Gene-Editing Unintentionally Adds Bovine DNA, Goat DNA, and Bacterial DNA, Mouse Researchers Find

The gene-editing of DNA inside living cells is considered by many to be the preeminent technological breakthrough of the new millennium. Researchers in medicine and agriculture have rapidly adopted it as a technique for discovering cell and organism functions. But its commercial prospects are much more complicated.

Gene-editing has many potential uses. These include altering cells to treat human disease, altering crops and livestock for breeding and agriculture. Furthermore, in a move that has been widely criticised, Chinese researcher He Jiankui claims to have edited human babies to resist HIV by altering a gene called CCR5.

For most commercial applications gene-editing’s [purported] appeal is simplicity and precision: it alters genomes at precise sites and without inserting foreign DNA. This is why, in popular articles, gene-editing is often referred to as ‘tweaking’.

The tweaking narrative, however, is an assumption and not an established fact. And it recently suffered a large dent. In late July researchers from the US Food and Drug Administration (FDA) analysed the whole genomes of two calves originally born in 2016. The calves were edited by the biotech startup Recombinetics using a gene-editing method called TALENS (Norris et al., 2019). The two Recombinetics animals had become biotech celebrities for having a genetic change that removed their horns. Cattle without horns are known as ‘polled’. The calves are well-known because Recombinetics has insisted that its two edited animals were extremely precisely altered to possess only the polled trait.

However, what the FDA researchers found was not precision. Each of Recombinetics’ calves possessed two antibiotic resistance genes, along with other segments of superfluous bacterial DNA. Thus, apparently unbeknownst to Recombinetics, adjacent to its edited site were 4,000 base pairs of DNA that originated from the plasmid vector used to introduce the DNA required for the hornless trait.

The FDA finding has attracted some media attention; mainly focussed on the incompetence of Recombinetics. The startup failed to find (or perhaps look for) DNA it had itself added as part of the editing process. Following the FDA findings, Brazil terminated a breeding program begun with the Recombinetics animals.

These findings have not been reported in the scientific or popular media. But they are of great consequence from a biosafety perspective and therefore for the commercial and regulatory landscape of gene-editing. They imply, at the very least, the need for strong measures to prevent contamination by stray DNA, along with thorough scrutiny of gene-edited cells and gene-edited organisms. And, as the Recombinetics case suggests, these are needs that developers themselves may not meet.But FDA’s findings are potentially trivial besides another recent discovery about gene-editing: that foreign DNA from surprising sources can routinely find its way into the genome of edited animals. This genetic material is not DNA that was put there on purpose, but rather, is a contaminant of standard editing procedures….

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