Diagnosis of Alzheimer's disease: defining genetic profiles (genotype vs phenotype)

Resting-state network disruption and APOE genotype in Alzheimer's disease: a lagged functional connectivity study

Background

The apolipoprotein E epsilon 4 (APOE-4) is associated with a genetic vulnerability to Alzheimer's disease (AD) and with AD-related abnormalities in cortical rhythms. However, it is unclear whether APOE-4 is linked to a specific pattern of intrinsic functional disintegration of the brain after the development of the disease or during its different stages. This study aimed at identifying spatial patterns and effects of APOE genotype on resting-state oscillations and functional connectivity in patients with AD, using a physiological connectivity index called “lagged phase synchronization”.

Methodology/Principal Findings

Resting EEG was recorded during awake, eyes-closed state in 125 patients with AD and 60 elderly controls. Source current density and functional connectivity were determined using eLORETA. Patients with AD exhibited reduced parieto-occipital alpha oscillations compared with controls, and those carrying the APOE-4 allele had reduced alpha activity in the left inferior parietal and temporo-occipital cortex relative to noncarriers. There was a decreased alpha2 connectivity pattern in AD, involving the left temporal and bilateral parietal cortex. Several brain regions exhibited increased lagged phase synchronization in low frequencies, specifically in the theta band, across and within hemispheres, where temporal lobe connections were particularly compromised. Areas with abnormal theta connectivity correlated with cognitive scores. In patients with early AD, we found an APOE-4-related decrease in interhemispheric alpha connectivity in frontal and parieto-temporal regions.

Conclusions/Significance

In addition to regional cortical dysfunction, as indicated by abnormal alpha oscillations, there are patterns of functional network disruption affecting theta and alpha bands in AD that associate with the level of cognitive disturbance or with the APOE genotype. These functional patterns of nonlinear connectivity may potentially represent neurophysiological or phenotypic markers of AD, and aid in early detection of the disorder.

The application of functional genomics to Alzheimer's disease

Alzheimer's disease (AD) is a polygenic/complex disorder in which more than 50 genetic loci are involved. Primary and secondary loci are potentially responsible for the phenotypic expression of the disease under the influence of both environmental factors and epigenetic phenomena. The construction of haplotypes as genomic clusters integrating the different genotype combinations of AD-related genes is a suitable strategy to investigate functional genomics in AD. It appears that AD patients show about 3-5 times higher genetic variation than the control population. The analysis of genotype-phenotype correlations has revealed that the presence of the APOE-4 allele in AD, in conjunction with other loci distributed across the genome, influence disease onset, brain atrophy, cerebrovascular perfusion, blood pressure, beta-amyloid deposition, ApoE secretion, lipid metabolism, brain bioelectrical activity, cognition, apoptosis and treatment outcome. Pharmacogenomics studies also indicate that the therapeutic response in AD is genotype-specific and that approximately 15% of the cases with efficacy and/or safety problems are associated with a defective CYP2D6 gene. Consequently, the understanding of functional genomics in AD will foster productive pharmacogenomic studies in the search for effective medications and preventive strategies in AD.

A pharmacogenomic approach to Alzheimer's disease

Single nucleotide polymorphisms (susceptibility genetics) and genomic point mutations (mendelian genetics) can be used in Alzheimer's disease (AD) for diagnostic, predictive and therapeutic purposes. Using a matrix genetic model, including APOE, PS1 and PS2 allelic variants, we have studied the distribution of 36 different genotypes in the AD population (N= 479) and the genotype-related cognitive response to a multifactorial therapy in AD patients with mild-to-moderate dementia. The 10 most frequent AD genotypes are the following: 1) E33P112P2 + (17.75%), 2) E33P112P2- (15.55%), 3) E33P111P2+ (10.85%), 4) E34P112P2+ (9.60%), 5) E34P112P2- (7.56%), 6) E33P111P2- (7.10%), 7) E34P111P2+ (4.80%), 8) E33P122P2+ (4.38%), 9) E34P111P2- (4.18%), and 10) E34P122P2+ (3.55%). APOE-4/4-related genotypes represent less than 3% in the following order: E44P112P2 + > E44P111P2+ = E44P111P2- > E44P112P2+ > E44P122P2+ = E44P122P2. Multifactorial therapy with CDP-choline (1,000 mg/day) + piracetam (2,400 mg/day) + anapsos (360 mg/day) did improve mental performance during the first 6-15 months in a genotype-specific fashion. The best responders in the APOE series were patients with APOE-3/4 genotype (r= +0.013), while the worst responders were APOE-4/4 patients (r= -0.93). PS1-related genotypes responded in a similar manner; and patients with a defective PS2 gene exon 5 (PS2+) always showed a poorer therapeutic response than PS2- patients. All these data suggest that the therapeutic outcome in AD exhibits a genotype-specific pattern, and that a pharmacogenomic approach to AD might be a valuable strategy for drug development and monitoring.

