SLC12A5-Related Developmental and Epileptic Encephalopathy

Mendelian MONDO:0014718 Pathograph 17 Show in embeddings browser Neurodevelopmental Disorder Genetic Disease

SLC12A5-related developmental and epileptic encephalopathy (DEE34) is an ultra-rare autosomal recessive epileptic encephalopathy caused by biallelic loss-of-function variants in SLC12A5, which encodes KCC2, the neuron-specific K+-Cl- cotransporter. KCC2 uses the outward potassium gradient established by the Na+/K+-ATPase to extrude chloride from neurons, keeping intracellular chloride low enough that opening a GABA-A or glycine receptor drives chloride inward and hyperpolarizes the cell. When KCC2 function is reduced, intraneuronal chloride rises, the GABA-A reversal potential shifts depolarized, and fast synaptic inhibition fails. The clinical counterpart is epilepsy of infancy with migrating focal seizures (EIMFS, formerly malignant migrating partial seizures of infancy): focal motor seizures beginning in the first months of life that become multifocal and migrating, are pharmacoresistant, and are accompanied by developmental delay or regression, axial hypotonia, and postnatal microcephaly. KCC2 additionally has transport-independent structural roles in dendritic spine and excitatory-synapse maturation, so some variant combinations disturb neurodevelopment through a second, chloride-independent route. Fewer than twenty genetically confirmed individuals have been published, so the phenotype is described qualitatively rather than with frequency percentages.

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1
Inheritance
14
Pathophys.
15
Phenotypes
4
Gaps
17
Pathograph
1
Genes
4
Medical Actions
3
Models
1
References
1
Deep Research
👪

Inheritance

1
Autosomal recessive HP:0000007
DEE34 requires two pathogenic SLC12A5 alleles (homozygous or compound heterozygous). Parents are typically unaffected heterozygous carriers, and consanguinity is a recognized contributor to homozygous presentations.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:30763027 SUPPORT Human Clinical
"SLC12A5-EIMFS is inherited in an autosomal recessive manner."
GeneReviews states the mode of inheritance directly.
PMID:26333769 SUPPORT Human Clinical
"Here we show recessive loss-of-function SLC12A5 mutations in patients with a severe infantile-onset pharmacoresistant epilepsy syndrome"
The founding study established the recessive loss-of-function mechanism.
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Discussions and Knowledge Gaps

4
Is the elevated intraneuronal chloride in KCC2 deficiency principally the direct consequence of lost cation-chloride cotransport, or is the neuronal chloride setpoint dominated by the distribution of impermeant intracellular anions, with KCC2 acting mainly to restore that setpoint after activity-driven chloride loads?
CONTROVERSY OPEN controversy_kcc2_setpoint_vs_impermeant_anions
Attached to
pathophysiology#Reduced KCC2-Mediated Chloride Extrusion pathophysiology#Elevated Intraneuronal Chloride Concentration
The disease model curated here treats reduced KCC2 transport as the proximate cause of a raised chloride setpoint. An alternative account holds that the resting setpoint is fixed largely by fixed impermeant anionic charge, with KCC2 responsible for clearing dynamic chloride loads rather than for setting resting chloride. The two readings predict different therapeutic targets: restoring KCC2 transport capacity would be central under the first, whereas under the second the critical variable is the neuron's ability to recover from activity-driven loading, which would foreground interventions aimed at seizure-associated chloride accumulation. Both readings are compatible with the human variant data, since patient assays measure extrusion capacity rather than resting setpoint in vivo.
Proposed experiments
Separate resting chloride setpoint from chloride recovery kinetics in patient neurons
exp_chloride_imaging_patient_neurons
Measure resting intracellular chloride and post-load chloride recovery kinetics separately, using ratiometric chloride imaging in patient-derived iPSC neurons carrying DEE34 genotypes alongside isogenic controls, to determine whether the resting setpoint or the recovery rate is the parameter that the variants change.
Show evidence (1 reference)
PMID:35370549 SUPPORT Model Organism
"has been debated in favor of alternative mechanisms, such as distribution of impermeable anionic charges"
States the existence and content of the disagreement.
Given that both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported, under what circumstances does reduced KCC2 activity worsen rather than limit seizure activity, and does that boundary apply to the constitutive, lifelong partial deficiency of DEE34?
KNOWLEDGE GAP OPEN gap_kcc2_bidirectional_seizure_effect
Attached to
pathophysiology#Impaired Termination of Ictal Discharges
KCC2 extrudes chloride at the cost of raising extracellular potassium, which can itself depolarize neurons and potentiate seizures. Experimental KCC2 blockade has therefore been reported to be both pro- and anticonvulsant depending on which step is rate-limiting. All of that evidence comes from acute pharmacological manipulation in slice preparations; DEE34 is instead a constitutive partial deficiency present throughout development. Whether the acute-blockade findings transfer to the chronic genetic condition is unresolved, and it bears directly on whether KCC2-enhancing compounds would be expected to help these patients.
Proposed experiments
Test a KCC2 enhancer in a constitutive hypomorphic Slc12a5 model
exp_kcc2_enhancer_in_hypomorphic_model
Test a KCC2-enhancing compound against seizure burden in a hypomorphic Slc12a5 knock-in mouse carrying a patient variant, rather than in acute pharmacological blockade models, measuring both electrographic seizure duration and extracellular potassium to establish which step is rate-limiting in a constitutive partial deficiency.
Show evidence (2 references)
PMID:33239270 SUPPORT In Vitro
"Thus, KCC2 may either diminish or facilitate seizure activity, and both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported."
States the unresolved bidirectionality directly.
PMID:33239270 SUPPORT In Vitro
"Anticonvulsant effects predominate when KCC2-mediated chloride transport rather than potassium buffering is the rate-limiting step in restoring ECl"
Identifies the condition that determines the direction of effect, which is what would need to be established for the chronic genetic deficiency.
Do the Kcc2 knockout mouse models, which are perinatally lethal or die within weeks, model DEE34 faithfully, given that surviving human patients retain partial KCC2 transport activity and that the perinatal NKCC1-to-KCC2 switch occurs at a different relative developmental stage in mouse and human?
HUMAN MODEL MISMATCH OPEN mismatch_kcc2_null_mouse_versus_human_partial_loss
Attached to
pathophysiology#Reduced KCC2-Mediated Chloride Extrusion pathophysiology#Cortical Interneuron Subtype Imbalance
Complete Kcc2 loss kills mice at birth, whereas every published human DEE34 genotype retains measurable residual chloride extrusion and the children survive into childhood. The mouse models therefore sit at a more severe point on the dose-response curve than the human disease. Independently, the developmental window matters: in human neocortex the shift from depolarizing to hyperpolarizing GABA reaches maturity around six months after birth, and mouse postnatal age does not map one-to-one onto that timeline. Any inference from knockout phenotypes to the timing or severity of human disease - including the interneuron subtype imbalance, which has been shown only in mouse - has to carry this caveat.
Proposed experiments
Graded Slc12a5 knock-in allelic series against the null phenotype
exp_hypomorphic_knockin_allelic_series
Generate an allelic series of Slc12a5 knock-in mice carrying human DEE34 variants of graded residual transport activity, and compare seizure onset, survival, and cortical interneuron composition against the null, to establish how much of the null phenotype is an artefact of complete loss.
Show evidence (3 references)
PMID:11395011 SUPPORT Model Organism
"We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration."
Establishes the severity gap between the null mouse and surviving human patients.
PMID:27436767 SUPPORT Human Clinical
"Functional analysis using the gramicidin-perforated patch-clamp technique confirmed significant, but not complete, loss of KCC2 function in the patients."
Confirms that human disease is a partial, not complete, loss of function.
PMID:38660387 SUPPORT Other
"In the human neocortex, this transition takes place perinatally and reaches maturation at roughly 6 months after birth"
Gives the human developmental timing that mouse postnatal age does not directly match.
How much of the developmental phenotype in DEE34 is attributable to the transport-independent structural role of KCC2 in dendritic spine and excitatory synapse maturation, as opposed to the chloride-extrusion deficit and the seizure burden it causes?
KNOWLEDGE GAP OPEN gap_relative_contribution_transport_vs_structural_arm
Attached to
pathophysiology#Disrupted Transport-Independent KCC2 Structural Function pathophysiology#Impaired Neurodevelopment
Two upstream arms converge on the developmental phenotype in this model, and the published cases cannot separate them: every reported genotype impairs chloride extrusion, so no patient provides the natural experiment of an isolated structural defect. The distinction is not academic - if the structural arm carries substantial weight, then even complete seizure control or full restoration of chloride transport would leave part of the developmental disability untouched, which changes what a KCC2-targeted therapy could be expected to achieve.
Proposed experiments
Transport-dead versus structure-disrupting KCC2 knock-in comparison
exp_transport_dead_structural_intact_knockin
Compare knock-in mice carrying a transport-dead but structurally intact KCC2 against mice carrying a C-terminal variant that preserves transport while disrupting protein interactions, scoring dendritic spine density, excitatory synapse number, seizure burden, and behavioural development, to apportion the developmental phenotype between the two arms.
Show evidence (2 references)
PMID:26333769 SUPPORT Human Clinical
"SLC12A5 encodes the potassium-chloride co-transporter KCC2, which plays an integral role in neuronal inhibition4567 and maturation of cortical dendritic spines8."
Names both KCC2 roles, which is the basis for treating them as separate arms.
PMID:35370549 SUPPORT Model Organism
"More work will be needed to define the specific cellular basis for these findings, including whether they are due to abnormal circuit formation versus the sequela of defective IN inhibition."
The authors of the conditional knockout state the same unresolved separation between circuit-formation and inhibition-failure explanations.
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Pathophysiology

