Molecular genetics of Alzheimer's disease: an update

Latest advances in mechanisms of epileptic activity in Alzheimer's disease and dementia with Lewy Bodies

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) stand as the prevailing sources of neurodegenerative dementia, impacting over 55 million individuals across the globe. Patients with AD and DLB exhibit a higher prevalence of epileptic activity compared to those with other forms of dementia. Seizures can accompany AD and DLB in early stages, and the associated epileptic activity can contribute to cognitive symptoms and exacerbate cognitive decline. Aberrant neuronal activity in AD and DLB may be caused by several mechanisms that are not yet understood. Hyperexcitability could be a biomarker for early detection of AD or DLB before the onset of dementia. In this review, we compare and contrast mechanisms of network hyperexcitability in AD and DLB. We examine the contributions of genetic risk factors, Ca2+ dysregulation, glutamate, AMPA and NMDA receptors, mTOR, pathological amyloid beta, tau and α-synuclein, altered microglial and astrocytic activity, and impaired inhibitory interneuron function. By gaining a deeper understanding of the molecular mechanisms that cause neuronal hyperexcitability, we might uncover therapeutic approaches to effectively ease symptoms and slow down the advancement of AD and DLB.

Minding the Gap: Exploring Neuroinflammatory and Microglial Sex Differences in Alzheimer's Disease

Research into Alzheimer's Disease (AD) describes a link between AD and the resident immune cells of the brain, the microglia. Further, this suspected link is thought to have underlying sex effects, although the mechanisms of these effects are only just beginning to be understood. Many of these insights are the result of policies put in place by funding agencies such as the National Institutes of Health (NIH) to consider sex as a biological variable (SABV) and the move towards precision medicine due to continued lackluster therapeutic options. The purpose of this review is to provide an updated assessment of the current research that summarizes sex differences and the research pertaining to microglia and their varied responses in AD.

Saccharomyces cerevisiae as a research tool for RNA-mediated human disease

The budding yeast, Saccharomyces cerevisiae, has been used for decades as a powerful genetic tool to study a broad spectrum of biological topics. With its ease of use, economic utility, well-studied genome, and a highly conserved proteome across eukaryotes, it has become one of the most used model organisms. Due to these advantages, it has been used to study an array of complex human diseases. From broad, complex pathological conditions such as aging and neurodegenerative disease to newer uses such as SARS-CoV-2, yeast continues to offer new insights into how cellular processes are affected by disease and how affected pathways might be targeted in therapeutic settings. At the same time, the roles of RNA and RNA-based processes have become increasingly prominent in the pathology of many of these same human diseases, and yeast has been utilized to investigate these mechanisms, from aberrant RNA-binding proteins in amyotrophic lateral sclerosis to translation regulation in cancer. Here we review some of the important insights that yeast models have yielded into the molecular pathology of complex, RNA-based human diseases. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Whole-exome sequencing detected a novel APP variant in a Han-Chinese family with Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is an incurable and debilitating neurodegenerative disease that results in the progressive degeneration and death of nerve cells. Mutations in the APP gene, which encodes an amyloid precursor protein, is the strongest genetic risk factor for sporadic AD.

METHODS AND RESULTS

We studied the APP gene (NM_000484.3: c.2045A > T; p.E682V) variants carried by members of a family suffering from AD using whole-exome sequencing and Sanger sequencing.

CONCLUSION

In this study, we identified a new variant of the APP gene (NM_000484.3: c.2045A > T; p.E682V) in members of a family with AD. This provides potential targets for subsequent studies and information that can be used in genetic counselling.

APP/PS1 Gene-Environmental Cadmium Interaction Aggravates the Progression of Alzheimer's Disease in Mice via the Blood-Brain Barrier, Amyloid-β, and Inflammation

BACKGROUND

There is limited information about gene-environment interaction on the occurrence and the progression of Alzheimer's disease.

OBJECTIVE

To explore the effect of environmental low-dose cadmium (Cd) exposure on the progress of Alzheimer's disease and the underlining mechanism.

METHODS

We administered 1 mg/L, 10 mg/L cadmium chloride (treated groups), and water (control group) to C57BL/6J and APP/PS1 mice through drinking water, from one week before mating, until the offspring were sacrificed at 6 months of age. The behaviors, Cd level, blood-brain barrier (BBB) leakage, Aβ1-42 deposition, and inflammation expression were evaluated in these mice.

RESULTS

Mice of both genotypes had similar blood Cd levels after exposure to the same dose of Cd. The toxic effects of Cd on the two genotypes differed little in terms of neuronal histomorphology and BBB permeability. Cd caused a series of pathological morphological changes in the mouse brains and more fluorescent dye leakage at higher doses. Furthermore, the APP/PS1 mice had more severe damage than the C57BL/6J mice, based on the following five criteria. They were increasing anxiety-like behavior and chaos movement, spatial reference memory damage, Aβ plaque deposition in mouse brains, increasing microglia expression in the brain, and IL-6 higher expression in the cortex and in the serum.

CONCLUSION

Low-dose Cd exposure for 6 months increases Aβ plaque deposition and BBB permeability, exacerbates inflammatory responses, and activates microglia, in APP/PS1 mice. APP/PS1 gene-environmental Cd interaction aggravates the progression of Alzheimer's disease in mice.

PSEN2 Mutation Spectrum and Novel Functionally Validated Mutations in Alzheimer’s Disease: Data from PUMCH Dementia Cohort

Background: The established causative mutations in the APP, PSEN1, and PSEN2 can explain less than 1%,Alzheimer’s disease (AD) patients. Of the identified variants, the PSEN2 mutations are even less common. Objective: With the genetic study from the dementia cohort of Peking Union Medical College Hospital (PUMCH), we aim to illustrate the PSEN2 mutation spectrum and novel functionally validated mutations in Chinese AD patients. Methods: 702 AD participants, aged 30–85, were identified in PUMCH dementia cohort. They all received history inquiry, physical examination, biochemical test, cognitive evaluation, brain CT/MRI, and next-generation DNA sequencing. Functional analysis was achieved by transfection of the HEK293 cells with plasmids harboring the wild-type PSEN2 or candidate mutations. Results: Nine PSEN2 rare variants were found, including two reported (M239T, R62C) and seven novel variants (N141S, I368F, L396I, G117X, I146T, S147N, H220Y). The HEK293 cells transfected with the PSEN2 N141S, M239T, I368F plasmids showed higher Aβ 42 and Aβ 42/Aβ 40 levels relative to the wild-type PSEN2. The PSEN2 L396I, G117X, S147N, H220Y, and R62C did not alter Aβ 42, Aβ 40 levels, or Aβ 42/Aβ 40 ratio. 1.9%,(13/702) subjects harbored rare PSEN2 variants. 0.4%,(3/702) subjects carried pathogenic/likely pathogenic PSEN2 mutations. The three subjects with the functionally validated PSEN2 mutations were all familial early-onset AD patients. The common symptoms included amnesia and mental symptom. Additionally, the M239T mutation carrier presented with dressing apraxia, visuospatial agraphia, dyscalculia and visual mislocalization. Conclusion: The PSEN2 N141S, M239T, and I368F are functionally validated mutations.

