Apolipoprotein E4, cholinergic integrity and the pharmacogenetics of Alzheimer's disease

Missense substitutions associated with behavioural disturbances in Alzheimer's disease (AD)

Behavioural and psychological symptoms in dementia, or BPSD, occur in the majority of Alzheimer's disease (AD) patients. They are associated with considerable patient morbidity and greater care-giver stress. There is some evidence suggesting that BPSD have a genetic component and a large number of studies have examined the association of candidate genes with these symptoms. This review provides a comprehensive summary of all the published studies investigating the association of candidate gene missense substitutions with BPSD. Missense substitutions could potentially alter protein function or render the protein non-functional, resulting in phenotypic consequences. More than 80 studies investigating the association of 8 missense substitutions in 7 genes with BPSD were identified. However, results of these studies are contradictory and do not provide firm support for these associations. Larger studies and more systematic approaches will delineate the association of missense substitutions with behavioural symptoms in AD.

Individual and combined effects of ApoE and MTHFR 677C/T polymorphisms on cognitive performance in Spanish adolescents: the AVENA study

OBJECTIVE

To examine the individual and combined associations of ApoE and MTHFR 677C/T polymorphisms with cognitive performance in adolescents.

STUDY DESIGN

The study comprised 412 Spanish adolescents (13 to 18.5 years of age). Cognitive performance (verbal, numeric and reasoning abilities, and an overall score) was measured by the Spanish-version of the SRA-Test of Educational-Ability.

RESULTS

We observed no differences in the cognitive performance study variables in adolescents carrying or not carrying the ApoE epsilon4 variant. Adolescents without the MTHFR 677TT genotype had significantly better cognitive performance than their TT peers. The analysis of the combined effect of these polymorphisms revealed that those individuals carrying both the ApoE epsilon4 variant and the MTHFR 677TT genotype had significantly worse cognitive performance than their peers with other genotype combinations. These findings were independent of sex, age pubertal status, socioeconomic status, physical activity, and skipping breakfast.

CONCLUSIONS

The results of the present study suggest that the ApoE epsilon4 alone is not associated with cognitive performance in adolescents. Individuals with the MTHFR 677TT genotype had slightly impaired cognitive performance, whereas we observed a combined effect of both the ApoE epsilon4 variant and the MTHFR 677TT genotype on cognitive performance. More research is needed in larger population samples to corroborate our findings.

PKR, the double stranded RNA-dependent protein kinase as a critical target in Alzheimer's disease

Amyloid β-peptide (Aβ) deposits and neurofibrillary tangles are key hallmarks in Alzheimer's disease (AD). Aβ stimulates many signal transducers involved in the neuronal death. However, many mechanisms remain to be elucidated because no definitive therapy of AD exists. Some studies have focused on the control of translation which involves eIF2 and eIF4E, main eukaryotic factors of initiation. The availability of these factors depends on the activation of the double-stranded RNA-dependent protein kinase (PKR) and the mammalian target of rapamycin (mTOR), respectively. mTOR positively regulates the translation while PKR results in a protein synthesis shutdown. Many studies demonstrated that the PKR signalling pathway is up-regulated in cellular and animal models of AD and in the brain of AD patients. Interestingly, our results showed that phosphorylated PKR and eIF2α levels were significantly increased in lymphocytes of AD patients. These modifications were significantly correlated with cognitive and memory test scores performed in AD patients. On the contrary, the mTOR signalling pathway is down-regulated in cellular and animal models of AD. Recently, we showed that p53, regulated protein in development and DNA damage response 1 and tuberous sclerosis complex 2 could represent molecular links between PKR and mTOR signalling pathways. PKR could be an early biomarker of the neuronal death and a critical target for a therapeutic programme in AD.

Revision of the apolipoprotein E compensatory mechanism recruitment hypothesis

The association between the apolipoprotein E epsilon4 allele and Alzheimer's disease (AD) is well-established. Functional neuroimaging research has supported a compensatory mechanism recruitment hypothesis whereby nondemented epsilon4 participants use additional cognitive resources to buffer against episodic memory declines in older age, a mechanism that is presumably associated with encroaching disease. However, recent studies have implicated a beneficial effect associated with the epsilon4 allele early in the life span. These studies suggest a revised hypothesis whereby epsilon4 persons perform better on cognitive measures early in the life span and then show greater recruitment of brain regions during performance to compensate for declines in older age caused by preclinical AD.

