Evidence that the APOE locus influences rate of disease progression in late onset familial Alzheimer's Disease but is not causative

Quantitative endophenotypes as an alternative approach to understanding genetic risk in neurodegenerative diseases

Endophenotypes, as measurable intermediate features of human diseases, reflect underlying molecular mechanisms. The use of quantitative endophenotypes in genetic studies has improved our understanding of pathophysiological changes associated with diseases. The main advantage of the quantitative endophenotypes approach to study human diseases over a classic case-control study design is the inferred biological context that can enable the development of effective disease-modifying treatments. Here, we summarize recent progress on biomarkers for neurodegenerative diseases, including cerebrospinal fluid and blood-based, neuroimaging, neuropathological, and clinical studies. This review focuses on how endophenotypic studies have successfully linked genetic modifiers to disease risk, disease onset, or progression rate and provided biological context to genes identified in genome-wide association studies. Finally, we review critical methodological considerations for implementing this approach and future directions.

Microglial signatures and their role in health and disease

Microglia are the primary innate immune cells in the CNS. In the healthy brain, they exhibit a unique molecular homeostatic 'signature', consisting of a specific transcriptional profile and surface protein expression pattern, which differs from that of tissue macrophages. In recent years, there have been a number of important advances in our understanding of the molecular signatures of homeostatic microglia and disease-associated microglia that have provided insight into how these cells are regulated in health and disease and how they contribute to the maintenance of the neural environment.

Variants regulating ZBTB4 are associated with age-at-onset of Alzheimer's disease

The identification of novel genetic modifiers of age-at-onset (AAO) of Alzheimer's disease (AD) could advance our understanding of AD and provide novel therapeutic targets. A previous genome scan for modifiers of AAO among families affected by early-onset AD caused by the PSEN2 N141I variant identified 2 loci with significant evidence for linkage: 1q23.3 and 17p13.2. Here, we describe the fine-mapping of these 2 linkage regions, and test for replication in 6 independent datasets. By fine-mapping these linkage signals in a single large family, we reduced the linkage regions to 11% their original size and nominated 54 candidate variants. Among the 11 variants associated with AAO of AD in a larger sample of Germans from Russia, the strongest evidence implicated promoter variants influencing NCSTN on 1q23.3 and ZBTB4 on 17p13.2. The association between ZBTB4 and AAO of AD was replicated by multiple variants in independent, trans-ethnic datasets. Our results show association between AAO of AD and both ZBTB4 and NCSTN. ZBTB4 is a transcriptional repressor that regulates the cell cycle, including the apoptotic response to amyloid beta, while NCSTN is part of the gamma secretase complex, known to influence amyloid beta production. These genes therefore suggest important roles for amyloid beta and cell cycle pathways in AAO of AD.

A common haplotype lowers PU.1 expression in myeloid cells and delays onset of Alzheimer's disease

A genome-wide survival analysis of 14,406 Alzheimer's disease (AD) cases and 25,849 controls identified eight previously reported AD risk loci and 14 novel loci associated with age at onset. Linkage disequilibrium score regression of 220 cell types implicated the regulation of myeloid gene expression in AD risk. The minor allele of rs1057233 (G), within the previously reported CELF1 AD risk locus, showed association with delayed AD onset and lower expression of SPI1 in monocytes and macrophages. SPI1 encodes PU.1, a transcription factor critical for myeloid cell development and function. AD heritability was enriched within the PU.1 cistrome, implicating a myeloid PU.1 target gene network in AD. Finally, experimentally altered PU.1 levels affected the expression of mouse orthologs of many AD risk genes and the phagocytic activity of mouse microglial cells. Our results suggest that lower SPI1 expression reduces AD risk by regulating myeloid gene expression and cell function.

Neuropsychiatric symptoms and Apolipoprotein E: Associations with eventual Alzheimer's disease development

OBJECTIVE

Alzheimer's disease (AD) is the result of neurodegeneration, which manifests clinically as deficits in memory, thinking, and behavior. It was hypothesized that neuropsychiatric symptoms and the apolipoprotein E genotype increase the likelihood of Alzheimer's disease development.

METHODS

Utilizing data from the National Alzheimer's Coordinating Center, information from evaluations of 11,453 cognitively intact participants was analyzed. Survival analysis was used to explore relationships between individual neuropsychiatric symptoms as determined by the Neuropsychiatric Inventory Questionnaire, apolipoprotein E, and eventual AD diagnosis. Cox proportional hazard models were utilized to explore the main effects and synergistic (additive and multiplicative) interactions.

RESULTS

This study provided evidence for an increased hazard of developing AD among participants with any of the symptoms assessed by the NPI-Q. The hazard of developing AD was almost thirteen times higher for ε4 carriers with delusions and eleven times greater for those with apathy and disinhibition. Statistically significant hazards (p>0.001) were also realized by ε4 carriers with hallucinations; agitation; depression; anxiety; elation; apathy; irritability; and motor, sleep, and appetite disturbances.

CONCLUSIONS

Findings suggest that neuropsychiatric symptoms are associated with eventual AD diagnosis among a group of cognitively asymptomatic participants at baseline. Many studies begin with a group of participants already impacted by AD diagnosis. The longitudinal analysis of a group of participants who, at baseline, demonstrated no observable signs of AD was a strength of this study. This investigation contributes to the literature exploring an increased hazard of AD due to potential modifiable risk factors and genetic biomarkers such as apolipoprotein E.

Genome scan in familial late-onset Alzheimer's disease: a locus on chromosome 6 contributes to age-at-onset

Alzheimer's disease (AD) is a common, genetically complex, fatal neurodegenerative disorder of late life. Although several genes are known to play a role in early-onset AD, identification of the genetic basis of late onset AD (LOAD) has been challenging, with only the APOE gene known to have a high contribution to both AD risk and age-at-onset. Here, we present the first genome-scan analysis of the complete, well-characterized University of Washington LOAD sample of 119 pedigrees, using age-at-onset as the trait of interest. The analysis approach used allows for a multilocus trait model while at the same time accommodating age censoring, effects of APOE as a known genetic covariate, and full pedigree and marker information. The results provide strong evidence for linkage of loci contributing to age-at-onset to genomic regions on chromosome 6q16.3, and to 19q13.42 in the region of the APOE locus. There was evidence for interaction between APOE and the locus on chromosome 6q and suggestive evidence for linkage to chromosomes 11p13, 15q12-14, and 19p13.12. These results provide the first independent confirmation of an AD age-at-onset locus on chromosome 6 and suggest that further efforts towards identifying the underlying causal locus or loci are warranted.

