Routine implementation of α-synuclein Seed Amplification Assays reveals high diagnostic performance and the limited value of Alzheimer disease fluid biomarkers for detecting α-synuclein co-pathology

Can We Spot Hidden Protein Overlap in Dementia?

Neurodegenerative diseases often involve the accumulation of misfolded proteins that disrupt brain function. Scientists have long known that different diseases rarely exist in isolation. Instead, they frequently overlap in the same patient.

Currently, clinicians rely on fluid biomarkers—proteins measured in cerebrospinal fluid (CSF) or blood—to identify these pathologies. These tests are highly effective at detecting the amyloid and tau proteins central to Alzheimer’s disease (AD). However, it remains unclear if they can also spot the presence of $\alpha$-synuclein (a protein associated with Lewy body dementia). A new study from the ALZAN multicenter cohort investigates whether a specialized assay can fill this diagnostic gap.

The search for $\alpha$-synuclein co-pathology

The researchers addressed a specific blind spot in modern neurology. They asked if we can detect $\alpha$-synuclein pathology in patients already diagnosed with Alzheimer's disease. In many cases, an AD patient may also harbor $\alpha$-synuclein aggregates. This condition is known as co-pathology.

Identifying these overlapping proteinopathies is critical for patient stratification. This is the process of grouping patients by their specific biological makeup. Such grouping ensures they receive the correct treatments. If a patient has both amyloid and $\alpha$-synuclein, their clinical progression might look very different from a patient with amyloid alone. The study determines if current gold-standard AD biomarkers can serve as proxies for this hidden pathology.

Cracks in the Alzheimer's biomarker toolkit

Before this study, the field relied heavily on measuring amyloid-$\beta$ (A$\beta$) and tau proteins. These biomarkers are remarkably accurate at identifying the hallmarks of AD. However, they are essentially "blind" to other protein types.

The authors note that $\alpha$-synuclein deposits are seen in 20–50% of neuropathologically confirmed AD patients. Despite this, existing fluid biomarkers provide almost no information regarding this concomitant pathology. This creates a disconnect. We can see the "amyloid smoke," but we cannot see the "synuclein fire" occurring simultaneously. Without a way to detect both, clinicians must rely on symptomatic observations. These observations can often be imprecise.

Testing the seed amplification assay

To solve this, the researchers employed an $\alpha$-synuclein seed amplification assay ($\alpha$SAA). They specifically used a technique called real-time quaking-induced conversion (RT-QuIC). Think of RT-QuIC as a biological "amplifier." If a tiny amount of misfolded $\alpha$-synuclein (the "seed") is present in a CSF sample, the assay triggers the misfolding of healthy proteins. This process is monitored via fluorescence over 168 to 225 hours.

The study analyzed 398 patients from the ALZAN cohort. They categorized them into clinical groups like AD, Lewy body dementia (LBD), and Frontotemporal Dementia (FTD) .

The researchers specifically isolated the AD subgroup. They wanted to see if existing biomarker profiles showed any correlation with $\alpha$SAA positivity. They compared these results against established markers of neurodegeneration and glial activation.

High performance meets biomarker silence

The results reveal a stark divide between what $\alpha$SAA can see and what traditional biomarkers can detect. The authors report that $\alpha$SAA demonstrated high diagnostic performance for LBD. It achieved a sensitivity of 95.0% and a specificity of 93.1% . This means the test is highly reliable at correctly identifying those with the disease and excluding those without it.

More importantly, the study found that 15.8% of patients clinically diagnosed with AD were actually $\alpha$SAA-positive. This indicates they harbored $\alpha$-synuclein co-pathology. When the researchers looked closely at these AD patients, they found a striking lack of signal in conventional tests. Aside from a marginal difference in the CSF A$\beta$42/40 ratio, most major markers showed no significant differences. These include CSF tau, CSF p-tau181, and plasma p-tau217. Even markers for neurodegeneration (NfL) and glial activation (GFAP) failed to differentiate the groups .

Essentially, the "standard" Alzheimer's tests passed right over the $\alpha$-synuclein pathology.

A new layer for precision medicine

These findings suggest that $\alpha$SAA provides unique biological information. This information is entirely independent of the current Alzheimer's biomarker suite. Therefore, $\alpha$SAA should be viewed as a complementary tool rather than a replacement. When a clinician suspects LBD or sees atypical symptoms in an AD patient, adding an $\alpha$SAA test could provide necessary biological clarity.

For the research community, this reinforces the necessity of multi-target approaches in clinical trials. If a drug targets $\alpha$-synuclein, enrolling patients based solely on AD biomarkers would be problematic. Roughly 15.8% of that population might actually be the intended target, while the rest are not. The paper does not explore how these combined pathologies affect the speed of cognitive decline over time. A logical next step would be a longitudinal study. Such a study could track whether $\alpha$SAA-positive AD patients experience faster neurodegeneration than their $\alpha$SAA-negative counterparts.