Gene therapy: The power of persistence: Nearly 50 years after the concept was first proposed, gene therapy is now considered a promising treatment option for several human diseases. The path to success has been long and tortuous. Serious adverse effects were encountered in early clinical studies, but this fueled basic research that led to safer and more efficient gene transfer vectors. Gene therapy in various forms has produced clinical benefits in patients with blindness, neuromuscular disease, hemophilia, immunodeficiencies, and cancer. Dunbar et al. review the pioneering work that led the gene therapy field to its current state, describe gene-editing technologies that are expected to play a major role in the field's future, and discuss practical challenges in getting these therapies to patients who need them. Science , this issue p. eaan4672

Holt et al. describe an anti-HIV strategy in which human hematopoietic stem/progenitor cells are modified with zinc-finger nucleases to knock out the viral co-receptor CCR5. Transplantation of these cells into mice confers resistance to HIV, as shown by higher human T-cell counts and lower viral loads compared with animals that received unmodified cells. CCR5 is the major HIV-1 co-receptor, and individuals homozygous for a 32-bp deletion in CCR5 are resistant to infection by CCR5-tropic HIV-1. Using engineered zinc-finger nucleases (ZFNs), we disrupted CCR5 in human CD34+ hematopoietic stem/progenitor cells (HSPCs) at a mean frequency of 17% of the total alleles in a population. This procedure produces both mono- and bi-allelically disrupted cells. ZFN-treated HSPCs retained the ability to engraft NOD/SCID/IL2rγnull mice and gave rise to polyclonal multi-lineage progeny in which CCR5 was permanently disrupted. Control mice receiving untreated HSPCs and challenged with CCR5-tropic HIV-1 showed profound CD4+ T-cell loss. In contrast, mice transplanted with ZFN-modified HSPCs underwent rapid selection for CCR5−/− cells, had significantly lower HIV-1 levels and preserved human cells throughout their tissues. The demonstration that a minority of CCR5−/− HSPCs can populate an infected animal with HIV-1-resistant, CCR5−/− progeny supports the use of ZFN-modified autologous hematopoietic stem cells as a clinical approach to treating HIV-1.

We compared the efficiency of transduction by an HIV-1-based lentiviral vector to that by a Moloney murine leukemia virus (MLV) retroviral vector, using stringentin vitroassays of primitive, quiescent human hematopoietic progenitor cells. Each construct contained the enhanced green fluorescent protein (GFP) as a reporter gene. The lentiviral vector, but not the MLV vector, expressed GFP in nondivided CD34⁺cells (45.5% GFP⁺) and in CD34⁺CD38⁻cells in G₀(12.4% GFP⁺), 48 hr after transduction. However, GFP could also be detected short-term in CD34⁺cells transduced with a lentiviral vector that contained a mutated integrase gene. The level of stable transduction from integrated vector was determined after extended long-term bone marrow culture. Both MLV vectors and lentiviral vectors efficiently transduced cytokine-stimulated CD34⁺cells. The MLV vector did not transduce more primitive, quiescent CD34⁺CD38⁻cells (n= 8). In contrast, stable transduction of CD34⁺CD38⁻cells by the lentiviral vector was seen for over 15 weeks of extended long-term culture (9.2 ± 5.2%,n= 7). GFP expression in clones from single CD34⁺CD38⁻cells confirmed efficient, stable lentiviral transduction in 29% of early and late-proliferating cells. In the absence of growth factors during transduction, only the lentiviral vector was able to transduce CD34⁺and CD34⁺CD38⁻cells (13.5 ± 2.5%,n= 11 and 12.2 ± 9.7%,n= 4, respectively). The lentiviral vector is clearly superior to the MLV vector for transduction of quiescent, primitive human hematopoietic progenitor cells and may provide therapeutically useful levels of gene transfer into human hematopoietic stem cells.

We describe studies of gene transfer and expression of the human glucocerebrosidase cDNA by a Moloney murine leukemia virus (MoMuLV)-based retroviral vector in a murine gene transfer/bone marrow transplant (BMT) model. Pluripotent hematopoietic stem cells (HSCs) were assayed as the colony-forming units, spleen (CFU-S) generated after serial transplantation. Transcriptional expression from the MoMuLV long-terminal repeat (LTR) was detected at a high level in the primary (1 degree) CFU-S and tissues of reconstituted BMT recipients. However, we observed transcriptional inactivity of the proviral MoMuLV-LTR in > 90% of the secondary (2 degrees) CFU-S and in 100% of the tertiary (3 degrees) CFU-S examined. We have compared the methylation status of the provirus in the 1 degree CFU-S, which show strong vector expression, to that of the transcriptionally inactive provirus in the 2 degrees and 3 degrees CFU-S by Southern blot analysis using the methylation-sensitive restriction enzyme Sma I. The studies demonstrated a 3- to 4-fold increase in methylation of the Sma I site in the proviral LTR of 2 degrees and 3 degrees CFU-S compared to the transcriptionally active 1 degree CFU-S. These observations may have important implications for future clinical applications of retroviral-mediated gene transfer into HSCs, where persistent gene expression would be needed for an enduring therapeutic effect.