GALC variants affect Glucocerebrosidase to Galactosylceramidase activity ratio

Objective: Genes within the lysosomal glycosphingolipid pathway, including GBA1 and GALC, are important genetic risk factors in Parkinson's disease (PD). We hypothesized that PD risk relates not only to individual enzyme effects but also to coordination between glucocerebrosidase (GCase) and galactosylceramidase (GalCase) activities, and examined how genetic variants influence their activity ratio. Methods: We studied 1,290 individuals from the Parkinson's Precision Medicine Initiative and Columbia University cohorts with genetic and dried-blood-spot enzymatic data. Genome-wide association analyses of the glucocerebrosidase/galactosylceramidase activity ratio were performed in the combined cohort, by sex, and per cohort, followed by meta-analysis. For lead variants, Pearson correlations between enzyme activities were compared across genotypes, and genotype-by-GCase interactions were tested. Results: The GALC locus was associated with glucocerebrosidase/galactosylceramidase activity ratio in the combined cohort (rs365448; beta=-1.90; P=5.91e-115), replicated across cohorts and meta-analysis (beta = -1.66, P=6.33e-191), and in linkage disequilibrium with the PD GWAS index variant rs979812. This association remained significant in both sexes. Correlation between glucocerebrosidase and galactosylceramidase activities differed by rs979812 genotype (TT r=0.33 versus GG r=0.13; P=0.01), with a significant genotype-by-GCase interaction (P=3.05e-5). Among 134 PD loci examined, ten showed nominal association with the ratio-GWAS, including GBA1, where risk-variant carriers showed a similar pattern, yet not statistically significant. Interpretation: GALC variants influence the balance between glucocerebrosidase and galactosylceramidase activity, and this relationship may differ by genotype at PD-associated variants. These findings suggest PD risk at lysosomal glycosphingolipid genes may partly reflect altered coordination between pathway enzymes rather than isolated single-enzyme effects, warranting functional validation.