Abnormal ultraviolet mutagenic spectrum in plasmid DNA replicated in cultured fibroblasts from a patient with the skin cancer-prone disease, xeroderma pigmentosum

Exome sequencing identifies novel mutation signatures of UV radiation and trichostatin A in primary human keratinocytes

Canonical ultraviolet (UV) mutation type and spectra are traditionally defined by direct sequencing-based approaches to map mutations in a limited number of representative DNA elements. To obtain an unbiased view of genome wide UV mutation features, we performed whole exome-sequencing (WES) to profile single nucleotide substitutions in UVB-irradiated primary human keratinocytes. Cross comparison of UV mutation profiles under different UVB radiation conditions revealed that T > C transition was highly prevalent in addition to C > T transition. We also identified 5'-ACG-3' as a common sequence motif of C > T transition. Furthermore, our analyses uncovered several recurring UV mutations following acute UVB radiation affecting multiple genes including HRNR, TRIOBP, KCNJ12, and KMT2C, which are frequently mutated in skin cancers, indicating their potential role as founding mutations in UV-induced skin tumorigenesis. Pretreatment with trichostatin A, a pan-histone deacetylase inhibitor that renders chromatin decondensation, significantly decreased the number of mutations in UVB-irradiated keratinocytes. Unexpectedly, we found trichostatin A to be a mutagen that caused DNA damage and mutagenesis at least partly through increased reactive oxidation. In summary, our study reveals new UV mutation features following acute UVB radiation and identifies novel UV mutation hotspots that may potentially represent founding driver mutations in skin cancer development.

UV signature mutations

Sequencing complete tumor genomes and exomes has sparked the cancer field's interest in mutation signatures for identifying the tumor's carcinogen. This review and meta-analysis discusses signatures and their proper use. We first distinguish between a mutagen's canonical mutations—deviations from a random distribution of base changes to create a pattern typical of that mutagen—and the subset of signature mutations, which are unique to that mutagen and permit inference backward from mutations to mutagen. To verify UV signature mutations, we assembled literature datasets on cells exposed to UVC, UVB, UVA, or solar simulator light (SSL) and tested canonical UV mutation features as criteria for clustering datasets. A confirmed UV signature was: ≥60% of mutations are C→T at a dipyrimidine site, with ≥5% CC→TT. Other canonical features such as a bias for mutations on the nontranscribed strand or at the 3' pyrimidine had limited application. The most robust classifier combined these features with criteria for the rarity of non-UV canonical mutations. In addition, several signatures proposed for specific UV wavelengths were limited to specific genes or species; UV's nonsignature mutations may cause melanoma BRAF mutations; and the mutagen for sunlight-related skin neoplasms may vary between continents.

Nonmelanoma skin cancer in organ transplant patients

Nonmelanoma skin cancer (NMSC) is more frequent in immunocompromised patients, for example, patients with organ transplants. A number of studies have been published from different countries that present a similar picture of tumors in transplant patients. In addition, the behavior of these tumors is often more aggressive in this group of high-risk patients. The multitude of NMSC and precancerous lesions presents a clinical diagnostic and therapeutic challenge to the managing dermatologists. Technology is being developed to cope with the clinical diagnosis and medical adjunct treatment to broaden the therapeutic options. It is suggested that the optimal use of these new developments occurs if patients are seen in specialized clinics aimed at providing preventive measures, diagnosis, and treatment.

Mutation spectra in supF: approaches to elucidating sequence context effects

Shuttle vectors carrying the supF suppressor tRNA gene were originally developed for mutagenesis experiments in primate and human cells. Since then, the supF gene has been used as a mutation reporter in other mammalian cells, yeast, Escherichia coli, and transgenic mice. The widespread use of the vector for studies of many DNA reactive agents has produced a large database of mutation spectra. These provide primary information on the kinds and distribution of mutations provoked by many agents and, in many instances, allow comparisons between related agents or the same agent in different cell backgrounds. In this review we will discuss some of these data with a primary focus on the interpretation of UV mutation spectra. We will also describe our development and application of custom supF marker genes as an approach to studying the effect of sequence context on mutation hotspots and cold spots. Our studies suggest that C-C photoproducts are not mutagenic in certain sequence contexts in which T-C photoproducts are mutation hotspots. In addition, we have found several examples of sequence context effects acting as much as 80 bases away from the site of mutation. We will consider some of the problems raised by these studies and the possible resolution of some of them offered by the newly discovered family of damage bypass DNA polymerases.

