Recent advances in the genetics of Alzheimer's disease

New approaches to genetic counseling and testing for Alzheimer's disease and frontotemporal degeneration

The discovery of new autosomal dominant and susceptibility genes for Alzheimer's disease (AD) and frontotemporal degeneration (FTD) is revealing important new information about the neurodegenerative process and the risk for acquiring these diseases. It is becoming increasingly clear that both the mechanisms that drive these diseases and their phenotypes overlap. New technologies will assist access to genetic testing but may increase difficulty with genetic test interpretation. Thus, the process of genetic counseling and testing for these diseases is becoming more complex. This article will review current knowledge on the genetics of AD and FTD and suggest clinical guidelines for helping families to navigate through these complexities. The implications of future discoveries will be offered.

Genetics of Alzheimer disease

Alzheimer disease (AD) is the most common causes of neurodegenerative disorder in the elderly individuals. Clinically, patients initially present with short-term memory loss, subsequently followed by executive dysfunction, confusion, agitation, and behavioral disturbances. Three causative genes have been associated with autosomal dominant familial AD (APP, PSEN1, and PSEN2) and 1 genetic risk factor (APOEε4 allele). Identification of these genes has led to a number of animal models that have been useful to study the pathogenesis underlying AD. In this article, we provide an overview of the clinical and genetic features of AD.

APOE alleles in Parkinson disease and their relationship to cognitive decline: a population-based, longitudinal study

Apolipoprotein E gene alleles have been linked to various cardiovascular and neurodegenerative disorders. There have been conflicting reports of associations between Apolipoprotein E alleles and Parkinson disease and Parkinson disease dementia. To investigate the role of Apolipoprotein E alleles in Parkinson disease and Parkinson disease dementia, we have determined Apolipoprotein E genotypes in a group of patients with Parkinson disease (n = 95) and compared them with those of healthy control participants (n = 73). Additionally, in 64 longitudinally followed patients with Parkinson disease, the allele types were correlated to development and progression of dementia and to time from onset of Parkinson disease to dementia using multivariate and survival analyses. The Apolipoprotein E e4e4 genotype was more common in patients with Parkinson disease (7.4%) than in healthy controls (1.4%; P = .03). No significant associations between the Apolipoprotein E genotype and development and progression of dementia or time to dementia were found. More studies with larger Parkinson disease samples are warranted.

Effect of apolipoprotein E on the cerebral load of latent herpes simplex virus type 1 DNA

Herpes simplex virus type 1 (HSV-1) is neurotropic and enters a latent state lasting the lifetime of the host. This pathogen has recently been proposed as a risk factor for Alzheimer's disease (AD) in conjunction with apolipoprotein E4 (ApoE4). In a murine acute infection model, we showed that viral neuroinvasiveness depends directly on the overall ApoE dosage and especially on the presence of isoform ApoE4. If an interaction between ApoE and HSV-1 is involved in AD, it may occur during latency rather than during acute infection. Certainly, ApoE plays an important role in late-onset AD, i.e., at a time in life when the majority of people harbor HSV-1 in their nervous system. In the present work, wild-type, APOE knockout, APOE3, and APOE4 transgenic mice were used to analyze the influence of the ApoE profile on the levels of latent virus DNA. The knockout mice had significantly lower concentrations of the virus in the nervous system than the wild-type mice, while the APOE4 mice had very high levels in the brain compared to the APOE3 animals. ApoE4 seems to facilitate HSV-1 latency in the brain much more so than ApoE3. The APOE dosage correlated directly with the HSV-1 DNA concentration in the brain, strengthening the hypothesis that HSV-1, together with ApoE, might be involved in AD.

Liquid chromatography with tandem mass spectrometry-based proteomic discovery in aging and Alzheimer's disease

Systems biology offers enormous potential to understand the complexity of human brain aging and neurodegenerative diseases. Proteomics has an important role in these investigations because of its unique strengths and because of the potential central pathogenic contribution of pathological protein to several of these diseases. Here we have reviewed the methods and presented some examples of liquid chromatography-electrospray ionization-tandem mass spectrometry-based proteomics, with and without quantification using isotope-coded affinity tags, in the investigation of aging and Alzheimer's disease. As protocols and methods for improved quantitative high-throughput proteomics constantly improve, this approach will likely continue to provide deeper insight into human brain aging and neurodegenerative diseases.

