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

Radiation Therapy in Alzheimer's Disease: A Systematic Review

PURPOSE

Pathophysiological hallmarks of Alzheimer's disease (AD) include extracellular amyloid plaques and intracellular neurofibrillary tangles. Recent studies also demonstrated a role of neuroinflammation in the progression of the disease. Clinical trials and animal studies using low-dose radiation therapy (LDRT) have shown therapeutic potential for AD. This systematic review summarizes the current evidence on the use of LDRT for the treatment of AD, outlines potential mechanisms of action, and discusses current challenges in the planning of future trials.

METHODS AND MATERIALS

A systematic review of human and animal studies as well as registered clinical trials describing outcomes for RT in the treatment of AD was conducted. We followed the 2020 Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Articles published until July 1, 2023, were included.

RESULTS

The initial search yielded 993 articles. After the removal of duplicates and ineligible publications, a total of 16 (12 animal, 4 human) studies were included. Various dose regimens were utilized in both animal and human trials. The results revealed that LDRT reduced the number of amyloid plaques and neurofibrillary tangles, and it has a role in the regulation of genes and protein expression involved in the pathological progression of AD. LDRT has demonstrated reduced astro- and microgliosis, anti-inflammatory and neuroprotective effects, and an alleviation of symptoms of cognitive deficits in animal models. Most studies in humans suggested improvements in cognition and behavior. None of the trials or studies described significant (>grade 2) toxicity.

CONCLUSIONS

Preclinical studies, animal studies, and early clinical trials in humans have shown a promising role for LDRT in the treatment of AD pathologies, although the underlying mechanisms are yet to be fully explored. Phase I/II/III trials are needed to assess the long-term safety, efficacy, and optimal treatment parameters of LDRT in AD treatment.

Toward an Early Diagnosis for Alzheimer's Disease Based on the Perinuclear Localization of the ATM Protein

Alzheimer's disease (AD) is the most common neurodegenerative dementia, for which the molecular origins, genetic predisposition and therapeutic approach are still debated. In the 1980s, cells from AD patients were reported to be sensitive to ionizing radiation. In order to examine the molecular basis of this radiosensitivity, the ATM-dependent DNA double-strand breaks (DSB) signaling and repair were investigated by applying an approach based on the radiation-induced ataxia telangiectasia-mutated (ATM) protein nucleoshuttling (RIANS) model. Early after irradiation, all ten AD fibroblast cell lines tested showed impaired DSB recognition and delayed RIANS. AD fibroblasts specifically showed spontaneous perinuclear localization of phosphorylated ATM (pATM) forms. To our knowledge, such observation has never been reported before, and by considering the role of the ATM kinase in the stress response, it may introduce a novel interpretation of accelerated aging. Our data and a mathematical approach through a brand-new model suggest that, in response to a progressive and cumulative stress, cytoplasmic ATM monomers phosphorylate the APOE protein (pAPOE) close to the nuclear membrane and aggregate around the nucleus, preventing their entry in the nucleus and thus the recognition and repair of spontaneous DSB, which contributes to the aging process. Our findings suggest that pATM and/or pAPOE may serve as biomarkers for an early reliable diagnosis of AD on any fibroblast sample.

Low-Dose Radiation Therapy (LDRT) against Cancer and Inflammatory or Degenerative Diseases: Three Parallel Stories with a Common Molecular Mechanism Involving the Nucleoshuttling of the ATM Protein?

Very early after their discovery, X-rays were used in multiple medical applications, such as treatments against cancer, inflammation and pain. Because of technological constraints, such applications involved X-ray doses lower than 1 Gy per session. Progressively, notably in oncology, the dose per session increased. However, the approach of delivering less than 1 Gy per session, now called low-dose radiation therapy (LDRT), was preserved and is still applied in very specific cases. More recently, LDRT has also been applied in some trials to protect against lung inflammation after COVID-19 infection or to treat degenerative syndromes such as Alzheimer's disease. LDRT illustrates well the discontinuity of the dose-response curve and the counterintuitive observation that a low dose may produce a biological effect higher than a certain higher dose. Even if further investigations are needed to document and optimize LDRT, the apparent paradox of some radiobiological effects specific to low dose may be explained by the same mechanistic model based on the radiation-induced nucleoshuttling of the ATM kinase, a protein involved in various stress response pathways.

Activated or Impaired: An Overview of DNA Repair in Neurodegenerative Diseases

As the population ages, age-related neurodegenerative diseases have become a major challenge in health science. Currently, the pathology of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, is still not fully understood. Remarkably, emerging evidence indicates a role of genomic DNA damage and repair in various neurodegenerative disorders. Here, we summarized the current understanding of the function of DNA damage repair, especially base excision repair and double strand break repair pathways, in a variety of neurodegenerative diseases. We concluded that exacerbation of DNA lesions is found in almost all types of neurodegenerative diseases, whereas the activities of different DNA repair pathways demonstrate distinct trends, depending on disease type and even brain region. Specifically, key enzymes involved in base excision repair are likely impaired in Alzheimer's disease and amyotrophic lateral sclerosis but activated in Parkinson's disease, while nonhomologous end joining is likely downregulated in most types of neurodegenerative diseases. Hence, impairment of nonhomologous end joining is likely a common etiology for most neurodegenerative diseases, while defects in base excision repair are likely involved in the pathology of Alzheimer's disease and amyotrophic lateral sclerosis but are Parkinson's disease, based on current findings. Although there are still discrepancies and further studies are required to completely elucidate the exact roles of DNA repair in neurodegeneration, the current studies summarized here provide crucial insights into the pathology of neurodegenerative diseases and may reveal novel drug targets for corresponding neurodegenerative diseases.

