Transient appearance of photolyase-induced break-sensitive sites in the DNA of ultraviolet light-irradiated Syrian hamster fetal cells

Syrian hamster fetal fibroblasts (HFC) were examined for photolyase-induced break-sensitive sites after ultraviolet light (UV) exposure and growth. These sites, observed in excision-defective human xeroderma pigmentosum (XP) cells, are due to cleavage of the internal phosphodiester bond of UV-induced pyrimidine dimers. Excision-inefficient HFC acquired photolyase-induced break-sensitive sites during incubation after UV (10 J/m2). However, these were observed transiently, with a maximum of 5% of the pyrimidine dimers at 9 h post UV; by 18 h they were undetectable. Caffeine (1 mM) delayed the peak of photolyase-induced break-sensitive sites by 2 h. In human XP cells photolyase-induced break-sensitive sites accumulate to a plateau level of about 20% of the pyrimidine dimers. The present results extend to rodent cells the observation that cleavage of the internal phosphodiester bond of pyrimidine dimers may be an early step in their excision repair. Furthermore, the data suggest that photolyase-induced break-sensitive sites might be necessary for replication bypass at pyrimidine dimers.     

Proximity of repair patches to persistent pyrimidine dimers in DNA of normal human and xeroderma pigmentosum cells

The proximity of repair patches to persistent pyrimidine dimers in normal human cells and xeroderma pigmentosum group C and D cells was analyzed by sequential digestion of repaired DNA with Micrococcus luteus UV-endonuclease and Escherichia coli DNA polymerase I. Although this enzymatic digestion removed one-third of the pyrimidine dimers, less than 3% of the label associated with repair patches and a similar amount of uniformly labeled DNA were removed. The repair patches therefore appear to be similarly distant from persistent dimers in all cell types, and, in particular, are not adjacent to unexcised dimers in xeroderma pigmentosum group D cells. A previous model that suggested that patches are inserted adjacent to dimers in xeroderma pigmentosum group D cells receives no support from these results.     

DNA Chain Elongation and Joining in Normal Human and Xeroderma Pigmentosum Cells after Ultraviolet Irradiation

DNA synthesized in human cells after ultraviolet (UV) irradiation is made in segments of lower molecular weight than in unirradiated cells. Within several hours after irradiation these smaller units are both elongated and joined together. This repair process has been observed in normal human fibroblasts, HeLa cells, and fibroblasts derived from three types of xeroderma pigmentosum patients—uncomplicated with respect to neurological problems, complicated (de Sanctis-Cacchione syndrome), and one with the clinical symptoms of xeroderma pigmentosum but with normal repair replication. The ability of human cells to elongate and to join DNA strands despite the presence of pyrimidine dimers enables them to divide without excising the dimers present in their DNA. It may be this mechanism which enables xeroderma pigmentosum cells to tolerate small doses of UV radiation.     

Detection and repair of a UV-induced photosensitive lesion in the DNA of human cells

Irradiation with UV light results in damage to the DNA of human cells. The most numerous lesions are pyrimidine dimers; however, other lesions are known to occur and may contribute to the overall deleterious effect of UV irradiation. We have observed evidence of a UV-induced lesion other than pyrimidine dimers in the DNA of human cells by measuring DNA strand breaks induced by irradiating with 313-nm light following UV (254-nm) irradiation. These breaks, measured by alkaline sucrose sedimentation, increased linearly with the dose of UV light over the range tested (10-40 J/m2). The breaks cannot be photolytically induced 5 h after a UV dose of 20 J/m2 in normal cells; however, in xeroderma pigmentosum variant cells, the breaks are inducible for up to 24 h after UV irradiation. Xeroderma pigmentosum group A cells in the same 5-h period show an increase in the number of strand breaks seen with 313-nm light photolysis from about 2 to 4 breaks/10(9) dalton DNA. These breaks can then be induced for up to 24 h. These data suggest that, in normal cells, the lesion responsible for this effect is rapidly repaired or altered; whereas, in xeroderma pigmentosum variant cells it seems to remain unchanged. Some change apparently occurs in the DNA of xeroderma pigmentosum group A cells which results in an increase in photolability. These data indicate a deficiency in DNA repair of xeroderma pigmentosum variant cells as well as in xeroderma pigmentosum group A cells.     

