Abstracts

Rationale: SLC13A5 Deficiency is a severe and rare form of epileptic encephalopathy that is due to autosomal recessive mutations in the SLC13A5 gene, which codes for a plasma membrane sodium-dependent citrate transporter. To date all tested mutations result in no or a much reduced amount of the citrate transported inside the cells. Affected children present with seizures beginning within a few days of birth that persist throughout life. They show difficulty with speech production, limited and slow motor progress and some never achieving walking independently. Currently there are no treatments for SLC13A5 deficiency that target the underlying cause of disease, and gene replacement therapy represents a therapeutic option for SLC13A5 Deficiency.

Methods: We developed a self-complementary vector encoding a codon-optimized human SLC13A5 gene (scAAV9/SLC13A5), the unaltered design of which could be appropriate for human use. We tested safety and efficacy of CSF-delivered scAAV9/SLC13A5 in Slc13a5 knockout (KO) mice and wild type (WT) littermates. Similar to patients, Slc13a5 KO mice have increased plasma citrate levels, EEG abnormalities and an increased susceptibility to seizure induction. Mice were treated with scAAV9/SLC13A5 via intrathecal (IT) delivery at post-natal day 10 or by IT or intracisternal-magna (ICM) delivery at ~ 3 months of age. Mice were monitored for weight and survival, and blood was collected at baseline and then monthly thereafter. Mice received telemetry implants to record baseline EEG and EMG activity and were then tested for susceptibility to seizure induction by pentylenetetrazol (PTZ). Tissues were collected at the study endpoint for assessing vector expression and distribution.

Results: KO mice treated with scAAV9/SLC13A5 had significantly decreased plasma citrate levels while KO mice treated with vehicle had sustained, high citrate levels. EEG assessments showed that vehicle treated KO mice had increased spike train activity and seizure frequency compared to their WT littermates and, importantly, that vector treatment reduced this epileptic activity with greater rescue achieved with ICM delivery than IT delivery in the older mice. Via the Racine scale, Slc13a5 KO mice had increased seizure susceptibility, which was attenuated with scAAV9/SLC13A5 treatment in both treatment age groups, with a greater benefit achieved with ICM delivery compared to IT delivery when mice were treated at 3 months of age. Importantly, these studies showed treatment benefits with CSF delivery of scAAV9/SLC13A5 when administered at an early age, during brain development, as well as in young adult mice when the brain was fully developed. Additionally, following treatment there were no adverse findings in KO or WT mice with either IT or ICM delivery and at both treatment ages.

Conclusions: Overall, our preclinical results suggest that gene replacement therapy with scAAV9/SLC13A5 could provide a meaningful benefit to SLC13A5 patients.

Funding: Please list any funding that was received in support of this abstract.: This work was supported by the TESS Foundation and Taysha Gene Therapies.