Familial Alzheimer's disease: progress and problems

Friend, Foe or Both? Immune Activity in Alzheimer's Disease

Alzheimer's disease (AD) is marked by the presence of amyloid beta (Aβ) plaques, neurofibrillary tangles (NFT), neuronal death and synaptic loss, and inflammation in the brain. AD research has, in large part, been dedicated to the understanding of Aβ and NFT deposition as well as to the pharmacological reduction of these hallmarks. However, recent GWAS data indicates neuroinflammation plays a critical role in AD development, thereby redirecting research efforts toward unveiling the complexities of AD-associated neuroinflammation. It is clear that the innate immune system is intimately associated with AD progression, however, the specific roles of glia and neuroinflammation in AD pathology remain to be described. Moreover, inflammatory processes have largely been painted as detrimental to AD pathology, when in fact, many immune mechanisms such as phagocytosis aid in the reduction of AD pathologies. In this review, we aim to outline the delicate balance between the beneficial and detrimental aspects of immune activation in AD as a more thorough understanding of these processes is critical to development of effective therapeutics for AD.

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.

Linkage analyses in Caribbean Hispanic families identify novel loci associated with familial late-onset Alzheimer's disease

INTRODUCTION

We performed linkage analyses in Caribbean Hispanic families with multiple late-onset Alzheimer's disease (LOAD) cases to identify regions that may contain disease causative variants.

METHODS

We selected 67 LOAD families to perform genome-wide linkage scan. Analysis of the linked regions was repeated using the entire sample of 282 families. Validated chromosomal regions were analyzed using joint linkage and association.

RESULTS

We identified 26 regions linked to LOAD (HLOD ≥3.6). We validated 13 of the regions (HLOD ≥2.5) using the entire family sample. The strongest signal was at 11q12.3 (rs2232932: HLODmax = 4.7, Pjoint = 6.6 × 10(-6)), a locus located ∼2 Mb upstream of the membrane-spanning 4A gene cluster. We additionally identified a locus at 7p14.3 (rs10255835: HLODmax = 4.9, Pjoint = 1.2 × 10(-5)), a region harboring genes associated with the nervous system (GARS, GHRHR, and NEUROD6).

DISCUSSION

Future sequencing efforts should focus on these regions because they may harbor familial LOAD causative mutations.

Genome-wide association of familial late-onset Alzheimer's disease replicates BIN1 and CLU and nominates CUGBP2 in interaction with APOE

Late-onset Alzheimer's disease (LOAD) is the most common form of dementia in the elderly. The National Institute of Aging-Late Onset Alzheimer's Disease Family Study and the National Cell Repository for Alzheimer's Disease conducted a joint genome-wide association study (GWAS) of multiplex LOAD families (3,839 affected and unaffected individuals from 992 families plus additional unrelated neurologically evaluated normal subjects) using the 610 IlluminaQuad panel. This cohort represents the largest family-based GWAS of LOAD to date, with analyses limited here to the European-American subjects. SNPs near APOE gave highly significant results (e.g., rs2075650, p = 3.2×10(-81)), but no other genome-wide significant evidence for association was obtained in the full sample. Analyses that stratified on APOE genotypes identified SNPs on chromosome 10p14 in CUGBP2 with genome-wide significant evidence for association within APOE ε4 homozygotes (e.g., rs201119, p = 1.5×10(-8)). Association in this gene was replicated in an independent sample consisting of three cohorts. There was evidence of association for recently-reported LOAD risk loci, including BIN1 (rs7561528, p = 0.009 with, and p = 0.03 without, APOE adjustment) and CLU (rs11136000, p = 0.023 with, and p = 0.008 without, APOE adjustment), with weaker support for CR1. However, our results provide strong evidence that association with PICALM (rs3851179, p = 0.69 with, and p = 0.039 without, APOE adjustment) and EXOC3L2 is affected by correlation with APOE, and thus may represent spurious association. Our results indicate that genetic structure coupled with ascertainment bias resulting from the strong APOE association affect genome-wide results and interpretation of some recently reported associations. We show that a locus such as APOE, with large effects and strong association with disease, can lead to samples that require appropriate adjustment for this locus to avoid both false positive and false negative evidence of association. We suggest that similar adjustments may also be needed for many other large multi-site studies.

