Background: HPDL is a recently identified enzyme that converts 4-hydroxymandelate (4-HMA) into 4-hydroxybenzoate (4-HB), the key precursor of the CoQ10 headgroup. Bi-allelic HPDL variants cause a strikingly broad neurodevelopmental spectrum—from early-onset encephalopathy to adolescent-onset SPG83—but the underlying mechanisms remain largely unexplored. Notably, 4-HB supplementation improved symptoms in one affected child, pointing to a potentially targetable metabolic bottleneck. To uncover the biology behind HPDL deficiency, we generated a CRISPR/Cas9 hpdl⁻/⁻ zebrafish model that recapitulates key disease features—including severe neurodevelopmental defects, mitochondrial dysfunction, and epilepsy-like behavior. These phenotypes offer a rapid, scalable platform to probe HPDL’s role in neuronal and mitochondrial homeostasis. We then performed targeted bypass therapies using 4-HMA, 4-HB, and MitoTempo. All three compounds achieved partial phenotypic rescue, strengthening the hypothesis of a disrupted CoQ10-related pathway and providing a foundation for therapeutic development. Crucially, I will present proteomic data from hpdl⁻/⁻ larval brains, revealing deep perturbations in mitochondrial, oxidative-stress, and synaptic pathways. These results offer the first mechanistic map connecting HPDL loss to neurodegeneration.