antisense oligonucleotide therapy

Researchers used a promising new therapy called antisense oligonucleotide therapy to control the expression of a mutant SCN8A gene, resulting in delayed seizure onset and survival in mouse models of SCN8A encephalopathy and Dravet syndrome. 

What is SCN8A encephalopathy?

Developmental and epileptic encephalopathies are a group of rare epilepsy syndromes that are characterized by early-onset seizures, abnormal electrical brain activity, and developmental delay or regression (loss of skills) in children.  These syndromes are often highly resistant to treatment and get worse with time.

The onset of seizures usually occurs around four months.

Seizures are sudden, uncontrolled electrical disturbances in the brain that cause changes in awareness, behavior, and/or abnormal movements.  

Types include generalized tonic-clonic seizures, infantile spasms, absence seizures, and focal seizures.

Other symptoms may include low muscle tone (hypotonia), movement disorders, mild to severe intellectual disability, sleep problems, and autistic-like features.

Although only one mutated gene from a parent is needed to manifest the disorder, interestingly, most patients acquire a new (de novo) mutation in the gene that was not inherited from a parent.

SCN8A encephalopathy results from a single gain-of-function mutation that disturbs electrical activity in nerve cells  

The SCN8A gene provides instructions for making one section of a voltage-gated sodium channel protein, the alpha (α8)-subunit called Nav1.6 located at the axon initial segment of a neuron.

SCN8A encephalopathy is associated with DNA base-pair (missense) mutations at this gene that cause gain-of-function disturbances.   

The body communicates with the brain through specialized nerve cells called neurons within the human nervous system. Electrical impulses called action potentials are generated in one neuron and travel forward to other neurons (or other excitable cells) with the release and uptake of neurotransmitters.

Nav1.6 channels are found in nerve cells of the brain and spinal cord (central nervous system) but also in peripheral sensory and motor neurons located outside the central nervous system that connects the body to the brain.

As its name implies, voltage-gated sodium channel proteins form a massive channel across the cell membrane that allow positive sodium atoms (ions) passage into the cell through the α-subunit pore.  Channels open and shut in response to voltage differences across the membrane. 

At rest, the channels stay closed but an action potential is generated when the channel momentarily opens and sodium ions flood into the cell.

SCN8A gain-of-function mutations cause the channel to open prematurely, or hinder closing resulting in increased electrical impulses that abnormally excite neuronal networks in the brain leading to seizures.

Note that SCN8A loss-of-function mutations are also possible that result in less active sodium channels leading to other less severe disorders.

Thus far, nine other (α-subunit) channel genes including SCN1A and SCNA3 have been identified with mutations leading to a wide range of disorders.

In four independent studies of individuals with epileptic encephalopathies, de novo mutations of SCN8A were identified in 1% of cases (13/1557).

Researchers used antisense oligonucleotide therapy as treatment for childhood epilepsy

The knowledge that a gene gets “turned on” when its DNA sequence is first made into an RNA copy (called a transcript), which then gets translated into a specific chain of amino acids called proteins, prompted scientists to use recently discovered antisense oligonucleotide therapy.

Researchers from the University of Michigan wanted to know if they could delay seizure onset and prolong survival in a mouse model of SCN8A encephalopathy by reducing the abundance of mutated RNA transcript.

They designed an antisense oligonucleotide (short single strands of synthetic DNA or RNA) that would complementarily bind to both the mutated and normal Scn8a transcript and block Nav1.6 sodium channel protein formation.

In a recent study published in Annals of Neurology, researchers developed a conditional mouse model of a commonly seen, de novo SCN8A patient mutation at amino acid number 1872 (R1872W).  This mutation impairs inactivation (closing) of the Nav1.6 channel leading to severe seizures. 

A litter was produced with 50% unaffected wild-type offspring and 50% SCN8A mutant mice. Mice were randomly assigned to treatment with Scn8a oligonucleotide (ASO) or a control ASO (on day two) that would not bind to the Scn8a transcript.  The abundance of Scn8a transcript was measured on day 21.

Researchers found that SCN8A mutant mice responded well to Scn8a ASO treatment by increasing their survival in a dose-dependent manner up to seven weeks of age.  They also found that Scn8a ASO treatment protected against low-level seizure activity in these mice.

However, mutant mice that did not receive treatment experienced a sudden onset of seizures on day 14 to 16 and died 24 hours later.

Furthermore, mutant mice that received a second dose of Scn8a ASO on day 30 increased their survival to about nine weeks demonstrating that Scn8a ASO has potential as a long-term treatment.

In terms of dosage, they found that reduction of Scn8a transcript level in mutant mice to about 50% of wild-type results in seizure protection that is well tolerated with only mild behavioral abnormalities.  It was previously shown that levels below 10% of wild-type can have detrimental effects.

By the time seizures began in mutant mice at about six weeks, Scn8a transcript levels had increased to levels similar as wild-type expression. 

Further investigations are necessary to determine if Scn8a ASO administration after seizure onset is also effective.

Antisense oligonucleotide therapy provides hope for childhood epilepsies

Researchers also tested Scn8a ASO therapy on Dravet syndrome, another developmental and epileptic encephalopathy caused by a mutation in the SCN1A voltage-gated sodium channel gene.

They found that a single treatment extended survival of Dravet syndrome mice from three weeks to over five months. 

The next step is to test other mouse models of seizure disorders with the potential to go beyond sodium channel disorders given the ability to synthetically design custom targeted ASOs for any genetic disorder. 

In support of this, ASO treatment has already been approved by the US Food and Drug Administration for the treatment of spinal muscular atrophy (which impairs motor neurons that control a muscle movement).

Written by Maria-Elena Bernal

References:

  1. Lenk, M., Jafar‐Nejad, P., Hill, S. F., Huffman, L. D., Smolen, C. E., Wagnon, J. L., … Meisler, M. H. (2020, February 6). Scn8a Antisense Oligonucleotide Is Protective in Mouse Models of SCN8A Encephalopathy and Dravet Syndrome. Retrieved from https://onlinelibrary.wiley.com/doi/full/10.1002/ana.25676#ana25676-fig-0002

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