Xeroderma pigmentosum (XP) patients with inherited defects in nucleotide excision repair (NER) are unable to excise from their DNA bulky photoproducts induced by UV radiation and therefore develop accelerated actinic damage, including cancer, on sun-exposed tissue. Some XP patients also develop a characteristic neurodegeneration believed to result from their inability to repair neuronal DNA damaged by endogenous metabolites since the harmful UV radiation in sunlight does not reach neurons. Free radicals, which are abundant in neurons, induce DNA lesions that, if unrepaired, might cause the XP neurodegeneration. Searching for such a lesion, we developed a synthesis for 8,5-(S)-cyclo-2-deoxyadenosine (cyclo-dA), a free radical-induced bulky lesion, and incorporated it into DNA to test its repair in mammalian cell extracts and living cells. Using extracts of normal and mutant Chinese hamster ovary (CHO) cells to test for NER and adult rat brain extracts to test for base excision repair, we found that cyclo-dA is repaired by NER and not by base excision repair. We measured host cell reactivation, which reflects a cell's capacity for NER, by transfecting CHO and XP cells with DNA constructs containing a single cyclo-dA or a cyclobutane thymine dimer at a specific site on the transcribed strand of a luciferase reporter gene. We found that, like the cyclobutane thymine dimer, cyclo-dA is a strong block to gene expression in CHO and human cells. Cyclo-dA was repaired extremely poorly in NER-deficient CHO cells and in cells from patients in XP complementation group A with neurodegeneration. Based on these findings, we propose that cyclo-dA is a candidate for an endogenous DNA lesion that might contribute to neurodegeneration in XP.

The effect of ionizing radiation on the expression of two DNA-damage-inducible genes, designated gadd45 and gaddl53, was examined in cultured human cells. These genes have previously been shown to be strongly and coordinately induced by UV radiation and alkylating agents in human and hamster cells. We found that the gadd45 but not the gaddl53 gene is strongly induced by X rays in human cells. The level of gadd45 mRNA increased rapidly after X rays at doses as low as 2 Gy. After 20 Gy of X rays, gadd45 induction, as measured by increased amounts of mRNA, was similar to that produced by the most effective dose of the alkylating agent methyl methanesulfonate. No induction was seen after treatment of either human or hamster cells with 12-O-tetradecanoylphorbol-13-acetate, a known activator of protein kinase C (PKC). Therefore, gadd45 represents the only known mammalian X-ray-responsive gene whose induction is not mediated by PKC. However, induction was blocked by the protein kinase inhibitor H7, indicating that induction is mediated by some other kinase(s). Sequence analysis of human and hamster cDNA clones demonstrated that this gene has been highly conserved and encodes a novel 165-amino-acid polypeptide which is 96% identical in the two species. This gene was localized to the short arm of human chromosome 1 between p12 and p34. When induction in lymphoblast lines from four normal individuals was compared with that in lines from four patients with ataxia telangiectasia, induction by X rays ofgadd45 mRNA was less in the cell lines from this cancer-prone radiosensitive disorder. Our results provide evidence for the existence of an X-ray stress response in human cells which is independent of PKC and which is abnormal in ataxia telangiectasia.There is increasing evidence that ionizing radiation can induce specific genes in mammalian and other eucaryotic cells. Such genes may have functions like those in procaryotes, in which genes encoding protective functions, such as DNA repair processes, can be induced (39). Prior treatment with a low dose of X rays induces a transient (usually small) protection, manifested as an increased survival or as a reduction in chromosomal damage, against a second, higher dose in human lymphocytes (25), Chinese hamster cells (12), and insect cells (15). These protective effects are blocked by inhibitors of protein (12) and RNA synthesis (15), indicating that such processes may be mediated by the induction of particular genes. The finding that certain proteins increase in abundance after X irradiation of human cells also provides evidence for X-ray-inducible genes (2). Recently, tumor necrosis factor a (TNF) mRNA has been reported to increase in human sarcoma cells after ionizing radiation (9). TNF has a role in a variety of cellular processes, including the acute-phase response and inflammation (1), and thus its induction probably represents a general response to cell injury rather than a specific response to X rays or other DNA-damaging agents. Transcription from the long terminal repeat o...

