Identification and functional characterization of two novel mutations in KCNJ10 and PI4KB in SeSAME syndrome without electrolyte imbalance

Abstract

Background: Dysfunction in inwardly rectifying potassium channel Kir4.1 has been implicated in SeSAME syndrome, an autosomal-recessive (AR), rare, multi-systemic disorder. However, not all neurological, intellectual disability, and comorbid phenotypes in SeSAME syndrome can be mechanistically linked solely to Kir4.1 dysfunction. Methods: We therefore performed whole-exome sequencing and identified additional genetic risk-elements that might exert causative effects either alone or in concert with Kir4.1 in a family diagnosed with SeSAME syndrome. Results: Two variant prioritization pipelines based on AR inheritance and runs of homozygosity (ROH), identified two novel homozygous variants in KCNJ10 and PI4KB and five rare homozygous variants in PVRL4, RORC, FLG2, FCRL1, NIT1 and one common homozygous variant in HSPA6 segregating in all four patients. The novel mutation in KCNJ10 resides in the cytoplasmic domain of Kir4.1, a seat of phosphatidylinositol bisphosphate (PIP2) binding. The mutation altered the subcellular localization and stability of Kir4.1 in patient-specific lymphoblastoid cells (LCLs) compared to parental controls. Barium-sensitive endogenous K + currents in patient-specific LCLs using whole-cell patch-clamp electrophysiology revealed membrane depolarization and defects in inward K + ion conductance across the membrane, thereby suggesting a loss-of-function effect of KCNJ10 variant. Conclusion: Altogether, our findings implicate the role of new genes in SeSAME syndrome without electrolyte imbalance and thereby speculate the regulation of Kir4.1 channel activity by PIP2 and integrin-mediated adhesion signaling mechanisms.

Characterizing Genotypes and Phenotypes Associated with Dysfunction of Channel-Encoding Genes in a Cohort of Patients with Intellectual Disability

“…Although this most frequently reported mutation is juxtaposed near the first transmembrane domain at the N-terminal cytoplasmic side, a large number of variants are seen intensively at cytoplasmic C-terminal pore domain. 26 In terms of zygosity, most variants were inherited in a homozygous state (42 of 54: 77.7%; Table S1.1). Patients with KCNJ10 mutation were mostly male (34 out of 53: 64%).…”

“…In this context, patients with the same mutation in KCNJ10 (p.T290A) showed different degrees of sensorineural hearing loss ranging from mild to severe. 26 The majority of reported mutations in KCNJ10 lead toward loss of function (76.2%; Table 3 ). This mechanism results in mild ID, 27 whereas, depending on the variant, gain of function mutations are associated with mild to severe ID.…”

“…It must be borne in mind that in channelopathies, seemingly, phenotypic heterogeneity even within family members with the same mutation does not solely emanate from variants in membrane ion channels and other unknown rare variants particularly in minor genes which change the effect of principal gene mutations are also involved. 26 , 32 Also, the variable clinical spectrum may stem from the numerous isoforms created by alternatively spliced exons, and changing expression of distinct transcripts in tissue. For instance, in some individuals with CACNA1C mutation, the expression of mutation-bearing transcript is predominantly confined to the heart (exon 8A), while in others, there is a predominant neurologic expression (exon 8A).…”