Kinetic analysis of UV-induced incision discriminates between fibroblasts from different xeroderma pigmentosum complementation groups, XPA heterozygotes and normal individuals

UV-enhanced reactivation of a UV-damaged reporter gene suggests transcription-coupled repair is UV-inducible in human cells

The genetic disorders xeroderma pigmentosum (XP) and Cockayne syndrome (CS) exhibit deficiencies in the repair of UV-induced DNA damage. CS fibroblasts retain proficient nucleotide excision repair (NER) of inactive (or bulk) DNA, but are deficient in the transcription-coupled repair (TCR) of active genes. In contrast, XP complementation group C (XP-C) fibroblasts retain proficient TCR, but are deficient in bulk DNA repair. The remaining NER-deficient XP groups exhibit deficiencies in both repair pathways. Ad5HCMVsp1lacZ is a recombinant adenovirus vector that is unable to replicate in human fibroblasts, but can efficiently infect and express the beta-galactosidase reporter gene in these cells. We have examined the host cell reactivation (HCR) of beta-galactosidase activity for UV-irradiated Ad5HCMVsp1lacZ in non-irradiated and UV-irradiated normal, XP-B, XP-C, XP-D, XP-F, XP-G, CS-A and CS-B fibroblasts. HCR of beta-galactosidase activity for UV-irradiated Ad5HCMVsp1lacZ was reduced in non-irradiated cells from each of the repair-deficient groups examined (including XP-C) relative to that in non-irradiated normal cells. Prior irradiation of cells with low UV fluences resulted in an enhancement of HCR for normal and XP-C strains, but not for the remaining XP and CS strains. HCR of the UV-damaged reporter gene in UV-irradiated XP and CS strains was similar to measurements of TCR reported previously for these cells. These results suggest that UV treatment results in an induced repair of UV-damaged DNA in the transcribed strand of an active gene in XP-C and normal cells through an enhancement of TCR or a mechanism which involves the TCR pathway.

Deficient DNA repair capacity, a predisposing factor in breast cancer

Women with breast cancer and a family history of breast cancer and some with sporadic breast cancer are deficient in the repair of radiation-induced DNA damage compared with normal donors with no family history of breast cancer. DNA repair was measured indirectly by quantifying chromatid breaks in phytohaemagglutinin (PHA)-stimulated blood lymphocytes after either X-irradiation or UV-C exposure, with or without post treatment with the DNA repair inhibitor, 1-beta-D-arabinofuranosylcytosine (ara-C). We have correlated chromatid breaks with unrepaired DNA strand breaks using responses to X-irradiation of cells from xeroderma pigmentosum patients with well-characterised DNA repair defects or responses of repair-deficient mutant Chinese hamster ovary (CHO) cells with or without transfected human DNA repair genes. Deficient DNA repair appears to be a predisposing factor in familial breast cancer and in some sporadic breast cancers.

G2 phase repair of X-ray-induced chromosomal DNA damage in trichothiodystrophy cells

The repair of X-ray-induced DNA damage during G2 cell-cycle phase has been examined in lines of skin fibroblasts from three patients with trichothiodystrophy (TTD), one with apparently normal and two with defective nucleotide excision repair (NER). These responses are compared with those of five lines from clinically normal controls, lines from xeroderma pigmentosum (XP), Cockayne syndrome (CS), Down syndrome (DS), and ataxia telangiectasia (AT) patients. Chromosomal DNA repair was measured as the chromatid aberration frequency (CAF) or total number of chromatid breaks and long gaps per 100 metaphase cells, determined 0.5-1.5 h after X-irradiation (53 rad). Chromatid breaks and gaps (as defined herein) represent unrepaired DNA strand breaks. Only one of the TTD lines, TTD 1BR, showed an abnormally high CAF. This line was shown subsequently to be of a different complementation group, representing a new nucleotide excision repair gene. An abnormally high CAF was also observed, as reported previously, in XP-C, AT and DS but not in CS skin fibroblasts. In addition, cell lines were examined for DNA incision activity by an indirect method in which chromatid aberrations were enumerated with or without ara-C, an inhibitor of repair synthesis, added after X-irradiation. All TTD lines had abnormally low incision activity.

