The review, led by Federico Garrou of the Division of Nuclear Medicine at ASST Papa Giovanni XXIII in Bergamo, Italy, together with colleagues from Novara, Exeter, and the University of Piemonte Orientale, focused on positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A, or SV2A. This protein sits in the membrane of nearly every synaptic vesicle in the brain, the tiny sacs that store and release neurotransmitters at the presynaptic terminal. Because SV2A is so abundant and so tightly linked to vesicle machinery, the amount of it that a radioactive tracer can bind to serves as a reliable proxy for the density of functioning synapses in any given region of the living brain.

The tracer story began in earnest in 2016, when researchers at Yale first demonstrated in Science Translational Medicine that the compound [11C]UCB-J could image synaptic density in the living human brain. A second-generation radioligand, [18F]SynVesT-1, followed with its first-in-human evaluation in 2021, offering the practical advantage of a fluorine-18 label, which extends the isotope’s usable half-life and makes the tracer far easier to distribute beyond on-site cyclotron facilities. Since then, a small but rapidly growing literature has applied these tools to the parkinsonian α-synucleinopathies, the family of diseases characterized by the pathological aggregation of α-synuclein protein: Parkinson’s disease itself, Parkinson’s disease dementia and dementia with Lewy bodies, and multiple system atrophy.

Garrou and colleagues systematically searched and screened the literature, ultimately including seventeen reports in their synthesis. The analysis followed established systematic review methodology, with independent screening and data extraction by multiple reviewers, and the authors explicitly note that no new datasets were generated; all evidence was drawn from previously published studies. That constraint matters, because it means the review’s conclusions are only as strong as the underlying primary studies, and the authors are candid about those limitations.

In Parkinson’s disease, the single most reproducible finding across studies was reduced SV2A binding in the substantia nigra, the midbrain structure where dopaminergic neurons die and where the disease process is known to begin. This is a technically remarkable result. The substantia nigra is small, deep in the brainstem, and notoriously difficult to image with precision, so detecting a measurable loss of synaptic terminals there in early disease validates the sensitivity of the method. But the nigra was not alone. Several studies reported additional reductions in the caudate nucleus, the broader striatum, the thalamus, the raphe nuclei, brainstem regions, and the cerebral cortex, with the extent of involvement varying considerably between cohorts.

That variability is itself informative. Parkinson’s disease is not a single uniform entity, and the spread of α-synuclein pathology follows patterns that differ between patients, potentially explaining why some cohorts show predominantly nigral synaptic loss while others reveal widespread cortical involvement. Notably, one study found that lower synaptic density in mood-related circuitry underlies depression in Parkinson’s disease, suggesting that SV2A imaging may connect specific non-motor symptoms to specific patterns of synaptic failure. Other work has linked synaptic loss to overall symptom severity, and dual-tracer studies comparing SV2A PET with dopamine transporter imaging have begun to disentangle how presynaptic terminal integrity relates to the dopaminergic deficits that conventional scans measure.

In the dementia spectrum, the abnormalities were broader and more cortical. Studies of patients with Lewy body dementia using [11C]UCB-J found reduced synaptic density relative to healthy controls, and subsequent work comparing SV2A PET with fluorodeoxyglucose PET showed relationships between synaptic density and cerebral glucose consumption. Critically, the review highlights evidence that cortical SV2A binding correlates with cognitive performance, which positions the tracer as a potential mechanistic bridge between the molecular pathology of α-synuclein and the cognitive decline that defines dementia with Lewy bodies and Parkinson’s disease dementia. If synaptic loss can be quantified before cognition deteriorates irreversibly, trials of synapse-protecting therapies would gain an outcome measure that reflects biology rather than compensation.

Multiple system atrophy, the rarest and often most aggressive member of the synucleinopathy family, provided perhaps the most diagnostically intriguing signal. One study using [18F]SynVesT-1 identified a distinct pattern of synaptic density reduction concentrated in infratentorial and cerebellar regions, a distribution that differs from the patterns seen in Parkinson’s disease and Lewy body dementias and that could eventually help stratify patients phenotypically. This matters clinically because distinguishing MSA from Parkinson’s disease early in the disease course remains one of the most difficult problems in movement disorder neurology, and a biomarker that captures disease-specific spatial signatures could sharpen both diagnosis and trial enrollment.

