Neurofilament Light Chain and Mutant Huntingtin Across Huntington's Disease Integrated Staging System Stages in Huntington Disease: A Cross‐Sectional Multimodal Analysis

BACKGROUND: The Huntington's Disease Integrated Staging System (HD-ISS) provides a framework for staging HD but does not incorporate fluid biomarkers. OBJECTIVE: The aim of this study was to examine neurofilament light chain (NfL) and mutant huntingtin (mHTT) and total tau across HD-ISS stages and their associations with genetic burden, neuroimaging, and clinical measures. METHODS: We performed cross-sectional analysis of the Barcelona-Huntington's Disease (BCN-HD) cohort with external replication in the multicenter HDClarity study. HD gene expansion carriers were classified as HD-ISS stages 0 to 3. NfL (cerebrospinal fluid [CSF] and plasma), CSF mHTT, total tau, striatal volumes, and Unified Huntington's Disease Rating Scale measures were quantified. Group differences and associations were analyzed using adjusted models. RESULTS: The BCN-HD cohort included 78 participants (50 HD gene expansion carriers [HDGECs], 28 controls) and HDClarity included 218 participants (166 HDGECs, 52 controls). CSF NfL increased across HD-ISS stages and discriminated premanifest from manifest individuals, as well as HD-ISS stage 0 from stage 1. Plasma NfL showed parallel increases and strongly correlated with CSF NfL (ρ = 0.88, P < 0.001). More than half of HD-ISS 0 individuals had NfL levels above the age-adjusted 95th percentile (P95) and shorter predicted time to onset (13 years for CSF and 10 years for plasma). From HD-ISS 1 onward, NfL levels exceeded P95 in most individuals. Higher NfL concentrations were associated with genetic burden, striatal atrophy, and clinical measures (all P < 0.001). CSF mHTT was detectable from HD-ISS 0 but showed weaker associations, whereas CSF total tau increased only from HD-ISS 3. CONCLUSIONS: NfL identifies biological activity across the HD-ISS continuum and shows neuroaxonal injury in a subset of individuals classified as stage 0. These findings suggest that fluid biomarkers may complement current HD-ISS criteria and support biological stratification in observational studies and clinical trials. © 2026 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.