Cerebellar neurodegeneration in human hereditary DNA repair disorders

DICER- and MMSET-catalyzed H4K20me2 recruits the nucleotide excision repair factor XPA to DNA damage sites

The endoribonuclease DICER facilitates chromatin decondensation during lesion recognition following UV exposure. Chitale and Richly show that DICER mediates the recruitment of the methyltransferase MMSET, which catalyzes the dimethylation of histone H4 at lysine 20 and facilitates the recruitment of the nucleotide excision repair factor XPA.

Ultraviolet (UV) irradiation triggers the recruitment of DNA repair factors to the lesion sites and the deposition of histone marks as part of the DNA damage response. The major DNA repair pathway removing DNA lesions caused by exposure to UV light is nucleotide excision repair (NER). We have previously demonstrated that the endoribonuclease DICER facilitates chromatin decondensation during lesion recognition in the global-genomic branch of NER. Here, we report that DICER mediates the recruitment of the methyltransferase MMSET to the DNA damage site. We show that MMSET is required for efficient NER and that it catalyzes the dimethylation of histone H4 at lysine 20 (H4K20me2). H4K20me2 at DNA damage sites facilitates the recruitment of the NER factor XPA. Our work thus provides evidence for an H4K20me2-dependent mechanism of XPA recruitment during lesion recognition in the global-genomic branch of NER.

The Immp2l mutation causes age-dependent degeneration of cerebellar granule neurons prevented by antioxidant treatment

Reactive oxygen species are implicated in age‐associated neurodegeneration, although direct in vivo evidence is lacking. We recently showed that mice with a mutation in the Inner Mitochondrial Membrane Peptidase 2‐like (Immp2l) gene had elevated levels of mitochondrial superoxide, impaired fertility and age‐associated phenotypes, including kyphosis and ataxia. Here we show that ataxia and cerebellar hypoplasia occur in old mutant mice (> 16 months). Cerebellar granule neurons (CGNs) are significantly underrepresented; Purkinje cells and cells in the molecular layer are not affected. Treating mutant mice with the mitochondria‐targeted antioxidant SkQ1 from 6 weeks to 21 months protected cerebellar granule neurons. Apoptotic granule neurons were observed in mutant mice but not in age‐matched normal control mice or SkQ1‐treated mice. Old mutant mice showed increased serum protein carbonyl content, cerebellar 4‐hydroxynonenal (HNE), and nitrotyrosine modification compared to old normal control mice. SOD2 expression was increased in Purkinje cells but decreased in granule neurons of old mutant mice. Mitochondrial marker protein VDAC1 also was decreased in CGNs of old mutant mice, suggesting decreased mitochondrial number. SkQ1 treatment decreased HNE and nitrotyrosine modification, and restored SOD2 and VDAC1 expression in CGNs of old mutant mice. Neuronal expression of nitric oxide synthase was increased in cerebella of young mutant mice but decreased in old mutant mice. Our work provides evidence for a causal role of oxidative stress in neurodegeneration of Immp2l mutant mice. The Immp2l mutant mouse model could be valuable in elucidating the role of oxidative stress in age‐associated neurodegeneration.

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.

Lesions of cortical GABAergic interneurons and acetylcholine neurons in xeroderma pigmentosum group A

Xeroderma pigmentosum (XP) is a rare genetic disorder caused by inherited disturbances in the nucleotide excision repair system; patients with XP groups A (XP-A), B, D, and G were shown to have progressive neurological disturbances. Particularly, XP-A patients, which account for approximately half of Japanese XP patients, show severe neurological disorders, including mental retardation and epilepsy. Herein, we performed an immunohistochemical analysis of the number of GABAergic interneurons (GABAis), including calbindin-D28K, parvalbumin, and calretinin, in the cerebral cortex and acetylcholinergic neurons (AchNs) in the nucleus basalis of Meynert (NM) and in the pedunculopontine tegmental nucleus (PPN) in six autopsy cases of XP-A in order to investigate the relationships between mental dysfunction and GABAis and AchNs. The density and percentages of neurons that were immunoreactive for calbindin-D28K and parvalbumin were significantly reduced in the frontal and temporal cortices in XP-A cases, although the density of neurons that were immunoreactive for MAP2 did not differ from that in controls. Additionally, XP-A cases showed reduced AchNs in both the NM and the PPN. The observed reductions of cortical GABAis and AchNs may be involved in the mental disturbances, the higher occurrence of epilepsy, and/or the abnormalities in rapid eye movement sleep in patients with XP-A.

