老年痴呆的遗传机制研究进展

老年痴呆症,又称阿尔茨海默病(Alzheimer’s disease,AD),是威胁老年人健康的主要疾病之一。根据发病年龄,AD可分为早发性(early-onset Alzheimer’s disease,EOAD)和迟发性(late-onset Alzheimer’s disease,LOAD)两种,两者均受到遗传因素的影响。目前已知3个致病基因导致家族性EOAD的发病:淀粉样前体蛋白基因(β-amyloid precursor protein,APP)、早老素1基因(presenilin 1,PSEN1)和早老素2基因(presenilin 2,PSEN2)。而近年来在全基因组关联分析(genome-wide association study,GWAS)等新技术的支持下,研究者相继发现并报道了一系列影响LOAD易感性的风险基因多态性位点。试对上述AD相关致病基因和主要风险基因加以简要介绍,深入探索这些基因的功能有助于对AD病理生理机制的认知。     

Mutations of presenilin genes in dilated cardiomyopathy and heart failure

Two common disorders of the elderly are heart failure and Alzheimer disease (AD). Heart failure usually results from dilated cardiomyopathy (DCM). DCM of unknown cause in families has recently been shown to result from genetic disease, highlighting newly discovered disease mechanisms. AD is the most frequent neurodegenerative disease of older Americans. Familial AD is caused most commonly by presenilin 1 (PSEN1) or presenilin 2 (PSEN2) mutations, a discovery that has greatly advanced the field. The presenilins are also expressed in the heart and are critical to cardiac development. We hypothesized that mutations in presenilins may also be associated with DCM and that their discovery could provide new insight into the pathogenesis of DCM and heart failure. A total of 315 index patients with DCM were evaluated for sequence variation in PSEN1 and PSEN2. Families positive for mutations underwent additional clinical, genetic, and functional studies. A novel PSEN1 missense mutation (Asp333Gly) was identified in one family, and a single PSEN2 missense mutation (Ser130Leu) was found in two other families. Both mutations segregated with DCM and heart failure. The PSEN1 mutation was associated with complete penetrance and progressive disease that resulted in the necessity of cardiac transplantation or in death. The PSEN2 mutation showed partial penetrance, milder disease, and a more favorable prognosis. Calcium signaling was altered in cultured skin fibroblasts from PSEN1 and PSEN2 mutation carriers. These data indicate that PSEN1 and PSEN2 mutations are associated with DCM and heart failure and implicate novel mechanisms of myocardial disease.     

Genetic study of familial cases of Alzheimer's disease

A small number (1-5%) of Alzheimer's disease (AD) cases associated with the early-onset form of the disease (EOAD) appears to be transmitted as a pure genetic, autosomal dominant trait. To date, three genes responsible for familial EOAD have been identified in the human genome: amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). Mutations in these genes account for a significant fraction (18 to 50%) of familial cases of early onset AD. The mutations affect APP processing causing increased production of the toxic Abeta42 peptide. According to the "amyloid cascade hypothesis", aggregation of the Abeta42 peptide in brain is a primary event in AD pathogenesis. In our study of twenty AD patients with a positive family history of dementia, 15% (3 of 20) of the cases could be explained by coding sequence mutations in the PS1 gene. Although a frequency of PS1 mutations is less than 2% in the whole population of AD patients, their detection has a significant diagnostic value for both genetic counseling and treatment in families with AD.     

Genetic markers for diagnosis and pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly and represents an important and increasing clinical challenge in terms of diagnosis and treatment. Mutations in the genes encoding amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) are responsible for early-onset autosomal dominant AD. The ε4 allele of the apolipoprotein E (APOE) gene has been recognized as a major genetic risk factor for the more common, complex, late-onset AD. Fibrillar deposits by phosphorylated tau are also a key pathological feature of AD. The retromer complex also has been reported to late-onset AD. More recently, genome-wide association studies (GWASs) identified putative novel candidate genes associated with late-onset AD. Lastly, several studies showed that circulating microRNAs (miRNAs) in the cerebrospinal fluid (CSF) and blood serum of AD patients can be used as biomarkers in AD diagnosis. This review addresses the advances and challenges in determining genetic and diagnostic markers for complex AD pathogenesis.     

