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The KCNMA1 gene encodes the α-subunit of the large conductance, voltage, and calcium-sensitive potassium channel (BK channels) that plays a critical role in neuronal excitability. Heterozygous mutations in KCNMA1 were first illustrated in a large family with generalized epilepsy and paroxysmal nonkinesigenic dyskinesia. Recent research has established homozygous KCNMA1 mutations accountable for the phenotype of cerebellar atrophy, developmental delay, and seizures.

Here, we report the case of a patient with a novel homozygous truncating mutation in KCNMA1 (p.Arg458Ter) presenting with both the loss- and gain-of-function phenotype with paroxysmal dyskinesia, epilepsy, intellectual delay, and corticospinal– cerebellar tract atrophy.

This report extends the KNCMA1 mutation phenotype with a patient who carries a novel frameshift variant, presenting with both the gain- and loss-of-function phenotypes along with spinal tract involvement as a novel characteristic