Pathogenic variants in CASK: Expanding the genotype–phenotype correlations

Abstract Pathogenic variants in CASK , an X‐linked gene that plays a role in brain development and synaptic function, are the cause of both microcephaly with pontine and cerebellar hypoplasia (MICPCH), and X‐linked intellectual disability (XLID) with or without nystagmus. MICPCH is caused by loss of function variants in CASK , typically affects females, and is associated with moderate‐to‐severe intellectual disability (ID). Additional findings, present in about one‐third of individuals, include feeding difficulties, ophthalmologic issues, hypertonicity, epilepsy, and sensorineural hearing loss. Only a few affected males with MICPCH phenotype have been reported and most have had profound developmental disability and intractable epilepsy. The XLID phenotype is typically caused by missense variants and most often manifests in males; carrier females are mildly affected or unaffected. Nystagmus is often present. In total, over 175 patients have been reported in the literature. We now report an additional 11 patients with pathogenic variants in CASK that expand these phenotypes and reported genotype–phenotype correlations.