Genome scan of age-at-onset in the NIMH Alzheimer disease sample uncovers multiple loci, along with evidence of both genetic and sample heterogeneity

Alzheimer's disease (AD) is a common neurodegenerative disorder of late life with a complex genetic basis. Although several genes are known to play a role in rare early onset AD, only the APOE gene is known to have a high contribution to risk of the common late-onset form of the disease (LOAD, onset >60 years). APOE genotypes vary in their AD risk as well as age-at-onset distributions, and it is likely that other loci will similarly affect AD age-at-onset. Here we present the first analysis of age-at-onset in the NIMH LOAD sample that allows for both a multilocus trait model and genetic heterogeneity among the contributing sites, while at the same time accommodating age censoring, effects of known genetic covariates, and full pedigree and marker information. The results provide evidence for genomic regions not previously implicated in this data set, including regions on chromosomes 7q, 15, and 19p. They also affirm evidence for loci on chromosomes 1q, 6p, 9q, 11, and, of course, the APOE locus on 19q, all of which have been reported previously in the same sample. The analyses failed to find evidence for linkage to chromosome 10 with inclusion of unaffected subjects and extended pedigrees. Several regions implicated in these analyses in the NIMH sample have been previously reported in genome scans of other AD samples. These results, therefore, provide independent confirmation of AD loci in family-based samples on chromosomes 1q, 7q, 19p, and suggest that further efforts towards identifying the underlying causal loci are warranted.

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.

Polymorphisms in LMNA and near a SERPINA gene cluster are associated with cognitive function in older people

A recent genome-wide association (GWA) study of late-onset Alzheimer's disease (LOAD) identified 15 novel single nucleotide polymorphisms (SNPs) independent of ApoE. We hypothesised that variants associated with LOAD are also associated with poor cognitive function in elderly populations. We measured additive associations between the five most strongly associated LOAD SNPs and grouped Mini Mental State Examination (MMSE) scores. Variants were genotyped in respondents (mean age 79 years) from the Oxford Healthy Ageing project (OHAP) and other sites of the MRC Cognitive Function and Ageing Study (MRC-CFAS). In adjusted ordinal logistic models, two variants were associated with poorer cognitive function: rs11622883 (OR=1.14, 95% CI: 1.01-1.28, p=0.040) and rs505058 (OR=1.29, 95% CI: 1.02-1.64, p=0.036). These SNPs are close to a SERPINA gene cluster and within LMNA, respectively. The mechanisms underlying the associations with cognitive impairment and LOAD require further elucidation, but both genes are interesting candidates for involvement in age-related cognitive impairment.

Evidence for three loci modifying age-at-onset of Alzheimer's disease in early-onset PSEN2 families

Families with early-onset Alzheimer's disease (AD) sharing a single PSEN2 mutation exhibit a wide range of age-at-onset, suggesting that modifier loci segregate within these families. While APOE is known to be an age-at-onset modifier, it does not explain all of this variation. We performed a genome scan within nine such families for loci influencing age-at-onset, while simultaneously controlling for variation in the primary PSEN2 mutation (N141I) and APOE. We found significant evidence of linkage between age-at-onset and chromosome 1q23.3 (P < 0.001) when analysis included all families, and to chromosomes 1q23.3 (P < 0.001), 17p13.2 (P = 0.0002), 7q33 (P = 0.017), and 11p14.2 (P = 0.017) in a single large pedigree. Simultaneous analysis of these four chromosomes maintained strong evidence of linkage to chromosomes 1q23.3 and 17p13.2 when all families were analyzed, and to chromosomes 1q23.3, 7q33, and 17p13.2 within the same single pedigree. Inclusion of major gene covariates proved essential to detect these linkage signals, as all linkage signals dissipated when PSEN2 and APOE were excluded from the model. The four chromosomal regions with evidence of linkage all coincide with previous linkage signals, associated SNPs, and/or candidate genes identified in independent AD study populations. This study establishes several candidate regions for further analysis and is consistent with an oligogenic model of AD risk and age-at-onset. More generally, this study also demonstrates the value of searching for modifier loci in existing datasets previously used to identify primary causal variants for complex disease traits.

Increased functional brain response during word retrieval in cognitively intact older adults at genetic risk for Alzheimer's disease

Recent language studies in aging and dementia provide two complementary lines of evidence that: (1) measures of semantic knowledge and word-finding ability show declines comparable to those of episodic memory, and greater impairment than executive function measures, during the prodromal period of Alzheimer's disease and (2) cognitively intact older adult carriers of the apolipoprotein E (APOE) epsilon4 allele also demonstrate poorer object naming than their low-risk peers. Given that possible changes in the neural substrates of word retrieval (e.g., Broca's area and fusiform gyrus) in at-risk adults may signal impending cognitive decline and serve as a prodromal marker of AD, we examined whether APOE epsilon4 carriers exhibit changes in brain response in regions subserving word retrieval and semantic knowledge. Eleven cognitively intact APOE epsilon4 older adults and 11 age, education, and family history of AD-matched APOE epsilon3 adults named aloud photographs of animals, tools, and vehicles during event-related fMRI. Results showed that, in the face of equivalent naming accuracy, APOE epsilon4 adults demonstrated more widespread brain response with greater signal change in the left fusiform gyrus, bilateral medial prefrontal cortex, and right perisylvian cortex. Findings are discussed in the context of possible compensatory mechanisms invoked to maintain performance in those at genetic risk for AD.

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.

Apolipoprotein E promoter polymorphisms (-491A/T and -427T/C) and Alzheimer's disease: no evidence of association in the Irish population

BACKGROUND

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder with as yet poorly understood aetiology. Both environmental and genetic factors have been implicated as predisposing factors. The APOE e4 allele is an established genetic susceptibility factor for AD for several populations including the Irish. Polymorphisms (-491A/T and -427T/C) at the promoter region of the APOE gene are postulated to affect the expression of the gene through differential binding of transcription factors.

