Severe combined immunodeficiency–X1 (SCID-X1) is an X-linked inherited disorder characterized by an early block in T and natural killer (NK) lymphocyte differentiation. This block is caused by mutations of the gene encoding the γc cytokine receptor subunit of interleukin-2, -4, -7, -9, and -15 receptors, which participates in the delivery of growth, survival, and differentiation signals to early lymphoid progenitors. After preclinical studies, a gene therapy trial for SCID-X1 was initiated, based on the use of complementary DNA containing a defective γc Moloney retrovirus–derived vector and ex vivo infection of CD34 ⁺ cells. After a 10-month follow-up period, γc transgene–expressing T and NK cells were detected in two patients. T, B, and NK cell counts and function, including antigen-specific responses, were comparable to those of age-matched controls. Thus, gene therapy was able to provide full correction of disease phenotype and, hence, clinical benefit.

Previously, several individuals with X-linked SCID (SCID-X1) were treated by gene therapy to restore the missing IL-2 receptor γ (IL2RG) gene to CD34⁺ BM precursor cells using gammaretroviral vectors. While 9 of 10 patients were successfully treated, 4 of the 9 developed T cell leukemia 31–68 months after gene therapy. In 2 of these cases, blast cells contained activating vector insertions near the LIM domain–only 2 (LMO2) proto-oncogene. Here, we report data on the 2 most recent adverse events, which occurred in patients 7 and 10. In patient 10, blast cells contained an integrated vector near LMO2 and a second integrated vector near the proto-oncogene BMI1. In patient 7, blast cells contained an integrated vector near a third proto-oncogene,CCND2. Additional genetic abnormalities in the patients’ blast cells included chromosomal translocations, gain-of-function mutations activating NOTCH1, and copy number changes, including deletion of tumor suppressor gene CDKN2A, 6q interstitial losses, and SIL-TAL1 rearrangement. These findings functionally specify a genetic network that controls growth in T cell progenitors. Chemotherapy led to sustained remission in 3 of the 4 cases of T cell leukemia, but failed in the fourth. Successful chemotherapy was associated with restoration of polyclonal transduced T cell populations. As a result, the treated patients continued to benefit from therapeutic gene transfer.

To the Editor: We recently reported (April 18 issue)¹ the sustained correction of X-linked severe combined immunodeficiency disease by ex vivo, retrovirally mediated transfer of the γc gene into CD34+ cells in four of five patients with the disease. These results have since been confirmed in four additional patients with typical X-linked severe combined immunodeficiency. Of the first four successfully treated patients, three continue to do well up to 3.6 years after gene therapy, whereas a serious adverse event occurred in the fourth patient. At a routine checkup 30 months after gene therapy, lymphocytosis consisting of a monoclonal population . . .

Slowing Brain Disease with Gene Therapy: X-linked adrenoleukodystrophy (ALD), the hereditary brain demyelinating disorder that was featured in the movie “ Lorenzo's Oil ,” is typically treated by transplantation of bone marrow from matched donors. This treatment slows progression of the disease by introducing cells that differentiate into myelin-producing cells. Cartier et al. (p. 818 ; see Perspective by Naldini ) tested an alternative gene therapy–based approach in two young patients without matched donors. A lentiviral vector was used to introduce a wild-type copy of the ALD gene into the patients' hematopoietic stem cells ex vivo. The modified cells were then infused back into the patients. Expression of the transferred gene was still detectable in the patients' blood cells 2 years later, and both patients showed neurological improvement and a delay in disease progression comparable to that seen with bone marrow transplants.

X-linked severe combined immunodeficiency due to a mutation in the gene encoding the common γ (γc) chain is a lethal condition that can be cured by allogeneic stem-cell transplantation. We investigated whether infusion of autologous hematopoietic stem cells that had been transduced in vitro with the γc gene can restore the immune system in patients with severe combined immunodeficiency.