The contribution of apolipoprotein E alleles on cognitive performance and dynamic neural activity over six decades

Neuroimaging shows brain-functional differences due to apolipoprotein E (APOE) polymorphisms may exist decades before the increased risk period for Alzheimer's disease, but little is known about their effect on cognition and brain function in children and young adults. This study assessed 415 healthy epsilon2 and epsilon4 carriers and matched epsilon3/epsilon3 controls, spanning ages 6-65, on a range of cognitive tests. Subjects were also compared on a new dynamical measure of EEG activity during a visual working memory task using alphabetical stimuli. epsilon4 subjects had better verbal fluency compared to epsilon3, an effect that was strongest in 51-65 year-olds. No epsilon4 deficits in cognition were found. In 6-15 year-olds, there were differences in total spatio-temporal wave activity between epsilon3 and epsilon4 subjects in the theta band, approximately 200ms post-stimulus. Differences in brain function in younger epsilon4 subjects and superior verbal fluency across the entire age range suggest that the APOE epsilon4 allele is an example of antagonistic pleiotropy.

Cumulative lead dose and cognitive function in adults: a review of studies that measured both blood lead and bone lead

OBJECTIVE

We review empirical evidence for the relations of recent and cumulative lead dose with cognitive function in adults.

DATA SOURCES

A systematic search of electronic databases resulted in 21 environmental and occupational studies from 1996 to 2006 that examined and compared associations of recent (in blood) and cumulative (in bone) lead doses with neurobehavioral outcomes.

DATA EXTRACTION

Data were abstracted after consideration of exclusion criteria and quality assessment, and then compiled into summary tables.

CONCLUSIONS

At exposure levels encountered after environmental exposure, associations with bio-markers of cumulative dose (mainly lead in tibia) were stronger and more consistent than associations with blood lead levels. Similarly, in studies of former workers with past occupational lead exposure, associations were also stronger and more consistent with cumulative dose than with recent dose (in blood). In contrast, studies of currently exposed workers generally found associations that were more apparent with blood lead levels; we speculate that the acute effects of high, recent dose may mask the chronic effects of cumulative dose. There is moderate evidence for an association between psychiatric symptoms and lead dose but only at high levels of current occupational lead exposure or with cumulative dose in environmentally exposed adults.

Premorbid effects of APOE on synaptic proteins in human temporal neocortex

APOE affects the risk of Alzheimer's disease (AD) and course of several other neurologic diseases. Experimental studies suggest that APOE influences synaptogenesis. We measured the concentration of two presynaptic proteins, synaptophysin and syntaxin 1, and also postsynaptic density-95 (PSD95), in superior temporal cortex from 42 AD and 160 normal brains, and determined the APOE genotypes. The concentration of both presynaptic proteins was approximately two-thirds lower in AD than normal brains and that of PSD95 one-third lower. No effect of APOE on synaptic proteins was found in advanced AD. However, in normal brain, epsilon4 was associated with lower concentrations of all three synaptic proteins and epsilon2 with significantly elevated PSD95 (p=0.03). A combined measure of synaptic proteins showed a significant linear decrease from epsilon2 through epsilon3 to varepsilon4 (p=0.01). APOE influences the concentration of synaptic proteins in normal superior temporal cortex and may thereby affect the response to injury, and the risk and outcome of a range of neurologic diseases.

Apolipoprotein E genotype predicts 24-month bayley scales infant development score

Apolipoprotein E (APOE) regulates cholesterol and fatty acid metabolism, and may mediate synaptogenesis during neurodevelopment. To our knowledge, the effects of APOE4 isoforms on infant development have not been studied. This study was nested within a cohort of mother-infant pairs living in and around Mexico City. A multiple linear regression model was constructed using the 24-mo Mental Development Index (MDI) of the Bayley Scale as the primary outcome and infant APOE genotype as the primary risk factor of interest. Regression models stratified on APOE genotype were constructed to explore effect modification. Of 311 subjects, 53 (17%) carried at least one copy of the APOE4 allele. Mean (SD) MDI scores among carriers with at least one copy of APOE4 were 94.1 (14.3) and among E3/E2 carriers were 91.2 (14.0). After adjustment for covariates, APOE4 carrier status was associated with a 4.4 point (95% confidence interval: 0.1-8.7; p = 0.04) higher 24-mo MDI. In the stratified regression models, the negative effects for umbilical cord blood lead level on 24-mo MDI score was approximately 4-fold greater among APOE3/APOE2 carriers than among APOE4 carriers. These results suggest that subjects with the E4 isoform of APOE may have advantages over those with the E2 or E3 isoforms with respect to early life neuronal/brain development.

