We identified exonic indels in patients and controls with comparable frequencies, although a number of patient and control specific variations were observed (). Indels in poly-glycine tracts were also observed with similar frequencies in patients (0.79%) and controls (0.56%). Although the indels observed in our control population could result from reduced disease penetrance, this is unlikely given similar indel frequencies in patients and controls. Despite previous reports of an enrichment of reduced poly-glycine tract lengths in ALS patients and a converse increase in the poly-glycine tract lengths in controls (Corrado et al. 2010), all 6 variants observed in controls in our study were deletions. The largest indel was an 18 base pair deletion in FUS exon 5 (c.412_429delGGACAGCAGCAAAGCTAT; p.Gly138_Tyr143del) identified in one control. Missense mutations and single amino acid deletions in this region of FUS were previously reported in ALS patients. In these patients, the disease was mostly sporadic or segregation of the mutation with disease could not be performed and autopsy confirming FUS pathology was not available. The identification of p.Gly138_Tyr143del in a control in this study therefore questions the pathogenicity of previously published FUS mutations outside the FUS C-terminal domain.
One patient was found to have a potentially pathogenic p.Gly475del mutation in exon 14, in a region of the gene where the ALS-causing missense mutations cluster. cDNA transcript analysis did not reveal alternative splicing events or degradation of the mutant transcript by nonsense-mediated decay. Cellular localization studies also revealed no obvious changes in the cellular localization of FUSGly475del. These findings suggest that p.Gly475del is not pathogenic due to FUS mislocalization, however we cannot exclude that this mutation may cause disease through an as yet unknown disease mechanism. Together this large study suggests that not all exonic indels in FUS cause disease. This finding has implications for genetic testing in ALS patients and should be taken into consideration when performing FUS mutation screening.