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

Abstract Background Mitochondrial dynamics are essential for neuronal function, with dynamin-related protein 1 (DRP1), encoded by the DNM1L gene, playing a key role in mitochondrial fission. Pathogenic variants in DNM1L 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 DNM1L variant and to elucidate its impact on DRP1 function and mitochondrial physiology. Results We integrated clinical, metabolic, neuroimaging, genetic, structural, neuropathological, and functional analyses in a 12-year-old girl with a heterozygous de novo DNM1L variant (c.1093 A > G; p.Arg365Gly). The patient presented with severe developmental delay, microcephaly, ataxia, refractory seizures, and behavioral disturbances. Brain MRI was unremarkable, whereas metabolic evaluation showed transient lactate elevation and EEG revealed generalized slowing with multifocal seizure activity. The p.Arg365Gly variant, located in the DRP1 middle domain, was classified as likely pathogenic. In silico structural modeling predicted altered local protein packing and disruption of stabilizing interactions, while multiple protein-stability predictors consistently indicated a destabilizing effect. Postmortem neuropathological examination revealed widespread neuronal loss, gliosis, and neurodegenerative changes affecting cortical and subcortical regions. Western blot analysis showed apparently reduced monomeric DRP1 and high-molecular-weight DRP1-immunoreactive species, although these findings were subject to limitations related to tissue composition and normalization. Immunofluorescence analysis revealed increased DRP1 immunoreactivity and pronounced mitochondrial hyperfusion in patient neurons. Together, these findings are consistent with altered DRP1 assembly and defective mitochondrial fission associated with the p.Arg365Gly variant. Conclusions We report a previously unreported de novo DNM1L variant associated with severe neurodevelopmental disease, refractory epilepsy, and widespread postmortem neuropathological abnormalities. Structural and functional findings support a detrimental effect of p.Arg365Gly on DRP1 function and mitochondrial dynamics. This case expands the clinical and molecular spectrum of DNM1L -related disorders and highlights the value of integrating genetic, structural, neuropathological, and functional analyses in the investigation of rare mitochondrial diseases.