ApoE genotype influences the biological effect of donepezil on APP metabolism in Alzheimer disease: evidence from a peripheral model

Three major amyloid precursor protein (APP) forms with apparent molecular weight ranging from 106 to 130 kDa are present in human platelets. Alzheimer disease (AD) is associated with a decreased APP forms ratio (APPr) between the three major forms. A total of 25 mild to moderate AD patients were investigated. Platelet APPr was studied before and after 30 days of acetylcholinesterase-inhibitor treatment (donepezil, 5 mg daily). Patients were grouped into non-epsilon4 carriers and epsilon4 carriers according to apolipoprotein E (ApoE) genotype. At baseline, all patients showed low APPr levels and no significant difference was found between the two ApoE subgroups. After treatment, although a marked improvement in APPr was observed in most patients, non-epsilon4 carriers displayed a higher increase compared to epsilon4 carriers (P<0.0001). The present study provides evidence that donepezil influences APP metabolism in platelets, and suggests that ApoE genotype might be an important modulating factor for drug responsiveness in AD.

Apoptosis as a general cell death pathway in neurodegenerative diseases

Neurodegenerative processes are generally characterized by the long-lasting course of neuronal death and the selectivity of the neuronal population or brain structure involved in the lesion. Two main common forms of cell death that have been described in neurons as in other vertebrate tissues i.e., necrosis and apoptosis. Necrosis is the result of cellular "accidents", such as those occurring in tissues subjected to chemical trauma. The necrotizing cells swell, rupture and provoke an inflammatory response. Apoptosis, on the other hand, is dependent on the cell's "decision" to commit suicide and die, and therefore is referred to as "programmed cell death" (PCD). The course of apoptotic death is characterized by a massive morphological change, including cell shrinkage, nuclear (chromosome) condensation and DNA degradation. Activation of PCD in an individual cell is based on its own internal metabolism, environment, developmental background and its genetic information. Such a situation occurs in most of the neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases and amyotrophic lateral sclerosis (ALS). In these pathological situations, specific neurons undergo apoptotic cell death characterized by DNA fragmentation, increased levels of pro-apoptotic genes and "apoptotic proteins" both, in human brain and in experimental models. It is of utmost importance to conclusively determine the mode of cell death in neurodegenerative diseases, because new "anti-apoptotic" compounds may offer a means of protecting neurons from cell death and of slowing the rate of cell degeneration and illness progression.

Recent advances in neurotrauma

The frequency of and outcome from acute traumatic brain injury (TBI) in humans are detailed together with a classification of the principal focal and diffuse pathologies, and their mechanisms in extract laboratory models are outlined. Particular emphasis is given to diffuse axonal injury, which is a major determinant of outcome. Cellular and molecular cascades triggered by injury are described with reference to the induction of axolemmal and cytoskeletal abnormalities, necrotic and apoptotic cell death, the role of Ca2+, cytokines and free radicals, and damage to DNA. It is concluded that TBI in humans is heterogeneous, reflecting various pathologies in differing proportions in patients whose genetic background (APOE gene polymorphisms) contributes to the outcome at 6 months. Although considerable progress has been made in the understanding of TBI, much remains to be determined. However, a deeper understanding of the pathophysiological events may lead to the possibility of improving outcome from rational targeted therapy.

The role of apolipoprotein E in Alzheimer's disease, acute brain injury and cerebrovascular disease: evidence of common mechanisms and utility of animal models

The epsilon 4 allele of apolipoprotein E (APOE denotes gene; apoE denotes protein) is a major risk factor for Alzheimer's disease (AD). More recent evidence indicates an association with a poor outcome after acute brain injury including that due to head trauma and intracerebral hemorrhage. APOE gene polymorphism also influences the risk of hemorrhage in cerebral amyloid angiopathy. These diverse brain disorders seem to have some mechanisms in common. The multiplicity of the roles of apoE within the central nervous system is currently being unraveled. For example, apoE can interact with amyloid beta-protein and tau, proteins central to the pathogenesis of AD. In addition to these effects, it is proposed that one of the major functions of apoE is to mediate neuronal protection, repair and remodeling. In all of the different roles proposed, there are marked apoE-isoform specific differences. Although it remains to be clarified which is the most important mechanism(s) in each disorder in which apoE is involved, these isoform specific differences seem to underly a genetically determined susceptibility to outcome from acute brain injury and to AD with APOE epsilon 4 conferring relative vulnerability. This review focuses on apoE research, from clinical studies to animal models, in AD, acute brain injury and cerebrovascular disease and explores the common mechanisms that may explain some of the complex underlying neurobiology.