As discussed in a recent interview between Krystal Jacques and Prof. Brian Bigger, a novel stem cell–based gene therapy is showing early promise as a potential one-time, lifelong treatment for Hunter syndrome, a rare and often fatal genetic disorder that primarily affects young boys. The approach, now in Phase I/II clinical trials, aims to overcome the key shortcomings of current standard therapy—most notably its inability to treat the brain.
Understanding Hunter Syndrome
Hunter syndrome, also known as mucopolysaccharidosis type II (MPS II), belongs to a group of inherited lysosomal storage diseases. It is caused by mutations in the IDS gene, located on the X chromosome, which result in insufficient production of the enzyme iduronate-2-sulfatase (IDS). Without functional IDS, complex sugars such as heparan sulfate and dermatan sulfate accumulate inside cells.
This progressive buildup damages multiple organs. Children with the severe form often experience cognitive decline, speech impairment, skeletal deformities, and heart and lung disease. Because the disorder is X-linked, it overwhelmingly affects males.
Limitations of Current Treatment
For decades, enzyme replacement therapy (ERT) has been the only approved treatment for Hunter syndrome. While ERT can reduce some systemic symptoms, it has significant drawbacks. The infused enzyme circulates in the bloodstream but has limited access to certain tissues, including bones, joints, and—most critically—the brain. The blood–brain barrier blocks large therapeutic proteins, leaving neurological disease unchecked.
ERT also places a heavy burden on families. Patients require weekly intravenous infusions lasting several hours, administered indefinitely in a hospital setting. In addition to the disruption of daily life, the financial cost is substantial, with annual drug expenses reaching hundreds of thousands of dollars per patient.
A Gene Therapy Designed to Reach the Brain
To address these challenges, researchers at the University of Edinburgh, led by Professor Brian Bigger, have developed a hematopoietic stem cell (HSC) gene therapy intended as a one-time intervention.
The process begins by collecting a patient’s own blood-forming stem cells. These cells are mobilized from the bone marrow into the bloodstream using established medications, then harvested and genetically modified in the laboratory. A lentiviral vector is used to insert a healthy copy of the IDS gene into the cells.
Before reinfusion, patients receive a conditioning chemotherapy regimen to make room in the bone marrow for the corrected cells to engraft. Once returned to the body, the modified stem cells give rise to circulating immune cells that continuously produce the missing enzyme.
Overcoming the Blood–Brain Barrier
A key innovation of this therapy lies in how it delivers IDS to the brain. Some of the immune cells derived from the engineered stem cells can cross the blood–brain barrier and take on microglia-like functions within the central nervous system. These cells release IDS directly where it is needed to clear toxic sugar buildup in neurons.
In addition, the research team enhanced the enzyme itself. The IDS protein is fused to a molecular tag known as ApoEII, which enables enzyme circulating in the bloodstream to enter the brain via receptor-mediated transport. This two-pronged strategy—cell-based delivery plus enhanced enzyme trafficking—significantly increases enzyme levels in the brain compared with earlier gene therapy approaches.
Early data suggest that this results in improved cognitive and behavioral outcomes, addressing the most devastating aspect of Hunter syndrome.
Safety and Enzyme Levels
One concern with gene therapy is whether producing high levels of an enzyme could be harmful. Preclinical studies have been reassuring. IDS is only active in the acidic environment of lysosomes, meaning excess enzyme in the bloodstream remains largely inactive. Furthermore, expression of the introduced gene is largely restricted to immune cells such as macrophages and microglia, limiting unintended effects.
While researchers remain cautious and continue close monitoring, early trial participants have tolerated the therapy well, even with markedly elevated enzyme levels.
Looking Beyond Early Childhood
The current clinical trial is focused on very young children, before extensive neurological damage has occurred. If outcomes remain positive, the team hopes to extend the approach to older patients. However, treating established brain degeneration presents a major challenge, as lost neurons cannot be restored by HSC gene therapy alone.
To address this unmet need, the Edinburgh group is developing a complementary strategy using neural stem cells derived from induced pluripotent stem cells. These cells are designed both to replace damaged neurons and to supply the missing enzyme. Although this work is still in preclinical stages, it represents a potential future option for adolescents and adults with Hunter syndrome.
A Shift in the Treatment Paradigm
If successful, this one-time gene therapy could mark a major shift in how Hunter syndrome is treated—moving away from lifelong enzyme infusions toward durable, systemic correction of the disease. While longer follow-up and larger studies are still needed, the approach offers renewed hope for families affected by this devastating condition.
