Origin and cell type specificity of mitochondrial DNA mutations in ALS-FTLD human brain organoids.

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are primarily genetic in ~20% of patients. Mutations in are the most frequent cause, but it is not understood why there is notable regional pathology. An increased burden of mitochondrial DNA (mtDNA) mutations in ALS-FTLD brains implicates mitochondrial mechanisms; however, it remains unclear how and when these mutations arise. To address this, we generated cerebral organoids derived from human-induced pluripotent stem cells (hiPSCs) of patients with ALS-FTLD harboring the hexanucleotide repeat expansion alongside CRISPR-corrected isogenic and healthy controls. Here, we show a higher mtDNA single-nucleotide variant (mtSNV) burden in astroglia derived from -mutant organoids, with some de novo mtSNVs likely due to the repeat and others evading selection to reach higher heteroplasmy levels. Thus, the functional consequences of the regional accumulation of mtSNVs in ALS-FTLD brains are likely to manifest through astroglial mitochondrial dysfunction.