AIMS

Two APOE promoter polymorphisms (-491A/T and -427T/C) are examined for possible association with AD.

METHODS

Using a case-control study design, a sample of 112 Irish late onset Alzheimer's (LOAD) patients and 107 ethnically matched controls were investigated for association with the above polymorphisms.

CONCLUSIONS

No evidence of association between any of the examined markers and AD was observed. Haplotype analysis using markers -491A/T and -427T/C in conjunction with the APOE (Hha I) polymorphism revealed significant associations of three haplotypes with AD. However, this association was mainly due to the highly significant association of the APOE e4 allele with AD and not of the promoter variants.

Effect of heterogeneity on the chromosome 10 risk in late-onset Alzheimer disease

With the exception of ApoE (APOE), no universally accepted genetic association has been identified with late-onset Alzheimer disease (AD). A broad region of chromosome 10 has engendered continued interest generated from both preliminary genetic linkage and candidate gene studies. To better examine this region, we combined unbiased genetic linkage with candidate gene association studies. We genotyped 36 SNPs evenly spaced across 80.2 Mb in a family-based data set containing 1,337 discordant sibling pairs in 567 multiplex families to narrow the peak region of linkage using both covariate and subset analyses. Simultaneously, we examined five functional candidate genes (VR22, LRRTM3, PLAU, TNFRSF6, and IDE) that also fell within the broad area of linkage. A total of 50 SNPs were genotyped across the genes in the family-based data set and an independent case-control data set containing 483 cases and 879 controls. Of the 50 SNPs in the five candidate genes, 22 gave nominally significant association results in at least one data set, with at least one positive SNP in each gene. SNPs rs2441718 and rs2456737 in VR22 (67.8 Mb) showed association in both family-based and case-control data sets (both P=0.03). A two-point logarithmic odds (LOD) score of 2.69 was obtained at SNP rs1890739 (45.1 Mb, P=0.03 in 21% of the families) when the families were ordered from low to high by ApoE LOD score using ordered subset analysis (OSA). These data continue to support a role for chromosome 10 loci in AD. However, the candidate gene and linkage analysis results did not converge, suggesting that there is more extensive heterogeneity on chromosome 10 than previously appreciated.

Apolipoprotein E epsilon4 genotype and gender: effects on memory

OBJECTIVE

Episodic recognition memory for odors and visual was assessed in apolipoprotein E (ApoE) epsilon4-positive and epsilon4-negative men and women diagnosed with Alzheimer disease (AD) and a healthy age- and gender-matched comparison group.

METHODS

A total of 38 AD patients and 38 age- and gender-matched healthy older adults completed a recognition memory task involving three categories of stimuli: odors, faces, and symbols.

RESULTS

In the healthy comparison group, men who were epsilon4 negative outperformed epsilon4-positive men in recognition memory for odors and committed fewer false-positive errors. However, there were no significant differences between epsilon4-negative and epsilon4-positive women in the comparison group. No significant gender or ApoE status differences were detected in recognition memory for faces or symbols in the comparison group. In patients with AD, epsilon4-negative women outperformed epsilon4-positive women in recognition memory for odors and committed significantly fewer false-positive errors. However, there were no significant differences between epsilon4-positive and epsilon4-negative men. There were no significant gender or ApoE status differences in recognition memory for faces or symbols in AD patients.

CONCLUSION

The results demonstrate that recognition memory for olfactory stimuli may be particularly impaired in healthy older men with the epsilon4 allele. In patients with AD, odor memory impairments may be less severe in women who are negative for the epsilon4 allele. The results offer new insight into how recognition memory is affected by gender, the epsilon4 allele, and the modality of the stimulus to be remembered in healthy older adults and patients with AD.

Opposite Transcriptional Regulation in Skeletal Muscle of AMP-activated Protein Kinase γ3 R225Q Transgenic Versus Knock-out Mice

AMP-activated protein kinase (AMPK) is an evolutionarily conserved heterotrimer important for metabolic sensing in all eukaryotes. The muscle-specific isoform of the regulatory gamma-subunit of the kinase, AMPK gamma3, has an important role in glucose uptake, glycogen synthesis, and fat oxidation in white skeletal muscle, as previously demonstrated by physiological characterization of AMPK gamma3 mutant (R225Q) transgenic (TgPrkag3(225Q)) and gamma3 knock-out (Prkag3(-/-)) mice. We determined AMPK gamma3-dependent regulation of gene expression by analyzing global transcription profiles in glycolytic skeletal muscle from gamma3 mutant transgenic and knock-out mice using oligonucleotide microarray technology. Evidence is provided for coordinated and reciprocal regulation of multiple key components in glucose and fat metabolism, as well as skeletal muscle ergogenics in TgPrkag3(225Q) and Prkag3(-/-) mice. The differential gene expression profile was consistent with the physiological differences between the models, providing a molecular mechanism for the observed phenotype. The striking pattern of opposing transcriptional changes between TgPrkag3(225Q) and Prkag3(-/-) mice identifies differentially expressed targets being truly regulated by AMPK and is consistent with the view that R225Q is an activating mutation, in terms of its downstream effects. Additionally, we identified a wide array of novel targets and regulatory pathways for AMPK in skeletal muscle.

APOA1 polymorphism influences risk for early-onset nonfamiliar AD

Alterations in cholesterol homeostasis influence the risk for Alzheimer's disease (AD). Apolipoprotein A1 is the major apolipoprotein of the high-density lipoprotein and is involved in reverse cholesterol transport. Variation in the apolipoprotein A1 gene (APOA1) might influence the function of the protein, and thus brain cholesterol metabolism, leading to an increased risk for AD. Two polymorphisms of APOA1, a G/A substitution at position -75bp and a C/T and G/A base substitution at position +83bp or +84bp, or both, in the APOA1 promoter, have been described. We investigated the effect of these polymorphisms on the risk for AD in 427 AD patients and 500 healthy control subjects of German and English descent. The A allele of the APOA1 -75bp G/A polymorphism was associated with an increased risk for AD in subjects with an age at onset of 66 years or younger. Further data analysis indicated that AD patients homozygous for the A allele at position -75bp presented with disease onset 8 years earlier than carriers of at least one G allele. No influence of the +83/84bp polymorphism on the risk for AD was observed. These results suggest that variants of APOA1 might influence the onset and the risk for AD.

