Elsevier

Journal of Psychiatric Research

Volume 92, September 2017, Pages 74-80
Journal of Psychiatric Research

DNA methylation changes at TREM2 intron 1 and TREM2 mRNA expression in patients with Alzheimer's disease

https://doi.org/10.1016/j.jpsychires.2017.04.003Get rights and content

Abstract

Objectives

Recent genome-wide association studies revealed that Triggering receptor expressed on myeloid cells 2 (TREM2) was associated with Alzheimer's disease (AD) and other neurodegenerative diseases. We previously reported that TREM2 mRNA is highly expressed in leukocytes of AD patients compared to those in healthy controls. However, the mechanism of TREM2 expression change is still not known. In this study, we examined the involvement of the DNA methylation status of TREM2 in its high gene expression.

Materials and methods

Fifty AD subjects and age- and sex-matched control subjects were recruited (25 males, 25 females; 79.9 ± 5.27 and 79.4 ± 3.92 years old, respectively). TREM2 mRNA expression and the percentage of DNA methylation at four CpG sites in intron 1 of TREM2 were studied using their peripheral leukocytes.

Results

We confirmed that TREM2 mRNA expression in leukocytes was significantly higher in AD patients than in controls (p = 0.007). The percentage methylation at three CpG sites in TREM2 intron 1 was significantly lower in AD subjects than in control: CpG1, 9.4 ± 3.2 vs 11.9 ± 4.0 (p = 0.001); CpG2, 15.4 ± 4.9 vs 19.1 ± 4.8 (p = 0.001); CpG3, 20.8 ± 5.5 vs 25.5 ± 5.4 (p < 0.001); and the average percentage methylation of all CpG sites: 13.5 ± 3.7 vs 16.1 ± 3.8 (p = 0.002), respectively. In addition, there were significant negative correlations between TREM2 mRNA expression and the percentage DNA methylation of each of CpG sites (CpG1, r = −0.416, p < 0.001; CpG2, r = −0.510, p < 0.001; CpG3, r = −0.504, p < 0.001; CpG4, r = −0.356, p < 0.001).

Conclusions

Lower DNA methylation at TREM2 intron 1 caused higher TREM2 mRNA expression in the leukocytes of AD subjects versus controls and may be a biomarker for AD.

Introduction

Alzheimer's disease (AD) is an irreversible neurodegenerative disease that is the most common cause of dementia and that is clinically characterized by progressive loss of the ability of learning and memory. In 2015, approximately 46.8 million people were diagnosed with AD in the world and this number is expected to increase to 131.5 million by 2050 (Prince et al., 2015).

AD is histopathologically characterized by extracellular amyloid plaques that are primarily composed of amyloid beta (Aβ) peptides and intercellular neurofibrillary tangles, which result from accumulation of the hyper-phosphorylated microtubule-associated protein tau (Castellani et al., 2010). Previous reports indicated that extracellular deposits of Aβ peptides are the principal cause of AD onset (Hardy and Higgins, 1992, Tanzi, 2005, De Strooper, 2010) and that the innate immune function of microglia could prevent Aβ accumulation (Wang et al., 2015).

It was recently reported that a functional single nucleotide polymorphism (rs75932628) within Triggering receptor expressed on myeloid cells 2 (TREM2) is associated with AD (Guerreiro et al., 2013). Homozygous loss of function mutations in TREM2 are also associated with an autosomal recessive form of early onset dementia, presenting with bone cysts and consequent fractures called polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, or Nasu-Hakola disease (Paloneva et al., 2003). TREM2 activates the innate immune response in macrophages and dendritic cells (N'Diaye et al., 2009). In the brain, TREM2 is expressed in microglia and promotes phagocytosis of apoptotic neurons, cellular debris, and misfolded proteins by the recognition of specific endogenous ligands on the surface of apoptotic cells (Jonsson and Stefansson, 2013). Subsequently, TREM2 delays the inflammatory response by suppressing microglial cytokine production (Hsieh et al., 2009). TREM2 deficiency augmented Aβ accumulation and neuronal loss in a mouse model of AD (Wang et al., 2015). In addition, higher TREM2 mRNA expression was predominantly associated with the increase of phosphorylated tau in AD patient brains (Lue et al., 2015).

Recently, peripheral leukocytes were reported to be indicated in AD pathogenesis (Zenaro et al., 2015). Gene expression and DNA methylation changes in leukocytes have been observed in neuropsychiatric conditions including AD (Yamazaki et al., 2016, Yoshino et al., 2016a, Yoshino et al., 2016b, Yoshino et al., 2016c, Funahashi et al., 2016). DNA methylation, a type of epigenetic modification, plays a key role in regulating gene expression (Abdolmaleky et al., 2004). Although we reported higher TREM2 mRNA expression in the leukocytes of AD subjects (Mori et al., 2015), methylation rates of TREM2 have not yet been examined.

In the present study, we examined the methylation rates of the TREM2 intron 1 in leukocytes of AD subjects and investigated the relationship between these methylation rates and TREM2 mRNA expression.

Section snippets

Samples

We recruited 50 AD subjects (n = 50; 25 males, 25 females; age 79.9 ± 5.27 years) and 50 age- and sex-matched healthy controls (n = 50; 25 males, 25 females; age 79.4 ± 3.92 years) from Ehime University Hospital and related community hospitals. Diagnosis of AD was made according to the Diagnostic and Statistical Manual of mental Disorders (DSM-5) by expert psychiatrists based on extensive clinical interviews and a review of medical records. AD subjects were assessed by the Mini-Mental State

TREM2 mRNA expression level and DNA methylation rates in AD and control subjects

TREM2 mRNA expression in the leukocytes of AD subjects was significantly higher than that in control subjects (p = 0.007, Fig. 2). Percentage methylation of CpG sites in TREM2 intron 1 in AD subjects was significantly lower than that in control subjects after Bonferroni corrections (p < 0.0125): CpG1, 9.4 ± 3.2 vs. 11.9 ± 4.0 (p = 0.001); CpG2, 15.4 ± 4.9 vs. 19.1 ± 4.8 (p = 0.001); CpG3, 20.8 ± 5.5 vs. 25.5 ± 5.4 (p < 0.001); and the average percentage methylation of all CpG sites: 13.5 ± 3.7

Discussion

This is the first study showing TREM2 DNA hypomethylation in AD subjects and significant correlation between TREM2 mRNA expression and TREM2 DNA methylation rates of intron 1 in peripheral leukocytes of AD subjects. Furthermore, we could replicate our preceding study with larger sample showing the elevated TREM2 mRNA expression in AD subjects compared to control subjects (Mori et al., 2015). In the present study, there are three major findings.

First, we confirmed that TREM2 mRNA expression in

Acknowledgments

We wish to thank Ms. Chiemi Onishi for technical assistance. This work was partially supported by a Health and Labor Science Research Grant from the Japanese Ministry of Health, Labour and Welfare and a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology, JSPS KAKENHI Grant Numbers 15K09808 and 16K21207.

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