Genome Editing

Genome Editing

Over the last few years, the exuberant development of genome editing has revolutionized research on the human genome, which has enabled investigators to better understand the contribution of a single-gene product to disease in an organism. In the 1970s, the development of genetic engineering (manipulation of DNA or RNA) established a novel frontier in genome editing.

Genome editing (also called gene editing) is a group of technologies that give scientists the ability to change an organism's DNA. These technologies allow genetic material to be added, removed, or altered at particular locations in the genome. Several approaches to genome editing have been developed.

Historically, homologous recombination (HR), in which undamaged homologous DNA fragments are used as templates, has been the approach to realize targeted gene addition, replacement, or inactivation; however, the utility of HR is heavily limited due to its inefficiency in mammalian cells and model organisms.8 After it was discovered that DSBs could raise the incidence of HDR by multiple orders of magnitude, targeted nucleases have been found as an alternative approach to increase the efficiency of HDR-mediated genetic alteration. Once a targeted DSB has been made, HDR may reconstruct the cleaved DNA using an exogenous DNA template analog to the break site sequence.

The development of gene-editing technology for gene therapy, however, proved difficult. Much early progress focused not on correcting genetic mistakes in the DNA but rather on attempting to minimize their consequence by providing a functional copy of the mutated gene, either inserted into the genome or maintained as an extrachromosomal unit (outside the genome). While that approach was effective for some conditions, it was complicated and limited in scope.

In February 2019, medical scientists working with Sangamo Therapeutics, headquartered in Richmond, California, announced the first-ever "in body" human gene editing therapy to permanently alter DNA - in a patient with Hunter Syndrome. Clinical trials by Sangamo involving gene editing using Zinc Finger Nuclease (ZFN) are ongoing.

In order to truly correct genetic mistakes, researchers needed to be able to create a double-stranded break in DNA at precisely the desired location in the more than three billion base pairs that constitute the human genome. Once created, the double-stranded break could be efficiently repaired by the cell using a template that directed the replacement of the “bad” sequence with the “good” sequence. However, making the initial break at precisely the desired location—and nowhere else—within the genome was not easy.

Thanks to the parallel development of single-cell transcriptomics, genome editing, and new stem cell models we are now entering a scientifically exciting period where functional genetics is no longer restricted to animal models but can be performed directly in human samples. Single-cell gene expression analysis has resolved a transcriptional road-map of human development from which key candidate genes are being identified for functional studies. Using global transcriptomics data to guide experimentation, the CRISPR-based genome-editing tool has made it feasible to disrupt or remove key genes in order to elucidate function in a human setting.