Rare Heterozygous Loss-of-Function Variants in MCOLN1 Identified in Two Sporadic Patients with α-Synucleinopathies.

BACKGROUND: Growing evidence links lysosomal dysfunction to parkinsonism. TRPML1, a lysosomal cation channel encoded by the MCOLN1 gene, is essential for lysosomal function. Biallelic loss-of-function variants in MCOLN1 cause mucolipidosis type IV. However, the role of heterozygous MCOLN1 variants remains unclear. METHODS: Two patients with α-synucleinopathies underwent clinical evaluation and whole-genome sequencing. The functional effects of TRPML1 variants were assessed using immunofluorescence, lysosomal patch-clamp recording, and autophagic flux assay. RESULTS: Two heterozygous MCOLN1 variants were identified: p.E376K in a patient with multiple system atrophy and p.L315del in a patient with early-onset Parkinson's disease. Functional analyses showed that p.L315del disrupted lysosomal localization, and both variants significantly reduced lysosomal currents upon TRPML1 agonist stimulation, with a trend toward impaired autophagic flux, indicating loss of function. CONCLUSIONS: These findings demonstrate that both variants impair TRPML1 function in vitro, identifying MCOLN1 as a candidate gene for α-synucleinopathies that warrants further investigation in larger cohorts. © 2026 International Parkinson and Movement Disorder Society.