High sensitivity of Xeroderma pigmentosum cells to the carcinogen 4-nitroguinoline-1-oxide

4-nitroquinoline-1-oxide-induced mutagen sensitivity and risk of cutaneous melanoma: a case-control analysis

Mutagen sensitivity assay, which measures the enhanced cellular response to DNA damage induced in vitro by mutagens/carcinogens, has been used in the study of cancer susceptibility. 4-Nitroquinoline-1-oxide (4-NQO), an ultraviolet (UV) radiation-mimetic chemical, can produce chromosomal breaks in mammalian cells and induce cancer. Given the potential role of 4-NQO as the experimental mutagen substituting for UV as the etiological carcinogen of cutaneous melanoma (CM), we tested the hypothesis that cellular sensitivity to 4-NQO is associated with the risk of developing CM in a case-control study of 133 patients with primary CM and 176 cancer-free controls. Short-term blood cultures were treated with 4-NQO at a final concentration of 10 μmol/l for 24 h and scored chromatid breaks in 50 well-spread metaphases. Multivariate logistic regression was used to calculate odds ratios and 95% confidence intervals. We found that the log-transformed frequency of chromatid breaks was significantly higher in 133 patients than in 176 controls (P=0.004) and was associated with an increased risk for CM (adjusted odds ratio=1.78, 95% confidence interval: 1.12-2.84) after adjustment for age and sex. Moreover, as the chromatid break values increased, the risk for CM increased in a dose-dependent manner (P(trend)=0.003). Further analysis explored a multiplicative interaction between the sensitivity to 4-NQO and a family history of skin cancer (P(interaction)=0.004) on the risk of CM. Therefore, our findings suggest that sensitivity to 4-NQO may be a risk factor for the risk of CM, which is more sensitive than UV-induced chromotid breaks.

Xeroderma pigmentosum group A gene action as a protection factor against 4-nitroquinoline 1-oxide-induced tongue carcinogenesis

To test the hypothesis that nucleotide excision repair (NER) plays a protective role in chemical carcinogenesis in internal organs, xeroderma pigmentosum group A gene-deficient (XPA(-/-)) mice, heterozygous (XPA(+/-)) and wild-type (XPA(+/+)) mice were orally administered 0.001% 4-nitroquinoline 1-oxide (4NQO) in their drinking water and compared. After 50 weeks of 4NQO exposure, tongue squamous cell carcinomas (SCCs) occurred in XPA(-/-) mice only, no tumors being observed in XPA(+/-) and XPA(+/+) animals. Of the XPA(-/-) mice 86% had tumors and 100% demonstrated multiple foci of dysplastic epithelium in the tongue. Accumulation of p53 protein was immunohistochemically detected in 56% of the SCCs. Mutational analysis of the p53 gene (exons 4-10) in carcinoma DNA revealed missense mutations in exons 5 and 9 in four of 20 samples. Our results clearly demonstrate that the NER gene XPA acts as a defensive factor against 4NQO-induced tongue carcinogenesis in vivo.

The ATPase domain but not the acidic region of Cockayne syndrome group B gene product is essential for DNA repair

Cockayne syndrome (CS) is a human genetic disorder characterized by UV sensitivity, developmental abnormalities, and premature aging. Two of the genes involved, CSA and CSB, are required for transcription-coupled repair (TCR), a subpathway of nucleotide excision repair that removes certain lesions rapidly and efficiently from the transcribed strand of active genes. CS proteins have also been implicated in the recovery of transcription after certain types of DNA damage such as those lesions induced by UV light. In this study, site-directed mutations have been introduced to the human CSB gene to investigate the functional significance of the conserved ATPase domain and of a highly acidic region of the protein. The CSB mutant alleles were tested for genetic complementation of UV-sensitive phenotypes in the human CS-B homologue of hamster UV61. In addition, the CSB mutant alleles were tested for their ability to complement the sensitivity of UV61 cells to the carcinogen 4-nitroquinoline-1-oxide (4-NQO), which introduces bulky DNA adducts repaired by global genome repair. Point mutation of a highly conserved glutamic acid residue in ATPase motif II abolished the ability of CSB protein to complement the UV-sensitive phenotypes of survival, RNA synthesis recovery, and gene-specific repair. These data indicate that the integrity of the ATPase domain is critical for CSB function in vivo. Likewise, the CSB ATPase point mutant failed to confer cellular resistance to 4-NQO, suggesting that ATP hydrolysis is required for CSB function in a TCR-independent pathway. On the contrary, a large deletion of the acidic region of CSB protein did not impair the genetic function in the processing of either UV- or 4-NQO-induced DNA damage. Thus the acidic region of CSB is likely to be dispensable for DNA repair, whereas the ATPase domain is essential for CSB function in both TCR-dependent and -independent pathways.

