Apolipoprotein E (APOE) ε genotype has previously been significantly associated with cognitive, brain imaging, and Alzheimer's disease-related phenotypes (e.g., age of onset). In the TOMM40 gene, the rs10524523 (“523”) variable length poly-T repeat polymorphism has more recently been associated with similar ph/enotypes, although the allelic directions of these associations have varied between initial reports. Using diffusion magnetic resonance imaging tractography, the present study aimed to investigate whether there are independent effects of apolipoprotein E (APOE) and TOMM40 genotypes on human brain white matter integrity in a community-dwelling sample of older adults, the Lothian Birth Cohort 1936 (mean age = 72.70 years, standard deviation = 0.74, N approximately = 640–650; for most analyses). Some nominally significant effects were observed (i.e., covariate-adjusted differences between genotype groups at p < 0.05). For APOE, deleterious effects of ε4 “risk” allele presence (vs. absence) were found in the right ventral cingulum and left inferior longitudinal fasciculus. To test for biologically independent effects of the TOMM40 523 repeat, participants were stratified into APOE genotype subgroups, so that any significant effects could not be attributed to APOE variation. In participants with the APOE ε3/ε4 genotype, effects of TOMM40 523 status were found in the left uncinate fasciculus, left rostral cingulum, left ventral cingulum, and a general factor of white matter integrity. In all 4 of these tractography measures, carriers of the TOMM40 523 “short” allele showed lower white matter integrity when compared with carriers of the “long” and “very-long” alleles. Most of these effects survived correction for childhood intelligence test scores and vascular disease history, though only the effect of TOMM40 523 on the left ventral cingulum integrity survived correction for false discovery rate. The effects of APOE in this older population are more specific and restricted compared with those reported in previous studies, and the effects of TOMM40 on white matter integrity appear to be novel, although replication is required in large independent samples.
White matter; Cognitive ageing; Diffusion MRI; Tractography; APOE; TOMM40; Alzheimer's disease
Apolipoprotein E (APOE) genotypes are associated with variable risk of developing late onset Alzheimer’s disease (LOAD), with APOE ε4 having higher risk. A variable poly-T length polymorphism at rs10524523, within intron 6 of the TOMM40 gene has been shown to influence age of onset in LOAD, with very long poly-T length associated with earlier disease onset, and short poly-T length associated with later onset. In this study, we tested the hypothesis that brain and cognitive changes suggestive of presymptomatic LOAD may be associated with this TOMM40 polymorphism.
Among N=117 healthy APOE ε3 homozygous adults (mean age 55), we compared those homozygous for very long (VL/VL; n=35) TOMM40 poly-T lengths (who are presumably at higher risk) to those homozygous for short (S/S; n=38) poly-T lengths, as well as those with heterozygous (S/VL; n=44) poly-T length polymorphisms, on measures of learning and memory and on structural brain imaging.
The VL/VL group exhibited lower performance than the S/S TOMM40 group on primacy retrieval from a verbal list learning task, a finding which is also seen in early AD. A dose-dependent increase in the VL TOMM40 polymorphism (from no VL alleles, to S/VL heterozygous, to VL/VL homozygous) was associated with decreasing gray matter volume in the ventral posterior cingulate and medial ventral precuneus, a region of the brain affected early in LOAD.
These findings among APOE ε3/ε3 late middle-aged adults suggest that a subgroup with very long TOMM40 poly-T lengths may be experiencing incipient LOAD-related cognitive and brain changes.
The ɛ4 allele of the apolipoprotein E (APOE) gene is currently the strongest and most highly replicated genetic factor for risk and age of onset of late-onset Alzheimer's disease (LOAD). Using phylogenetic analysis, we have identified a polymorphic poly-T variant, rs10524523, in the translocase of outer mitochondrial membrane 40 homolog (TOMM40) gene that provides greatly increased precision in the estimation of age of LOAD onset for APOE ɛ3 carriers. In two independent clinical cohorts, longer lengths of rs10524523 are associated with a higher risk for LOAD. For APOE ɛ3/4 patients who developed LOAD after 60 years of age, individuals with long poly-T repeats linked to APOE ɛ3 develop LOAD on an average of 7 years earlier than individuals with shorter poly-T repeats linked to APOE ɛ3 (70.5±1.2 years versus 77.6±2.1 years, P=0.02, n=34). Independent mutation events at rs10524523 that occurred during Caucasian evolution have given rise to multiple categories of poly-T length variants at this locus. On replication, these results will have clinical utility for predictive risk estimates for LOAD and for enabling clinical disease prevention studies. In addition, these results show the effective use of a phylogenetic approach for analysis of haplotypes of polymorphisms, including structural polymorphisms, which contribute to complex diseases.
AD genetics; phylogenetic analysis; TOMM40; APOE; poly-T variants
A highly polymorphic T-homopolymer was recently discovered to be associated with late onset Alzheimer’s disease (LOAD) risk and age of onset.
To explore the effects of the polymorphic polyT tract (rs10524523, referred as ‘523’) on cognitive performance in cognitively healthy elderly.