A Possible Pathogenic PSEN2 Gly56Ser Mutation in a Korean Patient with Early-Onset Alzheimer's Disease

Early-onset Alzheimer’s disease (EOAD) is characterized by the presence of neurological symptoms in patients with Alzheimer’s disease (AD) before 65 years of age. Mutations in pathological genes, including amyloid protein precursor (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2), were associated with EOAD. Seventy-six mutations in PSEN2 have been found around the world, which could affect the activity of γ-secretase in amyloid beta processing. Here, a heterozygous PSEN2 point mutation from G to A nucleotide change at position 166 (codon 56; c.166G>A, Gly56Ser) was identified in a 64-year-old Korean female with AD with progressive cognitive memory impairment for the 4 years prior to the hospital visit. Hippocampal atrophy was observed from magnetic resonance imaging-based neuroimaging analyses. Temporal and parietal cortex hypometabolisms were identified using fluorodeoxyglucose positron emission tomography. This mutation was at the N-terminal portion of the presenilin 2 protein on the cytosolic side. Therefore, the serine substitution may have promoted AD pathogenesis by perturbing to the mutation region through altered phosphorylation of presenilin. In silico analysis revealed that the mutation altered protein bulkiness with increased hydrophilicity and reduced flexibility of the mutated region of the protein. Structural changes were likely caused by intramolecular interactions between serine and other residues, which may have affected APP processing. The functional study will clarify the pathogenicity of the mutation in the future.

Evaluation of Utilizing the Distinct Genes as Predictive Biomarkers in Late-Onset Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease that is characterized by a devastating decline in cognitive activities among all types of dementia, and it severely affects the quality of life. Late-onset AD (LOAD) occurs after the age of 65 years and develops sporadically. Although aging comes first along the main risk factors underlying LOAD, disease-causing susceptibility genes have been associated with disease pathogenesis. In our study, we included the genes PARP1 , POLB , HTRA2 , SLC1A2 , HS1BP3 , and DRD3 to be investigated in LOAD patients based on their expression levels. Within this framework, we aimed to determine the possible functions of these genes in the pathophysiology of the disease. We investigated whether the utilization of these genes as biomarkers in the early diagnosis of LOAD may help the treatment scheme to be applied in the clinic. We involved 50 individuals in the study and collected peripheral blood samples from the patients and control groups for molecular genetic analysis. Subsequently, RNA was extracted from the peripheral blood samples, and expression analyzes were performed using qualitative reverse transcription polymerase chain reaction. The results obtained were evaluated by using proper statistical methods. Our results demonstrated that there was no difference between patient and control groups in terms of HTRA2 , DRD3 , HS1BP3 , and POLB genes. The expression levels of the SLC1A2 and PARP1 genes were significantly lower in the patient group compared with the control group. In conclusion, we presume that the PARP1 and SLC1A2 genes can be utilized as molecular biomarkers for LOAD.

Regulation of the Late Onset alzheimer's Disease Associated HLA-DQA1/DRB1 Expression

(Genome-wide Association Studies) GWAS have identified ∼42 late-onset Alzheimer's disease (LOAD)-associated loci, each of which contains multiple single nucleotide polymorphisms (SNPs) in linkage disequilibrium (LD) and most of these SNPs are in the non-coding region of human genome. However, how these SNPs regulate risk gene expression remains unknown. In this work, by using a set of novel techniques, we identified 6 functional SNPs (fSNPs) rs9271198, rs9271200, rs9281945, rs9271243, and rs9271247 on the LOAD-associated HLA-DRB1/DQA1 locus and 42 proteins specifically binding to five of these 6 fSNPs. As a proof of evidence, we verified the allele-specific binding of GATA2 and GATA3, ELAVL1 and HNRNPA0, ILF2 and ILF3, NFIB and NFIC, as well as CUX1 to these five fSNPs, respectively. Moreover, we demonstrate that all these nine proteins regulate the expression of both HLA-DQA1 and HLA-DRB1 in human microglial cells. The contribution of HLA class II to the susceptibility of LOAD is discussed.

Beta Amyloid, Tau Protein, and Neuroinflammation: An Attempt to Integrate Different Hypotheses of Alzheimer’s Disease Pathogenesis

Abstract

Alzheimer’s disease (AD) is a neurodegenerative disease that inevitably results in dementia and death. Currently, there are no pathogenetically grounded methods for the prevention and treatment of AD, and all current treatment regimens are symptomatic and unable to significantly delay the development of dementia. The accumulation of β-amyloid peptide (Aβ), which is a spontaneous, aggregation-prone, and neurotoxic product of the processing of signaling protein APP (Amyloid Precursor Protein), in brain tissues, primarily in the hippocampus and the frontal cortex, was for a long time considered the main cause of neurodegenerative changes in AD. However, attempts to treat AD based on decreasing Aβ production and aggregation did not bring significant clinical results. More and more arguments are arising in favor of the fact that the overproduction of Aβ in most cases of AD is not the initial cause, but a concomitant event of pathological processes in the course of the development of sporadic AD. The concept of neuroinflammation has come to the fore, suggesting that inflammatory responses play the leading role in the initiation and development of AD, both in brain tissue and in the periphery. The hypothesis about the key role of neuroinflammation in the pathogenesis of AD opens up new opportunities in the search for ways to treat and prevent this socially significant disease.

