Phoxing a myeloid-cell genetic defect

Chronic granulomatous disease (CGD) is a primary immunodeficiency disease that is caused by mutation of any of the four genes encoding the subunits of phagocytic oxidase and that is characterized by phagocytes lacking antimicrobial activity. Previous attempts to treat individuals with CGD using gene therapy have been unsuccessful. However, in a new study, published in Nature Medicine, successful correction of the gene defect was achieved using non-myeloablative conditioning before infusion of the genetically modified autologous haematopoietic progenitors.

The authors harvested CD34⁺ progenitors from the blood of two individuals with X-linked CGD (that is, individuals with CGD as a result of mutations in the gene encoding gp91^phox). These cells were transduced with a γ-retroviral vector expressing the gene encoding gp91^phox before being re-infused into the patients, who had been conditioned with a course of non-myeloablative chemotherapy. Gene-modified cells were detected in the peripheral-blood leukocytes of both individuals from day 21 after infusion until the latest time point analysed (542 days after infusion), as well as in the haematopoietic-progenitor population in the bone marrow. Most of the genetically modified peripheral-blood leukocytes were myeloid cells, in particular granulocytes.

Analysis of the sites of retroviral integration indicated that the clonal diversity of the genetically modified cells decreased over time but never became monoclonal. The clones that emerged showed retroviral integration in or near the upstream regulatory regions of at least one of three particular gene loci. Crucially, although these clones expanded preferentially in vivo, eventually forming up to 80% of the genetically modified cells, they were unable to proliferate in culture in the absence of growth factors, indicating that they were not able to self-renew.

... this trial resulted in the successful treatment of two individuals with X-linked CGD

Expression of gp91^phox was detected in granulocytes from the treated individuals, and NADPH-oxidase activity was reconstituted in these cells. Although the amount of superoxide produced by genetically modified neutrophils from the patients was less than that produced by neutrophils from healthy individuals, it was sufficient to mediate bacterial killing. Furthermore, the two individuals who had received gene therapy showed no evidence of recurrence of the bacterial and fungal infections that they had suffered before treatment.

Although this trial resulted in the successful treatment of two individuals with X-linked CGD, monitoring of the individuals will continue for 3–5 years because of the possibility that the abnormal clonal expansion observed could eventually result in leukaemogenesis.