Delineation of a Novel Mirror Syndrome: NFIC Variants Cause Syndromic Intellectual Disability With Macrocephaly.

Nuclear factor I (NFI) transcription factors regulate neural stem and progenitor differentiation during brain development. While NFIA, NFIB, and NFIX are linked to neurodevelopmental disorders, the role of NFIC (MIM: 600729) in human disease remains unclear. This study aimed to determine whether NFIC contributes to a neurodevelopmental syndrome, define its phenotype, and assess dosage-dependent effects. We established the first cohort of 11 individuals, including NFIC deletions and single nucleotide variants. Genotype-phenotype correlations, including critical region mapping, were performed. Murine data and bioinformatics were integrated to explore underlying pathomechanisms. We report 11 individuals with NFIC variants, including four with de novo SNVs and seven with deletions encompassing the gene, of whom nine have not been previously reported. A core phenotype of syndromic intellectual disability and macrocephaly was delineated. Opposing cranial phenotypes relative to proximal 19p13.3 duplication cases support a dosage-sensitive effect and a mirror-syndrome model. NFIC-related disorder represents a novel neurodevelopmental syndrome characterized by intellectual disability and macrocephaly, highlighting the importance of NFIC dosage supporting a mirror-syndrome model.