Expression of a mammalian DNA photolyase confers light-dependent repair activity and reduces mutations of UV-irradiated shuttle vectors in xeroderma pigmentosum cells

Photoreactivation is one of the DNA repair mechanisms to remove UV lesions from cellular DNA with a function of the DNA photolyase and visible light. Two types of photolyase specific for cyclobutane pyrimidine dimers (CPD) and for pyrimidine (6-4) pyrimidones (6-4PD) are found in nature, but neither is present in cells from placental mammals. To investigate the effect of the CPD-specific photolyase on killing and mutations induced by UV, we expressed a marsupial DNA photolyase in DNA repair-deficient group A xeroderma pigmentosum (XP-A) cells. Expression of the photolyase and visible light irradiation removed CPD from cellular DNA and elevated survival of the UV-irradiated XP-A cells, and also reduced mutation frequencies of UV-irradiated shuttle vector plasmids replicating in XP-A cells. The survival of UV-irradiated cells and mutation frequencies of UV-irradiated plasmids were not completely restored to the unirradiated levels by the removal of CPD. These results suggest that both CPD and other UV damage, probably 6-4PD, can lead to cell killing and mutations.

The effect of donor age on the processing of UV-damaged DNA by cultured human cells: reduced DNA repair capacity and increased DNA mutability

Aging in humans carries an increased risk of skin cancer, a disorder linked to somatic mutations in sun damaged skin. DNA repair plays a major role in protection against sun damage. We found an age-related decline in post-UV DNA repair capacity (measured by the ability to repair a UV-treated plasmid (pCMVcat)) of-0.6% per year (p = 0.0001) in cultured primary skin fibroblasts from normal donors from the first to the tenth decade of life. There was a corresponding age-related increase in post-UV mutability (measured as mutations introduced into a transfected, UV-treated plasmid (pSP189)) of +0.6% per year (p = 0.001) in lymphoblastoid cell lines from normal donors of the same age range. This study indicates that aging in humans is associated with decreasing ability to process new UV-induced DNA damage and this age-related reduction in DNA repair capacity and increase in DNA mutability is reflected in cultured skin and blood cells.

Inhibition of initiation of simian virus 40 DNA replication during acute response of cells irradiated by ultraviolet light

To study the mechanism by which ultraviolet (UV) light inhibits DNA replication, we examined the effects of UV 254 nm irradiation on the replication of simian virus 40 (SV40) DNA and SV40-based plasmid in monkey cells. The study was designed to determine the relative contributions made by inhibition of replication initiation and chain elongation to the immediate inhibition of DNA replication following UV irradiation. We used two-dimensional neutral-alkaline electrophoresis to examine the behaviour of replication intermediates unambiguously. Kinetic analysis using this technique showed that initiation of replication started to decline at 15 min post-irradiation. When the pulse label incorporated in SV40 replication intermediates before irradiation was chased for 1 h, most of the label was found in mature Form I and II molecules. This indicated that replication elongation took place on damaged template. We also used a transfection technique to show that heavily irradiated plasmids replicated efficiently in unirradiated transfected cells. By the transfection technique, we observed that UV irradiation of host cells dose-dependently inhibited replication of transfected non-irradiated plasmids, suggesting that the inhibition of DNA replication is due to a global change in cellular physiology induced by UV. This change was also apparent from poor staining of the chromatin by fluorescent-DNA-binding dyes immediately after UV irradiation of intact cells. We conclude that a significant fraction of chain elongation proceeds on damaged templates and DNA replication during the acute response of cells irradiated with UV is mainly controlled by the inhibition of replication initiation.

Analysis of mutagenesis in UV-sensitive mouse lymphoma L5178Y-R cells with a polyomavirus-based shuttle vector

A newly-developed polyomavirus-based shuttle vector, pPySLPT-2, was used to analyze mutations induced by UV radiation in the two related mouse lymphoma cell lines, L5178Y-R (LY-R) and L5178Y-S (LY-S). These well-studied cell lines differ in sensitivity to UV radiation, apparently due to differences in DNA repair capacity. Consistent with these differences, we found that replication of UV-irradiated vector is inhibited to a greater extent in the UV sensitive LY-R cells than in the LY-S cells. Mutations were induced in the vector's supF target gene at a higher frequency in the LY-R cells than in the LY-S cells, but the sequence characteristics of the base substitution mutations were similar in the two cell lines. The pattern of UV-induced mutations in the supF target gene of this polyomavirus-based vector, replicated in the mouse cells, was very similar to the pattern observed in the same target gene in the analogous simian virus 40-based vector, pZ189, replicated in monkey and human cells.

