Serotonin, Lipids and Parkinson’s disease Risk: A Retrospective Study and Mendelian Randomization Analysis

Technical Summary: Serotonin, Lipids, and Parkinson's Disease Risk

Problem Statement
Parkinson's disease (PD) is a complex neurodegenerative disorder involving the degeneration of dopaminergic and serotonergic neurons. While genetic factors contribute to PD, the majority of cases are sporadic, suggesting a significant interaction between genetic susceptibility and environmental factors. A key environmental factor is gut microbiota dysbiosis. Specifically, the genus Turicibacter has been observed to be enriched in PD patients and is known to influence gut serotonin levels and lipid metabolism. However, the specific causal mechanisms linking Turicibacter, its related metabolites (serotonin and serum lipids), and PD risk remain ambiguous. Furthermore, while peripheral serotonin and cholesterol levels are known to decrease in PD patients, the directionality of these relationships—whether they are causes or consequences of the disease—requires clarification.

Methodology
This study employed a dual-approach strategy combining a retrospective clinical study with Mendelian randomization (MR) analysis to investigate the causal relationships between serotonin, serum lipids, and PD risk.

1. Clinical Retrospective Study:

   • Cohort 1 (Microbiome): 20 PD patients and 20 age- and sex-matched healthy controls underwent 16S rRNA sequencing to assess Turicibacter abundance.
   • Cohort 2 (Lipids): An independent cohort of 80 PD patients and 80 matched healthy controls provided serum samples for biochemical analysis.
   • Measurements: Serum levels of Total Cholesterol (TC), Low-Density Lipoprotein Cholesterol (LDL-C), High-Density Lipoprotein Cholesterol (HDL-C), Triglycerides (TG), and Lipoprotein(a) [Lp(a)] were measured. Clinical severity was assessed using the Unified Parkinson's Disease Rating Scale (UPDRS part III), Hoehn-Yahr staging, and various non-motor symptom scales.
   • Analysis: Statistical comparisons (Student's t-test, Wilcoxon rank-sum) and Spearman's rank-correlation analyses were performed to evaluate differences between groups and correlations with disease severity.

2. Mendelian Randomization (MR) Analysis:

   • Data Sources: Summary statistics from publicly available Genome-Wide Association Studies (GWAS) were utilized. Exposure data for serotonin and lipids were sourced from the IEU Open GWAS project, while PD outcome data came from a dataset containing 1,570 cases and 1,259 controls.
   • Instrumental Variables (IVs): Single Nucleotide Polymorphisms (SNPs) were selected as IVs. Due to a lack of genome-wide significant SNPs for serotonin, the threshold was relaxed to $1 \times 10^{-5}$. Lipid SNPs were selected at the standard genome-wide significance threshold ($5 \times 10^{-8}$).
   • Statistical Methods: The primary analysis used the Inverse-Variance Weighted (IVW) method. Sensitivity analyses included MR-Egger, Weighted Median, Simple Mode, and Weighted Model approaches. Heterogeneity was assessed via Cochran's Q test, and horizontal pleiotropy was evaluated using the MR-Egger intercept test and MR-PRESSO global test. Leave-one-out analysis was conducted to identify influential SNPs.

Key Results

• Microbiome Alterations: The study observed an increase in the relative abundance of Turicibacter in PD patients compared to healthy controls, although this specific alteration did not reach statistical significance in the small microbiome cohort.
• Lipid Profile Differences: In the clinical cohort of 80 PD patients, serum levels of TC, TG, and LDL-C were significantly lower than in healthy controls ($P < 0.001$). These differences were present in both genders (all P values < 0.01 except for TG). Notably, among women, lipid levels (including TC, TG, LDL-C, and HDL-C) were significantly higher than in men. Additionally, a negative correlation was found between serum TC/LDL-C levels and motor severity (UPDRS part III scores).
• Mendelian Randomization Findings:
  • Serotonin: MR analysis suggested a causal inverse association between serotonin levels and PD risk. Higher serotonin levels were associated with a lower risk of PD (Odds Ratio [OR] = 0.119, 95% CI 0.017–0.847, $P = 0.034$).
  • Lipids: Similarly, higher levels of TC (OR = 0.602, 95% CI 0.377–0.961, $P = 0.033$) and LDL-C (OR = 0.495, 95% CI 0.323–0.757, $P = 0.001$) were causally associated with a reduced risk of PD.
  • Sensitivity: Sensitivity analyses (MR-Egger, MR-PRESSO, Cochran's Q) indicated no significant horizontal pleiotropy or heterogeneity, supporting the robustness of the causal estimates.

Key Contributions
The paper contributes to the field by:

1. Integrating Multi-Omics: Combining clinical microbiome sequencing, serum lipid profiling, and genetic epidemiology (MR) to construct a comprehensive view of the gut-microbiota-brain axis in PD.
2. Establishing Causality: Moving beyond correlation to provide evidence for a causal relationship where higher levels of serotonin and specific serum lipids (TC, LDL-C) are protective against PD risk.
3. Mechanistic Insight: Highlighting the potential role of Turicibacter as a mediator that consumes serotonin and alters lipid metabolism, thereby influencing PD pathogenesis.

Significance and Claims
The authors claim that their findings provide a theoretical foundation for understanding the roles of Turicibacter, serotonin, and lipids in PD. Specifically, the study emphasizes that:

• Higher levels of serotonin and lipids are causally linked to a lower future risk of PD.
• Turicibacter may act as a potential mediator in the gut-brain axis by modulating serotonin availability and lipid metabolism.
• Peripheral serotonin, despite not crossing the blood-brain barrier directly, may serve as a promising diagnostic biomarker and therapeutic target, potentially through mechanisms involving platelet transport or modulation of intestinal colonization.

The authors maintain a modest tone regarding the limitations, acknowledging the small sample size for lipid analysis, the lack of long-term follow-up, the absence of data on serotonin-cholesterol correlations, and the potential bias arising from racial inconsistencies between the European GWAS data and the Chinese clinical cohort. They conclude that further laboratory research is required to elucidate the specific mechanisms of lipid metabolism disorders and statin usage in PD.