Genetic associations with brain microbleeds: Systematic review and meta-analyses

Multifaceted roles of APOE in Alzheimer disease

For the past three decades, apolipoprotein E (APOE) has been known as the single greatest genetic modulator of sporadic Alzheimer disease (AD) risk, influencing both the average age of onset and the lifetime risk of developing AD. The APOEε4 allele significantly increases AD risk, whereas the ε2 allele is protective relative to the most common ε3 allele. However, large differences in effect size exist across ethnoracial groups that are likely to depend on both global genetic ancestry and local genetic ancestry, as well as gene-environment interactions. Although early studies linked APOE to amyloid-β - one of the two culprit aggregation-prone proteins that define AD - in the past decade, mounting work has associated APOE with other neurodegenerative proteinopathies and broader ageing-related brain changes, such as neuroinflammation, energy metabolism failure, loss of myelin integrity and increased blood-brain barrier permeability, with potential implications for longevity and resilience to pathological protein aggregates. Novel mouse models and other technological advances have also enabled a number of therapeutic approaches aimed at either attenuating the APOEε4-linked increased AD risk or enhancing the APOEε2-linked AD protection. This Review summarizes this progress and highlights areas for future research towards the development of APOE-directed therapeutics.

A comprehensive analysis of APOE genotype effects on human brain structure in the UK Biobank

Alzheimer's disease (AD) risk is increased in carriers of the apolipoprotein E (APOE) ε4 allele and decreased in ε2 allele carriers compared with the ε3ε3 genotype. The aim of this study was to determine whether: the APOE genotype affects brain grey (GM) or white matter (WM) structure; and if differences exist, the age when they become apparent and whether there are differential effects by sex. We used cross-sectional magnetic resonance imaging data from ~43,000 (28,494 after pre-processing) white British cognitively healthy participants (7,446 APOE ε4 carriers) aged 45-80 years from the UK Biobank cohort and investigated image-derived phenotypes (IDPs). We observed no statistically significant effects of APOE genotype on GM structure volumes or median T2* in subcortical structures, a measure related to iron content. The volume of white matter hyperintensities differed significantly between APOE genotype groups with higher volumes in APOE ε4ε4 (effect size 0.14 standard deviations [SD]) and ε3ε4 carriers (effect size 0.04 SD) but no differences in ε2 carriers compared with ε3ε3 carriers. WM integrity measures in the dorsal (mean diffusivity [MD]) and ventral cingulum (MD and intracellular volume fraction), posterior thalamic radiation (MD and isotropic volume fraction) and sagittal stratum (MD) indicated lower integrity in APOE ε4ε4 carriers (effect sizes around 0.2-0.3 SD) and ε3ε4 (effect sizes around 0.05 SD) carriers but no differences in ε2 carriers compared with the APOE ε3ε3 genotype. Effects did not differ between men and women. APOE ε4 homozygotes had lower WM integrity specifically at older ages with a steeper decline of WM integrity from the age of 60 that corresponds to around 5 years greater "brain age". APOE genotype affects various white matters measures, which might be indicative of preclinical AD processes. This hypothesis can be assessed in future when clinical outcomes become available.

MRI-Visible Perivascular Spaces and Risk of Incident Dementia: The Framingham Heart Study

BACKGROUND AND OBJECTIVES

Perivascular spaces (PVS) visible on MRI scans may represent key aspects in the pathophysiology of stroke and dementia, including cerebral small vessel disease and glymphatic dysfunction. This study aimed to determine the association between MRI-visible PVS burden and the risk of incident dementia.

METHODS

This study included community-dwelling Framingham Heart Study Original and Offspring cohort participants with available brain MRI-PVS ratings, free of stroke and dementia. Multivariable Cox proportional hazard regression was used to obtain hazard ratios (HRs) and 95% CIs of the association between MRI-visible PVS and incident dementia. PVS were rated using validated methods in the basal ganglia (BG) and centrum semiovale (CSO). The outcomes included all-cause dementia, Alzheimer dementia (AD), and vascular dementia (VaD).

RESULTS

One thousand four hundred forty-nine participants 50 years or older (46% male) were included. Over a median follow-up period of 8.3 years, the incidence of all-cause dementia, AD, and VaD was 15.8%, 12.5%, and 2.5%, respectively. In models that adjusted for vascular risk factors and cardiovascular disease, the hazard for dementia increased steadily as PVS burden increased, rising 2-fold for those with grade II PVS (HR 2.44, 95% CI 1.51-3.93) to 5-fold in participants with grade IV (HR 5.05, 95% CI 2.75-9.26) compared with grade I PVS in CSO. In the BG, hazards increased 1.6-fold (HR 1.62, 95% CI 1.15-2.27) for grade II to 2.6-fold (HR 2.67, 95% CI 1.04-6.88) for grade IV compared with grade I PVS. The association remained significant for CSO but not for BG, after adjustment for white matter hyperintensity volume (WMHV), covert infarcts, and total brain volume. Similar findings were observed for AD, but VaD, limited by a small number of events, was not statistically significant.

DISCUSSION

Higher burden of PVS in CSO was associated with increased risk of developing dementia, independent of vascular risk factors, total brain volume, WMHVs, and covert infarcts. This finding supports a role for PVS as a subclinical MRI marker to identify individuals in subclinical stages at high risk of developing dementia who may benefit from early intervention.

Genetics of common cerebral small vessel disease

Cerebral small vessel disease (cSVD) is a leading cause of ischaemic and haemorrhagic stroke and a major contributor to dementia. Covert cSVD, which is detectable with brain MRI but does not manifest as clinical stroke, is highly prevalent in the general population, particularly with increasing age. Advances in technologies and collaborative work have led to substantial progress in the identification of common genetic variants that are associated with cSVD-related stroke (ischaemic and haemorrhagic) and MRI-defined covert cSVD. In this Review, we provide an overview of collaborative studies - mostly genome-wide association studies (GWAS) - that have identified >50 independent genetic loci associated with the risk of cSVD. We describe how these associations have provided novel insights into the biological mechanisms involved in cSVD, revealed patterns of shared genetic variation across cSVD traits, and shed new light on the continuum between rare, monogenic and common, multifactorial cSVD. We consider how GWAS summary statistics have been leveraged for Mendelian randomization studies to explore causal pathways in cSVD and provide genetic evidence for drug effects, and how the combination of findings from GWAS with gene expression resources and drug target databases has enabled identification of putative causal genes and provided proof-of-concept for drug repositioning potential. We also discuss opportunities for polygenic risk prediction, multi-ancestry approaches and integration with other omics data.

Cerebrospinal fluid biomarkers and apolipoprotein E genotype in cerebral amyloid angiopathy. A narrative review

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Lower Cerebrospinal Fluid Levels of Aβ₄₀ and Aβ₄₂ in Cerebral Amyloid Angiopathy compared with Alzheimer Disease and Healthy Controls.

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Cortical Superficial Siderosis-extent and lower levels of Aβ₄₂ could be prognostic for the severity of Cerebral Amyloid Angiopathy.

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Apolipoprotein – E Genotype plays an important role in the pathophysiology of Cerebral Amyloid Angiopathy.

Sporadic cerebral amyloid angiopathy (CAA) is a cerebral small vessel disease, characterized by the deposition of β-amyloid within the cortical and leptomeningeal blood vessel walls. It has attracted interest concerning new therapeutic perspectives. However, there are scarce data regarding the cerebrospinal fluid biomarkers (CSF) and genetic factors in sporadic CAA.

