Enzyme replacement therapy in mucopolysaccharidosis type I: progress and emerging difficulties.

Mucopolysaccharidosis type I is due to a deficiency of the lysosomal enzyme alpha-L-iduronidase (EC 3.2.1.76) and is associated with a defect in the catabolism of the glycosaminoglycans heparan and dermatan sulphate. The clinical picture produced by this abnormal storage is diverse and ranges from a disorder that is fatal in the early months of life, due to cardiomyopathy, to a condition compatible with a normal life-span. It has become usual to describe the phenotypes associated with this spectrum of disorders after their eponymous names, Hurler (MPS IH, severe), Scheie (MPS IS, mild) and Hurler/Scheie (MPS IH/S, intermediate). Severely affected patients have progressive learning difficulties, facial and skeletal deformities, cardiac disease, corneal clouding, respiratory compromise and joint stiffness. Patients with MPS IH typically die in the first decade of life. MPS IH/S usually have normal intelligence and die in their twenties of cardiorespiratory disease. Patients with MPS IS may have joint stiffness, aortic valve disease and corneal clouding, but are often able to live a normal life-span. Enzyme replacement therapy has been developed as a potential therapy for some patients with MPS I. This process has been helped by the study of a naturally occurring canine model of the disease, which produces a phenotype similar to MPS IH/S in the human. This review details the progress that has been made in this area and also highlights some potential problems with the introduction of therapy.