Alzheimer's disease biomarker profiling in a memory clinic cohort without common comorbidities

Abstract

Alzheimer's disease is a multifactorial disorder with large heterogeneity. Comorbidities such as hypertension, hypercholesterolaemia and diabetes are known contributors to disease progression. However, less is known about their mechanistic contribution to Alzheimer's pathology and neurodegeneration. The aim of this study was to investigate the relationship of several biomarkers related to risk mechanisms in Alzheimer's disease with the well-established Alzheimer's disease markers in a memory clinic population without common comorbidities. We investigated 13 molecular markers representing key mechanisms underlying Alzheimer's disease pathogenesis in CSF from memory clinic patients without diagnosed hypertension, hypercholesterolaemia or diabetes nor other neurodegenerative disorders. An analysis of covariance was used to compare biomarker levels between clinical groups. Associations were analysed by linear regression. Two-step cluster analysis was used to determine patient clusters. Two key markers were analysed by immunofluorescence staining in the hippocampus of non-demented control and Alzheimer's disease individuals. CSF samples from a total of 90 participants were included in this study: 30 from patients with subjective cognitive decline (age 62.4 ± 4.38, female 60%), 30 with mild cognitive impairment (age 65.6 ± 7.48, female 50%) and 30 with Alzheimer's disease (age 68.2 ± 7.86, female 50%). Angiotensinogen, thioredoxin-1 and interleukin-15 had the most prominent associations with Alzheimer's disease pathology, synaptic and axonal damage markers. Synaptosomal-associated protein 25 kDa and neurofilament light chain were increased in mild cognitive impairment and Alzheimer's disease patients. Grouping biomarkers by biological function showed that inflammatory and survival components were associated with Alzheimer's disease pathology, synaptic dysfunction and axonal damage. Moreover, a vascular/metabolic component was associated with synaptic dysfunction. In the data-driven analysis, two patient clusters were identified: Cluster 1 had increased CSF markers of oxidative stress, vascular pathology and neuroinflammation and was characterized by elevated synaptic and axonal damage, compared with Cluster 2. Clinical groups were evenly distributed between the clusters. An analysis of post-mortem hippocampal tissue showed that compared with non-demented controls, angiotensinogen staining was higher in Alzheimer's disease and co-localized with phosphorylated-tau. The identification of biomarker-driven endophenotypes in cognitive disorder patients further highlights the biological heterogeneity of Alzheimer's disease and the importance of tailored prevention and treatment strategies.

Keywords: Alzheimer’s disease; biomarkers; cerebrospinal fluid; clustering; metabolic disorders.

Graphical Abstract

Figure 1

Scatter plots depicting biomarker levels across clinical groups (A–K). Biomarker concentrations are in y-axis. Median is shown as a horizontal line. P-values were calculated by ANCOVA, adjusting for age.

Figure 2

Correlation matrix of the analysed biomarkers. Correlation coefficients indicated in red/blue depending on the direction of the association (red: positive association, blue: negative association). Pearson test was applied for correlations between normally distributed variables, otherwise Spearman test was performed. A total of 90 samples were included in the analysis (1 value was missing for SNAP-25, NG, SYT-1, NFL and TRX-1 and 2 values for 27-OH). LP, lumbar puncture. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 3

Scatter plots of CSF biomarker composite scores in SCD, MCI and AD. Individual-level Z-scores of the composites in all included subjects are plotted: (A) vascular/metabolic composite score, (B) inflammatory composite score and (C) survival composite score. P-values were calculated by ANCOVA, adjusting for age.

Figure 4

Multivariate clustering of patients. (A) Clusters comparison showing horizontal box plots of the included markers. Large horizontal boxes represent the interquartile range and vertical line the median of each biomarker in the total cohort. Small boxes depict the median and their respective lines the interquartile range for each cluster. (B) Percentage of patients who are distributed in Clusters 1 and 2 within each clinical diagnosis. A total of 86 participants were included in the analysis.

Figure 5

Immunofluorescence co-staining of p-tau with thioredoxin-1 (TRX-1) or angiotensinogen (AGT) in human hippocampal sections. (A) Control and AD hippocampi sections immunostained for TRX-1 (green), p-tau (red) and DAPI for nuclei (blue). (B) Bar charts showing mean % TRX-1/p-tau colocalization in each clinical group, with individual values in dots. (C) Immunofluorescent staining of AGT (green), p-tau (red) and DAPI in control (upper panels) and AD (lower panels). (D) Bar charts of mean % AGT/p-tau colocalization in each clinical group, with individual values in dots. P values were calculated by two-tailed unpaired t test. Respective higher magnifications (dashed line) are presented below each panel. Scale bars, 50 mm.