The effect of APOE genotype on brain levels of oxysterols in young and old human APOE ε2, ε3 and ε4 knock-in mice

Despite apolipoprotein E's important role in cholesterol transport and metabolism in the brain as well as its influence on Alzheimer's disease, the impact of the human APOE genotype on cholesterol metabolism in brain has not been fully examined. This study was carried out to investigate APOE genotype effects on oxysterols measured. In this study the measurement of cholesterol and several oxysterols in the brains of human APOE ε2, ε3 and ε4 knock-in mice at 8 weeks and 1 year of age using gas chromatography mass spectrometry (GC–MS) demonstrated no APOE genotype or age effect on total brain cholesterol and the oxysterol 24-hydroxycholesterol. The level of 27-hydroxycholesterol was elevated in 1 year old animals for all APOE genotypes. Interestingly, lathosterol an indicator of cholesterol synthesis was significantly reduced in the 1 year old animals for all APOE genotypes. APOE ε4 expressing mice exhibited statistically lower levels of lathosterol compared to APOE ε2 in both the young and old mice. Oxidized cholesterol metabolites were significantly lower in APOE ε2 mice compared to other genotypes at 8 weeks old. Although minimal differences were observed between APOE E3 and E4 knock-in (KI) mice, these findings indicate that there are some clear APOE genotype specific effects on brain cholesterol synthesis and associated metabolic pathways, particularly in APOE ε2 KI mice.     

Association of cerebrospinal fluid Aβ42 with A2M gene in cognitively normal subjects

Low cerebrospinal fluid (CSF) Aβ₄₂ levels correlate with increased brain Aβ deposition in Alzheimer's disease (AD), which suggests a disruption in the degradation and clearance of Aβ from the brain. In addition, APOE ε4 carriers have lower CSF Aβ₄₂ levels than non-carriers. The hypothesis of this investigation was that CSF Aβ₄₂ levels would correlate with regulatory region variation in genes that are biologically associated with degradation or clearance of Aβ from the brain. CSF Aβ₄₂ levels were tested for associations with Aβ degradation and clearance genes and APOE ε4. Twenty-four SNPs located within the 5′ and 3′ regions of 12 genes were analyzed. The study sample consisted of 99 AD patients and 168 cognitively normal control subjects. CSF Aβ₄₂ levels were associated with APOE ε4 status in controls but not in AD patients; A2M regulatory region SNPs were also associated with CSF Aβ₄₂ levels in controls but not in AD patients, even after adjusting for APOE ε4. These results suggest that genetic variation within the A2M gene influences CSF Aβ₄₂ levels.     

Alzheimer's disease risk genes and the age-at-onset phenotype

Despite the recent identification of several novel risk genes for Alzheimer's disease (AD), little is known about their influence on the age at onset (AAO) of AD. The AAO is a phenotype with a heritable component distinct from disease risk and may be a useful trait to study in the context of developing interventions for delaying the onset of AD. We studied the influence of 10 recently identified AD risk genes and APOE in relation to AAO in a large cohort of AD patients (N = 2569). We find that the novel AD risk gene, PICALM, exerts a small effect on the AAO of AD with earlier disease onset in risk allele carriers. In addition, we confirmed the previously reported association between the APOE ε4 allele and earlier disease onset. None of the other AD risk genes influenced AAO of AD. Our results suggest that besides APOE, other genes associated with AD risk do not exert large effects on the AAO phenotype of AD.     

The complex interaction between APOE promoter and AD: an Italian case�Ccontrol study

The single nucleotide polymorphisms (SNPs) rs449647, rs769446 and rs405509 in the promoter region of the APOE gene have been variously suggested to be ɛ4-independent risk factors for Alzheimer''s disease (AD). A previous Italian study found that the rs449647 was significantly associated with late-onset AD. The aim of this study was to verify whether these APOE promoter SNPs are genetic risk factors for AD and to investigate their interaction with the common APOE polymorphism. A total of 169 clinically diagnosed AD patients and 99 cognitively intact age-matched controls were included in the study. Significant associations with AD independent from sex, age and APOE/ɛ4 status were found for rs449647 A/A and rs405509 G/G genotypes (positive), and rs449647 A/T and rs405509 T/T genotypes (negative). Haplotype frequency estimation at the APOE locus showed significant associations for the ATG4, ATT4 and ACG3 (positive) and ATT2, ATT3 and TCG3 (negative) haplotypes. Therefore this study confirms the role of the rs449647 A/A genotype as risk factor for AD in Italy and suggests that promoter genotypes and APOE haplotypes might have a complex function in AD-associated genetic risk factors.     

