Validation of amyloid-β peptides in CSF diagnosis of neurodegenerative dementias

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Abstract

Biomarkers for differential diagnosis of the three most frequent degenerative forms of dementia, Alzheimer's disease (AD), dementia with Lewy bodies (DLB) and frontotemporal dementias (FTD), are currently under intensive investigation, but disease-specific biomarkers for FTD and DLB are still lacking. We analyzed 303 cerebrospinal fluid (CSF) samples of 71 AD, 32 DLB and 36 FTD patients in comparison to 93 various other dementias (OD), 20 peripheral neurologic disease (PND) controls, 25 neurodegenerative disorders without dementia (ND) and 26 depressive cognitive complainers (DCC) for distinct CSF amyloid-β (Aβ) peptide patterns, using the quantitative Aβ-SDS-PAGE/immunoblot. Additionally, the novel electrochemiluminescence technique (MSD) was used to validate the measures on Aβ1-38. The main outcome measures were a striking decrease of Aβ1-42 in AD (P=7.4 × 10−19), and most interestingly a pronounced decrease of Aβ1-38 in FTD (P=9.6 × 10−7). Moreover, a novel peptide that most probably represents an oxidized α-helical form of Aβ1-40 (Aβ1-40ox) displayed a highly significant increase in DLB (P=3.7 × 10−3) as compared to non-demented disease controls. The overall diagnostic accuracy of percentage Aβ peptide abundances (Aβ1-X%) was clearly superior to absolute CSF Aβ levels. Aβ1-42% and Aβ1-38% enabled contrasts of 85% or beyond to distinguish AD and FTD, respectively, from all other investigated subjects. Aβ1-40ox% yielded a diagnostic sensitivity and specificity of 88 and 73% for the detection of DLB among all other investigated patients. We found a strong correlation between Aβ1-38 levels as measured by the Aβ-SDS-PAGE/immunoblot and MSD, respectively. CSF Aβ peptides may reflect disease-specific impact of distinct neurodegenerative processes on Aβ peptide metabolism and represent a potential diagnostic biomarker for AD, FTD and DLB.

Introduction

Amyloid plaques, one of the major neuropathological hallmarks of Alzheimer's disease (AD),1 mainly consist of aggregated amyloid-β (Aβ) peptides and also occur in other neurodegenerative diseases, such as dementia with Lewy bodies (DLB),2 but are rarely found in frontotemporal dementias (FTD).3

The recently established Aβ-SDS-PAGE/immunoblot enables measurement of a pattern of the carboxyterminally truncated (Ct-truncated) Aβ peptides 1–37, 1–38 and 1–39 in addition to 1–40 and 1-42 (Aβ peptide pattern) in cerebrospinal fluid (CSF). This approach can be assumed to reflect most adequately disease-specific changes of APP metabolism during ongoing neurodegenerative processes and has shown diagnostic value for differentiation of AD among other dementias (OD) or organic brain diseases.4, 5 In particular, a novel peptide that most probably represents an oxidized α-helical form of Aβ1-40 (Aβ1-40ox) was significantly increased in DLB in a smaller pilot study.5 Some of our own data already suggested a specific reduction of Aβ1-38 in FTLD as measured by Aβ-SDS-PAGE/immunoblot. However, these studies lack proper comparison groups for estimating test accuracies in a larger collective of differential diagnostic diagnoses.

To the best of our knowledge, the present study is the largest CSF-based biomarker investigation on Aβ peptide patterns directly comparing the three major degenerative dementias AD, DLB and FTD among various other dementia patients as well as peripheral neurologic diseases (PND), neurodegenerative disorders without dementia (ND) and depressive cognitive complainers (DCC).

Methods

Participants

We prospectively investigated 303 consecutive CSF samples referred to our laboratory between 1999 and 2004. Seventy-four of these patients had been investigated in previous studies under another objective and the results have been published.5, 6 The diagnoses thereof were non-demented disease controls (NDC) (n=15), AD (n=21), DLB (n=20) and OD (n=18).

CSF of AD, FTD and OD as well as three patients with depression came from the memory clinic of the University of Goettingen, other came from wards. Seven AD patients came from the dementia outpatient clinic of the University of Erlangen. The CSF of hospitalized patients with Parkinson's disease (PD), Parkinson's disease dementia (PDD) and DLB patients was recruited in the Paracelsus-Elena Klinik, Kassel, specialized in the diagnosis and treatment of movement disorders.

