Multiple system atrophy (MSA) is a prion disorder, according to data published online ahead of print August 31 in Proceedings of the National Academy of Sciences of the United States of America. A unique strain of α-synuclein prions causes MSA, and these prions are different from the prions believed to cause Parkinson’s disease.

In addition, α-synuclein is the first new human prion to be identified since the discovery 50 years ago that Creutzfeldt–Jakob disease was transmissible, said Stanley B. Prusiner, MD, Professor of Neurology and Director of the Institute for Neurodegenerative Diseases at University of California, San Francisco. “Establishing that MSA is an α-synuclein prion disorder sets the stage for a new therapeutic campaign,” he added.

Inoculating Mice With  Brain Specimens

In 2013, Dr. Prusiner and colleagues intracerebrally injected brain homogenates prepared from two people with MSA into TgM83⁺^/⁻ mice. After 120 days, the mice had progressive CNS dysfunction, and extensive phosphorylated α-synuclein deposits were evident in the cytoplasm and axons of neurons in the mice’s brains.

Dr. Prusiner and colleagues conducted an experiment to determine whether the transmission of MSA in the earlier study was anomalous. First, the researchers obtained brain specimens from 12 deceased patients with a clinical and neuropathologic diagnosis of MSA and from six patients with a diagnosis of Parkinson’s disease. The specimens were obtained from the Parkinson’s UK Brain Bank at Imperial College London, the Sydney Brain Bank in Australia, and the Massachusetts General Hospital Alzheimer’s Disease Research Center in Boston. The researchers confirmed the diagnoses of MSA through postmortem neuropathologic microscopic examination.

The investigators then intracerebrally inoculated TgM83⁺^/⁻ mice with samples from the 12 deceased patients with MSA. Inoculation with all of the MSA samples caused CNS dysfunction with mean incubation periods of between 100 and 150 days. The mice’s most common clinical signs were dysmetria and circling behavior.

When the researchers intracerebrally inoculated TgM83⁺^/⁻ mice with brain homogenates from six patients with Parkinson’s disease or a control, however, the intervention did not produce signs of neurologic dysfunction in more than 360 days. Dr. Prusiner and colleagues had had similar results when they bioassayed these samples in human embryonic kidney (HEK) cells expressing α-syn140*A53T–YFP fusion protein.

Distinct Patterns of  Protein Deposition

During a neuropathologic examination of the brains of the mice inoculated with MSA brain homogenates, the investigators found large aggregates of phosphorylated α-synuclein and widespread astrocytic gliosis. The aggregated α-synuclein primarily took the form of neuronal cytoplasmic inclusions. These mice developed phosphorylated α-synuclein deposits in several brain regions, mostly in the brainstem. The deposits were notably absent from cortical regions.

The brains of TgM83⁺^/⁻ mice inoculated with Parkinson’s disease brain, on the other hand, had a low and unspecific background signal of phosphorylated α-synuclein after more than 360 days, similar to that seen in the control. Inoculation with either control or Parkinson’s disease brain homogenate did not lead to deposition of appreciable phosphorylated α-synuclein in any brain region.

A Call for Heightened Vigilance

“Critical to the interpretation of our results is that TgM83⁺^/⁻ mice that were hemizygous for the human  α-synuclein^*A53T transgene did not develop CNS dysfunction spontaneously,” said Dr. Prusiner. “In addition, the brains of these mice did not infect cultured HEK cells expressing α-syn140*A53T fused to yellow fluorescent protein. In contrast, mouse brain homogenates from both the primary and secondary MSA prion transmissions did infect the cultured HEK cells.”

MSA prions can spread from cell to cell along the entire neuraxis, just like PrPSc prions, which cause Creutzfeldt–Jakob disease, do, according to the study. “Importantly, the ability of MSA to induce progressive neurologic disease in TgM83⁺^/⁻ mice represents the only other human prion disease apart from that caused by PrPSc to induce a lethal phenotype in an animal model,” said Dr. Prusiner.

The unique clinical presentations of the two disorders and the distinct locations in the CNS where they deposit α-synuclein support the hypothesis that the α-synuclein prions that putatively cause Parkinson’s disease may be different from those that cause MSA, said the investigators. Furthermore, the transmission of MSA prions requires Tg mice expressing the A53T mutation found in familial Parkinson’s disease, because mice expressing wild-type mouse or human α-synuclein could not support MSA prion propagation.

The study results suggest that electrodes used for deep brain stimulation and any other equipment that comes into contact with CNS tissue should not be reused. “You can’t kill a protein,” said Kurt Giles, DPhil, Associate Professor of Neurology at University of California, San Francisco and a coauthor of the study. “It can stick tightly to stainless steel, even when the surgical instrument is cleaned. We’re advocating a precautionary approach.” The extent to which MSA prions resist standard decontamination and sterilization procedures is unknown. Therefore, the increased vigilance used in brain biopsies of patients with suspected Creutzfeldt–Jakob disease should be applied to all patients with synucleinopathies, said the authors.

—Erik Greb

MSA May Be a Prion Disorder