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

Abstract

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 Alzheimer's 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.

Introduction

Current high-throughput sequencing technologies such as whole-exome and whole-genome sequencing enable the identification of rare genetic causes of disease. Using a whole-exome sequencing approach, homozygous mutations in the gene encoding triggering receptor expressed on myeloid cells 2 (TREM2; 6p21.1) were identified as a cause of behavioral variant frontotemporal dementia (FTD) in 3 consanguineous Turkish families (Guerreiro et al., 2012). TREM2 is a receptor of the innate immune system, expressed on the cell membrane of myeloid cells, especially on immature dendritic cells, microglia, and osteoclasts (Colonna 2003). Homozygous loss-of-function mutations in TREM2 (e.g., p.Q33X) were previously found in patients with Nasu-Hakola disease, also known as polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) (Klunemann et al., 2005, Paloneva et al., 2002, Soragna et al., 2003), a rare recessively inherited disease that is characterized by early-onset progressive dementia and bone cysts. In addition, a homozygous deletion affecting the consensus donor splice site in intron 1 has been reported in a Lebanese family with early-onset dementia, clinically resembling behavioral FTD, without bone involvement (Chouery et al., 2008).

More recently, an increased frequency of rare heterozygous TREM2 variations was detected in Alzheimer's disease (AD) patients in 2 independent studies (Guerreiro et al., 2013, Jonsson et al., 2013), suggesting that heterozygous TREM2 variations are risk factors for dementia with onset later in life. In line with this, 2 heterozygous carriers of a TREM2 loss-of-function mutation in a PLOSL family were reported to have subclinical evidence of impaired visuospatial memory and a selective metabolic deficit in the basal ganglia on functional neuroimaging compared to their homozygous wild-type relatives who presented normal neuropsychological and neuroimaging findings (Montalbetti et al., 2005).

Remarkable in AD patients was the increased genetic diversity across exon 2 of the gene compared to that in control individuals. Interestingly, 3 mutations were previously found in a homozygous state in context of FTD or PLOSL (p.T66M, p.Y38C and p.Q33X) (Guerreiro et al., 2012, Guerreiro et al., 2013, Soragna et al., 2003). One rare variant in particular, p.R47H (rs75932628), showed a strong association with late-onset AD in 2 studies (p = 9.0×10⁻⁹ (Guerreiro et al., 2013); p = 3.42×10⁻¹⁰) (Jonsson et al., 2013). The effect size of this variation was as high as APOE ε4 allele, although it occurs at much lower frequency (minor allele frequency [MAF] <1%). This association was confirmed in a Spanish (Benitez et al., 2013) and a French (Pottier et al., 2013) population including both late- and early-onset AD patients, focusing on TREM2 exon 2, and a similar trend was observed on imputed genotype data for p.R47H in a study population originating from the United States, United Kingdom, and Europe (Giraldo et al., 2013).

TREM2 is a membrane protein that forms a receptor-signaling complex with protein tyrosine kinase binding protein. This complex is involved in the immune response and activation of macrophages/microglia and dendritic cells, resulting in phagocytosis and elevated short-term production of reactive oxygen species (Neumann and Daly, 2012, Paloneva et al., 2002). TREM2 exon 2, in which both p.R47H and several homozygous FTD mutations were observed, encodes both the signal peptide and part of the extracellular domain containing an IgV-set domain. Hypothetically, non-synonymous variations in TREM2 may lead to a disturbed immune response with extensive inflammation or defective microglial survival or function, thus contributing to the neurodegenerative process (Guerreiro et al., 2013, Otero et al., 2009). Nevertheless, because studies on AD have mostly focused on TREM2 exon 2, the frequency and relevance of mutations outside of exon 2 remains unclear. Of note, 2 homozygous mutations outside of exon 2 have previously been detected in families segregating autosomal recessive behavioral variant FTD (Chouery et al., 2008, Giraldo et al., 2013).

In the present study, we extend previous reports by systematically investigating the entire TREM2 coding region for the contribution of rare variations to the occurrence of both AD and FTD in an extensive prospective study population of Belgian dementia patients (n = 1216 AD patients, n = 357 FTD patients) and non-affected individuals (n = 1094).