Although clinicians sometimes remark that a diagnosis of Parkinson’s disease (PD) can be made by walking a patient from the waiting room to an examination room, we are often stymied by surprising autopsy findings and other evidences contrary to our clinical impression.1 In addition, individual progression of the disease and its phenotypic variations such as cognitive deficit can be very difficult to predict. What constitutes disease severity and disease states has been also controversial. The traditional way of scoring motor deficits has significant limitations for the assessment of overall disease severity and is easily influenced by symptomatic treatment that obscures the underlying disease state. Likewise, therapeutic trials attempting to slow disease progression have faced significant challenges due to, at least in part, our inability to discern the heterogeneity of PD and monitor disease progression objectively. As such, the development of PD biomarkers has garnered significant and active interest with major investments by many public and private stakeholders.
Biomarkers are defined as characteristics that are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.2 There are now large consortia in the US, Europe, and internationally to establish cohorts for biomarker data collection to address these unmet needs. We invited prominent researchers who have been highly active in this field and asked them to comment on the current status of development and practical aspects of biomarkers for PD that may be of interest to clinicians.
Can we rely on biomarkers for PD clinical practice and research?
Dr. Plotkin:
I think we need to on the research front because otherwise the heterogeneity of evolution in PD really makes disease-modifying trials super-difficult. Additionally, I believe that we may in the not-too-distant future think of PD in more molecular terms. We would never talk about "blood cancer" now. We understand that the various cancers that occur in the blood should be defined by the type of cell/the markers it expresses/its signatures, because that definition actually impacts how we choose therapy and how we prognosticate.
Dr. Mollenhauer:
I think there are already biomarkers that support the clinical diagnosis (in clinical practice and for research studies), such as olfactory tests, polysomnography for REM sleep behavior disorder (and other non-motor symptoms), or dopamine transporter imaging. But if we are thinking about an objective biomarker such as a specific biochemical marker, which reflects disease onset, progression, or the risk of developing the disease, then we still need a lot of work to get there.
What is the most pressing need for biomarkers in PD (such as diagnosis, therapeutic monitoring)?
Dr. Plotkin:
Markers can guide therapeutic development, i.e. markers can be leads for identifying targets for therapy, and if therapies are developed on such pathways, then the marker itself is an efficacy readout for the intervention. It can make disease-modifying trials more efficient and incentivize the research to develop these therapies. Markers can also help doctors make good choices with the drugs they have, instead of the trial-and-error approach that is currently available.
Dr. Zhang:
The most pressing need is biomarkers for sub-populations of “PD” patients, given that the disease has many variants likely having different drivers for pathogenesis.
Dr. Mollenhauer:
I think the objective measurement of clinical progression of PD is the most pressing need. Biomarkers of disease progression would be helpful as an outcome measure in clinical trials. Previous clinical trials with putative neuroprotective compounds have failed, maybe solely due to the lack of objective biomarkers to show the effect slowing disease progression. Many new compounds will be advanced to clinical trials in the near future, and the key to success is to objectively demonstrate neuroprotection with biomarkers of PD progression.
What is the current status of biofluid biomarkers in PD?
Dr. Mollenhauer:
We see, very consistently across independent cohorts, a 10-15% decrease of total α-synuclein in cerebrospinal fluid in PD (and other α-synuclein aggregation disorders, such as multiple system atrophy and dementia with Lewy bodies) versus controls, but the overlap of single values is large and it does not show a meaningful change over time. I had high expectations in other α-synuclein species, but there are not many posttranslationally modified forms of α-synuclein in cerebrospinal fluid, and some assays are not widely replicable. More promising, the α-synuclein aggregation assays seem to have high sensitivity and specificity.3-5 Other biomarkers, such as those currently being identified by metabolome and other –omics technologies, have yet to be validated. Luckily, we now have great cohorts where we can systematically study progression markers in biological fluids. But one obstacle that I see in all cohorts I have had access to, and also in my clinical practice, is that PD is a very heterogeneous disorder, possibly with different molecular patterns, which will likely need identification of a variety or a panel of markers.
What is the most promising biomarker development in the pipeline?