A multigenerational pedigree of late-onset Alzheimer's disease implies new genetic causes

We describe the clinical phenotype and pathology of a new autosomal dominant late-onset familial form of Alzheimer's disease in four extensive kindred originated in a genetically isolated population. Twelve affected and 16 unaffected members of these kindred were examined clinically, and a brain post-mortem study was carried out in one case. The preliminary genetic assessment included complex segregation analysis, evaluation of the power to detect linkage, and exclusion of candidate genes. Dementia has been recorded for six generations in ancestors of examined cases. Review of death certificates allowed linking of all subjects in four extensive pedigrees. Affected individuals examined had progressive memory loss with onset between 57 and 74 years of age, along with seizures, myoclonus and parkinsonism in advanced stages. The brain of the case examined post-mortem showed widespread neocortical neuritic plaques and neurofibrillary tangles (stage VI of Braak), amyloid angiopathy, and Lewy bodies restricted to limbic areas. Sequencing exons 16 and 17 of amyloid precursor protein, and exons 4-12 of presenilin 1 and presenilin 2 genes did not disclose any mutations. Genotyping with markers D21S265, D14S71, D14S77, D1S2850 and D1S479 located 1-3 cM from the previously reported genes further excluded linkage to these genes. Seven out of 12 cases were apolipoprotein E (APOE) epsilon3/3, although the presence of an APOE epsilon4 allele was associated with an increased risk of dementia (odd ratio 6.17; 95% confidence interval: 1.15-33.15), but not to an earlier age of onset. Complex segregation analysis showed that the best model fitting the data was that of a major gene (dominant) with a gene frequency close to 3% in this population. Simulation analysis predicted an average logarithm of odds (LOD) of 2.2 at = 0.05. These four families, which seem to be part of a common extended pedigree originated by a founder arriving in this region in the 18th century, represent an autosomal dominant late-onset familial Alzheimer's disease not linked to previously known genetic loci. The simulation analysis suggests that it will be feasible to locate a novel responsible gene in these kindred.

Molecular genetics of Alzheimer's disease

Four genes have thus far been implicated in the etiology of Alzheimer's disease (AD). A series of mutations in the amyloid precursor protein (APP) gene on chromosome 21, which cause disease with a typical onset of 55 years of age, have been described. These include mutations at APP670/671, APP692, and APP717. The epsilon 4 allele of the apolipoprotein E (ApoE) gene on chromosome 19 is positively associated with disease, whereas the epsilon 2 allele is usually negatively associated with disease. Mutations in the S182 gene on chromosome 14 seem to cause disease with an onset age of < 50 years and mutations in a gene on chromosome 1 (S182 Like Protein: S182LP) cause disease with variable onset. These genetic findings are reviewed within the framework of the amyloid cascade hypothesis of AD etiology and pathogenesis. The occurrence and effects of the mutations in APP and the fact that the epsilon 4 allele of ApoE are genetic risk factors point to the hypothesis that the extracellular deposition of beta-amyloid is the key initiating event in the pathogenesis of AD.

Genetic susceptibility to Alzheimer disease

Alzheimer disease (AD) is the leading cause of dementia in the elderly. Less than a decade ago, it was questioned as to whether or not genes were even involved in anything but rare early onset AD. Since that time, using a variety of genetic epidemiological and molecular biological techniques, four loci have been identified that play a role in the genetic susceptibility of AD. AD presents as a prototype of the power of genetic techniques in defining the etiology of a complex disease.

