Biology, Look-alikes, and Diagnosis

While the symptoms of congenital enteropeptidase deficiency are visible, the cause is hidden deep within the biology of the small intestine. Getting to a diagnosis often requires ruling out more common conditions that look almost identical on the surface. Understanding the “domino effect” of protein digestion and the specific tests used to confirm the diagnosis can help you navigate your child’s medical reports.

The Missing “Starter” Enzyme

Normally, your child’s pancreas produces enzymes to digest protein, but it sends them to the small intestine in an “inactive” or sleeping state ^[1]. This is a safety measure to prevent the enzymes from digesting the pancreas itself.

The TMPRSS15 gene provides the instructions for making enteropeptidase (also called enterokinase), a specialized protein that sits on the lining of the small intestine ^[2][3]. Its only job is to act as a “starter” by cutting a small piece off an inactive enzyme called trypsinogen ^[1].

This single cut transforms trypsinogen into its active form, trypsin ^[1]. Once trypsin is active, it quickly activates the other protein-digesting enzymes. In children with this condition, pathogenic variants in the TMPRSS15 gene impair the function of the starter enzyme. Without it, the inactive trypsinogen passes through the body without ever waking up ^[2][1]. Because these enzymes remain inactive, the body cannot break down protein to absorb the nutrients it needs to grow.

Conditions That Look Similar

Because “failure to thrive” and chronic diarrhea are common in many childhood illnesses, doctors must rule out several “look-alike” conditions before confirming enteropeptidase deficiency ^[4][1]:

• Glucose-Galactose Malabsorption: A condition where the body cannot absorb certain sugars, leading to severe diarrhea and dehydration ^[4].
• Villous Atrophy: Damage to the lining of the small intestine (often seen in infections or other enteropathies) that prevents nutrient absorption ^[4].
• Primary Zinc Deficiency: This causes a skin rash (acrodermatitis enteropathica) that looks almost identical to the rash sometimes seen in enteropeptidase deficiency ^[5][3].
• Pancreatic Exocrine Insufficiency: A condition where the pancreas doesn’t make the enzymes at all, rather than just failing to “wake them up.”

How the Diagnosis is Confirmed

There are two main ways doctors confirm this diagnosis, though not every child will need both tests:

1. Genetic Testing (The Modern Standard): Doctors look for pathogenic or likely pathogenic variants in the TMPRSS15 gene ^[2][3]. To cause the disease, the child must have variants on both copies of the gene (one from each parent, known as being “in trans”). Finding two variants is not automatically diagnostic if one is a “variant of uncertain significance”—your geneticist will help interpret the results.
2. Functional Testing (Zymogen Activation): This is a highly specialized test that involves taking a small sample of fluid (aspirate) from the small intestine ^[1]. In a lab, scientists check if tryptic activity is absent. If they add lab-made enteropeptidase and the enzymes suddenly “wake up,” it proves the problem was specifically the missing starter enzyme ^[1]. This test is not available everywhere and is not always required to start treatment.

Your Medical Report

When you review your child’s medical records, a diagnostic workup may include several of the following components, selected by your specialists based on your child’s presentation:

• Genetic Report: Listing the specific TMPRSS15 variants ^[2][3]. Parental testing is often done to confirm the variants were inherited appropriately.
• Metabolic Markers: Blood tests showing levels of albumin and total protein, along with Complete Blood Counts (CBC) checking for anemia ^[3][1].
• Rule-Outs: Documentation that other causes of diarrhea, such as infections, zinc deficiency, or sugar malabsorption, were considered or tested ^[4].

By confirming the specific genetic or biochemical cause, your care team can tailor a treatment plan that directly addresses the missing starter step ^[3][5].