and persistence of 7-(2-hydroxyethyl)guanine in DNA following repeated exposures of rats and mice.

The formation of 7-(2-hydroxyethyl)guanine (7-HEG) in DNA of target and nontarget tissues was investigated in male B6C3F1 mice (20/group) and F344 rats (10/group) exposed to 0, 3, 10, 33, 100, or 300 (rats only) ppm ethylene oxide (ETO) by inhalation for 6 h/day for 4 weeks (5 days/week) and mice exposed to 100 ppm ETO for 1 or 3 days or 1, 2, or 4 weeks (5 days/week). The persistence of 7-HEG was studied in mice killed up to 7 days after cessation of the 4-week time-course study. In addition, the formation of O6-(2-hydroxyethyl)guanine and 3-(2-hydroxyethyl)adenine was evaluated in rats exposed to 300 ppm ETO. DNA samples from control and treated animals were analyzed for 7-HEG using neutral thermal hydrolysis, microconcentration, and high-performance liquid chromatography separation with fluorescence detection. Fluorescence-linked high-performance liquid chromatography was used for O6-(2-hydroxyethyl)guanine quantitation, and immunochromatography and gas chromatography-mass spectrometry were used for 3-(2-hydroxyethyl)adenine detection. Analysis of DNA from tissues of control mice and rats revealed the presence of peaks equivalent to 2-6 pmol 7-HEG/mg DNA. In mice exposed to 100 ppm ETO, 7-HEG accumulated to a similar extent in target and nontarget tissues, with adduct concentrations ranging from 17.5 +/- 3.0 (SE) (testis) to 32.9 +/- 1.9 (lung) pmol adduct/mg DNA after 4 weeks of exposure. Concurrent exposures of mice and rats to 100 ppm ETO for 4 weeks led to 2- to 3-fold lower concentrations of 7-HEG in mouse DNA in all tissues compared to rat DNA. 7-HEG disappeared slowly in a nearly linear fashion from the DNA of mouse kidney (t1/2 = 6.9 days) and rat brain and lung (t1/2 = 5.4-5.8 days), which was consistent with the loss of adduct mainly by chemical depurination. In contrast, a more rapid removal of 7-HEG from other mouse (t1/2 = 1.0-2.3 days) and rat (t1/2 = 2.9-4.8 days) tissues was consistent with adduct loss by depurination and DNA repair. Dose-response relationships for 7-HEG were nonlinear in both mice and rats, with the alkylating efficiency of ETO increasing at high exposures.(ABSTRACT TRUNCATED AT 400 WORDS)

Administration, Inhalation↗
PubMed1992

Modulation of ethylnitrosourea-induced toxicity and mutagenicity in human cells by O6-benzylguanine.

We have examined the contributions of O6-alkylguanine-DNA alkyl-transferase (AGT) and nucleotide excision repair to the protection of human cells from the toxic and mutagenic effects of ethylnitrosourea. Three human lymphoblastoid cell lines were used: one which possesses both of these DNA repair pathways; one derived from a xeroderma pigmentosum complementation group A patient, which expresses AGT but is deficient in nucleotide excision repair; and a third which does not express AGT but is capable of excision repair. The level of active AGT in the cells was further modulated with the use of the AGT inhibitor, O6-benzylguanine. These cells were exposed to ethylnitrosourea in both the presence and absence of O6-benzylguanine, and population survival, growth, and mutagenesis at the hypoxanthine-guanine phosphoribosyl-transferase locus were measured. The results for all three measurements indicated that the lack of either AGT or nucleotide excision repair significantly impairs the ability of human cells to withstand DNA ethylation damage. Furthermore, the inhibition of AGT in xeroderma pigmentosum group A cells did not increase toxicity or mutagenicity, suggesting that AGT and nucleotide excision repair cooperate in the removal of DNA ethyl adducts. Related studies in our laboratory have shown that AGT and nucleotide excision repair are both necessary for the efficient removal of O6-ethyldeoxyguanosine.

Alkylation↗
PubMed1992

In vitro mutational specificity of cisplatin in the human hypoxanthine guanine phosphoribosyltransferase gene.

