Increased epileptogenicity in a mouse model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is associated with higher rates of epilepsy compared to the general population. Some NF1 patients with epilepsy do not have intracranial lesions, suggesting the genetic mutation itself may contribute to higher rates of epilepsy in these patients. We have recently demonstrated increased seizure susceptibility in the Nf1^+/- mouse, but it is unknown whether this model displays altered epileptogenicity, as has been reported in patients with NF1. The aim of this study was to determine whether the Nf1^+/- mouse is more susceptible to electrical kindling-induced epileptogenesis.

Young male or female adult Nf1^+/- or Nf1^+/+ (wild-type; WT) mice were implanted with electrodes for neocortical or hippocampal kindling paradigms. Neocortical kindling was performed for 40 stimulation sessions followed by baseline EEG monitoring to detect possible SRSs. Hippocampal kindling was performed with a modified extended kindling paradigm, completed to a maximum of 80 sessions to try to induce spontaneous repetitive seizures (SRSs). Western blot assays were performed in naïve and kindled mice to compare levels of Akt and MAPK (ERK1/2), proteins downstream of the NF1 mutation.

The average initial neocortical after-discharge threshold (ADT) was significantly lower in the Nf1^+/- group, which also required fewer stimulations to reach stage 5 seizure, had greater average seizure severity across all kindling sessions, had a greater number of convulsive seizures, and had a faster progression of after-discharge duration and Racine score during kindling. No WT mice exhibited SRS after neocortical kindling, versus 33% of Nf1^+/- mice. The average initial hippocampal ADT was not significantly different between the WT and Nf1^+/- groups, nor was there a difference in the number of stimulations required to reach the kindled state. The WT group had a significantly higher average seizure severity across all kindling sessions as compared with the Nf1^+/- mice. The WT group also had faster progression of the Racine seizure score over the kindling sessions, mainly due to a faster increase in seizures severity early during the kindling process. However, SRSs were seen in 50% of Nf1^+/- mice after modified extended kindling and in no WT mice. Western blots showed hippocampal kindling increased the ratio of phosphorylated/total Akt in both the WT and Nf1^+/- mice, while neocortical kindling led to increased ratios of phosphorylated/total Akt and MAPK in Nf1^+/- mice only.

We have demonstrated for the first time an increased rate of epileptogenesis in an animal model of NF1 with no known macroscopic/neoplastic brain lesions. This work provides evidence for the genetic mutation itself playing a role in seizures and epilepsy in patients with NF1, and supports the use of the Nf1^+/- mouse model in future mechanistic studies.