A new UV-sensitive mutant that suggests a second excision repair pathway in Neurospora crassa

In an attempt to understand the relationship between photorepair and dark repair in Neurospora crassa, a new mutant was isolated, which showed defects in both repair processes. The new mutant, mus-38, is moderately sensitive to UV and shows imperfect photoreactivation following UV irradiation. DNA was purified from this mutant and the other UV-sensitive mutants, and analyzed for the removal of cyclobutane pyrimidine dimers (CPDs). UV-specific endonuclease-sensitive sites (ESS) completely disappeared with 1 h of photoreactivation in mus-38 DNA, although the survival recovery with photoreactivation was greatly reduced in this mutant. This suggests that the insufficient survival recovery with photoreactivation in mus-38 does not result from a failure of photo-reversal of CPDs. Removal of ESS during liquid holding (dark repair) was slower in mus-38 compared to wild type. To test the possibility that this mutant was involved in excision repair, the double mutant was made between mus-38 and mus-18, which encodes a UV-damage-specific endonuclease. CPD excision in the mus-18 null mutant was severely affected but not completely inhibited. The double mutant showed a complete loss of the excision activity and was super sensitive to UV. These results indicate that mus-38 participates in an excision pathway that is different from the mus-18 pathway. The mus-38 mutant was sensitive not only to UV but also to some chemical mutagens which make adducts on DNA. Thus, mus-38 is possibly involved in an excision-repair pathway that is related to the Saccharomyces cerevisiae RAD3 pathway.

High susceptibility to ultraviolet-induced carcinogenesis in mice lacking XPC

Compromise of genetic information by mutation may result in the dysregulation of cellular growth control and subsequent tumour formation. Xeroderma pigmentosum (XP) is a rare autosomal disease characterized by hypersensitivity of the skin to sunlight and > 1,000-fold increased risk of skin cancers in sun-exposed parts of the body. Cell fusion studies have revealed eight complementation groups in XP (A-G, and an XP-variant form); group C is one of the most common forms of the disease. We have isolated a mouse homologue of the human gene for XP group C and generated XPC-deficient mice by using embryonic stem cell technology. Mice homozygous for the XPC mutant allele (xpcm1/xpcm1) are viable and do not exhibit an increased susceptibility to spontaneous tumour generation at one year of age. However, xpcm1/xpcm1 mice were found to be highly susceptible to ultraviolet-induced carcinogenesis compared with mice heterozygous for the mutant allele (xpcm1/+) and wild-type controls. Homozygous xpcm1 mutant mice also display a spectrum of ultraviolet-exposure-related pathological skin and eye changes consistent with the human disease xeroderma pigmentosum group C.

Unique DNA repair property of an ultraviolet-sensitive (radC) mutant of Dictyostelium discoideum

Dictyostelium discoideum is an organism that shows higher UV resistance than other organisms, such as Escherichia coli and human cultured cells. We examined the removal of cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts from DNA in the radC mutant and the wild-type strain using an enzyme-linked immunosorbent assay with monoclonal antibodies. Wild-type cells excised more than 90% of both CPD and 6-4 photoproducts within 4 h. Dictyostelium discoideum appeared to have a special repair system, because 6-4 photoproducts were repaired faster than CPD in E. coli and human cultured cells. In radC mutant cells, although only 50% of CPD were excised from DNA within 8 h, effective removal of 6-4 photoproducts (80% in 8 h) was observed. Excision repair-deficient mutants generally cannot remove both CPD and 6-4 photoproducts. Though the radC mutant shows deficient excision repair, it can remove 6-4 photoproducts to a moderate degree. These results suggest that D. discoideum has two kinds of repair systems, one mainly for CPD and the other for 6-4 photoproducts, and that the radC mutant has a defect mainly in the repair enzyme for CPD.

Absence of DNA repair deficiency in the confirmed heterozygotes of xeroderma pigmentosum group A

This study was performed to elucidate whether xeroderma pigmentosum complementation group A (XPA) carrier has DNA repair abnormality against sun-exposure and ultraviolet (UV)-mimetic chemical carcinogen 4-nitroquinoline 1-oxide (4NQO). Here we report three sporadic cases of XP that were defined as group A by genetic complementation test as well as polymerase chain reaction (PCR) analysis to detect the point mutation in the responsible gene for XPA. DNA repair analyses in the skin fibroblasts revealed that the cells from the patients were much more sensitive to UV and 4NQO and had extremely low UV-induced unscheduled DNA synthesis (UDS) than control cells, whereas the cells from the carriers (heterozygotes of XP) had sensitivity to UV and 4NQO and levels of UV-induced UDS similar to normal cells. These results indicate that the obligate heterozygotes, despite having a mutated allele in XPA complementing gene demonstrated by PCR, have no DNA repair abnormality after UV irradiation and UV-mimetic 4NQO treatment. Our observations imply that XPA heterozygotes do not have higher risk of skin cancers than normal subjects based on their DNA repair abnormality.

Sequence effect on incision by (A)BC excinuclease of 4NQO adducts and UV photoproducts

Nucleotide excision repair in Escherichia coli is initiated by (A)BC excinuclease, an enzyme which incises DNA on both sides of bulky adducts and removes the damaged nucleotide as a 12-13 base long oligomer. The incision pattern of the enzyme was examined using DNA modified by 4-nitroquinoline 1-oxide (4NQO) and UV light. Similar to the cleavage pattern of UV photoproducts and other bulky adducts, the enzyme incises the 8th phosphodiester bond 5' and 5th phosphodiester bond 3' to the 4NQO-modifed base, primarily guanine. The extent of DNA damage by these agents was determined using techniques which quantitatively cleave the DNA or stop at the site of the adduct. By comparison of the intensity of gel bands created by (A)BC excinuclease and the specific cleavage at the damaged site, the efficiency of (A)BC excinuclease incision at 13 different 4NQO-induced adducts and 13 different photoproducts was determined by densitometric scanning. In general, incisions made at 4NQO-induced adducts are proportional to the extent of damage, though the efficiency of cutting throughout the sequence tested varies from 25 to 75%. Incisions made at pyrimidine dimers are less efficient than at 4NQO-adducts, ranging from 13 to 65% incision relative to modification, though most are around 50%. The two (6-4) photoproducts within the region tested are incised more efficiently than any pyrimidine dimer.

