Loss of repairability of DNA interstrand crosslinks in Fanconi's anemia cells with culture age

Optimization of the comet assay for the sensitive detection of PUVA-induced DNA interstrand cross-links

Psoralen plus ultraviolet A (PUVA), commonly used for the treatment of hyperproliferative skin disorders, has been found to be associated with an increased risk of squamous cell cancer. Interstrand cross-link (ICL) formation by PUVA treatment is considered the major factor contributing to the carcinogenesis. However, it remains unclear how PUVA causes, or promotes cancers, in humans. As an initial step in understanding the mechanisms of mutagenesis and carcinogenesis of PUVA photochemotherapy, we have optimized and subsequently utilized a modified alkaline comet assay involving a post-lysis gamma-irradiation at 9 Gy to sensitively measure the formation and repair of PUVA-induced ICLs in the immortalized human keratinocyte cell line HaCaT. A clear dose-dependent response of HaCaT cells to PUVA exposure was observed with a combination of a fixed UVA dose at 0.05 J/cm(2) and a dose of 8-methoxypsoralen ranging from 10 to 100 microM. Results also indicated that the ICL repair was concentration dependent. We have also demonstrated that PUVA-induced monoadduct formation, at an estimated ratio of 3:1 to ICLs in the present experimental conditions, does not interfere with the detection of the ICLs in the modified alkaline comet assay. Furthermore, comparison of the amount of ICL formation between the single-dose UVA treatment and a split-dose protocol was performed. The split-dose protocol was believed to generate more ICLs than the single-dose treatment, thus more effective in PUVA photochemotherapy. Our results demonstrate that comparable amounts of ICLs were formed in HaCaT cells for each dose of UVA, using either the split-dose or single-dose protocols.

Fanconi anemia revisited: old ideas and new advances

This review summarizes both historical and more recent data on the clinical, cellular and genetic features of Fanconi anemia (FA), a rare autosomal recessive disorder. FA patients are characterized by pancytopenia, congenital malformations, growth delay and an increased susceptibility to the development of malignancies, particularly acute myelogenous leukemia. FA cells show chromosomal fragility, slow growth and increased sensitivity to DNA crosslinking agents. FA can be caused by defects in any one of at least four genes. Two general hypotheses have been proposed to explain the underlying defect: loss of a DNA repair function or of a step in the defense toward oxygen toxicity. After many attempts to clone the FA genes, the first one, that defective in group C, has been cloned by complementation of the increased sensitivity of FA(C) cells to mitomycin C and diepoxybutane. This gene (FACC) codes for a novel protein and is ubiquitously expressed. Mutations in various FA(C) patients that cause loss of function have been identified. The review concludes by suggesting directions for future research in FA.

Induction and removal of interstrand crosslinks in the ribosomal RNA genes of lymphoblastoid cell lines from patients with Fanconi anemia

The repair of interstrand crosslinks has been investigated in Fanconi anemia (FA) and normal cells as there is evidence suggesting that FA patients have a defect in DNA repair. Lymphoblasts were treated with the crosslinking agent mitomycin C (MMC) and the removal of the induced DNA lesions investigated at the level of the actively transcribed ribosomal RNA (rRNA) genes. MMC-induced crosslinks appeared to be a rather stable lesion in the rRNA genes for all cell lines studied. Variable repair efficiencies were found between the different cells lines but they could not be used to distinguish normal cells from FA cells. Therefore, we propose that the specific sensitivity of FA cells towards MMC cannot be directly correlated with a deficient repair in interstrand crosslinks and that probably the complexity of the repair process is greater than previously described.

Defective DNA endonuclease activities in Fanconi's anemia cells, complementation groups A and B

Cells from patients with the inherited disorder, Fanconi's anemia (FA), were analyzed for endonucleases which recognize DNA interstrand cross-links and monoadducts produced by psoralen plus UVA irradiation. Two chromatin-associated DNA endonuclease activities, defective in their ability to incise DNA-containing adducts produced by psoralen plus UVA light, have been identified and isolated in nuclei of FA cells. In FA complementation group A (FA-A) cells, one endonuclease activity, pI 4.6, which recognizes psoralen intercalation and interstrand cross-links, has 25% of the activity of the normal human endonuclease, pI 4.6, on 8-methoxypsoralen (8-MOP) plus UVA-damaged DNA. In FA complementation group B (FA-B) cells, a second endonuclease activity, pI 7.6, which recognizes psoralen monoadducts, has 50% and 55% of the activity, respectively, of the corresponding normal endonuclease on 8-MOP or angelicin plus UVA-damaged DNA. Kinetic analysis reveals that both the FA-A endonuclease activity, pI 4.6, and the FA-B endonuclease activity, pI 7.6, have decreased affinity for psoralen plus UVA-damaged DNA. Both the normal and FA endonucleases showed approximately a 2.5-fold increase in activity on psoralen plus UVA-damaged reconstituted nucleosomal DNA compared to damaged non-nucleosomal DNA, indicating that interaction of these FA endonucleases with nucleosomal DNA is not impaired. These deficiencies in two nuclear DNA endonuclease activities from FA-A and FA-B cells correlate with decreased levels of unscheduled DNA synthesis (UDS), in response to 8-MOP or angelicin plus UVA irradiation, in these cells in culture.

