Amyotrophic lateral sclerosis caused by the C9orf72 expansion in Norway - prevalence, ancestry, clinical characteristics and sociodemographic status.

Objective: The most common genetic cause of amyotrophic lateral sclerosis (ALS) is the C9orf72 expansion. A high incidence of this expansion has been detected in Sweden and Finland. This Norwegian population-based study aimed to identify the prevalence, geographic distribution, ancestry, and relatedness of ALS patients with a C9orf72 expansion (C9_pos). Further, we compared C9_pos and C9_neg patients' clinical presentation, family history of ALS and other neurodegenerative disorders, and sociodemographic status.

Methods: We recruited ALS patients from all 17 Departments of neurology in Norway. Blood samples and questionnaires regarding clinical characteristics, sociodemographic status and family history of ALS, and other neurodegenerative disorders were collected. The C9orf72 expansion was examined for all patients.

Results: The study enrolled 500 ALS patients, 8.8% of whom were C9_pos, with half being sporadic ALS cases. The proportion of C9_pos cases differed between regions, ranging from 17.9% in the Northern region to 1.9% in the Western region. The majority of C9_pos patients had non-Finnish European descent and were not closely related. C9_pos patients exhibited a significantly shorter mean survival time, had a higher frequency of relatives with ALS or dementia, and were more often unmarried/single and childless than C9_neg patients.

Conclusion: C9_pos patients constitute a large portion of the Norwegian ALS population. Ancestry and relatedness do not adequately explain regional differences. Relying on clinical information to identify C9_pos patients has proven to be challenging. Half of C9_pos patients were reported as having sporadic ALS, underlining the importance of carefully assessing family history and the need for genetic testing.