181 participants were recruited from local independent-living retirement communities. Informed consent was obtained and participants completed demographic questionnaires, a conventional paper and pencil neuropsychological battery, and the computerized Cambridge Neuropsychological Test Automated Battery (CANTAB). Saliva samples were collected for determination of the TOMM40 ‘523’ (S, L, VL) and the APOE (ε2, 3, 4) genotypes. From the initial sample of 181 individuals, 127 participants were eligible for the association analysis. Participants were divided into three groups based on ‘523’ genotypes (S/S, S/L-S/VL, and L/L-L/VL-VL/VL) Generalized linear models were used to evaluate the association between the ‘523’ genotypes and neuropsychological test performance. Analyses were adjusted for age, sex, education, depression, and APOE ε4 status. A planned sub analysis was undertaken to evaluate the association between ‘523’ genotypes and test performance in a sample restricted to APOE ε3 homozygotes.
The S homozygotes performed better, although not significantly, than the S/L-S/VL and the VL/L-L/VL-VL/VL genotype groups on measures associated with memory (CANTAB Paired-Associate Learning, and VRM Free Recall) and executive function (CANTAB measures of Intra-Extradimensional set shifting). Follow-up analysis of APOEε 3 homozygotes only, showed that the S/S group performed significantly better than the S/VL group on measures of episodic memory (CANTAB Paired-Associate Learning and VRM Free Recall), attention (CANTAB RVP Latency) and executive function (Digit-Symbol substitution). The S/S group performed marginally better than the VL/VL group on Intra-Extradimensional set shifting. None of the associations remained significant after applying a Bonferroni correction for multiple testing.
Results suggest important APOE-independent associations between the TOMM40 ‘523’ polymorphism and specific cognitive domains of memory and executive control that are preferentially affected in early stage AD.
TOMM40; polyT polymorphism; cognition; aging; neuropsychological tests; CANTAB; Alzheimer’s disease
To evaluate the association of risk and age at onset (AAO) of Alzheimer disease (AD) with single-nucleotide polymorphisms (SNPs) in the chromosome 19 region including apolipoprotein E (APOE) and a repeat-length polymorphism in TOMM40 (poly-T, rs10524523).
Conditional logistic regression models and survival analysis.
Fifteen genome-wide association study data sets assembled by the Alzheimer's Disease Genetics Consortium.
Eleven thousand eight hundred forty AD cases and 10 931 cognitively normal elderly controls.
Main Outcome Measures
Association of AD risk and AAO with genotyped and imputed SNPs located in an 800-Mb region including APOE in the entire Alzheimer's Disease Genetics Consortium data set and with the TOMM40 poly-T marker genotyped in a subset of 1256 cases and 1605 controls.
In models adjusting for APOE ε4, no SNPs in the entire region were significantly associated with AAO at P<.001. Rs10524523 was not significantly associated with AD or AAO in models adjusting for APOE genotype or within the subset of ε3/ε3 subjects.
APOE alleles ε2, ε3, and ε4 account for essentially all the inherited risk of AD associated with this region. Other variants including a poly-T track in TOMM40 are not independent risk or AAO loci.
We previously discovered that a polymorphic, deoxythymidine-homopolymer (poly-T, rs10524523) in intron 6 of the TOMM40 gene is associated with age-of-onset of Alzheimer's disease and with cognitive performance in elderly. Three allele groups were defined for rs10524523, hereafter ‘523’, based on the number of ‘T’-residues: ‘Short’ (S, T≤19), ‘Long’ (L, 20≤T≤29) and ‘Very Long’ (VL, T≥30). Homopolymers, particularly long homopolymers like ‘523’, are difficult to genotype because ‘slippage’ occurs during PCR-amplification. We initially genotyped this locus by PCR-amplification followed by Sanger-sequencing. However, we recognized the need to develop a higher-throughput genotyping method that is also accurate and reliable. Here we describe a new ‘523’ genotyping assay that is simple and inexpensive to perform in a standard molecular genetics laboratory. The assay is based on the detection of differences in PCR-fragment length using capillary electrophoresis. We discuss technical problems, solutions, and the steps taken for validation. We employed the novel assay to investigate the ‘523’ allele frequencies in different ethnicities. Whites and Hispanics have similar frequencies of S/L/VL alleles (0.45/0.11/0.44 and 0.43/0.09/0.48, respectively). In African-Americans, the frequency of the L-allele (0.10) is similar to Whites and Hispanics; however, the S-allele is more prevalent (0.65) and the VL-allele is concomitantly less frequent (0.25). The allele frequencies determined using the new methodology are compared to previous reports for Ghanaian, Japanese, Korean and Han Chinese cohorts. Finally, we studied the linkage pattern between TOMM40-‘523’ and APOE alleles. In Whites and Hispanics, consistent with previous reports, the L is primarily linked to ε4, while the majority of the VL and S are linked to ε3. Interestingly, in African-Americans, Ghanaians and Japanese, there is an increased frequency of the ‘523’S-APOEε4 haplotype. These data may be used as references for ‘523’ allele and ‘523’-APOE haplotype frequencies in diverse populations for the design of research studies and clinical trials.