The in vitro mutational spectra of cisplatin [cis-diamminedichloroplatinum(II)] in exon 3 of the human hypoxanthine guanine phosphoribosyltransferase gene in B-lymphoblasts was examined by a combination of polymerase chain reaction and denaturing gradient gel electrophoresis. Several thousand independent mutants were induced at the hypoxanthine guanine phosphoribosyltransferase locus by cisplatin and were selected en masse by addition of 6-thioguanine to the bulk culture. Polymerase chain reaction was used to amplify exon 3 from the complex mutant population, and denaturing gradient gel electrophoresis was used to separate wild-type DNA sequences from mutant sequences. Mutational hotspots were visible as discrete bands on the denaturing gradient gel. Scanning densitometry was used to determine the fraction of the complex population represented by the novel bands. The mutant bands were excised from the denaturing gradient gel and sequenced. In this way, the nature and frequency of mutational hotspots in a population of several thousand mutants were determined. Cisplatin produced several mutational hotspots in exon 3. About 9-10% of the cisplatin-induced mutants had mutations in a GGGGGG sequence (base pairs 207-212). GC----AT substitutions at the second and third guanines in the 5'-GGGGGG-3' run made up about 2 and 4% of the induced mutants, respectively. About 4% of the induced mutants contained a GC----TA substitution at the sixth guanine. About 1% of the cisplatin-induced mutants had an AT----TA transversion in a TAGA sequence (base pair 271; mutated base is underlined). Our results are consistent with mutations occurring at GpG and ApG sites. These nucleotide sequences have been identified as the primary sites of cisplatin adduction.

Base Composition↗
PubMed1992

Efficient repair of O6-ethylguanine, but not O4-ethylthymine or O2-ethylthymine, is dependent upon O6-alkylguanine-DNA alkyltransferase and nucleotide excision repair activities in human cells.

The formation and persistence of O6-ethylguanine, O4-ethylthymine, and O2-ethylthymine were quantitated in the genomic DNA of human lymphoblasts exposed to 1.0 mM N-ethyl-N-nitrosourea using immunoslot-blot. The three cell lines used included one which lacks O6-alkylguanine-DNA alkyltransferase, one deficient in nucleotide excision repair, and a third which is competent in both of these repair pathways. The activity of O6-alkylguanine-DNA alkyltransferase was further modulated with O6-benzylguanine, a specific inhibitor of this protein. Repair of the O-ethylated thymines was slow and not related to either DNA repair phenotype. O6-Ethylguanine was repaired with a half-life of about 8 h in cells which expressed both O6-alkylguanine-DNA alkyltransferase and nucleotide excision repair functions. Cells expressing O6-alkylguanine-DNA alkyltransferase activity but lacking nucleotide excision repair showed only slow repair of O6-ethylguanine (half-life of O6-ethylguanine, 43 h), while cells lacking the alkyltransferase showed little or no repair of O6-ethylguanine regardless of nucleotide excision repair activity (half-lives of O6-ethylguanine, 53 to greater than 100 h). We conclude that O6-alkylguanine-DNA alkyltransferase and nucleotide excision repair cooperate in the repair of O6-ethylguanine in human cells.

B-Lymphocytes↗
PubMed1992

Human HPRT mutant database: software for data entry and retrieval.

We have developed a computer database containing information on over 1,000 human hypoxanthine guanine phosphoribosyl transferase (HPRT) mutants. Both published and unpublished data are present. The database itself is maintained in a dBASE format (.DBF) and we provide a set of programs to examine and extract information from the database. A program to input information into the database is also supplied. The database and programs are available directly from us or via remote FTP (file transfer protocol) using BITNET/INTERNET. All programs require an IBM-compatible computer, the MS-DOS operating system (version 3.3 or greater), and a hard disk with about 5 megabytes of free disk space. The purpose of the database is 1) to allow investigators to contribute their HPRT mutants directly to the database in a standardized fashion, and 2) to allow access to the entire database with a set of programs that allows manipulation and extraction of data. For example, using our programs it is possible to i) order the database by base pair position, ii) examine only information regarding mutagenesis by a particular agent, iii) search for a particular author, iv) create a report which contains selected portions of the database, the report can be printed or saved as a file. The database will be updated every several months and distributed.

Databases, Bibliographic↗
PubMed1991

Toxicity, mutagenicity, and mutational spectra of N-ethyl-N-nitrosourea in human cell lines with different DNA repair phenotypes.

