Since 1992, the amyloid cascade hypothesis has played the prominent role in explaining the etiology and pathogenesis of Alzheimer's disease (AD). It proposes that the deposition of β-amyloid (Aβ) is ...the initial pathological event in AD leading to the formation of senile plaques (SPs) and then to neurofibrillary tangles (NFTs), neuronal cell death, and ultimately dementia. While there is substantial evidence supporting the hypothesis, there are also limitations: (1) SP and NFT may develop independently, and (2) SPs and NFTs may be the products rather than the causes of neurodegeneration in AD. In addition, randomized clinical trials that tested drugs or antibodies targeting components of the amyloid pathway have been inconclusive. This paper provides a critical overview of the evidence for and against the amyloid cascade hypothesis in AD and provides suggestions for future directions.
Whether cholesterol is implicated in the pathogenesis of Alzheimer’s disease (AD) is still controversial. Several studies that explored the association between lipids and/or lipid-lowering treatment ...and AD indicate a harmful effect of dyslipidemia on AD risk. The findings are supported by genetic linkage and association studies that have clearly identified several genes involved in cholesterol metabolism or transport as AD susceptibility genes, including apolipoprotein E (
APOE
), apolipoprotein J (
APOJ
,
CLU
), ATP-binding cassette subfamily A member 7(
ABCA7
), and sortilin-related receptor (
SORL1
). Functional cell biology studies further support a critical involvement of lipid raft cholesterol in the modulation of Aβ precursor protein processing by β-secretase and γ-secretase resulting in altered Aβ production. However, conflicting evidence comes from epidemiological studies showing no or controversial association between dyslipidemia and AD risk, randomized clinical trials observing no beneficial effect of statin therapy, and cell biology studies suggesting that there is little exchange between circulating and brain cholesterol, that increased membrane cholesterol level is protective by inhibiting loss of membrane integrity through amyloid cytotoxicity, and that cellular cholesterol inhibits colocalization of β-secretase 1 and Aβ precursor protein in nonraft membrane domains, thereby increasing generation of plasmin, an Aβ-degrading enzyme. The aim of this article is to provide a comprehensive review of the findings of epidemiological, genetic, and cell biology studies aiming to elucidate the role of cholesterol in the pathogenesis of AD.
Alzheimer's disease (AD), the most common form of dementia in western societies, is a pathologically and clinically heterogeneous disease with a strong genetic component. The recent advances in ...high-throughput genome technologies allowing for the rapid analysis of millions of polymorphisms in thousands of subjects has significantly advanced our understanding of the genomic underpinnings of AD susceptibility. During the last 5 years, genome-wide association and whole-exome- and whole-genome sequencing studies have mapped more than 20 disease-associated loci, providing insights into the molecular pathways involved in AD pathogenesis and hinting at potential novel therapeutic targets. This review article summarizes the challenges and opportunities of when using genomic information for the diagnosis and prognosis of AD.
The retromer coat complex is a vital component of the intracellular trafficking mechanism sorting cargo from the endosomes to the
trans
-Golgi network or to the cell surface. In recent years, genes ...encoding components of the retromer coat complex and members of the vacuolar protein sorting 10 (Vps10) family of receptors, which play pleiotropic functions in protein trafficking and intracellular/intercellular signaling in neuronal and non-neuronal cells and are primary cargos of the retromer complex, have been implicated as genetic risk factors for sporadic and autosomal dominant forms of several neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease and frontotemporal lobar degeneration. In addition to their functions in protein trafficking, the members of the Vps10 receptor family (sortilin, SorL1, SorCS1, SorCS2, and SorCS3) modulate neurotrophic signaling pathways. Both sortilin and SorCS2 act as cell surface receptors to mediate acute responses to proneurotrophins. In addition, sortilin can modulate the intracellular response to brain-derived neurotrophic factor (BDNF) by direct control of BDNF levels and regulating anterograde trafficking of Trk receptors to the synapse. This review article summarizes the emerging data from this rapidly growing field of intracellular trafficking signaling in the pathogenesis of neurodegeneration.
