Juvenile myoclonic epilepsy is a condition characterized by recurrent seizures (epilepsy). This condition begins in childhood or adolescence, usually between ages 12 and 18, and lasts into adulthood. The most common type of seizure in people with this condition is myoclonic seizures, which cause rapid, uncontrolled muscle jerks. People with this condition may also have generalized tonic-clonic seizures (also known as grand mal seizures), which cause muscle rigidity, convulsions, and loss of consciousness. Sometimes, affected individuals have absence seizures, which cause loss of consciousness for a short period that appears as a staring spell. Typically, people with juvenile myoclonic epilepsy develop the characteristic myoclonic seizures in adolescence, then develop generalized tonic-clonic seizures a few years later. Although seizures can happen at any time, they occur most commonly in the morning, shortly after awakening. Seizures can be triggered by a lack of sleep, extreme tiredness, stress, or alcohol consumption.

The genetics of juvenile myoclonic epilepsy are complex and not completely understood. Mutations in one of several genes can cause or increase susceptibility to this condition. The most studied of these genes are the GABRA1 gene and the EFHC1 gene, although mutations in at least three other genes have been identified in people with this condition. Many people with juvenile myoclonic epilepsy do not have mutations in any of these genes. Changes in other, unidentified genes are likely involved in this condition.

A mutation in the GABRA1 gene has been identified in several members of a large family with juvenile myoclonic epilepsy. The GABRA1 gene provides instructions for making one piece, the alpha-1 (α1) subunit, of the GABAA receptor protein. The GABAA receptor acts as a channel that allows negatively charged chlorine atoms (chloride ions) to cross the cell membrane. After infancy, the influx of chloride ions creates an environment in the cell that inhibits signaling between nerve cells (neurons) and prevents the brain from being overloaded with too many signals. Mutations in the GABRA1 gene lead to an altered α1 subunit and a decrease in the number of GABAA receptors available. As a result, the signaling between neurons is not controlled, which can lead to overstimulation of neurons. Researchers believe that the overstimulation of certain neurons in the brain triggers the abnormal brain activity associated with seizures.

Mutations in the EFHC1 gene have been associated with juvenile myoclonic epilepsy in a small number of people. The EFHC1 gene provides instructions for making a protein that also plays a role in neuron activity, although its function is not completely understood. The EFHC1 protein is attached to another protein that acts as a calcium channel. This protein allows positively charged calcium ions to cross the cell membrane. The movement of these ions is critical for normal signaling between neurons. The EFHC1 protein is thought to help regulate the balance of calcium ions inside the cell, although the mechanism is unclear. In addition, studies show that the EFHC1 protein may be involved in the self-destruction of cells. EFHC1 gene mutations reduce the function of the EFHC1 protein. Researchers suggest that this reduction causes an increase in the number of neurons and disrupts the calcium balance. Together, these effects may lead to overstimulation of neurons and trigger seizures.

Normal Function

The GABRA1 gene provides instructions for making one piece, the alpha-1 (α1) subunit, of the GABAA receptor protein. GABAA receptors are made up of different combinations of five protein subunits, each produced from a different gene. (Nineteen different genes provide instructions for GABAA receptor subunits.) These subunits form a hole (pore) in the cell membrane through which negatively charged chlorine atoms (chloride ions) can flow.

A chemical that transmits signals in the brain (a neurotransmitter) called gamma-amino butyric acid (GABA) attaches to GABAA receptors. Once GABA attaches, the pore formed by the subunits opens, and chloride ions flow across the cell membrane. After infancy, chloride ions flow into the cell through the open pore, which creates an environment in the cell that blocks (inhibits) signaling between neurons. The primary role of GABA in children and adults is to prevent the brain from being overloaded with too many signals. In contrast, in newborns and infants, chloride ions flow out of the cell when the pore is opened, creating an environment that allows signaling between neurons.

Health Conditions Related to Genetic Changes

Juvenile myoclonic epilepsy

A mutation in the GABRA1 gene has been identified in at least one family with juvenile myoclonic epilepsy. This condition typically begins in childhood or adolescence and causes recurrent myoclonic seizures, which are characterized by rapid, uncontrolled muscle jerks. Affected individuals can also have other types of seizures called generalized tonic-clonic seizures (or grand mal seizures) and absence seizures. The mutation associated with this condition changes a single protein building block (amino acid) in the α1 subunit. The amino acid alanine at protein position 322 is replaced by the amino acid asparagine. This gene mutation is written as Ala322Asp or A322D.

