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

Early oxidative stress and DNA damage in Aβ-burdened hippocampal neurons in an Alzheimer's-like transgenic rat model

Oxidative stress is a key contributor to AD pathology. However, the earliest role of pre-plaque neuronal oxidative stress, remains elusive. Using laser microdissected hippocampal neurons extracted from McGill-R-Thy1-APP transgenic rats we found that intraneuronal amyloid beta (iAβ)-burdened neurons had increased expression of genes related to oxidative stress and DNA damage responses including Ercc2, Fancc, Sod2, Gsr, and Idh1. DNA damage was further evidenced by increased neuronal levels of XPD (Ercc2) and γH2AX foci, indicative of DNA double stranded breaks (DSBs), and by increased expression of Ercc6, Rad51, and Fen1, and decreased Sirt6 in hippocampal homogenates. We also found increased expression of synaptic plasticity genes (Grin2b (NR2B), CamkIIα, Bdnf, c-fos, and Homer1A) and increased protein levels of TOP2β. Our findings indicate that early accumulation of iAβ, prior to Aβ plaques, is accompanied by incipient oxidative stress and DSBs that may arise directly from oxidative stress or from maladaptive synaptic plasticity.

Modular transcriptional repertoire and MicroRNA target analyses characterize genomic dysregulation in the thymus of Down syndrome infants

Trisomy 21-driven transcriptional alterations in human thymus were characterized through gene coexpression network (GCN) and miRNA-target analyses. We used whole thymic tissue--obtained at heart surgery from Down syndrome (DS) and karyotipically normal subjects (CT)--and a network-based approach for GCN analysis that allows the identification of modular transcriptional repertoires (communities) and the interactions between all the system's constituents through community detection. Changes in the degree of connections observed for hierarchically important hubs/genes in CT and DS networks corresponded to community changes. Distinct communities of highly interconnected genes were topologically identified in these networks. The role of miRNAs in modulating the expression of highly connected genes in CT and DS was revealed through miRNA-target analysis. Trisomy 21 gene dysregulation in thymus may be depicted as the breakdown and altered reorganization of transcriptional modules. Leading networks acting in normal or disease states were identified. CT networks would depict the "canonical" way of thymus functioning. Conversely, DS networks represent a "non-canonical" way, i.e., thymic tissue adaptation under trisomy 21 genomic dysregulation. This adaptation is probably driven by epigenetic mechanisms acting at chromatin level and through the miRNA control of transcriptional programs involving the networks' high-hierarchy genes.

The hepatocyte as a microbial product-responsive cell

Much research has focused on the responses to microbial products of immune cells such as monocytes, macrophages, and neutrophils. Although the liver is a primary response organ in various infections, relatively little is known about the antimicrobial responses of its major cell type, the hepatocyte. It is now known that the recognition of bacteria occurs via cell-surface proteins that are members of the Toll-like receptor (TLR) family. In addition, lipopolysaccharide (LPS) is bound by circulating LPS-binding protein (LBP) and presented to cell-surface CD14, which in turn interacts with TLR and transduces an intracellular signal. We investigated the CD14 and TLR2 responses of whole liver and isolated hepatocytes, and demonstrated that these cells can be induced to express the molecules necessary for responses to both Gram-positive and Gram-negative bacteria. Our findings may have clinical implications for pathological states such as sepsis.

Base excision repair in physiology and pathology of the central nervous system

Relatively low levels of antioxidant enzymes and high oxygen metabolism result in formation of numerous oxidized DNA lesions in the tissues of the central nervous system. Accumulation of damage in the DNA, due to continuous genotoxic stress, has been linked to both aging and the development of various neurodegenerative disorders. Different DNA repair pathways have evolved to successfully act on damaged DNA and prevent genomic instability. The predominant and essential DNA repair pathway for the removal of small DNA base lesions is base excision repair (BER). In this review we will discuss the current knowledge on the involvement of BER proteins in the maintenance of genetic stability in different brain regions and how changes in the levels of these proteins contribute to aging and the onset of neurodegenerative disorders.

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.

Tau Ser262 phosphorylation is critical for Abeta42-induced tau toxicity in a transgenic Drosophila model of Alzheimer's disease

