Transmission and age-at-onset patterns in familial Alzheimer's disease: evidence for heterogeneity

Structural heterogeneity and intersubject variability of Aβ in familial and sporadic Alzheimer's disease

Point mutations in the amyloid-β (Aβ) coding region produce a combination of mutant and WT Aβ isoforms that yield unique clinicopathologies in familial Alzheimer's disease (fAD) and cerebral amyloid angiopathy (fCAA) patients. Here, we report a method to investigate the structural variability of amyloid deposits found in fAD, fCAA, and sporadic AD (sAD). Using this approach, we demonstrate that mutant Aβ determines WT Aβ conformation through prion template-directed misfolding. Using principal component analysis of multiple structure-sensitive fluorescent amyloid-binding dyes, we assessed the conformational variability of Aβ deposits in fAD, fCAA, and sAD patients. Comparing many deposits from a given patient with the overall population, we found that intrapatient variability is much lower than interpatient variability for both disease types. In a given brain, we observed one or two structurally distinct forms. When two forms coexist, they segregate between the parenchyma and cerebrovasculature, particularly in fAD patients. Compared with sAD samples, deposits from fAD patients show less intersubject variability, and little overlap exists between fAD and sAD deposits. Finally, we examined whether E22G (Arctic) or E22Q (Dutch) mutants direct the misfolding of WT Aβ, leading to fAD-like plaques in vivo. Intracerebrally injecting mutant Aβ40 fibrils into transgenic mice expressing only WT Aβ induced the deposition of plaques with many biochemical hallmarks of fAD. Thus, mutant Aβ40 prions induce a conformation of WT Aβ similar to that found in fAD deposits. These findings indicate that diverse AD phenotypes likely arise from one or more initial Aβ prion conformations, which kinetically dominate the spread of prions in the brain.

Genomic variants, genes, and pathways of Alzheimer's disease: An overview

Alzheimer's disease (AD) (MIM: 104300) is a highly heritable disease with great complexity in its genetic contributors, and represents the most common form of dementia. With the gradual aging of the world's population, leading to increased prevalence of AD, and the substantial cost of care for those afflicted, identifying the genetic causes of disease represents a critical effort in identifying therapeutic targets. Here we provide a comprehensive review of genomic studies of AD, from the earliest linkage studies identifying monogenic contributors to early-onset forms of AD to the genome-wide and rare variant association studies of recent years that are being used to characterize the mosaic of genetic contributors to late-onset AD (LOAD), and which have identified approximately ∼20 genes with common variants contributing to LOAD risk. In addition, we explore studies employing alternative approaches to identify genetic contributors to AD, including studies of AD-related phenotypes and multi-variant association studies such as pathway analyses. Finally, we introduce studies of next-generation sequencing, which have recently helped identify multiple low-frequency and rare variant contributors to AD, and discuss on-going efforts with next-generation sequencing studies to develop statistically well- powered and comprehensive genomic studies of AD. Through this review, we help uncover the many insights the genetics of AD have provided into the pathways and pathophysiology of AD. © 2016 Wiley Periodicals, Inc.

Inconsistencies and controversies surrounding the amyloid hypothesis of Alzheimer's disease

The amyloid hypothesis has driven drug development strategies for Alzheimer's disease for over 20 years. We review why accumulation of amyloid-beta (Aβ) oligomers is generally considered causal for synaptic loss and neurodegeneration in AD. We elaborate on and update arguments for and against the amyloid hypothesis with new data and interpretations, and consider why the amyloid hypothesis may be failing therapeutically. We note several unresolved issues in the field including the presence of Aβ deposition in cognitively normal individuals, the weak correlation between plaque load and cognition, questions regarding the biochemical nature, presence and role of Aβ oligomeric assemblies in vivo, the bias of pre-clinical AD models toward the amyloid hypothesis and the poorly explained pathological heterogeneity and comorbidities associated with AD. We also illustrate how extensive data cited in support of the amyloid hypothesis, including genetic links to disease, can be interpreted independently of a role for Aβ in AD. We conclude it is essential to expand our view of pathogenesis beyond Aβ and tau pathology and suggest several future directions for AD research, which we argue will be critical to understanding AD pathogenesis.

Autosomal recessive causes likely in early-onset Alzheimer disease

OBJECTIVES

To determine the genetic contribution to non-autosomal dominant early-onset Alzheimer disease (EOAD) (onset age ≤60 years) cases and identify the likely mechanism of inheritance in those cases.

DESIGN

A liability threshold model of disease was used to estimate heritability of EOAD and late-onset Alzheimer disease (AD) using concordance for AD among parent-offspring pairs.

SETTING

The Uniform Data Set, whose participants were collected from 32 US Alzheimer's Disease Centers, maintained by the National Alzheimer's Coordinating Center.

PARTICIPANTS

Individuals with probable AD and detailed parental history (n = 5370).

MAIN OUTCOME MEASURES

The concordance among relatives and heritability of EOAD and late-onset AD.

RESULTS

For late-onset AD (n = 4302), we found sex-specific parent-offspring concordance that ranged from approximately 10% to 30%, resulting in a heritability of 69.8% (95% confidence interval, 64.6%-75.0%), and equal heritability for both sexes regardless of parental sex. For EOAD (n = 702), we found that the parent-offspring concordance was 10% or less and concordance among siblings was 21.6%. Early-onset AD heritability was 92% to 100% for all likely values of EOAD prevalence.

