APOE and BDNF polymorphisms moderate amyloid ß-related cognitive decline in preclinical Alzheimer's disease.

Accumulation of ß-amyloid (Aß) in the brain is associated with memory decline in healthy individuals as a prelude to Alzheimer's disease (AD). Genetic factors may moderate this decline. We examined the role of apolipoprotein E (ɛ4 carrier[ɛ4(+)], ɛ4 non-carrier[ɛ4(-)]) and brain-derived neurotrophic factor (BDNF(Val/Val), BDNF(Met)) in the extent to which they moderate Aß-related memory decline. Healthy adults (n=333, Mage=70 years) enrolled in the Australian Imaging, Biomarkers and Lifestyle study underwent Aß neuroimaging. Neuropsychological assessments were conducted at baseline, 18-, 36- and 54-month follow-ups. Aß positron emission tomography neuroimaging was used to classify participants as Aß(-) or Aß(+). Relative to Aß(-)ɛ4(-), Aß(+)ɛ4(+) individuals showed significantly faster rates of cognitive decline over 54 months across all domains (d=0.40-1.22), while Aß(+)ɛ4(-) individuals showed significantly faster decline only on verbal episodic memory (EM). There were no differences in rates of cognitive change between Aß(-)ɛ4(-) and Aß(-)ɛ4(+) groups. Among Aß(+) individuals, ɛ4(+)/BDNF(Met) participants showed a significantly faster rate of decline on verbal and visual EM, and language over 54 months compared with ɛ4(-)/BDNF(Val/Val) participants (d=0.90-1.02). At least two genetic loci affect the rate of Aß-related cognitive decline. Aß(+)ɛ4(+)/BDNF(Met) individuals can expect to show clinically significant memory impairment after 3 years, whereas Aß(+)ɛ4(+)/BDNF(Val/Val) individuals can expect a similar degree of impairment after 10 years. Little decline over 54 months was observed in the Aß(-) and Aß(+) ɛ4(-) groups, irrespective of BDNF status. These data raise important prognostic issues in managing preclinical AD, and should be considered in designing secondary preventative clinical trials.

Associations between gonadotropins, testosterone and ß amyloid in men at risk of Alzheimer's disease.

Testosterone and gonadotropins have been associated with cognitive decline in men and the modulation of ß amyloid (Aß) metabolism. The relatively few studies that have investigated whether changes in one or a combination of these hormones influence Aß levels have focused primarily on plasma Aß(1-40) and not on the more pathogenic Aß(1-42). Currently, no study has investigated whether these hormones are associated with an increase in brain amyloid deposition, ante mortem. Through the highly characterised Australian imaging, biomarkers and lifestyle study, we have determined the impact of these hormones on plasma Aß levels and brain amyloid burden (Pittsburgh compound B (PiB) retention). Spearman's rank correlation and linear regression analysis was carried out across the cohort and within subclassifications. Luteinizing hormone (LH) was the only variable shown, in the total cohort, to have a significant impact on plasma Aß(1-40) and Aß(1-42) levels (beta=0.163, P<0.001; beta=0.446, P<0.001). This held in subjective memory complainers (SMC) (Aß(1-40); beta=0.208, P=0.017; Aß(1-42); beta=0.215, P=0.017) but was absent in mild cognitive impairment (MCI) and Alzheimer's disease (AD) groups. In SMC, increased frequency of the APOE-ɛ4 allele (beta=0.536, P<0.001) and increasing serum LH levels (beta=0.421, P=0.004) had a significant impact on PiB retention. Whereas in MCI, PiB retention was associated with increased APOE-ɛ4 allele copy number (beta=0.674, P<0.001) and decreasing calculated free testosterone (beta=-0.303, P=0.043). These findings suggest a potential progressive involvement of LH and testosterone in the early preclinical stages of AD. Furthermore, these hormones should be considered while attempting to predict AD at these earliest stages of the disease.

A blood-based predictor for neocortical Aß burden in Alzheimer's disease: results from the AIBL study.

