Total Cholesterol and APOE -Related Risk for Alzheimer’s Disease in the Alzheimer’s Disease Neuroimaging Initiative

INTRODUCTION

Alzheimer’s disease (AD), the most common cause of dementia [1], is the sixth leading cause of death in the United States [1, 2]. Despite decades of research, treatments currently available primarily offer symptom management rather than significant disease modification or reversal [3, 4]. A shift in focus from AD reversal to prevention may prove more successful in lowering AD incidence. Literature suggests that vascular risk factors, some of which are modifiable, may be important contributors to AD [5–8]. Given that AD pathology begins years prior to symptom presentation [3, 4], further research into relationships between modifiable vascular biomarkers and AD is critical to progress in AD risk reduction and disease prevention.

The Apolipoprotein E (APOE) gene is the best-known genetic risk factor for AD [4, 9]. Mechanisms underlying this genetic risk, however, remain poorly understood. Structural differences between apolipoprotein E (ApoE) isoforms coded by APOE ɛ2, ɛ3, and ɛ4 alleles correspond to different binding affinities of ApoE to lipoprotein receptors and amyloid-β (Aβ) [9–11]. The E4 isoform has a higher binding affinity to larger lipoprotein receptors and a lower binding affinity to Aβ than E2 and E3 isoforms, contributing to altered lipid metabolism and impaired Aβ clearance in APOE ɛ4 carriers [9–15]. Aβ and tau accumulations are the neuropathological hallmarks of AD [3, 16–18]. Aβ first aggregates between neurons, which in turn facilitates deposition of hyperphosphorylated tau protein in the form of neurofibrillary tangles within neurons [9, 18].

In addition to the impaired ability of APOE ɛ4 carriers to clear Aβ buildup, the role of APOE in cholesterol metabolism may relate to AD risk. APOE-related differences in cholesterol metabolism have been linked to differences in blood lipids—ɛ4 carriers tend to have the highest LDL and total blood cholesterol levels, while ɛ2 carriers have the lowest [11–13, 20]. Higher total and LDL cholesterol both in midlife [21–26] and late life [24, 27–29] have been associated with mild cognitive impairment (MCI) and AD. Literature also suggest that AD patients have higher LDL and total cholesterol, and lower HDL cholesterol compared to controls [13, 28–32]. High LDL and total cholesterol have also been associated with impaired global cognition [33–35] and greater Aβ pathology [30,36, 30,36]. However, literature on blood cholesterol and AD is equivocal [26, 37–39] and the mechanisms underlying this relationship remain elusive.

The purpose of the current study is to investigate total cholesterol as a potential mediator underlying APOE AD risk in a large sample of adults who participated in the Alzheimer’s Disease Neuroimaging Initiative. First, we aim to determine whether APOE ɛ4 carriers in our sample have higher total cholesterol compared to non-ɛ4 carriers, as should be the case based on the existent literature. We then examine whether total cholesterol is higher among those diagnosed with AD or MCI compared to controls. Finally, we investigate potential causality between total cholesterol and AD employing Mendelian randomization, using APOE as a genetic instrument.

MATERIALS AND METHODS

RESULTS

DISCUSSION

In agreement with the literature, we report significantly higher total cholesterol in APOE ɛ4 allele carriers compared to non-carriers [11–13, 45], and in those with AD and MCI diagnosis compared to the control group [27–29, 33]. Additionally, those with late, but not early, MCI had significantly higher total cholesterol compared to the non-demented control group. Further research is necessary to determine whether increasing total cholesterol values we observed across progression stages toward AD might contribute to the development of AD, or whether advancing AD pathology is causative of higher cholesterol, although the results of MR analysis suggest it may be the former. Mendelian randomization revealed higher risk for AD, late MCI, and early MCI in relation to higher total cholesterol levels in APOE4+ compared to APOE3. Collectively, our results suggest that high total cholesterol levels may contribute to AD risk and, furthermore, may be one mechanism through which APOE influences AD risk.

Traditionally, it was believed that cholesterol in the brain is produced locally and does not seem to relate directly to peripheral cholesterol levels [17, 55–58]. However, recent evidence suggests that cholesterol in the periphery may influence brain cholesterol levels through conversion into 27-hydroxycholesterol (27-OH), an oxysterol which can cross the blood-brain barrier and enter the brain from the periphery [17, 55–57]. Once in the brain, 27-OH promotes Aβ production and accumulation [17, 56] and may act as a critical link between unhealthy blood lipid profiles and AD. Production of 27-OH appears to increase in states of high blood cholesterol and following consumption of cholesterol-containing foods in which the cholesterol has already been oxidized [55, 56]. Cholesterol also accumulates in Aβ plaques and brain cholesterol levels positively correlate with AD severity, both of which may occur in part due to AD-related impairments in blood-brain barrier integrity [17, 58–60]. The modifiable nature of blood lipids through pharmaceutical and lifestyle approaches may therefore present an important opportunity to affect AD risk.

