Mutations identified by exome sequencing demonstrate that disruption of the gene SCN2A, which encodes the alpha subunit of the neuronal voltage-gated sodium channel 1.2 (Nav1.2), is strongly associated with autism spectrum disorder. Nav1.2 channels are important for action potential initiation and propagation in excitatory neurons. The expression levels of Nav1.2 channels changes throughout development, suggesting that they have distinct roles in developing and mature neurons.
Kevin Bender’s laboratory recently examined how Scn2a haploinsufficiency in mice affects excitability and synaptic connectivity of neurons in the prefrontal cortex. Their studies revealed that Nav1.2 channels have two developmentally distinct roles in pyramidal cells1. Early in development (before postnatal day 7), Scn2a haploinsufficiency impairs action potential initiation due to a loss of Nav1.2 channels in the axon initial segment. In more mature neurons, Nav1.2 loss in the dendrites impairs dendritic excitability, resulting in deficits in excitatory synaptic strength and synaptic plasticity, and learning and social impairments. Bender and colleagues also showed that loss of Nav1.2 channels late in life was sufficient to alter dendritic excitability and synaptic strength. This suggests that Nav1.2 channels are important for proper synaptic function throughout life, rather than being important only during a very early critical period of development. Furthermore, it raises an important question: would restoration of Nav1.2 function provide a therapeutic benefit for conditions associated with Scn2a haploinsufficiency, even in more developmentally mature systems?
Nadav Ahituv’s laboratory has recently shown that CRISPR-mediated activation (CRISPRa) tools could successfully be used to reverse obesity phenotypes in two distinct haploinsufficient mouse models of obesity2. Combining efforts, Bender and Ahituv have been exploring the possibility that CRISPRa-mediated restoration of normal Nav1.2 levels might restore the cellular, circuit and behavioral deficits seen in Scn2a haplosufficient mice. In a pilot study, funded by a SFARI Explorer Award, the team optimized adeno-associated viral CRISPRa titers, single-guide RNA targets and injection conditions to achieve activation levels in Scn2a heterozygous mice similar to that of wild-type mice.
Continuing on from this work, Bender and Ahituv aim to determine whether cellular and behavioral deficits can be restored in Scn2a heterozygous mice. If successful, work herein could pave the way towards utilizing CRISPRa as a therapeutic tool for a range of neurodevelopmental conditions associated with haploinsufficiency.