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H Vrieling

Publications and source records attributed to H Vrieling.

At least 55 records · Page 3Linked to original sources
PubMed1994

Neomorphic agouti mutations in obese yellow mice.

Several dominant mutations of the mouse agouti coat colour gene have pleiotropic effects that include obesity and a yellow coat. The Ay allele is caused by a large deletion that affects the expression of several contiguous genes. We show that three other obesity-associated agouti mutations, Aiy, Asy and Avy, are due to different molecular alterations that result in ubiquitous expression of a chimaeric RNA that encodes a normal agouti protein. The Aiy and Avy alleles are caused by insertion of an intracisternal A particle element 1 kb or 100 kb, respectively, upstream of agouti coding sequences. These results provide a model for other genes that show allele-specific imprinting, and demonstrate that molecular mechanisms typically responsible for activation of proto-oncogenes can also lead to other disease phenotypes.

Agouti Signaling Protein↗
PubMed1994

AT base pairs are the main target for mutations at the hprt locus of rat skin fibroblasts exposed in vitro to the monofunctional alkylating agent N-ethyl-N-nitrosourea.

Spectra of N-ethyl-N-nitrosourea (ENU)-induced mutations differ widely among various in vitro and in vivo mutational systems. To investigate possible reasons for these differences, a mutational system is needed in which the same target gene is used for comparison in the same type of cells in vitro and in vivo. In the present study, this was achieved by analysing at the molecular level 35 hprt mutant rat fibroblast clones obtained from cell populations exposed in vitro to ENU and comparing the mutational spectrum with the previously determined spectrum of ENU-induced hprt mutants in the same target cells exposed in vivo. Twenty-eight mutants contained a single base pair alteration in the hprt coding sequence. Most of these changes were found at AT base pairs (19/28), the AT to TA transversion being the most frequent kind of mutation (12/19), which is probably caused by O2-ethylthymine. Transversions at AT base pairs showed all mutated T's to be located in the nontranscribed strand of the hprt gene, suggesting a strand specific fixation of mutations induced by O2-ethylthymine, which appears to be a general feature of ENU- and ENNG-induced hprt mutations in mammalian cells. GC to AT transitions, probably caused by O6-ethylguanine, were detected at a lower frequency (7/28). This in vitro mutational spectrum was very similar to that of the same target cells exposed in vivo to ENU. A comparison of the mutational spectra in AGT-proficient and AGT-deficient rodent cells exposed to ethylating agents showed that in contrast to the situation in AGT-proficient rat fibroblasts, GC to AT base pair changes (and not AT to TA) are the predominant mutations in AGT-deficient hamster cells.

Alkylating Agents↗
PubMed1994

Pleiotropic effects of the mouse lethal yellow (Ay) mutation explained by deletion of a maternally expressed gene and the simultaneous production of agouti fusion RNAs.

Heterozygosity for the mouse lethal yellow (Ay) mutation leads to obesity, increased tumor susceptibility and increased activity of the agouti coat color gene; homozygosity for Ay results in embryonic death around the time of implantation. Although these pleiotropic effects have not been separated by recombination, previous studies have suggested that the dominant and recessive effects result from distinct genetic lesions. Here we use a combination of genomic and cDNA cloning experiments to demonstrate that the Ay mutation is caused by a 120 kb deletion which lies centromere-proximal to the agouti coat color gene. The deletion removes coding but not 5' untranslated sequences for a ubiquitously expressed gene predicted to encode a protein similar in sequence to an RNA-binding protein, which we named Merc, for maternally expressed hnRNP C-related gene, but have renamed Raly, since the gene is nearly identical to one reported recently by Michaud et al. (Gene Dev. 7, 1203-1213, 1993). The Ay deletion results in the splicing of Merc/Raly 5' untranslated sequences to agouti protein-coding sequences, which suggests that ectopic expression of the normal agouti protein by the Ay fusion RNA is responsible for the pleiotropic effects associated with heterozygosity for Ay. We find that Merc/Raly RNA is present in the unfertilized egg and is also transcribed in preimplantation embryos. Using a PCR-based assay to determine the genotype of individual embryos from an Ay/a x Ay/a intercross, we show that, in the absence of zygotic Merc/Raly expression, Ay/Ay embryos develop to the blastocyst stage, but do not hatch from the zona pellucida or form trophoblastic outgrowths. Injection of a Merc/Raly antisense oligonucleotide into non-mutant embryos blocks development prior to the blastocyst stage, and can be rescued by coinjection of a Merc/Raly transgene. These results suggest that maternal expression of Merc/Raly plays an important role in preimplantation development and that its deletion of is sufficient to explain Ay-associated embryonic lethality.

