The SCN2A gene encodes the alpha subunit of the neuronal voltage-gated sodium channel NaV1.2. SCN2A was one of the earliest identified genes associated with autism spectrum disorder (ASD) and loss-of-function mutations in SCN2A (e.g., Cys959X) are strongly correlated with ASD and developmental delay.
Loss-of-function mutations can result in missense codons whereby the incorrect amino acid produces a channel with diminished function or improper trafficking, or nonsense codons which introduce a premature termination codon (PTC) within the channel’s reading frame. In the current project, Christopher Ahern and his team are focused on the targeted repair of the later — PTCs which ablate the expression of NaV1.2 sodium channels.
Existing PTC small molecule therapies promote the encoding of a near-cognate amino acid at the site of the PTC, in effect, toggling the nonsense codon to a missense mutation. Given that NaV1.2 is poorly tolerant of missense mutations, it is likely that these small molecule approaches will yield channels with defective activity (i.e., leading to either hyper- or hypo- activity levels) and severe neurological side-effects.
Ahern’s group have recently expanded the utility of engineered suppressor transfer RNAs (tRNAs) for the repair of PTCs. Their findings show that these suppressor tRNAs encode the intended amino acid at the PTC, and high-resolution ribosomal profiling data argue for their limited interactions with the real termination codons that naturally halt protein synthesis1.
Here, the researchers, including co-investigator Aislinn Williams, propose to use adeno-associated viral (AAV) delivery of a tRNA (i.e., an engineered version that repairs TGA stop codons) into induced pluripotent stem cell (iPSC)-derived cortical neurons from individuals who are known to have nonsense mutations in SCN2A. Overall, they hypothesize that acute viral delivery of codon-edited tRNAs to these iPSC-derived neurons will restore NaV1.2 channel function and neurophysiology. Such an outcome would be a promising first-step towards a new gene-therapy approach for ASD and other sodium channelopathies that result from PTCs, such as Dravet syndrome.