Skin biopsies of patients with parkinsonism, mild cognitive impairment or dementia might offer a simple and relatively non-invasive means to determine whether α-synuclein aggregates might be the underlying pathology. Accurate biomarkers are critically needed for Lewy body disease (LBD) clinical trials but currently there are none that have undergone full regulatory scrutiny.

We sought to simulate the rigor of a diagnostic study done with regulatory oversight in this study of the accuracy of skin biopsies processed by α-synuclein seeding amplification assays (SAA) in predicting the clinical diagnoses of Parkinson disease (PD), PD with dementia (PDD) and dementia with Lewy bodies (DLB). Our study design utilized parallel blinded performances of assays in two independent, experienced SAA assay laboratories. For control subjects without clinical LBD, we used a clinically heterogeneous set, simulating a cross-section of elderly, in comparison with previous studies that have mainly used cases and controls that have been prescreened to accentuate group differences.

Subjects clinically diagnosed with PD, PDD and DLB as well as those without a suspected LBD were recruited from three sites, including the Mayo Clinic Arizona, Barrow Neurological Institute and Banner Sun Health Research Institute (BSHRI), all located in metropolitan Phoenix, Arizona. The LBD group was designated as Group 1 while the non-LBD groups were divided between a Group 2 and a Group 3, based on the absence (Group 2) or presence (Group 3) of potential clinical risk factors for LBD, including mild cognitive impairment, dementia, REM sleep behavior disorder and hyposmia. Skin punch biopsies were collected from the posterior cervical area and three biopsies were analyzed by SAA between the two labs.

Sensitivity across assays ranged between 50.0% and 60.0% while specificity ranged between 69.6% and 100%. Comparisons of Group 1, with the highest clinical diagnostic confidence for the presence of an LBD, versus Group 2, with no clinical indicators of relevant abnormalities, produced the greatest specificities, between 77.3% and 100%, while sensitivities were less variable, ranging between 50% and 60% on group comparisons. Specificities were lower when Group 3 subjects, with potential LBD risk factors, were included in the calculations.

Pairwise agreement between biopsy assays ranged from excellent, with a kappa of 0.816, to moderate, between 0.47 and 0.67. Agreement may have been affected by differing protocols, as substrate and testing strategies were different between the two laboratories. Also, it might be possible that variability in α-synuclein seed concentrations within different skin biopsies might have contributed to differences in the outcome of the testing by the two labs. The greater agreement for Biopsy 3 was perhaps to be expected given that a single biopsy was analyzed by both labs and the homogenates were prepared by a single lab (Lab 2) and then shared between labs; homogenates for Biopsies 1 and 2 were separately prepared and separately assayed in Labs 1 and 2.

A separate analysis of subjects diagnosed with PDD or DLB indicated that skin SAA may have a greater sensitivity for clinically advanced LBD. Nine of these eleven subjects had at least one positive assay, for a sensitivity of 81.82%, substantially better than the 57.1% sensitivity for probable PD subjects without dementia. This suggests that skin SAA may have greater utility as a diagnostic and progression biomarker of Lewy body dementias, as compared with PD subjects without dementia.

Byron Caughey and Christina Orru hold patents on RT-QuIC methods.

This study was funded by the National Institute of Neurological Disorders and Stroke award number R01NS118669, and by the National Institute on Aging, award number P30AG072980.

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Ethical approval for this work was granted by the WCG IRB of Puyallup, Washington, USA.

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