A zebrafish seizure model of cblX syndrome reveals a dose-dependent refractory response to mTor inhibition.

Mutations in the transcriptional cofactor HCFC1 cause methylmalonic aciduria and homocystinemia, cblX type (cblX) (MIM#309541), non-syndromic X-linked intellectual disability (XLID), and focal epilepsy. Zebrafish studies have revealed increased activation of the Akt/mTor signaling pathway after mutation of hcfc1a, one ortholog of HCFC1. mTOR hyperactivation is linked to seizures and its inhibition alleviates epilepsy in other preclinical models. We hypothesized that mTor overactivity in hcfc1a mutant zebrafish increases seizure susceptibility and/or severity. We employed a two-concentration model of the seizure inducing agent, pentylenetetrazol (PTZ), with or without pretreatment of the mTor inhibitor, torin1. Mutation of hcfc1a did not increase seizure susceptibility at sub-optimal concentrations of PTZ and the pharmaceutical inhibition of mTor reduced seizure severity when utilized at a dose of 250nM. Higher doses of torin1 treatment exacerbated seizure response in mutant larvae but not in wildtype siblings. These data suggest that in an hcfc1a deficient background, moderate inhibition of mTor signaling may partially alleviate seizure phenotypes, however, over-inhibition of the pathway causes a refractory response to PTZ. Collectively, we present a model that can be used to test dose response and for the development of combinatorial treatment approaches in a high throughput manner.