Delayed neuromotor recovery and increased memory acquisition dysfunction following experimental brain trauma in mice lacking the DNA repair gene XPA

OBJECT

This study investigates the outcome after traumatic brain injury (TBI) in mice lacking the essential DNA repair gene xeroderma pigmentosum group A (XPA). As damage to DNA has been implicated in neuronal cell death in various models, the authors sought to elucidate whether the absence of an essential DNA repair factor would affect the outcome of TBI in an experimental setting.

METHODS

Thirty-seven adult mice of either wild-type (n = 18) or XPA-deficient ("knock-out" [n = 19]) genotype were subjected to controlled cortical impact experimental brain trauma, which produced a focal brain injury. Sham-injured mice of both genotypes were used as controls (9 in each group). The mice were subjected to neurobehavoral tests evaluating learning/acquisition (Morris water maze) and motor dysfunction (Rotarod and composite neuroscore test), pre- and postinjury up to 4 weeks. The mice were killed after 1 or 4 weeks, and cortical lesion volume, as well as hippocampal and thalamic cell loss, was evaluated. Hippocampal staining with doublecortin antibody was used to evaluate neurogenesis after the insult.

RESULTS

Brain-injured XPA(-/-) mice exhibited delayed recovery from impairment in neurological motor function, as well as pronounced cognitive dysfunction in a spatial learning task (Morris water maze), compared with injured XPA(+/+) mice (p < 0.05). No differences in cortical lesion volume, hippocampal damage, or thalamic cell loss were detected between XPA(+/+) and XPA(-/-) mice after brain injury. Also, no difference in the number of cells stained with doublecortin in the hippocampus was detected.

CONCLUSIONS

The authors' results suggest that lack of the DNA repair factor XPA may delay neurobehavioral recovery after TBI, although they do not support the notion that this DNA repair deficiency results in increased cell or tissue death in the posttraumatic brain.

Age-related neuronal degeneration: complementary roles of nucleotide excision repair and transcription-coupled repair in preventing neuropathology

Neuronal degeneration is a hallmark of many DNA repair syndromes. Yet, how DNA damage causes neuronal degeneration and whether defects in different repair systems affect the brain differently is largely unknown. Here, we performed a systematic detailed analysis of neurodegenerative changes in mouse models deficient in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively, and that are associated with the UV-sensitive syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS). TCR–deficient Csa^−/− and Csb^−/− CS mice showed activated microglia cells surrounding oligodendrocytes in regions with myelinated axons throughout the nervous system. This white matter microglia activation was not observed in NER–deficient Xpa^−/− and Xpc^−/− XP mice, but also occurred in Xpd^XPCS mice carrying a point mutation (G602D) in the Xpd gene that is associated with a combined XPCS disorder and causes a partial NER and TCR defect. The white matter abnormalities in TCR–deficient mice are compatible with focal dysmyelination in CS patients. Both TCR–deficient and NER–deficient mice showed no evidence for neuronal degeneration apart from p53 activation in sporadic (Csa^−/−, Csb^−/−) or highly sporadic (Xpa^−/−, Xpc^−/−) neurons and astrocytes. To examine to what extent overlap occurs between both repair systems, we generated TCR–deficient mice with selective inactivation of NER in postnatal neurons. These mice develop dramatic age-related cumulative neuronal loss indicating DNA damage substrate overlap and synergism between TCR and NER pathways in neurons, and they uncover the occurrence of spontaneous DNA injury that may trigger neuronal degeneration. We propose that, while Csa^−/− and Csb^−/− TCR–deficient mice represent powerful animal models to study the mechanisms underlying myelin abnormalities in CS, neuron-specific inactivation of NER in TCR–deficient mice represents a valuable model for the role of NER in neuronal maintenance and survival.

Metabolism produces reactive oxygen species that damage our DNA and other cellular components, and as such it contributes to the aging process, including neuronal degeneration. Accordingly, genetic disorders associated with impaired DNA damage repair are frequently associated with premature onset of aging pathology in a variety of tissues, including the brain. This is well-illustrated by the progeroid DNA repair syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS), in which patients suffer from defects in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively. We have used a panel of XP and CS mice (including conditional double-mutant animals) to systematically investigate the impact of NER and TCR defects on neuronal degeneration. We have shown that, whereas a TCR defect causes white matter pathology, a NER defect can result in age related cumulative loss of neurons. These findings well match the neuropathology observed in CS and XP patients, underscoring the impact of spontaneous DNA damage in the onset of neuronal aging. Therefore, the XP and CS mouse models serve as valuable tools to delineate intervention strategies that combat age-associated pathology of the brain.

