Alzheimer's disease traits in Parkinson's disease without α-synuclein seeding

The α-synuclein (αSyn) seed amplification assay (αSyn-SAA) is an accurate tool to detect αSyn seeds in patients with Parkinson's disease (PD). However, a minority of clinically diagnosed PD patients are negative for αSyn.

The αSyn-SAA was performed in cerebrospinal fluid (CSF) of individuals with PD (n = 93), multiple system atrophy (MSA, n = 26), progressive supranuclear palsy (PSP, n = 18), corticobasal syndrome (n = 3), and healthy controls (n = 29).

The αSyn-SAA detected αSyn in 90% of PD and 81% of MSA patients, while exhibiting high specificity (97%). SAA- PD patients had a distinct phenotype compared to SAA+ PD, including a marked postural instability/gait disorder (P = 0.002), impaired episodic memory, and lower CSF amyloid beta₄₂ (P = 0.03). SAA+ PSP also displayed distinctive traits.

A negative αSyn-SAA in PD is associated with a distinct phenotype and pathological findings suggesting that these patients may have a motor subtype of Alzheimer's disease. This could influence future clinical trials.

The α-synuclein seed amplification assay (αSyn-SAA) is a robust assay. αSyn-SAA-negative Parkinson's disease shows a distinct motor-cognitive phenotype. Autopsy showed Alzheimer's disease (AD) pathology in parkinsonian diseases. AD stands as a major clinical confounder for the diagnosis of movement disorders.