Microglia in Neuroinflammation and Neurodegeneration: From Understanding to Therapy

Microglia are the resident macrophages of the central nervous system (CNS) acting as the first line of defense in the brain by phagocytosing harmful pathogens and cellular debris. Microglia emerge from early erythromyeloid progenitors of the yolk sac and enter the developing brain before the establishment of a fully mature blood-brain barrier. In physiological conditions, during brain development, microglia contribute to CNS homeostasis by supporting cell proliferation of neural precursors. In post-natal life, such cells contribute to preserving the integrity of neuronal circuits by sculpting synapses. After a CNS injury, microglia change their morphology and down-regulate those genes supporting homeostatic functions. However, it is still unclear whether such changes are accompanied by molecular and functional modifications that might contribute to the pathological process. While comprehensive transcriptome analyses at the single-cell level have identified specific gene perturbations occurring in the "pathological" microglia, still the precise protective/detrimental role of microglia in neurological disorders is far from being fully elucidated. In this review, the results so far obtained regarding the role of microglia in neurodegenerative disorders will be discussed. There is solid and sound evidence suggesting that regulating microglia functions during disease pathology might represent a strategy to develop future therapies aimed at counteracting brain degeneration in multiple sclerosis, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

The potential roles of genetic factors in predicting ageing-related cognitive change and Alzheimer's disease

Alzheimer's disease (AD) is a complex neurological disorder of uncertain aetiology, although substantial research has been conducted to explore important factors related to risk of onset and progression. Both lifestyle (e.g., complex mental stimulation, vascular health) and genetic factors (e.g., APOE, BDNF, PICALM, CLU, APP, PSEN1, PSEN2, and other genes) have been associated with AD risk. Despite more than thirty years of genetic research, much of the heritability of AD is not explained by measured loci. This suggests that the missing heritability of AD might be potentially related to rare variants, gene-environment and gene-gene interactions, and potentially epigenetic modulators. Moreover, while ageing is the most substantial factor risk for AD, there are limited longitudinal studies examining the association of genetic factors with decline in cognitive function due to ageing and the preclinical stages of this condition. This review summarises findings from currently available research on the genetic factors of ageing-related cognitive change and AD and suggests some future research directions.

Emerging genetic complexity and rare genetic variants in neurodegenerative brain diseases

Knowledge of the molecular etiology of neurodegenerative brain diseases (NBD) has substantially increased over the past three decades. Early genetic studies of NBD families identified rare and highly penetrant deleterious mutations in causal genes that segregate with disease. Large genome-wide association studies uncovered common genetic variants that influenced disease risk. Major developments in next-generation sequencing (NGS) technologies accelerated gene discoveries at an unprecedented rate and revealed novel pathways underlying NBD pathogenesis. NGS technology exposed large numbers of rare genetic variants of uncertain significance (VUS) in coding regions, highlighting the genetic complexity of NBD. Since experimental studies of these coding rare VUS are largely lacking, the potential contributions of VUS to NBD etiology remain unknown. In this review, we summarize novel findings in NBD genetic etiology driven by NGS and the impact of rare VUS on NBD etiology. We consider different mechanisms by which rare VUS can act and influence NBD pathophysiology and discuss why a better understanding of rare VUS is instrumental for deriving novel insights into the molecular complexity and heterogeneity of NBD. New knowledge might open avenues for effective personalized therapies.

Protein Homeostasis Networks and the Use of Yeast to Guide Interventions in Alzheimer's Disease

Alzheimer's Disease (AD) is a progressive multifactorial age-related neurodegenerative disorder that causes the majority of deaths due to dementia in the elderly. Although various risk factors have been found to be associated with AD progression, the cause of the disease is still unresolved. The loss of proteostasis is one of the major causes of AD: it is evident by aggregation of misfolded proteins, lipid homeostasis disruption, accumulation of autophagic vesicles, and oxidative damage during the disease progression. Different models have been developed to study AD, one of which is a yeast model. Yeasts are simple unicellular eukaryotic cells that have provided great insights into human cell biology. Various yeast models, including unmodified and genetically modified yeasts, have been established for studying AD and have provided significant amount of information on AD pathology and potential interventions. The conservation of various human biological processes, including signal transduction, energy metabolism, protein homeostasis, stress responses, oxidative phosphorylation, vesicle trafficking, apoptosis, endocytosis, and ageing, renders yeast a fascinating, powerful model for AD. In addition, the easy manipulation of the yeast genome and availability of methods to evaluate yeast cells rapidly in high throughput technological platforms strengthen the rationale of using yeast as a model. This review focuses on the description of the proteostasis network in yeast and its comparison with the human proteostasis network. It further elaborates on the AD-associated proteostasis failure and applications of the yeast proteostasis network to understand AD pathology and its potential to guide interventions against AD.

The Prevalence of Mild Cognitive Impairment in a Convenience Sample of 202 Gulf War Veterans

Gulf War Illness (GWI) is a chronic, multisymptom disorder estimated to affect approximately 25–32% of Gulf War veterans (GWVs). Cognitive dysfunction is a common symptom of GWI. On the continuum of cognitive decline, mild cognitive impairment (MCI) is conceptualized as a transitional phase between normal aging and dementia. Individuals with MCI exhibit cognitive decline but have relatively spared activities of daily function and do not meet criteria for dementia. The current study sought to investigate the prevalence of MCI in a convenience sample of 202 GWVs (median age: 52 years; 18% female). Twelve percent of the sample (median age: 48 years) had MCI according to an actuarial neuropsychological criterion, a rate materially higher than expected for this age group. GWVs with MCI also had a smaller hippocampal volume and a thinner parietal cortex, higher rates of current posttraumatic stress disorder and major depressive disorder compared to GWVs without MCI. Because people with MCI are more likely to progress to dementia compared to those with normal cognition, these results may portend future higher rates of dementia among deployed GWVs.

Contribution of homozygous and compound heterozygous missense mutations in VWA2 to Alzheimer's disease

Alzheimer's disease is the most frequent diagnosis of neurodegenerative dementia with early (≤65 years) and late (>65 years) onset ages in familial and sporadic patients. Causal mutations in 3 autosomal dominant Alzheimer genes, i.e. amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2), explain only 5%-10% of early-onset patients leaving the majority of patients genetically unresolved. To discover potential missing genetics, we used whole genome sequencing data of 17 early-onset patients with well-documented clinical diagnosis of Alzheimer's disease. In the discovery group, the mean onset age was 55.71 ± 6.83 years (range 37-65). Six patients had a brain autopsy and neuropathology confirmed Alzheimer's disease. Analysis of the genetic data identified in one patient a homozygous p.V366M missense mutation in the Von Willebrand factor A domain containing 2 gene (VWA2). Resequencing of the VWA2 coding region in an Alzheimer's disease patient cohort from Flanders-Belgium (n = 1148), including 152 early and 996 late onset patients, identified additional homozygous and compound heterozygous missense mutations in 1 early and 3 late-onset patients. Allele-sharing analysis identified common haplotypes among the compound heterozygous VWA2 mutation carriers, suggesting shared ancestors. Overall, we identified 5 patient carriers of homozygous or compound heterozygous missense mutations (5/1165; 0.43 %), 2 in early (2/169; 1.18 %) and 3 in late-onset (3/996; 0.30 %) patients. The frequencies of the homozygous and compound heterozygous missense mutations in patients are higher than expected from the frequencies calculated based on their combined single alleles. None of the homozygous/compound heterozygous missense mutation carriers had a family history of autosomal dominant Alzheimer's disease. Our findings suggest that homozygous and compound heterozygous missense mutations in VWA2 might contribute to the risk of Alzheimer's disease in sporadic patients.

