Tadokoro Award 2021 Best basic poster presentation

Best basic poster presentation

Abstract #318

Non-cell Autonomous Hyperexcitability Underlies Focal Epileptogenesis Mediated by Low-level Brain Somatic Mutations in MTOR
KOH H^1,2, JANG J¹, JU SH¹, KIM R¹, CHO G-B¹, KIM DS³, SOHN J-W¹, PAIK S-B¹, LEE JH^1,4
1Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, Republic of, ²Boston Children's Hospital, Boston, United States, ³Severance Children's Hospital, Yonsei University College of Medicine, Seoul, Korea, Republic of, ⁴SoVarGen, Inc., Daejeon, Korea, Republic of

Purpose: Low-level somatic mutation in neurons in the brain causes intractable focal epilepsy, including focal cortical dysplasia type 2 (FCD II) (Lim et al. Nat Med 2015;21:395-400.) and ganglioglioma (Koh et al. Nat Med 2018;24:1662-1668.). However, a specific mechanism, how these few mutation-carrying neurons induce epileptogenesis at the “local network level”, remains poorly understood.

Method: We generated FCD II model mice having a somatic mutation in MTOR L2427P presenting seizures by in utero electroporation and ascertained the number of mutated neurons (MTOR^L2427P) throughout the whole brain along with patients-derived tissue. To probe the origin of epileptogenesis, we measured the neuronal excitability in MTOR^L2427P and nearby non-mutated neurons (L2427P^nearby) by current injection and multi-electrode recording comparing the topographic distribution of mutation. Computational connectivity using the leaky-integrate-and-fire model recapitulates network changes based on measured properties. To examine the underlying mechanism, we measured inhibitory and excitatory (E-I) synaptic inputs in MTOR^L2427P and L2427P^nearby by electrophysiological and immunofluorescence studies. To explain non-cell autonomous hyperexcitability, an inhibitor of adenosine kinase (ADK), which translation is increased by MTOR^L2427P in ribosome-sequencing, was injected into mice in vivo.

Result: 1.85±0.80% of neurons carried the MTOR somatic mutation in the whole brain. Interestingly, the seizure-triggering hyperexcitability was originated from L2427P^nearby, but MTOR^L2427P was less excitable than L2427P^nearby(P< 0.0001). Consistent with estimation by the network simulation, functional and molecular study showed that the net balance between E-I synaptic inputs onto MTOR^L2427P remained unchanged in mice and FCD II patients' tissues, implying that intrinsic synaptic changes driven by MTOR mutation are less likely to be explanatory. Instead, we found that ADK inhibition reduced the hyperexcitability of L2427P^nearby(P< 0.0001), affecting adenosine metabolism in the local network.
Conclusions: Taken together, neurons carrying somatic mutations in MTOR lead to focal epileptogenesis via non-cell autonomous hyperexcitability of nearby non-mutated neurons.