R47H Variant of TREM2 Associated With Alzheimer Disease in a Large Late-Onset Family: Clinical, Genetic, and Neuropathological Study

TREM2 upregulation correlates with 5-hydroxymethycytosine enrichment in Alzheimer's disease hippocampus

Background

Recent genome-wide association studies revealed TREM2 rs75932628-T variant to be associated with Alzheimer’s disease (AD) and other neurodegenerative diseases. However, the role that TREM2 plays in sporadic AD is largely unknown. Our aim was to assess messenger RNA (mRNA) expression levels and DNA methylation profiling of TREM2 in human hippocampus in AD brain. We measured TREM2 mRNA levels in the hippocampus in a cohort of neuropathologically confirmed controls and pure AD cases showing no other protein deposits than β-amyloid and phosphorylated tau. We also examined DNA methylation levels in the TREM2 transcription start site (TSS)-associated region by bisulfite cloning sequencing and further extended the study by measuring 5-hydroxymethycytosine (5hmC) enrichment at different regions of TREM2 by 5hmC DNA immunoprecipitation combined with real-time qPCR.

Results

A 3.4-fold increase in TREM2 mRNA levels was observed in the hippocampus of AD cases compared to controls (p = 1.1E-05). Interestingly, TREM2 methylation was higher in AD cases compared to controls (76.2 % ± 15.5 versus 57.9 % ± 17.1; p = 0.0016). Moreover, TREM2 mRNA levels in the AD hippocampus correlated with enrichment in 5hmC at the TREM2 gene body (r = 0.771; p = 0.005).

Conclusions

TREM2 mRNA levels are increased in the human hippocampus in AD cases compared to controls. DNA methylation, and particularly 5hmC, may be involved in regulating TREM2 mRNA expression in the AD brain. Further studies are guaranteed to investigate in depth the role of 5hmC in AD and other neurodegenerative disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s13148-016-0202-9) contains supplementary material, which is available to authorized users.

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.

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.

Perturbation of the transcriptome: implications of the innate immune system in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease with broad impact. Although Aβ and tau have been proposed as the key molecules in the disease mechanism, comprehensive understanding of AD pathogenesis requires a systemic view at the genomic level. From studies on the brain transcriptome of AD, we have gradually realized the contribution of the immune system to AD development. Recent explorations on the blood transcriptome of AD patients have revealed robust immune activation in the peripheral blood. The combination of transcriptome studies and other types of studies has further elucidated the roles of specific immune pathways in distinct cell types during AD development and highlighted the critical contributions from immune genes such as TREM2.

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.

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.