A novel de novo DNM1L mutation linked to mitochondrial fission dysfunction in a pediatric patient with epileptic encephalopathy and refractory seizures

<title>Abstract</title> <p> Background Mitochondrial dynamics are essential for neuronal function, with dynamin-related protein 1 (DRP1), encoded by the <italic>DNM1L</italic> gene, playing a key role in mitochondrial fission. Pathogenic variants in <italic>DNM1L</italic> are associated with a spectrum of severe neurodevelopmental and neurodegenerative disorders. This study aimed to characterize the clinical, genetic, and pathological features of a novel <italic>DNM1L</italic> variant and to elucidate its impact on DRP1 function and mitochondrial physiology. Results A case study was performed by integrating clinical phenotyping, metabolic and neuroimaging evaluation, whole-exome sequencing, in silico modeling, postmortem histopathology, and functional analyses using western blotting and neuronal assays to assess DRP1 expression and mitochondrial morphology. The patient, a 12-year-old female, presented with developmental delay, microcephaly, ataxia, refractory seizures, and behavioral disturbances. Brain MRI was unremarkable, but metabolic testing revealed elevated lactate levels and abnormal EEG patterns. Whole-exome sequencing identified a novel heterozygous de novo <italic>DNM1L</italic> variant (c.1093A &gt; G; p.Arg365Gly) located in the DRP1 middle domain and classified as likely pathogenic. <italic>In silico</italic> modeling predicted disruption of stabilizing intramolecular interactions and altered protein packing. Postmortem neuropathology revealed widespread neurodegeneration, neuronal loss, and gliosis. Functional assays demonstrated decreased levels of monomeric and oligomeric DRP1 and pronounced mitochondrial hyperfusion, consistent with defective mitochondrial fission due to impaired DRP1 oligomerization. Conclusions This work identifies and functionally characterizes a novel <italic>DNM1L</italic> variant, broadening the clinical and genetic spectrum of DRP1-associated mitochondrial disorders. The findings underscore the importance of integrated clinical, genetic, structural, and functional analyses to elucidate the mechanistic basis of rare mitochondrial diseases. </p>