Apolipoprotein E-dependent load of white matter hyperintensities in Alzheimer's disease: a voxel-based lesion mapping study

Meta-Analysis of White Matter Hyperintensity Volume Differences Between APOE ε4 Carriers and Noncarriers

Several studies have suggested that white matter hyperintensity volume (WMHV) is increased among apolipoprotein E (APOE) ε4 carriers while others have reported contradictory findings. Although APOE ε4 carriage is associated with greater AD pathology, it remains unclear whether cerebrovascular damage is also associated with APOE ε4 carriage. The aim of this meta-analysis was to determine whether WMHV is associated with APOE ε4 carrier status. 12 studies that were included yielded a total sample size of 16,738 adult subjects (ε4 carrier n = 4,721; ε4 noncarrier n = 12,017). There were no significant differences in WMHV between ε4 carriers and noncarriers (Hedge's g = 0.07; 95% CI (-0.01 to 0.15), P = 0.09). Subgroup analysis of community-based studies (n = 8) indicated a small effect size where ε4 carriers had greater WMHV relative to noncarriers (Hedge's g = 0.09 95% CI (0.02 to 0.16), P = 0.008). Among clinic-based studies (n = 3) there was no significant difference in WMHV by APOE ε4 carrier status (Hedge's g = -0.09, 95% CI (-0.60 to 0.41), P = 0.70). Observed APOE ε4-associated WMHV differences may be context-dependent and may also be confounded by a lack of standardization for WMHV segmentation.

APOE Genotypes and Brain Imaging Classes in Normal Cognition, Mild Cognitive Impairment, and Alzheimer's Disease: A Longitudinal Study

OBJECTIVE

To evaluate in 419 stroke-free cognitively normal subjects (CN) aged 45-82 years covering during a long prospective study (11.54 ± 1.47 years) the preclinical to dementia spectrum: 1) the distribution of small vessel disease (V) and brain atrophy (A) aggregated as following: V-/A-, V-/A+, V+/A-, V+/A+; 2) the relationship of these imaging classes with individual apolipoprotein E (APOE) genotypes; 3) the risk of progression to Alzheimer Disease (AD) of the individual APOE genotypes.

METHODS

Participants underwent one baseline (t0), and 4 clinical and neuropsychological assessments (t1,t2,t3, and t4). Brain MRI was performed in all subjects at t0, t2, t3 and t4.. White matter hyperintensities were assessed through two visual rating scales. Lacunes were also rated. Subcortical and global brain atrophy were determined through the bicaudate ratio and the lateral ventricle to brain ratio, respectively. APOE genotypes were determined at t0 in all subjects. Cox proportional hazard model was used to evaluate the risk of progression to AD.

RESULTS

The imaging class of mixed type was very common in AD, and in non amnestic mild cognitive impaired APOE ε4 non carriers. In these subjects, frontal and parieto-occipital regions were most affected by small vessel disease.

CONCLUSION

Our findings suggest that the APOE ε3 allele is probably linked to the brain vascular pathology.

Spatial patterns of white matter hyperintensities associated with Alzheimer's disease risk factors in a cognitively healthy middle-aged cohort

Background

White matter hyperintensities (WMH) of presumed vascular origin have been associated with an increased risk of Alzheimer’s disease (AD). This study aims to describe the patterns of WMH associated with dementia risk estimates and individual risk factors in a cohort of middle-aged/late middle-aged individuals (mean 58 (interquartile range 51–64) years old).

Methods

Magnetic resonance imaging and AD risk factors were collected from 575 cognitively unimpaired participants. WMH load was automatically calculated in each brain lobe and in four equidistant layers from the ventricular surface to the cortical interface. Global volumes and regional patterns of WMH load were analyzed as a function of the Cardiovascular Risk Factors, Aging and Incidence of Dementia (CAIDE) dementia risk score, as well as family history of AD and Apolipoprotein E (APOE) genotype. Additional analyses were performed after correcting for the effect of age and hypertension.

Results

The studied cohort showed very low WMH burden (median 1.94 cm³) and 20-year dementia risk estimates (median 1.47 %). Even so, higher CAIDE scores were significantly associated with increased global WMH load. The main drivers of this association were age and hypertension, with hypercholesterolemia and body mass index also displaying a minor, albeit significant, influence. Regionally, CAIDE scores were positively associated with WMH in anterior areas, mostly in the frontal lobe. Age and hypertension showed significant association with WMH in almost all regions analyzed. The APOE-ε2 allele showed a protective effect over global WMH with a pattern that comprised juxtacortical temporo-occipital and fronto-parietal deep white matter regions. Participants with maternal family history of AD had higher WMH load than those without, especially in temporal and occipital lobes.

Conclusions

WMH load is associated with AD risk factors even in cognitively unimpaired subjects with very low WMH burden and dementia risk estimates. Our results suggest that tight control of modifiable risk factors in middle-age/late middle-age could have a significant impact on late-life dementia.

Electronic supplementary material

The online version of this article (10.1186/s13195-018-0460-1) contains supplementary material, which is available to authorized users.

