Heterozygous, pathogenic CUX1 variants are associated with global developmental delay or intellectual disability. This study delineates the clinical presentation in an extended cohort and investigates the molecular mechanism underlying the disorder in a Cux1+/− mouse model. Through international collaboration, we assembled the phenotypic and molecular information for 34 individuals (23 unpublished cases). A Cux1+/− mouse model was used to analyze CUX1 expression in the brain and evaluate susceptibility to epilepsy. We describe 34 patients with 26 different null and four missense variants. The leading symptoms were mild to moderate delayed speech and motor development, and intellectual disability. In Cux1+/− mice, we found delayed growth, histologically normal brains, and increased susceptibility to seizures. In Cux1+/− brains, the expression of Cux1 transcripts was half of WT animals. Expression of CUX1 proteins was also reduced, although in early postnatal animals significantly more than in adults. In summary, disease-causing CUX1 variants result in a non-syndromic phenotype of developmental delay and intellectual disability. In some patients, this phenotype ameliorates with age, resulting in a clinical catch-up and normal IQ in adulthood. Furthermore, the balance of CUX1 isoform expression in the brain during development appears to be important for this favorable clinical course.