In this narrative review, we investigated the literature regarding the cerebrospinal fluid core biomarkers profile of patients with probable or possible CAA and its subtype, the CAA- related inflammation (CAA-ri), taking into account the clinical and radiological characteristics of the patients. We also analyzed the Apolipoprotein E (APOE) genotype differentiations among the different subtypes of cerebral amyloid angiopathy.

Our results demonstrate specific CSF patterns of β-amyloid (Aβ₄₂ and Aβ₄₀) and tau-proteins (t-tau and p-tau) which may serve as molecular biomarkers for CAA/ CAA-ri and could prove helpful for novel therapeutic procedures. Specifically, decreased levels of Aβ₄₀ and Aβ₄₂ in both CAA and CAA-ri, mildly increased concentrations of tau protein in patients with CAA-ri and a strong association between APOE ε4/ε4 genotype and CAA-ri are the main findings.

Genetic Variants behind Cardiovascular Diseases and Dementia

Cardiovascular diseases (CVDs) and dementia are the leading causes of disability and mortality. Genetic connections between cardiovascular risk factors and dementia have not been elucidated. We conducted a scoping review and pathway analysis to reveal the genetic associations underlying both CVDs and dementia. In the PubMed database, literature was searched using keywords associated with diabetes mellitus, hypertension, dyslipidemia, white matter hyperintensities, cerebral microbleeds, and covert infarctions. Gene lists were extracted from these publications to identify shared genes and pathways for each group. This included high penetrance genes and single nucleotide polymorphisms (SNPs) identified through genome wide association studies. Most risk SNPs to both diabetes and dementia participate in the phospholipase C enzyme system and the downstream nositol 1,4,5-trisphosphate and diacylglycerol activities. Interestingly, AP-2 (TFAP2) transcription factor family and metabolism of vitamins and cofactors were associated with genetic variants that were shared by white matter hyperintensities and dementia, and by microbleeds and dementia. Variants shared by covert infarctions and dementia were related to VEGF ligand-receptor interactions and anti-inflammatory cytokine pathways. Our review sheds light on future investigations into the causative relationships behind CVDs and dementia, and can be a paradigm of the identification of dementia treatments.

Association of common genetic variants with brain microbleeds: A genome-wide association study

OBJECTIVE

To identify common genetic variants associated with the presence of brain microbleeds (BMBs).

METHODS

We performed genome-wide association studies in 11 population-based cohort studies and 3 case-control or case-only stroke cohorts. Genotypes were imputed to the Haplotype Reference Consortium or 1000 Genomes reference panel. BMBs were rated on susceptibility-weighted or T2*-weighted gradient echo MRI sequences, and further classified as lobar or mixed (including strictly deep and infratentorial, possibly with lobar BMB). In a subset, we assessed the effects of APOE ε2 and ε4 alleles on BMB counts. We also related previously identified cerebral small vessel disease variants to BMBs.

RESULTS

BMBs were detected in 3,556 of the 25,862 participants, of which 2,179 were strictly lobar and 1,293 mixed. One locus in the APOE region reached genome-wide significance for its association with BMB (lead single nucleotide polymorphism rs769449; odds ratio [OR]any BMB [95% confidence interval (CI)] 1.33 [1.21-1.45]; p = 2.5 × 10-10). APOE ε4 alleles were associated with strictly lobar (OR [95% CI] 1.34 [1.19-1.50]; p = 1.0 × 10-6) but not with mixed BMB counts (OR [95% CI] 1.04 [0.86-1.25]; p = 0.68). APOE ε2 alleles did not show associations with BMB counts. Variants previously related to deep intracerebral hemorrhage and lacunar stroke, and a risk score of cerebral white matter hyperintensity variants, were associated with BMB.

CONCLUSIONS

Genetic variants in the APOE region are associated with the presence of BMB, most likely due to the APOE ε4 allele count related to a higher number of strictly lobar BMBs. Genetic predisposition to small vessel disease confers risk of BMB, indicating genetic overlap with other cerebral small vessel disease markers.

Emerging insights from the genetics of cerebral small-vessel disease

Cerebral small-vessel disease (cSVD) is a common cause of stroke, functional decline, vascular cognitive impairment, and dementia. Pathological processes in the brain's microcirculation are tightly interwoven with pathology in the brain parenchyma, and this interaction has been conceptualized as the neurovascular unit (NVU). Despite intensive research efforts to decipher the NVU's structure and function to date, molecular mechanisms underlying cSVD remain poorly understood, which hampers the development of cSVD-specific therapies. Important steps forward in understanding the disease mechanisms underlying cSVD have been made using genetic approaches in studies of both monogenic and sporadic SVD. We provide an overview of the NVU's structure and function, the implications for cSVD, and the underlying molecular mechanisms of dysfunction that have emerged from recent genetic studies of both monogenic and sporadic diseases of the small cerebral vasculature.

Cortical superficial siderosis and bleeding risk in cerebral amyloid angiopathy: A meta-analysis

OBJECTIVE

To assess the association of cortical superficial siderosis (cSS) presence and extent with future bleeding risk in cerebral amyloid angiopathy (CAA).

METHODS

This was a meta-analysis of clinical cohorts of symptomatic patients with CAA who had T2*-MRI at baseline and clinical follow-up for future intracerebral hemorrhage (ICH). We pooled data in a 2-stage meta-analysis using random effects models. Covariate-adjusted hazard ratios (adjHR) from multivariable Cox proportional hazard models were used.

RESULTS

We included data from 6 eligible studies (n = 1,239). cSS pooled prevalence was 34% (95% confidence interval [CI] 26%-41%; I ² 87.94%; p < 0.001): focal cSS prevalence was 14% (95% CI 12%-16%; I ² 6.75%; p = 0.37), and disseminated cSS prevalence was 20% (95% CI 13%-26%; I ² 90.39%; p < 0.001). During a mean follow-up of 3.1 years (range 1-4 years), 162/1,239 patients experienced a symptomatic ICH-pooled incidence rate 6.9% per year (95% CI 3.9%-9.8% per year; I ² 83%; p < 0.001). ICH incidence rates per year according to cSS status were 3.9% (95% CI 1.7%-6.1%; I ² 70%; p = 0.018) for patients without cSS, 11.1% (95% CI 7%-15.2%; I ² 56.8%; p = 0.074) for cSS presence, 9.1% (95% CI 5.5%-12.8%; I ² 0%; p = 0.994) for focal cSS, and 12.5% (95% CI 5.3%-19.7%; I ² 73.2%; p = 0.011) for disseminated cSS. In adjusted pooled analysis, any cSS presence was independently associated with increased future ICH risk (adjHR 2.14; 95% CI 1.19-3.85; p < 0.0001). Focal cSS was linked with ICH risk (adjHR 2.11; 95% CI 1.31-2.41; p = 0.002), while disseminated cSS conferred the strongest bleeding risk (adjHR 4.28; 95% CI 2.91-6.30; p < 0.0001).

CONCLUSION

In patients with CAA, cSS presence and extent are the most important MRI prognostic risk factors for future ICH, likely useful in treatment planning.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that in symptomatic CAA survivors with baseline T2*-MRI, cSS (particularly if disseminated, i.e., affecting >3 sulci) increases the risk of future ICH.

APOE and cortical superficial siderosis in CAA: Meta-analysis and potential mechanisms

OBJECTIVE

To assess potential mechanisms of cortical superficial siderosis (cSS), a central MRI biomarker in cerebral amyloid angiopathy (CAA), we performed a collaborative meta-analysis of APOE associations with cSS presence and severity.

