By Rivka Sachdev, MD
Assistant Professor of Clinical Neurology, Weill Cornell Medicine
SYNOPSIS: This proof-of-concept study proposes that a noninvasive assay detecting pathology-associated α-synuclein extracted from blood may reveal a reliable biomarker for Parkinson’s disease.
SOURCE: Kluge A, Bunk J, Schaeffer E, et al. Detection of neuron-derived pathological α-synuclein in blood. Brain 2022;145:3058-3071.
An objective and reliable biomarker to improve accuracy in diagnosing Parkinson’s disease and measuring its progression is an urgent need that remains unmet. Currently, we diagnose Parkinson’s disease by assessing signs evident on physical examination after a history of motor and nonmotor symptoms is elicited. Progression of the disease also is assessed clinically using rating scales to document exam findings over time. But these methods have shortcomings, since clinician/rater experience and the psychological and motor status of the patient at the time of the examination are variable. Furthermore, abnormalities on exam only become apparent long after the process of neurodegeneration has begun. By the time motor signs are evident, it is estimated that up to 60% of dopaminergic neurons already have been lost. Currently, the diagnosis of Parkinson’s disease can only be confirmed with certainty by neuropathological findings from postmortem tissue analysis — specifically, the detection of misfolded α-synuclein that comprises Lewy bodies found in neurons in particular regions of the brain. Occasionally, postmortem analysis of brain tissue reveals clinical diagnoses of Parkinson’s disease in life that were misdiagnosed, showing that our current clinical methods have limitations in diagnostic accuracy.
One focus of research in recent years involves the migration of extracellular vesicles and the pathogenesis of neurodegenerative disorders. Extracellular vesicles released by cells of the central nervous system are called neuron-derived extracellular vesicles. These vesicles can cross the blood-brain barrier and can transport/spread neuropathological proteins, including misfolded α-synuclein. It is widely accepted that the accumulation and aggregation of misfolded α-synuclein has a primary role in the pathogenesis of Parkinson’s disease, and, therefore, much research on Parkinson’s disease focuses on this pathological protein form. Previous research studies have identified misfolded α-synuclein in peripheral tissues, such as in the gastrointestinal tract, skin, and salivary glands, but obtaining tissue samples requires invasive biopsies. Even studying α-synuclein in cerebrospinal fluid is invasive, since it requires a lumbar puncture.
The authors of this study revealed the potential of a biomarker, obtainable by a blood test, for reliably distinguishing Parkinson’s disease patients from controls. Their novel method for detecting and examining extracellular vesicle-derived α-synuclein from blood samples allowed them to accurately distinguish patients with Parkinson’s disease from the control participants. These methods may be the basis for developing a dependable, noninvasive blood-based test for diagnosing Parkinson’s disease.
In the study, 30 patients with Parkinson’s disease (all met requirements for diagnosis based on the UK Brain Bank Criteria) and 50 participants without Parkinson’s disease (controls) provided blood samples obtained by venipuncture. Samples underwent several steps of centrifugation, after which extracellular vesicle precipitation reagents were added and, ultimately, precipitated extracellular vesicles were isolated. Neuron-derived extracellular vesicles then were identified and filtered out based on surface protein markers. Then, the researchers tested the α-synuclein contents within these vesicles and detected the pathogenic misfolded form of α-synuclein using structure-specific antibodies. Quantitative analysis of the total α-synuclein in neuron-derived extracellular vesicles was carried out and, although there were no significant differences between Parkinson’s patients vs. controls, the Parkinson’s patients had significantly higher levels of the specific disease-associated abnormal form of α-synuclein vs. controls. This finding provides useful information for future research, which the authors argued should focus on detecting pathological α-synuclein conformers rather than quantitative levels of α-synuclein in body fluids or tissues. The last part of their protocol involved successfully reproducing and amplifying the extracted misfolded α-synuclein forms from the Parkinson’s patients.
COMMENTARY
The authors concluded that detection and amplification of pathological α-synuclein conformers in plasma neuron-derived extracellular vesicles is a promising biomarker for distinguishing Parkinson’s disease patients from healthy controls. However, further studies using larger cohorts and blinding are needed to validate this protocol. More investigations will be needed to confirm these findings for patients in various stages of disease, including early prodromal stages, such as in patients with confirmed rapid eye movement sleep behavior disorder. The authors suggested that future work should include how these biomarkers eventually can be used for distinguishing Parkinson’s disease from other synucleinopathies, such as dementia with Lewy bodies and multiple system atrophy. Although the diagnosis of Parkinson’s disease in current neurological practice remains a clinical one, this exciting research brings us a step closer to realizing a noninvasive biomarker that can reflect disease pathophysiology, progression, and severity and, therefore, a step closer to the development of new therapies for Parkinson’s disease patients.
