The Science of MED: Genes and Subtypes

Understanding the genetics of Multiple Epiphyseal Dysplasia (MED) is like looking at the architectural blueprints for a building. When there is a “typo” in the blueprints (a genetic mutation), the scaffolding of the joints is not built correctly. In MED, these blueprints affect the extracellular matrix (ECM)—the complex network of proteins that gives cartilage its strength and cushioning ^[1].

The Genes Involved

Several different genes can cause MED. Each gene is responsible for a different part of the cartilage “scaffolding.”

• COMP and MATN3: These genes provide instructions for proteins that help organize the cartilage matrix. When they are mutated, the proteins can become “clumped” or trapped inside the cartilage cells ^[2][3]. This causes the cells to become stressed and prevents them from building healthy bone ends (epiphyses) ^[4].
• COL9A1, COL9A2, and COL9A3: These genes make Type IX Collagen, which acts like the “glue” that holds other cartilage fibers together ^[5]. If this glue is weak, the cartilage is more likely to wear down under the pressure of daily activities.
• SLC26A2 (The Recessive Form): This gene works differently. It helps transport sulfate into the cartilage. Sulfate is what makes cartilage “slippery” and resistant to being squashed ^[6].

Autosomal Dominant vs. Recessive

The way MED is passed down through families depends on which gene is involved.

Autosomal Dominant (Most Common)

Most cases of MED (those involving COMP, MATN3, or COL9) are autosomal dominant. This means a person only needs to inherit one copy of the mutated gene from one parent to have the condition ^[7]. In many cases, this is a “new” mutation (de novo) that starts with the child and was not present in the parents.

Autosomal Recessive (MED Type 4)

MED Type 4, caused by the SLC26A2 gene, is autosomal recessive. This means the person must inherit two copies of the mutated gene—one from each parent ^[8]. Parents of a child with Type 4 MED usually do not have the condition themselves but are “carriers.” This type is often linked to a unique feature called a double-layered patella (kneecap) seen on X-rays ^[8][9].

A Modern Way to Classify

In the past, doctors used the terms Fairbank (for severe cases) and Ribbing (for milder cases) to describe MED. However, we now know that even people with the same “type” can have very different symptoms.

The medical community has shifted toward molecular classification ^[10]. Instead of “mild” or “severe,” doctors now prefer to name the condition based on the specific gene involved (e.g., “COMP-related MED”). This is more accurate and helps your care team better predict how the condition might progress ^[11].

The “Unknown” 10-20%

It is important to know that approximately 10% to 20% of people with clear physical and X-ray signs of MED do not have a mutation in any of the currently known genes ^[12]. This doesn’t mean the diagnosis is wrong; MED is primarily a clinical and radiographic diagnosis. A negative genetic test simply means science hasn’t discovered all the “blueprints” yet, but it does not invalidate the X-ray findings or the diagnosis itself ^[12][13].