Intellectual disabilityGene: SLC12A2
DD/ID was a feature in >= 6 individuals with monoallelic de novo SLC12A2. An individual with an exon 22 truncating variant was reported to have normal milestones and cognitive function. Exon 21 variants have been described in individuals with rather isolated hearing impairment (possibly some associated motor delay, but normal cognition). Hearing impairment was also reported in 2/6 patients with variants in other exons (1 missense / 1 frameshift).
DD/ID was reported in at least 3 individuals in literature. Hearing impairment has been reported on 2 occasions (although this was not probably evaluated in all subjects).
Monoallelic SLC12A2 mutations :
► Individuals with de novo mutations and developmental disorder were first identified by the DDD study (2017 - PMID: 28135719). 5 of them have been reported in detail by McNeill et al (below).
► McNeill et al (2020 - PMID: 32658972) report on 6 individuals with neurodevelopmental disorder due to de novo SLC12A2 mutation. All presented DD or ID ranging from mild to severe. ASD was reported in 3/6. Sensorineural hearing loss was a feature in 2/6 with the remaining having normal formal evaluations. Brain, cardiac and/or additional malformations were reported in a single individual. Following non-diagnostic prior work-up (CMA, FMR1 or other investigations) trio exome sequencing revealed missense (4/6) or truncating variants (2/6).
Three additional individuals (incl. a father and his son) with missense variants in exon 21 (NM_001046.3 / p.Glu979Lys and p.Glu980Lys) presented with bilateral sensorineural hearing loss. Speech and/or motor delay reported in these cases were attributed to the hearing impairment/vestibular arreflexia (cognitive abilities not tested).
SLC12A2 encodes sodium-potassium-chloride transporter 1 (also NKCC1).
The GTEx project has identified 8 isoforms. In brain both exon 21-containing/deleted isoforms are expressed (cited Morita et al 2014 - PMID: 24695712). As the authors discuss, RNA-seq of the developing mouse cochlea suggests that the exon 21 containing isoform is the single transcript expressed. Evidence from RNA-seq data (BrainSpan project) and literature suggests that the significant amounts of exon 21 lacking isoforms in fetal brain compensate for the deleterious effects of exon 21 variants and explain the lack of NDD in relevant patients.
Slc12a2 (NKCC1) null mouse model has demonstrated that the transporter plays a role in accumulation of the potassium rich endolymph in the inner ear, with NKCC1 absence causing sensorineural deafness and imbalance. Slc12a2 display cochlear malformations, loss of hair cells and hearing impairment (cited Delpire et al 1999 - PMID: 10369265). The brain phenotype has not been studied extensively, although loss of Slc12a2 has been shown to inhibit neurogenesis (cited: Magalhães and Rivera et al. - PMID: 27582690).
Slc12a2 null zebrafish display a collapse of the otic vesicle and reduced endolymph (Abbas and Whitfield, 2009 - PMID: 19633174) relevant to the human hearing disorder.
In vitro assessment of NKCC1 ion transporter function in Xenopus laevis, supported the deleterious effect of the identified variants (significant reduction in K+ influx). Using available single cell RNA-seq data the authors further demonstrated that SLC12A2 expressing cells display transcriptomic profiles reflective of active neurogenesis.
► Delpire et al (2016 - PMID: 27900370 - not reviewed in detail) described a 13 y.o. girl harboring a de novo 11-bp deletion in SLC12A2 exon 22. This individual reached developmental milestones on time and had a NORMAL cognitive function. Hearing was seemingly normal. Features included orthostatic intolerance, respiratory weakness, multiple endocrine abnormalities, pancreatic insufficiency and multiorgan failure incl. gut and bladder. Exome in the proband, parents and 3 unaffected sibs suggested SLC12A2 as the only candidate for her phenotype. Functional analyses in Xenopus laevis oocytes suggested that a non functional transporter was expressed and trafficked to the membrane as the wt. Detection of the truncated protein at higher molecular sizes suggested either enhanced dimerization or misfolded aggregate. There was no dominant-negative effect of mutant NKCC1. In patient fibroblasts a reduced total and NKCC1-mediated K+ influx.
