More evidence for association of a rare TREM2 mutation (R47H) with Alzheimer's disease risk

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.

TREM2 risk variants are associated with atypical Alzheimer's disease

Alzheimer's disease (AD) has multiple clinically and pathologically defined subtypes where the underlying causes of such heterogeneity are not well established. Rare TREM2 variants confer significantly increased risk for clinical AD in addition to other neurodegenerative disease clinical phenotypes. Whether TREM2 variants are associated with atypical clinical or pathologically defined subtypes of AD is not known. We studied here the clinical and pathological features associated with TREM2 risk variants in an autopsy-confirmed cohort. TREM2 variant cases were more frequently associated with non-amnestic clinical syndromes. Pathologically, TREM2 variant cases were associated with an atypical distribution of neurofibrillary tangle density with significantly lower hippocampal NFT burden relative to neocortical NFT accumulation. In addition, NFT density but not amyloid burden was associated with an increase of dystrophic microglia. TREM2 variant cases were not associated with an increased prevalence, extent, or severity of co-pathologies. These clinicopathological features suggest that TREM2 variants contribute to clinical and pathologic AD heterogeneity by altering the distribution of neurofibrillary degeneration and tau-dependent microglial dystrophy, resulting in hippocampal-sparing and non-amnestic AD phenotypes.

Alzheimer’s Disease Biomarkers Revisited From the Amyloid Cascade Hypothesis Standpoint

Alzheimer’s disease (AD) is the most common neurodegenerative disease worldwide. Amyloid beta (Aβ) is one of the proteins which aggregate in AD, and its key role in the disease pathogenesis is highlighted in the amyloid cascade hypothesis, which states that the deposition of Aβ in the brain parenchyma is a crucial initiating step in the future development of AD. The sensitivity of instruments used to measure proteins in blood and cerebrospinal fluid has significantly improved, such that Aβ can now successfully be measured in plasma. However, due to the peripheral production of Aβ, there is significant overlap between diagnostic groups. The presence of pathological Aβ within the AD brain has several effects on the cells and surrounding tissue. Therefore, there is a possibility that using markers of tissue responses to Aβ may reveal more information about Aβ pathology and pathogenesis than looking at plasma Aβ alone. In this manuscript, using the amyloid cascade hypothesis as a starting point, we will delve into how the effect of Aβ on the surrounding tissue can be monitored using biomarkers. In particular, we will consider whether glial fibrillary acidic protein, triggering receptor expressed on myeloid cells 2, phosphorylated tau, and neurofilament light chain could be used to phenotype and quantify the tissue response against Aβ pathology in AD.

Genomics and Functional Genomics of Alzheimer's Disease

Alzheimer's disease (AD) is a complex and multifactorial neurodegenerative disease. Due to its long clinical course and lack of an effective treatment, AD has become a major public health problem in the USA and worldwide. Due to variation in age-at-onset, AD is classified into early-onset (< 60 years) and late-onset (≥ 60 years) forms with early-onset accounting for only 5-10% of all cases. With the exception of a small number of early-onset cases that are afflicted because of high penetrant single gene mutations in APP, PSEN1, and PSEN2 genes, AD is genetically heterogeneous, especially the late-onset form having a polygenic or oligogenic risk inheritance. Since the identification of APOE as the most significant risk factor for late-onset AD in 1993, the path to the discovery of additional AD risk genes had been arduous until 2009 when the use of large genome-wide association studies opened up the discovery gateways that led the identification of ~ 95 additional risk loci from 2009 to early 2022. This article reviews the history of AD genetics followed by the potential molecular pathways and recent application of functional genomics methods to identify the causal AD gene(s) among the many genes that reside within a single locus. The ultimate goal of integrating genomics and functional genomics is to discover novel pathways underlying the AD pathobiology in order to identify drug targets for the therapeutic treatment of this heterogeneous disorder.