Diagnoses were rendered by a psychiatrist, a neurologist and a neuropsychologist, all very experienced in clinical differential diagnosis of dementias, on the basis of thorough anamnesis, clinical examination, results of neuropsychological assessment, clinical records and best clinical judgment. All three investigators were blinded to the neurochemical outcome measures. Investigations were carried out with the informed consent of patients or their authorized caregiver. If possible, neuropsychological assessment (Mini-Mental-Status Examination (MMSE) at minimum) was performed on patients suffering from cognitive impairments at the time of lumbar puncture. The study was conducted under the guidelines of the Declaration of Helsinki7 and approved by the ethics committee of the University of Goettingen, Erlangen-Nuremberg and Hessen.

Test methods

The preanalytical handling of CSF samples followed a standardized protocol according to previously published data.8

Aβ-SDS-PAGE/immunoblot

For analysis of Aβ peptides, 10 μl of CSF were boiled in SDS-PAGE sample buffer4, 8 and Aβ-SDS-PAGE/immunoblot was conducted as published elsewhere.4, 8

CSF samples of each individual patient were run as triplicates. Bands were quantified from individual blots of each patient relative to a four-point dilution series of synthetic Aβ peptides4 using a charge-coupled device camera. The detection sensitivity was 0.6 pg (Aβ1-38, Aβ1-40) and 1 pg (Aβ1-37, Aβ1-39, Aβ1-42), respectively. The inter- and intra-assay coefficients of variation for 20–80 pg of synthetic Aβ peptides were below 10%.4, 8

All neurochemical measurements and quantifications were performed in the neurobiology laboratory of the University of Goettingen between 2003 and 2006 by two very experienced technical assistants blinded to clinical diagnosis.

Electrochemiluminescence detection of Aβ1-38

The novel electrochemiluminescence detection technology (MSD) was applied to determine CSF Aβ1-38 levels independently of the Aβ-SDS-PAGE/immunoblot. It was conducted according to the manufacturer's recommendations (Meso Scale Discovery). In brief, Multi-Spot 4 96 well plates precoated with the N-terminal-specific anti-Aβ antibody 6E10 were blocked with solution A for 1 h. The plates were then incubated with peptide dilution series or 100 μl CSF sample, followed by C-terminal SULFO-TAG Aβ 1–38 detection antibody and Read Buffer, 1 h each. Washing with 1 × Tris buffer was performed between incubation steps. The measurement of emitted light was performed at 620 nm.

Statistical analysis

Patient groups were characterized by mean and standard deviation. Aβ peptide values are given in absolute (ng/ml) and percentage values relative to the sum of all investigated Aβ peptides (Aβ1-X%). The Mann–Whitney U-test was employed to determine significant differences of diagnostic groups (unpaired samples). Comparisons of multiple groups (age, MMSE) were evaluated by the Kruskal–Wallis test. Correlations of measured values were estimated by Spearman's ρ. The two-sided level of significance was taken as P<0.05. The global diagnostic accuracies were assessed by the area under the curve (AUC) of receiver operating characteristic curve. Cut-off points were determined at the maximum Youden index, providing a sensitivity of 85%. The statistical software packages SPSS, version 10.0 and SAS, version 8.2 served for computations.

Results

Participants

Non-demented disease controls

Patients with history of persistent cognitive decline for more than 6 months, MMSE score below 26 or clear focal atrophy in brain imaging, were excluded.

The NDC group consisted of three subgroups: peripheral neurological diseases without organic brain affection (PND)

Twenty patients (13 men and 7 women) underwent lumbar puncture for exclusion of central nervous affection in case of polyneuropathy (n=11), peripheral facial nerve palsy (n=3), benign paroxysmal positioning vertigo (n=2), intervertebral disk herniation (n=1), facial hemispasm (n=1), autosomal-dominant hereditary spastic spinal palsy (n=1) and Lyme disease without central nervous affection (n=1). Age of this subgroup was 63.0±10.3 years (mean±s.d.).

Neurodegenerative diseases without dementia syndrome (ND)

Twenty-five patients (14 men and 11 women) underwent lumbar puncture to exclude chronic inflammatory central nervous disease in case of genetically reconfirmed Huntington's disease (n=10), PD (n=7), multisystem atrophy (n=5), spinocerebellar ataxia (n=2) and amyotrophic lateral sclerosis (n=1). The MMSE score in patients with cognitive complaints (n=9) was 28.2±1.6 (mean±s.d.). None of these patients displayed clinical features of dementia syndrome. Age of this subgroup was 62.3±9.5 years (mean±s.d.).

Depressive cognitive complainers (DCC)

Twenty-six depressive patients (8 men and 18 women) underwent lumbar puncture for differential diagnosis of cognitive complaints during the course of disease. The diagnosis of depression was made according to the criteria of DSM IV and cognitive impairment was assessed by MMSE at minimum. The mean MMSE score was 28.6±1.4 (mean±s.d.). Age of this subgroup was 62.9±10.3 years (mean±s.d.).

