Could the key to detecting synucleinopathies be no more than skin-deep? In the March 20 JAMA, scientists led by Christopher Gibbons and Roy Freeman at Harvard Medical School in Boston detected phosphorylated synuclein in the skin of people clinically determined to have four such disorders. Skin p-serine 129 synuclein identified synucleinopathies with 93 to 100 percent accuracy. People at more advanced stages of disease harbored more p-synuclein. “The sensitivity and specificity of the procedure are both high (more than 90 percent), which is very encouraging," wrote Oskar Hansson of Lund University in Sweden (comment below). He was not involved in the work. "The method might, therefore, potentially be used to improve the diagnostic work-up in clinical practice but also when selecting appropriate patients for trials targeting α-synuclein.”

Synucleinopathies are typically diagnosed by their symptoms, which overlap with other diseases or can sometimes seem unrelated, such as REM sleep disorder or orthostatic hypotension. As such people often go undiagnosed or misdiagnosed for years, in the search for robust diagnostic marker, some scientists are using seed amplification assays. They pick up minuscule amounts of α-synuclein aggregates in tissue or cerebrospinal fluid samples, pegging Parkinson’s disease and Lewy body disorders with over 85 percent accuracy (Apr 2023 conference news; Aug 2023 conference news).

For their part, Gibbons and colleagues use skin samples taken via punch biopsy (image below). During this typically painless procedure, skin on the leg or back is numbed with local anesthetic. The physician pushes a tool with a hollow circle at the end into the skin, twists it to cut through the dermal layers, and then removes the 3mm circle of tissue. The patient is bandaged and sent on his or her way.

Punch Biopsy. Schematic of a punch biopsy to collect a skin sample. [Courtesy of Bruce Blaus on Wikimedia.]

Previously, the scientists detected phosphorylated α-synuclein in skin samples from 54 people with PD and 31 with multiple system atrophy (MSA) with 94 and 100 percent accuracy, respectively (Gibbons et al., 2023).

Now, Gibbons and colleagues turned to a larger sample size, including people with two other types of synucleinopathy—dementia with Lewy bodies (DLB) and pure autonomic failure (PAF), a degenerative disorder of the autonomic nervous system characterized by synuclein aggregates confined to those nerves. They analyzed skin samples from 343 older adults recruited from 30 academic and community neurology clinics across the U.S. Among participants, 120 were controls, 96 had PD, 50 DLB, 55 MSA, and 22 had PAF. Participants were 70, on average, and half were women.

The scientists measured p-synuclein in dermal neurons immunohistochemically using two antibodies, one that binds p-129 synuclein, and another that targets nerve fibers. The latter binds protein gene product 9.5 (PGP 9.5), aka UCH-L1, an abundant cytoplasmic protein within neurons and nerves. The authors took co-localization of the two antibodies to indicate p-S129-synuclein within neurons, and a sample with any overlap was deemed positive.

The assay detected cutaneous nerve p-S129-synuclein in 93 percent of people with PD. The assay did slightly better for DLB and MSA, picking out all but two of the former and one of the latter. It pegged all 22 cases of PAF. Gibbons chalked up negative tests to some forms of the diseases possibly having truncated α-synuclein or synuclein phosphorylated at a different residue. These are also not pathologically confirmed cases of synucleinopathy, so Gibbons suspects that a few might actually have another neurodegenerative disease, such as drug- or vascular-induced Parkinsonism, progressive supranuclear palsy, corticobasal degeneration, or Alzheimer’s disease.

Intriguingly, four of the 120 controls tested positive for p-synuclein. It is unclear if these represent false positives or if these people might be in the early stages of a synucleinopathy. In fact, upon closer review, Gibbons said three of four positives had signs of such a disorder, i.e., mild cognitive impairment or orthostatic hypotension.

Time since diagnosis did not sway biopsy results. For example, 93.5 or 92 percent of people with PD came up positive whether they had been diagnosed within the past three years or more than five years ago, respectively.

However, the amount of skin p-S129-synuclein did seem to change as the diseases progressed. People at a more advanced stage, as measured by symptom and neurological scale scores, had more p-synuclein.

The authors acknowledge that these results will need to be validated with larger sample sizes and in people without a clinical diagnosis of synucleinopathy to fully understand the potential of this biopsy test in clinical care.—Chelsea Weidman Burke

Comments

  1. This study confirms previous smaller studies showing that the levels of phosphorylated α-synuclein are increased in peripheral nerve terminals in the skin in neurological diseases characterized by accumulation of α-synuclein aggregates in the brain. The sensitivity and specificity of the procedure are both high (more than 90 percent), which is very encouraging. The method might therefore potentially be used to improve the diagnostic work-up in clinical practice, but also when selecting appropriate patients for trials targeting α-synuclein. Still, the method relies on immunohistochemistry and visual inspection of the stained sections using confocal microscopy to manually quantify the number of α-synuclein–positive nerve fibers, which is time-consuming and potentially user-dependent.

    Future advancements will likely include automated methods for actual quantification of the levels of phosphorylated α-synuclein in the tissue, potentially normalizing those levels to a marker representing the total amount of nerve fibers in the same tissue. Another interesting advancement would be to determine if there is a subtype of α-synuclein disease that starts in the periphery or not. In the BioFINDER study, we collect skin biopsies and CSF from many hundreds of healthy people, and it will be interesting to see whether there are cases with abnormal α-synuclein in the skin who do not yet have abnormal α-synuclein in the CSF, and if such individuals become positive in the CSF during follow-up. Alternatively, we might find that all study participants will be positive in CSF before skin, implying that the α-synuclein diseases always start in the brain before affecting peripheral nerves.  

  2. In Parkinson’s as in Alzheimer’s disease, biological markers are important to improve early diagnostics. This paper provides strong evidence that cutaneous phosphorylated α-synuclein may serve as diagnostic biomarker for Parkinson’s disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and pure autonomic failure.

    The large cohort is convincing, and the numbers of patients testing positive (more than 90 percent for PD, DLB, and MSA) are impressive. Even though the collection method (skin punch) is invasive, it is well-tolerated by most participants. The read-out evaluation is immunohistochemistry by an experienced histopathologists, is not that simple or automated for now, but automation can be foreseen. It will be relevant to know if one punch would suffice: They took three at different locations, and positivity was defined by at least one being positive.

    It is also unclear why no sensitivity or specificity—let alone PPV or NPV—has been provided. Such measures would allow easier comparison with other biomarker studies.

    As the authors also indicated, comparison between CSF seeding assay outcomes and skin biopsy studies within the same persons is relevant to determine if the reported more accurate diagnostic results obtained by skin biopsies can be replicated. It will become even more interesting if additional information can be obtained from these sections, for prognosis for example, analogous to being able to measure panels of biomarkers from one drop of CSF. 

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References

News Citations

  1. Synuclein Assay Passes the Sniff Test—What of Other Seeds?
  2. Finally, a Diagnostic Marker for Lewy Body Disease?

Paper Citations

  1. . Cutaneous α-Synuclein Signatures in Patients With Multiple System Atrophy and Parkinson Disease. Neurology. 2023 Apr 11;100(15):e1529-e1539. Epub 2023 Jan 19 PubMed. Correction.

Further Reading

No Available Further Reading

Primary Papers

  1. . Skin Biopsy Detection of Phosphorylated α-Synuclein in Patients With Synucleinopathies. JAMA. 2024 Mar 20; PubMed.