Amyloid-β1-43 cerebrospinal fluid levels and the interpretation of APP, PSEN1 and PSEN2 mutations

Background

Alzheimer’s disease (AD) mutations in amyloid precursor protein (APP) and presenilins (PSENs) could potentially lead to the production of longer amyloidogenic Aβ peptides. Amongst these, Aβ_1–43 is more prone to aggregation and has higher toxic properties than the long-known Aβ_1–42. However, a direct effect on Aβ_1–43 in biomaterials of individuals carrying genetic mutations in the known AD genes is yet to be determined.

Methods

N = 1431 AD patients (n = 280 early-onset (EO) and n = 1151 late-onset (LO) AD) and 809 control individuals were genetically screened for APP and PSENs. For the first time, Aβ_1–43 levels were analysed in cerebrospinal fluid (CSF) of 38 individuals carrying pathogenic or unclear rare mutations or the common PSEN1 p.E318G variant and compared with Aβ_1–42 and Aβ_1–40 CSF levels.

The soluble sAPPα and sAPPβ species were also measured for the first time in mutation carriers.

Results

A known pathogenic mutation was identified in 5.7% of EOAD patients (4.6% PSEN1, 1.07% APP) and in 0.3% of LOAD patients. Furthermore, 12 known variants with unclear pathogenicity and 11 novel were identified. Pathogenic and unclear mutation carriers showed a significant reduction in CSF Aβ_1–43 levels compared to controls (p = 0.037; < 0.001). CSF Aβ_1–43 levels positively correlated with CSF Aβ_1–42 in both pathogenic and unclear carriers and controls (all p < 0.001). The p.E318G carriers showed reduced Aβ_1–43 levels (p < 0.001), though genetic association with AD was not detected. sAPPα and sAPPβ CSF levels were significantly reduced in the group of unclear (p = 0.006; 0.005) and p.E318G carriers (p = 0.004; 0.039), suggesting their possible involvement in AD. Finally, using Aβ_1–43 and Aβ_1–42 levels, we could re-classify as “likely pathogenic” 3 of the unclear mutations.

Conclusion

This is the first time that Aβ_1–43 levels were analysed in CSF of AD patients with genetic mutations in the AD causal genes. The observed reduction of Aβ_1–43 in APP and PSENs carriers highlights the pathogenic role of longer Aβ peptides in AD pathogenesis. Alterations in Aβ_1–43 could prove useful in understanding the pathogenicity of unclear APP and PSENs variants, a critical step towards a more efficient genetic counselling.

Side-by-side comparison of Notch- and C83 binding to γ-secretase in a complete membrane model at physiological temperature

γ-Secretase cleaves the C99 fragment of the amyloid precursor protein, leading to formation of aggregated β-amyloid peptide central to Alzheimer's disease, and Notch, essential for cell regulation. Recent cryogenic electron microscopy (cryo-EM) structures indicate major changes upon substrate binding, a β-sheet recognition motif, and a possible helix unwinding to expose peptide bonds towards nucleophilic attack. Here we report side-by-side comparison of the 303 K dynamics of the two proteins in realistic membranes using molecular dynamics simulations. Our ensembles agree with the cryo-EM data (full-protein Cα-RMSD = 1.62–2.19 Å) but reveal distinct presenilin helix conformation states and thermal β-strand to coil transitions of C83 and Notch100. We identify distinct 303 K hydrogen bond dynamics and water accessibility of the catalytic sites. The RKRR motif (1758–1761) contributes significantly to Notch binding and serves as a “membrane anchor” that prevents Notch displacement. Water that transiently hydrogen bonds to G1753 and V1754 probably represents the catalytic nucleophile. At 303 K, Notch and C83 binding induce different conformation states, with Notch mostly present in a closed state with shorter Asp–Asp distance. This may explain the different outcome of Notch and C99 cleavage, as the latter is more imprecise with many products. Our identified conformation states may aid efforts to develop conformation-selective drugs that target C99 and Notch cleavage differently, e.g. Notch-sparing γ-secretase modulators.

Distinct membrane dynamics and conformations of C83- and Notch-bound γ-secretase may aid the development of Notch-sparing treatments of Alzheimer's disease.

The Association between Neighborhood Amenities and Cognitive Function: Role of Lifestyle Activities

Many of the modifiable risk factors for dementia are lifestyle-related, and multidomain interventions tailored to individual lifestyles are recommended to prevent cognitive decline and dementia. However, studies of the relationship between the environment and cognitive function have shown that cognitive disorders and dementia are more prevalent in rural areas than in urban areas. The purpose of this study was to clarify the role of lifestyle activities on the association between neighborhood amenities and cognitive function. Our data were measured between August 2011 and February 2012. Participants comprised 3786 older adults (mean age: 71.5 years, standard deviation (SD) = ±5.2). We categorized neighborhood amenities as institutional resources that promote cognitively beneficial activities such as physical activity. We calculated the Walk Score^® for all participants using their home address and divided them into three groups. We assessed their 12 lifestyle activities performed outdoors. Cognitive function was measured via Mini-Mental Status Exam, word list memory, attention, executive function, and processing speed. We found that participants who were more likely to report many lifestyle activities were more likely to have normal cognition, even in areas where neighborhood amenities were scarce. The clinical significance of this study is that increased lifestyle activity contributes to the prevention of cognitive decline.

Exploring the Role of PSEN Mutations in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. Mutations of presenilin (PSEN) genes that encode presenilin proteins have been found as the vital causal factors for early-onset familial AD (FAD). AD pathological features such as memory loss, synaptic dysfunction, and formation of plaques have been successfully mimicked in the transgenic mouse models that coexpress FAD-related presenilin and amyloid precursor protein (APP) variants. γ-Secretase (GS) is an enzyme that plays roles in catalyzing intramembranous APP proteolysis to release pathogenic amyloid beta (Aβ). It has been found that presenilins can play a role as the GS's catalytic subunit. FAD-related mutations in presenilins can modify the site of GS cleavage in a way that can elevate the production of longer and highly fibrillogenic Aβ. Presenilins can interact with β-catenin to generate presenilin complexes. Aforesaid interactions have also been studied to observe the mutational and physiological activities in the catenin signal transduction pathway. Along with APP, GS can catalyze intramembrane proteolysis of various substrates that play a vital role in synaptic function. PSEN mutations can cause FAD with autosomal dominant inheritance and early onset of the disease. In this article, we have reviewed the current progress in the analysis of PSENs and the correlation of PSEN mutations and AD pathogenesis.

