Overview – Dravet Syndrome

Hello Open Sciences World! In my very first post for the SGC’s Extreme Open Sciences project, I will start by providing an overview of my project – Gene therapy for Dravet Syndrome.

Dravet Syndrome is a catastrophic, rare form of infantile epilepsy. The disease onset occurs in the first year of life where children with Dravet Syndrome suffer from frequent and prolonged seizures, cognitive and motor dysfunction and a substantially elevated risk of coma and death. There is currently no cure for Dravet Syndrome. The current standard of care involves using anti-epileptic drugs to relieve its symptoms, but, unfortunately, most anti-epileptic drugs are not very effective.

The majority (>90%) of Dravet Syndrome is caused by mutations in SCN1A gene. This gene encodes a complex pore-forming protein that forms voltage-gated sodium channels on the surface of our neurons. The primary function of these channels is to initiate and transmit electrochemical communicative signals in neurons. They open in response to changes in electrical membrane potential across cell membranes and enable the influx of sodium ions into our neurons. Similar to the electrical currents that power our batteries, you can think of the inflow of ions as electrochemical signals that powers our neural circuit.

Most Dravet Syndrome related mutations result in haploinsufficiency of SCN1A where one copy of SCN1A does not function and having one healthy functioning copy of SCN1A gene is not enough for normal cellular function. One way to treating loss of function or haploinsufficiency disorders is via gene therapy where we make use of a vector, an engineered virus, to deliver back-up functional copies of genes into affected cells to increase the production and to restore functions of the missing proteins. While gene therapy methods have been approved to treat retinal dystrophy and several other diseases, the development of gene therapy for Dravet Syndrome remains a challenge. The engineered virus that we use as a vehicle to deliver gene can only fit ~4.7kb of DNA and SCN1A (~6kb) is too big for this vehicle.

My research focuses on developing a split-intein protein “gluing” system where the SCN1A gene can be split into two smaller fragments that are sufficiently small to be packaged into common gene therapy vehicles. Upon delivery in cells, two fragments of SCN1A protein, each fused to a split-intein, will be expressed. Inteins are protein sequences that can catalyze self-excision and concurrently glue its flanking protein fragments back together. When two fragments of SCN1A proteins fused to split-intein are next to each other, the split-intein can glue two parts of the proteins together, thereby reconstituting functional SCN1A proteins in targeted cells.

2 Replies to “Overview – Dravet Syndrome”

  1. Welcome Wen! I am very interested in your effort.

    One question: do you know of any patients carrying mutations that do not completely abrogate SCN1A glycoprotein function – and yet cause slight misfold and SCN1A glycoprotein retention in the Endoplasmic Reticulum?

    1. Hi Pietro! Sorry for the late reply – my notifications from openlabnotebooks ended up in the spam folder.

      The short answer is – YES. There are missense mutations that impair trafficking of SCN1A to the plasma membrane (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516746/).

      To be precise, a spectrum of phenotype can arise from SCN1A mutations, ranging from benign febrile seizures to Dravet, a very malignant and severe expression of epilepsy. The mutations that were identified at the more benign end of SCN1A related disorders were mostly missense mutations while most mutations (>80%) of SCN1A in Dravet patients were frame-shift and nonsense mutations. It will be interesting to perform a large-scale mutational analysis to see whether the remaining ~20% of those missense mutations cause protein misfolding and ER associated degradation.

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