Molecular genetics of Alzheimer's disease: presenilin 1 gene analysis in a cohort of patients from the Poznań region

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory loss and personality changes. Pathological hallmarks of AD are: deposition of amyloid plaques and neurofibrillary tangles in the brain, accompanied by neuronal and synaptic loss. The genetic background of AD is heterogeneous and strongly depends on the form of the disease. In most of the families with early-onset AD (EOAD) (10% of the total population of patients), the disease segregates as an autosomal dominant fully penetrant trait. To date, some missense mutations in three genes encoding the amyloid precursor protein, presenilin 1 (PS1) and 2 (PS2) have been found to cause familial EOAD. We screened for mutations in the presenilin genes in a sample of 55 patients with familial or sporadic form of EOAD from the Poznan region. We found 4 missense mutations in the PS1 gene: A246E in exon 7, P267L in exon 8, E318G in exon 9, and L424R in exon 12 among 5 unrelated patients. The frequency of PS1 mutations was 11% (5 of 55) in the whole sample of the patients with EOAD or 50% (3 of 6) if the analysis was restricted to familial cases with a positive history of dementia in the patient's family.     

Presenilin-1 but not amyloid precursor protein mutations present in mouse models of Alzheimer's disease attenuate the response of cultured cells to γ-secretase modulators regardless of their potency and structure

γ-Secretase modulators (GSMs) inhibit the generation of amyloidogenic Aβ42 peptides and are promising agents for treatment or prevention of Alzheimer's disease (AD). Recently, a second generation of GSMs with favorable pharmacological properties has emerged, but preclinical studies to assess their efficacy in vivo are lacking. Such studies rely on transgenic mouse models that express amyloid precursor protein (APP) and presenilin (PSEN) mutations associated with early-onset familial AD. Previously, we have shown that certain PSEN1 mutations attenuated the response of cultured cells to GSMs and potentially confound in vivo studies in AD mouse models. However, different combinations of familial AD mutations might have synergistic or opposing effects, and we have now systematically determined the response of APP and PSEN1 mutations present in current AD models. Using a potent acidic GSM, we found that APP mutations, either single mutations or in combination, did not affect the potency of GSMs. In contrast, all PSEN1 mutations that have been used to accelerate pathological changes in AD models strongly attenuated the Aβ42-lowering activity of GSMs with two exceptions (M146L, A246E). Similar results were obtained with potent non-acidic GSMs indicating that the attenuating effect of PSEN1 mutations cannot simply be overcome by increased potency or structural changes. Notably, two non-acidic compounds fully compensated the attenuating effect of the PSEN1-G384A mutation. Taken together, our findings indicate that most AD models with rapid pathology and advanced phenotypes are unsuitable for preclinical GSM studies. However, we also provide evidence that additional compound screens could discover GSMs that are able to break the attenuating effects of PSEN mutations.     

A Presenilin-1 Mutation Identified in Familial Alzheimer Disease with Cotton Wool Plaques Causes a Nearly Complete Loss of γ-Secretase Activity

Mutations in presenilin-1 and presenilin-2 (PS1 and PS2) are the most common cause of familial Alzheimer disease. PS1 and PS2 are the presumptive catalytic components of the multisubunit γ-secretase complex, which proteolyzes a number of type I transmembrane proteins, including the amyloid precursor protein (APP) and Notch. APP processing by γ-secretase produces β-amyloid peptides (Aβ40 and Aβ42) that accumulate in the Alzheimer disease brain. Here we identify a pathogenic L435F mutation in PS1 in two affected siblings with early-onset familial Alzheimer disease characterized by deposition of cerebral cotton wool plaques. The L435F mutation resides in a conserved C-terminal PAL sequence implicated in active site conformation and catalytic activity. The impact of PS1 mutations in and around the PAL motif on γ-secretase activity was assessed by expression of mutant PS1 in mouse embryo fibroblasts lacking endogenous PS1 and PS2. Surprisingly, the L435F mutation caused a nearly complete loss of γ-secretase activity, including >90% reductions in the generation of Aβ40, Aβ42, and the APP and Notch intracellular domains. Two nonpathogenic PS1 mutations, P433L and L435R, caused essentially complete loss of γ-secretase activity, whereas two previously identified pathogenic PS1 mutations, P436Q and P436S, caused partial loss of function with substantial reductions in production of Aβ40, Aβ42, and the APP and Notch intracellular domains. These results argue against overproduction of Aβ42 as an essential property of presenilin proteins bearing pathogenic mutations. Rather, our findings provide support for the hypothesis that pathogenic mutations cause a general loss of presenilin function.     

