Pathophysiology of Hurler syndrome (MPS I-H)
Hurler syndrome (MPS I-H) is a lysosomal storage disorder caused by severe deficiency of the enzyme alpha L iduronidase (IDUA). When IDUA is missing, the glycosaminoglycans (GAGs) dermatan sulphate and heparan sulphate cannot be fully broken down. They accumulate inside lysosomes, disrupt normal cell function and trigger a cascade of secondary changes that ultimately damage many organs, including the brain, skeleton, heart, lungs and blood vessels.
This page takes a more detailed, mechanism focused look at how IDUA deficiency leads from microscopic storage in cells to the complex, multisystem disease seen in Hurler syndrome.
Lysosomal biology in Hurler syndrome
Lysosomes are small compartments inside cells that act as recycling centres, breaking down complex molecules into simpler components. In MPS I, the key defect is loss of alpha L iduronidase, a lysosomal enzyme that removes specific residues from dermatan sulphate and heparan sulphate during their normal degradation. When IDUA activity is very low, these GAGs are only partially degraded and start to accumulate within lysosomes.
As storage increases, lysosomes enlarge and can occupy a large part of the cell. This distorts cell shape, crowds other organelles and alters intracellular trafficking. In connective tissues and organs where GAGs are naturally abundant, such as cartilage, heart valves and vessel walls, the cumulative effect is thickening and stiffening of tissues.
From GAG storage to widespread cellular stress
In Hurler syndrome, undegraded dermatan and heparan sulphate accumulate in lysosomes of many cell types, including fibroblasts, chondrocytes, endothelial cells, smooth muscle cells, neurons and glia. The primary storage of GAGs sets off a series of secondary disturbances within the lysosomal system.
Key secondary events described across MPS I studies include:
Secondary storage
of other substrates because the overloaded lysosome cannot process its usual range of molecules efficiently
Membrane changes
in lysosomal membrane composition and permeability, which disturb ion gradients and signalling
Impaired vesicle trafficking
and fusion in the endosomal lysosomal pathway
Disrupted autophagy
so damaged proteins and organelles are not cleared properly
Mitochondrial dysfunction
and increased oxidative stress, with excess reactive oxygen species
Altered calcium homeostasis
and cell signalling
Together, these changes create a state of chronic cellular stress that goes beyond simple accumulation of GAGs.
Inflammatory pathways in MPS I-H
Accumulated GAGs, particularly heparan sulphate, can act as damage associated molecular patterns. Experimental work suggests that they bind to pattern recognition receptors such as Toll like receptor 4 and trigger activation of innate immune pathways. This leads to release of pro inflammatory cytokines including TNF alpha and interleukin 1, and activation of inflammasome complexes such as NLRP3, especially within the brain and joints.
In tissues, this manifests as:- Chronic neuroinflammation in the central nervous system, driven by activated microglia and astrocytes
- Synovial and bone inflammation, contributing to pain and joint destruction
- Vascular inflammation, which may play a role in arterial wall disease in MPS I
Inflammation and oxidative stress appear to amplify tissue damage beyond what would be expected from mechanical storage alone.
Why the brain is affected in Hurler syndrome
In severe MPS I, GAG storage occurs in neurons, glia and cells of the meninges and cerebral vasculature. Brain imaging and animal models show enlarged perivascular spaces, white matter changes, cortical atrophy and spinal cord compression.
Mechanisms thought to contribute to neurological decline include:- GAG storage in neurons, interfering with synaptic function, axonal transport and neuronal survival
- Activation of microglia and astrocytes, leading to chronic release of inflammatory mediators
- Impaired neurogenesis and neuroplasticity, partly due to abnormal heparan sulphate disrupting growth factor signalling
- Hydrocephalus and raised intracranial pressure from impaired cerebrospinal fluid flow and thickened meninges
- Cervical spinal cord compression due to bony changes and dural thickening
Pathophysiology of bone and joint disease
Chondrocytes in growth plates and articular cartilage are naturally rich in GAGs, so they are heavily affected by IDUA deficiency. Storage within these cells disrupts normal endochondral ossification, alters signalling between cartilage and bone and leads to abnormal growth plate architecture.
