Lack of association between APOE gene polymorphisms and amyotrophic lateral sclerosis: a comprehensive meta-analysis

Association of APOE genotype and cerebrospinal fluid Aβ and tau biomarkers with cognitive and motor phenotype in amyotrophic lateral sclerosis

OBJECTIVE

Little is known about amyotrophic lateral sclerosis (ALS)-nonspecific cognitive deficits - most notably memory disturbance - and their biological underpinnings. We investigated the associations of the Alzheimer's disease (AD) genetic risk factor APOE and cerebrospinal fluid (CSF) biomarkers Aβ and tau proteins with cognitive and motor phenotype in ALS.

METHODS

APOE haplotype was determined in 281 ALS patients; for 105 of these, CSF levels of Aβ42, Aβ40, total tau (T-tau), and phosphorylated tau (P-tau181) were quantified by chemiluminescence enzyme immunoassay (CLEIA). The Edinburgh Cognitive and Behavioural ALS Screen (ECAS) was employed to evaluate the neuropsychological phenotype.

RESULTS

APOE-E4 allele was associated with worse ECAS memory score (median, 14.0 in carriers vs. 16.0 in non-carriers) and lower CSF Aβ42 (-0.8 vs. 0.1, log-transformed values) and Aβ42/40 ratio (-0.1 vs. 0.3). Some 37.1% of ALS patients showed low Aβ42 levels, possibly reflecting cerebral Aβ deposition. While lower Aβ42/40 correlated with lower memory score (β = 0.20), Aβ42 positively correlated with both ALS-specific (β = 0.24) and ALS-nonspecific (β = 0.24) scores. Although Aβ42/40 negatively correlated with T-tau (β = -0.29) and P-tau181 (β = -0.33), we found an unexpected positive association of Aβ42 and Aβ40 with both tau proteins. Regarding motor phenotype, lower levels of Aβ species were associated with lower motor neuron (LMN) signs (Aβ40: β = 0.34; Aβ42: β = 0.22).

CONCLUSIONS

APOE haplotype and CSF Aβ biomarkers are associated with cognitive deficits in ALS and particularly with memory impairment. This might partly reflect AD-like pathophysiological processes, but additional ALS-specific mechanisms could be involved.

Cholesterol dyshomeostasis in amyotrophic lateral sclerosis: cause, consequence, or epiphenomenon?

Amyotrophic lateral sclerosis (ALS), the most common adult-onset motor neuron disease, is characterized by the selective degeneration of motor neurons leading to paralysis and eventual death. Multiple pathogenic mechanisms, including systemic dysmetabolism, have been proposed to contribute to ALS. Among them, dyslipidemia, i.e., abnormal level of cholesterol and other lipids in the circulation and central nervous system (CNS), has been reported in ALS patients, but without a consensus. Cholesterol is a constituent of cellular membranes and a precursor of steroid hormones, oxysterols, and bile acids. Consequently, optimal cholesterol levels are essential for health. Due to the blood-brain barrier (BBB), cholesterol cannot move between the CNS and the rest of the body. As such, cholesterol metabolism in the CNS is proposed to operate autonomously. Despite its importance, it remains elusive how cholesterol dyshomeostasis may contribute to ALS. In this review, we aim to describe the current state of cholesterol metabolism research in ALS, identify unresolved issues, and provide potential directions.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Plasma apolipoprotein E levels in longitudinally followed patients with mild cognitive impairment and Alzheimer’s disease

Background

Low levels of plasma apolipoprotein E (apoE) and presence of the APOE ε4 allele are associated with an increased risk of Alzheimer’s disease (AD). Although the increased risk of AD in APOE ε4-carriers is well-established, the protein levels have received limited attention.

Methods

We here report the total plasma apoE and apoE isoform levels at baseline from a longitudinally (24 months) followed cohort including controls (n = 39), patients with stable amnestic mild cognitive impairment during 24 months follow up (MCI-MCI, n = 30), patients with amnestic MCI (aMCI) that during follow-up were clinically diagnosed with AD with dementia (ADD) (MCI-ADD, n = 28), and patients with AD with dementia (ADD) at baseline (ADD, n = 28). We furthermore assessed associations between plasma apoE levels with cerebrospinal fluid (CSF) AD biomarkers and α-synuclein, as well as both CSF and plasma neurofilament light chain (NfL), YKL-40 and kallikrein 6.

