Dravet syndrome (also known as severe myoclonic epilepsy of infancy) is a severe form of epilepsy, which usually appears during the first year of life as fever-related seizures. As the disease progresses, other types of seizures such as myoclonus and status epilepticus may also be seen.

What causes Dravet syndrome?

Around 70 to 85 percent of Dravet syndrome cases are caused by mutations in the SCN1A gene. Hundreds of mutations in this gene have been identified that are linked to seizures caused by high fever.

The SCN1A gene provides instructions to build a protein that forms a subunit of a sodium channel called NaV1.1. This sodium channel is found on the surface of nerve cells and controls the influx of sodium ions into the cells, activating them and allowing them to pass nerve signals onto other cells.

There are different types of nerve cells. Some are excitatory and send signals that activate other neurons, and some are inhibitory, sending signals that silence other neurons.

Scientists do not fully understand how the mutations in the SCN1A gene cause seizures. But they think one way could be by impairing the activity of a type of inhibitory neurons called GABA neurons leading to no inhibitory signals being fired. The result is the overactivation of the nervous system, firing signals uncontrollably and the development of seizures.

There may be other ways in which the mutations alter the functioning of the sodium channels and the nervous system, leading to seizures, but the molecular processes of these events are not currently known.

How is Dravet syndrome inherited?

Although most cases of Dravet syndrome are the result of de novo or new mutations, around 5 to 10 percent of cases are familial and inherited by children from their parents.

Dravet syndrome caused by mutations in the SCN1A gene follows an autosomal dominant inheritance pattern. This means that a single copy of the mutated SCN1A gene, inherited either from the mother or the father, is sufficient to cause Dravet. A person affected by the condition has a 50 percent risk of having a child affected by Dravet syndrome.

Other genetic causes of Dravet syndrome

Around 10 to 30 percent of Dravet syndrome cases occur with no apparent reason. However, other genes  such as GABRG2, SCN1B, SCN2A, CHD2, HCN1, GABRA, and STXBP1 are thought to also be implicated

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