Agouti Signaling Protein↗
PubMed1993

Mutagenic processing of ethylation damage in mammalian cells: the use of methoxyamine to study apurinic/apyrimidinic site-induced mutagenesis.

The aldehyde reagent methoxyamine is able to interact with apurinic/apyrimidinic sites formed in vivo within cells and displays both an anti-cytotoxic and an antimutagenic activity on N-ethyl-N'-nitro-N-nitrosoguanidine-induced DNA damage in Chinese hamster ovary cells. To clarify the underlying mechanism we have examined the mutational spectra induced by N-ethyl-N'-nitro-N-nitrosoguanidine alone and in the presence of methoxyamine in the hypoxanthine-guanine phosphoribosyltransferase gene of Chinese hamster ovary cells. In both cases all mutations were base pair substitutions, and their distribution among various classes did not differ significantly. Almost 60% were transitions, predominantly GC to AT, and the remaining 40% were transversions, mainly at AT base pairs. The analysis of the proportion of the different types of mutations showed that in the presence of methoxyamine, GC to AT transitions decreased by a factor of 1.8, and AT to CG transversions were reduced by a factor of 13. These data indicate that in mammalian cells the fixation of ethylation damage into mutations occurs by both (a) direct mutagenesis likely driven by O6-ethylguanine adducts and to a minor extent by O4-ethylthymine and (b) apurinic/apyrimidinic site-mediated mutagenesis. These apurinic/apyrimidinic sites are formed during the processing of ethylation at critical sites and are likely to involve O6-ethylguanine and O2-ethylthymine adducts.

Alkylation↗
PubMed1993

UV-induced photolesions, their repair and mutations.

UV-induced cyclobutane pyrimidine dimers (CPD) are selectively removed from the transcribed strand of transcriptionally active genes in V79 Chinese hamster cells. This strand specificity of repair corresponds well with the observation that UV-induced mutations in the HPRT gene are primarily generated by DNA photolesions in the non-transcribed strand. This strand bias for mutations is, however, much more pronounced at 2 J/m2 than at the higher dose of 12 J/m2. An alternative explanation for strand specificity of mutations would be that most of the mutations are caused by pyrimidone 6-4 pyrimidine photoproducts (6-4 PP). Indeed experiments with a V79-derived cell line capable of repairing 6-4 PP but not CPD have revealed direct evidence for 6-4 PP as the mutagenic lesions in UV-irradiated hamster cells. This implies that 6-4 PP are also preferentially repaired in the transcribed strand. We have investigated the repair of DNA photolesions in the HPRT gene by measuring the distribution of bromodeoxyuridine-labeled repair patches in the transcribed and non-transcribed strands of genes employing a newly developed immunoextraction procedure. Three cell lines with different capacities to remove CPD and 6-4 PP from the HPRT gene and from the genome overall were used. We found no evidence for preferential repair of 6-4 PP in the transcribed strand of the HPRT gene in cells exposed to 10 J/m2. These data are in favor of a lack of strand-specific repair of 6-4 PP underlying the much less pronounced strand bias for induced mutations at high UV dose. However, the conclusive test would be the demonstration of preferential repair of 6-4 PP in the transcribed strand of transcriptionally active genes in cells exposed to 2 J/m2.

Animals↗
PubMed1993

Cloning of the mouse agouti gene predicts a secreted protein ubiquitously expressed in mice carrying the lethal yellow mutation.

The mouse agouti gene controls the deposition of yellow and black pigment in developing hairs. Several dominant alleles, including lethal yellow (Ay), result in the exclusive production of yellow pigment and have pleiotropic effects that include obesity and increased tumor susceptibility. In an interspecific backcross, we established genetic limits for the agouti gene and found that the Ay and the lethal non-agouti (ax) allele were not separated from a previously identified probe at the breakpoint of the Is1GsO chromosomal rearrangement. Using the Is1GsO probe, we isolated the agouti gene, and find that it has the potential to code for a secreted protein expressed in hair follicles and the epidermis, and that the level of expression correlates with the synthesis of yellow pigment. In the Ay mutation, there is a chromosomal rearrangement that results in the production of a chimeric RNA expressed in nearly every tissue of the body. The 5' portion of this chimeric RNA contains highly expressed novel 5' sequences, but the 3' portion retains the protein-coding potential of the nonmutant allele. We speculate that dominant pleiotropic effects of Ay are caused by ectopic activation of a signaling pathway similar to that used during normal hair growth.