We examined the toxicity, mutagenicity, and mutational spectra of N-ethyl-N-nitrosourea (ENU) in three Epstein-Barr virus-transformed human lymphoblastoid cell lines, each with a different DNA repair phenotype. One cell line lacks O6-alkylguanine-DNA alkyltransferase (AGT) activity; another, derived from a patient with xeroderma pigmentosum, complementation group A, lacks nucleotide exicision repair (NER) capability, and the third is competent in both repair functions. ENU-induced toxicity and mutagenicity at the hypoxanthine-guanine phosphoribosyltransferase locus were increased to a similar degree relative to the repair-competent cells in both AGT-deficient and NER-deficient cells. We determined the mutational spectra for ENU by identifying DNA sequence changes at the hypoxanthine-guanine phosphoribosyltransferase locus in at least 26 clones resistant to 6-thioguanine from each cell line. Of the characterized mutations, 89% were single-base pair substitutions. Transitions and transversions were found at AT and GC base pairs in all three cell lines. The biggest difference within the spectra was in the rate of transitions at GC base pairs. Compared to the repair-competent cell line, this mutation was elevated about 8-fold in the AGT-deficient cells and about 3-fold in the NER-deficient cells. We conclude that both AGT and NER play an important role in protecting human cells from the toxic and mutagenic effects of ENU. Furthermore, the mutational spectra suggest that both of these repair systems participate in the repair of O6-ethylguanine adducts.

Adolescent↗
PubMed1991

Fidelity of Thermococcus litoralis DNA polymerase (Vent) in PCR determined by denaturing gradient gel electrophoresis.

DNA synthesis fidelities of two thermostable DNA polymerases, Thermus aquaticus (Taq) and Thermococcus litoralis (Tli, also known as Vent), and a non-thermostable enzyme, a modified T7 DNA polymerase (Sequenase), were determined by analyzing polymerase chain reaction (PCR) products using denaturing gradient gel electrophoresis (DGGE). The error rates were 4.4, 8.9, and 2.4 x 10(-5) errors/bp for modified T7, Taq, and Tli polymerase, respectively. Reducing the nucleotide triphosphate concentration for Tli polymerase during PCR did not alter the fidelity. The ability of DGGE to detect a mutant present at several percent in a wild type population is related to the polymerase fidelity. To examine the sensitivity of mutant detection, human genomic DNA containing a 1% fraction of a known base pair substitution mutant was PCR-amplified with the three enzymes using primers that flank the mutant sequence. The PCR products were analyzed by DGGE. The signal from the mutant present at 1% was visible in the samples amplified with modified T7 and Tli polymerase, but the higher error rate of Taq polymerase did not permit visualization of the signal in DNA amplified with Taq polymerase.

Base Sequence↗
PubMed1991

Deletion mutagenesis during polymerase chain reaction: dependence on DNA polymerase.

Polymerase chain reaction (PCR) was performed with two polymerases. Thermus aquaticus DNA polymerase (Taq), and modified T7 DNA polymerase (Sequenase). Both polymerases were used to amplify the same portion of the human 18S rRNA gene. We report a PCR artifact, namely a deletion of 54 bp, when Taq polymerase was used to amplify a portion of the human 18S rRNA gene. PCR performed with Sequenase did not produce this artifact. The deletion eliminated a potentially stable hairpin loop. Our data are consistent with the following model for generation of the deletion: (i) the formation of an intrastrand hairpin, and (ii) polymerization across the base of the hairpin, thus deleting the nucleotide sequence in the hairpin. Furthermore, we show that the deletion occurs mainly during synthesis of the (-)DNA strand. Our observations suggest that similar artifacts may occur in other sequences containing stable secondary structures.

Base Sequence↗
PubMed1991

Analysis of mutations using PCR and denaturing gradient gel electrophoresis.

Denaturing gradient gel electrophoresis (DGGE) separates DNA molecules based on primary sequence. Under the appropriate conditions, all base pair (bp) substitutions, frame-shifts, and deletions less than about 10 bp can be resolved from the wild type sequence using DGGE. Polymerase chain reaction (PCR) permits facile amplification of a given region of the genome. We have combined PCR and DGGE to: (i) Localize mutations in the X-linked human androgen receptor gene. PCR/DGGE was used to screen the individual exons in the 2757-bp coding region of the gene in afflicted individuals as well as in potential carriers. Inheritance of a mutant allele has been demonstrated in several cases; (ii) Analyze thousands of thioguanine-resistant mutants simultaneously. The in vitro mutational spectra of MNNG, ICR-191, and cisplatin at the human HPRT locus have been examined by this method. The compounds all have mutational hotspots in a GGGGGG sequence in exon 3; however, the particular mutations induced by the agents were different; (iii) Examine the fidelity of several DNA polymerases used in PCR. The fidelity of Thermus aquaticus DNA polymerase (Taq) is 1-2 x 10(-4) misincorporations/bp/replication. Problems with Taq polymerase arise in the analysis of complex mutant populations by DGGE because the Taq-induced errors reduce the sensitivity of the system. To circumvent this, it had been necessary to use Sequenase, a modified T7 DNA polymerase with a higher fidelity. However, Sequenase is not thermostable and must be added every PCR cycle. A thermostable DNA polymerase from Thermococcus litoralis (Vent) is now available, and we have examined the fidelity of Vent, Taq, and Sequenase polymerase in PCR using DGGE. The fidelity of Vent, Taq, and Sequenase polymerase was 2.4 x 10(-5), 8.9 x 10(-5), and 4.4 x 10(-5) errors/bp, respectively. Vent polymerase had the highest fidelity of the three enzymes tested.

