Dravet syndrome etiology

The diagnosis of Dravet syndrome is based on the patient’s clinical picture. He/she usually has no history of any significant brain injury to explain the epilepsy such as birth asphyxia or meningitis. Standard investigations such as blood biochemistry and brain imaging studies (CT and MRI) are at this point normal.

 

Molecular Diagnosis – Pr Le Guern

Genetics of Dravet Syndrome – Dr Christel Depienne

Since 2001, it has become clear that Dravet syndrome is caused in most cases by a genetic mutation that alters the normal electrical functioning of brain cells.

More than 70% of patients with Dravet syndrome have a mutation in the SCN1A gene on chromosome 2. Other Dravet patients may also have a mutation in other genes.

Boys and girls are equally affected by SCN1A mutations since it is not located on a sexual chromosome.

Patients with Dravet syndrome often have no family history of the disease and are in this case qualified as “sporadic” or “isolated”. This is due to the fact that the mutation has happened « de novo » in the patient, meaning that it is absent from any of their parents.

Examples of families with a child suffering from Dravet syndrome

Dravet syndrome affects children of both sexes and is usually an isolated disorder. It occurs independently of the rank of birth in the family.
Dravet syndrome affects children of both sexes and is usually an isolated disorder. It occurs independently of the rank of birth in the family.

When the mutation is “de novo”, there is little chance that future siblings of the Dravet child will be affected by Dravet syndrome. Despite this fact, it is recommended to undergo genetic testing, because siblings may also be affected in some rare cases.

The identification of the mutation is performed through molecular diagnosis as shown below:

Genetic counselling may help parents understand the risk of recurrence of Dravet syndrome in their other children.

If genetic testing is not available in your country, you may visit the following website which offers, under certain conditions, a genetic test free of charge. http://www.testdravet.info

1/ SCN1A gene

In 2001, a Belgium team showed that Dravet syndrome is in most cases due to a genetic mutation in the SCN1A gene on chromosome 2.

The SCN1A gene encodes the alpha 1 subunit of the voltage-dependant sodium channel, also called Nav1.1. channel. This channel is located on the postsynaptic membrane and permeable to sodium ions.

A mutation in this gene either modifies the proper working of the sodium ion channel or leads to a reduction in the number of these channels in brain cells.

The Nav1.1. channel is directly involved in the electrical signals generated by brain cells and the mutation is therefore suspected to have an impact on brain cell discharges, leading to epilepsy.

The SCN1A mutations are inherited in an autosomal dominant fashion, meaning that one mutation is sufficient to cause the disease.

The mutation can occur anywhere in the SCN1A gene. So far, more than 500 different mutations have been identified throughout this gene. These mutations are usually private, with each patient having a different mutation. If you wish to get more information on the SCN1A gene we invite you to visit the following websites:

Mutations found in patients with Dravet syndrome are of all types, including point DNA substitutions or small deletions-insertions (68% of patients). They are also large-scale microrearrangements (deletions or, more rarely, duplications) encompassing one or several exons or the whole gene.

A specific method can be needed to detect SCN1A microrearrangements depending on the method used to perform the genetic test.

A/ The de novo mutations in Dravet syndrome

  • Most SCN1A mutations, associated with Dravet syndrome, occur de novo. This means that the mutation is not present in the parents and occurs in the germ cell (egg or spermatozoid) of one of the parents.
  • Among the de novo mutations identified in patients, a great number of them are “private”, meaning that each patient has a different mutation.
  • When a novel SCN1A mutation is identified in a patient, it is important to confirm that it is the real cause of Dravet syndrome. Indeed, some types of mutations (for example mutations introducing a stop in the protein sequence) are always associated with Dravet syndrome whereas others (amino acid substitutions or missense mutations) are also found in other pathologies (milder familial forms of epilepsy called
    GEFS+Genetic (Generalised) Epilepsy with Febrile Seizures more
    , hemiplegic migraine) or can even be benign variants. The fact that the mutation is de novo in the patient usually confirms that it is the cause of Dravet syndrome.
  • An analysis of the parents is therefore recommended to confirm that the mutation is de novo. This information is also important for the geneticist in order to provide genetic counselling for the family.

