Dravet syndrome is a severe form of epilepsy that is characterized by seizures accompanied by high fever. Symptoms begin in infancy and are similar to febrile seizures. One result is that Dravet patients are often initially misdiagnosed.
Genetic testing is the surest method of diagnosing Dravet, although some cases may be caused by mutations in genes that have not yet been identified as disease-causing.
Genetics of Dravet syndrome
Around 70 to 85 percent of Dravet syndrome cases are caused by mutations in the SCN1A gene. The remaining 15 to 30 percent of cases relate to mutations in other genes, including GABRG2, SCN1B, SCN2A, CHD2, HCN1, GABRA, and STXBP1. Genes not yet identified might be involved as well.
Because the proteins these genes encode for are involved in nerve signaling, scientists think the genetic mutations result in faulty nerve signaling, leading to seizures and high fever.
Most cases of Dravet are caused by de novo mutations; that is, disease-causing mutations not inherited from a child’s parents.
What is a genetic test?
In genetic testing, a patient’s exact DNA sequence is determined in a laboratory, allowing scientists and health professionals to assess whether disease-causing mutations are present. The DNA may be sequenced completely, or, if a particular disease is suspected, only specific regions may be examined. In Dravet syndrome testing, only the SCN1A gene may be sequenced, rather than the patient’s entire genome.
An assay that detects deletions and duplications in the SCN1A gene or other genes may also be used. This assay, called a multiplex ligation-dependent probe amplification (MLPA), determines the copy number of a particular gene, meaning it determines whether a gene has been deleted or duplicated, not whether other mutations might be present.
In general, genetic testing is expensive, with each gene costing about $200 to $1,200 to sequence in the U.S. However, a new technique called next-generation sequencing is being developed to be faster, more accurate, and less costly.
What does the procedure involve?
A small blood sample, roughly that filling a 5 ml tube, is sufficient for genetic testing. The blood is collected at a hospital or clinic, and shipped to a diagnostic laboratory, where the DNA is isolated and sequenced. The results are then returned to the hospital or clinic where the sample was taken.
The results of genetic testing are usually available two to six weeks after the blood was submitted for analysis. The hospital or clinic will meet with the patient, usually together with a genetic counselor, to discuss test results and to plan treatment if necessary.
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