The N2-guanine adduct but not the C8-guanine or N6-adenine adducts formed by 4-nitroquinoline 1-oxide blocks the 3'-5' exonuclease action of T4 DNA polymerase

When O-acetyl-4-(hydroxyamino)quinoline 1-oxide (Ac-4HAQO) reacts with double-stranded DNA at 37 degrees C the major products, N2-guanine, C8-guanine, and N6-adenine adducts, are formed in the proportions of 5:3:2, respectively. When the reaction is carried out with single-stranded DNA at 0 degree C, the products are found in the ratio 1:7:2. Unique 174-bp DNA fragments were modified in these ways and used as substrates for the 3'-5' exonuclease activity of T4 DNA polymerase. The results obtained showed that the exonuclease is blocked by the N2-guanine adduct but not the other two adducts. Interpretation of the cleavage patterns suggested that the enzyme stopped 2 nucleotides before the N2-guanine adduct. The N2-guanine adduct lies in the minor groove of the DNA double helix, while the other two adducts are found in the major groove. Apparently, only the former hinders progression of the enzyme.

Cytogenetical characterization of UV-sensitive repair-deficient CHO cell line 43-3B. II. Induction of cell killing, chromosomal aberrations and sister-chromatid exchanges by 4NQO, mono- and bi-functional alkylating agents

An established cell line of Chinese hamster ovary (CHO-9) cells and its UV-sensitive mutant 43-3B have been studied for the induction of cell killing, chromosomal aberrations and sister-chromatid exchanges (SCEs) after exposure to different types of DNA-damaging agents such as 4-nitroquinoline-1-oxide (4NQO), mitomycin C (MMC), diepoxybutane (DEB), methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS) and ethyl nitrosourea (ENU). In comparison with the wild-type CHO cells, 43-3B cells showed very high sensitivity to the UV-mimetic agent 4NQO and the DNA cross-linking agents MMC and DEB. The 43-3B cells responded with higher sensitivity to the monofunctional alkylating agents (MMS, EMS and ENU). The increased cytotoxic effects of all these chemicals correlated well with the elevated increase in the frequency of chromosomal aberrations. In 43-3B cells exposed to 4NQO, MMC or DEB the increase in the frequency of chromosomal aberrations was much higher than the increase in the frequency of SCEs (4-10-fold) when compared to the wild-type CHO cells. This suggests that SCEs are results of fundamentally different cellular events. The responses of 43-3B cells to UV, 4NQO, MMC and DEB resemble those of 2 human syndromes, i.e., xeroderma pigmentosum and Fanconi's anemia. These data suggest that 43-3B cells are defective in excision repair as well as the other pathways involved in the repair of cross-links (MMC, DEB) and bulky DNA adducts (4NQO).

The molecular genetics of the incision step in the DNA excision repair process

This review describes the evolution of research into the genetic basis of how different organisms use the process of excision repair to recognize and remove lesions from their cellular DNA. One particular aspect of excision repair, DNA incision, and how it is controlled at the genetic level in bacteriophage, bacteria, S. cerevisae, D. melanogaster, rodent cells and humans is examined. In phage T4, DNA is incised by a DNA glycosylase-AP endonuclease that is coded for by the denV gene. In E. coli, the products of three genes, uvrA, uvrB and uvrC, are required to form the UVRABC excinuclease that cleaves DNA and releases a fragment 12-13 nucleotides long containing the site of damage. In S. cerevisiae, genes complementing five mutants of the RAD3 epistasis group, rad1, rad2, rad3, rad4 and rad10 have been cloned and analyzed. Rodent cells sensitive to a variety of mutagenic agents and deficient in excision repair are being used in molecular studies to identify and clone human repair genes (e.g. ERCC1) capable of complementing mammalian repair defects. Most studies of the human system, however, have been done with cells isolated from patients suffering from the repair defective, cancer-prone disorder, xeroderma pigmentosum, and these cells are now beginning to be characterized at the molecular level. Studies such as these that provide a greater understanding of the genetic basis of DNA repair should also offer new insights into other cellular processes, including genetic recombination, differentiation, mutagenesis, carcinogenesis and aging.

Evidence for a Micrococcus luteus gene homologous to uvrB of Escherichia coli

Restriction fragments of Micrococcus luteus DNA that contained the gene defined by the mutation of an excision repair-deficient mutant, UVsN1, were cloned from both the parental and mutant strains with the Escherichia coli host-vector system. The wild-type fragment was able to reverse the multiple sensitivity of the mutant to ultraviolet, mitomycin C, and 4-nitroquinoline-1-oxide by one-step transformation. Determination of the nucleotide sequences revealed an open reading frame potentially coding for a protein of 709 amino acid residues, within which the mutation was identified as a CG----TA transition causing a change from serine to phenylalanine. The putative product of the open reading frame showed an extensive amino acid sequence homology to the E. coli UvrB protein comprising 673 residues; the homologous region extended over the greater parts of both polypeptides, in which 55% and 17% of the 659 pairs of aligned amino acids were accounted for by conserved residues and conservative substitutions, respectively. This indicates that the gene defined by the UVsN1 mutation represents a homolog of the E. coli uvrB gene, implying the presence in M. luteus of an enzyme complex homologous to the E. coli UvrABC excinuclease.

