Elise Cuyvers

TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis

Loss of TREM2 function impairs phagocytosis and correlates with decreased soluble TREM2 in biological fluids of patients with neurodegenerative disorders. TREM2 and Neurodegeneration Little is known about how risk factors facilitate initiation and propagation of neurodegenerative disorders. Rare mutations in TREM2 increase the risk for several neurodegenerative disorders including Alzheimer’s disease (AD), Parkinson’s disease, and frontotemporal dementia (FTD). Kleinberger et al. now show that mutations associated with neurodegenerative diseases interfere with TREM2 function by preventing its maturation, transport to the cell surface, and shedding. Expression of mutant TREM2 led to reduced phagocytic activity by different cell types, suggesting that removal of cellular debris by, for example, microglia in the brain might be affected in patients with TREM2 mutations. In a patient with FTD-like syndrome carrying a homozygous TREM2 mutation, no soluble TREM2 was detected in the cerebrospinal fluid (CSF) and plasma. Patients with sporadic FTD and AD showed slightly reduced concentrations of soluble TREM2 in their CSF. Although much further testing and validation are needed, soluble TREM2 might be useful as a marker of neurodegeneration. Genetic variants in the triggering receptor expressed on myeloid cells 2 (TREM2) have been linked to Nasu-Hakola disease, Alzheimer’s disease (AD), Parkinson’s disease, amyotrophic lateral sclerosis, frontotemporal dementia (FTD), and FTD-like syndrome without bone involvement. TREM2 is an innate immune receptor preferentially expressed by microglia and is involved in inflammation and phagocytosis. Whether and how TREM2 missense mutations affect TREM2 function is unclear. We report that missense mutations associated with FTD and FTD-like syndrome reduce TREM2 maturation, abolish shedding by ADAM proteases, and impair the phagocytic activity of TREM2-expressing cells. As a consequence of reduced shedding, TREM2 is virtually absent in the cerebrospinal fluid (CSF) and plasma of a patient with FTD-like syndrome. A decrease in soluble TREM2 was also observed in the CSF of patients with AD and FTD, further suggesting that reduced TREM2 function may contribute to increased risk for two neurodegenerative disorders.

Investigating the role of rare heterozygous TREM2 variants in Alzheimer's disease and frontotemporal dementia

Homozygous mutations in exon 2 of TREM2, a gene involved in Nasu-Hakola disease, can cause frontotemporal dementia (FTD). Moreover, a rare TREM2 exon 2 variant (p.R47H) was reported to increase the risk of Alzheimers disease (AD) with an odds ratio as strong as that for APOEε4. We systematically screened the TREM2 coding region within a Belgian study on neurodegenerative brain diseases (1216 AD patients, 357 FTD patients, and 1094 controls). We observed an enrichment of rare variants across TREM2 in both AD and FTD patients compared to controls, most notably in the extracellular IgV-set domain (relative risk = 3.84 [95% confidence interval = 1.29-11.44]; p = 0.009 for AD; relative risk = 6.19 [95% confidence interval = 1.86-20.61]; p = 0.0007 for FTD). None of the rare variants individually reached significant association, but the frequency of p.R47H was increased ~ 3-fold in both AD and FTD patients compared to controls, in line with previous reports. Meta-analysis including 11 previously screened AD cohorts confirmed the association of p.R47H with AD (p = 2.93×10(-17)). Our data corroborate and extend previous findings to include an increased frequency of rare heterozygous TREM2 variations in AD and FTD, and show that TREM2 variants may play a role in neurodegenerative diseases in general.

Genetic variations underlying Alzheimer's disease: evidence from genome-wide association studies and beyond.

With the advent of genome-wide association studies (GWAS) and next-generation sequencing, more than 20 risk loci that affect Alzheimers disease have been identified. These loci are estimated to explain about 28% of the heritability of liability, 30% of familial risk, and over 50% of sibling recurrence risk of developing Alzheimers disease. These estimates are high in comparison with those for other complex diseases for which more risk loci have been discovered, such as type 2 diabetes, which is mostly a result of the strong effect of APOE ɛ4 and to a lesser extent the rare variant TREM2 p.Arg47His. The search for functionally relevant genetic variants in risk loci detected in GWAS has revealed that the genetic variations underlying Alzheimers disease include common variants affecting expression and splicing, a functional intragenic copy number variation, and rare pathogenic variants in risk loci, some of which might lead to familial Alzheimers disease. An understanding of the contribution of these variants to the development of Alzheimers disease has several clinical implications, including enhancing diagnostic accuracy and providing targets for the development of novel treatments.

Mutations in ABCA7 in a Belgian cohort of Alzheimer's disease patients: a targeted resequencing study

