The use of real-time quaking-induced conversion and protein misfolding cyclic amplification assays accurately allowed researchers to differentiate synucleinopathies from nonsynucleinopathies.
Results from a retrospective and prospective diagnostic study provided proof-of-concept that skin a-synuclein (aSynP) seeding activity may serve as a novel biomarker for antemortem diagnoses of Parkinson disease (PD) and other synucleinopathies, such as Lewy body dementia (LBD) and multiple system atrophy (MSA).
The researchers, led by Zerui Wang, MD, PhD, postdoctoral research associate, Case Western Reserve University School of Medicine, analyzed 160 autopsied skin specimens from 140 cadavers (85 male cadavers [60.7%]; mean age at death, 76.8 years [standard deviation (SD), 10.1]) and 41 antemortem skin biopsies (27 male participants [66%]; mean age at time of biopsy, 65.3 years [SD, 9.2]).
Real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA) assays were used to analyze skin aSynP seeding activity. Analysis of autopsy abdominal skin samples from 47 PD cadavers and 43 nonneurodegenerative controls (NNCs) using RT-QuIC revealed 94% sensitivity (95% CI, 85–99) and 98% specificity (95% CI, 89–100).
The data also showed that RT-QuIC yielded 93% sensitivity (95% CI, 85–97) and 93% specificity (95% CI, 83–97) among 57 cadavers with synucleinopathies (PD, LBD, and MSA), and 73 cadavers without synucleinopathies (Alzheimer disease [AD], progressive supranuclear palsy [PSP], corticobasal degeneration [CBD], and NNCs).
All told, the skin RT-QuIC assay was also able to highly accurately differentiate synucleinopathies (n = 57) from nonsynucleinopathies (n = 73; mean area under curve [AUC], 0.9696 [standard error (SE), 0.0135]; 95% CI, 0.9431–0.9961; P <.001).
"To our knowledge this study has demonstrated for the first time that skin αSynP has aggregation seeding activity that was significantly higher in individuals with PD and other synucleinopathies than in those with tauopathies and NNCs. The skin-based analyses provided comparable diagnostic sensitivity and specificity to CSF-based assays,” Wang and colleagues concluded.
The investigators examined autopsy skin samples from PD cadavers (n = 24), LBD cadavers (n = 5), MSA cadavers (n = 3), PSP cadavers (n = 5), CBD cadavers (n = 5), and NNCs (n = 8) using PMCA. The sensitivity and specificity of PMCA to differentiate synucleinopathies from nonsynucleinopathies were 82% (95% CI, 76–88) and 96% (95% CI, 85–100), respectively.
Consistent with RT-QuIC findings, PMCA maximum thioflavin T (ThT) fluorescence responses from skin samples were highly accurate in discriminating synucleinopathies (n = 32) from nonsynucleinopathies (n = 18; mean area under curve, 0.9444 [SE, 0.0298]; 95% CI, 0.8860–1.0000; P <.001).
Wang and colleagues noted that phosphorylated aSynP was detectable with pS129-Syn, otherwise known as the antibody directed against phosphorylated a-synuclein, colocalized with PGP9.5 (an axonal marker), revealing the presence of aSynP in the skin nerve fibers of PD cadavers but not cadavers without PD by immunohistochemistry (IHC) and immunofluorescence (IF) microscopy.
From posterior cervical and leg skin biopsy tissues from patients with PD and controls without PD, the sensitivity and specificity were 95% (95% CI, 77–100) and 100% (95% CI, 84–100), respectively, for RT-QuIC and 80% (95% CI, 49–96) and 90% (95% CI, 60–100) for PMCA.
When analyzing the agreement levels of RT-QuIC and PMCA for aSynP seeding activity in the cadavers who were examined by both assays, researchers found that the PD agreement percentage was 78.6% (95% CI, 62.2–95.0), with RT-QuIC having slightly higher observed sensitivity than PMCA. The percentage agreement for the other diagnosis groups was 100%. Among PD cases only, there was 75% agreement, and the McNemar test indicated no significant difference (P = .69), which also supports agreement between methods.
Wang and colleagues concluded, “To our knowledge, there is currently no antemortem diagnostic test to reliably differentiate tauopathies from synucleinopathies in living patients with parkinsonism. Such a test would aide in treatment planning, prognostication, and guiding enrollment in clinical studies. Here, we showed that RT-QuIC and PMCA analyses of skin aSynP seeding activity among patients with parkinsonism can differentiate synucleinopathies (PD, MSA, and LBD) from tauopathies (PSP and CBD), which is consistent with previous reports of detecting aSynP deposits in the skin nerve fibers of patients with LBD and MSA with IHC or IF microscopy.”