DNA repair synthesis in human heterokaryons. III. The rapid and slow complementing varieties of xeroderma pigmentosum.

Patients with Xeroderma pigmentosum and defective DNA excision repair can be distinguished as a rapid (r-XP) and slow (s-XP) complementing variety. When fused with normal cells, fibroblasts from the r-XP are complemented rapidly and in the absence of protein synthesis while those from the s-XP are complemented slowly by a process partly, but not entirely, dependent on protein synthesis. Heterokaryons with different ratios of r-XP to s-XP nuclei (i.e. 1:1-5 and 1-5:1) and control heterokaryons containing one normal and 1-5 r- or s-XP nuclei show that if cell fusion and incubation is conducted in medium preventing protein synthesis, the rXP cells do not complement the s-XP partner at all and, conversely, that the latter is not as effective as normal cells at complementing the rXP partner. On the contrary, if protein synthesis is permitted, the 2 types of XP cells complement each other in a gene dose-dependent manner and to an extent similar to that observed in the control heterokaryons. These findings indicate that the r- and s-XP varieties are caused by mutations at different loci and suggest that the products of these loci interact to produce a functional unit which is present in normal control cells but absent in the XP strains. The relationship between the complementation groups described here and those already reported in the literature being investigated.     

Complementation analysis of xeroderma pigmentosum variants

DNA repair after UV exposure was studied in multinucleate cells, obtained after fusion of excision-defective and variant xeroderma pigmentosum fibroblasts. Optimal fusion conditions were determined, facilitating the measurement of DNA replication in heterokaryons. In unirradiated multikaryons, entry into the S phase was depressed, when compared with unfused cells. The extent of the depression of S phase entry was dependent on the fusion conditions. In heterokaryons obtained after fusion of XP variant (6 different strains) with excision-defective XP (three cell strains from complementation groups A, C and D) both unscheduled DNA synthesis and postreplication repair after UV irradiation were restored to normal levels. In contrast, complementation was not observed after pairwise fusion of the XP variant cell strains. These results suggest that the XP variants comprise a single complementation group, different from complementation groups A, C and D.     

Interaction of human and chick DNA repair functions in UV-irradiated xeroderma pigmentosum-chick erythrocyte heterokaryons

Fusion of chick erythrocytes with human primary fibroblasts results in the formation of heterokaryons in which the inactive chick nuclei become reactivated. The expression of chick DNA repair functions was investigated by the analysis of the DNA repair capacity after exposure to ultraviolet (UV) irradiation of such heterokaryons obtained after fusion of chick erythrocytes with normal human or xeroderma pigmentosum (XP) cells of complementation groups A, B, C and D. Unscheduled DNA synthesis (UDS) in normal human nuclei in these heterokaryons is suppressed during the first 2–4 days after fusion. The extent and duration of this suppression is positively correlated with the number of chick nuclei in the heterokaryons. Suppression is absent in heterokaryons obtained after fusion of chicken embryonic fibroblasts with XP cells (complementation group A and C).Restoration of DNA repair synthesis is found after fusion in XP nuclei of all complementation groups studied. It occurs rapidly in XP group A nuclei, starting one day after fusion and reaching near normal human levels after 5–8 days. In nuclei of the B, C and D group increased levels of UDS are found 5 days after fusion. At 8 days after fusion the UDS level is about 50% of that found in normal human nuclei. The pattern of UDS observed in the chick nuclei parallels that of the human counterpart in the fusion. A fast complementation pattern is also observed in chick fibroblast-XP group A heterokaryons resulting within 24 h in a UDS level comparable with that in chick fibroblast-normal human heterokaryons. In heterokaryons obtained after fusion of chick fibroblasts with XP group C cells UDS remains at the level of chick cells. These data suggest that reactivation of chick erythrocyte nuclei results in expression of repair functions which are able to complement the defects in the XP complementation groups A, B, C and D.     

Caffeine inhibition of the repair of ultraviolet-irradiated adenovirus in human cells.