Results

Irrespective of clinical diagnosis, the highest versus the lowest apoE levels were found in APOE ε2/ε3 versus APOE ε4/ε4 subjects, with the most prominent differences exhibited in females. Total plasma apoE levels were 32% and 21% higher in the controls versus MCI-ADD and ADD patients, respectively. Interestingly, MCI-ADD patients exhibited a 30% reduction in plasma apoE compared to MCI-MCI patients. This decrease appeared to be associated with brain amyloid-β (Aβ42) pathology regardless of disease status as assessed using the Amyloid, Tau, and Neurodegeneration (A/T/N) classification. In addition to the association between low plasma apoE and low levels of CSF Aβ42, lower apoE levels were also related to higher levels of CSF total tau (t-tau) and tau phosphorylated at Threonine 181 residue (p-tau) and NfL as well as a worse performance on the mini-mental-state-examination. In MCI-ADD patients, low levels of plasma apoE were associated with higher levels of CSF α-synuclein and kallikrein 6. No significant correlations between plasma apoE and the astrocytic inflammatory marker YKL40 were observed.

Conclusions

Our results demonstrate important associations between low plasma apoE levels, Aβ pathology, and progression from aMCI to a clinical ADD diagnosis.

Association of apolipoprotein E ε4 allele and amyotrophic lateral sclerosis in Chinese population

Background: Amyotrophic lateral sclerosis (ALS) has a complex genetic origin, and how immune dysregulation may contribute to ALS etiology remain unclear. Given the roles played by apolipoprotein E (APOE) signaling in neuroinflammation and neurodegeneration, an improved knowledge of the association between APOE genotypes and ALS risk in Chinese population may help to understand the underlying etiology of the disease. Methods: A retrospective case-control study with participants of Chinese ancestry was conducted, with a total of 683 ALS patients and 369 healthy controls analyzed for APOE genotypes using Sanger sequencing. In addition, 282 of these patients were further analyzed for known ALS risk variants and rare deleterious variants related to immune disorders via whole exome sequencing. Results: Among the 683 ALS patients analyzed (346 males, 337 females; mean age at onset [SD]: 51.9 [10.9]), 145 patients (21.1%) carried ε4, the proportion of which was significantly higher than 16.0% in controls (59/369; OR, 1.42; 95%CI, 1.02-1.98; p = 0.02). There is no evidence supporting the association between APOE genotypes and disease phenotypes. We also didn't find any enrichment of currently known ALS risk variants or variants in genes related to immune abnormality in specific APOE genotypes. Conclusion: Our study highlighted the importance of trans-ethnic studies in identifying genetic risk factors, and the relevance of APOE in ALS etiopathogenesis in Chinese population.

APOE2: protective mechanism and therapeutic implications for Alzheimer's disease

Investigations of apolipoprotein E (APOE) gene, the major genetic risk modifier for Alzheimer's disease (AD), have yielded significant insights into the pathogenic mechanism. Among the three common coding variants, APOE*ε4 increases, whereas APOE*ε2 decreases the risk of late-onset AD compared with APOE*ε3. Despite increased understanding of the detrimental effect of APOE*ε4, it remains unclear how APOE*ε2 confers protection against AD. Accumulating evidence suggests that APOE*ε2 protects against AD through both amyloid-β (Aβ)-dependent and independent mechanisms. In addition, APOE*ε2 has been identified as a longevity gene, suggesting a systemic effect of APOE*ε2 on the aging process. However, APOE*ε2 is not entirely benign; APOE*ε2 carriers exhibit increased risk of certain cerebrovascular diseases and neurological disorders. Here, we review evidence from both human and animal studies demonstrating the protective effect of APOE*ε2 against AD and propose a working model depicting potential underlying mechanisms. Finally, we discuss potential therapeutic strategies designed to leverage the protective effect of APOE2 to treat AD.

