Jay H. Robbins

The Oxidative DNA Lesion 8,5′-(S)-Cyclo-2′-deoxyadenosine Is Repaired by the Nucleotide Excision Repair Pathway and Blocks Gene Expression in Mammalian Cells

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 cells 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.

Five complementation groups in xeroderma pigmentosum

A collaborative study was undertaken to determine the relationship between the three DNA repair complementation groups in xeroderma pigmentosum found at Erasmus University, Rotterdam, and the four groups found at the National Institutes of Health, Bethesda. The results of this study reveal that there are five currently known complementation groups in xeroderma pigmentosum.

Cockayne Syndrome: Clinicopathologic and Tissue Culture Studies of Affected Siblings

Two siblings with Cockayne syndrome (CS) had extremely severe and early onset cachectic dwarfism, developmental delay, cataracts, microcephaly, peripheral neuropathy, and spastic quadriplegia. In order to study the inherited DNA-repair defect known to be present in cultured CS cells, a lymphoblastoid line was established from the younger sibling. Tissue culture studies revealed the line to have a hypersensitivity to the lethal effects of 254-nm ultraviolet radiation (UV) equivalent to that of lymphoblastoid lines from CS patients who had either the usual severity or a very mild form of CS. Autopsy of the older sibling at six years of age showed the brain to be severely atrophic, with particularly severe cerebellar atrophy. There was a marked reduction in the number of granule cells in the cerebellum and irregular patchy myelination throughout the brain. Many astrocytes contained either a large, bizarre-shaped nucleus or multiple nuclei. Some Purkinje cells of the cerebellum and pyramidal neurons of the hippocampus were binucleated. It is suggested that the DNA-repair defect of CS caused abnormalities in nuclear DNA replication and cell division which result in cell death and in the observed nuclear abnormalities.

Evidence for detective repair of cyclobutane pyrimidine dimers with normal repair of other DNA photoproducts in a transcriptionally active gene transfected into Cockayne syndrome cells

Cockayne syndrome (CS) and xeroderma pigmentosum (XP), autosomal recessive diseases with clinical and cellular hypersensitivity to UV radiation, differ in ability to repair UV DNA photoproducts in their overall genome: normal repair in CS, defective repair in XP. In order to characterize a DNA repair defect in an active gene in CS, we measured the capacity of cells from patients with CS and XP to reactivate 2 major types of UV-induced DNA damage, photoreactivatable (i.e., cyclobutane pyrimidine dimers) and non-photoreactivatable (primarily pyrimidine-(6-4)pyrimidone photoproducts), in the actively transcribing chloramphenicol acetyltransferase (cat) gene of the plasmid expression vector pRSV-cat. Epstein-Barr virus-transformed lymphoblast lines from 4 normal persons and from 3 patients with CS and from two with XP were transiently transfected with the plasmid, and the cat activity in cell extracts was determined. When the cells were transfected with UV-irradiated plasmid, expression was abnormally decreased in both the CS and XP cells. When the cyclobutane pyrimidine dimers in the UV-irradiated plasmid were removed by photoreactivation prior to transfection, cat expression in the CS, but not in the XP, lines reached normal levels. These data imply that both the XP and CS cells are unable to repair normally the cyclobutane pyrimidine dimer photoproducts which block transcription of cat. However, the CS, but not XP, cells can repair normally the other UV-induced photoproducts which block transcription. The ability of CS, but not XP, cells to repair these non-dimer photoproducts indicates that the active gene repair mechanism treats the cyclobutane pyrimidine dimer differently from the non-dimer photoproducts.

Clinically Asymptomatic Xeroderma pigmentosum Neurological Disease in an Adult: Evidence for a Neurodegeneration in Later Life Caused by Defective DNA Repair

Xeroderma pigmentosum is a genetically heterogeneous disease caused by DNA repair defects resulting in skin cancer and, in some patients, a primary neuronal degeneration which has in all previous reports become symptomatic prior to age 21 years. A 47-year-old xeroderma pigmentosum patient is described who, although neurologically asymptomatic, has sensorineural hearing loss together with clinical signs and electrophysiologic evidence of a developing peripheral neuropathy. This case suggests that defective DNA repair may cause neurodegeneration in adults as well as in children.

Typical xeroderma pigmentosum complementation group A fibroblasts have detectable ultraviolet light-induced unscheduled DNA synthesis

