TREM2 is associated with the risk of Alzheimer's disease in Spanish population

Proteomics of brain, CSF, and plasma identifies molecular signatures for distinguishing sporadic and genetic Alzheimer's disease

Proteomic studies for Alzheimer's disease (AD) are instrumental in identifying AD pathways but often focus on single tissues and sporadic AD cases. Here, we present a proteomic study analyzing 1305 proteins in brain tissue, cerebrospinal fluid (CSF), and plasma from patients with sporadic AD, TREM2 risk variant carriers, patients with autosomal dominant AD (ADAD), and healthy individuals. We identified 8 brain, 40 CSF, and 9 plasma proteins that were altered in individuals with sporadic AD, and we replicated these findings in several external datasets. We identified a proteomic signature that differentiated TREM2 variant carriers from both individuals with sporadic AD and healthy individuals. The proteins associated with sporadic AD were also altered in patients with ADAD, but with a greater effect size. Brain-derived proteins associated with ADAD were also replicated in additional CSF samples. Enrichment analyses highlighted several pathways, including those implicated in AD (calcineurin and Apo E), Parkinson's disease (α-synuclein and LRRK2), and innate immune responses (SHC1, ERK-1, and SPP1). Our findings suggest that combined proteomics across brain tissue, CSF, and plasma can be used to identify markers for sporadic and genetically defined AD.

TREM2 R47H variant and risk for Alzheimer's disease: assessment in a Greek population and updated meta-analysis

INTRODUCTION

Rare coding variants in TREM2 and their association with the susceptibility towards Alzheimer's disease (AD) were recently studied in various ethnic groups with contradictory results. The T allele of the rs75932628 (p.R47H variant) has shown a positive risk association with AD in several studies; however, neither a study in Greece nor an updated meta-analysis have been conducted.

OBJECTIVE

To assess the association between TREM2 rs75932628 and late-onset (sporadic) AD in a Greek population, and perform a meta-analysis of current data.

MATERIALS AND METHODS

The rs75932628 was genotyped in a total of 327 patients with AD and 700 cognitively healthy controls. A systematic search and meta-analyses of studies presenting data regarding rs75932628 in AD cases and controls were also performed.

RESULTS

Three patients vs. none of the controls were found to carry the heterozygous risk allele of the rs75932628, yielding a significant association (p = 0.032), in the Greek sample. In the meta-analysis, the overall odds ratio (OR) under a fixed-effects model was 2.98 (Confidence Interval (CI):2.52-3.53) showing a significant association of the rs75932628-T allele with AD in the overall dataset, based on data from 27 studies (26200 AD cases and 142084controls). Caucasian population-only studies (n = 16) revealed a similar OR of 2.93 (CI:2.45-3.51), whereas Asian population-only studies (n = 5) had a non-significant OR of 0.84 (CI:0.19-3.74).

CONCLUSION

The rs75932628 was associated with AD in the Greek sample. Our meta-analysis, covering a total population of over 168,000 people, also showed a significant association of the allele with AD in Caucasian populations.

The Hidden Role of Non-Canonical Amyloid β Isoforms in Alzheimer's Disease

Recent advances have placed the pro-inflammatory activity of amyloid β (Aβ) on microglia cells as the focus of research on Alzheimer's Disease (AD). Researchers are confronted with an astonishing spectrum of over 100 different Aβ variants with variable length and chemical modifications. With the exception of Aβ1-42 and Aβ1-40, the biological significance of most peptides for AD is as yet insufficiently understood. We therefore aim to provide a comprehensive overview of the contributions of these neglected Aβ variants to microglia activation. First, the impact of Aβ receptors, signaling cascades, scavenger mechanisms, and genetic variations on the physiological responses towards various Aβ species is described. Furthermore, we discuss the importance of different types of amyloid precursor protein processing for the generation of these Aβ variants in microglia, astrocytes, oligodendrocytes, and neurons, and highlight how alterations in secondary structures and oligomerization affect Aβ neurotoxicity. In sum, the data indicate that gene polymorphisms in Aβ-driven signaling pathways in combination with the production and activity of different Aβ variants might be crucial factors for the initiation and progression of different forms of AD. A deeper assessment of their interplay with glial cells may pave the way towards novel therapeutic strategies for individualized medicine.

Cerebrospinal Fluid sTREM2 in Alzheimer’s Disease Is Associated with Both Amyloid and Tau Pathologies but not with Cognitive Status

Background: Cerebrospinal fluid (CSF) soluble triggering receptor expressed on myeloid cells 2 (sTREM2) is considered a biomarker of microglial activation. The relationships between CSF sTREM2 levels and Alzheimer’s disease (AD) CSF core biomarkers, cognitive status, and neurodegeneration remain unclear. Objective: To assess the association between CSF sTREM2 levels and AD progression and other AD hallmarks. Methods: Using the Alzheimer’s Disease Neuroimaging Initiative database, we investigated 1,035 participants, including 310 cognitively normal controls, 527 patients with mild cognitive impairment, and 198 patients with dementia. They were grouped according to CSF pathology (A/T profile) severity. CSF sTREM2 levels were compared between the groups, and linear regression analysis was performed to evaluate the factors affecting sTREM2 levels. The predictive effectiveness of sTREM2 levels was tested, and the correlation with other indicators was explored. The increase rate was assessed using linear mixed-effects models. Results: Higher CSF sTREM2 levels were associated with older age as well as higher CSF p-tau or t-tau and amyloid-β levels (all p <  0.001), but not with cognitive status. sTREM2 levels were not correlated with the baseline or longitudinal scale and neuroimaging result changes, and could not predict clinical conversion, but were correlated with multiple non-amyloid-β and non-tau CSF cytokines related to inflammation and neurodegeneration (p <  0.0001). The increased sTREM2 expression rate did not change among groups. Conclusion: CSF sTREM2 levels were jointly determined by age, amyloid-β, and tau pathologies, leading to complex AD cognitive continuum changes. Although sTREM2 levels could not predict cognitive deterioration and neurodegeneration, they could reflect the microglial state as a non-specific biomarker.

Glial cells in Alzheimer's disease: From neuropathological changes to therapeutic implications

Alzheimer's disease (AD) is a neurodegenerative disorder that usually develops slowly and progressively worsens over time. Although there has been increasing research interest in AD, its pathogenesis is still not well understood. Although most studies primarily focus on neurons, recent research findings suggest that glial cells (especially microglia and astrocytes) are associated with AD pathogenesis and might provide various possible therapeutic targets. Growing evidence suggests that microglia can provide protection against AD pathogenesis, as microglia with weakened functions and impaired responses to Aβ proteins are linked with elevated AD risk. Interestingly, numerous findings also suggest that microglial activation can be detrimental to neurons. Indeed, microglia can induce synapse loss via the engulfment of synapses, possibly through a complement-dependent process. Furthermore, they can worsen tau pathology and release inflammatory factors that cause neuronal damage directly or through the activation of neurotoxic astrocytes. Astrocytes play a significant role in various cerebral activities. Their impairment can mediate neurodegeneration and ultimately the retraction of synapses, resulting in AD-related cognitive deficits. Deposition of Aβ can result in astrocyte reactivity, which can further lead to neurotoxic effects and elevated secretion of inflammatory mediators and cytokines. Moreover, glial-induced inflammation in AD can exert both beneficial and harmful effects. Understanding the activities of astrocytes and microglia in the regulation of AD pathogenesis would facilitate the development of novel therapies. In this article, we address the implications of microglia and astrocytes in AD pathogenesis. We also discuss the mechanisms of therapeutic agents that exhibit anti-inflammatory effects against AD.

