Association between late-onset Alzheimer's disease and microsatellite polymorphisms in intron II of the human toll-like receptor 2 gene

The amyloid beta protein (Aβ) deposits in the brains of patients with Alzheimer's disease (AD) are closely associated with innate immune responses that were assumed to play a pivotal role in the pathogenesis of AD. Toll-like receptor 2 (TLR2) is thought to contribute to Aβ clearance. Studies have reported the presence and functional implications of guanine-thymine (GT) repeat microsatellite polymorphisms in intron II of the human TLR2 gene. The present study evaluated the association of the microsatellite polymorphism and sporadic late-onset AD (LOAD) in the Han Chinese population. The numbers of (GT) repeats were counted in 137 AD patients and in 137 non-AD control subjects, using polymerase chain reaction and genescan analysis. The alleles were divided into three subclasses: (GT)16 or less as the S allele, (GT)17 to (GT)22 as the M allele, and (GT)23 or more as the L allele. Patients with AD had more S alleles (P < 0.001; odds ratio (OR) = 2.32; 95% confidence interval (CI) = 1.57–3.42) and fewer L alleles (P = 0.02; OR = 0.66; 95% CI = 0.46–0.93) than did healthy controls. Genotypes SS and SM were more common, whereas ML and SL were less common in patients with AD. In subgroup analyses, the genotypes including S alleles were associated with an increased risk of LOAD (OR = 2.05, 95% CI = 1.26–3.34), and this association was influenced by the presence of APOE ?4 alleles. This study demonstrates an association of microsatellite polymorphisms in intron II of the human TLR2 gene with risk for LOAD in Han Chinese.     

Epigenetic mechanisms in Alzheimer's disease: Implications for pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) are late-onset forms (LOAD) likely due to the interplay of environmental influences and individual genetic susceptibility. Epigenetic mechanisms, including DNA methylation, histone modifications and non-coding RNAs, constitute dynamic intracellular processes for translating environmental stimuli into modifications in gene expression. Over the past decade it has become increasingly clear that epigenetic mechanisms play a pivotal role in aging the pathogenesis of AD. Here, we provide a review of the major mechanisms for epigenetic modification and how they are reportedly altered in aging and AD. Moreover, we also consider how aberrant epigenetic modifications may lead to AD pathogenesis, and we review the therapeutic potential of epigenetic treatments for AD.     

Cortical serotonin-S2 receptor binding abnormalities in patients with Alzheimer's disease: comparisons with Parkinson's disease

Reductions in the numbers of binding sites for the serotonergic S2-receptor antagonist, ketanserin, are, as previously reported, evident in Alzheimer's disease. New findings indicate that these sites are not affected in the cortex of patients with Parkinson's disease despite the presence of cognitive impairment. In contrast S1-receptor binding sites were reduced to a small but significant extent in both Alzheimer's and Parkinson's disease with cognitive deficit. The S2-receptor binding loss was not related to the cholinergic deficit (decreased choline acetyltransferase) common to both disorders nor to the presence of cortical senile plaques but did relate to the extent of cortical neurofibrillary tangle formation, evident in Alzheimer's but not generally in Parkinson's disease. These observations suggest that S2- but not S1-receptor binding abnormalities may reflect an important intrinsic cortical involvement specifically associated with the Alzheimer disease process.     

Association between somatostatin gene polymorphisms and sporadic Alzheimer's disease in Chinese population

Recent evidence has suggested that down-regulation of somatostatin (SST) expression in the human brain during early stages of aging leads to an elevation in the steady-state levels of Aβ and therefore may be involved in Alzheimer's disease (AD) progression. We hypothesized that alterations in the SST gene might alter its expression or function and also play a role in the pathogenesis of sporadic AD (SAD). First, we sequenced the entire SST gene in 25 randomly selected controls and 25 SAD patients and then screened for C/T polymorphisms (rs4988514) in the 3′ un-translated region. We genotyped rs4988514 polymorphisms as well as apolipoprotein ?4 (APOE ?4) status in 309 SAD patients and 276 normal controls with restriction fragment length polymorphism (RFLP) analysis. Results showed that the C allele of the rs4988514 polymorphism had an increased incidence in the SAD group compared to the control group (p = 0.042). In subjects with the APOE ?4 allele, the presence of both the CC genotype and the C allele of this polymorphism were elevated in the SAD group compared to the control group (genotype p = 0.027, allele p = 0.011). In the whole study group, the age, sex, and APOE ?4 adjusted OR for the risk of AD in C allele carriers was 1.313 (95%CI = 1.068–2.234, p = 0.027) whereas within only APOE ?4 allele carriers, the adjusted OR increased to 2.734 (95%CI = 1.236–5.862, p = 0.012). Our results supported the notion that the C allele of the rs4988514 polymorphism may increase the risk for AD in the Chinese population and possibly have additive effect with the APOE ?4 allele.     