Lipid Biomarkers for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal degenerative disease primarily characterized by the selective loss of upper and lower motor neurons. To date, there is still an unmet need for robust and practical biomarkers that could estimate the risk of the disease and its progression. Based on metabolic modifications observed at the level of the whole body, different classes of lipids have been proposed as potential biomarkers. This review summarizes investigations carried out over the last decade that focused on changes in three major lipid species, namely cholesterol, triglycerides and fatty acids. Despite some contradictory findings, it is becoming increasingly accepted that dyslipidemia, and related aberrant energy homeostasis, must be considered as essential components of the pathological process. Therefore, it is tempting to envisage dietary interventions as a means to counterbalance the metabolic disturbances and ameliorate the patient's quality of life.

A Quarter Century of APOE and Alzheimer's Disease: Progress to Date and the Path Forward

Alzheimer's disease (AD) is considered a polygenic disorder. This view is clouded, however, by lingering uncertainty over how to treat the quasi "monogenic" role of apolipoprotein E (APOE). The APOE4 allele is not only the strongest genetic risk factor for AD, it also affects risk for cardiovascular disease, stroke, and other neurodegenerative disorders. This review, based mostly on data from human studies, ranges across a variety of APOE-related pathologies, touching on evolutionary genetics and risk mitigation by ethnicity and sex. The authors also address one of the most fundamental question pertaining to APOE4 and AD: does APOE4 increase AD risk via a loss or gain of function? The answer will be of the utmost importance in guiding future research in AD.

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.

Apolipoprotein E and Alzheimer's disease: the influence of apolipoprotein E on amyloid-β and other amyloidogenic proteins

Alzheimer's disease (AD) is one of the fastest-growing causes of death and disability in persons 65 years of age or older, affecting more than 5 million Americans alone. Clinical manifestations of AD include progressive decline in memory, executive function, language, and other cognitive domains. Research efforts within the last three decades have identified APOE as the most significant genetic risk factor for late-onset AD, which accounts for >99% of cases. The apoE protein is hypothesized to affect AD pathogenesis through a variety of mechanisms, from its effects on the blood-brain barrier, the innate immune system, and synaptic function to the accumulation of amyloid-β (Aβ). Here, we discuss the role of apoE on the biophysical properties and metabolism of the Aβ peptide, the principal component of amyloid plaques and cerebral amyloid angiopathy (CAA). CAA is characterized by the deposition of amyloid proteins (including Aβ) in the leptomeningeal medium and small arteries, which is found in most AD cases but sometimes occurs as an independent entity. Accumulation of these pathologies in the brain is one of the pathological hallmarks of AD. Beyond Aβ, we will extend the discussion to the potential role of apoE on other amyloidogenic proteins found in AD, and also a number of diverse neurodegenerative diseases.

Role of apolipoprotein E polymorphism as a prognostic marker in traumatic brain injury and neurodegenerative disease: a critical review

OBJECT

The difference in course and outcome of several neurodegenerative conditions and traumatic injuries of the nervous system points toward a possible role of genetic and environmental factors as prognostic markers. Apolipoprotein E (Apo-E), a key player in lipid metabolism, is recognized as one of the most powerful genetic risk factors for dementia and other neurodegenerative diseases. In this article, the current understanding of APOE polymorphism in various neurological disorders is discussed.

METHODS

The English literature was searched for various studies describing the role of APOE polymorphism as a prognostic marker in neurodegenerative diseases and traumatic brain injury. The wide ethnic distribution of APOE polymorphism was discussed, and the recent meta-analyses of role of APOE polymorphism in multiple diseases were analyzed and summarized in tabular form.

RESULTS

Results from the review of literature revealed that the distribution of APOE is varied in different ethnic populations. APOE polymorphism plays a significant role in pathogenesis of neurodegeneration, particularly in Alzheimer’s disease. APOE ε4 is considered a marker for poor prognosis in various diseases, but APOE ε2 rather than APOE ε4 has been associated with cerebral amyloid angiopathy-related bleeding and sporadic Parkinson’s disease. The role of APOE polymorphism in various neurological diseases has not been conclusively elucidated.

CONCLUSIONS

Apo-E is a biomarker for various neurological and systemic diseases. Therefore, while analyzing the role of APOE polymorphism in neurological diseases, the interpretation should be done after adjusting all the confounding factors. A continuous quest to look for associations with various neurological diseases and wide knowledge of available literature are required to improve the understanding of the role of APOE polymorphism in these conditions and identify potential therapeutic targets.