Expanding the clinical and genetic spectrum of GLUL-related developmental and epileptic encephalopathy

The GLUL gene encodes glutamine synthetase (GS), which plays a crucial role in glutamine-glutamate homeostasis. Both loss-of-function and gain-of-function variants of GLUL are known to cause genetic disorders in humans. Biallelic loss-of-function variants cause congenital glutamine deficiency, leading to developmental and epileptic encephalopathy (DEE) in an autosomal recessive manner. In contrast, certain variants of GLUL that lead to the loss of the N-terminal degron exert a gain-of-function effect, causing an autosomal dominant DEE. Only six autosomal recessive cases and ten autosomal dominant cases have been reported to date, and knowledge about GLUL-related DEE remains limited. In this study, we identified three unrelated patients with DEE carrying heterozygous de novo GLUL variants. One patient carried a variant that had been reported previously in two patients (c.-13-2A > G), and the other two patients carried novel candidate variants (c.-13-1G > C and c.604T > C). An alternative splicing event causing loss of the N-terminal degron of GS was confirmed by RNA sequencing in a patient carrying c.-13-1G > C variant. A comparison of our patients with previously reported cases revealed common symptoms, including epilepsy and global developmental delay. However, our patients exhibited additional phenotypes, such as hypertonia, cerebral atrophy, and T2 hyperintensity in deep grey matter, which have not been described in patients with autosomal dominant GLUL-related DEE. The seizure patterns and responses to antiseizure medications varied among patients, reflecting their diverse phenotypic spectrum. Similarly, biochemical analyses of plasma and cerebrospinal fluid showed heterogeneous profiles. We present analyses of the GLUL-related DEE with detailed clinical descriptions and identified novel causal variants. Comparative analysis of genotypes and phenotypes revealed the diverse nature of the disease, expanding our knowledge about the genetic and clinical spectrum of GLUL-related DEE.