I coauthored a recently published research paper demonstrating that a variable length, poly-T polymorphism in the TOMM40 (Translocase of the Outer Mitochondrial Membrane 40 homolog (yeast)) gene, which lies adjacent to APOE on chromosome 19, accounts for the age of onset distribution for a complex disease, late-onset Alzheimer's disease (LOAD).1 These new data explain the average age of disease onset for patients with the APOE4/4 genotype, and differentiate two forms of TOMM40 poly-T polymorphisms linked to APOE with each form associated with a different age of disease onset distribution.2 When linked to APOE3, the longer TOMM40 poly-T repeats (19–39 nucleotides) at the rs10524523 (523) locus are associated with earlier age of onset and shorter TOMM40 alleles (11–16 nucleotides) with later onset. The data suggest that the poly-T alleles are co-dominant, with the age of onset phenotype determined by both of the two inherited alleles but with variable expressivity. Additional data will further refine the relationship between the length of the poly-T alleles and age of disease onset and determine if the relationship is linear.
The APOE ε and TOMM40 rs10524523 (‘523’) variable length poly-T repeat gene loci have been significantly and independently associated with Alzheimer’s disease (AD) related phenotypes such as age of clinical onset. Hippocampal atrophy has been significantly associated with memory impairment, a characteristic of AD. The current study aimed to test for independent effects of APOE ε and TOMM40 ‘523’ genotypes on hippocampal volumes as assessed by brain structural MRI in a relatively large sample of community-dwelling older adults. As part of a longitudinal study of cognitive ageing, participants in the Lothian Birth Cohort 1936 underwent genotyping for APOE ε2/ε3/ε4 status and TOMM40 ‘523’ poly-T repeat length, and detailed structural brain MRI at a mean age of 72.7 years (standard deviation = 0.7, N range = 624 to 636). No significant effects of APOE ε or TOMM40 523 genotype were found on hippocampal volumes when analysed raw, or when adjusted for either intracranial or total brain tissue volumes. In summary, in a large community-dwelling sample of older adults, we found no effects of APOE ε or TOMM40 523 genotypes on hippocampal volumes. This is discrepant with some previous reports of significant association between APOE and left/right hippocampal volumes, and instead echoes other reports that found no association. Previous significant findings may partly reflect type 1 error. Future studies should carefully consider: 1) their specific techniques in adjusting for brain size; 2) assessing more detailed sub-divisions of the hippocampal formation; and 3) testing whether significant APOE-hippocampal associations are independent of generalised brain atrophy.
Genetic variation within the apolipoprotein E gene (APOE) locus is associated with late-onset Alzheimer's disease risk and quantitative traits as well as apoE expression in multiple tissues. The aim of this investigation was to explore the influence of APOE locus cis-regulatory element enhancer region genetic variation on regional gene promoter activity. Luciferase reporter constructs containing haplotypes of APOE locus gene promoters; APOE, APOC1, and TOMM40, and regional putative enhancers; TOMM40 IVS2-4, TOMM40 IVS6 poly-T, as well as previously described enhancers; ME1, or BCR, were evaluated for their effects on luciferase activity in 3 human cell lines. Results of this investigation demonstrate that in SHSY5Y cells, the APOE promoter is significantly influenced by the TOMM40 IVS2-4 and ME1 and the TOMM40 promoter is significantly influenced by the TOMM40 IVS6 poly-T, ME1 and BCR. In HepG2 cells, theTOMM40 promoter is significantly influenced by all four enhancers, whereas the APOE promoter is not influenced by any of the enhancers. The main novel finding of this investigation was that multiple APOE locus cis-elements influence both APOE and TOMM40 promoter activity according to haplotype and cell type suggesting that a complex transcriptional regulatory structure modulates regional expression.
APOE; TOMM40; APOC1; ME1; BCR; Regulation; Enhancer; Luciferase Reporter Assay
Currently the ε4 allele of the apolipoprotein E gene (APOE) is the strongest genetic risk factor for late onset Alzheimer's disease (AD). However, inheritance of the APOE ε 4 allele is not necessary or sufficient for the development of AD. Genetic evidence suggests that multiple loci in a 70 kb region surrounding APOE are associated with AD risk. Even though these loci could represent surrogate markers in linkage disequilibrium with APOE ε4 allele, they could also contribute biological effects independent of the APOE ε4 allele. Our previous study identified multiple SNPs upstream from APOE that are associated with cerebrospinal fluid apoE levels, suggesting that a haplotype structure proximal to APOE can influence apoE expression. In this study, we examined apoE expression in human post-mortem brain (PMB), and constructed chromosome-phase-separated haplotypes of the APOE proximal region to evaluate their effect on PMB apoE expression. ApoE protein expression was found to differ among AD brain regions and to differ between AD and control hippocampus. In addition, an extended APOE proximal haplotype structure, spanning from the TOMM40 gene to the APOE promoter, may modulate apoE expression in a brain region-specific manner and may influence AD disease status. In conclusion, this haplotype-phenotype analysis of apoE expression in PMB suggests that either; (1) the cis-regulation of APOE expression levels extends far upstream of the APOE promoter or (2) an APOE ε4 allele independent mechanism involving the TOMM40 gene plays a role in the risk of AD.
