What is CRISPR in brief?
Derived from archaea and bacteria, CRISPR/Cas9 system consists of an endonuclease Cas9, CRISPR RNA (crRNA) and the trans-activating crRNA (tracrRNA). These molecules could recognize a target sequence and assemble together to form ribonucleoprotein complex for mediating gene editing. When used as an editing tool, a single guide RNA is designed and used in place of the crRNA and tracrRNA, which serve to enhance the robustness and convenience of targeting.
Now, we are empowered to edit the genome of our offspring – to correct them from inheritable genetic diseases (i.e. Down’s syndrome) and if we like, add in one or a few of our desirable traits. This has been made easier with the power of the newly-discovered CRISPR technology. Like other genome editing tools that have long been challenged with bioethical concerns, CRISPR emergence has once again cast unseen anxiety, caution and yet anticipation in the future of translational therapeutics for human diseases. Here, I shall briefly summed up what are the factors involved in the CRISPR dilemma.
1. Unforseen challenges: Low success rate and off-target effects
In April this year, a group from Sun Yat-sen University in China was the first to attempt CRISPR-editing on human embryos . The embryos used were non-viable to begin with, and so were selected for research. CRISPR/Cas9 tool was used to target the Human beta-globin gene (HBB) as a step towards correcting Beta-thalassaemia, a common blood disorder in China population and is potentially fatal. Despite their efforts, their work was largely unsuccessful. In their approach, a customized CRISPR/Cas enzyme complex specific to HBB gene was designed and injected into single-cell embryos to allow editing of several target regions of the HBB gene. However, only 52% of the embryos were edited by the enzyme complex and among them, only 25% (7 embryos) were successfully edited at the HBB gene [1, 2]. In addition, the group found uncontrollable off-targeting genetic modifications in these embryos, which may lead to harmful mutations and phenotypic changes. With these surprising findings, the group acknowledged that at this stage, CRISPR/Cas9 was too amateur for clinical applications. However, they remained undeterred and have come up with plans to resolve these technical challenges. Leading researchers have warned that such germline modifications are inheritable and may cause unpredictable harmful consequences to future generation .
2. An edge over other genome editing tools
i. Ease and Convenience
The CRISPR/Cas9 components can be applied to edit any gene as long as a guide RNA is designed accordingly to the target gene. This greatly simplifies the methodology of gene editing relative to other existing tools (i.e. TALEN and zinc-finger nucleases) which require the engineering of a guide protein. Moreover, questions on the specificity of CRISPR have driven studies to refine the functions and characteristics of Cas9 . This would help ensure higher precision in target gene modification and to reduce off-target mutation.
ii. Versatile and Widely Applicable
Acting at the cellular level, CRISPR/Cas9 has also found its way into the biological system of monocots and dicot plants [4, 5], drosophila , mice , rats  as well as in higher mammals like pig and sheep . Just to name a few of its vast applications, stronger resistance crops against fungal growth are under study by using CRISPR to target multiple copies of the plant gene [4, 5]. CRISPR-mediated knock-in mice with introduction of human genes to create humanized system can be used to study cancer, model Infectious diseases, correct blood disorders, as well as improve on antibiotic resistance . By tweaking the Myostatin gene of pig embryos, pigs with double-muscled can be produced and this alternative offers more meat to the consumers .
By Editor: Shirley Lam