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Mucopolysaccharidosis type I (Hurler)
This inherited disorder leads to the build-up of these molecules in cells, tissues, and organs throughout the body, causing progressive damage. It primarily affects infants and young children, impacting various body systems.
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Overview
Mucopolysaccharidosis type I, often referred to as Hurler syndrome, is a rare inherited metabolic disorder. It belongs to a group of conditions known as lysosomal storage disorders. Individuals with this condition lack a functioning enzyme called alpha-L-iduronidase, which is essential for breaking down specific complex sugar molecules, known as glycosaminoglycans (GAGs), within cells [PMID:32662899].
Without this enzyme, GAGs accumulate over time in various tissues and organs, leading to progressive cellular damage and a wide range of symptoms. The condition is progressive, meaning symptoms tend to worsen over time. Although rare, it can significantly impact a child's development and overall health.
Symptoms & clinical features
The symptoms of Hurler syndrome typically appear in infancy and can vary in severity. Common features include distinctive facial characteristics, such as a prominent forehead, flattened nasal bridge, and thick lips. Skeletal abnormalities are often observed, including short stature, joint stiffness, and a curved spine (scoliosis or kyphosis) [PMID:19385078].
Other symptoms may include an enlarged liver and spleen (hepatosplenomegaly), umbilical or inguinal hernias, and heart valve problems. Eye involvement can lead to clouding of the cornea. Neurodevelopmental delay is also characteristic, affecting cognitive and motor skills as the condition progresses. Breathing difficulties and recurrent ear infections are also frequently reported.
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Affected organs
Hurler syndrome can affect many different organ systems throughout the body due to the widespread accumulation of GAGs. The skeletal system is significantly impacted, leading to bone and joint abnormalities, including spinal deformities and restricted joint movement. The liver and spleen often become enlarged.
Cardiac involvement is common, primarily affecting heart valves and sometimes leading to issues with the heart muscle itself. The respiratory system can be affected by airway obstruction, and the eyes may develop corneal clouding. Neurological development is also a key area of concern, as the brain and spinal cord can be affected, leading to developmental delay.
Risks & severity
Hurler syndrome represents the most severe form of mucopolysaccharidosis type I. The condition is progressive, meaning that symptoms generally become more pronounced and severe with age if left untreated. Without intervention, individuals typically experience significant health complications affecting multiple organ systems.
The exact prevalence of Hurler syndrome is estimated to be around 1 in 100,000 live births. The prognosis without treatment is often poor, with severe neurological impairment and organ damage leading to a significantly reduced life expectancy. Early diagnosis and management are crucial for improving outcomes and quality of life.
Genetic causes
Mucopolysaccharidosis type I (Hurler syndrome) is caused by pathogenic variants in the IDUA gene. This gene provides instructions for making an enzyme called alpha-L-iduronidase. This enzyme is crucial for the normal functioning of lysosomes, which are cellular compartments responsible for breaking down and recycling waste products within cells.
Specifically, alpha-L-iduronidase is responsible for breaking down two types of glycosaminoglycans (GAGs): heparan sulphate and dermatan sulphate [PMID:32662899]. When pathogenic variants occur in the IDUA gene, the enzyme either isn't produced in sufficient quantities or is non-functional. This leads to the accumulation of heparan sulphate and dermatan sulphate within the lysosomes of cells throughout the body, causing the cellular and tissue damage observed in Hurler syndrome.
- IDUA alpha-L-iduronidaseThe IDUA gene provides instructions for producing the alpha-L-iduronidase enzyme, which is crucial for the breakdown of certain complex sugar molecules within cells.
Inheritance pattern
Hurler syndrome has an autosomal recessive inheritance pattern. This means that an individual must inherit two copies of a pathogenic IDUA gene variant - one from each parent - to develop the condition. If a person inherits only one copy of a pathogenic variant, they are considered a carrier. Carriers typically do not show any symptoms of the condition.
When both parents are carriers, there is a 25% (1 in 4) chance with each pregnancy that their child will inherit two copies of the pathogenic variant and develop Hurler syndrome. There is a 50% (2 in 4) chance that the child will be a carrier, and a 25% (1 in 4) chance that the child will inherit two healthy copies of the gene and be neither affected nor a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of Hurler syndrome typically begins with a clinical suspicion based on the characteristic symptoms. Initial screening tests may involve measuring GAG levels in urine, which are often elevated in individuals with the condition. However, these tests are not definitive for diagnosis.
A definitive diagnosis relies on biochemical testing to measure the activity of the alpha-L-iduronidase enzyme in blood samples (leukocytes) or skin cells (fibroblasts). If enzyme activity is found to be deficient, genetic testing for pathogenic variants in the IDUA gene is performed to confirm the diagnosis [PMID:19385078]. Genetic testing is available through the NHS Genomic Medicine Service, often initiated following referral to clinical genetics (NHS R-code R115 for Lysosomal Disorders).