Clinical phenotype, atrophy, and small vessel disease in APOEε2 carriers with Alzheimer disease

OBJECTIVE

To examine the clinical phenotype, gray matter atrophy patterns, and small vessel disease in patients who developed prodromal or probable Alzheimer disease dementia, despite carrying the protective APOEε2 allele.

METHODS

We included 36 β-amyloid-positive (by CSF or PET) APOEε2 carriers (all ε2/ε3) with mild cognitive impairment or dementia due to Alzheimer disease who were matched for age and diagnosis (ratio 1:2) to APOEε3 homozygotes and APOEε4 carriers (70% ε3/ε4 and 30% ε4/ε4). We assessed neuropsychological performance across 4 cognitive domains (memory, attention, executive, and language functions), performed voxelwise and region of interest analyses of gray matter atrophy on T1-weighted MRI, used fluid-attenuated inversion recovery images to automatically quantify white matter hyperintensity volumes, and assessed T2*-weighted images to identify microbleeds. Differences in cognitive domain scores, atrophy, and white matter hyperintensities between ε2 carriers, ε3 homozygotes, and ε4 carriers were assessed using analysis of variance analyses, and Pearson χ² tests were used to examine differences in prevalence of microbleeds.

RESULTS

We found that ε2 carriers performed worse on nonmemory domains compared to both ε3 homozygotes and ε4 carriers but better on memory compared to ε4 carriers. Voxelwise T1-weighted MRI analyses showed asymmetric (left > right) temporoparietal-predominant atrophy with subtly less involvement of medial-temporal structures in ε2 carriers compared to ε4 carriers. Finally, ε2 carriers had larger total white matter hyperintensity volumes compared to ε4 carriers (mean 10.4 vs 7.3 mL) and a higher prevalence of microbleeds compared to ε3 homozygotes (37.5% vs 18.3%).

CONCLUSION

APOEε2 carriers who develop Alzheimer disease despite carrying the protective allele display a nonamnestic clinical phenotype with more severe small vessel disease.

Effects of APOE ε4 on neuroimaging, cerebrospinal fluid biomarkers, and cognition in prodromal Alzheimer's disease

Apolipoprotein (APOE) ε4 is a major genetic risk factor for Alzheimer's disease (AD), but its importance for the clinical and biological heterogeneity in AD is unclear, particularly at the prodromal stage. We analyzed 151 prodromal AD patients (44 APOE ε4-negative and 107 APOE ε4-positive) from the BioFINDER study. We tested cognition, 18F-flutemetamol β-amyloid (Aβ) positron emission tomography, cerebrospinal fluid biomarkers of Aβ, tau and neurodegeneration, and magnetic resonance imaging of white matter pathology and brain structure. Despite having similar cortical Aβ-load and baseline global cognition (mini mental state examination), APOE ε4-negative prodromal AD had more nonamnestic cognitive impairment, higher cerebrospinal fluid levels of Aβ-peptides and neuronal injury biomarkers, more white matter pathology, more cortical atrophy, and faster decline of mini mental state examination, compared to APOE ε4-positive prodromal AD. The absence of APOE ε4 is associated with an atypical phenotype of prodromal AD. This suggests that APOE ε4 may impact both the diagnostics of AD in early stages and potentially also effects of disease-modifying treatments.

Higher prevalence of cerebral white matter hyperintensities in homozygous APOE-ɛ4 allele carriers aged 45-75: Results from the ALFA study

Cerebral white matter hyperintensities are believed the consequence of small vessel disease and are associated with risk and progression of Alzheimer's disease. The ɛ4 allele of the APOE gene is the major factor accountable for Alzheimer's disease heritability. However, the relationship between white matter hyperintensities and APOE genotype in healthy subjects remains controversial. We investigated the association between APOE-ɛ4 and vascular risk factors with white matter hyperintensities, and explored their interactions, in a cohort of cognitively healthy adults (45-75 years). White matter hyperintensities were assessed with the Fazekas Scale from magnetic resonance images (575 participants: 74 APOE-ɛ4 homozygotes, 220 heterozygotes and 281 noncarriers) and classified into normal (Fazekas < 2) and pathological (≥2). Stepwise logistic regression was used to study the association between pathological Fazekas and APOE genotype after correcting for cardiovascular and sociodemographic factors. APOE-ɛ4 homozygotes, but not heterozygotes, bear a significantly higher risk (OR 3.432; 95% CI [1.297-9.082]; p = 0.013) of displaying pathological white matter hyperintensities. As expected, aging, hypertension and cardiovascular and dementia risk scales were also positively associated to pathological white matter hyperintensities, but these did not modulate the effect of APOE-ɛ4/ɛ4. In subjects at genetic risk of developing Alzheimer's disease, the control of modifiable risk factors of white matter hyperintensities is of particular relevance to reduce or delay dementia's onset.