DNA repair deficiency in neurodegeneration

Deficiency in repair of nuclear and mitochondrial DNA damage has been linked to several neurodegenerative disorders. Many recent experimental results indicate that the post-mitotic neurons are particularly prone to accumulation of unrepaired DNA lesions potentially leading to progressive neurodegeneration. Nucleotide excision repair is the cellular pathway responsible for removing helix-distorting DNA damage and deficiency in such repair is found in a number of diseases with neurodegenerative phenotypes, including Xeroderma Pigmentosum and Cockayne syndrome. The main pathway for repairing oxidative base lesions is base excision repair, and such repair is crucial for neurons given their high rates of oxygen metabolism. Mismatch repair corrects base mispairs generated during replication and evidence indicates that oxidative DNA damage can cause this pathway to expand trinucleotide repeats, thereby causing Huntington's disease. Single-strand breaks are common DNA lesions and are associated with the neurodegenerative diseases, ataxia-oculomotor apraxia-1 and spinocerebellar ataxia with axonal neuropathy-1. DNA double-strand breaks are toxic lesions and two main pathways exist for their repair: homologous recombination and non-homologous end-joining. Ataxia telangiectasia and related disorders with defects in these pathways illustrate that such defects can lead to early childhood neurodegeneration. Aging is a risk factor for neurodegeneration and accumulation of oxidative mitochondrial DNA damage may be linked with the age-associated neurodegenerative disorders Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Mutation in the WRN protein leads to the premature aging disease Werner syndrome, a disorder that features neurodegeneration. In this article we review the evidence linking deficiencies in the DNA repair pathways with neurodegeneration.

Cockayne syndrome B protects against methamphetamine-enhanced oxidative DNA damage in murine fetal brain and postnatal neurodevelopmental deficits

Methamphetamine (METH) increases the oxidative DNA lesion 8-oxoguanine (8-oxoG) in fetal mouse brain, and causes postnatal motor coordination deficits after in utero exposure. Like oxoguanine glycosylase 1 (OGG1), the Cockayne syndrome B (CSB) protein is involved in the repair of oxidatively damaged DNA, although its function is unclear. Here we used CSB-deficient Csb(m/m) knockout mice to investigate the developmental role of DNA oxidation and CSB in METH-initiated neurodevelopmental deficits. METH (40 mg/kg intraperitoneally) administration to pregnant Csb females on gestational day 17 increased 8-oxoG levels in Csb(m/m) fetal brains (p < 0.05). CSB modulated 8-oxoG levels independent of OGG1 activity, as 8-oxoG incision activity in fetal nuclear extracts was identical in Csb(m/m) and Csb(+/+)mice. This CSB effect was evident despite 7.1-fold higher OGG1 activity in Csb(+/+) mice compared to outbred CD-1 mice. Female Csb(m/m) offspring exposed in utero to METH exhibited motor coordination deficits postnatally (p < 0.05). In utero METH exposure did not cause dopaminergic nerve terminal degeneration, in contrast to adult exposures. This is the first evidence that CSB protects the fetus from xenobiotic-enhanced DNA oxidation and postnatal functional deficits, suggesting that oxidatively damaged DNA is developmentally pathogenic, and that fetal CSB activity may modulate the risk of reactive oxygen species-mediated adverse developmental outcomes.

Neuropathology of Cockayne syndrome: Evidence for impaired development, premature aging, and neurodegeneration

Global growth and development failure, premature, accelerated, pathologic aging, and neurodegeneration characterize Cockayne syndrome (CS) and the cerebro-oculo-facial-skeletal and xeroderma pigmentosum/CS syndromes which overlap CS partially in their genetic, somatic, and neuropathologic features. Mutations of CSA or CSB genes jeopardize transcription-coupled repair of damaged nuclear and mitochondrial DNA and resumption of replication and transcription. Resultant defective proteins or gene silencing eventuate in profound dwarfism and micrencephaly, cachexia, vasculopathy, and neurodegeneration. Cellular effects are highly selective. Purkinje cells may die by apoptosis and have grossly dystrophic dendrites. Neuronal death and axonal spheroids indexing neuronal pathology predominate in, but are not limited to, the cerebellum. Progressive loss of retinal, cochlear, and vestibular sensory receptors foster degeneration of ganglion cells and transneuronal brain degeneration. Some proliferating astrocytes are multinucleated and bizarre. Primary damage of oligodendrocytes and Schwann cells may - or may not - explain severe patchy myelin loss ("tigroid leukodystrophy") and segmental demyelinating peripheral neuropathy. Age-related changes are minor in the brain, although precocious severe athero- and arteriolosclerosis are responsible for occasional strokes. Vasculopathology may contribute to myelin loss and to dystrophic mineralization of neurons and vessels, especially in basal ganglia and cerebellum. Understanding the genetics, biochemical, and cellular pathophysiology of these disorders remains fragmentary.