The expression of the Escherichia coli uvrA gene in human cells

Cells cultured from xeroderma pigmentosum (XP) patients are defective in excision repair of damaged DNA specifically at the incision step. In Escherichia coli this step is mediated by the UvrA, UvrB and UvrC gene products. Our goal is to express each of these genes in XP cells, singly or in combination, and to determine the most suitable conditions for generating faithful E. coli Uvr protein copies in functional concentrations and properly localized for the eventual repair of damaged chromosomal DNA or DNA which is introduced exogenously. The E. coli gpt gene in pSV2gpt is used as a selection marker for uvr gene transfection into XP cells. The uvr genes were cloned into composite pBR322, SV40 and gpt vectors in which each E. coli gene is flanked by individual SV40 regulatory elements. SV40-transformed XP-A cells were transfected with pSV2uvrASV2gpt, gpt+ colonies were selected, and cell lines established. Several lines were examined in detail. Cell lines 714 and 1511 contain uvrA together with flanking SV40 regulatory elements integrated intact in genomic DNA and express UvrA protein as well as a 95,000-dalton UvrA-related protein. The expression of uvrA was found to be 50-100-fold lower than the expression of gpt. Attempts were made to assay the mammalian UvrA protein for functionality, but endogenous activities interfered with assays for each of the UvrA protein's three activities. The peptide maps derived from partial proteolysis of the "mammalian" UvrA protein are identical to the E. coli UvrA protein. The sub-cellular location of UvrA protein in uvrA+ XP cells was investigated by fractionation of cell extracts in which an indirect immunofluorescence method revealed its location as being largely extra-nuclear. Two uvrA+ cell lines were examined for their UV-resistant phenotype and not unexpectedly were found not to be reverted to a state of repair proficiency.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

Lack of excision of 4HAQO adducts from DNA by cell extracts that excise pyrimidine dimers

A substrate of DNA containing 4HAQO adducts, suitable for studies of excision repair, was prepared by reacting calf thymus DNA with [3H]monoacetyl-4HAQO. A crude HeLa cell extract was prepared by the method of Mortelmans et al (Proc. Natl. Acad. Sci. U.S.A. 73, 2757, 1976). The cell extract would specifically excise pyrimidine dimers from UV-irradiated DNA but would not release 4HAQO adducts in an acid soluble form. This result points to different initial steps in the excision repair process for these two forms of damage even though much of the repair mechanism is common to both.

Studies on environmental chemical carcinogenesis in Japan

The historical background of studies in Japan on chemical carcinogenesis from environmental sources is described from personal experience.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

Toxicity of 4-nitroquinoline 1-oxide in Chinese hamster ovary cells: influence of cell density and of position in the cell cycle

Various factors influencing toxicity of 4-nitroquinoline 1-oxide (4-NQO) in Chinese hamster ovary cells were determined. Cell density during 4-NQO treatment and volume of treatment medium had a great effect on cell survival indicating that not the 4-NQO concentration per se, but the amount of 4-NQO per cell determines the toxic effect. When the cell-cycle response for 4-NQO-induced cell killing was measured in synchronous cells, a characteristic age response was seen in wild-type cells with greatly increased sensitivity in late G1 to early S and resistance increasing through the S-phase. In contrast, a UV-hypersensitive mutant, which is also more sensitive to 4-NQO showed only minor cell-cycle variations in its response to 4-NQO. Therefore, it appears that the cell-cycle pattern observed in the wild-type cells is associated with DNA repair.

Lack of effect of 4-nitroquinoline 1-oxide on cellular NAD levels

Human KB cells were treated with doses of 4-nitroquinoline 1-oxide (4NQO) or dimethyl sulfate (DMS) that produced equal numbers of DNA-strand breaks when measured by velocity sedimentation analysis in an alkaline sucrose gradient. The DMS treatment also caused a profound and sustained lowering of cellular NAD content. The 4NQO treatment had no effect on the cellular NAD content. This result with 4NQO was not expected because strand breaks in DNA activate poly(ADP-ribose)polymerase and in the ensuing reaction NAD is consumed. Since 4NQO adducts are removed by an excision-repair process it is postulated that the strand breaks formed during the repair process are not accessible to poly(ADP-ribose)polymerase.

Mechanism of increased susceptibility to 4-nitroquinoline-1-oxide in cultured skin fibroblasts from patients with familial polyposis coli

About 50% of the strains of cultured fibroblasts from patients with familial polyposis coli (FPC) exhibited increased susceptibility to cytotoxicity of 4-nitroquinoline-1-oxide (4NQO) compared with cells from normal individuals. The FPC cells that showed hyper-sensitivity to 4NQO were also hyper-sensitive to mitomycin C (MMC), but susceptibilities of these cells to UV radiation, methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were within the normal range. The extent of single-strand scission of DNA in the 4NQO-sensitive FPC cells was greater than in normal cells, and the amount of [14C]4NQO bound to DNA in the FPC cells was twice as high as in normal cells. The rate of release of [14C]4NQO from DNA by the post-culture was the same as in both FPC and normal cells. The 4NQO-sensitive FPC cells exhibited increased 4NQO-reductase activity; the level of this activity was consistent with the extent of the decrease in colony formation by 4NQO. These results suggest that the enhanced ability to activate 4NQO might be an important factor in the mechanism of susceptibility of FPC cells to 4NQO rather than the reduced ability to repair DNA.

