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Transporter Gene Mutations Implicated in Form of Infant Epilepsy

NEW YORK (GenomeWeb) – In Nature Communications, an international team led by investigators in Sweden and the UK described a new genetic cause for a severe infant-onset form of epilepsy.

Starting with dozens of families affected by a difficult-to-treat epilepsy syndrome called "epilepsy of infancy with migrating focal seizures" (EIMFS), the researchers focused in on two families for their exome sequencing and/or linkage assessments.

In all four affected children from these families, the analyses unearthed EIMFS-associated recessively inherited loss-of-function mutations in SLC12A5, a gene that encodes the KCC2 potassium chloride co-transporter, which aids activity of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by producing an appropriate chloride ion gradient.

"Mutations in the gene encoding KCC2 prevent this switch which makes GABA remain stimulatory, incapable of inhibiting the signals of the brain," co-corresponding author Anna Wedell, a molecular medicine, surgery, and inherited metabolic diseases researcher affiliated with the Karolinska Institute and Karolinska University Hospital, said in a statement. "The neurons then discharge at times, when they normally should not, giving rise to epilepsy."

Prior studies have pointed to KCC2 as a potential player in some types of epilepsy, she and her colleagues noted. But beyond decreased SLC12A5 expression, definitive ties to epileptic disease have been difficult to verify.

"Earlier associations with KCC2 have been observed, such as a down-regulation of the protein after brain damage that increases the tendency for seizures, but firm evidence for this disease mechanism has been lacking so far," Wedell said.

"[W]e have been able to prove that a defective function of the KCC2 protein causes epilepsy and hence that an imbalance in the brain's chloride ion regulation system can be the reason behind the disease," she explained, noting that additional research is needed to determine whether comparable ion imbalances contribute to other forms of epilepsy.

The current study uncovered KCC2 contributions to EIMFS through in-depth testing on members of a Swedish family with two affected children and unaffected, unrelated parents and a Pakistani family that included two affected children, two unaffected siblings, and consanguineous parents.

The researchers did autozygosity mapping on members of the Pakistani family using Illumina cytoSNP-12 arrays.

For both families, they also performed exome sequencing on affected children and their parents using Nimblegen, Agilent, or Illumina capture methods coupled with Illumina HiSeq 1000, 2000, or 2500 sequencing instruments.

After weeding out alterations that didn't fit the anticipated mode of inheritance for EIMFS in the families, the team was left with suspicious rare mutations in SLC12A5.

In affected children from the Pakistani family, the researchers found homozygous missense mutations in the gene, while the affected Swedish children appeared to have inherited alterations that affected each copy of SLC12A5 differently.

The researchers did not pick up other SLC12A5 mutations in the gene when it screened members of 37 more families affected by either EIMFS or other forms of early-infantile epileptic encephalopathy.

Their follow-up protein structure, cell line, and zebrafish experiments support the notion that the type of mutations identified in the original EIMFS-affected families dramatically alter functionality of the KCC2 transporter in ways that interfere with normal GABA signaling and affect early motor development.

"Genotype-phenotype correlations will become evident over time, as more cases are reported, but SLC12A5 gene dosage (recessive versus dominant disorders), mutation type, and the effect of mutations on protein activity may indeed influence epilepsy phenotype, including age of disease onset and clinical severity," Wedell and her co-authors concluded. 

"Identification of additional SLC12A5 mutations in humans will no doubt further expand the clinical spectrum," they explained, "and solidify genotype-phenotype observations in KCC2-related human diseases."