Biomarkers

Background: Distinct amyloid structures characterize specific proteinopathies, including tau and α-synuclein based neurodegenerative diseases. However, how protein seed co-occurrence and other pathologic features account for clinicopathological heterogeneity observed within and between proteinopathies is unclear. Here, we quantify α-synuclein and isoform-specific tau seeds across neurodegenerative diseases, including in Lewy body disease (LBD), AD neuropathologic change (ADNC), and 4R tauopathy cases to inform how co-occurring seeds may impact disease presentation and trajectory.

Method: Real-time quaking-induced conversion (RT-QuIC) assays with fM sensitivity were used to estimate α-synuclein and tau isoform specific seeding doses in LBD (n=21), ADNC (n=47) across a range of Braak stages, and in 4R tauopathy (CBD=6; PSP=15) in mid-frontal lobe and hippocampus.

Result: α-synuclein seeding doses were observed in the mid-frontal lobe of all LBD cases, increasing with Lewy body stage (F=69.8, p<0.001) and reaching up to 10⁵ and 10⁶ seeding doses. Co-occurring α-synuclein seeding was noted as prevalent (50%) in AD, albeit typically at multi-log lower levels than that quantified in LBD. 100% of AD with amygdala-predominant α-synuclein (N=4) and 30% of AD cases without Lewy bodies (N=10) had detectable α-synuclein seeding. 3R/4R tau seeds were nearly ubiquitously detected well before overt neuropathology across all cases, and α-synuclein seeding was also observed occasionally without α-synuclein neuropathology. Markedly, 3R/4R tau and α-synuclein seeding activities correlated in LBD while controlling for Braak stage (R²=0.72, p<0.001). 4R tau seeds confirmed 4R tauopathy and occurred to some extent, albeit with lower frequency than 3R/4R tau, in non-4R tauopathy.

Conclusion: Protein seed co-occurrence is prevalent, including at early stages of both tau and α-synuclein based disease. RT-QuIC assays can selectively amplify, with multi-log differentiation, α-synuclein or tau seeds related to distinct α-synuclein or tau structures characteristic of LBD, AD, and 4R tauopathies. Pervasive, but quantitatively distinct α-synuclein seeding activity occurred in frontal cortex in LBD and AD, including detectable α-synuclein seeds in some AD cases without identifiable Lewy body neuropathology. Regression analysis of seeding activities revealed quantitatively similar levels of α-synuclein and tau seeding in LBD suggesting a possible novel role for α-synuclein-tau interaction in LBD, which we will explore in future studies.