A rare variant in EZH2 is associated with prostate cancer risk

Prostate cancer (PrCa) is highly heritable, and although rare variants contribute significantly to PrCa risk, few have been identified to date. Herein, whole-genome sequencing was performed in a large PrCa family featuring multiple affected relatives spanning several generations. A rare, predicted splice site EZH2 variant, rs78589034 (G > A), was identified as segregating with disease in all but two individuals in the family, one of whom was affected with lymphoma and bowel cancer and a female relative. This variant was significantly associated with disease risk in combined familial and sporadic PrCa datasets (n = 1551; odds ratio [OR] = 3.55, P = 1.20 × 10^-5 ). Transcriptome analysis was performed on prostate tumour needle biopsies available for two rare variant carriers and two wild-type cases. Although no allele-dependent differences were detected in EZH2 transcripts, a distinct differential gene expression signature was observed when comparing prostate tissue from the rare variant carriers with the wild-type samples. The gene expression signature comprised known downstream targets of EZH2 and included the top-ranked genes, DUSP1, FOS, JUNB and EGR1, which were subsequently validated by qPCR. These data provide evidence that rs78589034 is associated with increased PrCa risk in Tasmanian men and further, that this variant may be associated with perturbed EZH2 function in prostate tissue. Disrupted EZH2 function is a driver of tumourigenesis in several cancers, including prostate, and is of significant interest as a therapeutic target.

Genetic Variability in Molecular Pathways Implicated in Alzheimer's Disease: A Comprehensive Review

Alzheimer's disease (AD) is a complex neurodegenerative disease, affecting a significant part of the population. The majority of AD cases occur in the elderly with a typical age of onset of the disease above 65 years. AD presents a major burden for the healthcare system and since population is rapidly aging, the burden of the disease will increase in the future. However, no effective drug treatment for a full-blown disease has been developed to date. The genetic background of AD is extensively studied; numerous genome-wide association studies (GWAS) identified significant genes associated with increased risk of AD development. This review summarizes more than 100 risk loci. Many of them may serve as biomarkers of AD progression, even in the preclinical stage of the disease. Furthermore, we used GWAS data to identify key pathways of AD pathogenesis: cellular processes, metabolic processes, biological regulation, localization, transport, regulation of cellular processes, and neurological system processes. Gene clustering into molecular pathways can provide background for identification of novel molecular targets and may support the development of tailored and personalized treatment of AD.

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.

Cerebrospinal Fluid Biomarkers of Alzheimer's Disease: Current Evidence and Future Perspectives

Alzheimer's disease is a progressive, clinically heterogeneous, and particularly complex neurodegenerative disease characterized by a decline in cognition. Over the last two decades, there has been significant growth in the investigation of cerebrospinal fluid (CSF) biomarkers for Alzheimer's disease. This review presents current evidence from many clinical neurochemical studies, with findings that attest to the efficacy of existing core CSF biomarkers such as total tau, phosphorylated tau, and amyloid-β (Aβ42), which diagnose Alzheimer's disease in the early and dementia stages of the disorder. The heterogeneity of the pathophysiology of the late-onset disease warrants the growth of the Alzheimer's disease CSF biomarker toolbox; more biomarkers showing other aspects of the disease mechanism are needed. This review focuses on new biomarkers that track Alzheimer's disease pathology, such as those that assess neuronal injury (VILIP-1 and neurofilament light), neuroinflammation (sTREM2, YKL-40, osteopontin, GFAP, progranulin, and MCP-1), synaptic dysfunction (SNAP-25 and GAP-43), vascular dysregulation (hFABP), as well as CSF α-synuclein levels and TDP-43 pathology. Some of these biomarkers are promising candidates as they are specific and predict future rates of cognitive decline. Findings from the combinations of subclasses of new Alzheimer's disease biomarkers that improve their diagnostic efficacy in detecting associated pathological changes are also presented.

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.

