Recessive mutations inSLC13A5result in a loss of citrate transport and cause neonatal epilepsy, developmental delay and teeth hypoplasia

Abstract

The epileptic encephalopathies are a clinically and aetiologically heterogeneous subgroup of epilepsy syndromes. Most epileptic encephalopathies have a genetic cause and patients are often found to carry a heterozygous de novo mutation in one of the genes associated with the disease entity. Occasionally recessive mutations are identified: a recent publication described a distinct neonatal epileptic encephalopathy (MIM 615905) caused by autosomal recessive mutations in the SLC13A5 gene. Here, we report eight additional patients belonging to four different families with autosomal recessive mutations in SLC13A5. SLC13A5 encodes a high affinity sodium-dependent citrate transporter, which is expressed in the brain. Neurons are considered incapable of de novo synthesis of tricarboxylic acid cycle intermediates; therefore they rely on the uptake of intermediates, such as citrate, to maintain their energy status and neurotransmitter production. The effect of all seven identified mutations (two premature stops and five amino acid substitutions) was studied in vitro, using immunocytochemistry, selective western blot and mass spectrometry. We hereby demonstrate that cells expressing mutant sodium-dependent citrate transporter have a complete loss of citrate uptake due to various cellular loss-of-function mechanisms. In addition, we provide independent proof of the involvement of autosomal recessive SLC13A5 mutations in the development of neonatal epileptic encephalopathies, and highlight teeth hypoplasia as a possible indicator for SLC13A5 screening. All three patients who tried the ketogenic diet responded well to this treatment, and future studies will allow us to ascertain whether this is a recurrent feature in this severe disorder.

Mutations in the Na+/Citrate Cotransporter NaCT (SLC13A5) in Pediatric Patients with Epilepsy and Developmental Delay

“…Our cell surface biotinylation results with untagged G219R using specific anti-NaCT antibodies agree with the previous study showing that tagged G219R is found on the plasma membrane (3), but unlike that study, our antibodies identified fully glycosylated G219R rather than just one lower mass band. Furthermore, we did not find protein expression of T227M, whereas the tagged T227M was found on the plasma membrane in the previous study (3).…”

“…The families are from the United States, the Netherlands and Brazil (Supplementary Table S1), and the children age range from 2 to 18.7 years ( Table 2). Three of the families had homozygous mutations: T227M and G219R, both reported previously in compound heterozygous form (2,3), and a new deletion mutation c511DelG (abbreviated DelG in this paper). The c511DelG mutation produces a frameshift leading to a premature stop codon, with a resulting (Table 3).…”

“…Mutations in the SLC13A5 gene that codes for the NaCT have been reported to cause epilepsy, developmental delay and tooth dysplasia in children (2,3). In this study, we identified six families with new SLC13A5 mutations.…”

“…The onset of seizures in the first weeks of life has been associated with a large number of metabolic and genetic causes (1) including recently identified heterozygous mutations in the SLC13A5 gene that codes for the Na + /citrate transporter, NaCT (2,3). Relatively few children with SLC13A5 disorder have been reported in the literature, and the full extent of their neurologic and epileptic phenotypes is only beginning to emerge.…”

“…This disorder is also characterized by limited and slow motor progress, with children described as unable to sit or walk independently. Reports of tone are also variable, and patients exhibit a range of symptoms: increased tone, decreased tone and poor motor coordination to choreoathetoid movements (2,3). Language is limited with a few patients…”