De Novo Mutations in SLC35A2 Encoding a UDP‐Galactose Transporter Cause Early‐Onset Epileptic Encephalopathy

Early‐onset epileptic encephalopathies (EOEE) are severe neurological disorders characterized by frequent seizures accompanied by developmental regression or retardation. Whole‐exome sequencing of 12 patients together with five pairs of parents and subsequent Sanger sequencing in additional 328 EOEE patients identified two de novo frameshift and one missense mutations in SLC35A2 at Xp11.23, respectively. The three patients are all females. X‐inactivation analysis of blood leukocyte DNA and mRNA analysis using lymphoblastoid cells derived from two patients with a frameshift mutation indicated that only the wild‐type SLC35A2 allele was expressed in these cell types, at least in part likely as a consequence of skewed X‐inactivation. SLC35A2 encodes a UDP‐galactose transporter (UGT), which selectively supplies UDP‐galactose from the cytosol to the Golgi lumen. Transient expression experiments revealed that the missense mutant protein was correctly localized in the Golgi apparatus. In contrast, the two frameshift mutant proteins were not properly expressed, suggesting that their function is severely impaired. Defects in the UGT can cause congenital disorders of glycosylation. Of note, no abnormalities of glycosylation were observed in three serum glycoproteins, which is consistent with favorably skewed X‐inactivation. We hypothesize that a substantial number of neurons might express the mutant SLC35A2 allele and suffer from defective galactosylation, resulting in EOEE.