Article In Brief
A study that analyzed the sensitivity and specificity of the alpha-synuclein seed amplification assay in 1,123 participants with Parkinson's disease and healthy controls found that the test can detect Parkinson's disease at the molecular level in cerebrospinal fluid before patients' symptoms appear.
A highly accurate test, the alpha-synuclein seed amplification assay (SAA), can now objectively and reliably detect Parkinson's disease (PD) at the molecular level in cerebrospinal fluid even before the onset of symptoms, according to new findings the Parkinson's Progression Markers Initiative (PPMI) published in May in Lancet Neurology.
It's a potential major breakthrough by a series of Parkinson's researchers in the field who have been working on the assay called RT-QuIC for several years. The assay was applied to samples in PPMI, created in 2010 by the Michael J. Fox Foundation with a core group of scientists and industry partners to identify biological markers of Parkinson's risk, onset and progression, experts told Neurology Today.
To date, PPMI has enrolled nearly 2,000 participants and is actively enrolling PD patients, at-risk individuals, and control volunteers at 51 clinical sites around the world.
The new study analyzed the sensitivity and specificity of the alpha-synuclein SAA in 1,123 participants with PD and healthy controls, including subgroups based on genetic and clinical features, between July 7, 2010, and July 4, 2019. Synuclein pathology is one of two biological hallmarks of PD; the other, dopaminergic transport dysfunction, can be visualized using the brain imaging test DaTScan.
This large-scale study confirms smaller previous reports that αSyn-SAA can distinguish PD from control volunteers—in this case, with a sensitivity of 88 percent and specificity of 96 percent.
The assay was particularly useful when combined with anosmia and/or rapid eye movement sleep behavior disorder (RBD), which earlier studies identified as prodromal signs of PD. Among prodromal and at-risk groups, 44 of 51 (86 percent) of participants with RBD or hyposmia had positive α-synuclein SAA (16 of 18 with hyposmia and 28 of 33 with RBD). The proportion of positive results was lower in people with PD and a LRRK2 mutation (68 percent) and in people with sporadic PD without olfactory deficit (78 percent).
“By the time a person with PD is symptomatic, we know that they've already lost 70 percent of their dopamine, so maybe part of the problem is that we have been starting too late. This assay will let us back up earlier and have a better shot at modifying the disease.”—DR. DAVID G. STANDAERT
Post-mortem examinations of the individuals enrolled in PPMI who died since joining the study further confirmed the findings; all those with positive assay results (14) showed typical alpha-synuclein aggregation pathology in brain cells, while the single PD participant who did not show positive assay results was a LRRK2 mutation carrier with preserved olfaction.
“Validation of this biomarker launches a new, biological era in Parkinson's research,” Kenneth Marek, MD, PPMI principal investigator and president and senior scientist at the Institute for Neurodegenerative Disorders, said in an April news briefing. “Using αSyn-SAA, we are already unlocking new understanding of Parkinson's, which will transform every aspect of drug development and ultimately clinical care.
“We will rapidly be in a position to test new therapies in the right populations, target the right therapy to the right patient at the right time, and launch studies of agents with potential to prevent Parkinson's disease altogether. This is a game-changing day for us in Parkinson's disease.”
“Collectively, over the past five years, we have observed many expert research groups using a RT-QuIC [cerebrospinal fluid real-time quaking-induced conversion] analysis technique to identify the presence of alpha-synuclein in multiple human body fluids,” said Michael Okun, MD, FAAN, professor of neurology and executive director of the Norman Fixel Institute for Neurological Diseases at UF Health in Gainesville, Florida.
“The recently published PPMI Michael J. Fox Foundation-funded cohort highlights the real potential for diagnosing at-risk and early-stage Parkinson's disease. This finding has implications for clinical trials, for research, and maybe, one day, even for clinical practice.”
Identifying PD Years Before Symptoms
The assay, commercialized by Amprion as the SYNTap Test, has been available for more than a year after being granted breakthrough designation by the US Food and Drug Administration (FDA) in 2022, but the new study confirms its accuracy in a large study cohort.
