The Biology and Genetics of MLC

While the name Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a mouthful, the biology behind it comes down to a simple problem: the brain’s “plumbing” system. To understand why your child’s brain appears swollen on an MRI, it helps to look at the specialized cells that manage fluid and salt in the brain.

The Biological Mechanism: A Simplified Model

The brain is protected by a network of cells called astrocytes. These cells act like guardians at the border between the blood vessels and the brain tissue ^[1]. One of their most important jobs is ion-water homeostasis—maintaining the perfect balance of water and salts (ions) so the brain doesn’t swell ^[2].

In a child with MLC, the mechanisms that these astrocytes use to regulate their own size are impaired.

• Astrocyte Swelling: Using a simplified analogy, think of the astrocytes as having a broken plumbing system. Because of a missing or broken protein complex (made of MLC1 and GlialCAM, also known as HEPACAM), the astrocytes have trouble shrinking back down after they naturally swell ^[3][4]. While researchers continue to study the precise molecular functions, this impaired fluid regulation is a primary defect.
• White Matter Swelling: These swollen cells create a backup of fluid. This fluid gets trapped within the layers of the white matter (the brain’s wiring), leading to the characteristic “swelling” seen on scans ^[5].
• Cyst Formation: Over time, this fluid can collect into small pockets called subcortical cysts, typically in the front and side (temporal) regions of the brain ^[6].

Understanding the Subtypes

Not all MLC is the same. The “type” your child has depends on which genes are affected and how they were inherited.

Classic MLC (MLC1 and MLC2A)

This is the most common form. It occurs when a child inherits two pathogenic variants of a gene—one from each parent. This is known as biallelic inheritance.

• Genetics: Most cases are caused by variants in the MLC1 gene, but some are caused by two variants in the GLIALCAM (also known as HEPACAM) gene (this is called MLC2A) ^[7][8].
• Course: These two types look identical. They both involve early head swelling and a slow, gradual decline in motor skills over many years ^[7].

Remitting MLC (MLC2B)

This is a more recently discovered and typically milder form of the disease.

• Genetics: This happens when a child has just one specific pathogenic variant (a heterozygous variant) in the GLIALCAM/HEPACAM gene with variable expression ^[8].
• Course: While the head is large in infancy, the brain swelling often “remits” or improves on later MRI scans ^[7].
• Motor vs. Cognitive: Children with MLC2B usually do not lose their ability to walk and have much milder physical symptoms ^[7]. However, parents should know that this type is more frequently linked to intellectual disability or autism than the classic form ^[7].

Inheritance: What It Means for Your Family

The way MLC is passed down determines the risk for future children and helps identify other family members who might carry the gene. A genetic counselor can explain these variant-specific risks, testing of relatives, and reproductive options.

In families with the dominant (MLC2B) form, you may find that a parent or relative also had a large head as a child but never knew why because their symptoms were so mild they essentially “outgrew” them—a phenomenon known as incomplete penetrance ^[9]. Understanding these patterns is a vital step in genetic counseling and family planning.