Glucose transporter 1 (GLUT1) deficiency syndrome is caused by a deficit in glucose transport to the brain during the pre- and postnatal periods. Here, we report two cases of GLUT1 deficiency syndrome diagnosed on the basis of clinical features, reduced GLUT1 activities, and mutations in the GLUT1 gene. Patient 1 had a novel heterozygous 1bp insertion in exon 7 that resulted in a shift of the reading frame and the introduction of a premature stop codon at amino acid position 380. His clinical phenotype appeared to be more severe than that of Patient 2 who had a missense mutation in exon 8 resulting in an arginine-to-tryptophan substitution at amino acid position 333. Patient 1 had no meaningful words and could not walk unassisted, while Patient 2 could speak and walk unassisted. Both the patients developed seizures of various types that have been successfully treated with zonisamide. Although several antiepileptic drugs, including barbiturates, diazepam, chloralhydrate, and valproic acid, have been shown to inhibit GLUT1 function, the present study demonstrated no inhibitory effect of zonisamide on GLUT1-mediated glucose transport. Our data suggested that zonisamide might be preferable if add-on anticonvulsant therapy is required to control the seizures in patients with this disorder.

Background: Rett syndrome (RTT) is a neurodevelopmental disorder that predominantly affects girls, resulting from a loss‐of‐function variant in X‐linked MECP2. Here, we report a rare case of a girl with RTT with an X chromosome mosaic karyotype (46,XX/47,XXX).Methods: Fluorescent in situ hybridization (FISH) was carried out to confirm the mosaic karyotype. Sanger sequencing was carried out to genetically diagnose RTT. Furthermore, we assessed the X chromosome inactivation (XCI) pattern. MECP2 expression levels were examined via RT‐PCR.Results: The patient presented with preserved speech variant, the milder form of RTT. Genetic examination revealed a de novo, heterozygous, truncating variant of MECP2. FISH revealed mosaicism in the 47,XXX karyotype in 6% of her cells. The XCI assay revealed unbalanced inactivation with skewing in favor of the paternal X chromosome. MECP2 was downregulated to only 84% of the control, indicating that the patient's variant was probably of paternal origin. Unbalanced XCI in this patient might have contributed to the alleviation of the phenotype. However, her supernumerary X chromosome was derived from maternal X chromosome harboring the wild‐type allele and might have had no preferential effect on her RTT‐related phenotype.Conclusion: The present results indicate that phenotypic effects of X chromosome aneuploidy depend on the nature of the supernumerary X chromosome, the pattern of mosaicism, and XCI status.