Objectives-To review genetic variants of Cockayne syndrome (CS) and xeroderma pigmentosum (XP), autosomal recessive disorders of DNA repair that affect the nervous system, and to illustrate them by the first case of xeroderma pigmentosum-Cockayne syndrome (XP-CS) complex to undergo neuropathologic examination.Methods-Published reports of clinical, pathologic, and molecular studies of CS, XP neurologic disease, and the XP-CS complex were reviewed, and a ninth case of XP-CS is summarized.Results-CS is a multisystem disorder that causes both profound growth failure of the soma and brain and progressive cachexia, retinal, cochlear, and neurologic degeneration, with a leukodystrophy and demyelinating neuropathy without an increase in cancer. XP presents as extreme photosensitivity of the skin and eyes with a 1000-fold increased frequency of cutaneous basal and squamous cell carcinomas and melanomas and a small increase in nervous system neoplasms. Some 20% of patients with XP incur progressive degeneration of previously normally developed neurons resulting in cortical, basal ganglia, cerebellar, and spinal atrophy, cochlear degeneration, and a mixed distal axonal neuropathy. Cultured cells from patients with CS or XP are hypersensitive to killing by ultraviolet (UV) radiation. Both CS and most XP cells have defective DNA nucleotide excision repair of actively transcribing genes; in addition, XP cells have defective repair of the global genome. There are two complementation groups in CS and seven in XP. Patients with the XP-CS complex fall into three XP complementation groups. Despite their XP genotype, six of nine individuals with the XP-CS complex, including the boy we followed up to his death at age 6, had the typical clinically and pathologically severe CS phenotype. HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript skin and blood cells had extreme sensitivity to killing by UV radiation, DNA repair was severely deficient, post-UV unscheduled DNA synthesis was reduced to less than 5%, and post-UV plasmid mutation frequency was increased.Conclusions-The paradoxical lack of parallelism of phenotype to genotype is unexplained in these disorders. Perhaps diverse mutations responsible for UV sensitivity and deficient DNA repair may also produce profound failure of brain and somatic growth, progressive cachexia and premature aging, and tissue-selective neurologic deterioration by their roles in regulation of transcription and repair of endogenous oxidative DNA damage.Cockayne syndrome (CS), 1 xeroderma pigmentosum (XP) neurologic disease, 2 the xeroderma pigmentosum-Cockayne syndrome (XP-CS) complex, 3,4 and others are rare genetic disorders with striking somatic and neurologic involvement. The hallmark of this group of diseases is inadequate DNA repair. Cells of both CS and XP are characterized in tissue culture by hypersensitivity to killing by ultraviolet (UV) radiation but differ in some of their cardinal laboratory characteristics (table 1). Although CS and most XP cel...

Xeroderma pigmentosum (XP) is an autosomal recessive, neurocutaneous disorder characterized by sunlight-induced skin cancers and defective DNA repair. Many XP children develop a primary neuronal degeneration. We describe 2 unusual XP patients who had a delayed onset of XP neurological disease. Somatic cell genetic studies indicated that they have the same defective DNA repair gene and are both in XP complementation group A. These 2 patients, together with a group A patient previously reported from London, establish as a distinct clinical entity the late onset type of the juvenile onset form of XP neurological disease. The functional capacity of these patients' cultured fibroblast strains to survive after treatment with ultraviolet radiation indicates that their DNA repair defect is less severe than that of typical group A patients who have a more severe neurodegeneration with an earlier symptomatic onset. The premature death of nerve cells in XP patients (which is presumably due to their inherited defects in DNA repair mechanisms) suggests that normal repair of damaged DNA in neurons is required to maintain integrity of the human nervous system.

Xeroderma pigmentosum is an autosomal recessive disease in which DNA repair processes are defective. All xeroderma piginentosum patients develop premature aging of sun-exposed skin, and some develop neurological abnormalities due to premature death of nerve cells. Sensitivity-to ultraviolet radiation of 24 xeroderma pigmentosum fibroblast strains was studied in vitro by measuring each strain's ability to divide and form colonies after irradiation. The most sensitive strains were derived from patients who had an early onset of neurological abnormalities; less sensitive strains were from patients with a later onset; and the most resistant strains were from patients without neurological abnormalities. The UV sensitivities of strains from each member of a sibling pair with xeroderma pigmentosum were identical, indicating that UV sensitivity of xeroderma pigmentosum strains is determined by the patient's inherited DNA repair defect. The results suggest that effective DNA repair is required to maintain the functional integrity of the human nervous system by preventing premature death of neurons. Cells from patients with xeroderma pigmentosum (XP), an autosomal recessive disease, have abnormal repair of DNA that has been damaged by ultraviolet (UV) radiation or certain chemical carcinogens (1). There are currently six known genetic forms of XP: the variant form, with a normal rate of UV-induced unscheduled DNA repair synthesis (a measure of excision repair) but abnormal postreplication repair (2), and five excision repair-deficient forms, which have been classified into complementation groups A-E by cell fusion of their strains (3). All XP patients develop excessive UV damage in sunlight-exposed areas at an early age. Some XP patients also develop neurological abnormalities due to the premature death of central nervous system neurons in the absence of recognizable and specific histopathology (1). We have, therefore, considered the neurological abnormalities of XP to be the result of an abnormal aging of the human nervous system (1).Post-UV colony-forming ability of human fibroblasts in vitro is markedly affected by the ability of the cells to repair their UV-damaged DNA to the functional state required for cell survival and division (4,5