Discovery of autism/intellectual disability somatic mutations in Alzheimer's brains: mutated ADNP cytoskeletal impairments and repair as a case study

With Alzheimer's disease (AD) exhibiting reduced ability of neural stem cell renewal, we hypothesized that de novo mutations controlling embryonic development, in the form of brain somatic mutations instigate the disease. A leading gene presenting heterozygous dominant de novo autism-intellectual disabilities (ID) causing mutations is activity-dependent neuroprotective protein (ADNP), with intact ADNP protecting against AD-tauopathy. We discovered a genomic autism ADNP mutation (c.2188C>T) in postmortem AD olfactory bulbs and hippocampi. RNA-Seq of olfactory bulbs also identified a novel ADNP hotspot mutation, c.2187_2188insA. Altogether, 665 mutations in 596 genes with 441 mutations in AD patients (389 genes, 38% AD-exclusive mutations) and 104 genes presenting disease-causing mutations (OMIM) were discovered. OMIM AD mutated genes converged on cytoskeletal mechanisms, autism and ID causing mutations (about 40% each). The number and average frequencies of AD-related mutations per subject were higher in AD subjects compared to controls. RNA-seq datamining (hippocampus, dorsolateral prefrontal cortex, fusiform gyrus and superior frontal gyrus-583 subjects) yielded similar results. Overlapping all tested brain areas identified unique and shared mutations, with ADNP singled out as a gene associated with autism/ID/AD and presenting several unique aging/AD mutations. The large fusiform gyrus library (117 subjects) with high sequencing coverage correlated the c.2187_2188insA ADNP mutation frequency to Braak stage (tauopathy) and showed more ADNP mutations in AD specimens. In cell cultures, the ADNP-derived snippet NAP inhibited mutated-ADNP-microtubule (MT) toxicity and enhanced Tau-MT association. We propose a paradigm-shifting concept in the perception of AD whereby accumulating mosaic somatic mutations promote brain pathology.

Role of Ionizing Radiation in Neurodegenerative Diseases

Ionizing radiation (IR) from terrestrial sources is continually an unprotected peril to human beings. However, the medical radiation and global radiation background are main contributors to human exposure and causes of radiation sickness. At high-dose exposures acute radiation sickness occurs, whereas chronic effects may persist for a number of years. Radiation can increase many circulatory, age related and neurodegenerative diseases. Neurodegenerative diseases occur a long time after exposure to radiation, as demonstrated in atomic bomb survivors, and are still controversial. This review discuss the role of IR in neurodegenerative diseases and proposes an association between neurodegenerative diseases and exposure to IR.

Review: Somatic mutations in neurodegeneration

Somatic mutations are postzygotic mutations which may lead to mosaicism, the presence of cells with genetic differences in an organism. Their role in cancer is well established, but detailed investigation in health and other diseases has only been recently possible. This has been empowered by the improvements of sequencing techniques, including single-cell sequencing, which can still be error-prone but is rapidly improving. Mosaicism appears relatively common in the human body, including the normal brain, probably arising in early development, but also potentially during ageing. In this review, we first discuss theoretical considerations and current evidence relevant to somatic mutations in the brain. We present a framework to explain how they may be integrated with current views on neurodegeneration, focusing mainly on sporadic late-onset neurodegenerative diseases (Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis). We review the relevant studies so far, with the first evidence emerging in Alzheimer's in particular. We also discuss the role of mosaicism in inherited neurodegenerative disorders, particularly somatic instability of tandem repeats. We summarize existing views and data to present a model whereby the time of origin and spatial distribution of relevant somatic mutations, combined with any additional risk factors, may partly determine the development and onset age of sporadic neurodegenerative diseases.

Shared molecular and cellular mechanisms of premature ageing and ageing-associated diseases

Ageing is the predominant risk factor for many common diseases. Human premature ageing diseases are powerful model systems to identify and characterize cellular mechanisms that underpin physiological ageing. Their study also leads to a better understanding of the causes, drivers and potential therapeutic strategies of common diseases associated with ageing, including neurological disorders, diabetes, cardiovascular diseases and cancer. Using the rare premature ageing disorder Hutchinson-Gilford progeria syndrome as a paradigm, we discuss here the shared mechanisms between premature ageing and ageing-associated diseases, including defects in genetic, epigenetic and metabolic pathways; mitochondrial and protein homeostasis; cell cycle; and stem cell-regenerative capacity.

Inefficient DNA Repair Is an Aging-Related Modifier of Parkinson's Disease

The underlying relation between Parkinson’s disease (PD) etiopathology and its major risk factor, aging, is largely unknown. In light of the causative link between genome stability and aging, we investigate a possible nexus between DNA damage accumulation, aging, and PD by assessing aging-related DNA repair pathways in laboratory animal models and humans. We demonstrate that dermal fibroblasts from PD patients display flawed nucleotide excision repair (NER) capacity and that Ercc1 mutant mice with mildly compromised NER exhibit typical PD-like pathological alterations, including decreased striatal dopaminergic innervation, increased phospho-synuclein levels, and defects in mitochondrial respiration. Ercc1 mouse mutants are also more sensitive to the prototypical PD toxin MPTP, and their transcriptomic landscape shares important similarities with that of PD patients. Our results demonstrate that specific defects in DNA repair impact the dopaminergic system and are associated with human PD pathology and might therefore constitute an age-related risk factor for PD.

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Ercc1-mediated DNA repair is necessary for preservation of dopaminergic neurons

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Mouse mutants with mild Ercc1 defects display signs of dopaminergic pathology

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Mild Ercc1 dysfunction is sensitized to the prototypical PD neurotoxin MPTP

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PD patients’ peripheral cells exhibit inefficient nucleotide excision repair

Chromatin regulation of DNA damage repair and genome integrity in the central nervous system

With the continued extension of lifespan, aging and age-related diseases have become a major medical challenge to our society. Aging is accompanied by changes in multiple systems. Among these, the aging process in the central nervous system is critically important but very poorly understood. Neurons, as post-mitotic cells, are devoid of replicative associated aging processes, such as senescence and telomere shortening. However, because of the inability to self-replenish, neurons have to withstand challenge from numerous stressors over their lifetime. Many of these stressors can lead to damage of the neurons' DNA. When the accumulation of DNA damage exceeds a neuron's capacity for repair, or when there are deficiencies in DNA repair machinery, genome instability can manifest. The increased mutation load associated with genome instability can lead to neuronal dysfunction and ultimately to neuron degeneration. In this review, we first briefly introduce the sources and types of DNA damage and the relevant repair pathways in the nervous system (summarized in Fig. 1). We then discuss the chromatin regulation of these processes and summarize our understanding of the contribution of genomic instability to neurodegenerative diseases.

