New Genetic Approaches to AD: Lessons from APOE-TOMM40 Phylogenetics

TOMM40 and APOC1 variants differentiate the impacts of the APOE ε4 allele on Alzheimer's disease risk across sexes, ages, and ancestries

INTRODUCTION

The variability in apolipoprotein E (APOE) ε4-attributed susceptibility to Alzheimer's disease (AD) across ancestries, sexes, and ages may stem from the modulating effects of other genetic variants.

METHODS

We examined associations of compound genotypes (CompGs) comprising the ε4-encoding rs429358, TOMM40 rs2075650, and APOC1 rs12721046 polymorphisms with AD in White (7181/16,356 AD-affected/unaffected), Hispanic/Latino (2305/2921), and Black American (547/1753) participants across sexes and ages.

RESULTS

The absence and presence of the rs2075650 and/or rs12721046 minor alleles in the ε4-bearing CompGs define lower- and higher-AD-risk profiles, respectively, in White participants. They differentially impact AD risks in men and women of different ancestries, exhibiting an increasing, decreasing, flat, and nonlinear-with lower risks at ages younger than 65/70 years and older than 85 years compared to the ages in between-patterns across ages.

DISCUSSION

The ε4-bearing CompGs have a potential to differentiate biological mechanisms of sex-, age-, and ancestry-specific AD risks and serve as AD biomarkers.

Highlights

Younger White women carrying the lower-risk (LR) CompG are at small risk of AD.Black carriers of the LR CompG are at negligible risk of AD at 85 years and older.The higher-risk (HR) CompGs confer high AD risk in Whites and Blacks at 70 to 85 years.AD risk decreases with age for Hispanic/Lation women carrying the HR CompGs.Hispanic/Lation carriers of the LR CompG but not HR CompGs have higher AD risk than Blacks.

Interaction Analysis Reveals Complex Genetic Associations with Alzheimer's Disease in the CLU and ABCA7 Gene Regions

Sporadic Alzheimer's disease (AD) is a polygenic neurodegenerative disorder. Single-nucleotide polymorphisms (SNPs) in multiple genes (e.g., CLU and ABCA7) have been associated with AD. However, none of them were characterized as causal variants that indicate the complex genetic architecture of AD, which is likely affected by individual variants and their interactions. We performed a meta-analysis of four independent cohorts to examine associations of 32 CLU and 50 ABCA7 polymorphisms as well as their 496 and 1225 pair-wise interactions with AD. The single SNP analyses revealed that six CLU and five ABCA7 SNPs were associated with AD. Ten of them were previously not reported. The interaction analyses identified AD-associated compound genotypes for 25 CLU and 24 ABCA7 SNP pairs, whose comprising SNPs were not associated with AD individually. Three and one additional CLU and ABCA7 pairs composed of the AD-associated SNPs showed partial interactions as the minor allele effect of one SNP in each pair was intensified in the absence of the minor allele of the other SNP. The interactions identified here may modulate associations of the CLU and ABCA7 variants with AD. Our analyses highlight the importance of the roles of combinations of genetic variants in AD risk assessment.

APOE ɛ4 allele and TOMM40-APOC1 variants jointly contribute to survival to older ages

Age-related diseases characteristic of post-reproductive life, aging, and life span are the examples of polygenic non-Mendelian traits with intricate genetic architectures. Polygenicity of these traits implies that multiple variants can impact their risks independently or jointly as combinations of specific variants. Here, we examined chances to live to older ages, 85 years and older, for carriers of compound genotypes comprised of combinations of genotypes of rs429358 (APOE ɛ4 encoding polymorphism), rs2075650 (TOMM40), and rs12721046 (APOC1) polymorphisms using data from four human studies. The choice of these polymorphisms was motivated by our prior results showing that the ɛ4 carriers having minor alleles of the other two polymorphisms were at exceptionally high risk of Alzheimer's disease (AD), compared with non-carriers of the minor alleles. Consistent with our prior findings for AD, we show here that the adverse effect of the ɛ4 allele on survival to older ages is significantly higher in carriers of minor alleles of rs2075650 and/or rs12721046 polymorphisms compared with their non-carriers. The exclusion of AD cases made this effect stronger. Our results provide compelling evidence that AD does not mediate the associations of the same compound genotypes with chances to survive until older ages, indicating the existence of genetically heterogeneous mechanisms. The survival chances can be mainly associated with lipid- and immunity-related mechanisms, whereas the AD risk, can be driven by the AD-biomarker-related mechanism, among others. Targeting heterogeneous polygenic profiles of individuals at high risks of complex traits is promising for the translation of genetic discoveries to health care.