MEK1/2 activity modulates TREM2 cell surface recruitment

Rare sequence variants in the microglial cell surface receptor TREM2 have been shown to increase the risk for Alzheimer’s disease (AD). Disease-linked TREM2 mutations seem to confer a partial loss of function, and increasing TREM2 cell surface expression and thereby its function(s) might have therapeutic benefit in AD. However, druggable targets that could modulate microglial TREM2 surface expression are not known. To identify such targets, we conducted a screen of small molecule compounds with known pharmacology using human myeloid cells, searching for those that enhance TREM2 protein at the cell surface. Inhibitors of the kinases MEK1/2 displayed the strongest and most consistent increases in cell surface TREM2 protein, identifying a previously unreported pathway for TREM2 regulation. Unexpectedly, inhibitors of the downstream effector ERK kinases did not have the same effect, suggesting that noncanonical MEK signaling regulates TREM2 trafficking. In addition, siRNA knockdown experiments confirmed that decreased MEK1 and MEK2 were required for this recruitment. In iPSC-derived microglia, MEK inhibition increased cell surface TREM2 only modestly, so various cytokines were used to alter iPSC microglia phenotype, making cells more sensitive to MEK inhibitor-induced TREM2 recruitment. Of those tested, only IFN-gamma priming prior to MEK inhibitor treatment resulted in greater TREM2 recruitment. These data identify the first known mechanisms for increasing surface TREM2 protein and TREM2-regulated function in human myeloid cells and are the first to show a role for MEK1/MEK2 signaling in TREM2 activity.

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.

Investigation of pathology, expression and proteomic profiles in human TREM2 variant postmortem brains with and without Alzheimer's disease

Triggering receptor expressed on myeloid cells 2 TREM2 was identified as a risk factor for late onset Alzheimer’s disease (AD). Here we compared TREM2 cases with a variant (TREM2⁺) and cases without a TREM2 variant (TREM2⁻), considering pathological burden, inflammatory response and altered canonical pathways and biochemical functions between the cohorts. We hypothesised that TREM2⁺ cases would have a loss of function, indicating an altered inflammatory profile compared to TREM2⁻ cases. Immunohistochemistry was performed using antibodies against Aβ, tau and microglia markers in TREM2⁺ cases, with and without AD, which were compared to sporadic TREM2⁻ AD, familial AD and neurologically normal control cases. Aβ and tau load were measured along with the composition of Aβ plaques, in addition to microglial load and circularity. Expression and proteomic profiles were determined from the frontal cortex of selected cases. TREM2⁺ control cases had no Aβ or tau deposition. No differences in the amount of Aβ or tau, or the composition of Aβ plaques were observed between TREM2⁺ and TREM2⁻ SAD cases. There were no differences in microglial load observed between disease groups. However, the TREM2⁺ SAD cases showed more amoeboid microglia than the TREM2⁻ SAD cases, although no differences in the spatial relationship of microglia and Aβ plaques were identified. Visualisation of the canonical pathways and biological functions showed differences between the disease groups and the normal controls, clearly showing a number of pathways upregulated in TREM2⁺ SAD, TREM2⁻ SAD and FAD groups whilst, the TREM2⁺ controls cases showed a downregulation of the majority of the represented pathways. These findings suggest that the TREM2⁺ control group, although carrying the TREM2⁺ variant, have no pathological hallmarks of AD, have altered microglial and expression profiles compared to the TREM2⁺ SAD cases. This indicates that other unknown factors may initiate the onset of AD, with TREM2 influencing the microglial involvement in disease pathogenesis.

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.

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.