The role of APOE in cerebrovascular dysfunction

The ε4 allele of the apolipoprotein E gene (APOE4) is associated with cognitive decline during aging, is the greatest genetic risk factor for Alzheimer's disease and has links to other neurodegenerative conditions that affect cognition. Increasing evidence indicates that APOE genotypes differentially modulate the function of the cerebrovasculature (CV), with apoE and its receptors expressed by different cell types at the CV interface (astrocytes, pericytes, smooth muscle cells, brain endothelial cells). However, research on the role of apoE in CV dysfunction has not advanced as quickly as other apoE-modulated pathways. This review will assess what aspects of the CV are modulated by APOE genotypes during aging and under disease states, discuss potential mechanisms, and summarize the therapeutic significance of the topic. We propose that APOE4 induces CV dysfunction through direct signaling at the CV, and indirectly via modulation of peripheral and central pathways. Further, that APOE4 predisposes the CV to damage by, and exacerbates the effects of, additional risk factors (such as sex, hypertension, and diabetes). ApoE4-induced detrimental CV changes include reduced cerebral blood flow (CBF), modified neuron-CBF coupling, increased blood-brain barrier leakiness, cerebral amyloid angiopathy, hemorrhages and disrupted transport of nutrients and toxins. The apoE4-induced detrimental changes may be linked to pericyte migration/activation, astrocyte activation, smooth muscle cell damage, basement membrane degradation and alterations in brain endothelial cells.

Cerebrovascular disease in ageing and Alzheimer's disease

Cerebrovascular disease (CVD) and Alzheimer’s disease (AD) have more in common than their association with ageing. They share risk factors and overlap neuropathologically. Most patients with AD have Aβ amyloid angiopathy and degenerative changes affecting capillaries, and many have ischaemic parenchymal abnormalities. Structural vascular disease contributes to the ischaemic abnormalities in some patients with AD. However, the stereotyped progression of hypoperfusion in this disease, affecting first the precuneus and cingulate gyrus, then the frontal and temporal cortex and lastly the occipital cortex, suggests that other factors are more important, particularly in early disease. Whilst demand for oxygen and glucose falls in late disease, functional MRI, near infrared spectroscopy to measure the saturation of haemoglobin by oxygen, and biochemical analysis of myelin proteins with differential susceptibility to reduced oxygenation have all shown that the reduction in blood flow in AD is primarily a problem of inadequate blood supply, not reduced metabolic demand. Increasing evidence points to non-structural vascular dysfunction rather than structural abnormalities of vessel walls as the main cause of cerebral hypoperfusion in AD. Several mediators are probably responsible. One that is emerging as a major contributor is the vasoconstrictor endothelin-1 (EDN1). Whilst there is clearly an additive component to the clinical and pathological effects of hypoperfusion and AD, experimental and clinical observations suggest that the disease processes also interact mechanistically at a cellular level in a manner that exacerbates both. The elucidation of some of the mechanisms responsible for hypoperfusion in AD and for the interactions between CVD and AD has led to the identification of several novel therapeutic approaches that have the potential to ameliorate ischaemic damage and slow the progression of neurodegenerative disease.

Peripheral (deep) but not periventricular MRI white matter hyperintensities are increased in clinical vascular dementia compared to Alzheimer's disease

BACKGROUND AND PURPOSE

Vascular dementia (VAD) is a complex diagnosis at times difficult to distinguish from Alzheimer's disease (AD). MRI scans often show white matter hyperintensities (WMH) in both conditions. WMH increase with age, and both VAD and AD are associated with aging, thus presenting an attribution conundrum. In this study, we sought to show whether the amount of WMH in deep white matter (dWMH), versus periventricular white matter (PVH), would aid in the distinction between VAD and AD, independent of age.

METHODS

Blinded semiquantitative ratings of WMH validated by objective quantitation of WMH volume from standardized MRI image acquisitions. PVH and dWMH were rated separately and independently by two different examiners using the Scheltens scale. Receiver operator characteristic (ROC) curves were generated using logistic regression to assess classification of VAD (13 patients) versus AD (129 patients). Clinical diagnoses were made in a specialty memory disorders clinic.

RESULTS

Using PVH rating alone, overall classification (area under the ROC curve, AUC) was 75%, due only to the difference in age between VAD and AD patients in our study and not PVH. In contrast, dWMH rating produced 86% classification accuracy with no independent contribution from age. A global Longstreth rating that combines dWMH and PVH gave an 88% AUC.

CONCLUSIONS

Increased dWMH indicate a higher likelihood of VAD versus AD. Assessment of dWMH on MRI scans using Scheltens and Longstreth scales may aid the clinician in distinguishing the two conditions.

Unraveling the potential co-contributions of cerebral small vessel vasculopathy to the pathogenesis of Alzheimer's dementia

Emerging evidence for the potential co-contributions of small vessel vasculopathy to dementia has resulted in a more nuanced view of Alzheimer's disease (AD) pathogenesis. Although cerebral small vessel disease, visualized on magnetic resonance imaging as hyperintense signal abnormalities, independently predicts the incidence and clinical progression of dementia, the relationships between AD pathology, white matter hyperintensity volume, genotype, and cognitive decline in AD remain unclear. The study by Morgen and colleagues, recently published in Alzheimer's Research & Therapy, presents important new findings on the associations between apolipoprotien E ε4 genotype, white matter hyperintensities, and cognition, independent of vascular risk, in a cohort of AD patients.