Dementia is a global epidemic with Alzheimer's disease (AD) being the leading cause. Early identification of patients at risk of developing AD is now becoming an international priority. Neocortical Aß (extracellular ß-amyloid) burden (NAB), as assessed by positron emission tomography (PET), represents one such marker for early identification. These scans are expensive and are not widely available, thus, there is a need for cheaper and more widely accessible alternatives. Addressing this need, a blood biomarker-based signature having efficacy for the prediction of NAB and which can be easily adapted for population screening is described. Blood data (176 analytes measured in plasma) and Pittsburgh Compound B (PiB)-PET measurements from 273 participants from the Australian Imaging, Biomarkers and Lifestyle (AIBL) study were utilised. Univariate analysis was conducted to assess the difference of plasma measures between high and low NAB groups, and cross-validated machine-learning models were generated for predicting NAB. These models were applied to 817 non-imaged AIBL subjects and 82 subjects from the Alzheimer's Disease Neuroimaging Initiative (ADNI) for validation. Five analytes showed significant difference between subjects with high compared to low NAB. A machine-learning model (based on nine markers) achieved sensitivity and specificity of 80 and 82%, respectively, for predicting NAB. Validation using the ADNI cohort yielded similar results (sensitivity 79% and specificity 76%). These results show that a panel of blood-based biomarkers is able to accurately predict NAB, supporting the hypothesis for a relationship between a blood-based signature and Aß accumulation, therefore, providing a platform for developing a population-based screen.

Physical activity and amyloid-ß plasma and brain levels: results from the Australian Imaging, Biomarkers and Lifestyle Study of Ageing.

Previous studies suggest physical activity improves cognition and lowers Alzheimer's disease (AD) risk. However, key AD pathogenic factors that are thought to be influenced by physical activity, particularly plasma amyloid-ß (Aß) and Aß brain load, have yet to be thoroughly investigated. The objective of this study was to determine if plasma Aß and amyloid brain deposition are associated with physical activity levels, and whether these associations differed between carriers and non-carriers of the apolipoprotein E (APOE) ε4 allele. Five-hundred and forty six cognitively intact participants (aged 60-95 years) from the Australian Imaging, Biomarkers and Lifestyle Study of Ageing (AIBL) were included in these analyses. Habitual physical activity levels were measured using the International Physical Activity Questionnaire (IPAQ). Serum insulin, glucose, cholesterol and plasma Aß levels were measured in fasting blood samples. A subgroup (n=116) underwent (11)C-Pittsburgh compound B (PiB) positron emission tomography (PET) scanning to quantify brain amyloid load. Higher levels of physical activity were associated with higher high density lipoprotein (HDL) (P=0.037), and lower insulin (P<0.001), triglycerides (P=0.019) and Aß1-42/1-40 ratio (P=0.001). After stratification of the cohort based on APOE ε4 allele carriage, it was evident that only non-carriers received the benefit of reduced plasma Aß from physical activity. Conversely, lower levels of PiB SUVR (standardised uptake value ratio) were observed in higher exercising APOE ε4 carriers. Lower plasma Aß1-42/1-40 and brain amyloid was observed in those reporting higher levels of physical activity, consistent with the hypothesis that physical activity may be involved in the modulation of pathogenic changes associated with AD.

Olfactory discrimination predicts cognitive decline among community-dwelling older adults.

The presence of olfactory dysfunction in individuals at higher risk of Alzheimer's disease has significant diagnostic and screening implications for preventive and ameliorative drug trials. Olfactory threshold, discrimination and identification can be reliably recorded in the early stages of neurodegenerative diseases. The current study has examined the ability of various olfactory functions in predicting cognitive decline in a community-dwelling sample. A group of 308 participants, aged 46-86 years old, were recruited for this study. After 3 years of follow-up, participants were divided into cognitively declined and non-declined groups based on their performance on a neuropsychological battery. Assessment of olfactory functions using the Sniffin' Sticks battery indicated that, contrary to previous findings, olfactory discrimination, but not olfactory identification, significantly predicted subsequent cognitive decline (odds ratio = 0.869; P<0.05; 95% confidence interval = 0.764-0.988). The current study findings confirm previously reported associations between olfactory and cognitive functions, and indicate that impairment in olfactory discrimination can predict future cognitive decline. These findings further our current understanding of the association between cognition and olfaction, and support olfactory assessment in screening those at higher risk of dementia.

Plasma apolipoprotein E and Alzheimer disease risk: the AIBL study of aging.