X-ray-induced chromatid damage in cells from Down syndrome and Alzheimer disease patients in relation to DNA repair and cancer proneness

Frequencies of chromatid aberrations in response to G2-phase x-irradiation were compared in PHA-stimulated blood lymphocytes from healthy control subjects, Down syndrome (DS) patients, and Alzheimer disease (AD) patients. In cells arrested with Colcemid immediately (0-30 min) after x-irradiation, DS, AD, and control cells showed similar high frequencies of chromatid breaks and gaps, representing unrepaired DNA strand breaks. Frequencies had decreased in AD and control cells arrested 30-90 min after irradiation. However, DS cells had two- to three-fold higher frequencies than AD or control cells. This result indicates deficient repair of the DNA damage in DS cells. Similar responses were obtained with lymphocytes from four of seven DS parents tested and with skin fibroblasts from DS patients compared to age-matched controls. Addition of 1-beta-D-arabinofuranosylcytosine (ara-C), an inhibitor of the repair polymerase, after x-irradiation during G2 phase increased the frequencies of chromatid breaks and gaps in lymphocytes from control and AD donors significantly more than in those from DS patients. This result indicates a deficiency in DS cells in incision at sites of x-ray-induced damage. Thus DS, like other cancer-prone genetic disorders, has a G2-phase DNA repair deficiency in strand break repair and also a second DNA repair deficiency in incision activity.

Cytogenetic evidence for differences in DNA incision activity in xeroderma pigmentosum group A, C and D cells after X-irradiation during G2 phase

The capacity of cells to incise DNA to remove altered sites after DNA damage can be determined from the rate of DNA-strand break accumulation in the presence of an inhibitor of DNA-repair synthesis, such as 1-beta-D-arabinofuranosylcytosine (ara-C). Because each chromatid contains a single continuous molecule of double-stranded DNA, chromatid breaks and gaps, i.e., non-displaced breaks, represent unrepaired DNA-strand breaks. The accumulation of chromatid breaks and gaps after X-irradiation in the presence of ara-C thus provides a measure of DNA incision activity. Addition of ara-C to skin fibroblasts or stimulated blood lymphocytes from normal individuals at intervals after X-irradiation significantly increased frequencies of chromatid breaks and/or gaps. In contrast, addition of ara-C to XP cells of complementation groups A and D had a negligible effect and a significant but less than normal effect on XP cells of complementation group C and one sample of blood lymphocytes of undetermined complementation group. The results thus show negligible incision activity after G2 phase X-irradiation in XP-A and XP-D cells and a level higher but less than normal in XP-C cells.

Repair of thymine dimers and (6-4) photoproducts in group A xeroderma pigmentosum cell lines harboring a transferred normal chromosome 9

Transfer of a normal chromosome 9 into a xeroderma pigmentosum (XP)-A cell line partially restored its DNA repair activity. XP-A cell lines harboring a transferred chromosome were much more UV-resistant than parental XP-A cells but still more UV-sensitive than normal cells. The amount of UV-induced unscheduled DNA synthesis was only one-third of that in normal cells. The repair of thymine dimers and (6-4) photoproducts in these cell lines was analyzed by using monoclonal antibodies raised against them. Although these XP-A cell lines carrying a normal chromosome 9 could repair (6-4) photoproduct with a little lower efficiency than normal cells, the repair of thymine dimers was completely absent in these cells. The present results suggest a gene-dosage effect in DNA excision repair mechanisms in human cells or a rather complicated mechanism which involves two or more pathways.

The XPD complementation group. Insights into xeroderma pigmentosum, Cockayne's syndrome and trichothiodystrophy

The xeroderma pigmentosum complementation group D is defined by more than 30 unrelated individuals of whom less than half show major abnormalities of the central nervous system, once considered to be the hallmark of the group. Fibroblasts from the great majority of these individuals show very considerable sensitivity to UV light in vitro despite the fact that the cells carry out what appears to be substantial excision repair, as judged from repair synthesis and incision activity. This article reviews the XPD group and the defects in cellular DNA repair and examines the lack of correlation between repair and the appearance of neurological abnormalities. The article also discusses the recent awareness that at least some members of two other inherited conditions, trichothiodystrophy and Cockayne's Syndrome, carry mutations in the XPD gene.