Roles of oxidative stress in xeroderma pigmentosum

Tissue damage caused by oxidative stress has been implicated in aging, carcinogenesis, atherosclerosis and neurodegeneration. In xeroderma pigmentosum (XP) and Cockayne syndrome (CS), oxidative stress is associated with promoted occurrence of skin cancers and progressive neurodegeneration, because decreased DNA repair and persistent DNA damage can result in augmented oxidative nucleotide damage. Oxidative nucleotide damage has been investigated mainly in isolated human skin and blood cells or their cell lines, in which CS cells may be more sensitive to oxidative DNA lesions than XP cells. However, cells from patients with XP group A (XPA) show defective repair of 8, 5'-(S)-cyclo-2'-deoxyadenosine, a free radical-induced endogenous DNA lesion and antioxidant system seems to be disturbed variously in cells from XP patients. We have neuropathologically investigated the involvement of oxidative stress in the brains of XPA and CS autopsy cases and clarified the enhanced lipid peroxidation and protein glycation in the pallidal and cerebellar degeneration. Also, oxidative nucleotide damage with reduced expression of superoxide dismutases has been identified in the basal ganglia lesions, lending further weight involvement of oxidative stress in neurodegeneration in XPA patients. Additionally, we are developing ELISA analysis of oxidative stress markers in the urine and cerebrospinal fluid from XP patients, which will aid with further data on oxidative stress in pathogenesis of XP.

Clinical implications of the basic defects in Cockayne syndrome and xeroderma pigmentosum and the DNA lesions responsible for cancer, neurodegeneration and aging

Cancer, aging, and neurodegeneration are all associated with DNA damage and repair in complex fashions. Aging appears to be a cell and tissue-wide process linked to the insulin-dependent pathway in several DNA repair deficient disorders, especially in mice. Cancer and neurodegeneration appear to have complementary relationships to DNA damage and repair. Cancer arises from surviving cells, or even stem cells, that have down-regulated many pathways, including apoptosis, that regulate genomic stability in a multi-step process. Neurodegeneration however occurs in nondividing neurons in which the persistence of apoptosis in response to reactive oxygen species is, itself, pathological. Questions that remain open concern: sources and chemical nature of naturally occurring DNA damaging agents, especially whether mitochondria are the true source; the target tissues for DNA damage and repair; do the human DNA repair deficient diseases delineate specific pathways of DNA damage relevant to clinical outcomes; if naturally occurring reactive oxygen species are pathological in human repair deficient disease, would anti-oxidants or anti-apoptotic agents be feasible therapeutic agent?

Tissue-specific accelerated aging in nucleotide excision repair deficiency

Nucleotide excision repair (NER) is a multi-step DNA repair mechanism that removes helix-distorting modified nucleotides from the genome. NER is divided into two subpathways depending on the location of DNA damage in the genome and how it is first detected. Global genome NER identifies and repairs DNA lesions throughout the genome. This subpathway of NER primarily protects against the accumulation of mutations in the genome. Transcription-coupled (TC)-NER rapidly repairs lesions in the transcribed strand of DNA that block transcription by RNA polymerase II. TC-NER prevents cell death in response to stalled transcription. Defects in NER cause three distinct human diseases: xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy. Each of these syndromes is characterized by premature onset of pathologies that overlap with those associated with old age in humans. This reveals the contribution of DNA damage to multiple age-related diseases. Tissues affected include the skin, eye, bone marrow, nervous system and endocrine axis. This review emphasizes accelerated aging associated with xeroderma pigmentosum and discusses the cause of these pathologies, either mutation accumulation or cell death as a consequence of failure to repair DNA damage.

Nucleotide excision repair deficient mouse models and neurological disease

Nucleotide excision repair (NER) is a highly conserved mechanism to remove helix-distorting DNA base damage. A major substrate for NER is DNA damage caused by environmental genotoxins, most notably ultraviolet radiation. Xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy are three human diseases caused by inherited defects in NER. The symptoms and severity of these diseases vary dramatically, ranging from profound developmental delay to cancer predisposition and accelerated aging. All three syndromes include neurological disease, indicating an important role for NER in protecting against spontaneous DNA damage as well. To study the pathophysiology caused by DNA damage, numerous mouse models of NER-deficiency were generated by knocking-out genes required for NER or knocking-in disease-causing human mutations. This review explores the utility of these mouse models to study neurological disease caused by NER-deficiency.

