C9orf72 repeat expansions modify risk for secondary motor and cognitive-behavioral symptoms in behavioral-variant frontotemporal degeneration and amyotrophic lateral sclerosis

In frontotemporal degeneration (FTD) and amyotrophic lateral sclerosis (ALS), subsequent motor or cognitive-behavioral features, respectively, are associated with shorter survival. However, factors influencing subsequent feature development remain largely unexplored. In this study, we examined whether the presence of a C9orf72 expansion or the initial clinical syndrome was associated with increased risk of subsequent feature development in individuals with ALS and FTD. We performed a retrospective evaluation of the entire disease course of individuals with ALS and FTD who had neuropathological confirmation of TDP-43 proteinopathy at autopsy or a C9orf72 hexanucleotide repeat expansion. We examined the odds and hazard of subsequent feature development and assessed whether each was modified by the presence of a C9orf72 expansion or initial clinical syndrome. At autopsy, we evaluated the association between TDP-43 pathology burden in characteristic brain regions and features across the FTD-ALS spectrum. For individuals with ALS (n=168) and FTD (n=73), binary logistic regression revealed increased odds (OR=3.49[95% CI 1.64-7.80], p=0.002) for developing subsequent features in those with a C9orf72 expansion compared to those without and decreased odds (OR=0.25[95% CI 0.12-0.53], p<0.001) for developing subsequent features in those with an initial ALS clinical syndrome compared to those with an initial FTD clinical syndrome. Cox proportional hazard analyses revealed an increased hazard (HR=3.78[95% CI 1.86-7.65], p<0.001) for developing subsequent features in those with a C9orf72 expansion compared to those without. We observed a 94-month difference in the time after symptom onset of the initial clinical syndrome that a given person without a C9orf72 expansion reached the highest probability of developing subsequent features (0.12[95% CI (0.03-0.19], 113.00 months) and a person with a C9orf72 expansion surpassed that probability (0.13[95% CI 0.06-0.19], 19.00 months). Beyond C9orf72 expansion status, cox proportional hazard analyses revealed a decreased hazard (HR=0.48[95% CI 0.25-0.95], p=0.03) for developing subsequent features in those with an initial ALS clinical syndrome compared to those with an initial FTD clinical syndrome. Age at symptom onset and sex were not associated with development of subsequent features. The distribution of TDP-43 pathology across characteristic brain regions reflected both the initial clinical syndrome and subsequent features, with relatively preserved spinal cord only in FTD cases without subsequent motor features (p<0.0001) and relatively preserved neocortical regions only in ALS cases without subsequent cognitive-behavioral features (p<0.0001). These data highlight the need for clinician vigilance to detect the onset of subsequent motor and cognitive-behavioral features in patients carrying a C9orf72 expansion, regardless of initial clinical syndrome. C9orf72 clinical care can be enhanced through coordination between cognitive and neuromuscular clinics. Abbreviated SummarySpencer et al. demonstrated both the presence of a C9orf72 expansion and the initial clinical syndrome modify risk of subsequent feature development in frontotemporal degeneration and amyotrophic lateral sclerosis, highlighting the need for clinician vigilance to detect the onset of subsequent motor and cognitive-behavioral features in this disease spectrum.