This GABRA1 gene mutation leads to the formation of an abnormal α1 subunit that reduces GABAA receptor function. GABAA receptors containing the abnormal subunit are broken down before they reach the cell membrane. Studies show that the altered receptors can also interfere with normal receptors inside the cell, leading to the additional loss of normal receptors. Because of the reduction of GABAA receptor function, signaling between neurons is not regulated, which can lead to overstimulation of neurons. Researchers believe that the overstimulation of certain neurons in the brain triggers the abnormal brain activity associated with seizures.

Childhood absence epilepsy

MedlinePlus Genetics provides information about Childhood absence epilepsy

Other Names for This Gene

• ECA4
• EJM
• EJM5
• GABA(A) receptor subunit alpha-1
• GABA(A) receptor, alpha 1
• gamma-aminobutyric acid (GABA) A receptor, alpha 1
• gamma-aminobutyric acid receptor subunit alpha-1
• gamma-aminobutyric acid receptor subunit alpha-1 precursor
• GBRA1_HUMAN

Genomic Location

Normal Function

The EFHC1 gene provides instructions for making a protein called EF-hand domain containing protein 1 (EFHC1). The EFHC1 protein interacts with another protein that acts as a calcium channel, allowing positively charged calcium atoms (calcium ions) to cross the cell membrane. The movement of these ions is critical for normal signaling between nerve cells (neurons) in the brain and other parts of the nervous system. The role of the EFHC1 protein is not well understood, although it is thought to help regulate the balance of calcium ions inside the cell (calcium homeostasis). Studies also show that the EFHC1 protein may stimulate the self-destruction of cells (apoptosis).

Health Conditions Related to Genetic Changes

Juvenile myoclonic epilepsy

Mutations in the EFHC1 gene have been identified in a small number of people with juvenile myoclonic epilepsy. This condition typically begins in childhood or adolescence and causes recurrent myoclonic seizures, which are characterized by rapid, uncontrolled muscle jerks. Affected individuals can also have other types of seizures called generalized tonic-clonic seizures (or grand mal seizures) and absence seizures. Most gene mutations associated with juvenile myoclonic epilepsy replace single protein building blocks (amino acids) in the EFHC1 protein. The function of the altered protein is thought to be reduced. Although it is unclear how EFHC1 gene mutations lead to juvenile myoclonic epilepsy, researchers have suggested that a decrease in EFHC1 protein function reduces apoptosis, leading to more neurons than normal, and disrupts calcium homeostasis. Together, these changes may lead to overstimulation of the neurons, causing seizures characteristic of juvenile myoclonic epilepsy.

Other Names for This Gene

• EF-hand domain (C-terminal) containing 1
• EF-hand domain-containing protein 1
• EFHC1_HUMAN
• myoclonin-1

Genomic Location

The inheritance pattern of juvenile myoclonic epilepsy is not completely understood. When the condition is caused by mutations in the GABRA1 gene, it is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. The inheritance pattern of juvenile myoclonic epilepsy caused by mutations in the EFHC1 gene is not known.

Although juvenile myoclonic epilepsy can run in families, many cases occur in people with no family history of the disorder.

• Myoclonic jerks or seizures, which are described as quick jerks of the arms and legs, and are the hallmark feature of the disease; they may be the only symptom in about 17% of the cases; in about 20% of the cases, the seizures occur in clusters, affecting only one side (unilateral) of the body, and start before a tonic-clonic seizure
• Generalized tonic-clonic seizures, appear a few months after onset of myoclonic jerks
• Absence seizures, usually the first symptom to present around 5 and 16 years of age
• Myoclonic status epilepticus is considered to be the most concerning problem of juvenile myoclonic epilepsy. It occurs when multiple myoclonic seizures do not readily stop and after sleep deprivation or missing medications.

Juvenile myoclonic epilepsy affects an estimated 1 in 1,000 people worldwide. Approximately 5 percent of people with epilepsy have juvenile myoclonic epilepsy.