Yet the review is equally clear about what stands between the current state of the field and clinical translation. The included studies involved small cohorts. Quantification strategies varied widely, including different reference region choices, kinetic modeling approaches, and scanning protocols, and recent methodological work has shown that reference region optimization directly affects test-retest reliability and the sensitivity with which patient-control differences can be detected. Longitudinal evidence remains scarce, with only limited follow-up data on how synaptic density evolves over time in individual patients, and the possibility of cohort overlap between publications complicates interpretation. Even the technical underpinnings are still being refined, with ongoing research into reduced injected doses, shorter scanning times, and direct four-dimensional PET reconstruction methods designed to reduce variance in synaptic density measurements.

The authors conclude that SV2A PET is a promising research biomarker for the biological characterization of synucleinopathies, but that multicentre validation and harmonized protocols are required before it can move into clinical practice. That verdict captures the field at an inflection point. The technology has proven it can see synapses fail in the living brain, and the patterns it reveals map onto what we know about how α-synucleinopathies unfold from early synaptic dysfunction to overt neurodegeneration. What remains is the unglamorous but essential work of standardization: agreeing on how to quantify binding, how to design scans, and how to pool data across centers so that the signal observed in a handful of patients in Novara, Exeter, or New Haven can become a diagnostic tool trusted everywhere. If that work succeeds, the synapse, long invisible to clinical neurology, may finally become the target that Parkinson’s disease diagnosis and drug development have been waiting for.

Subject of Research: SV2A PET imaging of synaptic density in parkinsonian α-synucleinopathies

Article Title: Synaptic vesicle glycoprotein 2A PET imaging in parkinsonian α-synucleinopathies: a systematic review

Article References: Synaptic vesicle glycoprotein 2A PET imaging in parkinsonian α-synucleinopathies: a systematic review. (n.d.). https://doi.org/10.1007/s00415-026-14123-2

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14123-2

Keywords: SV2A PET, Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, alpha-synucleinopathies, synaptic density, synaptic vesicle glycoprotein 2A, molecular neuroimaging, radiotracers, biomarkers, neurodegeneration, systematic review

The study, published as a short commentary in the Journal of Neurology, comes from a group led by Monica Gagliardi of the Neuroscience Research Center at Magna Graecia University in Catanzaro, in the southern Italian region of Calabria. The researchers measured two molecular features in whole blood from 58 patients with Parkinson’s disease, 35 patients with multiple system atrophy, and 62 healthy controls drawn from the same region. The first feature was the copy number of mitochondrial DNA, estimated by quantifying the mitochondrially encoded NADH dehydrogenase 1 gene, known as ND1, relative to the nuclear beta-actin gene. The second was telomere length, the protective caps at the ends of chromosomes that erode with each cell division and are widely regarded as a cellular clock of aging.

The technical approach relied on quantitative PCR, the workhorse method for amplifying and measuring specific DNA sequences. By comparing the abundance of a mitochondrial gene to a single-copy nuclear gene, the team obtained a relative estimate of how many mitochondrial genomes each blood sample carries per cell. Telomere length was measured with the same technique, comparing the amount of telomeric DNA to a reference gene. To guard against spurious results, the researchers applied multivariable linear regression adjusted for age and sex, two factors known to influence both mitochondrial DNA copy number and telomere length, and they demanded that findings survive correction for multiple comparisons using false discovery rate-adjusted Wald tests, a statistical safeguard that reduces the risk of false positives when many hypotheses are tested at once.

The results were striking. Mitochondrial DNA copy number, as indexed by the ND1 gene, was significantly reduced in both patient groups compared with healthy controls, with a false discovery rate-adjusted p-value of 1.63 times ten to the power of minus twenty-two for Parkinson’s disease and 2.56 times ten to the power of minus eight for multiple system atrophy. More importantly for the question of diagnosis, the marker also separated the two diseases from each other: Parkinson’s patients showed significantly lower ND1 copy number than multiple system atrophy patients, with an adjusted p-value of 8.43 times ten to the power of minus twenty-two. In a field where biomarkers that cleanly discriminate between overlapping neurodegenerative conditions are desperately scarce, a difference of that statistical magnitude in a peripheral blood test is remarkable.

Telomere length told a related but subtly different story. Both patient groups again showed significantly shorter telomeres than the controls, with adjusted p-values of 3.21 times ten to the power of minus sixteen for Parkinson’s disease and 1.44 times ten to the power of minus twelve for multiple system atrophy. But when the researchers compared the two patient groups directly against each other, the difference was far weaker, reaching only the threshold of statistical significance with an adjusted p-value of 5.47 times ten to the power of minus two. In other words, telomere shortening appears to be a shared feature of both alpha-synucleinopathies, a general signature of accelerated cellular aging, whereas mitochondrial DNA copy number carries disease-specific information that could, in principle, help clinicians tell the two conditions apart.

