α-Synuclein burden amplification in Parkinson's disease: a unified genetic, molecular, and cellular framework

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized pathologically by the accumulation and propagation of α-synuclein (α-syn). Although α-syn aggregation is considered central to PD pathogenesis, increasing evidence suggests that α-syn abundance may be as important as its conformational state. Genetic studies have demonstrated an SNCA dosage effect, with gene duplication and triplication associated with progressively more severe familial PD phenotypes. Complementary evidence indicates that dysfunction of protein clearance pathways, particularly the autophagy-lysosome system, promotes intracellular α-syn accumulation and increases its neurotoxic potential. In this review, we propose α-syn multiplication as an integrative framework for interpreting PD pathogenesis. This concept extends beyond SNCA copy-number variation to encompass processes that increase the effective α-syn burden within neurons or across neural networks, including increased gene expression, impaired degradation, disrupted proteostasis, and pathological propagation.

We summarize α-syn structural dynamics and the concentration-dependent distribution of monomeric, oligomeric, and fibrillar species. We then review evidence from SNCA gene-dosage studies and examine the role of the autophagy-lysosome pathway in regulating α-syn homeostasis, with particular emphasis on recent experimental findings demonstrating that autophagy deficiency exacerbates α-syn accumulation and neurodegeneration in human α-syn bacterial artificial chromosome transgenic mice.

Collectively, the available genetic, biochemical, and experimental evidence supports a model in which the balance between α-syn production and clearance influences disease progression alongside protein misfolding. The interaction between increased protein burden and impaired clearance capacity provides a unifying mechanism linking familial and sporadic forms of PD.

We propose that α-syn multiplication offers an integrative framework for understanding PD pathogenesis, provides a quantitative perspective on disease heterogeneity, and highlights therapeutic opportunities aimed at reducing α-syn burden and restoring proteostatic balance.