Glutamatergic receptor expression changes in the Alzheimer's disease hippocampus and entorhinal cortex

Alzheimer''s Disease (AD) is the leading form of dementia worldwide. Currently, the pathological mechanisms underlying AD are not well understood. Although the glutamatergic system is extensively implicated in its pathophysiology, there is a gap in knowledge regarding the expression of glutamate receptors in the AD brain. This study aimed to characterize the expression of specific glutamate receptor subunits in post‐mortem human brain tissue using immunohistochemistry and confocal microscopy. Free‐floating immunohistochemistry and confocal laser scanning microscopy were used to quantify the density of glutamate receptor subunits GluA2, GluN1, and GluN2A in specific cell layers of the hippocampal sub‐regions, subiculum, entorhinal cortex, and superior temporal gyrus. Quantification of GluA2 expression in human post‐mortem hippocampus revealed a significant increase in the stratum (str.) moleculare of the dentate gyrus (DG) in AD compared with control. Increased GluN1 receptor expression was found in the str. moleculare and hilus of the DG, str. oriens of the CA2 and CA3, str. pyramidale of the CA2, and str. radiatum of the CA1, CA2, and CA3 subregions and the entorhinal cortex. GluN2A expression was significantly increased in AD compared with control in the str. oriens, str. pyramidale, and str. radiatum of the CA1 subregion. These findings indicate that the expression of glutamatergic receptor subunits shows brain region‐specific changes in AD, suggesting possible pathological receptor functioning. These results provide evidence of specific glutamatergic receptor subunit changes in the AD hippocampus and entorhinal cortex, indicating the requirement for further research to elucidate the pathophysiological mechanisms it entails, and further highlight the potential of glutamatergic receptor subunits as therapeutic targets.     

Consistent decrease in global DNA methylation and hydroxymethylation in the hippocampus of Alzheimer's disease patients

Epigenetic dysregulation of gene expression is thought to be critically involved in the pathophysiology of Alzheimer's disease (AD). Recent studies indicate that DNA methylation and DNA hydroxymethylation are 2 important epigenetic mechanisms that regulate gene expression in the aging brain. However, very little is known about the levels of markers of DNA methylation and hydroxymethylation in the brains of patients with AD, the cell-type specificity of putative AD-related alterations in these markers, as well as the link between epigenetic alterations and the gross pathology of AD. The present quantitative immunohistochemical study investigated the levels of the 2 most important markers of DNA methylation and hydroxymethylation, that is, 5-methylcytidine (5-mC) and 5-hydroxymethylcytidine (5-hmC), in the hippocampus of AD patients (n = 10) and compared these to non-demented, age-matched controls (n = 10). In addition, the levels of 5-hmC in the hippocampus of a pair of monozygotic twins discordant for AD were assessed. The levels of 5-mC and 5-hmC were furthermore analyzed in a cell-type and hippocampal subregion–specific manner, and were correlated with amyloid plaque load and neurofibrillary tangle load. The results showed robust decreases in the hippocampal levels of 5-mC and 5-hmC in AD patients (19.6% and 20.2%, respectively). Similar results were obtained for the twin with AD when compared to the non-demented co-twin. Moreover, levels of 5-mC as well as the levels of 5-hmC showed a significant negative correlation with amyloid plaque load in the hippocampus (r_p = −0.539, p = 0.021 for 5-mC and r_p = −0.558, p = 0.016 for 5-hmC). These human postmortem results thus strengthen the notion that AD is associated with alterations in DNA methylation and hydroxymethylation, and provide a basis for further epigenetic studies identifying the exact genetic loci with aberrant epigenetic signatures.     

