Causes and genetics of Hurler syndrome (MPS I-H)
Hurler syndrome (mucopolysaccharidosis type I-H, MPS I-H) is caused by a deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). When IDUA activity is very low or absent, specific complex sugars called glycosaminoglycans (GAGs) build up inside cells and progressively damage many organs. This enzyme problem is the direct result of disease-causing variants in the IDUA gene, inherited in an autosomal recessive pattern.
This page explains, in more scientific detail, how IDUA deficiency leads to GAG storage, how Hurler syndrome is inherited within families and what this means for carrier testing and family planning.
The underlying cause: IDUA enzyme deficiency
Hurler syndrome is a lysosomal storage disorder caused by a marked reduction or absence of the enzyme alpha-L-iduronidase. This enzyme works inside lysosomes to help break down GAGs, especially dermatan sulphate and heparan sulphate. When IDUA activity is severely reduced, these GAGs are not properly degraded and gradually accumulate in cells and tissues across the body.
GAG accumulation causes cells to enlarge and tissues to thicken, stiffen or become infiltrated, which over time leads to the characteristic multisystem involvement of Hurler syndrome. Brain, skeleton, heart valves, airways, liver, spleen, eyes, ears and teeth can all be affected.
In simple terms
A critical enzyme that breaks down certain sugars in cells doesn’t work properly, causing those sugars to build up and damage organs throughout the body.
Quick Summary
•Primary cause: severe deficiency of alpha-L-iduronidase (IDUA)
•Substrates that build up: dermatan sulphate and heparan sulphate
•Site of storage: lysosomes in many cell types
•Result: progressive GAG storage and organ damage
The IDUA gene: instructions for alpha-L-iduronidase
The IDUA gene provides the instructions for making alpha-L-iduronidase. It is located on chromosome 4p16.3 and spans roughly 19 kilobases with 14 exons. The enzyme is synthesised in the endoplasmic reticulum, processed in the Golgi apparatus and then targeted to lysosomes, where it participates in the stepwise breakdown of dermatan and heparan sulphate.
In healthy cells, alpha-L-iduronidase removes terminal iduronic acid residues from GAG chains. This is an essential step in the normal recycling of these molecules. In Hurler syndrome, biallelic IDUA variants lead to minimal or absent enzyme activity and block this step, causing GAG storage and lysosomal enlargement.
Chromosome 4
IDUA gene location
GAG accumulation in Hurler syndrome
GAGs are long chains of sugars that help provide structure and resilience to connective tissues, cartilage, skin, blood vessels and many other tissues. In MPS I-H, the GAGs most affected are dermatan sulphate and heparan sulphate. When they cannot be fully broken down, partly degraded fragments build up inside lysosomes.
Over time, this storage leads to:
Thickened heart valves and blood vessel walls
Distorted bone growth and dysostosis multiplex
Airway narrowing and soft tissue enlargement
Hepatosplenomegaly
Corneal clouding and retinal changes
Central nervous system involvement from storage in brain and meninges
The distribution and degree of GAG storage help explain the wide range of Hurler syndrome symptoms seen in different organ systems.
Autosomal recessive inheritance
Hurler syndrome follows an autosomal recessive pattern of inheritance. A child is affected only when they inherit two non-working copies of the IDUA gene—one from each parent.
Parents who carry one non-working copy are usually healthy and are known as carriers.
When both parents are carriers:
- 25% chance of having Hurler syndrome
- 50% chance of being a healthy carrier
- 25% chance of being unaffected and not a carrier
Worldwide, the estimated carrier frequency for MPS I is approximately 1 in 158 people, though this may vary between populations.
Inheritance Pattern
For families
If you have a child with Hurler syndrome, both parents are almost always carriers. Brothers and sisters may be carriers or, less commonly, also affected. A clinical genetics service can help your family understand these risks and discuss testing options.
IDUA variants and disease severity
More than 200 different IDUA variants have been reported in people with MPS I, including nonsense, missense, splice site and small insertion or deletion variants. Some variants are strongly associated with severe disease, while others more often appear in attenuated forms, but there is considerable overlap and genotype–phenotype prediction is not always straightforward.
Two of the best studied severe variants are p.W402X and p.Q70X, which occur at appreciable frequencies in some populations and are usually associated with very low enzyme activity. Compound heterozygosity for one severe and one milder variant can lead to intermediate phenotypes, highlighting the continuum between severe Hurler and attenuated MPS I.
Key message
Genotype helps, but clinical assessment and enzyme activity remain essential for determining disease severity and treatment decisions.
Key point: Genetic testing can support diagnosis, carrier testing and family planning, but clinical features and enzyme activity still play a central role in assessing disease severity and treatment decisions.
Carrier testing, prenatal diagnosis and preimplantation testing
Once disease-causing IDUA variants have been identified in an affected child, targeted testing can be offered to parents and other at-risk relatives to clarify carrier status. This can be helpful for adult siblings or extended family members who may be planning a family.
Options that may be discussed with a clinical genetics team include:
Carrier testing
For at-risk relatives who may be considering starting a family
Prenatal diagnosis
Using chorionic villus sampling or amniocentesis to test IDUA variants or enzyme activity
Preimplantation genetic testing (PGT)
With in vitro fertilisation, to select embryos that are unaffected or not carriers
These decisions are personal and can be complex, so access to expert genetic counselling is important
Key points about causes and genetics
•Hurler syndrome is caused by severe deficiency of alpha-L-iduronidase, due to biallelic IDUA variants.
•GAGs (dermatan and heparan sulphate) accumulate in lysosomes, leading to progressive damage in many organs.
•The condition is inherited in an autosomal recessive way; parents are usually healthy carriers.
•The carrier frequency for MPS I is estimated around 1 in 158 people worldwide, with local variation.
•Many different IDUA variants exist, and genotype–phenotype correlations are helpful but not absolute.
•Genetic testing and counselling help families understand recurrence risks and consider options for future pregnancies.
Understanding the cause and inheritance of Hurler syndrome can be emotionally challenging. Genetic information is most useful when combined with clear clinical advice and support for the whole family.
What to read next
How Hurler syndrome affects the body
Organ by organ explanation of GAG storage consequences
Research and future therapies
Why new approaches such as gene therapy are being developed