*4.1. AD Pathology*

AD is a neurodegenerative disease and the number one leading cause of dementia in the elderly [43]. Most AD patients show a sporadic form of the disease (late-onset AD, LOAD) whereas almost 1% of them show a familial early-onset form (FAD), with Mendelian inheritance (<60 years). Nevertheless, these two forms of AD are both characterized by accumulation of amyloid-β (Aβ) peptides and hyperphosphorylated tau protein, both

provoking neuronal loss mainly in the hippocampus and cortex. Major AD symptoms are thus memory loss and behavioral complications [8].

Links between Aβ peptides and tau are still elusive despite a large number of studies, but it is now widely accepted that Aβ peptide deposition precedes the tau hyperphosphorylation by several decades [44]. In addition, FAD patients possess mutations in genes involved in Aβ peptide production, as, for example, in amyloid beta precursor protein (APP) sequence and in the enzyme responsible for APP cleavage. For this reason, it is compulsory to better understand the early molecular and cellular events controlling Aβ peptide synthesis and clearance, thus leading to the early step of the cerebral Aβ peptide accumulation.

All cells in the body are able to synthesize these peptides from APP but neurons remain the major sources of Aβ peptides [7]. APP is cleaved by at least three types of proteases (α-, β- and γ-secretases). To date, two processing pathways have been described—the amyloidogenic pathway generates Aβ peptides via β-secretase, whereas the anti-amyloidogenic pathway is initiated by α-secretase and prevents Aβ peptide generation. Interestingly, cellular cholesterol content can strongly influence these pathways and modulate Aβ synthesis, as discussed below and reviewed elsewhere [45].

Cerebral pools of Aβ peptides are cleared by several mechanisms, including phagocytosis processes occurring at the microglial cell level, and elimination across the blood–brain barrier (BBB). BBB physiology and functioning are summarized elsewhere and exhaustive reviews can be consulted for more details [7,10,46]. In brief, this physical and metabolic barrier is composed of the endothelial cells lining the brain microvessels. These cells are sealed together by tight junctions (TJs) in relation with adherent (AJs) and gap (GJ) junctions. Importantly, BBB endothelial cells also express a large panel of ABC transporters involved in the cerebral efflux of Aβ peptides but also restricting their entry to the CNS. Thus, a large body of evidence has confirmed in humans and in several animal models the involvement of P-gp (ABCB1), BCRP (ABCG2), MRP1 (ABCC1) and more recently ABCG4 (reviewed in [4,7]).

Noteworthy, some studies provide strong evidence that a decrease of amyloid clearance across the BBB or a decrease of phagocytosis by microglial cells might be involved in AD rather than an Aβ peptide overproduction by cleavage of the APP [47]. When analyzed in mouse models of AD and in patients, expression of ABCB1, ABCC1 and ABCG2 at the BBB are decreased. Restoration of these expressions increases amyloid clearance across the BBB, but also decreases the entry of peripheral Aβ peptides into the CNS, and thus reduces the brain Aβ burden and alleviates cognitive dysfunction [7,48–51]. However, despite these promising results, there is currently no cure for AD.

Because AD is also closely linked to the cholesterol metabolism and because the brain is one of the most cholesterol-rich organs of the body, researchers also focused their attention on ABCA1. In transgenic mice models, altering the cerebral cholesterol homeostasis by overexpressing or downregulating *Abca1* expression decreases or increases Aβ peptide synthesis and deposition, respectively [52–54].

It has also been largely demonstrated that environmental factors such as diet or lack of activity can promote AD onset. For example, animals fed with cholesterol-rich diets show increased production of Aβ peptides, whereas the use of statins decrease, in vivo and in vitro, this synthesis and deposition processes [45,55]. In addition, genetic factors are also largely involved as suggested since the early 2010s with the discovery of several gene polymorphisms in AD by GWAS. These studies confirmed that *ApoE4*, identified in the early 1990s, and is an important component of the low-density lipoproteins (LDL), remains the most strongly associated allele to AD. However, interestingly, almost 20 other genes have been identified, among them ABCA7, for which very little information in relation to its physiological and cellular functions has been identified so far.

Since 2011, genetic polymorphisms of *Abca7* have been reported in several dozens of genetic studies using samples coming from AD patients worldwide (reviewed in [56,57]). Further investigations of several of these variants demonstrated that they are responsible

for an alteration of ABCA7 expression in AD brains. While a protective ABCA7 allele was identified, deleterious alleles are responsible for protein loss-of-function or ABCA7 downregulation or upregulation [56–58]. These ABCA7 loss-of-function mutations increase risk of AD by 80% in African ancestry populations and, risk of early AD onset by 100% to 400% in European ancestry populations [59–61]. Table 1 summarizes the AD-associated ABCA7 epigenetic and genetic variations, but also the available data obtained in patients regarding the amyloid and tau pathology as well brain morphology and clinical symptoms.


**Table 1.** AD-associated ABCA7 (epi-)genetic variation and their effects in individuals.

Variants and CpG methylation sites in ABCA7 with their interpreted functional effect, and their association with AD and brain morphology and clinical symptoms when investigated. Adapted from [57]. AD: Alzheimer's disease; GWAS: Genome-wide association studies; PTC: premature termination codon; SNP: single nucleotide polymorphism; VNTR: variable number tandem repeat.
