A DNM1L GTPase Effector Domain Deletion Causes Intermediate Defects in Peroxisomal and Mitochondrial Dynamics and Reveals a Targetable Metabolic Vulnerability

DNM1L encodes dynamin-related protein 1 (DRP1), a key GTPase regulating mitochondrial and peroxisomal dynamics. Pathogenic DNM1L variants are associated with a broad spectrum of neurodevelopmental and neurodegenerative disorders, with phenotypic severity varying by affected protein domain. While variants in the GTPase domain are typically associated with milder phenotypes and middle-domain variants with severe developmental and epileptic encephalopathy, the functional impact of GTPase effector domain (GED) variants remains less defined. Here, we describe four individuals harboring a recurrent heterozygous GED deletion (L700del) presenting with movement disorders and lacking severe infantile encephalopathy, consistent with an intermediate clinical phenotype. Patient-derived fibroblasts exhibited tubular peroxisomes and enlarged mitochondria, while Drosophila models demonstrated that DNM1LL700del has intermediate functional effects that were more pronounced than those of a GTPase-domain variant A192E but less severe than those of the middle-domain variant G350R. Comparison with another GED variant, Y691C, further demonstrated variant-specific differences in severity within the GED. Metabolic stress assays showed that L700del fibroblasts exhibited impaired growth under galactose conditions, consistent with increased vulnerability under conditions dependent on mitochondrial oxidative metabolism. Bezafibrate, a PPAR agonist that promotes mitochondrial biogenesis, partially rescued this growth defect. Together, these findings support a functional allelic spectrum in DNM1L-associated disease in which protein domain contributes to, but does not completely determine, functional severity. L700del represents an intermediate-strength allele associated with a movement-disorder phenotype and a pharmacologically modifiable metabolic vulnerability.