New Insights into the Molecular Bases of Familial Alzheimer's Disease

Like several neurodegenerative disorders, such as Prion and Parkinson diseases, Alzheimer's disease (AD) is characterized by spreading mechanism of aggregated proteins in the brain in a typical "prion-like" manner. Recent genetic studies have identified in four genes associated with inherited AD (amyloid precursor protein-APP, Presenilin-1, Presenilin-2 and Apolipoprotein E), rare mutations which cause dysregulation of APP processing and alterations of folding of the derived amyloid beta peptide (A). Accumulation and aggregation of A in the brain can trigger a series of intracellular events, including hyperphosphorylation of tau protein, leading to the pathological features of AD. However, mutations in these four genes account for a small of the total genetic risk for familial AD (FAD). Genome-wide association studies have recently led to the identification of additional AD candidate genes. Here, we review an update of well-established, highly penetrant FAD-causing genes with correlation to the protein misfolding pathway, and novel emerging candidate FAD genes, as well as inherited risk factors. Knowledge of these genes and of their correlated biochemical cascade will provide several potential targets for treatment of AD and aging-related disorders.

Proteostasis Failure in Neurodegenerative Diseases: Focus on Oxidative Stress

Protein homeostasis or proteostasis is an essential balance of cellular protein levels mediated through an extensive network of biochemical pathways that regulate different steps of the protein quality control, from the synthesis to the degradation. All proteins in a cell continuously turn over, contributing to development, differentiation, and aging. Due to the multiple interactions and connections of proteostasis pathways, exposure to stress conditions may cause various types of protein damage, altering cellular homeostasis and disrupting the entire network with additional cellular stress. Furthermore, protein misfolding and/or alterations during protein synthesis results in inactive or toxic proteins, which may overload the degradation mechanisms. The maintenance of a balanced proteome, preventing the formation of impaired proteins, is accomplished by two major catabolic routes: the ubiquitin proteasomal system (UPS) and the autophagy-lysosomal system. The proteostasis network is particularly important in nondividing, long-lived cells, such as neurons, as its failure is implicated with the development of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. These neurological disorders share common risk factors such as aging, oxidative stress, environmental stress, and protein dysfunction, all of which alter cellular proteostasis, suggesting that general mechanisms controlling proteostasis may underlay the etiology of these diseases. In this review, we describe the major pathways of cellular proteostasis and discuss how their disruption contributes to the onset and progression of neurodegenerative diseases, focusing on the role of oxidative stress.

Genetic Study of Alzheimer's Disease in Saudi Population

BACKGROUND

Alzheimer's disease (AD) is a chronic neurological disorder associated with mental decline and dementia. Several studies focused on investigating the molecular basis of the disease that led to the identification of several causative genes and risk associated alleles. Replication of these studies and findings from different populations is very important.

OBJECTIVE

Molecular assessment of a cohort of 117 familial and sporadic AD cases from Saudi Arabia.

METHODS

Comprehensive screening for point mutations was carried out by direct sequencing of coding regions in the three known AD causative genes: PSEN1, PSEN2, APP, as well as the AD associated gene SORL1. All patients were also genotyped for APOE alleles. In silico 3D protein structure analysis was performed for two novel SORL1 variants.

RESULTS

We identified a total of eight potential pathogenic missense variants in all studied genes. Five of these variants were not previously reported including four in SORL1 (p.Val297Met, p.Arg1084Cys, p.Asp1100Asn, and p.Pro1213Ser) and one in APP (p.Glu380Lys). The frequency of APOE-ɛ4 allele was 21.37% of total investigated cases. In silico 3D protein structure analysis of two SORL1 novel missense variants (p.Pro1213Ser and p.Arg1084Cys) suggested that these variants may affect the folding of the proteins and disturb their structure.

CONCLUSIONS

Our comprehensive analysis of the open reading frame of the known genes have identified potential pathogenic rare variants in 18/117 cases. We found that point mutations in AD main genes (PSEN1, PSEN2, and APP) were underrepresented in our cohort of patients. Our results confirm involvement of SORL1 in familial and sporadic AD cases.

Behavioural and psychological symptoms of dementia in mouse models of Alzheimer's disease-related pathology

Transgenic mouse models have been used extensively to model the cognitive impairments arising from Alzheimer's disease (AD)-related pathology. However, less is known about the relationship between AD-related pathology and the behavioural and psychological symptoms of dementia (BPSD) commonly presented by patients. This review discusses the BPSD-like behaviours recapitulated by several mouse models of AD-related pathology, including the APP/PS1, Tg2576, 3xTg-AD, 5xFAD, and APP23 models. Current evidence suggests that social withdrawal and depressive-like behaviours increase with progressive neuropathology, and increased aggression and sleep-wake disturbances are present even at early stages; however, there is no clear evidence to support increased anxiety-like behaviours, agitation (hyperactivity), or general apathy. Overall, transgenic mouse models of AD-related pathology recapitulate some of the BPSD-like behaviours associated with AD, but these behaviours vary by model. This reflects the patient population, where AD patients typically exhibit one or more BPSD, but rarely all symptoms at once. As a result, we suggest that transgenic mouse models are an important tool to investigate the pathology underlying BPSD in human AD patients.

A Twin Study of Sex Differences in Genetic Risk for All Dementia, Alzheimer's Disease (AD), and Non-AD Dementia

BACKGROUND

While sex differences in incidence of Alzheimer's disease (AD) and potential explanations have received considerable attention, less attention has been paid to possible sex differences in genetic risk for AD.

OBJECTIVE

We examined sex differences in genetic and environmental influences on disease risk and age at onset for All Dementia, AD Only, and Non-AD Dementia.

METHODS

Twin pairs were drawn from the Swedish Twin Registry. All Dementia analysis included 9,467 pairs; AD only, 8,696 pairs; and non-AD dementia, 8,195 pairs. APOE analyses included 1,740 individual twins with measured ɛ4 alleles. Dementia diagnoses were based on clinical workup and national health registry linkage.

