CRISPR Code Cracked For Precise Human Genome Editing
CRISPR Cas9 has been a trending technique effective in editing genes in various organisms. Tons of experiments have been conducted using CRISPR, latest being the CRISPR baby born in China claimed by a Chinese scientist. However, there has been no lead in predicting the editing outcome in specific sites. In a major breakthrough, scientists at the Francis Crick Institute have unveiled a simple set of rules determining the precision of CRISPR/Cas9 genome editing in human cells. This new discovery will pave ways for genome editing both in labs as well as clinics with an increased efficiency and safety. The above findings were published in Molecular Cell.
CRISPR has been a topic of discussion Worldwide owing to its ethical usage. Pertaining to the unpredictable outcome of genome editing which may result in random insertions & deletions of DNA fragments. The effects of CRISPR editing was studied in 450 genes at 1491 target sites in human cells. The researchers concluded that the outcome of the CRISPR based genome editing depends on a simple rule, a single genetic ‘letter’ occupying a particular position in the region recognized by the ‘guide RNA’ to direct Cas9 for precision editing.
Synthetic molecules were designed called Guide RNAs, that comprises of 20
A,T,G,C’s designed specifically to bind to a specific fragment of the target gene DNA. The genetic letters in Guide RNA bind complementarily and stick firmly with A binding to T and C binding to G. Cas-9 enzyme guided by RNA molecule further scans the genome until the region of interest is located. Upon locating the correct DNA sequence the RNA molecule sticks to it, while cas9 cuts the DNA fragment into three letters from the end of the target sequence. Pieces of genetic code are either inserted or deleted randomly in an attempt by the cells to repair the break.
As per the above experiment, researchers concluded that the result of CRISPR genome editing depends on the fourth letter from the end of the RNA guide that is adjacent to the cutting site. They further noted that if this fourth letter is either A or T, precise genetic insertion can be expected. Whereas if it’s a C, it will lead to a relatively precise deletion and a G will lead to many more imprecise deletions. Thus by avoiding editing sites containing G as the fourth letter after cuts made will lead to predictable genome editing results.
The researchers also discovered the outcome of gene editing is also affected by how the target DNA open or close after the cut is made by cas9. Upon addition of compounds that force DNA to open and by allowing Cas9 to scan the genome, more efficient editing takes place, which could help when modifications are needed to be introduced in particularly closed genes.
Crick group leader, Paola Scaffidi said, “The effects of CRISPR were thought to be unpredictable and seemingly random, but by analyzing hundreds of edits we were shocked to find that there are actually simple, predictable patterns behind it all. This will fundamentally change the way we use CRISPR, allowing us to study gene function with greater precision and significantly accelerating our science.”
Read Further: Chakrabarti et al., Target-Specific Precision of CRISPR-Mediated Genome Editing, Molecular Cell (2018), https://doi.org/10.1016/j.molcel.2018.11.031