SUDEP Basic Science Mechanisms

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Observations from the PAME Conference
Wednesday, September 17, 2014

A meeting to understand mechanisms underlying mortality in people with epilepsy was held in Minneapolis from June 19-22. The PAME (Partners Against Mortality in Epilepsy) meeting was a unique opportunity for epileptologists, basic scientists and families affected by SUDEP (Sudden Unexpected Death in Epilepsy) to understand new research and support one another.  Below is a synopsis of the basic science topics that were covered in this four day-long meeting. Current research on this topic focuses on respiratory, cardiac and autonomic dysfunction that could possibly underlie SUDEP. An additional field of research is to understand the genetic underpinnings behind epilepsy and sudden death.

Respiratory Mechanisms Underlying SUDEP

Dr. Lisa Bateman started this session by talking about the mechanisms by which the brain controls respiration. The brainstem is the part of the brain responsible for this; and it is important to understand how the brain controls respiration so we can understand what could possibly go wrong in SUDEP. Seizures can contribute to SUDEP through numerous ways –there could be excessive buildup of saliva in the airway; a seizure could cause apnea (termination of breathing) by affecting neurons in the brainstem; seizures can cause sedation and suffocation by a surface e.g. a pillow. Dr. Bateman introduced a term known as PEGS (postictal generalized EEG suppression). Some research suggests that this overall flattening of EEG after a seizure could be a biomarker for SUDEP. While thinking about respiratory mechanisms, one cannot forget about cardiac mechanisms either. This is because the brain controls both respiration and cardiac function. The brain, heart and lungs, therefore form the ‘SUDEP triangle’.

Dr. George Richerson then talked about the various animal models that he uses in his laboratory to study the respiratory causes of SUDEP. He is especially interested in a neurotransmitter system called the serotonin (5-HT) system. The role of serotonin in SIDS (Sudden Infant Death Syndrome) has been shown, and several similarities between SUDEP and SIDS have been elucidated. Serotonin is interesting in epilepsy because it has also been implicated in depression – a condition that may co-occur in people with epilepsy. Dr. Richerson’s hypothesis that the serotonin system may be implicated in SUDEP comes from studies that have used mice with mutated serotonin receptors, and found they have a higher susceptibility to seizures.  A part of the brain called the raphe has neurons that express serotonin – these neurons are also sensitive to changes in pH of the blood. When these neurons detect a change in pH, they activate neurons in another part of the brain called the thalamus, which is responsible for arousal (i.e. waking up from sleep). Prolonged seizures could affect neurons in the raphe, rendering them no longer capable of detecting and responding to pH changes like they should.  Hence, when an individual experiences changes in blood pH because of a seizure, the mechanisms that should kick in to cause arousal don’t, ultimately leading to mortality.

Dr. Daniel Mulkey then talked about data from his lab, where he is interested in the brainstem – specifically the part of the brainstem called the retrotrapezoid nucleus (RTN). He focuses on KCNQ and HCN channels in the RTN. KCNQ channels are responsible for entry and exit of potassium (K+) in and out of neurons and are critical for maintaining neuronal excitability. HCN channels are also known as ‘pacemaker channels’ because they help generate rhythmic activity between groups of neurons. Dr. Mulkey found that KCNQ and HCN channels are important for regulating activity of RTN neurons. He proposed that if we found drugs and therapies to regulate the activity of KCNQ and HCN channels in the RTN nucleus, we may be able prevent mortality in people with epilepsy.

The session ended with a talk about adenosine by Dr. Detlev Boison. Adenosine is a molecule known as a nucleoside and plays an important role in a variety of physiological functions in the body by acting on adenosine receptors. Adenosine has gained popularity in SUDEP research lately, because seizures have found to increase levels of adenosine. Increase in adenosine can depress respiration and lead to apnea (termination of breathing). Dr. Boison hypothesized that death in SUDEP is caused by an increase in adenosine levels causing overactivation of adenosine receptors. He did experiments in mice to figure out whether this was true, and found that normalizing adenosine levels did indeed decrease incidence of SUDEP. Future experiments will be necessary to study this further, but for now, it seems that adenosine may play an important role in SUDEP.

An interesting fact is that caffeine found in coffee is an adenosine antagonist (meaning it naturally counteracts the effects of adenosine). It has been shown that sudden mortality observed in people with epilepsy is usually at night. Could this be because the body is lacking in caffeine during the nighttime?

Sleep and Arousal Mechanisms Underlying SUDEP

This session started off by a talk by Dr. Hal Blumenfeld who talked about the mechanisms that impair arousal and breathing after a seizure. He talked about the role of brain regions that regulate sleep and arousal; failure of these systems may ultimately lead to SUDEP. Possible mechanisms leading to SUDEP are - slowing down of cortical neurons following a seizure, involvement of the medulla (the part of the brain in the lower part of the brainstem, important for regulating cardiac and respiratory tone) and serotonin (5-HT) neurons in the raphe nucleus. Knowing these brain structures is important because we can potentially find therapies and drugs that could stimulate arousal, proving to be beneficial in preventing SUDEP.

