Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: GLUL
Cytogenetic location: 1q25.3 Genomic coordinates (GRCh38): 1:182,378,098-182,391,790 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 1q25.3 | Developmental and epileptic encephalopathy 116 | 620806 | 3 | |
| Glutamine deficiency, congenital | 610015 | Autosomal recessive | 3 |
Glutamine is a main source of energy and is involved in cell proliferation, inhibition of apoptosis, and cell signaling (Haberle et al., 2005). Fetal glutamine requirements are very high and depend largely on active glutamine synthesis and the release of glutamine into the fetal circulation by the placenta. Glutamine synthetase (EC 6.3.1.2), also called glutamate-ammonia ligase (GLUL), is expressed throughout the body and plays an important role in controlling body pH and in removing ammonia from the circulation. The enzyme clears L-glutamate, the major neurotransmitter in the central nervous system, from neuronal synapses (see references in Clancy et al., 1996).
Gibbs et al. (1987) reported the complete 1,119-bp coding sequence of glutamine synthetase, which they determined from a liver-derived cDNA.
Haberle et al. (2005) found that the glutamine synthetase mRNA consists of 1,122 basepairs encoding a 374-amino acid protein with an estimated molecular mass of 42 kD.
Haberle et al. (2005) determined that the GLUL gene consists of 6 exons.
Pesole et al. (1991) suggested that glutamine synthetase is a good molecular clock for determining times of divergence even as great as that which occurred between eukaryotes and prokaryotes. One conclusion reached by Pesole et al. (1991) was that organelle-specific enzymes, such as those of the mitochondria, may have originated from a duplication of nuclear genes. The endosymbiotic hypothesis suggests that a transfer of prokaryotic genes to nuclei occurred during the evolution of the primitive eukaryotic cell. In some cases, it is likely that the old prokaryotic gene could not be active in the new nuclear genome environment and was totally lost because its function in the organelle could be dispensed with. Subsequently, a new organelle-specific enzyme could have originated to serve specialized metabolic functions. The presence of glutamine synthetase in mitochondria is linked to the nitrogen metabolism of the species, and in particular to the need for glutamine as a source of ammonia and for particular biochemical pathways for ammonia detoxification.
Gunnersen and Haley (1992) found that the 42-kD ATP-binding protein present in the cerebrospinal fluid of Alzheimer disease (AD; 104300) patients is glutamine synthetase. It was detected in 38 of 39 AD CSF samples and in only 1 of 44 control samples. In brain, glutamine synthetase plays a key role in elimination of free ammonia and also converts the neurotransmitter and excitotoxic amino acid glutamate to glutamine, which is not neurotoxic.
In mice, Eelen et al. (2018) demonstrated that genetic deletion of Glul in endothelial cells impairs vessel sprouting during vascular development, whereas pharmacologic blockade of glutamine synthetase suppresses angiogenesis in ocular and inflammatory skin disease while only minimally affecting healthy adult quiescent endothelial cells. This relies on the inhibition of endothelial cell migration but not proliferation. Mechanistically, Eelen et al. (2018) showed that in human umbilical vein endothelial cells, GLUL knockdown reduces membrane localization and activation of the GTPase RHOJ (607653) while activating other Rho GTPases and Rho kinase (see ROCK1, 601702), thereby inducing actin stress fibers and impeding endothelial cell motility. Inhibition of Rho kinase rescues the defect in endothelial cell migration that is induced by GLUL knockdown. Notably, glutamine synthetase palmitoylates itself and interacts with RHOJ to sustain RHOJ palmitoylation, membrane localization, and activation. Eelen et al. (2018) concluded that, in addition to the known formation of glutamine, the enzyme glutamine synthetase shows activity in endothelial cell migration during pathologic angiogenesis through RHOJ palmitoylation.
Clancy et al. (1996) localized the GLUL gene to chromosome 1 by PCR analysis of a human/rodent somatic cell hybrid panel. They also localized a pseudogene to chromosome 9. Further localization of the functional gene to 1q23 was accomplished by fluorescence in situ hybridization. The glutamine synthetase gene was mapped to 5 CEPH mega-YACs between the polymorphic PCR markers D1S117 and D1S466 by analysis of the Whitehead Institute chromosome 1 contig map. By fluorescence in situ hybridization, Helou et al. (1997) placed the GLUL gene at 1q31.
