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NMDA receptors play crucial roles in excitatory synaptic transmission. Rare variants in genes that encode receptor subunits are associated with several intractable neurodevelopmental disorders, including developmental and epileptic encephalopathy (DEE). To extend understanding of these intractable childhood diseases, we have begun to model these variants in laboratory mice.
Models studied so far are a constitutive knockin for GRIN2A variant p.Ser644Gly (S644G) based on one case, and a GRIN2D variant, p.Val667Ile (V667I), noted in at least three unrelated cases. Homozygous and heterozygous Grin2a S644G mice exhibit altered hippocampal morphology at 2 weeks, and homozygotes exhibit lethal tonic-clonic seizures in week 3. Heterozygotes have a normal lifespan without spontaneous seizures, but display a variety of distinct features including
resistance to electrically induced limbic seizures, as well as hyperactivity and repetitive and reduced anxiety behaviors. Multielectrode recordings of mutant neuronal networks reveal hyperexcitability and altered bursting and synchronicity of both mutant genotypes. When expressed in heterologous cells, mutant receptors exhibit enhanced NMDAR agonist potency and slow deactivation following rapid removal of glutamate, as occurs at synapses. Consistent with this, NMDAR-mediated synaptic currents in hippocampal slices from mutant mice show a prolonged deactivation time course. Standard antiepileptic drug monotherapy was ineffective in the patient, but combined treatment of NMDAR antagonists with antiepileptic drugs substantially reduced the seizure burden albeit without appreciable developmental improvement. Chronic treatment of homozygous mutant mouse pups with NMDAR antagonists delayed the onset of lethal seizures but did not prevent them.
Grin2d V667I knockin mice are at an earlier stage of study. In contrast to Grin2a S644G, V667I heterozygous mice are severely impaired and suffer from lethal tonic-clonic seizures with an onset from about 18 days to 3 months. The impairment precludes natural mating such that the line needs to be propagated by ovary transplantation. Young heterozygotes adults in video-EEG also exhibit very frequent interictal epileptiform spiking and spike-wave discharge activity, resembling absence seizures. Preliminary histology shows significant pyknotic nuclei appearing to evidence
cell death in the cerebral cortex. Pup developmental milestones, while not evidencing significant developmental delay, show unusual features including excessive maternal separation induced pup vocalization. Further studies are ongoing. Together these efforts illustrate the power of modelling severe neurodevelopmental seizure disorders using multiple experimental modalities and suggest their utility in identifying and evaluating new therapies.