A mitochondrial DNA clone is associated with increased risk for Alzheimer disease

Severe mitochondrial genetic mutations lead to early degeneration of specific human tissues; milder mitochondrial mutations may cause degeneration at a later point in life. A mutation at position 4336 was reported to occur at increased frequency in individuals with Alzheimer disease (AD) and Parkinson disease [Shoffner, J. M., Brown, M. D., Torroni, A., Lott, M. T., Cabell, M. F., Mirra, S. S., Beal, M. F., Yang, C.-C., Gearing, M., Salvo, R., Watts, R. L., Juncos, J. L., Hansen, L. A., Crain, B. J., Fayad, M., Reckord, C. L. & Wallace, D. C. (1993) Genomics 17, 171-184]. We have investigated the notion that this mutation leads to excess risk of AD by using a case-control study design of 72 AD autopsies and 296 race- and age-matched controls. The 4336G mutation occurred at higher frequency in AD autopsies than age-matched controls, a statistically significant difference. Evolutionary analysis of mtDNAs bearing the 4336G mutation indicated they were more closely related to each other than to other mtDNAs, consistent with the model of a single origin for this mutation. The tight evolutionary relatedness and homoplasmy of mtDNAs that confer elevated risk for a late-onset disease contrast strikingly with the distant relatedness and heteroplasmy of mitochondrial genomes that cause early-onset disease. The dichotomy can be explained by a lack of selection against mutations that confer a phenotype at advanced age during most of the evolution of humans. We estimate that approximately 1.5 million Caucasians in the United States bear the 4336G mutation and are at significantly increased risk of developing mitochondrial AD in their lifetime. A mechanism for 4336G-mediated cell death is proposed.

Estimation of the proportion of genetic cases in late-onset diseases: an application to Alzheimer's disease

In diseases with a complex etiology including a genetic component, an important issue is to determine the proportion of cases really having inherited the disease. This is not easy in late-onset diseases where censoring might obscure the transmission pattern of the disease and give an apparently non-genetic distribution of the cases. We present a model that allows the estimation of the proportion of genetic cases in late-onset diseases. This model takes censoring into account by explicit modelling of the time dependency of the onset of the disease. The model is illustrated with an example based on an epidemiological survey in Alzheimer's disease and with simulated data.

Multicenter double-blind study examining the antidepressant effectiveness of the hypericum extract LI 160

Seventy-two depressive patients of 11 physicians' practices were treated in a double-blind study for a period of 6 weeks either with hypericum extract LI 160 or with placebo. Inclusion criterion was a major depression in accordance with DSM-III-R. The changes were assessed using four psychometric scales (HAMD, D-S, BEB, CGI). After 4 weeks of therapy, the statistical evaluation revealed a significant improvement in all four psychometric tests in the active group as compared to the placebo group. After switching the placebo group to active treatment (5th to 6th week of therapy), significant improvements were found in the original placebo group. No serious side effects were observed.

Chromosome 14-encoded Alzheimer's disease: genetic and clinicopathological description

A family of Finnish descent with very-early-onset Alzheimer's disease has been identified. Genetic analysis of this family eliminated the amyloid precursor protein gene as the pathogenic locus, but strongly implicated a locus on chromosome 14q23.4 between D14S52 and D14S55. The early age at onset of the disease (average, 36 years; range, 35-39 years), the rapid progression, and the early and prominent myoclonus, while they appear to be frequent findings in the chromosome 14-encoded form of Alzheimer's disease, raised the clinical suspicion of prion disease. However, sequencing the prion gene-coding region of 2 affected members of the pedigree failed to show any abnormality. Apart from the presence of modest cortical vacuolar change, the pathological features of our index patient appeared typical of Alzheimer's disease with abundant senile plaques immunoreactive with beta-amyloid, but not with prion protein antibodies.

Assessment of genetic polymorphisms in DNA from formalin fixed neurological tissues

The ability to analyze the genotype of deceased affected members of pedigrees segregating inherited neurological diseases considerably augments the informativeness of such pedigrees. This information has direct application in attempts to isolate disease genes by positional cloning strategies, and for genetic counselling. We show that the genotype at polymorphic simple sequence repeat loci can be determined from genomic DNA isolated from 10 micron thick paraffin embedded, formalin fixed neurological tissues. The critical constraint on this method is the size of the template target bearing the simple sequence repeat, which should ideally be less than 165 base pairs.

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.