APOE and other loci affect age-at-onset in Alzheimer's disease families with PS2 mutation

Several kindreds of Volga German (VG) ancestry have a single PS2 mutation that causes an autosomal dominant form of Alzheimer's disease (AD). These families show a wide range in age-at-onset, which suggests the existence of modifying factors other than the PS2 mutation. To examine evidence for a genetic basis of variation in onset age, we performed a Bayesian oligogenic segregation and linkage analysis on nine VG families confirmed to have at least one affected PS2 carrier. This analysis simultaneously estimated the effects of APOE and PS2 and the number and effects of additional loci affecting AD age-at-onset. In addition, a family effect accounted for shared environmental effects. This analysis approach has the advantage of full use of the complete pedigree structure, as well as use of information on unsampled individuals with phenotypic data. These analyses provide evidence that APOE plays a small, but significant, role in modifying the age-at-onset in these VG families. The effects estimated for the APOE epsilon3 and epsilon4 genotypes were consistent with those estimated in previous analysis of late-onset AD families, with evidence for a dose-dependent relationship between number of epsilon4 alleles and age-at-onset. We estimated an approximately 83% posterior probability of at least one modifier locus in addition to APOE, and that the fraction of the variance in age-at-onset attributable to PS2, APOE, other loci, and family effects is approximately 70, approximately 2, approximately 6.5, and approximately 8.5%, respectively. These results provide evidence that APOE and other loci modify onset in AD caused by PS2 mutation.

Evidence for a novel late-onset Alzheimer disease locus on chromosome 19p13.2

Late-onset familial Alzheimer disease (LOFAD) is a genetically heterogeneous and complex disease for which only one locus, APOE, has been definitively identified. Difficulties in identifying additional loci are likely to stem from inadequate linkage analysis methods. Nonparametric methods suffer from low power because of limited use of the data, and traditional parametric methods suffer from limitations in the complexity of the genetic model that can be feasibly used in analysis. Alternative methods that have recently been developed include Bayesian Markov chain-Monte Carlo methods. These methods allow multipoint linkage analysis under oligogenic trait models in pedigrees of arbitrary size; at the same time, they allow for inclusion of covariates in the analysis. We applied this approach to an analysis of LOFAD on five chromosomes with previous reports of linkage. We identified strong evidence of a second LOFAD gene on chromosome 19p13.2, which is distinct from APOE on 19q. We also obtained weak evidence of linkage to chromosome 10 at the same location as a previous report of linkage but found no evidence for linkage of LOFAD age-at-onset loci to chromosomes 9, 12, or 21.

The Role of Test Accuracy in Predicting Acceptance of Genetic Susceptibility Testing for Alzheimer's Disease

A survey questionnaire regarding perceptions of risk and genetic susceptibility to Alzheimer's disease (AD) was completed by 518 offspring of AD cases from families with multiple affected, ascertained as part of a genetic linkage study of late onset AD. The questionnaire focused on respondents' perceptions of their own risk for AD as well as on the properties of real and hypothetical susceptibility tests, including error rates for false-positive and false-negative test results. Our findings showed that about 20% of the sample would refuse a susceptibility test with zero error rates, about 40% would accept tests with very high error rates in both directions, and the remainder would exercise some discrimination. Acceptance of high test error rates was significantly associated with male gender, low education, and high perceived lifetime risk of AD. In a previous paper related to this work, we showed that physicians caring for these families exercised much more discrimination in judging the acceptability of genetic tests they would offer to these same respondents. The findings show that there is a pressing need to educate the public, particularly those with relatives affected by a complex disease, to expect standards of accuracy for genetic tests comparable to those that prevail in other diagnostic and prognostic testing efforts in the broad field of clinical medicine.

Evidence for parent of origin effect in late-onset Alzheimer disease

Evidence for a parent of origin effect in Alzheimer disease was obtained from a sample of 148 sibships on which affection status of the parents was sought using family history interviews. The parent study recruited families with two or more affected sibs for late onset AD utilizing rigorous diagnostic criteria. In 74 families, there was evidence of an affected parent, 49 maternal and 25 paternal. Genome scan data were analyzed for the sample as a whole and for the maternal and paternal families separately, using Genehunter-ASM. Seven regions with Z(lr) scores >or=2 were identified, four in maternal families (chr. 10,12,19,20) and three in paternal families (chr. 1,7,13). With the exception of the chromosome 10 finding, analysis by parent of origin greatly increased evidence of linkage in areas showing no linkage in the overall analyses. For example, a chr. 12 region reached a LOD = 2.29 among maternal families whereas the same region showed a LOD = 0.3 when all families were analyzed together. The strongest findings among maternal families (chr. 10 and 12) were followed up with fine mapping that resulted in an increase in maximum LOD scores from 2.7-3.2 on chr. 10, and 2.29-2.42 on chr. 12. These analyses highlight the importance of parent of origin effects in late-onset AD families and identify several genomic regions that may include genes linked to late-onset AD specific to disease transmission from the mother and require further investigation.

Confirmation of association between D10S583 and Alzheimer's disease in a case--control sample

Several independent studies have reported that loci on chromosome 10 are associated/linked with Alzheimer's disease (AD), including a family-based study demonstrating an association between the marker D10S583 and AD. We have examined the D10S583 polymorphic marker and apolipoprotein E (APOE) gene in a case-control study. We observed the expected association of the APOE allele varepsilon4 with AD, and an inverse association between the D10S583 allele 209 and AD. These data support the original findings that suggest the presence of a candidate gene for AD in this region of chromosome 10. The nearby insulin degrading enzyme gene has been previously proposed as a candidate gene; however, a number of other putative candidate genes are also located in this region. The ongoing investigation of the genetic source of association and linkage in this region is clearly warranted.