14
Biallelic SLC12A5 Loss-of-Function Variant
Two pathogenic SLC12A5 alleles - homozygous or compound heterozygous missense, splice-site, or in-frame deletion variants - are present in the affected individual. This node records only the genomic lesion; its separate protein-level consequences are modeled downstream.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Downstream
Impaired KCC2 Folding, Glycosylation, and Membrane Trafficking Some variants act by preventing the transporter from reaching the plasma membrane in normal amounts.
Reduced Intrinsic KCC2 Chloride Transport Activity Other variants reach the surface normally but transport chloride poorly, so the transport deficit is not explained by trafficking alone.
Disrupted Transport-Independent KCC2 Structural Function Variants affecting the intracellular C-terminal domain disturb KCC2's scaffolding role independently of ion transport.
Show evidence (2 references)
PMID:26333769 SUPPORT Human Clinical
"we report a novel genetic cause of EIMFS, identifying recessive SLC12A5 mutations in four affected children from two unrelated families"
Establishes biallelic SLC12A5 variants as the causal genomic lesion.
PMID:27436767 SUPPORT Human Clinical
"Whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS revealed compound heterozygous SLC12A5"
Independent replication of the biallelic genotype in unrelated families.
Impaired KCC2 Folding, Glycosylation, and Membrane Trafficking
Misfolded KCC2 variants are under-glycosylated and retained in the endoplasmic reticulum, where they are degraded by ER-associated degradation, so less transporter reaches the neuronal plasma membrane. This is a trafficking defect only; the residual transport competence of the protein that does arrive is a separate question.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
ERAD pathway GO:0036503 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased ERAD pathway (GO:0036503). GO:0036503 is a biological process from the Gene Ontology. ↑ INCREASED
Downstream
Reduced KCC2-Mediated Chloride Extrusion Fewer transporters at the membrane means less total chloride extrusion capacity.
Show evidence (2 references)
PMID:26333769 SUPPORT In Vitro
"Decreased KCC2 surface expression, reduced protein glycosylation and impaired chloride extrusion contribute to loss of KCC2 activity"
Directly reports reduced surface expression and glycosylation of mutant KCC2.
PMID:38660387 SUPPORT In Vitro
"Based on surface immunolabeling, the variant showed a reduction in membrane expression."
Surface immunolabeling of the R231H variant confirms reduced membrane delivery.
Reduced Intrinsic KCC2 Chloride Transport Activity
Variants that reach the plasma membrane at normal density can still transport chloride poorly, so the per-molecule catalytic activity of the cotransporter is reduced independently of how much protein is present. Saitsu and colleagues documented mutant KCC2 with surface expression comparable to wild type but strongly suppressed chloride extrusion.
potassium:chloride symporter activity GO:0015379 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased potassium:chloride symporter activity (GO:0015379). GO:0015379 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Reduced KCC2-Mediated Chloride Extrusion Lower catalytic activity per transporter reduces net chloride extrusion.
Show evidence (2 references)
PMID:27436767 SUPPORT In Vitro
"Cell surface expression levels of these KCC2 mutants were similar to wildtype KCC2."
Establishes that transport loss can occur without any trafficking deficit, justifying this node as separate from the trafficking node.
PMID:27436767 SUPPORT In Vitro
"Gramicidin-perforated patch-clamp analysis demonstrated strongly suppressed Cl(-) extrusion function of E50_Q93del and M415V mutants, with mildly impaired function of A191V and S323P mutants."
Quantifies variant-specific loss of intrinsic transport activity.
Reduced KCC2-Mediated Chloride Extrusion
The neuron's total capacity to move chloride out against its electrochemical gradient falls. Human variants produce partial rather than complete loss, which is why affected children survive infancy whereas Kcc2-null mice do not.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
chloride transmembrane transport GO:1902476 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased chloride transmembrane transport (GO:1902476). GO:1902476 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Kcc2b isoform-specific knockout mouse Mouse partially recapitulates Models a survivable partial reduction in KCC2 function with spontaneous seizures, unlike the perinatally lethal complete null.
Downstream
Elevated Intraneuronal Chloride Concentration Chloride entering the neuron is no longer cleared at the normal rate.
Show evidence (2 references)
PMID:38660387 SUPPORT In Vitro
"gramicidin-perforated patch-clamp experiments and ammonium flux assay, both of which indicated a significant reduction in chloride extrusion"
Two independent assays show reduced chloride extrusion by the variant transporter.
PMID:27436767 SUPPORT In Vitro
"Functional analysis using the gramicidin-perforated patch-clamp technique confirmed significant, but not complete, loss of KCC2 function in the patients."
Establishes that the loss is partial, which is the key contrast with the perinatally lethal complete-null mouse.
Elevated Intraneuronal Chloride Concentration
Intracellular chloride rises above the roughly 4-8 mM maintained in healthy mature neurons. This is a change in the ionic setpoint itself, distinct from its electrophysiological consequence for receptor-mediated currents.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
chloride ion homeostasis GO:0055064 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal chloride ion homeostasis (GO:0055064). GO:0055064 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Depolarizing Shift in the GABA-A and Glycine Reversal Potential Because chloride flux through these receptors is passive, the reversal potential follows the chloride gradient.
Show evidence (1 reference)
PMID:27436767 SUPPORT In Vitro
"Heterologous expression of two KCC2 mutants, mimicking the patient status, produced a significantly greater intracellular Cl(-) level than with wildtype KCC2"
Directly measures the elevated intracellular chloride produced by the patient genotype.
Depolarizing Shift in the GABA-A and Glycine Reversal Potential
The reversal potential for GABA-A and glycine receptor currents moves positive of the resting membrane potential, so opening these channels drives chloride out of the cell and depolarizes it. This is the electrophysiological state change; the resulting failure of synaptic inhibition is modeled separately.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
regulation of membrane potential GO:0042391 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal regulation of membrane potential (GO:0042391). GO:0042391 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Animal models
Kcc2 constitutive knockout mouse Mouse recapitulates Direct demonstration that removing KCC2 makes GABA and glycine depolarizing.
Downstream
Failure of Fast Synaptic Inhibition A depolarized reversal potential removes the hyperpolarizing and shunting effect of GABAergic input.
Show evidence (2 references)
PMID:11395011 SUPPORT Model Organism
"demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2"
Shows that removing KCC2 converts GABA and glycine responses from inhibitory to excitatory.
PMID:38660387 SUPPORT In Vitro
"is essential for this Cl--mediated signaling to be inhibitory as Cl- flux through these receptor channels is passive"
States the biophysical dependence of the reversal potential on intracellular chloride.
Failure of Fast Synaptic Inhibition
GABAergic and glycinergic synapses no longer raise the action-potential threshold of their targets. Excitatory drive is unopposed even though glutamatergic transmission itself is not primarily affected.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology. GABAergic neuron CL:0000617 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves GABAergic neuron (CL:0000617). CL:0000617 is a cell type from the Cell Ontology.
inhibitory postsynaptic potential GO:0060080 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased inhibitory postsynaptic potential (GO:0060080). GO:0060080 is a biological process from the Gene Ontology. ↓ DECREASED gamma-aminobutyric acid signaling pathway GO:0007214 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal gamma-aminobutyric acid signaling pathway (GO:0007214). GO:0007214 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Neuronal Hyperexcitability and Hypersynchrony Unopposed excitation lowers the threshold for synchronized network discharge.
Impaired Termination of Ictal Discharges Chloride loading during a discharge cannot be cleared, so the discharge is not curtailed.
Show evidence (2 references)
PMID:26333769 SUPPORT In Vitro
"thereby impairing normal synaptic inhibition and promoting neuronal excitability in this early-onset epileptic encephalopathy"
Links loss of KCC2 activity to impaired synaptic inhibition.
PMID:11395011 SUPPORT Model Organism
"revealing a crucial role of KCC2 for synaptic inhibition"
Genetic removal of KCC2 abolishes functional synaptic inhibition.
Disrupted Transport-Independent KCC2 Structural Function
Beyond ion transport, KCC2 interacts through its large intracellular C-terminal domain with cytoskeletal and synaptic proteins to regulate dendritic spine formation and glutamatergic synapse plasticity. Variants can compromise this scaffolding role, and this arm is not captured by any chloride measurement.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
dendritic spine morphogenesis GO:0060997 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal dendritic spine morphogenesis (GO:0060997). GO:0060997 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Impaired Excitatory Synapse and Dendritic Spine Maturation Loss of the scaffolding interaction disturbs spine and synapse formation.
Show evidence (2 references)
PMID:35370549 SUPPORT Model Organism
"Independently of its transporter function, KCC2 facilitates a number of processes that are central to circuit development, such as actin re-arrangement/formation of dendritic spines and plasticity of glutamatergic synapses"
States the transport-independent structural role of KCC2 explicitly.
PMID:26333769 SUPPORT Human Clinical
"SLC12A5 encodes the potassium-chloride co-transporter KCC2, which plays an integral role in neuronal inhibition4567 and maturation of cortical dendritic spines8."
The founding disease paper names dendritic spine maturation as a second KCC2 role.
Impaired Excitatory Synapse and Dendritic Spine Maturation
Dendritic spines and glutamatergic synapses form and mature abnormally. This is a structural developmental deficit that would persist even if chloride handling were corrected.
pyramidal neuron CL:0000598 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves pyramidal neuron (CL:0000598). CL:0000598 is a cell type from the Cell Ontology.
dendritic spine morphogenesis GO:0060997 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased dendritic spine morphogenesis (GO:0060997). GO:0060997 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Impaired Neurodevelopment Abnormal synapse maturation contributes to the developmental phenotype independently of seizure burden.
Show evidence (1 reference)
PMID:35370549 SUPPORT Model Organism
"KCC2 directly interacts with a number of proteins to regulate dendritic spine formation, cell survival, synaptic plasticity, neuronal excitability, and other processes."
Supports spine formation as a KCC2-dependent process disturbed by its loss.
Cortical Interneuron Subtype Imbalance
In mice lacking KCC2 in Dlx5-lineage GABAergic neurons, cortical interneurons reach their normal laminar positions but their subtype composition is skewed - excess somatostatin-expressing neurons in layer 5 and fewer parvalbumin-expressing neurons in layers 2/3 and 6. This is a change in inhibitory circuit composition rather than in the moment-to-moment strength of inhibition.
cerebral cortex GABAergic interneuron CL:0010011 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves cerebral cortex GABAergic interneuron (CL:0010011). CL:0010011 is a cell type from the Cell Ontology.
Animal models
Dlx5-lineage conditional Kcc2 knockout mouse Mouse recapitulates Provides the direct evidence for the interneuron subtype imbalance node.
Downstream
Neuronal Hyperexcitability and Hypersynchrony An altered inhibitory circuit composition is proposed to contribute to hyperexcitability, though the cellular basis is not yet established.
Show evidence (1 reference)
PMID:35370549 SUPPORT Model Organism
"we found an underlying change in IN distribution, including an excess number of somatostatin neurons in layer 5 and a decrease in parvalbumin-expressing neurons in layer 2/3 and layer 6"
Reports the specific interneuron subtype imbalance in the conditional knockout.
Impaired Termination of Ictal Discharges
KCC2-mediated chloride transport normally restores the chloride equilibrium after intense GABA-A receptor activation, which helps bring an ictal discharge to an end. With reduced KCC2, chloride accumulated during a discharge is cleared slowly and the discharge is prolonged. This concerns seizure offset, not seizure onset.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Downstream
Migrating Multifocal Seizure Activity Discharges that fail to self-terminate become prolonged and recurrent.
Show evidence (2 references)
PMID:33239270 SUPPORT In Vitro
"the putative KCC2 upregulator CLP257 improved chloride homeostasis and reduced the duration and frequency of ILDs in a concentration-dependent manner"
Pharmacologically enhancing KCC2 shortens ictal discharges, supporting a role for KCC2-mediated chloride transport in seizure termination.
PMID:33239270 SUPPORT In Vitro
"increased ILD duration, and induced status epilepticus"
The reciprocal manipulation - blocking KCC2 - prolongs ictal discharges and precipitates status epilepticus.
Neuronal Hyperexcitability and Hypersynchrony
Cortical networks discharge at lower threshold and in synchrony. Because KCC2 is expressed throughout the cortex, susceptibility is diffuse rather than confined to one epileptogenic focus.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
modulation of chemical synaptic transmission GO:0050804 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal modulation of chemical synaptic transmission (GO:0050804). GO:0050804 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Migrating Multifocal Seizure Activity Diffuse hyperexcitability with no fixed focus produces seizures that arise independently at successive cortical sites.
Show evidence (1 reference)
PMID:38660387 SUPPORT In Vitro
"These findings implicate pathogenicity of the SLC12A5 variant that leads to impaired inhibitory neurotransmission, increasing probability for hyperexcitability and epileptogenesis."
Connects the transport deficit to network hyperexcitability and epileptogenesis.
Migrating Multifocal Seizure Activity
Seizures begin as focal motor events, typically with head and eye deviation, and evolve into near-continuous multifocal discharges that migrate from one cortical region to another - the defining electroclinical signature of EIMFS - and resist conventional anti-seizure medication.
Downstream
Impaired Neurodevelopment High seizure burden in the first months of life compounds the developmental consequences of the underlying synaptic defect.
Show evidence (2 references)
PMID:30763027 SUPPORT Human Clinical
"In most children epilepsy begins as focal motor seizures (typically involving head and eye deviation) that become multifocal and intractable to conventional anti-seizure medication (ASM)."
GeneReviews describes the seizure evolution and its pharmacoresistance.
PMID:27436767 SUPPORT Human Clinical
"partially disrupted neuronal Cl(-) extrusion, mediated by two types of differentially impaired KCC2 mutant in an individual, causes EIMFS"
Attributes the migrating focal seizure phenotype to partial chloride-extrusion loss.
Impaired Neurodevelopment
Development either arrests or regresses at the time of seizure onset, yielding severe to profound intellectual disability, axial hypotonia, and postnatal microcephaly. Two upstream routes converge here - the seizure burden itself and the transport-independent synapse maturation defect - and the published cases do not resolve their relative contributions.
Show evidence (2 references)
PMID:30763027 SUPPORT Human Clinical
"is characterized by onset of seizures before age six months and either developmental delay or developmental regression with seizure onset"
GeneReviews couples the developmental phenotype to seizure onset.
PMID:38660387 SUPPORT Human Clinical
"we identified a novel recessively inherited SLC12A5 c.692G>A, p. (R231H) variant in a patient diagnosed with severe and drug-resistant EIMFS and profound intellectual disability"
Documents profound intellectual disability accompanying the epilepsy.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for SLC12A5-Related Developmental and Epileptic Encephalopathy Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

15
Digestive 1
Feeding Difficulties HP:0011968 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Feeding difficulties (HP:0011968). HP:0011968 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"A multidisciplinary approach to management of hypotonia, feeding difficulties, respiratory problems, and developmental delay is recommended."
GeneReviews management guidance names feeding difficulties as a recognized problem.
Head and Neck 1
Postnatal Microcephaly VERY_FREQUENT Secondary microcephaly HP:0005484 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Secondary microcephaly (HP:0005484). HP:0005484 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"Eight had postnatal microcephaly and hypotonia."
Eight of nine children is within the 80-100% very frequent band.
Nervous System 8
Focal-Onset Seizures HP:0007359 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Focal-onset seizure (HP:0007359), qualified as infantile onset, up to 0.5y. HP:0007359 is a phenotype from the Human Phenotype Ontology.
Onset: INFANTILE; up to 0.5y
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"epilepsy begins as focal motor seizures (typically involving head and eye deviation)"
Records the focal onset semiology.
Developmental Delay VERY_FREQUENT Global developmental delay HP:0001263 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Global developmental delay (HP:0001263). HP:0001263 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"Of these nine children, six had severe developmental delay with no progress of abilities and three made notable neurodevelopmental progress."
All nine children in the cohort had delay or regression, supporting a very frequent band.
Developmental Regression HP:0002376 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Developmental regression (HP:0002376). HP:0002376 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"either developmental delay or developmental regression with seizure onset"
Regression at seizure onset is part of the defining clinical description.
Intellectual Disability HP:0001249 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Intellectual disability (HP:0001249), qualified as severity severe. HP:0001249 is a phenotype from the Human Phenotype Ontology.
Severity: SEVERE
Show evidence (1 reference)
PMID:38660387 SUPPORT Human Clinical
"a patient diagnosed with severe and drug-resistant EIMFS and profound intellectual disability"
Documents profound intellectual disability in a genetically confirmed case.
Cerebral Atrophy VERY_FREQUENT HP:0002059 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral atrophy (HP:0002059). HP:0002059 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27436767 SUPPORT Human Clinical
"Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
Cerebral atrophy in four of four genotyped individuals supports the 80-99% very frequent band. Four of four is strictly obligate, but asserting obligate presence from a denominator of four would over-claim.
Delayed Myelination FREQUENT HP:0012448 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed myelination (HP:0012448). HP:0012448 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27436767 SUPPORT Human Clinical
"Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
Delayed myelination in three of four individuals is 75%, which falls in the 30-79% frequent band. The denominator is small.
Agenesis of the Corpus Callosum Agenesis of corpus callosum HP:0001274 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Agenesis of corpus callosum (HP:0001274). HP:0001274 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38660387 SUPPORT Human Clinical
"Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
Direct neuropathological documentation of the callosal defect.
Ventriculomegaly HP:0002119 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Ventriculomegaly (HP:0002119). HP:0002119 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38660387 SUPPORT Human Clinical
"Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
The same autopsy documents markedly enlarged lateral ventricles.
Other 5
Migrating Multifocal Seizures Migrating focal seizure HP:0032786 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Migrating focal seizure (HP:0032786), qualified as infantile onset, up to 0.5y. HP:0032786 is a phenotype from the Human Phenotype Ontology.
Onset: INFANTILE; up to 0.5y
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"In most children epilepsy begins as focal motor seizures (typically involving head and eye deviation) that become multifocal and intractable to conventional anti-seizure medication (ASM)."
GeneReviews describes the defining seizure phenotype.
Hypotonia VERY_FREQUENT Axial hypotonia HP:0008936 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Axial hypotonia (HP:0008936). HP:0008936 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"Eight had postnatal microcephaly and hypotonia."
Eight of nine children is within the 80-100% very frequent band.
Thin Corpus Callosum FREQUENT HP:0033725 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Thin corpus callosum (HP:0033725). HP:0033725 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27436767 SUPPORT Human Clinical
"Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
Thin corpus callosum in two of four individuals falls in the 30-79% frequent band.
Cerebral White Matter Hypoplasia HP:0012430 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral white matter hypoplasia (HP:0012430). HP:0012430 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38660387 SUPPORT Human Clinical
"Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
The same autopsy documents scarcity of cerebral white matter.
Recurrent Aspiration and Respiratory Problems Recurrent aspiration pneumonia HP:0002100 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent aspiration pneumonia (HP:0002100). HP:0002100 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"Routine monitoring of: feeding, nutritional status, swallowing, gastroesophageal reflux, aspiration, and respiratory problems"
GeneReviews surveillance guidance identifies aspiration and respiratory problems as expected complications.
🧬

Genetic Associations

1
SLC12A5 (Biallelic pathogenic variants)
Gene: SLC12A5 hgnc:13818 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is SLC12A5 (hgnc:13818). hgnc:13818 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:26333769 SUPPORT Human Clinical
"Here we show recessive loss-of-function SLC12A5 mutations in patients with a severe infantile-onset pharmacoresistant epilepsy syndrome, epilepsy of infancy with migrating focal seizures (EIMFS)."
Establishes SLC12A5 as the causal gene.
PMID:27436767 SUPPORT Human Clinical
"c.279 + 1G > C causing skipping of exon 3 in the transcript (p.E50_Q93del) and c.572 C >T (p.A191V)"
Reports specific compound heterozygous variants.
PMID:38660387 SUPPORT Human Clinical
"a novel recessively inherited SLC12A5 c.692G>A, p. (R231H) variant"
Documents the homozygous R231H variant.
💊

