Gain-of-function variants in GABRD reveal a novel pathway for neurodevelopmental disorders and epilepsy

Abstract

A potential link between GABRD encoding the δ subunit of extrasynaptic GABAA receptors and neurodevelopmental disorders has largely been disregarded due to conflicting conclusions from early studies. However, we identified seven heterozygous missense GABRD variants in 10 patients with neurodevelopmental disorders and generalized epilepsy. One variant occurred in two sibs of healthy parents with presumed somatic mosaicism, another segregated with the disease in three affected family members, and the remaining five occurred de novo in sporadic patients. Electrophysiological measurements were used to determine the functional consequence of the seven missense δ subunit variants in receptor combinations of α1β3δ and α4β2δ GABAA receptors. This was accompanied by analysis of electro-clinical phenotypes of the affected individuals. We determined that five of the seven variants caused altered function of the resulting α1β3δ and α4β2δ GABAA receptors. Surprisingly, four of the five variants led to gain-of-function effects whereas one led to a loss-of-function effect. The stark differences between the gain-of-function and loss-of function effects were mirrored by the clinical phenotypes. Six patients with gain-of-function variants shared common phenotypes: neurodevelopmental disorders with generalized epilepsy, behavioral issues, and various degrees of intellectual disability. Six patients with gain-of-function variants shared common phenotypes: neurodevelopmental disorders with behavioral issues, various degrees of intellectual disability, generalized epilepsy with atypical absences and generalized myoclonic and/or bilateral tonic-clonic seizures. The EEG showed qualitative analogies among the different gain-of-function variant carriers consisting of focal slowing in the occipital regions often preceding irregular generalized epileptiform discharges, with frontal predominance. In contrast, the one patient carrying a loss-of-function variant had normal intelligence, no seizure history but has a diagnosis of autism spectrum disorder and suffering from elevated internalizing psychiatric symptoms. We hypothesize that increase in tonic GABA-evoked current levels mediated by δ-containing extrasynaptic GABAA receptors lead to abnormal neurotransmission, which represent a novel mechanism for severe neurodevelopmental disorders. In support of this, the electro-clinical findings for the gain-of-function GABRD variants resemble the phenotypic spectrum reported in patients with missense SLC6A1 (GABA uptake transporter) variants. This also indicates that the phenomenon of extrasynaptic receptor over-activity is observed in a broader range of patients with neurodevelopmental disorders, since SLC6A1 loss-of-function variants also lead to overactive extrasynaptic δ-containing GABAA receptors. These findings have implications when selecting potential treatment options, since a substantial portion of available anti-seizure medication act by enhancing GABAergic function either directly or indirectly, which could exacerbate symptoms in patients with gain-of-function GABRD variants.

Molecular and Clinical Repercussions of GABA Transporter 1 Variants Gone Amiss: Links to Epilepsy and Developmental Spectrum Disorders

“…Abundantly available extracellular GABA acts on extrasynaptic GABA A receptors in the thalamocortical region to induce absence seizure. The δ subunit of extracellular GABA A receptors is linked to aberrant tonic inhibition, and gain-of-function mutations in the GABRD gene encoding the δ subunit, mimic the phenotypic spectrum of patients harboring SLC6A1 disease mutations ( AhriNg et al, 2021 ). Furthermore, GABA B receptor agonists are known to induce absence seizures and can even facilitate the extrasynaptic GABA A receptor-mediated tonic inhibition ( Cope et al, 2009 ).…”

GABA Receptors Can Depolarize the Neuronal Membrane Potential via Quantum Tunneling of Chloride Ions: A Quantum Mathematical Study

“…This yields a quantum gradient that produces a net chloride efflux that leads to membrane depolarization, which has been observed during early neuronal development [ 2 , 3 , 4 ]. The gain-of-function mutations are well-established genetic conditions that affect ion channels such as ligand-gated channels and voltage-gated channels [ 33 , 34 ]. If these gain-of-function mutations decrease the gate energy of the closed GABA receptor, this will enhance the quantum tunneling of chloride ions to depolarize the membrane potential.…”

“…Interestingly, this quantum tunneling-induced membrane depolarization occurs even without GABA binding to the receptor because the drop of the gate energy had occurred. Therefore, these mutations may be responsible for generating outward anion current that depolarizes membrane potential, which explains the increased tendency of seizures in patients with such mutations [ 33 , 34 ]. Acquired conditions such as stroke and trauma are implicated as pathological events that affect the function of GABA receptors [ 5 , 6 ].…”

“…The gain-of-function mutations are well-established genetic conditions that affect ion channels such as ligand-gated channels and voltage-gated channels [ 33 , 34 ]. If these gain-of-function mutations decrease the gate energy of the closed GABA receptor, this will enhance the quantum tunneling of chloride ions to depolarize the membrane potential.…”

A de novo missense variant in GABRA4 alters receptor function in an epileptic and neurodevelopmental phenotype

“…α4‐subunit‐containing GABA A receptors contribute to tonic inhibition mediated by extrasynaptic receptors, which have been associated with seizures and seizure‐protective effects of neurosteroids 6 . In addition, GABRD , which encodes the extrasynaptic δ‐subunit, is considered as an epilepsy susceptibility gene 7,8 …”