A synonymous SLC2A1 variant causes familial epilepsy and paroxysmal exercise-induced dyskinesia by creating aberrant mosaic splicing patterns

The glucose transporter type-1 deficiency syndrome (GLUT1-DS) arises from variants in the SLC2A1 gene encoding the glucose transporter type-1 (GLUT1). Genetic analysis of a GLUT1-DS family identified a recurrent heterozygous synonymous SLC2A1 variant, adjacent to a 5' donor splice site (NG₀08232. 1 (NM₀06516. 4): c. 972G > A, NP₀06507. 2: p. Ser324=). The splice site proximity and family segregation analysis warranted an investigation into SLC2A1 mRNA splicing. The same genotype has been published in two further GLUT1-DS multiplex families without functional biology validation. The proband exhibited juvenile onset focal epilepsy and paroxysmal exercise induced dyskinesia and family members underwent multiplex segregation analysis for c. 972G > A. Family members exhibited phenotypes including focal epilepsy, intellectual disability, early-onset absence epilepsy and paroxysmal exercise-induced dyskinesia. In silico and in vitro minigene analysis assessed the effect of c. 972G > A on SLC2A1 mRNA splicing. Segregation analysis revealed the synonymous variant associates with more severe epilepsy and GLUT1-DS phenotypes in the family. In silico analysis predicted a disruption of the 5' donor splice site of intron 7. In vitro minigene assays demonstrated the activation of two cryptic donor splice sites, generating three transcripts: WT, and two aberrantly spliced isoforms causing 4 bp and 32 bp deletions. This was previously undetectable by whole blood RNA analysis. Our report demonstrates the synonymous c. 972G > A, p. Ser324 = variant causes a leaky mosaic SLC2A1 splicing aberrancy, affecting ~40% of transcripts, with ~60% remaining WT spliced. These GLUT1 frameshift deletions result in a variable GLUT1 haploinsufficiency and phenotypic heterogeneity in 3 GLUT1-DS families and highlights the clinical importance of synonymous variants.