Epigenetic changes in the progression of Alzheimer's disease

The formation of 5-hydroxymethylcytosine (5hmC), a key intermediate of DNA demethylation, is driven by the ten eleven translocation (TET) family of proteins that oxidize 5-methylcytosine (5mC) to 5hmC. To determine whether methylation/demethylation status is altered during the progression of Alzheimer's disease (AD), levels of TET1, 5mC and subsequent intermediates, including 5hmC, 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) were quantified in nuclear DNA from the hippocampus/parahippocampal gyrus (HPG) and the cerebellum of 5 age-matched normal controls, 5 subjects with preclinical AD (PCAD) and 7 late-stage AD (LAD) subjects by immunochemistry. The results showed significantly (p < 0.05) increased levels of TET1, 5mC, and 5hmC in the HPG of PCAD and LAD subjects. In contrast, levels of 5fC and 5caC were significantly (p < 0.05) decreased in the HPG of PCAD and LAD subjects. Overall, the data suggest altered methylation/demethylation patterns in vulnerable brain regions prior to the onset of clinical symptoms in AD suggesting a role in the pathogenesis of the disease.     

Valproic acid stimulates clusterin expression in human astrocytes: Implications for Alzheimer's disease

Clusterin is a secreted molecular chaperone, also called apolipoprotein J. Recent genetic studies have demonstrated that clusterin is a significant susceptibility gene for late-onset Alzheimer's disease (AD). Clusterin shares several properties with apolipoprotein E, a well-known risk gene for AD, i.e. they bind to amyloid-β peptides and are present in neuritic plaques, enhance the clearance of amyloid-β peptides in brain, and are included in lipid particles and thus regulate cholesterol traffic. Biochemical studies indicate that clusterin can prevent the progress of AD pathogenesis. We have observed earlier that histone deacetylase (HDAC) inhibitors can induce the expression of clusterin in several neuroblastoma and glioma cell lines. Recent studies have revealed that valproic acid, a common and well-tolerated drug for epilepsy and bipolar disorders, is a potent HDAC inhibitor. In this study, we examined whether valproic acid can induce the expression of clusterin in human astrocytes. Our results demonstrated that valproic acid is a potent inducer of clusterin expression and secretion in human astrocytes at the therapeutical concentrations. Another clinically used HDAC inhibitor, the cancer drug, Vorinostat (SAHA, suberoylanilide hydroxamic acid), also robustly stimulated the expression of clusterin in human astrocytes. One could postulate that valproic acid may be able to prevent amyloid-β aggregation in AD, as observed in transgenic AD mice, by increasing clusterin expression.     

Association between a variant of the sigma-1 receptor gene and Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with complex etiology and strong genetic predisposition. A number of investigations support the possible involvement of sigma non-opioid intracellular receptor 1 (SIGMAR1) in the pathophysiology of AD. We aimed to investigate the association between SIGMAR1 polymorphisms and late-onset AD, therefore we genotyped rs1799729 (GC-241-240TT) and rs1800866 (Q2P) in 322 Hungarian late-onset AD patients and 250 ethnically matched, elderly control individuals. The investigated polymorphisms were in nearly complete linkage disequilibrium resulting in the GC-Q and TT-P predominant haplotypes that were subjected to the statistical analyses. Our data demonstrates an association between the SIGMAR1 TT-P variant and the risk for developing AD (p=0.019), and a potential modest interaction effect (p=0.058) of the co-presence of the TT-P haplotype with apolipoprotein E4 allele on the risk for AD. Based on this mild significance, we could not fully support the hypothesis that TT-P haplotype in interaction with APOE E4 allele confers risk for developing AD.     

Lymphocyte G-protein-coupled receptor kinase-2 is upregulated in patients with Alzheimer's disease

Alterations in signal transduction pathway of G-protein-coupled receptors (GPCRs) have been found in the cerebrocortex and in the peripheral cultured tissues of patients with Alzheimer's disease (AD). The G-protein-coupled receptor kinase-2 (GRK2) plays an important role in regulating the GPCRs signaling: its increased expression is associated with receptor desensitization. The aim of this study was to explore GRK2 levels in peripheral lymphocytes of AD patients and to establish a correlation between lymphocyte protein concentrations and the degree of cognitive impairment. GRK2 mRNA and protein expression were evaluated in the lymphocytes of AD patients with mild or moderate/severe cognitive impairment and in age-matched healthy subjects. Both GRK2 mRNA and protein expression were higher in AD patients lymphocytes compared to controls. Furthermore, lymphocyte GRK2 levels were significantly correlated to the degree of cognitive decline. Our preliminary data suggest that GRK2 is involved in GPCRs coupling dysfunction observed in AD patients. Further studies are needed in order to verify whether the lymphocyte GRK2 might be utilized as a novel biomarker in AD diagnosis and clinical monitoring.     