The development and physiological and pathophysiological functions of resident macrophages and glial cells

In the past, brain function and the onset and progression of neurological diseases have been studied in a neuron-centric manner. However, in recent years the focus of many neuroscientists has shifted to other cell types that promote neurodevelopment and contribute to the functionality of neuronal networks in health and disease. Particularly microglia and astrocytes have been implicated in actively contributing to and controlling neuronal development, neuroinflammation, and neurodegeneration. Here, we summarize the development of brain-resident macrophages and astrocytes and their core functions in the developing brain. We discuss their contribution and intercellular crosstalk during tissue homeostasis and pathophysiology. We argue that in-depth knowledge of non-neuronal cells in the brain could provide novel therapeutic targets to reverse or contain neurological diseases.

Alternative Splicing Regulation of Low-Frequency Genetic Variants in Exon 2 of TREM2 in Alzheimer's Disease by Splicing-Based Aggregation

TREM2 is among the most well-known Alzheimer’s disease (AD) risk genes; however, the functional roles of its AD-associated variants remain to be elucidated, and most known risk alleles are low-frequency variants whose investigation is challenging. Here, we utilized a splicing-guided aggregation method in which multiple low-frequency TREM2 variants were bundled together to investigate the functional impact of those variants on alternative splicing in AD. We analyzed whole genome sequencing (WGS) and RNA-seq data generated from cognitively normal elderly controls (CN) and AD patients in two independent cohorts, representing three regions in the frontal lobe of the human brain: the dorsolateral prefrontal cortex (CN = 213 and AD = 376), frontal pole (CN = 72 and AD = 175), and inferior frontal (CN = 63 and AD = 157). We observed an exon skipping event in the second exon of TREM2, with that exon tending to be more frequently skipped (p = 0.0012) in individuals having at least one low-frequency variant that caused loss-of-function for a splicing regulatory element. In addition, genes differentially expressed between AD patients with high vs. low skipping of the second exon (i.e., loss of a TREM2 functional domain) were significantly enriched in immune-related pathways. Our splicing-guided aggregation method thus provides new insight into the regulation of alternative splicing of the second exon of TREM2 by low-frequency variants and could be a useful tool for further exploring the potential molecular mechanisms of multiple, disease-associated, low-frequency variants.

TrendyGenes, a computational pipeline for the detection of literature trends in academia and drug discovery

Target identification and prioritisation are prominent first steps in modern drug discovery. Traditionally, individual scientists have used their expertise to manually interpret scientific literature and prioritise opportunities. However, increasing publication rates and the wider routine coverage of human genes by omic-scale research make it difficult to maintain meaningful overviews from which to identify promising new trends. Here we propose an automated yet flexible pipeline that identifies trends in the scientific corpus which align with the specific interests of a researcher and facilitate an initial prioritisation of opportunities. Using a procedure based on co-citation networks and machine learning, genes and diseases are first parsed from PubMed articles using a novel named entity recognition system together with publication date and supporting information. Then recurrent neural networks are trained to predict the publication dynamics of all human genes. For a user-defined therapeutic focus, genes generating more publications or citations are identified as high-interest targets. We also used topic detection routines to help understand why a gene is trendy and implement a system to propose the most prominent review articles for a potential target. This TrendyGenes pipeline detects emerging targets and pathways and provides a new way to explore the literature for individual researchers, pharmaceutical companies and funding agencies.

The Association between TREM2 Gene and Late-Onset Alzheimer's Disease in Chinese Han Population

OBJECTIVE

The objective of this study was to evaluate the impact of single nucleotide polymorphisms (SNPs) in triggering receptor expressed on the myeloid cells 2 protein (TREM2) gene and their interaction with environmental factors and haplotypes on late-onset Alzheimer's disease (LOAD).

METHODS

DNA was extracted from the whole blood of the participants and genotyped using PCR and followed by restriction fragment length polymorphism. The Hardy-Weinberg equilibrium test was used in the control group. Multivariate logistic regression analysis was used to determine the relationship between the 4 SNPs of the TREM2 gene and the risk of LOAD. Generalized multifactor dimensionality reduction was used to test the best interaction combination between SNPs and environmental factors.

RESULTS

Logistic regression analysis showed that the T allele of rs75932628 and the T allele of rs2234253 were independently associated with increased risk of LOAD, and adjusted odds ratios (ORs) were 1.81 (1.271-2.35) and 1.59 (1.15-2.03), respectively. However, there was no significant association with LOAD for rs142232675 and rs143332484. We found a best model significantly associated with LOAD risk that consisted of rs75932628 and smoking, which scored 10/10 for both the sign test and cross-validation consistency (p = 0.012). Stratified analysis indicated that current smokers with rs75932628-CT/TT genotype have the highest LOAD risk compared to never smokers with rs75932628 - CC genotype, OR (95% confidence interval) = 2.73 (1.72-3.79). Haplotypes of rs75932628 and rs2234253 were analyzed using the SHEsis online software. However, no haplotype was found to be significantly associated with the risk of LOAD.

CONCLUSIONS

The T allele of rs75932628 and the T allele of rs2234253 and interaction between rs75932628 and smoking were all correlated with increased risk of LOAD.

How understudied populations have contributed to our understanding of Alzheimer's disease genetics

The majority of genome-wide association studies have been conducted using samples with a broadly European genetic background. As a field, we acknowledge this limitation and the need to increase the diversity of populations studied. A major challenge when designing and conducting such studies is to assimilate large samples sizes so that we attain enough statistical power to detect variants associated with disease, particularly when trying to identify variants with low and rare minor allele frequencies. In this review, we aimed to illustrate the benefits to genetic characterization of Alzheimer's disease, in researching currently understudied populations. This is important for both fair representation of world populations and the translatability of findings. To that end, we conducted a literature search to understand the contributions of studies, on different populations, to Alzheimer's disease genetics. Using both PubMed and Alzforum Mutation Database, we systematically quantified the number of studies reporting variants in known disease-causing genes, in a worldwide manner, and discuss the contributions of research in understudied populations to the identification of novel genetic factors in this disease. Additionally, we compared the effects of genome-wide significant single nucleotide polymorphisms across populations by focusing on loci that show different association profiles between populations (a key example being APOE). Reports of variants in APP, PSEN1 and PSEN2 can initially determine whether patients from a country have been studied for Alzheimer's disease genetics. Most genome-wide significant associations in non-Hispanic white genome-wide association studies do not reach genome-wide significance in such studies of other populations, with some suggesting an opposite effect direction; this is likely due to much smaller sample sizes attained. There are, however, genome-wide significant associations first identified in understudied populations which have yet to be replicated. Familial studies in understudied populations have identified rare, high effect variants, which have been replicated in other populations. This work functions to both highlight how understudied populations have furthered our understanding of Alzheimer's disease genetics, and to help us gauge our progress in understanding the genetic architecture of this disease in all populations.

