UV damage causes uncontrolled DNA breakage in cells from patients with combined features of XP-D and Cockayne syndrome

Nucleotide excision repair (NER) removes damage from DNA in a tightly regulated multiprotein process. Defects in NER result in three different human disorders, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS). Two cases with the combined features of XP and CS have been assigned to the XP-D complementation group. Despite their extreme UV sensitivity, these cells appeared to incise their DNA as efficiently as normal cells in response to UV damage. These incisions were, however, uncoupled from the rest of the repair process. Using cell-free extracts, we were unable to detect any incision activity in the neighbourhood of the damage. When irradiated plasmids were introduced into unirradiated XP-D/CS cells, the ectopically introduced damage triggered the induction of breaks in the undamaged genomic DNA. XP-D/CS cells thus have a unique response to sensing UV damage, which results in the introduction of breaks into the DNA at sites distant from the damage. We propose that it is these spurious breaks that are responsible for the extreme UV sensitivity of these cells.     

Retrovirus-mediated DNA repair gene transfer into xeroderma pigmentosum cells: Perspectives for a gene therapy

The rare hereditary disease xeroderma pigmentosum (XP) is clinically characterized by extreme sun sensitivity and an increased predisposition for developing skin cancer. Cultured cells from XP patients exhibit hypersensitivity to ultraviolet (UV) radiation due to the defect in nucleotide excision repair (NER), and other cellular abnormalities. Seven genes identified in the classical XP forms, XPA to XPG, are involved in the NER pathway. In view of developing a strategy of gene therapy for XP, we devised recombinant retrovirus-carrying DNA repair genes for transfer and stable expression of these genes in cells from XP patients. Results showed that these retroviruses are efficient tools for transducing XP fibroblasts and correcting repair-defective cellular phenotypes by recovering normal UV survival, unscheduled DNA synthesis, and RNA synthesis after UV irradiation, and also other cellular abnormalities resulting from NER defects. These results imply that the first step of cellular gene therapy might be accomplished successfully.     

Cockayne syndrome complementation group B associated with xeroderma pigmentosum phenotype

Two siblings have been reported whose clinical manifestations (cutaneous photosensitivity and central nervous system dysfunction) are strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum (XP), a severe form of XP. Fibroblasts from the siblings showed UV sensitivity, a failure of recovery of RNA synthesis (RRS) after UV irradiation, and a normal level of unscheduled DNA synthesis (UDS), which were, unexpectedly, the biochemical characteristics usually associated with Cockayne syndrome (CS). However, no complementation group assignment in these cells has yet been performed. We here report that these patients can be assigned to CS complementation group B (CSB) by cell fusion complementation analysis. To our knowledge, these are the first patients with defects in the CSB gene to be associated with an XP phenotype. The results imply that the gene product from the CSB gene must interact with the gene products involved in excision repair and associated with XP.     

Xeroderma pigmentosum complementation group G associated with Cockayne syndrome.

Xeroderma pigmentosum (XP) and Cockayne syndrome (CS) are two rare inherited disorders with a clinical and cellular hypersensitivity to the UV component of the sunlight spectrum. Although the two traits are generally considered as clinically and genetically distinct entities, on the biochemical level a defect in the nucleotide excision-repair (NER) pathway is involved in both. Classical CS patients are primarily deficient in the preferential repair of DNA damage in actively transcribed genes, whereas in most XP patients the genetic defect affects both "preferential" and "overall" NER modalities. Here we report a genetic study of two unrelated, severely affected patients with the clinical characteristics of CS but with a biochemical defect typical of XP. By complementation analysis, using somatic cell fusion and nuclear microinjection of cloned repair genes, we assign these two patients to XP complementation group G, which previously was not associated with CS. This observation extends the earlier identification of two patients with a rare combined XP/CS phenotype within XP complementation groups B and D, respectively. It indicates that some mutations in at least three of the seven genes known to be involved in XP also can result in a picture of partial or even full-blown CS. We conclude that the syndromes XP and CS are biochemically closely related and may be part of a broader clinical disease spectrum. We suggest, as a possible molecular mechanism underlying this relation, that the XPGC repair gene has an additional vital function, as shown for some other NER genes.     

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.     

Xeroderma pigmentosum and the role of UV-induced DNA damage in skin cancer

Xeroderma pigmentosum (XP) is a rare, autosomal recessive disease that is characterized by the extreme sensitivity of the skin to sunlight. Compared to normal individuals, XP patients have a more than 1000-fold increased risk of developing cancer on sun-exposed areas of the skin. Genetic and molecular analyses have revealed that the repair of ultraviolet (UV)-induced DNA damage is impaired in XP patients owing to mutations in genes that form part of a DNA-repair pathway known as nucleotide excision repair (NER). Two other diseases, Cockayne syndrome (CS) and the photosensitive form of trichothiodystrophy (TTD), are linked to a defect in the NER pathway. Strikingly, although CS and TTD patients are UV-sensitive, they do not develop skin cancer. The recently developed animal models that mimic the human phenotypes of XP, CS and TTD will contribute to a better understanding of the etiology of these diseases and the role of UV-induced DNA damage in the development of skin cancer.     

