Evidence that lack of deoxyribonucleic acid repair causes death of neurons in xeroderma pigmentosum

Combination of Aβ Secretion and Oxidative Stress in an Alzheimer-Like Cell Line Leads to the Over-Expression of the Nucleotide Excision Repair Proteins DDB2 and XPC

Repair of oxidative DNA damage, particularly Base Excision Repair (BER), impairment is often associated with Alzheimer’s disease pathology. Here, we aimed at investigating the complete Nucleotide Excision Repair (NER), a DNA repair pathway involved in the removal of bulky DNA adducts, status in an Alzheimer-like cell line. The level of DNA damage was quantified using mass spectrometry, NER gene expression was assessed by qPCR, and the NER protein activity was analysed through a modified version of the COMET assay. Interestingly, we found that in the presence of the Amyloid β peptide (Aβ), NER factors were upregulated at the mRNA level and that NER capacities were also specifically increased following oxidative stress. Surprisingly, NER capacities were not differentially improved following a typical NER-triggering of ultraviolet C (UVC) stress. Oxidative stress generates a differential and specific DNA damage response in the presence of Aβ. We hypothesized that the release of NER components such as DNA damage binding protein 2 (DDB2) and Xeroderma Pigmentosum complementation group C protein (XPC) following oxidative stress might putatively involve their apoptotic role rather than DNA repair function.

The influence of DNA repair on neurological degeneration, cachexia, skin cancer and internal neoplasms: autopsy report of four xeroderma pigmentosum patients (XP-A, XP-C and XP-D)

Background

To investigate the association of DNA nucleotide excision repair (NER) defects with neurological degeneration, cachexia and cancer, we performed autopsies on 4 adult xeroderma pigmentosum (XP) patients with different clinical features and defects in NER complementation groups XP-A, XP-C or XP-D.

Results

The XP-A (XP12BE) and XP-D (XP18BE) patients exhibited progressive neurological deterioration with sensorineural hearing loss. The clinical spectrum encompassed severe cachexia in the XP-A (XP12BE) patient, numerous skin cancers in the XP-A and two XP-C (XP24BE and XP1BE) patients and only few skin cancers in the XP-D patient. Two XP-C patients developed internal neoplasms including glioblastoma in XP24BE and uterine adenocarcinoma in XP1BE. At autopsy, the brains of the 44 yr XP-A and the 45 yr XP-D patients were profoundly atrophic and characterized microscopically by diffuse neuronal loss, myelin pallor and gliosis. Unlike the XP-A patient, the XP-D patient had a thickened calvarium, and the brain showed vacuolization of the neuropil in the cerebrum, cerebellum and brainstem, and patchy Purkinje cell loss. Axonal neuropathy and chronic denervation atrophy of the skeletal muscles were observed in the XP-A patient, but not in the XP-D patient.

Conclusions

These clinical manifestations and autopsy findings indicate advanced involvement of the central and peripheral nervous system. Despite similar defects in DNA repair, different clinicopathological phenotypes are seen in the four cases, and therefore distinct patterns of neurodegeneration characterize XP-D, XP-A and XP-C patients.

Cancer and neurologic degeneration in xeroderma pigmentosum: long term follow-up characterises the role of DNA repair

BACKGROUND

The frequency of cancer, neurologic degeneration and mortality in xeroderma pigmentosum (XP) patients with defective DNA repair was determined in a four decade natural history study.

METHODS

All 106 XP patients admitted to the National Institutes of Health from 1971 to 2009 were evaluated from clinical records and follow-up.

RESULTS

In the 65 per cent (n=69) of patients with skin cancer, non-melanoma skin cancer (NMSC) was increased 10,000-fold and melanoma was increased 2000-fold in patients under age 20. The 9 year median age at diagnosis of first non-melanoma skin cancer (NMSC) (n=64) was significantly younger than the 22 year median age at diagnosis of first melanoma (n=38)-a relative age reversal from the general population suggesting different mechanisms of carcinogenesis between NMSC and melanoma. XP patients with pronounced burning on minimal sun exposure (n=65) were less likely to develop skin cancer than those who did not. This may be related to the extreme sun protection they receive from an earlier age, decreasing their total ultraviolet exposure. Progressive neurologic degeneration was present in 24% (n=25) with 16/25 in complementation group XP-D. The most common causes of death were skin cancer (34%, n=10), neurologic degeneration (31%, n=9), and internal cancer (17%, n=5). The median age at death (29 years) in XP patients with neurodegeneration was significantly younger than those XP patients without neurodegeneration (37 years) (p=0.02).

CONCLUSION

This 39 year follow-up study of XP patients indicates a major role of DNA repair genes in the aetiology of skin cancer and neurologic degeneration.

