Commentary
Article
Author(s):
George Tofaris, MBBChir, PhD, FRCP, professor of neurology and translational neuroscience at University of Oxford, talked about a recent study assessing α-synuclein in neuronally derived serum-extracellular vesicles identified patients at risk of Parkinson disease.
George Tofaris, MBBChir, PhD, FRCP
Credit: University of Oxford
Parkinson disease (PD), a common movement disorder, is characterized by a long prodromal phase that begins many years prior to clinical presentation of motor symptoms. Several nonmotor symptoms, such as REM sleep behavior disorder, can occur during the prodromal phase and are associated with higher risk of phenoconversion to PD. Since symptoms correlate broadly with the evolving of the α-synuclein pathology, accurate measurements of neuronally derived α-synuclein in at-risk patients has the potential to assist with the identification of those likely to develop PD.1
A new cross-sectional study published in JAMA Neurology showed an increase in serum L1CAM-positive extracellular vesicle (L1EV) α-synuclein levels among patients at risk of PD. Conducted by senior author George Tofaris, MBBChir, PhD, FRCP, and colleagues, the findings suggest a combination of prodromal markers and L1EV α-synuclein should be reviewed in the stratification of those at high risk of developing PD and related Lewy body diseases. In addition, the blood test used in the study could potentially identify patients who had evidence of neurodegeneration detected by imaging, or pathology detected by a spinal fluid assay, but did not develop a movement disorder or dementia.2
Tofaris, professor of neurology and translational Neuroscience at University of Oxford and consultant neurologist at John Radcliffe Hospital, had a conversation with NeurologyLive® to discuss how the unique composition of neuronal extracellular vesicles contributes to the accuracy of PD biomarker detection. He also talked about the significance of the observed association between elevated α-synuclein levels in extracellular vesicles and delayed disease onset holds for Parkinson research. Additionally, Tofaris spoke about how the development of a microfluidic platform can potentially enhance the scalability and efficiency of the blood test for wider clinical use.
George Tofaris, MBBChir, PhD, FRCP: α-synuclein is a sticky protein and in blood is mostly (~99%) derived from peripheral sources, especially red blood cells. For this reason, so far, measurements of α-synuclein directly in blood have not been useful for biomarker development as most of the protein that floats in the blood is not relevant to the disease process. Because of the sticky nature of α-synuclein it is also difficult to separate the disease-relevant fraction of this protein without contamination from the massive excess of peripherally derived α-synuclein.
In the blood stream there are also millions of tiny nanoparticles called extracellular vesicles that originate from all sorts of cells from different organs.A tiny fraction of these vesicles originates from nerve cells and have on their surface a telltale marker that enables us to trace their origin. Our assay utilizes antibodies against this neuronal marker that are hinged onto otherwise nonsticky magnetic beads. Antibody-bead composites bind to neuronal extracellular vesicles in the serum and are then extracted with a magnet with minimal contamination from other blood proteins or free floating α-synuclein. The extracellular vesicles are washed and opened up with detergents so that the fraction of α-synuclein in them can be measured. We believe that this fraction reflects changes in nerve cells and therefore offers a more accurate snapshot of the disease process.
Using this approach, we looked at 365 at-risk individuals from 4 clinical cohorts (Oxford Discovery, Marburg, Cologne and the US-based Parkinson’s Progression Markers Initiative), 282 healthy controls and 71 patients with genetic or sporadic PD. We found that those with the highest risk of developing PD (more than 80% probability based on research criteria) had a 2-fold increase in α-synuclein levels in neuronal extracellular vesicles and the test could accurately differentiate them from those with low risk (less than 5% probability) or healthy controls. Overall, the test could distinguish an individual with high risk of developing PD from a healthy control with 90% probability.
α-synuclein accumulates inside nerve cells and as mentioned above this process starts years before the main symptoms appear. We had previously shown that our test identified a 2-fold increase in α-synuclein levels in nerve-derived extracellular vesicles in PD but not Parkinson like conditions. In a way, in my view it is logical to look for and find the same change earlier in those at high risk of developing the disease. Perhaps the surprise comes from our pilot analysis suggesting an association between α-synuclein levels and internal to diagnosis. In other words, there was a trend for those with higher levels of alpha-synuclein in extracellular vesicles to develop the disease later. This finding suggests that extracellular vesicle α-synuclein in the blood is increased in PD and those at risk because nerve cells are trying to get rid of this protein.
The brain has a certain reserve and ability to compensate, so by the time patients with PD present to the clinic with the familiar movement disorder, it has been estimated that at least 60% of dopamine producing nerve cells have already died and neurodegeneration is often widespread. At this stage it is difficult to restore the neuronal networks. On the other hand, if we could detect the pathology early before this critical threshold is reached, then we are more likely to be effective with disease-modifying therapies and the impact of any benefit is likely to be longer lasting even if “cure" is not achieved. We know from different studies that the pathology starts a decade before the clinical presentation; what we are lacking is a scalable biomarker to identify these changes, ideally a blood test. Therefore, the need for such a test goes hand in hand with the urgent need for disease-modifying therapies.
We are performing further validation of the biomarker in longitudinal cohorts, i.e. studying serial samples in patients who went on to develop PD sometime in the future or are closer to developing the disease. In collaboration with colleagues in Oxford Chemistry, we have also developed a microfluidic platform for on-chip extraction of these extracellular vesicles in 30 minutes using minimal volume of blood and we continue to optimize and develop such platforms. This is needed to minimize manual handling (which may introduce variability) and increase scalability for wider use in clinical practice. My hope is that in the future, a blood test such as this one, possibly in combination with a questionnaire or a limited assessment, will be implemented for screening to identify Parkinson early for the instigation of a disease-modifying intervention as we currently do for certain cancers or screening programs for cardiovascular risk factors.
Transcript edited for clarity.
