Colony-stimulating factor 1 receptor-related disorder (CSF1R-RD) is a neurodegenerative condition characterized by rapid progression, leading to profound functional decline and ultimately resulting in a persistent vegetative state. Although an effective treatment option exists, there remains a lack of identified biomarkers capable of monitoring disease progression and detecting the earliest symptom onset in CSF1R pathogenic variant carriers, limiting the ability of clinicians to make informed decisions regarding patient care. This study aims to identify both fluid and neuroimaging biomarkers for CSF1R-RD that can inform the optimal timing of treatment administration to maximize therapeutic benefit, while also providing sensitive quantitative measurements to monitor disease progression. Our study compared neuroimaging and fluid (plasma and cerebrospinal fluid (CSF)) biomarkers across three distinct populations: asymptomatic CSF1R pathogenic variant carriers (N=14), symptomatic CSF1R pathogenic variant carriers (N=17), and healthy controls (N=30). We evaluated biomarker correlations with both an established (Montreal Cognitive Assessment (MoCA)) and a novel (CSF1R Clinical Severity Score (CCSS)) clinical diagnostic scale to investigate potential clinical utility. Additionally, we tested the relationship between select biomarkers and cortical thickness using 3D T1-weighted MPRAGE scans, providing a highly valuable physiological component to our analyses. Our results demonstrate that while plasma glial fibrillary acidic protein (GFAP) displays a high sensitivity for distinguishing early-stage CSF1R-RD patients from healthy controls, plasma neurofilament light chain (NfL) is more effective for tracking disease progression following the onset of symptoms. Overall, our study provides evidence for plasma NfL and GFAP as valuable biomarkers of earliest symptom onset and disease progression for CSF1R-RD

SG, GP, MBJ and RCS are employed by Savanna Biotherapeutics, ZKW serves as PI or Co-PI on Biohaven Pharmaceuticals, Inc. (BHV4157-206), Vigil Neuroscience, Inc. (VGL101-01.002, VGL101-01.201, Csf1r biomarker and repository project, and ultra-high field MRI in the diagnosis and management of CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia), ONO-2808-03, and Amylyx AMX0035-009 projects/grants. He serves as Co-PI of the Mayo Clinic APDA Center for Advanced Research and as a consultant for Savanna Bio, Eli Lilly & Company. All other authors declare no conflict of interest.

This work was supported by the National Institutes of Health [(R01NS120992: M.P.), (U54NS123743: L.P. and M.P.), (R35NS097273: L.P.)], the Target ALS Foundation (M.P. and L.P.), BrightFocus Foundation (M.P.), the Robert Packard Center for ALS Research at Johns Hopkins (L.P.), and the Kissick Family Foundation (M.P.). Z.K.W. is partially supported by the NIH/NIA and NIH/NINDS (1U19AG063911, FAIN: U19AG063911), the Haworth Family Professorship in Neurodegenerative Diseases fund, The Albertson Parkinson's Research Foundation, PPND Family Foundation, and Margaret N. and John Wilchek Family. This study was partially funded by Savanna Biotherapeutics, Inc.

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