Gliotoxicity of the cyanotoxin, β-methyl-amino-L-alanine (BMAA)

The amino acid variant β-methyl-amino-L-alanine (BMAA) has long been associated with the increased incidence and progression of the amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC). Previous studies have indicated that BMAA damages neurons via excitotoxic mechanisms. We have challenged rat olfactory ensheathing cells (OECs) with exogenous BMAA and found it to be cytotoxic. BMAA also induces a significant increase in Ca2+ influx, enhanced production of reactive oxygen species (ROS), and disrupts mitochondrial activity in OECs. This is the first study investigating BMAA toxicity using pure glial cells. These findings align BMAA with the three proposed mechanisms of degeneration in ALS, those being non-cell autonomous death, excitotoxicity and mitochondrial dysfunction.

Sublethal doses of β-amyloid peptide abrogate DNA-dependent protein kinase activity

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer disease (AD). In multicellular eukaryotes, double strand breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by the nonhomologous end joining pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains, but the molecular player causing DNA repair dysfunction is still undetermined. β-Amyloid (Aβ), a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by reactive oxygen species generation and oxidative stress induction, which may then cause DNA damage. Here, we show that in PC12 cells sublethal concentrations of aggregated Aβ(25-35) inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to nonlethal oxidative injury. The inhibition of DNA-PK activity is associated with down-regulation of the catalytic subunit DNA-PK (DNA-PKcs) protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of Aβ(1-42) oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species, and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall, these findings suggest that Aβ mediates inhibition of the DNA-PK-dependent nonhomologous end joining pathway contributing to the accumulation of DSBs that, if not efficiently repaired, may lead to the neuronal loss observed in AD.

Selective neuronal vulnerability to oxidative stress in the brain

Oxidative stress (OS), caused by the imbalance between the generation and detoxification of reactive oxygen and nitrogen species (ROS/RNS), plays an important role in brain aging, neurodegenerative diseases, and other related adverse conditions, such as ischemia. While ROS/RNS serve as signaling molecules at physiological levels, an excessive amount of these molecules leads to oxidative modification and, therefore, dysfunction of proteins, nucleic acids, and lipids. The response of neurons to this pervasive stress, however, is not uniform in the brain. While many brain neurons can cope with a rise in OS, there are select populations of neurons in the brain that are vulnerable. Because of their selective vulnerability, these neurons are usually the first to exhibit functional decline and cell death during normal aging, or in age-associated neurodegenerative diseases, such as Alzheimer's disease. Understanding the molecular and cellular mechanisms of selective neuronal vulnerability (SNV) to OS is important in the development of future intervention approaches to protect such vulnerable neurons from the stresses of the aging process and the pathological states that lead to neurodegeneration. In this review, the currently known molecular and cellular factors that contribute to SNV to OS are summarized. Included among the major underlying factors are high intrinsic OS, high demand for ROS/RNS-based signaling, low ATP production, mitochondrial dysfunction, and high inflammatory response in vulnerable neurons. The contribution to the selective vulnerability of neurons to OS by other intrinsic or extrinsic factors, such as deficient DNA damage repair, low calcium-buffering capacity, and glutamate excitotoxicity, are also discussed.

Nerve cell death in degenerative diseases of the central nervous system: clinical aspects

The origin of degenerative diseases of the central nervous system lies in genetic and acquired disorders. Analysis of the clinical characteristics of diseases affecting specific neuronal systems may help us to understand their pathogenesis. The stereotyped symptomatology characteristic of most degenerative diseases results from neuronal death in specific pathways: pyramidal tract and motor neurons in amyotrophic lateral sclerosis, nigrostriatal dopamine system in Parkinson's disease, posterior and lateral columns of the spinal cord in Friedreich's ataxia, etc. This suggests that these neurons are sensitive to pathological processes that are still unknown. Progression of the disease, whether linear or not, is slow, but it is more rapid than similar effects due to ageing. This indicates either that the environmental cause of degeneration (if it exists) is continuously present or that a vital process has been once and for all disrupted, perhaps at the level of the genome, causing insufficient production of essential proteins, or accumulation of eventually toxic metabolites. Symptoms generally appear during adulthood, i.e. after normal differentiation has taken place, and after a considerable number of neurons have already been damaged. The initiation of neuronal death precedes the appearance of the first symptoms.

Dieldrin elicits a widespread DNA repair and antioxidative response in mouse brain

Dieldrin is an organochlorine pesticide that is toxic for monoaminergic neurons. This study was designed to test the hypothesis that a weak DNA repair response to dieldrin by nigrostriatal dopaminergic (DA) neurons results in depletion of striatal DA. The activity of the mammalian base excision repair enzyme oxyguanosine glycosylase was utilized as the index of DNA repair. Other measures of oxidative stress were also studied, including the regional distribution of lipid peroxidation and superoxide dismutase (SOD) activity. The effects of acute and slow infusion of dieldrin on striatal DA levels were biphasic with a transient initial depression followed by increases beyond normal steady-:state levels. Dieldrin administration caused a global oxidative stress evidenced by increased levels of lipid peroxidation in all brain regions, an effect consistent with its capacity to affect mitochondrial bioenergetics. Dieldrin also elicited strong antioxidative and DNA repair responses across the entire mouse brain. Although mitochondrial SOD was not as increased in midbrain as it was in other regions following a cumulative dose of 24 mg/kg, this response, along with the robust DNA repair response, appeared to be sufficient to protect potentially vulnerable DA neurons from cytotoxicity. However, the long-:term consequences of chronic low-:dose dieldrin exposure remain to be studied, especially in light of the concept of "slow excitotoxicity,'' which postulates that even a mild bioenergetic compromise can over time result in the demise of neurons.

DNA repair, mitochondria, and neurodegeneration

Accumulation of nuclear and mitochondrial DNA damage is thought to be particularly deleterious in post-mitotic cells, which cannot be replaced through cell division. Recent experimental evidence demonstrates the importance of DNA damage responses for neuronal survival. Here, we summarize current literature on DNA damage responses in the mammalian CNS in aging and neurodegeneration. Base excision repair (BER) is the main pathway for the removal of small DNA base modifications, such as alkylation, deamination and oxidation, which are generated as by-products of normal metabolism and accumulate with age in various experimental models. Using neuronal cell cultures, human brain tissue and animal models, we and others have shown an active BER pathway functioning in the brain, both in the mitochondrial and nuclear compartments. Mitochondrial DNA repair may play a more essential role in neuronal cells because these cells depend largely on intact mitochondrial function for energy metabolism. We have characterized several BER enzymes in mammalian mitochondria and have shown that BER activities change with age in mitochondria from different brain regions. Together, the results reviewed here advocate that mitochondrial DNA damage response plays an important role in aging and in the pathogenesis of neurodegenerative diseases.