Associations of the APOE ε2 and ε4 alleles and polygenic profiles comprising APOE-TOMM40-APOC1 variants with Alzheimer's disease biomarkers

Capturing the genetic architecture of Alzheimer's disease (AD) is challenging because of the complex interplay of genetic and non-genetic factors in its etiology. It has been suggested that AD biomarkers may improve the characterization of AD pathology and its genetic architecture. Most studies have focused on connections of individual genetic variants with AD biomarkers, whereas the role of combinations of genetic variants is substantially underexplored. We examined the associations of the APOE ε2 and ε4 alleles and polygenic profiles comprising the ε4-encoding rs429358, TOMM40 rs2075650, and APOC1 rs12721046 polymorphisms with cerebrospinal fluid (CSF) and plasma amyloid β (Aβ40 and Aβ42) and tau biomarkers. Our findings support associations of the ε4 alleles with both plasma and CSF Aβ42 and CSF tau, and the ε2 alleles with baseline, but not longitudinal, CSF Aβ42 measurements. We found that the ε4-bearing polygenic profiles conferring higher and lower AD risks are differentially associated with tau but not Aβ42. Modulation of the effect of the ε4 alleles by TOMM40 and APOC1 variants indicates the potential genetic mechanism of differential roles of Aβ and tau in AD pathogenesis.

Genome-wide analysis identified abundant genetic modulators of contributions of the apolipoprotein E alleles to Alzheimer's disease risk

INTRODUCTION

The apolipoprotein E (APOE) ε2 and ε4 alleles have beneficial and adverse impacts on Alzheimer's disease (AD), respectively, with incomplete penetrance, which may be modulated by other genetic variants.

METHODS

We examined whether the associations of the APOE alleles with other polymorphisms in the genome can be sensitive to AD-affection status.

RESULTS

We identified associations of the ε2 and ε4 alleles with 314 and 232 polymorphisms, respectively. Of them, 35 and 31 polymorphisms had significantly different effects in AD-affected and -unaffected groups, suggesting their potential involvement in the AD pathogenesis by modulating the effects of the ε2 and ε4 alleles, respectively. Our survival-type analysis of the AD risk supported modulating roles of multiple group-specific polymorphisms. Our functional analysis identified gene enrichment in multiple immune-related biological processes, for example, B cell function.

DISCUSSION

These findings suggest involvement of local and inter-chromosomal modulators of the effects of the APOE alleles on the AD risk.

APOE alleles modulate associations of plasma metabolites with variants from multiple genes on chromosome 19q13.3

The APOE ε2, ε3, and ε4 alleles differentially impact various complex diseases and traits. We examined whether these alleles modulated associations of 94 single-nucleotide polymorphisms (SNPs) harbored by 26 genes in 19q13.3 region with 217 plasma metabolites using Framingham Heart Study data. The analyses were performed in the E2 (ε2ε2 or ε2ε3 genotype), E3 (ε3ε3 genotype), and E4 (ε3ε4 or ε4ε4 genotype) groups separately. We identified 31, 17, and 22 polymorphism-metabolite associations in the E2, E3, and E4 groups, respectively, at a false discovery rate P FDR < 0.05. These entailed 51 and 19 associations with 20 lipid and 12 polar analytes. Contrasting the effect sizes between the analyzed groups showed 20 associations with group-specific effects at Bonferroni-adjusted P < 7.14E-04. Three associations with glutamic acid or dimethylglycine had significantly larger effects in the E2 than E3 group and 12 associations with triacylglycerol 56:5, lysophosphatidylethanolamines 16:0, 18:0, 20:4, or phosphatidylcholine 38:6 had significantly larger effects in the E2 than E4 group. Two associations with isocitrate or propionate and three associations with phosphatidylcholines 32:0, 32:1, or 34:0 had significantly larger effects in the E4 than E3 group. Nine of 70 SNP-metabolite associations identified in either E2, E3, or E4 groups attained P FDR < 0.05 in the pooled sample of these groups. However, none of them were among the 20 group-specific associations. Consistent with the evolutionary history of the APOE alleles, plasma metabolites showed higher APOE-cluster-related variations in the E4 than E2 and E3 groups. Pathway enrichment mainly highlighted lipids and amino acids metabolism and citrate cycle, which can be differentially impacted by the APOE alleles. These novel findings expand insights into the genetic heterogeneity of plasma metabolites and highlight the importance of the APOE-allele-stratified genetic analyses of the APOE-related diseases and traits.