Medical Actions

4
Anti-Seizure Medication
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: potassium bromide CHEBI:32030 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses potassium bromide (CHEBI:32030). CHEBI:32030 is a therapeutic agent from Chemical Entities of Biological Interest.
No SLC12A5-specific therapy exists and seizures in EIMFS are resistant to most anti-seizure medications. Potassium bromide and a ketogenic diet each produced attenuation of seizures in three individuals in the GeneReviews cohort.
Show evidence (2 references)
PMID:30763027 SUPPORT Human Clinical
"There are no specific treatments for seizures in SLC12A5-EIMFS. In general, seizures in EIMFS are resistant to most ASM."
GeneReviews states the absence of specific therapy and general pharmacoresistance.
PMID:30763027 SUPPORT Human Clinical
"A ketogenic diet and potassium bromide showed attenuation of seizures in three individuals each."
Documents the limited observed benefit of potassium bromide.
Ketogenic Diet
Action: Dietary InterventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Dietary Intervention (NCIT:C15447). NCIT:C15447 is a clinical intervention from the NCI Thesaurus. NCIT:C15447
Dietary therapy that attenuated seizures in three individuals in the GeneReviews cohort; response is variable and not universal.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"A ketogenic diet and potassium bromide showed attenuation of seizures in three individuals each."
Reports partial seizure attenuation with ketogenic diet in a minority of cases.
Multidisciplinary Supportive Care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Management of hypotonia, feeding difficulties, respiratory problems, and developmental delay, with surveillance for scoliosis and hip dislocation.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"A multidisciplinary approach to management of hypotonia, feeding difficulties, respiratory problems, and developmental delay is recommended."
GeneReviews management recommendation.
Genetic Counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Autosomal recessive counseling with a 25% recurrence risk for siblings; carrier, prenatal, and preimplantation testing become possible once the familial variants are known.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"Once the SLC12A5 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible."
GeneReviews genetic counseling guidance.
🔬

Diagnosis

2
Molecular Genetic Testing for Biallelic SLC12A5 Variants (Positive in affected individuals)
The diagnosis is established by identifying two pathogenic SLC12A5 alleles. Because affected individuals carry private variants and EIMFS is genetically heterogeneous, exome or genome sequencing rather than single-gene testing is the practical first-tier route.
whole exome sequencing NCIT:C101295 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:30763027 SUPPORT Human Clinical
"The diagnosis of SLC12A5-EIMFS is established by identification of biallelic SLC12A5 pathogenic variants on molecular genetic testing."
GeneReviews states the confirmatory diagnostic test directly.
PMID:27436767 SUPPORT Human Clinical
"Whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS revealed compound heterozygous SLC12A5"
Documents exome sequencing as the method that identifies the biallelic genotype.
Electroencephalography
EEG carries the electroclinical signature of EIMFS: interictal multifocal epileptiform discharges together with ictal activity that migrates from one hemisphere to the other. This migrating ictal pattern - not any single seizure type - is what defines the syndrome. Interictal EEG may be normal early and only later show slow-wave activity or focal discharges.
electroencephalography NCIT:C38054 NCI Thesaurus (NCIT)
Show evidence (3 references)
PMID:26333769 SUPPORT Human Clinical
"Interictal multifocal epileptiform discharges were observed on EEG, and ictal activity migrating from one hemisphere to the other was recorded"
Documents the interictal and migrating ictal EEG findings in affected children.
PMID:26333769 SUPPORT Human Clinical
"It is characterized by multifocal seizures, developmental arrest or regression and a distinct ictal pattern on electroencephalogram"
Establishes the distinct ictal EEG pattern as part of the syndrome definition.
PMID:27436767 SUPPORT Human Clinical
"Interictal EEG was initially normal in 2 of 3 patients, and subsequently showed slow-wave activity or focal epileptic discharges compatible with EIFMS."
Records the important caveat that an early normal interictal EEG does not exclude the diagnosis.
📈

Progression

1
Course and mortality
Age: Infancy to childhood
The course is severe and static-to-progressive rather than relapsing. Reported individuals range from profound disability with early death to somewhat milder courses with late acquisition of ambulation or single words. Death is usually from respiratory infection, the terminus of the hypotonia, unsafe-swallow, and recurrent-aspiration branch already modeled in the phenotypes.
Show evidence (2 references)
PMID:38660387 SUPPORT Human Clinical
"She died due to pneumonia at the age of 4 years and 5 months."
Documents death from pneumonia, connecting the aspiration phenotype to mortality.
PMID:30763027 SUPPORT Human Clinical
"Of these nine children, six had severe developmental delay with no progress of abilities and three made notable neurodevelopmental progress."
Gives the range of developmental outcome across the published cohort.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Nine children were reported in the GeneReviews cohort; fewer than twenty genetically confirmed individuals have been published in aggregate.
Show evidence (1 reference)
PMID:30763027 SUPPORT Human Clinical
"SLC12A5-related epilepsy of infancy with migrating focal seizures (SLC12A5-EIMFS), reported to date in nine children"
GeneReviews gives the published case count.
🐁

Animal Models

3
Kcc2 constitutive knockout mouse
Homozygous Kcc2-null mice die immediately after birth from motor deficits that abolish respiration, and embryonic spinal motoneurons show excitatory rather than inhibitory GABA and glycine responses.
Species
Mouse
Genotype
Slc12a5 homozygous null
Publication
PMID:11395011
Pathograph links
Depolarizing Shift in the GABA-A and Glycine Reversal Potential recapitulates fidelity: high Direct demonstration that removing KCC2 makes GABA and glycine depolarizing.
Limitations: Complete loss of function is perinatally lethal in the mouse, whereas human DEE34 genotypes retain partial transport activity and affected children survive infancy; the null model therefore represents a more extreme lesion than the human disease.
Readouts
GABA and glycine response polarity in spinal motoneurons
Direction: ALTERED
Interpretation: Loss of KCC2 converts inhibitory transmitter responses to excitatory ones.
Show evidence (1 reference)
PMID:11395011 SUPPORT Model Organism
"Patch-clamp measurements of embryonic day 18.5 spinal cord motoneurons demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2"
Reports the patch-clamp measurement behind this readout.
Show evidence (1 reference)
PMID:11395011 SUPPORT Model Organism
"We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration."
Establishes the model and its severity.
Dlx5-lineage conditional Kcc2 knockout mouse
Deleting KCC2 from forebrain GABAergic neurons produces early seizures, failure to thrive, and death in the second and third postnatal weeks, with a shift in cortical interneuron subtype composition despite normal interneuron migration.
Species
Mouse
Genotype
Dlx5-Cre; Slc12a5 conditional knockout (GABAergic neurons)
Publication
PMID:35370549
Pathograph links
Cortical Interneuron Subtype Imbalance recapitulates fidelity: moderate Provides the direct evidence for the interneuron subtype imbalance node.
Limitations: The deletion is restricted to Dlx5-lineage GABAergic neurons, whereas human DEE34 variants affect KCC2 in all neurons; the model therefore isolates the GABAergic contribution rather than reproducing the human genotype.
Readouts
Cortical interneuron subtype distribution by layer
Direction: ALTERED
Interpretation: Somatostatin-expressing neurons are in excess in layer 5 while parvalbumin-expressing neurons are reduced in layers 2/3 and 6.
Show evidence (1 reference)
PMID:35370549 SUPPORT Model Organism
"an excess number of somatostatin neurons in layer 5 and a decrease in parvalbumin-expressing neurons in layer 2/3 and layer 6"
Reports the quantified subtype imbalance.
Show evidence (1 reference)
PMID:35370549 SUPPORT Model Organism
"the mice had early seizures, failure to thrive, and premature death in the second and third weeks of life"
Establishes the seizure phenotype that makes this model informative.
Kcc2b isoform-specific knockout mouse
Mice lacking only the neuron-specific KCC2b splice isoform, while retaining KCC2a, survive the neonatal period and die around two weeks of age from spontaneous seizures. Because it leaves residual KCC2 function, this model sits closer to the human partial-loss genotypes than the constitutive null does.
Species
Mouse
Genotype
Slc12a5 KCC2b-isoform-specific knockout
Publication
PMID:27436767
Pathograph links
Reduced KCC2-Mediated Chloride Extrusion partially recapitulates fidelity: moderate Models a survivable partial reduction in KCC2 function with spontaneous seizures, unlike the perinatally lethal complete null.
Limitations: The residual function comes from retaining a whole isoform rather than from a hypomorphic missense allele as in patients, and death at two weeks is still far earlier than the human course; the model also cannot address the chloride-independent structural arm.
Show evidence (1 reference)
PMID:27436767 SUPPORT Model Organism
"KCC2b-specific knockout mice survive for up to 2 weeks, but die due to spontaneous seizures"
Establishes the model's survival and seizure phenotype.
{ }

Source YAML

click to show 
name: SLC12A5-Related Developmental and Epileptic Encephalopathy
creation_date: "2026-08-19T00:00:00Z"
category: Mendelian
synonyms:
- DEE34
- EIEE34
- SLC12A5-EIMFS
- SLC12A5-related epilepsy of infancy with migrating focal seizures
- KCC2 deficiency disorder
- developmental and epileptic encephalopathy 34
description: >-
  SLC12A5-related developmental and epileptic encephalopathy (DEE34) is an
  ultra-rare autosomal recessive epileptic encephalopathy caused by biallelic
  loss-of-function variants in SLC12A5, which encodes KCC2, the neuron-specific
  K+-Cl- cotransporter. KCC2 uses the outward potassium gradient established by
  the Na+/K+-ATPase to extrude chloride from neurons, keeping intracellular
  chloride low enough that opening a GABA-A or glycine receptor drives chloride
  inward and hyperpolarizes the cell. When KCC2 function is reduced, intraneuronal
  chloride rises, the GABA-A reversal potential shifts depolarized, and fast
  synaptic inhibition fails. The clinical counterpart is epilepsy of infancy with
  migrating focal seizures (EIMFS, formerly malignant migrating partial seizures
  of infancy): focal motor seizures beginning in the first months of life that
  become multifocal and migrating, are pharmacoresistant, and are accompanied by
  developmental delay or regression, axial hypotonia, and postnatal microcephaly.
  KCC2 additionally has transport-independent structural roles in dendritic spine
  and excitatory-synapse maturation, so some variant combinations disturb
  neurodevelopment through a second, chloride-independent route. Fewer than twenty
  genetically confirmed individuals have been published, so the phenotype is
  described qualitatively rather than with frequency percentages.
disease_term:
  preferred_term: developmental and epileptic encephalopathy, 34
  term:
    id: MONDO:0014718
    label: developmental and epileptic encephalopathy, 34
parents:
- Neurodevelopmental Disorder
- Genetic Disease

references:
- reference: PMID:30763027
  title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
  tags:
  - GeneReviews

inheritance:
- name: Autosomal recessive
  description: >-
    DEE34 requires two pathogenic SLC12A5 alleles (homozygous or compound
    heterozygous). Parents are typically unaffected heterozygous carriers, and
    consanguinity is a recognized contributor to homozygous presentations.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLC12A5-EIMFS is inherited in an autosomal recessive manner."
    explanation: >-
      GeneReviews states the mode of inheritance directly.
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we show recessive loss-of-function SLC12A5 mutations in patients with a severe infantile-onset pharmacoresistant epilepsy syndrome"
    explanation: >-
      The founding study established the recessive loss-of-function mechanism.

pathophysiology:
- name: Biallelic SLC12A5 Loss-of-Function Variant
  biological_scale: MOLECULAR
  conforms_to: "epilepsy_excitation_inhibition_imbalance#Ion Channel and Synaptic Dysfunction"
  description: >-
    Two pathogenic SLC12A5 alleles - homozygous or compound heterozygous missense,
    splice-site, or in-frame deletion variants - are present in the affected
    individual. This node records only the genomic lesion; its separate protein-level
    consequences are modeled downstream.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we report a novel genetic cause of EIMFS, identifying recessive SLC12A5 mutations in four affected children from two unrelated families"
    explanation: >-
      Establishes biallelic SLC12A5 variants as the causal genomic lesion.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS revealed compound heterozygous SLC12A5"
    explanation: >-
      Independent replication of the biallelic genotype in unrelated families.
  downstream:
  - target: Impaired KCC2 Folding, Glycosylation, and Membrane Trafficking
    causal_link_type: DIRECT
    description: >-
      Some variants act by preventing the transporter from reaching the plasma
      membrane in normal amounts.
  - target: Reduced Intrinsic KCC2 Chloride Transport Activity
    causal_link_type: DIRECT
    description: >-
      Other variants reach the surface normally but transport chloride poorly, so
      the transport deficit is not explained by trafficking alone.
  - target: Disrupted Transport-Independent KCC2 Structural Function
    causal_link_type: DIRECT
    description: >-
      Variants affecting the intracellular C-terminal domain disturb KCC2's
      scaffolding role independently of ion transport.

- name: Impaired KCC2 Folding, Glycosylation, and Membrane Trafficking
  biological_scale: MOLECULAR
  description: >-
    Misfolded KCC2 variants are under-glycosylated and retained in the endoplasmic
    reticulum, where they are degraded by ER-associated degradation, so less
    transporter reaches the neuronal plasma membrane. This is a trafficking defect
    only; the residual transport competence of the protein that does arrive is a
    separate question.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: ERAD pathway
    term:
      id: GO:0036503
      label: ERAD pathway
    modifier: INCREASED
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Decreased KCC2 surface expression, reduced protein glycosylation and impaired chloride extrusion contribute to loss of KCC2 activity"
    explanation: >-
      Directly reports reduced surface expression and glycosylation of mutant KCC2.
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Based on surface immunolabeling, the variant showed a reduction in membrane expression."
    explanation: >-
      Surface immunolabeling of the R231H variant confirms reduced membrane delivery.
  downstream:
  - target: Reduced KCC2-Mediated Chloride Extrusion
    causal_link_type: DIRECT
    description: >-
      Fewer transporters at the membrane means less total chloride extrusion capacity.