Dose-dependent increase in repair of 1-beta-D-arabinofuranosylcytosine-detectable DNA lesions in UV-treated xeroderma pigmentosum (group A) fibroblasts

The extent of DNA-excision repair was determined in human fibroblast strains from clinically normal and xeroderma pigmentosum complementation group A (XP-A) donors after irradiation with 254-nm ultraviolet (UV) light. Repair was monitored by the use of 1-beta-D-arabinofuranosylcytosine (araC), a potent inhibitor of DNA synthesis, and alkaline sucrose velocity sedimentation to quantitate DNA single-strand breaks. In this approach, the number of araC-accumulated breaks in post-UV incubated cultures becomes a measure of the efficiency of a particular strain to perform long-patch excision repair. The maximal rate of removal of araC-detectable DNA lesions equalled approximately 1.8 sites/10(8) dalton/h in the normal strains (GM38, GM43), while it was more than 10-fold lower in both XP-A strains (XP4LO, XP12BE) examined. In normal fibroblasts the number of lesions removed during the first 4 h after irradiation saturated at approximately 10 J/m2. In contrast, the residual amount of repair in the excision-deficient cells increased as a linear function of UV fluence over a range 5-120 J/m2. Thus we conclude that the repair of araC-detectable UV photoproducts in XP group A fibroblasts is limited by availability of damaged regions in the genome to repair complexes.

DNA repair endonuclease activity during synchronous growth of diploid human fibroblasts

DNA-repair endonuclease activity in response to UV-induced DNA damage was quantified in diploid human fibroblasts after synchronizing cell cultures to selected stages of the cell cycle. Incubation of irradiated cells with aphidicolin, an inhibitor of DNA polymerases alpha and delta, delayed the sealing of repair patches and allowed estimation of rates of strand incision by the repair endonuclease. The apparent Vmax for endonucleolytic incision and Km for substrate utilization were determined by Lineweaver-Burk and Eadie-Hofstee analyses. For cells passing through G1, S or G2, Vmax for reparative incision was, respectively, 7.6, 8.4 and 8.4 breaks/10(10) Da per min, suggesting that there was little variation in incision activity during these cell-cycle phases. The Km values of 2.4-3.1 J/m2 for these cells indicate that the nucleotidyl DNA excision-repair pathway operates with maximal effectiveness after low fluences of UV that are in the shoulder region of survival curves. Fibroblasts in mitosis demonstrated a severe attenuation of reparative incision. Rates of incision were 11% of those seen in G2 cells. Disruption of nuclear structure during mitosis may reduce the effective concentration of endonuclease in the vicinity of damaged chromatin. The extreme condensation of chromatin during mitosis also may restrict the accessibility of reparative endonuclease to sites of DNA damage. Confluence-arrested fibroblasts in G0 expressed endonuclease activity with Vmax of 5.5 breaks/10(10) Da per min and a Km of 5.5 J/m2. The greater condensation of chromatin in quiescent cells may restrict the accessibility of endonuclease to dimers and so explain the elevated Km. When fibroblasts were synchronized by serum-deprivation, little variation in reparative endonuclease activity was discerned as released cells transited from early G1 through late G1 and early S. Proliferating fibroblasts in G1 were shown to express comparatively high numbers of reparative incision events in the absence of aphidicolin which was normally used to inhibit DNA polymerases and hold repair patches open. It was calculated that in G0, S and G2 phase cells, single-strand breaks at sites of repair remained open for 30, 19 and 14 sec, respectively. In G1 phase cells, repair sites remained open for 126 sec. Addition of deoxyribonucleosides to G1 cells reduced this time to 42 sec suggesting that the slower rate of synthesis and ligation of repair patches in G1 was due to a relative deficiency of deoxyribonucleotidyl precursors for DNA polymerase.(ABSTRACT TRUNCATED AT 400 WORDS)

Carrier detection in xeroderma pigmentosum

We were able to detect clinically normal carriers of xeroderma pigmentosum (XP) genes with coded samples of either peripheral blood lymphocytes or skin fibroblasts, using a cytogenetic assay shown previously to detect individuals with cancer-prone genetic disorders. Metaphase cells of phytohemagglutinin-stimulated T-lymphocytes from eight individuals who are obligate heterozygotes for XP were compared with those from nine normal controls at 1.3, 2.3, and 3.3 h after x-irradiation (58 R) during the G2 phase of the cell cycle. Lymphocytes from the XP heterozygotes had twofold higher frequencies of chromatid breaks or chromatid gaps than normal (P less than 10(-5)) when fixed at 2.3 or 3.3 h after irradiation. Lymphocytes from six XP homozygotes had frequencies of breaks and gaps threefold higher than normal. Skin fibroblasts from an additional obligate XP heterozygote, when fixed approximately 2 h after x-irradiation (68 R), had a twofold higher frequency of chromatid breaks and a fourfold higher frequency of gaps than fibroblasts from a normal control. This frequency of aberrations in cells from the XP heterozygote was approximately half that observed in the XP homozygote. The elevated frequencies of chromatid breaks and gaps after G2 phase x-irradiation may provide the basis of a test for identifying carriers of the XP gene(s) within known XP families.