BACKGROUND ABCA7 was identified as a risk gene for Alzheimers disease in genome-wide association studies (GWAS). It was one of the genes most strongly associated with risk of Alzheimers disease in a Belgian cohort. Using targeted resequencing, we investigated ABCA7 in this cohort with the aim to directly detect rare and common variations in this gene associated with Alzheimers disease pathogenesis. METHODS We did massive parallel resequencing of ABCA7 after HaloPlex target enrichment of the exons, introns, and regulatory regions in 772 unrelated patients with Alzheimers disease (mean age at onset 74·6 years [SD 8·9]) recruited at two memory clinics in Flanders, Belgium, and 757 geographically matched community-dwelling controls (mean age at inclusion 73·9 years [8·0]). After bioinformatic processing, common variants were analysed with conditional logistic regression and rare variant association analysis was done in Variant Association Tools. To explore an observed founder effect, additional unrelated patients with Alzheimers disease (n=183, mean age at onset 78·8 years [SD 6·0]) and control individuals (n=265, mean age at inclusion 56·9 years [10·8]) from the same cohort who had not been included in massive parallel resequencing because of insufficient biosamples were screened for the ABCA7 frameshift mutation Glu709fs with Sanger sequencing. The effect of loss-of-function mutations on ABCA7 expression was investigated with quantitative real-time PCR in post-mortem brains of patients (n=3) and control individuals (n=4); nonsense mediated mRNA decay was investigated in lymphoblast cell lines from three predicted loss-of-function mutation carriers from the cohort of 772 patients with Alzheimers disease. FINDINGS An intronic low-frequency variant rs78117248 (minor allele frequency 3·8% in 58 patients with Alzheimers disease and in controls 1·8% in 28 controls) showed strongest association with Alzheimers disease (odds ratio 2·07, 95% CI 1·31-3·27; p=0·0016), and remained significant after conditioning for the GWAS top single nucleotide polymorphisms rs3764650, rs4147929, and rs3752246 (2·00, 1·22-3·26; p=0·006). We identified an increased frequency of predicted loss-of-function mutations in the patients compared with the controls (relative risk 4·03, 95% CI 1·75-9·29; p=0·0002). One frameshift mutation (Glu709fs) showed a founder effect in the study population, and was found to segregate with disease in a family with autosomal dominant inheritance of Alzheimers disease. Expression of ABCA7 was reduced in the two carriers of loss-of-function mutations found only in patients with Alzheimers disease (Glu709fs and Trp1214*) compared with four non-carrier controls (relative expression 0·45, 95% CI 0·25-0·84; p=0·002) and in lymphoblast cell lines from three carriers of Glu709fs compared with those from two non-carrier controls. INTERPRETATION We propose that a low-frequency variant can explain the association between ABCA7 and Alzheimers disease, and the evidence of loss-of-function mutations in this risk gene suggests that partial loss-of-function of ABCA7 could be a potential pathogenetic mechanism of Alzheimers disease. FUNDING Belgian Science Policy Office Interuniversity Attraction Poles program P7/16, Alzheimer Research Foundation, King Baudouin Foundation AB Fund, Methusalem Excellence Program initiative of the Flemish Government, Flanders Impulse Program on Networks for Dementia Research, Research Foundation Flanders, Agency for Innovation by Science and Technology Flanders, University of Antwerp Research Fund, and European Unions Seventh Framework Programme for Research, Technological development and Demonstration (AgedBrainSYSBIO).

A 22-single nucleotide polymorphism Alzheimer's disease risk score correlates with family history, onset age, and cerebrospinal fluid Aβ42

The ability to identify individuals at increased genetic risk for Alzheimers disease (AD) may streamline biomarker and drug trials and aid clinical and personal decision making.

Genome-Wide Association Interaction Analysis for Alzheimer’s Disease

We propose a minimal protocol for exhaustive genome-wide association interaction analysis that involves screening for epistasis over large-scale genomic data combining strengths of different methods and statistical tools. The different steps of this protocol are illustrated on a real-life data application for Alzheimers disease (AD) (2259 patients and 6017 controls from France). Particularly, in the exhaustive genome-wide epistasis screening we identified AD-associated interacting SNPs-pair from chromosome 6q11.1 (rs6455128, the KHDRBS2 gene) and 13q12.11 (rs7989332, the CRYL1 gene) (p = 0.006, corrected for multiple testing). A replication analysis in the independent AD cohort from Germany (555 patients and 824 controls) confirmed the discovered epistasis signal (p = 0.036). This signal was also supported by a meta-analysis approach in 5 independent AD cohorts that was applied in the context of epistasis for the first time. Transcriptome analysis revealed negative correlation between expression levels of KHDRBS2 and CRYL1 in both the temporal cortex (β = -0.19, p = 0.0006) and cerebellum (β = -0.23, p < 0.0001) brain regions. This is the first time a replicable epistasis associated with AD was identified using a hypothesis free screening approach.

Phenotypic characteristics of Alzheimer patients carrying an ABCA7 mutation.

Objective: To generate a clinical and pathologic phenotype of patients carrying rare loss-of-function mutations in ABCA7, identified in a Belgian Alzheimer patient cohort and in an autosomal dominant family. Methods: We performed a retrospective review of available data records, medical records, results of CSF analyses and neuroimaging studies, and neuropathology data. Results: The mean onset age of the mutation carriers (n = 22) was 73.4 ± 8.4 years with a wide age range of 36 (54–90) years, which was independent of APOE genotype and cerebrovascular disease. The mean disease duration was 5.7 ± 3.0 years (range 2–12 years). A positive family history was recorded for 10 carriers (45.5%). All patient carriers except one presented with memory complaints. The 4 autopsied brains showed typical immunohistochemical changes of late-onset Alzheimer disease. Conclusions: All patients carrying a loss-of-function mutation in ABCA7 exhibited a classical Alzheimer disease phenotype, though with a striking wide onset age range, suggesting the influence of unknown modifying factors.

Lymphoblast-derived integration-free iPS cell line from a 65-year-old Alzheimer's disease patient expressing the TREM2 p.R47H variant

Human lymphoblast cells from a male patient diagnosed with Alzheimers disease (AD) expressing the TREM2 p.R47H variant were used to generate integration-free induced pluripotent stem (iPS) cells employing episomal plasmids expressing OCT4, SOX2, NANOG, LIN28, c-MYC and L-MYC. The iPS cells retained the TREM2 mutation, and were defined as pluripotent based on (i) expression of pluripotent-associated markers, (ii) embryoid body-based differentiation into cell types representative of the three germ layers and (iii) the similarity between the transcriptomes of the iPS cell line and the human embryonic stem cell line H1 with a Pearson correlation of 0.966.