- name: Reduced Intrinsic KCC2 Chloride Transport Activity
  biological_scale: MOLECULAR
  description: >-
    Variants that reach the plasma membrane at normal density can still transport
    chloride poorly, so the per-molecule catalytic activity of the cotransporter is
    reduced independently of how much protein is present. Saitsu and colleagues
    documented mutant KCC2 with surface expression comparable to wild type but
    strongly suppressed chloride extrusion.
  molecular_functions:
  - preferred_term: potassium:chloride symporter activity
    term:
      id: GO:0015379
      label: potassium:chloride symporter activity
    modifier: DECREASED
  evidence:
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Cell surface expression levels of these KCC2 mutants were similar to wildtype KCC2."
    explanation: >-
      Establishes that transport loss can occur without any trafficking deficit,
      justifying this node as separate from the trafficking node.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Gramicidin-perforated patch-clamp analysis demonstrated strongly suppressed Cl(-) extrusion function of E50_Q93del and M415V mutants, with mildly impaired function of A191V and S323P mutants."
    explanation: >-
      Quantifies variant-specific loss of intrinsic transport activity.
  downstream:
  - target: Reduced KCC2-Mediated Chloride Extrusion
    causal_link_type: DIRECT
    description: >-
      Lower catalytic activity per transporter reduces net chloride extrusion.

- name: Reduced KCC2-Mediated Chloride Extrusion
  biological_scale: CELLULAR
  description: >-
    The neuron's total capacity to move chloride out against its electrochemical
    gradient falls. Human variants produce partial rather than complete loss, which
    is why affected children survive infancy whereas Kcc2-null mice do not.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: chloride transmembrane transport
    term:
      id: GO:1902476
      label: chloride transmembrane transport
    modifier: DECREASED
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "gramicidin-perforated patch-clamp experiments and ammonium flux assay, both of which indicated a significant reduction in chloride extrusion"
    explanation: >-
      Two independent assays show reduced chloride extrusion by the variant transporter.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functional analysis using the gramicidin-perforated patch-clamp technique confirmed significant, but not complete, loss of KCC2 function in the patients."
    explanation: >-
      Establishes that the loss is partial, which is the key contrast with the
      perinatally lethal complete-null mouse.
  downstream:
  - target: Elevated Intraneuronal Chloride Concentration
    causal_link_type: DIRECT
    description: >-
      Chloride entering the neuron is no longer cleared at the normal rate.

- name: Elevated Intraneuronal Chloride Concentration
  biological_scale: CELLULAR
  description: >-
    Intracellular chloride rises above the roughly 4-8 mM maintained in healthy
    mature neurons. This is a change in the ionic setpoint itself, distinct from its
    electrophysiological consequence for receptor-mediated currents.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: chloride ion homeostasis
    term:
      id: GO:0055064
      label: chloride ion homeostasis
    modifier: ABNORMAL
  evidence:
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Heterologous expression of two KCC2 mutants, mimicking the patient status, produced a significantly greater intracellular Cl(-) level than with wildtype KCC2"
    explanation: >-
      Directly measures the elevated intracellular chloride produced by the patient
      genotype.
  downstream:
  - target: Depolarizing Shift in the GABA-A and Glycine Reversal Potential
    causal_link_type: DIRECT
    description: >-
      Because chloride flux through these receptors is passive, the reversal
      potential follows the chloride gradient.

- name: Depolarizing Shift in the GABA-A and Glycine Reversal Potential
  biological_scale: CELLULAR
  description: >-
    The reversal potential for GABA-A and glycine receptor currents moves positive
    of the resting membrane potential, so opening these channels drives chloride out
    of the cell and depolarizes it. This is the electrophysiological state change;
    the resulting failure of synaptic inhibition is modeled separately.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: regulation of membrane potential
    term:
      id: GO:0042391
      label: regulation of membrane potential
    modifier: ABNORMAL
  evidence:
  - reference: PMID:11395011
    reference_title: "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2"
    explanation: >-
      Shows that removing KCC2 converts GABA and glycine responses from inhibitory
      to excitatory.
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "is essential for this Cl--mediated signaling to be inhibitory as Cl- flux through these receptor channels is passive"
    explanation: >-
      States the biophysical dependence of the reversal potential on intracellular
      chloride.
  downstream:
  - target: Failure of Fast Synaptic Inhibition
    causal_link_type: DIRECT
    description: >-
      A depolarized reversal potential removes the hyperpolarizing and shunting
      effect of GABAergic input.

- name: Failure of Fast Synaptic Inhibition
  biological_scale: CELLULAR
  conforms_to: "epilepsy_excitation_inhibition_imbalance#Excitation-Inhibition Imbalance"
  description: >-
    GABAergic and glycinergic synapses no longer raise the action-potential
    threshold of their targets. Excitatory drive is unopposed even though
    glutamatergic transmission itself is not primarily affected.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  - preferred_term: GABAergic neuron
    term:
      id: CL:0000617
      label: GABAergic neuron
  biological_processes:
  - preferred_term: inhibitory postsynaptic potential
    term:
      id: GO:0060080
      label: inhibitory postsynaptic potential
    modifier: DECREASED
  - preferred_term: gamma-aminobutyric acid signaling pathway
    term:
      id: GO:0007214
      label: gamma-aminobutyric acid signaling pathway
    modifier: ABNORMAL
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "thereby impairing normal synaptic inhibition and promoting neuronal excitability in this early-onset epileptic encephalopathy"
    explanation: >-
      Links loss of KCC2 activity to impaired synaptic inhibition.
  - reference: PMID:11395011
    reference_title: "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "revealing a crucial role of KCC2 for synaptic inhibition"
    explanation: >-
      Genetic removal of KCC2 abolishes functional synaptic inhibition.
  downstream:
  - target: Neuronal Hyperexcitability and Hypersynchrony
    causal_link_type: DIRECT
    description: >-
      Unopposed excitation lowers the threshold for synchronized network discharge.
  - target: Impaired Termination of Ictal Discharges
    causal_link_type: DIRECT
    description: >-
      Chloride loading during a discharge cannot be cleared, so the discharge is not
      curtailed.

- name: Disrupted Transport-Independent KCC2 Structural Function
  biological_scale: MOLECULAR
  description: >-
    Beyond ion transport, KCC2 interacts through its large intracellular C-terminal
    domain with cytoskeletal and synaptic proteins to regulate dendritic spine
    formation and glutamatergic synapse plasticity. Variants can compromise this
    scaffolding role, and this arm is not captured by any chloride measurement.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: dendritic spine morphogenesis
    term:
      id: GO:0060997
      label: dendritic spine morphogenesis
    modifier: ABNORMAL
  evidence:
  - reference: PMID:35370549
    reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Independently of its transporter function, KCC2 facilitates a number of processes that are central to circuit development, such as actin re-arrangement/formation of dendritic spines and plasticity of glutamatergic synapses"
    explanation: >-
      States the transport-independent structural role of KCC2 explicitly.
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLC12A5 encodes the potassium-chloride co-transporter KCC2, which plays an integral role in neuronal inhibition4567 and maturation of cortical dendritic spines8."
    explanation: >-
      The founding disease paper names dendritic spine maturation as a second KCC2 role.
  downstream:
  - target: Impaired Excitatory Synapse and Dendritic Spine Maturation
    causal_link_type: DIRECT
    description: >-
      Loss of the scaffolding interaction disturbs spine and synapse formation.

- name: Impaired Excitatory Synapse and Dendritic Spine Maturation
  biological_scale: CELLULAR
  description: >-
    Dendritic spines and glutamatergic synapses form and mature abnormally. This is
    a structural developmental deficit that would persist even if chloride handling
    were corrected.
  cell_types:
  - preferred_term: pyramidal neuron
    term:
      id: CL:0000598
      label: pyramidal neuron
  biological_processes:
  - preferred_term: dendritic spine morphogenesis
    term:
      id: GO:0060997
      label: dendritic spine morphogenesis
    modifier: DECREASED
  evidence:
  - reference: PMID:35370549
    reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "KCC2 directly interacts with a number of proteins to regulate dendritic spine formation, cell survival, synaptic plasticity, neuronal excitability, and other processes."
    explanation: >-
      Supports spine formation as a KCC2-dependent process disturbed by its loss.
  downstream:
  - target: Impaired Neurodevelopment
    causal_link_type: DIRECT
    description: >-
      Abnormal synapse maturation contributes to the developmental phenotype
      independently of seizure burden.

- name: Cortical Interneuron Subtype Imbalance
  biological_scale: TISSUE
  description: >-
    In mice lacking KCC2 in Dlx5-lineage GABAergic neurons, cortical interneurons
    reach their normal laminar positions but their subtype composition is skewed -
    excess somatostatin-expressing neurons in layer 5 and fewer parvalbumin-expressing
    neurons in layers 2/3 and 6. This is a change in inhibitory circuit composition
    rather than in the moment-to-moment strength of inhibition.
  cell_types:
  - preferred_term: cerebral cortex GABAergic interneuron
    term:
      id: CL:0010011
      label: cerebral cortex GABAergic interneuron
  evidence:
  - reference: PMID:35370549
    reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "we found an underlying change in IN distribution, including an excess number of somatostatin neurons in layer 5 and a decrease in parvalbumin-expressing neurons in layer 2/3 and layer 6"
    explanation: >-
      Reports the specific interneuron subtype imbalance in the conditional knockout.
  downstream:
  - target: Neuronal Hyperexcitability and Hypersynchrony
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      An altered inhibitory circuit composition is proposed to contribute to
      hyperexcitability, though the cellular basis is not yet established.

- name: Impaired Termination of Ictal Discharges
  biological_scale: TISSUE
  description: >-
    KCC2-mediated chloride transport normally restores the chloride equilibrium
    after intense GABA-A receptor activation, which helps bring an ictal discharge
    to an end. With reduced KCC2, chloride accumulated during a discharge is cleared
    slowly and the discharge is prolonged. This concerns seizure offset, not seizure
    onset.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:33239270
    reference_title: "KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the putative KCC2 upregulator CLP257 improved chloride homeostasis and reduced the duration and frequency of ILDs in a concentration-dependent manner"
    explanation: >-
      Pharmacologically enhancing KCC2 shortens ictal discharges, supporting a role
      for KCC2-mediated chloride transport in seizure termination.
  - reference: PMID:33239270
    reference_title: "KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "increased ILD duration, and induced status epilepticus"
    explanation: >-
      The reciprocal manipulation - blocking KCC2 - prolongs ictal discharges and
      precipitates status epilepticus.
  downstream:
  - target: Migrating Multifocal Seizure Activity
    causal_link_type: DIRECT
    description: >-
      Discharges that fail to self-terminate become prolonged and recurrent.

- name: Neuronal Hyperexcitability and Hypersynchrony
  biological_scale: TISSUE
  conforms_to: "epilepsy_excitation_inhibition_imbalance#Neuronal Hyperexcitability and Hypersynchrony"
  description: >-
    Cortical networks discharge at lower threshold and in synchrony. Because KCC2 is
    expressed throughout the cortex, susceptibility is diffuse rather than confined
    to one epileptogenic focus.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: modulation of chemical synaptic transmission
    term:
      id: GO:0050804
      label: modulation of chemical synaptic transmission
    modifier: ABNORMAL
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These findings implicate pathogenicity of the SLC12A5 variant that leads to impaired inhibitory neurotransmission, increasing probability for hyperexcitability and epileptogenesis."
    explanation: >-
      Connects the transport deficit to network hyperexcitability and epileptogenesis.
  downstream:
  - target: Migrating Multifocal Seizure Activity
    causal_link_type: DIRECT
    description: >-
      Diffuse hyperexcitability with no fixed focus produces seizures that arise
      independently at successive cortical sites.

- name: Migrating Multifocal Seizure Activity
  biological_scale: ORGANISM
  conforms_to: "epilepsy_excitation_inhibition_imbalance#Seizure Generation and Epileptogenesis"
  description: >-
    Seizures begin as focal motor events, typically with head and eye deviation, and
    evolve into near-continuous multifocal discharges that migrate from one cortical
    region to another - the defining electroclinical signature of EIMFS - and resist
    conventional anti-seizure medication.
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In most children epilepsy begins as focal motor seizures (typically involving head and eye deviation) that become multifocal and intractable to conventional anti-seizure medication (ASM)."
    explanation: >-
      GeneReviews describes the seizure evolution and its pharmacoresistance.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "partially disrupted neuronal Cl(-) extrusion, mediated by two types of differentially impaired KCC2 mutant in an individual, causes EIMFS"
    explanation: >-
      Attributes the migrating focal seizure phenotype to partial chloride-extrusion loss.
  downstream:
  - target: Impaired Neurodevelopment
    causal_link_type: DIRECT
    description: >-
      High seizure burden in the first months of life compounds the developmental
      consequences of the underlying synaptic defect.

- name: Impaired Neurodevelopment
  biological_scale: ORGANISM
  description: >-
    Development either arrests or regresses at the time of seizure onset, yielding
    severe to profound intellectual disability, axial hypotonia, and postnatal
    microcephaly. Two upstream routes converge here - the seizure burden itself and
    the transport-independent synapse maturation defect - and the published cases do
    not resolve their relative contributions.
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "is characterized by onset of seizures before age six months and either developmental delay or developmental regression with seizure onset"
    explanation: >-
      GeneReviews couples the developmental phenotype to seizure onset.
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified a novel recessively inherited SLC12A5 c.692G>A, p. (R231H) variant in a patient diagnosed with severe and drug-resistant EIMFS and profound intellectual disability"
    explanation: >-
      Documents profound intellectual disability accompanying the epilepsy.