Inactivation by nitrogen mustard of plasmids introduced into normal and Fanconi's anaemia cells

An SV40-transformed Fanconi's anaemia (FA) cell line, GM6914, exhibits approximately 2.4-fold increased sensitivity to the cytotoxic effects of nitrogen mustard (NM) when compared with the normal line, MRC5-V1. Host cell reactivation of NM-treated plasmid has been investigated using transient expression vectors which contain the chloramphenicol acetyltransferase (CAT) gene. In both cell types there is a similar, dose-dependent reduction in CAT expression which correlates with an increase in NM-induced DNA-interstrand crosslinking. The data are consistent with two possible mechanisms for inactivation of the plasmid. Either a single crosslink anywhere within the plasmid is sufficient to prevent transcription of the cat gene. Alternatively, inactivation may result from some other more prevalent NM-induced lesions within the cat coding sequence.

The fate of 8-methoxypsoralen-photoinduced DNA interstrand crosslinks in Fanconi's anemia cells of defined genetic complementation groups

The fate of 8-methoxypsoralen (8-MOP)-photoinduced DNA interstrand crosslinks was followed by alkaline elution in Fanconi's anemia (FA) fibroblasts belonging to complementation groups A (FA 150 and FA 402) and B (FA 145) in comparison to a normal (1 BR/3) and a heterozygote (F 311) cell line. Clonogenic cell survival to 8-MOP photoaddition was established in parallel for all cell lines. In comparison to normal cells, group A FA cells demonstrated a higher photosensitivity than group B cells (sensitivity index 2.3 and 1.5, respectively), the heterozygote cell line being only slightly more sensitive. FA cells from both groups A and B demonstrated an incision capacity of crosslinks, the kinetics and extent of which being, however, different from that of normal or heterozygote cells. The incision is slower in FA cells and, at 24 h of post-treatment incubation, the amount of crosslinks incised is clearly lower than that observed in normal cells for group A cells, whereas in group B cells incision approaches the level of normal cells. These results correlate with survival as well as with rates of DNA semi-conservative synthesis after 8-MOP photoaddition.

Abnormal response to DNA crosslinking agents of Fanconi anemia fibroblasts can be corrected by transfection with normal human DNA

Primary skin fibroblast cell lines from patients with Fanconi anemia were cotransfected with UV-irradiated pSV2neo plasmids and high molecular weight DNA from normal human cells. Restoration of a normal cellular resistance to mitomycin C (MMC) was observed provided that a Fanconi anemia cell line is selected for DNA-mediated transformation (neo gene) and that at least two successive rounds of transfection are performed. Cells were selected by taking advantage of the higher proliferation rate and plating efficiency of the MMC resistant transformants. As estimated from reconstruction experiments, the frequency of transfer of MMC resistance lies between 1 and 30 X 10(-7). The MMC resistance phenotype was maintained for at least 10 generations following transfection. Evidence for DNA-mediated transformation also includes the recovery of a normal pattern of DNA semiconservative synthesis after treatment with 8-methoxypsoralen and 365-nm UV irradiation, and the presence of exogenous pSV2neo DNA sequences was shown by Southern blot analysis. The acquired MMC resistance is probably due to the presence of DNA from normal cells. Indeed, sensitivity to MMC was maintained when Fanconi anemia cells were cotransfected with the UV-irradiated pSV2neo plasmid mixed with their own DNA or with yeast or salmon sperm DNA. These negative results also render unlikely the selection of spontaneous MMC resistant revertants in transfection of Fanconi anemia cells with normal DNA. These experiments establish the prerequisites for the isolation of the gene(s) involved in the response to DNA crosslinking lesions in human cells.