Neurodegenerative Disease-Associated Variants in TREM2 Destabilize the Apical Ligand-Binding Region of the Immunoglobulin Domain

Single nucleotide variations in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) have been linked to both late-onset Alzheimer's disease and behavioral variant frontotemporal dementia (FTD), the latter presenting either in isolation or with cystic bone lesions in a condition called Nasu-Hakola disease. Models of the extracellular domain of TREM2 show that Nasu-Hakola disease-associated mutations are grossly inactivating by truncation, frameshift, or unfolding, that Alzheimer's disease (AD)-associated variants localize to a putative ligand-interacting region (PLIR) on the extracellular surface, and that FTD-associated variants are found in the hydrophobic core. However, while these disease-associated residues are predicted to play some role in disrupting ligand binding to the extracellular domain of TREM2, how they ultimately lead to disease remains unknown. Here, we used in silico molecular modeling to investigate all-atom models of TREM2 and characterize the effects on conformation and dynamical motion of AD-associated R47H and R62H as well as FTD-associated T96K, D86V, and T66M variants compared to the benign N68K variant and the common variant. Our model, which is based on a published 2.2 Å resolution crystal structure of the TREM2 extracellular domain, finds that both AD- and FTD-associated variants cause localized instability in three loops adjacent to the PLIR that correspond to the complementarity-determining regions (CDRs) of antibodies. This instability ultimately disrupts tethering between these CDRs and the core of the immunoglobulin domain, exposing a group of otherwise-buried, negatively charged residues. This instability and exposure of negatively charged residues is most severe following introduction of the T66M variant that has been described as causing FTD even in the heterozygous state and is less severe following introduction of variants that are less strongly tied to FTD or of those associated with AD. Thus, our results provide further evidence that the proposed loss-of-function caused by neurodegenerative disease-associated variants may be driven by altered conformational stability of the ligand-interacting CDR and, ultimately, loss of affinity or specificity for TREM2 ligands.

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.

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.

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.

The role of TREM2 in Alzheimer's disease and other neurodegenerative disorders

Alzheimer's disease is a genetically complex disorder; rare variants in the triggering receptor expressed on myeloid cells 2 (TREM2) gene have been shown to as much as triple an individual's risk of developing Alzheimer's disease. TREM2 is a transmembrane receptor expressed in cells of the myeloid lineage, and its association with Alzheimer's disease supports the involvement of immune and inflammatory pathways in the cause of the disease, rather than as a consequence of the disease. TREM2 variants associated with Alzheimer's disease induce partial loss of function of the TREM2 protein and alter the behaviour of microglial cells, including their response to amyloid plaques. TREM2 variants have also been shown to cause polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy and frontotemporal dementia. Although the low frequency of TREM2 variants makes it difficult to establish robust genotype-phenotype correlations, such studies are essential to enable a comprehensive understanding of the role of TREM2 in different neurological diseases, with the ultimate goal of developing novel therapeutic approaches.

The Microglial Response to Neurodegenerative Disease

Microglia are a subset of tissue macrophages that constitute the major immune cell type of the central nervous system. These cells have long been known to change their morphology and functions in response to various neurological insults. Recently, a plethora of unbiased transcriptomics studies have revealed that across a broad spectrum of neurodegeneration-like disease models, microglia adopt a similar activation signature and perform similar functions. Despite these commonalities in response, the role of microglia has been described as both positive and negative in different murine disease models. In humans, genetic association studies have revealed strong connections between microglia genes and various neurodegenerative diseases, and mechanistic investigations of these mutations have added another layer of complexity. Here, we provide an overview of studies that have built a case for a common microglial response to neurodegeneration and discuss pathways that may be important to initiate and sustain this response; delineate the multifaceted functions of activated microglia spanning different diseases; and discuss insights from studying genes associated with disease in humans. We argue that strong evidence causally links activated microglia function to neurodegeneration and discuss what seems to be a conflict between mouse models and human genetics.