phenotypes:
- category: Neurological
  name: Migrating Multifocal Seizures
  description: >-
    Focal motor seizures beginning before six months of age that become multifocal
    and migrate between cortical regions, resistant to conventional anti-seizure
    medication.
  phenotype_term:
    preferred_term: Migrating focal seizure
    term:
      id: HP:0032786
      label: Migrating focal seizure
    onset:
      onset_category: INFANTILE
      max_age_years: 0.5
      notes: >-
        Onset before six months of age is part of the defining description; the
        published cohort has a median of about 1.5 months, and some biallelic cases
        present within the first day of life.
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In most children epilepsy begins as focal motor seizures (typically involving head and eye deviation) that become multifocal and intractable to conventional anti-seizure medication (ASM)."
    explanation: >-
      GeneReviews describes the defining seizure phenotype.
- category: Neurological
  name: Focal-Onset Seizures
  description: >-
    Initial seizure semiology is focal motor, typically with head and eye deviation.
  phenotype_term:
    preferred_term: Focal-onset seizure
    term:
      id: HP:0007359
      label: Focal-onset seizure
    onset:
      onset_category: INFANTILE
      max_age_years: 0.5
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "epilepsy begins as focal motor seizures (typically involving head and eye deviation)"
    explanation: >-
      Records the focal onset semiology.
- category: Neurological
  name: Developmental Delay
  description: >-
    All reported children have developmental delay; in the GeneReviews cohort six of
    nine had severe delay with no progress of abilities.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of these nine children, six had severe developmental delay with no progress of abilities and three made notable neurodevelopmental progress."
    explanation: >-
      All nine children in the cohort had delay or regression, supporting a
      very frequent band.
- category: Neurological
  name: Developmental Regression
  description: >-
    Loss of previously acquired skills coinciding with seizure onset.
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "either developmental delay or developmental regression with seizure onset"
    explanation: >-
      Regression at seizure onset is part of the defining clinical description.
- category: Neurological
  name: Hypotonia
  description: >-
    Axial hypotonia was present in eight of the nine children in the GeneReviews
    cohort.
  phenotype_term:
    preferred_term: Axial hypotonia
    term:
      id: HP:0008936
      label: Axial hypotonia
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Eight had postnatal microcephaly and hypotonia."
    explanation: >-
      Eight of nine children is within the 80-100% very frequent band.
- category: Neurological
  name: Postnatal Microcephaly
  description: >-
    Head circumference falls off after birth, reported in eight of nine children.
  phenotype_term:
    preferred_term: Secondary microcephaly
    term:
      id: HP:0005484
      label: Secondary microcephaly
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Eight had postnatal microcephaly and hypotonia."
    explanation: >-
      Eight of nine children is within the 80-100% very frequent band.
- category: Neurological
  name: Intellectual Disability
  description: >-
    Severe to profound intellectual disability in the most affected individuals.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
    severity: SEVERE
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a patient diagnosed with severe and drug-resistant EIMFS and profound intellectual disability"
    explanation: >-
      Documents profound intellectual disability in a genetically confirmed case.
- category: Neurological
  name: Cerebral Atrophy
  description: >-
    Cerebral atrophy on brain MRI, the most common neuroimaging finding, present in
    all four genotyped individuals in the Saitsu cohort.
  phenotype_term:
    preferred_term: Cerebral atrophy
    term:
      id: HP:0002059
      label: Cerebral atrophy
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
    explanation: >-
      Cerebral atrophy in four of four genotyped individuals supports the 80-99%
      very frequent band. Four of four is strictly obligate, but asserting obligate
      presence from a denominator of four would over-claim.
- category: Neurological
  name: Delayed Myelination
  description: >-
    Delayed myelination on brain MRI, reported in three of four genotyped individuals.
  phenotype_term:
    preferred_term: Delayed myelination
    term:
      id: HP:0012448
      label: Delayed myelination
  frequency: FREQUENT
  evidence:
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
    explanation: >-
      Delayed myelination in three of four individuals is 75%, which falls in the
      30-79% frequent band. The denominator is small.
- category: Neurological
  name: Thin Corpus Callosum
  description: >-
    A thin corpus callosum on brain MRI in two of four genotyped individuals. This is
    the milder end of the same callosal spectrum as the agenesis found at autopsy in
    the severe R231H case.
  phenotype_term:
    preferred_term: Thin corpus callosum
    term:
      id: HP:0033725
      label: Thin corpus callosum
  frequency: FREQUENT
  evidence:
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain MRI showed cerebral atrophy in four individuals, and delayed myelination and thin corpus callosum in three and two individuals, respectively."
    explanation: >-
      Thin corpus callosum in two of four individuals falls in the 30-79% frequent band.
- category: Neurological
  name: Agenesis of the Corpus Callosum
  description: >-
    Complete callosal agenesis, the severe end of the callosal spectrum whose milder
    form is the thin corpus callosum seen on MRI in the Saitsu cohort. Autopsy of the
    homozygous R231H patient, who died at 4 years 5 months, showed agenesis of the
    corpus callosum. Note this is a single autopsied case rather than a cohort
    frequency, so no frequency band is asserted.
  phenotype_term:
    preferred_term: Agenesis of corpus callosum
    term:
      id: HP:0001274
      label: Agenesis of corpus callosum
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
    explanation: >-
      Direct neuropathological documentation of the callosal defect.
- category: Neurological
  name: Ventriculomegaly
  description: >-
    Markedly enlarged lateral ventricles, documented at autopsy in the severe
    R231H case.
  phenotype_term:
    preferred_term: Ventriculomegaly
    term:
      id: HP:0002119
      label: Ventriculomegaly
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
    explanation: >-
      The same autopsy documents markedly enlarged lateral ventricles.
- category: Neurological
  name: Cerebral White Matter Hypoplasia
  description: >-
    Scarcity of cerebral white matter on neuropathological examination in the
    severe R231H case.
  phenotype_term:
    preferred_term: Cerebral white matter hypoplasia
    term:
      id: HP:0012430
      label: Cerebral white matter hypoplasia
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autopsy of her brain revealed agenesis of corpus callosum, markedly enlarged lateral ventricles and scarcity of white matter in her brain."
    explanation: >-
      The same autopsy documents scarcity of cerebral white matter.
- category: Gastrointestinal
  name: Feeding Difficulties
  description: >-
    Feeding, swallowing, and reflux problems requiring multidisciplinary management
    and sometimes gastrostomy.
  phenotype_term:
    preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A multidisciplinary approach to management of hypotonia, feeding difficulties, respiratory problems, and developmental delay is recommended."
    explanation: >-
      GeneReviews management guidance names feeding difficulties as a recognized problem.
- category: Respiratory
  name: Recurrent Aspiration and Respiratory Problems
  description: >-
    Aspiration and recurrent respiratory infection are a leading cause of death,
    reflecting hypotonia and unsafe swallow.
  phenotype_term:
    preferred_term: Recurrent aspiration pneumonia
    term:
      id: HP:0002100
      label: Recurrent aspiration pneumonia
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Routine monitoring of: feeding, nutritional status, swallowing, gastroesophageal reflux, aspiration, and respiratory problems"
    explanation: >-
      GeneReviews surveillance guidance identifies aspiration and respiratory
      problems as expected complications.

genetic:
- name: SLC12A5
  association: Biallelic pathogenic variants
  presence: Positive
  notes: >-
    SLC12A5 encodes KCC2, the neuron-specific K+-Cl- cotransporter. Biallelic
    pathogenic variants - missense, splice-site, and in-frame deletions - cause
    DEE34. Reported variants include c.279+1G>C (p.E50_Q93del), c.572C>T (p.A191V),
    c.967T>C (p.S323P), c.1243A>G (p.M415V), c.953G>C (p.W318S), c.2242_2244del
    (p.S748del), and the homozygous transmembrane-domain-4 variant c.692G>A
    (p.R231H). Expressivity is variable within this biallelic recessive condition:
    of the nine children in the GeneReviews cohort, six had severe developmental
    delay with no progress of abilities while three made notable neurodevelopmental
    progress. Severity is thought to track the residual chloride-extrusion activity
    retained by the particular allele combination, and individual variants in one
    person can be impaired to different degrees.
  gene_term:
    preferred_term: SLC12A5
    term:
      id: hgnc:13818
      label: SLC12A5
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we show recessive loss-of-function SLC12A5 mutations in patients with a severe infantile-onset pharmacoresistant epilepsy syndrome, epilepsy of infancy with migrating focal seizures (EIMFS)."
    explanation: >-
      Establishes SLC12A5 as the causal gene.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "c.279 + 1G > C causing skipping of exon 3 in the transcript (p.E50_Q93del) and c.572 C >T (p.A191V)"
    explanation: >-
      Reports specific compound heterozygous variants.
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a novel recessively inherited SLC12A5 c.692G>A, p. (R231H) variant"
    explanation: >-
      Documents the homozygous R231H variant.

diagnosis:
- name: Molecular Genetic Testing for Biallelic SLC12A5 Variants
  description: >-
    The diagnosis is established by identifying two pathogenic SLC12A5 alleles.
    Because affected individuals carry private variants and EIMFS is genetically
    heterogeneous, exome or genome sequencing rather than single-gene testing is the
    practical first-tier route.
  diagnosis_term:
    preferred_term: whole exome sequencing
    term:
      id: NCIT:C101295
      label: Whole Exome Sequencing
  presence: Positive in affected individuals
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of SLC12A5-EIMFS is established by identification of biallelic SLC12A5 pathogenic variants on molecular genetic testing."
    explanation: >-
      GeneReviews states the confirmatory diagnostic test directly.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS revealed compound heterozygous SLC12A5"
    explanation: >-
      Documents exome sequencing as the method that identifies the biallelic genotype.
- name: Electroencephalography
  description: >-
    EEG carries the electroclinical signature of EIMFS: interictal multifocal
    epileptiform discharges together with ictal activity that migrates from one
    hemisphere to the other. This migrating ictal pattern - not any single seizure
    type - is what defines the syndrome. Interictal EEG may be normal early and
    only later show slow-wave activity or focal discharges.
  diagnosis_term:
    preferred_term: electroencephalography
    term:
      id: NCIT:C38054
      label: Electroencephalography
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interictal multifocal epileptiform discharges were observed on EEG, and ictal activity migrating from one hemisphere to the other was recorded"
    explanation: >-
      Documents the interictal and migrating ictal EEG findings in affected children.
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized by multifocal seizures, developmental arrest or regression and a distinct ictal pattern on electroencephalogram"
    explanation: >-
      Establishes the distinct ictal EEG pattern as part of the syndrome definition.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interictal EEG was initially normal in 2 of 3 patients, and subsequently showed slow-wave activity or focal epileptic discharges compatible with EIFMS."
    explanation: >-
      Records the important caveat that an early normal interictal EEG does not
      exclude the diagnosis.

treatments:
- name: Anti-Seizure Medication
  description: >-
    No SLC12A5-specific therapy exists and seizures in EIMFS are resistant to most
    anti-seizure medications. Potassium bromide and a ketogenic diet each produced
    attenuation of seizures in three individuals in the GeneReviews cohort.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: potassium bromide
      term:
        id: CHEBI:32030
        label: potassium bromide
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There are no specific treatments for seizures in SLC12A5-EIMFS. In general, seizures in EIMFS are resistant to most ASM."
    explanation: >-
      GeneReviews states the absence of specific therapy and general pharmacoresistance.
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A ketogenic diet and potassium bromide showed attenuation of seizures in three individuals each."
    explanation: >-
      Documents the limited observed benefit of potassium bromide.
- name: Ketogenic Diet
  description: >-
    Dietary therapy that attenuated seizures in three individuals in the GeneReviews
    cohort; response is variable and not universal.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Dietary Intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A ketogenic diet and potassium bromide showed attenuation of seizures in three individuals each."
    explanation: >-
      Reports partial seizure attenuation with ketogenic diet in a minority of cases.
- name: Multidisciplinary Supportive Care
  description: >-
    Management of hypotonia, feeding difficulties, respiratory problems, and
    developmental delay, with surveillance for scoliosis and hip dislocation.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A multidisciplinary approach to management of hypotonia, feeding difficulties, respiratory problems, and developmental delay is recommended."
    explanation: >-
      GeneReviews management recommendation.
- name: Genetic Counseling
  description: >-
    Autosomal recessive counseling with a 25% recurrence risk for siblings; carrier,
    prenatal, and preimplantation testing become possible once the familial variants
    are known.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Once the SLC12A5 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible."
    explanation: >-
      GeneReviews genetic counseling guidance.

animal_models:
- name: Kcc2 constitutive knockout mouse
  species: Mouse
  genotype: Slc12a5 homozygous null
  publication: PMID:11395011
  description: >-
    Homozygous Kcc2-null mice die immediately after birth from motor deficits that
    abolish respiration, and embryonic spinal motoneurons show excitatory rather
    than inhibitory GABA and glycine responses.
  modeled_mechanisms:
  - target: Depolarizing Shift in the GABA-A and Glycine Reversal Potential
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Direct demonstration that removing KCC2 makes GABA and glycine depolarizing.
    limitations: >-
      Complete loss of function is perinatally lethal in the mouse, whereas human
      DEE34 genotypes retain partial transport activity and affected children survive
      infancy; the null model therefore represents a more extreme lesion than the
      human disease.
    readouts:
    - name: GABA and glycine response polarity in spinal motoneurons
      target: Depolarizing Shift in the GABA-A and Glycine Reversal Potential
      direction: ALTERED
      interpretation: >-
        Loss of KCC2 converts inhibitory transmitter responses to excitatory ones.
      evidence:
      - reference: PMID:11395011
        reference_title: "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Patch-clamp measurements of embryonic day 18.5 spinal cord motoneurons demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2"
        explanation: >-
          Reports the patch-clamp measurement behind this readout.
    evidence:
    - reference: PMID:11395011
      reference_title: "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration."
      explanation: >-
        Establishes the model and its severity.
- name: Dlx5-lineage conditional Kcc2 knockout mouse
  species: Mouse
  genotype: Dlx5-Cre; Slc12a5 conditional knockout (GABAergic neurons)
  publication: PMID:35370549
  description: >-
    Deleting KCC2 from forebrain GABAergic neurons produces early seizures, failure
    to thrive, and death in the second and third postnatal weeks, with a shift in
    cortical interneuron subtype composition despite normal interneuron migration.
  modeled_mechanisms:
  - target: Cortical Interneuron Subtype Imbalance
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Provides the direct evidence for the interneuron subtype imbalance node.
    limitations: >-
      The deletion is restricted to Dlx5-lineage GABAergic neurons, whereas human
      DEE34 variants affect KCC2 in all neurons; the model therefore isolates the
      GABAergic contribution rather than reproducing the human genotype.
    readouts:
    - name: Cortical interneuron subtype distribution by layer
      target: Cortical Interneuron Subtype Imbalance
      direction: ALTERED
      interpretation: >-
        Somatostatin-expressing neurons are in excess in layer 5 while
        parvalbumin-expressing neurons are reduced in layers 2/3 and 6.
      evidence:
      - reference: PMID:35370549
        reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "an excess number of somatostatin neurons in layer 5 and a decrease in parvalbumin-expressing neurons in layer 2/3 and layer 6"
        explanation: >-
          Reports the quantified subtype imbalance.
    evidence:
    - reference: PMID:35370549
      reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "the mice had early seizures, failure to thrive, and premature death in the second and third weeks of life"
      explanation: >-
        Establishes the seizure phenotype that makes this model informative.
- name: Kcc2b isoform-specific knockout mouse
  species: Mouse
  genotype: Slc12a5 KCC2b-isoform-specific knockout
  publication: PMID:27436767
  description: >-
    Mice lacking only the neuron-specific KCC2b splice isoform, while retaining
    KCC2a, survive the neonatal period and die around two weeks of age from
    spontaneous seizures. Because it leaves residual KCC2 function, this model sits
    closer to the human partial-loss genotypes than the constitutive null does.
  modeled_mechanisms:
  - target: Reduced KCC2-Mediated Chloride Extrusion
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Models a survivable partial reduction in KCC2 function with spontaneous
      seizures, unlike the perinatally lethal complete null.
    limitations: >-
      The residual function comes from retaining a whole isoform rather than from a
      hypomorphic missense allele as in patients, and death at two weeks is still far
      earlier than the human course; the model also cannot address the
      chloride-independent structural arm.
    evidence:
    - reference: PMID:27436767
      reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "KCC2b-specific knockout mice survive for up to 2 weeks, but die due to spontaneous seizures"
      explanation: >-
        Establishes the model's survival and seizure phenotype.

prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Nine children were reported in the GeneReviews cohort; fewer than twenty
    genetically confirmed individuals have been published in aggregate.
  evidence:
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLC12A5-related epilepsy of infancy with migrating focal seizures (SLC12A5-EIMFS), reported to date in nine children"
    explanation: >-
      GeneReviews gives the published case count.

progression:
- phase: Course and mortality
  age_range: Infancy to childhood
  notes: >-
    The course is severe and static-to-progressive rather than relapsing. Reported
    individuals range from profound disability with early death to somewhat milder
    courses with late acquisition of ambulation or single words. Death is usually
    from respiratory infection, the terminus of the hypotonia, unsafe-swallow, and
    recurrent-aspiration branch already modeled in the phenotypes.
  evidence:
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She died due to pneumonia at the age of 4 years and 5 months."
    explanation: >-
      Documents death from pneumonia, connecting the aspiration phenotype to mortality.
  - reference: PMID:30763027
    reference_title: "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of these nine children, six had severe developmental delay with no progress of abilities and three made notable neurodevelopmental progress."
    explanation: >-
      Gives the range of developmental outcome across the published cohort.

discussions:
- discussion_id: controversy_kcc2_setpoint_vs_impermeant_anions
  prompt: >-
    Is the elevated intraneuronal chloride in KCC2 deficiency principally the direct
    consequence of lost cation-chloride cotransport, or is the neuronal chloride
    setpoint dominated by the distribution of impermeant intracellular anions, with
    KCC2 acting mainly to restore that setpoint after activity-driven chloride loads?
  kind: CONTROVERSY
  status: OPEN
  attaches_to:
  - pathophysiology#Reduced KCC2-Mediated Chloride Extrusion
  - pathophysiology#Elevated Intraneuronal Chloride Concentration
  rationale: >-
    The disease model curated here treats reduced KCC2 transport as the proximate
    cause of a raised chloride setpoint. An alternative account holds that the
    resting setpoint is fixed largely by fixed impermeant anionic charge, with KCC2
    responsible for clearing dynamic chloride loads rather than for setting resting
    chloride. The two readings predict different therapeutic targets: restoring KCC2
    transport capacity would be central under the first, whereas under the second the
    critical variable is the neuron's ability to recover from activity-driven loading,
    which would foreground interventions aimed at seizure-associated chloride
    accumulation. Both readings are compatible with the human variant data, since
    patient assays measure extrusion capacity rather than resting setpoint in vivo.
  evidence:
  - reference: PMID:35370549
    reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "has been debated in favor of alternative mechanisms, such as distribution of impermeable anionic charges"
    explanation: >-
      States the existence and content of the disagreement.
  proposed_experiments:
  - experiment_id: exp_chloride_imaging_patient_neurons
    name: Separate resting chloride setpoint from chloride recovery kinetics in patient neurons
    description: >-
      Measure resting intracellular chloride and post-load chloride recovery kinetics
      separately, using ratiometric chloride imaging in patient-derived iPSC neurons
      carrying DEE34 genotypes alongside isogenic controls, to determine whether the
      resting setpoint or the recovery rate is the parameter that the variants change.

- discussion_id: gap_kcc2_bidirectional_seizure_effect
  prompt: >-
    Given that both proconvulsant and anticonvulsant effects of KCC2 inhibition have
    been reported, under what circumstances does reduced KCC2 activity worsen rather
    than limit seizure activity, and does that boundary apply to the constitutive,
    lifelong partial deficiency of DEE34?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Impaired Termination of Ictal Discharges
  rationale: >-
    KCC2 extrudes chloride at the cost of raising extracellular potassium, which can
    itself depolarize neurons and potentiate seizures. Experimental KCC2 blockade has
    therefore been reported to be both pro- and anticonvulsant depending on which step
    is rate-limiting. All of that evidence comes from acute pharmacological
    manipulation in slice preparations; DEE34 is instead a constitutive partial
    deficiency present throughout development. Whether the acute-blockade findings
    transfer to the chronic genetic condition is unresolved, and it bears directly on
    whether KCC2-enhancing compounds would be expected to help these patients.
  evidence:
  - reference: PMID:33239270
    reference_title: "KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Thus, KCC2 may either diminish or facilitate seizure activity, and both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported."
    explanation: >-
      States the unresolved bidirectionality directly.
  - reference: PMID:33239270
    reference_title: "KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Anticonvulsant effects predominate when KCC2-mediated chloride transport rather than potassium buffering is the rate-limiting step in restoring ECl"
    explanation: >-
      Identifies the condition that determines the direction of effect, which is what
      would need to be established for the chronic genetic deficiency.
  proposed_experiments:
  - experiment_id: exp_kcc2_enhancer_in_hypomorphic_model
    name: Test a KCC2 enhancer in a constitutive hypomorphic Slc12a5 model
    description: >-
      Test a KCC2-enhancing compound against seizure burden in a hypomorphic Slc12a5
      knock-in mouse carrying a patient variant, rather than in acute pharmacological
      blockade models, measuring both electrographic seizure duration and
      extracellular potassium to establish which step is rate-limiting in a
      constitutive partial deficiency.

- discussion_id: mismatch_kcc2_null_mouse_versus_human_partial_loss
  prompt: >-
    Do the Kcc2 knockout mouse models, which are perinatally lethal or die within
    weeks, model DEE34 faithfully, given that surviving human patients retain partial
    KCC2 transport activity and that the perinatal NKCC1-to-KCC2 switch occurs at a
    different relative developmental stage in mouse and human?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Reduced KCC2-Mediated Chloride Extrusion
  - pathophysiology#Cortical Interneuron Subtype Imbalance
  rationale: >-
    Complete Kcc2 loss kills mice at birth, whereas every published human DEE34
    genotype retains measurable residual chloride extrusion and the children survive
    into childhood. The mouse models therefore sit at a more severe point on the
    dose-response curve than the human disease. Independently, the developmental
    window matters: in human neocortex the shift from depolarizing to hyperpolarizing
    GABA reaches maturity around six months after birth, and mouse postnatal age does
    not map one-to-one onto that timeline. Any inference from knockout phenotypes to
    the timing or severity of human disease - including the interneuron subtype
    imbalance, which has been shown only in mouse - has to carry this caveat.
  evidence:
  - reference: PMID:11395011
    reference_title: "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration."
    explanation: >-
      Establishes the severity gap between the null mouse and surviving human patients.
  - reference: PMID:27436767
    reference_title: "Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Functional analysis using the gramicidin-perforated patch-clamp technique confirmed significant, but not complete, loss of KCC2 function in the patients."
    explanation: >-
      Confirms that human disease is a partial, not complete, loss of function.
  - reference: PMID:38660387
    reference_title: "A novel pathogenic SLC12A5 missense variant in epilepsy of infancy with migrating focal seizures causes impaired KCC2 chloride extrusion."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In the human neocortex, this transition takes place perinatally and reaches maturation at roughly 6 months after birth"
    explanation: >-
      Gives the human developmental timing that mouse postnatal age does not directly
      match.
  proposed_experiments:
  - experiment_id: exp_hypomorphic_knockin_allelic_series
    name: Graded Slc12a5 knock-in allelic series against the null phenotype
    description: >-
      Generate an allelic series of Slc12a5 knock-in mice carrying human DEE34
      variants of graded residual transport activity, and compare seizure onset,
      survival, and cortical interneuron composition against the null, to establish
      how much of the null phenotype is an artefact of complete loss.

- discussion_id: gap_relative_contribution_transport_vs_structural_arm
  prompt: >-
    How much of the developmental phenotype in DEE34 is attributable to the
    transport-independent structural role of KCC2 in dendritic spine and excitatory
    synapse maturation, as opposed to the chloride-extrusion deficit and the seizure
    burden it causes?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Disrupted Transport-Independent KCC2 Structural Function
  - pathophysiology#Impaired Neurodevelopment
  rationale: >-
    Two upstream arms converge on the developmental phenotype in this model, and the
    published cases cannot separate them: every reported genotype impairs chloride
    extrusion, so no patient provides the natural experiment of an isolated structural
    defect. The distinction is not academic - if the structural arm carries substantial
    weight, then even complete seizure control or full restoration of chloride
    transport would leave part of the developmental disability untouched, which
    changes what a KCC2-targeted therapy could be expected to achieve.
  evidence:
  - reference: PMID:26333769
    reference_title: "Mutations in SLC12A5 in epilepsy of infancy with migrating focal seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLC12A5 encodes the potassium-chloride co-transporter KCC2, which plays an integral role in neuronal inhibition4567 and maturation of cortical dendritic spines8."
    explanation: >-
      Names both KCC2 roles, which is the basis for treating them as separate arms.
  - reference: PMID:35370549
    reference_title: "Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "More work will be needed to define the specific cellular basis for these findings, including whether they are due to abnormal circuit formation versus the sequela of defective IN inhibition."
    explanation: >-
      The authors of the conditional knockout state the same unresolved separation
      between circuit-formation and inhibition-failure explanations.
  proposed_experiments:
  - experiment_id: exp_transport_dead_structural_intact_knockin
    name: Transport-dead versus structure-disrupting KCC2 knock-in comparison
    description: >-
      Compare knock-in mice carrying a transport-dead but structurally intact KCC2
      against mice carrying a C-terminal variant that preserves transport while
      disrupting protein interactions, scoring dendritic spine density, excitatory
      synapse number, seizure burden, and behavioural development, to apportion the
      developmental phenotype between the two arms.

datasets: []
📚

References & Deep Research

References

1
PMID:30763027
SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures.
No top-level findings curated for this source.

Deep Research

1
Claude Code ▸
SLC12A5-Related Developmental and Epileptic Encephalopathy: Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 18 citations 2026-08-19T13:25:03.253621

SLC12A5-Related Developmental and Epileptic Encephalopathy: Comprehensive Research Report

1. Disease Information

Overview. SLC12A5-related disease is a severe, early-infantile developmental and epileptic encephalopathy (DEE) caused by biallelic (autosomal recessive) loss-of-function variants in SLC12A5, the gene encoding the neuron-specific potassium-chloride cotransporter KCC2. The canonical, most severe presentation is epilepsy of infancy with migrating focal seizures (EIMFS) — formerly called "malignant migrating partial seizures of infancy (MMPSI)" — characterized by onset of intractable, multifocal, migrating seizures typically before 6 months of age, accompanied by developmental delay or regression, hypotonia, and postnatal microcephaly (Stödberg et al. 2015, PMID:26333769; GeneReviews NBK537476). A milder heterozygous-variant spectrum has also been reported in idiopathic generalized epilepsy, febrile seizures, autism, and schizophrenia, though causality there is less firmly established (Kahle lab, PMC4600830).

Key identifiers: - Gene: SLC12A5 (HGNC:13818), chromosome 20q13.12, 24 exons, NCBI Gene ID 57468 - Protein: KCC2 (K-Cl cotransporter 2 / SLC12A5), UniProt Q9H2X9, 1139 amino acids, ~126 kDa - OMIM gene: 606726 (SLC12A5) - OMIM phenotype: #616645 — Developmental and Epileptic Encephalopathy 34 (DEE34), also historically "EIEE34" (early infantile epileptic encephalopathy 34) - Orphanet: ORPHA:293181 (Epilepsy of infancy with migrating focal seizures) - Inheritance: Autosomal recessive (biallelic — homozygous or compound heterozygous) - GeneReviews: NBK537476 — "SLC12A5-Related Epilepsy of Infancy with Migrating Focal Seizures" - MONDO:* corresponds to DEE34 (recommend cross-checking exact MONDO CURIE against OMIM 616645 mapping)

Synonyms: SLC12A5-EIMFS; EIEE34; DEE34; KCC2-related epileptic encephalopathy; KCC2 deficiency disorder; (historically) malignant migrating partial seizures of infancy (MMPSI) when caused by SLC12A5.

Evidence basis: Nearly all published data derive from aggregated case series/case reports (fewer than 20 published individuals across all reports as of 2024–2026) plus mechanistic cellular/animal-model studies — this is a very rare, individually-reported-patient literature rather than large-registry epidemiology.

Sources: GeneReviews SLC12A5-EIMFS, OMIM #616645, OMIM *606726, Nature Communications 2015

2. Etiology

Disease causal factor: Biallelic loss-of-function pathogenic variants (homozygous or compound heterozygous) in SLC12A5, encoding KCC2. This is a purely monogenic, Mendelian etiology — no infectious, toxic, or acquired trigger is implicated in the primary disease process.

Genetic risk factors: - Missense, splice-site, and in-frame deletion variants have been reported; missense predominates (GeneReviews NBK537476). - Recent case reports (Hamze et al. 2026, Epilepsia) describe compound heterozygous variants that impair both the canonical chloride-extrusion function of KCC2 and separate chloride-independent developmental functions of KCC2 (e.g., structural/scaffolding roles in synapse and dendritic spine maturation), broadening the molecular mechanism spectrum beyond simple transport loss-of-function. - A specific missense variant, p.(R231H) in transmembrane domain 4 (TM4) — the first pathogenic missense variant described in that domain — was identified in a Finnish patient from consanguineous parents and functionally characterized (PMC11039960, 2024). - Consanguinity is a recognized risk factor given the recessive, biallelic requirement (illustrated in the R231H homozygous case). - Heterozygous SLC12A5 variants (distinct from the biallelic DEE34 mechanism) have been separately associated with idiopathic generalized epilepsy (IGE) and febrile seizures, and rare regulatory-domain/CpG-site variants (e.g., R952H, R1049C) with autism spectrum disorder and schizophrenia (PMC4600830) — these represent a different, milder, likely partial-penetrance/susceptibility mechanism, not the severe biallelic DEE34 phenotype.

Environmental risk factors: None specifically established; this is a monogenic disorder. No known toxin, infection, or lifestyle exposure modifies risk.

Protective factors: No specific genetic or environmental protective factors are documented in the literature.

Gene-environment interaction: Not established/reported for this ultra-rare monogenic condition.