Caffeine is shown to block repair of ultraviolet-irradiated adenovirus 2 when the irradiated virus infects normal human fibroblasts from a xeroderma pigmentosum (XP) variant. Such blockage is not observed when the irradiated virus infects XP fibroblasts belonging to XP complementation group A. Thus normal and XP variant cells have a caffeine-sensitive repair process. This may be either excision or an excision dependent repair process because fibroblasts belonging to XP complementation group A are believed to lack the excision repair process.     

Interspecies complementation analysis of xeroderma pigmentosum and UV-sensitive chinese hamster cells

Complementation analysis was performed 24 h after fusion of UV-sensitive CHO cells (CHO 12 RO) with XP cells of complementation groups A, B, C, D, F and G. The parental cells are characterized by low levels of unscheduled DNA synthesis (UDS). In all combinations, the UDS levels observed in heterokaryons were higher than those in parental mutant cells, clearly indicating cooperation of human and Chinese hamster repair functions. In heterokaryons of CHO 12 RO with XP-A and XP-C cells, the UDS values reached about the normal human level, whereas in heterokaryons with XP-B, XP-D and XP-F, UDS was restored at a level approaching that in wild-type CHO cells. The results obtained after fusion of CHO cells with two representative cell strains from the XP-G group, XP 2 BI and XP 3 BR, were inconsistent. Fusion with XP 3 BR cells yielded UDS levels ranging from wild-type Chinese hamster to normal human, whereas fusion with XP 2 BI cells resulted in a slight increase in UDS which even after 48 h remained below the level found in wild-type CHO cells. The occurrence of complementation in these interspecies heterokaryons indicates that the genetic defect in the CHO 12 RO cells is different from the defects in the XP complementation groups tested.     

Genetic complementation between UV-sensitive CHO mutants and xeroderma pigmentosum fibroblasts

The purpose of this study was to determine the feasibility of doing complementation analysis between DNA-repair mutants of CHO cells and human fibroblasts based on the recovery of hybrid cells resistant to DNA damage. Two UV-sensitive CHO mutant lines, UV20 and UV41, which belong to different genetic complementation groups, were fused with fibroblasts of xeroderma pigmentosum in various complementation groups. Selection for complementing hybrids was performed using a combination of ouabain to kill the XP cells and mitomycin C to kill the CHO mutants. Because the frequency of viable hybrid clones was generally < 10⁻⁶ and the frequency of revertants of each CHO mutant was 2×10⁻⁷, putative hybrids required verification. The hybrid character of clones was established by testing for the presence of human DNA in a dot-blot procedure.

Hybrid clones were obtained from 9 of the 10 different crosses involving 5 complementation groups of XP cells. The 4 attempted crosses with 2 other XP groups yielded no hybrid colonies. Thus, a definitive complementation analysis was not possible. Hybrids were evaluated for their UV resistance using a rapid assay that measures differential cytotoxicity (DC). All 9 hybrids were more resistant than the parental mutant CHO and XP cells, indicating that in each case complementation of the CHO repair defect by a human gene had occurred. 3 hybrids were analyzed for their UV-radiation survival curves and shown to be much more resistant that the CHO mutants but less resistant than normal CHO cells. With 2 of these hybrids, sensitive subclones, which had presumably lost the complementing gene, were found to have similar sensitivity to the parental CHO mutants. We conclude that the extremely low frequency of viable hybrids in this system limits the usefulness of the approach. The possibility remains that each of the nonhybridizing XP strains could be altered in the same locus as one of the CHO mutants.     

U.V. enhanced reactivation of U.V.-and gamma-irradiated adenovirus in Cockayne syndrome and Xeroderma pigmentosum fibroblasts

U.V.-enhanced reactivation (UVER) of both U.V.-irradiated and gamma-irradiated human adenovirus type 2 (Ad 2) was examined following the infection of a variety of Cockayne Syndrome (CS) and Xeroderma pigmentosum (XP) fibroblast strains which had been pre-irradiated with U.V. light. U.V.-irradiated or non-irradiated fibroblasts were infected with either non-irradiated or irradiated Ad2, and at 48 hours after infection cells were examined for the presence of viral structural antigens (Vag) using immunofluorescent staining. Normal levels of UVER (i.e. 2-4 fold) of U.V.- and of gamma-irradiated Ad 2 were detected in 2 CS strains (CS IBE and CS 3BE), 2 XP complementation group A strains (XP 12BE and XP 25RO), and 2 XP complementation group D strains (XP 5BE and XP 6BE), although the U.V. doses to these mutant cells which resulted in peak UVER values (0 . 2 Jm-2 for XP 25RO, 0 . 14 Jm-2 for XP 12BE, 0 . 8 Jm-2 for XP 5BE and XP 6BE, and 1 . 6-5 . 0 Jm-2 for CS 1BE and CS 3BE) were considerably lower than those yielding peak UVER in normal strains (10-15 Jm-2). XP variant strains (XP 4BE and XP 115LO), however, showed substantially lower levels of UVER than normal strains.     