CONCLUSIONS

We confirm late-onset AD is a highly polygenic disease. By contrast, the data for EOAD suggest it is an almost entirely genetically based disease, and the patterns of observed concordance for parent-offspring pairs and among siblings lead us to reject the hypotheses that EOAD is a purely dominant, mitochondrial, X-linked, or polygenic disorder. The most likely explanation of the data is that approximately 90% of EOAD cases are due to autosomal recessive causes.

Glucose metabolism and PIB binding in carriers of a His163Tyr presenilin 1 mutation

Six young related pre-symptomatic carriers of a His163Tyr mutation in the presenilin 1 gene who will develop early onset familial Alzheimer's disease (eoFAD), and a control group of 23 non-carriers underwent (18)F-fluorodeoxyglucose positron emission tomography (FDG PET). The mutation carriers were followed-up after 2 years. Multivariate analysis showed clear separation of carriers from non-carriers on both occasions, with the right thalamus being the region contributing most to group differentiation. Statistical parametric mapping (SPM) revealed in the carriers non-significantly lower thalamic cerebral glucose metabolism (CMRglc) at baseline and significantly decreased CMRglc in the right thalamus at follow-up. One mutation carrier was followed-up with FDG PET 10 years after baseline and showed reductions in cognition and CMRglc in the posterior cingulate and the frontal cortex. This subject was diagnosed with AD 1 year later and assessed with an additional FDG as well as an (11)C-PIB PET scan 12 years after baseline. Global cortical CMRglc and cognition were distinctly decreased. PIB binding was comparable with sporadic AD patterns but showing slightly higher striatal levels.

Pharmacogenomics in Alzheimer's disease

Pharmacological treatment in Alzheimer's disease (AD) accounts for 10-20% of direct costs, and fewer than 20% of AD patients are moderate responders to conventional drugs (donepezil, rivastigmine, galantamine, memantine), with doubtful cost-effectiveness. Both AD pathogenesis and drug metabolism are genetically regulated complex traits in which hundreds of genes cooperatively participate. Structural genomics studies demonstrated that more than 200 genes might be involved in AD pathogenesis regulating dysfunctional genetic networks leading to premature neuronal death. The AD population exhibits a higher genetic variation rate than the control population, with absolute and relative genetic variations of 40-60% and 0.85-1.89%, respectively. AD patients also differ in their genomic architecture from patients with other forms of dementia. Functional genomics studies in AD revealed that age of onset, brain atrophy, cerebrovascular hemodynamics, brain bioelectrical activity, cognitive decline, apoptosis, immune function, lipid metabolism dyshomeostasis, and amyloid deposition are associated with AD-related genes. Pioneering pharmacogenomics studies also demonstrated that the therapeutic response in AD is genotype-specific, with apolipoprotein E (APOE) 4/4 carriers the worst responders to conventional treatments. About 10-20% of Caucasians are carriers of defective cytochrome P450 (CYP) 2D6 polymorphic variants that alter the metabolism and effects of AD drugs and many psychotropic agents currently administered to patients with dementia. There is a moderate accumulation of AD-related genetic variants of risk in CYP2D6 poor metabolizers (PMs) and ultrarapid metabolizers (UMs), who are the worst responders to conventional drugs. The association of the APOE-4 allele with specific genetic variants of other genes (e.g., CYP2D6, angiotensin-converting enzyme [ACE]) negatively modulates the therapeutic response to multifactorial treatments affecting cognition, mood, and behavior. Pharmacogenetic and pharmacogenomic factors may account for 60-90% of drug variability in drug disposition and pharmacodynamics. The incorporation of pharmacogenetic/pharmacogenomic protocols to AD research and clinical practice can foster therapeutics optimization by helping to develop cost-effective pharmaceuticals and improving drug efficacy and safety.

Genetics of Alzheimer's disease: a centennial review

Alzheimer's disease (AD) genetics may be one of the most prolifically published areas in medicine and biology. Three early-onset AD genes with causative mutations (APP, PSEN1, PSEN2) and one late-onset AD susceptibility gene, apolipoprotein E (APOE), exist with ample biologic, genetic, and epidemiologic data. Evidence suggests a significant genetic component underlying AD that is not explained by the known genetic risk factors. This article summarizes the evidence for the genetic component in AD and the identification of the early-onset familial AD genes and APOE, and examines the current state of knowledge about additional AD susceptibility loci and alleles. The future directions for genetic research in AD as a common and complex condition are also discussed.

Genetics and dementia nosology

In preparation for the development of the Diagnostic and Statistical Manual of Mental Disorders (5th ed), the American Psychiatric Association convened workshops reviewing scientific evidence relevant to diagnosis of dementia. One of the domains covered was genetics, which is reviewed here. The following areas are reviewed: genetic data on Alzheimer's disease and other dementias; the impact of nosology on genetic research in terms of its potential to improve diagnostic sensitivity and specificity and to decrease heterogeneity; the impact of genetic research findings on nosology, specific diagnostic criteria, and subtypes; and recommendations and future directions. The focus is on Alzheimer's disease, where more genetic data are available, and other dementias are reviewed more briefly. In addition, a separate section reviews the relationship of genetic findings and mild cognitive impairment, a boundary zone between normal aging and dementia, particularly Alzheimer's disease.