Inter- and intra-chromosomal modulators of the APOE ɛ2 and ɛ4 effects on the Alzheimer's disease risk

The mechanisms of incomplete penetrance of risk-modifying impacts of apolipoprotein E (APOE) ε2 and ε4 alleles on Alzheimer's disease (AD) have not been fully understood. We performed genome-wide analysis of differences in linkage disequilibrium (LD) patterns between 6,136 AD-affected and 10,555 AD-unaffected subjects from five independent studies to explore whether the association of the APOE ε2 allele (encoded by rs7412 polymorphism) and ε4 allele (encoded by rs429358 polymorphism) with AD was modulated by autosomal polymorphisms. The LD analysis identified 24 (mostly inter-chromosomal) and 57 (primarily intra-chromosomal) autosomal polymorphisms with significant differences in LD with either rs7412 or rs429358, respectively, between AD-affected and AD-unaffected subjects, indicating their potential modulatory roles. Our Cox regression analysis showed that minor alleles of four inter-chromosomal and ten intra-chromosomal polymorphisms exerted significant modulating effects on the ε2- and ε4-associated AD risks, respectively, and identified ε2-independent (rs2884183 polymorphism, 11q22.3) and ε4-independent (rs483082 polymorphism, 19q13.32) associations with AD. Our functional analysis highlighted ε2- and/or ε4-linked processes affecting the lipid and lipoprotein metabolism and cell junction organization which may contribute to AD pathogenesis. These findings provide insights into the ε2- and ε4-associated mechanisms of AD pathogenesis, underlying their incomplete penetrance.

Definitive roles of TOMM40-APOE-APOC1 variants in the Alzheimer's risk

Despite advances, the roles of genetic variants from the APOE-harboring 19q13.32 region in Alzheimer's disease (AD) remain controversial. We leverage a comprehensive approach to gain insights into a more homogeneous genetic architecture of AD in this region. We use a sample of 2,673 AD-affected and 16,246 unaffected subjects from 4 studies and validate our main findings in the landmark Alzheimer's Disease Genetics Consortium cohort (3,662 AD-cases and 1,541 controls). We report the remarkably high excesses of the AD risk for carriers of the ε4 allele who also carry minor alleles of rs2075650 (TOMM40) and rs12721046 (APOC1) polymorphisms compared to carriers of their major alleles. The exceptionally high 4.37-fold (p=1.34 × 10^-3) excess was particularly identified for the minor allele homozygotes. The beneficial and adverse variants were significantly depleted and enriched, respectively, in the AD-affected families. This study provides compelling evidence for the definitive roles of the APOE-TOMM40-APOC1 variants in the AD risk.

Alzheimer's disease: a tale of two diseases?

Sporadic late-onset Alzheimer's disease (SLOAD) and familial early-onset Alzheimer's disease (FEOAD) associated with dominant mutations in APP, PSEN1 and PSEN2, are thought to represent a spectrum of the same disorder based on near identical behavioral and histopathological features. Hence, FEOAD transgenic mouse models have been used in past decades as a surrogate to study SLOAD pathogenic mechanisms and as the gold standard to validate drugs used in clinical trials. Unfortunately, such research has yielded little output in terms of therapeutics targeting the disease's development and progression. In this short review, we interrogate the widely accepted view of one, dimorphic disease through the prism of the Bmi1+/- mouse model and the distinct chromatin signatures observed between SLOAD and FEOAD brains.

Protective association of the ε2/ε3 heterozygote with Alzheimer's disease is strengthened by TOMM40-APOE variants in men

INTRODUCTION

Despite advances, understanding the protective role of the apolipoprotein E (APOE) ε2 allele in Alzheimer's disease (AD) remains elusive.

METHODS

We examined associations of variants comprised of the TOMM40 rs8106922 and APOE rs405509, rs440446, and ε2-encoding rs7412 polymorphisms with AD in a sample of 2862 AD-affected and 169,516 AD-unaffected non-carriers of the ε4 allele.

RESULTS

Association of the ε2/ε3 heterozygote of men with AD is 38% (P = 1.65 × 10^-2 ) more beneficial when it is accompanied by rs8106922 major allele homozygote and rs405509 and rs440446 heterozygotes than by rs8106922 heterozygote and rs405509 and rs440446 major allele homozygotes. No difference in the beneficial associations of these two most common ε2/ε3-bearing variants with AD was identified in women. The role of ε2/ε3 heterozygote may be affected by different immunomodulation functions of rs8106922, rs405509, and rs440446 variants in a sex-specific manner.