Pharmacogenomics for the treatment of dementia

Alzheimer's disease (AD) is a genetically complex disorder associated with multiple genetic defects either mutational or of susceptibility. Current AD genetics does not explain in full the etiopathogenesis of AD, suggesting that environmental factors and/or epigenetic phenomena may also contribute to AD pathology and phenotypic expression of dementia. The genomics of AD is still in its infancy, but is helping us to understand novel aspects of the disease including genetic epidemiology, multifactorial risk factors, pathogenic mechanisms associated with genetic networks and genetically-regulated metabolic cascades. AD genomics is also fostering new strategies in pharmacogenomic research and prevention. Functional genomics, proteomics, pharmacogenomics, high-throughput methods, combinatorial chemistry and modern bioinformatics will greatly contribute to accelerating drug development for AD and other complex disorders. The multifactorial genetic dysfunction in AD includes mutational loci (APP, PS1, PS2) and diverse susceptibility loci (APOE, A2M, AACT, LRP1, IL1A, TNF, ACE, BACE, BCHE, CST3, MTHFR, GSK3B, NOS3) distributed across the human genome, probably converging in common pathogenic mechanisms that lead to premature neuronal death. Genomic associations integrate polygenic matrix models to elucidate the genomic organization of AD in comparison to the control population. Using APOE-related monogenic models it has been demonstrated that the therapeutic response to drugs (e.g., cholinesterase inhibitors, non-cholinergic compounds) in AD is genotype-specific. A multifactorial therapy combining three different drugs yielded positive results during 6-12 months in approximately 60% of the patients. With this therapeutic strategy, APOE-4/4 carriers were the worst responders and patients with the APOE-3/4 genotype were the best responders. Other polymorphic variants (PS1, PS2) also influence the therapeutic response to different drugs in AD patients, suggesting that the final pharmacological outcome is the result of multiple genomic interactions, including AD-related genes and genes associated with drug metabolism, disposition, and elimination. The pharmacogenomics of AD may contribute in the future to optimise drug development and therapeutics, increasing efficacy and safety, and reducing side-effects and unnecessary costs.

A unifying hypothesis of Alzheimer's disease. IV. Causation and sequence of events

Contrary to common concepts, the brain in Alzheimer's disease (AD) does not follow a suicide but a rescue program. Widely shared features of metabolism in starvation, hibernation and various conditions of energy deprivation, e.g. ischemia, allow the definition of a deprivation syndrome which is a phylogenetically conserved adaptive response to energetic stress. It is characterized by hypometabolism, oxidative stress and adjustments of the glucose-fatty acid cycle. Cumulative evidence suggests that the brain in aging and AD actively adapts to the progressive fuel deprivation. The counterregulatory mechanisms aim to preserve glucose for anabolic needs and promote the oxidative utilization of ketone bodies. The agent mediating the metabolic switch is soluble Abeta which inhibits glucose utilization and stimulates ketone body utilization at various levels. These processes, which are initiated during normal aging, include inhibition of pro-glycolytic neurohormones, cholinergic transmission, and pyruvate dehydrogenase, the key transmitter and effector systems regulating glucose metabolism. Hormonal and effector systems which promote ketone body utilization, such as glucocorticosteroid and galanin activity, GABAergic transmission, nitric oxide, lipid transport, Ca2+ elevation, and ketone body metabolizing enzymes, are enhanced. A multitude of risk factors feed into this pathophysiological cascade at a variety of levels. Taking into account its pleiotropic regulatory actions in the deprivation response, a new name for Abeta is suggested: deprivin. On the other hand, cumulative evidence, taken together compelling, suggests that senile plaques are the dump rather than the driving force of AD. Moreover, the neurotoxic action of fibrillar Abeta is a likely in vitro artifact but does not contribute significantly to the in vivo pathophysiological events. This archaic program, conserved from bacteria to man, aims to ensure the survival of a deprived organism and controls such divergent processes as sporulation, hibernation, aging and aging-related diseases. In contrast to the immature brain, ketone body utilization of the aged brain is no longer sufficient to meet the energetic demands and is later supplemented by lactate, thus recapitulating in reverse order the sequential fuel utilization of the immature brain. The transduction pathways which operate to switch metabolism also convey the programming and balancing of the de-/redifferentiation/apoptosis cell cycle decisions. This encompasses the reiteration of developmental processes such as transcription factor activation, tau hyperphosphorylation, and establishment of growth factor independence by means of Ca2+ set point shift. Thus, the increasing energetic insufficiency results in the progressive centralization of metabolic activity to the neuronal soma, leading to pruning of the axonal/dendritic trees, loss of neuronal polarity, downregulation of neuronal plasticity and, eventually, depending on the Ca2+ -energy-redox homeostasis, degeneration of vulnerable neurons. Finally, it is outlined that genetic (e.g. Down's syndrome, APP and presenilin mutations and apoE4) and environmental risk factors represent progeroid factors which accelerate the aging process and precipitate the manifestation of AD as a progeroid systemic disease. Aging and AD are related to each other by threshold phenomena, corresponding to stage 2, the stage of resistance, and stage 3, exhaustion, of a metabolic stress response.

Age-dependent cytokine responses: trimethyltin hippocampal injury in wild-type, APOE knockout, and APOE4 mice

In this study, the hippocampal neurotoxicant trimethyltin (TMT) was used to examine possible differential susceptibility associated with the apolipoprotein E genotype. Mice-wild type (C57BL6J), APOE knockout, and APOE4 transgenic-received either saline or TMT (2 mg/kg, ip) at either 21 days or 8 months of age. At both ages, similar mRNA levels were seen in the hippocampus across genotypes for ICAM-1, A20, and MAC-1. GFAP mRNA was higher in the APOE knockouts and APOE4 as compared to wild-type mice. Within 24 h, TMT produced cell death of hippocampal dentate granule neurons and mild astrogliosis in all animals. In 21-day-old mice, TMT exposure significantly increased mRNA levels for ICAM-1 and MIP-1alpha in all genotypes. EB-22, GFAP, TNFalpha, and TGF-beta1 levels were significantly elevated in both wild-type and APOE knockout mice following TMT. At 8 months of age, genotype specific differences were observed. mRNA levels for GFAP, TNFbeta, TNFalpha, and MIP-1alpha were increased in both APOE knockout and APOE4 mice compared to wild-type mice. TMT exposure significantly increased mRNA levels for GFAP and MIP-1alpha in all animals. TNFalpha mRNA levels were increased in wild-type and APOE4 mice while EB22 mRNA levels were increased in both the APOE knockout and APOE4 mice but not wild-type mice. These data suggest an age-dependent effect on both microglia early inflammatory responses to injury associated with the APOE genotype.