A novel polymorphism in SEL1L confers susceptibility to Alzheimer's disease

Alzheimer's disease (AD) is considered to be a conformational disease arising from the accumulation of misfolded and unfolded proteins in the endoplasmic reticulum (ER). SEL1L is a component of the ER stress degradation system, which serves to remove unfolded proteins by retrograde degradation using the ubiquitin-proteosome system. In order to identify genetic variations possibly involved in the disease, we analysed the entire SEL1L gene sequence in Italian sporadic AD patients. Here we report on the identification of a new polymorphism within the SEL1L intron 3 (IVS3-88 A>G), which contains potential binding sites for transcription factors involved in ER-induced stress. Our statistical analysis shows a possible role of the novel polymorphism as independent susceptibility factor of Alzheimer's dementia.

Proteomic analysis of neurofibrillary tangles in Alzheimer disease identifies GAPDH as a detergent-insoluble paired helical filament tau binding protein

We performed proteomic analysis of neurofibrillary tangles (NFTs) obtained by laser capture microdissection from pyramidal neurons in hippocampal sector CA1 in patients with Alzheimer disease (AD) using liquid chromatography (LC)-mass spectrometry (MS)/MS. We discovered a total of 155 proteins in laser captured NFT's, 72 of which were identified by multiple unique peptides. Of these 72 proteins, 63 had previously unknown association with NFTs; one of these was glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We validated by immunohistochemistry that GAPDH co-localized with the majority of NFTs as well as plaque-like structures in AD brain and was co-immunoprecipitated by antibodies to abnormal forms of tau in AD, but not tau from AD temporal cortex. Characterization of GAPDH showed that it, along with phosphorylated tau and Abeta peptides, was present in detergent-insoluble fractions from AD temporal cortex but not from age-matched controls. These data are the first proteomic investigation of NFTs. Moreover, our results validate this approach by demonstrating that GAPDH, a glycolytic and microtubule binding protein, not only co-localized to NFTs and immunoprecipitated with PHF-tau, but also is one of the few proteins known to undergo conversion to a detergent-insoluble form in AD.

Amyloid-beta metal interaction and metal chelation

Alzheimer's disease (AD) is associated with the abnormal aggregation of amyloid-beta (Abeta) protein. Abeta and its precursor protein (APP) interact with metal ions such as zinc, copper and iron. Evidence shows that these metals play a role in the precipitation and cytotoxicity of Abeta. Despite recent advances in AD research, there is a lack of therapeutic agents to hinder the apparent aggregation and toxicity of Abeta. Recent studies show that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. Here we discuss the interaction of Abeta with metals, metal dyshomeostasis in the CNS of patients with AD, and the potential therapeutic effects of metal chelators.

A review of the evidence from family, twin and adoption studies for a genetic contribution to adult psychiatric disorders

Family, twin and adoption studies have provided major evidence for the role of genetics in numerous psychiatric disorders including obsessive-compulsive disorder, panic disorder, major depressive disorder, bipolar disorder, schizophrenia and Alzheimer's disease. As the search for patterns of inheritance and candidate genes of these complex disorders continues, we review relevant findings from quantitative genetic studies and outline the main challenges for the field of psychiatric genetics to focus on in order to more definitively establish the underpinnings of genetic and environmental influences of adult psychopathology.

Analysis of neurological disease in four dimensions: insight from ALS-PDC epidemiology and animal models

The causal factor(s) responsible for sporadic neurological diseases are unknown and the stages of disease progression remain undefined and poorly understood. We have developed an animal model of amyotrophic lateral sclerosis-parkinsonism dementia complex which mimics all the essential features of the disease with the initial neurological insult arising from neurotoxins contained in washed cycad seeds. Animals fed washed cycad develop deficits in motor, cognitive, and sensory behaviors that correlate with the loss of neurons in specific regions of the central nervous system. The ability to recreate the disease by exposure to cycad allows us to extend the model in multiple dimensions by analyzing behavioral, cellular, and biochemical changes over time. In addition, the ability to induce toxin-based neurodegeneration allows us to probe the interactions between genetic and epigenetic factors. Our results show that the impact of both genetic causal and susceptibility factors with the cycad neurotoxins are complex. The article describes the features of the model and suggests ways that our understanding of cycad-induced neurodegeneration can be used to decipher and identify the early events in various human neurological diseases.