Microglia-Mediated Neuroprotection, TREM2, and Alzheimer's Disease: Evidence From Optical Imaging

Recent genetic studies have provided overwhelming evidence of the involvement of microglia-related molecular networks in the pathophysiology of Alzheimer's disease (AD). However, the precise mechanisms by which microglia alter the course of AD neuropathology remain poorly understood. Here we discuss current evidence of the neuroprotective functions of microglia with a focus on optical imaging studies that have revealed a role of these cells in the encapsulation of amyloid deposits ("microglia barrier"). This barrier modulates the degree of plaque compaction, amyloid fibril surface area, and insulation from adjacent axons thereby reducing neurotoxicity. We discuss findings implicating genetic variants of the microglia receptor, triggering receptor expressed on myeloid cells 2, in the increased risk of late onset AD. We provide evidence that increased AD risk may be at least partly mediated by deficient microglia polarization toward amyloid deposits, resulting in ineffective plaque encapsulation and reduced plaque compaction, which is associated with worsened axonal pathology. Finally, we propose possible avenues for therapeutic targeting of plaque-associated microglia with the goal of enhancing the microglia barrier and potentially reducing disease progression.

Genetics of Alzheimer's disease: an update

It is clear that late-onset Alzheimer's disease (AD), the most common form of dementia in western societies, has a significant genetic component. The recent technological advances in high-throughput genome technologies have enabled the identification of more than 20 novel susceptibility loci. These findings have significantly advanced the understanding of the molecular mechanisms potentially underlying AD etiology, and have therefore provided valuable information for the development of targets for genetic testing, prevention and treatment. This article reviews these recent findings in AD genomics and discusses their implications for understanding the molecular underpinnings of the disease.

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.

From Common to Rare Variants: The Genetic Component of Alzheimer Disease

Alzheimer disease (AD) is a remarkable example of genetic heterogeneity. Extremely rare variants in the APP, PSEN1, or PSEN2 genes, or duplications of the APP gene cause autosomal dominant forms, generally with complete penetrance by the age of 65 years. Nonautosomal dominant forms are considered as a complex disorder with a high genetic component, whatever the age of onset. Although genetically heterogeneous, AD is defined by the same neuropathological criteria in all configurations. According to the amyloid cascade hypothesis, the Aβ peptide, which aggregates in AD brains, is a key player. APP, PSEN1, or PSEN2 gene mutations increase the production of more aggregation-prone forms of the Aβ peptide, triggering the pathological process. Several risk factors identified in association studies hit genes involved in Aβ production/secretion, aggregation, clearance, or toxicity. Among them, the APOE ε4 allele is a rare example of a common allele with a large effect size, the ORs ranging from 4 to 11-14 for heterozygous and homozygous carriers, respectively. In addition, genome-wide association studies have identified more than two dozen loci with a weak but significant association, the OR of the at-risk allele ranging from 1.08 to 1.30. Recently, the use of massive parallel sequencing has enabled the analysis of rare variants in a genome-wide manner. Two rare variants have been nominally associated with AD risk or protection (TREM2 p.R47H, MAF approximately 0.002, OR approximately 4 and APP p.A673T, MAF approximately 0.0005, OR approximately 0.2). Association analyses at the gene level identified rare loss-of-function and missense, predicted damaging, variants (MAF <0.01) in the SORL1 and ABCA7 genes associated with a moderate relative risk (OR approximately 5 and approximately 2.8, respectively). Although the latter analyses revealed association signals with moderately rare variants by collapsing them, the power to detect genes hit by extremely rare variants is still limited. An alternative approach is to consider the de novo paradigm, stating that de novo variants may contribute to AD genetics in sporadic patients. Here, we critically review AD genetics reports with a special focus on rare variants.

TREMs in Alzheimer's disease: Genetic and clinical investigations

Triggering receptor expressed on myeloid cells (TREMs) receptors constitute a family modulators in human innate immunity system that encode by a gene cluster. Rare variants in TREM2 were reported to be associated with significant Alzheimer's disease (AD) risk. However, inconsistent results were also reported in some studies of Non-European descents. Recently, the other TREM family members are also considered to involve in AD and cerebrospinal fluid (CSF) soluble form of TREM2 (sTREM2) levels has also been associated with respond to progression of disease. In this review, we converged the data of genetic and clinical investigations to identify the clearer role of TREMs in AD. Here, comprehensively analyze of multidisciplinary fields highlights the contribution of TREMs locus to AD development.