No association between the very low density lipoprotein receptor gene and late-onset Alzheimer's disease nor interaction with the apolipoprotein E gene in population-based and clinic samples

It is now commonly known that possession of one of the three common alleles of the apolipoprotein E (APOE) gene (allele epsilon 4) confers an increased risk for both familial and sporadic Alzheimer's disease (AD), and that this risk is dose-dependent. Other genes that may play a role in AD, either through independent association with the disease or through modification of the existing APOE risk, are under investigation. One such gene, the very low density lipoprotein receptor (VLDL-R) gene, was reported by Okuizumi et al. to be independently associated with AD in a Japanese population, but not interactive with the APOE4 conferred risk. Their clinic-based data set demonstrated a 2-fold increased risk conferred by the 5-repeat allele of a polymorphism in VLDL-R. As recruitment from a clinic rather than a population-based sample may result in a distortion of allele frequencies, as has been shown with APOE allele frequencies, it is important to investigate this association in a population-based study. We have genotyped both population and clinic-based AD data sets at this VLDL-R polymorphism, and we find no independent association between the VLDL-R gene and the occurrence of AD in either sample. Further, despite the biochemical relationship between the VLDL-R and APOE proteins, we find no significant statistical interaction between the alleles at these loci.

A genealogical study of Alzheimer disease in the Saguenay region of Quebec

We performed an analysis of inbreeding and kinship among the ascending genealogies of 205 autopsy-confirmed Alzheimer disease (AD) subjects recruited in the Saguenay area of Québec. We hypothesized that if some traits pertaining to the disease were determined by inherited factors, and if the corresponding genes were not too frequent in the population, it might be possible to detect some clusters of patients related to common ancestors and presenting a level of kinship and/or inbreeding higher than is observed in the unaffected population of the same age. In view of the heterogeneity of the disease, we also verified if some of the factors investigated could be associated more specifically with subsets of cases based on age of onset and on apolipoprotein E (APOE) genotype. Results were compared with those obtained on 205 controls matched for gender, place and year of birth. We found that late-onset AD cases with an APOE-epsilon 4 were significantly more inbred than controls and that this increase was explained by the high level of inbreeding of a few cases whose parents were related at the first-cousin level. This could possibly indicate the implication of a recessive element in a small subset of AD cases in the Saguenay population. We also found that late-onset epsilon 4+ cases were significantly more closely related among themselves than with controls. This increase in kinship may be attributable to the presence of the epsilon 4 allele or to some other unidentified genetic factor possibly acting in conjunction with APOE-epsilon 4.

Mixed effect models of longitudinal Alzheimer's disease data: a cautionary note

Longitudinal studies of cognitive function in Alzheimer's disease (AD) patients are powerful tools to better understand the biology and natural history of the disease, but the attributes of the studies that make them valuable also pose special challenges to analysts. A fundamental problem is the accurate measure of time at which cognitive decline begins. Investigators typically use the date of AD diagnosis or the date of enrollment in an AD study. If the rate of cognitive decline is non-linear, variables associated with the time of diagnosis or enrollment might artificially be associated with the rate of decline. Unlike the mixed effects models typically used to analyse cognitive decline, summary measure analyses do not directly compare the rate of decline with time since decline began, and, therefore, are less sensitive to biased measures of time of decline. We simulated trajectories of cognitive decline using the multivariate normal random effect model and tested the ability of the two analytic techniques to discriminate between true and spurious associations. Our analyses suggest summary measure models are less likely to detect spurious associations generated by biased measures of time at which decline begins, and more likely to detect true associations concealed by biased time measurement.

The number of trait loci in late-onset Alzheimer disease

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

Key Words Collapse

MESH Headings

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

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Grants

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

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Affiliation(s)

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

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Influence of APOE genotype on familial aggregation of AD in an urban population

OBJECTIVE

To examine the influence of the proband's APOE genotype on AD among first-degree relatives in a community-based study of African Americans, whites, and Caribbean Hispanics.

METHODS

History of AD and demographic information were obtained on 1,073 siblings and parents of 312 patients with AD and 2,722 siblings and parents of 802 nondemented controls. APOE genotyping was performed on all 1,114 patients and controls.

RESULTS

A higher proportion of patients with AD (35%) than controls (27%) had one or more APOE-epsilon4 alleles (p = 0.03). When compared with relatives of controls without an APOE-epsilon4 allele, the risk for AD was increased in first-degree relatives of both patients (rate ratio [RR] = 1.9, 95% confidence interval [CI] = 1.2 to 3.1) and controls (RR = 1.8, 95% CI = 1.2 to 2.6) with one or more APOE-epsilone alleles, regardless of ethnic group. There was a similar trend of increased risk in relatives of patients without an APOE-epsilon4 allele, but this was limited to Hispanics and African Americans.

CONCLUSIONS

The presence of an APOE-epsilon4 allele increases risk for AD among first-degree relatives, regardless of the probands' disease status, among all ethnic groups. Relatives of patients without an APOE-epsilon4 allele were also at increased risk for AD among Hispanics and African Americans, suggesting that other genes or risk factors may influence risk.