The analysis of survival data with a non-susceptible fraction and dual censoring mechanisms

It is known that the ages of onset of many diseases are determined by both a genetic predisposition to disease as well as environmental risk factors that are capable of either triggering or hastening the onset of disease. Difficulties in modelling onset ages arise when a large fraction fail to inherit the disease-causing gene, and multiple reasons for censoring result in unobserved onset ages. We present a parametric Bayesian model that includes subjects with missing age information, non-susceptible subjects and allows for regression on risk factor information. The model is fit using Markov chain Monte Carlo simulation from the posterior distribution, and allows the simultaneous estimation of the proportion of the population at risk of disease, the mean onset age of disease, survival after disease onset, and the association of risk factors with susceptibility, onset age and survival after onset. An example employing Huntington's disease data is presented.

Linkage heterogeneity for the IBD1 locus in Crohn's disease pedigrees by disease onset and severity

BACKGROUND & AIMS

There is evidence for the IBD1 Crohn's disease (CD) susceptibility locus on chromosome 16 in several but not all populations studied. Genetic and phenotypic heterogeneity may underlie ability to replicate IBD1. We determined if age and severity stratification could identify a clinical subgroup at risk for IBD1.

METHODS

Linkage analysis at microsatellites spanning chromosome 16 was performed in 2 groups of CD pedigrees: group 1, 57 pedigrees with at least one affected relative classified as having "severe" disease, by history of surgical resection or immunomodulator therapy, and with disease diagnosed before age 22; and group 2, 33 pedigrees with no history of early-onset, severe CD.

RESULTS

Group 1 pedigrees demonstrated genomewide significant linkage evidence for the IBD1 locus (nonparametric multipoint logarithm of the odds [Mlod], 3.84; P = 1.3 x 10(-5)) with linkage evidence greater than all 90 pedigrees (Mlod, 2.12; P = 9.0 x 10(-4)). Group 2 pedigrees had near zero nonparametric 2-point and Mlod scores for the IBD1 region. Heterogeneity between groups 1 and 2 was significant (P = 0.002).

CONCLUSIONS

Presence of early-onset, more severe CD identifies pedigrees at high risk for IBD1. These pedigrees will have more power to refine the IBD1 locus and identify the causative gene.

Decision-making within the social course of dementia: accounts by Chinese-American caregivers

The goal of this retrospective pilot study was to provide a naturalistic description of the decision-making process regarding dementia, a chronic illness. The hypothesis was that in-depth descriptions by caregivers in a community setting would provide a more comprehensive and realistic representation of decision-making in the case of chronic illness than has been provided by most models. Data were gathered from in-depth, qualitative interviews with seven Chinese-American families living in the Boston area and caring for an elderly family member with dementia. These were supplemented with interviews with medical professionals and ethnography done in the community regarding the care of elders with dementia. Based on the findings, this paper proposes a dynamic, more comprehensive model for the social process of decision-making which is particularly applicable to clinical and life situations of decision-making in the case of chronic illness. The model describes: 1) the decision-maker constellation, including multiple family members, professionals and service systems; 2) relationships between decision-makers as "allies" or "competitors"; 3) variations in the nature of the decision-making process; and 4) the interaction of decision-making with larger social, economic and cultural forces. Decision-making in the case of dementia is placed within the conceptual framework of the "social course" of chronic illness.

Alpha-2 macroglobulin is genetically associated with Alzheimer disease

Alpha-2-macroglobulin (alpha-2M; encoded by the gene A2M) is a serum pan-protease inhibitor that has been implicated in Alzheimer disease (AD) based on its ability to mediate the clearance and degradation of A beta, the major component of beta-amyloid deposits. Analysis of a deletion in the A2M gene at the 5' splice site of 'exon II' of the bait region (exon 18) revealed that inheritance of the deletion (A2M-2) confers increased risk for AD (Mantel-Haenzel odds ratio=3.56, P=0.001). The sibship disequilibrium test (SDT) also revealed a significant association between A2M and AD (P=0.00009). These values were comparable to those obtained for the APOE-epsilon4 allele in the same sample, but in contrast to APOE-epsilon4, A2M-2 did not affect age of onset. The observed association of A2M with AD did not appear to account for the previously published linkage of AD to chromosome 12, which we were unable to confirm in this sample. A2M, LRP1 (encoding the alpha-2M receptor) and the genes for two other LRP ligands, APOE and APP (encoding the amyloid beta-protein precursor), have now all been genetically linked to AD, suggesting that these proteins may participate in a common neuropathogenic pathway leading to AD.

Aging, increasing genomic entropy, and neurodegenerative disease

The genome, as biologic information, can be conceptualized in terms of entropy. The second law of thermodynamics dictates that entropy must increase over time. Consequently, aging can be viewed as increasing genomic entropy. Genetic instability is the biophysical correlate of increasing genomic entropy. Rates of increasing genomic entropy can be determined from age-specific mortality rate dynamics (e.g., Parkinson's disease and amyotrophic lateral sclerosis). These observations are consistent with a model of neurodegenerative disease as a manifestation of increasing genomic entropy with aging.