Increased apoptosis, p53 up-regulation, and cerebellar neuronal degeneration in repair-deficient Cockayne syndrome mice

Cockayne syndrome (CS) is a rare recessive childhood-onset neurodegenerative disease, characterized by a deficiency in the DNA repair pathway of transcription-coupled nucleotide excision repair. Mice with a targeted deletion of the CSB gene (Csb-/-) exhibit a much milder ataxic phenotype than human patients. Csb-/- mice that are also deficient in global genomic repair [Csb-/-/xeroderma pigmentosum C (Xpc)-/-] are more profoundly affected, exhibiting whole-body wasting, ataxia, and neural loss by postnatal day 21. Cerebellar granule cells demonstrated high TUNEL staining indicative of apoptosis. Purkinje cells, identified by the marker calbindin, were severely depleted and, although not TUNEL-positive, displayed strong immunoreactivity for p53, indicating cellular stress. A subset of animals heterozygous for Csb and Xpc deficiencies was more mildly affected, demonstrating ataxia and Purkinje cell loss at 3 months of age. Mouse, Csb-/-, and Xpc-/- embryonic fibroblasts each exhibited increased sensitivity to UV light, which generates bulky DNA damage that is a substrate for excision repair. Whereas Csb-/-/Xpc-/- fibroblasts were more UV-sensitive than either single knockout, double-heterozygote fibroblasts had normal UV sensitivity. Csb-/- mice crossed with a strain defective in base excision repair (Ogg1) demonstrated no enhanced neurodegenerative phenotype. Complete deficiency in nucleotide excision repair therefore renders the brain profoundly sensitive to neurodegeneration in specific cell types of the cerebellum, possibly because of unrepaired endogenous DNA damage that is a substrate for nucleotide but not base excision repair.

Cockayne syndrome in adults: review with clinical and pathologic study of a new case

Cockayne syndrome and xeroderma pigmentosum-Cockayne syndrome complex are rare autosomal recessive disorders with poorly understood biology. They are characterized by profound postnatal brain and somatic growth failure and by degeneration of multiple tissues resulting in cachexia, dementia, and premature aging. They result in premature death, usually in childhood, exceptionally in adults. This study compares the clinical course and pathology of a man with Cockayne syndrome group A who died at age 31(1/2) years with 15 adequately documented other adults with Cockayne syndrome and 5 with xeroderma pigmentosum-Cockayne syndrome complex. Slowing of head and somatic growth was apparent before age 2 years, mental retardation and slowly progressive spasticity at 4 years, ataxia and hearing loss at 9 years, visual impairment at 14 years, typical Cockayne facies at 17 years, and cachexia and dementia in his twenties, with a retained outgoing personality. He experienced several transient right and left hemipareses and two episodes of status epilepticus following falls. Neuropathology disclosed profound microencephaly, bilateral old subdural hematomas, white-matter atrophy, tigroid leukodystrophy with string vessels, oligodendrocyte proliferation, bizarre reactive astrocytes, multifocal dystrophic calcification that was most marked in the basal ganglia, advanced atherosclerosis, mixed demyelinating and axonal neuropathy, and neurogenic muscular atrophy. Cellular degeneration of the organ of Corti, spiral and vestibular ganglia, and all chambers of the eye was severe. Rarely, and for unexplained reasons, in some patients with Cockayne syndrome the course is slower than usual, resulting in survival into adulthood. The profound dwarfing, failure of brain growth, cachexia, selectivity of tissue degeneration, and poor correlation between genotypes and phenotypes are not understood. Deficient repair of DNA can increase vulnerability to oxidative stress and play a role in the premature aging, but why patients with mutations in xeroderma pigmentosum genes present with the Cockayne syndrome phenotype is still not known.

Oxidative nucleotide damage and superoxide dismutase expression in the brains of xeroderma pigmentosum group A and Cockayne syndrome

Xeroderma pigmentosum group A (XPA) and Cockayne syndrome (CS) are caused by a genetic defect of nucleotide excision repair mechanisms, showing cutaneous hypersensitivity to sunlight and progressive neurological disturbances. The cause of neurological abnormalities has yet to be clarified and fundamental treatments have never been established in both disorders. In order to investigate neurodegeneration of XPA and CS, we immunohistochemically examined deposition of oxidative stress-related materials of nucleotides and expression of two types of superoxide dismutase (SOD) in the brains from autopsy cases of XPA and CS. Cases of XPA but not CS demonstrated nuclear deposition of 8-hydroxy-2'-deoxyguanosine and cytoplasmic deposition of 8-hydroxyguanosine, being speculated as oxidative stress-related materials of DNA and RNA, respectively, in the globus pallidus. Four of five XPA cases exhibited reduced neuronal immunoreactivity for Cu/ZnSOD in the cerebral and cerebellar corteces in addition to the basal ganglia, and two XPA cases showed reduced immunoreactivity for MnSOD in the brain regions examined. In contrast, five CS cases demonstrated comparatively preserved immunoreactivity for Cu/ZnSOD and MnSOD. Both XPA and CS cases showed increased cytoplasmic immunoreactivity for Cu/ZnSOD and/or MnSOD in the microglial cells in the cerebral and cerebellar white matters. These findings suggest that oxidative damage to nucleotides and disturbed SOD expression can be involved in neurodegeneration in XPA but not CS.

