Genetic Analysis of Prosaposin, the Lysosomal Storage Disorder Gene in Parkinson's Disease

Recent genetic studies clearly indicate that variants in several lysosomal genes act as risk factors for idiopathic Parkinson's disease (PD). Variants in the co-activator of glucocerebrosidase gene (GBA) and the four active saposins (Sap A-D) which are encoded by the prosaposin gene (PSAP) are of particular interest; however, their genetic roles in PD are unknown. Whole-exome sequencing and Sanger sequencing were used to assess the genetic etiology of 400 autosomal dominant inherited PD (ADPD) and 300 sporadic PD (SPD) patients. Variants from public databases, including Genome Aggregation Database-East Asian (GnomAD_EAS) and Chinese Millionome Database (CMDB), were used as control groups. Burden analysis based on gene and domains level were performed to investigate the role of rare PSAP variants in PD. Six rare and likely pathogenic variants, located in the Sap A-D domains, were identified and accounted for 0.75% (3/400) of ADPD and 1.33% (4/300) of SPD in the Chinese population. Based on the gene or domain, burden analysis showed that damaging missense variants in SapC had statistical significance on the risk of developing PD. Interestingly, rs4747203, an intronic variant potentially linked to PSAP expression, was associated with reduced risk for PD (p = 8.6e-7 in GnomAD EAS and p = 0.002 in Chinese). Clinically, patients carrying the likely pathogenic variants presented typical PD motor symptoms and responded well to levodopa treatment. Six out of seven patients carrying the likely pathogenic variants of PSAP presented slow disease progression, and none of the patients developed cognitive impairment. Our study expands the spectrum of mutations associated with the risk of developing PD and enhances the understanding of the relationship of the clinical phenotype of PD with PSAP variants.