Regulation of microglial survival and proliferation in health and diseases

Microglia play an important role in the development and maintenance of the central nervous system (CNS) under homeostatic conditions as well as during neurodegenerative diseases. Recent observations in human genomics and advances in genetic mouse models have provided insights into signaling pathways that control development, survival, proliferation and function of microglia. Alteration of these pathways contributes to the pathogenesis of CNS diseases. Here we review the current literature regarding the roles of these microglial pathways in both the normal and diseased brain and discuss areas that require further investigation.

Mutation analysis of the MS4A and TREM gene clusters in a case-control Alzheimer's disease data set

Genome wide association studies have identified an association between Alzheimer's disease (AD) and common polymorphisms in the MS4A and TREM loci (each containing a cluster of homologous genes) and should be thoroughly investigated for the presence of potentially functional variations. We conducted a mutation analysis by next generation sequencing of 15 genes within the MS4A and TREM gene clusters; and catalogued rare coding variants detected in a North American data set of 210 cases and 233 controls. Investigation of the 5 homologues genes in the TREM locus revealed potentially damaging rare variants in TREM2, TREML1, TREML2, and TREML4. In agreement with a previous report, we observed a significant enrichment of TREM2-damaging missense substitutions in cases (N = 9; 4.2%) compared with controls (N=2; 0.9%; p = 0.010; after Yates' correction p = 0.022). Among known AD-associated TREM2 substitutions, we detected p.R47H, p.D87N, and p.H157Y affecting both TREM2 isoforms (NM_018965 and NM_001271821). In addition, we identified 2 cases with novel TREM2 variants (p.L205P and p.G219C), which mapped only to the isoform NM_001271821 at the C-terminus. Investigation of the MS4A gene cluster revealed that potentially damaging missense substitutions and loss-of-function variants were twice as frequent in controls (N = 19; 8.2%) than cases (N = 9; 4.3%), generating a nominally significant result (p = 0.047; after Yates' correction p = 0.07). Validation of our observation in large data sets might address the question whether such variants could contribute to the protective effect of the minor alleles of Genome wide association study-significant single nucleotide polymorphisms at the MS4A locus.

Association study of TREM2 polymorphism rs75932628 with leucoaraiosis or Parkinson's disease in the Han Chinese population

OBJECTIVES

The previously reported functional mutation rs75932628-T (p.R47H) in the triggering receptor expressed on myeloid cells 2 (TREM2) is a genetic risk factor for Alzheimer's disease, Parkinson's disease (PD) and frontotemporal dementia, in European populations. This study aims to assess the genetic association of the variant rs75932628-T with PD and leucoaraiosis (LA) in a Han Chinese population.

SETTING

This population-based study was conducted in China by Xiamen University and its affiliated hospital.

PARTICIPANTS

308 patients with LA, 342 patients with PD and 198 healthy blood donors were recruited from the First Affiliated Hospital of Xiamen University.

OUTCOME MEASURES

Genotyping was performed by molecular beacon real-time PCR and Sanger sequencing.

RESULTS

None of our participants carried the rs75932628-T mutation.

CONCLUSIONS

Our results corroborate and extend previous findings, concluding that the variant rs75932628-T (p.R47H) in TREM2 is not a risk factor for LA or PD in the Han Chinese population.

