Abstracts

Rationale: The SCN8A gene encodes the voltage-gated sodium channel alpha-subunit Na_v1.6, which is critical for generation and propagation of neuronal action potentials. Missense gain-of-function variants in SCN8A cause a severe developmental and epileptic encephalopathy (DEE) characterized by seizures, developmental delay, motor impairment, and elevated risk of death. While motor impairment is the most common comorbidity of SCN8A DEE and significantly impacts quality of life, its pathophysiology is not clear. Previous mouse models of SCN8A DEE recapitulated seizures but lacked motor dysfunction.

Methods: Using CRISPR-Cas9 system, we developed a new mouse model of SCN8A DEE with an inducible allele of the patient variant p.Thr767Ile (T767I). Global expression of T767I allele with Sox2-cre (Scn8a-T767I/+) results in early-onset seizures and premature lethality (within 3-weeks after birth). Using a battery of behavioral assessments, we show that Scn8a-T767I/+ mice also exhibit significant motor impairment, including righting delay, muscle weakness, and altered gait. To study the relationship between motor impairment and motor neuron activity in our epilepsy model we assessed the electrophysiology of motor neurons by recording compound muscle action potentials (CMAPs) from mouse hindlimbs and also estimated functional motor units in sciatic nerve innervated muscles. Additionally, we used motor network specific Cre lines to delineate the role of neuronal circuits that contribute to motor impairment in T767I/+ mice.


Results: Mice expressing Scn8a T767I allele only in excitatory neurons (Emx1-Cre) have seizures without motor impairment, indicating that the neuronal mechanisms underlying seizures are distinct from those underlying motor impairment. Electrophysiological recordings from muscles of Scn8a-T767I/+ (Sox2-Cre) mice revealed reduced compound muscle action potentials and fewer functional motor units, suggesting that abnormal motor neuron activity contributes to motor impairment. Further, with other network-specific Cre mouse lines we show that T767I expression in lower motor neurons (ChAT-Cre) causes righting delay, indicating that motor neuron dysfunction contributes to delayed motor development. In contrast, expression of T767I in cerebellar Purkinje neurons (Pcp2-Cre) results in uncoordinated gait, indicating that cerebellar dysfunction contributes to movement comorbidities in SCN8A DEE.



Conclusions: Overall, our studies show that the Scn8a-T767I/+ mouse is the first rodent model of SCN8A DEE to recapitulate both early-onset spontaneous seizures and motor impairment. In this study we identified distinct neuronal populations contributing to seizures and motor impairment in Scn8a-T767I/+ mice. Our ongoing studies will help determine the diverse pathogenic neuronal mechanisms underlying seizures, motor impairment, and other comorbidities in SCN8A DEE.





Funding: R01 – NINDS NS131319

R21 – NINDS NS098003