METHODS

We pooled data from published studies reporting APOE genotype and MRI assessment of cSS in 3 distinct settings: (1) stroke clinic patients with symptomatic CAA (i.e., lobar intracerebral hemorrhage, transient focal neurologic episodes) according to the Boston criteria; (2) memory clinic patients; and (3) population-based studies. We compared cSS presence and severity (focal or disseminated vs no cSS) in participants with ε2+ or ε4+ genotype vs the ε3/ε3 genotype, by calculating study-specific and random effects pooled, unadjusted odds ratios (ORs).

RESULTS

Thirteen studies fulfilled inclusion criteria: 7 memory clinic cohorts (n = 2,587), 5 symptomatic CAA cohorts (n = 402), and 1 population-based study (n = 1,379). There was no significant overall association between APOE ε4+ and cSS presence or severity. When stratified by clinical setting, APOE ε4+ was associated with cSS in memory clinic (OR 2.10; 95% confidence interval [CI] 1.11-3.99) but not symptomatic CAA patients. The pooled OR showed significantly increased odds of having cSS for APOE ε2+ genotypes (OR 2.42, 95% CI 1.48-3.95) in both patient populations. This association was stronger for disseminated cSS in symptomatic CAA cohorts. In detailed subgroup analyses, APOE ε2/ε2 and APOE ε2/ε4 genotypes were most consistently and strongly associated with cSS presence and severity.

CONCLUSION

CAA-related vasculopathic changes and fragility associated with APOE ε2+ allele might have a biologically meaningful role in the pathophysiology and severity of cSS.

Genome-Wide Association Study of Cerebral Microbleeds on MRI

Cerebral microbleeds are the presence of a group of pathological processes affecting the small arteries, arterioles, capillaries, and venules of the brain. Previous studies showed that cerebral microbleeds were associated with higher risk of dementia and stroke. We conducted a genome-wide association study of cerebral microbleeds to identify novel loci associated with the presence and progression of cerebral microbleeds. This study included 454 individuals composed by 176 subjects with cerebral microbleeds and 278 subjects without cerebral microbleeds in a non-Hispanic/Latino white population. Association of genetic variants with the presence and progression of cerebral microbleeds was assessed by logistic regression model. Potential genetic risk variants Apolipoprotein E (ApoE) polymorphisms were independently genotyped and checked the association with the presence and progression of cerebral microbleeds. No single-nucleotide polymorphisms (SNPs) associated with the presence or progression of cerebral microbleeds were identified at genome-wide significant level (P < 1 × 10^-8). A total of 19 SNPs were associated with the presence of microbleeds at suggestive level (P < 1 × 10^-5). One SNP was associated with lower progression risk for cerebral microbleeds with suggestive evidence (P < 1 × 10^-5). ApoE ε4ε4 was independently associated with the presence and progression of cerebral microbleeds (odds ratio = 2.54, 95% confidence interval 1.08-6.00 and odds ratio = 5.1, 95% confidence interval 1.36-19.16). We highlighted 19 novel SNPs associated with the presence of cerebral microbleeds and one novel SNP associated with the progression of cerebral microbleeds for the first time. ApoE ε4ε4 was confirmed independently associated with the presence and progression of cerebral microbleeds.

Associations of Pulse and Blood Pressure with Hippocampal Volume by APOE and Cognitive Phenotype: The Alzheimer's Disease Neuroimaging Initiative (ADNI)

BACKGROUND

It is increasingly evident that high blood pressure can promote reduction in global and regional brain volumes. While these effects may preferentially affect the hippocampus, reports are inconsistent.

METHODS

Using data from the Alzheimer's Disease Neuroimaging Initiative (ADNI), we examined the relationships of hippocampal volume to pulse pressure (PPR) and systolic (SBP) and diastolic (DBP) blood pressure according to apolipoprotein (APOE) ɛ4 positivity and cognitive status. The ADNI data included 1,308 participants: Alzheimer disease (AD = 237), late mild cognitive impairment (LMCI = 454), early mild cognitive impairment (EMCI = 254), and cognitively normal (CN = 365), with up to 24 months of follow-up.

RESULTS

Higher quartiles of PPR were significantly associated with lower hippocampal volumes (Q1 vs. Q4, p = 0.034) in the CN and AD groups, but with increasing hippocampal volume (Q1, p = 0.008; Q2, p = 0.020; Q3, p = 0.017; Q4 = reference) in the MCI groups. In adjusted stratified analyses among non-APOE ɛ4 carriers, the effects in the CN (Q1 vs. Q4, p = 0.006) and EMCI groups (Q1, p = 0.002; Q2, p = 0.013; Q3, p = 0.002; Q4 = reference) remained statistically significant. Also, higher DBP was significantly associated with higher hippocampal volume (p = 0.002) while higher SBP was significantly associated with decreasing hippocampal volume in the EMCI group (p = 0.015).

CONCLUSION

Changes in PPR, SBP, and DBP differentially influenced hippocampal volumes depending on the cognitive and APOE genotypic categories.

Risk factors of cerebral microbleeds in young and middle-aged patients with hypertension

Objective This study aimed to investigate the incidence and related risk factors of cerebral microbleeds (CMBs) in young and middle-aged patients with hypertension. Methods The study included 232 young and middle-aged (18-59 years-old) patients with hypertension from September 2014 to December 2016 in the Department of Neurology, Affiliated Hospital of Guizhou Medical University, China. The data were recorded which included demographics, vascular risk factors, medication history, and imaging data of patients. CMBs were evaluated based on the microbleeds anatomical rating scale. Results Of the enrolled participants, 115 were CMB positive, accounting for 49.6%. CMBs were more prone to occur in deep regions than in others (39.13%). Multiple cerebral microbleeds were associated with white matter hyperintensities(WMH), dyslipidemia, hyperhomocysteine, and uric acid. Moreover, WMH, dyslipidemia, ever smoker, antiplatelets use, and hyperhomocysteine were found to be risk factors for deep or infratentorial CMBs in young and middle-aged patients with hypertension. However, the lobar CMBs only had an independent correlation with dyslipidemia in these participants. Conclusions The incidence of CMBs in patients with hypertension was relatively high. It mostly occurred in a deep or infratentorial area with more vascular-associated risk factors. However, in patients with lobar CMBs, factors associated with lipid metabolism, such as amyloid deposition and unidentified genotype variation, may be crucial. Screening and regular follow-ups of CMBs by Susceptibility Weighted Imaging and active prevention in young and middle-aged patients with hypertension have clinical significance for timely understanding and predicting the occurrence and development of related cerebrovascular disease events.

APOE gene and neuropsychiatric disorders and endophenotypes: A comprehensive review

The Apolipoprotein E (APOE) gene is one of the main candidates in neuropsychiatric genetics, with hundreds of studies carried out in order to explore the possible role of polymorphisms in the APOE gene in a large number of neurological diseases, psychiatric disorders, and related endophenotypes. In the current article, we provide a comprehensive review of the structural and functional aspects of the APOE gene and its relationship with brain disorders. Evidence from genome-wide association studies and meta-analyses shows that the APOE gene has been significantly associated with several neurodegenerative disorders. Cellular and animal models show growing evidence of the key role of APOE in mechanisms of brain plasticity and behavior. Future analyses of the APOE gene might find a possible role in other neurological diseases and psychiatric disorders and related endophenotypes. © 2016 Wiley Periodicals, Inc.

