BACKGROUND: Amyotrophic lateral sclerosis (ALS) shows a strong genetic basis, 
with SOD1, FUS, TARDBP, and C9ORF72 being the genes most frequently involved. 
This has allowed identification of asymptomatic mutation carriers, which may be 
of help in understanding the molecular changes preceding disease onset.
OBJECTIVES: We studied the cellular expression of FUS protein and the effect of 
heat-shock- and dithiothreitol-induced stress in fibroblasts from FUS P525L 
mutation carriers, healthy controls, and patients with sporadic ALS.
METHODS: Western blots and immunocytochemistry were performed to study the 
subcellular localization of FUS protein. Control and stressed cells were double 
stained with FUS and the stress marker TIA-R.
RESULTS: Fibroblasts from healthy controls and sporadic ALS cases showed a 
prominent nuclear FUS expression. In the 2 FUS P525L mutation carriers, instead, 
most cells showed a protein localization in both nucleus and cytoplasm, or 
exclusively in the cytoplasm. Stress prompted the formation of cytoplasmic 
granules in all subjects and in sporadic ALS FUS mislocalization to the 
cytoplasm. Cytoplasmic FUS was recruited into stress granules, which showed a 
time-dependent decrease in all subjects. However, in the FUS P525L fibroblasts, 
the granules persisted longer, and they were more numerous than those detected 
in the cells from controls and sporadic ALS patients.
CONCLUSIONS: We show that in fibroblasts of FUS P525L mutation carriers, FUS 
mislocalized to the cytoplasm where it redistributed into stress granules with 
likely a dose effect, i.e. a higher number of cells with granules, which persist 
longer, than in controls and ALS cases. These data represent an early molecular 
change occurring before ALS onset, suggesting a transient preaggregative state.