Bacteria↗
PubMed1990

Molecular analyses of a Lesch-Nyhan syndrome mutation (hprtMontreal) by use of T-lymphocyte cultures.

The frequency of hprt mutants in peripheral blood T-lymphocytes of two putative Lesch-Nyhan individuals and their parents was determined by a cell cloning assay to quantify the frequency of thioguanine-resistant mutants. The results confirmed the Lesch-Nyhan diagnosis and demonstrated that the mother has an elevated mutant frequency consistent with being heterozygous for an hprt mutation. Mass cultures of T-lymphocytes from both the children and their mother, as well as cultures of hprt mutant clones from the mother, were employed as sources of mRNA for cDNA sequence analysis. These hprt mutants show a single base substitution (T----C transition) at position 170 (exon 3). The predicted amino acid change is the substitution of threonine for methionine56. We have designated this new Lesch-Nyhan mutation hprtMontreal. The use of T-lymphocyte cultures allows rapid sequence analyses of hprt mutations, as well as family studies to define the origin of a particular mutation.

Adult↗
PubMed1990

Molecular analysis of hprt mutants induced by 2-cyanoethylene oxide in human lymphoblastoid cells.

The mutagenic epoxide metabolite of acrylonitrile, 2-cyanoethylene oxide (ANO), was used to treat human TK6 lymphoblasts (150 microM x 2 h ANO). A collection of hypoxanthine-phosphoribosyltransferase (hprt) mutants was isolated and characterized by dideoxy sequencing of cloned hprt cDNA. Base-pair substitution mutations in the hprt coding region were observed in 19/39 of hprt mutants: 11 occurred at AT base pairs and 8 at GC base pairs. Two -1 frameshift mutations involving GC bases were also observed. Approximately half (17/39) of the hprt mutants displayed the complete loss of single and multiple exons from hprt cDNA, as well as small deletions, some extending from exon/exon junctions. Southern blot analysis of 5 mutants with single exon losses revealed no visible alterations. Analysis of 1 mutant missing exons 3-6 in its hprt mRNA revealed a visible deletion in the corresponding region in its genomic DNA. The missing exon regions of 4 mutants (one each with exons 6, 7 and 8 loss and one mutant with a 17-base deletion of the 5' region of exon 9) were PCR amplified from genomic DNA and analyzed by Southern blot using exon-specific probes. The exons missing from the hprt mRNA were present in the genomic hprt sequence. DNA sequencing of the appropriate intron/exon regions of hprt genomic DNA from a mutant with exon 8 loss and a mutant exhibiting aberrant splicing in exon 9 revealed point mutations in the splice acceptor site of exon 8 (T----A) and exon 9 (A----G), respectively.

Base Composition↗
PubMed1990

DNA sequence analysis of in vivo hprt mutation in human T lymphocytes.

We have determined the molecular basis of hypoxanthine-guanine phosphoribosyltransferase (hprt) mutations that arose in vivo in the T lymphocytes of a normal male subject. In previous studies approximately 16% (23/141) of the mutants from this individual analyzed by Southern blot displayed large structural alterations in hprt. Thirty-two mutants without these large hprt structural alterations produced sufficient hprt cDNA for polymerase chain reaction amplification and DNA sequence analysis. Base substitutions in hprt cDNA resulting in missense mutations and one mRNA splicing aberration (inclusion of intron sequences) were observed in 18/32 of these these mutants; substitutions occurred at both AT and GC base pairs. Small deletions (3/32), a tandem change and a single base insertion were also observed among the hprt cDNAs. Exon skipping and inclusion of hprt intron sequences in the hprt cDNA were observed in an additional 9/32 of the mutants. Analysis of T cell receptor (TCR) gene rearrangements revealed that six of eight mutants with an identical hprt T----A transversion displayed the same TCR rearrangement pattern, indicating that they were clonally related and arose from a single in vivo mutational event.