Implication of IL-33 gene polymorphism in Chinese patients with Alzheimer's disease

Interleukin-33 (IL-33), a newly described member of the IL-1 family, is located on chromosome 9p24, a chromosomal region of interest in Alzheimer's disease (AD) defined by many genome-wide studies. Three intronic rs1157505, rs11792633, and rs7044343 single nucleotide polymorphisms (SNPs) within IL-33 have recently been reported to be associated with risk of AD in Caucasian populations. In order to assess the involvement of the IL-33 polymorphisms in the risk of developing late onset AD (LOAD), we analyzed the genotype and allele distributions of these 3 polymorphisms in 704 Han Chinese subjects. The minor alleles of the rs11792633 polymorphism within IL-33 was significantly associated with a reduced risk of LOAD (odds ratio [OR] = 0.73, p = 0.005). Furthermore, rs11792633 polymorphism was still strongly associated with LOAD (dominant model: OR = 0.67, p = 0.015; recessive model: OR 0.57, p = 0.021; additive model: OR = 0.71, p = 0.004) after adjusting for age, gender, and the apolipoprotein E (APOE) ε4 status. Our results support the evidence that genetic variants of IL-33 affect susceptibility to LOAD in Han Chinese.     

Inflammasomes as therapeutic targets for Alzheimer's disease

Alzheimer's disease is the most common form of progressive dementia, typified initially by short term memory deficits which develop into a dramatic global cognitive decline. The classical hall marks of Alzheimer's disease include the accumulation of amyloid oligomers and fibrils, and the intracellular formation of neurofibrillary tangles of hyperphosphorylated tau. It is now clear that inflammation also plays a central role in the pathogenesis of the disease through a number of neurotoxic mechanisms. Microglia are the key immune regulators of the CNS which detect amyloidopathy through cell surface and cytosolic pattern recognition receptors (PRRs) and respond by initiating inflammation through the secretion of cytokines such as interleukin‐1β (IL‐1β). Inflammasomes, which regulate IL‐1β release, are formed following activation of cytosolic PRRs, and using genetic and pharmacological approaches, NLRP3 and NLRP1 inflammasomes have been found to be integral in pathogenic neuroinflammation in animal models of Alzheimer's disease. Therefore, the inflammasomes are very promising novel pharmacological targets which merit further research in the continued endeavor for efficacious therapeutics for Alzheimer's disease.     

Effect of the functional toll-like receptor 4 Asp299Gly polymorphism on susceptibility to late-onset Alzheimer's disease

Experimental data have shown an upregulated expression of toll-like receptors, particularly toll-like receptor 4 (TLR4), in neurodegeneration. The Asp299Gly polymorphism of the TLR4 gene has been associated with an attenuated receptor signalling and a blunted inflammatory response. In the present study, we sought to determine whether this common genetic variant could influence susceptibility to late-onset Alzheimer's disease (LOAD) in an Italian population sample. A cohort of 277 LOAD patients and 300 cognitively healthy controls were genotyped for the TLR4 Asp299Gly polymorphism using restriction isotyping. The frequency of the minor 299Gly allele was significantly higher in the controls than in the LOAD cases (7.2% versus 3.1%, respectively, P=0.003). Additionally, the frequency of the variant genotypes (Asp/Gly and Gly/Gly) was 13.0% in the controls and 5.4% in LOAD patients (P=0.002). After adjustment for age, gender, and the APOE varepsilon4 carrier status, the odds ratio for the development of LOAD associated with the Asp/Gly and Gly/Gly versus Asp/Asp genotype was 0.37 (95% CI: 0.20-0.69, P=0.002). Our data further support a role for innate immunity in neurodegeneration and give the first evidence that the TLR4 Asp299Gly variant may be protective toward the development of LOAD.     