The amyloid-beta 42 (Abeta42) peptide has been suggested to promote tau phosphorylation and toxicity in Alzheimer's disease (AD) pathogenesis; however, the underlying mechanisms are not fully understood. Using transgenic Drosophila expressing both human Abeta42 and tau, we show here that tau phosphorylation at Ser262 plays a critical role in Abeta42-induced tau toxicity. Co-expression of Abeta42 increased tau phosphorylation at AD-related sites including Ser262, and enhanced tau-induced neurodegeneration. In contrast, formation of either sarkosyl-insoluble tau or paired helical filaments was not induced by Abeta42. Co-expression of Abeta42 and tau carrying the non-phosphorylatable Ser262Ala mutation did not cause neurodegeneration, suggesting that the Ser262 phosphorylation site is required for the pathogenic interaction between Abeta42 and tau. We have recently reported that the DNA damage-activated Checkpoint kinase 2 (Chk2) phosphorylates tau at Ser262 and enhances tau toxicity in a transgenic Drosophila model. We detected that expression of Chk2, as well as a number of genes involved in DNA repair pathways, was increased in the Abeta42 fly brains. The induction of a DNA repair response is protective against Abeta42 toxicity, since blocking the function of the tumor suppressor p53, a key transcription factor for the induction of DNA repair genes, in neurons exacerbated Abeta42-induced neuronal dysfunction. Our results demonstrate that tau phosphorylation at Ser262 is crucial for Abeta42-induced tau toxicity in vivo, and suggest a new model of AD progression in which activation of DNA repair pathways is protective against Abeta42 toxicity but may trigger tau phosphorylation and toxicity in AD pathogenesis.

A DNA damage-activated checkpoint kinase phosphorylates tau and enhances tau-induced neurodegeneration

Hyperphosphorylation of the microtubule associated protein tau is detected in the brains of individuals with a range of neurodegenerative diseases including Alzheimer's disease (AD). An imbalance in phosphorylation and/or dephosphorylation of tau at disease-related sites has been suggested to initiate the abnormal metabolism and toxicity of tau in disease pathogenesis. However, the mechanisms underlying abnormal phosphorylation of tau in AD are not fully understood. Here, we show that the DNA damage-activated Checkpoint kinase 2 (Chk2) is a novel tau kinase and enhances tau toxicity in a transgenic Drosophila model. Overexpression of Drosophila Chk2 increases tau phosphorylation at Ser262 and enhances tau-induced neurodegeneration in transgenic flies expressing human tau. The non-phosphorylatable Ser262Ala mutation abolishes Chk2-induced enhancement of tau toxicity, suggesting that the Ser262 phosphorylation site is involved in the enhancement of tau toxicity by Chk2. In vitro kinase assays revealed that human Chk2 and a closely related checkpoint kinase 1 (Chk1) directly phosphorylate human tau at Ser262. We also demonstrate that Drosophila Chk2 does not modulate the activity of the fly homolog of microtubule affinity regulating kinase, which has been shown to be a physiological tau Ser262 kinase. Since accumulation of DNA damage has been detected in the brains of AD patients, our results suggest that the DNA damage-activated kinases Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in the pathogenesis of AD.

DNA damage and repair: relevance to mechanisms of neurodegeneration

DNA damage is a form of cell stress and injury that has been implicated in the pathogenesis of many neurologic disorders, including amyotrophic lateral sclerosis, Alzheimer disease, Down syndrome, Parkinson disease, cerebral ischemia, and head trauma. However, most data reveal only associations, and the role for DNA damage in direct mechanisms of neurodegeneration is vague with respect to being a definitive upstream cause of neuron cell death, rather than a consequence of the degeneration. Although neurons seem inclined to develop DNA damage during oxidative stress, most of the existing work on DNA damage and repair mechanisms has been done in the context of cancer biology using cycling nonneuronal cells but not nondividing (i.e. postmitotic) neurons. Nevertheless, the identification of mutations in genes that encode proteins that function in DNA repair and DNA damage response in human hereditary DNA repair deficiency syndromes and ataxic disorders is establishing a mechanistic precedent that clearly links DNA damage and DNA repair abnormalities with progressive neurodegeneration. This review summarizes DNA damage and repair mechanisms and their potential relevance to the evolution of degeneration in postmitotic neurons.

Mechanisms of Disease: DNA repair defects and neurological disease

In this Review, familial and sporadic neurological disorders reported to have an etiological link with DNA repair defects are discussed, with special emphasis placed on the molecular link between the disease phenotype and the precise DNA repair defect. Of the 15 neurological disorders listed, some of which have symptoms of progeria, six--spinocerebellar ataxia with axonal neuropathy-1, Huntington's disease, Alzheimer's disease, Parkinson's disease, Down syndrome and amyotrophic lateral sclerosis--seem to result from increased oxidative stress, and the inability of the base excision repair pathway to handle the damage to DNA that this induces. Five of the conditions (xeroderma pigmentosum, Cockayne's syndrome, trichothiodystrophy, Down syndrome, and triple-A syndrome) display a defect in the nucleotide excision repair pathway, four (Huntington's disease, various spinocerebellar ataxias, Friedreich's ataxia and myotonic dystrophy types 1 and 2) exhibit an unusual expansion of repeat sequences in DNA, and four (ataxia-telangiectasia, ataxia-telangiectasia-like disorder, Nijmegen breakage syndrome and Alzheimer's disease) exhibit defects in genes involved in repairing double-strand breaks. The current overall picture indicates that oxidative stress is a major causative factor in genomic instability in the brain, and that the nature of the resulting neurological phenotype depends on the pathway through which the instability is normally repaired.

