Hunter syndrome, the blood-brain barrier problem

The history of mucopolysaccharidosis type II is the history of a question pediatric metabolism has been trying to answer for fifty years: how do you deliver a missing enzyme to the brain. Idursulfase, the recombinant enzyme replacement therapy approved by the FDA in 2006 as Elaprase, addresses peripheral disease. It does not cross the blood-brain barrier. For boys with the neuronopathic form of Hunter syndrome, who comprise roughly two-thirds of cases, the central nervous system disease that determines their cognitive trajectory has been largely outside the reach of approved therapy until very recently. Two clinical-development programs have changed that picture, with results that the field is still digesting.

What Hunter is

Mucopolysaccharidosis type II is an X-linked recessive lysosomal storage disorder caused by deficiency of iduronate-2-sulfatase, IDS, encoded by IDS on Xq28. IDS removes a sulfate group from the glycosaminoglycans dermatan sulfate and heparan sulfate as part of their lysosomal degradation. When IDS activity is reduced or absent, dermatan sulfate and heparan sulfate accumulate in lysosomes throughout the body. The clinical phenotype overlaps with Hurler syndrome (MPS I) but with characteristic differences: corneal clouding is absent or minimal in Hunter, the inheritance is X-linked rather than autosomal recessive (so affected individuals are predominantly male), and survival to adulthood is more common in attenuated forms.

Two clinical phenotypes are recognized. Neuronopathic Hunter, the severe form, presents in early childhood with developmental delay, progressive cognitive decline, behavioral disturbance, hyperactivity, sleep disturbance, and a clinical picture that resembles a degenerative encephalopathy alongside the somatic features (coarse facial features, hepatosplenomegaly, dysostosis multiplex, recurrent respiratory infections, cardiac valve thickening). Without treatment, neuronopathic Hunter typically leads to death in the second decade. Attenuated Hunter, the form with preserved cognition, presents with somatic features alone and longer survival, sometimes into adulthood with progressive functional disability.

Reported incidence runs roughly 1 in 100,000 to 1 in 170,000 male births. The condition is panethnic. Female heterozygotes can occasionally manifest features when X-inactivation is skewed.

Detection

Newborn screening for MPS II measures IDS enzyme activity on the dried blood spot. MPS II was added to the federal Recommended Uniform Screening Panel in 2022 after a long advocacy effort by the Hunter community. State adoption has expanded. Confirmation includes urinary glycosaminoglycan profile (with elevated dermatan sulfate and heparan sulfate), plasma IDS activity, and IDS gene sequencing.

The newborn screening problem in MPS II is similar to that in MPS I and Krabbe: the assay reliably identifies a population enriched for affected infants, but predicting at birth which boys will develop the neuronopathic versus attenuated form is imperfect. Genotype-phenotype correlation for IDS is partial. Combined enzyme activity, glycosaminoglycan profile, and variant analysis improve the prediction but do not resolve it completely.

What management looks like

For attenuated MPS II, intravenous enzyme replacement therapy with idursulfase (Elaprase, FDA 2006) is the standard of care. Weekly infusions reduce somatic glycosaminoglycan accumulation, slow progression of hepatosplenomegaly and joint stiffness, improve airway function in some recipients, and modestly improve walk distance in functional assessments. The therapy does not address cognitive disease because IDS does not cross the blood-brain barrier in meaningful quantities.

For neuronopathic MPS II, the clinical question for two decades has been whether any approved therapy could address the central nervous system disease. Allogeneic HSCT, which works for Hurler (MPS I), has been less consistently effective for Hunter, with mixed long-term cognitive outcomes in published cohorts. Intrathecal idursulfase, delivered via implanted intraventricular device, was trialed but did not produce the cognitive benefit the protocol design anticipated.

In 2024, two clinical development paths reached pivotal data. JR-141 (pabinafusp alfa), a fusion protein in which IDS is linked to an antibody against the human transferrin receptor that crosses the blood-brain barrier through receptor-mediated transcytosis, was approved in Japan in 2021 and is in clinical use there. Similar receptor-mediated delivery approaches and AAV-delivered IDS gene therapy are in clinical trials in the United States and Europe. The field is in the period where multiple programs are competing for the central-nervous-system-active treatment slot, and the regulatory and access landscape is evolving quickly.

Adjunctive surgeries and supportive care for MPS II include cardiac valve management, spinal decompression, hip and knee surgery, airway management for upper airway obstruction (sleep apnea is common and severe), and supportive care for behavioral and cognitive features in the neuronopathic form.

Genetic counseling addresses the X-linked inheritance pattern. Female carriers have a 50 percent risk of an affected son and a 50 percent risk of a carrier daughter. Identification of carrier status in female relatives of an affected individual is an important part of the genetic counseling pathway.

What this looks like for a family

A baby boy is born and the heel-prick is sent. On day 5, the state lab reports a low IDS activity. On day 7, urinary glycosaminoglycans, plasma IDS, and IDS sequencing confirm MPS II. The variant is one previously associated with the neuronopathic form. The family meets the metabolic team, who lays out the current options: idursulfase for somatic disease, with the caveat that it does not address the cognitive course; the question of HSCT, weighed against the published outcome data; and the question of clinical trial enrollment in one of the brain-penetrant programs, weighed against the timeline of trial availability and the timeline of cognitive disease.

The family chooses idursulfase plus enrollment in a brain-penetrant ERT trial for which the child meets eligibility criteria. Years pass. Cognitive function is monitored with standardized assessments. Somatic disease is partially controlled. Whether the trial therapy is changing the cognitive trajectory will be a question that takes years to answer.

That is what Hunter care looks like in practice in 2026. The somatic disease has a treatment. The cognitive disease has clinical trials. The decision the family makes today will be different from the decision the family makes in five years, because the field is moving.