Update on Vascular Cognitive Impairment Associated with Subcortical Small-Vessel Disease

Subcortical small-vessel disease (SSVD) is a disorder well characterized from the clinical, imaging, and neuropathological viewpoints. SSVD is considered the most prevalent ischemic brain disorder, increasing in frequency with age. Vascular risk factors include hypertension, diabetes, hyperlipidemia, elevated homocysteine, and obstructive sleep apnea. Ischemic white matter lesions are the hallmark of SSVD; other pathological lesions include arteriolosclerosis, dilatation of perivascular spaces, venous collagenosis, cerebral amyloid angiopathy, microbleeds, microinfarcts, lacunes, and large infarcts. The pathogenesis of SSVD is incompletely understood but includes endothelial changes and blood-brain barrier alterations involving metalloproteinases, vascular endothelial growth factors, angiotensin II, mindin/spondin, and the mammalian target of rapamycin pathway. Metabolic and genetic conditions may also play a role but hitherto there are few conclusive studies. Clinical diagnosis of SSVD includes early executive dysfunction manifested by impaired capacity to use complex information, to formulate strategies, and to exercise self-control. In comparison with Alzheimer's disease (AD), patients with SSVD show less pronounced episodic memory deficits. Brain imaging has advanced substantially the diagnostic tools for SSVD. With the exception of cortical microinfarcts, all other lesions are well visualized with MRI. Diagnostic biomarkers that separate AD from SSVD include reduction of cerebrospinal fluid amyloid-β (Aβ)42 and of the ratio Aβ42/Aβ40 often with increased total tau levels. However, better markers of small-vessel function of intracerebral blood vessels are needed. The treatment of SSVD remains unsatisfactory other than control of vascular risk factors. There is an urgent need of finding targets to slow down and potentially halt the progression of this prevalent, but often unrecognized, disorder.

Incident risk and progression of cerebral microbleeds in healthy adults: a multi-occasion longitudinal study

Decline in cerebrovascular health complicates brain aging, and development of cerebral microbleeds (CMBs) is one of its prominent indicators. In a large sample of healthy adults (N = 251, age 18-78 years at baseline, 70% women), the contributions of chronological age and vascular health indicators to the risk of developing a CMB, as well as the change in CMB size and iron content, were examined in a prospective 8-year longitudinal study using susceptibility weighted imaging. Twenty-six persons (10.4%), most of whom were 40 years of age or older, had at least 1 CMB during the study. Older age was associated with greater risk for developing a CMB (odds ratio 1.03). Elevation of combined metabolic syndrome indicators (b = 0.15, p = 0.001) conferred additional risk (odds ratio 1.02). High body mass index exacerbated the risk associated with poor vascular health (b = 0.75, p < 0.001) and frequent exercise mitigated it (b = -0.46, p = 0.03). CMBs persisted over time, yet their volume decreased (mean change = -0.32, p < 0.05), whereas their relative iron content remained stable (mean change = -0.14, p = 0.05). We conclude that although developing a CMB is unlikely during normal aging, risk increases with declining vascular health, which is modifiable via behavioral and pharmaceutical intervention.

Background and distribution of lobar microbleeds in cognitive dysfunction

OBJECTIVES

Cerebral microbleeds (CMBs) are often observed in memory clinic patients. It has been generally accepted that deep CMBs (D-CMBs) result from hypertensive vasculopathy (HV), whereas strictly lobar CMBs (SL-CMBs) result from cerebral amyloid angiopathy (CAA) which frequently coexists with Alzheimer's disease (AD). Mixed CMBs (M-CMBs) have been partially attributed to HV and also partially attributed to CAA. The aim of this study was to elucidate the differences between SL-CMBs and M-CMBs in terms of clinical features and regional distribution.

MATERIALS

We examined 176 sequential patients in our memory clinic for clinical features and CMB location using susceptibility-weighted images obtained on a 3T-MRI. The number of lobar CMBs in SL-CMBs and M-CMBs was counted in each cerebral lobe and their regional density was adjusted according to the volume of each lobe.

RESULTS

Of the total 176 patients, 111 patients (63.1%) had CMBs. Within the patients who had CMBs, M-CMBs were found in 54 patients (48.6%), followed by SL-CMBs in 35 (31.5%) and D-CMBs in 19 (17.1%). The SL-CMB group showed a significantly higher prevalence of family history of dementia, whereas the M-CMB group showed an increasing trend toward hypertension and smoking. The prevalence of AD was significantly higher in the SL-CMBs group, whereas the prevalence of AD with cerebrovascular disease was higher in the M-CMBs group. The regional density of lobar CMBs was significantly higher in the occipital lobe in the M-CMB group, whereas the SL-CMB group showed higher regional density between regions an increasing tendency in the parietal and occipital lobe.

CONCLUSION

The between-group differences in clinical features and regional distribution indicate there to be an etiological relationship of SL-CMBs to AD and CAA, and M-CMBs to both HV and CAA.

Cerebral microbleeds and risk of incident dementia: the Framingham Heart Study

Cerebral microbleeds (CMBs) are MRI markers attributed to the most common cerebral angiopathies in the elderly and in patients with dementia: hypertensive and cerebral amyloid angiopathy. CMB detection in asymptomatic persons may help identify those at risk for dementia and may influence preventive strategies and design of clinical trials testing treatments for dementia. We studied the association of CMB with risk of incident dementia in community dwelling individuals. A total of 1296 dementia-free Framingham Heart Study participants (mean age 72 years; 54% women) with available brain MRI and incident dementia data during a mean follow-up period of 6.7 years were included. Using Cox proportional hazards models, we related CMB presence to incident dementia. Multivariable models were adjusted for age, sex, APOE status, and education, with additional models adjusting for vascular risk factors and MRI markers of ischemic brain injury. CMBs were observed in 10.8% and incident dementia in 85 participants (6.6% over study period). Participants with any CMB had 1.74 times higher risk of dementia (hazard ratio [HR] 1.74, 95% confidence interval [CI] 1.00-3.01), whereas those with deep and mixed CMB had a three-fold increased risk (HR 2.99, 95% CI 1.52-5.90). The associations were independent of vascular risk factors, and for deep and mixed CMB also independent of MRI markers of ischemia (HR 2.44, 95% CI 1.22-4.88). Purely lobar CMBs were not associated with incident dementia. Our findings support a role for hypertensive vasculopathy and the interplay of hypertensive and cerebral amyloid angiopathy in risk of dementia and suggest that CMB presence can identify individuals at risk of dementia.

Oral Cnm-positive Streptococcus Mutans Expressing Collagen Binding Activity is a Risk Factor for Cerebral Microbleeds and Cognitive Impairment

Cerebral microbleeds (CMBs) are an important risk factor for stroke and dementia. We have shown that the collagen binding surface Cnm protein expressed on cnm-positive Streptococcus mutans is involved in the development of CMBs. However, whether the collagen binding activity of cnm-positive S. mutans is related to the nature of the CMBs or to cognitive impairment is unclear. Two-hundred seventy nine community residents (70.0 years) were examined for the presence or absence of cnm-positive S. mutans in the saliva by PCR and collagen binding activity, CMBs, and cognitive function were evaluated. Cnm-positive S. mutans was detected more often among subjects with CMBs (p < 0.01) than those without. The risk of CMBs was significantly higher (odds ratio = 14.3) in the group with S. mutans expressing collagen binding activity, as compared to the group without that finding. Deep CMBs were more frequent (67%) and cognitive function was lower among subjects with cnm-positive S. mutans expressing collagen binding activity. This work supports the role of oral health in stroke and dementia and proposes a molecular mechanism for the interaction.