Presenilins: how much more than γ-secretase?!

AD (Alzheimer's disease) is a neurodegenerative disease characterized by a gradual loss of neurons and the accumulation of neurotoxic Aβ (amyloid β-peptide) and hyperphosphorylated tau. The discovery of mutations in three genes, PSEN1 (presenilin 1), PSEN2 (presenilin 2) and APP (amyloid precursor protein), in patients with FAD (familial AD) has made an important contribution towards an understanding of the disease aetiology; however, a complete molecular mechanism is still lacking. Both presenilins belong to the γ-secretase complex, and serve as the catalytic entity needed for the final cleavage of APP into Aβ. PSEN only functions within the γ-secretase complex through intra- and inter-molecular interactions with three other membrane components, including nicastrin, Aph-1 (anterior pharynx defective-1) and Pen-2 (PSEN enhancer-2). However, although the list of γ-secretase substrates is still expanding, other non-catalytic activities of presenilins are also increasing the complexity behind its molecular contribution towards AD. These γ-secretase-independent roles are so far mainly attributed to PSEN1, including the transport of membrane proteins, cell adhesion, ER (endoplasmic reticulum) Ca(2+) regulation and cell signalling. In the present minireview, we discuss the current understanding of the γ-secretase-independent roles of PSENs and their possible implications in respect of AD.     

Alzheimer's disease. Present and future role of genetics

Research in the field of Alzheimer's disease has shown that genetic factors play an important role in the aetiology of the disease. Until now, four genes have been found to be implicated in Alzheimer's disease. Mutations in the amyloid precursor protein gene (APP) and the presenilin genes (PSEN1 en PSEN2) cause early onset Alzheimer's disease. These mutations segregate in an autosomal dominant pattern. The fourth gene involved in Alzheimer's disease is the apolipoprotein E gene (APOE). Carriers of the E4 variant of APOE have an increased risk of Alzheimer's disease. Being a carrier of this E4 variant increases the risk of both early- and late-onset Alzheimer's disease. Of the four Alzheimer-genes, APOE plays the most important role in the general population. Mutations in APP and the presenilin genes account for less than 1% of the prevalence of the disease in the general population compared to 10-17% for the APOE variation. Up till now the impact of genetics in daily clinical practice is very limited. However, genetics has caused major progress in molecular-biological knowledge, especially of the amyloid metabolism, creating optimism about novel biological markers and eventually therapeutic strategies. In Alzheimer's genetics break-throughs are to be expected using classical methods such as the candidate-gene or linkage approach. Novel strategies such as genetic research in isolated populations are promising.     

APP717, APP693, and PRIP gene mutations are rare in Alzheimer disease

The amyloid precursor protein (APP) gene codes for the precursor to the beta-protein found in the amyloid deposits of Alzheimer disease (AD). Recently Goate et al. identified in codon 717 of this gene a missense mutation which segregates with AD in a familial AD (FAD) kindred. The same mutation was also found in affected subjects from a second FAD family but not in other FAD families or in normal controls. The following work was undertaken to determine the frequency of the codon 717 mutation in FAD and nonfamilial AD cases and in normal controls. We tested 76 FAD families, 127 "sporadic" AD subjects, 16 Down syndrome cases, and 256 normal controls for this mutation, and none were positive. We also tested for the APP codon 693 mutation associated with hereditary cerebral hemorrhage with amyloidosis-Dutch type, for PRIP gene missense mutations at codons 102, 117, and 200, and for the PRIP insertion mutations which are associated with Creutzfeld-Jakob disease and Gerstmann-Straussler Scheinker syndrome. No examples of these mutations were found in our population. Thus these APP and PRIP mutations are rare in both FAD and nonfamilial AD.     