Defective responses to DNA single- and double-strand breaks in spinocerebellar ataxia

Failure to maintain the integrity of DNA/chromatin can result in genome instability and an increased risk of cancer. The description of a number of human genetic disorders characterised not only by cancer predisposition but by a broader phenotype including neurodegeneration suggests that maintaining genome stability is also important for preserving post-mitotic neurons. The identification of genes associated with other neurodegenerative disorders provides further evidence for the importance of DNA damage response and DNA repair genes in protecting against neurodegeneration. This theme is further developed in this review.

DNA repair in neural cells: basic science and clinical implications

As one part of a distinguished scientific career, Dr. Bryn Bridges focused his attention on the issue of DNA damage and repair in stationary phase bacteria. His work in this area led to his interest in DNA repair and mutagenesis in another non-dividing cell population, the neurons in the mammalian nervous system. He has specifically taken an interest in the magnocellular neurons of the central nervous system, and the possibility that somatic mutations may be occurring in these neurons. As part of this special issue dedicated to Bryn Bridges upon his retirement, I will discuss the various DNA repair pathways known to be active in the nervous system. The importance of DNA repair to the nervous system is most graphically illustrated by the neurological abnormalities observed in patients with hereditary diseases associated with defects in DNA repair. I will consider the mechanisms underlying the neurological abnormalities observed in patients with four of these diseases: xeroderma pigmentosum (XP), Cockayne's syndrome (CS), ataxia telangectasia (AT) and AT-like disorder (ATLD). I will also propose a mechanism for one of the observations indicating that somatic mutation can occur in the magnocellular neurons of the aging rat brain. Finally, as a parallel to Bridges inquiry into how much DNA synthesis is going on in stationary phase bacteria, I will address the question of how much DNA synthesis in going on in neurons, and the implications of the answer to this question for recent studies of neurogenesis in adult mammals.

Role of DNA-dependent protein kinase in neuronal survival

DNA-dependent protein kinase (DNA-PK) is a DNA repair enzyme composed of a DNA-binding component called Ku70/80 and a catalytic subunit called DNA-PKcs. Many investigators have utilized DNA-PKcs-deficient cells and cell lines derived from severe combined immunodeficiency (scid) mice to study DNA repair and apoptosis. However, little is known about the CNS of these mice. This study was carried out using primary neuronal cultures derived from the cerebral hemispheres of new-born wild-type and scid mice to investigate the effects of loss of DNA-PK function on neuronal maturation and survival. Purified neuronal cultures developed comparably in terms of neurite formation and expression of neuronal markers, but scid cultures showed a significant increase in the percentage of dying cells. Furthermore, when apoptosis was induced by staurosporine, scid neurons died more rapidly and in higher numbers. Apoptotic scid neurons exhibited nuclear condensation, DNA fragmentation and caspase-3 activation, but treatment with the general caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone did not prevent staurosporine-induced apoptosis. We conclude that a DNA-PK deficiency in cultured scid neurons may cause an accumulation of DNA damage and increased susceptibility to caspase-independent forms of programmed cell death.

Xeroderma pigmentosum and related disorders: defects in DNA repair and transcription

The genetic disorders xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD) are all associated with defects in nucleotide excision repair (NER) of DNA damage. Their clinical features are very different, however, XP being a highly cancer-prone skin disorder, whereas CS and TTD are cancer-free multisystem disorders. All three are genetically complex, with at least eight complementation groups for XP (XP-A to -G and variant), five for CS (CS-A, CS-B, XP-B, XP-D, and XP-G), and three for TTD (XP-B, XP-D, and TTD-A). With the exception of the variant, the products of the XP genes are proteins involved in the different steps of NER, and comprise three damage-recognition proteins, two helicases, and two nucleases. The two helicases, XPB and XPD, are components of the basal transcription factor TFIIH, which has a dual role in NER and initiation of transcription. Different mutations in these genes can affect NER and transcription differentially, and this accounts for the different clinical phenotypes. Mutations resulting in defective repair without affecting transcription result in XP, whereas if transcription is also affected, TTD is the outcome. CS proteins are only involved in transcription-coupled repair, a subpathway of NER in which damage in the transcribed strands of active genes is rapidly and preferentially repaired. Current evidence suggests that they also have an important but not essential role in transcription. The variant form of XP is defective in a novel DNA polymerase, which is able to synthesise DNA past UV-damaged sites.

Brain atrophy and neuronal loss in alcoholism: a role for DNA damage?

