Abnormal amount of a protein called alpha-synuclein. AND Multiple System Atrophy.

https://www.mayoclinic.org/diseases-conditions/multiple-system-atrophy/symptoms-causes/syc-20356153

MSA causes deterioration and shrinkage (atrophy) of portions of your brain (cerebellum, basal ganglia and brainstem) that regulate internal body functions, digestion and motor control.

Under a microscope, the damaged brain tissue of people with MSA shows nerve cells (neurons) that contain an abnormal amount of a protein called alpha-synuclein. Some research suggests that this protein may be overexpressed in multiple system atrophy.