Patients with Alzheimer's disease (AD)

Seventy-one patients (29 men and 42 women) fulfilled DSM IV criteria and NINCDS-ADRDA criteria for clinical diagnosis of AD.9 Structural (CT or MRI) or functional (SPECT or PET) brain imaging, respectively, displayed global cortical atrophy or temporal, parietotemporal, frontotemporal focal atrophy or marked hypometabolism of these regions.

Patients with dementia with Lewy bodies (DLB)

Thirty-two patients (19 men and 13 women) fulfilled the DSM IV criteria for dementia and the McKeith criteria for clinical diagnosis of DLB.10

Patients presented with at least two core features according to the criteria10 and with parkinsonism less than 1 year before onset of dementia. Enrolled patients were hospitalized for several days to evaluate fluctuating cognition, extrapyramidal symptoms and visual hallucinations.

Patients with frontotemporal dementias (FTD)

All 36 patients (22 men and 14 women) of this group fulfilled the DSM IV and the consensus criteria for FTD.11 Detailed neuropsychological testing additional to MMSE, including clock drawing and CERAD, was carried out on 23 patients. Neuropsychological assessment was hindered in five patients by severe lingual or cognitive deficits. Structural (CT or MRI) or functional (SPECT or PET) brain imaging revealed frontal or frontotemporal focal atrophy or marked hypometabolism.

Patients with other dementias (OD)

Ninety-three patients (57 men and 36 women) fulfilled the DSM IV criteria for dementia. Patients with primary progressive aphasia (n=10) fulfilled the consensus criteria of Neary et al.11 Structural or functional (SPECT or PET) brain imaging revealed left anterior temporal focal atrophy or marked hypometabolism.

The diagnosis of vascular dementia (VAD) was made in 27 patients according to NINDS-AIREN criteria.12 All patients exhibited signs of relevant vascular disease in structural brain imaging (CT or MRI).

PDD was diagnosed in 21 patients according to UK Parkinson's Disease Society Brain Bank clinical diagnostic criteria for idiopathic PD13 and the consensus criteria.10 All patients presented parkinsonism at least 1 year before onset of dementia.

Normal pressure hydrocephalus according to the proposed criteria of Ishikawa14 was diagnosed in nine patients. All these patients exhibited at least two symptoms of the typical triad and improved after spinal tap.

Six patients fulfilled the criteria of probable progressive supranuclear palsy according to established NINDS-SPSP criteria.15

Six patients were diagnosed as corticobasal degeneration according to the established criteria.15

Seven patients suffering from sporadic Creutzfeld-Jakob's disease (CJD) were evaluated according to established criteria16 at the national surveillance unit for transmissible spongiform encephalopathies in Goettingen, Germany.

Seven patients with Korsakow's syndrome were evaluated according to the criteria of Oslin et al.17

The mean age and MMSE score of each patient group is given in Table 1.

Table 1 Absolute (ng/ml) and percentage abundances of CSF Aβ peptides in the diagnostic groups

Test results

The mean age of FTD and NDC was significantly younger than all other patient groups. The mean MMSE score did not significantly differ between the dementia groups. The Aβ-SDS-PAGE/immunoblot revealed a regular abundant pattern of six peptides: Aβ1-40, Aβ1-38, Aβ1-42, Aβ1-39, Aβ1-37 and Aβ1-40ox. All Aβ peptides were strongly correlated to each other throughout the whole group of patients (P<2.2 × 10−5). A correlation of the investigated Aβ peptides with age was not found in any of the diagnostic groups.

Neurochemical phenotype in NDC

There was no significant difference among PND and ND. In contrast, DCC exhibited higher absolute levels of Aβ1-37 (P=1.3 × 10−3), Aβ1-38 (P=4.9 × 10−2) and Aβ1-42 (P=4.0 × 10−3) than a combined group of PND and ND. In percentage terms, Aβ1-37% (P=2.9 × 10−4) and Aβ1-42% (P=1.5 × 10−3) were increased, paralleled by diminished Aβ1-40% (P=3.3 × 10−3) and Aβ1-40ox% (P=1.4 × 10−2) values in DCC.

Neurochemical phenotype in AD

AD versus NDC AD presented with clearly decreased Aβ1-42 levels in absolute (P=7.4 × 10−19) and percentage terms (P=3.8 × 10−24), whereas Aβ1-40ox levels were increased in absolute (P=1.1 × 10−2) and percentage (P=1.8 × 10−5) terms. Additionally, there was a percentage increase of all peptides C-terminally shorter than Aβ1-42, which failed the level of significance for Aβ1-37% and Aβ1-38%. The elevation of Aβ1-38% was highly significant compared with the ND group (P=4.6 × 10−3).