Far less frequent mutations in ras genes than in the p53 gene in skin tumors of xeroderma pigmentosum patients

Mutations in Ha-ras, Ki-ras, and N-ras genes in squamous and basal cell carcinomas in patients with xeroderma pigmentosum (XP) were examined by the polymerase chain reaction followed by single-strand conformation polymorphism analysis and direct base sequencing. No mutation was detected in codons 12, 13, and 61 of the ras genes in XP skin tumors. This was in contrast with previous findings of a high frequency of mutation in the p53 gene in skin tumors in XP patients. A novel mutation in codon 6 of the Ki-ras gene was detected in a squamous cell carcinoma. The mutation was a C-->T transition at a dipyrimidine (5'-CT) sequence and could have been produced by solar ultraviolet light. The mutated ras gene did not have the ability to transform NIH/3T3 cells. In three tumors, multiple base substitutions were detected in exon 1 of the Ki-ras and N-ras genes. These results and our previous work on p53 gene mutations suggest that mutations in ras genes are far less frequent than in the p53 gene in the skin tumors in XP patients and that ras genes are less important in skin tumorigenesis in XP patients than is the p53 gene.

A potential laboratory test for dysplastic nevus syndrome: ultraviolet hypermutability of a shuttle vector plasmid

The diagnosis of the melanoma-prone disorder dysplastic nevus syndrome (DNS) is based currently on a combination of clinical and histopathologic examinations of patients. To develop a potential laboratory test for DNS, we utilized the observation that an ultraviolet light (UV)-treated mutagenesis plasmid shuttle vector has an abnormally increased frequency of mutations after transfection into lymphoblastoid cells from a patient with familial DNS. pSP189 (containing the bacterial suppressor tRNA gene supF as a marker for mutations and a gene for ampicillin resistance for selection) was treated with UV and transfected into familial DNS, xeroderma pigmentosum complementation group A (XP-A), and normal lymphoblastoid cells by electroporation or diethylaminoethyl (DEAE) dextran. Untreated plasmid pZ189K (containing a gene for kanamycin resistance) was co-transfected as an internal standard to reduce the variability of plasmid survival measurements. After 2 d, plasmids were extracted, used to transform an indicator strain of Escherichia coli, and assayed on plates containing ampicillin or kanamycin. Counting light blue or white colonies (containing mutated supF in the plasmid) and blue colonies (with wild type supF) permitted measurement of the plasmid survival and mutation frequency. Transfection by electroporation or DEAE dextran resulted in abnormally reduced survival of UV-treated plasmid after passage through the XP-A but normal survival in the three DNS lines. Transfection of UV-treated plasmid by DEAE dextran yielded a greater hypermutability with the familial DNS lines than by electroporation. These results suggest that pSP189 UV hypermutability with normal UV survival using DEAE dextran transfection may form the basis of a potential laboratory assay for familial DNS.

A reduced level of multiple mutation in a shuttle vector passaged in Sjögren's syndrome cells

Sjögren's syndrome is a systemic disorder with unknown etiology, displaying many signs of autoimmunity. Although the basic mechanism of this disease is unknown, a defect in somatic mutagenesis of antibody genes has been suggested. Using a shuttle vector plasmid, we here show that the number of vectors with multiple base changes in a marker gene was reduced in B cell lines from two patients with Sjögren's syndrome (8% in both), as compared with values reported for cell lines from normal human donors (16-27%). This finding suggests that a reduction of the rate of somatic mutagenesis may influence the development of symptoms in Sjögren patients.