Relationship of the Ubiquilin 1 gene with Alzheimer's and Parkinson's disease and cognitive function

Ubiquilin 1 (UBQLN1) is involved in the ubiquitination machinery, which has been implicated in Alzheimer's disease (AD) as well as Parkinson's disease (PD). A polymorphism in the gene encoding for UBQLN1 has been previously associated with a higher risk of AD. We studied the role of the SNP rs12344615 on the UBQLN 1 gene in AD, PD and cognitive function in a population-based study, the Rotterdam Study, and a family-based study embedded in the genetic research in isolated population (GRIP) program. The Rotterdam Study includes 549 patients with AD and 157 patients with PD. The GRIP program includes a series of 123 patients with AD and a study of 1049 persons who are characterized for cognitive function. Data were analysed using logistic and multiple regression analysis. We found no significant difference in risk of AD or PD by the UBQLN1 SNP rs12344615 in our overall and stratified analyses in the Rotterdam Study. In our family-based study, we did not find evidence for linkage of AD to the region including the UBQLN1 gene. In the family-based study we also failed to detect an effect of this polymorphism on cognitive function. Our results suggest that it is unlikely that the SNP rs12344615 of the UBQLN1 gene is related to the onset of AD, PD or cognitive function.     

Choosing Alzheimer's disease prevention clinical trial populations

To assist investigators in making design choices, we modeled Alzheimer's disease prevention clinical trials. We used longitudinal Clinical Dementia Rating Scale Sum of Boxes data, retention rates, and the proportions of trial-eligible cognitively normal participants age 65 and older in the National Alzheimer's Coordinating Center Uniform Data Set to model trial sample sizes, the numbers needed to enroll to account for drop out, and the numbers needed to screen to successfully complete enrollment. We examined how enrichment strategies affected each component of the model. Relative to trials enrolling 65-year-old individuals, trials enriching for older (minimum 70 or 75) age required reduced sample sizes, numbers needed to enroll, and numbers needed to screen. Enriching for subjective memory complaints reduced sample sizes and numbers needed to enroll more than age enrichment, but increased the number needed to screen. We conclude that Alzheimer's disease prevention trials can enroll elderly participants with minimal effect on trial retention and that enriching for older individuals with memory complaints might afford efficient trial designs.     

Genomic mechanisms in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease and, owing to its increasing prevalence, represents one of the leading public health problems in aging populations. The molecular causes underlying the onset and progression of AD are manifold and hitherto still incompletely understood. Research over nearly four decades has clearly delineated genetics to play a crucial role in AD susceptibility, likely in concert with non‐genetic factors. The field has gained considerable momentum and novel insights over the past 10 years owing to the advent and application of high‐throughput genomics technologies in datasets of increasing size. In this contribution to the Mini‐Symposium on the Molecular Etiology of Alzheimer's Disease, we review the current status of genomics research in AD. To this end, we scrutinize and discuss the main findings from the two largest and most current genome‐wide association studies (GWAS) in the field, that is, the papers published by Jansen et al (Nat Genet 51:404–413) and Kunkle et al (Nat Genet 51:414–430). Particular focus is laid on genomics findings overlapping across both studies and on the novel insights they provide in terms of improving our understanding of the “genomic mechanisms” underlying this devastating disease.     

Association between neprilysin polymorphisms and sporadic Alzheimer's disease

The deposition of amyloid beta peptide (Abeta) in the form of plaques in the brain is a hallmark of Alzheimer's disease (AD). Neprilysin is the major Abeta-degradating enzyme and reduction in neprilysin activity could contribute to Alzheimer's by increasing the steady-state level of Abeta. To provide further evidence for the role of neprilysin in AD we genotyped 22 polymorphisms, 21 SNPs and the GT repeat in the promoter region, across the neprilysin gene in 298 Caucasian sporadic Alzheimer's patients and 298 age-matched controls. Several SNPs showed genotypic and allelic association to AD. SNP rs1836915, in linkage disequilibrium block 2, showed the greatest extent of genotypic association with AD (p=0.0076). We were unable to replicate any of the SNPs that were previously reported as putatively associated with AD. However, these novel findings add to the weight of evidence supporting the involvement of neprilysin in the aetiology of Alzheimer's disease.