Heterogeneity in Alzheimer's disease

The genetic data implicating mutations framing the beta-amyloid segment of the amyloid precursor protein as causes of Alzheimer's disease are reviewed and integrated with information on the normal processing of the amyloid precursor protein. The data indicating that there is a second and quantitatively major locus for early-onset Alzheimer's disease on the long arm of chromosome 14 are reviewed. The prediction that this second genetic locus will produce a protein intimately involved in the metabolism of the amyloid precursor protein is reiterated, together with the prediction that all causes of Alzheimer's disease will directly involve this process.

Potassium channel dysfunction in fibroblasts identifies patients with Alzheimer disease

Since memory loss is characteristic of Alzheimer disease (AD), and since K+ channels change during acquisition of memory in both molluscs and mammals, we investigated K+ channel function as a possible site of AD pathology and, therefore, as a possible diagnostic index as well. A 113-pS tetraethylammonium (TEA)-sensitive K+ channel was consistently absent from AD fibroblasts, while it was often present in young and aged control fibroblasts. A second (166-pS) K+ channel was present in all three groups. Elevated external potassium raised intracellular Ca2+ in all cases. TEA depolarized and caused intracellular Ca2+ elevation in young and aged control fibroblasts but not AD fibroblasts. The invariable absence of a 113-pS TEA-sensitive K+ channel and TEA-induced Ca2+ signal indicate K+ channel dysfunction in AD fibroblasts. These results suggest the possibility of a laboratory method that would diagnostically distinguish AD patients, with or without a family history of AD, from normal age-matched controls and also from patients with non-AD neurological and psychiatric disorders.

Isolation and characterization of APLP2 encoding a homologue of the Alzheimer's associated amyloid beta protein precursor

Familial Alzheimer's disease (FAD) is a genetically heterogeneous disorder that includes a rare early-onset form linked to mutations in the amyloid b protein precursor (APP) gene. Clues to the function of APP derive from the recent finding that it is a member of a highly conserved protein family that includes the mammalian amyloid precursor-like protein (APLP1) gene which maps to the same general region of human chromosome 19 linked to late-onset FAD. Here we report the isolation of the human APLP2 gene. We show that APLP2 is a close relative of APP and exhibits a very similar pattern of expression in the brain and throughout the body. Like APP, APLP2 contains a cytoplasmic domain predicted to couple with the GTP-binding protein G(o) indicating that it may be an additional cell surface activator of this G protein.

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.

Mutation of the gene for the human lysosomal serine protease cathepsin G is not the cause of aberrant APP processing in familial Alzheimer disease

Recent genetic linkage studies have implicated a gene on chromosome 14 in the pathogenesis of FAD. The identity of this gene remains unknown but it has been speculated that it may be involved in the cellular processing of the amyloid precursor protein (APP). We have analyzed the nucleotide sequence of the entire open reading frame of the cathepsin G gene located on chromosome 14q. No mutations were observed, suggesting that defects in this lysosomal protease are not responsible for aberrant accumulation of proteolytic products of APP in FAD brain tissue.

The power of systematic genealogical study in familial Alzheimer disease

Several kindreds (N, C, To and RB) with familial Alzheimer disease (FAD) from the same small area of Calabria are currently under study. Recently two of us (F.M. and L.F-S.) identified a family in Milan (FJ01) made up of 3 siblings whose parents were of Calabrian origin. Through a subsequent systematic or blanket genealogical study a link has been traced between kindreds To and FJ01. We discuss the relevance of these results to genetic studies.

Genetic heterogeneity of gene defects responsible for familial Alzheimer disease

Inherited Alzheimer's disease is a genetically heterogeneous disorder that involves gene defects on at least five chromosomal loci. Three of these loci have been found by genetic linkage studies to reside on chromosomes 21, 19, and 14. On chromosomes 21, the gene encoding the precursor protein of Alzheimer-associated amyloid (APP) has been shown to contain several mutations in exons 16 and 17 which account for roughly 2-3% of familial Alzheimer's disease (FAD). The other loci include what appears to be a susceptibility gene on chromosome 19 associated with late-onset (> 65 years) FAD, and a major early-onset FAD gene defect on the long arm of chromosome 14. In other early- and late-onset FAD kindreds, the gene defects involved do not appear to be linked to any of these three loci, indicating the existence of additional and as of yet unlocalized FAD genes. This review provides a historical perspective of the search for FAD gene defects and summarizes the progress made in world-wide attempts to isolate and characterize the genes responsible for this disorder.