AD versus all other dementias A specific decrease of Aβ1-42 was evident in AD, whereas other dementias with low-Aβ1-42 levels displayed an overall decrease of all Aβ peptides. Correspondingly, Aβ1-42 was reduced in absolute (P=4.1 × 10−7) and percentage (P=7.6 × 10−22) terms.

In contrast, absolute levels of Aβ1-37 (P=2.5 × 10−2), Aβ1-38 (P=5.0 × 10−5), Aβ1-39 (P=2.4 × 10−3) and Aβ1-40 (P=6.3 × 10−3) were elevated. In percentage terms, the aforementioned alterations were only present for Aβ 1–38%, 1–39% and 1–42%, respectively.

Neurochemical phenotype in DLB

DLB versus NDC DLB presented with higher Aβ1-40ox absolute (P=1.7 × 10−3) and percentage (P=3.3 × 10−11) levels. Conversely, absolute levels of Aβ1-37 (P=1.5 × 10−4), 1–38 (P=6.7 × 10−6), 1–39 (P=3.4 × 10−4), 1–40 (P=4.3 × 10−5) and 1–42 (P=4.8 × 10−9) were lowered, although not evident in percentage terms for Aβ1-37 and Aβ1-39.

DLB versus all other dementias Aβ1-40ox was elevated in absolute (P=3.7 × 10−3) and percentage (8.7 × 10−9) terms. The overall decrease of other Aβ peptides was still evident, but less pronounced than in NDC.

Neurochemical phenotype in FTD

FTD versus NDC FTD showed lower levels of Aβ1-37 (P=2.3 × 10−4), Aβ1-38 (P=9.6 × 10−7) and Aβ1-42 (P=6 × 10−5). In percentage terms, there was an additional increase of Aβ1-40% values (P=7 × 10−10).

FTD versus all other dementias FTD presented lower Aβ1-38 (P=2.1 × 10−2) and Aβ1-40ox (P=5.0 × 10−3) levels, whereas Aβ1-42 levels were elevated (P=3.8 × 10−3). In percentage terms, there were drops in Aβ1-37% (P=2.5 × 10−4), Aβ1-38% (P=3.5 × 10−15), Aβ1-39% (P=1.0 × 10−2) and Aβ1-40ox% (P=2.6 × 10−4), paralleled by elevated levels for Aβ1-40% (P=1.9 × 10−5) and Aβ1-42% (P=6.2 × 10−4).

Compared to PPA, there were decreased absolute Aβ1-37 (P=1.1 × 10−2), Aβ1-38 (P=1 × 10−2) and Aβ1-39 (P=1.8 × 10−2) levels in FTD. Additionally, Aβ1-40% was elevated (P=1.8 × 10−3) in FTD (see Figures 1, 2 and 3).

Figure 1
figure1

Mean and 95% confidence interval (CI) of Aβ1-42% for each diagnostic group.

Figure 2
figure2

Mean and 95% confidence interval (CI) of Aβ1-40ox% for each diagnostic group.

Figure 3
figure3

Mean and 95% confidence interval (CI) of Aβ1-38% for each diagnostic group.

Aβ1-38 in the Aβ-SDS-PAGE/immunoblot and electrochemiluminescence detection

A total of 150 patients were reevaluated using novel electrochemiluminescence detection technology (MSD). Diagnoses thereof were NDC (n=37), AD (n=31), DLB (n=2), OD (n=47) and FTD (n=33). Absolute levels of peptide concentration were considerably lower in MSD compared with the Aβ-SDS-PAGE/immunoblot (see Tables 1 and 2). Conversely, there was a strong correlation of values between the two independent methods of measurement (Spearman's ρ=0.45, P=7.9 × 10−9) (see Figure 4).

Table 2 Cut-off points and specificities at a minimum sensitivity of 85% of the best discriminating factor for each differential diagnostic testing
Figure 4
figure4

Correlation of Aβ1-38 levels as measured by the Aβ-SDS-PAGE/immunoblot and electrochemiluminescence detection (MSD).