B/ The case of parental mosaicism

  • In some rare cases, children with Dravet syndrome have inherited the SCN1A mutation. Indeed, one parent may have this SCN1A mutation, without being affected, or presenting only with benign epilepsy.
  • Parental mosaicism is where one of the parents has in their body both cells with and cells without the mutation. This phenomenon happens when the mutation arises during the parent's own foetal development. These parents are said to be “mosaic”. As shown by the illustration below, the possibility to detect the mutation from a blood sample of the mosaic parent depends on the time when the mutation has occurred during the parent's development.
  • If the mutation appeared at a late stage of cell multiplication, the mutation will be present in low number of the parent's body cells, and therefore will not be detectable in blood cells.
  • On the other hand, if the mutation has appeared at an early stage of cell multiplication, the mutation will be much more present in the parent’s organism, increasing its chance to be detectable in blood cells. Parents with a high number of mutated cells may also be epileptic, depending on the number.
  • In patients with Dravet syndrome, the identification of an SCN1A mutation along with the analysis of both parents is necessary to determine whether the mutation is de novo or inherited.
  • To prevent passing on the mutation and the risk of having another child with Dravet syndrome, a prenatal diagnosis may be proposed during genetic counselling.

C/ Dravet syndrome and Genetic (Generalised) Epilepsy with Febrile Seizures plus (GEFS+) families (missense mutations)

  • Different types of mutations exist in the SCN1A gene. Some are also found in other forms of epilepsy. These more benign forms are known as Genetic (formerly known as Generalised) Epilepsy with
    febrileWith fever
    Seizures more (
    GEFS+Genetic (Generalised) Epilepsy with Febrile Seizures more
    ).
  • Children with Dravet syndrome, who have a SCN1A mutation may be part of families with a GEFS+ condition. In such cases, one of the parents has the same SCN1A missense mutation as his child. This parent is usually less severely or even not affected. The mutation is transmitted to the children following an autosomal dominant inheritance mode, meaning that all children will have a 50% risk of receivinge the mutation from the mutated parent. How and why the same mutation leads to severe epilepsy in the child while it does only mildly or not at all for the parent remains, so far, not understood. The explanation could possibly come from other genetic or environmental factors yet unknown that contribute to this huge difference.

Examples of GEFS+ families including a child with Dravet syndrome

The other family members who have the mutation can be unaffected or only mildly affected compared to the child with Dravet syndrome. The reason for this clinical variability remains so far unknown but genetic or non-genetic factors other than the mutation itself can contribute to this difference.
The other family members who have the mutation can be unaffected or only mildly affected compared to the child with Dravet syndrome. The reason for this clinical variability remains so far unknown but genetic or non-genetic factors other than the mutation itself can contribute to this difference.

A prenatal diagnosis can be carried out in GEFS+ families. However since missense mutations found in GEFS+ families are usually associated with benign phenotypes, the request of prenatal diagnosis should undergo ethics approval.

2/ Other genes

  • About 70% of Dravet syndrome patients have a SCN1A gene mutation. Therefore around 30% of patients clinically diagnosed with Dravet syndrome may have mutations in other genes.

A/ PCDH19 gene

In 2009, a French team showed that mutations or deletions in PCDH19, located on chromosome X, lead to a severe epilepsy that resembled Dravet syndrome.

Mutations in PCDH19 mainly affect females who have a normal and a mutated copy of the gene, whereas males who only have one copy of the mutated gene are unaffected. For this reason, the epilepsy related to PCDH19 mutations was first described in 2008 as Epilepsy in Females with Mental Retardation (EFMR) by an Australian team.

The PCDH19 gene encodes a transmembrane protein called Protocadherin 19. This protein is probably involved in cell adhesion and the establishment of synaptic connections during brain development, although this has not yet been demonstrated.

1- PCDH19 transmission

PCDH19 mutations follow an unusual X-linked inheritance mode.

For diseases caused by mutations on the X chromosome, mutated males are usually more affected than mutated females since they only carry one copy of the gene while females carry two copies. For some diseases, only males are affected and females are unaffected (recessive X-linked inheritance).

The expression mode of PCDH19 related disease is different:

Males carrying the mutation are usually unaffected (or asymptomatic).

Females carrying one normal and one mutated X chromosome are affected.

Example of pedigree with several family members affected with PCDH19-related epilepsy

Characteristics of the unusual inheritance associated with PCDH19-related epilepsy

It is important to keep in mind that a little girl affected by a PCDH19-related Dravet syndrome may have inherited it through an asymptomatic father, but it can also be a de novo mutation.

Examples of family with a girl affected with PCDH19-related epilepsy

Males carrying a PCDH19 mutation in a mosaic state can also be affected like mutated girls, although such cases are probably rare.

