Siblings of FBXL4-related mitochondrial DNA depletion syndrome, leading to fatal fulminant pneumonia

The F-box and leucine-rich repeat protein 4 (<i>FBXL4</i>) is a nuclear encoded mitochondrial protein essential for mitochondrial DNA (mtDNA) maintenance. Biallelic variants in <i>FBXL4</i> cause FBXL4-related mitochondrial DNA depletion syndrome (FBXL4-MTDPS), characterized by lactic acidosis and developmental delay. We report two siblings diagnosed with FBXL4-MTDPS who died of fulminant pneumonia in infancy; autopsy revealed extensive pulmonary inflammation consistent with severe bacterial infection. FBXL4-MTDPS may involve intrinsic defects in pulmonary infection defense, increasing susceptibility to fatal infection such as pneumonia.