Information for patients
This page is written to provide background information on STXBP1-Encephalopathy, intended for patient families and others interested. Here you can find more information on the symptoms, the cause and what kind of research we, the VU University Amsterdam, do to advance our understanding.
What is STXBP1-Encephalopathy?
STXBP1 is one of the many thousands of genes in our DNA. In 2008, researchers concluded that small changes in the DNA sequence (mutations) of this gene cause Ohtahara syndrome (Saitsu et al., 2008). Since then, more reports have come out identifying STXBP1 mutations as a cause for several neurodevelopmental diseases. Collectively these diseases are now called STXBP1 encephalopathy or STXBP1-E (most of these reports are nicely summarized in the review paper Stamberger et al., 2016).
Patients with an STXBP1 mutation can experience a variety of symptoms. The most characteristic symptoms are intellectual disability and epilepsy (95% of the patients), but many patients suffer from additional symptoms, such as movement disorders (e.g. muscle hypotonia, ataxia) or neuropsychiatric features such as autism spectrum disorder. Strikingly, some STXBP1 mutations have been detected in healthy individuals. It is currently unknown why some mutations lead to the symptoms described above and others do not. To find an answer to this question is one of the main goal in our research (see below).
Mutations in STXBP1 are rare. A recent estimation (based on the Danish population) is that 1 in approximately 90.000 children are affected by a disease-causing change in STXBP1 (Stamberger et al., 2016). Patients are diagnosed by genetic testing after visiting a clinic, typically soon after birth.
To date, there is no therapy known that treats STXBP1-E patients beyond symptomatic treatments, for example using antiepileptic drugs. Providing new strategies for developing therapies for STXBP1 patients is another long term goal in our research (see below).
STXBP1 encephalopathy are typically caused by a heterozygous mutation
Humans, as well as most animals and plants, have two copies of each of their genes. STXBP1-E patients generally have a mutation only in one of the two copies, while the other copy is unaffected. This is called a heterozygous mutation.
STXBP1 mutations are typically ‘de novo’
STXBP1 disorders typically arise in families ‘de novo’ (Latin for new). This means that the mutation in the STXBP1 gene occurs for the first time in the affected patient, and is not present in the (grand)parents. If the mutation is de novo, the mutation has occurred spontaneously in the germ cells of one of the parents or in the fertilized egg. De novo mutations in our genome occur randomly. Every person carries many de novo mutation, but very often these mutations are not in essential parts of our DNA and do not affect our health. It is relatively rare that de novo mutations occur in an essential part of our DNA and that a single mutation has a large impact on our health, as is the case of STXBP1 mutations.
STXBP1 encephalopathy is caused by mutations in the genetic code that encodes the MUNC18-1 protein
Genes contain the genetic code to produce proteins, in the case of the STXBP1 gene, the STXBP1 protein, also known as MUNC18-1. Proteins like MUNC18-1 are the molecules to make our cells and tissues work. Mutations in genes often lead to the production of abnormal proteins that cannot perform their normal function. In some cases, the effects of mutations are really subtle and the mutant protein functions almost as well as the normal protein, but in other cases the protein is not even produced at all.
In addition to the genetic code to produce a protein, genes also contain much more code, for instance information on when to produce protein during development and/or in adulthood. This ‘non-protein-coding’ code instructs our cells to produce the MUNC18-1 protein in all nerve cells of the brain, already during early (prenatal) development until old age.
Hence, only a fraction of a gene’s genetic code encodes a protein, most of the code is to guide protein production and for much of the code we do not yet understand the function. In the case of STXBP1, mutations that cause STXBP1 encephalopathy are inside those parts of gene that encodes the protein (called ‘exons’) and are found across the whole length of the STXBP1 gene (see figure above). In addition, mutations in the STXBP1 genetic code that does not encode the protein have also been linked to disease, especially autism, but these links are still poorly understood. Finally, several other links between STXBP1/Munc18-1 function and brain disease have been reported, for instance with schizophrenia and Alzheimer’s disease. Understanding such links to other diseases is one of the aims of our research (see below).
Mutations in STXBP1 may cause haploinsufficiency
Mutations that are predicted to have a mild effect on protein function (mild impact) occur as well as mutations where no protein is predicted to be made at all (severe impact). Based on the clinical information currently available, mutations with predicted mild and severe impact lead to the same disease and there is no evidence that certain symptoms are associated with mild or severe mutations. However, a more systematic comparison between predicted impact and symptoms is required. It is our ambition to contribute to such systematic comparisons.