Base excision repair and the central nervous system

Reactive oxygen species generated during normal cellular metabolism react with lipids, proteins, and nucleic acid. Evidence indicates that the accumulation of oxidative damage results in cellular dysfunction or deterioration. In particular, oxidative DNA damage can induce mutagenic replicative outcomes, leading to altered cellular function and/or cellular transformation. Additionally, oxidative DNA modifications can block essential biological processes, namely replication and transcription, triggering cell death responses. The major pathway responsible for removing oxidative DNA damage and restoring the integrity of the genome is base excision repair (BER). We highlight herein what is known about BER protein function(s) in the CNS, which in cooperation with the peripheral nervous system operates to control physical responses, motor coordination, and brain operation. Moreover, we describe evidence indicating that defective BER processing can promote post-mitotic (i.e. non-dividing) neuronal cell death and neurodegenerative disease. The focus of the review is on the core mammalian BER participants, i.e. the DNA glycosylases, AP endonuclease 1, DNA polymerase beta, X-ray cross-complementing 1, and the DNA ligases.

Reduction of chromatin assembly factor 1 p60 and C21orf2 protein, encoded on chromosome 21, in Down syndrome brain

Trisomy 21 (Down syndrome, DS) is the most common genetic cause of mental retardation, resulting from triplication of the whole or distal part of human chromosome 21. Overexpression of genes located on chromosome 21, as a result of extra gene load, has been considered a central hypothesis for the explanation of the DS phenotype. This gene dosage hypothesis has been challenged, however. We have therefore decided to study proteins whose genes are encoded on chromosome 21 in brain of patients with DS and Alzheimer's disease (AD), as all patients with DS from the fourth decade show Alzheimer-related neuropathology. Using immunoblotting we determined Coxsackievirus and adenovirus receptor (CAR), Claudin-8, C21orf2, Chromatin assembly factor 1 p60 subunit (CAF-1 p60) in frontal cortex from DS, AD and control patients. Significant reduction of C21orf2 and CAF-1 p60, but comparable expression of CAR and claudin-8 was observed in DS but all proteins were comparable to controls in AD, even when related to NSE levels to rule out neuronal cell loss or actin to normalise versus a housekeeping protein. Reduced CAF-1 p60 may reflect impaired DNA repair most probably due to oxidative stress found as early as in fetal life continuing into adulthood. The decrease of C21orf2 may represent mitochondrial dysfunction that has been reported repeatedly and also data on CAR and claudin-8 are not supporting the gene-dosage hypothesis at the protein level. As aberrant expression of the four proteins was not found in brains of patients with AD, decreased CAF and C21orf2 can be considered specific for DS.

Aberrant protein expression of transcription factors BACH1 and ERG, both encoded on chromosome 21, in brains of patients with Down syndrome and Alzheimer’s disease

Down syndrome (DS; trisomy 21) is a genetic disorder associated with early mental retardation and patients inevitably develop Alzheimer's disease (AD)-like neuropathological changes. The molecular defects underlying the DS-phenotype may be due to overexpression of genes encoded on chromosome 21. This so-called gene dosage hypothesis is still controversial and demands systematic work on protein expression. A series of transcription factors (TF) are encoded on chromosome 21 and are considered to play a pathogenetic role in DS. We therefore decided to study brain expression of TF encoded on chromosome 21 in patients with DS and AD compared to controls: Frontal cortex of 6 male DS patients, 6 male patients with AD and 6 male controls were used for the experiments. Immunoblotting was used to determine protein levels of TF BACH1, ERG, SIM2 and RUNX1. SIM2 and RUNX1 were comparable between groups, while BACH1 was significantly reduced in DS, and ERG was increased in DS and AD as compared to controls. These findings may indicate that DS pathogenesis cannot be simply explained by the gene dosage effect hypothesis and that results of ERG expression in DS were paralleling those in AD probably reflecting a common pathogenetic mechanism possibly explaining why all DS patients develop AD like neuropathology from the fourth decade. We conclude that TF derangement is not only due to the process of neurodegeneration and propose that TFs BACH1 and ERG play a role for the development of AD-like neuropathology in DS and pathogenesis of AD per se and the manifold increase of ERG in both disorders may form a pivotal pathogenetic link.

