Primary lateral sclerosis: Upper‐motor‐predominant amyotrophic lateral sclerosis with frontotemporal lobar degeneration – immunohistochemical and biochemical analyses of TDP‐43

Primary lateral sclerosis (PLS) is clinically defined as a disorder selectively affecting the upper motor neuron (UMN) system. However, recently it has also been considered that PLS is heterogeneous in its clinical presentation. To elucidate the association of PLS, or disorders mimicking PLS, with 43‐kDa TAR DNA‐binding protein (TDP‐43) abnormality, we examined two adult patients with motor neuron disease, which clinically was limited almost entirely to the UMN system, and was followed by progressive frontotemporal atrophy. In the present study, the distribution and severity, and biochemical profile of phosphorylated TDP‐43 (pTDP‐43) in the brains and spinal cords were examined immunohistochemically and biochemically. Pathologically, in both cases, frontotemporal lobar degeneration with ubiquitin inclusions (FTLD‐U) was evident, with the most severe degeneration in the motor cortex. An important feature in both cases was the presence of Bunina bodies and/or ubiquitin inclusions, albeit very rarely, in the well preserved lower motor neurons. The amygdala and neostriatum were also affected. pTDP‐43 immunohistochemistry revealed the presence of many positively stained neuronal cytoplamic inclusions (NCIs) and dystrophic neurites/neuropil threads in the affected frontotemporal cortex and subcortical gray matter. By contrast, such pTDP‐43 lesions, including NCIs, were observed in only a few lower motor neurons. pTDP‐43 immunoblotting revealed that fragments of ～25‐kDa were present in the cortices, but not in the spinal cord in both cases. Genetically, neither of the patients had any mutation in the TDP‐43 gene. In conclusion, we consider that although PLS may be a clinically significant disease entity, at autopsy, the majority of such clinical cases would present as upper‐motor‐predominant amyotrophic lateral sclerosis with FTLD‐TDP.