Global Identification of Human Modifier Genes of Alpha-synuclein Toxicity

Alpha-synuclein is a small lipid binding protein abundantly expressed in the brain. Lewy body or Lewy-like pathology, primarily composed of misfolded alpha-synuclein, is a pathological feature shared by several neurodegenerative disorders, including Parkinson\u27s disease (PD). Both missense mutations and increased copy numbers of the SNCA gene, encoding the alpha-synuclein protein, have been genetically linked to autosome dominant PD. Other genetic variations affecting the expression of the SNCA gene have been associated with sporadic PD. Although the physiological function of alpha-synuclein is not well understood, its localization to plasma and vesicular membranes at the presynaptic terminals suggests a role in neurotransmission. How alpha-synuclein mis-folds and deposits into the Lewy bodies or causes cytotoxicity to neurons remains unclear. Both membrane association and dosage-dependent toxicity of alpha-synuclein can be recapitulated in the budding yeast, Saccharomyces cerevisiae. Using yeast genetic screens, hundreds of suppressor and enhancer genes have been discovered, revealing complex cellular processes underlying the toxicity of alpha-synuclein. Mammalian homologs of several yeast suppressor genes exhibit similar protective effect in neurons, supporting the existence of conserved mechanisms pertinent to alpha-synuclein toxicity in yeast. We reasoned that screening yeast models expressing toxic high levels of alpha-synuclein may not uncover genes that promote aggregation and toxicity of the protein expressed at a low dosage. Furthermore, human genes without yeast homologs cannot be identified in genetic screens using only yeast genes. To address these limitations, we expressed SNCA at a range of levels in yeast and found distinct membrane localizations of alpha-synuclein corresponding to various degrees of toxicity. Next, we constructed an overexpression library containing ~15,000 human-gene clones. Using this library, we identified new genes that enhance or suppress the toxicity of alpha-synuclein. Characterization of some modifier genes suggest that alpha-synuclein targets conserved cellular machineries in yeast, impairing endocytosis, actin cytoskeleton and cell polarity. Future validation of these modifier genes in neurons and animal models may provide new insights in alpha-synuclein pathology and the genetic basis of neurodegenerative disease