DISCUSSION

Combination of TOMM40 and APOE variants defines a more homogeneous AD-protective ε2/ε3-bearing profile in men.

TOMM40 and APOE variants synergistically increase the risk of Alzheimer's disease in a Chinese population

BACKGROUND

The apolipoprotein E (APOE) ε4 allele is a strong risk factor for Alzheimer's disease (AD) in Caucasian and African American populations. It suggests that other genetic factors may modulate AD pathogenesis in Chinese populations, among which the frequency of this allele is reduced but the AD prevalence is maintained. The translocase of outer mitochondrial membrane 40 (TOMM40), which is located adjacent to APOE, may play an APOE-dependent role in modulating AD pathogenesis.

AIMS

This work aimed to investigate whether TOMM40 polymorphisms modulate AD risk independently of, or in conjunction with APOE polymorphisms in Chinese populations.

METHODS

We conducted a case-control study including 834 patients with AD recruited from the Memory Clinic and 643 cognitively normal participants recruited from the community. The Taqman SNP method was used for APOE genotyping, while TOMM40 polymorphism genotyping was conducted via a polymerase chain reaction-ligase detection reaction.

RESULTS

The TOMM40 rs10119 and rs71352238 alleles were associated with AD independently of the patient APOE status. The rs10119 AA genotype and rs71352238 CC genotype were risk genotypes of AD. Individuals carrying a TOMM40 rs10119 GG/APOE ε4+ (OR, 3.73; 95% CI 1.49-9.37; P = 0.005), TOMM40 rs10119 AG/APOE ε4+ (OR, 4.16; 95% CI 3.30-5.24; P < 0.001), or TOMM40 rs10119 AA/APOE ε4+ (OR, 14.78; 95% CI 8.56-25.54; P < 0.001) genotype exhibited a significantly higher AD risk. Those carrying a TOMM40 rs71352238 TT/APOE ε4+ (OR, 3.82; 95% CI 2.32-6.29; P < 0.001), TOMM40 rs71352238 CT/APOE ε4+ (OR, 4.40; 95% CI 3.46-5.56; P < 0.001), or TOMM40 rs71352238 CC/APOE ε4+ (OR, 14.02; 95% CI 7.81-25.17; P < 0.001) genotype also exhibited a significantly increased AD risk.

DISCUSSION AND CONCLUSIONS

This study provides invaluable insights into the mechanisms underlying the prevalence of AD in Chinese populations, and supports that simultaneous TOMM40 and APOE genotyping in the clinical setting may identify individuals at high risk of developing AD.

Transcript Variants of Genes Involved in Neurodegeneration Are Differentially Regulated by the APOE and MAPT Haplotypes

Genetic variations at the Apolipoprotein E (ApoE) and microtubule-associated protein tau (MAPT) loci have been implicated in multiple neurogenerative diseases, but their exact molecular mechanisms are unclear. In this study, we performed transcript level linear modelling using the blood whole transcriptome data and genotypes of the 570 subjects in the Parkinson’s Progression Markers Initiative (PPMI) cohort. ApoE, MAPT haplotypes and two SNPs at the SNCA locus (rs356181, rs3910105) were used to detect expression quantitative trait loci eQTLs associated with the transcriptome and differential usage of transcript isoforms. As a result, we identified 151 genes associated with the genotypic variations, 29 cis and 122 trans eQTL positions. Profound effect with genome-wide significance of ApoE e4 haplotype on the expression of TOMM40 transcripts was identified. This finding potentially explains in part the frequently established genetic association with the APOE e4 haplotypes in neurodegenerative diseases. Moreover, MAPT haplotypes had significant differential impact on 23 transcripts from the 17q21.31 and 17q24.1 loci. MAPT haplotypes had also the largest up-regulating (256) and the largest down-regulating (−178) effect sizes measured as β values on two different transcripts from the same gene (LRRC37A2). Intronic SNP in the SNCA gene, rs3910105, differentially induced expression of three SNCA isoforms. In conclusion, this study established clear association between well-known haplotypic variance and transcript specific regulation in the blood. APOE e4 and MAPT H1/H2 haplotypic variants are associated with the expression of several genes related to the neurodegeneration.

Haplotype architecture of the Alzheimer's risk in the APOE region via co-skewness

INTRODUCTION

As a multifactorial polygenic disorder, Alzheimer's disease (AD) can be associated with complex haplotypes or compound genotypes.

