The enzyme at the centre of the story is LRRK2, a large protein kinase encoded by one of the most important genes in Parkinson’s research. Pathogenic variants in LRRK2, such as the common G2019S substitution, drive disease by increasing the kinase activity of the protein. Kinases act as molecular switches, attaching phosphate groups to target proteins, and when LRRK2 runs too hot, its downstream targets are over-phosphorylated. The most informative of these targets are members of the Rab family, small GTPases that regulate traffic between cellular compartments. Rab10, in particular, carries a threonine at position 73 that is directly phosphorylated by LRRK2, making pRab10-Thr73 a widely used readout of LRRK2 pathway activity in accessible cells.

Why does this matter therapeutically? Because a wave of LRRK2 kinase inhibitors is advancing through clinical development, and these drugs are designed for patients whose disease is driven by excessive LRRK2 activity. If clinicians select patients purely on the basis of known pathogenic mutations, they may miss a much larger pool of individuals whose LRRK2 pathway is equally activated by other mechanisms. The Dundee-led study was designed to answer exactly that question: how common is elevated LRRK2 pathway activity among Parkinson’s patients who lack recognised pathogenic variants, and what might be driving it in those cases?

The researchers assembled a genetically enriched cohort of 181 Parkinson’s disease patients alongside 71 healthy controls. Rather than relying on targeted panels that test only a shortlist of known mutations, the team performed whole-exome sequencing, which reads the protein-coding regions of essentially every gene. This allowed them to identify both established pathogenic variants and rare variants of uncertain significance across the genome. In parallel, they quantified LRRK2-dependent phosphorylation of Rab10 at threonine 73 in blood neutrophils, the granulocytes that have become the standard peripheral cell type for this assay because they express LRRK2 at high levels and can be obtained from a simple blood draw.

The results provided a striking biological benchmark. Patients carrying the VPS35 p.D620N variant, a mutation in the retromer trafficking machinery that is known to activate LRRK2 signalling indirectly, showed marked elevation of phosphorylated Rab10 in their neutrophils. Because VPS35 p.D620N is an established cause of familial Parkinson’s disease that acts through LRRK2, the team used these carriers to define a reference threshold for what biologically meaningful pathway activation looks like. This is a subtle but important methodological advance: instead of relying on statistical outliers among controls, the threshold is anchored to patients whose disease mechanism is well understood.

Against that benchmark, the headline finding emerged. Fourteen percent of the Parkinson’s patients who had no recognised pathogenic variant showed LRRK2 pathway activation that was similar to, or even greater than, that of the VPS35 p.D620N carriers. In other words, roughly one in seven genetically unsolved patients in this cohort carried a molecular fingerprint indistinguishable from a known genetic cause of the disease. These individuals would be invisible to standard genetic counselling and would not be flagged by mutation-based screening, yet their biology points squarely at the LRRK2 pathway as a driver.

What could be pushing LRRK2 signalling into overdrive in patients without mutations in LRRK2 or VPS35? To begin answering this, the researchers took a functional genomics approach. They selected genes harbouring rare variants found in the subset of patients with increased LRRK2 pathway activity and tested each candidate experimentally. Using small interfering RNA to knock down the expression of these genes one at a time in A549 cells, a human lung carcinoma cell line that has become a workhorse for LRRK2 Rab phosphorylation assays, the team assessed whether reducing each gene’s activity changed LRRK2-dependent Rab10 phosphorylation. This exploratory screen identified several candidate genetic modifiers of LRRK2 signalling, among them NECAP2, a gene involved in clathrin-mediated endocytosis and adaptor protein trafficking.

The identification of NECAP2 and other candidates is best understood as hypothesis-generating rather than definitive. A knockdown screen in a cell line cannot prove that a rare variant in a patient caused their elevated pathway activity, and the authors are careful to frame the functional assessments as exploratory. Nevertheless, the logic of the approach is compelling: rather than guessing which rare variants matter based on evolutionary conservation or predicted protein damage alone, the team prioritised variants by asking whether perturbing the corresponding genes actually moves the needle on the disease-relevant molecular pathway. That functional filter is exactly what the field needs as whole-exome sequencing floods clinics with variants of uncertain significance.

The broader implication is a shift in how patients with Parkinson’s disease might be stratified for precision therapy. The authors argue for integrating functional pathway phenotyping, the direct measurement of LRRK2 activity in patient cells, with genomic analysis, so that trial enrolment and eventually treatment decisions rest on biology rather than on mutation status alone. As LRRK2-targeted therapies advance toward the clinic, this combined approach could identify the patients most likely to benefit, including those whose elevated kinase activity arises from undiscovered genetic modifiers rather than from the handful of variants currently listed as pathogenic. It also raises the possibility that some cases labelled idiopathic are, at the molecular level, genetic after all, simply through mechanisms that current diagnostic tests do not interrogate.

There are, of course, caveats to keep in view. The cohort was genetically enriched, meaning patients were selected partly because of family history or other genetic signals, so the fourteen percent figure may not generalise to the unselected sporadic population. Neutrophil pRab10 measurements reflect peripheral blood cells, not the dopaminergic neurons that degenerate in the disease, although the assay has been validated extensively as a surrogate of systemic LRRK2 activity. And the candidate modifiers, including NECAP2, require independent replication before they can inform diagnosis. Even so, the study delivers a clear and consequential message: measuring pathway activity, not just reading the genetic code, reveals a hidden layer of Parkinson’s disease biology, and that layer is large enough to matter for the next generation of LRRK2-targeted clinical trials.

Subject of Research: LRRK2 kinase pathway activity in genetically unsolved Parkinson's disease

Article Title: Functional pathway phenotyping reveals increased LRRK2 activity beyond recognised genetic causes of Parkinson’s disease

Article References: Pratuseviciute, N., Pirker, W., Huber, J., Gomes, S., Squires, I., Filipe Soares, R., Brücke, C., Zimprich, A., & Sammler, E. (2026). Functional pathway phenotyping reveals increased LRRK2 activity beyond recognised genetic causes of Parkinson’s disease. npj Parkinson's Disease. https://doi.org/10.1038/s41531-026-01575-6

Image Credits: AI Generated

DOI: 10.1038/s41531-026-01575-6

Keywords: Parkinson's disease, LRRK2, Rab10 phosphorylation, VPS35 D620N, whole-exome sequencing, kinase activity, NECAP2, siRNA screen, precision medicine, genetic modifiers, neutrophils, patient stratification