Microglial activation induced by factor(s) contained in sera from Alzheimer-related ApoE genotypes

Several factors that increase the likelihood of developing Alzheimer's disease (AD) have already been identified. A correct evaluation of these may contribute to a better understanding of the etiology of the disease. The risk of developing AD definitely increases with (a) age, (b) head injuries, (c) family history of AD or Down syndrome, (d) sex (higher prevalence of AD in women), (e) vascular disease, (f) exposure to environmental toxins, (g) infectious processes, or (h) changes in immune function, and recent advances in molecular genetics have suggested that genetic predisposition (i) can be considered one of the most important risk factors in the development of AD. A significant increase in the number of amyloid plaques in AD patients with an apolipoprotein E4 (ApoE) allele has been observed and the results of several genetic studies indicate that the etiology of this neurodegenerative disease is associated with the presence of the allele E4 of ApoE. A potential source of damage in the AD brain is an altered response triggered by microglial activation, which is associated with amyloid plaques. It has become evident that a dysregulation of cytokine release appears within lesions of many types of brain disorders including infection, trauma, stroke, and neurodegenerative diseases. Many studies have shown that microglia secrete both cytokines and cytotoxins and since reactive microglia appears in nearly every type of brain damage, it is likely that their secreted products ultimately help to determine the rate of damaged brain tissue. In this study, in vitro cell cultures were established to investigate the effect of different concentrations of human sera (2.5% and 10%) with specific ApoE genotypes from Alzheimer's and non-Alzheimer's subjects on ameboid and flat microglial cells obtained from neonatal rat hippocampi. Results show that a modulation in the proliferation and activation of microglial cells was obtained and that AD sera, mainly in the ApoE 3/4 and 4/4 genotype contain factor(s) which are able to induce morphological changes, as measured by an increase in the ameboid cell type. In addition, major histocompatibility complex (MHC) class II antigen expression, as measured by flow cytometric analysis, and interleukin-1beta (IL-1beta) release as measured by enzyme linked immunoadsorbent assay (ELISA), in comparison with control groups and lipopolysaccharide (LPS)-treated cells, clearly demonstrate a direct effect of ApoE 3/4 and 4/4 and/or an indirect effect mediated by the release of IL-1beta on microglia activation. These results strongly suggest that primary in vitro microglial cell cultures can be used as a screening model to test human sera as well as the effect of new potential drugs aimed at down-regulating microglia activation.

Loss of somatostatin-like immunoreactivity in the frontal cortex of Alzheimer patients carrying the apolipoprotein epsilon 4 allele

We measured somatostatin-like immunoreactivity, using a radioimmunoassay which does not cross react with cortistatin-like immunoreactivity, in postmortem frontal cortex (Brodmann area 9) from 32 patients, of different apolipoprotein E genotypes, and presenting with different degrees of cognitive impairment. Eleven subjects and eight patients presented with no (controls) or limited memory impairments (Borderline), respectively. Six patients with clinical criteria for possible Alzheimer's disease also presented with clinical or brain imaging of cerebrovascular disease (mixed dementia) and seven patients were classified as Alzheimer's disease (AD). In the 6 months preceeding their deaths, all subjects had been evaluated by Folstein's Mini Mental State examination (MMS). Sixty nine percent of patients with MMS >20 did not carry the epsilon 4 allele while 66% of patients with MMS <10 did. Somatostatin concentrations (ng/mg wet weight) were significantly lower in the patients carrying the epsilon 4 allele (E2/3: 0.71 +/- 0.05, n = 19 vs. E4: 0.42 +/- 0.06, n = 13; mean +/- SEM, P < 0.001). These results, which are reminiscent of those obtained on cholinergic markers, suggest that apolipoprotein E4 is involved in the somatostatinergic dysfunction early after the onset in AD.

A polymorphism in the regulatory region of APOE associated with risk for Alzheimer's dementia

The epsilon4 allele of the apolipoprotein E gene (APOE) has been associated with an increased risk of developing Alzheimer's disease (AD; refs 1,2). However, it is apparent that the APOEepsilon4 allele alone is neither necessary nor sufficient to cause the disease. We have recently found three new polymorphisms within the APOE transcriptional regulatory region (M.J.A. et al., manuscript submitted) and now establish an association between one of these polymorphisms (-491A/T) and dementia as observed in Alzheimer's disease, in two independent clinical populations. The results suggest that homozygosity of a common variant (-491A) is associated with increased risk for AD, and that this association is independent of APOEepsilon4 status. In vitro studies suggest that the -491A/T polymorphism may increase risk for AD by altering the level of ApoE protein expression.

Plastic neuronal remodeling is impaired in patients with Alzheimer's disease carrying apolipoprotein epsilon 4 allele

A relationship between the apolipoprotein E (apoE) genotype and the risk to develop Alzheimer's disease has been established recently. Apolipoprotein synthesis is implicated in developmental processes and in neuronal repair of the adult nervous system. In the present study, we investigated the influence of the apolipoprotein polymorphism on the severity of neuronal degeneration and the extent of plastic dendritic remodeling in Alzheimer's disease. Changes in length and arborization of dendrites of Golgi-impregnated neurons in the basal nucleus of Meynert, locus coeruleus, raphe magnus nucleus, medial amygdaloid nucleus, pedunculopontine tegmental nucleus, and substantia nigra were analyzed after three-dimensional reconstruction. Patients with either one or two apoE epsilon 4 alleles not only showed a more severe degeneration in all areas investigated than in patients lacking the apoE 4 allele but also revealed significantly less plastic dendritic changes. ApoE epsilon 4 allele copy number, furthermore, had a significant effect on the pattern of dendritic arborization. Moreover, the relationship between the intensity of dendritic growth and both the extent of neuronal degeneration and the stage of the disease seen in patients lacking the apoE epsilon 4 allele was very weak in the presence of one epsilon 4 allele and completely lost in patients homozygous for the epsilon 4 allele. The results provide direct evidence that neuronal reorganization is affected severely in patients with Alzheimer's disease carrying the apoE epsilon 4 allele. This impairment of neuronal repair might lead to a more rapid functional decompensation, thereby contributing to an earlier onset and more rapid progression of the disease.

No interaction between the APOE and the alpha-1-antichymotrypsin genes on risk for Alzheimer's disease

It is now known that possession of one of the three common forms of the apolipoprotein E gene (allele epsilon 4) confers an increased risk for Alzheimer's disease (AD), both familial and sporadic, and that this risk is dose-dependent. Other genes that may play a role in AD, either through independent association with the disease or through modification of, or interaction with, the existing apolipoprotein E (APOE) risk, are now under investigation including the alpha-1-antichymotrypsin (ACT) gene, the very low density lipoprotein receptor (VLDL-R) gene, and the presenilin-1 (PS-1) gene. Kamboh et al. [1995] reported that a polymorphism in the alpha-1-antichymotrypsin gene could modify the risk for AD conferred by the APOE locus, specifically by increasing the risk for AD among epsilon 4 homozygotes. The ACT gene, which is found on chromosome 14, has previously been proposed as a candidate for AD due to the presence of the ACT protein in senile plaques and the reported elevation of the protein in the cerebro-spinal fluid (CSF) and serum of AD cases. We have investigated this reported association within our familial and sporadic AD dataset, where we find no independent association between ACT and the occurrence of AD. Logistic regression analysis excludes ACT or the interaction between ACT and APOE as significant contributors in the prediction of disease status. By this analysis, ACT genotyping does not provide additional information about an individual's risk of Alzheimer's disease beyond the risk information conferred by APOE genotype alone.