METHODS

We examined associations of 4960 single nucleotide polymorphism (SNP) triples, comprising 32 SNPs from five genes in the apolipoprotein E gene (APOE) region with AD in a sample of 2789 AD-affected and 16,334 unaffected subjects.

RESULTS

We identified a large number of 1127 AD-associated triples, comprising SNPs from all five genes, in support of definitive roles of complex haplotypes in predisposition to AD. These haplotypes may not include the APOE ε4 and ε2 alleles. For triples with rs429358 or rs7412, which encode these alleles, AD is characterized mainly by strengthening connections of the ε4 allele and weakening connections of the ε2 allele with the other alleles in this region.

DISCUSSION

Dissecting heterogeneity attributed to AD-associated complex haplotypes in the APOE region will target more homogeneous polygenic profiles of people at high risk of AD.

Circulating levels of Irisin in obese individuals at genetic risk for Alzheimer's disease: Correlations with amyloid-β, metabolic, and neurocognitive indices

Irisin is involved in various metabolic pathways and is suggested to be a potential agent capable of preventing onset of Alzheimer's disease (AD) and ameliorating AD neuropathology and cognitive deficits. In the present study, the serum levels of Irisin and Amyloid-β (Aβ) peptides and the neurocognitive performance among obese individuals at genetic risk for AD were investigated. The correlations between Irisin and AD-related neuropathological and neurocognitive indices were also explored. Thirty-two individuals with a family history of AD (ADFH) and obesity (ADFH-obesity group) and 32 controls (ADFH-non-obesity group) were recruited. Circulating levels of Irisin, Aβ peptides, and metabolic biomarkers, as well as neurocognitive performance [e.g., behavior and brain even-related potentials (ERP)] were measured during a visuospatial working memory task. Although the ADFH-obesity group exhibited comparable reaction times, ERP N2 latency and amplitudes, and P3 latency as compared to the ADFH-non-obesity group when performing the cognitive task, they exhibited significantly lower rates of accuracy and smaller P3 amplitudes in the higher memory-load condition, even when controlling for the blood pressure and cardiorespiratory fitness co-variables. The serum levels of leptin, insulin, and glucose, and HOMA-IR were significantly higher in the ADFH-obesity group relative to the ADFH-non-obesity group, but this was not the case for the levels of Aβ1-40 and Aβ1-42. The Irisin levels approached between-group significance. Partial correlations adjusting for cardiorespiratory fitness and blood pressure showed that Irisin levels were positively associated with neurophysiological (i.e., P3 amplitude) performance in the ADFH-obesity group. The Irisin levels were not significantly correlated with the levels of Aβ1-40 and Aβ1-42. The present findings suggest that ADFH individuals with obesity exhibited neurocognitive deficits when performing the visuospatial working memory task, and serum Irisin levels could be one of the influencing factors. However, the relationship between the circulating levels of Irisin and Aβ peptides needs more evidence to support this assumption.

The Role of Apolipoprotein E Isoforms in Alzheimer's Disease

Alzheimer's disease (AD), the most common type of dementia worldwide, is characterized by high levels of amyloid-β (Aβ) peptide and hyperphosphorylated tau protein. Genetically, the ɛ4 allele of apolipoprotein E (ApoE) has been established as the major risk factor for developing late-onset AD (LOAD), the most common form of the disease. Although the role ApoE plays in AD is still not completely understood, a differential role of its isoforms has long been known. The current review compiles the involvement of ApoE isoforms in amyloid-β protein precursor transcription, Aβ aggregation and clearance, synaptic plasticity, neuroinflammation, lipid metabolism, mitochondrial function, and tau hyperphosphorylation. Due to the complexity of LOAD, an accurate description of the interdependence among all the related molecular mechanisms involved in the disease is needed for developing successful therapies.

MAPT haplotype-stratified GWAS reveals differential association for AD risk variants

INTRODUCTION

MAPT H1 haplotype is implicated as a risk factor for neurodegenerative diseases including Alzheimer's disease (AD).

METHODS

Using Alzheimer's Disease Genetics Consortium (ADGC) genome-wide association study (GWAS) data (n = 18,841), we conducted a MAPT H1/H2 haplotype-stratified association to discover MAPT haplotype-specific AD risk loci.