Agouti Signaling Protein↗
PubMed1992

Spectrum of spontaneously occurring mutations in the hprt gene of V79 Chinese hamster cells.

A total of 76 independent spontaneous mutants in the hprt gene of V79 Chinese hamster cells have been analyzed. These mutants were obtained in two different laboratories, 17 and 59 mutants in sets 1 and 2, respectively, under different cell culture conditions. Mutation analysis was performed by amplification of hprt cDNA with the polymerase chain reaction and direct sequencing of the products. The data obtained showed similar spectra of spontaneous mutations in both sets of mutants, suggesting that culture does not play a major role in spontaneous mutagenesis. The majority of the mutations were base substitutions (greater than 60%), with twice as many transversions as transitions. Base changes were evenly distributed throughout the structural gene, including the splice junctions. All types of base substitutions appeared in comparable frequencies, except for A.T to T.A transversions, which were almost absent. The fraction of deletion mutations was low (13%). A striking feature of the observed mutation spectra is that one third of the spontaneous mutations analyzed involved aberrant splicing of the hprt primary transcript, with exon 4 being affected most frequently, indicating that splice mutations are a common mechanism of mutation in the hprt gene.

Animals↗
PubMed1992

Molecular analysis of mutations affecting hprt mRNA splicing in human T-lymphocytes in vivo.

Molecular analysis of hypoxanthine-guanine phosphoribosyltransferase (hprt) cDNA from 6-thioguanine-resistant T-lymphocytes cloned from smoking and non-smoking adult donors showed that 35% of these mutants were defective in splicing of hprt mRNA. Among a set of 42 hprt splice mutants, we observed i) complete loss of one or more exons, ii) partial loss of one exon, or iii) inclusion of part of an intron sequence between adjacent exons. Loss of exon 4 was significantly more frequent than of the other exons, suggesting that the sequences that regulate splicing of this exon are either larger than those of the other exons or especially prone to mutation. In order to identify the molecular nature of DNA alterations causing aberrant splicing of hprt mRNA, 17 splice mutants were analyzed in more detail by sequencing the genomic regions flanking the mis-spliced exon. Base pair substitutions or small deletions causing defective splicing were either detected in exon sequences or in splice site consensus sequences of introns. Furthermore, genomic deletions encompassing entire exons were found. In some mutants, the alteration responsible for incorrect splicing could not be identified, suggesting that the target sequence for splice mutations is larger than merely the splice junctions. Molecular characterization of hprt splice mutations will lead to the identification of specific sequences regulating splicing of hprt mRNA and will reveal whether the mutational spectrum in splice mutants is similar to that found in the hprt coding region.

Adult↗
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↗
PubMed1992

The gene encoding hypoxanthine-guanine phosphoribosyltransferase as target for mutational analysis: PCR cloning and sequencing of the cDNA from the rat.

In this paper, the cloning and nucleotide sequence of the cDNA of the rat gene coding for hypoxanthine-guanine phosphoribosyltransferase (hprt) is reported. Knowledge of the cDNA sequence is needed, among other reasons, for the molecular analysis of hprt mutations occurring in rat cells, such as skin fibroblasts isolated according to the granuloma pouch assay. The rat hprt cDNA was synthesized and used as a template for in vitro amplification by PCR. For this purpose, oligonucleotide primers were used, the nucleotide sequences of which were based on mouse and hamster hprt cDNA sequences. Sequence analysis of 1146 bp of the amplified rat hprt cDNA showed a single open reading frame of 654 bp, encoding a protein of 218 amino acids. In the predicted rat hprt amino acid sequence, the proposed functional domains for 5'-phosphoribosyl-1-pyrophosphate (PRPP) and nucleotide binding in phosphoribosylating enzymes as well as a region near the carboxyl terminal part were highly conserved when compared with amino acid sequences of other mammalian hprt proteins. Analysis of hprt amino acid sequences of 727 independent hprt mutants from human, mouse, hamster and rat cells bearing single amino acid substitutions revealed that a large variety of amino acid changes were located in these highly conserved regions, suggesting that all 3 domains are important for proper catalytic activity. The suitability of the hprt gene as target for mutational analysis is demonstrated by the fact that amino acid changes in at least 151 of the 218 amino acid residues of the hprt protein result in a 6-thioguanine-resistant phenotype.

Amino Acid Sequence↗
PubMed1992

(6-4) photoproducts and not cyclobutane pyrimidine dimers are the main UV-induced mutagenic lesions in Chinese hamster cells.