Alzheimer's disease; APOE; post-mortem brain; TOMM40
Apolipoprotein E (APOE) ε4 alleles increase the risk for late-onset Alzheimer disease (LOAD) and decrease the age of onset. Recently, sequencing the APOE region in a small sample of LOAD subjects identified a variable length poly-T repeat sequence in the nearby gene, TOMM40, which may affect age of onset. We genotyped the TOMM40 poly-T repeat using a novel statistical approach to refine the identification of allele length in 892 LOAD subjects and evaluated its effects on age of onset. Because psychosis in LOAD is a heritable phenotype which has shown conflicting associations with APOE genotype, we also evaluated the association of poly-T repeat length with psychosis. Poly-T repeat lengths had a trimodal distribution which differed between APOE genotype groups. After accounting for APOE ε4 there was no association of poly-T repeat length with age of onset. Neither APOE ε4 nor poly-T repeat length was associated with psychosis. Our findings do not support the association of poly-T repeat length with age of onset in LOAD. The clinical implications of this repeat length polymorphism remain to be elucidated.
Apolipoprotein E (APOE) ε4; late-onset Alzheimer disease (LOAD); psychosis; TOMM40; variable length poly-T repeat sequence
Mitochondrial dysfunction is implicated in neurodegenerative disorders, such as Alzheimer's disease (AD). Translocase of outer mitochondrial membrane 40 (TOMM40) may be influential in this regard by influencing mitochondrial neurotoxicity. Little is known about the influence of the TOMM40 gene on hippocampal (HC) volume and episodic memory (EM), particularly in healthy older adults. Thus, we sought to discern the influence of TOMM40 single nucleotide polymorphisms (SNPs), which have previously been associated with medial temporal lobe integrity (rs11556505 and rs2075650), on HC volume and EM. The study sample consisted of individuals from the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K) who were free of dementia and known neurological disorders, and 60–87 years of age (n = 424). EM was measured by using a 16-item word list with a 2-min free recall period and delineation of the HC was performed manually. The influence of Apolipoprotein E (APOE) and TOMM40 was assessed by 2 × 2 ANOVAs and partial correlations. There was no effect of APOE and TOMM40 on EM performance and HC volume. However, partial correlations revealed that HC volume was positively associated with free recall performance (r = 0.21, p < 0.01, r2 = 0.04). When further stratified for TOMM40, the observed association between HC volume and free recall in APOE ε4 carriers was present in combination with TOMM40 rs11556505 any T (r = 0.28, p < 0.01, R2 = 0.08) and rs2075650 any G (r = 0.28, p < 0.01, R2 = 0.08) “risk” alleles. This pattern might reflect higher reliance on HC volume for adequate EM performance among APOE ε4 carriers with additional TOMM40 “risk” alleles suggesting that the TOMM40 gene cannot merely be considered a marker of APOE genotype. Nevertheless, neither APOE nor TOMM40 influenced HC volume or EM in this population-based sample of cognitively intact individuals over the age of 60.
APOE; TOMM40; episodic memory; hippocampus; cognitive aging
This study investigates the association between TOMM40 poly-T length, age-at-onset, and neuropathology in Alzheimer’s disease (AD) individuals with the APOE ε3/ε3 allele.
Thirty-two PSEN1 mutation carriers with AD, 27 PSEN2 mutation carriers with AD, 59 participants with late-onset AD (LOAD), and 168 participants with autopsies from a community-based cohort were genotyped for TOMM40 intron 6 poly-T (rs10524523) length using short tandem repeat assays.
Among AD patients with PSEN2 mutations, the presence of a long poly-T was associated with an earlier age-at-onset, whereas there were no such associations for patients with PSEN1 mutations or LOAD. In community-based participants, the presence of a long poly-T was associated with increased neuritic tangles and a higher likelihood of pathologically diagnosed AD.
TOMM40 intron 6 poly-T length may explain some of the variation in age-at-onset in PSEN2 familial AD and may be associated with AD neuropathology in persons with APOE ε3/ε3.
Alzheimer’s disease; age-at-onset; genetic; APOE; TOMM40; PSEN1 mutation; PSEN2 mutation; neuropathology
TOMM40 (translocase of the outer mitochondrial membrane pore subunit) is in linkage disequilibrium with apolipoprotein E (APOE). APOE e4 is linked to long (L; 21–29 T residues) poly-T variants within intron 6 of TOMM40 while APOE e3 can be associated with either with a short (S; <21 T residues) or very long (VL; >29 T residues) variant. To assess the possible contribution of TOMM40 to Alzheimer’s disease (AD) onset, we compared the effects of TOMM40 and APOE genotype on preclinical longitudinal memory decline.