Evidence that the beta-catenin nuclear translocation assay allows for measuring presenilin 1 dysfunction

BACKGROUND: Mutations in the presenilin (PSEN) genes are responsible for the majority of early-onset Alzheimer disease (AD) cases. PSEN1 is a component of a high molecular weight, endoplasmic reticulum, membrane-bound protein complex, including beta-catenin. Pathogenic PSEN1 mutations were demonstrated to have an effect on beta-catenin and glycogen synthase kinase-3beta(GSK-3beta), two members of the wingless Wnt pathway. The nuclear translocation and the stability of beta-catenin, and the interaction between GSK3beta and PSEN1 were influenced. MATERIALS AND METHODS: Stably transfected human embryonic kidney (HEK) 293 cells overexpressing wild-type (wt) and mutant (mt) PSEN1, treated with and without LiCl, were used to isolate cytoplasmic and nuclear fractions. By Western blot analysis, endogenous beta-catenin levels were examined. By analyzing cytosolic fractions of PSEN1, transfected and nontransfected HEK 293 cells, and total brain extracts of AD patients and controls, we evaluated the effect of PSEN1 overexpression on beta-catenin stability. Finally, we analyzed the effect of pathogenic PSEN1 mutations on the interaction between PSEN1 and GSK3beta by co-immunoprecipitation experiments. RESULTS: We report reduced nuclear translocation of beta-catenin in cells stably expressing I143T, G384A, and T113-114ins PSEN1. The G384A PSEN1 mutation showed a similar pronounced effect on nuclear translocation of beta-catenin, as reported for processing of amyloid precursor protein (APP) into amyloid beta(Abeta). Overexpression of PSEN1 and the presence of pathogenic mutations in PSEN1 had no significant effect on the stability of beta-catenin. Nonspecific binding of overexpressed PSEN1 to endogenous GSK3beta was observed when GSK3beta was immunoprecipitated. Immunoprecipitation of PSEN1 in cells overexpressing PSEN1 and in native cells, however, did not result in co-immunoprecipitation of endogenous GSK3beta. CONCLUSION: Our results further establish the nuclear translocation assay of beta-catenin as an adequate alternative for traditional Abeta measurement to evaluate the effect of PSEN1 mutations on biochemical processes. We detected no significant effect of overexpressed wt or mt PSEN1 on the stability of beta-catenin. Finally, co-immunoprecipitation between PSEN1 and GSK3beta was not observed in our experimental setup.     

Genetic discoveries and advances in late-onset Alzheimer’s disease

Alzheimer’s disease (AD) is a heterogeneous disorder with multiple patterns of clinical manifestations. Recently, due to the advance of linkage studies, next-generation sequencing and genome-wide association studies, a large number of putative risk genes for AD have been identified using acquired genome mega data. The genetic association between three causal genes, including amyloid precursor protein, presenilin1, and presenilin2 in early-onset AD (EOAD), was discovered over the past few decades. These discoveries showed that there should be additional genetic risk factors for both EOAD and late-onset AD (LOAD) to help fully explain the leading molecular mechanisms in a single pathophysiological entity. This study reviews the clinical features and genetic etiology of LOAD and discusses a variety of AD-mediated genes that are involved in cholesterol and lipid metabolism, endocytosis, and immune response according to their mutations for more efficient selection of functional candidate genes for LOAD. New mechanisms and pathways have been identified as a result.     

Linkage and mutational analysis of familial Alzheimer disease kindreds for the APP gene region

A large number of familial Alzheimer disease (FAD) kindreds were examined to determine whether mutations in the amyloid precursor protein (APP) gene could be responsible for the disease. Previous studies have identified three mutations at APP codon 717 which are pathogenic for Alzheimer disease (AD). Samples from affected subjects were examined for mutations in exons 16 and 17 of the APP gene. A combination of direct sequencing and single-strand conformational polymorphism analysis was used. Sporadic AD and normal controls were also examined by the same methods. Five sequence variants were identified. One variant at APP codon 693 resulted in a Glu-->Gly change. This is the same codon as the hereditary cerebral hemorrhage with amyloidosis-Dutch type Glu-->Gln mutation. Another single-base change at APP codon 708 did not alter the amino acid encoded at this site. Two point mutations and a 6-bp deletion were identified in the intronic sequences surrounding exon 17. None of the variants could be unambiguously determined to be responsible for FAD. The larger families were also analyzed by testing for linkage of FAD to a highly polymorphic short tandem repeat marker (D21S210) that is tightly linked to APP. Highly negative LOD scores were obtained for the family groups tested, and linkage was formally excluded beyond theta = .10 for the Volga German kindreds, theta = .20 for early-onset non-Volga Germans, and theta = .10 for late-onset families. LOD scores for linkage of FAD to markers centromeric to APP (D21S1/S11, D21S13, and D21S215) were also negative in the three family groups. These studies show that APP mutations account for AD in only a small fraction of FAD kindreds.     

