Previous clinical studies of hematopoietic gene therapy have reported sustained or temporary correction of lymphocytes in patients with severe combined immunodeficiency or adrenoleukodystrophy8,9,24
and of myeloid cells in patients with chronic granulomatous disease or adrenoleukodystrophy. 12,24
Our study shows that gene therapy for the Wiskott–Aldrich syndrome is feasible and has not been associated with treatment-limiting adverse events up to 3 years after gene therapy. These findings indicate the successful correction of early hematopoietic progenitor or stem cells that contributed quantitatively to all cell lineages of blood. We also confirmed correction of platelets on transplantation of retrovirus-transduced HSCs in humans.
In line with a proliferative advantage of WASP-positive cells,14
our results show increasing proportions of corrected lymphocytes over time. Since the reconstitution of the lymphoid system reaches a plateau, there continues to be homeostatic control. Engraftment of progenitor cells was stable, similar to the results achieved in a recent trial of lentivirus-based gene therapy for adrenoleukodystrophy, 24
in which a completely myeloablative conditioning regimen was used.
The pathophysiology of thrombocytopenia in WAS is complex and involves decreased production of platelets and increased immune and nonimmune mechanisms of platelet destruction.25
This study provides evidence that gene transfer can correct thrombocytopenia in humans, a major cause of death and complications in patients with WAS.
The assessment of the clonal inventory in patients whose disease has been successfully corrected uncovers vector influence on key regulators of hematopoiesis. In addition, we observed preferential targeting of genes implicated in autoimmunity control, suggesting an unexpected, disease-related gene targeting. Most interestingly, insertion-site recovery in sorted lymphoid and myeloid fractions provides novel molecular evidence that the molecular signatures contributing to long-term production of hematologic cells are distinct in both cell lineages of WAS patients. These data suggest that distinctive mechanisms control selective proliferative advantage or differentiation of myeloid and lymphoid cells.
Despite targeting potential oncogenes, no persistent clonal imbalance has yet been observed. Deep sequencing revealed a fluctuating pattern of molecularly defined hematopoietic clones and evidence of initial transduction of HSCs with multilineage capacity, suggesting that hematopoiesis continues to follow a cyclic pattern. Clinically, both patients had marked improvement in their susceptibility to infection, autoimmunity, and bleeding. Longer follow-up and in-depth analyses in additional patients are warranted in light of the considerable clinical benefit derived from genetic correction of this disease. Hematopoietic stem-cell gene transfer may achieve complete immunologic correction without producing the toxic effects implicit in the concept of retrovirus-based stem-cell gene transfer.