Chromosomal hypersensitivity in mutant M10 and Q31 mouse cells exposed to ultraviolet radiation (UV) and 4-nitroquinoline-1-oxide (4NQO)

2 mutant mouse cells M10 and Q31 were examined for chromosomal aberrations induced by ultraviolet radiation (UV) and 4-nitroquinoline-1-oxide (4NQO), as compared with mouse lymphoma L5178Y cells. Q31 cells are UV- and 4NQO-sensitive cells isolated from L5178Y cells. M10 cells are similar but are sensitive to ionizing radiation and 4NQO. After treatment with UV or 4NQO, chromatid-type aberrations in these cell strains were induced more frequently in the first mitotic cells, at late fixation times. After UV exposure (2.4 J/m2), the maximal frequencies of chromatid-type breaks in Q31 cells were about 5 times higher than in L5178Y cells. In M10 cells such breaks were only as frequent as in L5178Y cells. After 4NQO treatment (50 ng/ml) the frequencies of chromatid-type breaks in M10 and Q31 cells were significantly higher than in L5178Y cells. From these results and those of previous studies (Takahashi et al., 1982), M10 cells may be considered hypersensitive to gamma-rays and 4NQO, but not to UV, and thus react similarly to L5178Y cells. The hypersensitivity of M10 cells to 4NQO may result from a defect in the ionizing-radiation repair mechanism as has been suggested to occur in ataxia telangiectasia (AT) cells. Q31 cells are hypersensitive to UV and 4NQO, but not to gamma-rays. Q31 cells may be considered to be deficient in a UV-like repair pathway. In conclusion, characteristics of murine M10 and Q31 cells are compared with those of human AT and xeroderma pigmentosum (XP) cells.

UV- and X-ray-sensitive double mutants of mouse L5178Y cells are synergistically more sensitive to 4-nitroquinoline-1-oxide than is either of the single mutants

The X-ray-sensitive mutant M10 and the UV-sensitive mutant Q31 of mouse lymphoma L5178Y cells are both sensitive to killing by 4-nitroquinoline-1-oxide (4NQO). Since cell hybridization experiments showed that the 4NQO sensitivities in M10 and Q31 cells behaved as codominant traits (Shiomi et al., 1982c), it is not possible to determine by complementation test whether the M10 and the Q31 mutations responsible for 4NQO sensitivities are allelic. We have obviated this difficulty by selecting double mutants that are sensitive to both X-rays and UV. From X-ray-sensitive M10 cells, two UV-sensitive mutants (XU 1 and XU 2) were isolated by a cell-suspension spotting method. XU 1 and XU 2 were found to belong to the same complementation group as Q31 (group I). Double mutants XU 1 and XU 2 were 30-37-fold more sensitive to 4NQO than parental L5178Y cells, whereas the single mutants M10 and Q31 were only 6-8-fold more sensitive to 4NQO than L5178Y cells in terms of D10 values (dose required to reduce survival to 10%). These results show that the M10-Q31-double mutations enhance 4NQO sensitivity synergistically, indicating that the M10 and the Q31 mutations relevant to 4NQO sensitivities are non-allelic. The implications of this finding are discussed.

Hypersensitivity of ataxia telangiectasia skin fibroblasts to DNA alkylating agents

3 ataxia telangiectasia (AT) fibroblast cell strains, AT4BI, AT5BI and AT2BE (CRL1343) were studied for their colony-forming ability after treatment with various concentrations of 4 different DNA alkylating agents. The results were compared to the response of fibroblast strains from 3 normal individuals. None of the AT strains were abnormally sensitive to N-methyl-N'-nitro-N-nitrosoguanidine. 1 strain (AT5BI) was significantly more sensitive to treatment with methyl methanesulfonate (MMS) based on a survival curve D0 value of 0.29 mM vs. the normal average D0 of 0.38 mM (P less than 0.02) and a D10 value of 0.85 mM vs. the normal average D10 of 1.2 mM (P less than 0.025). Strain AT4BI was also significantly more sensitive to MMS treatment when D10 values were compared (0.73 mM, P less than 0.01). All 3 AT cell strains were significantly more sensitive to treatment with ethyl methanesulfonate when D10 values were the criterion of sensitivity, AT4BI 16 mM, AT5BI 13 mM and AT2BE 15 mM vs. the normal human fibroblast average D10 value of 28 mM (P less than 0.01 for all 3 AT strains). 2 of the 3 AT cell strains (AT4BI and AT2BE) were abnormally sensitive to treatment with 4-nitroquinoline-1-oxide; the D0 values were 0.045 microM and 0.05 microM, respectively, vs. the normal average D0 value of 0.11 microM (P less than 0.01 for both AT strains). The corresponding D10 values were 0.08 microM and 0.11 microM, respectively, vs. the normal average D10 value of 0.27 microM (P less than 0.01 for AT4BI and P less than 0.025 for AT2BE). These results indicate that there is a heterogeneity in the response of AT fibroblast cell strains to treatment with DNA alkylating agents, except possibly in the case of ethylating compounds.