Sources: PMC11039960 (R231H variant), Hamze et al. 2026 Epilepsia, PMC4600830 (regulatory variants, autism/schizophrenia)

3. Phenotypes

Core seizure phenotype (symptoms/signs): - Onset before 6 months of age; median seizure onset 1.5 months, mean 1.8 months (range: 1 day–4 months) in the published GeneReviews cohort of 9 children (NBK537476). - Some biallelic cases present even more acutely, with bilateral migratory focal seizures within the first 24 hours of life in severe neonatal-onset presentations. - Initial seizure semiology: focal motor seizures with prominent head and eye deviation, apnea, and autonomic features (facial flushing, salivation). - Seizures become multifocal, "migrating" across cortical regions (the defining EIMFS electroclinical signature) and are typically pharmacoresistant to standard anti-seizure medications (ASMs). - Suggested HPO terms: HP:0032792 (migrating seizures — if available) or more generally HP:0002133 (focal-onset seizure), HP:0011097 (epileptic spasm, if present), HP:0002197 (generalized-onset seizure for later evolution), HP:0032799 (developmental regression with seizures pattern), HP:0011451 (drug-resistant epilepsy), HP:0032207 (apneic episode/autonomic seizure feature).

Developmental/neurological phenotype: - Developmental delay in all affected children; developmental regression (loss of previously acquired skills) at seizure onset in a majority (5/9 in the GeneReviews cohort). - Profound intellectual disability in the most severely affected (e.g., developmental age of 2 months at chronological age 2 years in the R231H case). - Axial hypotonia — a recurrent, near-universal feature (HP:0008936 Hypotonia; HP:0002490 Axial hypotonia). - Extrapyramidal features reported in some neonatal-onset cases (dystonia/dyskinetic movements) — HP:0002071 (extrapyramidal motor findings). - Postnatal microcephaly in most patients — HP:0005484. - Variable outcomes: some children achieve independent ambulation (ages 2.9–4 years) or single words (by age 6); others remain profoundly disabled or die in early childhood.

Neuroimaging findings: - Nonspecific: delayed myelination, thin corpus callosum, cerebral atrophy — HP:0002119 (Ventriculomegaly), HP:0002079 (Hypoplasia of the corpus callosum), HP:0002505 (Loss of ambulation, later stage), HP:0012444 (Brain atrophy). - One autopsy case (R231H, deceased at 4y5m) showed agenesis of the corpus callosum, markedly enlarged lateral ventricles, and scarcity of white matter — an extreme end of the imaging spectrum.

Systemic/secondary complications: - Recurrent aspiration pneumonia and respiratory infections (a leading cause of death — one child died at 2.5 years from respiratory infection; the R231H patient died at 4y5m from pneumonia). - Feeding difficulties requiring gastrostomy. - Osteopenia (reported in the R231H case, likely related to immobility/anticonvulsant use).

Frequency/severity notes: Given the very small published cohort (fewer than 20 confirmed cases in aggregate across all reports as of 2026), formal frequency percentages (e.g., "80% of patients") are not statistically robust; qualitative descriptors ("most," "all," "some") are used throughout the primary literature rather than quantitative frequency bands.

Quality of life impact: Severely affected — profound intellectual disability, non-ambulation in many cases, high caregiver burden, and reduced life expectancy in the most severe neonatal-onset cases. No disease-specific EQ-5D/SF-36 data are published; QoL burden is inferred from the DEE literature generally (comparable to other severe infantile DEEs).

Sources: GeneReviews NBK537476, PMC11039960, Hamze et al. 2026

4. Genetic/Molecular Information

Causal gene: SLC12A5 (HGNC:13818; OMIM *606726), chromosome 20q13.12, 24 exons.

Variant spectrum: - Primarily missense variants; also splice-site variants (e.g., ClinVar RCV001230522 c.2787+6G>A; RCV000652718 c.3126-6C>A — both classified in association with DEE34) and in-frame deletions. - Notable characterized variant: c.692G>A, p.(R231H) — homozygous, in TM4, first pathogenic missense variant in that transmembrane domain, from consanguineous Finnish parents (PMC11039960). - Compound heterozygous genotypes reported combining variants that separately impair chloride-transport function and chloride-independent (developmental/structural) KCC2 functions (Hamze et al. 2026). - Original Stödberg et al. 2015 cohort (PMID:26333769) established recessive loss-of-function as the mechanism via biallelic variants in multiple unrelated families, functionally validated in vitro. - Saitsu et al. 2016 (Scientific Reports, PMC4951812) independently identified biallelic SLC12A5 mutations causing impaired KCC2 function in migrating focal seizures with severe developmental delay.

Variant classification (ACMG/ClinVar): Multiple variants classified as Pathogenic or Likely Pathogenic in ClinVar in association with "Developmental and epileptic encephalopathy, 34." Functional data (electrophysiology, surface expression) have been used to reclassify VUS variants as pathogenic (e.g., R231H, originally VUS, reclassified pathogenic per ACMG criteria after functional study — PMC11039960).

Allele frequency: Pathogenic SLC12A5 DEE34 variants are extremely rare/private in population databases (gnomAD), consistent with an ultra-rare severe recessive DEE; no specific population allele-frequency statistics for individual pathogenic variants were retrieved in this search, but the extreme rarity of the disease (estimated prevalence 0.11/100,000 children — see Epidemiology) implies very low carrier frequency.

Functional consequences (loss of function): - Decreased KCC2 surface/membrane expression — R231H showed ~5-fold lower membrane-bound fluorescence vs. wild type. - Reduced protein glycosylation and impaired post-translational trafficking. - Enhanced ER-associated degradation (ERAD) — R231H "undergoes ERAD more efficiently than wild-type." - Impaired chloride extrusion: gramicidin-perforated patch-clamp shows depolarized glycine-receptor reversal potential (E_Gly) — wild-type median −79.5 mV vs. R231H −58.5 mV vs. mock (no KCC2) −49.5 mV (p=0.0329) — indicating substantially but not completely abolished transport function. - Reduced ion (K+) flux: NH4+/pHluorin flux assay showed 52% reduction in acidification rate for R231H vs. wild-type (p=0.00067). - Net result: elevated intracellular chloride, depolarizing (excitatory) GABA_A responses, impaired synaptic inhibition, and neuronal hyperexcitability.

Epigenetic/regulatory variation: Regulatory-domain or CpG-site variants in SLC12A5 (distinct from coding loss-of-function) reported in autism and schizophrenia cohorts (PMC4600830) — a separate, milder mechanistic category from DEE34.

Modifier genes: None specifically established for SLC12A5-DEE34.

Chromosomal abnormalities: Not a copy-number/structural-variant disease mechanism — point mutations and small indels/splice variants predominate; no recurrent CNV etiology reported.

Ontology suggestions: HGNC:13818 (SLC12A5); GO:1902476 (chloride transmembrane transport) / GO:0055064 (chloride ion homeostasis); GO:0008511 (sodium:potassium:chloride symporter activity — for family context) or more precisely potassium:chloride symporter activity; CHEBI:17996 (chloride).

Sources: Stödberg 2015, Saitsu 2016 PMC4951812, PMC11039960, Hamze 2026

5. Environmental Information

No specific environmental, lifestyle, or infectious contributory factors are documented for the primary (biallelic) SLC12A5-DEE34 disease process — this is a purely monogenic disorder. Secondary environmental factors are relevant only to disease complications (e.g., recurrent aspiration pneumonia as a cause of mortality relates to hypotonia/dysphagia rather than being a disease cause). No infectious agent is implicated in etiology.

6. Mechanism / Pathophysiology

Core causal chain: 1. Trigger: Biallelic loss-of-function SLC12A5 variant → reduced KCC2 protein synthesis, trafficking, membrane insertion, and/or transport activity (molecular scale). 2. Molecular consequence: Impaired KCC2-mediated K+-Cl− cotransport → failure to extrude intracellular chloride against its electrochemical gradient (KCC2 normally uses the outward K+ gradient set by Na+/K+-ATPase to drive Cl− out of neurons). 3. Cellular consequence: Elevated intraneuronal [Cl−] → depolarizing shift in the GABA_A/glycine receptor reversal potential (E_GABA/E_Gly) → GABA and glycine signaling become excitatory rather than inhibitory (the classic "GABA excitatory shift"). 4. Circuit consequence: Loss of fast synaptic inhibition → neuronal hyperexcitability, hypersynchronization, and failure of seizure termination mechanisms (KCC2 chloride transport also contributes to terminating ictal activity, per PMC7986536). 5. Developmental consequence (chloride-independent arm, per Hamze et al. 2026): KCC2 additionally serves structural/scaffolding roles (via its large intracellular C-terminal domain interacting with cytoskeletal and synaptic proteins) important for dendritic spine and excitatory synapse maturation; variants disrupting this arm independently impair neurodevelopment, compounding the chloride-transport deficit. 6. Clinical manifestation: Neonatal/early-infantile onset migrating, multifocal, pharmacoresistant seizures, developmental arrest/regression, hypotonia, and progressive encephalopathy.

Molecular pathway: Cation-chloride cotransporter (CCC) family signaling; KCC2 is the principal neuronal Cl− extruder, counterbalanced developmentally by NKCC1 (SLC12A2, the Cl− importer) — the NKCC1-to-KCC2 developmental switch underlies the well-known perinatal shift from depolarizing to hyperpolarizing GABA action. In SLC12A5-DEE34, this maturational switch fails or is incomplete.

Protein dysfunction: KCC2 misfolding/reduced glycosylation → ER retention and enhanced ER-associated degradation (ERAD) → reduced surface expression; for missense variants retaining some surface expression (e.g., R231H), residual but markedly reduced transport activity persists (partial/hypomorphic loss of function rather than complete null).

Cellular processes: Impaired GABAergic/glycinergic inhibitory neurotransmission; secondary effects on interneuron circuit maturation — conditional Kcc2 knockout in GABAergic neurons in mice causes an imbalance of cortical interneuron subtypes (excess somatostatin+ neurons in layer 5, reduced parvalbumin+ neurons in layers 2/3 and 6) (PMC8966887), suggesting KCC2 loss disrupts interneuron network assembly, not merely acute inhibition.

Immune involvement: Not a primary mechanism, though downstream KCC2 inhibition/neuronal hyperexcitability has been linked in unrelated contexts to complement (C1q)-dependent extrinsic apoptotic signaling (PMC12399595) — relevance to SLC12A5-DEE34 specifically is not established and should be treated as a tangential mechanistic note, not disease-specific evidence.

Suggested GO terms: GO:0006821 (chloride transport), GO:0034765 (regulation of ion transmembrane transport), GO:0007214 (gamma-aminobutyric acid signaling pathway), GO:0060080 (inhibitory postsynaptic potential), GO:0050804 (modulation of chemical synaptic transmission). Suggested CL terms: CL:0000540 (neuron), CL:0000601 (GABAergic interneuron), CL:0000679 (glutamatergic neuron, as postsynaptic partner).

Sources: Journal of Molecular Neuroscience 2022, PMC6873151, PMC7986536 (KCC2 and ictal termination), PMC8966887 (conditional Kcc2 KO, interneuron imbalance), Hamze 2026

7. Anatomical Structures Affected

Organ level: Central nervous system (brain) is the primary and essentially exclusive site of disease; KCC2 is neuron-specific (not expressed in most peripheral tissues). Secondary/complication-level organ involvement includes the respiratory system (aspiration pneumonia) and skeletal system (osteopenia, likely secondary to immobility).

Body systems: Nervous system (primary); musculoskeletal (secondary, hypotonia-related); respiratory (secondary, aspiration risk).

Tissue/cell level: Cortical and subcortical neurons broadly; particular relevance to GABAergic interneurons (given KCC2's centrality to GABAergic inhibitory signaling) and to postsynaptic neurons receiving GABAergic/glycinergic input generally. Corpus callosum white matter is structurally affected (thin/agenesis on imaging).

Subcellular level: Plasma membrane (KCC2 is a 12-transmembrane-domain integral membrane transporter); large intracellular C-terminal cytoplasmic domain (site of chloride-independent structural/scaffolding interactions); endoplasmic reticulum (site of misfolding/ERAD for trafficking-defective variants).

Anatomical ontology suggestions: UBERON:0000955 (brain), UBERON:0001872 (cerebral cortex), UBERON:0002336 (corpus callosum), CL:0000601 (GABAergic interneuron); GO Cellular Component: GO:0005886 (plasma membrane), GO:0005783 (endoplasmic reticulum), GO:0043005 (neuron projection).

Localization: Bilateral, diffuse/multifocal cortical involvement (consistent with the "migrating" multifocal seizure semiology, implying no single fixed epileptogenic focus but rather a global susceptibility to hyperexcitability).

8. Temporal Development

Onset: Neonatal to early infantile — median 1.5 months, mean 1.8 months, range 1 day to 4 months in the largest published case series; some biallelic cases present within the first 24 hours of life. Onset pattern is typically acute/subacute, often abrupt.

Progression: - Rapid evolution from focal to multifocal, migrating, drug-resistant seizures within weeks of onset. - Developmental trajectory: either static delay from birth or regression (loss of previously acquired skills) coinciding with seizure onset in the majority of reported cases. - Disease course is generally progressive/static-severe rather than truly relapsing-remitting, though seizure burden can fluctuate with treatment response (e.g., partial response to ketogenic diet or potassium bromide in some). - No formal staging system exists (unlike, e.g., cancer staging) — severity is generally described qualitatively (mild/attenuated vs. severe neonatal-onset).

Patterns: - No spontaneous remission reported; some children show treatment-associated partial seizure reduction (ketogenic diet, potassium bromide) without full seizure freedom. - The first months of life represent a critical developmental window — this is biologically consistent with the normal developmental NKCC1-to-KCC2 chloride-transporter switch that occurs perinatally in humans; disruption of KCC2 function during this exact window is thought to be maximally deleterious, which may explain the strict early-infantile onset window of EIMFS phenotypes.

Sources: GeneReviews NBK537476

9. Inheritance and Population

Epidemiology: - SLC12A5-EIMFS is exceedingly rare. EIMFS overall (all genetic causes combined) was estimated at a prevalence of 0.11 per 100,000 children in the UK (non-population-based estimate). SLC12A5 accounts for only a small fraction of EIMFS cases — the most common EIMFS gene is KCNT1 (~27% of a 135-case genetic cohort), with SCN2A second (~7%); SLC12A5 is one of several rarer causal genes (alongside SCN1A, SCN8A, PLCB1, SLC25A22, TBC1D24, and 16p11.2 duplication, plus more recently described genes GABRA1, GABRB1, ATP1A3, CDKL5, PIGA, ITPA, AIMP1, KARS, WWOX). - For context, DEE overall (all causes) has a cumulative incidence of ~169/100,000 children and point prevalence of ~112/100,000 children — underscoring that SLC12A5-DEE34 represents a very small subset of this broader category. - Fewer than 20 genetically confirmed SLC12A5-DEE34 patients have been published in aggregate across all case series as of the 2026 literature.

Inheritance pattern: Autosomal recessive. Each sibling of an affected individual has a 25% chance of being affected, 50% chance of being an asymptomatic carrier, and 25% chance of being unaffected. Parents are typically unaffected heterozygous carriers.

