Summary While motor and cortical neurons are affected in C9orf72 ALS/FTD, it remains still largely unknown if and how non-neuronal cells induce or exacerbate neuronal damage. We generated C9orf72 ALS/FTD patient-derived induced pluripotent stem cells differentiated into microglia (iPSC-MG) and examined their intrinsic phenotypes. Similar to iPSC motor neurons, C9orf72 ALS/FTD iPSC-MG mono-cultures form G 4 C 2 repeat RNA foci, exhibit reduced C9orf72 protein levels and generate dipeptide repeat proteins. Healthy control and C9orf72 iPSC-MG equivalently express microglial specific genes and display microglial functions including inflammatory cytokine release and phagocytosis of extracellular toxic cargos such as synthetic amyloid beta peptides and healthy human brain synaptoneurosomes. Select C9orf72 iPSC-MG patient lines show inability to efficiently remove phagocytosed contents, suggesting dysfunction of the endosomal-lysosomal pathways. Finally, RNA sequencing revealed overall transcriptional changes in diseased microglia yet no significant differentially expressed microglial-enriched genes. These minimal differences in cellular, molecular and functional characteristics of microglial mono-cultures suggest that a diseased microenvironment is associated with microglial activation and subsequent regulation of neuronal dysfunction.

Cellular and molecular phenotypes of C9orf72 ALS/FTD patient derived iPSC-microglia mono-cultures