Linkage analysis in familial Alzheimer disease: description of the Duke and Boston data sets

Familial Alzheimer diseases is a neurological disorder of adult onset. Three research centers have each contributed their families and genetic linkage data for combined analyses. The data from the Duke and Boston centers, comprising 73 pedigrees for whom numerous markers on chromosomes 19 and 21 were typed are described.

Sib-pair linkage analyses of Alzheimer's disease

Sib-pair linkage analyses were used to search for linkage to a set of chromosome 19 and 21 marker loci in two sets of families with Alzheimer's disease. The advantage of this technique is that no assumption is made about the mode of inheritance of the disease. Some mild suggestions of linkage were found in early-onset families for a chromosome 21 marker and in a set of late-onset families for a chromosome 19 marker.

Genetic evidence for a novel familial Alzheimer's disease locus on chromosome 14

Familial Alzheimer's disease (FAD) has been shown to be genetically heterogeneous, with a very small proportion of early onset pedigrees being associated with mutations in the amyloid precursor protein (APP) gene on chromosome 21, and some late onset pedigrees showing associations with markers on chromosome 19. We now provide evidence for a major early onset FAD locus on the long arm of chromosome 14 near the markers D14S43 and D14S53 (multipoint lod score z = 23.4) and suggest that the inheritance of FAD may be more complex than had initially been suspected.

Monozygotic twins concordant for late-onset probable Alzheimer disease with suspected Alzheimer disease in four sibs

Probable Alzheimer disease (AD) is described in 79-year-old male twins with monozygosity confirmed by DNA examination. The first twin to be affected began to show signs of intellectual deterioration at age 70. In the other, onset was at age 72. Four of their living sibs (current age range = 75-92) are also suspected to have AD. The possible roles of genetic and environmental factors in the development of AD in this sibship are discussed.

Nature of mental retardation and dementia in Down syndrome: study with PET, CT, and neuropsychology

Recent evidence suggests that Alzheimer's disease is an etiologically heterogeneous disorder. A human model of Alzheimer's disease exists that avoids such problems of etiologic heterogeneity. Down syndrome (DS), trisomy 21, is a genetic disorder in which an extra portion of chromosome 21 leads to mental retardation, short stature, and phenotypic abnormalities. Prior investigations by others have shown that DS subjects over 40 years of age demonstrate neuropathologic and neurochemical defects postmortem that are virtually indistinguishable from those found in brains of Alzheimer's disease patients and a universal cognitive deterioration more severe in demented than nondemented older DS subjects. In our study, these nondemented older DS subjects show a distinctive pattern of age-related deficits, while a more global pattern is seen in demented older DS subjects. Dementia occurs in 40% of older DS subjects. We find that in older demented DS subjects positron emission tomography (PET) shows identical patterns of abnormal glucose metabolism as those described previously in Alzheimer's disease patients, selectively involving the phylogenetically newer association areas of parietal and temporal neocortices but sparing primary sensory and motor regions. Further, we find in older demented DS patients quantitative computer-assisted tomography (CT) indicates accelerated neuronal loss and brain atrophy, similar to that previously shown in Alzheimer's disease patients. As a potential use of the DS model, we observed a case of DS with dementia but without mental retardation. This case suggests that expression of dementia in DS may involve genes on chromosome 21 other than in the "obligatory" distal segment of the q arm. Alternatively, differential expression of genes on the q arm of chromosome 21 might cause dementia without phenotypic features and mental retardation.

Risk factors for cognitive aging in adult twins

Monozygotic (MZ) and dizygotic (DZ) twins in later adulthood were studied in order to examine genetic and environmental contributions to the decline of cognitive performance. In this study, 118 twin pairs took a comprehensive medical examination at a university hospital. Cognitive function was measured by the Wechsler Adult Intelligence Scale (WAIS). The intraclass correlation coefficients on Digit Span (D) and Digit Symbol (DS) subtests of the WAIS did not show any significant difference between MZ and DZ twins although Block Design (BD) showed a significant difference. The values of the intraclass correlation coefficients were mostly around 0.5 and showed significant within-pair similarity of test scores. The mean score of D, DS and BD declined with advancing age. The intraclass correlation coefficients for D, DS and BD were around 0.2 in the MZ twins reared apart, and around 0.6 in the MZ twins reared together. These results indicated a significant environmental influence on cognitive aging in later adulthood.