An APOE e4 enriched cohort of 639 cognitively normal individuals age 21–97 years of known TOMM40 genotype underwent longitudinal neuropsychological testing every two years. We estimated the longitudinal effect of age on memory using statistical models that simultaneously modeled cross sectional and longitudinal effects of age on the auditory verbal learning test long term memory score (AVLT) by APOE, TOMM40, and the interaction between the two.
There were significant effects overall for both TOMM40 (p=0.04 linear effect, p=0.03 quadratic effect) and APOE (p=0.06 linear effect, p=0.008 quadratic effect) with no significant interaction (p=0.63). These differences were age-dependent: there was a significant TOMM40 effect prior to age 60 (p=0.009) characterized by flattened test-retest improvement (VL/VL subgroup only) but no significant APOE effect; and a significant APOE effect after age 60 (p=0.006) characterized by accelerated memory decline (e4 carriers) but no significant TOMM40 effect.
Both TOMM40 and APOE significantly influence age-related memory performance, but appear to do so independently of each other.
TOMM40; APOE; preclinical Alzheimer’s disease; cognitive aging; age-related memory loss; mitochondria; very long term memory; test-retest effects
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.
Genetic factors are well-established to play a role in risk of Alzheimer's disease (AD). However, it has been difficult to find genes that are involved in AD susceptibility, other than a small number of genes that play a role in early-onset, high-penetrant disease risk, and the APOE ε4 allele, which increases risk of late-onset disease. Here we use a European-American family-based sample to examine the role of common genetic variants on late-onset AD. We show that variants in CUGBP2 on chromosome 10p, along with nearby variants, are associated with AD in those highest-risk APOE ε4 homozygotes. We have replicated this interaction in an independent sample. CUGBP2 has one isoform that is expressed predominantly in neurons, and identification of such a new risk locus is important because of the severity of AD. We also provide support for recently proposed associated variants (BIN1, CLU, and partly CR1) and show that there are markers throughout the genome that are correlated with APOE. This emphasizes the need to adjust for APOE for such markers to avoid false associations and suggests that there may be confounding for other diseases with similar strong risk loci.
With the exception of APOE ε4 allele, the common genetic risk factors for sporadic Alzheimer's Disease (AD) are unknown.
Methods and Findings
We completed a genome-wide association study on 381 participants in the ADNI (Alzheimer's Disease Neuroimaging Initiative) study. Samples were genotyped using the Illumina Human610-Quad BeadChip. 516,645 unique Single Nucleotide Polymorphisms (SNPs) were included in the analysis following quality control measures. The genotype data and raw genetic data are freely available for download (LONI, http://www.loni.ucla.edu/ADNI/Data/). Two analyses were completed: a standard case-control analysis, and a novel approach using hippocampal atrophy measured on MRI as an objectively defined, quantitative phenotype. A General Linear Model was applied to identify SNPs for which there was an interaction between the genotype and diagnosis on the quantitative trait. The case-control analysis identified APOE and a new risk gene, TOMM40 (translocase of outer mitochondrial membrane 40), at a genome-wide significance level of≤10−6 (10−11 for a haplotype). TOMM40 risk alleles were approximately twice as frequent in AD subjects as controls. The quantitative trait analysis identified 21 genes or chromosomal areas with at least one SNP with a p-value≤10−6, which can be considered potential “new” candidate loci to explore in the etiology of sporadic AD. These candidates included EFNA5, CAND1, MAGI2, ARSB, and PRUNE2, genes involved in the regulation of protein degradation, apoptosis, neuronal loss and neurodevelopment. Thus, we identified common genetic variants associated with the increased risk of developing AD in the ADNI cohort, and present publicly available genome-wide data. Supportive evidence based on case-control studies and biological plausibility by gene annotation is provided. Currently no available sample with both imaging and genetic data is available for replication.
Using hippocampal atrophy as a quantitative phenotype in a genome-wide scan, we have identified candidate risk genes for sporadic Alzheimer's disease that merit further investigation.
Human longevity results from a number of factors, including genetic background, favorable environmental, social factors and chance. In this study, we aimed to elucidate the association of human longevity with genetic variations in several major candidate genes in a Han Chinese population.
A case-control association study of 1015 long-lived individuals (aged 90 years or older) and 1725 younger controls (30–70 years old) was undertaken. Rs2075650 in TOMM40 was firstly genotyped using the ABI SNaPshot method in an initial cohort consisted of 597 unrelated long-lived individuals and 1275 younger controls enrolled from Sichuan. Secondly, eighteen tag single-nucleotide polymorphisms (SNPs) in the PVRL2-TOMM40-APOE locus were genotyped for extensive study in the same cohort. Finally, 5 associated SNPs were genotyped in a replication cohort including 418 older individuals and 450 younger controls. The genotype and allele frequencies were evaluated using the χ2 tests. The linkage disequilibrium (LD) block structure was examined using the program Haploview.
