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Synaptic Proteins as Fluid Biomarkers in Alzheimer's Disease: A Systematic Review and Meta-Analysis.

BACKGROUND
Synaptic disruption precedes neuronal death and correlates with clinical features of Alzheimer's disease (AD). The identification of fluid biomarkers of synaptic damage is emerging as a goal for early and accurate diagnosis of the disease.


OBJECTIVE
To perform a systematic review and meta-analysis to determine whether fluid biomarkers of synaptic damage are impaired in AD.


METHODS
PubMed, Scopus, EMBASE, and Web of Science were searched for articles reporting synaptic proteins as fluid biomarkers in AD and cognitively unimpaired (CU) individuals. Pooled effect sizes were determined using the Hedge G method with random effects. Questions adapted from the Quality Assessment of Diagnostic Accuracy Studies were applied for quality assessment. A protocol for this study has been previously registered in PROSPERO (registration number: CRD42021277487).


RESULTS
The search strategy identified 204 articles that were assessed for eligibility. A total of 23 studies were included in the systematic review and 15 were included in the meta-analysis. For Neurogranin, 827 AD and 1,237 CU subjects were included in the meta-analysis, showing a significant increase in cerebrospinal fluid of patients with AD compared to CU individuals, with an effect size of 1.01 (p <  0.001). A significant increase in SNAP-25 and GAP-43 levels in CSF of patients with AD was observed.


CONCLUSION
Neurogranin, SNAP-25, and GAP-43 are possible biomarkers of synaptic damage in AD, and other potential synaptic biomarkers are emerging. This meta-analysis also revealed that there are still relatively few studies investigating these biomarkers in patients with AD or other dementias and showed wide heterogeneity in literature.