Haploinsufficiency of brain‐specific kinase BRSK1 causes epilepsy and neurodevelopmental disorders

Objective The BRSK1 gene encodes brain‐specific serine/threonine kinase 1 (also known as SAD‐B kinase), which is almost exclusively expressed in the brain and plays critical roles in neuronal polarization, neurotransmitter release, mitochondrial dynamics, and neuronal maturation. This study aimed to investigate the pathogenicity of BRSK1 variants in epilepsy and neurodevelopmental disorders. Methods Trio‐based exome sequencing was performed in 394 probands with epilepsy. Expression analyses and functional validation were conducted using a newly generated Brsk1 exon 4–7 knockout mouse line ( Brsk1 Δ4–7 ), with subsequent behavioral, electrophysiological, proteomic, and phosphoproteomic assessments. Results Six novel BRSK1 variants were identified in seven probands, including four single nucleotide variants (SNVs) and two indels; five were de novo, one inherited, and one recurrent. Frameshift or nonsense variants led to complete loss of detectable BRSK1 protein, whereas one missense variant reduced protein levels. Both heterozygous and homozygous Brsk1 Δ4–7 mice exhibit increased seizure susceptibility, neuronal hyperexcitability, and neurobehavioral impairments—recapitulating key clinical features associated with BRSK1 haploinsufficiency in humans. Proteomic and phosphoproteomic analyses revealed dysregulation of pathways critical for axonal development and synaptic function. Significance Our genomic and functional studies strongly support BRSK1 haploinsufficiency as a pathogenic mechanism in a human epilepsy syndrome. To our knowledge, this is the first study to implicate the brain‐specific serine/threonine kinase BRSK1 in epilepsy and neurodevelopmental disorders.