Blood lipids can be managed and are modifiable through pharmacological and lifestyle interventions [13, 61–82]. Aside from statins and other cholesterol-lowering medications, lifestyle interventions reported to be effective in maintaining optimal lipid profiles include minimal consumption of foods containing saturated fat, trans fat, and cholesterol, as well as exercise and stress management [13, 64–72]. Specific dietary approaches that may help establish healthy cholesterol levels and potentially also influence dementia risk include Dietary Approach to Stop Hypertension (DASH) [73], Mediterranean [74–76], Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND; a combination of Mediterranean and DASH diets) [77], Okinawan [78], and plant-based diets [68, 79–82]. All of these diets involve lower consumption of cholesterol, saturated fat, and trans fat compared to Western diets given their emphasis on plant foods and healthy fats while reducing or avoiding intake of red meat, high-fat dairy products, and fried and processed foods [68, 73–82].

Greater AD risk in relation to consumption of saturated fat, trans fat, and cholesterol has been previously established [13, 83–89]. Population studies also report that developing, non-Western countries have very low rates of AD in APOE ɛ4 and non-ɛ4 carriers alike, but that these rates are expected to rise—possibly due to increasing incorporation of the Western diet which is rich in cholesterol, saturated fat, and trans fat [13, 90–93]. Furthermore, blood cholesterol levels in APOE ɛ4 allele carriers appear to be more responsive to changes in cholesterol and fat consumption compared to non-carriers [71, 72], suggesting that AD risk conferred by APOE may be at least partially modifiable. While the literature on statin use and dementia risk is currently inconclusive [94–98], perhaps lifestyle interventions alone or in combination with pharmacological treatments may achieve greater success. It is exciting to think that such interventions could potentially positively affect AD risk in addition to beneficial consequences on health in general.

This study is not without limitations. A significantly greater proportion of participants in late MCI and AD groups were on cholesterol-lowering medication compared to controls (Table 1). Nevertheless, this would only have biased our results toward the null. Further, LDL levels tend to be higher in relation to both APOE4 and AD risk [13, 30–32], which we could not investigate because LDL cholesterol is not measured in ADNI. An additional research question that we did not explore in the current study due to the lack of data is whether duration of elevated blood cholesterol levels is important. The sample is not representative of the general population considering that ADNI participants are highly educated and predominantly white, with a selection bias toward MCI and AD diagnoses (74%). The distribution of APOE genotypes in this sample also differs from that found in the general population, although this is not entirely surprising given that the majority of participants have been selectively recruited for their MCI or AD diagnosis. The estimated global frequencies of the APOE ɛ2, ɛ3, and ɛ4 alleles are approximately 8%, 78%, and 14%, respectively [9]. Our sample includes fewer ɛ3 (45.47%) and more ɛ4 (46.94%) allele carriers in comparison. Finally, while our overall sample size is on the smaller end of the spectrum for MR analyses, all of our significant findings exceed an F-statistic of 10, indicating that APOE is a sufficiently strong genetic instrumental variable for use in MR [51]. Existing reports of associations and potential causality between APOE, total cholesterol, and AD further justify use of MR in our approach [9, 99].

This should not undermine the unique strengths of the current study, including a large sample of participants from across both the U.S. and Canada. Our sample also has additional diagnostic information regarding early versus late stage MCI, which is rarely available. Further, MR provides important information regarding potential causality of associations between total cholesterol, APOE genotype, and AD risk. Given the lack of understanding of mechanisms underlying APOE’s conferral of AD risk, a key contribution of our study is the exploration of this mechanistic link via Mendelian randomization.

Identification of exact processes by which APOE influences AD pathology and confers risk is critical for the development of successful prevention and treatment methods. Our findings add to the growing literature suggesting that lipid profiles may be a clinically meaningful feature of AD and provide support for a potential mechanism by which APOE gene confers AD risk. Maintaining low total cholesterol may therefore be a critical aspect of preventative care for AD, particularly for APOE ɛ4 allele carriers.