Diagnostic Imaging and Genetic Testing for MLC

Confirming a diagnosis of Megalencephalic leukoencephalopathy with subcortical cysts (MLC) requires a two-step process: specialized brain imaging and detailed genetic testing. Because MLC is so rare, it is important to ensure your child’s diagnostic workup is thorough and that the results are interpreted by specialists familiar with leukodystrophies (disorders of the brain’s white matter) ^[1].

Step 1: The MRI Findings

A Magnetic Resonance Imaging (MRI) scan is a critical tool for identifying MLC. While the MRI pattern is strongly suggestive of the condition, it overlaps with other white-matter disorders, meaning molecular testing and expert clinical interpretation are always needed for a confirmed diagnosis.

Characteristic Findings

• Diffuse White-Matter Swelling: The brain’s white matter (the “wiring”) appears swollen and brighter than usual on specific MRI views, such as T2/FLAIR images ^[2]. This swelling is typically symmetric, meaning it looks the same on both the left and right sides of the brain ^[3].
• Subcortical Cysts: These are small, fluid-filled pockets located just beneath the brain’s outer layer (cortex). They are most commonly found in the anterior temporal (front-side) and frontoparietal (top-front) regions ^[4].
• Relative Sparing of Central Structures: A key clue for doctors is that the very center of the brain’s white matter often looks relatively normal (or “spared”) compared to the outer layers, though this is not absolute ^[4].

Identifying the Subtypes Early

While early MRI features do not reliably establish MLC2B or predict an individual child’s course, there are hints that might point toward the Remitting (MLC2B) form:

• Rarefaction: Instead of clear, distinct cysts, the white matter in early MLC2B scans might just look “thin” or “rarefied” ^[5].
• Clearer Centers: In the early stages of MLC2B, the cerebellum (the balance center at the back of the brain) and the internal capsule (a major pathway for movement) often look normal, whereas they are more likely to show abnormalities in the classic form ^[5].

Step 2: Genetic Confirmation

Once the MRI suggests MLC, genetic testing is used to confirm which gene is involved. Note that a “Variant of Uncertain Significance” (VUS) does not establish a diagnosis; parental testing and specialized interpretation are required.

The Two-Part Genetic Test

It is not enough to just “sequence” the genes. A complete genetic report should include:

1. Sequencing: This looks for small “spelling errors” in the MLC1 and GLIALCAM (also known as HEPACAM) genes ^[6].
2. Deletion/Duplication Analysis: Also called Copy-Number Analysis or MLPA, this test looks for large chunks of DNA that are missing or extra ^[7]. Some children have the correct “spelling” but are missing a whole “page” of the gene, which a standard sequence test might miss ^[8].

The Completeness Checklist

When you receive your child’s diagnostic reports, ensure the following have been addressed:

• [ ] MRI Review: Has a pediatric neuroradiologist evaluated the white-matter swelling and subcortical cysts? ^[3]
• [ ] Gene Coverage: Did the genetic test cover both the MLC1 and GLIALCAM/HEPACAM genes? ^[6]
• [ ] Test Type: Does the report confirm that both sequencing and deletion/duplication (CNV/MLPA) analysis were performed? ^[8]
• [ ] Parental Testing: Were both parents tested? While not universally required for diagnosis, this helps interpret variants and confirms if inheritance is “recessive” (both parents are carriers) or “dominant” ^[9].

What if the Genetics are Negative?

If the MRI looks like MLC but the tests for MLC1 and GLIALCAM are normal, do not lose hope. Your clinical geneticist will guide the next steps using broader panels or exome/genome sequencing.

• Other Genes: Researchers have noted rare case reports of newer genes, such as AQP4 and GPRC5B, presenting with an MLC-like phenotype ^[6][10]. However, testing for these is selected and interpreted by a clinical geneticist, rather than routinely ordered next.
• Differential Diagnoses: Your doctor may need to rule out other leukodystrophies, such as Alexander disease (which often shows a “rim” around the brain’s fluid spaces) or Canavan disease ^[11][2].
• Re-analysis: Sometimes, simply asking a laboratory to re-examine the “raw data” from a previous exome test can reveal variants that were missed the first time ^[8].