RESULTS

Although within-pair correlations for All Dementia and AD Only were higher for women than for men, sex differences did not statistically differ for genetic or environmental etiology of All Dementia, AD Only, and Non-AD dementia. Similar results were observed when looking at specific genetic effects (APOEɛ4). Co-twin control analyses indicated that among twin pairs discordant for dementia, female twins without dementia had approximately 40% greater risk of developing dementia, compared with their male counterparts, in the 2-5 years following the first twin's diagnosis.

CONCLUSION

For All Dementia, AD Only, and Non-AD Dementia, genetic influences could be equated across sex. Co-twin analyses, however, suggest greater risk to female than to male co-twins of dementia cases even though sex differences in either genetic or shared environmental influences on the risk of dementia could not be differentiated.

A Review of the Familial Alzheimer's Disease Locus PRESENILIN 2 and Its Relationship to PRESENILIN 1

PRESENILIN 1 (PSEN1) and PRESENILIN 2 (PSEN2) genes are loci for mutations causing familial Alzheimer's disease (fAD). However, the function of these genes and how they contribute to fAD pathogenesis has not been fully determined. This review provides a summary of the overlapping and independent functions of the PRESENILINS with a focus on the lesser studied PSEN2. As a core component of the γ-secretase complex, the PSEN2 protein is involved in many γ-secretase-related physiological activities, including innate immunity, Notch signaling, autophagy, and mitochondrial function. These physiological activities have all been associated with AD progression, indicating that PSEN2 plays a particular role in AD pathogenesis.

APP, PSEN1, and PSEN2 Mutations in Asian Patients with Early-Onset Alzheimer Disease

The number of patients with Alzheimer's disease (AD) is rapidly increasing in Asia. Mutations in the amyloid protein precursor (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2) genes can cause autosomal dominant forms of early-onset AD (EOAD). Although these genes have been extensively studied, variant classification remains a challenge, highlighting the need to colligate mutations across populations. In this study, we performed a genetic screening for mutations in the APP, PSEN1, and PSEN2 genes in 200 clinically diagnosed EOAD patients across four Asian countries, including Thailand, Malaysia, the Philippines, and Korea, between 2009 and 2018. Thirty-two (16%) patients presented pathogenic APP, PSEN1, or PSEN2 variants; eight (25%), 19 (59%), and five (16%) of the 32 patients presented APP, PSEN1, and PSEN2 variants, respectively. Among the 21 novel and known non-synonymous variants, five APP variants were found in Korean patients and one APP variant was identified in a Thai patient with EOAD. Nine, two, and one PSEN1 mutation was found in a Korean patient, Malaysian siblings, and a Thai patient, respectively. Unlike PSEN1 mutations, PSEN2 mutations were rare in patients with EOAD; only three variants were found in Korean patients with EOAD. Comparison of AD-causative point mutations in Asian countries; our findings explained only a small fraction of patients, leaving approximately 84% (p = 0.01) of autosomal dominant pedigrees genetically unexplained. We suggest that the use of high-throughput sequencing technologies for EOAD patients can potentially improve our understanding of the molecular mechanisms of AD.

A novel V272D presenilin mutation associated with logopenia, disorientation, and apraxia in an autosomal-dominant Alzheimer's disease family

In the present study, a novel mutation in the presenilin 1 gene was discovered in an Iraq-native patient with early-onset Alzheimer's disease, who presented with speech impairment and memory decline at age 46 years. Magnetic resonance imaging showed a frontotemporal atrophy. Sanger sequencing identified a heterozygous T to A transversion at position 815 (c.815T>A) in the presenilin 1 gene (PSEN1), resulting in a novel missense mutation at codon 272 from valine to aspartate (V272D). We tested this PSEN1 mutation in vitro and found V272D resulted in an altered Aβ42/40 ratio.

Genetic analyses of early-onset Alzheimer's disease using next generation sequencing

Alzheimer’s disease (AD) is the most common type of neurodegenerative dementia, but the cause of AD remained poorly understood. Many mutations in the amyloid precursor protein (APP) and presenilin 1 and 2 (PSEN1 and PSEN2) have been reported as the pathogenic causes of early-onset AD (EOAD), which accounts for up to 5% of all AD cases. In this study, we screened familiar/de novo EOAD (n = 67) samples by next-generation sequencing (NGS) of a 50-gene panel, which included causative and possible pathogenic variants linked to neurodegenerative disorders. Remarkably, three missense mutations in PSEN1 (T119I, G209A, and G417A) and one known variant in PSEN2 (H169N) were discovered in 6% of the cases. Additionally, 67 missense mutations in susceptibility genes for late-onset AD were identified, which may be involved in cholesterol transport, inflammatory response, and β-amyloid modulation. We identified 70 additional novel and missense variants in other genes, such as MAPT, GRN, CSF1R, and PRNP, related to neurodegenerative diseases, which may represent overlapping clinical and neuropathological features with AD. Extensive genetic screening of Korean patients with EOAD identified multiple rare variants with potential roles in AD pathogenesis. This study suggests that individuals diagnosed with AD should be screened for other neurodegenerative disease-associated genes. Our findings expand the classic set of genes involved in neurodegenerative pathogenesis, which should be screened for in clinical trials. Main limitation of this study was the absence of functional assessment for possibly and probably pathogenic variants. Additional issues were that we could not perform studies on copy number variants, and we could not verify the segregation of mutations.

Plasma amyloid beta levels and platelet mitochondrial respiration in patients with Alzheimer's disease

OBJECTIVES

Altered amyloid metabolism and mitochondrial dysfunction play key roles in the development of Alzheimer's disease (AD). We asked whether an association exists between disturbed platelet mitochondrial respiration and the plasma concentrations of Aβ₄₀ and Aβ₄₂ in patients with AD.

DESIGN AND METHODS

Plasma Aβ₄₀ and Aβ₄₂ concentrations and mitochondrial respiration in intact and permeabilized platelets were measured in 50 patients with AD, 15 patients with vascular dementia and 25 control subjects. A pilot longitudinal study was performed to monitor the progression of AD in a subgroup 11 patients with AD.

RESULTS

The mean Aβ₄₀, Aβ₄₂ and Aβ₄₂/Aβ₄₀ levels were not significantly altered in patients with AD compared with controls. The mitochondrial respiratory rate in intact platelets was significantly reduced in patients with AD compared to controls, particularly the basal respiratory rate, maximum respiratory capacity, and respiratory reserve; however, the flux control ratio for basal respiration was increased. A correlation between the plasma Aβ₄₂ concentration and mitochondrial respiration in both intact and permeabilized platelets differs in controls and patients with AD.

