Identification of a Loss-of-Function Mutation in the Context of Glutaminase Deficiency and Neonatal Epileptic Encephalopathy

Abstract

IMPORTANCE The identification and understanding of the monogenic causes of neurodevelopmental disorders are of high importance for personalized treatment and genetic counseling. OBJECTIVE To identify and characterize novel genes for a specific neurodevelopmental disorder characterized by refractory seizures, respiratory failure, brain abnormalities, and death in the neonatal period; describe the outcome of glutaminase deficiency in humans; and understand the underlying pathological mechanisms. DESIGN, SETTING, AND PARTICIPANTS We performed exome sequencing of cases of neurodevelopmental disorders without a clear genetic diagnosis, followed by genetic and bioinformatic evaluation of candidate variants and genes. Establishing pathogenicity of the variants was achieved by measuring metabolites in dried blood spots by a hydrophilic interaction liquid chromatography method coupled with tandem mass spectrometry. The participants are 2 families with a total of 4 children who each had lethal, therapy-refractory early neonatal seizures with status epilepticus and suppression bursts, respiratory insufficiency, simplified gyral structures, diffuse volume loss of the brain, and cerebral edema. Data analysis occurred from October 2017 to June 2018. MAIN OUTCOMES AND MEASURES Early neonatal epileptic encephalopathy with glutaminase deficiency and lethal outcome. RESULTS A total of 4 infants from 2 unrelated families, each of whom died less than 40 days after birth, were included. We identified a homozygous frameshift variant p.(Asp232Glufs*2) in GLS in the first family, as well as compound heterozygous variants p.(Gln81*) and p.(Arg272Lys) in GLS in the second family. The GLS gene encodes glutaminase (Enzyme Commission 3.5.1.2), which plays a major role in the conversion of glutamine into glutamate, the main excitatory neurotransmitter of the central nervous system. All 3 variants probably lead to a loss of function and thus glutaminase deficiency. Indeed, glutamine was increased in affected children (available z scores, 3.2 and 11.7). We theorize that the potential reduction of glutamate and the excess of glutamine were a probable cause of the described physiological and structural abnormalities of the central nervous system. CONCLUSIONS AND RELEVANCE We identified a novel autosomal recessive neurometabolic disorder of loss of function of glutaminase that leads to lethal early neonatal encephalopathy. This inborn error of metabolism underlines the importance of GLS for appropriate glutamine homeostasis and respiratory regulation, signal transduction, and survival.

GLS hyperactivity causes glutamate excess, infantile cataract and profound developmental delay

“…In line, a disturbed equilibrium of glutamate and glutamine was described in patients with GS deficiency, clinically resulting in glutamine deficiency, neonatal epilepsy and early death (7). Recently, GLS loss of function has been described to cause lethal epileptic encephalopathy and glutamine excess in two families (8). The description of patients with spastic ataxia and optic atrophy harbouring bi-allelic hypomorphic variants in GLS, suggests a phenotypic spectrum -presumably depending on the degree of residual activity-that is yet to be uncovered (9).…”

“…The first description was of a late childhood onset disease, including optic atrophy and spastic ataxia (9). Recently, bi-allelic loss of function variants in GLS were described to cause lethal, neonatal onset encephalopathy characterized by respiratory failure, status epilepticus and early death within weeks after birth (8). These patients had simplified gyral patterns and showed destructions of initially normal appearing brain structures.…”

Altered Glutaminase 1 Activity During Neurulation and Its Potential Implications in Neural Tube Defects

“…However, the most likely alternative is through the immediate product of catalytic activity: glutamate. On one hand, glutamate is transformed into a-ketoglutarate, a TCA intermediary, regulating energy metabolism and promoting cell proliferation, an essential event in neurulation (Lukey et al, 2016;Rumping et al, 2019). Moreover, it was recently discovered that glutamate plays an important role in neurulation, acting through NMDA receptors (Sequerra et al, 2018).…”

Inborn errors of enzymes in glutamate metabolism

“…Awareness of this phenotypic spectrum is needed for recognition and diagnosis of these patients. 22,24,25 Glutamate and glutamine concentrations are most aberrant in defects of the two enzymes directly interconverting these amino acids: GLS and GS. Interestingly, in GLS hyperactivity, glutamate and glutamine levels are normal in plasma and CSF, while both are deviant when analysed using brain MRS and in urine in line with tissue specific expression of GLS.…”

“…In two families, patients clinically presented with neonatal respiratory dysfunction and fatal, refractory status epilepticus. 22 MRI of the brain showed extensive cerebral oedema and white matter involvement. The GLS variants were discovered by post-mortem exome sequencing.…”