The genetics of seizure disorders, including epilepsy, has recently come into the spotlight (see the Nature Outlook on epilepsy). Epilepsy is a complex disease with many different subtypes, both sporadic and familial. While epilepsy is one of the most common neurological disorders, and it has been studied for a very long time, the underlying mechanisms of seizure disorders remain largely elusive. Identifying the genetic causes of different subtypes of the disorder can help to illuminate the gene networks involved and lead to a deeper understanding overall. Importantly, the genetic tools now exist to identify causal mutations for the many different subtypes of seizure disorders.
Febrile seizures, which are induced by fever, affect approximately 2-4% of children worldwide. This type of epileptic seizure is often triggered by infectious disease, but there is strong evidence that it has a genetic basis. A paper recently published in Nature Genetics by Bjarke Feenstra identified two genes associated with vaccine-induced febrile seizures (vaccines, such as MMR, are an extremely rare cause of febrile seizure).
Now, a study by Holger Lerche, Camila Esguerra and colleagues identifies variants in the gene STX1B as causing a familial form of febrile seizure disorder. STX1B encodes a protein called syntaxin-1B. Syntaxin-1 is a key component of a protein complex necessary for the release of neurotransmitters from the presynaptic membrane.
The authors first identified two families in Germany with a history of febrile seizures. They used a combination of whole-exome and whole-genome sequencing to identify the gene most likely to harbor pathogenic mutations causing the disorder. Targeted sequencing in an extended cohort identified further variants in STX1B in patients who had experienced febrile seizures.
To validate these findings, the authors tested the function of stx1b in zebrafish, and showed that a reduction in syntaxin-1B led to behavioral defects in the fish, such as lack of touch response, fin fluttering and jerking movements. Recordings of brain activity confirmed that the fish were experiencing epilepsy-like symptoms. You can read a more in-depth summary of the paper in a blog post at Beyond the Ion Channel by one of the study’s co-authors.
We asked one of the study’s senior authors, Holger Lerche, to tell us a little more about the background of this study:
How did you initially become interested in studying seizure disorders?
I was working during my thesis with mutated ion channels in rare muscle diseases. When I started with my Neurology training, epilepsy emerged as a highly interesting topic in that field as well, and also clinically I became very interested in epilepsy.
How did the two families in this study first come to your attention?
The index case of the first family was referred to me during a cooperation with the Children’s Hospital (at that time at the University of Ulm), when I was looking for familial cases with epilepsy for genetic studies. When I called his grandmother, it turned out to be a large pedigree further increasing when contacting and visiting the different branches of the families. The second family was referred to my colleague Yvonne Weber for similar reasons from another Children’s Hospital in Germany.
STX1B mutations have been associated with other forms of epilepsy. How does the association with febrile seizures further the understanding of this gene’s function?
The function of this gene has been explored very well already by Nobel Laureate Thomas Südhof and his group. The mutations we detected may teach us more about the functional role of different protein domains and their interaction with other proteins in the vesicle release machinery. It is not surprising that mutations in STX1B cause epilepsy, but how febrile seizures develop is still an enigma. Follow-up studies of our discovery may shed light on the unknown temperature-sensitive mechanisms leading to febrile seizures.
Do you think there is the potential for developing drugs targeting STX1B in these patients?
The question is how the loss of function of one allele of STX1B could be compensated. If targeting STX1B to enhance its production or activity is possible, and if this may help these patients, is difficult to predict. However, the zebrafish model can also help us to find therapies which work in a completely different way to compensate for STX1B failure (see answer to next question).
Can you say a little about why you chose zebrafish as a model, and what you learned from this model organism that you wouldn’t have been able to learn otherwise?
We started only recently to collaborate with Camila Esguerra and Alex Crawford who have the zebrafish facilities and expertise. It is a vertebrate, easy to study and very quick to manipulate (much quicker and easier than mice).
To establish a cellular model for functional proof of these mutations would have been more difficult in our case. And the zebrafish is an in vivo model, so we can study behavior and EEG, which is not possible in a cellular assay. Also the temperature effect could be studied very nicely with an effect on EEG in an in vivo system.Last but not least, and most important when thinking of the impact of our work: zebrafish models can be used to find new drugs in medium to high throughput screens using seizure-like behaviour or EEG as read-outs. This allows us to find different kinds of drugs that are able to antagonize the consequences of the STX1B defect on a system-wide level.