DBA/2J Genetic Background Exacerbates Spontaneous Lethal Seizures but Lessens Amyloid Deposition in a Mouse Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is a leading cause of dementia in the elderly and is characterized by amyloid plaques, neurofibrillary tangles (NFTs) and neuronal dysfunction. Early onset AD (EOAD) is commonly caused by mutations in amyloid precursor protein (APP) or genes involved in the processing of APP including the presenilins (e.g. PSEN1 or PSEN2). In general, mouse models relevant to EOAD recapitulate amyloidosis, show only limited amounts of NFTs and neuronal cell dysfunction and low but significant levels of seizure susceptibility. To investigate the effect of genetic background on these phenotypes, we generated APP^swe and PSEN1^de9 transgenic mice on the seizure prone inbred strain background, DBA/2J. Previous studies show that the DBA/2J genetic background modifies plaque deposition in the presence of mutant APP but the impact of PSEN1^de9 has not been tested. Our study shows that DBA/2J.APP^swePSEN1^de9 mice are significantly more prone to premature lethality, likely to due to lethal seizures, compared to B6.APP^swePSEN1^de9 mice—70% of DBA/2J.APP^swePSEN1^de9 mice die between 2-3 months of age. Of the DBA/2J.APP^swePSEN1^de9 mice that survived to 6 months of age, plaque deposition was greatly reduced compared to age-matched B6.APP^swePSEN1^de9 mice. The reduction in plaque deposition appears to be independent of microglia numbers, reactive astrocytosis and complement C5 activity.     

A population-based study of familial Alzheimer disease: linkage to chromosomes 14, 19, and 21.

Linkage of Alzheimer disease (AD) to DNA markers on chromosomes 14, 19, and 21 was studied in 10 families in which the disease was apparently inherited as an autosomal dominant trait. Families were derived from a Dutch population-based epidemiologic study of early-onset AD. Although in all probands the onset of AD was at or before age 65 years, the mean age at onset was after age 65 years in four families (referred to as "LOAD"). Among the six families with early-onset AD (referred to as "EOAD," i.e., mean age of onset of AD of relatives was at or before age 65 years), conclusive linkage to 14q24.3 was found in one family with a very early onset (around 47 years), while linkage to the same region was excluded in two other families. For the LOAD families, predominantly negative lod scores were obtained, and the overall lod score excluded linkage to chromosome 14. The results with markers on chromosome 19 and chromosome 21 were not conclusive for EOAD and LOAD. The findings of our study confirm genetic heterogeneity within familial EOAD.     

The gene encoding nicastrin, a major gamma-secretase component, modifies risk for familial early-onset Alzheimer disease in a Dutch population-based sample

Nicastrin regulates gamma-secretase cleavage of the amyloid precursor protein by forming complexes with presenilins, in which most mutations causing familial early-onset Alzheimer disease (EOAD) have been found. The gene encoding nicastrin (NCSTN) maps to 1q23, a region that has been linked and associated with late-onset Alzheimer disease (LOAD) in various genome screens. In 78 familial EOAD cases, we found 14 NCSTN single-nucleotide polymorphisms (SNPs): 10 intronic SNPs, 3 silent mutations, and 1 missense mutation (N417Y). N417Y is unlikely to be pathogenic, since it did not alter amyloid beta secretion in an in vitro assay and its frequency was similar in case and control subjects. However, SNP haplotype estimation in two population-based series of Dutch patients with EOAD (n=116) and LOAD (n=240) indicated that the frequency of one SNP haplotype (HapB) was higher in the group with familial EOAD (7%), compared with the LOAD group (3%) and control group (3%). In patients with familial EOAD without the APOE epsilon4 allele, the HapB frequency further increased, to 14%, resulting in a fourfold increased risk (odds ratio = 4.1; 95% confidence interval 1.2-13.3; P=.01). These results are compatible with an important role of gamma-secretase dysfunction in the etiology of familial EOAD.     

