Identification of a novel loss-of-function C9orf72 splice site mutation in a patient with amyotrophic lateral sclerosis

Abnormal expansion of a hexanucleotide GGGGCC repeat in the C9orf72 gene is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia in Caucasians. However, the underlying pathologic mechanisms remain controversial, and both loss-of-function and gain-of-function models have been proposed. To gain further insight into these mechanisms, we performed mutation analysis of C9orf72 in 276 Han Chinese patients with ALS. We identified GGGGCC expansions in 2 cases of sporadic ALS with 38 and 63 repeats, as well as a novel splice site mutation (c.601-2A>G) in a third case. These genetic alterations were not detected in 332 control patients without neurological disease. Intriguingly, functional analysis revealed that the splice site mutation disrupted the reading frame, creating a premature stop codon (p.I201fsX235). Decreased levels of the mutant messenger RNA were detected in patient cells, suggesting that it may undergo nonsense-mediated messenger RNA decay. Taken together, these results demonstrate that C9orf72 mutation is infrequently associated with ALS in Han Chinese patients and suggest that a splice site mutation in C9orf72 may lead to loss of function due to haploinsufficiency of the resulting protein.