Efficacy of Gene Therapy for Wiskott-Aldrich-Syndrome

Abstract 165^FN2

CB and KB contributed equally and should be considered aequo loco.

Wiskott-Aldrich-Syndrome (WAS) is a rare and life-threatening immune-disorder characterized by autoimmunity, microthrombocytopenia, immunodeficiency and susceptibility to lymphoma. WAS is caused by mutations in the WAS gene which encodes WASP, a key regulator of actin polymerization exclusively expressed in hematopoietic cells. WASP deficiency causes defects in lymphocytes, myeloid cells, and platelets.

We here report a comprehensive analysis of ten patients treated by hematopoietic stem cell gene therapy between 2006 and 2009 (median follow up time 29.6 months, range 15 to 58 months).

Patients were mobilized with G-CSF alone (3/10) or G-CSF combined with anti-CXCR4/AMD3100 (7/10), conditioned with busulfan (8mg/kg body weight) and received between 2.8×10⁶ and 24.9×10⁶ cells/kg bw, with a median transduction efficacy of 52%. Upon transplantation of retrovirus-transduced WASP-expressing progenitor cells, the proportion of corrected platelets and lymphocytes increased steadily over time reaching 85–90% and 80–85%, respectively. Interestingly, also myeloid cells showed a continuously increasing percentage of WASP-expressing fractions (20 to 70%).

Due to transplantation of insufficient numbers of WASP-transduced HSC, one patient failed to engraft. He had no evidence of corrected myeloid/hematopoietic progenitor cells and continued to suffer from life-threatening infections and autoimmunity. He was successfully treated by haploidentical HSCT. All other patients had marked improvement of their clinical status. Bleeding diatheses, susceptibility to infections, and autoimmunity resolved. Patients had evidence of significant and sustained increase in platelet counts (p=0.01) together with a reconstituted WASP expression and normalization of thrombocyte size (p<0.001).

After gene therapy, we observed a normalization of the T cell compartment (T cell proliferative responses and TCR Vb spectratyping), NK cell function (assessed by in vitro cytotoxicity and formation of NK cell immunological synapse). The B cell compartment showed consistent expression of WASP. In 4 of 7 patients, IgG substitution could be discontinued. These patients and also those without initial IgG substitution mounted a specific immune response to vaccines, as evidenced by determination of specific antibody titers.

One patient developed a T-cell acute lymphoid leukemia 488 days after gene therapy associated with vector integration close to the LMO2 locus. Chemotherapeutic treatment induced remission that is documented since d33 after initiation of induction therapy.

Long-term follow up observation indicated that gene therapy for WAS, although not without toxicity, is feasible and provides an effective alternative treatment strategy to allogeneic HSC transplantation.