Clinical characterization and natural history of ALS8/VAPB p.Pro56Ser: upper motor neurone signs, survival, and functional milestones in 78 patients.

OBJECTIVE

Amyotrophic lateral sclerosis type 8 (ALS8), caused by the VAPB p.Pro56Ser mutation, is a rare familial motor neurone disease with an incompletely characterized profile. We aimed to characterize the clinical phenotype, upper motor neurone (UMN) sign prevalence, survival, and functional milestones.

METHODS

We retrospectively analyzed 78 patients with ALS8 confirmed via molecular testing or familial linkage analysis from 57 apparently unrelated families. UMN signs were assessed using a five-item composite of pyramidal signs. Survival and milestones were estimated using Kaplan-Meier analysis.

RESULTS

Median age at onset was 44.9 years; 51% were men. Onset was lumbar in 94%, proximally predominant. UMN signs were present in 53 patients; none exhibited clonus. At admission, 51% had spinal-onset ALS, 42% progressive muscular atrophy (PMA) and 6% flail leg; 30% of patients with PMA subsequently developed UMN signs. Survival was 21.9 years; times to wheelchair dependence and noninvasive ventilation were 7.0 and 10.0 years, respectively. Bulbar involvement occurred in 17 (21.8%) patients, predominantly as dysphonia. UMN status did not affect survival (p = 0.312). The standardized mortality ratio was 4.54 (95% CI 2.77-7.01), supporting disease-related excess mortality.

CONCLUSIONS

ALS8 is a slowly progressive motor neurone disease with lumbar onset, ascending progression, and frequent but subtle UMN signs. Survival was markedly prolonged but functional decline followed a predictable sequence. These findings expand the phenotypic characterization of ALS8 and support genetic counseling and anticipatory management.