Misfolded α-synuclein co-occurrence with Alzheimer's disease proteinopathy

Multi-etiology dementia necessitates in vivo markers of copathologies including misfolded $\alpha$ -synuclein (syn). We measured misfolded syn aggregates (syn-seeds) via qualitative seed amplification assays (synSAA) and examined relationships with markers of Alzheimer's disease (AD).

Cerebrospinal fluid (CSF) was obtained from 420 participants in two AD risk cohorts (35% male; 91% cognitively unimpaired; mean [standard deviation] age, 65.42 [7.78] years; education, 16.17 [2.23]) years). synSAA results were compared to phosphorylated tau (T), amyloid beta (A), and clinical outcomes. Longitudinal cognition was modeled with mixed effects.

Syn positivity (synSAA+) co-occurred with T (in synSAA+ vs. synSAA-, 36% vs. 20% T+; Pp = 0.011) and with cognitive impairment (10% vs. 7% mild cognitive impairment; 10% vs. 0% dementia; p = 0.00050). synSAA+ participants' cognitive performance declined ≈ 40% faster than synSAA- for Digit Symbol Substitution, but not other tests.

Findings support prevalent syn copathology in a mostly unimpaired AD risk cohort. Relationships with progression should be evaluated once more have declined.

In a middle-aged sample, misfolded $\alpha$ -synuclein (syn) co-occurred with phosphorylated tau181 (T). syn+/T+ status was linked with higher levels of other cerebrospinal fluid biomarkers. syn+ individuals were more likely than syn- to be cognitively impaired. syn+ status was linked to faster decline on an executive function task.