DNA-binding protein from HeLa cells that binds preferentially to supercoiled DNA damaged by ultraviolet light or N-acetoxy-N-acetyl-2-aminofluorene

A DNA-binding protein was partially purified from extracts of HeLa cells by high-speed centrifugation and chromatography on DEAE-cellulose, phosphocellulose and ultraviolet light-irradiated DNA-cellulose columns. It eluted from the phosphocellulose column with 0.375 M potassium phosphate and from the ultraviolet light-irradiated DNA-cellulose column between 0.5 M and 1 M NaCl. The protein binds preferentially to supercoiled PM2 DNA treated with ultraviolet light or N-acetoxy-N-acetyl-2-aminofluorene, as compared to native supercoiled PM2 DNA. The binding is non-cooperative. Nicked or linear forms of PM2 DNA (damaged or untreated) are not efficient substrates, indicating a requirement of DNA supercoiling for DNA binding. The sedimentation coefficient of the protein estimated by glycerol gradient centrifugation is 2.0-2.5 S, corresponding to a molecular weight of about 20000-25000 if the protein is spherical. The binding to DNA irradiated with ultraviolet light or treated with acetoxyacetylaminofluorene is optimal at around 100-200 mM NaCl and is relatively independent of temperature and pH. MgCl2 and MnCl2 at concentrations between 1 and 5 mM do not markedly affect the binding, but it is inhibited by sucrose, ATP and caffeine. The biological significance of the DNA-binding protein remains to be determined. It does not possess significant glycosylase, endonuclease or exonuclease activities. The dissociation equilibrium constant for the binding reaction of the protein to the ultraviolet light or acetoxyacetylaminofluorene-induced binding sites on DNA is estimated to be 4.10(-11) M. There are at least 1.10(5) DNA-binding protein molecules/HeLa cell.

Distribution of DNA-bound carcinogen 4-nitroquinoline 1-oxide and of repair-synthesized DNA in chromatin of WI-38 cells

Distribution of DNA-bound 4-nitroquinoline 1-oxide (4NQO) and repair-synthesized DNA after treatment with 4NQO in chromatin was investigated with human diploid fibroblast WI-38. Cells were incubated with [3H] 4NQO, and sites of binding on chromatin DNA were analyzed either by sensitivity to micrococcal nuclease or DNase I, or by fractionation of chromatin on sucrose gradient centrifugation. The results showed that 4NQO preferentially binds to DNA of linker region of chromatin, but the binding was random with respect to transcriptional activity of chromatin. The distribution of repair-synthesized DNA in chromatin damaged by 4NQO was also studied by similar experiments. [3H] Thymidine incorporated into repaired DNA was more sensitive to nucleases, but distributed almost equally among DNAs of chromatin subfractions with different transcriptional activity.

Unscheduled DNA synthesis induced by 4-nitroquinoline-1-oxide in xeroderma pigmentosum cells and their complementing heterodikaryons

Unscheduled DNA synthesis (UDS) induced by 4-nitroquinoline-1-oxide (rNQO) in the complementation groups A to E and variant xeroderma pigmentosum (XP) cells, and various pairs of complementing XP heterokaryons were investigated. The pattern of UDS induced by various concentrations of 4NQO in normal cells was quite different from those in groups A to D XP cells. The patterns of UDS in group E and variant XP cells were indistinguishable from those of normal cells under our experimental conditions. The levels of UDS induced by 5 x 10(-6) M 4NQO were 13% of normal in group A, 9% in group B, 17% in group C, 25% in group D. The heterokaryons obtained by pair-wise fusions between different complementation groups of XP strains showed restored UDS induced by 5 x 10(-6) M 4NQO, while the dikaryons obtained from fusion between the same groups did not.

Defective DNA repair and increased lethality in ataxia telangiectasia cells exposed to 4-nitroquinoline-1-oxide

Ataxia telangiectasia (AT) is a rare autosomal recessive disorder in man characterized by progressive loss of muscular coordination (due to neurodegeneration) and permanent dilation of the small blood vessels of the eyes and skin. AT patients also have repeated sinopulmonary infections, immune defects associated with thymus underdevelopment, and abnormal endocrine functions. Affected patients are at high risk of developing malignancy, particularly lymphomas and lymphatic leukaemias, and there are clinical indications that AT patients are hypersensitive to conventional radiotherapy administered for treatment of malignancy. Cultured diploid fibroblasts from AT donors are consistently hypersensitive to ionizing radiation, apparently due to defective enzymatic repair of radiogenic DNA damage. We have determined the survival responses and DNA repair abilities of AT cells exposed to 4-nitroquinoline-1-oxide (4NQO), a chemical carcinogen whose DNA-damaging properties partially mimic those of ionizing radiation. We report here that certain AT cell strains show hypersensitivity to inactivation by 4NQO and defective repair of 4NQO-induced adducts in DNA.

Analyses of differential sensitivities of synchronized HeLa S3 cells to radiations and chemical carcinogens during the cell cycle. Par V. Radiation- and chemical carcinogen-induced mutagenesis

8-Azaguanine(8AG)-resistant mutations induced by X-rays, ultraviolet radiation (UV) and a chemical carcinogen, 4-hydroxyaminoquinoline 1-oxide (4-HAQO) were examined during the cell cycle of synchronized HeLa S3 cells. Mutants induced by 400 R of X-rays occurred in a higher frequency in the X-ray-SENSITIVE Gl-S boundary phase than in the X-ray-resistant G1, S and early G2 phases. 8AG-resistant mutants induced by treatment with 10(-5) M 4-HAQO for 20 min appeared in a higher frequency in the early to middle S phases than in the other phases. In the case of UV, however, we found no significant difference in the induced mutation frequencies during the cell cycle, because the mutation frequencies induced by the UV doses (0-20 J/m2) used were too low for detection of the difference. These results suggest that there is a close correlation between the critical damage induced in DNA molecule(s) at the DNA-synthetic phase in the cell cycle and mutagenesis, because mitotic cells have a low mutability in spite of their high radio-sensitivity.