Can low level exposure to ochratoxin-A cause parkinsonism?

Mycotoxins are fungal metabolites with pharmacological activities that have been utilized in the production of antibiotics, growth promoters, and other classes of drugs. Some mycotoxins have been developed as biological and chemical warfare agents. Bombs and ballistic missiles loaded with aflatoxin were stockpiled and may have been deployed by Iraq during the first Gulf War. In light of the excess incidence of amyotrophic lateral sclerosis (ALS) in veterans from Operation Desert Storm, the potential for delayed neurotoxic effects of low doses of mycotoxins should not be overlooked. Ochratoxin-A (OTA) is a common mycotoxin with complex mechanisms of action, similar to that of the aflatoxins. Acute administration of OTA at non-lethal doses (10% of the LD(50)) have been shown to increase oxidative DNA damage in brain up to 72 h, with peak effects noted at 24 h in midbrain (MB), caudate/putamen (CP) and hippocampus (HP). Levels of dopamine (DA) and its metabolites in the striatum (e.g., CP) were shown to be decreased in a dose-dependent manner. The present study focused on the effects of chronic low dose OTA exposure on regional brain oxidative stress and striatal DA metabolism. Continuous administration of low doses of OTA with implanted subcutaneous Alzet minipumps caused a small but significant decrease in striatal DA levels and an upregulation of anti-oxidative systems and DNA repair. It is possible that low dose exposure to OTA will result in an earlier onset of parkinsonism when normal age-dependent decline in striatal DA levels are superimposed on the mycotoxin-induced lesion.

Neuroanatomical mapping of DNA repair and antioxidative responses in mouse brain: Effects of a single dose of MPTP

The primary objective of this study was to map the normal distribution of the base excision enzyme oxyguanosine glycosylase (OGG1) across mouse-brain regions as a prelude to assessing the effects of various neurotoxicants, ranging from highly selective molecules like MPTP to more global toxic agents. This research is based on the hypothesis that regional brain vulnerability to a toxicant is determined, in part, by variation in the intrinsic capacity of cellular populations to successfully repair oxidative DNA damage. After mapping the normal distributions of OGG1 and superoxide dismutase (SOD) across 44 loci dissected from mouse brain, MPTP, a mitochondrial toxicant with selective dopamine (DA) neuron cytotoxicity was used to elicit focal oxidative stress and DNA repair responses. A single dose of MPTP (20mg/kg, i.p.) elicited time- and region-dependent changes in both SOD and OGG1, with early increases in DNA repair and anti-oxidant activities throughout all regions of brain. In some sampled loci, notably the substantia nigra (SN) and hippocampus, the heightened DNA repair and antioxidant responses were not maintained beyond 48h. Other loci from cerebellum, cerebral cortex and pons maintained high levels of activity up to 72h. Levels of dopamine (DA) were decreased significantly at all time points and remained below control levels in nigro-striatal and mesolimbic systems (ventral tegmental area and nucleus accumbens). Assessment of apoptosis by TUNEL staining revealed a significant increase in number of apoptotic nuclei in the substantia nigra at 72h and not in other loci. The marked degree of apoptosis that became evident in SN at 72h was associated with large decreases in SOD and DNA repair activity at that locus. In conclusion, MPTP elicited global effects on DNA repair and antioxidant activity in all regions of brain, but the most vulnerable loci were unable to maintain elevated DNA repair and antioxidant responses.

Alzheimer's lymphocytes are resistant to ultraviolet B-induced apoptosis

In the present pilot investigation, the susceptibility of T-lymphocytes from Alzheimer's disease (AD) subjects (n=22) and aged-matched, non-demented controls (CNT) (n=12) was examined with ultraviolet (UV) B light-induced apoptosis in vitro. The basal apoptotic ratios were similar in both groups. However, the AD lymphocytes displayed significantly (p<0.0001) lower apoptotic levels than those of the CNT lymphocytes at all of the applied UVB exposure doses (100, 200 and 300 mJ/cm(2)). These observations indicate that AD lymphocytes are more resistant than CNT lymphocytes to UVB irradiation.

Radio-sensitivity of the cells from amyotrophic lateral sclerosis model mice transfected with human mutant SOD1

In order to clarify the possible involvement of oxidative damage induced by ionizing radiation in the onset and/or progression of familial amyotrophic lateral sclerosis (ALS), we studied radio-sensitivity in primary cells derived from ALS model mice expressing human mutant SOD1. The primary mouse cells expressed both mouse and the mutant human SOD1. The cell survival of the transgenic mice (with mutant SOD1), determined by counting cell numbers at a scheduled time after X-irradiation, is very similar to that of cells from wild type animals. The induction and repair of DNA damage in the transgenic cells, measured by single cell gel electrophoresis and pulsed field gel electrophoresis, are also similar to those of wild type cells. These results indicate that the human mutant SOD1 gene does not seem to contribute to the alteration of radio-sensitivity, at least in the fibroblastic cells used here. Although it is necessary to consider the difference in cell types between fibroblastic and neuronal cells, the present results may suggest that ionizing radiation is not primarily responsible for the onset of familial ALS with the SOD1 mutation, and that the excess risks are probably not a concern for radiation diagnosis and therapy in familial ALS patients.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to the genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation, offering a better likelihood of cure for malignant tumours. Although only a small percentage of individuals are "hypersensitive" to radiation effects, all medical specialists using ionising radiation should be aware of the aforementioned progress in medical knowledge. The present paper, the second of two parts, reviews human disorders known or strongly suspected to be associated with hypersensitivity to ionising radiation. The main tests capable of detecting such pathologies in advance are analysed, and ethical issues regarding genetic testing are considered. The implications for radiation protection of possible hypersensitivity to radiation in a part of the population are discussed, and some guidelines for nuclear medicine professionals are proposed.