Psoralen-DNA crosslink repair in human lymphocytes. Comparison of alkaline elution with electron microscopy

Much interest has surrounded the question of the removal of psoralen interstrand crosslinks in DNA of eukaryotic organisms. A commonly employed method for the study of psoralen repair is alkaline elution. In this study we have used alkaline elution to assess psoralen crosslink repair in human lymphocytes. The lymphocytes were treated with 8-methoxypsoralen or 4,5',8-trimethylpsoralen and allowed to repair for different periods of time. Analysis by alkaline elution showed elution patterns compatible with crosslink removal. When the crosslink removal under comparable conditions was studied by the use of electron microscopy under totally denaturing conditions, no repair of the crosslinks could be detected.

Psoralen photoinactivation of herpes simplex virus: monoadduct and cross-link repair by xeroderma pigmentosum and Fanconi's anemia cells

Furocoumarins (psoralen and its derivatives) are used to photoinactivate a variety of viruses and cell types. In the presence of long-wavelength ultraviolet light (UVA), furocoumarins bind covalently with pyrimidine residues via a cyclobutane ring. A second photoevent allows pyrimidines located on the opposite DNA strand in an adjacent base pair to react, forming a cross-link. In the experiments in this report, psoralen photoinactivation is employed to investigate human DNA repair pathways by analyzing the ability of xeroderma pigmentosum (XP) and Fanconi's anemia (FA) cells to rescue psoraleninactivated herpes simplex virus (HSV). Comparison of several XP complementation groups and one XP variant with normal human fibroblasts demonstrates that the ability of all cells to repair damage by 4,5',8-trimethylpsoralen (TMP), a derivative that forms cross-links efficiently, is similar. However, HSV photochemically reacted with 5-methylangelicin (5-MA), an isopsoralen that forms only monoadducts, is repaired at significantly lower levels in several XP complementation groups than in control fibroblast cells, which indicates that the XP repair deficiency resides in the removal of monoadducts and not of cross-links in these cell lines. Surprisingly, the FA cells rescue both TMP- and 5-MA-treated virus with slightly greater efficiency than that observed in normal human fibroblasts.

Fanconi anaemia cells are not uniformly deficient in unhooking of DNA interstrand crosslinks, induced by mitomycin C or 8-methoxypsoralen plus UVA

Fanconi anaemia (FA) cells are extremely sensitive to crosslinking agents, e.g. mitomycin C, but only moderately sensitive to trimethylpsoralen plus UVA. Evidence has been reported suggesting that there is a deficient DNA crosslink repair mechanism in FA cells, but others failed to confirm this conclusion using other methods and other crosslinking agents. We reinvestigated the mitomycin C and 8-methoxypsoralen crosslink repair in FA cells with a high sensitivity to mitomycin C. Although an essentially similar methodology was used to that previously described, no difference between the control and FA cell strains was observed, neither for mitomycin C- nor for 8-methoxypsoralen-induced crosslinks.

DNA interstrand cross-linking, repair, and SCE mechanism in human cells in special reference to Fanconi anemia

The relation of DNA cross-linking and repair to sister chromatid exchange (SCE) formation was studied in normal human, Fanconi anemia (FA), and xeroderma pigmentosum (XP) cells. Despite a hypersensitive lethality response in FA cells, the SCE induction rates by mitomycin C (MMC), trimethylpsoralen (TMP)-light, cisplatin, and diepoxybutane were twice as high as in normal cells. For MMC, the induced SCE frequency in normal cells was reduced in a biphasic fashion with a repair incubation time (the first decline t1/2 = 2 hr; the second t1/2 = 14-18 hr) which corresponds exactly to the molecular kinetics of cross-link and monoadduct removal. However, FA cells lack the first half-excision process and exhibit a lack of the first rapid decline SCE component. The slow decline component is present, and a higher SCE frequency is observed 24 to 48 hr after treatment. By contrast, XP cells capable of the half-excision process reveal the first rapid decline component, followed by an extremely slow second-reduction component (t1/2 = 48 hr) due to defective monoadduct repair. The endoreduplication-SCE method revealed that rates of both twin (first cycle) and single (second cycle) SCE formations by MMC and TMP-light were higher in FA cells than in normal cells. These results indicate that cross-links remaining unrepaired induce SCEs as do monoadducts. The probabilistic SCE induction occurs at a rate of 1 SCE per 35 MMC cross-links in FA cells. Further, a non-SCE-forming tolerance mechanism also operates in hypersensitive FA cells. These molecular and cytogenetic results allow us to construct a new probabilistic model for cross-link-induced SCE into which the replication-fork model, random cross-link transfer to both chromatids, and chromatid breakage-reunion are incorporated.