CURE Epilepsy Discovery: CURE Epilepsy Grantee Discovers Specific Alterations in the Inhibitory Neurotransmitter System in Infantile Spasms (IS)

Key Points:

• Infantile spasms (IS), also called West syndrome, is a rare epilepsy syndrome associated with stereotypical spasms, developmental delay, and a telltale brainwave pattern. Medications used to treat IS are not effective in everyone with IS and are associated with side effects.
• CURE Epilepsy launched the Infantile Spasms Initiative (IS Initiative) in 2013 with a team science approach to bring together groups of investigators working on diverse topics related to IS; this one-of-a-kind initiative in epilepsy research contributed immensely to today’s understanding of IS and its mechanisms.
• One of the Initiative’s grantees, Dr. Chris Dulla, developed a mouse model that simulates the neuronal excitation and inhibition relevant to IS. Animal models are incredibly useful to understanding the biological mechanisms underlying IS, and by better understanding the interplay between neural excitation and inhibition in IS, there is hope that we can develop targeted therapies.

Deep dive

Infantile spasms (IS) is a devastating and rare epilepsy syndrome that is typically seen in the first year of a child’s life, most commonly between four and eight months of age.[1, 2] One in 2,000 children is affected by infantile spasms, and worldwide it is estimated that one baby is diagnosed with IS every 12 minutes.[3] IS consists of the following characteristics: subtle seizures consisting of repetitive, but often subtle movements—such as jerking of the mid-section, dropping of the head, raising of the arms or wide-eyed blinks; developmental delay and cognitive and physical deterioration; and a signature disorganized, atypical brainwave pattern called “hypsarrhythmia.”[4, 5] Potential causes include brain injuries or infections, issues with brain development and malformations, gene variants, or metabolic conditions. IS can sometimes have an underlying genetic cause as well.[2, 6] Often, infants appear to develop normally until spasms start, but then show signs of regression. Some infants may have hundreds of such seizures a day.

Current treatment for IS consists of hormonal therapy such as adrenocorticotropic hormone and prednisone, and antiseizure medications such as vigabatrin. These medications are effective in approximately half of the patients with IS.[7, 8] Even infants who have been diagnosed in a timely fashion may not respond to the available treatments, or they may suffer adverse side effects. There is no reliable way to predict who will respond favorably to medications.

As there was a dire need to better understand and treat IS, because there was no advocacy group or organization dedicated to IS and no organization was focusing on finding treatments or cures, CURE Epilepsy stepped in and leveraged our expertise as well as our resources to assemble the Infantile Spasms (IS) Initiative in 2013. Committing $4 million, the Initiative brought together eight research teams working on various aspects of IS. [9] Team science” is a unique way of conducting research that leverages the strengths and expertise of scientists trained in different but related fields to solve a single, complex problem. CURE Epilepsy’s IS Initiative was the first team science approach in the epilepsy research community, and teams in the Initiative benefitted from sharing knowledge and resources to expedite understanding of IS. [9] Collectively, the Initiative studied the basic biology that may explain what causes IS, searched for biomarkers and novel drug targets, and explored ideas for improved treatments for the condition. [9]

One of the eight teams involved in the Initiative was led by Dr. Chris Dulla and his laboratory at Tufts University. Dr. Dulla’s team developed an animal model for IS by targeting a gene called Adenomatous polyposis coli (APC). For the epilepsies in general, animal models are crucial to understanding the biological mechanisms that cause seizures. Dr. Dulla’s team developed an animal model for IS by targeting a gene called Adenomatous polyposis coli (APC). Mice that were genetically altered to have a decrease in the activity of APC exhibited many of the features that were reminiscent of human IS.[10] The development of this mouse model (called “APC cKO”) was an important step in IS research as it provided a way for scientists to study IS and the mechanisms that may cause it.