OBJECTIVE: There is mounting evidence for the contribution of apoE to the pathophysiology of Alzheimer disease (AD). Studies also indicate that plasma apoE levels may reflect disease status, suggesting that apoE is a potential AD biomarker. However, while some studies of apoE levels in plasma have presented correlations with AD pathology, others have not. Thus, there is a lack of consensus as to the suitability of plasma apoE as an AD biomarker. The major objective of this cross-sectional study was to investigate total plasma apoE as well as levels of the apoE4 form in a large, highly characterized cohort which included both healthy controls and participants with early-stage AD. METHODS: Total apoE and apoE4 were measured in 1,079 individuals drawn from the highly characterized Australian Imaging, Biomarkers and Lifestyle (AIBL) study. Total and isoform-specific plasma apoE levels were then compared with cerebral Aß load, as assessed by PET using Pittsburgh compound B (PiB). RESULTS: Total apoE and apoE4 levels were found to be significantly lower in patients with AD in the entire cohort, and decrease with Aß load in the PiB-PET subset. ApoE levels were significantly lower among ε4 homozygous individuals. In APOE ε3/ε4 heterozygote carriers, apoE4 levels decrease, indicating that apoE3 levels increase with disease. CONCLUSION: Analysis of cross-sectional data from the AIBL study indicates that plasma apoE levels are altered in AD and correlate with AD pathology level. The significance of these findings will be determined in the AIBL longitudinal study of aging.

Lack of evidence to support the association of polymorphisms within the alpha- and beta-secretase genes (ADAM10/BACE1) with Alzheimer's disease.

Cleavage of the amyloid precursor protein (APP) occurs through either an amyloidogenic or a non-amyloidogenic pathway. The first results in the generation of beta-amyloid (Aß) and is initiated through cleavage by the beta-site amyloid beta A4 precursor protein-cleaving enzyme 1 (BACE1). The second precludes the formation of Aß through cleavage by alpha-secretase, an enzyme's activity demonstrated in a disintegrin metalloproteinase, ADAM10. To assess the contribution of variants in the BACE1 and ADAM10 genes we used a detailed fine mapping approach. Genotyping of 11 single nucleotide polymorphisms covering the complete BACE1 gene, and 27 covering the entire ADAM10 gene, revealed no single-marker or haplotypic association with AD. We conclude that, in this present study, neither ADAM10 nor BACE1 present with any evidence to suggest that they are major candidate genes involved in conferring risk for AD.

No association of Tachykinin receptor 2 (TACR2) polymorphisms with Alzheimer's disease.

The Tachykinin Receptor 2 (TACR2) located at chromosome 10q21.3 belongs to a class of receptors that bind members of the tachykinin neurotransmitter family. The TACR2 binds neurokinin A, also known as substance K, and is expressed in distinct parts of the human brain. Functionally, the TACR2 has been implicated in stress induced hippocampal acetylcholine release and the gene TACR2 is located within a previously identified linkage region for Alzheimer's disease (AD) on chromosome 10q21. Together, both facts make the TACR2 a reasonable positional and functional candidate gene for AD. Genotyping of 13 single nucleotide polymorphisms (SNPs) covering the entire gene and haplotypic analysis revealed no association with AD. Thus, we conclude that TACR2 can be excluded as a major susceptibility gene conferring risk to AD.

Novel phage peptides attenuate beta amyloid-42 catalysed hydrogen peroxide production and associated neurotoxicity.

Amyloid-beta (Abeta) peptides play a central role in the pathogenesis of Alzheimer's disease. There is accumulating evidence that supports the notion that the toxicity associated with human Abeta (both 40 and 42) is dependent on its superoxide dismutase (SOD)-like activity. We developed a novel screening method involving phage display technology to identify novel peptides capable of inhibiting Abeta's neurotoxicity. Two random peptide libraries containing 6-mer and 15-mer peptide inserts were used and resulted in the identification of 25 peptides that bound human Abeta (40 or 42). Here, we show that two of the three most enriched peptides obtained significantly reduced Abeta42's SOD-like activity. A 15-mer peptide reduced Abeta42 neurotoxicity in a dose-dependent manner as evidenced by a reduction in LDH release. These findings were confirmed in the independent MTT assay. Furthermore, comparative analysis of the 15-mer peptide with Clioquinol, a known inhibitor of Abeta's metal-mediated redox activity, showed the 15-mer peptide to be equipotent to this metal chelator, under the same experimental conditions. These agents represent novel peptides that selectively target and neutralise Abeta-induced neurotoxicity and thus provide promising leads for rational drug development.

