Historic Overview of Genetic Engineering Technologies for Human Gene Therapy

Abstract

The concepts of gene therapy were initially introduced during the 1960s. Since the early 1990s, more than 1900 clinical trials have been conducted for the treatment of genetic diseases and cancers mainly using viral vectors. Although a variety of methods have also been performed for the treatment of malignant gliomas, it has been difficult to target invasive glioma cells. To overcome this problem, immortalized neural stem cell (NSC) and a nonlytic, amphotropic retroviral replicating vector (RRV) have attracted attention for gene delivery to invasive glioma. Recently, genome editing technology targeting insertions at site-specific locations has advanced; in particular, the clustered regularly interspaced palindromic repeats/CRISPR-associated-9 (CRISPR/Cas9) has been developed. Since 2015, more than 30 clinical trials have been conducted using genome editing technologies, and the results have shown the potential to achieve positive patient outcomes. Gene therapy using CRISPR technologies for the treatment of a wide range of diseases is expected to continuously advance well into the future.

Keywords: CRISPR/Cas9; TALEN; ZFN; gene therapy; genome editing.

Fig. 1

Genome editing tools. Three types of genome editing tools including ZFNs, TALENs, and CRISPR/Cas9 are shown. ZFNs are hybrid proteins using zinc-finger arrays and the catalytic domain of FokI endonuclease. TALENs are hybrid proteins containing the TAL effector backbone and the catalytic domain of FokI endonuclease. The CRISPR/Cas9 system is composed of Cas9 endonuclease and sgRNA. Cas9: CRISPR-associated-9, CRISPR: clustered regularly interspaced palindromic repeats, sgRNA: single-guide RNA, TALENs: transcription activator-like effector nucleases, ZFNs: zinc-finger nucleases.

Fig. 2

In- vivo and ex- vivo strategies of gene therapy. In- vivo and ex- vivo gene transfer strategies are shown. For in- vivo gene transfer, genetic materials containing therapeutic genes, such as viral vectors, nanoparticles, and ribosomes, are delivered directly to the patient, and genetic modification occurs in situ. For ex- vivo gene transfer, the harvested cells are modified by the appropriate gene delivery tools in vitro (e.g., recombinant viruses genome editing technologies). The modified cells are then delivered back to the patient via autologous or allogeneic transplantation after the evaluation of off-target effects.