The Biology and Genetics of FCD: Why Blood Tests Are Often Normal

When a child is diagnosed with Focal Cortical Dysplasia (FCD), one of the most confusing moments for parents is receiving a “normal” genetic test result. You may wonder how a physical malformation in the brain could not be genetic.

The answer often lies in mosaicism—a biological phenomenon where a genetic change is only present in some cells of the body, like a few tiles in a mosaic, rather than in every cell ^[1][2]. In isolated FCD, the genetic “typo” often exists only within the small, abnormal area of the brain and nowhere else ^[3][4].

The Blueprint Disrupted: How FCD Forms

During pregnancy, a baby’s brain develops as cells move from the center of the brain outward to form the cortex (the outer layer). This is a highly organized process, like building a skyscraper from a complex blueprint.

In many cases of FCD, a genetic mutation occurs in a progenitor cell (a “starter” cell) while the brain is still forming ^[1]. Because this mutation happens after conception—known as a somatic mutation—it is not inherited from the parents and is frequently not present in the child’s blood or saliva ^[3][5]. Cells that descend from that mutated starter cell will carry the same “typo” and fail to follow the blueprint. These cells may stop in the wrong place, grow too large, or wire themselves incorrectly, creating the “focus” of dysplasia that causes seizures ^[6][7].

The mTOR Pathway: The Brain’s Growth Switch

A significant number of genetic causes of FCD Type II involve a specific biological highway called the mTOR pathway. Think of mTOR as a master “growth switch” for cells ^[6].

• Normal Growth: When the switch is working correctly, it tells cells when to grow and when to stop.
• Hyperactivation: In FCD, a mutation can cause this switch to get stuck in the “ON” position ^[8][7].

When the mTOR pathway is hyperactive, it produces two types of abnormal cells that are the hallmarks of FCD:

1. Dysmorphic Neurons: These are brain cells that have grown much larger than normal and have a chaotic, disorganized shape ^[4][7].
2. Balloon Cells: Found specifically in FCD Type IIb, these are massive, pale cells named for their inflated, balloon-like appearance ^[7][9].

Because these cells are physically different and incorrectly “wired,” they become hyperexcitable, firing electrical signals when they shouldn’t and triggering seizures ^[6][10].

Why Blood Tests Are Often Negative

Standard genetic tests use blood or saliva. These tests look for germline mutations—changes that are present in every cell of the body ^[2]. However, research shows that in many cases of isolated FCD, the mutation is only found in the brain tissue ^[3].

In one specific high-depth sequencing study, researchers found that among patients who had a confirmed genetic mutation in their brain tissue, only 6% of those mutations could be detected in their blood ^[2]. This means a “negative” blood test is actually a very common result in FCD and does not mean the condition isn’t genetic. It simply means the “typo” is hidden deep within the brain’s “mosaic” ^[4]. Because recurrence risk cannot be inferred from a negative blood test alone, consulting a genetic counselor is recommended.

Key Genes Involved in FCD

Scientists have identified several genes that, when mutated, can over-activate the mTOR pathway. These include:

The Future: Testing Brain Tissue

Because blood tests are so often negative, the only way to get a definitive genetic answer for many children is to test the brain tissue itself. This is becoming more common when a child undergoes epilepsy surgery for clinical reasons to remove the dysplastic area, though even tissue testing can be negative ^[16].

Newer techniques are also being explored, such as:

• Deep Sequencing: Using ultra-sensitive technology to find mutations that make up less than 1% of the cells in a sample ^[17].
• SEEG Testing: In some specialized centers, doctors are exploring whether they can find these mutations using DNA from the tiny bits of tissue that cling to SEEG electrodes (wires placed in the brain to map seizures), though this remains largely experimental ^[18][19].

Finding the specific gene involved may eventually help doctors choose targeted medications (like mTOR inhibitors) that can “turn down” the hyperactive mTOR pathway, though these targeted medicines are currently established for conditions like tuberous sclerosis and remain largely experimental for most isolated FCD ^[8][20].