A Zebrafish Seizure Model of cblX Syndrome Reveals a Dose-Dependent Response to mTor Inhibition

Mutations in the transcriptional co-factor HCFC1 cause methylmalonic aciduria and homocystinemia, cblX type (<i>cblX</i>) (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 <i>hcfc1a</i>, one ortholog of <i>HCFC1</i>. mTOR hyperactivation is linked to seizures, and its inhibition alleviates epilepsy in other preclinical models. We hypothesized that mTor overactivity in <i>hcfc1a</i> 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 <i>hcfc1a</i> did not alter the response to PTZ at sub-optimal concentrations, and the pharmaceutical inhibition of mTor using the compound Torin1 reduced response to 1 µM PTZ, but only in a dose-dependent manner. Higher doses of mTor inhibition did not reduce the seizure response in mutant larvae but were effective in wildtype siblings. These data suggest that inhibition of mTor in an <i>hcfc1a</i>-deficient background leads to a reaction that differs from the traditional response observed in wildtype siblings. Collectively, we present a model that can be used to test dose-response and the development of combinatorial treatment approaches in a high-throughput manner.