CRISPR/Cas9 gene editing technology has greatly advanced the field of biomedicine as exemplified by Casgevy for the treatment of sickle cell disease. All technologies have inherent limitations. CRISPR/Cas9 is no exception.
Among different DNA silencing technologies, CRISPR is by far the most advanced platform in its reduced off-targeting effect, enhanced DNA excision, and gene silencing capabilities. Most significant of all, CRISPR is not just a tool that cuts DNA, it is also capable of editing genes through the introduction of donor DNA to produce the correct copy of a gene of interest to replace the faulty gene. Despite these advantages, CRISPR confronts challenges, especially in terms of efficient delivery into complex mammalian cells and on-target editing of intended genes.35
Among many reasons, one contributing to the challenge of on-target gene editing in mammalian systems stems from the complexity of the very mammalian system. The extensive size of the human genome plays a role in CRISPR's off-target effect. The 23 pairs of chromosomes harbor a wide range of sizes from 50 to 300 million base pairs that collectively make up approximately 3 billion base pairs in total. The size of the human genome matters, for the bigger the genome size, the greater the possibility for identical or near-identical sequences vis-à-vis the sequence designed to be targeted by CRISPR/Cas9.
35. Peng, R., Lin, G., & Li, J. (2016). Potential pitfalls of CRISPR/Cas9-mediated genome editing. The FEBS journal, 283(7), 1218–1231. https://doi.org/10.1111/febs.13586
