Hyperkinetic Movement Disorder Caused by the Recurrent c.892C>T NACC1 Variant

Background Genetic syndromes of hyperkinetic movement disorders associated with epileptic encephalopathy and intellectual disability are becoming increasingly recognized. Recently, a de novo heterozygous NACC1 ( nucleus accumbens‐associated 1) missense variant was described in a patient cohort including one patient with a combined mitochondrial oxidative phosphorylation (OXPHOS) deficiency. Objectives The objective is to characterize the movement disorder in affected patients with the recurrent c.892C>T NACC1 variant and study the NACC1 protein and mitochondrial function at the cellular level. Methods The movement disorder was analyzed on four patients with the NACC1 c.892C>T (p.Arg298Trp) variant. Studies on NACC1 protein and mitochondrial function were performed on patient‐derived fibroblasts. Results All patients had a generalized hyperkinetic movement disorder with chorea and dystonia, which occurred cyclically and during sleep. Complex I was found altered, whereas the other OXPHOS enzymes and the mitochondria network seemed intact in one patient. Conclusions The movement disorder is a prominent feature of NACC1‐related disease.