Chromosome 14 familial Alzheimer's disease: the clinical and neuropathological characteristics of a family with a leucine-->serine (L250S) substitution at codon 250 of the presenilin 1 gene

BACKGROUND

Seven affected members are described from a kindred with autosomal dominant familial Alzheimer's disease associated with a novel mutation in the presenilin 1 (PS1) gene on chromosome 14 that results in a leucine to serine substitution at codon 250 (L250S).

METHOD

Clinical information on the pedigree was collected directly from family members including affected members and their carers and also from hospital records.

RESULTS

Detailed clinical information was available on five members. All had an early age at onset with a median age of 52 (95% confidence interval (95% CI) 49.4-54.9). Age at onset varied between 49 and 56 years, with duration of illness varying between six years and 15 years. Myoclonus, depression, and psychosis were features of this pedigree; seizures were not reported.

CONCLUSIONS

PS1 L250S familial Alzheimer's disease is an early onset form of Alzheimer's disease with clinical features similar to other reported familial Alzheimer's disease pedigrees, except that seizures were absent.

Models of Alzheimer's disease: cellular and molecular aspects

Glucose metabolism in the brain is an important process that influences many normal cellular processes, from neurotransmitter synthesis to ATP production. While cortisol and insulin have opposing effects on glucose metabolism, desensitization of the neuronal insulin receptor results in metabolic abnormalities. In the normal aging brain, glucose/energy metabolism is decreased slightly. In the majority of cases. Alzheimer's disease is sporadic and has a late onset. Therefore, age-related variations in cellular metabolism following the principle of self-organized criticality may come into focus with respect to the etiopathogenesis of this neurodegenerative disorder. As a possible primary abnormal event in late-onset sporadic DAT, a prolonged desensitization of the neuronal insulin receptor is assumed to be responsible for cascade-like abnormalities in oxidative energy metabolism and related metabolism with impacts on amyloid formation.

Allelic association but only weak evidence for linkage to the apolipoprotein E locus in late-onset Swedish Alzheimer families

An association between the epsilon 4 allele of the apolipoprotein E gene (APOE) and late-onset Alzheimer's disease (AD) was recently demonstrated. In order to confirm the association and to gauge the ability of standard genetic linkage methods to identify susceptibility genes, we investigated 15 Swedish late-onset Ad families. We found an association of familial AD to the APOE epsilon 4 allele (P = 0.01) but no indication of linkage to the APOE region using 2-point linkage analysis, and only weak evidence using the affected pedigree-member (APM) method. Our results confirm an APOE epsilon 4 association with late-onset familial AD and indicate that susceptibility genes can easily be missed when using standard lod score and APM genetic linkage analysis.

A beta A4 amyloid precursor protein gene and Alzheimer's disease

Alzheimer's disease is a senile dementia caused by progressive neurodegeneration of the central nervous system. One of the most prominent pathological characteristics is beta A4 amyloid deposition in senile plaques in the brain parenchyma and in cerebral blood vessels. beta A4 amyloid is processed from a larger integral membrane protein, the beta A4 amyloid precursor protein. Different pathogenic mutations in this protein have been detected in a small number of Alzheimer's disease families. Here functional implications of these mutations on the processing of the precursor protein and the beta A4 amyloid deposition will be discussed with respect to the pathogenesis of Alzheimer's disease and related disorders.

Oxidative metabolism deficiencies in brains of patients with Alzheimer's disease

Glucose metabolism in the brain has an important influence on many normal cellular processes. It contributes to the synthesis of acetylcholine, glutamate, aspartate, gamma-aminobutyric acid, glycine, and ATP production (the driving force behind almost all cellular and molecular activity). Neuronal glucose metabolism is controlled antagonistically by insulin and cortisol. Desensitization of the neuronal insulin receptor causes abnormalities in oxidative energy metabolism. During normal aging, the cerebral energy pool is slightly diminished, but its level increases after stressful events. In age-related sporadic late-onset dementia of the Alzheimer type (SDAT), glucose metabolism and formation of cellular energy are severely reduced. Desensitization of the neuronal insulin receptor seems to be an early event in the pathogenesis or even etiology of SDAT causing disturbances in oxidative glucose metabolism and energy failure in insulin-sensitive brain structures. These abnormalities appear to induce a cascade of disturbances that leads to abnormal APP processing and amyloid formation, membrane damage, and neuronal death.

Mutational screening of APP gene in patients with early-onset Alzheimer disease utilizing mismatched PCR-RFLP

To elucidate the frequency of mutations of the beta/A4 amyloid protein precursor (APP) gene in early-onset Alzheimer disease, we designed a mismatched PCR-RFLP that can identify all kinds of missense mutations at codon 717 in addition to the seven kinds of known mutations at exon 17. When we screened mutations at exon 17 utilizing this method and the double missense mutations at exon 16 of the APP gene by PCR-RFLP, no cases revealed mutations of the APP gene among 13 familial and 54 sporadic cases, except one family (OS-1) that had previously been reported and used as a positive control of APP717(Val-->Ile). Our results support the hypothesis that mutations in the APP gene are not major causes in early-onset Alzheimer disease.