Ultraviolet hypermutability of a shuttle vector propagated in xeroderma pigmentosum variant cells

Patients with the variant form of xeroderma pigmentosum (XP) have clinical XP including a high frequency of skin cancer but, in contrast to the other forms of XP, have normal post-ultraviolet (UV) DNA excision repair and nearly normal post-UV survival. However, like excision repair-deficient XP cells, the XP variant cells are UV hypermutable. We used a UV-treated plasmid shuttle vector, pZ189, to examine the DNA repair defect in lymphoblastoid cells from an XP variant patient, XPPHBE, and a normal control. Plasmid repair, mutagenesis, and replication occur within transfected cells in a process dependent on the cells' repair capacity. With the XP variant cells post-UV, plasmid survival was normal with but there was an abnormally increased post-UV plasmid mutation frequency. Sequence analysis of the mutated plasmids revealed an increased frequency of plasmids with single base substitution mutations with the XP variant cells. As in earlier studies with UV mutagenesis, there was a predominance of G:C-->A:T base substitution mutations with plasmids recovered from both cell lines. The frequency of G:C-->C:G transversions was significantly higher with plasmids recovered from the XP variant cells than from normal cells. The location of mutations in the marker gene was non-random with different mutagenic hotspots found in plasmids recovered from the XP variant cells and from the normal cells. This study suggests that plasmid UV hypermutability in the presence of normal UV survival may be related to the increased UV skin cancer susceptibility of XP variant patients.

Analysis of mutations caused by DNA double-strand breaks produced by a restriction enzyme in shuttle vector plasmids propagated in ataxia telangiectasia cells

Rejoining of DNA double-strand breaks (DSB) produced by a restriction endonuclease AvaI in the supF gene in a plasmid pZ189Ava, and mutations presumably due to the altered rejoinings were analyzed. After allowing the rejoining and replication of the plasmids in human cells originating from normal subjects and ataxia telangiectasia (AT) patients, the plasmids were retrieved and those containing mutated supF were screened in an indicator strain of Escherichia coli. The proportion of correctly rejoined plasmids was significantly lower in AT cells than in normal cells, suggesting that AT cells have lower fidelity in rejoining DSB. DNA sequencing of the mutated supF genes revealed that all mutations were deletions or insertions occurring exactly or closely at the rejoining site in both normal and AT cells. In AT cells, the majority of mutations were deletions, while deletions and insertions were evenly formed in normal cells. AT cells may be deficient in the mechanism to protect the broken ends of DNA strands from the exonucleolytic digestion.

Ultraviolet mutagenesis in human lymphocytes: the effect of cellular transformation

We have assessed the role of cellular transformation in ultraviolet (uv)-induced mutagenic events in human cells. To maintain uniformity of genetic background and to eliminate the effect of DNA repair, primary nontransformed lymphocytes (T-cells) and Epstein-Barr virus-transformed lymphocytes (B-cells) from one patient (XP12Be) with the DNA repair-deficient disorder xeroderma pigmentosum (group A) were transfected with the mutagenesis shuttle vector pZ189. Parallel control experiments were performed with primary, nontransformed lymphocytes from a normal individual and with a repair-proficient Epstein-Barr virus-transformed lymphocyte line (KR6058). pZ189 was treated with uv and introduced into the four cell lines by electroporation. Plasmid survival and mutations inactivating the marker supF suppressor tRNA gene in the recovered pZ189 were scored by transforming an indicator strain of Escherichia coli. Plasmid survival was reduced and mutation frequency elevated equally with both XP-A cell lines compared to both normal cell lines. Base sequence analysis of more than 250 independent plasmids showed that while the G:C----A:T base substitution mutation was found in at least 60% of plasmids with single or tandem mutations with all four cell lines, the frequency with the transformed XP-A (93%) cells was significantly higher (P less than 0.01) than that with the nontransformed XP-A cells (77%). In addition, with the transformed XP-A cells, there were significantly fewer plasmids with transversions and with mutations at a transversion hotspot (base pair 134) than with plasmids recovered from nontransformed XP-A cells. Interleukin-2 and phytohemagglutinin (used to maintain growth of the nontransformed lymphocytes) treatment of transformed XP12Be cells did not change overall plasmid survival or mutation frequency, but increased the transversion frequency and induced a mutational hotspot (at base pair 159), while another mutational hotspot (at base pair 123) disappeared. Thus we have demonstrated that in repair-deficient human cells, cellular transformation, while not affecting overall postuv plasmid survival and mutation frequency, does increase the susceptibility to G:C----A:T transition mutations, a type of mutation associated with uv-induced neoplasia.

