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Objective: Most paroxysmal kinesigenic dyskinesia (PKD) cases are hereditary, yet approximately 60% of patients remain genetically undiagnosed. We undertook the present study to uncover the genetic basis for undiagnosed PKD patients. Methods: Whole‐exome sequencing was performed for 106 PRRT2‐negative PKD probands. The functional impact of the genetic variants was investigated in HEK293T cells and Drosophila. Results: Heterozygous variants in KCNJ10 were identified in 11 individuals from 8 unrelated families, which accounted for 7.5% (8/106) of the PRRT2‐negative probands. Both co‐segregation of the identified variants and the significantly higher frequency of rare KCNJ10 variants in PKD cases supported impacts from the detected KCNJ10 heterozygous variants on PKD pathogenesis. Moreover, a KCNJ10 mutation‐carrying father from a typical EAST/SeSAME family was identified as a PKD patient. All patients manifested dystonia attacks triggered by sudden movement with a short episodic duration. Patch‐clamp recordings in HEK293T cells revealed apparent reductions in K+ currents of the patient‐derived variants, indicating a loss‐of‐function. In Drosophila, milder hyperexcitability phenotypes were observed in heterozygous Irk2 knock‐in flies compared to homozygotes, supporting haploinsufficiency as the mechanism for the detected heterozygous variants. Electrophysiological recordings showed that excitatory neurons in Irk2 haploinsufficiency flies exhibited increased excitability, and glia‐specific complementation with human Kir4.1 rescued the Irk2 mutant phenotypes. Interpretation: Our study established haploinsufficiency resulting from heterozygous variants in KCNJ10 can be understood as a previously unrecognized genetic cause for PKD and provided evidence of glial involvement in the pathophysiology of PKD. ANN NEUROL 2024;96:758–773 [ABSTRACT FROM AUTHOR]

Background: Hereditary spastic paraplegias (HSP) are neurologic disorders characterized by progressive lower‐extremity spasticity. Despite the identification of several HSP‐related genes, many patients lack a genetic diagnosis. Objectives: The aims were to confirm the pathogenic role of biallelic COQ4 mutations in HSP and elucidate the clinical, genetic, and functional molecular features of COQ4‐associated HSP. Methods: Whole exome sequences of 310 index patients with HSP of unknown cause from three distinct populations were analyzed to identify potential HSP causal genes. Clinical data obtained from patients harboring candidate causal mutations were examined. Functional characterization of COQ4 variants was performed using bioinformatic tools, single‐cell RNA sequencing, biochemical assays in cell lines, primary fibroblasts, induced pluripotent stem cell–derived pyramidal neurons, and zebrafish. Results: Compound heterozygous variants in COQ4, which cosegregated with HSP in pedigrees, were identified in 7 patients from six unrelated families. Patients from four of the six families presented with pure HSP, whereas probands of the other two families exhibited complicated HSP with epilepsy or with cerebellar ataxia. In patient‐derived fibroblasts and COQ4 knockout complementation lines, stable expression of these missense variants exerted loss‐of‐function effects, including mitochondrial reactive oxygen species accumulation, decreased mitochondrial membrane potential, and lower ubiquinone biosynthesis. Whereas differentiated pyramidal neurons expressed high COQ4 levels, coq4 knockdown zebrafish displayed severe motor dysfunction, reflecting motor neuron dysregulation. Conclusions: Our study confirms that loss‐of‐function, compound heterozygous, pathogenic COQ4 variants are causal for autosomal recessive pure and complicated HSP. Moreover, reduced COQ4 levels attributable to variants correspond with decreased ubiquinone biosynthesis, impaired mitochondrial function, and higher phenotypic disease severity. © 2023 International Parkinson and Movement Disorder Society. [ABSTRACT FROM AUTHOR]