Mucopolysaccharidosis type II (Hunter syndrome) is a lysosomal storage disorder characterized by a deficiency of the enzyme iduronate-2-sulfatase, which leads to the accumulation of glycosaminoglycans in visceral organs and connective tissue throughout the body. Patients with the severe form of Hunter syndrome also experience progressive neurological deterioration, which leads to severe cognitive impairment and behavioral problems. Although intravenous enzyme replacement therapy (ERT) can successfully treat many of the somatic symptoms of Hunter syndrome, it has shown little success in treating the neurological symptoms, presumably because the blood–brain barrier limits enzyme distribution to the central nervous system.

Allogeneic hematopoietic stem cell transplantation has been effective in preventing neurological deterioration in other lysosomal storage disorders; however, its role in Hunter syndrome has been controversial until recently. Bone marrow transplantation has been shown to stabilize or improve cardiovascular outcomes, hepatosplenomegaly and joint mobility in Hunter syndrome, but patients with severe disease typically show continued neuropsychological decline after treatment.1 However, a recent study in Japan evaluated 17 patients with Hunter syndrome who underwent hematopoietic stem cell transplantation using bone marrow or umbilical cord blood from 1990 to 2003 (mean follow-up, 9.6±3.5 years). The results showed stabilization or improvement of brain MRI abnormalities (brain atrophy, cribriform lesions and ventricular enlargement) in 6/10 patients with the severe form of the disease.2 In addition, some patients with severe disease showed stabilized or improved ability to carry out activities of daily living (7/9 patients) and/or functional independence measure scores (3/9 patients), an indicator of physical and cognitive disability.2

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