Mitomycin C-induced postmitotic fibroblasts retain the capacity to repair pyrimidine photodimers formed after UV-irradiation

The formation and excision of UV-C light-induced cyclobutane-type pyrimidine photodimers were determined in cultures of human skin fibroblasts at time zero and several weeks following treatment with mitomycin C (MMC). Characteristic morphological changes of the fibroblasts and specific shifts in the [35S]methionine polypeptide pattern of total cellular proteins support the notion that MMC accelerates the differentiation pathway from mitotic (MF) to post-mitotic fibroblasts (PMF). No discernible difference could be detected between the fluence-response curves of pyrimidine dimers for untreated and MMC-treated repair-deficient xeroderma pigmentosum cells of group A. Furthermore we investigated the removal of pyrimidine dimers in 3 normal human skin fibroblast strains frequently used in mutation, transformation and aging research. We were able to demonstrate that no significant difference exists in the rate and extent of the excision-repair response to thymine-containing pyrimidine dimers following UV-irradiation shortly after MMC treatment of fibroblasts and in the MMC-induced PMF stage of these cells.     

UV stimulation of DNA-mediated transformation of human cells.

Irradiation of dominant marker DNA with UV light (150 to 1,000 J/m2) was found to stimulate the transformation of human cells by this marker from two- to more than fourfold. This phenomenon is also displayed by xeroderma pigmentosum cells (complementation groups A and F), which are deficient in the excision repair of UV-induced pyrimidine dimers in the DNA. Also, exposure to UV of the transfected (xeroderma pigmentosum) cells enhanced the transfection efficiency. Removal of the pyrimidine dimers from the DNA by photoreactivating enzyme before transfection completely abolished the stimulatory effect, indicating that dimer lesions are mainly responsible for the observed enhancement. A similar stimulation of the transformation efficiency is exerted by 2-acetoxy-2-acetylaminofluorene modification of the DNA. No stimulation was found after damaging vector DNA by treatment with DNase or gamma rays. These findings suggest that lesions which are targets for the excision repair pathway induce the increase in transformation frequency. The stimulation was found to be independent of sequence homology between the irradiated DNA and the host chromosomal DNA. Therefore, the increase of the transformation frequency is not caused by a mechanism inducing homologous recombination between these two DNAs. UV treatment of DNA before transfection did not have a significant effect on the amount of DNA integrated into the xeroderma pigmentosum genome.     

Photoreactivation of pyrimidine dimers in the DNA of normal and xeroderma pigmentosum cells.

Photoproducts formed in the DNA of human cells irradiated with ultraviolet light (uv) were identified as cyclobuytl pyrimidine dimers by their chromatographic mobility, reversibility to monomers upon short wavelength uv irradiation, and comparison of the kinetics of this monomerization with that of authentic cis-syn thymine-thymine dimers prepared by irradiation of thymine in ice. The level of cellular photoreactivation of these dimers reflects the level of photoreactivating enzyme measured in cell extracts. Action spectra for cellular dimer photoreactivation in the xeroderma pigmentosum line XP12BE agree in range (300 nm to at least 577 nm) and maximum (near 400 nm) with that for photoreactivation by purified human photoreactivating enzyme. Normal human cells can also photoreactivate dimers in their DNA. The action spectrum for the cellular monomerization of dimers is similar to that for photoreactivation by the photoreactivating enzyme in extracts of normal human fibroblasts.     

Correction of the ultraviolet light induced DNA-repair defect in xeroderma pigmentosum cells by electroporation of a normal human endonuclease

Cells from patients with xeroderma pigmentosum, complementation group A (XPA), are known to be defective in repair of pyrimidine dimers and other forms of damage produced by 254-nm ultraviolet (UVC) radiation. We have isolated a DNA endonuclease, pI 7.6, from the chromatin of normal human lymphoblastoid cells which recognizes damage produced by UVC light, and have introduced this endonuclease into UVC-irradiated XPA cells in culture to determine whether it can restore their markedly deficient DNA repair-related unscheduled DNA synthesis (UDS). Introduction of the normal endonuclease, which recognizes predominantly pyrimidine dimers, but not the corresponding XPA endonuclease into UVC-irradiated XPA cells restored their levels of UDS to approximately 80% of normal values. Electroporation of both the normal and the XPA endonuclease into normal human cells increases UDS in normal cells to higher than normal values. These results indicate that the normal endonuclease can restore UDS in UVC-irradiated XPA cells. They also indicate that XPA cells have an endonuclease capable of increasing the efficiency of repair of UVC damage in normal cells.     

Recovery of the Ability to Synthesize DNA in Segments of Normal Size at Long Times After Ultraviolet Irradiation of Human Cells

DNA synthesized in human cells within the first hour after ultraviolet (UV) irradiation is made in segments of lower molecular weight than in nonirradiated cells. The size of these segments approximates the average distance between pyrimidine dimers in the parental DNA. This suggests that the dimers interrupt normal DNA synthesis and result in gaps in the newly synthesized DNA. However, DNA synthesized in human cells at long times after irradiation is made in segments equal or nearly equal to those synthesized by nonirradiated cells. The recovery of the ability to synthesize DNA in segments of normal size occurs in normal human cells, where the dimers are excised, and also in cells of the human mutants xeroderma pigmentosum (XP), where the dimers remain in the DNA. This observation implies that the pyrimidine dimer may not be the lesion that causes DNA to be synthesized in smaller than normal segments.     