CONCLUSIONS

Based on our data, (1) mitochondrial respiration in intact platelets, but not the Aβ level itself, may be included in a panel of biomarkers for AD; (2) dysfunctional mitochondrial respiration in platelets is not explained by changes in plasma Aβ concentrations; and (3) the association between mitochondrial respiration in platelets and plasma Aβ levels differs in patients with AD and controls. The results supported the hypothesis that mitochondrial dysfunction is the primary factor contributing to the development of AD.

BACE-1 and γ-Secretase as Therapeutic Targets for Alzheimer's Disease

Alzheimer’s disease (AD) is a growing global health concern with a massive impact on affected individuals and society. Despite the considerable advances achieved in the understanding of AD pathogenesis, researchers have not been successful in fully identifying the mechanisms involved in disease progression. The amyloid hypothesis, currently the prevalent theory for AD, defends the deposition of β-amyloid protein (Aβ) aggregates as the trigger of a series of events leading to neuronal dysfunction and dementia. Hence, several research and development (R&D) programs have been led by the pharmaceutical industry in an effort to discover effective and safety anti-amyloid agents as disease modifying agents for AD. Among 19 drug candidates identified in the AD pipeline, nine have their mechanism of action centered in the activity of β or γ-secretase proteases, covering almost 50% of the identified agents. These drug candidates must fulfill the general rigid prerequisites for a drug aimed for central nervous system (CNS) penetration and selectivity toward different aspartyl proteases. This review presents the classes of γ-secretase and beta-site APP cleaving enzyme 1 (BACE-1) inhibitors under development, highlighting their structure-activity relationship, among other physical-chemistry aspects important for the successful development of new anti-AD pharmacological agents.

Perspectives on ethnic and racial disparities in Alzheimer's disease and related dementias: Update and areas of immediate need

Alzheimer's disease and related dementias (ADRDs) are a global crisis facing the aging population and society as a whole. With the numbers of people with ADRDs predicted to rise dramatically across the world, the scientific community can no longer neglect the need for research focusing on ADRDs among underrepresented ethnoracial diverse groups. The Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment (ISTAART; alz.org/ISTAART) comprises a number of professional interest areas (PIAs), each focusing on a major scientific area associated with ADRDs. We leverage the expertise of the existing international cadre of ISTAART scientists and experts to synthesize a cross-PIA white paper that provides both a concise "state-of-the-science" report of ethnoracial factors across PIA foci and updated recommendations to address immediate needs to advance ADRD science across ethnoracial populations.

Genetic screening in early-onset Alzheimer's disease identified three novel presenilin mutations

Mutations in presenilin 1 (PSEN1), presenilin 2 (PSEN2), and amyloid precursor protein (APP) are major genetic causes of early-onset Alzheimer's disease (EOAD). Clinical heterogeneity is frequently observed in patients with PSEN1 and PSEN2 mutations. Using whole exome sequencing, we screened a Dutch cohort of 68 patients with EOAD for rare variants in Mendelian Alzheimer's disease, frontotemporal dementia, and prion disease genes. We identified 3 PSEN1 and 2 PSEN2 variants. Three variants, 1 in PSEN1 (p.H21Profs*2) and both PSEN2 (p.A415S and p.M174I), were novel and absent in control exomes. These novel variants can be classified as probable pathogenic, except for PSEN1 (p.H21Profs*2) in which the pathogenicity is uncertain. The initial clinical symptoms between mutation carriers varied from behavioral problems to memory impairment. Our findings extend the mutation spectrum of EOAD and underline the clinical heterogeneity among PSEN1 and PSEN2 mutation carriers. Screening for Alzheimer's disease-causing genes is indicated in presenile dementia with an overlapping clinical diagnosis.

The intact postsynaptic protein neurogranin is reduced in brain tissue from patients with familial and sporadic Alzheimer's disease

Synaptic degeneration and neuronal loss are early events in Alzheimer's disease (AD), occurring long before symptom onset, thus making synaptic biomarkers relevant for enabling early diagnosis. The postsynaptic protein neurogranin (Ng) is a cerebrospinal fluid (CSF) biomarker for AD, also in the prodromal phase. Here we tested the hypothesis that during AD neurodegeneration, processing of full-length Ng into endogenous peptides in the brain is increased. We characterized Ng in post-mortem brain tissue and investigated the levels of endogenous Ng peptides in relation to full-length protein in brain tissue of patients with sporadic (sAD) and familial Alzheimer's disease (fAD), healthy controls and individuals who were cognitively unaffected but amyloid-positive (CU-AP) in two different brain regions. Brain tissue from parietal cortex [sAD (n = 10) and age-matched controls (n = 10)] and temporal cortex [sAD (n = 9), fAD (n = 10), CU-AP (n = 13) and controls (n = 9)] were included and all the samples were analyzed by three different methods. Using high-resolution mass spectrometry, 39 endogenous Ng peptides were identified while full-length Ng was found to be modified including disulfide bridges or glutathione. In sAD parietal cortex, the ratio of peptide-to-total full-length Ng was significantly increased for eight endogenous Ng peptides compared to controls. In the temporal cortex, several of the peptide-to-total full-length Ng ratios were increased in both sAD and fAD cases compared to controls and CU-AP. This finding was confirmed by western blot, which mainly detects full-length Ng, and enzyme-linked immunosorbent assay, most likely detecting a mix of peptides and full-length Ng. In addition, Ng was significantly associated with the degree of amyloid and tau pathology. These results suggest that processing of Ng into peptides is increased in AD brain tissue, which may reflect the ongoing synaptic degeneration, and which is also mirrored as increased levels of Ng peptides in CSF.

Genetic Markers of Alzheimer's Disease

Alzheimer's disease is a complex and heterogeneous, severe neurodegenerative disorder and the predominant form of dementia, characterized by cognitive disturbances, behavioral and psychotic symptoms, progressive cognitive decline, disorientation, behavioral changes, and death. Genetic background of Alzheimer's disease differs between early-onset familial Alzheimer's disease, other cases of early-onset Alzheimer's disease, and late-onset Alzheimer's disease. Rare cases of early-onset familial Alzheimer's diseases are caused by high-penetrant mutations in genes coding for amyloid precursor protein, presenilin 1, and presenilin 2. Late-onset Alzheimer's disease is multifactorial and associated with many different genetic risk loci (>20), with the apolipoprotein E ε4 allele being a major genetic risk factor for late-onset Alzheimer's disease. Genetic and genomic studies offer insight into many additional genetic risk loci involved in the genetically complex nature of late-onset Alzheimer's disease. This review highlights the contributions of individual loci to the pathogenesis of Alzheimer's disease and suggests that their exact contribution is still not clear. Therefore, the use of genetic markers of Alzheimer's disease, for monitoring development, time course, treatment response, and prognosis of Alzheimer's disease, is still far away from the clinical application, because the contribution of genetic variations to the relative risk of developing Alzheimer's disease is limited. In the light of prediction and prevention of Alzheimer's disease, a novel approach could be found in the form of additive genetic risk scores, which combine additive effects of numerous susceptibility loci.