In cases where mild and severe impact mutations lead to the same disease, it is often the case that proteins produced from the mutated copy of the gene are not stable and are rapidly degraded inside our cells. As a consequence, individuals with such mutations only have proteins made using the other (normal) copy of the gene in their cells. This situation is different from healthy individuals, who have proteins made by both (healthy) copies of a gene. The situation where one healthy copy of a gene is not enough to maintain healthy functioning is called haploinsufficiency. In the case of STXBP1 encephalopathy, this would mean that half of the usual amount of MUNC18-1 protein is not enough for its normal function in our nerve cells and this may be responsible for the symptoms. Haploinsufficiency was already proposed as a likely scenario in the first paper on STXBP1 encephalopathy and is still the most likely explanation for the disease, but direct proof is lacking. One of the main goals of our research is to understand how genetic mutations in STXBP1 lead to the clinical symptoms.
Please visit our research page for more information.
Biological function of MUNC18-1 protein
Thousands of different proteins, encoded by thousands of genes, are present in every nerve cell (neuron) of our brain. Proteins are essential for the structure, maintenance and functionality of neurons. For a neuron, one of the main tasks is to receive and send information to other neurons and to integrate information coming from different sources (other neurons, sensory organs). This communication and integration is a central aspect in the the way we think, feel and move.
STXBP1 has an essential function in synaptic transmission
Neuronal communication happens at specialized contact points between neurons, the synapse. It has been estimated that we have around 100 billion synapses in our brain. When at a particular synapse the sending neuron conveys a message, vesicles filled with signalling molecules are released. These signalling molecules are called neurotransmitters. Neurotransmitters will cross the distance to the receiving neuron. The receiving neuron will subsequently propagate the message to other neurons (see Figure). This form of communication is called synaptic transmission. MUNC18-1 plays an essential role in fusing the neurotransmitter-filled vesicles with the membrane. In addition, MUNC18-1 probably has other functions in neurons and possibly other cells in our body. Understanding the biological functions of Munc18-1 has been a long term goal in our research.
Our research at the VU University Amsterdam
Our research at the Functional Genomics Department at the VU University Amsterdam and VU Medical Center (VUmc) has focused for many years on understanding the function of STXBP1/MUNC18-1 in nerve cells.
We have used experimental mouse and cell models in which STXBP1/MUNC18-1 protein cannot be produced due to genetic modifications that alter the stxbp1 gene in such a way that the gene cannot be translated into protein (‘knock out’ technology; see Figure). It is important to note that these models do not exactly mimic the situation in (human) patients. Patients carry one healthy STXBP1 gene copy and one copy with a mutation (see above), while in knock out mice and neurons both copies are defective. However, these studies using mouse and cell models provide invaluable information on the function of STXBP1 in the brain. We have unraveled how STXBP1/MUNC18-1 organizes the initial docking of transmitter vesicles at the synapse, how it helps to prepare vesicles for fusion with the plasma membrane and with which other proteins it collaborates to do this. We have also discovered several pathways that modulate these functions of STXBP1/MUNC18-1, promoting or inhibiting its role in synaptic transmission. Please visit our research page to find out more detailed information about what our lab discovered on the function of STXBP1/MUNC18-1.
Translational approaches to STXBP1-E
In addition, we have recently published a scientific paper where we present a mouse model that recapitulates the the clinical symptoms seen in patients carrying a STXBP1 mutation (see full paper here: Kovacevic et al., 2018).
In this paper, we first provide evidence that haploinsufficiency is the most likely explanation for STXBP1-E. We show that disease-causing mutations in STXBP1 lead to reduced stability of the STXBP1 protein in nerve cells. Next, we have modeled haploinsufficiency in mice by heterozygous deletion of STXBP1. This means that one of the gene copies of STXBP1 is disabled, leaving only one healthy copy. Seizures, tonic spasms (especially during sleep) and EEG-abnormalities observed in patients with STXBP1-E are all observed in these mice. The anti-epileptic drug levetiracetam, often subscribed to STXBP1-E patients, suppressed these symptoms. In addition, these mice displayed cognitive impairments and hyperactivity. Together, our study shows that heterozygous deletion of STXBP1 in mice provides a valid model for development of therapeutic interventions for STXBP1-E.
