Genetics and Gene Therapy in Hunter Disease

Mucopolysaccharidosis type II or Hunter syndrome is an X-linked lysosomal storage disease caused by a mutation in the gene encoding the lysosomal enzyme iduronate-2-sulfatase. The consequent enzyme deficiency causes a progressive, multisystem accumulation of glycosaminoglycans, which is the cause of the clinical manifestations involving also Central Nervous System for patients with the severe form of disease. The limits of the currently available therapies for Hunter syndrome, hematopoietic stem cell transplantation and recombinant enzyme replacement therapy, mainly regarding brain achievement, have encouraged several studies which recognized gene therapy as a potential therapeutic option for this condition. In vitro studies firstly aimed at the demonstration that viral vector- mediated IDS gene expression could lead to high levels of enzyme activity in transduced cells. The encouraging results obtained allowed the realization of many preclinical studies investigating the utilization of gene therapy vectors in animal models of Mucopolysaccharidosis II, together with a phase I clinical trial approved for Hunter patients affected by the mild form of the disease. Together to in vivo studies in which recombinant vectors are directly administered, systematically or by direct injection into Central Nervous System, also ex vivo gene therapy, consisting in transplantation of autologous hematopoietic stem cells, modified in vitro, into the animal or patient, has been tested. A wider clinical application of the results obtained so far is essential to ensure that gene therapy can be definitively validated as a therapeutic option available and usable for this rare but life-threatening disorder.