The most prevalent rare coding variants of TREM2 conferring risk of Alzheimer's disease: A systematic review and meta-analysis

Rare variants in the coding sequence of triggering receptor expressed on myeloid cells 2 (TREM2) have been identified in Alzheimer's disease (AD). They have been reported to be causative or confer risk of AD in several populations. However, the results are not conclusive. Therefore, a meta-analysis was performed to investigate the association between rare variants of TREM2 and the susceptibility to AD. Case-control studies meeting the inclusion criteria were identified by searching the PubMed, Embase and Web of Science databases. The association between four commonly analyzed variants of TREM2, p.Arg47His (R47H), p.Arg62His (R62H), p.Asp87Asn (D87N) and p.His157Tyr (H157Y), and the risk of AD were evaluated by meta-analyses with the fixed-effects model. Finally, a total of 26 datasets comprising 28,007 cases and 45,121 controls were included. There was no or low between-study heterogeneity in all comparisons. A significantly increased risk of AD was observed in carriers of R47H compared with non-carriers [odds ratio (OR)=3.88, 95% CI: 3.17-4.76, P<0.001], R62H (OR=1.37, 95% CI: 1.11-1.70, P=0.004) and H157Y (OR=4.22, 95% CI: 1.93-9.21, P<0.001). However, R62H only conferred a mild risk compared to R47H and H157Y (OR=1.37 vs. 3.88 and 4.22, respectively). D87N was not associated with AD susceptibility. Sensitivity analysis indicated that the association identified for R62H was not significant (P=0.192) when excluding a large-sample study. Subgroup analysis according to ethnicity revealed significant associations (R47H and H157Y) in Caucasians but not in Asians. In conclusion, rare coding variants of TREM2 were associated with an elevated risk of AD, particularly in Caucasians.

TREM2, microglia, and Alzheimer's disease

Triggering receptor expressed on myeloid cells 2 (TREM2) has been suggested to play a crucial role in Alzheimer's disease (AD) pathogenesis, as revealed by genome-wide association studies (GWAS). Since then, rapidly increasing literature related to TREM2 has focused on elucidating its role in AD pathology. In this review, we summarize our understanding of TREM2 biology, explore TREM2 functions in microglia, address the multiple mechanisms of TREM2 in AD, and raise key questions for further investigations to elucidate the detailed roles and molecular mechanisms of TREM2 in microglial responses. A major breakthrough in our understanding of TREM2 is based on our hypothesis suggesting that TREM2 may act as a multifaceted player in microglial functions in AD brain homeostasis. We conclude that TREM2 can not only influence microglial functions in amyloid and tau pathologies but also participate in inflammatory responses and metabolism, acting alone or with other molecules, such as apolipoprotein E (APOE). This review provides novel insight into the broad role of TREM2 in microglial function in AD and enables us to develop new strategies aimed at the immune system to treat AD pathogenesis.

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

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

Genetic architecture of Alzheimer's disease

Advances in genetic and genomic technologies over the last thirty years have greatly enhanced our knowledge concerning the genetic architecture of Alzheimer's disease (AD). Several genes including APP, PSEN1, PSEN2, and APOE have been shown to exhibit large effects on disease susceptibility, with the remaining risk loci having much smaller effects on AD risk. Notably, common genetic variants impacting AD are not randomly distributed across the genome. Instead, these variants are enriched within regulatory elements active in human myeloid cells, and to a lesser extent liver cells, implicating these cell and tissue types as critical to disease etiology. Integrative approaches are emerging as highly effective for identifying the specific target genes through which AD risk variants act and will likely yield important insights related to potential therapeutic targets in the coming years. In the future, additional consideration of sex- and ethnicity-specific contributions to risk as well as the contribution of complex gene-gene and gene-environment interactions will likely be necessary to further improve our understanding of AD genetic architecture.

TREM2 Variants and Neurodegenerative Diseases: A Systematic Review and Meta-Analysis

TREM2 (triggering receptor expressed on myeloid cells 2) gene variants were reported to increase the risk of Alzheimer's disease (AD) and even other neurodegenerative diseases (frontotemporal dementia (FTD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS)), but so far, no definite conclusion has been drawn. The aim of our systematic review and meta-analysis was to investigate the role of TREM2 variants in neurodegenerative diseases. A total of 39 papers (including 26 case-control studies and 13 case reports) were retrieved from PubMed, MEDLINE, EMBASE, and the Cochrane library in this study. A fixed effect model was used to pool results in the analysis. Three variants in TREM2 (rs75932628 (R47H), rs2234255 (H157Y), and rs143332484 (R62H)) were significantly associated with AD risk, but the similar associations between rs104894002 (Q33X), rs2234253 (T96K), rs142232675 (D87N), rs2234256 (L211P), and AD were not proven. Rs75932628 also increased risk of PD in North Americans and FTD, but not PD in Europeans or ALS. In the systematic review, 12 biallelic TREM2 mutations (e.g., rs104894002, rs201258663 (T66M), and rs386834144, etc.) have been described to cause Polycystic Lipomembranous Osteodysplasia with Sclerosing Leukoencephalopathy (PLOSL) in 14 families. And homozygous mutations also have been reported to cause FTD without typical bone phenotypes in 7 families. This study demonstrates that multiple variants in TREM2 have association with the onset of AD, FTD, and PD in North Americans and also play a key role in the phenotypes of the rare familial genetic disorder.

Synonymous variants associated with Alzheimer disease in multiplex families

OBJECTIVE

Synonymous variants can lead to disease; nevertheless, the majority of sequencing studies conducted in Alzheimer disease (AD) only assessed coding variation.

METHODS

To detect synonymous variants modulating AD risk, we conducted a whole-genome sequencing study on 67 Caribbean Hispanic (CH) families multiply affected by AD. Identified disease-associated variants were further assessed in an independent cohort of CHs, expression quantitative trait locus (eQTL) data, brain autopsy data, and functional experiments.

RESULTS

Rare synonymous variants in 4 genes (CDH23, SLC9A3R1, RHBDD2, and ITIH2) segregated with AD status in multiplex families and had a significantly higher frequency in these families compared with reference populations of similar ancestry. In comparison to subjects without dementia, expression of CDH23 (β = 0.53, p = 0.006) and SLC9A3R1 (β = 0.50, p = 0.02) was increased, and expression of RHBDD2 (β = -0.70, p = 0.02) decreased in individuals with AD at death. In line with this finding, increased expression of CDH23 (β = 0.26 ± 0.08, p = 4.9E-4) and decreased expression of RHBDD2 (β = -0.60 ± 0.12, p = 5.5E-7) were related to brain amyloid load (p = 0.0025). SLC9A3R1 expression was associated with burden of TDP43 pathology (β = 0.58 ± 0.17, p = 5.9E-4). Using eQTL data, the CDH23 variant was in linkage disequilibrium with variants modulating CDH23 expression levels (top single nucleotide polymorphism: rs11000035, p = 4.85E-6, D' = 1.0). Using minigene splicing assays, the CDH23 and SLC9A3R1 variants affected splicing efficiency.

CONCLUSIONS

These findings suggest that CDH23, SLC9A3R1, RHBDD2, and possibly ITIH2, which are involved in synaptic function, the glutamatergic system, and innate immunity, contribute to AD etiology. In addition, this study supports the notion that synonymous variants contribute to AD risk and that comprehensive scrutinization of this type of genetic variation is warranted and critical.

Frequency of the TREM2 R47H Variant in Various Neurodegenerative Disorders

OBJECTIVE

A rare variant in TREM2 (p.R47H, rs75932628) has been consistently reported to increase the risk for Alzheimer disease (AD), while mixed evidence has been reported for association of the variant with other neurodegenerative diseases. Here, we investigated the frequency of the R47H variant in a diverse and well-characterized multicenter neurodegenerative disease cohort.

METHODS

We examined the frequency of the R47H variant in a diverse neurodegenerative disease cohort, including a total of 3058 patients clinically diagnosed with AD, frontotemporal dementia spectrum syndromes, mild cognitive impairment, progressive supranuclear palsy syndrome, corticobasal syndrome, or amyotrophic lateral sclerosis and 5089 control subjects.

RESULTS

We observed a significant association between the R47H variant and AD, while no association was observed with any other neurodegenerative disease included in this study.

