Recovery from inhibition by radiation of transcriptionally controlled enzyme induction: a possible probe for DNA repair

A Drosophila model for xeroderma pigmentosum and Cockayne's syndrome: haywire encodes the fly homolog of ERCC3, a human excision repair gene

The haywire gene of Drosophila encodes a protein with 66% identity to the product of the human ERCC3 gene, associated with xeroderma pigmentosum B (XP-B) and Cockayne's syndrome (CS). XP is a human autosomal recessive disease characterized by extreme sensitivity to ultraviolet irradiation and marked susceptibility to skin cancer. In addition, XP and CS patients often exhibit a variety of defects, ranging from central nervous system disorders to hypogonadism. Phenotypes of haywire mutants mimic some of the effects of XP. Many haywire alleles are recessive lethal, viable alleles cause ultraviolet sensitivity, and files expressing marginal levels of haywire display motor defects and reduced life span. Progeny of females carrying a maternal effect allele show central nervous system defects.

Ultraviolet light inactivation of zinc-mediated metallothionein induction in normal and repair-deficient human cells

Synthesis of the low molecular weight, thiol-rich, metal-binding metallothioneins (MTS) is undetectable in normal human (NF) or xeroderma pigmentosum (XP) fibroblasts grown in the absence of excess ZnCl2. Addition of 200 microM ZnCl2 to the growth medium produces an increased MT synthesis rising from the basal rate to a rate at least 50-fold greater than basal rate within 7 h. MT induction kinetics in confluent and in exponentially growing subconfluent monolayers were indistinguishable. Zn2+-mediated MT induction is sensitive to actinomycin D suggesting that the induction process is under transcriptional control. Ultraviolet light irradiation causes a dose-dependent inactivation of Zn2+-mediated MT induction in both NF and XP cells. Post-irradiation incubation of UV-irradiated cells using liquid holding techniques leads to reactivation of Zn2+-mediated MT induction in NF cells but not in XP cells. These findings suggest the utility of MT induction produce transcription-terminating lesions, and (b) in evaluating cellular repair capacity for this class of DNA lesions.

Extent of excision repair before DNA synthesis determines the mutagenic but not the lethal effect of UV radiation

Excision repair-proficient diploid fibroblasts from normal persons (NF) and repair-deficient cells from a xeroderma pigmentosum patient (XP12BE, group A) were grown to confluence and allowed to enter the G0 state. Autoradiography studies of cells released from G0 after 72 h and replated at lower densities (3-9 x 10(3) cells/cm2) in fresh medium containing 15% fetal bovine serum showed that semi-conservative DNA synthesis (S phage) began approximately 24 h after the replating. To determine whether the time available for DNA excision repair between ultraviolet irradiation (254 nm) and the onset of DNA synthesis was critical in determining the cytotoxic and/or mutagenic effect of UV in human fibroblasts, we released cultures of NF or XP12BE cells from G0, allowed them to reattach at lower densities, irradiated them in early G1 (approximately 18 h prior to the onset of S) or just prior to S phase, and assayed the frequency of mutations to 6-thioguanine resistance and the survival of colony-forming ability. The XP12BE cells, which are virtually incapable of excising UV-induced DNA lesions, showed approximately the same frequency of mutations and survival regardless of the time of UV irradiation. In NF cells, the slope of the dose response for mutations induced in cells irradiated just prior to S was about 7-fold steeper than that of cells irradiated 18 h earlier. However, the two sets of NF cells showed no significant difference in survival. Neither were there significant differences in the survival of NF cells released from G0, plated at cloning densities and irradiated as soon as they had attached and flattened out. (approximately 20 h prior to S) or 4, 8, 12, 16, 20, or 24 h later. We conclude that the frequency of mutations induced by UV is dependent upon the number of unexcised lesions remaining at the time of semi-conservative DNA replication. However, the amount of time available for excision of potentially cytotoxic lesions is not determined primarily by the period between irradiation and the onset of S phase.

Relationship between excision repair and the cytotoxic and mutagenic effect of the 'anti' 7,8-diol-9,10-epoxide of benzo[a]pyrene in human cells

The cytotoxic and mutagenic effect of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti BPDE) in normally excising diploid human cells treated just prior to onset of S was compared with that of cells allowed approximately 16 h for excision repair before onset of S and with that observed in excision-deficient xeroderma pigmentosum (XP12BE) cells. The cells were synchronized by release from density inhibition of cell replication. DNA synthesis began approximately 22 h after the cells were plated at lower density (i.e., 1.4 x 10(4) cells/cm2). The frequency of thioguanine-resistant mutants induced in normal cells treated just prior to onset of S was approximately 12- to 16-fold higher than that observed in cells treated in early G1 or treated in G0 (confluence) and then plated at lower density. The frequency approximated that expected for XP12BE cells from extrapolation of data obtained at lower doses. The frequency of mutants measured in normal cells treated in exponential growth was also much higher than that in the cells treated in early G1 or in G0. No such difference could be seen in XP12BE cells treated in exponential growth or in G0. In contrast to the mutagenicity data in the normal cells, there was no significant difference in the slope of the survival curve of normal cells treated at various times prior to S phase at low densities. However, normal cells treated even at the onset of S exhibited survival equal to XP12BE cells given a 4- to 5-fold lower dose. The data support the hypothesis that DNA synthesis is the cellular event which converts unexcised DNA lesions into mutations. However, they indicate that S is not the event primarily responsible for translating DNA damage into cell death. Accompanying studies on the rate of excision of anti BPDE adducts from the normal cells during the period prior to S support the conclusions.

Fractionated low-level gamma irradiation of Chinese Hamster cells: cellular and biochemical effects

Chinese hamster V79 cells in log-phase were exposed daily to 0.6 Gy of gamma radiation for 3-6 months. After such an exposure the population doubling time increased from 10 to 15 h. When irradiation was discontinued doubling time gradually decreased. Cell survival following acute radiation dose of the low-level irradiated cells remained the same as that of untreated cells. The fractionated irradiation did not affect the capacity of the cells to perform DNA repair synthesis. Likewise, the sensitivity to inhibition by acute radiation exposure of the ability to induce ornithine decarboxylase activity was similar in cells exposed to fractionated irradiation and in untreated cells. It is concluded that there is no apparent effect of sublethal radiation dose received in one generation on the radiation sensitivity of the succeeding generations during the log-phase of growth.

An apparent correlation between the inhibition of induced ornithine decarboxylase activity by gamma radiation and the capacity for DNA repair synthesis in normal and ataxia telangiectasia human fibroblasts: no correlation with cell survival

Exposure of normal human fibroblasts (F107) in stationary phase to gamma radiation inhibited the appearance of induced ornithine decarboxylase (ODC) activity. Skin fibroblasts derived from two ataxia telangiectasia (AT) patients (F184 and F182) displayed a similar response. The level of DNA repair synthesis was also similar in the three cell strains. Fibroblasts from another apparently normal donor (F196) were very sensitive to inhibition of induced ODC activity by gamma radiation and were also deficient in radiation-induced DNA repair synthesis. However, the two strains derived from normal donors displayed the same degree of cellular sensitivity towards X-rays, whereas the two AT strains showed the typical hypersensitivity to the cytotoxic effect of X-irradiation. The results suggest a possible correlation between the inhibition of induced ODC activity by gamma radiation and the extent of DNA repair synthesis at high radiation doses, but there is no correlation between these two parameters and cellular survival at low radiation doses.