RESULTS

We identified 11 loci-5 in H2-non-carriers and 6 in H2-carriers-although none of the MAPT haplotype-specific associations achieved genome-wide significance. The most significant H2 non-carrier-specific association was with a NECTIN2 intronic (P = 1.33E-07) variant, and that for H2 carriers was near NKX6-1 (P = 1.99E-06). The GABRG2 locus had the strongest epistasis with MAPT H1/H2 variant rs8070723 (P = 3.91E-06). Eight of the 12 genes at these loci had transcriptome-wide significant differential expression in AD versus control temporal cortex (q < 0.05). Six genes were members of the brain transcriptional co-expression network implicated in "synaptic transmission" (P = 9.85E-59), which is also enriched for neuronal genes (P = 1.0E-164), including MAPT.

DISCUSSION

This stratified GWAS identified loci that may confer AD risk in a MAPT haplotype-specific manner. This approach may preferentially enrich for neuronal genes implicated in synaptic transmission.

THE EXPOSOME IN HUMAN EVOLUTION: FROM DUST TO DIESEL

Global exposures to air pollution and cigarette smoke are novel in human evolutionary history and are associated with about 16 million premature deaths per year. We investigate the history of the human exposome for relationships between novel environmental toxins and genetic changes during human evolution in six phases. Phase I: With increased walking on savannas, early human ancestors inhaled crustal dust, fecal aerosols, and spores; carrion scavenging introduced new infectious pathogens. Phase II: Domestic fire exposed early Homo to novel toxins from smoke and cooking. Phases III and IV: Neolithic to preindustrial Homo sapiens incurred infectious pathogens from domestic animals and dense communities with limited sanitation. Phase V: Industrialization introduced novel toxins from fossil fuels, industrial chemicals, and tobacco at the same time infectious pathogens were diminishing. Thereby, pathogen-driven causes of mortality were replaced by chronic diseases driven by sterile inflammogens, exogenous and endogenous. Phase VI: Considers future health during global warming with increased air pollution and infections. We hypothesize that adaptation to some ancient toxins persists in genetic variations associated with inflammation and longevity.

The Contribution of Genetic Factors to Cognitive Impairment and Dementia: Apolipoprotein E Gene, Gene Interactions, and Polygenic Risk

Alzheimer's disease (AD) is a progressive neurodegenerative disease. Although it has been studied for years, the pathogenesis of AD is still controversial. Genetic factors may play an important role in pathogenesis, with the apolipoprotein E (APOE) gene among the greatest risk factors for AD. In this review, we focus on the influence of genetic factors, including the APOE gene, the interaction between APOE and other genes, and the polygenic risk factors for cognitive function and dementia. The presence of the APOE ε4 allele is associated with increased AD risk and reduced age of AD onset. Accelerated cognitive decline and abnormal internal environment, structure, and function of the brain were also found in ε4 carriers. The effect of the APOE promoter on cognition and the brain was confirmed by some studies, but further investigation is still needed. We also describe the effects of the associations between APOE and other genetic risk factors on cognition and the brain that exhibit a complex gene⁻gene interaction, and we consider the importance of using a polygenic risk score to investigate the association between genetic variance and phenotype.

Longer TOMM40 poly-T variants associated with higher FDDNP-PET medial temporal tau and amyloid binding

BACKGROUND

The translocase of outer mitochondrial membrane 40 (TOMM40), which lies in linkage disequilibrium with the apolipoprotein E (APOE) gene, has been implicated in Alzheimer's disease (AD). TOMM40 influences AD pathology through mitochondrial neurotoxicity, and the medial temporal lobe (MTL) is the most likely brain region for identifying early manifestations of AD-related morphology changes. While early reports indicated that the longer length poly-T allele of TOMM40 increases risk for AD, these findings have not been consistently replicated in further studies. We examined the effect of TOMM40 and APOE on regional brain positron emission tomography (PET) 2-(1-{6-[(2 [F18]fluoroethyl) (methyl) amino]-2-naphthyl}ethylidene)malononitrile (FDDNP) binding values in MTL.

METHODS

A total of 73 non-demented older adults (42 females; mean age: 62.9(10.9) completed genotyping for both APOE and TOMM40 and received FDDNP-PET scans. For TOMM40, the lengths of the poly-T sequence were classified as short (14-20 repeats; S), long (21-29 repeats, L) or very long (>29 repeats, VL). Using general linear models, we examined medial temporal lobe FDDNP binding and cognitive functioning between TOMM40 and APOE-4 groups, with age, sex, and education as covariates.