When the test is positive, a person likely has PD or a related disorder of alpha-synuclein, such as dementia with Lewy bodies. “This is a story that has been building for some years now,” said David G. Standaert, MD, PhD, FAAN, the John N. Whitaker Professor and Chair of Neurology at the Heersink School of Medicine at the University of Alabama at Birmingham (UAB) and director of the NIH-funded Alabama Morris K. Udall Center of Excellence in Parkinson's Research.
He noted that the idea that abnormal forms of synuclein are an important part of the biology of PD began more than 10 years ago in research led by Virginia Lee, MD, PhD, and John Trojanowski, MD, PhD, in the Center for Neurodegenerative Disease at the University of Pennsylvania.
“Along with Laura Volpicelli-Daley, PhD, who is now here at UAB, they demonstrated, first in cell culture and then in mice, how abnormal synucleins could propagate across the brain,” he said. “More recent research has shown that we can detect these abnormal synucleins in the spinal fluid of PD patients. But prior studies were pretty small. This is a large, definitive cohort, and it shows very clearly that the vast majority of people with PD have abnormal synuclein in spinal fluid that can be detected by this method.”
The study also confirmed that, in individuals 60 and older, significant and enduring loss of smell is an important sign of risk for PD, as is diagnosis with RBD.
These findings are particularly important because they offer a way to identify individuals likely to develop PD far in advance of when symptoms appear. “As you think about the lifespan of an individual with Parkinson's disease, we've always sort of measured that from their clinical diagnosis, but we've known that there really is 20 years before that that they have had degeneration in their brain. We just couldn't identify it before,” Dr. Marek said. “Now we can begin to plan therapeutics during that period. So the promise of preventive therapy is really in our grasp.”
Clinical Trials and Drug Development
The findings provide a major boost to fine-tuning PD clinical trials after some recent disappointments, said Lorraine Kalia, MD, PhD, associate professor in the department of medicine (neurology) and the Tanz Centre for Research in Neurodegenerative Diseases at the University of Toronto and senior scientist in the Krembil Brain Institute at Toronto Western Hospital.
“There has been very little available to allow us to identify abnormalities in protein aggregation in people with Parkinson's,” she said. “We don't have the equivalent of amyloid-beta PET or tau PET, which are used to image protein accumulation in other neurodegenerative diseases. While we are optimistic that an α-synuclein PET tracer is on the horizon for Parkinson's, it's still far away from use in clinical trials.”
In one of the recent anti-α-synuclein antibody trials published Aug. 4, 2022, in the New England Journal of Medicine, CSF samples from a subset of participants were tested using α-synuclein SAA, and 7 percent were non-seeders, Dr. Kalia said. “So there was an important minority of individuals in that trial who may not have been the right candidates to be receiving this therapy, and it is possible that this could have contributed to the trial's negative results,” she said. “Therefore, the ability to be able to accurately identify the right populations for clinical trials is extremely important, especially with the expected wave of trials that will test new therapies that target harmful forms of α-synuclein or their upstream pathways.”
“The recently published PPMI Michael J. Fox Foundation-funded cohort highlights the real potential for diagnosing at-risk and early-stage Parkinson's disease. This finding has implications for clinical trials, for research, and maybe, one day, even for clinical practice.”—DR. MICHAEL OKUN
“We have been trying to find a treatment to slow or prevent PD for a long time now, and we have failed in a lot of different trials,” Dr. Standaert said. “One reason may be the heterogeneity among patients with the disease. If you enroll a mixed group of patients, you get mixed results, and it's hard to have a successful trial. I suspect this assay will soon be incorporated as an entry criterion into future clinical trials of PD therapies.”
The assay can be used to help drug companies refine their clinical trial populations almost immediately, said Debi Brooks, co-founder and CEO of the Michael J. Fox Foundation and a control participant in the PPMI study. “We have a really robust pipeline of drug programs that are looking to address underlying biology and interfere with the progression of the disease, and this will help make those trials smarter, faster, and cheaper. Patients living with PD today will benefit immediately from the value of such a tool in terms of applying it to the current pipeline.”
The fact that the assay also provides an early signal in presymptomatic individuals could open up possibilities around disease prevention, added Brooks. “This is already something that can change the trajectory of progress for anybody with Parkinson's waiting for better drugs, as well as people who will learn over time that they may be at risk for Parkinson's disease.”
