Prevalence of SOD1 and C9orf72 Variants Among French ALS Population: The GENIALS Study

Amyotrophic Lateral Sclerosis (ALS) is a fatal motoneuron disease in which genetics plays a central role for both familial and sporadic ALS cases. Systematic genetic analysis for all ALS patients is recommended at the time of diagnosis, leading to an early proposal of specific genetic therapy. Currently, C9orf72 is considered the most frequently mutated gene in ALS. Patients with a SOD1 pathogenic or probably pathogenic variants (ACMG classification) are eligible for SOD1 antisense oligonucleotide therapy.

To determine the frequency of SOD1 variants and C9orf72 G4C2 repeats in a French ALS population and to describe genotype-phenotype relationships.

One thousand incident ALS patients were enrolled from 22 ALS centers in France and followed up for 12 months. Epidemiological, familial history, neurological data, and genetic status were collected.

C9orf72 G4C2 repeats and SOD1 variants were observed in 7.6% and 1.6%, respectively. Fifty percent of SOD1 patients and 51% of C9orf72 patients had sporadic ALS. Fifteen different SOD1 variants were identified within the five exons and one intron. C9orf72 patients had a significantly younger age at onset and a trend toward a faster progression compared to non-expanded C9orf72 patients. Moreover, among the non-SOD1 non-C9orf72 population, patients with at least one C9orf72 copy with two G4C2 repeats had a shorter disease duration.

This study confirms SOD1 variants low frequency in the French population and highlights the more rapid disease progression observed in patients carrying C9orf72 expansions. These findings underscore the importance of systematic genetic screening at diagnosis.