CONCLUSIONS

Our results support the consensus that the R47H variant is significantly associated with AD. However, we did not find evidence for association of the R47H variant with other neurodegenerative diseases.

GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease

OBJECTIVE

To propose a new hypothesis that GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease (AD).

BACKGROUND

Synaptic dysfunction and E/I imbalance emerge decades before the appearance of cognitive decline in AD patients, which contribute to neurodegeneration. Initially, E/I imbalance was thought to occur first, due to dysfunction of the glutamatergic and cholinergic systems. However, new evidence has demonstrated that the GABAergic system, the counterpart of E/I balance and the major inhibitory neurotransmitter system in the central nervous system, is altered enormously and that this contributes to E/I imbalance and further AD pathogenesis.

NEW HYPOTHESIS

Alterations to the GABAergic system, induced by multiple AD pathogenic or risk factors, contribute to E/I imbalance and AD pathogenesis.

MAJOR CHALLENGES FOR THE HYPOTHESIS

This GABAergic hypothesis accounts for many critical questions and common challenges confronting a new hypothesis of AD pathogenesis. More specifically, it explains why amyloid beta (Aβ), β-secretase (BACE1), apolipoprotein E4 gene (APOE ε4), hyperactive glia cells, contributes to AD pathogenesis and why age and sex are the risk factors of AD. GABAergic dysfunction promotes the spread of Aβ pathology throughout the AD brain and associated cognitive impairments, and the induction of dysfunction induced by these varied risk factors shares this common neurobiology leading to E/I imbalance. In turn, some of these factors exacerbate GABAergic dysfunction and E/I imbalance. Moreover, the GABAergic system modulates various brain functions and thus, the GABAergic hypothesis accounts for nonamnestic manifestations. Furthermore, corrections of E/I balance through manipulation of GABAergic functions have shown positive outcomes in preclinical and clinical studies, suggesting the potential of the GABAergic system as a therapeutic target in AD.

LINKAGE TO OTHER MAJOR THEORIES

Dysfunction of the GABAergic system is induced by multiple critical signaling pathways, which include the existing major theories of AD pathogenesis, such as the Aβ and neuroinflammation hypotheses. In a new perspective, this GABAergic hypothesis accounts for the E/I imbalance and related excitotoxicity, which contribute to cognitive decline and AD pathogenesis. Therefore, the GABAergic system could be a key target to restore, at least partially, the E/I balance and cognitive function in AD patients.

Shifting paradigms: The central role of microglia in Alzheimer's disease

Recent human genetic studies have challenged long standing hypotheses about the chain of events in Alzheimer's disease (AD), as the identification of genetic risk factors in microglial genes supports a causative role for microglia in the disease. Parallel transcriptome and histology studies at the single-cell level revealed a rich palette of microglial states affected by disease status and genetic risk factors. Taken together, those findings support microglia dysfunction as a central mechanism in AD etiology and thus the therapeutic potential of modulating microglial activity for AD treatment. Here we review how human genetic studies discovered microglial AD risk genes, such as TREM2, CD33, MS4A and APOE, and how experimental studies are beginning to decipher the cellular functions of some of these genes. Our review also focuses on recent transcriptomic studies of human microglia from postmortem tissue to critically assess areas of similarity and dissimilarity between human and mouse models currently in use in order to better understand the biology of innate immunity in AD.

A TREML2 missense variant influences specific hippocampal subfield volumes in cognitively normal elderly subjects

INTRODUCTION

Triggering receptor expressed on myeloid cells-like transcript 2 gene (TREML2) is a newly identified AD susceptibility gene. Its missense variant rs3747742-C substantially decreases AD risk in both Caucasians and Han Chinese, but the underlying mechanisms remain elusive. In the present study, to uncover the possible mechanisms by which TREML2 rs3747742-C reduces AD risk, we investigated the possible relation of this variant with AD-related brain structures using a cognitively normal elderly population from Alzheimer's Disease Neuroimaging Initiative (ADNI) database.

METHODS

In total, 158 cognitively normal elders from ADNI database with complete data for brain structures and TREML2 rs3747742 genotype were included in this study. The association of TREML2 rs3747742 genotype with the structures of three cerebral cortices (entorhinal cortex, middle temporal gyrus, and parahippocampal gyrus), two subcortical regions (amygdala and hippocampus), and three subfields of hippocampus (CA1, CA2 + CA3, and CA4 + dentate gyrus) was investigated.

RESULTS

A significant difference was noted in the volume of right CA1 subfield among three genotypes of TREML2 rs3747742 (p = .0364). In the multivariate analysis, TREML2 rs3747742-C significantly increased right CA1 subfield volume after adjusting for age, gender, education years, APOE ε4 status, and intracranial volume under the recessive genetic model (Bonferroni corrected p = .003586).

CONCLUSION

The present study provides the first evidence that TREML2 rs3747742-C carriers have larger volumes of hippocampal CA1 subfield in a cognitively normal elderly population. These findings imply that enhancement of brain reserve may contribute to the protection of TREML2 rs3747742-C in AD susceptibility.

Next Generation Sequencing Analysis in Early Onset Dementia Patients

Background:

Early onset dementias (EOD) are rare neurodegenerative dementias that present before 65 years. Genetic factors have a substantially higher pathogenetic contribution in EOD patients than in late onset dementia.

Objective:

To identify known and/or novel rare variants in major candidate genes associated to EOD by high-throughput sequencing. Common-risk variants of apolipoprotein E (APOE) and prion protein (PRNP) genes were also assessed.

Methods:

We studied 22 EOD patients recruited in Memory Clinics, in the context of studies investigating genetic forms of dementia. Two methodological approaches were applied for the target-Next Generation Sequencing (NGS) analysis of these patients. In addition, we performed progranulin plasma dosage, C9Orf72 hexanucleotide repeat expansion analysis, and APOE genotyping.

Results:

We detected three rare known pathogenic mutations in the GRN and PSEN2 genes and eleven unknown-impact mutations in the GRN, VCP, MAPT, FUS, TREM2, and NOTCH3 genes. Six patients were carriers of only common risk variants (APOE and PRNP), and one did not show any risk mutation/variant. Overall, 69% (n = 9) of our early onset Alzheimer’s disease (EAOD) patients, compared with 34% (n = 13) of sporadic late onset Alzheimer’s disease (LOAD) patients and 27% (n = 73) of non-affected controls (ADNI, whole genome data), were carriers of at least two rare/common risk variants in the analyzed candidate genes panel, excluding the full penetrant mutations.

Conclusion:

This study suggests that EOD patients without full penetrant mutations are characterized by higher probability to carry polygenic risk alleles that patients with LOAD forms. This finding is in line with recently reported evidence, thus suggesting that the genetic risk factors identified in LOAD might modulate the risk also in EOAD.

Association Between Toll-Like Receptor 4 (TLR4) and Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) Genetic Variants and Clinical Progression of Huntington's Disease

BACKGROUND

Although Huntington's disease (HD) is caused by a single dominant gene, it is clear that there are genetic modifiers that may influence the age of onset and disease progression.

OBJECTIVES

We sought to investigate whether new inflammation-related genetic variants may contribute to the onset and progression of HD.

METHODS

We first used postmortem brain material from patients at different stages of HD to look at the protein expression of toll-like receptor 4 (TLR4) and triggering receptor expressed on myeloid cells 2 (TREM2). We then genotyped the TREM2 R47H gene variant and 3 TLR4 single nucleotide polymorphisms in a large cohort of HD patients from the European Huntington's Disease Network REGISTRY.