Management & lifestyle
Management of Hurler syndrome focuses on supportive care and specific therapies aimed at reducing the accumulation of GAGs. Haematopoietic stem cell transplantation (HSCT) is a primary treatment option, particularly when performed early in life, and can significantly improve neurological and skeletal outcomes [PMID:32662899].
Enzyme replacement therapy (ERT) involves regular infusions of the missing alpha-L-iduronidase enzyme. While ERT can help manage many non-neurological symptoms and improve quality of life, it generally does not cross the blood-brain barrier effectively enough to reverse neurological damage. Ongoing management involves a multidisciplinary team, including paediatricians, geneticists, neurologists, orthopaedic specialists, and physiotherapists, to address the diverse range of symptoms and complications. Regular monitoring of heart, respiratory, and skeletal health is essential.
UK care pathway
In the UK, individuals suspected of having genetic conditions like Hurler syndrome would typically be referred by their GP or another specialist to a clinical genetics service. These services are part of the NHS Genomic Medicine Service and can provide expert assessment, genetic counselling, and access to appropriate diagnostic testing. Genetic testing for lysosomal disorders, including Hurler syndrome, is available through specific NHS R-codes, such as R115.
Genetic counsellors play a vital role in explaining the condition, its inheritance pattern, and the implications for families. They can also help navigate management options and provide support. The care pathway involves a network of specialist centres across the UK that collaborate to provide comprehensive care for rare conditions.
Frequently asked questions
What is the difference between Hurler syndrome and other types of MPS I?
Hurler syndrome is the most severe form of Mucopolysaccharidosis type I (MPS I). Other forms, like Hurler-Scheie and Scheie syndromes, are caused by different variants in the same IDUA gene but lead to less severe symptoms, a later onset, and slower disease progression.
If I am a carrier, will my child definitely have Hurler syndrome?
No, if you are a carrier of an IDUA gene variant, your child will not necessarily have Hurler syndrome. The condition is autosomal recessive, meaning a child must inherit a pathogenic variant from both parents to be affected. If both parents are carriers, there is a 25% chance with each pregnancy that their child will have the condition.
Is there a cure for Hurler syndrome?
Currently, there isn't a complete cure for Hurler syndrome, but treatments like haematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT) can significantly improve symptoms and quality of life. Early intervention is key to achieving the best possible outcomes.
How is Hurler syndrome diagnosed in the UK?
In the UK, diagnosis typically involves a referral to a clinical genetics service. This may involve urine tests for GAGs, followed by blood or skin cell tests to measure the alpha-L-iduronidase enzyme activity. Genetic testing of the IDUA gene confirms the diagnosis.
What kind of medical team will look after a child with Hurler syndrome?
A child with Hurler syndrome is typically cared for by a multidisciplinary team. This can include paediatricians, geneticists, neurologists, orthopaedic specialists, cardiologists, and physiotherapists, all working together to manage the wide range of symptoms.
References
- Parini R, Deodato F, Di Rocco M. Open issues in Mucopolysaccharidosis type I-Hurler. Orphanet journal of rare diseases. 2017. PMID: 28619065
- Consiglieri G, Tucci F, De Pellegrin M. Early skeletal outcomes after hematopoietic stem and progenitor cell gene therapy for Hurler syndrome. Science translational medicine. 2024. PMID: 38691622
- Gentner B, Tucci F, Galimberti S. Hematopoietic Stem- and Progenitor-Cell Gene Therapy for Hurler Syndrome. The New England journal of medicine. 2021. PMID: 34788506
- De Pellegrin M, Filisetti C, Cossutta M. Ultrasonographic hip morphology in mucopolysaccharidosis type I Hurler after hematopoietic stem cell gene therapy. Journal of children's orthopaedics. 2025. PMID: 40809356
- Lamichhane S, Sapkota A, Sapkota S. Mucopolysaccharidosis type I Hurler-Scheie syndrome: a case report. Annals of medicine and surgery (2012). 2024. PMID: 38222696
- Kirkpatrick K, Ellwood J, Walker RW. Mucopolysaccharidosis type I (Hurler syndrome) and anesthesia: the impact of bone marrow transplantation, enzyme replacement therapy, and fiberoptic intubation on airway management. Paediatric anaesthesia. 2012. PMID: 22672476
- Kariya S, Schachern PA, Nishizaki K. Inner ear changes in mucopolysaccharidosis type I/Hurler syndrome. Otology & neurotology : official publication of the American Otological Society, American Neurotology Society [and] European Academy of Otology and Neurotology. 2012. PMID: 22918113
- Schmidt M, Breyer S, Löbel U. Musculoskeletal manifestations in mucopolysaccharidosis type I (Hurler syndrome) following hematopoietic stem cell transplantation. Orphanet journal of rare diseases. 2016. PMID: 27392569