Examination of the current top candidate genes for AD in a genome-wide association study.

With the advent of technologies that allow simultaneous genotyping of thousands of single-nucleotide polymorphisms (SNPs) across the genome, the genetic contributions to complex diseases can be explored at an unprecedented detail. This study is among the first to apply the genome-wide association study (GWAS) approach to Alzheimer disease (AD). We present our GWAS results from the German population for genes included in the 'Top Results' list on the AlzGene database website. In addition to the apolipoprotein E locus, we identified nominally significant association signals in six of the ten genes investigated, albeit predominantly for SNPs other than those already published as being disease associated. Further, all of the four AD genes previously identified through GWAS also showed nominally significant association signals in our data. The results of our comparative study reinforce the necessity for replication and validation, not only of GWAS but also of candidate gene case-control studies, in different populations. Furthermore, cross-platform comparison of genotyping results can also identify new association signals. Finally, our data confirm that GWAS, regardless of the platform, are valuable for the identification of genetic variants associated with AD.

No association of lipase C polymorphisms with Alzheimer's disease.

Hepatic lipase, also known as hepatic triglyceride lipase (LIPC), much like the major genetic risk factor for Alzheimer's disease (AD), apolipoprotein E (APOE), is associated with altered lipid metabolism. As such this link makes LIPC a potential functional candidate for AD risk. Previously, three single nucleotide polymorphisms (SNPs) have been investigated in AD with a lack of association reported. To rule out a possible contribution of other variants in LIPC, located at 15q21-q23, we used a detailed fine mapping approach in a German case-control sample. Genotyping of 25 single nucleotide polymorphisms covering the complete LIPC gene and haplotypic analysis revealed no association with AD. Thus, we conclude that LIPC can be excluded as a major functional candidate gene conferring risk to AD.

Genetic analysis of MAPT haplotype diversity in frontotemporal dementia.

The H1 haplotype of the tau gene, MAPT, has been linked to the sporadic tauopathies corticobasal degeneration and progressive supranuclear palsy; however, there have been inconsistent findings regarding association with frontotemporal dementia (FTD). We investigated MAPT haplotype diversity, in 171 sporadic FTD and 186 healthy controls individuals, and report no single marker or haplotype association with increased risk or changes in age at onset. These findings do not support an association of MAPT with FTD but do not rule out its association with other tauopathies.

No association of vacuolar protein sorting 26 polymorphisms with Alzheimer's disease.

Using a case-control sample we evaluated a possible involvement of the Vacuolar protein sorting 26 (VPS26) gene in the pathogenesis of AD. VPS26 located at 10q22.1 denotes a retromer subunit functionally involved in the cellular trafficking of Memapsin 2 (BACE). Genotyping of eight single nucleotide polymorphisms covering the complete VPS26 gene and haplotypic analysis revealed no association with AD. Thus, we conclude that VPS26 can be excluded as a major positional and functional candidate gene conferring risk to AD.

Fine mapping of the MAPT locus using quantitative trait analysis identifies possible causal variants in Alzheimer's disease.

In addition to senile plaques, neurofibrillary tangles are characteristic of Alzheimer's disease (AD) pathology, suggesting a clear involvement of the microtubule-associated protein tau (MAPT) in AD. Recent findings, suggesting that the H1c haplotype is associated with increased risk, now also implicate MAPT genetically. In this study, we aim to clarify this association by a fine mapping approach using both a traditional phenotypic association analysis and a quantitative trait (QT) analysis using cerebrospinal fluid (CSF) tau protein levels in the German population. Here, we report that both methodologies identify that the H1c haplotype may play important role in AD (AD risk, P=0.007, uncorrected; CSF tau levels, P=0.027, uncorrected). Further, the use of a sliding window approach in the QT analysis allowed for the narrowing down of the region where a probable causal variant may be located. The data suggest that this may lie at or within close proximity to the rs242557 single nucleotide polymorphism as association with CSF tau levels seems to be primarily driven by rs242557 in a gene dosage-dependent manner (trend model: P=0.002, uncorrected). These findings provide functional evidence to support the genetic association of MAPT with AD.