Estimates

Neurochemically supported differential diagnosis

Diagnosis of AD The striking drop of Aβ1-42% enabled contrasts beyond 85% for discrimination of AD among the total of all investigated patients. A sensitivity of 85% gave a specificity of 81% for exclusion of all non-Alzheimer dementias. Owing to decreased values of Aβ1-38 and a less marked drop of Aβ1-42 in dementias other than AD, the ratio of Aβ1-38 to Aβ1-42 (Aβ1-42/1-38) improved the test slightly to contrasts of 85% or beyond for all investigated differential diagnostic questions. Otherwise, the sole absolute values of Aβ1-42 yielded a specificity of 50% for exclusion of non-Alzheimer dementias, when the sensitivity for AD detection was set to a minimum of 85%.

Diagnosis of DLB DLB could be detected with a sensitivity and specificity of 88 and 83% among NDC with the use of Aβ1-40ox% levels. The discrimination of DLB from all other dementias was less accurate mainly owing to an overlap with increased Aβ1-40ox% in PSP, CBD and CJD. The percentage portion of Aβ1-40ox relative to Aβ1-40 levels exhibited similar test accuracies for detection of DLB among NDC and all other dementias.

Diagnosis of FTD The pronounced percentage reduction of Aβ1-38% in FTD exhibited satisfactory accuracies above 85% for discrimination among all other dementias and NDC. The combination of decreased Aβ1-38 and elevated Aβ1-40 (Aβ1-38/1–40) levels just failed to fulfill the requirements for differentiation of FTD and all other dementias, but still exhibited contrasts above 85% for detection of FTD among NDC. The loss of accuracy of the Aβ1-38/1-40 ratio as compared to Aβ1-38% was mainly because of elevated Aβ1-40 levels in single AD, DLB and PPA patients. Table 2 summarizes the diagnostic accuracies and cut off points of the afore mentioned peptides for each differential diagnostic testing.

Discussion

The Aβ-SDS-PAGE/immunoblot revealed the regular abundance of the Aβ peptides Aβ1-37, Aβ1-38, Aβ1-39, Aβ1-40, Aβ1-40ox and Aβ1-42 in all investigated CSF samples. In line with previous studies,18, 4 the absolute abundances of these peptides were strongly correlated to each other, suggesting a close regulation of the Aβ peptides’ enzymatic processing and their post-translational modification.

The most prominent alterations of the investigated Aβ patterns were the expected decrease of Aβ1-42 in AD, the increase of Aβ1-40ox in DLB and the decrease of Aβ1-38 in FTD.

Aβ1-42 levels in CSF: neurochemical detection of AD

The CSF pattern of diminished Aβ1-42 and elevated tau concentration can be considered as typical, and has recently been reviewed by experts as an applicable AD biomarker.19, 20 Nevertheless, levels of decreased Aβ 1-42 and increased tau can also be found in other dementias, such as VAD,21 DLB,22 FTLD23 and CJD.24 At a sensitivity of 85% for AD detection, the diagnostic value of these biomarkers for the exclusion of non-Alzheimer dementias was therefore limited to 58%.21

The detailed characterization of CSF Aβ peptides by the Aβ-SDS-PAGE/immunoblot has shown additional diagnostic value to discriminate AD from DLB, PDD and CJD.5, 24 In AD, the pronounced decrease of Aβ1-42 seems to be counteracted by an upregulation of Ct shortened Aβ peptides, leading to a highly selective reduction of Aβ1-42 in percentage terms. In contrast, decreased absolute CSF Aβ1-42 levels, paralleled by an overall reduction of all investigated Aβ peptides, could be found for patients with DLB, PDD, CJD, PSP, CBD and in part VAD. This may particularly contribute to the loss of accuracy, when absolute Aβ1-42 values are employed to discriminate AD from these dementias. Moreover, the evaluation of Aβ1-42 relative to the sum of all investigated Aβ peptides led to a reduced interindividual variance of values.24 This may be explained by the assumption that the abundances of single Aβ peptide species are closely correlated to each other and are thus regulated in narrow limits, whereas the total amount of Aβ peptides varies interindividually.4, 24 The discriminative power of the selective Aβ1-42% decrease for AD in a combined non-demented control and non-Alzheimer dementia group was similar to what was reported for the p-tau-ELISA 231.25 Moreover, it was superior to reported accuracies of the widely accepted and established measurement of tau and Aβ1-42.4, 5, 24

