Disruption of the ATXN1–CIC complex causes a spectrum of neurobehavioral phenotypes in mice and humans

Huda Zoghbi and colleagues report that loss of the ATXN1–CIC protein complex in the developing mouse forebrain results in hyperactivity and defects in learning and memory. Loss of Cic in specific brain regions causes social interaction defects, and patients with de novo CIC mutations present signs of hyperactivity, autism spectrum disorder and intellectual disability. Gain-of-function mutations in some genes underlie neurodegenerative conditions, whereas loss-of-function mutations in the same genes have distinct phenotypes. This appears to be the case with the protein ataxin 1 (ATXN1), which forms a transcriptional repressor complex with capicua (CIC). Gain of function of the complex leads to neurodegeneration, but ATXN1–CIC is also essential for survival. We set out to understand the functions of the ATXN1–CIC complex in the developing forebrain and found that losing this complex results in hyperactivity, impaired learning and memory, and abnormal maturation and maintenance of upper-layer cortical neurons. We also found that CIC activity in the hypothalamus and medial amygdala modulates social interactions. Informed by these neurobehavioral features in mouse mutants, we identified five individuals with de novo heterozygous truncating mutations in CIC who share similar clinical features, including intellectual disability, attention deficit/hyperactivity disorder (ADHD), and autism spectrum disorder. Our study demonstrates that loss of ATXN1–CIC complexes causes a spectrum of neurobehavioral phenotypes.