Editorial: Ex-vivo and in-vivo genome engineering for metabolic and neurometabolic diseases.

Recent advances in genome modification tools have led to a growing interest in using genome engineering as a therapeutic solution for many diseases. At the forefront of this revolution is the CRISPR-Cas9 technology, which made genome editing broadly accessible and engendered the development of chimeric genome editing tools like base editors and prime editors. To achieve the desired DNA modifications, nucleasebased platforms use cellular DNA repair pathways, such as Homology Directed Repair (HDR), Non-Homologous End Joining (NHEJ), and Microhomology-Mediated End Joining (MMEJ), while prime editors employ an RNA-based reverse transcription mechanism. For therapeutic applications, genome engineering platforms can be used ex vivo and in vivo and can either disrupt coding or regulatory sequences (therapeutic NHEJ) or make precise sequence changes (therapeutic HDR, Base editing, and Prime editing). The most advanced applications of genome editing for human monogenic diseases involve therapeutic NHEJ, which uses Cas9 endonuclease and guide RNAs (gRNAs) to create site-specific double-strand breaks (DSBs), which NHEJ then repairs. This process often results in the insertion/deletion of a few nucleotides (INDELs) or larger deletions, depending on the gRNA design, mostly disrupting, or inactivating the target gene. Therapeutic NHEJ has been successfully applied ex vivo to modify CD34 hematopoietic stem and progenitor cells (HSPCs) from individuals affected by beta-Thalassemia (b-Thal) and Sickle cell disease (SCD), both caused bymutations in the β-globin gene (HBB) (Ledford, 2020; Frangoul et al., 2021). In this strategy, Cas9/gRNAs are used to reactivate the expression of the fetal γ-globin by knocking down the erythroid expression of BCL11A, its key transcriptional repressor. Data from clinical trials confirmed that γ-globin could functionally complement the deficiency of β-globin in the hemoglobin tetramers and exert an anti-sickling function. This approach can be applied to β-Thal and SCD independently from the underlying beta-globin mutations. It is also proving to be safe and effective in OPEN ACCESS