Exercise and the Aging Brain: Considerations for Sex Differences

Engaging in targeted exercise interventions is a promising, non-pharmacological strategy to mitigate the deleterious effects of aging and disease on brain health. However, despite its therapeutic potential, a large amount of variation exists in exercise efficacy in older adults aged 55 and older. In this review, we present the argument that biological sex may be an important moderator of the relationship between physical activity and cognition. Sex differences exist in dementia as well as in several associated risk factors, including genetics, cardiovascular factors, inflammation, hormones and social and psychological factors. Different exercise interventions, such as aerobic training and resistance training, influence cognition and brain health in older adults and these effects may be sex-dependent. The biological mechanisms underlying the beneficial effects of exercise on the brain may be different in males and females. Specifically, we examine sex differences in neuroplasticity, neurotrophic factors and physiological effects of exercise to highlight the possible mediators of sex differences in exercise efficacy on cognition. Future studies should address the potential sex difference in exercise efficacy if we are to develop effective, evidence-based exercise interventions to promote healthy brain aging for all individuals.

Identification of a novel mutation in APP gene in a Thai subject with early-onset Alzheimer's disease

INTRODUCTION

Early-onset Alzheimer's disease (AD) accounts for than less 1% of all AD cases, with large variation in the reported genetic contributions of known dementia genes. Mutations in the amyloid precursor protein (APP) gene were the first to be recognized as the cause of AD.

METHODS

Here, a male patient with probable early-onset AD at the age of 55 years from Thailand was investigated by next-generation sequencing.

RESULTS

A novel mutation in exon 14 of APP (c.1810C>T, p.V604M) was found. He initially illustrated the clinical manifestations of progressive nonfluent aphasia in 2011. However, he was finally diagnosed with AD presenting logopenic aphasia in 2013. The follow-up magnetic resonance imaging scan showed progression of hippocampal trophy compared with the initial image. A 3D protein structure model revealed that V604M exchange could result in significant changes in the APP protein due to the increased hydrophobicity of methionine in the helix, which could result in altering of the APP functions.

CONCLUSION

Additional studies to characterize APP p.V604M are necessary to further understand the effects of this mutation.

Linkage analysis of multiplex Caribbean Hispanic families loaded for unexplained early-onset cases identifies novel Alzheimer's disease loci

Introduction

Less than 10% of early-onset Alzheimer's disease (EOAD) is explained by known mutations.

Methods

We conducted genetic linkage analysis of 68 well-phenotyped Caribbean Hispanic families without clear inheritance patterns or mutations in APP, PSEN1, and PSEN2 and with two or more individuals with EOAD.

Results

We identified 16 (logarithm of odds > 3.6) linked regions, including eight novel loci for EOAD (2p15, 5q14.1, 11p15.1, 13q21.22, 13q33.1, 16p12.1, 20p12.1, and 20q11.21) and eight regions previously associated with late-onset Alzheimer's disease. The strongest signal was observed at 16p12.1 (25 cM, 33 Mb; heterogeneity logarithm of odds = 5.3), ∼3 Mb upstream of the ceroid lipofuscinosis 3 (CLN3) gene associated with juvenile neuronal ceroid lipofuscinosis (JNCL), which functions in retromer trafficking and has been reported to alter intracellular processing of the amyloid precursor protein.

Discussion

This study supports the notion that the genetic architectures of unexplained EOAD and late-onset AD overlap partially, but not fully.

A pathogenic PSEN2 p.His169Asn mutation associated with early-onset Alzheimer's disease

Background

Autosomal dominant early-onset Alzheimer’s disease (EOAD) is genetically heterogeneous and has been associated with mutations in 3 different genes, coding for amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2). Most frequent cases are associated with mutations in the PSEN1 gene, whereas mutations in the APP and PSEN2 genes are rare.

Methods

Patient who presented progressive memory decline in her 50s was enrolled in this study. A broad battery of neuropsychological tests and neuroimaging was applied to make the diagnosis. Genetic tests were performed in the patient to evaluate possible mutations using next-generation sequencing (NGS). The pathogenic nature of missense mutation and its 3D protein structure prediction were performed by in silico prediction programs.

Results

A pathogenic mutation in the PSEN2 gene in a Korean patient associated with EOAD was identified. Targeted Next-generation sequencing and Sanger sequencing revealed a heterozygous C to A transition at position 505 (c.505C>A), resulting in a probably missense mutation at codon 169 (p.His169Asn) in PSEN2. PolyPhen-2 and SIFT software analyses predicted this mutation to be a probable damaging variant. This hypothesis was supported by the results of 3D in silico modelling analyses that predicted the p.His169Asn may result in major helix torsion due to histidine to asparagine substitution. Mutation may cause additional stresses with hydrophobic residues on the surface that interact inside the transmembrane domain III, which is a conserved domain in PSEN2 His169.

Conclusion

These findings revealed that the p.His169Asn might be an important residue in PSEN2, which may alter the functions of PSEN2, suggesting its potential involvement with AD phenotype. Future functional studies are needed to evaluate the role of PSEN2 p.His169Asn mutation in AD disease progression.

The genes associated with early-onset Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that accounts for the most cases of dementia, which is characterized by the deposition of dense plaques of amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of hyperphosphorylated tau. The two main types of AD can be classified as early-onset AD (EOAD, onset < 65 years) and late-onset AD (LOAD, onset ≥ 65 years). Evidence from family and twin studies indicate that genetic factors are estimated to play a role in at least 80% of AD cases. The first milestone with linkage analysis revealed the mutations in APP, PSEN1, and PSEN2 genes that cause EOAD. But pathogenic mutations in these three genes can only explain a small fraction of EOAD families. The additional disease-causing genes have not yet been identified. This review provides an overview of the genetic basis of EOAD and the relationship between the functions of these risk genes and the neuropathologic features of AD. A better understanding of genetic mechanisms underlying EOAD pathogenesis and the potentially molecular mechanisms of neurodegeneration will lead to the development of effective diagnosis and treatment strategies for this devastating disease.