Repair of DNA damage after exposure to 4-nitroquinoline-1-oxide in heterokaryons derived from xeroderma pigmentosum cells

Xeroderma pigmentosum (XP) cells are deficient in the repair of damage induced by ultraviolet irradiation. Excision-repair-deficient XP cell strains have been classified into 7 distinct complementation groups, according to results of studies on cell fusion and UV irradiation. XP cells are not only abnormally sensitive to UV, but also to a variety of chemical carcinogens, including 4-nitro-quinoline-1-oxide (4NQO). Complementation analysis with XP strains from 4 different complementation groups with respect to the repair of 4NQO-induced DNA damage revealed that the classification of the strains into complementation groups with respect to 4NQO-induced repair coincides with the classification based on the repair of UV damage.

A screening method for isolating DNA repair-deficient mutants of CHO cells

A simple procedure for isolating mutagen-sensitive clones of CHO cells was developed and applied in mutant hunts in which colonies were screened for hypersensitivity to killing by ultraviolet radiation (UV, ethyl methanesulfonate (EMS), or mitomycin C (MMC). Each of two UV-sensitive clones studied in detail had a D37 dose of 1.0 J/m2 compared to 7.0 J/m2 for the wild-type cells, and each was shown to have no detectable repair replication following exposure to UV doses of up to 26 J/m2. Although these mutants resemble xeroderma pigmentosum human mutants with respect to their repair defect and cross-sensitivity to the carcinogen 4-nitroquinoline-1-oxide, one of two clones (UV-20) is characterized by extreme hypersensitivity to MMC (80-fold as compared to the wild type). Clones having hypersensitivity to alkylating agents, but not UV, were obtained using MMC and EMS. In the latter case the two clones had significantly increased sensitivity to the killing action of 60Co gamma-rays.

DNA excision in repair proficient and deficient human cells treated with a combination of ultraviolet radiation and acridine mustard (ICR-170) or 4-nitroquinoline 1-oxide

Excision repair was measured in normal human and xeroderma pigmentosum group C fibroblasts treated with ultraviolet radiation and the carcinogens acridine mustard (ICR-170) or 4-nitroquinoline 1-oxide (4NQO) by the techniques of unscheduled synthesis, photolysis of bromodeoxyuridine incorporated into parental DNA during repair, and assays of sites sensitive to ultraviolet (UV)-endonuclease. Doses of ICR-170 and 4NQO, low enough not to inhibit unscheduled DNA synthesis (UDS), caused damage to DNA that was repaired by a long patch type mechanism and the rates of UDS decreased rapidly in the first 12 h after treatment. Repair after a combined action of UV plus ICR-170 or UV plus 4NQO was additive in normal cells and no inhibition of loss of endonuclease sensitive sites was detected. In xeroderma pigmentosum (XP) C cells there was less repair after UV plus ICR-170 than after each treatment separately; whereas there was an additive effect after UV plus 4NQO and no inhibition of loss of endonuclease sensitive sites. The results indicate that in normal human fibroblasts there are different rate limiting steps for removal of chemical and physical damages from DNA and that XP cells have a different repair system for ICR-170, not just a lower level, than normal cells. Possibly the same long patch repair system works on 4NQO damage in both normal and XP cells.

Isolation of UV-sensitive variants of human FL cells by a viral suicide method

A new method (viral suicide method) for the isolation of UV-sensitive mutants is described. Colonies of mutagenized human FL cells were infected with UV-irradiated Herpes simplex viruses and surviving ones which seemed to be deficient in host cell reactivation (HCR) were examined for their UV sensitivity. Nineteen of 238 clones examined were sensitive to UV irradiation at the time of the isolation. After recloning, four of these clones have been studied and two (UVS-1 and UVS-2) of them are stable in their UV sensitivity for 4 months in culture. UV sensitivity of UVS-1, UVS-2, and the parental FL cells are as follows: the extrapolation numbers (n) are 2.2, 2.1, and 1.8 and mean lethal doses (D0) are 2.9, 3.7, and 7.8 J/m2 for UVS-1, UVS-2, and the parental FL cells, respectively- They are no more sensitive than FL cells to X-irradiation. The ability of HCR in UVS-2 cells is apparently lower than that in FL cells, whereas UVS-1 cells are the same as FL cells in the ability.

Effects of DNA damaging agents on cultured fibroblasts derived from patients with Cockayne syndrome

The cytotoxic action of physical and chemical agents on 10 skin fibroblast strains in culture derived from individuals with Cockayne's syndrome was measured in terms of colony-forming ability. As compared to fibroblasts from normal donors, all Cockayne cell strains tested exhibited a significantly increased sensitivity to UV light and a normal sensitivity to X-rays. Cells from two sets of parents of unrelated Cockayne children showed an intermediate level of UV sensitivity. There was no effect of 0.5 mM caffeine on UV survival in normal and two Cockayne strains tested, indicating that postreplicational repair in Cockayne cells as measured by caffeine sensitivity was probably normal. Sensitivity of normal and Cockayne cells to the chemical carcinogens and mutagens 4NQO, N-AcO-AAF, ICR-170 and EMS was also compared. An increased sensitivity of Cockayne cells to 4NQO or N-AcO-AAF, but not the ICR-170 or EMS, was observed. However, unlike the intermediate UV sensitivity, the cell strains from two parents of Cockayne patients showed the same sensitivity to N-AcO-AAF or 4NQO as fibroblasts from normal individuals. Quantiation of damage to the DNA after 20 J . m-2 UV irradiation indicates normal levels of [3H] thymidine incorporation in the Cockayne cells, in contrast to UV-irradiated xeroderma pigmentosum cells (XP 12BE) in which there was a very low level of repari synthesis. Moreover, we have shown previously that excision of UV-induced pyrimidine dimers in 2 of the 10 Cockayne cell strains was normal.

