Background: Gain-of-kinase-function variants in LRRK2 are a leading cause of monogenic Parkinson's disease (PD). Objectives: We tested the functional impact of a novel LRRK2 variant p.V1447L identified in a young-onset PD patient in vivo in peripheral blood, as well as in a robust cellular assay, alongside other variants in close proximity to V1447. Methods: We measured LRRK2-dependent Rab10 phosphorylation in neutrophils and monocytes of a LRRK2 p.V1447L carrier with PD. We performed structural mapping and evaluated the potential impact of other LRRK2 variants at and around LRRK2 V1447. Results: LRRK2 p.V1447L strongly increases LRRK2 kinase activity. We identified additional variants in the LRRK2 ROC:COR B interface with critical impact on kinase activity and demonstrated that different substitutions at the same residue can have opposing effects. Conclusions: We recommend reclassifying LRRK2 p.V1447L from variant of uncertain significance to likely pathogenic. Our study expands the range of putative loss-of-kinase function variants to LRRK2 missense variants.