Abstracts

This is a Late-Breaking abstract.

Rationale: Tuberous sclerosis complex (TSC) is a multi-system disorder caused by loss-of-function (LoF) mutations in the TSC1 or TSC2 genes. TSC is inherited in an autosomal dominant pattern indicating that one functional copy of the TSC1 or TSC2 gene is insufficient to compensate for the LoF mutation, a situation known as haploinsufficiency. The clinical presentation of TSC is highly heterogenous, but the majority of patients have epilepsy. Mouse models are an ideal system to dissect the mechanisms of genetic disorders like TSC, but to date no mouse models with Tsc1 haploinsufficiency present with epilepsy. To generate seizures in these models, a homozygous Tsc1 knockout (KO) is typically induced conditionally in neurons. Such second-hit systems do not model the pattern of human inheritance of TSC. Because mouse models of TSC are created on a limited number of genetic backgrounds, it is unknown whether mice in general are a poor model of epilepsy in TSC, or if the genetic backgrounds studied are particularly resilient to epilepsy. We hypothesized that by varying the genetic background with a heterozygous Tsc1 LoF mutation, we would indeed be able to recapitulate TSC-associated epilepsy, without having to resort to conditional homozygous knockouts.

Methods: To test this hypothesis, we generated a new congenic strain by backcrossing an LoF allele onto a pure C57BL/6J (B6) background to create B6-Tsc1^+/- mice. We then bred B6-Tsc1^+/- mice to make F1 hybrids (Figure 1A) with the seizure-susceptible DBA/2J (D2) strain (D2-Tsc1^+/-) and the BXD87/RwwJ (BXD87) strain (BXD87-Tsc1^+/-). The BXD87 strain is a recombinant inbred strain generated from the B6 and D2 parent strains that has a mosaic genome containing alleles from both parents. We performed long-term electrophysiological monitoring for seizures and other epileptiform activity in these mice and their littermate controls (N = 49).

Results: In the BXD87-Tsc1^+/- mice we observed multiple spontaneous generalized tonic-clonic seizures (3/11 mice) confirmed with video, including seizures captured on video prior to electrode implantation surgery (Figure 1B, example trace). We did not observe spontaneous seizures in B6-Tsc1^+/- or D2-Tsc1^+/- mice, despite the known seizure susceptibility of the D2 strain. Together, these results demonstrate for the first time that seizures caused by Tsc1 haploinsufficiency can be modeled in mice, providing the first construct-valid and face-valid model of TSC-associated epilepsy. Moreover, the strain dependence demonstrates the existence of genetic modifiers of epilepsy in this model. Indeed, the lack of seizures in both the B6-Tsc1^+/- and D2-Tsc1^+/- mice shows that there must be at least two such modifiers in BXD87-Tsc1^+/- mice, implicating a complex inheritance pattern.

Conclusions: Our BXD87-Tsc1^+/- mouse is a unique new resource for modeling TSC-associated epilepsy that can be generated by breeding alone. In future work, we hope to expand the strain survey to identify susceptible strains for other TSC outcomes and use the BXD87-Tsc1^+/- model to identify risk and resilience factors for TSC-associated epilepsy.

Funding: DOD Tuberous Sclerosis Complex Research Program (DOD-TS180087)