Alzheimer's disease (AD) is an age-related progressive neurodegenerative disorder that causes impairments in memory and thinking. The strongest genetic risk factor for AD is apolipoprotein E (APOE) genotype . In comparison to people who are homozygous for the common ε3 allele, people who carry the ε4 allele are at higher risk for AD and generally have an earlier age of onset, while people who carry the ε2 allele are at lower risk and have a later age of onset [2–6]. ApoE is a chaperone for amyloid-β (Aβ) peptide, which deposits in the brain and is thought to initiate a cascade of events that causes AD [7, 8]. Mouse models have shown that the time of onset and amount of Aβ deposition depends not only on APOE genotype but also on apoE levels. Interestingly, higher expression of mouse apoE increases the amount of Aβ deposition [9, 10], while higher expression of the human ε3 isoform of APOE knocked into the mouse Apoe locus decreases levels of amyloid deposition . Additionally, expression of human apoE in mice delays the onset of Aβ deposition in an isoform-specific fashion, with ε2 expression decreasing Aβ deposition the most and ε4 expression decreasing Aβ deposition the least [12, 13].
Despite evidence from animal studies suggesting that apoE levels affect Aβ deposition, there is no consensus regarding levels of apoE expression and its effects on Aβ deposition in human studies. The examination of whether apoE levels affect AD risk in humans has focused on APOE promoter polymorphisms. Over 50 studies listed on the Alzforum website tested for an association between AD and one or more polymorphisms within the APOE promoter . Meta-analyses on this website support the notion that APOE promoter variation is associated with risk for AD. However, it is unclear whether this association is due to linkage disequilibrium with the coding polymorphisms or whether there are independent effects on risk due to the level of APOE expression. Some studies have examined the effect of APOE promoter polymorphisms on APOE expression in vitro [15, 16]. More recently, allele specific gene expression has been used in post-mortem brain samples to measure the relative expression of APOE ε3 and ε4 isoforms . However, even these studies do not directly examine the effect of the promoter polymorphisms on levels of apoE protein.
Previous studies of CSF apoE levels in humans have reached varying conclusions. Some report that CSF apoE levels are lower in AD subjects than in control subjects [18–20], other studies find no association between CSF apoE levels and AD [21, 22], and one study shows that CSF apoE levels are higher in AD subjects than in control subjects . Multiple studies found that the APOE genotype was not associated with differing CSF apoE levels [19–22]. In contrast, plasma apoE levels are clearly dependent on APOE genotype [24, 25], which suggests that apoE is metabolized differently in the CSF and plasma. Gender and age do not appear to affect CSF apoE levels .
Recently, our laboratory and others reported that apoE levels were greatly reduced in mice lacking functional ATP-binding cassette A1 transporter (ABCA1) [26–28]. Within the CNS of ABCA1 knock-out mice, CSF apoE was 2% of normal levels and apoE in the cortex was 20% of normal levels . ABCA1 transfers cholesterol and phospholipids from the cell membrane to apolipoproteins (including apoE) to form nascent high density lipoproteins (HDL). In the rare case that both copies of ABCA1 are non-functional, as occurs in Tangier's disease, apoE and other lipoproteins do not receive normal amounts of lipid and are rapidly degraded . Multiple studies have shown that levels of plasma HDL-C and associated apolipoproteins are affected by single nucleotide polymorphisms (SNPs) in ABCA1 [30–34]. In particular, studies have implicated the following SNPs in affecting levels of plasma HDL-C: rs2230806 (R219K) , rs2066718 (V771M) [31, 32], rs2066715 (V825I) , rs4149313 (I883M) , rs2230808 (R1587K) . Since ABCA1 appears to have a similar role in the CNS and in the periphery, we hypothesized that these ABCA1 SNPs would also have an effect on CSF apoE levels since apoE is the major apoprotein component of HDL produced in the CNS. Additionally, studies by others have reported that the ABCA1 SNP rs2230806 (R219K) affects risk for AD [35–38]. This is particularly interesting because ABCA1 falls within a region of chromosome 9 that is linked to late-onset AD [39–43]. The profound effect of ABCA1 levels on CNS apoE levels in mice, in addition to reports that an ABCA1 SNP may affect risk for AD, suggested that ABCA1 may be involved in the genetic control of CNS apoE levels in humans.
Given the contrasting results and small sample sizes used in some studies of apoE levels in human CSF, we chose to begin our study by characterizing CSF apoE levels in a relatively large sample of 168 individuals with respect to AD status, APOE genotype, gender, race and age. We next examined whether ten ABCA1 SNPs, including five SNPs shown to affect plasma HDL-C, affected levels of apoE in the CSF. Finally, in a large sample of 1225 AD cases and 1431 controls, we attempted to replicate the previously reported association between the ABCA1 SNP rs2230806 and AD.