Aβ1-42 levels in CSF: pathophysiological considerations

The relationship between decreased levels of Aβ1-42 in CSF and the occurrence of senile amyloid plaques needs to be clarified. The reduction of Aβ1-42 levels in AD has long been explained by an increased clearance of the peptide from CSF into plaques.26 Otherwise, dementias, where plaque formation is a rare feature, also exhibit low absolute levels of Aβ1-42.24 In addition, the amount of measured Aβ1-42 levels strongly depended on the method of measurement and preanalytical sample handling. For example, denaturizing of samples before measurement (e.g., SDS-heat pretreatment or presence of detergents) led to higher absolute Aβ1-42 levels and demonstrated a more pronounced reduction of Aβ 1-42 in AD.8, 26 The freezing of samples before denaturizing resulted in an irreversible loss of Aβ peptides, markedly in the case of Aβ1-42.8 Accordingly, we hypothesize that Aβ1-42 may be bound to a specific carrier in an either SDS-labile or SDS-stable manner. In AD, the portion of SDS-stable peptide bindings may be increased at the cost of both the SDS-labile bound and the free peptide. The mutual dysfunction of a specific carrier for the peptide could contribute to a loss of its solubility, sufficient metabolism and consequently promote its aggregation. Thus far, candidates reported for such a carrier include apolipoprotein E and J,27 SDS-stable oligomers28 and supramolecular aggregates of Aβ peptides.29

Whatever mechanisms are responsible for the reduction of CSF Aβ peptides, they seem to be restricted to Aβ1-42 only in AD. In case of an overall reduction of Aβ, either they also apply to other Aβ peptide species or a reduced production of Aβ must be considered.

Aβ1-38 levels in CSF: neurochemically supported diagnosis of FTD and PPA

For lack of promising disease-specific biomarkers for FTD, previous studies have mainly focused on pronounced alterations of Aβ1-42 and tau in AD as compared to FTD. Results on the performance of these biomarkers for differential diagnosis of FTD and AD have been inconsistent.30, 31, 32 However, the pattern of mildly increased tau and moderately decreased Aβ 1-42 is not unique for FTD, and it can be assumed that the diagnostic value of these biomarkers to discriminate FTD among dementias other than AD is limited.33 A disease-specific biomarker test for FTD among dementias other than AD and differential diagnostic non-dementive disorders, such as depressive patients with cognitive complaints, has not yet been established.

Analogous to the selective decrease of Aβ1-42 in AD, the decrease in Aβ1-38 was unique for FTD, only after determination of its percentage value (Aβ1-38%).

Aβ1-38 levels in CSF: pathophysiological considerations

As little is known about the role of Aβ peptides in FTD, the pathogenic relevance of the above findings remains speculative. CSF Aβ1-42 is reduced in FTD, whereas amyloid plaques are uncommon. This phenomenon has recently contributed to a putative binding protein that masks its epitopes to antibodies during immunologic detection.4, 8, 32 Given that similar mechanisms may apply to CSF Aβ1-38, the postulated carrier may possess a disease-specific affinity to Aβ1-38 in FTD. However, the analogy between the characteristics of the reduction of Aβ1-42 and Aβ1-38 in AD and FTD, respectively, points to similar disease-specific mechanisms causing this phenomenon.

Alternatively, an overall reduction of Aβ peptides could be counteracted by upregulating other Aβ peptides, such as Aβ1-40. The mutual involvement of Ct-truncated Aβ peptides in pathogenic events of FTD is supported by a previous publication reporting a correlation between CSF Aβ1-40 levels and the degree of frontal lobe atrophy in FTD.34 In line with their results, we have found elevated Aβ1-40% levels in FTD, when compared to all other dementias, even to PPA. Most noteworthy, presenilin1 mutations have been shown to be associated with FTD-like clinical phenotypes, one of which causes a Pick-type tauopathy without the development of amyloid plaques in neuropathological confirmation.35 The molecular impact of presenilin1 mutations on APP processing and the etiology of FTD has been attributed to an altered or dysfunctional Notch processing.35 For further elucidation of this aspect, especially with regard to CSF neurochemical phenotypes, we propose to investigate Aβ peptide patterns in affected families and transgenic cell culture models.

Aβ1-40ox levels in CSF: neurochemical detection of DLB

The combined assay of tau and Aβ1-42 ELISA revealed sensitivities and specificities of around 75% for discrimination of DLB among AD and non-demented controls, respectively.36 The discrimination of these two dementias is mainly based on strikingly elevated tau levels in AD as compared to mild elevations in probable DLB.37

A disease-specific CSF biomarker for applicable diagnostic testing of DLB among other dementias is not yet available. In a pilot study, we have shown promising results about discrimination of DLB from PDD using a pronounced increase of an oxidized and presumably α-helical form of Aβ1-40 (Aβ1-40ox) in DLB.5 Likewise, the increase of Aβ1-40ox% was most specific for DLB in contrast to all other investigated patients in the present study. This enabled a detection of DLB with a sensitivity of 87% at a specificity of 70%. A prominent overlap of values with PSP, CBD and CJD hampered a more accurate discrimination.

