Why systemic gene therapy for Hurler syndrome?
Hurler syndrome (severe MPS I, MPS I-H) remains a progressive, multisystem disease even after haematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT). These treatments have improved survival and somatic outcomes, but they do not fully correct brain, bone and cartilage involvement and they place a lifelong burden on patients, families and health systems.
Systemic in vivo gene therapy aims to provide durable, body wide expression of alpha L iduronidase from the patient’s own cells, with the goal of addressing key limitations of HSCT and ERT, especially in the central nervous system and skeleton.
Note for clinicians and researchers: This page is intended for clinicians and researchers and describes the scientific and clinical rationale for systemic gene therapy. All gene therapy approaches for Hurler syndrome are investigational and not approved for routine clinical use.
Unresolved problems with current standard of care
HSCT and ERT have transformed the outlook for many children with MPS I-H, but major gaps remain.
Biological limitations
CNS access
- Systemic ERT does not cross the blood brain barrier in meaningful amounts.
- HSCT offers partial CNS correction via donor derived microglia, but effect is time dependent and incomplete, especially if transplant is delayed.
Bone and cartilage access
- Avascular cartilage and growth plate regions are poorly accessible to both HSCT derived enzyme and circulating ERT.
- Skeletal dysplasia and joint disease usually progress despite standard treatment.
Established damage
Structural changes in brain, bone, valves and airways that occur before treatment are only partly reversible, even with optimal care.
Practical and clinical limitations
- HSCT carries significant transplant related mortality and long term toxicity.
- ERT requires regular lifelong infusions, venous access, and carries risk of infusion reactions and anti drug antibodies.
- Residual multisystem morbidity is common, with ongoing neurocognitive, musculoskeletal, cardiac, respiratory, hearing and visual complications.
These limitations define the unmet need and point directly to domains where gene therapy might add value.
From intermittent enzyme replacement to endogenous production
Systemic in vivo gene therapy aims to change the source and distribution of alpha L iduronidase:
- Instead of giving exogenous enzyme intermittently (ERT), or relying solely on donor cells (HSCT), a gene therapy vector delivers a functional IDUA transgene to the patient’s own cells.
- Transduced cells produce enzyme continuously, which can be secreted and taken up by neighbouring cells through mannose 6 phosphate receptors (cross correction).
- Depending on the platform and route, gene therapy may achieve higher and more stable enzyme exposure in blood and tissues than standard ERT, without the need for lifelong infusions.
Potential targets for systemic delivery
- Hepatocytes – liver directed gene therapy, with liver as a factory for systemic enzyme
- Haematopoietic cells or other long lived cell populations
- Combinations that aim to influence both systemic and CNS compartments
Mechanism and biodistribution depend on the vector platform and route of administration used.
Theoretical advantages over HSCT and ERT
Systemic gene therapy is being explored because it could, in principle, address several key challenges.
Continuous, high level enzyme exposure
- Gene therapy can provide sustained enzyme production rather than intermittent peaks and troughs.
- This may reduce substrate re accumulation between ERT infusions and maintain more stable biochemical correction.
- A single or limited number of administrations might replace decades of intravenous infusions.
Enhanced tissue penetration
- Higher steady state plasma enzyme levels, supporting better uptake in difficult to reach tissues.
- Design options that aim to improve exposure to bone, cartilage and other sites that are poorly corrected by standard treatment.
If CNS targeting is included, additional mechanisms (for example brain penetrant vectors or delivery strategies) are needed and are usually considered separately from purely systemic liver directed approaches.
Reduced procedural burden
- Avoidance of myeloablative conditioning and donor search, in contrast to HSCT.
- Avoidance of regular hospital based ERT infusions in settings where gene therapy achieves adequate systemic exposure.
- Potentially fewer episodes of general anaesthesia related to line placement, ERT access issues and repeated infusions.
Matching delivery strategy to pathophysiology
The scientific rationale for systemic gene therapy is grounded in the underlying biology:
- MPS I-H is caused by lack of functional alpha L iduronidase, not by an abnormal substrate structure, so restoring enzyme activity is conceptually straightforward.
- Cross correction allows secreted enzyme from transduced cells to be taken up by non transduced cells, amplifying the effect of gene transfer.
- Even modest increases in residual enzyme activity can significantly change disease course, especially if therapy begins early in life.
Gene therapy design strategy
Systemic gene therapy strategies are designed to:
- Raise enzyme activity above the critical threshold required to prevent progressive accumulation of dermatan sulfate and heparan sulfate.