Depletion of potential A2M risk haplotype for Alzheimer's disease in long-lived individuals

Risk alleles for age-related diseases are expected to decrease in frequency in the population strata of increasing age. Consistent with this hypothesis, earlier studies showed a depletion of the Alzheimer''s disease risk factor APOE^*ɛ4 in long-lived individuals (LLIs). To evaluate whether this observation also holds for a previously suggested Alzheimer''s disease risk haplotype in the A2M gene, we analyzed this particular haplotype in 1042 German LLIs (aged 95–100 years) and 1040 younger individuals (aged 60–75 years). Our results show a significant depletion of this haplotype in LLIs, thus confirming it as a mortality factor in the elderly. Consequently, our data support an involvement of the suggested A2M risk haplotype in the pathogenesis of Alzheimer''s disease and adds new evidence to the risk-allele depletion hypothesis.     

F-fluorodeoxyglucose positron emission tomography,aging, and apolipoprotein E genotype in cognitively normal persons

Our objective was to examine associations between glucose metabolism, as measured by ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG PET), and age and to evaluate the impact of carriage of an apolipoprotein E (APOE) ε4 allele on glucose metabolism and on the associations between glucose metabolism and age. We studied 806 cognitively normal (CN) and 70 amyloid-imaging-positive cognitively impaired participants (35 with mild cognitive impairment and 35 with Alzheimer's disease [AD] dementia) from the Mayo Clinic Study of Aging, Mayo Alzheimer's Disease Research Center and an ancillary study who had undergone structural MRI, FDG PET, and ¹¹C-Pittsburgh compound B (PiB) PET. Using partial volume corrected and uncorrected FDG PET glucose uptake ratios, we evaluated associations of regional FDG ratios with age and carriage of an APOE ε4 allele in CN participants between the ages of 30 and 95 years, and compared those findings with the cognitively impaired participants. In region-of-interest (ROI) analyses, we found modest but statistically significant declines in FDG ratio in most cortical and subcortical regions as a function of age. We also found a main effect of APOE ε4 genotype on FDG ratio, with greater uptake in ε4 noncarriers compared with carriers but only in the posterior cingulate and/or precuneus, lateral parietal, and AD-signature meta-ROI. The latter consisted of voxels from posterior cingulate and/or precuneus, lateral parietal, and inferior temporal. In age- and sex-matched CN participants the magnitude of the difference in partial volume corrected FDG ratio in the AD-signature meta-ROI for APOE ε4 carriers compared with noncarriers was about 4 times smaller than the magnitude of the difference between age- and sex-matched elderly APOE ε4 carrier CN compared with AD dementia participants. In an analysis in participants older than 70 years (31.3% of whom had elevated PiB), there was no interaction between PiB status and APOE ε4 genotype with respect to glucose metabolism. Glucose metabolism declines with age in many brain regions. Carriage of an APOE ε4 allele was associated with reductions in FDG ratio in the posterior cingulate and/or precuneus, lateral parietal, and AD-signature ROIs, and there was no interaction between age and APOE ε4 status. The posterior cingulate and/or precuneus and lateral parietal regions have a unique vulnerability to reductions in glucose metabolic rate as a function both of age and carriage of an APOE ε4 allele.     

Alzheimer's disease susceptibility genes APOE and TOMM40, and brain white matter integrity in the Lothian Birth Cohort 1936

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.     

Evaluating NAT2PRED for inferring the individual acetylation status from unphased genotype data

Background

Genome-wide linkage studies for Alzheimer's disease have implicated several chromosomal regions as potential loci for susceptibility genes.

Methods

In the present study, we have combined a selection of affected relative pairs (ARPs) from the UK and the USA included in a previous linkage study by Myers et al. (Am J Med Genet, 2002), with ARPs from Sweden and Washington University. In this total sample collection of 397 ARPs, we have analyzed linkage to chromosomes 1, 9, 10, 12, 19 and 21, implicated in the previous scan.

Results

The analysis revealed that linkage to chromosome 19q13 close to the APOE locus increased considerably as compared to the earlier scan. However, linkage to chromosome 10q21, which provided the strongest linkage in the previous scan could not be detected.