Why would mitochondrial DNA copy number be depleted in the blood of patients with these brain diseases? The answer likely lies in the intimate relationship between mitochondria and neurodegeneration. Mitochondria are the organelles that generate most of the chemical energy a cell needs, and they carry their own small circular genome, separate from the DNA in the nucleus. Cells adjust the number of mitochondrial genomes they harbor in response to energy demands and stress. Decades of research have implicated mitochondrial dysfunction in Parkinson’s disease, from studies showing impaired mitochondrial respiration in the blood cells of people with early and even prodromal Parkinson’s, to work demonstrating mitochondrial defects in the fibroblasts of patients with multiple system atrophy. Previous studies have also reported reduced mitochondrial DNA copy number as a biomarker of Parkinson’s disease, although the picture is complicated: some investigations, including one in individuals of African ancestry, found increased blood-derived mitochondrial DNA copy number, and recent work has even documented elevated mitochondrial DNA copy number in the cerebellum of people with Parkinson’s. The direction and meaning of the signal may depend on tissue, disease stage, ancestry, and the specific cell populations being counted.

That last caveat is an important one. Whole blood is a mixture of cell types, and different immune cells carry different amounts of mitochondrial DNA. Recent research has suggested that differences in the abundance of peripheral immune cell populations may link blood mitochondrial DNA copy number to Parkinson’s disease, meaning that what looks like a mitochondrial defect could partly reflect shifts in blood cell composition driven by inflammation. Neuroinflammation is increasingly recognized as a player in both Parkinson’s disease and multiple system atrophy, so the reduced copy number observed in this study could reflect either genuine mitochondrial depletion within cells or an altered mix of circulating leukocytes, or both. The authors themselves frame their findings as preliminary and call for longitudinal studies in larger, independent cohorts to validate the diagnostic and prognostic utility of the marker before it could enter clinical practice.

The telomere findings also fit into a rich and sometimes contradictory literature. Telomere dysfunction is known to trigger metabolic and mitochondrial compromise through a stress-response network involving the p53 protein, creating a mechanistic bridge between the two markers measured in this study. Mouse experiments have shown that telomere shortening accelerates synucleinopathy and impairs the response of microglia, the brain’s immune cells, in genetic models of Parkinson-like disease. Human studies of leukocyte telomere length in Parkinson’s disease have produced mixed results over the years, with some groups reporting shorter telomeres in patients, others finding no difference, and longitudinal work suggesting that longer telomeres at diagnosis may paradoxically predict faster progression to dementia in idiopathic parkinsonism. A recent prospective analysis of the UK Biobank linked leukocyte telomere length to the risk of neurodegenerative diseases more broadly, reinforcing the idea that biological aging markers capture something real about vulnerability to these conditions, even if the details remain contested.

What makes the new study noteworthy is not any single result but the head-to-head comparison of the two markers within the same cohort, using the same methods and the same statistical framework. By showing that mitochondrial DNA copy number separates Parkinson’s disease from multiple system atrophy while telomere length does not, the researchers provide a concrete argument for prioritizing mitochondrial DNA copy number as a non-invasive, blood-based biomarker of disease-specific mitochondrial alterations in alpha-synucleinopathies. If the finding holds up in larger and more diverse populations, it could eventually help solve one of clinical neurology’s persistent puzzles: distinguishing Parkinson’s disease from multiple system atrophy early in the disease course, when the distinction matters most for counseling patients, planning treatment, and selecting the right participants for clinical trials of disease-modifying therapies. For now, the measurement requires nothing more exotic than a blood draw and quantitative PCR, a combination that, if validated, would make this one of the most practical biomarker candidates to emerge from the intersection of mitochondrial biology and neurodegeneration research.

Subject of Research: Mitochondrial DNA copy number and telomere length as blood biomarkers in Parkinson's disease and multiple system atrophy

Article Title: Preliminary insights into whole-blood mitochondrial DNA copy number and telomere length in Parkinson’s disease and multiple system atrophy

Article References: Preliminary insights into whole-blood mitochondrial DNA copy number and telomere length in Parkinson’s disease and multiple system atrophy. (n.d.). https://doi.org/10.1007/s00415-026-14172-7

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14172-7

Keywords: Parkinson's disease, multiple system atrophy, mitochondrial DNA copy number, telomere length, alpha-synucleinopathies, biomarkers, neurodegeneration, mitochondrial dysfunction, quantitative PCR, cellular aging, blood biomarker, Journal of Neurology