In recent years, genomic, animal and cell biology studies have implicated deficiencies in
retromer-mediated trafficking of proteins in an increasing number of neurodegenerative diseases including
...Alzheimer's Disease (AD), Parkinson's Disease (PD) and Frontotemporal Lobar Degeneration
(FTLD). The retromer complex, which is highly conserved across all eukaryotes, regulates the sorting
of transmembrane proteins out of endosomes to the cell surface or to the trans-Golgi network. Within
retromer, cargo selection and binding are performed by a trimer of the Vps26, Vps29 and Vps35 proteins,
named the “Cargo-Selective Complex (CSC)”. Sorting of cargo into tubules for distribution to
the trans-Golgi network or the cell surface is achieved through the dimeric sorting nexin (SNX) component
of retromer and accessory proteins such as the WASH complex which mediates the formation
of discrete endosomal tubules enabling the sorting of cargo into distinct pathways through production
of filamentous actin patches. In the present article, we review the molecular structure and function of
the retromer and summarize the evidence linking retromer dysfunction to neurodegenerative disease.
Purpose of Review
Early-onset Alzheimer’s disease (EOAD), defined as Alzheimer’s disease (AD) occurring before age 65, is significantly less well studied than the late-onset form (LOAD) despite EOAD ...often presenting with a more aggressive disease progression. The aim of this review is to summarize the current understanding of the etiology of EOAD, their translation into clinical practice, and to suggest steps to be taken to move our understanding forward.
Recent Findings
EOAD cases make up 5–10% of AD cases but only 10–15% of these cases show known mutations in the
APP
,
PSEN1
, and
PSEN2
, which are linked to EOAD. New data suggests that these unexplained cases following a non-Mendelian pattern of inheritance is potentially caused by a mix of common and newly discovered rare variants. However, only a fraction of this genetic variation has been identified to date leaving the molecular mechanisms underlying this type of AD and their association with clinical, biomarker, and neuropathological changes unclear.
Summary
While great advancements have been made in characterizing EOAD, much work is needed to disentangle the molecular mechanisms underlying this type of AD and to identify putative targets for more precise disease screening, diagnosis, prevention, and treatment.
The global prevalence of dementia is as high as 24 million, and has been predicted to quadruple by the year 2050. In the US alone, Alzheimer disease (AD) – the most frequent cause of dementia ...characterized by a progressive decline in cognitive function in particular the memory domain – causes estimated health-care costs of $ 172 billion per year. Key neuropathological hallmarks of the AD brain are diffuse and neuritic extracellular amyloid plaques – often surrounded by dystrophic neurites – and intracellular neurofibrillary tangles. These pathological changes are frequently accompanied by reactive microgliosis and loss of neurons, white matter and synapses. The etiological mechanisms underlying these neuropathological changes remain unclear, but are probably caused by both environmental and genetic factors. In this review article, we provide an overview of the epidemiology of AD, review the biomarkers that may be used for risk assessment and in diagnosis, and give suggestions for future research.
Objective
Common single nucleotide polymorphisms in the SORL1 gene have been associated with late onset Alzheimer disease (LOAD), but causal variants have not been fully characterized nor has the ...mechanism been established. The study was undertaken to identify functional SORL1 mutations in patients with LOAD.
Methods
This was a family‐ and cohort‐based genetic association study. Caribbean Hispanics with familial and sporadic LOAD and similarly aged controls were recruited from the United States and the Dominican Republic, and patients with sporadic disease of Northern European origin were recruited from Canada. Prioritized coding variants in SORL1 were detected by targeted resequencing and validated by genotyping in additional family members and unrelated healthy controls. Variants transfected into human embryonic kidney 293 cell lines were tested for Aβ40 and Aβ42 secretion, and the amount of the amyloid precursor protein (APP) secreted at the cell surface was determined.
Results
Seventeen coding exonic variants were significantly associated with disease. Two rare variants (rs117260922‐E270K and rs143571823‐T947M) with minor allele frequency (MAF) < 1% and 1 common variant (rs2298813‐A528T) with MAF = 14.9% segregated within families and were deemed deleterious to the coding protein. Transfected cell lines showed increased Aβ40 and Aβ42 secretion for the rare variants (E270K and T947M) and increased Aβ42 secretion for the common variant (A528T). All mutants increased the amount of APP at the cell surface, although in slightly different ways, thereby failing to direct full‐length APP into the retromer‐recycling endosome pathway.