SEL-10 interacts with presenilin 1, facilitates its ubiquitination,and alters A-beta peptide production

Mutations in the human presenilin genes (PS1 or PS2) have been linked to autosomal dominant, early onset Alzheimer's disease (AD). Presenilins, probably as an essential part of gamma-secretase, modulate gamma-cleavage of the amyloid protein precursor (APP) to the amyloid beta-peptide (Abeta). Mutations in sel-12, a Caenorhabditis elegans presenilin homologue, cause a defect in egg laying that can be suppressed by loss of function mutations in a second gene, SEL-10. SEL-10 protein is a homologue of yeast Cdc4, a member of the SCF (Skp1-Cdc53/CUL1-F-box protein) E2-E3 ubiquitin ligase family. In this study, we show that human SEL-10 interacts with PS1 and enhances PS1 ubiquitination, thus altering cellular levels of unprocessed PS1 and its N- and C-terminal fragments. Co-transfection of sel-10 and APP cDNAs in HEK293 cells leads to an alteration in the metabolism of APP and to an increase in the production of amyloid beta-peptide, the principal component of amyloid plaque in Alzheimer's disease.     

New frontiers in Alzheimer's disease genetics.

Alzheimer's disease (AD) is a genetically complex disorder that accounts for the majority of dementia in the elderly population. Over 100 rare, highly penetrant mutations have been described in three genes (APP, PSEN1, PSEN2) for early-onset familial AD. In the more common late-onset form, a polymorphism in the apolipoprotein E gene has been associated with increased susceptibility. However, recent studies suggest that these four genes account for less than 30% of the genetic variance for AD and that more genetic factors remain to be identified. In this review, we present a brief history of AD genetics and preview some of the next frontiers in Alzheimer gene discovery primarily focusing on chromosomes 12, 10, and 9.     

Cholesterol impairs autophagy-mediated clearance of amyloid beta while promoting its secretion

Macroautophagy/autophagy failure with the accumulation of autophagosomes is an early neuropathological feature of Alzheimer disease (AD) that directly affects amyloid beta (Aβ) metabolism. Although loss of presenilin 1 function has been reported to impair lysosomal function and prevent autophagy flux, the detailed mechanism leading to autophagy dysfunction in AD remains to be elucidated. The resemblance between pathological hallmarks of AD and Niemann-Pick Type C disease, including endosome-lysosome abnormalities and impaired autophagy, suggests cholesterol accumulation as a common link. Using a mouse model of AD (APP-PSEN1-SREBF2 mice), expressing chimeric mouse-human amyloid precursor protein with the familial Alzheimer Swedish mutation (APP695swe) and mutant presenilin 1 (PSEN1-dE9), together with a dominant-positive, truncated and active form of SREBF2/SREBP2 (sterol regulatory element binding factor 2), we demonstrated that high brain cholesterol enhanced autophagosome formation, but disrupted its fusion with endosomal-lysosomal vesicles. The combination of these alterations resulted in impaired degradation of Aβ and endogenous MAPT (microtubule associated protein tau), and stimulated autophagy-dependent Aβ secretion. Exacerbated Aβ-induced oxidative stress in APP-PSEN1-SREBF2 mice, due to cholesterol-mediated depletion of mitochondrial glutathione/mGSH, is critical for autophagy induction. In agreement, in vivo mitochondrial GSH recovery with GSH ethyl ester, inhibited autophagosome synthesis by preventing the oxidative inhibition of ATG4B deconjugation activity exerted by Aβ. Moreover, cholesterol-enrichment within the endosomes-lysosomes modified the levels and membrane distribution of RAB7A and SNAP receptors (SNAREs), which affected its fusogenic ability. Accordingly, in vivo treatment with 2-hydroxypropyl-β-cyclodextrin completely rescued these alterations, making it a potential therapeutic tool for AD.