Repair by human cells of adenovirus-2 damaged by psoralen plus near ultraviolet light treatment.

Adenovirus-2 is damaged by treatment with psoralen plus near-ultraviolet (UV) light is shown by reduced ability to infect human fibroblasts. The apparent sensitivity of the virus to this treatment depended upon the strain of cells used. The virus was 3-4 times more sensitive to the treatment when infecting xeroderma pigmentosum (XP complementation groups A or D) fibroblasts than when infecting normal fibroblasts. DNA extracted from virus preparations that had undergone such treatment was analyzed for treatment-induced crosslinks by gel electrophoresis and sedimentation in alkaline sucrose gradients. The fraction of adenovirus DNA molecules remaining non-crosslinked after treatment was found to correlate with the survival of the virus in normal fibroblasts. This result showed that the psoralen plus near-UV treatment gave rise to non-crosslink lesions (presumably psoralen-DNA mono-adducts), that were repairable by normal but not by XP fibroblasts, and suggested the possibility that normal fibroblasts cannot repair this type of crosslink in the DNA of an infecting adenovirion.     

Host-cell reactivation of UV-irradiated and chemically-treated herpes simplex virus-1 by xeroderma pigmentosum, XP heterozygotes and normal skin fibroblasts

The host-cell reactivation of UV-irradiated and N-acetoxy-2-acetylamino-fluorene-treated herpes simplex virus type 1 strain MP was studied in normal and xeroderma pigmentosum human skin fibroblasts. Virus treated with either agent demonstrated lower survival in XP cells from complementation groups A, B, C and D than in normal fibroblasts. The relative reactivation ability of XP cells from the different genetic complementation groups was found to be the same for both irradiated and chemically treated virus. In addition, the inactivation kinetics for virus treated with either agent in the XP variant were comparable to that seen in normal skin fibroblasts. The addition of 2 or 4 mmoles caffeine to the post-infection assay medium had no effect on the inactivation kinetics of virus treated by either agent in the XP variant or in XP cells from the different genetic complementation groups. Treatment of the virus with nitrogen mustard resulted in equivalent survival in normal and XP genetic complementation group D cells. No apparent defect was observed in the ability of XP heterozygous skin fibroblasts to repair virus damaged with up to 100 μg N-acetoxy-2-acetylaminofluorene per ml. These findings indicate that the repair of UV-irradiated and N-acetoxy-2-acetylaminofluorene-treated virus is accomplished by the same pathway or different pathways sharing a common intermediate step and that the excision defect of XP cells plays little if any role in the reactivation of nitrogen mustard treated virus.     

Complementation studies with enucleated fibroblasts from different variants of beta-galactosidase deficiency.

Somatic cell hybridization of β-galactosidase fibroblasts derived from patients with the infantile type 1 and the adult type 4 G_M1-gangliosidosis results in restoration of the β-galactosidase activity. The kinetic properties of the enzyme activity in heterokaryons were found to be similar as in controls. Genetic complementation did not occur after inhibition of protein synthesis by cycloheximide indicating the necessity of de novo protein synthesis. When fibroblasts of the adult type 4 patient were enucleated and fused with cells from an infantile type 1 variant no restoration of β-galactosidase in the hybrids was observed. Fusion of enucleated type 1 cells with nucleated type 4 cells, however, did result in genetic complementation. The results obtained in this study and observations by others fit with the hypothesis of intergenic complementation. In heterokaryons the adult type 4 genome codes for normal β-galactosidase monomers and the infantile type 1 cells or cytoplasts provide a protein factor necessary for the hydrolytic activity and aggregation of the molecule.     