Brainstem and Basal Ganglia Lesions in Xeroderma Pigmentosum Group A

Xeroderma pigmentosum group A (XPA) is a hereditary disorder characterized by cutaneous symptoms and progressive neurodegeneration. Since XPA patients exhibit peripheral neuropathy, neuronal deafness, rigidity, dysphagia, and laryngeal dystonia, it is indispensable for investigation of the neurodegeneration to analyze brainstem and basal ganglia lesions clinically and pathologically; we have previously shown the role of oxidative stress in the development of basal ganglia lesions. Here we immunohistochemically examined the expression of neurotransmitters, calcium-binding proteins, and neuropeptides in the brainstem, basal ganglia, and thalamus in 5 XPA autopsy cases. In the brainstem, immunoreactivity for tyrosine hydroxylase, tryptophan hydroxylase, and calbindin-D28K was severely reduced throughout the brainstem in all the XPA cases. Nevertheless, the expressions of parvalbumin, substance P, and methionine-enkephalin in the brainstem were comparatively preserved; the exception being reduced immunoreactivity for them in the cochlear and dorsal column nuclei in 3 cases. The large cell neurons in the putamen were preferentially reduced, the immunoreactivity for tyrosine hydroxylase reflecting the dopaminergic afferent and efferent pathways was severely affected, and the expression of 3 calcium binding proteins (i.e. parvalbumin, calbindin-D28K, and calretinin) was disturbed in various ways. The expression of substance P and methionine-enkephalin, which are involved in the efferent pathways in the basal ganglia, in the globus pallidus and substantia nigra was spared. It is speculated that the selective damage to the dopamine system in the basal ganglia and the disturbed monoaminergic expression in the brainstem could be related to clinical abnormalities such as the rigidity, laryngeal dystonia, and several neurophysiological changes. Functional analysis of autopsy brains will facilitate clarification of the pathogenesis of the neurodegeneration in XPA.

Deficiency of the Mre11 DNA repair complex in Alzheimer's disease brains

The hallmarks of Alzheimer's disease (AD) brains are deposition of Abeta plaques, appearance of neurofibrillary tangles, and extensive loss of neuronal cells. While Abeta plaques and neurofibrillary tangles play a significant role in the pathogenic dysfunction of neurons, factors that accelerate the neurodegenerating process remain to be defined. One set of factors examined in this study is the Mre11 protein complex, composed of Rad50, Mre11 and Nbs1. This protein complex plays an essential role in cellular responses to DNA damage, such as initiating cell cycle checkpoints and repairing damaged DNA. A defect in any component of this protein complex is detrimental to cells. Recently, several groups have observed abnormal cell cycle regulation and/or accumulated DNA damage in AD neurons. These pathological alterations could conceivably be caused or exacerbated by any compromise in the Mre11 complex. In this study, we compared the levels of the Mre11 complex proteins in brain samples from AD and age-matched non-dementia controls. We show for the first time that the Mre11 complex proteins are present in neurons of the adult human cortex and cerebellum. These proteins were found substantially reduced in the neurons of AD cortex. Our finding suggests that the loss of the Mre11 complex may be associated with the pathogenesis of AD.

The contributions of excitotoxicity, glutathione depletion and DNA repair in chemically induced injury to neurones: exemplified with toxic effects on cerebellar granule cells

Six chemicals, 2-halopropionic acids, thiophene, methylhalides, methylmercury, methylazoxymethanol (MAM) and trichlorfon (Fig. 1), that cause selective necrosis to the cerebellum, in particular to cerebellar granule cells, have been reviewed. The basis for the selective toxicity to these neurones is not fully understood, but mechanisms known to contribute to the neuronal cell death are discussed. All six compounds decrease cerebral glutathione (GSH), due to conjugation with the xenobiotic, thereby reducing cellular antioxidant status and making the cells more vulnerable to reactive oxygen species. 2-Halopropionic acids and methylmercury appear to also act via an excitotoxic mechanism leading to elevated intracellular Ca2+, increased reactive oxygen species and ultimately impaired mitochondrial function. In contrast, the methylhalides, trichlorfon and MAM all methylate DNA and inhibit O6-guanine-DNA methyltransferase (OGMT), an important DNA repair enzyme. We propose that a combination of reduced antioxidant status plus excitotoxicity or DNA damage is required to cause cerebellar neuronal cell death with these chemicals. The small size of cerebellar granule cells, the unique subunit composition of their N-methyl-d-aspartate (NMDA) receptors, their low DNA repair ability, low levels of calcium-binding proteins and vulnerability during postnatal brain development and distribution of glutathione and its conjugating and metabolizing enzymes are all important factors in determining the sensitivity of cerebellar granule cells to toxic compounds.