A partial revertant (RH1-26) of the UV-sensitive Chinese hamster V79 cell mutant V-H1 (complementation group 2) was isolated and characterized. It was used to analyze the mutagenic potency of the 2 major UV-induced lesions, cyclobutane pyrimidine dimers and (6-4) photoproducts. Both V-H1 and RH1-26 did not repair pyrimidine dimers measured in the genome overall as well as in the active hprt gene. Repair of (6-4) photoproducts from the genome overall was slower in V-H1 than in wild-type V79 cells, but was restored to normal in RH1-26. Although V-H1 cells have a 7-fold enhanced mutagenicity, RH1-26 cells, despite the absence of pyrimidine dimer repair, have a slightly lower level of UV-induced mutagenesis than observed in wild-type V79 cells. The molecular nature of hprt mutations and the DNA-strand specificity were similar in V79 and RH1-26 cells but different from that of V-H1 cells. Since in RH1-26 as well as in V79 cells most hprt mutations were induced by lesions in the non-transcribed DNA strand, in contrast to the transcribed DNA strand in V-H1, the observed mutation-strand bias suggests that normally (6-4) photoproducts are preferentially repaired in the transcribed DNA strand. The dramatic influence of the impaired (6-4) photoproduct repair in V-H1 on UV-induced mutability and the molecular nature of hprt mutations indicate that the (6-4) photoproduct is the main UV-induced mutagenic lesion.

Animals↗
PubMed1992

UV-induced hprt mutations in a UV-sensitive hamster cell line from complementation group 3 are biased towards the transcribed strand.

The molecular nature of 254 nm ultraviolet light (UV)-induced mutations at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus in UV24 Chinese hamster ovary (CHO) cells, which are defective in nucleotide excision repair, was determined. Sequence analysis of 19 hprt mutants showed that single base substitutions (9 mutants) and tandem base changes (7 mutants) dominated the UV mutation spectrum in this cell line. Sixty-five percent of the base substitutions were GC greater than AT transitions, whereas the rest consisted of transitions and transversions at AT base pairs. Analysis of the distribution of dipyrimidine sites over the two DNA strands, where the photoproducts causing these mutations presumably were formed, showed that 12 out of 14 mutations were located in the transcribed strand of the hprt gene. A similar strand distribution of mutagenic photoproducts as in UV24 has previously been found in two other UV-sensitive Chinese hamster cell lines (V-H1 and UV5), indicating that under defective nucleotide excision repair conditions the induction of mutations is strongly biased towards lesions in the transcribed strand of the hprt gene. A plausible explanation for this phenomenon is that during DNA replication large differences exist in the error rate with which DNA polymerase(s) bypass lesions in the templates for the leading and lagging strand, respectively.

Animals↗
PubMed1992

N-acetylated and deacetylated 4'-fluoro-4-aminobiphenyl and 4-aminobiphenyl adducts differ in their ability to inhibit DNA replication of single-stranded M13 in vitro and of single-stranded phi X174 in Escherichia coli.

Calf thymus single-stranded (ss) DNA was modified with the N-sulfate conjugate of N-hydroxy-2-acetylaminofluorene (N-OH-AAF), N-hydroxy-4'-fluoro-4-acetylaminobiphenyl (N-OH-FAABP) or N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) to yield predominantly N-acetylated adducts of 2-aminofluorene, 4-aminobiphenyl and 4'-fluoro-4-amino-biphenyl respectively to C8 of deoxyguanosine (dG-C8-AAF, dG-C8-AABP and dG-C8-FAABP). The modified DNAs were used as templates for in vitro DNA synthesis. DNA replication on the randomly primed template was inhibited as compared to control (unmodified) DNA to the same extent by all three types of adducts, irrespective of whether polymerization was performed by Escherichia coli DNA polymerase I, modified T7 DNA polymerase or Thermus aquaticus (Taq) DNA polymerase. In addition, all three types of adducts completely blocked replication of ss phi X174 in an E. coli host: on average one adduct per DNA molecule was sufficient to inactivate the bacteriophage. Polyacrylamide gel electrophoresis of DNA fragments synthesized by E. coli DNA polymerase I on FAABP- and AABP-modified ss M13mp9 DNA templates, showed that termination occurred predominantly one nucleotide before (and occasionally opposite) a modified deoxyguanosine in the template. However, the deacetylated adducts, dG-C8-AF, dG-C8-ABP and dG-C8-FABP (obtained by reacting DNA with their N-trifluoroacetyl-N-acetoxy esters) were frequently bypassed during replication of ss phi X174 in E. coli, though with different efficiencies: 1 out 7, 1 out of 2 and 1 out of 3 adducts on average respectively caused bacteriophage inactivation. Polyacrylamide gel electrophoresis showed that termination of DNA synthesis occurred at least as frequently opposite as 3' to a modified deoxyguanosine in the template.

