Compound heterozygous mutations in the <i>SSPOP</i> gene lead to epilepsy and developmental disorders

The SSPOP gene, currently classified as a pseudogene in the human genome, encodes the SCO-spondin protein, which plays an important role in human neurodevelopment, though its function remains poorly understood. In this study, we used trio-based whole exome sequencing to identify compound heterozygous SSPOP variants in four children from three unrelated families, including one pair of dizygotic twins. These children exhibited variable phenotypes, including variation in age of onset, seizure semiology, and response to antiseizure medications, along with neurodevelopmental disorders. We demonstrated that SSPOP is a functional gene by confirming its expression at both the transcriptional and protein levels. We analysed ten brain tissue samples from seven pediatric patients and brain organoids derived from human iPSCs to confirm its expression via qRT-PCR, immunofluorescence and Western blotting, supporting its biological function during both prenatal and postnatal stages of brain development. In addition, CRISPR-mediated sspo knockout zebrafish demonstrated abnormal neurodevelopment and epileptic discharges in vivo. Together, these findings suggest that SSPOP is a functional gene and a potential contributor to neurodevelopmental disorders and epilepsy.