RESULTS

We found an increase in the number of cells expressing TREM2 and TLR4 in postmortem brain samples from patients dying with HD. We also found that the TREM2 R47H gene variant was associated with changes in cognitive decline in the large cohort of HD patients, whereas 2 of 3 TLR4 single nucleotide polymorphisms assessed were associated with changes in motor progression in this same group.

CONCLUSIONS

These findings identify TREM2 and TLR4 as potential genetic modifiers for HD and suggest that inflammation influences disease progression in this condition. © 2019 International Parkinson and Movement Disorder Society.

Soluble TREM2 is elevated in Parkinson's disease subgroups with increased CSF tau

Parkinson's disease is the second most common neurodegenerative disease after Alzheimer's disease and affects 1% of the population above 60 years old. Although Parkinson's disease commonly manifests with motor symptoms, a majority of patients with Parkinson's disease subsequently develop cognitive impairment, which often progresses to dementia, a major cause of morbidity and disability. Parkinson's disease is characterized by α-synuclein accumulation that frequently associates with amyloid-β and tau fibrils, the hallmarks of Alzheimer's disease neuropathological changes; this co-occurrence suggests that onset of cognitive decline in Parkinson's disease may be associated with appearance of pathological amyloid-β and/or tau. Recent studies have highlighted the appearance of the soluble form of the triggering receptor expressed on myeloid cells 2 (sTREM2) receptor in CSF during development of Alzheimer's disease. Given the known association of microglial activation with advancing Parkinson's disease, we investigated whether CSF and/or plasma sTREM2 differed between CSF biomarker-defined Parkinson's disease participant subgroups. In this cross-sectional study, we examined 165 participants consisting of 17 cognitively normal elderly subjects, 45 patients with Parkinson's disease with no cognitive impairment, 86 with mild cognitive impairment, and 17 with dementia. Stratification of subjects by CSF amyloid-β and tau levels revealed that CSF sTREM2 concentrations were elevated in Parkinson's disease subgroups with a positive tau CSF biomarker signature, but not in Parkinson's disease subgroups with a positive CSF amyloid-β biomarker signature. These findings indicate that CSF sTREM2 could serve as a surrogate immune biomarker of neuronal injury in Parkinson's disease.

Differential role of triggering receptors expressed on myeloid cells 2 R47H in 3 neurodegenerative diseases based on a systematic review and meta-analysis

BACKGROUND

Recent studies have suggested that the potential functional polymorphism R47H in triggering receptors expressed on myeloid cells 2 (TREM2) is associated with several neurodegenerative diseases, however, the results remain inconclusive. This meta-analysis aimed to investigate the association between TREM2 R47H and the risk for 3 typical neurodegenerative diseases: Alzheimer disease (AD), Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS).

METHODS

A literature review was carried out using PubMed, Medline, and Embase. Data analysis was conducted using Stata 15.0 software. The pooled odds ratio (ORs) and 95% confidence interval (CIs) were calculated.

RESULTS

A total of 35 articles were identified as eligible: 22 on AD, 3 on ALS, 7 on PD, 2 on AD and ALS, and 1 on ALS and PD. The AD set included 23,092 cases and 30,920 controls, the ALS set included 7391 cases and 12,442 controls, and the PD set included 8498 patients and 9161 controls. We found that R47H was associated with an increased risk of AD in the total pooled population (P < .001, OR = 4.02, 95% CI = 3.15-5.13). However, this significant difference existed for Caucasian people (OR = 4.16, 95% CI = 3.24-5.33) but not for Asian or African people. Moreover, we did not find any significant differences in minor allele frequency distribution between the PD and control groups or between the ALS and control groups, not only for the total pooled population but also for the subgroups of different ethnicities.

CONCLUSION

Our study suggested that R47H in the TREM2 gene leads to an increased risk for developing AD, but not for ALS and PD, which adds evidence to the notion that diverse pathogenesis may be involved in different neurogenerative diseases.

Examination of the Effect of Rare Variants in TREM2, ABI3, and PLCG2 in LOAD Through Multiple Phenotypes

BACKGROUND

Rare variants in PLCG2 (p.P522R), ABI3 (p.S209F), and TREM2 (p.R47H, p.R62H) have been associated with late onset Alzheimer's disease (LOAD) risk in Caucasians. After the initial report, several studies have found positive results in cohorts of different ethnic background and with different phenotype.

OBJECTIVE

In this study, we aim to evaluate the association of rare coding variants in PLCG2, ABI3, and TREM2 with LOAD risk and their effect at different time points of the disease.

METHODS

We used a European American cohort to assess the association of the variants prior onset (using CSF Aβ42, tau, and pTau levels, and amyloid imaging as endophenotypes) and after onset (measured as rate of memory decline).

RESULTS

We confirm the association with LOAD risk of TREM2 p.R47H, p.R62H and ABI3 p.S209F variants, and the protective effect of PLCG2 p.P522R. In addition, ABI3 and TREM2 gene-sets showed significant association with LOAD risk. TREM2 p.R47H and PLCG2 p.P522R variants were also statistically associated with increase of amyloid imaging and AD progression, respectively. We did not observe any association of ABI3 p.S209F with any of the other AD endophenotypes.

CONCLUSION

The results of this study highlight the importance of including biomarkers and alternative phenotypes to better understand the role of novel candidate genes with the disease.

Plasma levels of soluble TREM2 and neurofilament light chain in TREM2 rare variant carriers

Background

Results from recent clinical studies suggest that cerebrospinal fluid (CSF) biomarkers that are indicative of Alzheimer’s disease (AD) can be replicated in blood, e.g. amyloid-beta peptides (Aβ₄₂ and Aβ₄₀) and neurofilament light chain (NFL). Such data proposes that blood is a rich source of potential biomarkers reflecting central nervous system pathophysiology and should be fully explored for biomarkers that show promise in CSF. Recently, soluble fragments of the triggering receptor expressed on myeloid cells 2 (sTREM2) protein in CSF have been reported to be increased in prodromal AD and also in individuals with TREM2 rare genetic variants that increase the likelihood of developing dementia.

Methods

In this study, we measured the levels of plasma sTREM2 and plasma NFL using the MesoScale Discovery and single molecule array platforms, respectively, in 48 confirmed TREM2 rare variant carriers and 49 non-carriers.

Results

Our results indicate that there are no changes in plasma sTREM2 and NFL concentrations between TREM2 rare variant carriers and non-carriers. Furthermore, plasma sTREM2 is not different between healthy controls, mild cognitive impairment (MCI) or AD.

Conclusion

Concentrations of plasma sTREM2 do not mimic the recent changes found in CSF sTREM2.

The p.R47H Variant of TREM2 Gene is Associated With Late-onset Alzheimer Disease in Colombian Population

OBJECTIVE

We evaluated the association of several single-nucleotide polymorphisms in the triggering receptor expressed on myeloid cells 2 (TREM2) gene in a Colombian sample of late-onset Alzheimer disease (LOAD).

METHODS

The p.Q33* (rs104894002), p.R47H (rs75932628), p.R62H (rs143332484), and p.D87N (rs142232675) variants of TREM2 gene were directly genotyped using KASPar technology in 358 cases and 329 healthy controls. Sanger sequencing was used to validate >10% of KASPar's results. The Fisher exact test was used to compare the distribution of allelic and genotype frequency between cases and controls, and the Bonferroni correction was set at P<0.05.

RESULTS

The minor allele frequency of rs75932628-T was 0.009 in cases and was not found in any healthy controls which suggests a significant association between rs75932628-T and LOAD risk in our sample (P=0.010). The rs143332484-T variant did not exhibit a significant association (P=0.160), whereas rs104894002 and rs142232675 were not found.

CONCLUSIONS

Our findings suggest that the rs75932628-T variant of TREM2 is an important risk factor for LOAD in the Colombian population.