Wang et al. (1996) screened a bacterial artificial chromosome (BAC) library with a GLUL probe and isolated 18 clones. Southern blotting of human genomic DNA revealed that all bands could be accounted for by 5 loci, suggesting that humans have a family of 5 glutamine synthetase genes. Wang et al. (1996) used fluorescence in situ hybridization to map the GLUL gene to chromosome 1q25 and mapped a GLUL processed pseudogene (GLULP) to chromosome 9p13. Three related genes were also named and mapped: GLULL1 to 5q33, GLULL2 to 11p15, and GLULL3 to 11q24. Because of the small size of the 3 related genes, Wang et al. (1996) thought they may be intronless pseudogenes.
Congenital Glutamine Deficiency
Haberle et al. (2005) described 2 unrelated newborns with congenital glutamine synthetase deficiency (610015) with severe brain malformations resulting in multiorgan failure and neonatal death. Glutamine was largely absent from their serum, urine, and cerebrospinal fluid. Each infant had a homozygous mutation in the glutamine synthetase gene, R324C (138290.0001) and R341C (138290.0002). Studies that used immortalized lymphocytes expressing R324C glutamine synthetase and COS-7 cells expressing R341C glutamine synthetase suggested that these mutations are associated with reduced glutamine synthetase activity.
Kolker et al. (2006) suggested that the chronic depletion of neuronal glutamate stores described by Haberle et al. (2005) is a result of the defective glutamine-glutamate cycle. Haberle et al. (2006) responded that because the low glutamate concentrations in cerebrospinal fluid were established with the use of chromatography, which is not sufficiently sensitive to define glutamate concentrations accurately in the very low range, it was not possible to include a deficiency of glutamate in cerebrospinal fluid as one of the features of glutamine synthetase deficiency.
Rose and Jalan (2006) suggested the possibility that inhibition of glutamine synthetase in the brain may have lead to increased levels of ammonia and extracellular glutamate in the brain, partly explaining the clinical observations of Haberle et al. (2005). Haberle et al. (2006) noted that during the course of the disease, plasma ammonia levels were normal, and therefore hyperammonemia could not be regarded as a sign of glutamine synthetase deficiency. Hyperammonemia would also not explain the severe malformation of the brain and extracerebral manifestations.
In a Sudanese boy with congenital glutamine deficiency, Haberle et al. (2011) identified a homozygous mutation in the GLUL gene (R324S; 138290.0003). The patient was alive at age 3 years but had severe epileptic encephalopathy and severe psychomotor retardation.
Developmental and Epileptic Encephalopathy 116
Jones et al. (2024) identified 6 different heterozygous mutations in the GLUL1 gene (138290.0004-138290.0009) in 9 unrelated patients with developmental and epileptic encephalopathy-16 (DEE116; 620806). All of the mutations disrupted the canonical methionine start site either directly or by causing alternative splicing in the untranslated region of GLUL. All of the mutations were identified by whole-exome sequencing and all were shown to occur de novo, except for patient 5 whose parents were not tested. Western blot analysis against the glutamine synthetase (GS) protein in fibroblast cells from patients 1 and 6 demonstrated 2 bands, an expected full-length band and a smaller band corresponding to GS translated from a downstream alternative methionine start site. Expression of GLUL constructs with different potential downstream methionine start sites resolved that met18 of the GS was the alternative start site. This new methionine start site excludes the lys11 and lys13 degron residues, which are acetylated in high glutamine conditions. Jones et al. (2024) therefore tested stability of full-length GS and GS without the first 18 residues (GS-met18) when expressed in HEK293 cells in low- and high-glutamine culture conditions. Full-length GS had appropriate reduction in protein expression under high glutamine conditions, whereas GS-met18 did not, indicating loss of glutamine feedback in the mutant GS.
In an infant with congenital glutamine deficiency (610015), the offspring of consanguineous Turkish parents, Haberle et al. (2005) found a homozygous C-to-T transition at nucleotide 970 of the GLUL gene that resulted in an arg324-to-cys (R324C) substitution in glutamine synthetase. The patient was resuscitated at birth and found to be neurologically compromised, with marked flaccidity and cardiac insufficiency. He died at 2 days of age. On postmortem examination the brain weighed only 202 g (335 g expected for gestational age), but no visceral malformations were evident. Another mutation in this codon (R324S; 138290.0003) was found in a Sudanese patient with the disorder.