Age-dependent cytokine responses: trimethyltin hippocampal injury in wild-type, APOE knockout, and APOE4 mice

In this study, the hippocampal neurotoxicant trimethyltin (TMT) was used to examine possible differential susceptibility associated with the apolipoprotein E genotype. Mice-wild type (C57BL6J), APOE knockout, and APOE4 transgenic-received either saline or TMT (2 mg/kg, ip) at either 21 days or 8 months of age. At both ages, similar mRNA levels were seen in the hippocampus across genotypes for ICAM-1, A20, and MAC-1. GFAP mRNA was higher in the APOE knockouts and APOE4 as compared to wild-type mice. Within 24 h, TMT produced cell death of hippocampal dentate granule neurons and mild astrogliosis in all animals. In 21-day-old mice, TMT exposure significantly increased mRNA levels for ICAM-1 and MIP-1alpha in all genotypes. EB-22, GFAP, TNFalpha, and TGF-beta1 levels were significantly elevated in both wild-type and APOE knockout mice following TMT. At 8 months of age, genotype specific differences were observed. mRNA levels for GFAP, TNFbeta, TNFalpha, and MIP-1alpha were increased in both APOE knockout and APOE4 mice compared to wild-type mice. TMT exposure significantly increased mRNA levels for GFAP and MIP-1alpha in all animals. TNFalpha mRNA levels were increased in wild-type and APOE4 mice while EB22 mRNA levels were increased in both the APOE knockout and APOE4 mice but not wild-type mice. These data suggest an age-dependent effect on both microglia early inflammatory responses to injury associated with the APOE genotype.

Disturbance of cerebral function in people exposed to drinking water contaminated with aluminium sulphate: retrospective study of the Camelford water incident

OBJECTIVE

To establish whether people exposed to drinking water contaminated with 20 tonnes of aluminium sulphate in the Camelford area of Cornwall in the south west of England in July 1988 had suffered organic brain damage as opposed to psychological trauma only.

DESIGN

Retrospective study of affected people.

PARTICIPANTS

55 affected people and 15 siblings nearest in age to one of the group but who had not been exposed to the contaminated water were studied.

MAIN OUTCOME MEASURES

Various clinical and psychological tests to determine medical condition and anxiety levels in affected people. Assessment of premorbid IQ (pFSIQ) with the national adult reading test, a computerised battery of psychomotor testing, and measurement of the difference in latencies between the flash and pattern visual evoked potentials in all participants.

RESULTS

The mean (SE) pFSIQ was above average at 114.4 (1.1). The most sensitive of the psychomotor tests for organic brain disease was the symbol digit coding (SDC) test (normal score 100, abnormal <85). PARTICIPANTS performed less well on this test (54.5 (6.0)) than expected from their pFSIQ (P<0.0001) and a little less poorly on the averaged less discriminating tests within the battery (86.1 (2.5), P<0.0001). In a comparison with the 15 sibling pairs (affected people's age 41.0 (3.3) years v sibling age of 42.7 (3.1) years (P=0.36) the exposed people had similar pFSIQ (114.7 (2.1)) to their siblings (116.3 (2.1), (P=0.59) but performed badly on the symbol digit coding test (51.8 (16.6)) v (87.5 (4.9) for siblings, P=0.03). The flash-pattern differences in exposed people were greater than in 42 unrelated control subjects of similar age (27.33 (1.64) ms v 18. 57 (1.47) ms, P=0.0002). The 15 unexposed siblings had significantly better flash-pattern differences than their affected siblings (13.4 (2.4) ms v 29.6 (2.9) ms, P=0.0002). No effect of anxiety could be shown on these measurements from the analysis of the anxiety scores of exposed people.

