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Key Points Delivery of ZFNs and donor templates results in high levels of gene correction in human CD34+ cells from multiple sources, including SCD BM. Modified CD34+ cells are capable of engrafting immunocompromised NSG mice and produce cells from multiple lineages.

Summary X-linked hyper-IgM syndrome (XHIM) is a primary immunodeficiency due to mutations in CD40 ligand that affect immunoglobulin class switch recombination and somatic hypermutation. The disease is amenable to gene therapy using retroviral vectors, but dysregulated gene expression resulted in abnormal lymphoproliferation in mouse models, highlighting the need for alternative strategies. Here, we demonstrate the ability of both the TALEN and CRISPR/Cas9 platforms to efficiently drive integration of a normal copy of the CD40L cDNA delivered by Adeno-Associated Virus. Site-specific insertion of the donor sequence downstream of the endogenous CD40L promoter maintained physiologic expression of CD40L while overriding all reported downstream mutations. High levels of gene modification were achieved in primary human hematopoietic stem cells (HSC) as well as in cell lines and XHIM patient-derived T cells. Notably, gene corrected HSC engrafted in immunodeficient mice at clinically-relevant frequencies. These studies provide the foundation for a permanent curative therapy in XHIM.

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Effective medical management for sickle cell disease (SCD) remains elusive. As a prevalent and severe monogenic disorder, SCD has been long considered a logical candidate for gene therapy. Significant progress has been made in moving toward this goal. These efforts have provided substantial insight into the natural regulation of the globin genes and illuminated challenges for genetic manipulation of the hematopoietic system. The initial γ-retroviral vectors, next-generation lentiviral vectors, and novel genome engineering and gene regulation approaches each share the goal of preventing erythrocyte sickling. After years of preclinical studies, several clinical trials for SCD gene therapies are now open. This review focuses on progress made toward achieving gene therapy, the current state of the field, consideration of factors that may determine clinical success, and prospects for future development.

The use of engineered nucleases combined with a homologous DNA donor template can result in targeted gene correction of the sickle cell disease mutation in hematopoietic stem and progenitor cells. However, because of the high homology between the adjacent human β- and δ-globin genes, off-target cleavage is observed at δ-globin when using some endonucleases targeted to the sickle mutation in β-globin. Introduction of multiple double-stranded breaks by endonucleases has the potential to induce intergenic alterations. Using a novel droplet digital PCR assay and high-throughput sequencing, we characterized the frequency of rearrangements between the β- and δ-globin paralogs when delivering these nucleases. Pooled CD34 cells and colony-forming units from sickle bone marrow were treated with nuclease only or including a donor template and then analyzed for potential gene rearrangements. It was observed that, in pooled CD34 cells and colony-forming units, the intergenic β-δ-globin deletion was the most frequent rearrangement, followed by inversion of the intergenic fragment, with the inter-chromosomal translocation as the least frequent. No rearrangements were observed when endonuclease activity was restricted to on-target β-globin cleavage. These findings demonstrate the need to develop site-specific endonucleases with high specificity to avoid unwanted gene alterations.

Gene editing enables the site-specific modification of the genome. These technologies have rapidly advanced such that they have entered common use in experimental hematology to investigate genetic function. In addition, genome editing is becoming increasingly plausible as a treatment modality to rectify genetic blood disorders and improve cellular therapies. Genome modification typically ensues from site-specific double-strand breaks and may result in a myriad of outcomes. Even single-strand nicks and targeted biochemical modifications that do not permanently alter the DNA sequence (epigenome editing) may be powerful instruments. In this review, we examine the various technologies, describe their advantages and shortcomings for engendering useful genetic alterations, and consider future prospects for genome editing to impact hematology.