The Genetics of GEFS+: Understanding the Spectrum

Behind many cases of Genetic Epilepsy with Febrile Seizures Plus (GEFS+) is a set of biological instructions—genes—that have been slightly altered. These instructions tell the brain how to build and operate the “channels” and “receptors” that control electrical activity ^[1]. When these instructions change, the brain’s balance between “go” signals (excitement) and “stop” signals (inhibition) can shift, making seizures more likely ^[2][3].

However, GEFS+ is primarily a clinical diagnosis based on family history and seizure types. Many families will not have an identifiable pathogenic variant, and a negative genetic test does not rule out the condition.

The Master Regulators: Sodium and GABA

When a genetic cause is found, the most common genes linked to GEFS+ are involved in two critical systems in the brain: sodium channels and GABA receptors.

• SCN1A and SCN1B (Sodium Channels): These genes provide the blueprint for sodium channels, which act like gates that let electricity flow into brain cells (neurons) ^[2]. The SCN1A gene is the most frequent identified cause of GEFS+ ^[1]. Some SCN1A variants cause a “loss-of-function” (weakening the brain’s “brakes”), while others cause a “gain-of-function.” Clinical reports do not always establish which mechanism is at play ^[3][4].
• GABRG2 (GABA Receptors): This gene helps build GABA receptors, which are the primary “stop” signs in the brain ^[5]. When GABA (a chemical messenger) binds to these receptors, it tells the neuron to quiet down. Mutations in GABRG2 can prevent these receptors from reaching the surface of the cell or stop them from working correctly, leading to an overactive brain environment ^[5][6].
• Other Genes: Researchers have also identified other genes like SLC32A1 (which helps package GABA for use) and SCN2A in broader epilepsy cohorts, though these are less common in the GEFS+ spectrum ^[7][8].

Why Families Look So Different

One of the most confusing parts of GEFS+ is how one gene change can look so different in two different people. This is due to two key genetic concepts:

Variable Expressivity

This term explains why symptoms vary in severity. In the same family, one person might only have a single febrile seizure at age two, while their sibling has several seizures a year that continue into adulthood ^[9]. They both have the exact same genetic change, but their brains respond to it differently ^[10].

Incomplete Penetrance

This means that even if a person carries a familial GEFS+ gene mutation, they might never have a seizure in their entire life ^[1]. They are “silent carriers” who can still pass the gene to their children, who may then develop symptoms ^[11]. In some specific families studied, as many as 12% to 15% of people with the mutation never experience a seizure, though this percentage varies by gene and kindred ^[1].

Patterns of Inheritance

In most cases, GEFS+ follows an autosomal dominant pattern ^[12]. This means a child only needs to inherit one copy of the changed gene from one parent to be at risk for the condition.

However, a family history is not always present. Sometimes, a mutation occurs for the very first time in a child—this is called a de novo mutation ^[13]. In these cases, neither parent carries the gene in their blood, and there is no history of seizures in the extended family.

What Influences the Outcome?

While a primary gene variant (like SCN1A) sets the stage, other factors influence how the condition plays out. A Variant of Uncertain Significance (VUS) on a genetic test means the lab found a change but doesn’t know if it causes disease; a VUS should never be used alone to make predictions or alter treatment. Recent research also suggests that a person’s “polygenic background”—thousands of other tiny genetic variations they inherit—can act as “modifiers,” either making them more resilient or more susceptible to seizures ^[10]. This is why your child’s journey may look very different from a cousin’s, even if they share the same primary diagnosis.