The case-control study of rs2075650 in TOMM40 showed significant difference in allele frequencies between cases and controls (P = 0.006) in an initial study. Of the 18 SNPs genotyped, rs405509 in APOE and another three SNPs (rs12978931, rs519825 and rs395908) in the PVRL2 gene also showed significant association with human longevity in extensive study in the same cohort. Rs2075650 in TOMM40, rs405509 in APOE and rs519825 in PVRL2 showed a significant association with human longevity in a replication cohort.
These results suggested that PVRL2, TOMM40 and APOE might be associated with human longevity. However, further research is needed to identify the causal variants and determine which of these genes are involved in the progress of human longevity.
The isoform-specific role of human apolipoprotein E (apoE) has been assessed in a mouse model of ocular herpes. Female, age-matched transgenic mice knocked-in for the human allele apoE3 or apoE4 and their parent C57Bl/6 mice were inoculated corneally with HSV-1 strain KOS. Ocular HSV-1 pathogenesis was monitored through viral replication and clinical progression of stromal opacity and neovascularization by slit-lamp examination. Establishment of latency was determined by analysis of HSV-1 DNA (copy number) by specific real-time PCR in the cornea, trigeminal ganglia (TG), and brain. Representative groups of transgenic mice were sacrificed for the analysis of gene expression of vascular endothelial growth factor (VEGF) by reverse-transcription PCR, and apoE expression by Western blot analysis. At 6 days post-infection (P.I.), the ocular infectious HSV-1 titer was significantly higher (p < 0.05) in apoE4 mice compared with apoE3 and C57Bl/6 mice. Corneal neovascularization in apoE4 mice was significantly higher (p < 0.05) than apoE3 and C57Bl/6 mice. The onset of corneal opacity in apoE4 mice was accelerated during days 9--11 P.I.; however, no significant difference in severity was seen on P.I. days 15 and beyond. At 28 days P.I., infected mice of all genotypes had no significant differences in copy numbers (range 0--15) of HSV-1 DNA in their corneas, indicating that HSV-1 DNA copy numbers in cornea are independent of apoE isoform regulation. At 28 days P.I., both apoE4 and C57Bl/6 mice had a significantly higher (p = 0.001) number of copies of HSV-1 DNA in TG compared with apoE3. ApoE4 mice also had significantly higher (p = 0.001) copies of HSV-1 DNA in their TGs compared with C57Bl/6 mice. In brain, both apoE4 and C57Bl/6 mice had significantly higher numbers (p ≤ 0.03) of copies of HSV-1 DNA compared with apoE3 mice. However, the number of HSV-1 DNA copies in the brain of C57Bl/6 mice was not significantly different than that of apoE4 (p = 0.1). Comparative molecular analysis between apoE3 and apoE4 mice on selected days between 7 and 28 P.I., inclusive, revealed that the corneas of apoE4 mice expressed VEGF. None of the corneas in the apoE3 mice expressed VEGF during this time. Western blot analysis showed proteolytic cleavage of the apoE protein in the corneas of the apoE4 mice. Through days 14 to 28 P.I., a ~29 kDa C-terminal truncated apoE fragment was present in the corneas of apoE4 mice, but not in apoE3 mice. ApoE4 is a risk factor for ocular herpes, in part, through increased replication of virus in the eye, an earlier onset in clinical opacity, significantly higher neovascularization, and increased HSV-1 DNA load in TG and brain than that of apoE3. Increased pathogenesis of ocular herpes in apoE4 mice was also mediated, in part through up-regulated expression of VEGF and apoE proteolysis in the cornea. This is the first report linking a human gene, apoE4, as a risk factor for ocular herpes pathogenesis in a transgenic mouse model.
HSV-1; apolipoprotein E; brain; cornea; herpetic stromal keratitis; trigeminal ganglia; transgenic mouse model; vascular endothelial growth factor
Recent genome-wide association studies (GWAS) of late-onset Alzheimer’s disease (LOAD) have identified single nucleotide polymorphisms (SNPs) which show significant association at the well-known APOE locus and at nineteen additional loci. Among the functional, disease-associated variants at these loci, missense variants are particularly important because they can be readily investigated in model systems to search for novel therapeutic targets. It is now possible to perform a low-cost search for these “actionable” variants by genotyping the missense variants at known LOAD loci already cataloged on the Exome Variant Server (EVS). In this proof-of-principle study designed to explore the efficacy of this approach, we analyzed three rare EVS variants in APOE, p.L28P, p.R145C and p.V236E, in our case control series of 9114 subjects. p.R145C proved to be too rare to analyze effectively. The minor allele of p.L28P, which was in complete linkage disequilibrium (D’ = 1) with the far more common APOE ϵ4 allele, showed no association with LOAD (P = 0.75) independent of the APOE ϵ4 allele. p.V236E was significantly associated with a marked reduction in risk of LOAD (P = 7.5×10−05; OR = 0.10, 0.03 to 0.45). The minor allele of p.V236E, which was in complete linkage disequilibrium (D’ = 1) with the common APOE ϵ3 allele, identifies a novel LOAD-associated haplotype (APOE ϵ3b) which is associated with decreased risk of LOAD independent of the more abundant APOE ϵ2, ϵ3 and ϵ4 haplotypes. Follow-up studies will be important to confirm the significance of this association and to better define its odds ratio. The ApoE p.V236E substitution is the first disease-associated change located in the lipid-binding, C-terminal domain of the protein. Thus our study (i) identifies a novel APOE missense variant which may profitably be studied to better understand how ApoE function may be modified to reduce risk of LOAD and (ii) indicates that analysis of protein-altering variants cataloged on the EVS can be a cost-effective way to identify actionable functional variants at recently discovered LOAD loci.
