Highlight
- α-synuclein seed amplification assay (SAA) positivity accurately distinguishes Parkinson’s disease (PD) from progressive supranuclear palsy (PSP), particularly identifying Lewy body co-pathology in PSP via kinetic profiles.
- Faster time to threshold (TTT) in α-synuclein SAA kinetics is characteristic of GBA1-associated PD compared to sporadic PD.
- TTT at baseline robustly predicts cognitive decline in PD patients, even in those negative for Alzheimer’s disease biomarkers.
Study Background and Disease Burden
Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by motor symptoms and a heterogeneous non-motor symptom profile, including cognitive impairment. Diagnostic and prognostic biomarkers are urgently needed to improve patient stratification, aid early diagnosis, and predict disease trajectories, particularly cognitive decline which strongly impacts quality of life and care needs. α-synuclein aggregation in the central nervous system is a pathological hallmark of PD, and α-synuclein seed amplification assay (SAA) has emerged as a promising diagnostic tool detecting misfolded α-synuclein aggregates in cerebrospinal fluid (CSF). While SAA positivity has been established in small cohorts as a diagnostic marker, its prognostic utility, especially using quantitative kinetic measures of seeding, has not been sufficiently explored in large, longitudinal studies.
Study Design
This longitudinal cohort study compiled data from 1631 participants derived from three cohorts: the UK Parkinsonism cohort (including PD, PSP, and controls), the Parkinson’s Progression Markers Initiative (PPMI) international observational study, and the Tübingen PD cohort. Participant enrollment spanned from January 1, 2005 to November 1, 2023, with baseline CSF samples assessed for α-synuclein SAA. Kinetic measures evaluated included time to threshold (TTT), maximum Thioflavin T fluorescence (MaxThT), and area under the fluorescence curve (AUC). Seeding kinetics were compared between sporadic PD, monogenic PD (notably GBA1-associated), and PSP to investigate diagnostic discrimination and the linkage with Lewy body co-pathology. Time-to-event analyses adjusted for sex, age, and disease duration evaluated the association of SAA kinetics with unfavorable clinical outcomes, focusing notably on cognitive decline measured by Montreal Cognitive Assessment (MoCA).
Key Findings
Data analysis revealed that α-synuclein SAA was positive in 96% of PD cases and 15% of PSP cases within the UK Parkinsonism cohort, with distinct slow seeding kinetics (low MaxThT, high TTT) in the majority of α-synuclein-positive PSP cases, indicating Lewy body co-pathology. Across PPMI and Tübingen cohorts, TTT was significantly faster in GBA1-associated PD compared to sporadic PD (p=0.04 and p=0.01, respectively).
Focusing on prognosis, 810 α-synuclein SAA positive PD participants without unfavourable outcomes at baseline were followed for a median of 4.5 years. Among these, 73% developed an unfavorable outcome, primarily cognitive decline. Baseline TTT predicted cognitive decline (MoCA ≤21) with hazard ratios of 2.36 in PPMI and 2.17 in Tübingen cohorts, maintaining statistical significance after adjustment. Notably, TTT retained predictive value for cognitive decline in a subgroup of PD patients negative for Alzheimer’s disease biomarkers (HR 1.80), evidencing independent prognostic relevance beyond coexistent Alzheimer’s pathology.
Expert Commentary
These findings support α-synuclein SAA not only as a diagnostic biomarker but also as a prognostic tool providing insights into disease heterogeneity. The kinetic parameters, particularly TTT, correlate with genetic subtypes and predict cognitive trajectories, underscoring the biological plausibility of aggregation dynamics impacting downstream neurodegeneration. The ability to differentiate PD from PSP with Lewy body co-pathology proposes a valuable clinical application. Limitations include variable cohort sizes and follow-up durations, yet the consistency across independent cohorts strengthens validity. Future integration of α-synuclein SAA kinetics with multimodal biomarkers may enhance personalized medicine approaches.
Conclusion
Quantitative α-synuclein SAA kinetic measures refine diagnostic specificity between PD and PSP with Lewy pathology and serve as a robust predictor of cognitive decline in PD, especially in genetically defined subgroups such as GBA1-associated PD. Their predictive power is independent of Alzheimer’s disease co-pathology, positioning α-synuclein SAA kinetics as a crucial advancement in biomarker-guided prognosis and clinical decision-making in Parkinson’s disease.
References
Orrú CD, Vaughan DP, Vijiaratnam N, et al. Diagnostic and prognostic value of α-synuclein seed amplification assay kinetic measures in Parkinson’s disease: a longitudinal cohort study. Lancet Neurol. 2025 Jul;24(7):580-590. doi:10.1016/S1474-4422(25)00157-7.