Cerebral microbleeds in early Alzheimer's disease

We hypothesize that cerebral microbleeds (CMB) in patients with different neuropsychological profiles (amnestic or non-amnestic) and MRI features of vascular damage could provide important information on the underlying pathological process in early Alzheimer's disease. The study was performed at two trial sites. We studied 136 outpatients with cognitive decline. MRI was performed using a magnetic field of 1.5 and 3 T. Neuropsychological assessment included Mini-Mental State Examination (MMSE), Montreal Cognitive Assessment scale (MoCA), Addenbrooke's Cognitive Examination (ACE-R), Cambridge Cognitive Examination battery (CAMCOG) (Part 3), Clock Drawing Test, fluency test and the visual memory test (SCT). CSF was examined for standard parameters such as tau, phosphorylated tau, amyloid-β 1-40 and 42 and Qalbumin, in accordance with established protocols and genotype. In 61 patients (45 %), at least 1 CMB was found. Most of the CMBs were described in the amnestic profile (67 %). In 86 % of the cases, multiple CMB were observed. The ratio of Aβ1-40/42 in non-amnestic patients with CMB was significantly lower (mean 0.6) than in patients without CMB (mean 1.2). A notable difference in the albumin ratio as an indicator of the BBB was observed between groups with and without CMB. In the CMP-positive group, the E2 genotype was observed more frequently, and the E4 genotype less frequently, than in the CMB-negative group. Based on the cerebrospinal fluid-serum albumin ratio, we were able to show that patients with CMB present several features of BBB dysfunction. According to logistic regression, the predictive factors for CMB in patients with cognitive decline were age, WMHs score and albumin ratio. We found a significant reduction in the Aβ-amyloid ratio in the non-amnestic profile group with CMB (particularly in the cortical region) in comparison to those without CMB. While this is an interesting finding, its significance needs to be assessed in a prospective follow-up.

ApoA1, ApoJ and ApoE Plasma Levels and Genotype Frequencies in Cerebral Amyloid Angiopathy

The involvement of apolipoproteins, such as the ApoE4 isoform, in Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA) highlights the fact that certain lipid carriers may participate in soluble β-amyloid (Aβ) transport. Our general aim was to characterize the soluble levels of the apolipoproteins apoE, apoA1 and apoJ/clusterin and their genotype status in patients with CAA. We analyzed the genotypes frequency of APOA1 (rs5069, rs670), CLU (rs11136000, rs1532278, rs7012010, rs9331888) and APOE (rs429358, rs7412) in a cohort of patients with CAA-associated intracerebral hemorrhage (ICH) (n = 59) and compared the results with those from hypertension-associated ICH (n = 42), AD patients (n = 73) and controls (n = 88). In a subgroup of patients, we also determined the plasma concentrations of apoE, apoA1 and apoJ/clusterin. We found increased plasma apoJ/clusterin levels in CAA patients compared to AD patients or controls after adjusting for sex and age (CAA vs. controls, p = 0.033; CAA vs. AD, p = 0.013). ApoA1 levels were not altered between groups, although a strong correlation was observed between plasma Aβ(1-40) and apoA1 among CAA patients (r = 0.583, p = 0.007). Regarding plasma apoE concentration, a robust association between circulating levels and genotype status was confirmed (p < 0.001). Whereas the APOE4 frequency was higher in AD (p < 0.001) and CAA (p = 0.013), the APOA1 and CLU genotypes were not different among groups. In the CAA cohort, the risk-linked CLU variant (C) rs11136000 was associated with white matter hyperintensities (p = 0.045) and the presence of lobar microbleeds (p = 0.023) on MRI. In summary, our findings suggest that apoA1 may act as a physiological transporter of Aβ(1-40) and that apoJ/clusterin appears to be a chaperone related to distinctive lesions in CAA brains.

APOE/TOMM40 genetic loci, white matter hyperintensities, and cerebral microbleeds

Background

Two markers of cerebral small vessel disease are white matter hyperintensities and cerebral microbleeds, which commonly occur in people with Alzheimer's disease.

Aim and/or hypothesis

To test for independent associations between two Alzheimer's disease‐susceptibility gene loci – APOE ε and the TOMM40 ‘523’ poly‐T repeat – and white matter hyperintensities/cerebral microbleed burden in community‐dwelling older adults.

Methods

Participants in the Lothian Birth Cohort 1936 underwent genotyping for APOE ε and TOMM40 523, and detailed structural brain magnetic resonance imaging at a mean age of 72·70 years (standard deviation = 0·7; range = 71–74).

Results

No significant effects of APOE ε or TOMM40 523 genotypes on white matter hyperintensities or cerebral microbleed burden were found amongst 624 participants.

Conclusions

Lack of association between two Alzheimer's disease susceptibility gene loci and markers of cerebral small vessel disease may reflect the relative health of this population compared with those in other studies in the literature.

No neuropathological evidence for a direct topographical relation between microbleeds and cerebral amyloid angiopathy

Introduction

Cerebral microbleeds correspond to blood breakdown products, including hemosiderin-containing macrophages around small vessels on histological examination. Superficial lobar cerebral microbleeds are increasingly recognized on MRI as a biomarker of cerebral amyloid angiopathy but the direct association between amyloid-laden vessels burden and cerebral microbleeds has yet to be validated neuropathologically. To address this issue, we examined the frequency of histopathologically-defined cerebral microbleeds in different brain regions and their relationship with cerebral amyloid angiopathy in a large autopsy population.

Results

The frontal, parietal and occipital cortex as well as the adjacent white matter and basal ganglia of 113 consecutive autopsies were examined. Cerebral microbleedss were identified on haematoxylin-eosin-stained histological slides, cerebral amyloid angiopathy using anti-amyloid antibody. Cerebral microbleeds were present in 92.9 % of the cases and cerebral amyloid angiopathy in 44.3 % of them. Cerebral microbleeds were more frequent in parietal and frontal lobes followed by the occipital region and basal ganglia. In contrast, cerebral amyloid angiopathy was most frequent in the occipital lobe. There was no significant topographical association between cerebral amyloid angiopathy presence or severity and cerebral microbleeds in any brain region. In lobar areas, cerebral amyloid angiopathy was found in the cortex, predominantly affecting pial arteries and their superficial cortical branches, in contrast to microbleeds which were mainly in the white matter and occurred around deeper arteries and arterioles, including the subcortical segment of long penetrating branches of pial vessels.

Conclusions

Our study does not support a direct relation between cerebral microbleeds and cerebral amyloid angiopathy burden at the neuropathological level, raising intriguing questions on the potential pathophysiological mechanisms of cerebral microbleeds in the context of cerebral amyloid angiopathy or other small vessel disease pathology.

