Gene Editing in Adult Hematopoietic Stem Cells

Over the last years, an important development has allowed the scientific community to address a precise and accurate modification of the genome. The first probe of concept appeared with the design and use of engineered zinc-finger nucleases (ZFNs), which was expanded later on with the discovery and engineering of meganucleases and transcrip‐ tion activator-like effector nucleases (TALENs) and finally democratized and made easily available to the whole scientific community with the discovery of the clustered regular‐ ly interspaced short palindromic repeats (CRISPR)/Cas9 nuclease combination technol‐ ogy. The availability of these tools has allowed a precise gene editing, such as knockout of a specific gene or the correction of a defective gene by means of homologous recombi‐ nation (HR), taking advantage of the endogenous cell repair machinery. This process was already known and used but was inefficient—efficiency that has been increased more than 100-fold with the addition of the mentioned specific nucleases to the process. Apart from the proper design of the nucleases to recognize and cut the selected site in the cell genome, two main goals need to be adequately addressed to optimize its function: the delivery of the tools into the desired cells and the selection of those where the gene editing process has occurred correctly. Both steps can be easily solved when the source of cells is extensive or can be expanded and manipulated in vitro extensively, such as immortal‐ ized cell lines or pluripotent stem cells (embryonic stem cells and induced pluripotent stem cells). However, both steps are critical in the case of primary cells, replenishment of all hematopoietic cells for the lifespan of the animals by means of two fundamental properties: self-renewal and multipotency. HSC population is then the ideal target for the correction of hematopoietic genetic diseases and also for the knockout of the responsible genes to in vitro and in vivo model those hematopoietic diseases. This rare population cannot be expanded and its in vitro manipulation and culture negatively affects their fundamental properties of self-renewal and multipotency. These factors challenge the application of gene editing to HSCs. Important efforts are now ongoing trying to optimize the protocols of gene delivery and selection for HSCs. This chapter will review and discuss how researchers are trying to solve them, all attempts that are ongoing and the potential application of the technology to the patients affected with hematopoietic genetic diseases.