Elevated Cerebrospinal Fluid Ubiquitin Carboxyl‐Terminal Hydrolase Isozyme L1 in Asymptomatic C9orf72 Hexanucleotide Repeat Expansion Carriers

Objective To identify biochemical changes in individuals at higher risk of developing amyotrophic lateral sclerosis (ALS) or frontotemporal dementia (FTD) via C9orf72 hexanucleotide repeat expansion (HRE) heterozygosity. Methods Cross‐sectional observational study of 48 asymptomatic C9orf72 HRE carriers, 39 asymptomatic non‐carrier controls, 19 people with sporadic ALS, 10 with C9orf72 ALS, 14 with sporadic FTD, and 10 with C9orf72 FTD. Relative abundance of 30 pre‐defined cerebrospinal fluid biomarkers of ALS and FTD were compared in asymptomatic C9orf72 HRE carriers and age‐matched non‐carrier controls. Differential abundance of these proteins was quantified using data independent acquisition mass spectrometry or electro chemiluminescent assay for neurofilament light chain. Unbiased analysis of the entire cerebrospinal fluid proteome was then carried out. Results Ubiquitin carboxyl‐hydrolase isozyme L1 levels were higher in asymptomatic C9orf72 HRE carriers compared with age‐matched non‐carriers (log 2 fold change 0.20, FDR‐adjusted p ‐value = 0.034), whereas neurofilament light chain levels did not significantly differ. Ubiquitin carboxyl‐hydrolase isozyme L1 levels remained elevated after matching of groups by neurofilament levels ( p = 0.011), and after adjusting for age, sex, and neurofilament levels. A significant difference was also observed when restricting analysis to younger participants (<37) matched by neurofilament level ( p = 0.007). Interpretation Elevated cerebrospinal fluid ubiquitin carboxyl‐hydrolase isozyme L1 levels in C9orf72 HRE carriers can occur in the absence of increased neurofilament levels, potentially reflecting either compensatory or pathogenic mechanisms preceding rapid neuronal loss. This brings forward the window on changes associated with the C9orf72 HRE carrier state, with potential to inform understanding of penetrance and approaches to prevention. ANN NEUROL 2024