Infection of UV-irradiated xeroderma pigmentosum fibroblasts by herpes simplex virus: study of capacity and Weigle reactivation.

The capacity of monolayers of both normal human and xeroderma pigmentosum (XP) filbroblasts to support plaque formation by herpes simplex virus was decreased when the monolayers were ultraviolet (UV) irradiated and infected with virus. Fibroblasts of XP complementation groups A, B, and D were sensitive to UV, being 4-6 fold more sensitive than either fibroblasts of XP complementation group C or fibroblasts from a normal individual. When the monolayers were irradiated 4 days prior to infection, the capacity of normal fibroblasts to support herpes virus growth recovered, whereas the capacity of the XP strains decreased further compared to that measured when infection immediately followed irradiation. Concurrent experiments with UV-irradiated herpes virus showed that the survival of this virus did not increase when infection by irradiated virus immediately followed irradiation of the monolayers. However, if the monolayers were irradiated 4 days prior to infection, the survival of this virus increased by a factor of nearly 2. Such Weigle reactivation (WR) occurred at lower fluences to the XP fibroblasts than to normal fibroblasts, suggesting that WR results from residual cellular DNA damage left after excision repair.     

Repair of damage by ultraviolet radiation in xeroderma pigmentosum cell strains of complementation groups E and F

The xeroderma pigmentosum fibroblast strains XP2RO, complementation group E, and XP23OS, group F, were compared with normal human primary fibroblasts with regard to repair of damage induced by 254-nm UV. In XP2RO cells, repair DNA synthesis, measured by autoradiography (unscheduled DNA synthesis = UDS), was about 50% of the value found in normal human cells. In these cells also the removal of UV-induced sites recognized by a specific UV-endonuclease proceeds at a reduced rate. By having BUdR incorporated into the repaired regions, followed by the induction of breaks in these patches by 313-nm UV, it was shown that the reduced repair synthesis is not caused by a shorter length of the repair regions in XP2RO, but is solely due to a reduction in the number of sites removed by excision repair. In XP23OS a discrepancy was observed between the level of UDS, which was about 10% of the normal value, and other repair-dependent properties such as UV survival, host-cell reactivation and removal of UV-endonuclease-susceptible sites, which were less reduced than could be expected from the UDS level. However, when UDS was followed over a longer period than the 2 or 3 h normally used in UDS analysis, it appeared that in XP23OS cells, the rate of UDS remained constant whereas the rate decreased in normal control cells. Consequently, the residual level of UDS varies with the period over which it is studied.     

Genetic complementation of propionyl-CoA carboxylase deficiency in cultured human fibroblasts.

Propionyl-CoA carboxylase (PCC) deficiency is an inherited metabolic disorder showing considerable variability of expression. We have investigated the possibility that there is a genetic basis for the clinical heterogeneity in this disorder by examining complementation in Sendai virus mediated heterokaryons of mutant fibroblast strains. Restoration of PCC activity was monitored in individual multinucleate cells in situ using a radioautographic procedure which detects the incorporation of 14C-propionate into trichloracetic acid precipitable material. Each mutant strain incorporated negligible amounts of radioactivity compared to control strains. Activity was not restored when different mutants were mixed without virus or when homokaryons were produced by self-fusion. Seven mutant strains were fused in all pairwise combinations and examined for increased 14C-propionate incorporation in heterokaryons. Two main complementation groups were revealed. One group was composed of three mutants. The other was a complex group composed of four mutants in which intragroup complementation was demonstrated. Two mutants showing excellent complementation by radioautography were examined for complementation by the direct assay of PCC ACTIVITY. The enzyme activity of virus-treated preparations with 23% multinucleate cells was 183 U (pmol/min/mg protein) compared to 16 U for the untreated mixture (normal range 450-850 u). We conclude that PCC deficiency resulted from mutations of heterogeneous origin, although the classification of mutants into complementation groups did not correlate with patterns of clinical heterogeneity.     

