Microglia in frontotemporal lobar degeneration with progranulin or C9ORF72 mutations.

Objective: To identify clinicopathological differences between frontotemporal lobar degeneration (FTLD) due to mutations in progranulin (FTLD-GRN) and chromosome 9 open reading frame 72 (FTLD-C9ORF72).

Methods: We performed quantitative neuropathologic comparison of 17 FTLD-C9ORF72 and 15 FTLD-GRN with a focus on microglia. For clinical comparisons, only cases with high quality medical documentation and concurring diagnoses by at least two neurologists were included (14 FTLD-GRN and 13 FTLD-C9ORF72). Neuropathological analyses were limited to TDP-43 Type A to assure consistent assessment between the groups, acknowledging that Type A is a minority of C9ORF72 patients. Furthermore, only cases with sufficient tissue from all regions were studied (11 FTLD-GRN and 11 FTLD-C9ORF72). FTLD cases were also compared to age- and sex-matched normal controls. Immunohistochemistry was performed for pTDP-43, IBA-1, CD68, and GFAP. Morphological characterization of microglia was performed in sections of cortex blinded to clinical and genetic information.

Results: FTLD-GRN patients had frequent asymmetric clinical features, including aphasia and apraxia, as well as more asymmetric cortical atrophy. Neuropathologically, FTLD-C9ORF72 had greater hippocampal tau pathology and more TDP-43 neuronal cytoplasmic inclusions. FTLD-GRN had more neocortical microvacuolation, as well as more IBA-1-positive ameboid microglia in superficial cortical layers and in subcortical white matter. FTLD-GRN also had more microglia with nuclear condensation, possibly indicating apoptosis. Microglial morphology with CD68 immunohistochemistry in FTLD-GRN and FTLD-C9ORF72 differed from controls.

Interpretation: Our findings underscore differences in microglial response in FTLD-C9ORF72 and FTLD-GRN as shown by significant differences in ameboid microglia in gray and white matter. These results suggest the differential contribution of microglial dysfunction in FTLD-GRN and FTLD-C9ORF72 and suggest that clinical, neuroimaging and pathologic differences could in part be related to differences in microglia response.