Most PD therapies in the pipeline have been tested in already symptomatic patients. But Dr. Standaert pointed to recent advances in Alzheimer's disease drug development as an indicator of where PD therapy trials should go.
“In Alzheimer's, treatments were originally developed to target later-stage disease, and they were largely unsuccessful,” he said. “The only success is when you back up to treat patients with very early disease, mild cognitive impairment, or an unimpaired state with biomarker positivity, as we've seen with lecanemab. By the time a person with PD is symptomatic, we know that they've already lost 70 percent of their dopamine, so maybe part of the problem is that we have been starting too late. This assay will let us back up earlier and have a better shot at modifying the disease.”
At present, the assay is done using cerebrospinal fluid, but efforts are underway to develop similar assays in skin, olfactory mucosa and blood.
“In order to really scale this to make it broadly useful and more acceptable to patients, it would be really ideal to be able to do seed amplification in tissue other than CSF,” said lead investigator Andrew Siderowf, MD, the Hurtig-Stern Professor of Neurology and chief of the movement disorders division in the Penn Perelman School of Medicine department of neurology.
“These other matrices where the SAA would be more accessible offers the opportunity for more widespread application of the test to broader populations and easier to do in a routine clinical setting.”
Applying the test outside the research setting would also likely require a lower price tag, Dr. Standaert said. “When we ordered the kits last week, we got a notice that we will receive them, but they are not yet paid for by Medicare, and the cost is $1,500,” he said.
More Questions Raised
“This is the first really convincing result from this study, which has taken more than 10 years now but is pretty exciting. Like any great result, it raises even more questions,” Dr. Standaert said. One of the biggest: the proportion of positive results for the assay was lower in people with PD who had a LRRK2 mutation (68 percent), and also lower in people with sporadic PD without olfactory deficit (78 percent).
People with PD, a LRRK2 mutation and normal smell ability were even less likely to show positive assay results, a finding particularly true among females, he said, noting only 12.5 percent of females in this population showed synuclein pathology on the test. What does this mean?
“More than 30 percent of individuals with a genetic variant for the LRRK2 gene are SAA-negative, which is both puzzling and exciting,” Dr. Marek said. “They don't have synuclein. And so this offers us enormous possibilities for research to try to figure out what they do have and how it's important.”
“It is critical that we understand that Parkinson's is not one disease,” Dr. Okun said. “We see postmortem autopsy brains without synuclein, especially in those with gene mutations, and therefore we must not look at these tests as a one-size-fits-all. The pathology and biology are likely different across patients.”
Whatever these patients do have, it is still likely a dopamine cell loss disease, Dr. Standaert said. “We know from autopsy that people with LRRK2 lose dopamine and clinically behave very similarly to those with sporadic PD, responding to levodopa in a similar way,” he said. “Yet at least a significant subset of them has a biology that is different in some way. That's a real mystery that needs to be unraveled.”
The study also has some limitations. For example, it is a fairly homogeneous population—approximately 95 percent of participants were White. Asian individuals were particularly under-represented, with only a handful of participants in two cohorts and none among healthy controls, prodromal patients, or non-manifesting carriers. Depending on the cohort, Black participants ranged from 1 to 5 percent.
“PPMI is making important efforts to improve the heterogeneity of their study population, but this current study primarily includes White individuals from the US,” Dr. Kalia said. “Will these findings also be applicable to individuals from Asia, Latin America, or Africa, for example? This we don't know, and it's an important next step for our field, to understand how this assay performs globally.”
Dr. Kalia also cautioned that the Lancet Neurology publication is a cross-sectional study, not a longitudinal study. “We don't yet know the longitudinal performance of this test. If someone is a positive seeder today, is there a time when they become a negative seeder, or vice versa? That is something that is going to have to be studied. Will this be a surrogate endpoint to evaluate efficacy in clinical trials testing new therapeutics? That's still unknown. As we observe how seeding changes over time and in response to interventions, we will better understand how we can use about this test and also about the biology of Parkinson's.”
Dr. Standaert agreed. “A significant fraction of the prodromal group with RBD or hyposmia are positive, and we would think that this marks the ones who are likely to develop PD, but the study has not followed them long enough to prove that yet,” he says. “You may have to follow them for 10 years or more. PPMI is now in its second decade, and the intent is to keep going—particularly if it's going to yield discoveries like this.”