Current research goals
Our long standing research on STXBP1 and MUNC18-1 has sparked new ideas on how STXBP1 mutations cause the symptoms that are experienced by the patient. Currently, we are exploring these ideas in three research lines:
1. Investigating the (unclear) relationship between mutations and symptoms
Mutations in STXBP1 typically cause changes in the structure of the STXBP1 protein which is produced by our nerve cells using the STXBP1 gene as a template. Based on similarities and dissimilarities between the normal and mutated protein, scientists can predict how strong a given mutation will affect the function of the STXBP1 protein inside our cells. Strikingly, for STXBP-E patients studied so far, no correlation was observed between the predicted severity of mutations and the severity of the symptoms. In other words, if a mutation in STXBP1 is predicted to have a strong effect on protein function, the symptoms are not more severe than for a mutation with predicted mild effect. However, it might be that certain characteristic features or symptoms experienced by multiple patients are currently overlooked because information tends to be gathered from multiple sources (such as separate clinical reports).
We think it is important to study these relationships between mutations and symptoms more systematically in a large group of STXBP1-E patients. Therefore, our goal is to obtain a more complete and standardized picture of all symptoms for each individual patient, with each specific mutation and make better comparisons between symptoms and mutations. This will reveal whether there are correlations between a particular mutation (genotype) and particular features or symptoms experienced by the patient.
Information about characteristic symptoms can help reach a faster diagnosis for new STXBP1-E patients. Moreover, a systematic overview of any correlations between particular mutation (genotype) and features/symptoms could help physicians to better predict prognosis and the occurrence of particular symptoms for their patients, based on their mutation.
2. Performing non-invasive EEG recordings to analyze excitation/inhibition balance
As described earlier, communication between neurons happens at synapses. Two dominant types of synapses exist: excitatory synapses, which activate other nerve cells, and inhibitory synapses who inhibit other nerve cells. For optimal brain function a delicate balance between excitation and inhibition is maintained. Too much excitation can lead to epileptic activity, whereas too much inhibition can suppress brain activity. This delicate balance is called the Excitation/Inhibition (E/I) balance. Our work in mouse models for STXBP1-encephalopathy shows that the loss of functional STXBP1 may affect the E/I balance by affecting the performance of inhibitory synapses more than excitatory synapses. This might imply that there is reduced inhibition in patients carrying STXBP1 mutations, thereby shifting the E/I balance towards over-excitation, leading to epileptic seizures and disturbances in cognitive performance.
We aim to test this idea by measuring brain activity in STXBP1-E patients and compare this to recordings from individuals without an STXBP1 mutation. Brain activity will be measured using electroencephalography (EEG) recordings.
Understanding whether an imbalance between excitation and inhibition occurs in the brain of a STXBP1-E patient could give valuable information on mechanisms that may be underlying the symptoms experienced by the patient. In the future, this could also be used to screen for, and develop, new therapeutic strategies.
3. Studying the effects of STXBP1 mutations in single and small networks of nerve cells
In addition to studying the symptoms experienced by patients and the brain activity as measured using EEG, we also seek to identify the consequences of STXBP1 mutations in brain cells. An important step we have recently taken is to study STXBP1 mutations in cells from patients. We can re-program human cells such as skin cells to a stem cell state (induced pluripotent stem cells, iPSCs) and subsequently differentiate these to neurons in a culture dish. This technique is widely used in the scientific community and received the Nobel prize in 2012. The iPSC technique allows us to investigate human neurons, derived from patients and compare them to neurons from healthy individuals. For more information, see here3.
Using patient-derived cells, we may be able to identify cellular processes that are altered as a consequence of a mutation in STXBP1. Knowing which cellular processes are affected will allow us to gain a more thorough understanding of pathological mechanisms underlying the symptoms that patients suffer from, and in turn may lead to new hypotheses about ways to help restore these cellular processes. This cell model could in the future be used to screen new treatment options.
Get in touch with us
We hope to gather information about STXBP1 mutations observed in patients and their symptoms from many STXBP1-E patients. We work with local clinicians as well as with clinicians worldwide to gather such information and we are very interested to get in touch with patients carrying an STXBP1 mutation and their caregivers. If you are interested in our research and would like to know how you can help further it, please send us an email at firstname.lastname@example.org.
Collaboration with STXBP1 disorders foundation
The STXBP1 disorders foundation is dedicated to raising awareness of STXBP1-E among parents, physicians, scientists, and pharmaceutical innovators. The foundation was created by a group of dedicated parents. They are focused on advocacy, driving research, and providing the families and their physicians with information and resources.
We work together to increase awareness and to provide resources to the STXBP1-E community. Please visit their website for more information and events.
Reference list primary clinical diagnoses
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