The ε4 allele of APOE confers a two- to four-fold increased risk for late-onset Alzheimer’s disease (LOAD), but LOAD pathology does not all fit neatly around APOE. It is conceivable that genetic variation proximate to APOE contributes to LOAD risk. Therefore, we investigated the degree of linkage disequilibrium (LD) for a comprehensive set of 50 SNPs in and surrounding the APOE using a substantial Caucasian sample of 1100 chromosomes. SNPs in APOE were further molecular haplotyped to determine their phases. One set of SNPs in TOMM40, roughly 15 Kb upstream of APOE, showed intriguing LD with the ε4 allele, and were strongly associated with the risk for developing AD. However, when all the SNPs were entered into a logit model, only the effect of APOE ε4 remained significant. These observations diminish the possibility that loci in the TOMM40 may have a major effect on the risk of LOAD in Caucasians.
molecular haplotyping; apolipoprotein E; selection; linkage disequilibrium; genetic association; Alzheimer’s disease
The ε4 allele of the apolipoprotein E gene (APOE) is associated with increased risk and earlier age at onset in late onset Alzheimer’s disease (AD). Other factors, such as expression level of apolipoprotein E protein (apoE), have been postulated to modify the APOE related risk of developing AD. Multiple loci in and outside of APOE are associated with a high risk of AD. The aim of this exploratory hypothesis generating investigation was to determine if some of these loci predict cerebrospinal fluid (CSF) apoE levels in healthy non-demented subjects. CSF apoE levels were measured from healthy non-demented subjects 21–87 years of age (n = 134). Backward regression models were used to evaluate the influence of 21 SNPs, within and surrounding APOE, on CSF apoE levels while taking into account age, gender, APOE ε4 and correlation between SNPs (linkage disequilibrium). APOE ε4 genotype does not predict CSF apoE levels. Three SNPs within the TOMM40 gene, one APOE promoter SNP and two SNPs within distal APOE enhancer elements (ME1 and BCR) predict CSF apoE levels. Further investigation of the genetic influence of these loci on apoE expression levels in the central nervous system is likely to provide new insight into apoE regulation as well as AD pathogenesis.
Apolipoprotein E gene; apolipoprotein E protein; cerebroshinal fluid; enhancer; promoter; SNP
Abnormalities of the hypothalamic-pituitary-adrenal (HPA) axis have been reported in subjects with Alzheimer’s disease (AD) and may include increased cerebrospinal fluid (CSF) cortisol concentrations. Moreover, presence of the APOE ε4 allele, which is an established risk factor for the development of AD, has been shown to associate with higher CSF cortisol levels, especially in AD sufferers. In this study, we examined whether TOMM40 variants, which have been reported to influence age of onset of AD, also had an effect on CSF cortisol levels, in healthy, cognitively intact individuals with or without APOE ε4. In our results, the increase in CSF cortisol associated with the presence of the APOE ε4 allele was only detected when a short TOMM40 poly-T variant, shown to associate with later age of onset of AD in ε4 carriers, was not when present. These results are consistent with previous reports (e.g., Roses et al. 2009) suggesting that TOMM40 poly-T variants influence the effects of APOE alleles.
TOMM40 poly-T; APOE; cerebrospinal fluid; cortisol; Alzheimer’s disease; hippocampus
Genetic polymorphisms in the APOE ɛ and TOMM40 ‘523' poly-T repeat gene loci have been associated with significantly increased risk of Alzheimer's disease. This study investigated the independent effects of these polymorphisms on human cognitive ageing, and the extent to which nominally significant associations with cognitive ageing were mediated by previously reported genetic associations with brain white matter tract integrity in this sample. Most participants in the Lothian Birth Cohort 1936 completed a reasoning-type intelligence test at age 11 years, and detailed cognitive/physical assessments and structural diffusion tensor brain magnetic resonance imaging at a mean age of 72.70 years (s.d.=0.74). Participants were genotyped for APOE ɛ2/ɛ3/ɛ4 status and TOMM40 523 poly-T repeat length. Data were available from 758–814 subjects for cognitive analysis, and 522–543 for mediation analysis with brain imaging data. APOE genotype was significantly associated with performance on several different tests of cognitive ability, including general factors of intelligence, information processing speed and memory (raw P-values all<0.05), independently of childhood IQ and vascular disease history. Formal tests of mediation showed that several significant APOE-cognitive ageing associations—particularly those related to tests of information processing speed—were partially mediated by white matter tract integrity. TOMM40 523 genotype was not associated with cognitive ageing. A range of brain phenotypes are likely to form the anatomical basis for significant associations between APOE genotype and cognitive ageing, including white matter tract microstructural integrity.
