Genetic epidemiology of C9orf72 repeat expansion associated amyotrophic lateral sclerosis in Hungary

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive motor neuron loss. The most common genetic cause of ALS is the hexanucleotide repeat expansion in the C9orf72 gene, which is associated with earlier disease onset, faster progression, and an increased frequency of cognitive and psychiatric involvement. Data on population-specific characteristics of C9orf72-associated ALS remains limited in Central and Eastern Europe. METHODS: Between 2011 and 2024, a total of 959 ALS patients fulfilling established diagnostic criteria were screened for C9orf72 repeat expansions at two Hungarian centers. Hexanucleotide repeat expansions were analyzed using repeat-primed long-read PCR. Repeat numbers exceeding 30 were considered pathogenic. Clinical, demographic, and disease course data were retrospectively collected and analyzed. RESULTS: Pathogenic C9orf72 repeat expansions were identified in 63 of 959 patients, corresponding to a prevalence of 6.57% among Hungarian ALS patients. Bulbar onset was the most common presentation and was associated with faster progression and shorter survival (mean survival: 27.8 months). Cognitive impairment and psychiatric comorbidities were present in a substantial proportion of patients and were associated with slower functional decline. Regional differences in survival were observed, likely reflecting disparities in healthcare access rather than biological factors. CONCLUSIONS: This study provides the first comprehensive national characterization of C9orf72 repeat expansion-associated ALS in Hungary, based on a genetically defined cohort assembled over 13 years. Despite limitations related to retrospective data collection and cohort size, this ethnically homogeneous dataset offers valuable insight into population-specific clinical and epidemiological features and complements larger international studies. Systematic characterization and longitudinal follow-up of genetically defined, trial-ready ALS cohorts will be essential as targeted therapies for C9orf72-associated ALS approach clinical implementation.