Segregation analysis of phenotypic components of learning disabilities. II. Phonological decoding

Dyslexia is a common, complex disorder, which is thought to have a genetic component. The study of the genetics of dyslexia is complicated by a lack of consensus on diagnostic criteria, and the probability of genetic heterogeneity-it is possible that deficits in different language processes are caused by different underlying genes. In order to address these difficulties, we study continuous phenotypes that are part of the psychometric test batteries often used to diagnose dyslexia. Prior to embarking on a linkage study, it is helpful to employ segregation analysis, both to identify phenotypes that may be amenable to mapping by linkage analysis, and to determine the best models to use for model based analyses. We study 409 people in 102 nuclear families, and employ (1) oligogenic segregation analysis to estimate the number of quantitative trait loci (QTLs) contributing to each phenotype, and (2) complex segregation analysis in order to identify the most parsimonious inheritance model. In this paper, we consider two measures of phonological decoding ability-word attack and phonemic decoding efficiency. We find evidence for one or two genes of at least modest effect contributing to phonemic decoding efficiency, and the best fitting model is a dominant major gene model with residual familial correlations. For word attack, we find evidence for one or two genes of at least modest effect, and the variation in the trait is best explained by a polygenic model.

Enhanced production of amyloid precursor protein mRNA by peripheral mononuclear blood cell in Alzheimer's disease

Previous studies have suggested the involvement of amyloid precursor protein (APP) in Alzheimer's disease (AD), as exons 16 and 17 of the APP gene mutations have been found in some familial AD patients. Furthermore, overexpression and deposition of the beta amyloid peptide, a proteolytic product of APP, have been considered as a pathological hallmark of Alzheimer's disease. Therefore, it is of particular interest to determine the expression of APP gene at the transcription level for better understanding of the roles of APP gene in AD pathogenesis. In this work, we employed the quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to quantify APP mRNA transcripts in the peripheral mononuclear blood cells (PMBC) of 52 Alzheimer's patients, 28 vascular dementia (VD) patients, and 60 healthy elderly controls. The results showed that the amount (mean +/- SEM) of APP transcripts per microgram of total cDNA was 4.05 +/- 0.27, 2.73 +/- 0.33, and 2.59 +/- 0.27 amole in AD, VD, and healthy controls, respectively. There was a significant increase (P < 0.05) in the expression of APP mRNA transcripts in AD compared with that in VD or in healthy controls. Thus, our data indicated that variation of APP gene expression in PMBC might be a pathogenic source of Alzheimer's disease.

Reduction of beta-amyloid plaques in brain of transgenic mouse model of Alzheimer's disease by EFRH-phage immunization

Antibodies to the epitope EFRH, representing residues 3-6 within the beta-amyloid (Abeta) sequence, were previously shown to affect the solubility and disaggregation of Abeta fibrils in vitro. Here, we describe a novel method of immunization, using as antigen the EFRH peptide displayed on the surface of the filamentous phage. The EFRH phage evoked effective auto-immune antibodies in amyloid precursor protein [V717I] (APP[V717I]) transgenic mice that recapitulate the amyloid plaques and vascular pathology of Alzheimer's disease (AD). The immunization provoked a considerable reduction in the number of Abeta amyloid plaques in the brain of the transgenic mice and may serve as the basis for anti-Abeta vaccine.

Dementia epidemiology

Determining the incidence and prevalence of dementia is an inexact science. Dementia is difficult to define and detect in the population. Even with the difficulties of determining prevalence and incidence, it is clear that dementia causes a substantial burden on our society. Problems with diagnostic inaccuracy and insidious disease onset influence our ability to observe risk factor associations; factors related to survival may be mistaken for risk/protective factors. Current studies suggest that factors influencing brain development or cognitive reserve may delay the onset of AD, perhaps through a protective mechanism or a delay in diagnosis caused by improved performance on cognitive tests. The recent identification of genes that cause dementia suggests that these genes or their biochemical pathways may be involved in the pathogenesis of nonfamilial cases. The contribution of genes that cause disease in and of themselves may be smaller than that of genes that act to metabolize or potentiate environmental exposures. The interaction between gene and environment should be increasingly well studied in the future. Epidemiology must take advantage of these molecular advances. The tasks of public health and epidemiology should still involve prevention, the nonrandom occurrence of disease, and its environmental context in addition to heredity. The tools to address these tasks should continue to be refined.