Dr. Plotkin:
I think from the clinical perspective, the most useful markers are related to the GBA genetic pattern. Here we have a genetic marker – does the patient have a GBA mutation or not – that guides whether specific therapies are appropriate for that individual (GBA targeting drugs) and can be read out biochemically to determine efficacy of targeting. However, the problem is that most (>90%) PD patients do not have GBA mutations, and thus, we need to identify markers that are appropriate for those patients too.
Dr. Zhang:
Total a-synuclein in CSF (as well as other body fluids) has been measured by multiple groups, with decent sample sizes, demonstrating that its sensitivity and specificity are not even close to what has been achieved in Alzheimer’s disease when using A-beta and tau. On the other hand, variants of a-synuclein (e.g. oligomeric a-synuclein) or a-synuclein in certain fractions of a body fluid, e.g. exosomes in CSF or blood, remain to be investigated further. Adding tau and A-beta species to PD biomarker panel is also encouraging, given recent observations about potential roles of these proteins in PD.
Dr. Mollenhauer:
There are many good developments in the pipeline. My focus is biological fluids, but I know that imaging and other tools can also be applied to identify biomarkers. With respect to biological fluids –omics and the “cross-omics” analyses (proteomics, metabolomics, lipidomics) will be the future of biofluid markers. These have shown great potential, but different sample handling leads to high variability and lack of reproducibility. With improved methodologies (sample handling, analyses of comorbidities, genetics, etc.) as well as larger cohorts, we will be able to overcome this variability in the future. As mentioned above, I also expected more from the aggregation assays and posttranslational modifications of α-synuclein, but here we are still hampered by technical drawbacks. Some assays are extremely difficult to replicate, and we need robust high throughput assays optimally independent of antibodies. New developments include studies in peripheral tissue and microbiome studies in PD, these may not only lead to a biomarker development, but also help us understand the pathogenesis of the disease.
Summary
There is a clear consensus among experts that biomarkers are needed to improve clinical care and advance research in PD. Examples from other diseases and disorders are abundant. As reliable biomarkers are essential for the development of new therapies, the two most pressing needs are for defining disease heterogeneity and tracking disease progression. Additionally, the incorporation of non-traditional clinical parameters6 and imaging can aid in the diagnosis of PD. The most promising biochemical biomarkers in current development seek to identify α-synuclein protein variants, such as α-synuclein seeding/aggregation assays,4-5 although technical hurdles remain to be overcome. Importantly, there is a surge in new initiatives such as the AMP-PD program to perform non-biased analyses of proteins (proteomics), metabolites (metabolomics), and lipids (lipidomics) in combination with RNA and DNA variant analyses that may prove to be a powerful tool for biomarker identification in the future. However, much work is needed to validate the reliability and reproducibility of any potential biomarker, and to develop them for useful practical application.
References
1. Adler CH, Beach TG, Hentz JG, et al. Low clinical diagnostic accuracy of early vs advanced Parkinson disease: Clinicopathologic study. Neurology 2014;83:406-412.
2. Biomarkers and surrogate endpoints: preferred definitions and conceptual framework. Clinical pharmacology and therapeutics 2001;69:89-95.
3. Fairfoul G, McGuire LI, Pal S, et al. Alpha-synuclein RT-QuIC in the CSF of patients with alpha-synucleinopathies. Ann Clin Transl Neurol 2016;3:812-818.
4. Shahnawaz M, Tokuda T, Waragai M, et al. Development of a Biochemical Diagnosis of Parkinson Disease by Detection of alpha-Synuclein Misfolded Aggregates in Cerebrospinal Fluid. JAMA Neurol 2017;74:163-172.
5. Groveman BR, Orru CD, Hughson AG, et al. Rapid and ultra-sensitive quantitation of disease-associated alpha-synuclein seeds in brain and cerebrospinal fluid by alphaSyn RT-QuIC. Acta neuropathologica communications 2018;6:7.
6. Doty RL, Deems DA, Stellar S. Olfactory dysfunction in parkinsonism: a general deficit unrelated to neurologic signs, disease stage, or disease duration. Neurology 1988;38:1237-1244.