Gene Therapy for Wiskott-Aldrich Syndrome: Benefits and Risks

An international team headed by Dr. Christoph Klein in Germany has reported clinical improvement after gene therapy in two children with Wiskott-Aldrich Syndrome (WAS), a disease caused by mutation of the gene encoding Wiskott Aldrich syndrome protein (WASp). In patients with this condition, the lack of functional WASp, which plays a critical role in actin polymerization in blood cells, leads to thrombocytopenia as well as immunodeficiency with recurrent infections and a tendency toward development of autoimmune diseases. WAS is a promising candidate disease for gene therapy, because the mutated gene is expressed uniquely in hematopoietic cells, and previous animal studies have shown that WASp-corrected cells have a survival advantage over non-corrected cells. Thus, even low levels of corrected cells may produce clinical benefit.

The study included patients over one year of age with documented severe WAS. After receiving a nonmyeloablative dose of busulfan (4 x 4 mg/kg), the children underwent autologous transplantation of gene-corrected cells. Prior to transplantation, CD34+ G-CSF mobilized peripheral blood stem cells were infected in vitro with a retrovirus encoding the normal WASp gene, with a five-fold multiplicity of infection (number of virus particles per cell), such that some cells would likely be infected with more than one copy of the retrovirus. The final CD34+ dose in these patients was 13 and 18 million CD34/Kg, with approximately 50 percent transduction efficiency.

Both patients in the study showed functional immune reconstitution and significant increases in platelet counts from below 20,000 to 80,000-200,000 per microliter. Within six months of transplantation, both children had significant levels of WASp+ blood cells with 80 to 90 percent of their T cells (CD4, CD8, and Treg) expressing WASp, and these increases were maintained for the length of follow-up (three years). Clinically, both patients experienced fewer and less severe infections and bleeding episodes. Both had decreased autoimmunity (autoimmune anemia and eczema, respectively) and normalized immunoglobulin levels, and after immunization each developed effective titers against tetanus, diphtheria, and H. flu.

With retroviral gene therapy, retroviral DNA permanently inserts into the genome, usually near promoters of active genes. The biggest safety concern in these WAS patients is that permanent insertion may occur in or near an oncogene to promote leukemia. Retroviral insertion into the LMO2 locus in children with X-SCID severe combined immunodeficiency was associated with development of leukemia after gene therapy.¹  The investigators therefore performed extensive insertion site analyses in the bone marrow cells and found increased representation of cells having insertions near genes critical for cell proliferation, including LMO2.

In this published report, there were no adverse effects and the clinical benefits persisted. At the 2010 ASH annual meeting, Dr. Klein reported on results from 10 children treated with this protocol. Of these patients, nine (including the two published) showed clinical improvement after gene therapy. One patient failed treatment due to low CD34+ cell counts and instead underwent haploidentical transplantation. One patient developed acute lymphocytic leukemia; this patient, like those patients with X-SCID who developed leukemia after gene therapy, had a leukemia clone with retroviral insertion near the LMO2 gene.