Aβ1-40ox levels in CSF: pathophysiological considerations

Given an oxidized and α-helical structure of the peptide, this would imply a particular relevance of oxidation and hydrophobic interactions in the pathogenesis of DLB. The occurrence of oxidative stress and amyloid pathology is well documented for DLB, and oxidized Aβ comprises a major component of amyloid plaques.38 α-Synuclein facilitates the aggregation of Aβ owing to hydrophobic interactions,39 which have been shown to induce an α-helical structure of Aβ in vitro.40 Thus, α-synuclein may also induce an enhanced conformational shift of Aβ into an α-helix in vivo. The α-helical Aβ is specifically prone to aggregation, as the assembly of toxic oligomers41 and β-sheet formation of Aβ involve the transient formation of an α-helix, suggesting it to be an on-pathway to aggregation.42 Moreover, the α-helix predisposes the peptide to oxidation.43 The oxidized Aβ is more hydrophilic44 and less toxic43 to neurons than the reduced peptide. Otherwise, metal ions may particularly trigger its aggregation owing to a facilitated release from the cell membrane.44 This may result in Aβ precipitation as a kind of seed around synapses, a quite sensitive site of the neuron.44 Furthermore, one-electron oxidation of Aβ by metal ions at met-35 produces a sulfuramyl free radical on the sulfur atom.43

Taken together, the hydrophobic interaction between Aβ peptides and α-synuclein may cause an overproduction of α-helical Aβ that is prone to both fibril formation in its reduced and metal-dependent precipitation in its oxidized form. Additionally, α-helical Aβ may undergo facilitated one-electron oxidation resulting in a toxic sulfuramyl radical. Thus, the increased abundance of Aβ1-40ox may indicate a disease-specific mechanism of amyloid toxicity in DLB.

Aβ-peptide patterns in differential diagnosis of dementias

Tau, Aβ1-42 and p-tau may differentiate AD from DLB, FTD and controls, but a specific biomarker for diagnosis of DLB or FTD among other dementias and differential diagnostic non-dementive disorders is still lacking. The expression of CSF Aβ peptides varies among AD, DLB and FTD, resulting in unique peptide patterns as compared to NDC and other dementias. Aβ peptide patterns came closest to the requirements for a useful biomarker in these dementias.20

Previous reports of our group investigated considerably smaller groups of patients and focused on one or two crucial differential diagnoses. First, AD patients were evaluated in comparison to various non-dementive neuropsychiatric disorders and chronic inflammatory diseases (CID) of the central nervous system.4 Although AD and CID shared a percentage increase of Aβ1-38, AD could be separated from the other groups without any overlap by a selective decrease of Aβ1-42.4 With respect to the differentiation of AD from non-dementive neuropsychiatric disorders, we were able to reproduce this in several subsequent studies,5, 6, 8, 24, 36 including the present one. The examination of CJD in comparison to AD revealed a more pronounced selective drop of Aβ1-42 in AD and the ratio of Aβ1-42/1-39 yielded a sensitivity and specificity of 100% and 79%, respectively, in differentiating the two disorders.24 Accordingly, the overlap of absolute Aβ1-42 values could largely be reduced by the introduction of a ratio of Aβ1-42 relative to Ct-truncated Aβ peptides.24 A similar finding was obtained in comparing Alzheimer's to Huntington's disease.6 Next, the evaluation of DLB in comparison to AD and PDD revealed a novel potential biomarker (i.e., Aβ1-40ox), which was markedly elevated in DLB and to a lesser degree in AD.5 In fact, there was an overlap among the latter two disorders, but the sensitivity for detection of DLB and specificity for exclusion of all other investigated disorders were still 81 and 70%, respectively.5 The slight improvement of accuracy of this marker for DLB in the present study may rely on (i) the larger study size, especially for DLB and AD, and (ii) the inclusion of other differential diagnostically relevant disorders that do not share the elevation of Aβ1-40ox in CSF (e.g., FTD).

