How is MPS I Inherited? Genetics & Carrier Risks

Receiving a diagnosis of Mucopolysaccharidosis type 1 (MPS I) for your child can be overwhelming, and it is natural to immediately worry about your other children or future family plans. The most likely scenario is that both you and your partner are carriers of a non-working gene. Because MPS I is an autosomal recessive genetic condition, there is a 25% (1 in 4) chance in every future pregnancy that the child will also have MPS I ^[1][1]. There is a 50% chance the child will be a carrier like you (unaffected), and a 25% chance they will not inherit the non-working gene at all ^[1].

The IDUA Gene and Autosomal Recessive Inheritance

To understand these odds, it helps to look at the specific gene involved. MPS I is caused by changes (mutations) in the IDUA gene ^[1][2]. This gene provides the body’s instruction manual for making a crucial enzyme called alpha-L-iduronidase, which clears away cellular waste (such as complex sugars known as GAGs) ^[2][3]. We all inherit two copies of the IDUA gene—one from our mother and one from our father ^[1].

In an autosomal recessive condition like MPS I, a person must inherit two non-working copies of the IDUA gene (one from each parent) to actually develop the disease ^[1][1]. If you have only one non-working copy, you are considered a “carrier.” Carriers generally do not show any symptoms because their one working gene produces enough of the enzyme to keep them healthy.

What This Means for Future Pregnancies

Because parents of a child with MPS I both carry one non-working IDUA gene, the outcome of each future pregnancy depends on chance ^[1]. For each individual pregnancy, the probabilities are exactly the same:

• 25% (1 in 4) chance the child inherits two non-working genes and has MPS I.
• 50% (2 in 4) chance the child inherits one working and one non-working gene, making them a carrier (like the parents) but generally healthy.
• 25% (1 in 4) chance the child inherits two fully working genes, meaning they do not have MPS I and are not a carrier.

These odds do not change based on your previous children ^[1]. Having a child with MPS I does not mean your next child is “safe” or “destined” to have it—each pregnancy rolls the same 25% risk. Furthermore, children who inherit the condition from the same parents will generally share the same genetic mutations, meaning the severity of the disease will likely be very similar to your currently diagnosed child ^[3][4].

What This Means for Existing Siblings

It is common to worry about the siblings of a newly diagnosed child. If you have an older or younger child who appears completely healthy, it is still crucial to have them tested. They could potentially have the condition but not yet show obvious signs ^[1]. If testing confirms that a sibling definitely does not have MPS I, the mathematics of being a carrier shift slightly: they have a 2 in 3 (roughly 66%) chance of being a carrier, and a 1 in 3 (roughly 33%) chance of not being a carrier at all ^[1].

The Role of Genetic Counseling

Learning about genetic risks is difficult, which is why meeting with a genetic counselor is strongly recommended for families affected by MPS I ^[5]. A genetic counselor can help you:

• Understand your family’s specific genetic test results.
• Coordinate testing for the affected child’s siblings to see if they are carriers or if they also have the condition.
• Discuss options for future family planning, such as prenatal testing or preimplantation genetic testing for monogenic disorders (PGT-M), which allows embryos to be tested for the condition before a pregnancy begins ^[5]. (Note: PGT-M is sometimes still referred to in older medical literature as PGD).