Chronic alcohol abuse has deleterious effects on several organs in the body including the brain. Neuroradiological studies have demonstrated that the brains of chronic alcoholics undergo loss of both gray and white matter volumes. Neuropathological studies using unbiased stereological methods have provided evidence for loss of neurons in specific parts of the brain in chronic alcoholics. The purpose of this paper is to propose a mechanism for this alcohol related neuronal loss. The hypothesis is based on the neurodegeneration observed in patients with the genetic disorder xeroderma pigmentosum (XP), who lack the capacity to carry out a specific type of DNA repair called nucleotide excision repair (NER). Some XP patients develop a progressive atrophic neurodegeneration, termed XP neurological disease, indicating that endogenous DNA damage that is normally repaired by NER has the capacity to cause neuronal death. Accumulating evidence indicates that the neurodegenerative DNA damage that is responsible for neuronal loss in XP patients results from reactive oxygen species (ROS) and lipid peroxidation products, and has the capacity to inhibit gene expression by RNA polymerase II. Therefore, the following model is proposed: chronic alcohol abuse results in increased levels of ROS and lipid peroxidation products in neurons, which results in an overwhelming burden on the NER pathway, and increased steady state levels of DNA lesions that inhibit gene expression. This results in neuronal death either by reduction in the levels of essential gene products or by apoptosis. The implications of this model for future studies are discussed.

Compound heterozygous group A xeroderma pigmentosum patient with a novel mutation and an inherited reciprocal translocation

The severity of neurological abnormalities in Japanese group A xeroderma pigmentosum (XP-A) patients correlates with the sites of non-sense mutation in the XP-A gene. We describe a patient who presented with a more severe photosensitivity and neurological abnormality than those in typical Japanese XP-A patients with a splicing mutation in intron 3. The patient was compound heterozygous for the splicing mutation in intron 3, which resulted in formation of a non-sense codon in exon 4, and a novel non-sense mutation at codon 208 in exon 5, a C to T transition creating a stop codon TAG. Although the combination of these mutations might have been thought to cause only mild neurological signs, the longer truncated XP-A proteins than those of typical XP-A patients may have resulted in severe neurological symptoms. This phenomenon may be explained by a translocation of chromosome (1;10)(q25.3;q22.3) inherited from his father.

Oxidative damage to the c-fos gene and reduction of its transcription after focal cerebral ischemia

We investigated oxidative damage to the c-fos gene and to its transcription in the brain of Long-Evans rats using a transient focal cerebral ischemia and reperfusion (FCIR) model. We observed a significant (p < 0.001) increase in the immunoreactivity to 8-hydroxy-2'-guanine (oh8G) and its deoxy form (oh8dG) in the ischemic cortex at 0-30 min of reperfusion in all 27 animals treated with 15-90 min of ischemia. Treatment with a neuronal nitric oxide synthase (nNOS) inhibitor, 3-bromo-7-nitroindazole (60 mg/kg, i.p.), abolished the majority but not all of the oh8G/oh8dG immunoreactivity. Treatment with RNase A reduced the oh8G immunoreactivity, suggesting that RNA may be targeted. This observation was further supported by decreased levels of mRNA transcripts of the c-fos and actin genes in the ischemic core within 30 min of reperfusion using in situ hybridization. The reduction in mRNA transcription occurred at a time when nuclear gene damage, detected as sensitive sites to Escherichia coli Fpg protein in the transcribed strand of the c-fos gene, was increased 13-fold (p < 0.01). Our results suggest that inhibiting nNOS partially attenuates FCIR-induced oxidative damage and that nNOS or other mechanisms induce nuclear gene damage that interferes with gene transcription in the brain.

Changes in proliferating cell nuclear antigen, a protein involved in DNA repair, in vulnerable hippocampal neurons following global cerebral ischemia

Proliferating cell nuclear antigen (PCNA) is required for completion of the DNA synthesis step of DNA replication as well as nucleotide excision repair (NER) of damaged DNA. We investigated the expression of PCNA mRNA and the levels of PCNA protein in the adult rat hippocampus following normo- and hypothermic global forebrain ischemia. Hypothermia protected the CA1 neurons from ischemic damage. A constitutive expression of PCNA mRNA and protein was detected in all hippocampal subfields, as well as in other brain regions. During reperfusion, PCNA mRNA levels were up-regulated in the vulnerable CA1 subfield at 36 h following normothermic ischemia. In hypothermia, this induction appeared already after 18 h. Following normothermic ischemia, nuclear PCNA immunoreactivity was largely abolished during reperfusion in the vulnerable CA1 neurons, prior to cell death. In contrast, total PCNA protein content of this region, as measured by Western blotting, remained largely unchanged. In the CA3 region, a transient decrease in nuclear PCNA immunoreactivity was observed. In the dentate gyrus region, no down-regulation of nuclear or total PCNA protein was observed during reperfusion. Following hypothermic ischemia, the PCNA protein levels did not decrease in any of the hippocampal subregions. In contrast, no change in the levels of Ref-1, a protein involved in base excision DNA repair (BER), was observed following normo- or hypothermic ischemia. Our findings indicate an altered functional state of PCNA protein in the ischemia-sensitive CA1 neurons suggesting that DNA repair processes are affected in these post-mitotic cells following ischemia. Impaired DNA repair may play a role in the development of postischemic neuronal damage.