► Mutai et al (2020 - PMID: 32294086) report on several individuals from 4 families, harboring variants within exon 21 or - in one case - at it's 3' splice-site (leading to skipping oe this exon at the mRNA level). All subjects were investigated for severe/profound hearing loss (in line with the role of exon 21-included isoforms in cochlea. The variant segregated with hearing impairment in 3 generations of a family while in all other subjects the variant had occured as de novo event. Despite motor delays (e.g. the subject from fam2 could not hold head or sit at the age of 10m / the proband in Fam3 was able to hold his head and walk at 6 and 20 m respectively) behavior and cognition were commented to be within normal range.
Biallelic SLC12A2 mutations:
► Anazi et al (2017 - PMID: 29288388) briefly reported on a 3 y.o. boy (17DG0776) with central hypotonia, neonatal respiratory distress, failure to thrive, global DD and microcephaly and a skeletal survey suggestive of osteopenia. After non-diagnostic prior investigations (CMA revealing a 1p duplication classified as VUS, extensive metabolic workup), WES revealed a homozygous SLC12A2 splicing variant [NM_001046.2:c.2617-2A>G].
► Macnamara et al (2019 - PMID: 30740830) described a 5.5 y.o. male with sensorineural hearing loss, profound delays in all developmental areas among several other features (choanal atresia, failure to thrive, respiratory problems, absent sweat and tear production or salivation, GI abnormalities). Genetic testing for several disorders considered (cystic fibrosis, spinal muscular atrophy, sequencing and del/dup analysis of mtDNA) was normal. CMA revealed paternal uniparental isodisomy for chr. 5 and WGS a homozygous 22kb deletion in SLC12A2. This was followed by confirmation of homozygosity in the proband, heterozygosity of the unaffected father, delineation of breakpoints (chr5:127441491-127471419). mRNA studies in patient fibroblasts confirmed deletion of ex2-7, splicing of ex1 directly to ex8 and introduction of a premature stop codon in ex9. qRT-PCR confirmed that mRNA is likely subjected to NMD (expression ~80% of control). Western blot confirmed absence of the protein in the patient's fibroblasts. Again mouse models are thought to recapitulate the hearing defect but also the deficient saliva production (cited Evans et al 2000 - PMID: 10831596). Again the authors speculate a role of SLC12A2 in brain development based on evidence from murine models (migration, dendritic growth, increse in neuron density through regulation of GABAergic signalling (Young et al 2012 - PMID: 23015452). Hypotheses are also made on a regulatory relationship between NKCC1 and CFTR based on mRNA data from the ko mouse model.
► Stödberg et al (2020 - PMID: 32754646) reported 2 sibs with a complex neurodevelopmental disorder due to compound heterozygosity for a frameshift SLC12A2 variant and a splicing one (NM_001046:c.1431delT and c.2006-1G>A). Both presented hypotonia, neonatal S. aureus parotitis and respiratory problems (incl. apneas). While the older sib died at the age of 22 days, the younger one had persistent respiratory issues incl. a dry respiratory mucosa motivating metabolic, immunology investigations and testing for CF. She displayed microcephaly (OFC -2.5 SD, H was also -3.5SD), severe intellectual disability. MRI was suggestive of white matter and basal ganglia abnormalities. Other features incl. hearing impairment, and lack of tears,saliva and sweat, constipation and intestinal malrotation. There was facial dysmorphism. The variants were the only retained following WGS of the 2 affected sisters, parents and an unaffected brother. The splicing variant was shown to result in skipping of exon 13, while the indel in NMD. Again the authors discuss that the deficient saliva production, impaired hearing and GI problems are recapitulated in the mouse model (several refs provided).
Created: 20 Sep 2020, 7:24 p.m. | Last Modified: 20 Sep 2020, 7:24 p.m.
