The Biology of MPS I and Understanding Your Diagnosis

Understanding the biology of Mucopolysaccharidosis type 1 (MPS I) is an important first step in managing your child’s health. While the terms and processes may seem complex, they describe a specific breakdown in how your child’s body recycles natural materials.

The Cellular Recycling System

Inside almost every cell in the body, there are small compartments called lysosomes ^[1]. You can think of these as the cell’s “recycling centers” ^[1]. Their job is to break down large molecules into smaller pieces that the body can reuse or discard.

In children with MPS I, a specific worker in this recycling center—an enzyme called alpha-L-iduronidase (IDUA)—is either missing or not working correctly ^[1][2]. This enzyme is responsible for breaking down complex sugar chains called glycosaminoglycans (GAGs) ^[1]. When the enzyme is deficient, two specific GAGs—heparan sulfate and dermatan sulfate—begin to pile up inside the lysosomes ^[2][3]. Over time, this “cellular clutter” builds up in tissues and organs, leading to the symptoms of MPS I ^[2][4].

How MPS I is Inherited

MPS I is an autosomal recessive condition ^[5]. This means that for a child to have the disorder, they must inherit two copies of a non-working IDUA gene—one from each parent ^[5].

• Carriers: Parents are typically “carriers,” meaning they have one working gene and one non-working gene ^[5]. Because they have one working gene, their bodies produce enough enzyme to stay healthy.
• The 25% Rule: When two carriers have a child, there is a 25% (1 in 4) chance the child will inherit both non-working genes and have MPS I ^[5]. There is a 50% chance the child will be a carrier like their parents, and a 25% chance they will inherit two working genes.
• Family Carrier Testing: Because MPS I is inherited, it is highly recommended to consult a genetic counselor to determine if your other children or extended family members should be tested for the condition or carrier status ^[1].

The Path to a Diagnosis

In many regions, children are increasingly identified through Newborn Screening (NBS), a public health program that tests babies shortly after birth ^[6][7]. The diagnostic process typically follows a “two-tier” approach:

1. Enzyme Activity Assay: A small drop of blood is tested to see how much alpha-L-iduronidase enzyme is active ^[6]. If the level is very low, the lab moves to the second step.
2. DNA Sequencing: The lab looks directly at the IDUA gene to find specific mutations (changes) that confirm a diagnosis ^[6][7].

Beyond the blood test, doctors often perform a urinary GAG analysis ^[2]. This test measures the amount of heparan and dermatan sulfate being “spilled” into the urine ^[2][3]. High levels of these sugars in the urine are a strong indicator of a true MPS I diagnosis ^[3].

Understanding “Pseudodeficiency”

Sometimes, a newborn screening test comes back with low enzyme levels, but the child does not actually have the disease. This is often due to pseudodeficiency alleles ^[8][9].

A pseudodeficiency allele is a genetic variation (such as p.Ala79Thr or p.His82Gln) that makes the enzyme look “weak” in a laboratory test tube, but it still works well enough inside the human body to prevent GAGs from building up ^[8][2]. These are considered “false alarms” ^[8]. If a child has these alleles, their urinary GAG levels will typically be normal, and they will not develop the symptoms of MPS I ^[8][7]. Distinguishing between these harmless variants and true disease-causing mutations is a critical part of the diagnostic journey ^[9][10].

A Spectrum of Results

The specific mutations found in the IDUA gene can help doctors predict whether a child has the severe (Hurler) form or the attenuated (Scheie) form of the disease ^[1][11].

• Severe Mutations: Some mutations, called “null” mutations, tell the body to stop making the enzyme entirely, which usually leads to the more severe form ^[11][12].
• Attenuated Mutations: Other mutations allow the body to make a small amount of working enzyme, which often leads to the milder, more slowly progressing forms ^[11][13].

Your medical team will use all of these pieces—enzyme levels, urine tests, and genetic results—to build a complete picture of your child’s health.