RESULTS

Data from 30 individuals with APOE-4 and L TOMM40 poly-T length, 11 non E4 TOMM40 S/S, 14 non E4 TOMM40 S/VL and 13 non E4 TOMM40 VL/VL were analyzed. Medial temporal FDDNP binding differed significantly between TOMM40/APOE groups (F(3,62) = 3.3,p = .03). Participants with TOMM40 S/S exhibited significantly lower binding compared to TOMM40 S/VL and APOE-4 carriers. We did not find a significant relationship between TOMM40 poly-T lengths/APOE risk groups and cognitive functioning.

CONCLUSIONS

This is the first report to demonstrate a significant association between longer TOMM40 poly-T lengths and higher medial temporal plaque and tangle burden in non-demented older adults. Identifying biomarkers that are risk factors for AD will enhance our ability to identify subjects likely to benefit from novel AD treatments.

The Role of Irisin in Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by progressive memory dysfunction, oxidative stress, and presence of senile plaques formed by amyloid beta (Aβ) accumulation in the brain. AD is one of the most important causes of morbidity and mortality worldwide. AD has a variety of risk factors, including environmental factors, metabolic dysfunction, and genetic background. Recent research has highlighted the relationship between AD and systemic metabolic changes such as glucose and lipid imbalance and insulin resistance. Irisin, a myokine closely linked to exercise, has been associated with glucose metabolism, insulin sensitivity, and fat browning. Recent studies have suggested that irisin is involved in the process in central nervous system (CNS) such as neurogenesis and has reported the effects of irisin on AD as one of the neurodegenerative disease. Here, we review the roles of irisin with respect to AD and suggest that irisin highlight therapeutic important roles in AD. Thus, we propose that irisin could be a potential future target for ameliorating AD pathology and preventing AD onset.

Dual-Task Gait and Alzheimer's Disease Genetic Risk in Cognitively Normal Adults: A Pilot Study

BACKGROUND

Dual-task paradigms, in which an individual performs tasks separately and then concurrently, often demonstrate that people with neurodegenerative disorders experience more dual-task interference, defined as worse performance in the dual-task condition compared to the single-task condition.

OBJECTIVE

To examine how gait-cognition dual-task performance differs between cognitively normal older adults with and without an APOE ɛ4 allele.

METHODS

Twenty-nine individuals ages 60 to 72 with normal cognition completed a dual-task protocol in which walking and cognitive tasks (executive function, memory) were performed separately and concurrently. Fourteen participants carried APOE ɛ4 alleles (ɛ3/ɛ4 or ɛ2/ɛ4); fifteen had APOE genotypes (ɛ2/ɛ2, ɛ2/ɛ3, or ɛ3/ɛ3) associated with lower risk of Alzheimer's disease (AD).

RESULTS

The two risk groups did not differ by age, sex, race, education, or gait or cognitive measures under single-task conditions. Compared to low risk participants, APOE ɛ4 carriers tended to exhibit greater dual-task interference. Both the memory and executive function tasks resulted in dual-task interference on gait, but effect sizes for a group difference were larger when the cognitive task was executive function. In the dual-task protocol that combined walking and the executive function task, effect sizes for group difference in gait interference were larger (0.62- 0.70) than for cognitive interference (0.45- 0.47).

DISCUSSION

Dual-task paradigms may reveal subtle changes in brain function in asymptomatic individuals at heightened risk of AD.

Effects of apolipoprotein E gene polymorphism on the intracellular Ca2+ concentration of astrocytes in the early stages post injury

The present study aimed to investigate the correlation between apolipoprotein E (APOE) polymorphisms and the intracellular concentration of Ca²⁺ in astrocytes in the early stages after an injury. The chondroitin sulfate region of three APOE alleles (ε2, ε3 and ε4) was obtained by reverse transcription-polymerase chain reaction (RT-PCR). A recombinant plasmid, pEGFP-N1-APOE, was constructed and identified by sequencing, while astrocytes were isolated from APOE gene-knockout mice and examined using immunocytochemistry. The recombinant plasmid was transfected into the astrocytes using the liposome-mediated method and cell injury models were constructed by a scratch assay. Laser confocal scanning microscopy (LCSM) was used to detect dynamic alterations in intracellular Ca²⁺ concentration at 12, 24, 48 and 72 h after injury. Compared with the control group, cells transfected with any of the three alleles demonstrated significant increases in the fluorescence intensity of Ca²⁺ (P<0.05). The fluorescence intensity of Ca²⁺ was weak at 12 h after injury, with no statistically significant difference detected between any two groups at this time point (P>0.05). However, the fluorescence intensity increased in a time-dependent manner and at 24, 48 and 72 h post injury, the fluorescence intensity of the ε4 allele-containing cells was significantly higher when compared with that of cells harboring the other two alleles (P<0.05). These results indicate that intracellular Ca²⁺ overloading may contribute to the deterioration of brain cells and poor outcome subsequent to traumatic brain injury in APOE ε4 carriers.