A DNA binding protein from human placenta specific for ultraviolet damaged DNA.

A DNA-binding protein specific for ultraviolet irradiated DNA has been purified extensively from human placenta. The binding preparation is free of exonuclease, polymerase, endonuclease, and N-glycosidase activity. The binding activity is salt dependent and is specific for double-stranded irradiated DNA. DNA from which the pyrimidine dimers have been monomerized by the action of photolyase (photoreactivating enzyme) remains an effective substrate for the binding protein, suggesting that the protein recognizes photoproducts other than pyrimidine dimers. This is supported by the finding that DNA irradiated under conditions which introduce only pyrimidine dimers is not a substrate for the binding protein. Examination of three of the xeroderma pigmentosum complementation groups has revealed no deficiency in this binding activity.     

Unique DNA repair properties of a xeroderma pigmentosum revertant.

A group A xeroderma pigmentosum revertant with normal sensitivity was created by chemical mutagenesis. It repaired (6-4) photoproducts normally but not pyrimidine dimers and had near normal levels of repair replication, sister chromatid exchange, and mutagenesis from UV light. The rate of UV-induced mutation in a shuttle vector, however, was as high as the rate in the parental xeroderma pigmentosum cell line.     

Photoreactivation and dark repair of ultraviolet light-induced pyrimidine dimers in chloroplast DNA.

A UV-specific endonuclease was used to detect ultraviolet light-induced pyrimidine dimers in chloroplast DNA of Chlamydomonas reinhardi that was specifically labeled with tritiated thymidine. All of the dimers induced by 100 J/m2 of 254 nm light are removed by photoreaction. Wild-type cells exposed to 50 J/m2 of UF light removed over 80% of the dimers from chloroplast DNA after 24 h of incubation in growth medium in the dark. A UV- sensitive mutant, UVS1, defective in the excision of pyrimidine dimers from nuclear DNA is capable of removing pyrimidine dimers from chloroplast DNA nearly as well as wild-type, suggesting that nuclear and chloroplast DNA dark-repair systems are under separate genetic control.     

A sensitive, enzymatic assay for the detection of closely opposed cyclobutyl pyrimidine dimers induced in human diploid fibroblasts

A sensitive, enzymatic assay has been developed for the detection of closely opposed cyclobutyl pyrimidine dimers induced in UV-irradiated human diploid fibroblasts. In this assay closely opposed dimers are detected as bifilar enzyme-sensitive sites. Single-strand incisions are made at the positions of individual pyrimidine dimers through the action of M. luteus pyrimidine dimer-DNA glycosylase. Incisions at closely opposed dimers, effectively expressed as double-strand breaks, are quantified from the resulting reduction in DNA double-strand molecular weight as determined by velocity sedimentation through neutral sucrose density gradients. The stability of the bacteriophage lambda cos site under our reaction conditions indicates that opposed incisions must be relatively close to be expressed as a double-strand break. The dose response for the induction of bifilar enzyme-sensitive sites in mammalian cells was found to be complex but can be approximated by a function that increases as the 1.2-1.4 power of UV dose. The frequency of bifilar enzyme-sensitive sites observed decreased during postirradiation incubation of excision-repair-proficient human diploid fibroblasts with less than 20% still detectable at 24 h after irradiation with 5 J/m2 (254 nm). By comparison, over 80% of the bifilar enzyme-sensitive sites induced in fibroblasts assigned to xeroderma pigmentosum complementation group A remained detectable 24 h after irradiation. The implications of these results for models addressing the induction and repair of closely opposed pyrimidine dimers are discussed.     

Repair of closely opposed cyclobutyl pyrimidine dimers in UV-sensitive human diploid fibroblasts

An enzyme-sensitive site assay has been used to examine the fate of closely opposed pyrimidine dimers (bifilar enzyme-sensitive sites) in fibroblasts from individuals afflicted with various genetic disorders that confer increased cellular sensitivity to UV radiation. The disappearance of bifilar enzyme-sensitive sites was found to be normal in cells from individuals with Fanconi's anemia, Cockayne's syndrome, dyskeratosis congenita and the variant form of xeroderma pigmentosum. The rate of bifilar enzyme-sensitive site removal in XP cells assigned to complementation group C was reduced by an amount similar to that observed for the repair of isolated dimers. Our results indicate that the initiation of repair at closely opposed dimers is slow in XP-C cells but normal in all other cells examined.     