Prevention and Management of Cerebral Small Vessel Disease

Lacunar infarcts/lacunes, white matter hyperintensities (WMH), and cerebral microbleeds (CMBs) are considered various manifestations of cerebral small vessel disease (SVD). Since the exact mechanisms of these manifestations differ, their associated risk factors differ. High blood pressure is the most consistent risk factor for all of these manifestations. However, a "J curve" phenomenon in terms of blood pressure probably exists for WMH. The association between cholesterol levels and lacunar infarcts/lacunes or WMH was less consistent and sometimes conflicting; a low cholesterol level probably increases the risk of CMBs. Homocysteinemia appears to be associated with WMH. It is noteworthy that the risk factors profile may also differ between different lacunar patterns and CMBs located at different parts of the brain. Thrombolysis, antihypertensives, and statins are used to treat patients with symptomatic lacunar infarction, just as in those with other stroke subtypes. However, it should be remembered that bleeding risks increase in patients with extensive WMH and CMBs after thrombolysis therapy. According to the Secondary Prevention of Small Subcortical Strokes trial results, a blood pressure reduction to <130 mmHg is recommended in patients with symptomatic lacunar infarction. However, an excessive blood pressure decrease may induce cognitive decline in older patients with extensive WMH. Dual antiplatelet therapy (aspirin plus clopidogrel) should be avoided because of the excessive risk of intracerebral hemorrhage. Although no particular antiplatelet is recommended, drugs such as cilostazol or triflusal may have advantages for patients with SVD since they are associated with less frequent bleeding complications than aspirin.

Cerebral amyloid angiopathy: emerging concepts

Cerebral amyloid angiopathy (CAA) involves cerebrovascular amyloid deposition and is classified into several types according to the amyloid protein involved. Of these, sporadic amyloid β-protein (Aβ)-type CAA is most commonly found in older individuals and in patients with Alzheimer's disease (AD). Cerebrovascular Aβ deposits accompany functional and pathological changes in cerebral blood vessels (CAA-associated vasculopathies). CAA-associated vasculopathies lead to development of hemorrhagic lesions [lobar intracerebral macrohemorrhage, cortical microhemorrhage, and cortical superficial siderosis (cSS)/focal convexity subarachnoid hemorrhage (SAH)], ischemic lesions (cortical infarction and ischemic changes of the white matter), and encephalopathies that include subacute leukoencephalopathy caused by CAA-associated inflammation/angiitis. Thus, CAA is related to dementia, stroke, and encephalopathies. Recent advances in diagnostic procedures, particularly neuroimaging, have enabled us to establish a clinical diagnosis of CAA without brain biopsies. Sensitive magnetic resonance imaging (MRI) methods, such as gradient-echo T2^* imaging and susceptibility-weighted imaging, are useful for detecting cortical microhemorrhages and cSS. Amyloid imaging with amyloid-binding positron emission tomography (PET) ligands, such as Pittsburgh Compound B, can detect CAA, although they cannot discriminate vascular from parenchymal amyloid deposits. In addition, cerebrospinal fluid markers may be useful, including levels of Aβ40 for CAA and anti-Aβ antibody for CAA-related inflammation. Moreover, cSS is closely associated with transient focal neurological episodes (TFNE). CAA-related inflammation/angiitis shares pathophysiology with amyloid-related imaging abnormalities (ARIA) induced by Aβ immunotherapies in AD patients. This article reviews CAA and CAA-related disorders with respect to their epidemiology, pathology, pathophysiology, clinical features, biomarkers, diagnosis, treatment, risk factors, and future perspectives.

Genetics of cerebral small vessel disease

Cerebral small vessel disease (SVD) is an important cause of stroke and cognitive impairment among the elderly and is a more frequent cause of stroke in Asia than in the US or Europe. Although traditional risk factors such as hypertension or diabetes mellitus are important in the development of cerebral SVD, the exact pathogenesis is still uncertain. Both, twin and family history studies suggest heritability of sporadic cerebral SVD, while the candidate gene study and the genome-wide association study (GWAS) are mainly used in genetic research. Robust associations between the candidate genes and occurrence of various features of sporadic cerebral SVD, such as lacunar infarction, intracerebral hemorrhage, or white matter hyperintensities, have not yet been elucidated. GWAS, a relatively new technique, overcomes several shortcomings of previous genetic techniques, enabling the detection of several important genetic loci associated with cerebral SVD. In addition to the more common, sporadic cerebral SVD, several single-gene disorders causing cerebral SVD have been identified. The number of reported cases is increasing as the clinical features become clear and diagnostic examinations are more readily available. These include cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy, COL4A1-related cerebral SVD, autosomal dominant retinal vasculopathy with cerebral leukodystrophy, and Fabry disease. These rare single-gene disorders are expected to play a crucial role in our understanding of cerebral SVD pathogenesis by providing animal models for the identification of cellular, molecular, and biochemical changes underlying cerebral small vessel damage.

A systematic appraisal of field synopses in genetic epidemiology: a HuGE review

Evidence from genetic association studies is accumulating rapidly. Field synopses have recently arisen as an unbiased way of systematically synthesizing this evidence. We performed a systematic review and appraisal of published field synopses in genetic epidemiology and assessed their main findings and methodological characteristics. We identified 61 eligible field synopses, published between January 1, 2007, and October 31, 2013, on 52 outcomes reporting 734 significant associations at the P < 0.05 level. The median odds ratio for these associations was 1.25 (interquartile range, 1.15-1.43). Egger's test was the most common method (n = 30 synopses) of assessing publication bias. Only 12 synopses (20%) used the Venice criteria to evaluate the epidemiologic credibility of their findings (n = 449 variants). Eleven synopses (18%) were accompanied by an online database that has been regularly updated. These synopses received more citations (P = 0.01) and needed a larger research team (P = 0.02) than synopses without an online database. Overall, field synopses are becoming a valuable tool for the identification of common genetic variants, especially when researchers follow relevant methodological guidelines. Our work provides a summary of the current status of the field synopses published to date and may help interested readers efficiently identify the online resources containing the relevant genetic evidence.

Cerebral microbleeds in a multiethnic elderly community: demographic and clinical correlates

BACKGROUND

Microbleeds, small perivascular collections of hemosiderin manifested radiologically as hypointensities on gradient-echo magnetic resonance imaging (MRI), are important markers of small vessel pathology. Despite their clinical relevance, little is known about their prevalence and demographic correlates, particularly among ethnically diverse older adults. We examined demographic and clinical correlates of regional microbleeds in a multi-ethnic cohort and examined categorization schemes of microbleed distribution and severity.

METHODS

Between 2005 and 2007, 769 individuals participated in a MRI study as part of the Washington Heights/Inwood Columbia Aging Project. Approximately four years later, 243 out of 339 participants (mean age=84.50) who returned for a repeat MRI had gradient-echo scans for microbleed assessment and comprised the sample. We examined the association of deep and lobar microbleeds with age, sex, education, vascular factors, cognitive status and markers of small vessel disease.

RESULTS

Sixty-seven of the 243 (27%) participants had at least one microbleed. Individuals with microbleeds were more likely to have a history of stroke than individuals without. When categorized as having either no microbleeds, microbleeds in deep regions only, in lobar regions only, and both deep and lobar microbleeds, hypertension, proportion of strokes, and white matter hyperintensity volume (WMH) increased monotonically across the four groups. The number of lobar microbleeds correlated with WMH volume and diastolic blood pressure.

CONCLUSIONS

Microbleeds in deep and lobar locations are associated with worse outcomes than microbleeds in either location alone, although the presence of lobar microbleeds appears to be more clinically relevant.

Clinical predictors of severe cerebral amyloid angiopathy and influence of APOE genotype in persons with pathologically verified Alzheimer disease

IMPORTANCE

Although cerebral amyloid angiopathy (CAA) has important clinical implications, our understanding of it and ability to diagnose it are limited.

OBJECTIVE

To determine pathological correlates and clinical factors identifiable during life that predict the presence of severe CAA in persons with pathologically confirmed Alzheimer disease (AD).