Analysis of age dependent changes of Topoisomerase II α and β in rat brain

Eukaryotic Topoisomerase II (Topo II) is present in two isoforms alpha and beta. The alpha isoform is predominantly localized in proliferative tissue, while beta isoform is present in all tissues. In the present study we report the activity and protein levels of Topoisomerase II alpha and beta in rat brains of different age groups viz.: E11 (Embryo day 11), E18 (Embryo day 18), post-natal day 1, young (<10 days), adult (<6 months) and old (>2 years). Topoisomerase II beta isoform is found to be the predominant form in brain tissue but Topoisomerase II alpha is found in embryos up to post-natal day 1. The studies to examine the regional distribution of Topoisomerase II beta in brain showed highest activity in cerebellar region and that too only neuronal cell fraction. There was a significant age-dependent decline in this activity. Hence, Topoisomerase II beta may have some unknown function in cerebellum and the low levels of Topoisomerase II beta activity in ageing cerebellum may contribute to the genomic instability in cerebellar region of ageing brain.

p53-dependent cell death signaling in neurons

The p53 tumor suppressor gene is a sequence-specific transcription factor that activates the expression of genes engaged in promoting growth arrest or cell death in response to multiple forms of cellular stress. p53 expression is elevated in damaged neurons in acute models of injury such as ischemia and epilepsy and in brain tissue samples derived from animal models and patients with chronic neurodegenerative diseases. p53 deficiency or p53 inhibition protects neurons from a wide variety of acute toxic insults. Signal transduction pathways associated with p53-induced neuronal cell death are being characterized, suggesting that intervention may prove effective in maintaining neuronal viability and restoring function following neural injury and disease.

Immature and mature cortical neurons engage different apoptotic mechanisms involving caspase-3 and the mitogen-activated protein kinase pathway

The authors used cultured mouse cortical neurons to study mechanisms of DNA damage-induced apoptosis in immature and mature neurons. Neurons were maintained viably for 60 days in vitro (DIV60). The increased levels of glutamate receptors, synaptic proteins, and glycolytic enzyme were used to track maturation. Exposure of neurons to the DNA-damaging agent camptothecin induced apoptosis in immature (DIV5) and mature (DIV25-30) neurons. Internucleosomal fragmentation of DNA emerged more rapidly in mature neurons than in immature neurons. Immunoblotting revealed that cleaved caspase-3 increased in apoptotic DIV5 neurons but not in DIV30 neurons, but immunolocalization showed accumulation of cleaved caspase-3 in DIV5 and DIV30 neurons. A reversible caspase-3 inhibitor blocked apoptosis in DIV5 neurons but not in DIV30 neurons. Phosphorylation of extracellular signal-regulated kinase/mitogen-activated protein kinase (Erk/MAP kinase)-42/44 occurred preapoptotically in mature but not immature neurons, while Erk54 nuclear translocation and MAP kinase kinase kinase-1 cleavage into putative caspase-3-generated proapoptotic fragments occurred in DIV5 but not DIV30 neurons. Inhibition of Erk activation with MAP kinase kinase inhibitor blocked apoptosis at both ages. The results show that immature and mature cortical neurons engage different signaling mechanisms in MAP kinase and caspase pathways during apoptosis; thus, neuron age influences the mechanisms and progression of apoptosis.

Early postnatal ataxia and abnormal cerebellar development in mice lacking Xeroderma pigmentosum Group A and Cockayne syndrome Group B DNA repair genes

Xeroderma pigmentosum (XP) and Cockayne syndrome (CS) are rare autosomal recessive disorders associated with a defect in the nucleotide excision repair (NER) pathway required for the removal of DNA damage induced by UV light and distorting chemical adducts. Although progressive neurological dysfunction is one of the hallmarks of CS and of some groups of XP patients, the causative mechanisms are largely unknown. Here we show that mice lacking both the XPA (XP-group A) and CSB (CS-group B) genes in contrast to the single mutants display severe growth retardation, ataxia, and motor dysfunction during early postnatal development. Their cerebella are hypoplastic and showed impaired foliation and stunted Purkinje cell dendrites. Reduced neurogenesis and increased apoptotic cell death occur in the cerebellar external granular layer. These findings suggest that XPA and CSB have additive roles in the mouse nervous system and support a crucial role for these genes in normal brain development.

