CRISPR/Cas9-Based Disease Modeling and Functional Correction of Interleukin 7 Receptor Alpha Severe Combined Immunodeficiency in T-Lymphocytes and Hematopoietic Stem Cells.

Interleukin 7 Receptor alpha Severe Combined Immunodeficiency (IL7R-SCID) is a life-threatening disorder caused by homozygous mutations in the IL7RA gene. Defective IL7R expression in humans hampers T cell precursors' proliferation and differentiation during lymphopoiesis resulting in the absence of T cells in newborns, who succumb to severe infections and death early after birth. Previous attempts to tackle IL7R-SCID by viral gene therapy have shown that unregulated IL7R expression predisposes to leukemia, suggesting the application of targeted gene editing to insert a correct copy of the IL7RA gene in its genomic locus and mediate its physiological expression as a more feasible therapeutic approach. To this aim, we have first developed a CRISPR/Cas9-based IL7R-SCID disease modeling system that recapitulates the disease phenotype in primary human T cells and hematopoietic stem and progenitor cells (HSPCs). Then, we have designed a knockin strategy that targets IL7RA exon 1 and introduces through homology-directed repair a corrective, promoterless IL7RA cDNA followed by a reporter cassette through AAV6 transduction. Targeted integration of the corrective cassette in primary T cells restored IL7R expression and rescued functional downstream IL7R signaling. When applied to HSPCs further induced to differentiate into T cells in an Artificial Thymic Organoid system, our gene editing strategy overcame the T cell developmental block observed in IL7R-SCID patients, while promoting full maturation of T cells with physiological and developmentally regulated IL7R expression. Finally, genotoxicity assessment of the CRISPR/Cas9 platform in HSPCs using biased and unbiased technologies confirmed the safety of the strategy, paving the way for a new, efficient, and safe therapeutic option for IL7R-SCID patients.

Preclinical Evaluation of a Novel TALEN Targeting CCR5 Confirms Efficacy and Safety in Conferring Resistance to HIV-1 Infection.

Therapies to treat patients infected with human immunodeficiency virus (HIV) aim at preventing viral replication but fail to eliminate the virus. Although transplantation of allogeneic CCR5Δ32 homozygous stem cell grafts provided a cure for a few patients, this approach is not considered a general therapeutic strategy because of potential side effects. Conversely, gene editing to disrupt the C-C chemokine receptor type 5 (CCR5) locus, which encodes the major HIV coreceptor, has shown to confer resistance to CCR5-tropic HIV strains. Here, an engineered transcription activator-like effector nuclease (TALEN) that enables efficient CCR5 editing in hematopoietic cells is presented. After transferring TALEN-encoding mRNA into primary CD4+ T cells, up to 89% of CCR5 alleles are disrupted. Genotyping confirms the genetic stability of the CCR5-edited cells, and genome-wide off-target analyses established the absence of relevant mutagenic events. When challenging the edited T cells with CCR5-tropic HIV, protection in a dose-dependent manner is observed. Functional assessments reveal no significant differences between edited and control cells in terms of proliferation and their ability to secrete cytokines upon exogenous stimuli. In conclusion, a highly active and specific TALEN to disrupt CCR5 is successfully engineered, paving the way for its clinical application in hematopoietic stem cell grafts.

Targeted gene correction of human hematopoietic stem cells for the treatment of Wiskott - Aldrich Syndrome.

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency with severe platelet abnormalities and complex immunodeficiency. Although clinical gene therapy approaches using lentiviral vectors have produced encouraging results, full immune and platelet reconstitution is not always achieved. Here we show that a CRISPR/Cas9-based genome editing strategy allows the precise correction of WAS mutations in up to 60% of human hematopoietic stem and progenitor cells (HSPCs), without impairing cell viability and differentiation potential. Delivery of the editing reagents to WAS HSPCs led to full rescue of WASp expression and correction of functional defects in myeloid and lymphoid cells. Primary and secondary transplantation of corrected WAS HSPCs into immunodeficient mice showed persistence of edited cells for up to 26 weeks and efficient targeting of long-term repopulating stem cells. Finally, no major genotoxicity was associated with the gene editing process, paving the way for an alternative, yet highly efficient and safe therapy.

Genome editing for blood disorders: state of the art and recent advances.

In recent years, tremendous advances have been made in the use of gene editing to precisely engineer the genome. This technology relies on the activity of a wide range of nuclease platforms - such as zinc-finger nucleases, transcription activator-like effector nucleases, and the CRISPR-Cas system - that can cleave and repair specific DNA regions, providing a unique and flexible tool to study gene function and correct disease-causing mutations. Preclinical studies using gene editing to tackle genetic and infectious diseases have highlighted the therapeutic potential of this technology. This review summarizes the progresses made towards the development of gene editing tools for the treatment of haematological disorders and the hurdles that need to be overcome to achieve clinical success.

Delivery of Designer Epigenome Modifiers into Primary Human T Cells.

The development of tools which allow for the precise alterations of the epigenetic landscape in desired genomic locations presents exciting possibilities toward further understanding how gene expression is regulated and opportunities to harness these properties for therapeutic purposes. In contrast to gene knockout strategies, targeted epigenome modifications, such as editing of DNA methylation, can mediate gene expression modulation without changing the genomic sequence. Thereby, in a therapeutic context, this strategy may offer a safer route as compared to gene disruption using designer nucleases that, to reach high efficiencies, relies on the occurrence of random mutations to inactivate the target gene. In addition, therapeutic benefit is influenced not only by the intrinsic safety and efficacy of the tools used but also by methods that allow efficient and non-toxic transfer of the selected reagents in the target cells. Here, we describe a detailed protocol, for safe delivery of TALE-based designer epigenome modifiers in the form of in vitro transcribed mRNA into primary human CD4+ T cells to efficiently silence the expression of an exemplary human gene (i.e., CCR5).

Designer epigenome modifiers enable robust and sustained gene silencing in clinically relevant human cells.

Targeted modulation of gene expression represents a valuable approach to understand the mechanisms governing gene regulation. In a therapeutic context, it can be exploited to selectively modify the aberrant expression of a disease-causing gene or to provide the target cells with a new function. Here, we have established a novel platform for achieving precision epigenome editing using designer epigenome modifiers (DEMs). DEMs combine in a single molecule a DNA binding domain based on highly specific transcription activator-like effectors (TALEs) and several effector domains capable of inducing DNA methylation and locally altering the chromatin structure to silence target gene expression. We designed DEMs to target two human genes, CCR5 and CXCR4, with the aim of epigenetically silencing their expression in primary human T lymphocytes. We observed robust and sustained target gene silencing associated with reduced chromatin accessibility, increased promoter methylation at the target sites and undetectable changes in global gene expression. Our results demonstrate that DEMs can be successfully used to silence target gene expression in primary human cells with remarkably high specificity, paving the way for the establishment of a potential new class of therapeutics.