We have assessed the frequency of alpha-synuclein (SNCA) mutations in Japanese patients with familial or sporadic Parkinson's disease (PD) and surveyed their associated clinical manifestations. We screened SNCA exon 3 in 988 patients without SNCA multiplications (430 with autosomal dominant PD and 558 with sporadic PD). We detected 1 patient harboring a homozygous SNCA p.A53V substitution albeit with an autosomal dominant pattern of disease inheritance (frequency 2/860 = 0.2%). The proband manifested slow and progressive parkinsonism at 55 years. Later she complicated with cognitive decline and hallucinations. Several of her immediate family members also presented with parkinsonism, cognitive decline, and psychosis. Positron emission tomography imaging of F-18-6-fluoro-L-dopa (F-18-DOPA) uptake, C-11(+) dihydrotetrabenzine (type 2 vesicular monoamine transporter), and 11C-d-threo-methylphenidate (a plasmalemmal dopamine transporter marker) binding in the striatum were significantly reduced. Hence, alpha-synuclein p.A53V homozygous mutation leads to a distinct phenotype of progressive parkinsonism and cognitive decline, commonly observed in patients with SNCA missense mutation or multiplications. (C) 2017 Elsevier Inc. All rights reserved.