Forough Shams
1 
, Hadi Bayat
2,3, Omid Mohammadian
4,3, Somayeh Mahboudi
5, Hassan Vahidnezhad
6,7, Mohsen Soosanabadi
8, Azam Rahimpour
4,3* 1 Department of Medical Biotechnology, School of Advanced Technologies in Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
2 Department of Molecular Genetics, Faculty of Biological Sciences, Tarbiat Modares University, Tehran, Iran
3 Department of Tissue Engineering and Applied Cell Sciences, School of Advanced Technologies in Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
4 Medical Nano-Technology and Tissue Engineering Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran
5 Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran
6 Department of Dermatology and Cutaneous Biology, Sidney Kimmel Medical College, Thomas Jefferson University, Philadelphia, PA, USA
7 Jefferson Institute of Molecular Medicine, Thomas Jefferson University, Philadelphia, PA, USA
8 Department of Medical Genetics, Semnan University of Medical Sciences, Semnan, Iran
Abstract
Introduction: Clustered regularly interspaced short palindromic repeat and its associated protein (CRISPR-Cas)-based technologies generate targeted modifications in host genome by inducing site-specific double-strand breaks (DSBs) that can serve as a substrate for homology-directed repair (HDR) in both in vitro and in vivo models. HDR pathway could enhance incorporation of exogenous DNA templates into the CRISPR-Cas9-mediated DSB site. Owing to low rate of HDR pathway, the efficiency of accurate genome editing is diminished. Enhancing the efficiency of HDR can provide fast, easy, and accurate technologies based on CRISPR-Cas9 technologies.
Methods: The current study presents an overview of attempts conducted on the precise genome editing strategies based on small molecules and modified CRISPR-Cas9 systems.
Results: In order to increase HDR rate in targeted cells, several logical strategies have been introduced such as generating CRISPR effector chimeric proteins, anti-CRISPR proteins, modified Cas9 with donor template, and using validated synthetic or natural small molecules for either inhibiting non-homologous end joining (NHEJ), stimulating HDR, or synchronizing cell cycle. Recently, high-throughput screening methods have been applied for identification of small molecules which along with the CRISPR system can regulate precise genome editing through HDR.
Conclusion: The stimulation of HDR components or inhibiting NHEJ can increase the accuracy of CRISPR-Cas-mediated engineering systems. Generating chimeric programmable endonucleases provide this opportunity to direct DNA template close proximity of CRISPR-Cas-mediated DSB. Small molecules and their derivatives can also proficiently block or activate certain DNA repair pathways and bring up novel perspectives for increasing HDR efficiency, especially in human cells. Further, high throughput screening of small molecule libraries could result in more discoveries of promising chemicals that improve HDR efficiency and CRISPR-Cas9 systems.