Thymidine kinase deficient human cells have increased UV sensitivity in their capacity to support herpes simplex virus but normal UV sensitivity for colony formation

A thymidine kinase deficient (tk-) and two thymidine kinase proficient (tk+) human cell lines were compared for UV sensitivity using colony-forming ability as well as their capacity to support the plaque formation of herpes simplex type 1 (HSV-1). The tk- line (143 cells) was a derivative of one of the tk+ lines (R970-5), whereas the other tk+ line (AC4 cells) was a derivative of the 143 cells obtained by transfection with purified sheared HSV-2 DNA encoding the viral tk gene. 143, R970-5 and AC4 cells showed a similar UV sensitivity for colony-forming ability. In contrast, the capacity to support HSV-1 plaque formation immediately (within 1 h) after UV-irradiation was reduced to a greater extent in the 143 cells compared to the R970-5 and AC4 cells. Capacity curves for plaque formation of the HSV-1: KOS wild-type (tk+) strain were similar to those for the HSV-1: PTK3B mutant (tk-) strain in the 3 cell strains, indicating that the viral tk gene does not influence the ability of HSV-1 to form plaques in UV-irradiated compared to unirradiated human cells. Cellular capacity for HSV-1 plaque formation was found to recover in both tk- and tk+ cells for cultures infected 24 h after UV-irradiation. These results suggest that repair of UV-damaged DNA takes place to a similar extent in both tk- and tk+ human cells, but the kinetics of repair are initially slower in tk- compared to tk+ human cells.

Factors affecting and significance of G2 chromatin radiosensitivity in predisposition to cancer

The frequencies of chromatid breaks and gaps in metaphase cells fixed 2 h after G2 phase X-irradiation (1 Gy) were in almost all cases at least two- to three-fold higher in skin fibroblasts from individuals with genetic conditions predisposing to cancer than in comparable cells from clinically normal controls. Previously, we reported this response in all cancer-prone genetic disorders tested including ataxia telangiectasia, Bloom's syndrome, Fanconi's anemia, xeroderma pigmentosum (XP), familial polyposis, Gardner's syndrome, hereditary malignant melanoma, dysplastic nevus syndrome and cancer family members. One exception was XP-A. In this report we add information on skin fibroblasts from retinoblastoma, Wilms' tumor and XP-C patients, 13 clinically normal controls and six cell lines from fetal or infant cells. Factors affecting the response are identified and include pH, temperature, cell density, culture medium or serum, microbial contamination and visible light exposure (effective wavelength 405 nm). Because of experimental variability, known normal controls should be used in each group of assays. With adequate control of the above factors this response could provide the basis of a test for detecting individuals carrying genes that predispose to a high risk of cancer.

Lack of complementation between xeroderma pigmentosum complementation groups D and H

The construction of permanent hybrid cell lines between xeroderma pigmentosum (XP) cells from different complementation groups allows analysis not only of the degree of repair correction but also of the restoration of biological activity to the UV-irradiated cells. With use of an immortal human cell line (HD2) that expresses excision repair defects typical of XP group D, a series of permanent hybrid cells has been produced with XP cells from groups A to H. Excision repair, as measured by incision analysis and unscheduled DNA synthesis, is restored to normal or near normal levels in crosses involving HD2 and cells from XP groups A, B, C, E, F, G, and I. All these hybrids show complementation for the recovery of normal UV resistance. As expected, hybrids expressing poor incision and hypersensitivity to UV were produced in crosses between HD2 and XPD fibroblasts, but they were also produced without exception when XPH was the partner. In the permanent HD2 x XPD or XPH hybrids, analysis of incision capacity reveals abnormally low activity and therefore that there has been no complementation. The true hybrid nature of HD2 x XPH cells has been confirmed by HL-A and -B tissue typing; moreover, detailed kinetic analysis of incision in these cells shows that the XPH phenotype, rather than the XPD, is expressed, i.e. breaks accumulate at low UV fluence of 1 J/m2. To help confirm these findings, another immortal XPD cell line was used in fusions involving HD2, XPH, or XPI. Cells resistant to ultraviolet were produced only with XPI fibroblasts. These data are discussed in terms of whether XPD and H mutations are likely to be allelic with respect to incision.