The findings, published in the journal Nature Biotechnology on Monday, have safety implications for gene therapies that are being developed using CRISPR/Cas9 - a type of molecular scissor technology that can be used to edit DNA.
He urged "anyone thinking of using this technology for gene therapy" to proceed with great caution, and to look very carefully "to check for possible harmful effects".
It is already used widely in scientific research and is seen by many as a promising way to create potential genome editing treatments for diseases such as HIV, cancer or sickle cell disease.
Bradley's team carried out a full systematic study in both mouse and human cells and discovered that CRISPR/Cas9 frequently caused extensive mutations including large genetic rearrangements such as DNA deletions and insertions.
These could lead to important genes being switched on or off - as intended by the therapies - but could also have major unexpected implications, the scientists said.
Commenting on the findings, Robin Lovell-Badge, a stem cell expert at Britain's Francis Crick Institute, said the work highlighted the need for very careful work when using to genome editing "to verify that the alterations to the DNA sequence are those, and only those, that had been designed to occur".
"But the results give no reason to panic or to lose faith in the methods when they are carried out by those who know what they are doing," he added.
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