Effects of diethylmaleate on DNA damage and repair in the mouse brain

The enzyme 8-oxoguanine DNA glycosylase 1 participates in the repair of damaged DNA by excising the oxidized base 8-hydroxy-2'-deoxyguanosine. We have previously demonstrated that enzymatic activity of this enzyme is inversely related to the levels of the damaged base in specific brain regions. We now report that the activity of 8-oxoguanine DNA glycosylase 1 is increased in a region-specific manner following treatment with diethylmaleate, a compound that reduces glutathione levels in the cell. A single treatment with diethylmaleate elicited a significant increase ( approximately 2-fold) in the activity of 8-oxoguanine DNA glycosylase 1 in three brain regions with low basal levels of activity (cerebellum, cortex, and pons/medulla). There was no change in the activity of 8-oxoguanine DNA glycosylase 1 in those regions with high basal levels of activity (hippocampus, caudate/putamen, and midbrain). This is the first report to demonstrate that DNA repair capacity can be upregulated in the CNS, and the increased repair activity correlates with a reduction in the levels of DNA damage. The brain region-specific capacity to deal with increased oxidative damage to DNA may be responsible, in part, for the vulnerability of specific neuronal populations with aging, sources of oxidative stress, and neurodegenerative diseases.

Comparison of base-excision repair capacity in proliferating and differentiated PC 12 cells following acute challenge with dieldrin

Dieldrin, an organochlorine pesticide and known neurotoxicant, is ubiquitously distributed in the environment. Dieldrin depletes brain monoamines in some animal species and is toxic for dopaminergic neurons in vitro. Dieldrin interferes with mitochondrial electron transport and increases generation of superoxide anion. Reactive oxygen species have been shown to produce oxidative lesions to DNA bases, i.e., 8-hydroxy-2'-deoxyguanosine (8-oxodGuo). Accumulation of 8-oxodGuo has been shown to be promutagenic in proliferating cells, and can lead to degeneration in fully differentiated cells. The objective of this study was to determine the effects of dieldrin exposure on the activity of the enzyme responsible for removing 8-oxodGuo, OGG1, from undifferentiated (untreated with NGF) and differentiated (NGF-treated) PC 12 cells. Proliferating PC 12 cells exhibited a mild upregulation of glycosylase activity, reaching a maximum by 1 h and returning to baseline by 6 h. Differentiated (+) NGF cells showed a time-dependent decline in activity reaching a nadir at 3 h with a return towards baseline by 6 h. Levels of the damaged base, 8-oxodGuo, in the differentiated PC12 cells appeared to be regulated by the activity of OGG1. In contrast, levels of the damaged base in actively proliferating cells were independent of the OGG1 activity. This difference between actively dividing and differentiated cells in the regulation of base-excision repair and DNA damage accumulation explains, in part, the vulnerability of postmitotic neurons to oxidative stresses and neurotoxins.

An application of the challenge assay in boat builders exposed to low levels of styrene--a feasibility study of a possible biomarker for acquired susceptibility

Sensitivity to carcinogens and susceptibility for malignant diseases may be related to genetic predisposition, e.g. polymorphisms in toxicant-metabolizing enzymes or DNA repair deficiencies. The latter may also be acquired by exposure to substances that interfere with DNA repair processes. Application of the challenge assay to an exposed population may allow scientists to study the interference of DNA repair as an acquired susceptibility phenomenon. The assay was therefore used in a feasibility study to evaluate its application. A group of 14 workers exposed to low levels of styrene (mean < 100 mg/m3 styrene in air; 35 micrograms/l styrene in blood) and a reference of seven controls were investigated for structural chromosomal aberrations using FISH. The rate of exchange-type aberrations per 100 metaphases was 0.14 (95% CI, 0.05-0.31) in controls and 0.22 (95% CI, 0.13-0.36) in exposed workers. The difference is not statistically significant. Interaction with DNA repair was measured in the 14 workers and 2 historical controls using the challenge assay. Exchange-type aberrations per 100 metaphases after X-ray challenge of 1.66 Gy were 13.26 (10.53-16.50) and 16.19 (15.00-17.40) for the controls and exposed, respectively. The difference is statistically significant (p < 0.038). Among the exposed group, the challenge response was also significantly correlated with the cumulative lifetime exposure to styrene (R2 = 0.3996; p < 0.015) but not with the current exposure as measured in blood (R2 = 0.0226; p = 0.700). The challenge responses in the short-term and long-term exposed subgroups were 15.55 (14.23-16.96) and 17.90 (15.64-20.39), respectively, based on sample sizes of 5 and 9, respectively. The difference was not significant. Hence, data from our study are consistent with the hypothesis that long-term exposure to styrene can interfere with DNA repair activities. The lack of statistically significant differences in some of the data may be due to the small sample size and a possible confounding by age in our investigation. Additional data from our ongoing study should clarify this uncertainty.

Differential alterations in antioxidant capacity in cells from Alzheimer patients

Oxidative stress occurs in brains of Alzheimer's disease (AD) patients. A major question in AD research is whether the oxidative stress is just secondary to neurodegeneration. To test whether oxidative stress is an inherent property of AD tissues, the ability of cultured fibroblasts bearing the AD Presenilin-1 246 Ala-->Glu mutation to handle reactive oxygen species (ROS) was compared to controls. Although ROS in cells from AD subjects were only slightly less than cells from controls under basal conditions (-10%) or after exposure to H(2)O(2) (-16%), treatment with antioxidants revealed clear differences. Pretreatment with DMSO, a hydroxyl radical scavenger, reduced basal and H(2)O(2)-induced ROS levels significantly more in cells from controls (-22%, -22%) than in those from AD subjects (-4%, +14%). On the other hand, pretreatment with Trolox diminished H(2)O(2)-induced ROS significantly more in cells from AD (-60%) than control subjects (-39%). In summary, cells from AD patients have greater Trolox sensitive ROS and less DMSO sensitive ROS than controls. The results demonstrate that fibroblasts bearing this PS-1 mutation have altered means of handling oxidative stress and appear useful for determining the mechanism underlying the altered redox metabolism.

