LAUSR

BACKGROUND The number of mutations in nuclear encoded genes causing mitochondrial disease is ever increasing. Identification of these mutations is particularly important in the diagnosis of neuromuscular disorders as their presentation may mimic other acquired disorders.We present a novel heterozygous variant in mitochondrial fission factor (MFF) which mimics myasthenia gravis. OBJECTIVE To determine if the MFF c.937G>A, p.E313K variant causes a mild mitochondrial phenotype. METHODS We used whole exome sequencing (WES) to identify a novel heterozygous variant in MFF in a patient with ptosis, fatigue and muscle weakness. Using patient derived fibroblasts, we performed assays to evaluate mitochondrial and peroxisome dynamics. RESULTS We show that fibroblasts derived from this patient are defective in mitochondrial fission, despite normal recruitment of Drp1 to the mitochondria. CONCLUSIONS The MFF c.937G>A, p.E313K variant leads to a mild mitochondrial phenotype and is associated with defective mitochondrial fission in patient-derived fibroblasts.