UV-induced base substitution mutations in a shuttle vector plasmid propagated in group C xeroderma pigmentosum cells

To assess the contribution to mutagenesis of human DNA repair defects, the UV-irradiated shuttle vector plasmid pZ189 was propagated in fibroblasts derived from a xeroderma pigmentosum (XP) patient in DNA repair complementation group C. In comparison to results with DNA repair-proficient human cells (WI-38 VA13), UV-irradiated pZ189 propagated in the XP-C (XP4PA(SV)) cells showed fewer surviving plasmids and a higher frequency of mutated plasmids. Base sequence analysis of 67 mutated plasmids recovered from the XP-C cells revealed similar classes of point mutations and mutation spectrum, and a higher frequency of G:C to A:T transitions along with a lower frequency of transversions among plasmids with single or tandem mutations compared to plasmids recovered from the normal line. Most single-base substitution mutations (83%) occurred at G:C base pairs in which the 5'-adjacent base of the cytosine was thymine or cytosine. These results indicate that the DNA repair defects in XP-C, in comparison to data previously reported for XP-A, XP-D and XP-F, result in different UV survival and mutation frequency but in similar types of base substitution mutations.

Modulation of an ultraviolet mutational hotspot in a shuttle vector Xeroderma cells

Ultraviolet mutagenesis of the shuttle vector plasmid pZ189 in Xeroderma Pigmentosum cells yields a mutational pattern marked by hotspots at photoproduct sites on both strands of the supF marker gene. In order to test the influence of strand orientation on the appearance of hotspots the mutagenesis study was repeated on a vector with the supF gene in the inverted orientation. We recovered a pattern the same as that in the earlier work and conclude that the nature of the DNA polymerase involved in the replication of specific strands is not a primary determinant of hotspot occurrence in this system. One of the hotspots lies in an 8 base palindrome while the corresponding site on the other strand was not a hotspot. These results were obtained with calcium phosphate transfection of the UV treated vector. When DEAE dextran was used as a transfection agent both sites in the palindrome were hotspots. In a mixing experiment the calcium phosphate pattern was recovered. Our data suggest that the sequence determinants of mutational probability at these two sites lie outside the 8 bases of the palindrome and that mutagenesis at one, but not the other, site is sensitive to perturbation of cellular calcium levels.

UV mutation spectra in cell lines from patients with Cockayne's syndrome and ataxia telangiectasia, using the shuttle vector pZ189

We have used the SV40-based shuttle vector pZ189 to determine ultraviolet mutation spectra in SV40-transformed cell lines from two patients with Cockayne's syndrome (CS) and ataxia telangiectasia (AT). The shuttle vector was UV-irradiated, transfected into the cells and recovered two days later, after many rounds of replication had occurred. Plasmid DNA was used to transform indicator bacteria in which plasmids containing a mutation in the supF gene resulted in white colonies. Mutant plasmids were analysed both by agarose gels and by DNA sequencing. In contrast to published spectra for xeroderma pigmentosum cells, the types of mutation induced by UV mutation in the CS and AT cell lines were similar to each other and to published spectra for normal cell lines. There were however, some differences in the sequence distribution of the mutations.

Molecular analysis of ultraviolet-induced mutations in a xeroderma pigmentosum cell line

We have isolated and characterized 47 ultraviolet light-induced hprt mutants from a simian virus 40-transformed excision-repair-deficient xeroderma pigmentosum cell line (complementation group A). Twenty-one independent mutations were found, of which the majority were point mutations. Eleven of these were identified as base changes, nine of which could be attributed to ultraviolet damage on the transcribed DNA strand. Both transitions and transversions were found among the single base changes. A large proportion of the mutations (13/21) resulted in aberrant splicing of the hprt gene, suggesting that the target size for mutations resulting in aberrant splicing must be quite large. A small number of spontaneous mutations were identified, most of which were large deletions. Our data provide a spectrum for the intrinsic mutations resulting from ultraviolet damage in human cells in the absence of repair.

