Combinatorial CRISPR/Cas12a excises targeted loci in the mammalian genome at very low efficiency
This study demonstrates that while CRISPR/Cas12a can process multiple gRNAs from a single promoter, its efficiency in excising targeted genomic loci via paired guide-induced double-strand breaks is too low to support sensitive negative selection assays in mammalian cells.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Most of the human genome is not a blueprint for making proteins, the building blocks of our bodies. Instead, the vast majority of our DNA consists of noncoding sequences that act more like switches, dimmers, and instruction manuals, telling genes when to turn on or off. Scientists have long suspected that these hidden regions hold the keys to understanding many diseases and traits, yet studying them is difficult. The standard tool for editing genes, known as CRISPR, works by making a single cut in the DNA, which usually results in a small, messy error that breaks a gene's function. While this is excellent for turning off a protein-coding gene, it is often too blunt an instrument for noncoding regions, where the goal is to remove a specific segment of DNA entirely to see what happens when it is gone. To truly understand these regions, researchers need a way to snip out a precise chunk of the genome and watch the cell react, a process that requires making two cuts at once and hoping the cell deletes the piece between them.
A team of researchers set out to test whether a specific version of the CRISPR system, called Cas12a, could perform this delicate double-cut operation efficiently enough to be used in large-scale experiments. Unlike the more common CRISPR tool, Cas12a has a unique ability to process a long string of instructions into many separate cutting guides from a single starting point. This feature suggested it might be the perfect candidate for a high-throughput screen, where thousands of different DNA segments are tested simultaneously to find which ones are essential for a cell's survival. The researchers designed a massive library of genetic instructions intended to target the regions just before and just after specific sections of DNA in both essential genes, which cells cannot live without, and nonessential genes, which cells can survive without. The plan was simple: if the system worked, the Cas12a enzyme would make two cuts, the cell would delete the segment between them, and the cells carrying these instructions would die if that segment was vital.
The team introduced these instructions into millions of human skin cancer cells and watched how the population changed over three weeks. They expected that if the system successfully removed the essential segments, the cells carrying those specific instructions would disappear from the population, just as they do when a gene is directly broken. However, the results were far from what they hoped. While the controls worked perfectly—cells with instructions to directly break essential genes died as expected—the cells with instructions to remove the segments between two cuts showed almost no change in their numbers. The DNA segments were rarely being removed. The researchers analyzed the data to see if they could find a pattern that might make the system work better. They looked at the specific sequences of the instructions, the distance between the two cuts, the direction the cuts faced, and even the chemical composition of the DNA nearby. None of these factors made a meaningful difference. The efficiency of the double-cut method remained so low that it was indistinguishable from random chance, failing to produce the clear signal needed to identify which DNA segments were critical.
The study also compared their results with a similar approach used by another research group that combined two different CRISPR enzymes to achieve the same goal. Even with that more complex setup, the success rate for removing DNA segments was too low to be useful for large-scale screening. The authors concluded that while the Cas12a system is powerful for making single cuts, it does not currently possess the reliability required to excise targeted loci in a way that allows scientists to screen thousands of genetic regions at once. The failure likely stems from the fact that the two cuts needed to remove a segment do not happen at the exact same time in every cell, or that the cell's repair mechanisms do not always join the ends in a way that deletes the middle piece. Until the tools can be improved to make these simultaneous cuts more consistently, this specific method of removing DNA segments remains unsuitable for the broad, systematic exploration of the noncoding genome that scientists had envisioned.
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