Deterioration on the Blessed test in Alzheimer's disease: longitudinal data and their implications for clinical trials and identification of subtypes

One hundred eleven patients with probable Alzheimer's disease (AD) were given the Blessed test (BT) of information, memory, and concentration (scored 0-33) at 6-month intervals over periods of 6-96 months. For each patient, the change in the total BT score between pairs of visits at 6- and 12-month intervals was measured. Mean deterioration scores over 6 and 12 months were 2.2 (SD = 3.2) and 4.1 (SD = 4.1) points, respectively. There was no significant correlation between degree of dementia on the BT and the rate of deterioration. Gender, age of onset, and family history had no significant effect on the rate of deterioration. The implications of the results for treatment trials and investigations of clinical heterogeneity are discussed.

Spectrum of amyloid beta-protein immunoreactivity in hereditary Alzheimer disease with a guanine to thymine missense change at position 1924 of the APP gene

We studied neuropathologically 3 patients of a previously unreported kindred of presenile Alzheimer disease (AD), characterized by a G to T mutation at base pair 1924 (695 transcript) of the amyloid precursor protein gene. Classic features of presenile AD are observed. Neurofibrillary tangles with paired helical filaments as well as neuritic plaques are found in large number in neocortex and hippocampus. beta-Protein deposits in meningeal and parenchymal vessels are present, but not severe. Prominent subpial ribbon-like deposits are detected with antibodies to a 28-residue synthetic peptide; however, only occasionally can they be seen in thioflavin S treated sections. Along with a mild involvement of vessels, as demonstrated by beta-protein immunolabeling, parenchymal involvement is also seen in the cerebellar molecular layer. In the course of the study, we have not detected neuropathologic changes, which are mutation specific. Further investigations of familial Alzheimer disease with known genetic mutations will clarify whether correlations exist between specific mutations and neuropathologic phenotypes.

Commingling analysis of memory performance in offspring of Alzheimer patients

Dementia of the Alzheimer type (DAT) is a neurodegenerative disorder which afflicts approximately 3% of the population. Genetic influences are indicated from twin and family studies although genetic heterogeneity has been suggested from both pedigree analyses and linkage investigations. Autosomal dominant inheritance with age-dependent penetrance has been suggested in at least some families with DAT. In the present investigation, we examine memory and nonmemory task performance in 106 asymptomatic offspring (mean age 40.6 years) of 54 DAT probands. Intraclass sibling correlations revealed little evidence of sibling similarity for performance on three memory tasks which have been reported to be relatively sensitive to the memory losses accompanying DAT. Subsequent investigations of the distributions of the cognitive task scores in the offspring revealed evidence for a commingling of two distributions for the three memory tasks but not for the nonmemory measures. These findings are consistent with a hypothesis that these distributions reflect genotypic subgroups, carriers, and noncarriers, of a presumed DAT gene.

A family with multiple instances of definite, probable and possible early-onset Alzheimer's disease

A family with a multigenerational history of proven or suspected early-onset Alzheimer's disease (AD) consistent with autosomal-dominant inheritance is described. To date, the pedigree comprises five generations in which there are 13 known affected individuals. The mean age of onset of cognitive deficits in those for whom data are available (n = 11) is 47.6 (s.d. 3.0) years and the mean age of death (n = 10) is 58.8 (s.d. 4.0) years. The variability in the extent and quality of available data illustrates the diagnostic difficulties encountered in ascertaining such an extended pedigree, and the need for caution in interpreting the evidence.

Genetic linkage studies of human neurodegenerative disorders

Recombinant DNA technology has the ability to delineate the causes of several neurodegenerative disorders. Genetic linkage studies have been used successfully to localize gene defects and it is likely that in the near future the exact loci will be determined.

