Numerous FUS-positive inclusions in an elderly woman with motor neuron disease

We report an autopsy case of a 75-year-old Japanese woman with motor neuron disease (MND) showing numerous neuronal and glial inclusions immunostained with anti-fused in sarcoma (FUS) antibody. At 73 years, she received a diagnosis of MND and died of respiratory insufficiency 2 years later. No mutation was found in all exons of the FUS gene. Neuropathological examination revealed a reduced number of anterior horn cells and degeneration of the pyramidal tracts. Neither Bunina bodies nor inclusions positive for ubiquitin/phosphorylated TAR DNA binding protein of 43 kD (pTDP-43), such as skein-like or round inclusions, were observed. However, basophilic inclusions (BIs) were frequently observed in the remaining neurons of the anterior horns, facial nuclei, hypoglossal nuclei, vestibular nuclei, dentate nuclei and inferior olivary nuclei. In an immunohistochemical analysis, the BIs showed strong immunoreactivity with anti-FUS and anti-ubiquitin-binding protein p62 (p62) antibodies. The nuclear staining of FUS was preserved in some neurons with FUS-positive inclusions, and a few FUS-positive glial inclusions were found. FUS-positive inclusions were more common than p62-positive inclusions in some anatomical regions, and in some neurons, p62 immunoreactivity was observed in only parts of the BIs. These results suggest that BI formation and TDP-43 aggregation have different pathogenic mechanisms, and FUS may play an important role in the pathogenesis of MND with BIs. This patient has the oldest reported age of onset for MND with BIs, and clinical features observed in this patient were indistinguishable from those of classic sporadic MND. Therefore, we consider that the age of onset and clinical features of FUS-related disorders may be variable.