XPC initiation codon mutation in xeroderma pigmentosum patients with and without neurological symptoms

Two unrelated xeroderma pigmentosum (XP) patients, with and without neurological abnormalities, respectively, had identical defects in the XPC DNA nucleotide excision repair (NER) gene. Patient XP21BE, a 27-year-old woman, had developmental delay and early onset of sensorineural hearing loss. In contrast, patient XP329BE, a 13-year-old boy, had a normal neurological examination. Both patients had marked lentiginous hyperpigmentation and multiple skin cancers at an early age. Their cultured fibroblasts showed similar hypersensitivity to killing by UV and reduced repair of DNA photoproducts. Cells from both patients had a homozygous c.2T>G mutation in the XPC gene which changed the ATG initiation codon to arginine (AGG). Both had low levels of XPC message and no detectable XPC protein on Western blotting. There was no functional XPC activity in both as revealed by the failure of localization of XPC and other NER proteins at the sites of UV-induced DNA damage in a sensitive in vivo immunofluorescence assay. XPC cDNA containing the initiation codon mutation was functionally inactive in a post-UV host cell reactivation (HCR) assay. Microsatellite markers flanking the XPC gene showed only a small region of identity ( approximately 30kBP), indicating that the patients were not closely related. Thus, the initiation codon mutation resulted in DNA repair deficiency in cells from both patients and greatly increased cancer susceptibility. The neurological abnormalities in patient XP21BE may be related to close consanguinity and simultaneous inheritance of other recessive genes or other gene modifying effects rather than the influence of XPC gene itself.     

Defective repair of gamma-ray induced DNA damage in xeroderma pigmentosum cells.

We used the bromouracil-photolysis technique to estimate the sizes of the repaired regions in normal human and xeroderma pigmentosum (XP) cells irradiated by gamma-rays aerobically or anoxically. After 1 1/2 hours of incubation, single-strand breaks were repaired and the repaired regions were small--one to two BrUra residues--for cells irradiated aerobically or anoxically. After a 20-hour incubation, the repaired region in normal cells showed a component mimicking U.V.-repair. There were large patches (approximately 30 BrUra residues) in the approximate ratios of one per six chain breaks for aerobic irradiation and one per three chain breaks for anoxic irradiation. XP cells, however, only showed large patches at 20 hours if they had been irradiated aerobically. We could not detect such regions in XP cells irradiated anoxically. These results indicate (1) that some part of ionizing damage mimics excision of U.V. damage in that the repair patches are large and the repair takes an appreciable time; (2) the types of such damage depend on whether the irradiation is done aerobically or anoxically; and (3) XP cells are defective in repairing a component of anoxic damage.     

Expression of the cDNA for the beta subunit of human casein kinase II confers partial UV resistance on xeroderma pigmentosum cells.

An immortalized xeroderma pigmentosum cell line belonging to the complementation group D (XP-D) was transfected with a normal human cDNA clone library constructed in a mammalian expression vector. Following UV-irradiation-selection, a transformant having a stable, partially UV-resistant phenotype was isolated. A transfected cDNA of partial length was rescued from the transformant's cellular DNA by in vitro amplification, using expression-vector specific oligonucleotides as primers in a polymerase chain reaction (PCR). Expression of this cDNA complemented the UV sensitivity of the XP-D cell line to the UV-resistance levels characteristic of the primary transformant. The nucleotide sequence of the cDNA was determined. The deduced protein identified the cDNA as encoding for the beta subunit of casein kinase II (CKII-beta). Similar to the effect exerted by the truncated CKII-beta cDNA, expression of a cDNA clone encompassing the complete translated region of CKII-beta leads to XP-D cells partially resistant to UV-irradiation. However, transfection of CKII-beta cDNA could also partially complement the UV-sensitivity of a xeroderma pigmentosum cell line belonging to group C (XP-C). Analysis by Southern, Northern and RNAase mismatch cleavage techniques did not reveal any functional defect in the CKII-beta gene of cell lines derived from either 7 XP-D or 10 XP-C families. We therefore consider it unlikely that either the XP-D or the XP-C DNA repair deficiency is associated with a defect in the beta subunit of casein kinase II. Nevertheless, our findings suggest the possibility that the cell's response to DNA damage is modulated by CKII-dependent protein phosphorylation.     

Ultraviolet light-induced chromosomal aberrations in cultured cells from Cockayne syndrome and complementation group C xeroderma pigmentosum patients: lack of correlation with cancer susceptibility.

Both Cockayne syndrome (CS) and xeroderma pigmentosum (XP) are inherited diseases with defective repair of damage induced in DNA by UV. Patients with XP, but not those with CS, have an increased susceptibility to formation of sunlight-induced skin tumors. We determined the frequency of UV-induced chromosomal aberrations in cultured lymphoblastoid cell lines from five CS patients and three complementation-group-C XP patients to determine whether such aberrations were abnormally increased only in the XP cells. We found that CS cells had the same abnormally increased number of induced aberrations as the XP cells, indicating that the number of UV-induced aberrations in XP group C cells does not account for the susceptibility of these XP patients to sunlight-induced skin cancer.