CRISPR-Cas9 has made gene editing very easy. However, the tool has been
found to be less precise than previously assumed as it leads to unwanted
mutations in a cell's DNA. Researchers at Delft University of Technology
(TUDelft) have now identified a mechanism that causes such mutations when
CRISPR-Cas9 is used incorrectly. This can cause dormant genes to become
expressed, which is potentially very dangerous. The researchers created a
checklist based on their findings, and this checklist will prevent the
harmful mechanism from being activated and makes gene editing using
CRISPR-Cas9 safer.

Gene editing experts using CRISPR-Cas9 can change part of the genome by
introducing a new DNA sequence. The Cas protein cuts open the DNA at a
targeted spot, after which the cell is expected to repair its DNA using this
new string of genetic material. Thus, new genes can be introduced. "When the
repair mechanism uses the other chromosome as a template instead of the
newly introduced string of DNA, the editing will not be successful", said
researcher Dr. Jean-Marc Daran. Since repair using the other chromosome is
far more efficient, repair using the intended DNA fragment almost never
occurs. Worse than that, loss of heterozygosity can occur, which can have
serious consequences.

The discovery of the repair mechanism that blocks gene editing was made
through the research of Arthur Gorter de Vries, a PhD candidate at Delft
University of Technology who was working on brewer's yeast. He tried to
determine how domestication of brewer's yeast ultimately led to the modern
strains of yeast that beer producers now use. As he was trying to remove a
certain gene to determine its function, he could not confirm that the gene
was removed successfully and noticed that the cells were behaving
erratically. Gorter de Vries was targeting only one of the chromosomes in
yeast, and further experiments showed that the yeast cells were using the
other copy of the targeted chromosome as a template to repair the DNA that
he had been trying to remove.

[www.tudelft.nl]
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