Two of the studies published so far have reported a direct comparison between absolute CSF values of Aβ1-42 as measured by ELISA and the Aβ-SDS-PAGE/immunoblot and consistently found higher levels using the latter method.8, 37 This has led to the assumption that Aβ1-42 may be bound to specific carrier proteins in CSF. In the present study, Aβ1-38 levels were determined by both Aβ-SDS-PAGE/immunoblot and electrochemiluminescence detection. The principle of detection in the latter method is comparable to a sandwich ELISA, as it relies on antibody-mediated binding of Aβ. Analogous to Aβ1-42, the Aβ-SDS-PAGE/immunoblot yielded considerably higher values for Aβ1-38 than electrochemiluminescence detection, suggesting that there might also be a portion of CSF Aβ1-38, which is not freely accessible to antibodies. Obviously, SDS-heat denaturizing of samples increases the accessibility of their epitopes to respective antibodies in the immnuoblot, probably by stripping off the peptide from a putative binding protein.8 Up to now, we have not directly compared the amounts of Aβ1-40 in the same CSF samples as obtained by the Aβ-SDS-PAGE/immunoblot and ELISA, respectively. However, the values measured by ELISA (The Genetics, Salzburg, Austria) in a previous study45 are comparable to what the Aβ-SDS-PAGE/immunoblot revealed in the present report. Given the above considerations, it can be assumed that, in comparison to Aβ1-42 and Aβ1-38, a larger portion of Aβ1-40 is freely accessible to antibodies during the ELISA.

In contrast, studies of others have shown discrepant results with regard to Aβ1-40 and Aβ1-38, respectively. Using their ELISA, Schoonenboom et al.46 found higher absolute values for Aβ1-40 in non-demented control subjects and AD. Furthermore, they were the first to report CSF Aβ1-38 levels as measured by ELISA.46 Their results for Aβ1-38 largely match what we found using the Aβ-SDS-PAGE/immunoblot, but levels were considerably higher as compared to electrochemiluminescence detection. The reason for these discrepancies may include the application of different Ct and/or N-terminally specific anti-Aβ antibodies with distinct affinities and specificities, respectively, in each assay.

However, whereas the measured absolute values of Aβ peptides may depend on preanalytical sample handling and the method of measurement, their percentage amount has been widely found to be stable among these factors.8 Once more, this argues in favour of percentage Aβ peptide values instead of absolute ones for neurochemical dementia diagnostic use.

Limitations of the study

Although the Aβ-SDS-PAGE/immunoblot can be considered as a quantitative and highly reproducible method4, 5, 8, 24 and has meanwhile also been used successfully by other groups,47 the method is time consuming and requires specially trained personnel. This limits its application for high throughput screening (HTS) in routine neurochemical work-up to specialized centers. However, HTS-capable platforms and multiplex assays are upcoming neurochemical diagnostic tools that may include measurement of different Aβ peptide species in addition to other biomarker candidates. In this respect, it is of tremendous value that we could demonstrate a good correlation between the measurement of Aβ1-38 by Aβ-SDS-PAGE/immunoblot and electrochemiluminescence detection.

Further limitations of the study arise from the reliance on clinical diagnosis, which is claimed to misclassify 15–20% of dementia cases. The study size is limited for some rarer dementia forms, namely PSP, PPA, CBD, CJD and Korsakow's syndrome, but the high level of the AUCs shows the global diagnostic accuracy of the investigated factors.

Conclusions

The patterns of differentially expressed distinct Aβ peptide species represent a biomarker candidate with disease-specific alterations among the three most frequent neurodegenerative causes of dementia, AD, DLB and FTD. The combined evaluation of the species Aβ1-42, Aβ1-38 and Aβ1-40ox relative to the total CSF Aβ amount came close to the accuracy recommendations of an international consensus work group on applicable biomarkers for dementias and could be demonstrated to be clearly superior to absolute peptide values. Nevertheless, this study awaits confirmation from other work groups and validation by independent methods of measurement. In particular, HTS-capable methods will have to be evaluated for a future application of Aβ peptide patterns in routine diagnostic neurochemical work-up. Moreover, the data will have to be correlated with neuropathological data to draw more exact conclusions on their relationship to the specific pathogenic events in the respective dementias.

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Acknowledgements

MB, PL, HE, SW, JK, MO and JW were supported by grants from the German Federal Ministry of Education and Research (Competence Net Dementia, grant O1 GI 0420); MB was supported by the Research Program, Faculty of Medicine, Georg-August-Universität Göttingen; JK, JW and PL were supported by grants from the German Federal Ministry of Education and Research CJK (01 GI 0301) and HBPP-NGFN2 (01 GR 0447). MO was supported by grants from the German Federal Ministry of Education (German CJD therapy study FK 01KO0506), EU (anteprion 019090) and Landesstiftung Baden Württemberg).

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Correspondence to M Bibl or J Wiltfang.

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Bibl, M., Mollenhauer, B., Lewczuk, P. et al. Validation of amyloid-β peptides in CSF diagnosis of neurodegenerative dementias. Mol Psychiatry 12, 671–680 (2007) doi:10.1038/sj.mp.4001967

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Keywords

  • Alzheimer's disease
  • frontotemporal dementia
  • dementia with Lewy bodies
  • cerebrospinal fluid
  • amyloid-β peptides
  • biomarkers

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