- Sustain that correction long term, ideally for years, without repeated dosing.
- This is particularly attractive for a disease where lifelong storage of substrate drives cumulative damage.
Complement, alternative or future primary therapy
Several conceptual positions for systemic gene therapy in Hurler syndrome are being considered in the field:
Adjunct to HSCT
Use gene therapy to boost systemic enzyme exposure after HSCT, particularly in tissues that remain under corrected.
Aim to reduce long term skeletal and cardiac morbidity while preserving CNS benefit from HSCT.
Alternative where HSCT not feasible
- For children in whom HSCT risk is high, donor options are limited, or access to transplant is restricted, gene therapy could theoretically provide a less invasive disease modifying option if efficacy and safety are demonstrated.
Replacement for long term ERT
- If systemic gene therapy achieves stable enzyme levels, it could eliminate the need for regular ERT infusions, reducing burden and cost over a lifetime.
The exact positioning will ultimately depend on clinical trial data, safety profiles, regulatory decisions and real world experience.
Why the rationale must be balanced with caution
Although the theoretical advantages are strong, systemic gene therapy carries important uncertainties and risks:
- Vector related toxicity (for example immune responses, infusion related reactions, potential off target effects)
- Long term durability of expression, especially in growing children where liver and other tissues are still expanding
- Integration or insertional risks depending on platform and vector design
- Unknown interactions with future therapies or procedures
- Need for intensive short term monitoring and long term follow up, potentially for life
MPS I-H specific questions
- How much systemic enzyme activity is enough to meaningfully change CNS and skeletal trajectories, if CNS is not directly targeted
- The best age window for maximal benefit and acceptable risk
- How gene therapy interacts with prior or planned HSCT and ERT
These questions are central to ongoing preclinical and clinical research.
Information for different audiences
For families and adults
Hopeful but cautious
Although this page is written with professionals in mind, many families and adults will encounter the idea of gene therapy through news stories or discussions.
Key messages:
- Gene therapy for Hurler syndrome is a promising research area but remains experimental.
- Early results in rare disease gene therapy are often based on small numbers and short follow up.
- No treatment, including gene therapy, is without risk.
- Decisions about research participation should always be made through detailed discussion with your own specialist tea
Families can ask:
- What is known so far about safety and benefit in this specific approach?
- How does gene therapy compare with HSCT and ERT for a child in our situation?
- What would participation in a trial involve in practical terms?
For clinicians and researchers
Key questions when assessing systemic gene therapy studies:
Target population
Pre symptomatic vs early symptomatic infants, children post HSCT with residual disease, patients not eligible for transplant
Vector and delivery
Target tissue, route and dose, with attention to anticipated exposure in brain, bone and cartilage
Endpoints
Biochemical: plasma and urinary GAGs, enzyme activity
Clinical: neurocognition, growth, skeletal outcomes, cardiac and respiratory measures, quality of life
Safety: acute events, immune responses, liver function, long term oncogenic risk
Comparator
Realistic standard of care baseline (HSCT plus or minus ERT and structured follow up) rather than historical untreated cohorts alone
This framework helps situate gene therapy data within the real world of Hurler syndrome management.
Key points on the rationale for systemic gene therapy
- HSCT and ERT have significantly improved outcomes in Hurler syndrome but leave substantial residual CNS, skeletal and multisystem morbidity and carry important burdens and risks.
- Systemic in vivo gene therapy aims to provide sustained endogenous production of alpha L iduronidase, with the potential to deliver higher and more stable enzyme exposure across the body and reduce reliance on lifelong infusions.
- The scientific rationale is strong because even modest increases in enzyme activity can change disease trajectory, and cross correction allows benefit beyond transduced cells.
- Gene therapy may ultimately complement HSCT, replace ERT in some settings, or serve as an alternative where transplant is not feasible, but its exact role awaits robust clinical data.
- Careful evaluation of safety, durability, tissue targeting and clinically meaningful outcomes is essential before systemic gene therapy can be considered part of routine standard of care.
What to read next
Research hub
Main research themes in Hurler syndrome
Scientific background
Molecular and cellular mechanisms of MPS I-H
Unmet need
Residual morbidity after HSCT and ERT
Current standard of care
HSCT, ERT and multidisciplinary management
Treatments and care
Patient facing overview of existing options
Glossary
Definitions of gene therapy and research terminology