Panel Version: 3.315
Mode of inheritance
BOTH monoallelic and biallelic, autosomal or pseudoautosomal
Comment on list classification: There is sufficient evidence to rate this gene GREEN at the next major review - at least nine unrelated cases presenting a relevant phenotype in association with variants in SLC12A2.
Created: 7 Sep 2020, 4:41 p.m. | Last Modified: 8 Sep 2020, 10:03 a.m.
Panel Version: 3.289
- PMID: 28940097 (2017) - SLC12A2 first identified as a novel candidate gene in a 3.3-year-old male with GDD, failure to thrive, hypotonia, microcephaly, neonatal respiratory distress, recurrent aspiration pneumonia, and osteopenia. Sequencing revealed a homozygous variant (c.2617-2A>G) that segregated within the family.
- PMID: 30740830 (2019) - Homozygous 22kb deletion identified in a 5-year-old male with GDD, sensorineural hearing loss, gastrointestinal abnormalities, early postnatal respiratory distress, generalised hypotonia, and absent salivation. Neuropsychological testing demonstrated profound delays in all developmental areas, with skills ranging from 1 to 6 months. The deletion was the result of uniparental paternal isodisomy.
Functional studies using patient-derived fibroblasts showed truncated SLC12A2 transcripts and markedly reduced protein levels compared to control. Knockout mouse model recapitulated phenotypic features such as deafness, abnormal neuronal growth and migration, gastrointestinal abnormalities, and absent salivation.
- PMID: 32754646 (2020) - Compound heterozygous variants (c.1431delT and c.2006-1G>A) were identified in two sibs. The proband, an 8-year-old girl, presented a severe neurodevelopmental disorder (including severe ID), hearing impairment, gastrointestinal problems, hypotonia, and absent tear fluid, saliva, and sweat. Phenotypic overlap was noted in an affected older sister, who died at 22 days of age.
- PMID: 32658972 (2020) - Six unrelated children, all with mild-severe ID/DD, associated with de novo variants in SLC12A2. Additional clinical features included bilateral sensorineural hearing loss (2/6), abnormalities on brain MRI (2/4), and cerebral palsy (2/6). Some functional data in Xenopus laevis oocytes, indicating a role of SLC12A2 in neurogenesis.
Created: 7 Sep 2020, 4:37 p.m. | Last Modified: 8 Sep 2020, 10:02 a.m.
Panel Version: 3.289
Mode of inheritance
BOTH monoallelic and biallelic, autosomal or pseudoautosomal
Two families reported and a mouse model. New report is not on PubMed yet: (https://doi.org/10.1212/NXG.0000000000000478)
Created: 6 Jul 2020, 7:32 a.m.
Mode of inheritance
BIALLELIC, autosomal or pseudoautosomal
Kilquist syndrome; deafness; intellectual disability; dysmorphic features; absent salivation; ectodermal dysplasia; constipation; intestinal malrotation; multiple congenital anomalies
Variants in this GENE are reported as part of current diagnostic practice
Publications for gene: SLC12A2 were set to 28940097; 30740830; 32754646
Mode of inheritance for gene: SLC12A2 was changed from BIALLELIC, autosomal or pseudoautosomal to BOTH monoallelic and biallelic, autosomal or pseudoautosomal
Publications for gene: SLC12A2 were set to 30740830
Gene: slc12a2 has been classified as Amber List (Moderate Evidence).
Tag for-review tag was added to gene: SLC12A2.
gene: SLC12A2 was added gene: SLC12A2 was added to Intellectual disability. Sources: Literature Mode of inheritance for gene: SLC12A2 was set to BIALLELIC, autosomal or pseudoautosomal Publications for gene: SLC12A2 were set to 30740830 Phenotypes for gene: SLC12A2 were set to Kilquist syndrome; deafness; intellectual disability; dysmorphic features; absent salivation; ectodermal dysplasia; constipation; intestinal malrotation; multiple congenital anomalies Review for gene: SLC12A2 was set to GREEN gene: SLC12A2 was marked as current diagnostic