Parkinson’s
disease (PD) is a complex and increasingly prevalent neurodegenerative
disease of the central nervous system (CNS). It is clinically
characterised by progressive motor and non-motor symptoms that are
caused by α-synuclein aggregation predominantly in dopaminergic cells,
which leads to Lewy body (LB) formation1.
The failure of neuroprotective strategies in preventing disease
progression is due, in part, to the clinical heterogeneity of the
disease—it has several phenotypes—and to the lack of objective biomarker
readouts2.
To facilitate the approval of neuroprotective strategies, governing
agencies and pharmaceutical companies need regulatory pathways that use
objectively measurable markers—potential therapeutical targets as well
as state and rate biomarkers—directly associated with PD pathophysiology
and clinical phenotypes3.
The
recently emerged α-synuclein seed amplification assays (SAA) can
identify α-synuclein pathology in vivo and support stratification
purposes but still rely on cerebrospinal fluid (CSF) obtained through
relatively invasive lumbar punctures4.
Therefore, this test remains specialised and not readily suitable for
large-scale clinical use. As peripheral fluid biomarkers are less
invasive and easier to obtain, they could be used in repeated and
long-term monitoring, which is necessary for population-based screenings
for upcoming neuroprotective trials. While the only emerged serum
biomarker in the last years, axonal marker neurofilament light chain
(NfL), increases longitudinally and correlates with motor and cognitive
PD progression5, it is non-specific to the disease process.
Growing
data support evidence of PD pathology in the peripheral system, which
increases the likelihood of finding a source of matrices for less
invasive biomarkers. We know α-synuclein aggregation induces
neurodegeneration, which is propagated throughout the CNS. Evidence
indicates that additional inflammatory events are an early and
potentially initial step in a pathophysiological cascade leading to
downstream α-synuclein aggregation that activates the immune system6.
Inflammatory risk factors in circulating blood (i.e. C-reactive-protein
and Interleukin-6 and α-synuclein-specific T-cells) are associated with
motor deterioration and cognitive decline in PD7,8.
These inflammatory blood markers can even be identified in plasma/serum
samples of individuals with isolated REM sleep behaviour disorder
(iRBD), the early stage of a neuronal synuclein disease (NSD), and the
most specific predictor for PD and dementia with Lewy bodies (DLB)6.
NSD was recently proposed as a biologically defined term, for a
spectrum of clinical syndromes, including iRBD, PD and DLB, that follow
an integrated clinical staging system of progressing neuronal
α-synuclein pathology (NSD-ISS)9.
In
this study, we used mass spectrometry-based proteomic phenotyping to
identify a panel of blood biomarkers in early PD. In the initial
discovery stage, we analysed samples from a well-characterised cohort of
de novo PD patients and healthy controls (HC) who had been subjected to
rigorous collection protocols10.
Using unbiased state-of-the-art mass spectrometry, we identified
putatively involved proteins, suggesting an early inflammatory profile
in plasma. We thereafter moved on to the validation phase by creating a
high-throughput and targeted proteomic assay that was applied to samples
from an independent replication cohort, consisting of de novo PD, HC
and iRBD patients. Finally, after refining the targeted proteomic panel
to include a multiplex of only the biomarkers which were reliably
measured, an independent analysis was performed on a larger and
independent cohort of longitudinal, high-risk subjects who had been
confirmed as iRBD by state-of-the-art video-recorded polysomnography
(vPSG), including follow-up sampling of up to 7 years.
In summary,
using a panel of eight blood biomarkers identified in a
machine-learning approach, we were able to differentiate between PD and
HC with a specificity of 100%, and to identify 79% of the iRBD subjects,
up to 7 years before the development of either DLB or motor PD (NSD
stage 3). Our identified panel of biomarkers significantly advances NSD
research by providing potential screening and detection markers for use
in the earliest stages of NSD for subject identification/stratification
for the upcoming prevention trials.
