Cognitive and Neuroimaging Divergence Between Juvenile and Adult FUS Amyotrophic Lateral Sclerosis

OBJECTIVE: Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive motor neuron degeneration. Fused in sarcoma (FUS)-associated juvenile ALS (jALS) represents a distinct and aggressive subgroup with rapid deterioration and poor prognosis. Certain FUS mutations have been associated with comorbid intellectual disability, suggesting neurodevelopmental involvement. We compared FUS-jALS with adult-onset FUS-ALS cases (aALS) to evaluate the association between premorbid cognitive impairment, genetic and clinical features incorporating neuroimaging data. METHODS: Patients with genetically confirmed FUS-ALS were classified as jALS (onset < 25 years) or aALS (onset ≥ 25 years). Neuropsychological assessment used Mehrfachwahl-Wortschatz-Test (MWT) for verbal IQ, and the Edinburgh Cognitive and Behavioral ALS Screen (ECAS), with cognitive impairment classified according to Strong criteria. Volumetric analysis was conducted on structural MRI and FDG-PET data. RESULTS: All three jALS (P525L [n = 2], H517_Q519del [n = 1]) showed rapid progression with early severe clinical events. Neuropsychological assessment revealed global cognitive deficits (ALS-ci) with widespread dysfunction beyond typical ALS-specific patterns and reduced verbal IQ, pointing towards premorbid cognitive impairment. aALS demonstrated slower progression and were predominantly cognitively unimpaired (ALS-ni) or showed an ALS-specific impairment. Neuroimaging revealed distinct patterns: jALS cases demonstrated posterior cortical atrophy and hypometabolism on FDG-PET, while aALS showed largely preserved brain volumes and limbic-subcortical hypometabolism. INTERPRETATION: Specific FUS mutations (P525L, H517_Q519del) predispose to jALS with severe progression and premorbid cognitive impairments, supporting a genotype-phenotype association. Posterior cortical findings suggest neurodevelopmental delay rather than disease-related neurodegeneration. Genetic FUS screening may be warranted in patients with intellectual disability and motor signs, given emerging targeted therapies.