2- PCDH19 cellular interference

  • In normal males or females, normal Protocadherin 19 is expressed in all neurons and therefore the individual is unaffected.
  • In healthy male carriers, the mutated Protocadherin 19 protein is expressed in all neurons but dose not lead to pathology.
  • In females carrying one normal and one mutated PCDH19 gene, random X-inactivation leads to the co-existence of two brain cell populations, one expressing normal Protocadherin 19, the other expressing mutated Protocadherin 19. It is the abnormal coexistence of these two cell populations that causes severe epilepsy and
    intellectual disabilityDefined by the World Health Organisation as a significantly reduced ability to understand new or complex information and to learn and apply new skills (impaired intelligence). This results in a reduced ability to cope independently (impaired social functioning), and begins before adulthood, with a lasting effect on development.
    . Mosaic males who have cells expressing normal and mutated Protocadherin 19 can also be affected, although these cases are probably very rare.

b/ SCN1B

  • Autosomal recessive inherited mutations in the SCN1B gene cause severe epilepsy whose features are similar to Dravet syndrome.
  • Two mutations (each on one allele) are necessary to lead to the disease and are each inherited by healthy parents. The mutations causing these severe epilepsies are suspected to be very rare.
  • SCN1B encodes the Sodium channel subunit beta-1 (ß1), which is an accessory subunit of voltage-dependent sodium channels, interacting in particular with Nav1.1 and regulating its activity in the brain.
  • Missense SCN1B mutations in the heterozygous state are also responsible for milder familial epilepsies (
    GEFS+Genetic (Generalised) Epilepsy with Febrile Seizures more
    condition) with autosomal dominant inheritance.

c/ CHD2

  • Autosomal dominant mutations in CHD2 are responsible for severe epilepsy associated with
    intellectual disabilityDefined by the World Health Organisation as a significantly reduced ability to understand new or complex information and to learn and apply new skills (impaired intelligence). This results in a reduced ability to cope independently (impaired social functioning), and begins before adulthood, with a lasting effect on development.
    , which also shares many similar features with Dravet syndrome.
  • So far, all CHD2 mutations described in patients with severe epilepsy have occurred de novo. The risk of parental mosaicism is unknown.
  • CHD2 encodes an ATP-Dependent Helicase on chromosome 15 that could regulate the expression other genes by interacting with the chromatin.

d/ Ongoing research

Twenty to thirty per cent of diagnosed Dravet patients do not carry the SCN1A mutation. Research is ongoing worldwide in order to identify other genes and/or factors which may cause this type of epilepsy.

3/ Genetic counselling

Genetic counselling is the process by which patients and relatives are advised of the nature and consequences of the disorder, the probability of developing or transmitting it, and the options available for family planning.

This counselling can be separated in two parts:

  • First the diagnosis (search for the causative mutation, its identification and the analysis of the parents, therefore allowing an estimation of the risk of recurrence).
  • Then the supportive aspects (psychological support).
  • According to the type of mutation identified, the following may be performed:
  • Presymptomatic testing : The process by which, in a family, a mutation identified in an affected member, is tested in another member that is at-risk but until then unaffected.

Not all genetic diseases show their effect immediately at birth or early in childhood. Although the gene mutation is present at birth, some diseases do not appear until adulthood. If a specific mutated gene responsible for a late-onset disease has been identified, a person from an affected family can be tested before symptoms appear.

For severe diseases with no possibility of treatment or cure, individuals undergoing presymptomatic testing should be aged ≥18 years. The process includes a time for reflection and a psychological support.

  • Or prenatal diagnosis : The process by which a mutation identified in an affected individual of a family is tested in an at-risk foetus during pregnancy.

It can require the advice of an ethical committee in some countries.

Prenatal testing is done on foetal cells sampled from chorionic villus (i.e. baby’s developing placenta) at ~12 weeks or from cells present in the amniotic fluid (the fluid surrounding the baby) after 15-16 weeks of pregnancy.

The issue of prenatal testing is the interruption of the pregnancy in case the foetus has the mutation causing the disease

 

A/ SCN1A

The following illustration shows the risk of recurrence of Dravet syndrome related to a SCN1A mutation, according to the origin of the mutation.

 

B/ PCDH19

The following illustration shows the risk of recurrence of Dravet syndrome/EFMR related to a PCDH19 mutation, according to the origin of the mutation.