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The Role of ABCA7 in Alzheimer's Disease: Evidence from Genomics

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The Role of ABCA7 in Alzheimer's Disease: Evidence from Genomics Acta Neuropathologica https://doi.org/10.1007/s00401-019-01994-1 REVIEW The role of ABCA7 in Alzheimer’s disease: evidence from genomics, transcriptomics and methylomics Arne De Roeck1,2 · Christine Van Broeckhoven1,2 · Kristel Sleegers1,2 Received: 1 February 2019 / Revised: 18 March 2019 / Accepted: 18 March 2019 © The Author(s) 2019 Abstract Genome-wide association studies (GWAS) originally identifed ATP-binding cassette, sub-family A, member 7 (ABCA7), as a novel risk gene of Alzheimer’s disease (AD). Since then, accumulating evidence from in vitro, in vivo, and human-based studies has corroborated and extended this association, promoting ABCA7 as one of the most important risk genes of both early-onset and late-onset AD, harboring both common and rare risk variants with relatively large efect on AD risk. Within this review, we provide a comprehensive assessment of the literature on ABCA7, with a focus on AD-related human -omics studies (e.g. genomics, transcriptomics, and methylomics). In European and African American populations, indirect ABCA7 GWAS associations are explained by expansion of an ABCA7 variable number tandem repeat (VNTR), and a common pre- mature termination codon (PTC) variant, respectively. Rare ABCA7 PTC variants are strongly enriched in AD patients, and some of these have displayed inheritance patterns resembling autosomal dominant AD. In addition, rare missense variants are more frequent in AD patients than healthy controls, whereas a common ABCA7 missense variant may protect from disease. Methylation at several CpG sites in the ABCA7 locus is signifcantly associated with AD. Furthermore, ABCA7 contains many diferent isoforms and ABCA7 splicing has been shown to associate with AD. Besides associations with disease status, these genetic and epigenetic ABCA7 markers also showed signifcant correlations with AD endophenotypes; in particular amyloid deposition and brain morphology. In conclusion, human-based –omics studies provide converging evidence of (partial) ABCA7 loss as an AD pathomechanism, and future studies should make clear if interventions on ABCA7 expression can serve as a valuable therapeutic target for AD. Keywords ATP-binding cassette, sub-family A, member 7 (ABCA7) · Alzheimer’s disease · Meta-analysis · Loss-of- function · Endophenotype · Rare variants · Variable number tandem repeat (VNTR) Introduction AD pathology hypothesis, which postulates that amyloid β (Aβ) accumulation leads to synaptic defcits and tau aggre- Dementia—and in particular Alzheimer’s disease (AD), gation which in turn causes neuronal death. While early tau which makes up 50–70% of dementia patients—is one of pathology in the entorhinal and limbic system can manifest the major medical challenges of our time. There is a dire without any Aβ, severe neocortical AD-like tau pathology need for drugs to prevent, delay the onset, slow the progres- in the form of neurofbrillary tangles and neuritic plaques is sion, or treat the (symptoms of) disease. So far, AD drug not seen in absence of Aβ, indicating that Aβ is crucial for development has been unusually difcult. Most eforts have the development of clinically overt dementia [1]. Among focused on interventions in the amyloid cascade, the main other brain regions, pathological hallmarks often originate in the hippocampus, a brain region involved in the genera- tion of memories. As a consequence, short-term memory * Kristel Sleegers [email protected] loss is often one of the frst and principal symptoms of AD [2]. Nevertheless, there is increasing awareness that current 1 Neurodegenerative Brain Diseases Group, VIB Center attempts to intervene in the amyloid cascade are insuf- for Molecular Neurology, University of Antwerp, CDE, cient. To overcome these issues, several eforts are needed, Universiteitsplein 1, 2610 Antwerp, Belgium which include, but are not limited to: new therapeutic tar- 2 Biomedical Sciences, University of Antwerp, Antwerp, gets and better clinical trial design through development of Belgium Vol.:(0123456789)1 3 Acta Neuropathologica biomarkers with high predictive value in the pre-dementia insights into the molecular features of AD. Many of these stage, readily accessible biomarkers, and better markers for novel fndings trace back to ABCA7. In this review, we will reliable and timely diagnosis [1]. frst briefy discuss the main in vitro and in vivo fndings, Generally, when studying the molecular biology of AD, followed by a comprehensive assessment of human-based it is not trivial to diferentiate the disease-causing pathways ABCA7 studies. from complementary processes. Identifcation of a gene with AD-associated variants, however, provides certainty about ABCA7, evidence from in vitro and in vivo models an early pathological role of that gene which forms a poten- tially interesting target for drugs. Additionally, the variants The ABCA7 gene consists of 47 exons and encodes a com- themselves are easily assessed using peripheral tissue (e.g. plete transmembrane transporter protein of 2146 amino acids blood) and provide early markers which can be useful for with a molecular weight of 220 kDa [11]. As a typical ABC prognosis and diagnosis. AD is split into two categories transporter, ABCA7 contains two intracellular nucleotide- based on the disease onset age: early-onset AD (EOAD) binding domains (NBD) with conserved Walker A and B and late-onset AD (LOAD), respectively, corresponding to motifs for ATP hydrolysis, two transmembrane domains, and