PCR-RFLP analysis as an aid to genetic counseling of families of Japanese patients with group A xeroderma pigmentosum

Because Japanese patients with complementation group A xeroderma pigmentosum (XP-A) show early skin cancer and severe neurologic dysfunction, their family members are greatly concerned about the risk of inherited disease. In contrast to western XP-A patients, almost all Japanese XP-A patients have two of the three mutations (nonsense mutation in exon 3, splicing mutation in intron 3, and non-sense mutation in exon 6), which are easily detected by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. This work was aimed to see whether PCR-RFLP analysis is useful for genetic counseling of XP patients' siblings who are potential carriers of an XP-A gene mutation. In two of the three case studies presented, the probands were homozygous for the splicing mutation in intron 3 of the gene. In their siblings receiving genetic counseling, no mutation was found in the mutation site in one case, and one splicing mutation was found in the second case. In the third case, the proband was a compound heterozygote for the splicing mutation and for an unidentified mutation; in her sibling, no mutation was found in either of these mutation sites. No mutation was found in the siblings' spouses. On the basis of these findings, we reassured the prospective parents that there was little probability of having XP children, but in the second and third cases, we told them that their apparently unaffected children might be carriers. Each couple subsequently had one unaffected child. Thus, PCR-RFLP analysis is useful for genetic counseling of family members of XP-A patients.

Truncated XPA protein detected in atypical group A xeroderma pigmentosum

XPA protein from a patient with typical group A xeroderma pigmentosum (XP) and three atypical group-A XP patients were analysed. Immunoblot analysis of XPA proteins revealed that a typical group-A XP patient showed no XPA protein band, while a smaller, truncated XPA protein, which appears to be responsible for mid skin lesions and minimal neurological abnormalities, was detected in cells from three atypical group-A XP patients. Furthermore, the difference in the amount of truncated XPA protein correlated with the mildness of neurological manifestations in these three atypical group-A XP patients. The results suggest a correlation between clinical manifestations and qualitative and quantitative abnormalities of XPA protein products.

Chronological difference in walking impairment among Japanese group A xeroderma pigmentosum (XP-A) patients with various combinations of mutation sites

Almost all Japanese group A xeroderma pigmentosum (XP-A) patients have nonsense and/or nonsense codon-leading mutations in the XP group A (XPA) gene, and develop neurological abnormalities. Walking ability is one of the most important neuromuscular functions of the patients, because it determines their daily activities. We studied the correlation between the various combinations of mutations found by PCR-RFLP in Japanese XP-A patients and their chronological walking impairment. We classified these patients into six groups. Group I: A patient who was homozygous for the mutation at codon 116 in exon 3 (Type 1 mutation) could never walk unaided. Group III: Typical patients who were homozygous for the mutation at intron 3 (Type 2 mutation) could walk unaided till 7-16 years of age. Group V: Patients who were compound heterozygous for Type 2 mutation and for the mutation at codon 228 in exon 6 (Type 3 mutation) began to develop some walking difficulty at 5-13 years of age and became unable to walk at 25-28 years of age. Group VI: A patient who was homozygous for Type 3 mutation could walk unaided without any difficulty till the age of 21. The walking ability of group II and IV patients is not known yet.

DNA damage and repair in telomeres: relation to aging

We have established a method for the detection of DNA damage and its repair in human telomeres, the natural ends of chromosomes which are necessary for replication and critical for chromosomal stability. We find that ultraviolet light-induced pyrimidine dimers in telomeric DNA are repaired less efficiently than endogenous genes but more efficiently than inactive, noncoding regions. We have also measured telomeric length, telomeric DNA damage, and its repair in relation to the progression of aging. Telomeres are shorter in fibroblasts from an old donor compared to fibroblasts from a young donor, shortest in cells from a patient with the progeroid disorder Werner syndrome, and relatively long in fibroblasts from a patient with Alzheimer disease. Telomeric DNA repair efficiency is lower in cells from an old donor than in cells from a young donor, normal in Alzheimer cells, and slightly lower in Werner cells. It is possible that this decline in telomeric repair with aging is of functional significance to an age-related decline in genomic stability.

Severe neurological abnormalities associated with a mutation in the zinc-finger domain in a group A xeroderma pigmentosum patient

All the reported Japanese patients with group A xeroderma pigmentosum (XP) have two or three mutations at codon 116 in exon 3, codon 228 in exon 6, and the splicing acceptor site of intron 3 of XP group A complementing (XPAC) gene. A homozygote (XP39OS) with a nonsense mutation at codon 228 has less severe neurological abnormalities than patients with the splicing mutation at the acceptor site of intron 3. As homozygotes for the nonsense mutation at codon 116, which truncates a carboxyl-terminal site of XPAC protein at an early part of its zinc-finger domain, have not been reported previously, the possible severity of associated neurological abnormalities was not known. We report a group A XP patient, XP18OS, who had neurological abnormalities which were more severe than those in patients homozygous for the splicing mutation. The polymerase chain reaction product from exon 3 of the patient's XPAC gene was digested completely into three fragments by MseI restriction endonuclease. Thus, the patient was homozygous for the mutation at codon 116.