Abstract Ultraviolet-induced nuclear uptake of tritiated thymidine [ 3 H]dThd demonstrable by autoradiography in non-synthesis phases of the cell cycle is known as unscheduled DNA synthesis and reflects repair replication of ultraviolet-damaged DNA. We have reported that the rate of any such unscheduled DNA synthesis in typical group A xeroderma pigmentosum fibroblasts, if present, is less than 2% of the normal rate. We have now performed experiments to determine whether these fibroblasts have any unscheduled DNA synthesis. Fibroblast coverslip cultures of four xeroderma pigmentosum group A strains were prepared. Irradiated (254 nm ultraviolet light) and unirradiated cultures from each strain were incubated with [ 3 H]dThd at 37°C, and autoradiograms were prepared using NTB-3 emulsion. A nuclear grain count was made of 100 consecutive nuclei of non-S-phase irradiated and unirradiated cells. A slide background grain count was simultaneously made from an acellular area adjacent to each cell analyzed. When a strains irradiated and unirradiated autoradiograms having similar slide background grain count averages were compared, the nuclear grain count average of the irradiated cells was always higher than that of the unirradiated cells. This ultraviolet-induced increase in the mean nuclear grain count ranged from 0.4 to 1.3% of that given by normal non-xeroderma pigmentosum fibroblasts and was not reduced by 10 −2 M hydroxyurea. Planimetric studies showed that the ultraviolet-induced increase in nuclear grain count is not due to an increased nuclear area in irradiated cells. We conclude that these typical group A xeroderma pigmentosum strains perform very low, but detectable, ultraviolet-induced unscheduled DNA synthesis which probably reflects repair replication. We cannot, however, determine if there are significantly different rates of ultraviolet-induced unscheduled DNA synthesis among these ultraviolet strains.

Colony-forming ability of ultraviolet-irradiated xeroderma pigmentosum fibroblasts from different DNA repair complementation groups

Patients with xeroderma pigmentosum develop severe sunlight-induced damage, including malignant neoplasms, on sun-exposed skin. Some patients also have neurological abnormalities. Xeroderma pigmentosum cells are known to have impaired ability to repair ultraviolet light- or chemical mutagen-induced damage to their DNA, and cell-fusion studies have shown five complementation groups among the DNA excision repair-deficient strains. All xeroderma pigmentosum fibroblast strains we tested had lower colony-forming abilities after ultraviolet irradiation than normal strains. Furthermore, we have found that strains from different complementation groups can have different post-ultraviolet colony-forming abilities and that strains from patients with neurological abnormalities are the most sensitive to ultraviolet light. These results suggest that extremely ineffective repair of damaged DNA in central nervous system neurons may be the cause of the neurological abnormalities.

Hypersensitivity to DNA-Damaging Agents in Abiotrophies: A New Explanation for Degeneration of Neurons, Photoreceptors, and Muscle in Alzheimer, Parkinson and Huntington Diseases, Retinitis Pigmentosa, and Duchenne Muscular Dystrophy

Gowers (1902) introduced the term ‘abiotrophy’ to signify the premature death of neurons and skeletal muscle in primary neuronal degenerations and in muscular dystrophies respectively. Collins (1919) classified retinitis pigmentosa, with its premature degeneration of photoreceptors, as an abiotrophy. Abiotrophies share several characteristics in addition to the primary degeneration of excitable tissue which occurs in the absence of histopathologic evidence of the etiology (Gowers, 1902; Collins, 1919; Blackwood and Corsellis, 1976; Richardson and Adams, 1977). The abiotrophic degenerations: 1) become evident after the excitable tissue has attained a normal, mature development; 2) are relentlessly progressive; 3) selectively affect certain excitable tissues but not others; 4) have either a clear hereditary or a sporadic basis; and 5) may be variable in their clinical and pathological features and often overlap with one another. Examples of abiotrophies include xeroderma pigmentosum (XP), ataxia telangiectasia, Cockayne syndrome, Alzheimer disease, Parkinson disease, Huntington disease, Friedreich ataxia, Duchenne muscular dystrophy, and retinitis pigmentosa.

Hypersensitivity to ionizing radiation in cultured cells from Down syndrome patients

Down syndrome is caused by trisomy of chromosome 21 and is comprised of a constellation of abnormalities including neuropathological features that closely resemble those characterizing the neurodegeneration of Alzheimer disease. Because cultured cell lines from patients with Alzheimer disease and other neurodegenerations have a hypersensitivity to the lethal effects of DNA-damaging agents, we studied the response of Down syndrome lymphoblastoid lines to the lethal effects of ionizing and ultraviolet radiation. Lines from the four Down syndrome patients were more sensitive to X-rays than lines from 28 normal donors (P = 10(-4)), while survival of the Down syndrome lines after ultraviolet irradiation was not significantly different from normal. This hypersensitivity to X-rays, which may reflect defective repair of X-ray-induced DNA damage, represents the first abnormality common to cultured cells from both Down syndrome and Alzheimer disease patients.

Prolonged ultraviolet-induced thymidine incorporation into xeroderma pigmentosum lymphocytes: Studies on its duration, amount, localization and relationship to hydroxyurea

Abstract Xeroderma pigmentosum lymphocytes which have a lower than normal rate of ultraviolet-induced [ 3 H]thymidine incorporation during the hours immediately after irradiation continue their incorporation for a longer time than normal lymphocytes. This difference in the duration of incorporation into xeroderma pigmentosum and normal lymphocytes is not caused by the continuous presence of hydroxyurea in the cultures. The prolonged incorporation ceases after the xeroderma pigmentosum lymphocytes have incorporated as much thymidine as normal lymphocytes. Autoradiograms show that the prolonged incorporation is nuclear and occurs in the majority of lymphocytes. The results indicate that these xeroderma pigmentosum lymphocytes can perform as much repair of ultraviolet-damaged DNA as normal lymphocytes but require a longer period in which to do so.