Novel susceptibility loci for Alzheimer's disease

Late-onset Alzheimer's disease (AD), a highly prevalent neurodegenerative disorder characterized by progressive deterioration in cognition, function and behavior terminating in incapacity and death, is a clinically and pathologically heterogeneous disease with a substantial heritable component. During the past 5 years, the technological developments in next-generation high-throughput genome technologies have led to the identification of more than 20 novel susceptibility loci for AD, and have implicated specific pathways in the disease, in particular intracellular trafficking/endocytosis, inflammation and immune response and lipid metabolism. These observations have significantly advanced our understanding of underlying pathogenic mechanisms and potential therapeutic targets. This review article summarizes these recent advances in AD genomics and discusses the value of identified susceptibility loci for diagnosis and prognosis of AD.

Apolipoprotein E Is a Ligand for Triggering Receptor Expressed on Myeloid Cells 2 (TREM2)

Several heterozygous missense mutations in the triggering receptor expressed on myeloid cells 2 (TREM2) have recently been linked to risk for a number of neurological disorders including Alzheimer disease (AD), Parkinson disease, and frontotemporal dementia. These discoveries have re-ignited interest in the role of neuroinflammation in the pathogenesis of neurodegenerative diseases. TREM2 is highly expressed in microglia, the resident immune cells of the central nervous system. Along with its adaptor protein, DAP12, TREM2 regulates inflammatory cytokine release and phagocytosis of apoptotic neurons. Here, we report apolipoprotein E (apoE) as a novel ligand for TREM2. Using a biochemical assay, we demonstrated high-affinity binding of apoE to human TREM2. The functional significance of this binding was highlighted by increased phagocytosis of apoE-bound apoptotic N2a cells by primary microglia in a manner that depends on TREM2 expression. Moreover, when the AD-associated TREM2-R47H mutant was used in biochemical assays, apoE binding was vastly reduced. Our data demonstrate that apoE-TREM2 interaction in microglia plays critical roles in modulating phagocytosis of apoE-bound apoptotic neurons and establish a critical link between two proteins whose genes are strongly linked to the risk for AD.

The Triggering Receptor Expressed on Myeloid Cells 2 Binds Apolipoprotein E

The triggering receptor expressed on myeloid cells 2 (TREM2) is an Ig-like V-type receptor expressed by populations of myeloid cells in the central nervous system and periphery. Loss-of-function mutations in TREM2 cause a progressive, fatal neurodegenerative disorder called Nasu-Hakola disease. In addition, a TREM2 R47H coding variant was recently identified as a risk factor for late-onset Alzheimer disease. TREM2 binds various polyanionic molecules but no specific protein ligands have been identified. Here we show that TREM2 specifically binds apolipoprotein E, a well established participant in Alzheimer disease. TREM2-Ig fusions efficiently precipitate ApoE from cerebrospinal fluid and serum. TREM2 also binds recombinant ApoE in solution and immobilized ApoE as detected by ELISA. Furthermore, the Alzheimer disease-associated R47H mutation, and other artificial mutations introduced in the same location, markedly reduced the affinity of TREM2 for ApoE. These findings reveal a link between two Alzheimer disease risk factors and may provide important clues to the pathogenesis of Nasu-Hakola disease and other neurodegenerative disorders.

TREM2 in CNS homeostasis and neurodegenerative disease

Myeloid-lineage cells accomplish a myriad of homeostatic tasks including the recognition of pathogens, regulation of the inflammatory milieu, and mediation of tissue repair and regeneration. The innate immune receptor and its adaptor protein—triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12)—possess the ability to modulate critical cellular functions via crosstalk with diverse signaling pathways. As such, mutations in TREM2 and DAP12 have been found to be associated with a range of disease phenotypes. In particular, mutations in TREM2 increase the risk for Alzheimer's disease and other neurodegenerative disorders. The leading hypothesis is that microglia, the resident immune cells of the central nervous system, are the major myeloid cells affected by dysregulated TREM2-DAP12 function. Here, we review how impaired signaling by the TREM2-DAP12 pathway leads to altered immune responses in phagocytosis, cytokine production, and microglial proliferation and survival, thus contributing to disease pathogenesis.