A rare heterozygous TREM2 coding variant identified in familial clustering of dementia affects an intrinsically disordered protein region and function of TREM2

Rare coding variants in the triggering receptor expressed on myeloid cells-2 (TREM2) gene have been associated with Alzheimer disease (AD) and homozygous TREM2 loss-of-function variants have been reported in families with monogenic frontotemporal-like dementia with/without bone abnormalities. In a whole-exome sequencing study of a family with probable AD-type dementia without pathogenic variants in known autosomal dominant dementia disease genes and negative for the apolipoprotein E (APOE) ε4 allele, we identified an extremely rare TREM2 coding variant, that is, a glycine-to-tryptophan substitution at amino acid position 145 (NM_018965.3:c.433G>T/p.[Gly145Trp]). This alteration is found in only 1 of 251,150 control alleles in gnomAD. It was present in both severely affected as well as in another putatively affected and one 61 years old as yet unaffected family member suggesting incomplete penetrance and/or a variable age of onset. Gly145 maps to an intrinsically disordered region (IDR) of TREM2 between the immunoglobulin-like and transmembrane domain. Subsequent cellular studies showed that the variant led to IDR shortening and structural changes of the mutant protein resulting in an impairment of cellular responses upon receptor activation. Our results, suggest that a p.(Gly145Trp)-induced structural disturbance and functional impairment of TREM2 may contribute to the pathogenesis of an AD-like form of dementia.

The MS4A gene cluster is a key modulator of soluble TREM2 and Alzheimer's disease risk

Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in cerebrospinal fluid (CSF) has been associated with Alzheimer's disease (AD). TREM2 plays a critical role in microglial activation, survival, and phagocytosis; however, the pathophysiological role of sTREM2 in AD is not well understood. Understanding the role of sTREM2 in AD may reveal new pathological mechanisms and lead to the identification of therapeutic targets. We performed a genome-wide association study (GWAS) to identify genetic modifiers of CSF sTREM2 obtained from the Alzheimer's Disease Neuroimaging Initiative. Common variants in the membrane-spanning 4-domains subfamily A (MS4A) gene region were associated with CSF sTREM2 concentrations (rs1582763; P = 1.15 × 10-15); this was replicated in independent datasets. The variants associated with increased CSF sTREM2 concentrations were associated with reduced AD risk and delayed age at onset of disease. The single-nucleotide polymorphism rs1582763 modified expression of the MS4A4A and MS4A6A genes in multiple tissues, suggesting that one or both of these genes are important for modulating sTREM2 production. Using human macrophages as a proxy for microglia, we found that MS4A4A and TREM2 colocalized on lipid rafts at the plasma membrane, that sTREM2 increased with MS4A4A overexpression, and that silencing of MS4A4A reduced sTREM2 production. These genetic, molecular, and cellular findings suggest that MS4A4A modulates sTREM2. These findings also provide a mechanistic explanation for the original GWAS signal in the MS4A locus for AD risk and indicate that TREM2 may be involved in AD pathogenesis not only in TREM2 risk-variant carriers but also in those with sporadic disease.

TREM2 brain transcript-specific studies in AD and TREM2 mutation carriers

BACKGROUND

Low frequency coding variants in TREM2 are associated with Alzheimer disease (AD) risk and cerebrospinal fluid (CSF) TREM2 protein levels are different between AD cases and controls. Similarly, TREM2 risk variant carriers also exhibit differential CSF TREM2 levels. TREM2 has three different alternative transcripts, but most of the functional studies only model the longest transcript. No studies have analyzed TREM2 expression levels or alternative splicing in brains from AD and cognitively normal individuals. We wanted to determine whether there was differential expression of TREM2 in sporadic-AD cases versus AD-TREM2 carriers vs sex- and aged-matched normal controls; and if this differential expression was due to a particular TREM2 transcript.

METHODS

We analyzed RNA-Seq data from parietal lobe brain tissue from AD cases with TREM2 variants (n = 33), AD cases (n = 195) and healthy controls (n = 118), from three independent datasets using Kallisto and the R package tximport to determine the read count for each transcript and quantified transcript abundance as transcripts per million.

RESULTS

The three TREM2 transcripts were expressed in brain cortex in the three datasets. We demonstrate for the first time that the transcript that lacks the transmembrane domain and encodes a soluble form of TREM2 (sTREM2) has an expression level around 60% of the canonical transcript, suggesting that around 25% of the sTREM2 protein levels could be explained by this transcript. We did not observe a difference in the overall TREM2 expression level between cases and controls. However, the isoform which lacks the 5' exon, but includes the transmembrane domain, was significantly lower in TREM2- p.R62H carriers than in AD cases (p = 0.007).

CONCLUSION

Using bulk RNA-Seq data from three different cohorts, we were able to quantify the expression level of the three TREM2 transcripts, demonstrating: (1) all three transcripts of them are highly expressed in the human cortex, (2) that up to 25% of the sTREM2 may be due to the expression of a specific isoform and not TREM2 cleavage; and (3) that TREM2 risk variants do not affect expression levels, suggesting that the effect of the TREM2 variants on CSF levels occurs at post-transcriptional level.

Transethnic meta-analysis of rare coding variants in PLCG2, ABI3, and TREM2 supports their general contribution to Alzheimer's disease

Rare coding variants in TREM2, PLCG2, and ABI3 were recently associated with the susceptibility to Alzheimer's disease (AD) in Caucasians. Frequencies and AD-associated effects of variants differ across ethnicities. To start filling the gap on AD genetics in South America and assess the impact of these variants across ethnicity, we studied these variants in Argentinian population in association with ancestry. TREM2 (rs143332484 and rs75932628), PLCG2 (rs72824905), and ABI3 (rs616338) were genotyped in 419 AD cases and 486 controls. Meta-analysis with European population was performed. Ancestry was estimated from genome-wide genotyping results. All variants show similar frequencies and odds ratios to those previously reported. Their association with AD reach statistical significance by meta-analysis. Although the Argentinian population is an admixture, variant carriers presented mainly Caucasian ancestry. Rare coding variants in TREM2, PLCG2, and ABI3 also modulate susceptibility to AD in populations from Argentina, and they may have a European heritage.

Up-Regulation of Trem2 Inhibits Hippocampal Neuronal Apoptosis and Alleviates Oxidative Stress in Epilepsy via the PI3K/Akt Pathway in Mice

Epilepsy is a chronic and severe neurological disorder that has negative effects on the autonomous activities of patients. Functionally, Trem2 (triggering receptor expressed on myeloid cells-2) is an immunoglobulin receptor that affects neurological and psychiatric genetic diseases. Based on this rationale, we aimed to assess the potential role of Trem2 integration with the PI3K/Akt pathway in epilepsy. We used microarray-based gene expression profiling to identify epilepsy-related differentially-expressed genes. In a mouse hippocampal neuron model of epilepsy, neurons were treated with low-Mg²⁺ extracellular fluid, and the protein and mRNA expression of Trem2 were determined. Using a gain-of-function approach with Trem2, neuronal apoptosis and its related factors were assessed by flow cytometry, RT-qPCR, and Western blot analysis. In a pilocarpine-induced epileptic mouse model, the malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) content and superoxide dismutase (SOD) and glutathione-peroxidase (GSH-Px) activity in the hippocampus were determined, and the protein expression of Trem2 was measured. In addition, the regulatory effect of Trem2 on the PI3K/Akt pathway was analyzed by inhibiting this pathway in both the cell and mouse models of epilepsy. Trem2 was found to occupy a core position and was correlated with epilepsy. Trem2 was decreased in the hippocampus of epileptic mice and epileptic hippocampal neurons. Of crucial importance, overexpression of Trem2 activated the PI3K/Akt pathway to inhibit neuronal apoptosis. Moreover, activation of the PI3K/Akt pathway through over-expression of Trem2 alleviated oxidative stress, as shown by the increased expression of SOD and GSH-Px and the decreased expression of MDA and 8-OHdG. The current study defines the potential role of Trem2 in inhibiting the development of epilepsy, indicating that Trem2 up-regulation alleviates hippocampal neuronal injury and oxidative stress, and inhibits neuronal apoptosis in epilepsy by activating the PI3K/Akt pathway.