Amyloid deposition as the central event in the aetiology of Alzheimer's disease

While there may be many causes of Alzheimer's disease (AD), the same pathological sequence of events, described here by John Hardy and David Allsop, is likely to occur in all cases. The recent discovery of a pathogenic mutation in the beta-amyloid precursor protein (APP) gene on chromosome 21 suggests that APP Mismetabolism and beta-amyloid deposition are the primary events in the disease process. The occurrence of AD in Down syndrome is consistent with this hypothesis. The pathological cascade for the disease process is most likely to be: beta-amyloid deposition----tau phosphorylation and tangle formation----neuronal death. The development of a biochemical understanding of this pathological cascade will facilitate rational design of drugs to intervene in this process.

Genetic linkage analysis of nerve growth factor (beta) in familial Alzheimer's disease

Recent studies have not shown linkage of late-onset (mean age, greater than 60 years) familial Alzheimer's disease (FAD) to the chromosome 21 locus reported linked to early-onset FAD. Beta nerve growth factor (beta-NGF) has been considered a candidate gene in the pathogenesis and therapy of FAD, based on its localization in the cortex and hippocampus and its ability to enhance the growth and survival of cholinergic neurons. A 1.5-kb fragment of the beta-NGF gene was used to detect a BglII restriction fragment length polymorphism, which was then used for linkage analysis. A total of 30 families (27 late onset) with 147 affected members were studied. Close linkage (theta less than or equal to 0.03, z less than or equal to -2.00) of late-onset FAD with beta-NGF was excluded. Two apparent obligate crossovers between affected members were detected in different autopsy-confirmed families. Based on these results, beta-NGF is not the gene responsible for late-onset FAD in the families analyzed.

The distribution of tangles, plaques and related immunohistochemical markers in healthy aging and Alzheimer's disease

Neurofibrillary tangles and senile plaques, together with cells immunoreactive for the Alz-50 antibody (A50-ir cells) or for an antibody against paired helical filaments (PHF-ir cells), and amyloid deposits stained with antibodies against beta-(or A4)-amyloid, have been mapped throughout the ventral forebrains of 25 old people. The cognitive status of each individual was assessed and a "Clinical Dementia Rating" (CDR) assigned, either before death in the Memory and Aging Project of Washington University, or by a postmortem interview, with an appropriate collateral source. The cases studied included 13 nondemented cases (CDR = 0), six very mildly to mildly demented cases (CDR = 0/0.5 to 1) and six more severely demented cases (CDR = 2 to 3). Because even the very mildly demented brains showed substantial pathological change, emphasis was placed on examining the nondemented cases for the earliest changes that could be associated with Alzheimer's disease. Different distributions were found for tangles and plaques. Tangles (and A50-ir and PHF-ir cells) were present in all of the brains examined. In the younger nondemented cases (aged 54 to 63) there were a few affected cells in the anterior olfactory nucleus and the parahippocampal gyrus. In older nondemented cases (aged 73-89) more tangles were found in the same areas, and also in hippocampal field CA1. The very mildly demented cases had many more tangles, but their distribution was similar. Only in the severely demented cases were large numbers of tangles present in the neocortex. In contrast, no plaques (or beta-amyloid immunoreactivity) were found in any of the younger nondemented cases or in four of the eight older nondemented cases. In three older nondemented cases there were a few primitive plaques, which were restricted to localized regions of the neocortex (e.g., a portion of the inferior temporal cortex). In one nondemented case and all of the very mildly to mildly demented cases there were very large numbers of mostly primitive plaques, particularly in the neocortex. With greater severity of dementia there is a shift from primitive to mature plaques. These results were interpreted to imply that the first development of tangles and plaques occurs in different parts of the brain. Tangles appear during aging in the anterior olfactory nucleus, the parahippocampal gyrus and the hippocampus, but are rare in the neocortex except in demented brains. Conversely plaques may develop first in the neocortex. Unlike tangles, plaques are not a consistent feature of aging, at least up to age 80.