DNA damage, repair, and antioxidant systems in brain regions: a correlative study

8-Hydroxy-2'-deoxyguanosine (oxo(8)dG) has been used as a marker of free radical damage to DNA and has been shown to accumulate during aging. Oxidative stress affects some brain regions more than others as demonstrated by regional differences in steady state oxo(8)dG levels in mouse brain. In our study, we have shown that regions such as the midbrain, caudate putamen, and hippocampus show high levels of oxo(8)dG in total DNA, although regions such as the cerebellum, cortex, and pons and medulla have lower levels. These regional differences in basal levels of DNA damage inversely correlate with the regional capacity to remove oxo(8)dG from DNA. Additionally, the activities of antioxidant enzymes (Cu/Zn superoxide dismutase, mitochondrial superoxide dismutase, and glutathione peroxidase) and the levels of the endogenous antioxidant glutathione are not predictors of the degree of free radical induced damage to DNA in different brain regions. Although each brain region has significant differences in antioxidant defenses, the capacity to excise the oxidized base from DNA seems to be the major determinant of the steady state levels of oxo(8)dG in each brain region.

Distribution and kainate-mediated induction of the DNA mismatch repair protein MSH2 in rat brain

DNA repair is one of the most essential systems for maintaining the inherited nucleotide sequence of genomic DNA over time. Repair of DNA damage would be particularly important in neurons, because these cells are among the longest-living cells in the body. MSH2 is one of the proteins which are involved in the recognition and repair of a specific type of DNA damage that is characterized by pair mismatches. We studied the distribution of MSH2 in rat brain by immunohistochemical analysis. We found the level of MSH2 expression in rat brain to be clearly heterogeneous. The highest intensity of staining was found in the pyramidal neurons of the hippocampus and in the entorhinal and frontoparietal cortices. Positive cells were observed in the substantia nigra pars compacta, in cerebellar granular and Purkinje cells, and in the motor neurons of the spinal cord. We investigated the possible modulation of MSH2 expression after injection of kainate. Systemic administration of kainate induces various behavioural alterations and a typical pattern of neuropathology, with cell death in the hippocampal pyramidal neurons of the CA3/CA4 fields. Kainate injection also resulted in a marked, dose-dependent increase of MSH2 immunoreactivity in the hippocampal neurons of the CA3/CA4 fields. The effect was specific, since no changes in immunoreactivity were detected in the dentate gyrus nor in other brain areas. In summary, our data suggest that a mismatch DNA repair system, of which MSH2 protein is a representative component, is heterogeneously expressed in the rat brain and specifically induced by an experimental paradigm of excitotoxicity.

A novel neuron-specific DNA end-binding factor in the murine brain

To characterize the distribution of transcription factor AP-1 and YY1 DNA-binding activities in the rat brain, the labeled target oligonucleotides were loaded on brain sections and after incubation and washing, the residual signal was registered by autoradiography. The binding was predominantly associated with neurons and was regionally specific with highest levels in the cerebellum, hippocampus, and piriform cortex. The identified binding factor was not, however, sequence-specific, but apparently recognized DNA ends and was activated by long double-stranded DNA. UV cross-linking identified the molecular mass of the factor to be about 80 kDa. The factor was not found in soluble brain extracts, suggesting its association with membranes or the nuclear matrix. Despite apparent similarities with Ku protein, which targets DNA-ends, the DNA end-binding activity was present in brains of Ku86- and Ku70-deficient mice. Since DNA end-binding factors are generally involved in DNA repair, the same function may be suggested for the novel factor identified in the present study.

Pathogenesis and preclinical course of Parkinson's disease

Idiopathic parkinsonism (IP) is defined by its classic symptomology, its responsiveness to therapies which elevate dopamine levels, and by the failure to identify a specific etiological factor. The progressive and irreversible degeneration of dopaminergic neurons projecting from the substantia nigra pars compacta (SNc) to the striatum and the presence of SNc Lewy bodies are regarded as the essential pathological bases of IP, but neither the initiator(s) nor the nature of the degeneration have been determined, nor its relationship with degenerative changes in other parts of the IP brain. This paper discusses the various hypotheses that have been proposed to explain these phenomena, arguing that IP be regarded as a multisystem disorder, both at the level of individual neurons and at the whole brain level. It is probable that IP is the result of a multifactorial process, and that a cascade of interacting and overlapping biochemical mechanisms determine the course of the disease.

Increased steady state mRNA levels of DNA-repair genes XRCC1, ERCC2 and ERCC3 in brain of patients with Down syndrome

Although deficient DNA-repair was proposed for neurodegenerative disorders including Down Syndrome (DS), repair genes for nucleotide excision repair or X-ray repair have not been studied in brain yet. As one of the hypotheses for the pathogenesis of brain damage in DS is oxidative stress and cells of patients with DS are more susceptible to ionizing irradiation, we decided to study ERCC2, ERCC3 and XRCC1, representatives of repair genes known to be involved in the repair of oxidative DNA-damage. mRNA steady state levels of ERCC2, ERCC3, XRCC1, a transcription activator (TAF-DBP) and an elongation factor (EF1A) were determined and normalized versus the housekeeping gene beta-actin in five individual brain regions of nine controls and nine DS patients. Although different in the individual regions, DNA-repair genes were consistently higher in temporal, parietal and occipital lobes of patients with DS accompanied by comparable changes of TFA-DBP and EF1A. Our results are the first to describe DNA-repair gene patterns in human brain regions providing the basis for further studies in this area. We showed that DNA-repair genes ERCC2 and ERCC3 (excision-repair-cross-complementing-) for nucleotide excision repair and XRCC1 (X-ray-repair-cross-complementing-) for X-ray-repair, were increased at the transcriptional level with the possible biological meaning that this increase may be compatible with permanent (oxidative?) DNA damage.

Induction of two DNA mismatch repair proteins, MSH2 and MSH6, in differentiated human neuroblastoma SH-SY5Y cells exposed to doxorubicin

The MutS homologues MSH2 and MSH6 form a heterodimeric protein complex that is involved in the recognition of base/base mismatches and insertion/deletion loops, as well as some other types of DNA damage. We investigated the expression of these proteins in undifferentiated and retinoic acid-differentiated human neuroblastoma SH-SY5Y cells by immunocytochemistry, western blot analysis, and RT-PCR. Nuclei from undifferentiated SH-SY5Y cells were found to be immunoreactive to anti-MSH2 and anti-MSH6 antibodies. Following differentiation, the cells stop dividing and change morphology to acquire a neuron-like phenotype. Under these conditions, both anti-MSH2 and anti-MSH6 immunoreactivities were still detectable, although the signals were somewhat less intense. When these cells were exposed for 2 h to neurotoxic concentrations of doxorubicin (50 nM), they exhibited a marked and homogeneous increase of both anti-MSH2 and anti-MSH6 immunoreactivities. As revealed by western blot analysis, these effects were associated with increased protein content and were dose-dependent. Using RT-PCR technology, we also found that doxorubicin treatment did not change MSH2 or MSH6 mRNA levels. Our data indicate that human postmitotic, neuron-like cells constitutively express the molecular machinery devoted to recognition of DNA mismatches and that this system is activated by specific treatment leading to cell death. These findings might help clarify the molecular mechanisms underlying various human neurological diseases that are associated with deficiencies in DNA repair and/or a high rate of DNA damage acquisition.