Apolipoprotein E genotype in patients with Alzheimer's disease: implications for the risk of dementia among relatives

Numerous studies have shown that the risk of Alzheimer's disease (AD) is associated with the dose of the epsilon 4 allele of apolipoprotein E (ApoE). However, more than one third of AD patients lack epsilon 4 and many persons having epsilon 4 survive cognitively intact to old age. We evaluated the lifetime risk of disease in 3,999 first-degree relatives of 549 probands who met the criteria for probable or definite AD and whose ApoE genotypes were known. ApoE genotypes for relatives were not determined. After age 65 the risk among relatives was proportional, as much as 7 to 10% at age 85, to the number of epsilon 4 alleles present in the proband. Risks to relatives of ApoE 2/2 and 2/3 probands were nearly identical at all ages to risks for relatives of ApoE 3/3 probands. The expected proportion of relatives having at least one epsilon 4 allele was calculated for each genotype group based on the distribution of parents, sibs, and offspring in the sample. Among relatives in the ApoE 3/3 group, the lifetime risk for AD by age 90 was three times greater than the expected proportion of epsilon 4 carriers, suggesting that factors other than ApoE contribute to AD susceptibility. Furthermore, the 44% risk of AD by age 93 among relatives of ApoE 4/4 probands indicates that as many as 50% of people having at least one epsilon 4 allele do not develop AD. We also found that among male relatives, risk of AD in the ApoE 3/4 group was similar to that for the ApoE 3/3 group but significantly less than the risk for the ApoE 4/4 group. In contrast, among female relatives the risk for the ApoE 3/4 group was nearly twice that for the ApoE 3/3 group and identical to the risk for the ApoE 4/4 group. These findings are consistent with a sex-modification effect of the E4 isoform on disease susceptibility.

Desensitization of the neuronal insulin receptor: a new approach in the etiopathogenesis of late-onset sporadic dementia of the Alzheimer type (SDAT)?

Even in its incipient stage, late-onset sporadic dementia of the Alzheimer type (SDAT) is characterized by an abnormal reduction in brain glucose consumption and energy formation. Gathering evidence indicates that cerebral glucose metabolism is controlled by brain insulin/insulin receptors. This led us to hypothesize that the abnormal reduction in glucose utilization found in Alzheimer brains is preceded by a desensitization of cerebral insulin receptors which might be due to enhanced levels of stress factors such as cortisol and catecholamines. The hypothesis is supported by clinical findings of an abnormal response to the oral glucose tolerance test in AD patients. Furthermore, experimental desensitization of the cerebral insulin receptor resulted in both cognitive deficits and metabolic abnormalities in cerebral oxidative glucose metabolism resembling those described in incipient late-onset SDAT. Glucose is the major source of energy in the CNS, and any impairment in cerebral glucose oxidation can be expected to result in deficits in both acetylcholine synthesis and ATP formation, which might contribute to altered APP processing and enhanced susceptibility to neurotoxicity.

Characteristics of familial aggregation in early-onset Alzheimer's disease: evidence of subgroups

Characteristics of familial aggregation of Alzheimer's Disease were studied in 92 families ascertained through a clinically diagnosed proband with an onset below age 60 years. In each family data were systematically collected on the sibships of the proband, of his father, and of his mother. A total of 926 relatives were included and 81% of the living relatives (i.e., 251 individuals) were directly examined. The estimated cumulative risk among first degree relatives was equal to 35% by age 89 years (95% confidence interval 22 to 47%). This result does not support the hypothesis that an autosomal dominant gene, fully penetrant by age 90 years, is segregating within all these pedigrees. Despite the fact that all probands were selected for an onset before age 60 years it was shown that two types of families could be delineated with respect to age at onset among affected relatives: all secondary cases with an onset below age 60 years were contributed by a particular group of families (type 1 families), whereas all secondary cases with an onset after age 60 years were contributed by another group of families (type 2 families). Although genetic interpretation of these findings is not straightforward, they support the hypothesis of etiologic heterogeneity in the determination of early-onset Alzheimer's disease.

A large pedigree with early-onset Alzheimer's disease: clinical, neuropathologic, and genetic characterization

We present clinical, neuropsychological, and neuropathologic data on a large pedigree including 34 subjects with early-onset progressive dementia. The mean (+/- SD) age at onset was 46 +/- 3.5 years and the mean age at death 52.6 +/- 5.7 years. Twelve patients were clinically diagnosed as having probable Alzheimer's disease (AD) according to the NINCDS-ADRDA criteria. Neuropsychological evaluation, performed at a moderate stage of the disease, was available in six subjects and showed a classic pattern of cognitive deficit. Myoclonus and extrapyramidal signs were common, and seizures were present in all affected subjects. There were neuropathologic changes typical of AD in two brains. A significant lod score of 5.48 was observed at a recombination fraction of theta = 0.0 with the genetic marker D14S43, thereby establishing that the responsible gene was located on chromosome 14q24.3. These results suggest that epilepsy could represent a particular feature in AD families linked to chromosome 14q.

Heredity in dementia of the Alzheimer type

Family, twin and linkage studies in Alzheimer's disease are reviewed. Several of these studies appear to be burdened with sources of error, mainly related to sampling and clinical diagnoses. It was previously thought that early-onset cases of Alzheimer's disease were heavily genetically influenced, whereas late-onset cases were sporadic. The author questions whether this difference is mainly due to methodological error.

Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease

Gene dosage of the apolipoprotein E (APOE) epsilon 4 allele is a major risk factor for familial Alzheimer disease (AD) of late onset (after age 60). Here we studied a large series of 115 AD case subjects and 243 controls as well as 150 affected and 197 unaffected members of 66 AD families. Our data demonstrate a protective effect of the epsilon 2 allele, in addition to the dose effect of the epsilon 4 allele in sporadic AD. Although a substantial proportion (65%) of AD is attributable to the presence of epsilon 4 alleles, risk of AD is lowest in subjects with the epsilon 2/epsilon 3 genotype, with an additional 23% of AD attributable to the absence of an epsilon 2 allele. The opposite actions of the epsilon 2 and epsilon 4 alleles further support the direct involvement of APOE in the pathogenesis of AD.

A comparison of multiplex and simplex families with Alzheimer's disease/senile dementia of Alzheimer type within a well defined population

A study was made on 150 clinically demented patients presenting at autopsy at Umeå University Hospital in Sweden. In 90 of the cases dementia was considered to be primary in nature and of these forty six per cent (41 cases), fulfilled both the clinical and histopathological criteria for the diagnosis of Alzheimer's disease/Senile dementia of Alzheimer type (AD/SDAT). The families of these 41 AD/SDAT cases were then studied, and a family history obtained through interviews with multiple family informants and from civil and medical records. Additional diseased family members suffering from progressive dementia (multiplex families) were observed in 12 probands out of 41 (29%). Multiplex families exhibited similar clinical and histopathological characteristics as simplex families containing a single affected individual. The secondary cases in the multiplex families exhibited similar demographic and clinical characteristics as the probands. 39% of the multiplex and 14% of the simplex cases had an early age of onset of the disease, that was under 65 years. The overall prevalence of progressive dementia disorders in the 41 families was 5.9%. The prevalence of a progressive dementia disorder was 11% in the multiplex families (14% for the early onset cases) and 3.5% in the simplex families (2% for the early onset cases). The prevalence of progressive dementia disorder for family members who had passed the mean age of the onset of the disease for their family, was 45% for multiplex and 18% for simplex families. Furthermore the incidence rate for dementia was significantly higher (p < 0.005) in multiplex families (5.5 per 1,000 person years) when compared to simplex families (2.5 per 1,000 person years). No differences could be seen in parental age at birth of the diseased when comparing the two sets of families. However in multiplex families the duration of the disease was significantly (p < 0.025) shorter, in subjects with parental age at birth over 35 years compared to those with a parental age under 35 years. The multiplex families contained significantly (p < 0.025) larger sibships; and showed a significantly lower age of onset for the disease (p < 0.001), and a significantly longer duration of disease (p < 0.05) compared to the simplex families. A significant intra familial correlation of age at disease onset was observed in both sets of the families.

Molecular genetics of Alzheimer's disease

At present it is not clear whether Alzheimer's disease is a single disease, a complex syndrome, or a heterogeneous ill-defined group of disorders. In the last few years significant progress has been made in identifying and describing its different manifestations, as well as the underlying biological mechanisms. Modern molecular biology techniques have provided new insights into possible etiological mechanisms. Linkage analysis and gene sequencing studies have produced evidence of a possible locus on chromosome 21 in a small group of families with early onset familial Alzheimer's disease (FAD). It was shown that another small group of early onset FAD families develops the disease as a result of mutations in the gene coding for the beta-amyloid precursor protein, and that in a larger subgroup of early onset families the disease appears to be caused by an unidentified gene on chromosome 14. Several other early onset FAD families are clearly not linked to any of these loci, suggesting that other abnormal genes, probably on different chromosomes, might be the cause of the disease in these families. Finally, it was recently shown that the epsilon 4 allele of apolipoprotein E (ApoE) gene, which has been mapped to chromosome 19, is associated with an increased risk of developing the disease in late onset FAD families and sporadic cases. These results not only evidence that Alzheimer's disease is a genetically heterogeneous disorder, but also delineate new approaches in the study of the etiological and pathogenetic mechanisms of Alzheimer's disease.

Assessing the risk of Alzheimer's disease in first-degree relatives of Alzheimer's disease cases

Family history of Alzheimer's disease (AD) was investigated using a telephone reinterview of 99 cases and 116 controls selected from a case-control study of 170 matched pairs. It was found that the family history method used in the initial interview was satisfactory in identifying first-degree relatives and assessing their ages of birth and death, but the number of first-degree relatives suffering from AD was probably under-estimated. Family history of AD was confirmed as a risk factor for AD. Higher estimates of cumulative incidence were obtained among case relatives than among control relatives. No evidence was found to support the hypothesis that a familial form of AD is more common in those with earlier onset AD (before age 75) in those who display early, prominent features of aphasia or apraxia, or that an AD gene may be sex-linked. The curves for cumulative incidence showed no tendency to reach an asymptote, as is implied by the theory that some forms of AD are due to the action of an autosomal dominant gene. Estimates of lifetime risk by age 90 were within the range found by other investigators. Much larger samples of the very old are needed to obtain estimates of total lifetime risk with smaller standard errors.

Sporadic dementia of Alzheimer type: role of amyloid in etiology is challenged

Alzheimer's disease is a heterogeneous neurodegenerative disorder. Whereas only a minority is due to genetic abnormalities and mostly with early onset, the majority of all Alzheimer cases is sporadic and with late onset. Therefore, in the latter, age-related disturbances in cellular metabolism may come into focus with respect to the etiopathogenesis rather than the primary formation of amyloid. In this "Editor's note for debate", the role of amyloid as a causative factor of sporadic Alzheimer's disease is challenged. Instead, as a possible primary abnormal event in sporadic Alzheimer's disease, the perturbations in neuronal glucose metabolism and the subsequent ATP deficit with its impacts on the secondary amyloid formation are discussed to open a new field of research and another aspect for debate.