Association study between the D10S1423 microsatellite marker and Alzheimer's disease.

Several studies have reported conflicting results concerning the genetic association between Alzheimer's disease (AD) and the microsatellite marker D10S1423 on chromosome 10p12-14. In an ethnically homogeneous German population of 422 patients with AD and 254 cognitively healthy controls, the 238-bp allele of the D10S1423 marker showed a weak, but after correction for multiple testing no longer significant association with AD (p = 0.015, uncorrected; p = 0.11, corrected). These findings do not support the presence of a relevant susceptibility locus for AD on chromosome 10p12-14.

Neurological soft signs are associated with APOE genotype, age and cognitive performance.

Neurodegeneration is associated with increased frequency of neurological soft signs (NSS). We designed the present study to investigate the association between NSS and subjective memory complaints, cognitive function and apolipoprotein E genotype in a community-dwelling sample of volunteers participating in an ongoing longitudinal program investigating predictors of cognitive decline. NSS were found to be associated with apolipoprotein E (APOE) epsilon4 genotype (p = 0.015), age (p = 0.012) and poor cognitive performance, as assessed by the Mini Mental State Examination (p = 0.053). There was no significant difference between subjects with and without memory complaints in relation to the frequency of NSS (p = 0.130). The association with age and the APOE epsilon4 genotype suggests that the systematic investigation of NSS may contribute to identify subjects at risk of clinically significant cognitive decline in later life.

Oestrogen replacement therapy may improve memory functioning in the absence of APOE epsilon4.

There is currently intense controversy regarding the use of hormone replacement therapy (HRT) in postmenopausal women, in relation to its therapeutic efficacy in Alzheimer's disease (AD). It has been suggested that the benefits of HRT may be modified by apolipoprotein E (APOE) genotype (the major genetic risk factor for AD). Here we report the findings of the first study designed to systematically explore the interaction of (a) oestrogen replacement therapy (ERT) and (b) possession of an epsilon4 allele of APOE on specific elements of episodic learning and memory that are commonly used indices of age-related cognitive decline. This data represents a cross-sectional analysis of the interaction of ERT and APOE genotype on learning and memory in a cohort of 181 healthy postmenopausal women [ERT users (n = 101, mean age 65.40 +/- 6.34); ERT non-users (n = 80, mean age 67.03 +/- 6.80)] residing in Perth, Western Australia. The highest level of learning (trials 2-5; P < 0.05) and memory (e.g. total number of items recalled; P < 0.05) performance was observed in women taking ERT who were not carriers of the APOE epsilon4 allele. APOEepsilon4 carriers receiving ERT performed no better on episodic memory testing than APOE epsilon4 carriers who were not receiving ERT. These cognitive differences related to genetic profile, were noted on both recall and recognition (P = 0.005) tests of memory. The findings have significance for evaluating whether and when ERT may be clinically indicated. Specifically, ERT may benefit the cognitive functioning of women not carrying the APOE epsilon4 allele.

Variation at the APOE -491 promoter locus is associated with altered brain levels of apolipoprotein E.

The apolipoprotein E (APOE, gene; apoE, protein) type 4 isoform is a well-established risk factor for late-onset Alzheimer's disease (AD), and new data suggest that APOE promoter polymorphisms might also modulate AD risk, perhaps by altering transcription of the APOE gene. The current study was undertaken to determine whether the presence of the APOE promoter -491AA genotype (that appears to increase the risk for AD) is associated with an increase in the levels of apoE in brain tissue. Among 40 control and 20 autopsy-confirmed AD brain samples, levels of apoE were increased in the frontal cortex of AD cases (P < 0.001), consistent with the well-recognized up-regulation of APOE expression in reactive astrocytes. Among controls, the -491A allele appeared to impart a gene dose-dependent effect on the levels of apoE in frontal cortex. The levels of apoE in the brains of AD patients with the -491AA genotype were increased as compared to control subjects with the same genotype (P< 0.001). These data support the notion that the -491AA APOE promoter genotype is associated with elevated brain apolipoprotein E levels, suggesting that the risk for AD may be modulated by the apoE protein level as well as by the apoE protein isoform.