Conclusion

The present investigation provides yet further evidence that 19q13 is the only chromosomal region consistently linked to Alzheimer's disease.     

Disentangling the effects of age and APOE on neuropathology and late life cognitive decline

Age and APOE are the most robust risk factors for dementia and cognitive decline, but the underlying neurobiology remains unclear. We examined the extent to which the hallmark pathologies of Alzheimer's disease, Lewy body disease, and cerebrovascular diseases account for the association of age and APOE with decline in episodic memory versus nonepisodic cognitive abilities. Up to 20 waves of longitudinal cognitive data were collected from 858 autopsied participants in 2 ongoing clinical-pathologic cohort studies of aging. Neuropathologic examinations quantified measures of beta amyloid (Aβ) plaque, mesial temporal and neocortical neurofibrillary tangles, macro- and microinfarcts, and neocortical Lewy bodies. Random coefficient models estimated person-specific slopes of decline in episodic memory and nonepisodic cognition. Path analysis examined the relation of age, APOE, and the 6 pathologic indices to the slopes of cognitive decline. The effect of age on decline in episodic memory was mediated by Aβ, mesial temporal and neocortical tau tangles, and macroscopic infarcts; age on decline in nonepisodic cognition was mediated by Aβ, neocortical tangles, and macroscopic infarcts. The effect of APOE on decline in episodic memory was mediated by Aβ, mesial temporal and neocortical tangles, and neocortical Lewy bodies; APOE on nonepisodic cognition was mediated by Aβ, neocortical tangles, and neocortical Lewy bodies. There were no direct effects of age and APOE on decline after accounting for these pathologic pathways.     

Lack of association between PICALM rs3851179 polymorphism and Alzheimer's disease in Chinese population and APOEε4-negative subgroup

Recently, the association between PICALM rs3851179 polymorphism and Alzheimer's disease (AD) was investigated in the Chinese population by 3 independent studies. However, both allele and genotype tests failed to reveal any association. The association was identified only in the APOEε4-negative subgroup. We think that the failure to replicate the association may be because of the relatively small sample size. In this research, we reinvestigated the association using all the samples from these 3 studies (n = 2486, and 1202 cases and 1284 control subjects). We failed to replicate this association between the rs3851179 polymorphism and AD in all samples and the APOEε4-negative subgroup. Our results indicate that rs3851179 may not be an AD susceptibility locus in the Chinese population and the APOEε4-negative subgroup.     

Insulin-like growth factor 1 (IGF1) polymorphism is associated with Alzheimer's disease in Han Chinese

A review of pathogenic findings in Alzheimer's brains and the functional consequences of altered insulin-like growth factor 1 (IGF1) input to the brain suggest the association between Alzheimer's disease (AD) and the disrupted IGF1 signaling. Recently, the identification of polymorphism rs972936 that was associated with both an increased risk of AD and high circulating levels of IGF1 was reported in Southern European population. In order to evaluate the involvement of the IGF1 polymorphism in the risk of developing late-onset Alzheimer's disease (LOAD) in Chinese, we performed an independent case-control association study in a Han Chinese population (794 LOAD cases and 796 controls). There were significant differences in genotype and allele frequencies between LOAD cases and controls (genotype P = 0.006, allele P = 0.047). The T allele of rs972936 demonstrated a 1.16-fold risk for developing LOAD when compared with the C allele, which diverges to the report in the Caucasian population. After stratification by apolipoprotein E (APOE) ?4-carrying status, rs972936 polymorphism was only significantly associated with LOAD in non-ApoE ?4 allele carriers (genotype P = 0.002, allele P = 0.039). Multivariate logistic regression analysis also conferred this positive association between the SNP rs972936 and LOAD in the recessive and additive model after adjustment for age, gender, and the ApoE ?4 carrier status. These results suggest that IGF1 polymorphism has a possible role in changing the genetic susceptibility to LOAD in a Han Chinese population.     

