Quantitative biochemical profiling of GCase activity and α-synuclein proteoforms in postmortem human brains from GBA-related and idiopathic Parkinson’s disease

Parkinson’s disease (PD) is characterized by α-synuclein (αSyn) deposition and lysosomal dysfunction. Variants in the GBA1 gene, which encodes for lysosomal glucocerebrosidase (GCase), are PD risk factors (GBA-PD) and have been associated with increased cortical involvement compared to idiopathic PD (IPD). Nonetheless, the relationship between αSyn accumulation and GCase deficiency remains unclear. This study aims to quantitatively define the biochemical relationship between GCase deficiency and αSyn proteoforms across brain regions in GBA-related and IPD. Here, we sequenced GBA1 in 160 postmortem brains (25 iLBD, 114 PD, 21 controls) and conducted a comprehensive region-resolved quantitative biochemical analysis of the Locus coeruleus (LC), substantia nigra (SN) and medial temporal gyrus (GTM). The tissue was sequentially extracted to yield Soluble and Insoluble fractions for the measurement of Total, Ser129-phosphorylated (pSer129), and C-terminally truncated at aa122 (CTT122) αSyn proteoforms, and for the quantification of GCase activity and GCase protein levels. GBA1 variants were detected in 21.9% of PD cases, including a novel frameshift variant. Disease-associated αSyn accumulation was observed only in the Insoluble pool. Insoluble pSer129 and CTT122 αSyn were markedly increased in both iLBD and PD, whereas Soluble species were unchanged. Insoluble pSer129 αSyn was undetectable in controls. Cortical, as well as midbrain αSyn burden did not differ between GBA-PD and IPD. Interestingly, GCase activity was substantially reduced in GBA-PD and in IPD across regions. pSer129 αSyn burden inversely correlated with GCase activity, both in the presence (GBA-PD) and absence (IPD) of GBA1 variants. Overall, we demonstrate that aggregated, pSer129-enriched αSyn and GCase deficiency are biochemically linked across the PD spectrum independently of GBA1 status and support therapies enhancing lysosomal/GCase function in both genetic and idiopathic PD.