Diagnostic and pharmacological approaches in Alzheimer's disease

Alzheimer's disease is a chronic progressive disease affecting higher intellectual functioning. The clinical diagnosis is made when the onset of illness is insidious, the course slowly progressive and all the treatable causes of dementia have been ruled out. The use of more stringent criteria has improved clinical diagnosis, but at best only 80% of patients are accurately diagnosed. Ultimately the diagnosis depends upon pathological confirmation. The neuritic plaques and neurofibrillary tangles described by Alzheimer, although not pathognomonic for the disease, continue to be the basis for pathological diagnosis. The aetiology and pathophysiology of Alzheimer's disease are presently unknown. Epidemiological studies have suggested a genetic basis for the disorder, and many biochemical studies have linked it to degeneration of central cholinergic neurons, and possibly to abnormalities of other neurotransmitter systems. A marker which would permit accurate diagnosis early in the course of disease would be of major importance to researchers and clinicians alike. No marker has been found to date, although recent research results are promising. Various pharmacological strategies have been employed in the treatment of Alzheimer's disease. More recently attempts have focused on enhancing central cholinergic transmission. Despite the well-founded rationale for these studies, results have been modest.

Twin studies of Alzheimer's disease: an approach to etiology and prevention

Epidemiologic studies of environmental factors associated with risk of Alzheimer's disease (AD) have produced inconsistent and disappointing results. By contrast, family/genetic studies and case control investigations suggest that genetic causes of AD are important. The investigation of such genetic causes remains an important aim in all forms of AD including typical, late-onset disease where linkage work is impractical. But the public health burden of AD creates an especially urgent need to identify environment risk factors, if these exist, since they will more likely be susceptible to intervention. Such environmental factors may interact with genetic susceptibility to accelerate or retard disease expression, and environmental interventions that delay onset may constitute an important strategy for prevention. All these issues may be addressed by twin studies of AD, but the few such studies to date have been limited by small samples and other methodologic difficulties. This paper reviews the rationale for twin studies of AD, and describes briefly the work in this area to date. It also discusses a number of suggestions for methodologic improvements. We conclude that the time is ripe for twin studies of AD, and that such work holds considerable potential for the investigation of etiology and, possibly, for the identification of strategies for prevention.

Genetic linkage studies suggest that Alzheimer's disease is not a single homogeneous disorder

Alzheimer's disease, a fatal neurodegenerative disorder of unknown aetiology, is usually considered to be a single disorder because of the general uniformity of the disease phenotype. Two recent genetic linkage studies revealed co-segregation of familial Alzheimer disease with the D21S1/S11 and D21S16 loci on chromosome 21. But two other studies, one of predominantly multiplex kindreds with a late age-of-onset, the other of a cadre of kindreds with a unique Volga German ethnic origin, found absence of linkage at least to D21S1/S11. So far it has not been possible to discern whether these conflicting reports reflect aetiological heterogeneity, differences in methods of pedigree selection, effects of confounding variables in the analysis (for example, diagnostic errors, assortative matings), or true non-replication. To resolve this issue, we have now examined the inheritance of five polymorphic DNA markers from the proximal long arm of chromosome 21 in a large unselected series of pedigrees with familial Alzheimer's disease. Our data suggest that Alzheimer's disease is not a single entity, but rather results from genetic defects on chromosome 21 and from other genetic or nongenetic factors.

Familial Alzheimer's disease: genetic analysis related to disease heterogeneity, Down syndrome and human brain evolution

Etiologically heterogeneous subgroups of patients with Alzheimer's disease (AD) exist and need to be distinguished so as to better identify genetic causes of familial cases. Furthermore, the presence of AD neuropathology in Down syndrome (trisomy 21) subjects older than 35 years suggests that AD in some cases is caused by dysregulation of expression of genes on chromosome 21. Cerebral metabolic abnormalities in life, and the distribution of AD neuropathology in the post-mortem brain, indicate that AD involves the association neocortices and subcortical regions with which they evolved during evolution of the human brain. Accordingly, understanding the molecular basis of this evolution should elucidate the genetic basis of AD, whereas knowing the genetics of AD should be informative about the genomic changes which promoted brain evolution.