Altered closed state inactivation gating in Kv4.2 channels results in developmental and epileptic encephalopathies in human patients

Abstract

Kv4.2 subunits, encoded by KCND2, serve as the pore‐forming components of voltage‐gated, inactivating ISA K+ channels expressed in the brain. ISA channels inactivate without opening in response to subthreshold excitatory input, temporarily increasing neuronal excitability, the back propagation of action potentials, and Ca2+ influx into dendrites, thereby regulating mechanisms of spike timing‐dependent synaptic plasticity. As previously described, a de novo variant in Kv4.2, p.Val404Met, is associated with an infant‐onset developmental and epileptic encephalopathy in monozygotic twin boys. The p.Val404Met variant enhances inactivation directly from closed states, but dramatically impairs inactivation after channel opening. We now report the identification of a closely related, novel, de novo variant in Kv4.2, p.Val402Leu, in a boy with an early‐onset pharmacoresistant epilepsy that evolved to an epileptic aphasia syndrome (Continuous Spike Wave during Sleep Syndrome). Like p.Val404Met, the p.Val402Leu variant increases the rate of inactivation from closed states, but significantly slows inactivation after the pore opens. Although quantitatively the p.Val402Leu mutation alters channel kinetics less dramatically than p.Val404Met, our results strongly support the conclusion that p.Val402Leu and p.Val404Met cause the clinical features seen in the affected individuals and underscore the importance of closed state inactivation in ISA channels in normal brain development and function.

Etiological involvement of KCND1 variants in an X-linked neurodevelopmental disorder with variable expressivity

“… 39 , 40 This classification system utilizes the evidence code PS3 based on the results of “well-established” functional assays, which are suitable to discriminate between abnormal and normal gene/protein function. In the past, the highly reproducible and robust two-electrode voltage-clamp technique in Xenopus oocytes has been used successfully to assess the biophysical features of ion channels, including variants of KCND2 /Kv4.2 8 , 9 , 10 , 41 , 42 and KCND3 /Kv4.3, 43 , 44 , 45 as well as other voltage-gated potassium channels. 46 Therefore, we performed PS3 scoring of KCND1 variants on the basis of the results from this functional assay system.…”

“…In contrast, published heterozygous missense variants in KCND2 associated with a neurodevelopmental disease phenotype alter sites within the S4-S5 linker and the distal S6 segment, core regions of the protein known to play a pivotal role in the operation of the cytoplasmic gate, with a prominent impact on Kv4.2 channel gating. 8 , 9 , 10 , 11 This obvious difference may reflect natural selection due to the gonosomal and autosomal localization of KCND1 and KCND2 , respectively. 34 Putative KCND1 missense variants affecting the S4-S5 linker or the distal S6 segment may be selected against because homomeric variant channels may lead to embryonic lethality of males harboring these variants.…”

“…Regions highlighted in gray indicate the tetramerization (T1) domain and transmembrane segments S1–S6, as indicated below. Sequence motifs critical for channel assembly trafficking and function are indicated by bold letters (HX 5 CX 20 CC motif in the T1-domain: Zn 2+ coordination site; 6 positively charged residues in S4: voltage sensor; 7 potassium channel signature sequence GYGD in the pore loop [P]: selectivity filter; 5 critical dynamic coupling residues, glutamate [E] in the S4-S5 linker, and proline-valine [PV] in the distal S6 segment: operation of the cytoplasmic gate; 8 , 9 , 10 , 11 C-terminal di-leucine motif: dendritic targeting 12 ). Numbers on the right specify amino acid residue positions.…”

“… 26 In particular, a number of heterozygous missense variants in KCND2 (MIM: 605410) that alter Kv4.2 channel gating have been linked to early-onset global developmental delay of distinct severity, often in combination with seizures, muscular hypotonia, and/or visual impairment. 8 , 9 , 10 , 27 , 28 Also, a hemizygous nonsense variant of X-chromosomal KCND1 (MIM: 300281 ) has been suggested as a candidate pathogenic variant in a single individual with focal epilepsy, however, with no reference to Kv4.1 channel function. 29 …”

The role of dendritic spines in epileptogenesis

“…Ion channels are critical modulators of electrical activity and synaptic transmission. Kv4.2, a voltage gated potassium channel, has been implicated in epilepsy and seizure onset ( Gross et al, 2016 ; Hong et al, 2018 ; Bavan et al, 2022 ). In a mouse model with reduced expression of Kv4.2, there was notable decreased dendritic spine density in CA1 despite increased seizure susceptibility using models of epilepsy.…”