Penetrance: Full penetrance is assumed for biallelic loss-of-function variants based on published cases (no reported unaffected biallelic carriers), though the very small sample size limits confidence in this estimate.

Expressivity: Variable — clinical severity ranges from profound neonatal-onset encephalopathy with early death to somewhat milder courses with eventual acquisition of ambulation or single words, suggesting genotype-dependent (hypomorphic vs. null) variable expressivity.

Consanguinity: A recognized contributing factor for homozygous presentations (e.g., the Finnish R231H case arose from consanguineous parents).

Founder effects / population-specific variants: Not specifically documented for SLC12A5-DEE34 in the literature retrieved; each family generally carries private variants.

Sex ratio: No sex predilection reported (autosomal, not X-linked).

Carrier frequency: Not established at a population level given the extreme rarity and diversity of pathogenic alleles (essentially private variants rather than a small recurrent set).

Sources: GeneReviews NBK537476, Epidemiology of DEE, Neurology 2023, PMID:36581463, PMC6878841 (KCNT1 EIMFS landscape)

10. Diagnostics

Genetic testing (primary diagnostic modality): - Exome or genome sequencing is the preferred first-tier approach, given the broad genetic heterogeneity of EIMFS (many candidate genes) — GeneReviews explicitly recommends this over single-gene testing. - Diagnosis requires identification of biallelic pathogenic/likely pathogenic SLC12A5 variants. - Functional validation (electrophysiology, surface trafficking assays) can be used to reclassify variants of uncertain significance, as demonstrated for p.(R231H). - Multi-gene epilepsy panels covering EIMFS-associated genes (KCNT1, SCN2A, SCN1A, SCN8A, PLCB1, SLC25A22, TBC1D24, SLC12A5, GABRA1, GABRB1, ATP1A3, CDKL5, PIGA, and others) are a reasonable alternative when exome/genome sequencing is unavailable.

EEG: - Interictal: multifocal spikes. - Ictal: characteristic migrating pattern — seizure activity involving varying cortical areas over time with clinical-EEG correlation (the defining electroclinical signature of EIMFS, common to all genetic causes).

Neuroimaging (MRI): Nonspecific findings — delayed myelination, thin/absent corpus callosum, cerebral atrophy, enlarged ventricles in severe cases.

Clinical diagnostic criteria: EIMFS is a clinically defined electroclinical syndrome (onset <6 months, migrating multifocal seizures, developmental delay/regression) that requires genetic testing to identify the specific causal gene, since the electroclinical phenotype is shared across multiple genetic etiologies.

Differential diagnosis: Other EIMFS-causing genes (see above), particularly KCNT1 (most common cause, ~27%) and SCN2A; broader neonatal/infantile DEE differentials include other early-infantile epileptic encephalopathies (e.g., Ohtahara syndrome genes, STXBP1, KCNQ2).

Screening: No population newborn-screening or carrier-screening program exists specifically for SLC12A5, given its extreme rarity; testing is reactive (diagnostic) rather than population-screening based. Prenatal and preimplantation genetic testing become available once familial variants are identified in an affected proband.

Suggested ontology: NCIT:C63846 or similar for "Whole Exome Sequencing"; LOINC codes for EEG and MRI brain studies would be assigned per standard clinical coding, not disease-specific.

Sources: GeneReviews NBK537476, PMC11039960

11. Outcome/Prognosis

Survival/mortality: Mortality is significant in the most severely affected. Reported deaths: one child died at age 2.5 years from respiratory infection complications; the R231H homozygous patient died at 4 years 5 months from pneumonia. Other reported patients survive into childhood/early adulthood (ages 3–22 years in the GeneReviews cohort) with varying functional status.

Morbidity/function: Profound to severe intellectual disability is typical; motor outcomes range from non-ambulation to independent ambulation achieved late (ages 2.9–4 years) in less severely affected children; some achieve single-word speech by age 6.

Complications: Recurrent aspiration pneumonia/respiratory infections (leading cause of death), feeding difficulties/failure to thrive (often requiring gastrostomy), osteopenia, progressive encephalopathy.

Recovery potential: No cure or disease-modifying therapy exists; developmental gains are possible during periods of improved seizure control but the underlying encephalopathy is not reversible with current treatments.

Prognostic factors: Disease severity appears to correlate with the degree of residual KCC2 function retained by the specific variant(s) (null/complete loss-of-function vs. hypomorphic variants retaining partial transport activity), and possibly with whether chloride-independent developmental KCC2 functions are also disrupted (per Hamze et al. 2026) — though formal genotype-phenotype correlation studies with adequate sample size are not yet available given the rarity of the disorder.

Sources: GeneReviews NBK537476, PMC11039960

12. Treatment

Pharmacotherapy (anti-seizure medications): - No specific/disease-modifying treatment exists. GeneReviews states plainly: "There are no specific treatments for seizures in SLC12A5-EIMFS. In general, seizures in EIMFS are resistant to most ASM." - Documented ASM trials with limited/no efficacy in reported cases: phenytoin, phenobarbital, midazolam, ketamine, sodium thiopental, levetiracetam, topiramate, lacosamide, lidocaine (from the R231H case, largely ineffective). - Modest benefit reported with levetiracetam, rufinamide, and stiripentol in some EIMFS patients generally (GeneReviews); note these are general EIMFS management options, not SLC12A5-specific evidence in all cases. - Potassium bromide: achieved partial response in the R231H patient (40 mg/kg/day at age 2 years), reducing focal tonic-clonic seizures from >10/day to a few per day — the most clearly documented partial-responder therapy in the literature reviewed.

Dietary therapy: - Ketogenic diet: reported to produce seizure reduction in some individuals; however in the R231H case it was discontinued after 3.5 months due to lack of response — response is variable.

Investigational/mechanism-targeted approaches (not yet clinical for SLC12A5-DEE34 specifically): - NKCC1 inhibition (bumetanide): The broader chloride-cotransporter therapeutic literature has explored bumetanide (an NKCC1 inhibitor) to rebalance the NKCC1/KCC2 ratio and restore inhibitory GABA signaling in various neurodevelopmental/epilepsy conditions (autism, schizophrenia, fragile X, Down syndrome); this has NOT been reported as a validated SLC12A5-DEE34-specific therapy in the literature retrieved, and its rationale is somewhat paradoxical for KCC2 loss-of-function (bumetanide blocks Cl− import via NKCC1, but the core defect here is impaired Cl− export via KCC2 — theoretically bumetanide could still help by reducing the chloride load that a deficient KCC2 cannot clear, but empirical human data for this specific gene were not found). - KCC2-activating small molecules: Under active pharmaceutical development (patent literature: "KCC2 expression enhancing compounds," US11331313 and US12053465) but not yet in clinical use; reviewed in "Development of KCC2 therapeutics to treat neurological disorders" (PMC11666659) and "The Expanding Therapeutic Potential of Neuronal KCC2" (PMC7016893). These represent a rational future precision-therapy direction directly targeting the causal deficiency but are preclinical/early-stage.

Supportive/rehabilitative care: - Physical therapy for hypotonia. - Swallowing assessment and gastrostomy for feeding difficulties. - Preventive respiratory care given high pneumonia/aspiration risk. - Early intervention and developmental therapy programs.

Experimental treatments: No SLC12A5-DEE34-specific registered clinical trials were identified in this search; KCC2-targeted small-molecule programs remain preclinical.

Suggested NCIT terms: NCIT:C15632 (Chemotherapy — n/a here), NCIT:C15986 (Pharmacotherapy, for ASMs and potassium bromide), NCIT:C15447 (Dietary Intervention, for ketogenic diet), NCIT:C15302 (Physical Therapy), NCIT:C15315 (Rehabilitation), NCIT:C15329 (Surgical Procedure, for gastrostomy placement).

Sources: GeneReviews NBK537476, PMC11039960, PMC11666659 (KCC2 therapeutics development), PMC7016893 (KCC2 therapeutic potential)

13. Prevention

Primary prevention: Not applicable in the traditional sense (no modifiable risk factor); the only "primary prevention" available is reproductive: genetic counseling and carrier testing for families with a known affected proband, plus prenatal diagnosis or preimplantation genetic testing (PGT) once familial pathogenic variants are identified.

Secondary prevention: Early genetic diagnosis (via exome/genome sequencing) allows earlier prognostic counseling and avoidance of a prolonged, costly "diagnostic odyssey," though it does not currently alter seizure outcomes given the lack of disease-modifying therapy.

Genetic counseling: Standard autosomal recessive counseling applies — 25% recurrence risk for future pregnancies of carrier parents; extended family carrier testing can be offered once the familial variants are known.

Screening: No population-based newborn or carrier screening program exists for SLC12A5 given its rarity; this remains a reactive, proband-driven diagnostic pathway.

Public health/behavioral/prophylaxis: Not applicable — this is a non-preventable monogenic disorder; management focuses on secondary complication prevention (aspiration precautions, respiratory infection prophylaxis) rather than primary disease prevention.

14. Other Species / Natural Disease

No naturally occurring SLC12A5/KCC2-deficient disease has been reported in non-human species (e.g., no OMIA entry for a spontaneous veterinary KCC2 disorder was identified in this search). KCC2 orthologs are broadly conserved across vertebrates (mouse Slc12a5, MGI:1862037), and the gene's fundamental role in neuronal chloride homeostasis is evolutionarily conserved, but disease relevance in non-human species comes exclusively from engineered/induced models (see Section 15), not natural disease.

15. Model Organisms

Mouse models (the dominant model system): - Complete Kcc2 knockout mice (Hübner et al. 2001; Woo et al. 2002): homozygous null mice exhibit frequent generalized seizures and die shortly after birth — demonstrating that complete loss of KCC2 is perinatally lethal in mice, consistent with KCC2's essential role in establishing inhibitory GABA/glycine signaling. - Kcc2b isoform-specific knockout: mice lacking the neuron-specific KCC2b splice isoform (while retaining the KCC2a isoform expressed more broadly/earlier) survive longer but die in the third postnatal week from seizures — this partial/isoform-specific model better approximates a survivable, seizure-prone phenotype useful for mechanistic study (Tao et al., and related isoform-specific knockout literature). - Heterozygous Kcc2+/− mice: show altered seizure threshold and increased susceptibility to chemoconvulsant-induced seizures without the severe neonatal lethality of the homozygous null — modeling milder/partial KCC2 deficiency states, potentially relevant to the heterozygous IGE/febrile-seizure-associated human variants. - Conditional (GABAergic-neuron-specific) Kcc2 knockout mice (PMC8966887, Frontiers in Molecular Neuroscience 2022): early seizures, failure to thrive, premature death in the second/third postnatal week; underlying imbalance of cortical interneuron subtypes — excess somatostatin+ interneurons in cortical layer 5, decreased parvalbumin+ interneurons in layers 2/3 and 6 — providing a developmental circuit-level mechanistic link between KCC2 loss and cortical hyperexcitability distinct from the acute chloride-transport deficit alone.

Model characteristics and fidelity: - These knockout models recapitulate the core human phenotype of severe, early-onset, lethal/near-lethal seizures with loss of GABAergic inhibition, supporting strong construct and face validity for the acute chloride-dysregulation mechanism. - Limitation: complete knockouts are more severe and more rapidly lethal than most human cases (which, as compound heterozygotes or hypomorphic homozygotes, typically retain partial KCC2 function and survive infancy), so complete-null mouse models likely overrepresent the most extreme end of the human phenotypic spectrum; isoform-specific and conditional models better approximate survivable human disease. - Human-specific developmental timing (the NKCC1-to-KCC2 perinatal switch occurs at different relative developmental stages in mouse vs. human) is a recognized HUMAN_MODEL_MISMATCH-type caveat: the precise correspondence between mouse postnatal age and human gestational/neonatal timing for KCC2 maturation is not one-to-one, which should be considered when interpreting knockout-model timing as directly analogous to human neonatal-onset EIMFS.

Cellular/heterologous expression models: - HEK293/other heterologous cell systems expressing wild-type vs. mutant KCC2 (used for R231H functional characterization): gramicidin-perforated patch-clamp (E_Gly reversal potential), NH4+/pHluorin flux assays, and surface immunolabeling — these are the primary in vitro functional validation platforms used across nearly all published SLC12A5 variant characterization studies (Stödberg 2015, Saitsu 2016, PMC11039960).

Resources: MGI:1862037 (mouse Slc12a5 gene record, Jackson Laboratory Mouse Genome Informatics).

Sources: PMC8966887, Nature Scientific Reports 2017, MGI:1862037, PMC11039960

Summary Table: Key Evidence-Ready Citations

Topic PMID/Reference Key Finding
Original disease description PMID:26333769 (Stödberg 2015, Nat Commun) Recessive LOF SLC12A5 variants cause EIMFS; functional loss of KCC2 transport demonstrated
Independent confirmation PMC4951812 (Saitsu 2016, Sci Rep) Biallelic SLC12A5 mutations impair KCC2 function in migrating focal seizures + severe DD
Recent variant/mechanism study PMC11039960 (2024) p.(R231H) TM4 variant; detailed electrophysiology/trafficking functional data; ACMG reclassification
Expanded mechanistic spectrum Hamze et al. 2026, Epilepsia, doi:10.1002/epi.70258 Compound het variants disrupt both Cl−-dependent and Cl−-independent KCC2 functions
Clinical synopsis/management GeneReviews NBK537476 Comprehensive clinical, diagnostic, and management summary (9-patient cohort)
Disease/gene identifiers OMIM #616645 (DEE34); OMIM *606726 (SLC12A5) Formal phenotype/gene MIM entries
Mouse knockout mechanism Hübner 2001; Woo 2002; PMC8966887 Kcc2-null perinatal lethal seizures; conditional KO shows interneuron subtype imbalance
Heterozygous variant spectrum PMC4600830 Regulatory/CpG variants linked to autism, schizophrenia, IGE, febrile seizures
EIMFS genetic landscape/differential PMC6878841 KCNT1 most common (27%) EIMFS gene; SLC12A5 one of several rarer causes

Notes on data gaps: This is an ultra-rare disorder with a published literature base of well under 20 confirmed cases; formal quantitative frequency data (phenotype penetrance percentages, precise prevalence/incidence, standardized QoL scores) are largely unavailable and should be represented in a knowledge base as qualitative/descriptive rather than fabricated numeric estimates. Ontology term suggestions above (particularly specific HPO codes) should be verified against current HPO/OAK term lookups before curation, as exact CURIEs for some described features (e.g., "migrating seizures") may require confirmation of the precise current term ID and label.

Reference Validation

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References checked 14
Resolved 14
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 2
Quoted claims found in source 2
Quoted claims not found in source 0
References weighed for topical relevance 14
On topic 5
Off topic 0

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