The G4C2 hexanucleotide repeat expansion (HRE) mutation in the C9orf72 gene is the most common genetic cause underlying both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Pathologically, these two neurodegenerative disorders are also linked by the common presence of abnormal TDP-43 neuronal cytoplasmic inclusions. However, the different characteristics of inclusion bodies in forebrains from patients dying with C9orf72-associated disease and sporadic disease brain have not been well studied.

We compared the number of TDP-43 inclusions, their morphology, and their immunohistochemical characteristics in post-mortem brain tissue from control, sporadic FTLD/ALS and C9orf72-associated FTLD/ALS. These comparisons were performed with an automated algorithm to quantitatively determine the abundance, size and morphological differences of TDP-43 inclusions between these groups. 

In sporadic FTLD/ALS, there were numerous TDP-43 inclusions that were variable in size, whereas inclusions in C9orf72 FTLD/ALS brain were less abundant and were smaller in size. TDP-43 immunostaining also differed between these two groups, with sporadic FTLD/ALS showing consolidated inclusions while C9orf72 showing more diffuse TDP-43 immunostaining in the neuronal cytoplasm. We were able to demonstrate these phenomena using multiple antibodies to confirm these differences in post-mortem brain tissues.  In addition, TDP-43 inclusions co-localized with autophagosome and lysosome markers, suggesting that TDP-43 inclusions are substrates for autophagic degradation. 

The smaller and less abundant TDP-43 inclusion bodies in C9orf72 FTLD/ALS compared to sporadic disease suggest differences in TDP-43 sequestration into inclusion bodies and degradation by autophagy which may have important pathophysiological implications for C9orf72-related diseases.