Genetics of dementia

Many neurodegenerative diseases are exceedingly complex disorders (Fig. 6). In the past decade, we have made tremendous advances in our understanding [figure: see text] of the genetic basis of these disorders. One common characteristic of these disorders is the existence of rare families in which a given disease is inherited as a Mendelian trait. In this article, we have reviewed the genetics of several common neurodegenerative disorders that are associated with cognitive disturbances and for which causative genes have been identified. Further genetic analysis should clarify the roles of known genes in the pathogenesis of common sporadic forms of these various diseases. Investigation of the normal and aberrant functions of these genes should provide insight into the underlying mechanisms of these disorders. Such research should facilitate new strategies for therapeutic interventions. Although molecular genetics has helped to clarify the etiology of these disorders, clinicians have played a critical role in the careful identification and classification of many families who were involved in the eventual mapping and cloning of causative mutations. The role of the clinician should not be underestimated. Future clinical and molecular genetics findings hold many clinical implications. It is likely that new diagnostic and therapeutic strategies for dementing disorders are just on the horizon.

Pathogenic theories and intrathecal analysis of the sporadic form of Alzheimer's disease

Alzheimer's disease (AD) is an age-dependent dementia characterized by progressive loss of cognitive functions and by characteristic pathological changes in the brain: the formation of aggregates extracellularly by beta-amyloid (Abeta) peptide and intracellularly by tau proteins. The disease presents several major diagnostic difficulties: (1) AD develops slowly; (2) analysis of damaged brain tissues is difficult, requiring a biopsy which poses ethical problems; (3) no biochemical markers are available for the diagnosis and monitoring of the disease progression. Since the cerebrospinal fluid (CSF) is in contact with the extracellular space of the brain, many studies have tried to correlate the levels of the intrathecal peptides and amino acids and the development of dementia. The present review analyzes the main results of intrathecal content analyses in light of pathogenic theories proposed to explain the damage associated with AD and observed in the brain of patients by postmortem examination.

Age at onset of Parkinson disease and apolipoprotein E genotypes

Several lines of evidence suggest that the variable age at onset of Parkinson disease (PD) is likely influenced by genes. The apolipoprotein E (APOE) gene is associated with onset of Alzheimer disease, and possibly other neurodegenerative disorders. APOE has been investigated in relation to onset of PD, but results have been inconsistent. The aim of the present study was to determine if APOE genotypes are associated with onset age of PD, using a patient population large enough to assure sufficient power. We studied 521 unrelated Caucasian patients with idiopathic PD from movement disorder clinics in Oregon and Washington. Genotyping and statistical analyses were carried out using standard methods. Age at onset of PD was significantly earlier in patients with the varepsilon3varepsilon4/varepsilon4varepsilon4 genotype than in patients with the varepsilon3varepsilon3 genotype (56.1 +/- 10.9 vs. 59.6 +/- 11.0, P = 0.003). The significantly earlier onset of PD was not influenced by the possible effects of recruitment site, family history and gender. The effect of the varepsilon2varepsilon3 genotype on onset of PD differed between the two recruitment sites. There was a trend for earlier onset of PD in varepsilon2varepsilon3 patients than in varepsilon3varepsilon3 patients only in the Oregon sample. In conclusion, APOE is associated with age at onset of PD.

Defining the genetic contribution of type 2 diabetes mellitus

Type 2 diabetes mellitus is a common multifactorial genetic syndrome, which is determined by several different genes and environmental factors. It now affects 150 million people world wide but its incidence is increasing rapidly because of secondary factors, such as obesity, hypertension, and lack of physical activity. Many studies have been carried out to determine the genetic factors involved in type 2 diabetes mellitus. In this review we look at the different strategies used and discuss the genome wide scans performed so far in more detail. New technologies, such as microarrays, and the discovery of SNPs will lead to a greater understanding of the pathogenesis of type 2 diabetes mellitus and to better diagnostics, treatment, and eventually prevention.

Array analysis of the genes regulated during neuronal differentiation of human embryonal cells

Recent advances in genetic technology have provided a new platform on which the simultaneous analysis of a large number of genes is possible in a rapid and efficient fashion. To assess the differential expression of human genes during neuronal differentiation, we compared the transcript profiles of undifferentiated, partially differentiated, and fully differentiated NT2/D1 cultures with cDNA expression arrays. Approximately 75 genes (13% of the gene array pool) were differentially expressed during neuronal development of NT2/D1 cells. Genes coding for pyruvate kinase M2 isozyme, clathrin assembly proteins, calmodulin, fibronectin, laminin, thymosin β-10, and many others were upregulated as NT2/D1 cells differentiated into neurons. In contrast, several kinases, phosphatases, and G-protein coupled receptor genes showed downregulation upon neuronal differentiation. The information provided here is an invaluable reference for characterizing the phenotype of these cells. This information can also be used in cell therapy and transplantation in which the graft microenvironment and interaction with the host tissue is crucial.Key words: Atlas cDNA expression arrays, differentiation, neurodevelopment, neuron, NT2/D1 cells.

