Andreas Reichert

APOO/MIC26 is a subunit of the MICOS complex required for mitochondrial cristae morphology and function. Here, we report a novel variant of the APOO/MIC26 gene that causes a severe mitochondrial disease with overall progeria‐like phenotypes in two patients. Both patients developed partial agenesis of the corpus callosum, bilateral congenital cataract, hypothyroidism, and severe immune deficiencies. The patients died at an early age of 12 or 18 months. Exome sequencing revealed a mutation (NM_024122.5): c.532G>T (p.E178*) in the APOO/MIC26 gene that causes a nonsense mutation leading to the loss of 20 C‐terminal amino acids. This mutation resulted in a highly unstable and degradation prone MIC26 protein, yet the remaining minute amounts of mutant MIC26 correctly localized to mitochondria and interacted physically with other MICOS subunits. MIC26 KO cells expressing MIC26 harboring the respective APOO/MIC26 mutation showed mitochondria with perturbed cristae architecture and fragmented morphology resembling MIC26 KO cells. We conclude that the novel mutation found in the APOO/MIC26 gene is a loss‐of‐function mutation impairing mitochondrial morphology and cristae morphogenesis.