Fasting plasma insulin and the default mode network in women at risk for Alzheimer's disease

Brain imaging studies in Alzheimer's disease research have demonstrated structural and functional perturbations in the hippocampus and default mode network (DMN). Additional evidence suggests risk for pathological brain aging in association with insulin resistance (IR). This study piloted investigation of associations of IR with DMN-hippocampal functional connectivity among postmenopausal women at risk for Alzheimer's disease. Twenty middle-aged women underwent resting state functional magnetic resonance imaging. Subjects were dichotomized relative to fasting plasma insulin levels (i.e., > 8 μIU/mL [n = 10] and < 8 μIU/mL [n = 10]), and functional connectivity analysis contrasted their respective blood oxygen level-dependent signal correlation between DMN and hippocampal regions. Higher-insulin women had significantly reduced positive associations between the medial prefrontal cortex and bilateral parahippocampal regions extending to the right hippocampus, and conversely, between the left and right hippocampus and medial prefrontal cortex. Neuropsychological data (all within normal ranges) also showed significant differences with respect to executive functioning and global intelligence. The results provide further evidence of deleterious effects of IR on the hippocampus and cognition. Further imaging studies of the IR-related perturbations in DMN-hippocampal functional connectivity are needed.     

LRP1 expression in cerebral cortex,choroid plexus and meningeal blood vessels: Relationship to cerebral amyloid angiopathy and APOE status

APOE genotype is a risk factor for Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The risk and severity of CAA increase with possession of APOE ?4, whereas APOE ?2 increases the risk of vessel rupture. Uptake of Aβ by cerebrovascular smooth muscle cells (CVSMCs) is mediated by low-density lipoprotein receptor-related protein-1 (LRP1). To determine whether APOE influences CAA by altering LRP1 expression, particularly by CVSMCs, we analysed APOE genotype, CAA severity, and LRP1 levels in post-mortem cerebral cortex, choroid plexus and meningeal vessels. LRP1 mRNA and protein were not related to CAA severity and presence. LRP1 mRNA was increased in meningeal vessels, but not cortex or choroid plexus, in AD and in association with APOE ?4, and was decreased in association with APOE ?3. In brains with CAA, APOE ?2 was associated with decreased LRP1 protein in meningeal vessels, and ?3 with increased LRP1 in choroid plexus. These findings suggest that APOE may influence the severity of CAA through altered expression of LRP1.     

Endophenotypes in normal brain morphology and Alzheimer's disease: a review

Late-onset Alzheimer's disease is a common complex disorder of old age. Though these types of disorders can be highly heritable, they differ from single-gene (Mendelian) diseases in that their causes are often multifactorial with both genetic and environmental components. Genetic risk factors that have been firmly implicated in the cause are mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) genes, which are found in large multi-generational families with an autosomal dominant pattern of disease inheritance, the apolipoprotein E (APOE)ε4 allele and the sortilin-related receptor (SORL1) gene. Environmental factors that have been associated with late-onset Alzheimer's disease include depressive illness, various vascular risk factors, level of education, head trauma and estrogen replacement therapy. This complexity may help explain their high prevalence from an evolutionary perspective, but the etiologic complexity makes identification of disease-related genes much more difficult. The “endophenotype” approach is an alternative method for measuring phenotypic variation that may facilitate the identification of susceptibility genes for complexly inherited traits. The usefulness of endophenotypes in genetic analyses of normal brain morphology and, in particular for Alzheimer's disease will be reviewed as will the implications of these findings for models of disease causation. Given that the pathways from genotypes to end-stage phenotypes are circuitous at best, identifying endophenotypes more proximal to the effects of genetic variation may expedite the attempts to link genetic variants to disorders.     

Pharmacogenetic analysis of the effects of polymorphisms in APOE, IDE and IL1B on a ketone body based therapeutic on cognition in mild to moderate Alzheimer's disease; a randomized, double-blind, placebo-controlled study

Background

To examine the effect of genetic variation in APOE, IDE and IL1B on the response to induced ketosis in the Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-Cog) in subjects with mild to moderate Alzheimer's disease (AD).

Methods

Genotype effects on ADAS-Cog scores from a randomized, double-blind, placebo-controlled study in mild to moderate AD were examined by an overall two way analysis of variance. In addition, interactions with the carriage status of the epsilon 4 allele of the APOE gene (APOE4) were examined.

Results

Significant differences in response to induced ketosis were found among non-carriers of putative gain-of-function polymorphisms in rs1143627 and rs16944 in the IL1B gene and among variants of the polymorphism rs2251101 in the IDE gene. Significant differences were found among non-carriers of the APOE4 gene, with notable improvement among the E3/E3 genotype group.

Conclusions

Variants in APOE, IL1B and IDE may influence the cognitive response to induced ketosis in patients with mild to moderate AD.