Repair of N-methylpurines in DNA from lymphocytes of patients with amyotrophic lateral sclerosis

We have previously reported reduced ability of ALS fibroblasts to repair genomic DNA damage produced by alkylating agents. This report presents our experience of studying DNA repair in lymphocytes from ALS patients. The repair of N-methylpurines produced by treatment with the alkylating agent, methyl methanesulfonate, was studied in T-lymphocytes from patients with sporadic and familial ALS, and appropriate controls. Repair of damage was quantitated by using alkaline elution for genomic DNA repair, and methoxyamine protection of abasic sites in DNA fragments for gene-specific repair in the dihydrofolate reductase (dhfr) gene, at time points 0, 6 h and 24 h. No significant repair rate differences were observed between ALS and control lymphocytes in either genomic or gene-specific DNA repair. The possible reasons for the discrepancy with our earlier results in lymphocytes and fibroblasts are discussed.

DNA repair mechanisms in neurological diseases: facts and hypotheses

DNA repair mechanisms usually consist of a complex network of enzymatic reactions catalyzed by a large family of mutually interacting gene products. Thus deficiency, alteration or low levels of a single enzyme and/or of auxiliary proteins might impair a repair process. There are several indications suggesting that some enzymes involved both in DNA replication and repair are less abundant if not completely absent in stationary and non replicating cells. Postmitotic brain cell does not replicate its genome and has lower levels of several DNA repair enzymes. This could impair the DNA repair capacity and render the nervous system prone to the accumulation of DNA lesions. Some human diseases clearly characterized by a DNA repair deficiency, such as xeroderma pigmentosum, ataxia-telangiectasia and Cockayne syndrome, show neurodegeneration as one of the main clinical and pathological features. On the other hand there is evidence that some diseases characterized by primary neuronal degeneration (such as amyotrophic lateral sclerosis and Alzheimer disease) may have alterations in the DNA repair systems as well. DNA repair thus appears important to maintain the functional integrity of the nervous system and an accumulation of DNA damages in neurons as a result of impaired DNA repair mechanisms may lead to neuronal degenerations.

Allelic heterogeneity in group A xeroderma pigmentosum

The molecular basis of Group A xeroderma pigmentosum was investigated by restriction fragment length polymorphism analysis of PCR-amplified DNA sequences using the two restriction enzymes, endonucleases AlwN I and Hph I. The clones of a patient with Group A xeroderma pigmentosum who had typical symptoms showed a G-C substitution at the 3' splice acceptor site of intron 3. However, of the two atypical Group A xeroderma pigmentosum patients with mild skin lesions and minimal neurological abnormalities, the milder one showed homozygosity for the nonsense mutation of exon 6, while the other patient with slightly greater central nervous involvement was shown to be a compound heterozygote for the splicing mutation of intron 3 and the nonsense mutation of exon 6, thus indicating an allelic heterogeneity in group A xeroderma pigmentosum.

Identification of splicing mutations of the last nucleotides of exons, a nonsense mutation, and a missense mutation of the XPAC gene as causes of group A xeroderma pigmentosum

Four mutations of the XPAC gene were identified as molecular bases of different UV-sensitive subgroups of xeroderma pigmentosum (XP) group A. One was a G to C transversion at the last nucleotide of exon 4 in GM1630/GM2062, a little less hypersensitive subgroup than the most sensitive XP2OS/XP12RO. The second mutation was a G to A transition at the last nucleotide of exon 3 in GM2033/GM2090, an intermediate subgroup. Both mutations caused almost complete inactivation of the canonical 5' splice donor site and aberrant RNA splicing. The third mutation was a nucleotide transition altering the Arg-211 codon (CGA) to a nonsense codon (TGA) in another allele of GM2062. The fourth mutation was a nucleotide transversion altering the His-244 codon (CAT) to an Arg codon (CGT) in XP8LO, an intermediate subgroup. Our results strongly suggest that the clinical heterogeneity in XP-A is due to different mutations in the XPAC gene.