The National Institute on Aging/Alzheimer's Association recommendations on the application of apolipoprotein E genotyping to Alzheimer's disease

In a conference held in Chicago during October 1995, a working group of the National Institute of Aging (NIA) and the Alzheimer's Association (AlzA) drafted consensus recommendations on research and clinical applications of APOE genetic susceptibility testing for Alzheimer's disease (AD). The NIA/AlzA Working Group concluded that in considering future applications of APOE genotyping and other knowledge that has been gained about the genetic basis of AD, the interests of AD patients and their family members must be held paramount. The group acknowledged that a robust association exists between possession of the APOE epsilon 4 allele and the risk of late-onset AD and cited evidence that this allele is more strongly associated with AD than any other form of dementia. They recommended against the use of APOE genotyping to predict the-future development of AD in asymptomatic individuals at this time, and warned against the use of the test in isolation as the sole means for diagnosing AD. The group endorsed the concept of discretionary use of APOE genotyping as an adjunct to other AD diagnostic procedures. However, routine clinical use of the test for this purpose was not recommended at this time. Physicians were advised to weigh any potential benefits of testing against the possibility that genotype disclosure could adversely affect the insurability, employability, and social standing of AD patients and their family members. Adequate provisions for pre-test and post-test counseling and psychosocial support were advised for all future clinical and research applications of APOE genotyping. The group called for the development of improved protocols for AD genetic counseling as well as supplemental measures to assure genetic privacy for AD patients and their family members.

Familial and population-based studies of apolipoprotein E and Alzheimer's disease

The finding of an association between the epsilon 4 allele of the APOE locus and the early expression of late-onset Alzheimer's disease (AD) is robust. However, the estimates of the proportion of AD cases carrying one or more copies of the epsilon 4 allele vary dramatically between studies (highest estimates being 180% of lowest ones). Here we compare the results of association studies in samples drawn from an epidemiologically based study design and samples drawn from families selected for linkage studies. The significant differences between results probably point to the unwitting selection of familial factors other than the APOE locus in the family history positive samples. We conclude that any selection procedure tending to enrich samples for positive family history will also tend to artificially increase APOE epsilon 4 allele frequencies in probands. This is of significance in samples drawn from clinical settings where referral may be influenced by previous known family history. Further work is needed to clarify the nature of the additional factors operating within families. We also report data showing an association between late-onset AD and a polymorphism in an adjacent locus to APOE-the APOCI locus. As no additional risk for AD can be attributed to the APOCI locus, the most likely explanation for the association between AD and APOCI is the disequilibrium between the APOCI and APOE loci. Therefore, there are likely to be other genetic markers in the area that can be used in the same way as APOE as a marker of risk for the disease.

Apolipoprotein E genotype and concomitant clinical features in early-onset Alzheimer's disease

We have studied the relationship between the apolipoprotein E gene (APOE) and the development of myoclonus, tremors, rigidity and seizures in 168 patients with probable early-onset Alzheimer's disease (AD). There was a statistically significantly lower risk of tremor for carriers of the epsilon 4 allele of APOE. This allele was also associated with an increased risk of myoclonus. Our findings suggest that there may be differences in progression and clinical appearance in early-onset AD related to the APOE genotype.

Genetic analysis of mechanisms of aging

A wide range of genetic models with postponed aging are now available, from selected mice and Drosophilia to mutant Caenorhabditis elegans and Saccharomyces cerevisiae. These systems allow efficient testing of alternative mechanistic hypotheses for aging. Genetic analysis is forging stronger connections between particular alleles and susceptibility to particular 'diseases of aging'; for example, two different genes for Alzheimer disease have been identified.

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.

Rational design of an animal model for Alzheimer's disease: introduction of multiple human genomic transgenes to reproduce AD pathology in a rodent

A major obstacle to understanding the pathogenesis of Alzheimer's disease is the lack of easily studied animal models. Our approach is to apply transgenic methods to humanize mice and rats, employing methods that introduce large genomic transgenes, because this improves the level of transgene protein expression and the tissue specificity of expression. Our plan is to reproduce AD pathology in rodents by making them transgenic for several human proteins involved in AD. This report describes transgenic animal lines that we have produced, and summarizes our current approach and future plans. Two human genes known to be involved in AD pathology are the amyloid precursor protein (APP) and the E4 isoform of apolipoprotein E (apoE4). So far, we have produced and analyzed a transgenic line carrying the entire human APP gene cloned in a yeast artificial chromosome. We have also produced but not yet analyzed a mouse carrying the human apoE4 gene. Work is in progress to produce a transgenic line carrying a disease-causing mutation in the human APP gene. As we produce these animals, we are breeding them together, and also breeding them with a mouse line that lacks endogenous apoE, to produce an animal model carrying several human proteins whose interaction is believed to be instrumental in development of AD pathology. These transgenic animals will be useful for dissecting the biochemical and physiological steps leading to AD, and for development of therapies for disease intervention.

A mitochondrial bottleneck hypothesis of Alzheimer's disease

Alzheimer's disease, in its early onset familial form, is known to be a heterogeneous disorder. This suggests that the different degenerative mechanisms, initiated by different genetic causes and ending in the shared phenotype of the disease, should intersect at some point in the degenerative cascade to form a 'bottleneck' from which the pathological features that are common to each of the genetic forms emerge. A growing body of evidence suggests that disturbances of energy metabolism may play a fundamental role in the onset and progression of Alzheimer's disease. In light of this, we propose a 'mitochondrial bottleneck hypothesis', which unifies the various forms of the disease in which different causes lead to the disorder via disturbances of mitochondrial function. The characterization of such a bottleneck would present a unique target for therapeutic intervention because it would be the earliest point in a neurodegenerative cascade shared by all forms of Alzheimer's disease, independent of cause.