Somatic variants in SLC35A2 leading to defects in N-glycosylation in mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy (MOGHE).

Mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy (MOGHE) is a new histopathological entity identified in the surgically resected brain tissue of patients with drug-resistant epilepsy. Somatic variants in SLC35A2 have been increasingly identified in MOGHE brain resections. SLC35A2 protein transports uridine 5'-diphosphogalactose (UDP-Gal) into the Golgi lumen, playing a crucial role in the process of N-glycosylation. Currently, research on the pathogenic mechanism of SLC35A2 variants in MOGHE is limited. Here we conducted genetic testing on brain samples and paired blood samples from 28 pediatric patients pathologically diagnosed with MOGHE. We performed an in-depth functional analysis of somatic variants identified in SLC35A2, integrating glycan labeling and intact glycopeptide profiling to assess N-glycosylation defects. With whole-exome sequencing and validation with ultra-deep amplicon sequencing, we identified 101 potentially pathogenic somatic variants (PPSVs) across 87 genes. Nine PPSVs in SLC35A2 were found in 10 samples. The 9 identified variants of SLC35A2, characterized by various mutation types (4 frameshift, 3 missense and 2 nonsense variants), were all confirmed to be loss-of-function via altered glycan chains. Intact glycopeptide analysis at the cellular level indicated an increase in truncated N-glycan glycoforms. Analysis of brain tissue revealed N-glycosylated proteins and glycosites modified with agalactosylated glycoforms, and glycoproteins bearing agalactosylated N-glycans were significantly enriched in cell adhesion and axon guidance-related pathways. Additionally, chemoenzymatic glycan labeling in lesions demonstrated N-glycan damage of heterotopic neurons, suggesting a potential diagnostic approach for MOGHE. Our findings provide a comprehensive somatic landscape of MOGHE and a rich resource of somatic SLC35A2 variant-related glycoform and glycoprotein abnormalities, thereby unveiling valuable insights into compromised N-glycosylation and MOGHE formation.