Oxidative stress and disturbed glutamate transport in hereditary nucleotide repair disorders

Xeroderma pigmentosum group A (XPA) and Cockayne syndrome (CS) are hereditary DNA repair disorders complicated by progressive neurodegeneration. Here we immunohistochemically examine the in situ expression of materials that are produced by oxidative stress and glutamate transporters (which can contribute to prevention of glutamate neurotoxicity) in the brains of 5 autopsied patients each of XPA, CS, and control groups. All oxidative products, including nitrotyrosine, advanced glycation end product, and 4-hydroxy-2-nonenal-modified protein (HNE) were deposited in large amounts in the globus pallidus of CS patients compared to XPA patients. They were frequently recognized in the pseudocalcified foci and free minerals in the neuropil, and more rarely in foamy spheroids. In addition, the deposition of HNE was observed also in hippocampal and cerebellar dentate neurons of both CS and XPA patients. The expression of glial glutamate transporters, EAAT1 and GLT-1, was affected in the globus pallidus in 5 CS patients and 3 XPA patients. They were also altered in the cerebellar cortex in most of the CS patients. These data suggest that oxidative stress and disturbed glutamate transport may be involved in pallidal and/or cerebellar degeneration in hereditary nucleotide repair disorders.

Cockayne syndrome and xeroderma pigmentosum

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. Cultured 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.

The role of p53 in neuronal cell death

The p53 tumor suppressor gene is a sequence-specific transcription factor that activates the expression of genes engaged in promoting growth arrest or cell death in response to genotoxic stress. A possible role for p53-related modulation of neuronal viability has been suggested by the finding that p53 expression is elevated in damaged neurons in acute models of injury such as ischemia and epilepsy and in brain tissue samples derived from patients with chronic neurodegenerative diseases. Moreover, the absence of p53 has been shown to protect neurons from a wide variety of acute toxic insults. Signal transduction pathways associated with p53-induced cell death are being unraveled and suggest that intervention may prove fruitful in maintaining neuronal viability and restoring function following cytopathic insults.

Binding of pigment epithelium-derived factor (PEDF) to retinoblastoma cells and cerebellar granule neurons. Evidence for a PEDF receptor

Pigment epithelium-derived factor (PEDF) has neuronal differentiation and survival activity on retinoblastoma and cerebellar granule (CG) cells. Here, we investigated the presence of PEDF receptors on retinoblastoma Y-79 and CG cells. PEDF radiolabeled with (l25)I remained biologically active and was used for radioligand binding analysis. The binding was saturable and specific to a single class of receptors on both cells and with similar affinities (K(d) = 1.7-3.6 nM, B(max) = 0.5-2.7 x 10(5) sites/Y-79 cell; and K(d) = 3.2 nM, B(max) = 1.1 x 10(3) sites/CG cell). A polyclonal antiserum to PEDF, previously shown to block the PEDF neurotrophic activity, prevented the (125)I-PEDF binding. We designed two peptides from a region previously shown to confer the neurotrophic property to human PEDF, synthetic peptides 34-mer (positions 44-77) and 44-mer (positions 78-121). Only peptide 44-mer competed for the binding to Y-79 cell receptors (EC(50) = 5 nM) and exhibited neuronal differentiating activity. PEDF affinity column chromatography of membrane proteins from both cell types revealed a PEDF-binding protein of approximately 80 kDa. These results are the first demonstration of a PEDF-binding protein with characteristics of a PEDF receptor and suggest that the region comprising amino acid positions 78-121 of PEDF might be involved in ligand-receptor interactions.

Neurodegeneration in hereditary nucleotide repair disorders

Both xeroderma pigmentosum group A (XPA) and Cockayne syndrome (CS) are rare autosomal disorders, have a genetic defect in the step of nucleotide repair, and involve various neurological abnormalities caused by progressive neurodegeneration. We performed comprehensive neuropathological analysis of five cases of XPA and four cases of CS. The XPA cases showed widespread neuronal loss throughout the central nervous system, in sharp contrast to the comparative preservation of neurons in the CS cases, who rather exhibited patchy demyelination in the cerebral and cerebellar white matter, and multifocal calcium deposition in the basal ganglia and cerebral white matter, respectively. Exceptionally in the cerebellar cortex, neuronal loss was more severe in CS than in XPA. Grumose or foamy spheroid bodies occurred in the globus pallidus and substantia nigra, and axonal torpedoes were increased in the cerebellar cortex in both disorders. Neither silver impregnation nor immunohistochemistry for ubiquitin or tau succeeded in visualizing neurofibrillary tangles, senile plaques or augmented ubiquitination in either disorder, and these findings did not support the involvement of facilitated aging in the neurodegeneration in XPA or CS.