Consequences include:
- Dysostosis multiplex, with characteristic changes in skull, spine, ribs, pelvis and long bones
- Short stature and disproportionate growth, due to impaired growth plate function
- Hip dysplasia and joint malalignment, increasing the risk of pain and early degenerative changes
- Joint stiffness and contractures, from GAG laden connective tissue, capsular thickening and synovial involvement
Because cartilage is relatively avascular and has limited cell turnover, it is particularly difficult to correct once established. This contributes to the persistence of skeletal problems even in patients transplanted at an early age.
Heart, vessels and airway pathology
In the cardiovascular system, GAG storage occurs in heart valves, myocardium, coronary arteries and large vessel walls. Valves become thickened and dysfunctional, leading to regurgitation and, less commonly, stenosis. Coronary artery involvement and myocardial infiltration can contribute to heart failure and arrhythmias.
In the respiratory tract, storage in the upper airway, tongue, tonsils, adenoids and tracheal wall causes airway narrowing and obstruction. Combined with chest wall deformity and weak respiratory muscles, this leads to obstructive sleep apnoea, recurrent infections and restrictive lung disease. Thickened tissues around the craniocervical junction and spine can further compromise the airway, especially under anaesthesia.
Heart valves
- Thickened and dysfunctional
- Regurgitation and stenosis
- Myocardial infiltration
Vessels
- Coronary artery involvement
- Large vessel wall storage
- Contributes to heart failure
Upper airway and chest
- Airway narrowing
- Obstructive sleep apnoea
- Restrictive lung disease
Why some problems persist despite HSCT and ERT
Haematopoietic stem cell transplantation and enzyme replacement therapy provide systemic sources of IDUA and can clear storage in many organs. However, several features of the underlying pathophysiology help explain why residual disease remains in many patients even after successful treatment.
Key factors include:
- Blood brain barrier – circulating enzyme and donor derived cells only partially correct CNS storage, so neuroinflammation and subtle neurological deficits may persist
- Avascular cartilage and growth plates – limited vascular supply and slow turnover make it hard for enzyme to reach and remodel these tissues
- Established structural damage – bone deformities, valvular thickening and airway narrowing that have already developed are often only partially reversible
- Ongoing secondary cascades – inflammation, oxidative stress and disrupted autophagy may continue even after primary GAG levels fall
These limitations are a major reason why new approaches, including gene and cell based therapies that can deliver higher and more sustained enzyme levels to difficult to reach sites such as brain and bone, are being investigated.
Therapeutic blind spots
Brain (blood-brain barrier)
Bone and cartilage (avascular)
Airway structures (established damage)
Inflammatory cascades (ongoing)
Key points about Hurler syndrome pathophysiology
- Hurler syndrome is caused by severe IDUA deficiency, leading to accumulation of dermatan and heparan sulphate in lysosomes.
- Primary GAG storage triggers secondary lysosomal dysfunction, impaired autophagy, mitochondrial damage, oxidative stress and disturbed calcium signalling.
- GAGs and secondary substrates activate innate immune pathways, driving chronic inflammation in brain, joints and vessels.
- Organ specific effects in CNS, skeleton, heart and airways arise from a combination of storage, inflammation and mechanical distortion of tissues.
- These mechanisms explain both the clinical picture of Hurler syndrome and the limitations of current treatments, highlighting the need for therapies that can better reach brain and bone.
Reassurance for families: This page covers mechanisms at a cellular and tissue level. Families do not need to understand every detail. The key message is that Hurler syndrome is a whole body condition and that early, intensive and ongoing treatment is needed to limit long term damage.
What to read next
How Hurler syndrome affects the body
Organ by organ explanation for families and clinicians
Research and future therapies
How understanding pathophysiology shapes new treatment approaches