Three nonsense mutations responsible for group A xeroderma pigmentosum

The molecular basis of xeroderma pigmentosum (XP) group A was studied and 3 nonsense mutations of the XP-A complementing gene (XPAC) were identified. One was a nucleotide transition altering the Arg-228 codon (CGA) to a nonsense codon (TGA). This transition creates a new cleavage site for the restriction endonuclease HphI. Of 21 unrelated Japanese XP-A patients examined, 1 (XP39OS) was a homozygote for this mutation and 3 were compound heterozygotes for this mutation and for the splicing mutation of intron 3 reported previously which is the most common mutation in Japanese patients and creates a new cleavage site for the restriction endonuclease AlwNI. The second mutation was a nucleotide transition altering the Arg-207 codon (CGA) to a nonsense codon (TGA). A Palestinian patient (XP12RO) who had severe symptoms of XP was homozygous for this mutation. The third mutation was a nucleotide transversion altering the Tyr-116 codon (TAT) to a nonsense codon (TAA). This transversion creates a new cleavage site for the restriction endonuclease MseI. Of the Japanese patients, 2 with severe clinical symptoms had this mutant allele. One was a compound heterozygote for this mutation and for the splicing mutation, and the other was heterozygous for this mutation and homozygous for the splicing mutation. Although most XP-A patients such as XP12RO have severe skin symptoms and neurological abnormalities of the de Sanctis-Cacchione syndrome, patient XP39OS was an atypical XP-A patient who had mild skin symptoms and minimal neurological abnormalities. Our results suggest that the clinical heterogeneity in XP-A is due to different mutations in the XPAC gene. Moreover, our data indicate that almost all Japanese cases of XP-A are caused by one or more of the 3 mutations, i.e., the splicing mutation of intron 3 and the 2 nonsense mutations of codons 116 and 228. Therefore, by restriction fragment length polymorphism analysis of PCR-amplified DNA sequences using the 3 restriction enzymes described above, rapid and reliable diagnosis of XP-A can be achieved in almost all Japanese subjects including prenatal cases and carriers.

Age-dependent axonal loss in nerve biopsy of patients with xeroderma pigmentosum

We describe the histomorphometric changes in the superficial peroneal nerve biopsy from 13 patients with xeroderma pigmentosum (XP). The mean age at time of biopsy was 15 yr (range 2.5-24 yr). The clinical examination was normal in the two youngest patients and showed absence of the deep tendon reflexes, with choreo-athetosis in ten patients. In addition, in the three oldest patients there were cerebellar signs, ataxia and Babinski signs. The nerve biopsy showed an age-dependent decrease of myelinated fibres, which was mild in the youngest and severe in the oldest patients. This was associated with rare acute axonal degeneration, sparse axonal regeneration, rare axonal atrophy and few onion bulb formations. These findings suggest a neuropathic process. This neuronal degeneration seems to be a progressive and stereotyped phenomenon in XP.

Comparison of cellular sensitivity to UV killing with neuropsychological impairment in Cockayne syndrome patients

We studied the neuropsychological function including mental status and motor development, and cellular susceptibility to UV killing in four Cockayne syndrome (CS), of whom three were classic form (type I) and one was congenital form (type II). The results showed that there was no correlation between the age at symptomatic onset of CS neurological disorders and the levels of cellular UV-hypersensitivity and that neuropsychological impairment did not parallel cellular hypersensitivity to UV killing. It was suggested that the cellular UV-hypersensitivity might not be the essential cause of neurodegeneration in CS.

Heterogeneity of DNA repair: implications for human disease and oncology

DNA repair studies used to be confined to measurements representing an average over the entire mammalian genome. It is now possible to study repair processes at subgenomic levels including specific genes. We will describe such results and discuss the impact they may have on our understanding of important oncological processes. Also, we will describe and discuss some clinical conditions that may have some effect in DNA damage processing.

Somatic mosaicism for DNA repair capacity in fibroblasts derived from a group A xeroderma pigmentosum patient

A female Japanese xeroderma pigmentosum (XP) patient with severe skin lesions and various neurologic abnormalities was assigned to complementation group A by conventional cell fusion studies. Ultraviolet (UV)-irradiated skin fibroblasts showed a biphasic survival curve, as measured by colony-forming ability. The surviving fraction decreased rapidly up to 2 J/m2 of UV, with a steep slope of D(O) (mean lethal dose) = 0.95 J/m2. At much higher doses it decreased more slowly, with D(O) = 3.5 J/m2. To èlucidate the cause of this unique survival response, we isolated a large number of independent clones from single colonies and measured their responses to UV. Of 81 clones analyzed, ten showed a marked resistance to killing by UV, which was only slightly more sensitive than normal cells, and these clones had a rate of unscheduled DNA synthesis (UDS) that was about 45% of normal cells. By contrast, the remaining 71 clones were extremely sensitive to UV, typical of XP group A strains, and had a UDS level 1%-3% of normals. Analysis of restriction fragment length polymorphism using seven polymorphic DNA probes indicated that the UV-resistant clones were derived from the same individual as the UV-sensitive clones. These results clearly demonstrate that this patient's fibroblast cells consist of two types with differing responses to UV, and provide direct evidence of somatic mosaicism for DNA repair capacity in an XP patient.

DNA repair, sensitivity to gamma radiation and to heat shock in lymphocytes from acute, untreated multiple sclerosis patients

Increases in spontaneous sister chromatid exchange (SCE) and gamma radiation-induced chromosome aberrations have been reported in peripheral blood lymphocytes (PBL) from multiple sclerosis (MS) patients, suggesting the presence of an abnormality in repair in this disease. We tested this hypothesis by measuring the ability to repair DNA and survival, after exposure to low (2-12 Gy) and high (100 Gy) gamma ray doses or to a high temperature (37-45 degrees C), of freshly isolated PBL from 15 patients affected by definite MS and 15 healthy subjects. The MS patients were untreated and in the acute phase of the disease. No significant difference was found between the two groups. We suggest that the previously reported genomic instability may be of viral origin and not due to a genetic defect in repair of DNA in these patients.