Genetic transmission of Alzheimer's disease among families in a Dutch population based study

We evaluated age at onset and transmission patterns of Alzheimer's disease (AD) in families of 198 patients who had onset of symptoms before the age of 65 years and were diagnosed before the age of 70 years. Patients were ascertained in a population based study in The Netherlands. The results suggest that the risk of AD by the age of 90 in first degree relatives is 39% (95% confidence interval 27 to 51). By the age of 90, this risk is 2.8 (95% confidence interval 1.5-5.2) times greater than the corresponding risk of 14% among relatives of age and sex matched control subjects. Segregation analysis indicated that patterns of familial clustering are best explained by transmission of a major autosomal dominant gene with reduced penetrance and a multifactorial component. However, the single major locus model could be rejected in favour of the mixed model only when a cohort effect for heritability was allowed for. The frequency of the AD susceptibility allele was estimated to be 0.48% in the single major locus model and 0.31% in the mixed model. Although our study confirms that a dominant major gene is implicated in early onset AD, the results suggest that other genetic or perhaps non-genetic factors may account for the disease in a considerable number of patients.

Heterogeneity of brain gene expression in Alzheimer's disease

We have examined the expression of several genes whose transcripts have increased levels in Alzheimer's disease and have found heterogeneity in these levels in different patients with this condition. The level of expression of these genes was compared to different clinical and pathological aspects of the disease. A case with markedly elevated alpha 1-antichymotrypsin mRNA levels demonstrated prominent neuronal accumulation of this protein. Many of the neurons which demonstrated alpha 1-antichymotrypsin staining did not have neurofibrillary tangles, and vice versa. This suggests that alpha 1-antichymotrypsin staining might identify a different facet of the pathology of Alzheimer's disease than does neurofibrillary tangle staining and may provide new information in the study of this condition.

The Cystatin-C gene is not linked to early onset familial Alzheimer's disease

The APP717 mutations discovered in only a few early onset Alzheimer's disease (AD) families have confirmed the genetic heterogeneity of this disorder. To identify the other gene(s) involved in the disease we selected the protease inhibitor, Cystatin-C, as a candidate gene. Cystatin-C is an amyloidogenic protein causing hereditary cerebral haemorrhage with amyloidosis-Icelandic type (HCHWA-I). It is localised with the beta-amyloid peptide in the arterial walls of AD brains. We have analysed the segregation of a polymorphic marker in this gene in 8 early onset AD families. Two early onset families showed clear non-segregation of the marker with the disease. When the 8 families are analysed together (assuming only one other gene is involved), they present exclusion linkage criteria. These data indicate that Cystatin-C is not the site of the defect in 2 families and is not likely to be in the other families analysed. We conclude that the deposition of Cystatin-C in AD is a secondary event in the disease process, and that this gene is not pathogenic in familial AD.

Genetic susceptibility and head injury as risk factors for Alzheimer's disease among community-dwelling elderly persons and their first-degree relatives

We performed a community-based study to investigate the relationship of genetic susceptibility and head injury to Alzheimer's disease (AD) in 138 patients with AD and 193 healthy elderly control subjects. Data concerning presence or absence of dementia and certain exposures were also obtained from 799 first-degree relatives of the patients and 1,238 first-degree relatives of the control subjects. Adjusting for age, gender, and other risk factors, the odds ratio for AD associated with head injury was 3.7 (95% confidence interval [CI], 1.4-9.7). The association was highest for head injuries that occurred after age 70. The risk of AD was higher in first-degree relatives of patients with onset prior to age 70 than in relatives of control subjects (risk ratio [RR] = 2.5; 95% CI, 1.1-5.6). The risk was not increased for relatives of patients with onset of AD at age 70 or older. Compared with relatives without head injury, the risk of AD was increased among both head-injured relatives of patients (RR = 5.9; 95% CI, 2.3-14.8) and head-injured relatives of control subjects (RR = 6.9; 95% CI, 2.5-18.9). Our results are consistent with the hypothesis that severe head injury and genetic susceptibility are associated with AD. Both associations concur with current concepts regarding the role of amyloid in AD. Although we regard head injury, like genetic susceptibility, to be a putative risk factor for AD, the temporal relationship between head injury and AD warrants further investigation.

Intermediary metabolism disturbance in AD/SDAT and its relation to molecular events

1. Early-onset dementia of Alzheimer type (EODAT; AD) and late-onset dementia of Alzheimer type (LODAT; SDAT) are heterogenous in origin. 2. A common superordinate pathobiochemical principle in the etiopathogenesis of both types of dementia is neuronal energy failure with subsequent abnormalities in cellular Ca2+ homeostasis and glucose-related amino acid metabolism. 3. These metabolic abnormalities are assumed to occur first at axodendritic terminals of the acetylcholinergic-glutamatergic circuit and to cause morphological damage at synaptic sites. 4. Metabolic stress and structural damage at synaptic sites may induce enhanced formation of APP and its cleavage product amyloid. 5. Energy-metabolism related abnormalities along with functional and structural changes at synaptic sites of the acetylcholinergic-glutamatergic circuit may precede the formation of amyloid in DAT brain.