Ultraviolet mutational spectrum in a shuttle vector propagated in xeroderma pigmentosum lymphoblastoid cells and fibroblasts

In order to examine possible cell-type specificity in mutagenic events, a shuttle-vector plasmid, pZ189, carrying a bacterial suppressor tRNA marker gene, was treated with ultraviolet radiation and propagated in Epstein-Barr virus transformed lymphoblastoid cell lines from a patient, XP12BE, with xeroderma pigmentosum (XP), group A, and a normal control. XP is a skin-cancer-prone disorder with UV hypersensitivity and defective DNA repair. Plasmid survival and mutations inactivating the marker gene were scored by transforming an indicator strain of E. coli. An earlier report on this data [Seetharam et al., (1990) J. Mol. Biol., 212, 433] indicated lower survival and higher mutation frequency with the UV-treated plasmid passed through the XP12Be(EBV) line. In the present report, sequence analysis of 198 mutant plasmids revealed a predominance of G:C----A:T transitions with both lymphoblastoid cell lines. This finding is consistent with the bias of polymerases toward insertion of an adenine opposite non-coding photoproducts (dinucleotides or other lesions). Transversion mutagenesis, non-adjacent double mutations, and triple-base mutations may involve other mechanisms. These results were compared to similar data from a fibroblast line from the same patient [Bredberg et al., (1986) Proc. Natl. Acad. Sci. (U.S.A.), 83, 8273]. The frequency of G:C----A:T transitions was higher, and there were fewer plasmids with multiple-base substitutions and with transversion mutations with both XP lymphoblasts and fibroblasts than with the normal lymphoblasts and fibroblasts. There were no significant differences in classes or types of mutations in the UV-treated plasmid replicated in the XP lymphoblasts and the XP fibroblasts. This suggests that the major features of UV mutagenesis in different cell types from the same individual are similar.

Mutational hotspot variability in an ultraviolet-treated shuttle vector plasmid propagated in xeroderma pigmentosum and normal human lymphoblasts and fibroblasts

The mutagenesis shuttle vector, pZ189, was treated with ultraviolet (u.v.) radiation in vitro and passed through a DNA repair-deficient lymphoblastoid cell line derived from a patient with xeroderma pigmentosum complementation group A (XP-A) (XP12BE(EBV)) and a DNA repair-proficient lymphoblastoid cell line (GM606(EBV)). After u.v. treatment, plasmid survival was lower and mutation frequency higher with the XP-A cells mirroring the survival and mutagenesis of the host cells. The nature of the mutations in the suppressor tRNA marker gene was determined by direct sequence analysis. The G.C to A.T transition was the dominant (85%) base substitution mutation with the XP lymphoblasts and was the major (56%) base substitution mutation with the repair-proficient lymphoblasts. We found a G.C to A.T transition mutational hotspot with the XP lymphoblasts not seen in our previous experiments with fibroblasts from the same patient. Comparison of the data presented here with our results with DNA repair-deficient and DNA repair-proficient fibroblasts suggests that hotspot variability is not due to genetic polymorphism or repair capacity of the cells. Instead it appears that cellular factors can influence the probability of mutagenesis of modified DNA at particular sites.

DNA base sequence changes induced by ultraviolet light mutagenesis of a gene on a chromosome in Chinese hamster ovary cells

The DNA base sequence changes induced by mutagenesis with ultraviolet light have been determined in a gene on a chromosome of cultured Chinese hamster ovary (CHO) cells. The gene was the Escherichia coli gpt gene, of which a single copy was stably incorporated and expressed in the CHO cell genome. The cells were irradiated with ultraviolet light and gpt- colonies were selected by resistance to 6-thioguanine. The gpt gene was amplified from chromosomal DNA by use of the polymerase chain reaction (PCR), and the amplified DNA sequenced directly by the dideoxy method. Of the 58 sequenced mutants of independent origin 53 were base change mutations. Forty-one base substitutions were single base changes, ten had two adjacent (or tandem) base changes, and one had two base changes separated by a single base-pair. Only one mutant had a multiple base change mutation with two or more well separated base changes. In contrast much higher levels of such mutations were reported in ultraviolet mutagenesis of genes on a shuttle vector in primate cells. Two deletions of a single base-pair were observed and three deletions ranging from 6 to 37 base-pairs. The mutation spectrum in the gpt gene had similarities to the ultraviolet mutation spectra for several genes in prokaryotes, which suggests similarities in mutational mechanisms in prokaryotes and eukaryotes.