Peripheral neuropathy in four cases of group A xeroderma pigmentosum

We describe the clinical features and findings of biopsied sural nerves of 4 cases of xeroderma pigmentosum. Nine genetic forms of xeroderma pigmentosum have been reported by complementation studies. These four cases were diagnosed as Group A xeroderma pigmentosum by complementation studies using cultured skin fibroblasts. All cases had delayed mental and motor development in areas such as head control over 4 months of age and walking without support over 18 months of age. Three cases had the gradual onset of a gait disturbance between 6 and 9 years of age. Motor conduction velocity and sensory conduction velocity of the ulnar nerve were slightly delayed. The sural nerve of the slightly impaired patient showed a normal density of myelinated fibers, but a selective reduction of the large myelinated fibers with zebra-body-like structures in Schwann cell cytoplasm. The population density of all nerve fibers was severely diminished in the severely impaired cases. Ultrastructural observation disclosed many denervated Schwann cells and pockets of collagen isolated by loops of denervated Schwann cell cytoplasm. These findings suggest that the degenerative process in peripheral nerves of xeroderma pigmentosum is axonal. Peripheral neuropathy in Group A xeroderma pigmentosum resembles that of patients with ataxia telangiectasia who are known to have a defect in the repair mechanisms of their DNA in cultured skin fibroblasts.

Quantitative histologic study of sural nerves in xeroderma pigmentosum

The sural nerves of 2 siblings, 7 and 6 years of age with Group A xeroderma pigmentosum, were biopsied. The densities of myelinated fibers, 5,808/mm2 and 5,163/mm2, respectively, were strikingly decreased in comparison to control data. Both large and small myelinated fibers were reduced. Electron microscopy demonstrated many collagen fibers in the endoneurium and some collagen pockets. The loss of myelinated fibers was less severe than in previously reported patients. This discrepancy may be due to age differences at biopsy; our patients were biopsied at the ages of 7 and 6 years, while those patients reported previously were 10 years of age or older. The incidence of neurologic manifestations in xeroderma pigmentosum may increase after 6 years of age.

Deficient DNA repair in amyotrophic lateral sclerosis cells

We studied survival and DNA repair capacity in cultured sporadic ALS and control skin fibroblasts after treatment with DNA damaging agents producing different types of lesions. Mean survival in ALS and control fibroblasts was similar after exposure to ultraviolet (UV) light, x-rays and mitomycin C (MMC). Both mean survival and mean unscheduled (repair) DNA synthesis (UDS) were significantly reduced in ALS fibroblasts following treatment with the alkylating agent methyl methane sulfonate (MMS). These data suggest that ALS cells are relatively deficient in the repair of alkylation damage, possibly of apurinic/apyrimidinic sites, and that they are not unduly sensitive to DNA damage produced by UV light, x-rays and MMC. Normal survival and UDS seen in some patients' cells after MMS treatment indicate a spectrum of repair efficiency, and suggest heterogeneity of the biochemical defect in ALS.

Sensitivity to UV radiation of fibroblasts from a Japanese group A xeroderma pigmentosum patient with mild neurological abnormalities

A 12-year-old girl, suffering from xeroderma pigmentosum (XP), had mild cutaneous and neurological abnormalities. She showed no neurological abnormalities at the age of seven, but areflexia of the patellar tendons at II. She had no malignant tumours. The skin fibroblasts from the patient were about twice as sensitive to the lethal effects of 254 nm ultraviolet (UV) radiation as those of Group C XP patients, and about twice as resistant as those of typical Group A XP patients. The ability of these fibroblasts to reactivate UV-damage adenovirus 5 was intermediate between those of Group C and typical Group A patients. The patient's cells were assigned to genetic complementation Group A by use of the cell-fusion technique. This is the first case in Japan of a Group A XP patient with mild neurological abnormalities. The mild cutaneous manifestations of this patient may be explained by the residual ability of the cells to repair UV-damaged DNA.

Xeroderma pigmentosum patients from Germany: repair capacity of 45 XP fibroblast strains of the Mannheim XP Collection as measured by colony-forming ability and unscheduled DNA synthesis following treatment with methyl methanesulfonate and N-methyl-N-nitrosourea