The primary constituents of plaques (Aβ42/Aβ40) and neurofibrillary tangles (tau and phosphorylated forms of tau [ptau]) are the current leading diagnostic and prognostic cerebrospinal fluid (CSF) biomarkers for AD. In this study, we performed deep sequencing of APP, PSEN1, PSEN2, GRN, APOE and MAPT genes in individuals with extreme CSF Aβ42, tau, or ptau levels. One known pathogenic mutation (PSEN1 p.A426P), four high-risk variants for AD (APOE p.L46P, MAPT p.A152T, PSEN2 p.R62H and p.R71W) and nine novel variants were identified. Surprisingly, a coding variant in PSEN1, p.E318G (rs17125721-G) exhibited a significant association with high CSF tau (p = 9.2×10−4) and ptau (p = 1.8×10−3) levels. The association of the p.E318G variant with Aβ deposition was observed in APOE-ε4 allele carriers. Furthermore, we found that in a large case-control series (n = 5,161) individuals who are APOE-ε4 carriers and carry the p.E318G variant are at a risk of developing AD (OR = 10.7, 95% CI = 4.7–24.6) that is similar to APOE-ε4 homozygous (OR = 9.9, 95% CI = 7.2.9–13.6), and double the risk for APOE-ε4 carriers that do not carry p.E318G (OR = 3.9, 95% CI = 3.4–4.4). The p.E318G variant is present in 5.3% (n = 30) of the families from a large clinical series of LOAD families (n = 565) and exhibited a higher frequency in familial LOAD (MAF = 2.5%) than in sporadic LOAD (MAF = 1.6%) (p = 0.02). Additionally, we found that in the presence of at least one APOE-ε4 allele, p.E318G is associated with more Aβ plaques and faster cognitive decline. We demonstrate that the effect of PSEN1, p.E318G on AD susceptibility is largely dependent on an interaction with APOE-ε4 and mediated by an increased burden of Aβ deposition.
Alzheimer's disease (AD) is the most common neurodegenerative disease affecting more than 5.3 million people in the US. AD-causing mutations have been identified in APP, PSEN1 and PSEN2 genes. Heterozygous carriers of APOE-ε4 allele exhibit a 3-fold increased risk for developing AD, while homozygous carriers show a 10-fold greater risk than non-carriers. Here, we sequenced individuals with extreme levels of well-established AD cerebrospinal fluid (CSF) biomarkers in order to identify variants in APOE, APP, PSEN1, PSEN2, GRN and MAPT genes associated with AD risk. This approach allowed us to identify known pathogenic variants, additional AD risk genetic factors and identify a low frequency variant in PSEN1, p.E318G (rs17125721-G) that increases risk for AD in a gene-gene interaction with APOE. These findings were replicated in three large (>4,000 individuals) and independent datasets. This finding is particularly important because we demonstrated that a currently considered non-pathogenic variant is associated with higher levels of neuronal degeneration, and with Aβ deposition, more Aβ plaques and faster cognitive decline in an APOE-ε4-dependent fashion. APOE-ε4 heterozygous individuals who carry this variant are at similar AD risk as APOE-ε4 homozygous individuals.
Toll-like receptors (TLRs), as major innate immune mediators, may be involved in clearance of cerebral amyloid-β (Aβ) deposits. Recently, a novel TLR9 signaling pathway has been uncovered, which is functionally associated with the immune inflammatory response and reducing Aβ burden in Alzheimer’s disease (AD) mice. Therefore, TLR9 might represent a reasonable functional candidate gene for AD.
Our study investigated 1,133 sporadic late-onset AD (LOAD) and 1,159 healthy controls matched for sex and age in a large Han Chinese population. One selected functional rs187084 polymorphism within the TLR9 gene was genotyped by polymerase chain reaction-ligase detection reaction in a case–control associated study. The TLR9 rs187084 variant homozygote GG was significantly associated with a decreased LOAD risk after adjusting for age, gender, and ApoE ϵ4 status by logistic regression analysis (P = 0.035). Our result showed significant evidence of the interaction of ApoE ϵ4 with rs187084. When we further stratified our data by the ApoE ϵ4 status, we detected significant differences in the genotype and allele distributions of rs187084 between LOAD patients and controls in ApoE ϵ4 carriers (P < 0.001, P = 0.003, respectively). Moreover, we examined TLR9 expression in peripheral blood monocytes by flow cytometry, and the GG genotype of the TLR9 rs187084 polymorphism was associated with a higher TLR9 expression than two other genotypes in LOAD patients.
Our findings support the hypothesis that the TLR9 polymorphism may modify LOAD risk in the Han Chinese population.
Alzheimer’s disease; Polymorphisms; TLR9; rs187084; Expression; Association study