Base Sequence↗
PubMed1989

Formation of O6-methyldeoxyguanosine at specific sites in a synthetic oligonucleotide designed to resemble a known mutagenic hotspot.

Four synthetic oligodeoxyribonucleotides of the sequence 5'-CCG1TG2G3G4ATATGGGCTG-3' were constructed with a 1',2'-[3H]deoxyguanosine located at one of the four sites indicated (1, 2, 3, or 4). This sequence was derived from a region of the Escherichia coli xanthine-guanine phosphoribosyltransferase gene where position 4 is a site frequently mutated by N-methyl-N'-nitrosourea as compared to sites 1-3. These four oligomers were alkylated in both single- and double-stranded form with N-methyl-N'-nitrosourea, and the relative amount of O6-methyldeoxyguanosine (O6-MedGuo) formed at each position was quantitated. Up to a 5-6-fold greater formation of O6-MedGuo was observed at positions 3 and 4 as compared to positions 1 and 2. This uneven distribution was only observed in oligomers in the double-stranded form, suggesting that secondary structure was an important determinant in generating the uneven distribution of O6-MedGuo. Comparisons between the extent of O6-MedGuo formation and mutation frequency at the four positions suggest that a difference in the formation of promutagenic adducts at specific sites is just one of the factors involved in the generation of mutagenic "hotspots." The novel method developed was applied to the study of formation of O6-MedGuo at specific sites; however, it should be suitable for studying the formation and repair of DNA adducts generated by a variety of chemicals in a wide variety of DNA sequences.

Alkylation↗
PubMed1989

Cytogenetic analysis of spontaneous and 2-cyanoethylene oxide-induced tk-/- mutants in TK6 human lymphoblastoid cultures.

Two phenotypic classes of selectable tk-/- mutants have been isolated from the TK6 human lymphoblast cell line; one has a normal growth rate (tkn) relative to the parental cell line; the other has a slow growth rate (tks). Complete karyotypes of metaphase chromosomes were prepared and analyzed from 16 tks mutants (eight spontaneous and eight 2-cyanoethylene oxide [CNEtO]-induced), two spontaneous tkn mutants, and the parental TK6 cell line. Southern blot analysis of these tk-/- mutants indicated that all had lost a 14.8 kb polymorphic band corresponding to the active tk allele. No chromosome abnormalities with respect to the parental cell line TK6 were observed in eight spontaneous tks mutants. Chromosome abnormalities that may have been related to CNEtO treatment were observed in four of eight CNEtO mutants. However, chromosome 17, containing the tk locus in man, was cytologically normal with respect to the parental TK6 cell line in 15 of 16 tks mutants. A visible abnormality of chromosome 17 was present in one CNEtO-induced tks mutant. The abnormality was a duplication of the long arm of the chromosome 17, with break points at q11 and q21. The latter break point is close to the site of the tk locus, suggesting that the aberration observed may be associated with tk-/- phenotype. These observations contrast with the relatively high incidence (greater than or equal to 59%) of chromosome 11 abnormalities reported in tks (small colony) mouse lymphoma L5178Y/TK +/- mutants.

Cell Division↗
PubMed1989

Formaldehyde-induced and spontaneous alterations in human hprt DNA sequence and mRNA expression.

Human lymphoblast mutants at the X-linked hprt locus have been examined by Southern blot, Northern blot and DNA sequence analysis. A previous study had shown that approximately a third of the spontaneously-arising mutants and half those induced by formaldehyde showed no alteration in restriction fragment pattern and thus were classified as point mutations. In this report, Northern blot analysis was used to show that these point mutants fall into 4 categories: normal size and amount of RNA, normal size but reduced amounts, reduced size of RNA or no RNA. Sequence analyses of cDNAs prepared from hprt mRNAs were performed on 1 spontaneous and 7 formaldehyde-induced mutants with normal Northern blots. The spontaneous mutant was caused by an AT----GC transition. 6 of the formaldehyde-induced mutants were base substitutions, all of which occurred at AT base-pairs. There was an apparent hot spot, in that 4/6 independent mutants were AT----CG transversions at one specific site. The remaining mutant had lost exon 8.

Base Sequence↗