A total of 45 XP fibroblast strains from the Mannheim XP Collection (representatives of XP complementation groups A, C, D, E, F or G, I, and XP variants) were investigated for colony-forming ability (term: D0) after treatment with up to ten doses of the methylating carcinogen MeSO2OMe. As controls 16 fibroblast strains from normal donors were used. Except for 4 XP strains (1 from group C and 3 from group D) which, however, were borderline cases, none of the remaining 41 XP strains was found to be more sensitive than normal controls. This held true within the limits of an experimental accuracy (experimental variability of D0 values) of +/- 7%. When weighted means were calculated for XP complementation groups and compared with that of normal donors at a significance level of 5%, no significant difference was detected. In contrast, after exposure of 6 XP group D strains to MeNOUr, a weighted mean D0 value was obtained which was significantly decreased by 27%. Unscheduled DNA synthesis (term: G0 which serves as a measure of excision repair) after exposure to MeNOUr was quantitatively the same (experimental variability: +/- 8%) both in the group of normal strains and in most of the XP complementation groups. Exceptions were group E and group F (or G) which had higher, and group I which had lower repair. Analogous G0 values measured after exposure to MeSO2OMe (experimental variability: +/- 13%), however, differed from that of the control strains: they were lower in XP complementation groups A, D, E, F (or G), and I. However, groups A, E, F (or G), and I including only 3 individual strains or less may be considered to be possibly ill-represented. Yet, group D including 11 XP strains did show reduction of the mean G0 value by 35%. From this it is concluded that there are repair defects in XP group D strains with regard to MeSO2OMe-induced adducts. These defects seem to be small.

Parkinson's disease and Alzheimer's disease: hypersensitivity to X rays in cultured cell lines

Fibroblast and/or lymphoblastoid lines from patients with several inherited primary neuronal degenerations are hypersensitive to DNA-damaging agents. Therefore, lymphoblastoid lines were irradiated from patients with sporadic Parkinson's disease (PD), Alzheimer's disease, and amyotrophic lateral sclerosis. The mean survival values of the eight Parkinson's disease and of the six Alzheimer's disease lines, but not of the five amyotrophic lateral sclerosis lines, were less than that of the 28 normal lines. Our results with Parkinson's disease and Alzheimer's disease cells can be explained by a genetic defect arising as a somatic mutation during embryogenesis, causing defective repair of the X-ray type of DNA damage. Such a DNA repair defect could cause an abnormal accumulation of spontaneously occurring DNA damage in Parkinson's disease and Alzheimer's disease neurons in vivo, resulting in their premature death.

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.

A sensitive assay for detecting hypersensitivity to ionizing radiation in lymphoblastoid lines from patients with Duchenne muscular dystrophy and primary neuronal degenerations

Hypersensitivity to the lethal effects of DNA-damaging agents is usually demonstrated using the classical colony-forming ability assay with cultured fibroblast lines. Based on the ability of viable cells in lymphoblastoid lines (Epstein-Barr virus-transformed B lymphocytes) to exclude the vital dye trypan blue, we have developed a more rapid survival assay which has been useful in detecting hypersensitivity to ionizing radiation in certain diseases characterized by primary degeneration of excitable tissue. We now present a complete description of this post-X-ray survival assay. We also demonstrate the suitability of both our assay and our method of data analysis for detecting hypersensitivity to ionizing radiation. This demonstration is based on a detailed analysis of assay results with lymphoblastoid lines from 28 normal donors, 3 ataxia telangiectasia (AT) patients, 2 obligate AT heterozygotes, 7 patients with diseases characterized by cellular hypersensitivity to ultraviolet radiation (UV), and 10 Duchenne muscular dystrophy (DMD) patients.

Use of lymphoblastoid cell lines to evaluate the hypersensitivity to ultraviolet radiation in Cockayne syndrome

Cockayne syndrome (CS) is a rare autosomal recessive disease characterized by acute sun sensitivity, cachectic dwarfism, and neurologic and skeletal abnormalities. Cultured skin fibroblasts from patients with this disease are known to be hypersensitive to the lethal effects of 254-nm UV radiation. We have studied the sensitivity of 254-nm UV radiation of lymphoblastoid lines derived from 3 typical CS patients, 1 atypical CS patient who had a very late age of onset of clinical manifestations, 2 patients who had both xeroderma pigmentosum (XP) and typical CS, and 3 heterozygous parents of these patients. Post-UV survival was determined by the trypan-blue dye-exclusion method. The lymphoblastoid lines from the 3 typical CS patients, the atypical CS patient, and the 2 patients with both CS and XP had decreased post-UV viability in comparison with lines from normal donors. Lines from the heterozygous parents had normal post-UV viability. The post-UV viability of the typical CS lines was similar to that of a XP complementation group C line. The relative post-UV viability of lymphoblastoid lines from the typical CS patients was similar to the relative post-UV survival of their fibroblast lines. The lymphoblastoid line from the atypical CS patient had a post-UV viability similar to that of the typical CS patients. Thus, the relative hypersensitivity of CS patients' cells in vitro does not reflect the severity or age of onset of the patients' clinical manifestations. The lymphoblastoid lines from the 2 patients who had both CS and XP were significantly more sensitive to the UV radiation than those from patients with only CS. Our studies demonstrate that lymphoblastoid lines from patients with CS are appropriate and useful cell lines for the study of the inherited hypersensitivity to UV radiation.