The Association Between Alzheimer's Disease-Related Biomarkers And Parkinson's Disease Based On Single-Cell Sequencing Analysis Combined With Mendelian Randomization.

Parkinson's disease (PD) and Alzheimer's disease (AD) are both neurodegenerative disorders sharing overlapping pathological mechanisms. Several studies suggest that AD biomarkers may have diagnostic and predictive value in PD, but causal evidence remains insufficient. This study aimed to explore the association between AD-related biomarkers and PD. Bidirectional two-sample Mendelian randomization (MR) was performed using GWAS summary statistics of AD-related biomarkers and PD to assess causal relationships. Subsequently, single-nucleus RNA sequencing (snRNA-seq) of postmortem midbrain tissues was employed to examine the expression of AD biomarkers in PD patients and to explore underlying cellular signaling pathways. Forward MR revealed a significant causal association between PD and decreased CSF p-tau levels. PD progression showed significant causal associations with decreased CSF Aβ42 and increased CSF total tau. Reverse MR indicated that elevated CSF total tau may increase PD risk. SnRNA-seq analysis demonstrated that MAPT (encoding tau) was predominantly expressed in dopaminergic neurons, with differentially expressed genes enriched in PD- and neurodegeneration-related pathways. Our findings provide preliminary, hypothesis-generating evidence at the genetic level suggesting potential associations between CSF tau, Aβ42, and PD or its progression. Bidirectional MR analyses suggest that CSF total tau may be associated with increased PD risk, while exploratory snRNA-seq analysis indicates that MAPT is predominantly expressed in dopaminergic neurons with enrichment in PD-related pathways. Given the limited number of instrumental variables for several exposures, the small snRNA-seq cohort, and the absence of protein-level validation, these findings should be interpreted with caution. Large-scale prospective clinical studies with protein-level biomarker assessment, tau-PET imaging, and independent multi-cohort transcriptomic validation are warranted before CSF total tau can be considered a candidate biomarker for PD progression.