Is heart disease a risk factor for low dementia test battery scores in older persons with Down syndrome? Exploratory, pilot study, and commentary

OBJECTIVES

Certain heart conditions and diseases are common in Down syndrome (DS; trisomy 21), but their role in early onset dementia that is prevalent in older adults with DS has not been evaluated. To address this knowledge gap, we conducted a study of risk factors for low neurocognitive/behavioral scores obtained with a published dementia test battery (DTB). Participants were adults with DS living in New York (N = 29; average age 46 years). We asked three questions. 1. Does having any type of heart disease affect the association between DTB scores and chronological age? 2. Does thyroid status affect the association between heart disease and DTB scores? 3. Are the E4 or E2 alleles of apolipoprotein E (APOE) associated with DTB scores or with heart disease?

METHOD

The study was retrospective, pilot, and exploratory. It involved analysis of information in a database previously established for the study of aging in DS. Participants had moderate intellectual disability on average. Information for each person included: gender, age, a single DTB score obtained by combining results from individual subscales of the DTB, the presence or absence of heart disease, thyroid status (treated hypothyroidism or normal), and APOE genotype. Trends were visualized by inspection of graphs and contingency tables. Statistical methods used to evaluate associations included Pearson correlation analysis, Fisher's exact tests (2-tailed), and odds ratio analysis. P values were interpreted at the 95% confidence level without Bonferroni correction. P values >.05<.1 were considered trends.

RESULTS

The negative correlation between DTB scores and age was significant in those with heart disease but not in those without. Heart disease was significantly associated with DTB scores >1 SD below the sample mean; there was a strong association between heart disease and low DTB scores in those with treated hypothyroidism but not in those with normal thyroid status. The APOE genotype was weakly associated with heart disease (E4, predisposing; E2, protective) in males.

CONCLUSIONS

On the basis of the potentially important findings from the present study, large prospective studies are warranted to confirm and extend the observations. In these, particular heart conditions or diseases and other medical comorbidities in individuals should be documented.

The Alu neurodegeneration hypothesis: A primate-specific mechanism for neuronal transcription noise, mitochondrial dysfunction, and manifestation of neurodegenerative disease

It is hypothesized that retrotransposons have played a fundamental role in primate evolution and that enhanced neurologic retrotransposon activity in humans may underlie the origin of higher cognitive function. As a potential consequence of this enhanced activity, it is likely that neurons are susceptible to deleterious retrotransposon pathways that can disrupt mitochondrial function. An example is observed in the TOMM40 gene, encoding a β-barrel protein critical for mitochondrial preprotein transport. Primate-specific Alu retrotransposons have repeatedly inserted into TOMM40 introns, and at least one variant associated with late-onset Alzheimer’s disease originated from an Alu insertion event. We provide evidence of enriched Alu content in mitochondrial genes and postulate that Alus can disrupt mitochondrial populations in neurons, thereby setting the stage for progressive neurologic dysfunction. This Alu neurodegeneration hypothesis is compatible with decades of research and offers a plausible mechanism for the disruption of neuronal mitochondrial homeostasis, ultimately cascading into neurodegenerative disease.

Hippocampal thinning linked to longer TOMM40 poly-T variant lengths in the absence of the APOE ε4 variant

INTRODUCTION

The translocase of outer mitochondrial membrane 40 (TOMM40), which lies in linkage disequilibrium with apolipoprotein E (APOE), has received attention more recently as a promising gene in Alzheimer's disease (AD) risk. TOMM40 influences AD pathology through mitochondrial neurotoxicity, and the medial temporal lobe (MTL) is the most likely brain region for identifying early manifestations of AD-related morphology changes.

METHODS

In this study, we examined the effects of TOMM40 using high-resolution magnetic resonance imaging in 65 healthy, older subjects with and without the APOE ε4 AD-risk variant.

RESULTS

Examining individual subregions within the MTL, we found a significant relationship between increasing poly-T lengths of the TOMM40 variant and thickness of the entorhinal cortex only in subjects who did not carry the APOE ε4 allele.

DISCUSSION

Our data provide support for TOMM40 variant repeat length as an important contributor to AD-like MTL pathology in the absence of APOE ε4.