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

Abstract

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.