Scientists recently demonstrated that rare inherited mismatches in SLC32A1, a gene encoding the vesicular gamma-aminobutyric acid transporter, can cause genetic epilepsies with febrile convulsions. Now the question was whether these miscoded genes could also be responsible for epileptic encephalopathy.
Members of the solute carrier ( SLC ) protein family transport a variety of substrates, with amino acids being the most common. SCL32A1 has been shown to transport gamma-aminobutyric acid (GABA) and glycine into synaptic vesicles (SVs). Therefore, it is also called vesicular GABA transporter (VGAT) or vesicular inhibitory amino acid transporter (VIAAT). GABA is an important inhibitory neurotransmitter that is critical for proper functioning of the mature brain. Together with glycine, GABA also acts as an inhibitory neurotransmitter in the spinal cord and brainstem. Because disruption of GABA-ergic neurotransmission is an established cause of epilepsy and neurodevelopmental disorders, impaired VGAT function is a plausible cause of developmental and epileptic encephalopathy (DEE) [1]. Recently, rare heterozygous missense variants in SLC32A1 have been described that have been isolated in families with the phenotype of genetic epilepsy with febrile seizures plus (GEFS+) and idiopathic generalized epilepsy [2]. Affected family members exhibited a wide range of seizure types – for example, febrile seizures, focal seizures, generalized seizures, and unclassified seizures, but no developmental delay (DD) or intellectual disability (ID). A study has now presented the causality of the variants by in silico modeling and their functional assessment in a neuronal cell culture model [3].
Four patient cases under the microscope
Using exome sequencing, four individuals with developmental and epileptic encephalopathy and de novo missense variants in SLC32A1 were identified. To assess causality, functional evaluation of the identified variants was also performed in a mouse neuronal cell culture model. The results showed global developmental delay and moderate to severe intellectual disability in all four patients. Seizures began at 4 months and 1 month of age in subjects 1 and 4, respectively, and in early infancy in subjects 2 and 3 (at 15 months and 18 months of age, respectively). Individuals 1 and 4 initially exhibited focal and focal impaired consciousness, whereas generalized tonic-clonic seizures predominated in individuals 2 and 3. In addition, the subject showed 2 myoclonic seizures and status epilepticus with suspected myoclonic atonic epilepsy. She also showed focal seizures with impaired consciousness. Despite the initial focal seizure semiology of Individual 4, this individual later developed generalized epilepsy with generalized atonic, myoclonic, and generalized tonic-clonic seizures, as well as eyelid myoclonia.
During the course of the epilepsy, all four subjects experienced at least some long-lasting periods of seizure freedom. During periods when seizures intensified, all individuals had periods of developmental slowing and also regression. All four de novo variants in SLC32A1 identified in this study included missense alterations.
Mouse model should provide information
To investigate the effects of the four VGAT variants on inhibitory neurotransmission, lentiviral constructs of each variant were expressed in mouse striatal GABAergic neurons in microisland cultures. Single neurons growing on astrocyte microislands form synapses only with themselves. Electrophysiological recordings from such isolated autaptic neurons allow detailed analysis of neurotransmission of individual neurons under defined conditions and are ideally suited for structure-function analyses of synaptic proteins. Human and mouse VGAT has 98.5% identity at the amino acid level.
In silico modeling and functional analyses showed that three of the variants located in helices lead to decreased quantum size, which is associated with impaired filling of synaptic vesicles with GABA. The fourth variant, located in the N-terminus of vesicular gamma-aminobutyric acid, has no effect on quantum size but increases presynaptic release probability, resulting in greater synaptic depression upon high-frequency stimulation.
Thus, variants of vesicular gamma-aminobutyric acid may affect GABAergic neurotransmission by at least two mechanisms-that of synaptic vesicle filling and by altering synaptic short-term plasticity. Thus, de novo missense variants in SLC32A1 were detected as a cause of developmental and epileptic encephalopathy.
Literature:
- Maljevic S, Møller RS, Reid CA, et al: Spectrum of GABAA receptor variants in epilepsy. Curr Opin Neurol 2019; 32: 183-190.
- Heron SE, Regan BM, Harris RV, et al: Association of SLC32A1 missense variants with genetic epilepsy with febrile seizures plus. Neurology 2021; 96: e2251-e2260.
- Platzer K, Sticht H, Bupp C, et al: De Novo Missense Variants in SLC32A1 Cause a Developmental and Epileptic Encephalopathy Due to Impaired GABAergic Neurotransmission. Ann Neurol 2022; 92: 958-973.
InFo NEUROLOGY & PSYCHIATRY 2023; 21(2): 32.