Expression of DNA excision-repair-cross-complementing proteins p80 and p89 in brain of patients with Down Syndrome and Alzheimer's disease

Although deficient DNA repair was proposed for neurodegenerative disorders including Down syndrome (DS), repair proteins for nucleotide excision repair have not been studied in brain yet. As one of the hypotheses for the pathogenesis of brain damage in DS and Alzheimer's disease (AD), is oxidative stress, and cells of patients with DS were shown to be more susceptible to ionizing irradiation. We decided to study expression of excision repair-cross-complementing (ERCC) gene products, proteins 80 and 89, representatives of repair genes known to be involved in the repair of different types of DNA damage. ERCC2-protein 80 kDa and ERCC3-protein p89 were determined in five individual brain regions of controls, aged DS and AD patients. Although different in the individual regions, DNA repair proteins were consistently higher in temporal and frontal lobes of patients with DS and higher in all brain regions of patients with AD. Our results are the first to describe DNA repair gene protein patterns in human brain regions providing the basis for further studies in this area. We showed that DNA repair genes ERCC2 and ERCC3 (excision-repair-cross-complementing) for nucleotide excision repair were increased at the protein level with the possible biological meaning that this increase may be compatible with and indicate ongoing (oxidative?) DNA damage.

Nigrostriatal neuronal death in Parkinson's disease--a passive or an active genetically-controlled process?

The cause for the rather selective degeneration of the nigrostriatal dopaminergic (DA) neurons in Parkinson's disease (PD) is still enigmatic. The major current hypothesis suggests that nigral neuronal death in PD is due to excessive oxidant stress generated by auto- and enzymatic oxidation of DA, formation of neuromelanin and presence of high concentrations of iron. Such cell death is generally regarded as a passive, necrotic process, mainly resulting from membrane lipid peroxidation, leading to its dysfunction and rupture and then to neuronal disintegration. We suggest a novel approach, that views neuronal degeneration in PD as an active process that occurs mainly the nuclear level. Our concept is based on the following observations: (1) Nigral histopathology in PD is characterized by a slow, protracted degeneration of individual neurons. We propose that it may be due to apoptosis [programmed cell-death (PCD), an active, genetically-controlled, intrinsic program of cell "suicide"] rather than to necrotic cell death. (2) DA exerts antitumor effect on melanoma and neuroblastoma cells. (3) Many anticancer drugs, trigger PCD by causing DNA damage. (4) DA has been shown to be genotoxic. (5) We recently first showed that DA, the endogenous neurotransmitter in the nigra, can trigger apoptosis in cultured, postmitotic sympathetic neurons. (6) We have also shown that PC-12 cells, transfected with the bcl-2 gene (a proto-oncogene that inhibits PCD) are relatively resistant to DA-apoptotic effect. Degeneration of nigrostriatal neurons in PD may therefore be linked to dysregulation of the control mechanisms that normally restrain the PCD-triggering-potential of their own neurotransmitter.

Fluorescent light-induced chromatid breaks distinguish Alzheimer disease cells from normal cells in tissue culture

The neurodegeneration and amyloid deposition of sporadic Alzheimer disease (AD) also occur in familial AD and in all trisomy-21 Down syndrome (DS) patients, suggesting a common pathogenetic mechanism. We investigated whether defective processing of damaged DNA might be that mechanism, as postulated for the neurodegeneration in xeroderma pigmentosum, a disease with defective repair not only of UV radiation-induced, but also of some oxygen free radical-induced, DNA lesions. We irradiated AD and DS skin fibroblasts or blood lymphocytes with fluorescent light, which is known to cause free radical-induced DNA damage. The cells were then treated with either beta-cytosine arabinoside (araC) or caffeine, and chromatid breaks were quantified. At least 28 of 31 normal donors and 10 of 11 donors with nonamyloid neurodegenerations gave normal test results. All 12 DS, 11 sporadic AD, and 16 familial AD patients tested had abnormal araC and caffeine tests, as did XP-A cells. In one of our four AD families, an abnormal caffeine test was found in all 10 afflicted individuals (including 3 asymptomatic when their skin biopsies were obtained) and in 8 of 11 offspring at a 50% risk for AD. Our tests could prove useful in predicting inheritance of familial AD and in supporting, or rendering unlikely, the diagnosis of sporadic AD in patients suspected of having the disease.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.

Gene specific DNA repair of damage induced in familial Alzheimer disease cells by ultraviolet irradiation or by nitrogen mustard

We have measured gene specific DNA repair in a normal human fibroblast cell line, and in fibroblast lines from two patients with familial Alzheimer disease (AD). Cells were treated with either ultraviolet radiation (UV) or the chemotherapeutic alkylating agent, nitrogen mustard (HN2). DNA damage formation and repair were studied in the active dihydrofolate reductase (DHFR) gene for the main lesions introduced by each of these two types of DNA damaging agents. The gene specific repair of UV induced cyclobutane pyrimidine dimers in the human DHFR gene was 86% complete in the AD cells after 24 h of repair incubation. This repair efficiency was similar to what we and others have found in normal human fibroblasts. After treatment of the AD cells with HN2, we found the frequency of HN2 induced lesions in the DHFR gene to be similar to the frequency in the transcriptionally inactive delta-globin gene. The gene specific repair of HN2 induced lesions in the DHFR gene was completed within 8-24 h in the normal fibroblast line and in the familial AD line, and the repair kinetics were similar for both cell lines. These results indicate that familial AD fibroblasts have normal gene specific repair of both UV induced and HN2 induced DNA damage in active genes.

