HTT and Parkinson disease: The transneuronal spreading of oligomers or fibrillar aggregates is increasingly recognized in a variety of neurodegenerative disorders including tau protein and amyloid-β peptide in Alzheimer’s disease, superoxide dismutase 1 (SOD1) in amyotrophic lateral sclerosis (ALS), huntingtin in Huntington’s disease (HD) and α-synuclein in Parkinson’s disease (PD).