Pathological heterogeneity in amyotrophic lateral sclerosis with FUS mutations: two distinct patterns correlating with disease severity and mutation

Abstract

Mutations in the gene encoding the fused in sarcoma (FUS) protein are responsible for ~3% of familial amyotrophic lateral sclerosis (ALS) and <1% of sporadic ALS (ALS-FUS). Descriptions of the associated neuropathology are few and largely restricted to individual case reports. To better define the neuropathology associated with FUS mutations, we have undertaken a detailed comparative analysis of six cases of ALS-FUS that include sporadic and familial cases, with both juvenile and adult onset, and with four different FUS mutations. We found significant pathological heterogeneity among our cases, with two distinct patterns that correlated with the disease severity and the specific mutation. Frequent basophilic inclusions and round FUS-immunoreactive (FUS-ir) neuronal cytoplasmic inclusions (NCI) were a consistent feature of our early-onset cases, including two with the p.P525L mutation. In contrast, our late-onset cases that included two with the p.R521C mutation had tangle-like NCI and numerous FUS-ir glial cytoplasmic inclusions. Double-labeling experiments demonstrated that many of the glial inclusions were in oligodendrocytes. Comparison with the neuropathology of cases of frontotemporal lobar degeneration with FUS-ir pathology showed significant differences and suggests that FUS mutations are associated with a distinct pathobiology.

Fig. 1

Basophilic inclusions (BI). Most BI were well-defined, single or multiple, round or multilobulated structures with a pale blue-grey color (a). In early-onset cases, multiple BI were present in all sections of spinal cord, medulla and motor cortex (b). Some UMN and LMN contained smaller, densely basophilic deposits that resembled abnormally clumped Nissl substance (c). Hematoxylin and eosin. Scale bar 30 µm (a); 50 µm (b); 20 µm (c)

Fig. 2

FUS-immunoreactive (FUS-ir) neuronal cytoplasmic inclusions in late-onset ALS-FUS cases most often appeared as a collection of thick filaments with either a globose or flame shape (a – c; lower motor neuron (a), upper motor neuron (b), basal ganglia (c)). Non-compact collections of FUS-ir cytoplasmic granules were also common in both late- and early-onset cases (d). FUS immunohistochemistry, developed with AEC. Scale bar 15 µm (a); 25 µm (b); 75 µm (c); 20 µm (d)

Fig. 3

FUS-immunoreactive (FUS-ir) neuronal cytoplasmic inclusions in early-onset ALS-FUS cases usually had a compact, round morphology and were often of similar size and shape to BI (a – d; spinal cord lower motor neuron (a), primary motor cortex (b), basis pontis (c), hypoglossal nucleus (d)). In some early-onset cases, neurons of the hypoglossal nucleus contained small round FUS-ir intranuclear inclusions (d). FUS immunohistochemistry, developed with DAB. Scale bar 20 µm (a, d); 100 µm (b, c)

Fig. 4

FUS-immunoreactive glial cytoplasmic inclusions (GCI) had variable morphology, including small round or crescentic bodies (a) that often extended into a few (b) or multiple ramified processes (c, d). GCI were only numerous in the late-onset cases where they had a wide neuroanatomical distribution (basal ganglia (e), cerebral white matter (f)). FUS immunohistochemistry, developed with AEC. Scale bar 10 µm (a – c); 15 µm (d); 20 µm (µ); 40 µm (f)

Fig. 5

Double-labeling immunofluorescence of FUS-immunoreactive (FUS-ir) glial inclusions. Glial cells with FUS-ir inclusions were often labeled by the oligodendrocyte marker CNPase (a, b). FUS-ir inclusions were not found in GFAP-positive astrocytes (c) or CR3/43 positive microglia cells (d). Double-labeling immunofluorescence; glial markers red, FUS green. Scale bar 10 µm (a–c); 40 µm