DNA strand specificity for UV-induced mutations in mammalian cells

The influence of DNA repair on the molecular nature of mutations induced by UV light (254 nm) was investigated in UV-induced hprt mutants from UV-sensitive Chinese hamster cells (V-H1) and the parental line (V79). The nature of point mutations in hprt exon sequences was determined for 19 hprt mutants of V79 and for 17 hprt mutants of V-H1 cells by sequence analysis of in vitro-amplified hprt cDNA. The mutation spectrum in V79 cells consisted of single- and tandem double-base pair changes, while in V-H1 cells three frameshift mutations were also detected. All base pair changes in V-H1 mutants were due to GC----AT transitions. In contrast, in V79 all possible classes of base pair changes except the GC----CG transversion were present. In this group, 70% of the mutations were transversions. Since all mutations except one did occur at dipyrimidine sites, the assumption was made that they were caused by UV-induced photoproducts at these sites. In V79 cells, 11 out of 17 base pair changes were caused by photoproducts in the nontranscribed strand of the hprt gene. However, in V-H1 cells, which are completely deficient in the removal of pyrimidine dimers from the hprt gene and which show a UV-induced mutation frequency enhanced seven times, 10 out of 11 base pair changes were caused by photoproducts in the transcribed strand of the hprt gene. We hypothesize that this extreme strand specificity in V-H1 cells is due to differences in fidelity of DNA replication of the leading and the lagging strand. Furthermore, we propose that in normal V79 cells two processes determine the strand specificity of UV-induced mutations in the hprt gene, namely preferential repair of the transcribed strand of the hprt gene and a higher fidelity of DNA replication of the nontranscribed strand compared with the transcribed strand.

DNA strand specificity for UV-induced mutations in mammalian cells

The influence of DNA repair on the molecular nature of mutations induced by UV light (254 nm) was investigated in UV-induced hprt mutants from UV-sensitive Chinese hamster cells (V-H1) and the parental line (V79). The nature of point mutations in hprt exon sequences was determined for 19 hprt mutants of V79 and for 17 hprt mutants of V-H1 cells by sequence analysis of in vitro-amplified hprt cDNA. The mutation spectrum in V79 cells consisted of single- and tandem double-base pair changes, while in V-H1 cells three frameshift mutations were also detected. All base pair changes in V-H1 mutants were due to GC----AT transitions. In contrast, in V79 all possible classes of base pair changes except the GC----CG transversion were present. In this group, 70% of the mutations were transversions. Since all mutations except one did occur at dipyrimidine sites, the assumption was made that they were caused by UV-induced photoproducts at these sites. In V79 cells, 11 out of 17 base pair changes were caused by photoproducts in the nontranscribed strand of the hprt gene. However, in V-H1 cells, which are completely deficient in the removal of pyrimidine dimers from the hprt gene and which show a UV-induced mutation frequency enhanced seven times, 10 out of 11 base pair changes were caused by photoproducts in the transcribed strand of the hprt gene. We hypothesize that this extreme strand specificity in V-H1 cells is due to differences in fidelity of DNA replication of the leading and the lagging strand. Furthermore, we propose that in normal V79 cells two processes determine the strand specificity of UV-induced mutations in the hprt gene, namely preferential repair of the transcribed strand of the hprt gene and a higher fidelity of DNA replication of the nontranscribed strand compared with the transcribed strand.

Use of supF, an Escherichia coli tyrosine suppressor tRNA gene, as a mutagenic target in shuttle-vector plasmids

The Escherichia coli tyrosine amber suppressor tRNA gene, supF, has been utilized as a mutagenic target in several shuttle-vector plasmids. Data on mutagenic inactivation of suppressor activity was obtained from induced mutagenesis experiments with plasmids pZ189 and p3AC, and from studies on alterations of the supF gene transduced into E. coli. 162 single or tandem base-substitution mutations that reduce or eliminate suppressor activity were identified at 86 sites within 158 base pairs. The 2 transition and 4 transversion mutations possible in double-stranded DNA were all detectable. At 56 sites two different inactivating mutations were found; and at 20 sites all 3 possible base substitution mutations inactivated suppressor function. Most of the mutations were clustered within the mature tRNA region: 144 of the base-substitution mutations were found at 74 sites within the 85-bp mature tRNA region. Insertions of 1 or 2 bases at 4 sites and deletions of 1 to 3 bases at 15 sites were found to inactivate supF function. A few silent mutations which do not inactivate suppressor function were found: single base-substitutions at 4 sites, 14 pairs of silent double mutations, and a large deletion including the promoter region. The supF gene is thus an extremely sensitive target for mutagenic inactivation in shuttle-vector plasmids.