Whole-exome analysis in Parkinson's disease reveals a high burden of ultra-rare variants in early-onset cases

Parkinsons disease (PD) is a complex neurodegenerative disorder with a strong genetic component. We performed a "hypothesis-free" exome-wide burden-based analysis of different variant frequencies, predicted functional impact and age of onset classes, in order to expand the understanding of rare variants in PD. Analyzing whole-exome data from a total of 1,425 PD cases and 596 controls, we found a significantly increased burden of ultra-rare (URV= private variants absent from gnomAD) protein altering variants (PAV) in early-onset PD cases (EOPD, <40 years old; P=3.95x10-26, beta=0.16, SE=0.02), compared to LOPD cases (>60 years old, late-onset), where more common PAVs (allele frequencies <0.001) showed the highest significance and effect (P=0.026, beta=0.15, SE=0.07). Gene-set burden analysis of URVs in EOPD highlighted significant disease- and tissue-relevant genes, pathways and protein-protein interaction networks that were different to that observed in non-EOPD cases. Heritability estimates revealed that URVs account for 15.9% of the genetic component in EOPD individuals. Our results suggest that URVs play a significant role in EOPD and that distinct etiological bases may exist for EOPD and sporadic PD. By providing new insights into the genetic architecture of PD, our study may inform approaches aimed at novel gene discovery and provide new directions for genetic risk assessment based on disease age of onset.