Broadly speaking, there are two kinds of neurons (brain cells): “excitatory” neurons that activate other neurons, and “inhibitory” neurons that restrain other neurons. A fine balance between excitation and inhibition is critical for the brain to function, and in epilepsy, this delicate balance may be disturbed. Inhibitory neurons are modulated by a neurotransmitter called gamma amino butyric acid (GABA). In the current study, Dr. Dulla’s team wanted to study GABAergic neurotransmission in the APC cKO model. Previous studies have shown a link between GABAergic neurotransmission and IS; specifically, alterations in GABAergic transmission have been found in animal models of IS [11, 12] and in human patients with IS.[13] The ultimate goal is to better understand the interplay between excitatory and inhibitory neurotransmission in IS.[14]

In a recent study that was published in February 2023, Dr. Dulla’s team studied inhibitory neurons (also called “interneurons,” abbreviated to “INs”). They looked at a certain kind of interneuron, called a parvalbumin-positive interneuron (PV+ IN) and studied the way these interneurons looked under the microscope (i.e., their anatomy) as well as the way they functioned (i.e., their physiology). In humans, IS has a time course in terms of when seizures start. To recreate this in their mouse model, the team studied APC cKO mice at multiple time points: in infancy (postnatal days 9 and 14), and then later, as adults, and compared them to mice that did not have the genetic mutation (i.e., “wild-type” mice). The goal of the study was to understand what happens to PV+ INs in the APC cKO mouse model of IS.[14]

In normal brain development, an excess of PV+ INs are made, but then they disappear over time. The first discovery Dr. Dulla’s team made was that in APC cKO mice, there is an excess of PV+ IN death. The second important finding was that in APC cKO mice, the death of these PV+ INs occurred earlier in development as compared to wild-type mice.[14] This change in the pattern of death of PV+ INs could mean that there are subtle changes taking place in the neural circuit. Since the primary role of interneurons is to keep brain activity in check, an excess of interneurons dying very quickly may mean that the excitatory neurons run amok. In contrast, other types of interneurons did not show accelerated dying, but this change was specific to PV+ INs.[14] The change in the pattern of PV+ IN cell death in APC cKO mice was also reflected in the functioning of the brain as studied by looking at the electrical activity in the brain.14 The changes seen in the interneuron death and associated function were more pronounced at postnatal day 9 (as compared to postnatal day 14), which suggests that this is the critical period in brain development in this model, when, if GABAergic inhibition is perturbed, may lead to IS and associated symptoms.

In totality, Dr. Dulla’s findings regarding GABAergic neurotransmission and PV+ INs are complicated with respect to the sequence and timing of events. However, there is a variation in the way the inhibitory GABAergic neuronal circuitry develops and matures in APC cKO mice that suggests a critical window of events that if perturbed, may lead to spasms and behavioral impacts later on. This work positions PV + INs as a potential target to treat IS, and perhaps even offers avenues for timely diagnosis.[14] This perturbation of inhibition during a critical period in development may contribute to spasms and seizures later in life. The development of the GABAergic circuitry depends on brain activity; hence, the changes in activity of the neural circuitry seen in APC cKO mice interneurons may contribute to long-term changes in the brain. The exact link between the PV+ IN death in early development and behavioral spasms needs to be investigated in more detail, but this work lays the foundation to continue studying inhibitory transmission in IS. Future studies will reveal if stopping this excess PV+ IN death may be a therapeutic target or a cure for Infantile Spasms.

Literature Cited:

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10. Pirone A, Alexander J, Lau LA, Hampton D, Zayachkivsky A, Yee A, et al. APC conditional knock-out mouse is a model of infantile spasms with elevated neuronal ?-catenin levels, neonatal spasms, and chronic seizures Neurobiol Dis. 2017 Feb;98:149-157.
11. Marsh E, Fulp C, Gomez E, Nasrallah I, Minarcik J, Sudi J, et al. Targeted loss of Arx results in a developmental epilepsy mouse model and recapitulates the human phenotype in heterozygous females Brain. 2009 Jun;132:1563-1576.
12. Katsarou AM, Li Q, Liu W, Moshé SL, Galanopoulou AS. Acquired parvalbumin-selective interneuronopathy in the multiple-hit model of infantile spasms: A putative basis for the partial responsiveness to vigabatrin analogs? Epilepsia Open. 2018 Dec;3:155-164.
13. Bonneau D, Toutain A, Laquerrière A, Marret S, Saugier-Veber P, Barthez MA, et al. X-linked lissencephaly with absent corpus callosum and ambiguous genitalia (XLAG): clinical, magnetic resonance imaging, and neuropathological findings Ann Neurol. 2002 Mar;51:340-349.
14. Ryner RF, Derera ID, Armbruster M, Kansara A, Sommer ME, Pirone A, et al. Cortical Parvalbumin-Positive Interneuron Development and Function Are Altered in the APC Conditional Knockout Mouse Model of Infantile and Epileptic Spasms Syndrome J Neurosci. 2023 Feb 22;43:1422-1440.