α-Synuclein in cerebrospinal fluid of Alzheimer's disease and mild cognitive impairment

Abstract

In addition to amyloid-β (Aβ) and tau, α-synuclein, best known for its role in Parkinson's disease (PD), has been suggested to be involved in cognition and pathogenesis of Alzheimer's disease (AD). We investigate the potential of α-synuclein in cerebrospinal fluid (CSF) as a biomarker of cognitive decline in AD, and its prodromal phase, mild cognitive impairment (MCI). Using an established, sensitive Luminex assay, we measured α-synuclein levels in the CSF of a cohort of close to 400 healthy control, MCI, and AD subjects obtained from the Alzheimer's Disease Neuroimaging Initiative (ADNI) and factored in APOE genotype in data analysis. CSF α-synuclein levels were significantly higher in the MCI (p = 0.005) and AD (p < 0.001) groups, compared to controls. However, receiver operating characteristic (ROC) curve analysis suggests that CSF α-synuclein level on its own only offered modest sensitivity (65%) and specificity (74%) as a diagnostic marker of AD, with an area under the curve (AUC) value of 0.719 for AD versus controls. The effect of APOE genotype, if any, was quite subtle. However, there was a significant correlation between α-synuclein and cognition (p = 0.001), with increased α-synuclein levels associated with decreased Mini-Mental State Exam scores. Our results support a role for α-synuclein even in MCI, the early phase of AD, in addition to being a potential contributor in MCI and AD diagnosis or monitoring of disease progression.

Figure 1

Flowchart depicting for control, MCI and AD subjects: the target enrollment number of ADNI-1, the number of CSF samples received for α-synuclein measurement in the current study, the number of CSF samples remaining after controlling for blood contamination and the number of CSF samples from patients with APOE genotype ε4/ε4 and ε4/ε3. Of note, among the subjects enrolled in ADNI, initial CSF tap rate is only about 50%, and we received all cases with >10 aliquots CSF samples (the cut-off set by ADNI Repository).

Figure 2. Correlation of CSF α-synuclein levels with hemoglobin levels

CSF α-synuclein and hemoglobin levels were measured in individual healthy controls and cases with mild cognitive impairment (MCI) and Alzheimer’s disease (AD). Data shown are before (A) or after (B) elimination of samples with blood contamination (200 ng/ml hemoglobin was used as a cut-off). Spearman’s correlation coefficients (rho) and P-values were rho = 0.550 (P<0.001) and rho = 0.066 (P = 0.292), for before and after elimination of contaminated cases, respectively.

Figure 3. Cross-sectional examination of CSF α-synuclein levels

Quantitative Luminex analysis was performed to measure CSF α-synuclein in healthy controls (CTL) and patients with mild cognitive impairment (MCI) and Alzheimer’s disease (AD). Data shown are after elimination of samples with blood contamination (200 ng/ml hemoglobin was used as a cut-off). To obtain a normal distribution, α-synuclein levels were log transformed. Data are shown for all cases irrespective of APOE genotype (clear bars) and for cases with APOE genotype ε4/ε4 and ε4/ε3 (solid bars). Data shown are mean ± SEM. *P = 0.005 and **P<0.001 versus control group in all cases. Number of cases can be found in Table 1.

Figure 4. ROC curves to evaluate CSF α-synuclein level as a biomarker of MCI and AD

ROC curve for α-synuclein of (A) mild cognitive impairment (MCI) and (B) Alzheimer’s disease (AD) versus controls, after elimination of blood contaminated samples (200 ng/ml hemoglobin was used as a cut-off). To obtain a normal distribution, α-synuclein levels were log transformed. AUCs, P values, as well as sensitivity and specificity can be found in Supplementary Table 1.

Figure 5. Correlation of CSF α-synuclein levels with cognition as measured by MMSE

CSF α-synuclein levels were measured in healthy controls (CTL) and patients with mild cognitive impairment (MCI) and Alzheimer’s disease (AD). Data shown are after elimination of samples with blood contamination (200 ng/ml hemoglobin was used as a cut-off). To obtain a normal distribution, α-synuclein levels were log transformed. Data are shown for all cases irrespective of APOE genotype (A) and for cases with APOE genotype ε4/ε4 and ε4/ε3 (B). The correlation coefficients (R) and P-values were R = −0.204 (P<0.01) and R = −0.71 (P = 0.449), for all cases and cases with APOE genotype ε4/ε4 and ε4/ε3, respectively.