Trial registration

This trial was registered with ClinicalTrials.gov, registry number NCT00142805.     

Age-related downregulation of the CaV3.1 T-type calcium channel as a mediator of amyloid beta production

Alzheimer's is a crippling neurodegenerative disease that largely affects aged individuals. Decades of research have highlighted age-related changes in calcium homeostasis that occur before and throughout the duration of the disease, and the contributions of such dysregulation to Alzheimer's disease pathogenesis. We report an age-related decrease in expression of the CaV_3.1 T-type calcium channel at the level of messenger RNA and protein in both humans and mice that is exacerbated with the presence of Alzheimer's disease. Downregulating T-type calcium channels in N2a cells and the 3xTg-AD mouse model of Alzheimer's disease, by way of pharmacologic inhibition with NNC-55-0396, results in a rapid increase in amyloid beta production via reductions in non-amyloidogenic processing, whereas genetic overexpression of the channel in human embryonic kidney cells expressing amyloid precursor protein produces complementary effects. The age-related decline in CaV_3.1 expression may therefore contribute to a pro-amyloidogenic environment in the aging brain and represents a novel opportunity to intervene in the course of Alzheimer's disease pathogenesis.     

Butyrylcholinesterase,ApoE and Alzheimer's disease in a population from the Canary Islands (Spain)

Aim: Cholinergic dysfunction is a major neurochemical feature in Alzheimer's disease (AD), accountable for many cognitive dysfunctions and some psychiatric symptoms. Butyrylcholinesterase (BChE) is one of the cholinesterases with increased activity in the brain of Alzheimer's patients. Several mutations code for different BChE, such as the K variant, which is the most common and is capable of reducing BChE activity by 30%. We studied the relationship between this K variant and Alzheimer's disease in our population from the Canary Islands (Spain). Patients and methods: We used DNA PCR-RFLP techniques to compare 282 patients who had been diagnosed with probable Alzheimer's disease – according to NINCS-ADRDA criteria – with 312 control subjects confirmed to be free of cognitive impairment as assessed by using the CAMDEX cognitive subscale CAMCOG. Results: In our population the K variant of BChE is linked to the age of diagnosis of Alzheimer's disease, since AD individuals with this allele presented the disease at a later stage. No other susceptibility relations are exposed in this study. In addition, the BChE allelic frequencies in our population are higher than those previously reported.     

Amyloid β concentrations in older people with Down syndrome and dementia

People with Down syndrome develop Alzheimer's disease with an early age of onset. Plasma amyloid beta (Aβ) levels were measured in individuals with Down syndrome who were over the age of 40. No associations between age and Aβ1–40 and Aβ1–42 concentrations were found and nor were Aβ1–40 and Aβ1–42 levels found to vary between those with Alzheimer's-type dementia and those without dementia. The APOE genotype was not found to have an impact upon Aβ1–40 or Aβ1–42 concentrations. These data suggest that other factors play important roles in determining the onset and progression of dementia in the Down syndrome population.     

Accuracy of imputation to infer unobserved APOE epsilon alleles in genome-wide genotyping data

Apolipoprotein E, encoded by APOE, is the main apoprotein for catabolism of chylomicrons and very low density lipoprotein. Two common single-nucleotide polymorphisms (SNPs) in APOE, rs429358 and rs7412, determine the three epsilon alleles that are established genetic risk factors for late-onset Alzheimer''s disease (AD), cerebral amyloid angiopathy, and intracerebral hemorrhage (ICH). These two SNPs are not present in most commercially available genome-wide genotyping arrays and cannot be inferred through imputation using HapMap reference panels. Therefore, these SNPs are often separately genotyped. Introduction of reference panels compiled from the 1000 Genomes project has made imputation of these variants possible. We compared the directly genotyped and imputed SNPs that define the APOE epsilon alleles to determine the accuracy of imputation for inference of unobserved epsilon alleles. We utilized genome-wide genotype data obtained from two cohorts of ICH and AD constituting subjects of European ancestry. Our data suggest that imputation is highly accurate, yields an acceptable proportion of missing data that is non-differentially distributed across case and control groups, and generates comparable results to genotyped data for hypothesis testing. Further, we explored the effect of imputation algorithm parameters and demonstrated that customization of these parameters yields an improved balance between accuracy and missing data for inferred genotypes.