Genetic counseling for psychiatric disorders

Like other medical conditions, some psychiatric disorders are inherited, whereas others are not. Human genetics research is moving at a rapid pace. Genes for over 450 genetic disorders have been cloned and many disease-causing mutations have also been identified. The explosion of this new knowledge has created many new exciting opportunities in the diagnosis of these heritable disorders. The rapid pace of gene discovery will aid the identification of susceptibility genes for psychiatric disorders. Indeed, we can look forward to answers to many clinical and research questions. These are some of the gifts that the expanding field of human genetics research will continue to bring to medical science. However, as genetic tests for the detection of psychiatric disorders become available, many ethical, legal, and social implications will need to be considered. In this article, we review the principles of genetic counseling for psychiatric disorders, as well as the social and ethical dilemmas that genetic testing may bring. Although medical and scientific advances may bring many gifts, we should approach this new knowledge with caution, as one of the gifts may be a Pandora's box.

Segregation analysis of phenotypic components of learning disabilities. I. Nonword memory and digit span

Dyslexia is a common and complex disorder with evidence for a genetic component. Multiple loci (i.e., quantitative-trait loci [QTLs]) are likely to be involved, but the number is unknown. Diagnosis is complicated by the lack of a standard protocol, and many diagnostic measures have been proposed as understanding of the component processes has evolved. One or more genes may, in turn, influence these measures. To date, little work has been done to evaluate the mode of inheritance of individual component-as opposed to composite-phenotypes, beyond family or twin correlation studies that initially demonstrate evidence for a genetic basis of such components. Here we use two approaches to segregation analysis in 102 nuclear families to estimate genetic models for component phenotypes associated with dyslexia: digit span and a nonword-repetition task. Both measures are related to phonological skills, one of the key component processes in dyslexia. We use oligogenic-trait segregation analysis to estimate the number of QTLs contributing to each phenotype, and we use complex segregation analysis to identify the most parsimonious inheritance models. We provide evidence in support of both a major-gene mode of inheritance for the nonword-repetition task, with approximately 2.4 contributing QTLs, and for a genetic basis of digit span, with approximately 1.9 contributing QTLs. Results obtained by reciprocal adjustment of measures suggest that genes contributing to digit span may contribute to the nonword-repetition score but that there are additional QTLs involved in nonword repetition. Our study adds to existing studies of the genetic basis of composite phenotypes related to dyslexia, by providing evidence for major-gene modes of inheritance of these single-measure component phenotypes.

The number of trait loci in late-onset Alzheimer disease

Although it is clear that apoE plays an important role in the genetics of late-onset Alzheimer disease (AD), evidence exists that additional genes may play a role in AD, and estimates of the total contribution of apoE to the variance in onset of AD vary widely. Unfortunately, little information is available on the number and contribution of additional genes. We estimated the number of additional quantitative-trait loci and their contribution to the variance in age at onset of AD, as well as the contribution of apoE and sex, in an oligogenic segregation analysis of 75 families (742 individuals) ascertained for members with late-onset AD. We found evidence that four additional loci make a contribution to the variance in age at onset of late-onset AD that is similar to or greater in magnitude than that made by apoE, with one locus making a contribution several times greater than that of apoE. Additionally, we confirmed previous findings of a dose effect for the apoE varepsilon4 allele, a protective effect for the varepsilon2 allele, evidence for allelic interactions at the apoE locus, and a small protective effect for males. Furthermore, although we estimate that the apoE genotype can make a difference of Collapse

Key Words Collapse

MESH Headings

• Adult
• Age of Onset
• Aged
• Aged, 80 and over
• Alzheimer Disease/genetics
• Apolipoproteins E/genetics
• Female
• Genotype
• Humans
• Male
• Middle Aged
• Monte Carlo Method
• Quantitative Trait, Heritable
• Sex Factors
• Survival Analysis

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Grants

• U24 AG021886 NIA NIH HHS
• R01 GM046255 NIGMS NIH HHS
• GM 46255 NIGMS NIH HHS
• P50 AG005136 NIA NIH HHS
• P30 AG008017 NIA NIH HHS
• AG 08017 NIA NIH HHS
• R37 GM046255 NIGMS NIH HHS
• AG 05136 NIA NIH HHS

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Affiliation(s)

E W Daw Division of Medical Genetics, Department of Medicine and Department of Statistics, University of Washington, Seattle, WA 98195-7720, USA.
H Payami
E J Nemens
D Nochlin
T D Bird
G D Schellenberg
E M Wijsman

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