Sex differences in the psychiatric manifestations of Alzheimer's disease

OBJECTIVE

To test the null hypothesis, ie, that there are no gender differences in psychiatric problems manifest in patients with Alzheimer's disease.

DESIGN

Survey.

SETTING

Patients living in the community and evaluated at Alzheimer's disease and geriatric outpatient programs.

PATIENTS

Three hundred twenty-eight women and 186 men clinically diagnosed with Alzheimer's disease using NINCDS/ADRDA or DSM-III-R criteria.

MEASUREMENTS

Psychiatric signs and symptoms recorded following a psychiatric interview, including the Hamilton Depression Rating Scale.

RESULTS

Approximately two-thirds of both men and women had psychiatric problems, but women had significantly more multiple symptoms. When pairs of symptoms were analyzed for independence, agitation was only significantly associated with paranoia in men, whereas in women agitation was significantly associated with most other psychiatric problems.

CONCLUSION

The higher prevalence of multiple psychiatric problems in women may be due to many factors, including sociodemographic influences, physician bias, and/or other differences between men and women. The finding of a different pattern of association of symptoms with agitation in men and women deserves replication.

Risk factors for late-onset Alzheimer's disease: a population-based, case-control study

Our current knowledge of risk factors for Alzheimer's disease is limited and primarily addresses early-onset disease. This study aimed to determine the risk factors for late-onset Alzheimer's disease using a case-control approach. Ninety-eight cases and 216 controls were gathered from an ongoing population survey on aging and dementia in Stockholm (the Kungsholmen Project). We found a high relative risk (3.2; 95% confidence interval, 1.8-5.7) with the presence of at least one first-degree relative affected by dementia. Among all the other risk factors, alcohol abuse (relative risk, 4.4; 95% confidence interval, 1.4-13.8) and manual work (relative risk for men of 5.3; 95% confidence interval, 1.1-25.5) emerged as positively associated. No clear association was found with a family history of Parkinson disease, advanced parental age at index delivery, season of birth, or previous head trauma. In conclusion, our data suggest that the main risk factor for late-onset Alzheimer's disease is a family history of dementia, as has been previously reported for early-onset disease. Moreover, alcohol abuse and occupational exposure might play a specific role for this form of the disease.

Linkage of Alzheimer's disease to chromosome 21 and chromosome 19 markers: effect of age of onset assumptions

Age of onset heterogeneity in Alzheimer's disease families was modelled by allowing for different liability classes for affected individuals according to their age of onset when calculating lod scores to chromosome 21 and chromosome 19 markers. Linkage to chromosome 21 was supported in the Boston data set, and the method of age correction did not greatly change the lod scores when only affected individuals were analyzed. The location of a gene on chromosome 19 for late age of onset illness was affected by the assumptions about early onset individuals.

A two-locus model for familial Alzheimer's disease?

The present findings for familial Alzheimer's disease suggest a possible linkage to gene(s) on chromosome 21 for the early onset form and to chromosome 19 for the late onset. Since these results are not unequivocal, possible alternative hypotheses include the effect of genetic heterogeneity or of an oligogenic model of segregation for FAD.

Brain oxidative energy and related metabolism, neuronal stress, and Alzheimer's disease: a speculative synthesis

A reduction in the cerebral metabolic rate of glucose is one of the most predominant abnormalities generally found in the Alzheimer brain, whereas the cerebral metabolic rate of oxygen is diminished only slightly or not at all at the beginning of this dementive disorder. From the cerebral metabolic rates of oxidized glucose and oxygen, the cerebral adenosine triphosphate (ATP) formation rate was calculated in incipient early-onset, incipient late-onset, and stable advanced dementia of the Alzheimer type (DAT). A reduction in ATP formation by various amounts was found, ranging from at least 7% in incipient early-onset DAT, from around 20% in incipient late-onset DAT, and from 35% up to more than 50% in stable advanced dementia. The cerebral diminution in energy availability, along with a loss of functionally important amino acids, ammonia toxicity, supposed membrane damage, dysregulation of Ca2+ homeostasis, and glycogen accumulation in the incipient stages of DAT are assumed to be stress-related abnormalities capable of inducing the formation of heat shock proteins. These events may lead to an enhanced generation of amyloid precursor protein in earlier states of DAT. If abnormally cleaved, amyloid A4 protein may be produced in increased amounts. From the results discussed in this article it is deduced as a speculative synthesis that perturbations in brain oxidative energy and related metabolism may precede the generation of amyloid precursor protein and the formation of plaques in the brain affected by incipient DAT.

Susceptibility genes for familial Alzheimer's disease on chromosomes 19 and 21: a reality check

Linkage data from 92 FAD kindreds were analyzed by lod score analysis under various assumptions of disease penetrance, marker allele frequencies, and heterogeneity. Multilocus linkage analysis supports the existence of a gene in 40%-65% of families with predominantly late-onset illness (after age 65) on chromosome 19 between D19S13 and ATP1A3. Evidence for a second FAD gene on chromosome 21 is weaker and stems primarily from a few families with early-onset disease. Our findings also indicate that choice of the genetic model for FAD and marker allele frequencies may be crucial to conclusions about linkage and heterogeneity.