Deterioration of the transcriptional, splicing and elongation machinery in brain of fetal Down syndrome

Perturbation of brain development i.e. regulation of gene expression, differentiation, growth and migration in Down Syndrome (DS) has been reported to occur early in life pointing to impairment of the complex system of transcription and or translation and indeed, altered expression of transcription factors has been reported in adult DS brain. We therefore decided to compare the transcriptional and translational machinery in cortex of brains of controls and fetuses with Down syndrome in the second trimenon of gestation. We determined a series of transcription/translation factors by 2 D-electrophoresis followed by MALDI--identification and quantification with specific software. The protooncogene C-CRK, CRK-like protein, elongation factor 1-alpha 1, elongation factor 2, elongation factor tu and two out of four spots representing PTB-associated splicing factor PSF were significantly downregulated in brain of fetal DS fetuses as compared to controls. The finding of reduced transcription and translation factors may indicate deranged protein synthesis. The underlying cause for individual reduced transcription, splicing and translation factors may be explained by chromosomal imbalance or by posttranslational modifications as e.g. phosphorylation, known to be aberrant in DS. Reduced expression of transcription factors in fetal DS during early life may be responsible or reflecting impaired brain development and deficient wiring of the brain in DS.

Downregulation of the transcription factor scleraxis in brain of patients with Down syndrome

Performing gene hunting in fetal Down Syndrome (DS) brain, we found a downregulated sequence with 100% homology to the basic-helix-loop-helix transcription factor (TF) scleraxis (Scl). It was the aim of the study to evaluate Scl-mRNA steady state levels in adult DS brain with Alzheimer's disease (AD) neuropathological changes, brain of patients with AD, and controls in order to find out whether Scl-downregulation is linked to DS per se or simply to neurodegeneration, common to both disorders. Determination of Scl-mRNA steady state levels was carried out by a blotting method in frontal, parietal, temporal, occipital lobe and cerebellum. We found significantly decreased Scl-transcripts in brain of DS and AD, both, when normalized versus the house-keeping gene beta actin or total RNA. We demonstrate the significant decrease of Scl-mRNA steady state levels in the pathogenesis of DS and AD suggesting a tentative role for this transcription factor in the development of the neurodegenerative processes known to occur in both disorders. More specifically, the biological meaning of the downregulation of Scl may be the involvement in the pathogenesis of impaired neuronal plasticity and wiring observed in DS and AD, phenomena regulated by the concerted action of the many transcription factors expressed in human brain.

Altered gene expression in fetal Down syndrome brain as revealed by the gene hunting technique of subtractive hybridization

Information on gene expression in brain of patients with Down Syndrome (DS, trisomy 21) is limited and molecular biological research is focussing on mapping and sequencing chromosome 21. The information on gene expression in DS available follows the current concept of a gene dosage effect due to a third copy of chromosome 21 claiming overexpression of genes encoded on this chromosome. Based upon the availability of fetal brain and recent technology of gene hunting, we decided to use subtractive hybridization to evaluate differences in gene expression between DS and control brains. Subtractive hybridization was applied on two fetal brains with DS and two age and sex matched controls, 23rd week of gestation, and mRNA steady state levels were evaluated generating a subtractive library. Subtracted sequences were identified by gene bank and assigned by alignments to individual genes. We found a series of up- and downregulated sequences consisting of chromosomal transcripts, enzymes of intermediary metabolism, hormones, transporters/channels and transcription factors (TFs). We show that trisomy 21 or aneuploidy leads to the deterioration of gene expression and the derangement of transcripts describes the impairment of transport, carriers, channels, signaling, known metabolic and hormone imbalances. The dys-coordinated expression of transcription factors including homeobox genes, POU-domain TFs, helix-loop-helix-motifs, LIM domain containing TFs, leucine zippers, forkhead genes, maybe of pathophysiological significance for abnormal brain development and wiring found in patients with DS. This is the first description of the concomitant expression of a large series of sequences indicating disruption of the concerted action of genes in this disorder.

Decreased transcription factor junD in brains of patients with Down syndrome

JunD is a member of the Jun family of transcription factors (TF), recently shown to negatively regulate cell growth and antagonizes transformation by the protooncogene ras: c-jun decreases while junD is accumulating when fibroblasts become quiescent. Furthermore, overexpression of junD resulted in slower growth and an increase in cells in G0/G1. Performing gene hunting on fetal Down syndrome (DS) brain we found a sequence downregulated and homologous to junD. This observation made us examine junD protein levels in adult brain specimens. Western blot experiments were carried out in five brain regions of aged patients with DS (n = 9), controls (n = 9) and patients with Alzheimer's disease (AD, n = 9). We found that junD in AD brains were comparable to controls, whereas junD levels were significantly and remarkably reduced in frontal, temporal lobe and cerebellum of patients with DS. These findings may indicate a specific finding in DS and were not linked to the AD-like-neuropathological changes of plaques and tangles, observed in DS from the fourth decade, which is also suggested by the findings of downregulated junD at the mRNA level revealed by the gene hunting technique (subtractive hybridization) in fetal DS brain. We propose that junD plays a role for the impaired development and wiring of DS brain, maybe already early in life.