Increased sensitivity of UV-repair-deficient human cells to DNA bound platinum products which unlike thymine dimers are not recognized by an endonuclease extracted from Micrococcus luteus

We have studied the response of human cells in culture to cis platinum[II] diammine dichloride (cis Pt[II]) induced DNA damage. The survival data, measured as a function of cis Pt[II] dose were similar in a normal cell line (Human foetal lung) compared to a UV-sensitive, thymine dimer excision repair-deficient cell line (Xeroderma pigmentosum). However, there was a marked difference between the two cell lines when binding to DNA was plotted against dose of cis Pt[II] given for 1 h. When these findings were expressed as cell survival versus binding to DNA, a 4.1--fold difference between the slopes of the survival curves for the two cell lines was obtained. These findings are consistent with the notion that normal cells are able to excise cis Pt[II] induced damage from their genome and thus increase their ability to survive as compared to excision-deficient cells. An endonuclease preparation from Micrococcus luteus is able to recognise UV damage in DNA, but did not recognise cis Pt[II] induced damage. These results possibly indicate differences in the pathways of repair of damage caused by the two agents.

Excision repair by human fibroblasts of DNA damaged by r-7, t-8-dihyroxy-t-9,10-oxy-7,8,9,10- tetrahydrobenzo(a)pyrene

Benzo(a)pyrene diol-epoxide I (r-7,t-8,dihydroxy-t-9,10 oxy-7,8,9,10 tetrahydrobenzo(a)pyrene) was used to treat either human adenovirus 5 or cultures of human fibroblasts. The survival of diol-epoxide I treated adenovirus was greater when infecting fibroblasts from normal persons than when infecting fibroblasts from patients with xeroderma pigmentosum (XP). One diol-epoxide I molecule bound per viral genome correlated with one lethal hit as measured using XP fibroblasts. Normal fibroblasts blocked in semi-conservative DNA synthesis incorporated into their DNA more [3H]thymidine in response to diol-epoxide I treatment than did XP fibroblasts, and also excised more diol-epoxide I from their DNA. All of the effects described above were similar to those obtained when the inactivating agent was ultraviolet light rather than benzo(a)pyrene diol-epoxide I.

Pesticide induced DNA damage and its repair in cultured human cells

The effects of pesticides on the induction of unscheduled DNA synthesis in SV-40 transformed human cells (VA-4) in culture with and without metabolic activation by liver microsomes was studied. Results showed that ten of the thirteen compounds examined either directly or upon metabolic activation induced unscheduled DNA synthesis in the human cell system used. The DNA repair kinetics and size of the repaired regions resulting from treatment with four of the chemicals (Carbaryl, Chlordane, Dieldrin and 2.4-D Fluid) were studied by 313 nm photolysis of repaired regions containing bromodeoxyuridine (BUdR). The size of the repaired regions differed between compounds but could generally be classified as either of the X-ray (short) or UV-type (long).

Somatic mutation as the basis for malignant transformation of BHK cells by chemical carcinogens

The chemical induction of malignant transformation in BHK cells seems to result from a somatic mutation. Stable transformants, whose frequency is significantly increased by mutagenic carcinogens, can revert to normal and often display temperature-restricted phenotypes indicative of an altered gene product.

Main binding sites of the carcinogen, 4-nitroquinoline 1-oxide in nucleic acids

4-Hydroxyaminoquinoline 1-oxide, the reduced metabolite of 4-nitroquinoline 1-oxide, was reacted with homopolyribonucleotides through the catalysis of an activating enzyme. It bound specifically to poly(G), poly(A) and poly(X) but negligibly to poly(C), poly (U) and poly(I). Chromatographic analysis of the acid hydrolysates of carcinogen-bound polynucleotides revealed that the reaction of the carcinogen with polynucleotides yielded two guanine, one adenine and two xanthine adducts. The same kinds of guanine and adenine adducts were found in DNA or RNA isolated from Escherichia coli and mammalian cells that had been exposed to the carcinogen. Analysis of nucleic acids isolated from 4-hydroxyaminoquinoline 1-oxide-treated cells revealed that 4-hydroxy-aminoquinoline reacts in vivo preferentially with guanines, to a less, but significant, extent with adenines and not significantly with pyrimidines.

Colony-forming ability of ultraviolet-irradiated xeroderma pigmentosum fibroblasts from different DNA repair complementation groups

Patients with xeroderma pigmentosum develop severe sunlight-induced damage, including malignant neoplasms, on sun-exposed skin. Some patients also have neurological abnormalities. Xeroderma pigmentosum cells are known to have impaired ability to repair ultraviolet light- or chemical mutagen-induced damage to their DNA, and cell-fusion studies have shown five complementation groups among the DNA excision repair-deficient strains. All xeroderma pigmentosum fibroblast strains we tested had lower colony-forming abilities after ultraviolet irradiation than normal strains. Furthermore, we have found that strains from different complementation groups can have different post-ultraviolet colony-forming abilities and that strains from patients with neurological abnormalities are the most sensitive to ultraviolet light. These results suggest that extremely ineffective repair of damaged DNA in central nervous system neurons may be the cause of the neurological abnormalities.