Increased burden of rare risk variants across gene expression networks predisposes to sporadic Parkinson's disease
Alpha-synuclein (Syn) is an intrinsically disordered protein that accumulates in the brains of patients with Parkinsons disease and forms intraneuronal inclusions called Lewy Bodies. While the mechanism underlying the dysregulation of Syn in Parkinsons disease is unclear, it is thought that prionoid cell-to-cell propagation of Syn has an important role. Through a high throughput screen, we recently identified 38 genes whose knock down modulates Syn propagation. Follow up experiments were undertaken for two of those genes, TAX1BP1 and ADAMTS19, to study the mechanism with which they regulate Syn homeostasis. We used a recently developed M17D neuroblastoma cell line expressing triple mutant (E35K+E46K+E61K) "3K" Syn under doxycycline induction. 3K Syn spontaneously forms inclusions that show ultrastructural similarities to Lewy Bodies. Experiments using that cell line showed that TAX1BP1 and ADAMTS19 regulate how Syn interacts with lipids and phase separates into inclusions, respectively, adding to the growing body of evidence implicating those processes in Parkinsons disease. Through RNA sequencing, we identified several genes that are differentially expressed after knock-down of TAX1BP1 or ADAMTS19. Burden analysis revealed that those differentially expressed genes (DEGs) carry an increased frequency of rare risk variants in Parkinsons disease patients versus healthy controls, an effect that was independently replicated across two separate cohorts (GP2 and AMP-PD). Weighted gene co-expression network analysis (WGCNA) showed that the DEGs cluster within modules in regions of the brain that develop high degrees of Syn pathology (basal ganglia, cortex). We propose a novel model for the genetic architecture of sporadic Parkinsons disease: increased burden of risk variants across genetic networks dysregulates pathways underlying Syn homeostasis, thereby leading to pathology and neurodegeneration.