Dr. Brian Dhlouhy then talked about his work with human subjects that had electrodes implanted in the amygdala – the almond-shaped part of the brain important for memory, emotional reaction and decision-making. He showed that stimulation of the amygdala led to apnea (termination of breathing); surprisingly, the patients weren’t aware of this. These results are interesting because they show a potential connection between the amygdala and respiratory systems in the brainstem. In theory, it could be that seizures in the amygdala could cause cessation of respiration and death. Once we know more about the mechanism by which the amygdala controls breathing centers, we can find strategies to recognize this and prevent SUDEP.

Dr. Gordon Buchanan then talked about his work trying to understand the role of serotonin in arousal, specifically right after a seizure (the postictal period). He used a certain kind of transgenic mice called Lmx1b where all serotonin neurons are genetically deleted. He found that these mice do not show normal arousal in response to changes in blood pH. Dr. Buchanan’s team found that these transgenic mice with no serotonin are more susceptible to seizures and also show greater mortality in response to seizures. These studies also help explain the role of serotonin in seizures and seizure-induced death.

Dr. Carl Faingold then talked about the role of adenosine in respiratory arrest. Seizures have been shown to cause an increase in levels of adenosine. To understand the role of adenosine in respiratory arrest, he used a special type of mice called DBA/1 mice. These mice, when exposed to high frequency sounds, can develop convulsions (known as audiogenic seizures). Dr. Faingold discovered that when DBA/1 mice were exposed to a high frequency sound after they were given a drug that blocks or reduces adenosine had fewer incidence of SUDEP. Hence, it may be worthwhile to investigate adenosine blockers in greater detail.

Autonomic and Cardiac Mechanisms Underlying SUDEP

The autonomic nervous system (also known as the visceral or involuntary nervous system) is responsible for critical functions such as respiration, heart rate, salivation and digestion. The autonomic nervous system is called ‘involuntary’ because it controls the body below the level of consciousness – this is the reason we are not consciously aware of our breathing or our food being digested. Dr Jeffrey Britton started this session by talking about how the autonomic nervous system regulates cardiac function, and what could go wrong in SUDEP. The possibility that the autonomic nervous system may be involved in seizure-induced mortality is not new –decades ago, physicians observed that patients report of auras, cardiac and gastrointestinal symptoms such as nausea and vomiting right after a seizure. Since SUDEP is usually seen in people with refractory epilepsy (i.e. epilepsy that is not well controlled by drugs), Dr. Britton proposed that rampant autonomic nervous system activation caused by seizures could be a reason that explains morality in epilepsy.

Switching gears a bit, Dr. Lori Isom talked about SUDEP in Dravet Syndrome. Dravet Syndrome is a form of intractable and catastrophic epilepsy and is usually seen in infancy.  Dravet Syndrome is caused by mutations in a gene called SCN1A that codes for sodium (Na+) channels. Unfortunately, Dravet Syndrome is also associated with a high rate of SUDEP. Dr. Isom focuses on understanding SUDEP in Dravet Syndrome. By using a transgenic mouse model that has the name mutation in SCN1A gene as do humans with Dravet Syndrome, she showed that these mice showed abnormal electrical activity in their cardiac cells (myocytes).

In more recent experiments, she made use of a technique known as the iPSC (induced pluripotent stem cell) method to understand this better. Briefly, in this method, one can take cells from a patient, reprogram them in the lab, and study their properties. The reason this method is so popular is because one can take cells from any part of the body e.g. the skin. In this case, Dr. Isom took cells from patients with Dravet Syndrome, reprogrammed them into neurons and cardiac cells and studied their properties. Her team found that neurons and cardiac cells derived from Dravet Syndrome patients had abnormal sodium channels, and showed signs of hyperexcitabilty (too much excitation). This study is important because it tells us that infants with Dravet Syndrome may have cardiac dysfunction (arrhythmias) as well. Hence, in these individuals, it may be worthwhile to check for cardiac function even if there are no overt signs of cardiac failure. Since the iPSC technique models Dravet Syndrome in a lab, the hope is that this method may be used to screen new and better drugs. 

Dr. Franck Kalume talked more about SUDEP in Dravet Syndrome. Dr. Kalume studied a phenomenon called Heart Rate Variability, or variation in time intervals between heartbeats. A reduced Heart Rate Variability can be an indicator of cardiac dysfunction. By using mice that had the same mutation as people with Dravet Syndrome do, Dr. Kalume found that these mice have a reduced Heart Rate Variability. Building from past knowledge that the autonomic nervous system regulates cardiac tone, he gave mice a drug that blocks the cholinergic system, and found a reduced incidence of SUDEP in these mice. The clinical implication of these studies is that by understanding what exactly goes wrong in SUDEP, we can develop therapies that detect aberrant autonomic activity and stimulate cardiac centers in the brain to correct the problem.