Interpretation
Common and rare variants in SORL1 elevate the risk of LOAD by directly affecting APP processing, which in turn can result in increased Aβ40 and Aβ42 secretion. Ann Neurol 2015;77:215–227
The ApoE ε4 allele is the most significant genetic risk factor for late-onset Alzheimer disease. The risk conferred by ε4, however, differs across populations, with populations of African ancestry ...showing lower ε4 risk compared to those of European or Asian ancestry. The cause of this heterogeneity in risk effect is currently unknown; it may be due to environmental or cultural factors correlated with ancestry, or it may be due to genetic variation local to the ApoE region that differs among populations. Exploring these hypotheses may lead to novel, population-specific therapeutics and risk predictions. To test these hypotheses, we analyzed ApoE genotypes and genome-wide array data in individuals from African American and Puerto Rican populations. A total of 1,766 African American and 220 Puerto Rican individuals with late-onset Alzheimer disease, and 3,730 African American and 169 Puerto Rican cognitively healthy individuals (> 65 years) participated in the study. We first assessed average ancestry across the genome ("global" ancestry) and then tested it for interaction with ApoE genotypes. Next, we assessed the ancestral background of ApoE alleles ("local" ancestry) and tested if ancestry local to ApoE influenced Alzheimer disease risk while controlling for global ancestry. Measures of global ancestry showed no interaction with ApoE risk (Puerto Rican: p-value = 0.49; African American: p-value = 0.65). Conversely, ancestry local to the ApoE region showed an interaction with the ApoE ε4 allele in both populations (Puerto Rican: p-value = 0.019; African American: p-value = 0.005). ApoE ε4 alleles on an African background conferred a lower risk than those with a European ancestral background, regardless of population (Puerto Rican: OR = 1.26 on African background, OR = 4.49 on European; African American: OR = 2.34 on African background, OR = 3.05 on European background). Factors contributing to the lower risk effect in the ApoE gene ε4 allele are likely due to ancestry-specific genetic factors near ApoE rather than non-genetic ethnic, cultural, and environmental factors.
Objective
To detect rare coding variants underlying loci detected by genome‐wide association studies (GWAS) of late onset Alzheimer disease (LOAD).
Methods
We conducted targeted sequencing of ABCA7, ...BIN1, CD2AP, CLU, CR1, EPHA1, MS4A4A/MS4A6A, and PICALM in 3 independent LOAD cohorts: 176 patients from 124 Caribbean Hispanics families, 120 patients and 33 unaffected individuals from the 129 National Institute on Aging LOAD Family Study; and 263 unrelated Canadian individuals of European ancestry (210 sporadic patients and 53 controls). Rare coding variants found in at least 2 data sets were genotyped in independent groups of ancestry‐matched controls. Additionally, the Exome Aggregation Consortium was used as a reference data set for population‐based allele frequencies.
Results
Overall we detected a statistically significant 3.1‐fold enrichment of the nonsynonymous mutations in the Caucasian LOAD cases compared with controls (p = 0.002) and no difference in synonymous variants. A stop‐gain mutation in ABCA7 (E1679X) and missense mutation in CD2AP (K633R) were highly significant in Caucasian LOAD cases, and mutations in EPHA1 (P460L) and BIN1 (K358R) were significant in Caribbean Hispanic families with LOAD. The EPHA1 variant segregated completely in an extended Caribbean Hispanic family and was also nominally significant in the Caucasians. Additionally, BIN1 (K358R) segregated in 2 of the 6 Caribbean Hispanic families where the mutations were discovered.
Interpretation
Targeted sequencing of confirmed GWAS loci revealed an excess burden of deleterious coding mutations in LOAD, with the greatest burden observed in ABCA7 and BIN1. Identifying coding variants in LOAD will facilitate the creation of tractable models for investigation of disease‐related mechanisms and potential therapies. Ann Neurol 2015;78:487–498
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