New insights into the role of TREM2 in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia. The two histopathological markers of AD are amyloid plaques composed of the amyloid-β (Aβ) peptide, and neurofibrillary tangles of aggregated, abnormally hyperphosphorylated tau protein. The majority of AD cases are late-onset, after the age of 65, where a clear cause is still unknown. However, there are likely different multifactorial contributors including age, enviornment, biology and genetics which can increase risk for the disease. Genetic predisposition is considerable, with heritability estimates of 60-80%. Genetic factors such as rare variants of TREM2 (triggering receptor expressed on myeloid cells-2) strongly increase the risk of developing AD, confirming the role of microglia in AD pathogenesis. In the last 5 years, several studies have dissected the mechanisms by which TREM2, as well as its rare variants affect amyloid and tau pathologies and their consequences in both animal models and in human studies. In this review, we summarize increases in our understanding of the involvement of TREM2 and microglia in AD development that may open new therapeutic strategies targeting the immune system to influence AD pathogenesis.

Lack of association between triggering receptor expressed on myeloid cells 2 polymorphism rs75932628 and late-onset Alzheimer's disease in a Chinese Han population

Recently, several studies have investigated the association between the rare mutation of triggering receptor expressed on myeloid cells 2 (TREM2) gene (rs75932628-T) and the risk of late-onset Alzheimer’s disease (LOAD), but they did not draw the same conclusion. Our aim was to investigate the link between the TREM2 polymorphism and LOAD in the Chinese Han population. We examined 786 patients and 803 controls in this study. The rs75932628 polymorphism was evaluated using high-resolution melting analysis and direct sequencing. rs75932628-T (predicted to cause an R47H substitution) was absent in our cohort. We did not find an association between the rs75932628 single nucleotide polymorphism of TREM2 and LOAD in this study. Thus, rs75932628 is unlikely to be related to LOAD in the Chinese Han population.

Triggering receptor expressed on myeloid cells 2 (TREM2): a potential therapeutic target for Alzheimer disease?

INTRODUCTION

There are currently no effective therapeutics for Alzheimer disease (AD). Clinical trials targeting amyloid beta thus far have shown very little benefit and only in the earliest stages of disease. These limitations have driven research to identify alternative therapeutic targets, one of the most promising is the triggering receptor expressed on myeloid cells 2 (TREM2). Areas covered: Here, we review the literature to-date and discuss the potentials and pitfalls for targeting TREM2 as a potential therapeutic for AD. We focus on research in animal and cell models for AD and central nervous system injury models which may help in understanding the role of TREM2 in disease. Expert opinion: Studies suggest TREM2 plays a key role in AD pathology; however, results have been conflicting about whether TREM2 is beneficial or harmful. More research is necessary before designing TREM2-targeting therapies. Successful therapeutics will most likely be administered early in disease.

TREM2 regulates innate immunity in Alzheimer's disease

Recent research has shown that the triggering receptor expressed on myeloid cells 2 (TREM2) in microglia is closely related to the pathogenesis of Alzheimer's disease (AD). The mechanism of this relationship, however, remains unclear. TREM2 is part of the TREM family of receptors, which are expressed primarily in myeloid cells, including monocytes, dendritic cells, and microglia. The TREM family members are cell surface glycoproteins with an immunoglobulin-like extracellular domain, a transmembrane region and a short cytoplasmic tail region. The present article reviews the following: (1) the structure, function, and variant site analysis of the Trem2 gene; (2) the metabolism of TREM2 in peripheral blood and cerebrospinal fluid; and (3) the possible underlying mechanism by which TREM2 regulates innate immunity and participates in AD.

Whole genome sequencing of Caribbean Hispanic families with late-onset Alzheimer's disease

OBJECTIVE

To identify rare causal variants underlying known loci that segregate with late-onset Alzheimer's disease (LOAD) in multiplex families.

METHODS

We analyzed whole genome sequences (WGS) from 351 members of 67 Caribbean Hispanic (CH) families from Dominican Republic and New York multiply affected by LOAD. Members of 67 CH and additional 47 Caucasian families underwent WGS as a part of the Alzheimer's Disease Sequencing Project (ADSP). All members of 67 CH families, an additional 48 CH families and an independent CH case-control cohort were subsequently genotyped for validation. Patients met criteria for LOAD, and controls were determined to be dementia free. We investigated rare variants segregating within families and gene-based associations with disease within LOAD GWAS loci.

RESULTS

A variant in AKAP9, p.R434W, segregated significantly with LOAD in two large families (OR = 5.77, 95% CI: 1.07-30.9, P = 0.041). In addition, missense mutations in MYRF and ASRGL1 under previously reported linkage peaks at 7q14.3 and 11q12.3 segregated completely in one family and in follow-up genotyping both were nominally significant (P < 0.05). We also identified rare variants in a number of genes associated with LOAD in prior genome wide association studies, including CR1 (P = 0.049), BIN1 (P = 0.0098) and SLC24A4 (P = 0.040).

CONCLUSIONS AND RELEVANCE

Rare variants in multiple genes influence the risk of LOAD disease in multiplex families. These results suggest that rare variants may underlie loci identified in genome wide association studies.

Polygenic risk score of sporadic late-onset Alzheimer's disease reveals a shared architecture with the familial and early-onset forms

OBJECTIVE

To determine whether the extent of overlap of the genetic architecture among the sporadic late-onset Alzheimer's Disease (sLOAD), familial late-onset AD (fLOAD), sporadic early-onset AD (sEOAD), and autosomal dominant early-onset AD (eADAD).

METHODS

Polygenic risk scores (PRSs) were constructed using previously identified 21 genome-wide significant loci for LOAD risk.

RESULTS

We found that there is an overlap in the genetic architecture among sEOAD, fLOAD, and sLOAD. The highest association of the PRS and risk (odds ratio [OR] = 2.27; P = 1.29 × 10-7) was observed in sEOAD, followed by fLOAD (OR = 1.75; P = 1.12 × 10-7) and sLOAD (OR = 1.40; P = 1.21 × 10-3). The PRS was associated with cerebrospinal fluid ptau181-Aβ42 on eADAD (P = 4.36 × 10-2).

CONCLUSION

Our analysis confirms that the genetic factors identified for LOAD modulate risk in sLOAD and fLOAD and also sEOAD cohorts. Specifically, our results suggest that the burden of these risk variants is associated with familial clustering and earlier onset of AD. Although these variants are not associated with risk in the eADAD, they may be modulating age at onset.