After learning about how seizures can affect cardiac function, Dr. Rainer Surger talked about the fact that even anti-epileptic drugs can produce cardiac dysfunction. Even when prescribed in daily prescribed doses, anti-epileptic drugs can cause AV block (impairment of conduction in the heart), especially when prescribed in combination with other anti-epileptic drugs. Since people who are susceptible to SUDEP usually have refractory epilepsy (and are presumably on multiple drugs), it is important to take into account the effect of anti-epileptic drugs on cardiac function.

Genetic Mechanisms Underlying SUDEP

In addition to understanding respiratory, cardiac and autonomic mechanisms underlying SUDEP, another way to study mechanisms that can cause mortality in epilepsy is to ask whether genetic mechanisms may be at play. Since mortality in epilepsy is mainly due to cardiac or autonomic reasons, scientists are trying to figure out whether there are genes that can increase susceptibility to epilepsy and to cardiac or autonomic dysfunction. The thought would be that such a gene would make an individual more susceptible to suffering mortality due to epilepsy.

Ion channels are proteins that form pores on the membranes of neurons. Proper functioning of ion channels is critical for neurotransmission, and mutations in these channels may render the neuronal circuit capable of generating and sustaining seizures. Dr. Alicia Goldman presented recent research from her lab trying to elucidate genetic mechanisms epilepsy. Dr. Goldman was able to use samples from a child that suffered from Dravet Syndrome, and unfortunately, SUDEP as well. Using cutting-edge genetic techniques, she and her lab members were to dissect out mutations in a number of genes that code for ion channels. Some of these genes were SCN1A (that codes for sodium channels), KCNA1 (codes for potassium channels) and RYR3 (codes for calcium channels). In addition to genes coding for ion channels, her team also found mutations in HTR2C gene, which codes for a serotonin (5-HT) receptor subunit. Although these results cannot tell us whether these mutations were responsible for SUDEP, it makes sense that these mutations could in theory, cause neurotransmission to go awry, and potentially cause seizures. Identifying these novel genes will help us understand novel pathways that could help better understand the genetics underlying mortality in subjects with epilepsy. The most likely scenario is that there are multiple genes that each confer a little susceptibility to SUDEP, and the combined effect of all these mutations may lead to mortally.

This was followed by Dr. Dan Lowenstein’s talk about two projects that aim to elucidate in greater detail genetic factors that can underlie epilepsy - the Epilepsy Phenome/Genome Project (EPGP) and Epi4K. The goal of these multi-center, international consortia is to create a repository of tissue and DNA from people with epilepsy with a detailed history of their phenotypes and genotypes in order to understand the genetic basis of epilepsy. These projects are made possible by the outstanding advances in genetic sequencing techniques. Already, a shared genetic component between epilepsy and migraine has been shown thanks to these efforts. Dr. Lowenstein stressed that the collaborative approach enables one to make more progress than any one person or lab alone. Large-scale collaborations such as these are critical to start finding genes that can be implicated in epilepsy with the hopes of providing better clinical care to affected individuals.

Dr. Jeffrey Noebels then talked the need to for research to identify genes that could be implicated in conferring susceptibility to epilepsy and sudden death. Similar to Dr. Goldman’s work on ion channels, Dr. Noebels talked about a gene called kvLQT1 that codes for potassium channels. A syndrome known as long QT syndrome has been shown to be responsible for cardiac arrhythmia, and Dr. Noebels’ team wanted to see if long QT syndrome caused by a mutation in kvLQT1 gene could be responsible for SUDEP as well. They identified mutations in the kvLQT1 gene, and using a transgenic technique, inserted that into mice instead of their normal kvLQT1 gene. This was done to ask how a mutation in this ion channel affects behavior and neuronal circuits. True to their hypothesis, they found seizures in the mouse, leading them to propose that perhaps a mutation in this gene may be causing sudden death. By identifying such new genes, and by using mouse models, Dr. Noebels and other scientists can ask important questions – is this mutation deleterious? How does a certain mutation lead to seizures and mortality? Such questions are not possible to ask with human subjects, whereas mice models give us the unique opportunity to study these genes in its entirety.

This session concluded with a talk by Dr. Miriam Meisler who talked about genes that code for sodium ion channels. SCN1A and SCN8A are genes that are responsible for regulating transmission of sodium (Na+), and a mutation or deficiency in these genes could lead to impaired neurotransmission. Both these ion channels are found in the axon initial segment. These channels are found in a part of the neuron called the ‘axon initial segment’ – this is the part of the nerve cell that is responsible for conducting electric signals to other neurons. Dr. Meisler identified mutations in SCN1A and SCN8A genes, and in an experimental system, found that this gene leads to excessive excitation of nerve cells. Similar to Dr. Noebels, her team is now in the process of creating a transgenic mouse with the mutated ion channels instead of the normal one. This will allow her to ask some very important questions - what drugs work in mice that have a mutation in these genes?  Is there dysfunction in cardiac and autonomic systems of the mice? How do these mice respond to novel therapies?

Authored by: Sloka Iyengar PhD on 9/2014

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