patients with an onset age younger or older than 65 years of two extracellular loops (Fig. 1) [12]. A detailed ABCA7 age. By studying rare autosomal dominant forms of EOAD, structure (e.g. obtained through X-ray crystallography or three genes (APP, PSEN1, and PSEN2) were found to harbor cryogenic electron microscopy), however, is currently not causal mutations. All of them are directly involved in the available [RCSB PDB id: Q8IZY2, rcsb.org, accessed Jan- production of Aβ [3]. Most AD cases, however, are caused uary 2019] [13]. Under physiological conditions, ABCA7 by a multifactorial etiology. APOE, a gene with pleiotropic is expressed in brain tissue at a generally low abundance, efects, was identifed as the frst common genetic risk factor which is comparable to other human tissues and cell types with high penetrance in EOAD and LOAD. Subsequently, [Genotype-Tissue Expression (GTEx) project, accessed genome-wide association studies (GWAS) have been con- January 2019]. Two studies examining ABCA7 expres- ducted on large cohorts of unrelated LOAD patients and sion in mice brain, or human primary neurons from fetal cognitively healthy controls. This led to the identifcation brain, respectively, denoted the principal ABCA7 expressing of common genetic variants with low AD-associated efect cell type as neurons [14] or microglia [15]. More recently sizes in more than 20 loci. These common variants, however, though, with the advent of large-scale single-cell sequenc- only provide indirect association signals, and to understand ing projects, ABCA7 expression was observed to be rela- the biology and clinical applicability, additional research tively comparable across all diferent cell types in mouse is needed [4]. One of the loci with the highest post-GWAS brain [16]. A similar experimental set-up with human brain, research success rate is the ATP-binding cassette, sub-family unfortunately, did not detect sufcient ABCA7 RNA levels A, member 7 (ABCA7) gene, with the identifcation of both to determine cell type-specifc expression [17]. common risk variants with a direct functional consequence In analogy to ABCA1—the closest homolog to ABCA7 on ABCA7, as well as rare coding variants of intermediate and a more extensively studied protein—ABCA7 is pre- to high penetrance providing compelling evidence of the dicted to regulate lipid metabolism. In vitro studies show involvement of ABCA7 in AD risk. that both ABCA1 and ABCA7 conduct apolipoprotein A Further substantiation of the downstream molecular (apoA)-I mediated export of cholesterol and phospholip- efects of ABCA7 and the role of ABCA7 in AD was initially ids, but with diferent preferred substrates and efciencies obtained through in vitro and in vivo studies in model sys- [19–22]. Additionally, ABCA7 expression is upregulated tems, and this has been the main topic of prior reviews on when cholesterol is depleted via the sterol regulatory ele- ABCA7 [5–9]. While these in vitro and in vivo fndings have ment-binding protein 2 (SREBP2) pathway, which has an been crucial to our understanding of ABCA7, they need to inverse regulatory efect on ABCA1 [23]. Furthermore, be translated to humans. No in vitro or in vivo model fully in vivo Abca7 knockout mice have shown lower serum levels captures the entire AD spectrum, and in the past, this has led of cholesterol and high-density lipoprotein (HDL) [14], an to a lack of concordance between animal models and human altered phospholipid profle in the brain [24], and disruption clinical trials [10]. While research based on human indi- of lipid rafts in thymocytes and antigen-presenting cells [25]. viduals has several practical limitations, they are paramount When analyzing Abca7 defciency in mouse primary mac- to understand the contribution and therapeutic potential for rophages, however, no efect on cholesterol and phospholipid risk factors, such as ABCA7. Fortunately, many large-scale efux was observed [14]. Hence, while a role of ABCA7 in collaborations combining AD patient cohorts and bioma- lipid metabolism seems
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    MAF aa/Aa/AA aa/Aa/AA GENE POSITION cDNA change Aa change rs ID ExAC MA SIFT POLYPHEN P-VALUE OR cases-ctrls cases ctrls 0.04669- ABCA7 19:1043103 c.G643A p.G215S rs72973581 0.04316 A Tolerated Benign 0/31/301 1/96/579 0.026 0.615 0.07249 0.003012- ABCA7 19:1050996 c.G2629A p.A877T rs74176364 0.01692 A Deleterious Benign 0/2/330 0/16/660 0.072 0.25 0.01183 0.01506- ABCA7 19:1059056 c.G5435A p.R1812H rs114782266 0.01057 A Tolerated Benign 0/10/322 0/11/665 0.162 1.87 0.008136 0.006024- ABCA7 19:1057343 c.G4795A p.V1599M rs117187003 0.003085 A Deleterious Probably damaging 0/4/328 0/3/673 0.226 2.73 0.002219 0.01657- ABCA7 19:1047537 c.A2153C p.N718T rs3752239 0.07028 C Deleterious Benign 0/11/321 0/33/641 0.325 0.665 0.02448 ABCA7 19:1043794 c.G1001A p.R334Q rs147846250 0.001506-0 0.0001995 A Tolerated Benign 0/1/331 0/0/676 0.329 Inf ABCA7 19:1044619 c.C1091G p.P364R rs146982710 0.001506-0 0.0003843 G Tolerated Possibly damaging 0/1/331 0/0/676 0.329 Inf ABCA7 19:1046239 c.C1456G p.P486A rs141428162 0.001506-0 0.0003212 G Deleterious Possibly damaging 0/1/331 0/0/676 0.329 Inf ABCA7 19:1047372 c.G2062A p.A688T rs376686030 0.001506-0 A Tolerated Possibly damaging 0/1/331 0/0/676 0.329 Inf ABCA7 19:1053521 c.C3414G p.S1138R rs1053521 0.001506-0 0.00002865 G Deleterious Possibly damaging 0/1/331 0/0/676 0.329 Inf ABCA7 19:1056372 c.T4460G p.V1487G rs200825702 0.001506-0 0.0001245 G Deleterious Probably damaging 0/1/331 0/0/676 0.329 inf ABCA7 19:1056941 c.G4622A p.C1541Y rs145632609 0.001506-0 0.00004131 A Deleterious Probably damaging 0/1/331
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