Chromosomal fragility associated with familial Alzheimer's disease

To test whether chromosomal instability is associated with familial Alzheimer's disease, we examined breakage on X chromosomes of fibroblasts derived from patients with familial Alzheimer's disease, using gene cotransfer methodology. The X chromosome is a convenient target for analyzing DNA breakage because of its numerous markers and ease of selection in rodent-human hybrid cells. Patients with familial Alzheimer's disease, including the large Nova Scotia Alzheimer's kindred, show a significantly lower cotransfer of the X-linked glucose-6-phosphate dehydrogenase (G6PD) gene with the selected HPRT gene in hybrid cells, indicating breakage between the markers. Lower cotransfer of the more distant X-linked gene, MIC-2, was statistically significant in this kindred, but not in other patients with familial Alzheimer's disease. The distance between MIC2 and HPRT is sixfold to ninefold greater than that between HPRT and G6PD, suggesting that there may be a "hot spot" for breakage in the latter interval on the X chromosome of patients with familial Alzheimer's disease. The somatic cell hybrid model provides insights into underlying mechanisms for chromosomal breakage induced by the Alzheimer defect. A hypothesis implicating a candidate gene, C1-THF synthase, in the generation of chromosome instability in the pathogenesis of familial Alzheimer's disease, is presented.

Possible factors in the etiology of Alzheimer's disease

Inherited cases of Alzheimer's disease (AD) comprise only a very small proportion of the total. The remainder are of unknown etiopathogenesis, but they are very probably multifactorial in origin. This article describes studies on four possible factors: aluminum; viruses--in particular, herpes simplex type I virus (HSV1); defective DNA repair; and head trauma. Specific problems associated with aluminum, such as inadvertent contamination and its insolubility, have led to some controversy over its usage. Nonetheless, the effects of aluminum on animals and neuronal cells in culture have been studied intensively. Changes in protein structure and location in the cell are described, including the finding in this laboratory of a change in tau resembling that in AD neurofibrillary tangles, and also the lack of appreciable binding of aluminum to DNA. As for HSV1, there has previously been uncertainty about whether HSV1 DNA is present in human brain. Work in this laboratory using polymerase chain reaction has shown that HSV1 DNA is present in many normal aged brains and AD brains, but is absent in brains from younger people. Studies on DNA damage and repair in AD and normal cells are described, and finally, the possible involvement of head trauma is discussed.

Gas chromatographic-mass spectrometric method for the assessment of oxidative damage to double-stranded dna by quantification of thymine glycol residues

A technique for the measurement of thymine glycol at parts per million concentrations in double-stranded polymeric DNA is described. The procedure utilizes base to ring-open DNA-bound thymine glycol in the presence of monomeric [(2)H4]thymine glycol as an internal standard, followed by reduction, solvolytic cleavage, and quantification of the characteristic methyl-2-methylglycerate released from polymeric DNA. Methyl-2-methyl-glycerate is derivatized to form the di-tert-butyldimethylsilyl [(TBDMS)2] ether to enhance its gas chromatographic properties and electron ionization detection. This assay was tested by measuring thymine gIyco1 levels in native, undamaged DNA (not purposefully oxidized). The measured quantities of thymine glycol are proportional to the amount of DNA analyzed. Components of DNA not containing oxidizable thymine do not contribute to the measured signal from methyl-2-methylglycerate-(TBDMS)2. These results indicate that there is approximately one thymine glycol per lo6 bases in undamaged DNA and that this value increases with storage of DNA in refrigerated aqueous solutions.

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.

Slowed synthesis of DNA and methionine is a pathogenetic mechanism common to dementia in Down's syndrome, AIDS and Alzheimer's disease?

This is a presentation of the hypothesis of a pathogenetic mechanism common to the dementia seen in Alzheimer's disease (AD), Down's Syndrome (DS) and the acquired immunodeficiency syndrome (AIDS). As there is experimental evidence of defective DNA repair capacity in AD and DS, unrepaired damage to DNA occurs in these diseases and may lead to complete breakdown of cellular function and ultimate cell death. Cobalamin and folate are coordinated in a vulnerable key position in the synthesis of DNA and S-adenosylmethionine (SAM). Cobalamin/folate deficiency, a significant feature in senile dementia of Alzheimer type and in AIDS-related dementia complex, will result in concomitant slowed synthesis of DNA and SAM. The enzyme cystathionine-beta-synthetase (CBS) has been localized to the chromosome band 21q22.3. Owing to gene dosage, CBS activity is increased in trisomy 21. As a consequence, cobalamin/folate metabolism is inhibited, which leads to slowing of DNA and SAM synthesis in DS patients. Amyloidosis is a hallmark of AD and DS brain neuropathology and recent experimental findings support the view that amyloid or amyloid precursors stimulate DNA synthesis, which is in agreement with the hypothesis presented in this paper. In summary, demented patients with cobalamin/folate deficiency, trisomy 21 and human immunodeficiency virus (HIV) infection display a simultaneous downregulation of DNA and SAM synthesis, which may indicate a pathway common to the dementia seen in AD, DS and AIDS.

Gender differences and the interpretation of genetic instability in Alzheimer's disease

The neuronal degeneration and death which characterize Alzheimer's disease (AD) may stem from a constitutive genetic instability related to DNA repair deficits. To test this hypothesis, we treated peripheral blood lymphocytes from persons with AD, age-matched controls, and young controls with two drugs that induce chromosome breakage. Bleomycin, a radiomimetic antineoplastic drug, causes single- and double-stranded DNA breaks through the generation of activated oxygen radicals. Methyl methane-sulfonate (MMS) is a monofunctional alkylating agent that binds covalently to DNA. Cells were grown in culture for 72 h, with drug treatments for 4 h (bleomycin) or 24 h (MMS) prior to harvest. Fifty cells per subject per drug were scored for chromosome breakage. Breakage rates for both drugs in AD women were significantly higher than those in age-matched control women. This was not the case in men, due to the very high induced breakage rates seen in the age-matched normal control men. Because the induced breakage rates in AD women and AD men are equivalent, it seems likely that an independent factor may be contributing to genetic instability in the normal control men. Our findings indicate that the interpretation of the response of AD lymphocyte chromosomes to DNA-damaging chemicals can be strongly confounded by the effects of gender ratio in the control population sampled. These findings have important implications for the design of future studies of Alzheimer's disease, as well as for the assessment of health risks in unaffected elderly populations.