Size and frequency of gaps in newly synthesized DNA of xeroderma pigmentosum human cells irradiated with ultraviolet light

Native newly synthesized DNA from human cells (xeroderma pigmentosum type) irradiated with ultraviolet light releases short pieces of DNA (L-DNA) when incubated with the single-strand specific S1 nuclease. This is not observed in the case of unirradiated cells. Previous experiments had shown that the L-DNA resulted from the action of S1 nuclease upon gaps, i.e., single-stranded DNA discontinuities in larger pieces of double-stranded DNA. We verified that the duplex L-DNA, that arises from the inter-gap regions upon S1 nuclease treatment, has a size which approximates the distance between two pyrimidine dimers on the same strand; this has been observed at different fluences of ultraviolet-light and indicates that the gap is related to or opposite the dimer. A method was devised to measure the size of the gaps. A Poisson distribution analysis of the percentage of the L-DNA produced as a function of S1 nuclease concentration made this possible. 65% of the gaps corresponded to stretches of 1,250 nucleotides and 35% to stretches of 150 nucleotides. These parameters have been considered in the proposition of a model for DNA synthesis on a template containing pyrimidine dimers.     

Transformation of ultraviolet-irradiated human fibroblasts by simian virus 40 is enhanced by cellular DNA repair functions

Human fibroblasts irradiated with ultraviolet light were either tested for survival (colony formation) or infected with simian virus 40 and examined for transformation (foci formation). For normal cell cultures, the fractions of surviving colonies which were also transformed increased with increasing irradiation dose. In contrast, little increase in the transformation of ultraviolet-irradiated repair-deficient (xeroderma pigmentosum and xeroderma pigmentosum variant) cells was observed. Similar experiments with xeroderma pigmentosum variant cells treated with caffeine following irradiation indicated that, under these conditions, the deficient cells produced more transformants among the survivors of ultraviolet irradiation than did unirradiated cells. These results suggest (1) that DNA repair functions, not DNA damage per se, are required for enhanced viral transformation in normal cells; (2) that functions involved in excision repair and functions needed for replication of ultraviolet-damaged DNA appear necessary for this stimulation; and (3) that blocking DNA replication in ultraviolet-irradiated xeroderma pigmentosum variant cells by caffeine enhances viral transformation.     

Postreplication repair of DNA in human fibroblasts after UV irradiation or treatment with metabolites of benzo(a)pyrene

We have examined the ability of normal fibroblasts and of excision-deficient xeroderma pigmentosum (XP) and XP variant fibroblasts to perform postreplication DNA repair after increasing doses of either ultraviolet (UV) irradiation or mutagenic benzo(a)pyrene derivatives. XP cells defective in the excision of both UV-induced pyrimidine dimers and guanine adducts induced by treatment with the 7,8-diol-9,10-epoxides of benzo(a)pyrene were partially defective in their ability to synthesize high molecular weight DNA after the induction of both classes of DNA lesions. This defect was more marked in XP variant cells, despite their ability to remove by excision repair both pyrimidine dimers and the diol epoxide-induced lesions to the same degree as observed in normal cells. The benzo(a)pyrene 9,10-oxide had no effect in any of the 3 cell lines. The response of the excision and postreplication DNA repair mechanisms operating in human fibroblasts treated with benzo(a)pyrene 7,8-diol-9,10-epoxides, therefore, appears to resemble closely that seen after the induction of pyrimidine dimers by UV irradiation.     

Cyclic GMP and cyclic GMP-dependent protein kinase in brown adipose tissue of developing rats.

Human cells (normal and xeroderma pigmentosum variant) irradiated with ultraviolet light and pulse-labelled with [3H]thymidine underwent transient decline and recovery of molecular weights of newly synthesized DNA and rates of [3H]thymidine incorporation. The ability to synthesize normal-sized DNA recovered more rapidly in both cell types than thymidine incorporation. During recovery cells steadily increased in their ability to replicate normal-sized DNA on damaged templates. The molecular weight versus time curves fitted exponential functions with similar rate constants in normal and heterozygous xeroderma pigmentosum cells, but with a slower rate in two xeroderma pigmentosum variant cell lines. Caffeine added during the post-irradiation period eliminated the recovery of molecular weights in xeroderma pigmentosum variant but not in normal cells. The recovery of the ability to synthesize normal-sized DNA represents a combination of a number of cellular regulatory processes, some of which are constitutive, and one of which is altered in the xeroderma pigmentosum variant such that recovery becomes slow and caffeine sensitive.