Aminobiphenyl Compounds↗
PubMed1992

Enhanced hprt mutant frequency but no significant difference in mutation spectrum between a smoking and a non-smoking human population.

Recently, we have observed a small (36%), but significant, enhancement of the frequency of 6-thioguanine (6-TG)-resistant T-lymphocytes in blood from smokers. The molecular nature of 43 hypoxanthine-guanine phosphoribosyltransferase (hprt) mutant T-lymphocyte clones from nine smoking individuals was determined to investigate whether the increase in hprt mutant frequency would lead to a changed mutation spectrum. The types and distribution of hprt mutations in smokers was compared with those found in 55 6-TGr T-lymphocyte clones from 12 members of a control group of non-smokers. From this control group 25 hprt mutants were novel, whereas 31 have been described previously. Among smokers and non-smokers, a similar proportion of base substitutions (approximately 35%), mutations causing aberrant splicing (approximately 37%), frameshifts (approximately 16%) and deletions (approximately 9%) was found. In both groups, GC----AT base pair changes were found to be predominant among transitions. However, whereas all types of transversions were about equally represented in non-smokers, GC----TA transversions were not recovered among smokers. Investigation of the distribution of base substitutions over the hprt coding region showed no differences between the two groups. These data provide no clues on the nature of DNA adducts induced by smoking, which are thought to be responsible for the increased mutation frequency at the hprt locus in T-lymphocytes from smokers.

Adult↗
PubMed1991

Strand specificity for UV-induced DNA repair and mutations in the Chinese hamster HPRT gene.

DNA excision repair modulates the mutagenic effect of many genotoxic agents. The recently observed strand specificity for removal of UV-induced cyclobutane dimers from actively transcribed genes in mammalian cells could influence the nature and distribution of mutations in a particular gene. To investigate this, we have analyzed UV-induced DNA repair and mutagenesis in the same gene, i.e. the hypoxanthine phosphoribosyl-transferase (hprt) gene. In 23 hprt mutants from V79 Chinese hamster cells induced by 2 J/m2 UV we found a strong strand bias for mutation induction: assuming that pre-mutagenic lesions occur at dipyrimidine sequences, 85% of the mutations could be attributed to lesions in the nontranscribed strand. Analysis of DNA repair in the hprt gene revealed that more than 90% of the cyclobutane dimers were removed from the transcribed strand within 8 hours after irradiation with 10 J/m2 UV, whereas virtually no dimer removal could be detected from the nontranscribed strand even up to 24 hr after UV. These data present the first proof that strand specific repair of DNA lesions in an expressed mammalian gene is associated with a strand specificity for mutation induction.

Animals↗
PubMed1991

Strand-specific mutation spectra in repair-proficient and repair-deficient hamster cells.

The mutation spectrum induced by UV light has been determined at the hprt locus for both cultured normal (AA8) and UV-sensitive (UV-5) Chinese hamster ovary cells to investigate the effect of DNA repair on the nature of induced mutations. DNA base-pair changes of 23 hprt mutants of AA8 and of 28 hprt mutants of UV-5 were determined by sequence analysis of in vitro amplified hprt cDNA. Almost all mutants in AA8 carried single-base substitutions, transitions and transversions accounting for 38% and 62% of the base changes, respectively. In contrast, in repair-deficient cells (UV-5) tandem and nontandem double mutations represented a considerable portion of the mutations observed (30%), whereas the vast majority of base-pair substitutions were GC greater than AT transitions (87%). Moreover, 5 splice mutants and 2 frameshift mutations were found in the UV-5 collection. In almost all mutants analyzed base changes were located at dipyrimidine sites where UV photoproducts could have been formed. In AA8 the photolesions causing mutations were predominantly located in the nontranscribed strand whereas a strong bias for mutation induction towards photolesions in the transcribed strand was found in UV-5. We hypothesize that preferential removal of lesions from the transcribed strand of the hprt gene accounts for the observed DNA strand specificity of mutations in repair-proficient cells. Furthermore, differences in the degree of misincorporation opposite a lesion for lagging and leading strand DNA synthesis may dictate the pattern of UV-induced mutations in the absence of DNA repair.

Animals↗