DESIGN, SETTING, AND PARTICIPANTS

We compared demographic and clinical variables at the earliest visit during life at which participants were found to have cognitive impairment and compared pathological variables between persons ultimately found to have no or severe CAA at autopsy using logistic regression. Analyses were repeated separately for carriers and noncarriers of the APOE ε4 allele. Data were obtained from the Uniform Data Set, which comprises longitudinal clinical assessments performed in the Alzheimer's Disease Centers funded by the National Institute on Aging. Participants included 193 persons with AD and severe CAA and 232 persons with AD and no CAA. All participants had cognitive impairment and met National Institute on Aging-Reagan Institute neuropathological criteria for AD.

MAIN OUTCOMES AND MEASURES

Prevalence of demographic characteristics and the APOE ε4 allele and odds ratios (ORs) of clinical variables for the prediction of severe CAA.

RESULTS

Persons with severe CAA compared with those without CAA were more likely to carry an APOE ε4 allele (64.9% vs 42.8%, respectively; P < .001), to be Hispanic (6.8% vs 1.3%, respectively; P = .003), to have had a transient ischemic attack (12.5% vs 6.1%, respectively; OR = 2.1; 95% CI, 1.0-4.4), and to have lower degrees of diffuse amyloid plaque pathology (mean [SD] Consortium to Establish a Registry for Alzheimer's Disease score, 1.2 [0.5] vs 1.4 [0.8], respectively; P = .01). Those with CAA compared with those without CAA more commonly had intracerebral hemorrhage (9.3% vs 3.5%, respectively; P = .01), cortical microinfarcts (20.7% vs 12.9%, respectively; P = .03), and subcortical leukoencephalopathy (20.5% vs 12.1%, respectively; P = .02). Noncarriers of the APOE ε4 allele with severe CAA compared with those without CAA had a higher prevalence of stroke (11.1% vs 3.9%, respectively; OR = 3.8; 95% CI, 1.0-14.6) and hypercholesterolemia (50.0% vs 32.7%, respectively; OR = 2.3; 95% CI, 1.1-4.7).

CONCLUSIONS AND RELEVANCE

Being Hispanic and having had a transient ischemic attack-like episode were predictors of CAA in persons with AD. Less diffuse parenchymal amyloid pathology in persons with severe CAA suggests a difference in β-amyloid trafficking.

Pattern of and risk factors for brain microbleeds in neurodegenerative dementia

OBJECTIVE

A cross-sectional study was conducted to describe the prevalence, locations, and risk factors for brain microbleeds (BMBs) in neurodegenerative dementia.

METHODS

The database of the Alzheimer Center Reina Sofía Foundation was searched, BMBs were described, and the potential associations of BMBs were investigated using univariate statistics.

RESULTS

A total of 148 patients (age 81.6 [standard deviation 6.7], 79.1% female) were studied. Prevalence of BMBs was 44.6%. A group of patients with unusually high (ie, ≥4) number of BMBs were identified, which displayed higher number of vascular risk factors and vascular diseases. Brain microbleeds were also associated with ischemic lesions in the basal ganglia (r = .39), clinical diagnosis of Alzheimer's disease (AD) and cerebrovascular disease (r = .33), cortical infarction (r = .20), and antiaggregant or anticoagulant treatment duration (r = .20).

CONCLUSIONS

Brain microbleeds are associated with vascular burden and AD diagnosis in old patients with neurodegenerative dementia. More research is warranted regarding the mechanisms and potential clinical implications of these results.

Incidence of cerebral microbleeds in preclinical Alzheimer disease

OBJECTIVE

We sought to determine the incidence and associations of lobar microbleeds (LMBs) in a longitudinal cohort with (11)C-Pittsburgh compound B (PiB) PET imaging.

METHODS

One hundred seventy-four participants from the observational Australian Imaging, Biomarkers and Lifestyle Study of Ageing (97 with normal cognition [NC], 37 with mild cognitive impairment [MCI], and 40 with Alzheimer disease [AD] dementia) were assessed at 3 time points over 3 years with 3-tesla susceptibility-weighted MRI and (11)C-PiB PET. MRIs were inspected for microbleeds, siderosis, infarction, and white matter hyperintensity severity, blind to clinical and PiB findings. Neocortical PiB standardized uptake value ratio, normalized to cerebellar cortex, was dichotomized as positive or negative (PiB+/-, standardized uptake value ratio >1.5). Annualized LMB incidence was calculated, and logistic regression was used to determine the association of incident LMBs with PiB, APOE ε4+ status, and cerebrovascular disease.

RESULTS

LMBs were present in 18.6% of NC, 24.3% of MCI, and 40% of AD participants (p < 0.05 vs NC). LMB incidence was 0.2 ± 0.6 per year in NC participants, 0.2 ± 0.5 in MCI, and 0.7 ± 1.4 in AD (p < 0.03 vs NC) and was 6-fold higher in PiB+ than PiB-NC. Incident LMBs were associated with age, APOE ε4+, PiB+, and baseline LMBs. Incidence of multiple LMBs was also associated with lacunar infarction and white matter hyperintensity severity.

CONCLUSIONS

Older age, baseline LMBs, higher β-amyloid burden, and concomitant cerebrovascular disease may all confer higher risk of incident LMBs. This should be considered when designing protocols for amyloid-modifying clinical trials.

Cerebral microbleeds: a review of clinical, genetic, and neuroimaging associations

Cerebral microbleeds (microbleeds) are small, punctuate hypointense lesions seen in T2* Gradient-Recall Echo (GRE) and Susceptibility-Weighted (SWI) Magnetic Resonance Imaging (MRI) sequences, corresponding to areas of hemosiderin breakdown products from prior microscopic hemorrhages. They occur in the setting of impaired small vessel integrity, commonly due to either hypertensive vasculopathy or cerebral amyloid angiopathy. Microbleeds are more prevalent in individuals with Alzheimer’s disease (AD) dementia and in those with both ischemic and hemorrhagic stroke. However they are also found in asymptomatic individuals, with increasing prevalence with age, particularly in carriers of the Apolipoprotein (APOE) ε4 allele. Other neuroimaging findings that have been linked with microbleeds include lacunar infarcts and white matter hyperintensities on MRI, and increased cerebral β-amyloid burden using ¹¹C-PiB Positron Emission Tomography. The presence of microbleeds has been suggested to confer increased risk of incident intracerebral hemorrhage – particularly in the setting of anticoagulation – and of complications of immunotherapy for AD. Prospective data regarding the natural history and sequelae of microbleeds are currently limited, however there is a growing evidence base that will serve to inform clinical decision-making in the future.

Cerebral microbleeds: Spatial distribution implications

Cerebral microbleeds are considered an imaging marker of cerebral small vessel disease. The location of microbleeds is thought to reflect the underlying pathology. Microbleeds in the deep and infratentorial region are thought to reflect hypertensive arteriopathy whereas lobar microbleeds are associated clinically with cerebral amyloid angiopathy (CAA). Aside from patient populations, microbleeds are frequently observed in seemingly asymptomatic populations. Moreover, many elderly, both in clinical and preclinical populations, have multiple coexisting pathologies in their brains, which complicates the interpretation of cerebral microbleeds, especially early in the clinical course. In this commentary, we discuss the influence of the strongest genetic risk factor for CAA, Apolipoprotein E (APOE), in the spatial distribution of microbleeds, and we additionally address issues in interpretation and implication of the location of microbleeds in clinical and asymptomatic populations.

APOE genotype and MRI markers of cerebrovascular disease: systematic review and meta-analysis

OBJECTIVE

We aimed to examine the association of APOE ε genotype with MRI markers of cerebrovascular disease (CVD): white matter hyperintensities, brain infarcts, and cerebral microbleeds.

