Longitudinal Magnetic Resonance Imaging in Asymptomatic C9orf72 Mutation Carriers Distinguishes Phenoconverters to Amyotrophic Lateral Sclerosis or Amyotrophic Lateral Sclerosis With Frontotemporal Dementia

Objective We prospectively studied asymptomatic C9orf72 mutation carriers, identifying those developing amyotrophic lateral sclerosis (ALS) or frontotemporal dementia (FTD). Methods We enrolled 56 asymptomatic family members (AFM) with a C9orf72 mutation (AFM C9+), 132 non‐carriers (AFM C9−), and 359 population‐based controls. Using 3 T magnetic resonance imaging, we measured cortical thickness, gyrification, and subcortical volumes longitudinally. Linear mixed‐effects models on non‐converting AFM C9+ scans (n = 107) created a reference for these measurements, establishing individual atrophy patterns. Atrophy patterns from presymptomatic phenoconverters (n = 10 scans) served as a template for group comparisons and similarity assessments. Similarity with phenoconverters was quantified using Dice similarity coefficient (DSC) for cortical and Kullback–Leibler similarity (KLS) for subcortical measures. Using longitudinal similarity assessments, we predicted when participants would reach the average similarity level of phenoconverters at their first post‐onset scan. Results Five AFM C9+ converted to ALS or ALS‐FTD. Up to 6 years before symptoms, these phenoconverters exhibited significant atrophy in frontal, temporal, parietal, and cingulate cortex, along with smaller thalamus, hippocampus, and amygdala compared to other AFM C9+. Some non‐converted AFM C9+ had high DSC and KLS, approaching values of phenoconverters, whereas others, along with AFM C9− and controls, had lower values. At age 80, we predicted 27.9% (95% confidence interval, 13.2–40.1%) of AFM C9+ and no AFM C9− would reach the same DSC as phenoconverters. Interpretation Distinctive atrophy patterns are visible years before symptom onset on presymptomatic scans of phenoconverters. Combining baseline and follow‐up similarity measures may serve as a promising imaging biomarker for identifying those at risk of ALS or ALS‐FTD. ANN NEUROL 2024