Calcium modulating ligand confers risk for Parkinson's disease and impacts lysosomes.

Objective: Several genetic loci known to confer risk for Parkinson's disease (PD) function in lysosomal pathways. We systematically screened common variants linked to PD risk by genome-wide association studies (GWAS) for impact on cerebrospinal fluid (CSF) proteins reflecting lysosomal function.

Methods: Starting with 525 candidate gene-single nucleotide polymorphism (SNPs) pairs nominated by Mendelian randomization from published PD GWAS, we filtered SNPs for downstream evaluation, based on strength of association with PD and impact on brain gene expression. We genotyped top SNPs in 173 PD participants, adding three SNPs capturing variation at the TMEM106B, CTSB, and RAB29 loci, encoding genes with known lysosomal function. In the same 173 individuals, we measured 15 CSF proteins (nine lysosomal proteins and six other proteins implicated in neurodegeneration) by parallel reaction monitoring mass spectrometry. We tested SNPs for association with lysosomal proteins. For our top SNP associating with multiple lysosomal proteins, we characterized expression of its target gene CAMLG in human brain tissue.

Results: Sixteen SNPs emerged from our analysis of GWAS-nominated loci. Genotypes at rs12657663 (CAMLG) associated with CSF levels of multiple lysosomal markers (cathepsin F, cathepsin L, hexosaminidase B, and tripeptidyl peptidase I) and genotypes at rs7910668 (ITGA8) with CSF levels of cathepsin B. The protein encoded by CAMLG, calcium modulating ligand (CAML), is highly expressed in neurons of multiple human brain regions, with higher expression in Lewy body disease cases.

Interpretation: Systematic analysis of PD risk loci nominates CAMLG as a neuronally expressed risk gene with effects on lysosomes.