METHODS

We performed a systematic review and meta-analysis of 42 cross-sectional or longitudinal studies identified in PubMed from 1966 to June 2012 (n = 29,965). This included unpublished data from 3 population-based studies: 3C-Dijon, Framingham Heart Study, and Sydney Memory and Ageing Study. When necessary, authors were contacted to provide effect estimates for the meta-analysis.

RESULTS

APOE ε4 carrier status and APOE ε44 genotype were associated with increasing white matter hyperintensity burden (sample size-weighted z score meta-analysis [meta]-p = 0.0034 and 0.0030) and presence of cerebral microbleeds (meta odds ratio [OR] = 1.24, 95% confidence interval [CI] [1.07, 1.43], p = 0.004, and 1.87 [1.26, 2.78], p = 0.002), especially lobar. APOE ε2 carrier status was associated with increasing white matter hyperintensity load (z score meta-p = 0.00053) and risk of brain infarct (meta OR = 1.41[1.09, 1.81], p = 0.008).

CONCLUSIONS

APOE ε4 and APOE ε2 were associated with increasing burden in MRI markers for both hemorrhagic and ischemic CVD. While the association of APOE ε4 with an increased burden of CVD could be partly contributing to the relationship between APOE ε4 and AD, APOE ε2 was associated with MRI markers of CVD in the opposite direction compared to AD.

Genetics of cerebral amyloid angiopathy: systematic review and meta-analysis

BACKGROUND AND PURPOSE

Cerebral amyloid angiopathy (CAA) is common in the ageing brain and is associated with dementia and lobar intracerebral haemorrhage. We systematically reviewed genetic associations with CAA to better understand its pathogenesis.

METHODS

We comprehensively sought and critically appraised published studies of associations between any genetic polymorphism and histopathologically confirmed CAA. We assessed the effects of genotype by calculating study specific and pooled odds ratios (ORs) in meta-analyses, and assessed small study bias.

RESULTS

58 studies (6855 participants) investigated apolipoprotein E (APOE) genotype and sporadic CAA. Meta-analysis of 24 (3520 participants) of these showed an association of APOE ε4 with CAA (ε4 present vs absent, pooled OR 2.7, 95% CI 2.3 to 3.1, p<0.00001), which was dose dependent, robust to potential small study biases and occurred irrespective of dementia status. There was no significant association between APOE ε2 and CAA. Among 24 studies (4703 participants) of other genetic polymorphisms, there was preliminary evidence of an association with CAA of polymorphisms in the transforming growth factor β1 gene (two studies, 449 participants), translocase of outer mitochondrial membrane 40 gene (one study, 723 participants) and the complement component receptor 1 gene (one study, 544 participants). There were insufficient data to draw conclusions from 24 studies (∼200 participants) of APOE and hereditary CAA or familial Alzheimer's disease.

CONCLUSIONS

There is convincing evidence for a dose dependent association between APOE ε4 and sporadic CAA. Further work is needed to better understand the mechanism of this association and to further investigate other genetic associations with CAA.

Lower susceptibility to cerebral small vessel disease in human familial longevity: the Leiden Longevity Study

BACKGROUND AND PURPOSE

On MRI, cerebral white matter lesions, lacunar infarcts, and cerebral microbleeds are common imaging correlates of cerebral small vessel damage in apparently healthy elderly individuals. We investigated whether middle-aged to elderly offspring of nonagenarian siblings, who are predisposed to become long-lived as well, have a lower prevalence of white matter lesions, lacunar infarcts, and cerebral microbleeds than control subjects.

METHODS

All subjects were from the Leiden Longevity Study. In this study, middle-aged to elderly offspring of nonagenarian siblings, who are predisposed to become long-lived as well, were contrasted to their spouses. Cerebral small vessel disease was assessed using 3-T MRI.

RESULTS

Offspring were less likely to have severe periventricular frontal caps (odds ratio [OR], 0.3; 95% confidence interval [CI], 0.1-1.1; P trend=0.01) and severe periventricular bands (OR, 0.4; 95% CI, 0.2-0.8; P trend=0.02). Moreover, offspring were less likely to have frontal (OR, 0.4; 95% CI, 0.2-0.9; P trend=0.05), parietal (OR, 0.4; 95% CI, 0.1-0.9; P trend=0.001), temporal (OR, 0.3; 95% CI, 0.1-0.8]; P trend=0.004), and occipital subcortical white matter lesions (OR, 0.3; 95% CI, 0.1-0.6; P trend=0.001). Prevalence of lacunar infarcts also was lower in offspring (OR, 0.3; 95% CI, 0.1-1.1; P=0.07). Prevalence of microbleeds was not significantly different in offspring and control subjects.

CONCLUSIONS

Exceptional familial longevity is associated with a lower susceptibility to white matter lesions and lacunar infarcts, but not cerebral microbleeds.

Cerebral microbleeds and cognition in cerebrovascular disease: an update

Sporadic cerebral small vessel disease is a major cause of cognitive impairment. MRI is an important tool for detecting and mapping cerebral small vessel disease in vivo. Lacunes and white matter changes are recognized as characteristic MRI manifestations of small vessel disease. Cerebral microbleeds (CMBs) - small, perivascular haemorrhages seen as well-demarcated, hypointense, rounded lesions on MRI sequences sensitive to magnetic susceptibility - are a more recently recognized MRI marker of small vessel pathology. CMBs are increasingly found in various patient populations and disease settings, including first-ever and recurrent ischaemic or haemorrhagic stroke, Alzheimer's disease, vascular cognitive impairment and healthy elderly individuals. Increasing evidence suggests that the anatomical distribution of CMBs (lobar or deep) may have diagnostic value in detecting small vessel disease subtypes including hypertensive arteriopathy and cerebral amyloid angiopathy. However, the relevance of CMBs for cognitive impairment remains uncertain. The study of CMBs and cognition in populations with cerebrovascular disease presents a special challenge as they coexist and correlate with other cerebrovascular pathologies. This review updates current thinking on how CMBs may be relevant in the study of cognitive impairment in populations with cerebrovascular disease, and how they can contribute in understanding the links between cerebrovascular and degenerative pathologies.

Amyloid imaging in the differential diagnosis of dementia: review and potential clinical applications

In the past decade, positron emission tomography (PET) with carbon-11-labeled Pittsburgh Compound B (PIB) has revolutionized the neuroimaging of aging and dementia by enabling in vivo detection of amyloid plaques, a core pathologic feature of Alzheimer's disease (AD). Studies suggest that PIB-PET is sensitive for AD pathology, can distinguish AD from non-AD dementia (for example, frontotemporal lobar degeneration), and can help determine whether mild cognitive impairment is due to AD. Although the short half-life of the carbon-11 radiolabel has thus far limited the use of PIB to research, a second generation of tracers labeled with fluorine-18 has made it possible for amyloid PET to enter the clinical era. In the present review, we summarize the literature on amyloid imaging in a range of neurodegenerative conditions. We focus on potential clinical applications of amyloid PET and its role in the differential diagnosis of dementia. We suggest that amyloid imaging will be particularly useful in the evaluation of mildly affected, clinically atypical or early age-at-onset patients, and illustrate this with case vignettes from our practice. We emphasize that amyloid imaging should supplement (not replace) a detailed clinical evaluation. We caution against screening asymptomatic individuals, and discuss the limited positive predictive value in older populations. Finally, we review limitations and unresolved questions related to this exciting new technique.