Phenotypic correction of the defect in xeroderma pigmentosum cells after fusion with isolated cytoplasts

The cybridization technique was used to study the role of cytoplasmic and nuclear factors in complementation of the repair defects in xeroderma pigmentosum (XP) cells. Cybrids were prepared by fusion of UV-exposed XP cells with cytoplasts derived from normal human or complementing XP cells. Phenotypic correction of the DNA repair defect measured by unscheduled DNA synthesis (UDS) occurred in these cybrids. The results show that the correcting factors are present in the cytoplasts and can move into the nucleus of the UV-exposed XP cell almost immediately after fusion. The defective repair in the nuclei of XP complementation group A cell strains is corrected with fast kinetics reaching normal UDS levels within 2 h after fusion. In the A-group cybrids the correcting activity decreased with a half-time of about 12 h. Correction of the XP group C defect occurred at a much slower rate, indicating that different factors are involved in the correction of the XP-A and XP-C defects.     

Decreased repair of gamma-irradiated adenovirus in Xeroderma pigmentosum fibroblasts.

The ability of gamma-irradiated adenovirus to produce viral structural antigens (Vag) was examined in several normal and Xeroderma pigmentosum (XP) fibroblast strains. The fibroblast cultures were infected with either irradiated or nonirradiated adenovirus and at 48 hours after infection, cells were examined for the presence of Vag using immunofluorescent staining. Survival of Vag synthesis for gamma-irradiated adenovirus had a D37 value of 47 +/- 4 x 10(4) rad following the infection of seven normal fibroblast strains. The survival of this viral function was found to be significantly less following infection of the XP strains. D37 values for Vag synthesis expressed as a percentage of that obtained on normal strains were obtained for a representative strain from each of the XP complementation groups: group A, 57 per cent; group B, 61 per cent; group C, 61 per cent, group D, 59 per cent; group E, 73 per cent; and variant, 75 per cent. These results indicate that XP cells have a reduced repair capacity for some type of gamma-ray-induced DNA damage.     

A comparison of the response of unstimulated and stimulated T-lymphocytes and fibroblasts from normal, xeroderma pigmentosum and trichothiodystrophy donors to the lethal action of UV-C.

Unstimulated T-lymphocytes from normal donors are significantly more sensitive to the lethal effects of UV-C than either stimulated T-lymphocytes or fibroblasts as judged by colony-forming ability. Data from other studies suggest that excision repair is more effective in stimulated than unstimulated T-lymphocytes leading to the prediction that these differences in survival should be minimal in cells established from excision defective donors. The prediction was met with XP6BR, a donor of unknown complementation group. For 3 XP's from complementation group D, however, enhanced survival in stimulated T-cells was observed. With cells from an excision-defective TTD who was included in complementation group D of XP both fibroblasts and unstimulated T-lymphocytes were hypersensitive. For a second excision defective TTD patient who was excluded from complementation group D, the unstimulated T-lymphocytes were more resistant than those of normal donors although the fibroblasts were hypersensitive. These results suggest that the in vitro response of stimulated T-lymphocytes or fibroblasts may not reflect the in vivo response of cells, as measured by the response of unstimulated T-lymphocytes.     

Transient correction of excision repair defects in fibroblasts of 9 xeroderma pigmentosum complementation groups by microinjection of crude human cell extracts

Crude extracts from human cells were microinjected into the cytoplasm of cultured fibroblasts from 9 excision-deficient xeroderma pigmentosum (XP) complementation groups. The level of UV-induced unscheduled DNA synthesis (UDS) was measured to determine the effect of the extract on the repair capacity of the injected cells. With a sensitive UDS assay procedure a (transient) increase in UV-induced UDS level was found in fibroblasts from all complementation groups after injection of extracts from repair-proficient (HeLa) or complementing XP cells (except in the case of XP-G), but not after introduction of extracts from cells belonging to the same complementation group. This indicates that the phenotypic correction is exerted by complementation-group-specific factors in the extract, a conclusion that is in agreement with the observation that different levels of correction are found for different complementation groups. The XP-G-correcting factor was shown to be sensitive to proteolytic degradation, suggesting that it is a protein like the XP-A factor.     

