The Fate of Transgenes
in the Human Gut
Nature Biotechnology v.22, n.2, 1feb2004
Gut microbes that cannot be recovered in artificial culture may acquire and harbor genes from genetically modified plants.
John Heritage is in the Division of Microbiology, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK. e-mail; email@example.com
Can transgenic DNA in a genetically modified (GM) crop be transferred to the people or animals that eat the crop or to their intestinal microflora (Fig. 1)? In this issue, Netherwood et al1 report that microbes found in the small bowel of people with ileostomies (that is, resection of the terminal ileum and diversion of digesta to a colostomy bag) are capable of acquiring and harboring DNA sequences from GM plants. This finding raises important questions for those charged with risk assessment of transgenic plants destined for food use.
Recently, much attention has been focused on the potential of transgenes to escape from GM plants to wild relatives2-4. But relatively little work has been devoted to the possibility of transgene transfer to the animals consuming the plant or to their commensal micro-flora. Even less work has been carried out on the fate of plant DNA when crops are eaten by humans. It has been argued that transgenes are the same as any other DNA that we consume and that if gene transfer from plants to humans or animals were commonplace, we should all be green by now. Intriguingly, it is chloroplast DNA that is found in the lymphocytes of animals eating both GM and conventionally bred plant material5, and there is evidence that bacteria in the oral flora remain competent for genetic transformation when suspended in saliva6.
In its 2003 GM Science Review, the UK government concluded that trans-kingdom transfer of DNA from GM plants to bacteria is "unlikely to occur because of a series of well-established barriers" 7 and illustrated support for this position from experimental evidence in peer-reviewed literature. The review also concluded that transgenic DNA is no different from other DNA consumed as part of the diet and that it will have a similar fate. Again, these conclusions were based on a review of the published scientific evidence, which indicates that ingested DNA is subject to degradation but that the degradative process is not necessarily complete. Much of this work has been done using animal studies, and very little is known about the process in humans.
Netherwood et al. have taken a significant step to address the lack of human studies. They studied 12 healthy volunteers and seven ileostomists given a meal of GM soya that contained the 5-enol-pyruvyl shikimate-3-phosphate synthase (epsps) transgene, which encodes resistance to the herbicide glyphosate. In the healthy subjects, DNA from the transgenic plant material was degraded completely after passage through the colon.
Figure 1 A possible route for transfer of DNA from plant cells in the human diet to bacteria. Some DNA in food is degraded during cooking and processing, but the remainder is ingested intact. Consumed DNA is largely hydrolyzed during digestion. Netherwood et al. provide evidence that intact transgenic DNA can be recovered in the human ileum and taken up by bacteria in this environment.
Interestingly, however, transgenic DNA sequences were recovered by PCR from the digesta of all seven ileostomists. In six of these subjects, a PCR product spanning the entire gene was detected. In one case, it was estimated that nearly 4% of the transgenic DNA in the test meal was recovered from the ileostomy fluid. Thus, quite large fragments of DNA can survive passage through the stomach-if shown only for people with a gastrointestinal pathology.
Ileostomists clearly may not be representative of the general population as they have suffered a disease requiring radical surgery. Nevertheless, they have been validated previously as a faithful model for studying other aspects of digestion. Further research will be needed to determine the relevance of the new results for the population at large.
Perhaps a more significant and puzzling finding of this work is evidence that transgenic DNA had crossed kingdom barriers from a GM plant source to intestinal microflora, even before the subjects' participation in the study. The transgenic sequences were not detected in the original samples of ileostomy fluid taken before consumption of the GM soya meal, but they could be detected at very low copy number once bacteria in these same samples were enriched through serial passage in Luria broth.
Great care was taken to avoid sample contamination; although contamination cannot be ruled out, the pattern of results makes this explanation unlikely. The epsps gene used in GM soya has a bacterial origin, but the sequence was modified to optimize expression in plants. Sequence analysis of the PCR products revealed that the target amplified from cultivated digesta was identical to the plant epsps gene rather than to its bacterial counterpart.
Microbiological experiments, including differential cultivation, provided good evidence that in all probability the bacterium harboring the transgenic epsps sequence is indifferent to the presence of oxygen in the culture medium and is dependent for its nutrition upon another organism present in the sample and subsequent liquid subcultures. Is it likely that a bacterium that cannot be isolated in pure culture is capable of acquiring transgenic DNA and harboring it through several passages in liquid culture, albeit at very low frequency? It should be remembered that conventional culture techniques cannot recover more than a tiny minority of the microbes in the gastrointestinal tract. We are only just beginning to learn how to characterize such organisms.
Netherwood et al. could have done more to characterize the putative trans-kingdom transfer. They do not report attempts to see whether the transgenic DNA present in cultured digesta, and presumably found in the bacterium that will not grow in pure culture on a solid medium, was being expressed-although no full-length genes were amplified from these samples This could have been accomplished with relative ease using RT-PCR.
There is also no description of the sequences flanking the epsps gene, so it is impossible to determine the context in which the PCR target is found in these cultures. Nevertheless, on balance, the data presented in the paper support the conclusion that gene flow from transgenic plants to the gut microflora does occur. Furthermore, because transfer events seem to have occurred in three of the seven subjects examined, it maybe that trans-kingdom gene transfers are not as rare as suggested by the UK GM Science Review Panel7. This observation is significant, and it is imperative that the transfer events be characterized more fully, particularly with a view to understanding the stability in cultivated ileal digesta of plant transgenes and native genes, the context in which these genes are found and their ability to be expressed.
Should the findings of Netherwood et al. influence risk assessment of GM crops? I believe that the authors strike exactly the right position here. They propose that the gene transfer events from transgenic plants to gut microflora for which they provide evidence are highly unlikely to alter gastrointestinal function or endanger human health. I would conclude, however, that whereas this may be true for the construct examined by Gilbert's group, it may not be true in other cases, such as genes that encode resistance to antibiotics used in human medicine.
Netherwood et al. call on risk assessors to consider the possibility of trans-kingdom gene flow in the future safety assessment of GM foods1. I endorse that conclusion and would extend it by adding that every case must be considered on its own merits.
- Netherwood T, et al. Nat. Biotechnol. 22, 204-209 (2004).
- Wilkinson, M.J. et al. Science 302, 457-459 (2003).
- Anon. Phil. Trans. R. Soc. Lond B 358, 1775-1913 (2003).
- Andow, D.A. Nat. Biotechnol. 21, 1453-1454 (2003).
- Einspanier, R. et al. Eur. Food Res. Technol. 212, 129-134 (2001).
- Mercer, D.K., Scott, K.P., Bruce-Johnson, W.A., Glover, L.A. & Flint, H.J. Appl. Environ. Microbiol. 65, 6-10 (1999).
- UK GM Science Review Panel. GM Science Review. First Report. An Open
Review of the Science Relevant to GM Crops and Food Based on Interests and
Concerns of the Public (UK Government, London, 2003). Available online (
http://www.gmsciencedebate.org.uk/report/pdf/gmsci-reportl-full.pdf). Last accessed 22 December 2003.
Updated URL: http://www.gmsciencedebate.org.uk/report/ 9aug2007