Assessment of the Genetic Architecture of Alzheimer's Disease Risk in Rate of Memory Decline

Many genetic studies for Alzheimer's disease (AD) have been focused on the identification of common genetic variants associated with AD risk and not on other aspects of the disease, such as age at onset or rate of dementia progression. There are multiple approaches to untangling the genetic architecture of these phenotypes. We hypothesized that the genetic architecture of rate of progression is different than the risk for developing AD dementia. To test this hypothesis, we used longitudinal clinical data from ADNI and the Knight-ADRC at Washington University, and we calculated PRS (polygenic risk score) based on the IGAP study to compare the genetic architecture of AD risk and dementia progression. Dementia progression was measured by the change of Clinical Dementia Rating Sum of Boxes (CDR)-SB per year. Out of the 21 loci for AD risk, no association with the rate of dementia progression was found. The PRS rate was significantly associated with the rate of dementia progression (β= 0.146, p = 0.03). In the case of rare variants, TREM2 (β= 0.309, p = 0.02) was also associated with the rate of dementia progression. TREM2 variant carriers showed a 23% faster rate of dementia compared with non-variant carriers. In conclusion, our results indicate that the recently identified common and rare variants for AD susceptibility have a limited impact on the rate of dementia progression in AD patients.

Intracellular trafficking of TREM2 is regulated by presenilin 1

Genetic mutations in triggering receptor expressed on myeloid cells 2 (TREM2) have been linked to a variety of neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, frontotemporal dementia and Parkinson's disease. In the brain, TREM2 is highly expressed on the cell surface of microglia, where it can transduce signals to regulate microglial functions such as phagocytosis. To date, mechanisms underlying intracellular trafficking of TREM2 remain elusive. Mutations in the presenilin 1 (PS1) catalytic subunit of the γ-secretase complex have been associated with increased generation of the amyloidogenic Aβ (amyloid-β) 42 peptide through cleavage of the Aβ precursor amyloid precursor protein. Here we found that TREM2 interacts with PS1 in a manner independent of γ-secretase activity. Mutations in TREM2 alter its subcellular localization and affects its interaction with PS1. Upregulation of PS1 reduces, whereas downregulation of PS1 increases, steady-state levels of cell surface TREM2. Furthermore, PS1 overexpression results in attenuated phagocytic uptake of Aβ by microglia, which is reversed by TREM2 overexpression. Our data indicate a novel role for PS1 in regulating TREM2 intracellular trafficking and pathophysiological function.

Genetics of Alzheimer's Disease: the Importance of Polygenic and Epistatic Components

PURPOSE OF REVIEW

We aimed to summarize the recent advances in genetic findings of Alzheimer's disease (AD), focusing on traditional single-marker and gene approaches and non-traditional ones, i.e., polygenic and epistatic components.

RECENT FINDINGS

Genetic studies have progressed over the last few decades from linkage to genome-wide association studies (GWAS), and most recently studies utilizing high-throughput sequencing. So far, GWASs have identified several common variants characterized by small effect sizes (besides APOE-ε4). Sequencing has facilitated the study of rare variants with larger effects. Nevertheless, missing heritability for AD remains extensive; a possible explanation might lie in the existence of polygenic and epistatic components. We review findings achieved by single-marker approaches, but also polygenic and epistatic associations. The latter two are critical, yet-underexplored mechanisms. Genes involved in complex diseases are likely regulated by mechanisms and pathways involving many other genes, an aspect potentially missed by traditional approaches.

TREM2 in Neurodegenerative Diseases

TREM2 variants have been identified as risk factors for Alzheimer's disease (AD) and other neurodegenerative diseases (NDDs). Because TREM2 encodes a receptor exclusively expressed on immune cells, identification of these variants conclusively demonstrates that the immune response can play an active role in the pathogenesis of NDDs. These TREM2 variants also confer the highest risk for developing Alzheimer's disease of any risk factor identified in nearly two decades, suggesting that understanding more about TREM2 function could provide key insights into NDD pathology and provide avenues for novel immune-related NDD biomarkers and therapeutics. The expression, signaling and function of TREM2 in NDDs have been extensively investigated in an effort to understand the role of immune function in disease pathogenesis and progression. We provide a comprehensive review of our current understanding of TREM2 biology, including new insights into the regulation of TREM2 expression, and TREM2 signaling and function across NDDs. While many open questions remain, the current body of literature provides clarity on several issues. While it is still often cited that TREM2 expression is decreased by pro-inflammatory stimuli, it is now clear that this is true in vitro, but inflammatory stimuli in vivo almost universally increase TREM2 expression. Likewise, while TREM2 function is classically described as promoting an anti-inflammatory phenotype, more than half of published studies demonstrate a pro-inflammatory role for TREM2, suggesting that its role in inflammation is much more complex. Finally, these components of TREM2 biology are applied to a discussion of how TREM2 impacts NDD pathologies and the latest assessment of how these findings might be applied to immune-directed clinical biomarkers and therapeutics.

Neocortical and hippocampal TREM2 protein levels during the progression of Alzheimer's disease

Heterozygous triggering receptor expressed on myeloid cells (TREM2) mutations are an Alzheimer's disease (AD) risk factor. Nonmutated TREM2 dysregulation occurs in AD brain. Whether TREM2 is altered in prodromal AD remains unknown. Western blotting was used to determine levels of TREM2 (∼25 kDa) and Iba1 in the frontal cortex and TREM2 in the hippocampus from people who died with an ante-mortem clinical diagnosis of non- and mild-cognitive impairment, mild/moderate AD, and severe AD (sAD). Immunohistochemistry defined the relationship between amyloid and Iba1 profiles. Polymerase chain reaction analysis revealed that all subjects did not carry the most common R47H TREM2 variant. TREM2 was significantly upregulated in sAD frontal cortex but stable in hippocampus. Frontal TREM2 mRNA and protein level patterns were similar but not significantly different. Iba1 immunopositive microglia counts increased significantly in frontal cortex containing plaques in sAD. TREM2 and Iba1 levels were not associated with plaques, tangles, neuropathological criteria, or cognitive performance. Frontal cortex TREM2 upregulation is a late event and may not play a major role early in the pathogenesis of the disease.

Genome-wide association study identifies four novel loci associated with Alzheimer's endophenotypes and disease modifiers

More than 20 genetic loci have been associated with risk for Alzheimer's disease (AD), but reported genome-wide significant loci do not account for all the estimated heritability and provide little information about underlying biological mechanisms. Genetic studies using intermediate quantitative traits such as biomarkers, or endophenotypes, benefit from increased statistical power to identify variants that may not pass the stringent multiple test correction in case-control studies. Endophenotypes also contain additional information helpful for identifying variants and genes associated with other aspects of disease, such as rate of progression or onset, and provide context to interpret the results from genome-wide association studies (GWAS). We conducted GWAS of amyloid beta (Aβ42), tau, and phosphorylated tau (ptau181) levels in cerebrospinal fluid (CSF) from 3146 participants across nine studies to identify novel variants associated with AD. Five genome-wide significant loci (two novel) were associated with ptau181, including loci that have also been associated with AD risk or brain-related phenotypes. Two novel loci associated with Aβ42 near GLIS1 on 1p32.3 (β = -0.059, P = 2.08 × 10-8) and within SERPINB1 on 6p25 (β = -0.025, P = 1.72 × 10-8) were also associated with AD risk (GLIS1: OR = 1.105, P = 3.43 × 10-2), disease progression (GLIS1: β = 0.277, P = 1.92 × 10-2), and age at onset (SERPINB1: β = 0.043, P = 4.62 × 10-3). Bioinformatics indicate that the intronic SERPINB1 variant (rs316341) affects expression of SERPINB1 in various tissues, including the hippocampus, suggesting that SERPINB1 influences AD through an Aβ-associated mechanism. Analyses of known AD risk loci suggest CLU and FERMT2 may influence CSF Aβ42 (P = 0.001 and P = 0.009, respectively) and the INPP5D locus may affect ptau181 levels (P = 0.009); larger studies are necessary to verify these results. Together the findings from this study can be used to inform future AD studies.