The rate of removal of pyrimidine dimers in quiescent cultures of normal human and xeroderma pigmentosum cells

The rate of removal of pyrimidine dimers from DNA of UV (254 nm)-irradiated (1 J/m2) normal and xeroderma pigmentosum (XP) cells maintained in culture as nondividing populations was determined. Several normal and XP strains from complementation groups A, C and D were studied. The excision rates and survival ability of nondividing cells were examined to determine if an abnormal sensitivity was associated with a decreased rate of dimer excision. The results show that all normal strains studied excise pyrimidine dimers at the same rate, with the rate curve characterized by two components. All 'excision-deficient' XP strains excise dimers at a slower-than-normal rate, with the rate curves also characterized by two components. The rate constants for the first components of all of the XP strains (group A, C and D) are the same, one tenth of the normal rate constant, except for XP8LO (group A). XP8LO has a first-component rate constant similar to that of normal strains and a second component rate constant similar to that of other group A strains (XP12BE, XP25RO). Thus, the slower rate of dimer excision in XP8LO is due to a defect in the mechanism responsible for the second component of the excision-rate curve. In general, an abnormal sensitivity of nondividing cells to UV is associated with a reduced dimer-excision rate. A notable exception to this is the group C strain XP1BE which has an initial repair rate similar to that of group A XP12BE but is considerably more resistant when survival is measured.     

Roles of DNA interstrand crosslinking and its repair in the induction of sister-chromatid exchange and a higher induction in fanconi's anemia cells

The roles of DNA crosslink and its repair in the induction of sister-chromatid exchanges (SCEs) were studied in normal, xeroderma pigmentosum (XP) complementation group A, and Fanconi's anemia (FA) fibroblasts after treatment with mitomycin C (MC) or decarbamoyl mitomycin C (DMC) for 1 h. FA strains were 5—30-fold more sensitive to MC killing than normal cells, but normally responded to DMC killing. XP group-A cells were twice and only slightly more sensitive to DMC and MC killings, respectively, than normal cells. The induction rate of immediate SCEs by MC was 1.7 times higher, despite a normal SCE rate by DMC, in FA strains than that in normal cells. Alternatively, SCE rates by DMC and MC were 6 times and only 1.3 times higher, respectively, in XP cells than in normal cells. In normal cells, the reduction of MC-induced SCEs as a function of repair time followed a biphasic curve of the first rapid (half-life, 2 h) and the second slow (half-life, 14 h) components. Such components corresponded exactly to the first half-excision and the second slow repair processes of molecular crosslink repair. In MC-induced SCEs, FA17JTO cells exhibited only the slow reduction component without the first rapid component and a higher saturation level in the time-dependent reduction in SCEs. This indicates that SCEs are produced by crosslinks remaining unrepaired for long times (24—48 h) after treatment of FA cells. Conversely, XP group-A cells capable of the first half-excision manifested the first rapid reduction in SCEs, although the second component declined at the slowest rate (half-life, 48 h) owing to a defect in the second mono-adduct repair. The reduction in DMC-induced SCEs followed only the slow component. Thus, these results demonstrate that crosslink can be the lesion leading to SCE, and the MC-induced SCE frequency is higher in FA cells than in normal cells. In the FA20JTO strain, such a repair defect seemed to be less than in FA17JTO cells, judged from the survival and SCE characteristics.     

Transformation and immortalization of diploid xeroderma pigmentosum fibroblasts

Diploid xeroderma pigmentosum (XP) skin fibroblast strains from various XP-complementation groups (B, C, G, and H) were transformed with an origin-defective SV40 early region or with the pSV3 gpt plasmid. In the latter case, transfected cells were selected for their ability to express the dominant xgpt gene. Immortalized cell lines were obtained, from XP-complementation groups C (8CA, 3MA, and 20MA; XP3MA and XP20MA were formerly considered to belong to complementation group I), G (2BI and 3BR), and H (2CS). No immortalized cells could be isolated from complementation group B (11BE). The immortalization frequency of wild-type diploid fibroblasts and diploid cultures from XP patients was not significantly increased by cotransfection with the SV40 early region plus several selected viral and cellular oncogenes. In fact, co-transfection with some of the oncogenes caused a marked decrease of the transformation frequency. The observed immortalization occurred at a frequency of approximately 5 x 10(-8).