In an infant with congenital glutamine deficiency (610015), the daughter of consanguineous Turkish parents, Haberle et al. (2005) found homozygosity for a C-to-T transition at nucleotide 1021 of the GLUL gene that caused an arg341-to-cys (R341C) mutation in glutamine synthetase. On the first day of life, convulsions and respiratory failure required intubation and ventilation. During the first weeks of life, the infant had voluminous yellowish stools and progressive weight loss, despite enteral feeding. After 2 weeks, a generalized blistering erythematous rash developed that on histologic examination supported a diagnosis of epidermal necrolysis. Brain MRI showed markedly attenuated gyri and subependymal cysts. She died during the fourth week of life from multiorgan failure. The only material available from the patient postmortem was her DNA, since her skin fibroblasts failed to grow.
In a Sudanese boy, born of consanguineous parents, with congenital glutamine deficiency (610015), Haberle et al. (2011) identified a homozygous mutation in the GLUL gene, resulting in an arg324-to-ser (R324S) substitution in a conserved residue in the active ATP-binding site. The mutation resulted in upregulation of GLUL protein expression in patient fibroblasts, but increased expression could not compensate for the functional deficits of the mutant enzyme. Another mutation in this codon (R324C; 138290.0001) was found in a Turkish patient with the disorder. The patient reported by Haberle et al. (2011) was alive at age 3 years but had severe epileptic encephalopathy and severe psychomotor retardation.
In a patient (patient 1) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.3G-A mutation (c.3G-A, NM_001033044.4) in the GLUL gene, leading to a met1-to-? start loss. The mutation, which was identified by trio whole-exome sequencing, was not present in the gnomAD database (v3.1.2). Western blot analysis against the glutamine synthase (GS) protein in fibroblast cells from the patient demonstrated 2 bands, an expected full-length band and a smaller band corresponding to GS translated from a downstream alternative methionine start site.
In a patient (patient 2) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.1A-T mutation (c.1A-T, NM_001033044.4) in the GLUL gene, resulting in a met1-to-? start loss. The mutation, which was identified by trio whole-exome sequencing, was not present in the gnomAD database (v3.1.2).
In a patient (patient 3) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.1A-C mutation in the GLUL gene (c.1A-C, NM_001033044.4), resulting in a met1-to-? start loss. The mutation, which was identified by trio whole-exome sequencing, was not present in the gnomAD database (v3.1.2).
In a 4 patients (patients 4, 5, 6, and 9) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.1A-G mutation in the GLUL gene (c.1A-G, NM_001033044.4), resulting in a met1-to-? start loss. The mutation, which was identified by trio whole-exome sequencing, was not present in the gnomAD database. Western blot analysis against the glutamine synthase (GS) protein in fibroblast cells from patient 6 demonstrated 2 bands, an expected full-length band and a smaller band corresponding to GS translated from a downstream alternative methionine start site.
In a patient (patient 7) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.-13-1G-A intronic transition (c.-13-1G-A, NM_001033044.4), located upstream of a GLUL 5-prime splice site. The mutation was identified by trio whole-exome sequencing. The mutation was not present in the gnomAD database (v3.1.2).
In a patient (patient 8) with developmental and epileptic encephalopathy-116 (DEE116; 620806), Jones et al. (2024) identified a de novo heterozygous c.-13-1G-A intronic transition (c.-13-1G-A, NM_001033044.4) in the GLUL gene. The mutation, which was identified by trio whole-exome sequencing, was not present in the gnomAD database (v3.1.2). cDNA analysis in fibroblasts from the patient showed that the mutation resulted in alternative splicing, which excluded the first 13 bases of exon 1, including the canonical initiation codon.
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Eelen, G., Dubois, C., Cantelmo, A. R., Goveia, J., Bruning, U., DeRan, M., Jarugumilli, G., van Rijssel, J., Saladino, G., Comitani, F., Zecchin, A., Rocha, S., and 27 others. Role of glutamine synthetase in angiogenesis beyond glutamine synthesis. Nature 561: 63-69, 2018. [PubMed: 30158707] [Full Text: https://doi.org/10.1038/s41586-018-0466-7]
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Jones, A. G., Aquilino, M., Tinker, R. J., Duncan, L., Jenkins, Z., Carvill, G. L., DeWard, S. J., Grange, D. K., Hajianpour, M. J., Halliday, B. J., Holder-Espinasse, M., Horvath, J., and 16 others. Clustered de novo start-loss variants in GLUL result in a developmental and epileptic encephalopathy via stabilization of glutamine synthetase. Am. J. Hum. Genet. 111: 729-741, 2024. [PubMed: 38579670] [Full Text: https://doi.org/10.1016/j.ajhg.2024.03.005]
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