CONCLUSION

People who were exposed to the contaminated water at Camelford suffered considerable damage to cerebral function, which was not related to anxiety. Follow up studies would be required to determine the longer term prognosis for affected individuals.

19/20 kDa low molecular weight serum protein pattern: a novel potential biochemical prognostic marker for different types of dementia

In recent years, several attempts have been made to identify biochemical and/or genetic markers which might have diagnostic and prognostic uses for Alzheimer's disease (AD). To look for possible new blood markers, a longitudinal study was carried out in our Central Nervous System Biomedical Research Center between October 1996 and July 1998. A total of 30 AD subjects were diagnosed as AD patients according to the DSM-IV and NINCDS-ADRDA criteria. Vascular dementia (VD, n= 17), mixed dementia (MXD, n = 18) and healthy age-matched control subjects (n = 15) have been included in the study. Serum samples have been analysed by SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) and the resulting protein patterns have been compared. The objectives of this study were to examine the presence of peripheral markers among patients with AD, VD and MXD and to explore the relationship between serum markers and APOE genotype. The findings suggest that two proteins of 19 and 20kDa molecular weight, respectively, might be associated with disease progression in different types of dementia.

Microglial activation induced by factor(s) contained in sera from Alzheimer-related ApoE genotypes

Several factors that increase the likelihood of developing Alzheimer's disease (AD) have already been identified. A correct evaluation of these may contribute to a better understanding of the etiology of the disease. The risk of developing AD definitely increases with (a) age, (b) head injuries, (c) family history of AD or Down syndrome, (d) sex (higher prevalence of AD in women), (e) vascular disease, (f) exposure to environmental toxins, (g) infectious processes, or (h) changes in immune function, and recent advances in molecular genetics have suggested that genetic predisposition (i) can be considered one of the most important risk factors in the development of AD. A significant increase in the number of amyloid plaques in AD patients with an apolipoprotein E4 (ApoE) allele has been observed and the results of several genetic studies indicate that the etiology of this neurodegenerative disease is associated with the presence of the allele E4 of ApoE. A potential source of damage in the AD brain is an altered response triggered by microglial activation, which is associated with amyloid plaques. It has become evident that a dysregulation of cytokine release appears within lesions of many types of brain disorders including infection, trauma, stroke, and neurodegenerative diseases. Many studies have shown that microglia secrete both cytokines and cytotoxins and since reactive microglia appears in nearly every type of brain damage, it is likely that their secreted products ultimately help to determine the rate of damaged brain tissue. In this study, in vitro cell cultures were established to investigate the effect of different concentrations of human sera (2.5% and 10%) with specific ApoE genotypes from Alzheimer's and non-Alzheimer's subjects on ameboid and flat microglial cells obtained from neonatal rat hippocampi. Results show that a modulation in the proliferation and activation of microglial cells was obtained and that AD sera, mainly in the ApoE 3/4 and 4/4 genotype contain factor(s) which are able to induce morphological changes, as measured by an increase in the ameboid cell type. In addition, major histocompatibility complex (MHC) class II antigen expression, as measured by flow cytometric analysis, and interleukin-1beta (IL-1beta) release as measured by enzyme linked immunoadsorbent assay (ELISA), in comparison with control groups and lipopolysaccharide (LPS)-treated cells, clearly demonstrate a direct effect of ApoE 3/4 and 4/4 and/or an indirect effect mediated by the release of IL-1beta on microglia activation. These results strongly suggest that primary in vitro microglial cell cultures can be used as a screening model to test human sera as well as the effect of new potential drugs aimed at down-regulating microglia activation.