ClinVar Genomic variation as it relates to human health

Based on the classification scheme VCGS_Germline_v1.1.1, this variant is classified as Pathogenic. Following criteria are met: 0101 - Gain-of-function is a known mechanism of disease for this gene (PMID 26784557). (N) 0107 - This gene is known to be associated with autosomal dominant disease. (N) 0200 - Variant is predicted to result in a missense amino acid change from a glycine to a serine (exon 11). (N) 0251 - Variant is heterozygous. (N) 0301 - Variant is absent from gnomAD. (P) 0501 - Missense variant consistently predicted to be damaging by multiple in silico tools and is very highly conserved with a minor amino acid change. (P) 0600 - Variant is located in an annotated domain or motif (helical region of the ion transport domain). (N) 0705 - No comparable variants have previous evidence for pathogenicity. (N) 0801 - Strong previous evidence of pathogenicity in unrelated individuals (ClinVar, PMID 26140313, PMID 26784557). (P) 0905 - No segregation evidence has been identified for this variant. (N) 1002 - Moderate functional evidence supporting abnormal protein function. Functional studies with this variant demonstrated gain of function effects on channel function (PMID 26784557). (P) 1205 - Variant is maternally inherited. (N) Legend: (P) - Pathogenic, (N) - Neutral, (B) - Benign

This mutation has been previously reported as disease-causing and was found twice in our laboratory de novo in individuals with infantile onset epileptic encephalopathy

KCNT1 NM_020822.2 exon 11 p.Gly288Ser (c.862G>A): This variant has been reported in the literature in multiple individuals affected with epilepsy including MMPSI, ADNFLE and hypomyelinating leukodystrophy, several of whom were reported to be de novo (Selected publications: Ishii 2013 PMID:24029708, Kim 2014 PMID:25482562, Arai-Ichinoi 2016 PMID:26597493, Rizzo 2016 PMID:26784557, Liu 2018 PMID:30185235, Routier 2019 PMID:31170314).This variant is not present in large control database but is present in ClinVar, with several labs classifying this variant as pathogenic (Variation ID:126421). Evolutionary conservation supports that this variant may impact the protein; computational predictive tools are unclear. Additionally, in vitro functional studies also suggest that this variant will impact the protein's activity (Kim 2014 PMID:25482562, Rizzo 2016 PMID:26784557). In summary, this variant is classified as pathogenic based on the data above.

KCNT1: PS2:Very Strong, PM2, PS3:Supporting, PS4:Supporting

20A3406

This variant is a recurrent gain of function alteration at a CG dinucleotide sequence in the pore region of the KCNT1 channel. The c.862G>A variant has been reported either as a de novo change or with unknown inheritance in several children with typical migrating malignant partial seizures in infancy (MMPSI) (PMID: 24029078, 26122718, 26140313, 30185235, 26993267, 25482562). Severe delayed myelination was reported in three of these patients (PMID: 26597493, 25482562). Two of these children had seizures that were initially resistant to different drugs, and responded to vagus nerve stimulation or clorazepate (PMID: 24029078). The c.862G>A, p.Gly288Ser variant was also identified in a child with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) (PMID: 25482562). Other KCNT1 missense variants, including a recurrent c.1283G>A alteration identified in three sporadic cases, are located at CG dinucleotides, suggesting that the CpG dinucleotide at these various KCNT1 positions, may be hot spots for point mutations. Functional characterization of the c.862G>A, p.Gly288Ser alteration using heterologous systems showed an increased KCNT1 current (PMID: 26784557, 25482562). The c.862G>A, p.Gly288Ser is absent from the ExAC and gnomAD population databases and thus is presumed to be rare. In addition, it affects a highly conserved amino acid and is predicted to have a deleterious effect on protein function by the majority of in silico tools used for the analysis. The c.862G>A, p.Gly288Ser variant is reported in ClinVar (Variation ID: 126421). Based on the available evidence, the c.862G>A, p.Gly288Ser variant is classified as Pathogenic.

Published functional studies demonstrate a damaging effect; KCNT1 channels with the G288S variant displayed increased maximal current density, a larger instantaneous component of current activation, and more hyperpolarized voltages, suggesting a gain-of-function effect (Rizzo et al., 2016); Not observed at significant frequency in large population cohorts (gnomAD); In silico analysis supports that this missense variant has a deleterious effect on protein structure/function; In silico analysis is inconclusive as to whether the variant alters gene splicing. In the absence of RNA/functional studies, the actual effect of this sequence change is unknown.; This variant is associated with the following publications: (PMID: 26784557, 24029078, 25482562, 26993267, 26597493, 30185235, 31532594, 31054490, 30782581, 31872048, 32167590, 31216405, 31170314, 36007526, 31440721, 35715422, 34114611, 34489640, 33851778, 33726816, 34020146)

This sequence change replaces glycine, which is neutral and non-polar, with serine, which is neutral and polar, at codon 288 of the KCNT1 protein (p.Gly288Ser). This variant is not present in population databases (gnomAD no frequency). This missense change has been observed in individual(s) with epilepsy, most of whom displayed malignant migrating partial seizures in infancy (PMID: 24029078, 26122718, 26140313, 26597493). In at least one individual the variant was observed to be de novo. ClinVar contains an entry for this variant (Variation ID: 126421). Advanced modeling of protein sequence and biophysical properties (such as structural, functional, and spatial information, amino acid conservation, physicochemical variation, residue mobility, and thermodynamic stability) has been performed at Invitae for this missense variant, however the output from this modeling did not meet the statistical confidence thresholds required to predict the impact of this variant on KCNT1 protein function. For these reasons, this variant has been classified as Pathogenic.

PS2, PS3, PS4, PM1, PM2