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Recent Evidence Confirms Risks
of Horizontal Gene Transfer
by Mae-Wan Ho, Institute of Science in Society
Sexually reproducing organisms pass their DNA only “vertically,” from one generation to the next. But bacteria and viruses exchange bits of DNA “horizontally,” from one organism to another. What happens when artificially introduced genes get transferred horizontally? Mae-Wan Ho of the Institute of Science in Society summarizes the evidence.
The oft-repeated refrain that “transgenic DNA is just like ordinary DNA” is false. Transgenic DNA is in many respects optimized for horizontal gene transfer. It is designed to cross species barriers and to jump into genomes, and it has homologies to the DNA of many species and their genetic parasites (plasmids, transposons and viruses), thereby enhancing recombination with all of them.  Transgenic constructs contain new combinations of genes that have never existed, and they also amplify gene products that have never been part of our food chain. 
The health risks of horizontal gene transfer include:
- Antibiotic resistance genes spreading to pathogenic bacteria;
- Disease-associated genes spreading and recombining to create new viruses and bacteria that cause diseases;
- Transgenic DNA inserting into human cells, triggering cancer.
The risk of cancer is highlighted by the recent report that gene therapy—genetic modification of human cells—claimed its first cancer victim.  The procedure, in which bone marrow cells are genetically modified outside the body and re-implanted, was previously thought to avoid creating infectious viruses and causing cancer, both recognized major hazards of gene therapy.
The risk of cancer is highlighted by the…report that gene therapy—genetic modification of human cells—claimed its first cancer victim.
The transgenic constructs used in genetic modification are basically the same whether it is of human cells or of other animals and plants. An aggressive promoter from a virus is often used to boost the expression of the transgene—in animal and human cells from the cytomegalovirus that infects mammalian cells, and in plants the 35S promoter from the cauliflower mosaic virus (CaMV) that infects Cruciferae plants.
Unfortunately, although the CaMV virus is specific for plants, its 35S promoter is active in species across the living world, human cells included, as we discovered in the scientific literature dating back to 1989. Plant geneticists who have incorporated the promoter into practically all GM crops now grown commercially are apparently unaware of this crucial information. 
In 1999, another problem with the CaMV 35S promoter was identified: it has a “recombination hotspot” where it tends to break and join up with other DNA.  Since then, we have continued to warn our regulators that the CaMV 35S promoter will be extra prone to spread by horizontal gene transfer and recombination [6–8]. The recent controversy over the transgenic contamination of the Mexican landraces  hinges on observations suggesting that the transgenic DNA with the CaMV 35S promoter is “fragmenting and promiscuously scattering throughout the genome” of the landraces, observations that would be consistent with our expectations. 
Research results released early in 2002 by the Food Standards Agency  indicate that transgenic DNA from GM soya flour, eaten in a single hamburger and milk shake meal, was found transferred to the bacteria in the gut contents from the colostomy bags of human volunteers.
…although the CaMV virus is specific for plants, its 35S promoter is active in species across the living world…
The Agency dismissed the findings and downplayed the risks. The comments, “it is extremely unlikely that genes from genetically modified (GM) food can end up in bacteria in the gut of people who eat them,” and “the findings had been assessed by several Government experts who had ruled that humans were not at risk,” are seriously misleading.
First the experimental design stacked the odds heavily against finding a positive result. For example, the probe for transgenic DNA covered only a tiny fraction of the entire construct. So only a correspondingly tiny fraction of the actual transfers would ever be detected, especially given the well-known tendency of transgenic constructs to fragment and rearrange.
Second, there was no attempt to check for transgenic DNA in the blood and blood cells, although scientific reports dating back to the early 1990s indicated transgenic DNA could pass through the intestine and the placenta, and become incorporated into the blood cells, liver and spleen cells and cells of the foetus and newborn. 
The observation in the FSA report  that no transgenic DNA was found in the faeces of the “healthy volunteers,” far from being reassuring, raises the worrying possibility that the transgenic DNA has all been taken up into the intestinal cells and/or passed into the bloodstream.
Research results…indicate that transgenic DNA from GM soya flour, eaten in a single hamburger…was found transferred to the bacteria in the gut…
Third, no attempt was made to address the limitations of the detection method and the scope of the investigation failed completely in assessing the real risks. False assurances were made that “humans were not at risk.”
Another research project on horizontal gene transfer commissioned by the Ministry of Agriculture, Fisheries and Food (MAFF), the predecessor to the Food Standards Agency, concerns Agrobacterium tumefaciens, the soil bacterium that causes crown gall disease, which has been developed as a major gene transfer vector for making transgenic plants. Foreign genes are typically spliced into T-DNA—part of a plasmid called Ti (tumour-inducing)—that’s integrated into plant genome.bh.
It turns out that Agrobacterium injects T-DNA into plant cells in a process that strongly resembles conjugation, i.e., mating between bacterial cells, and all the necessary signals and genes involved are interchangeable with those for conjugation .
That means transgenic plants created by the T-DNA vector system have a ready route for horizontal gene escape, via Agrobacterium, helped by the ordinary conjugative mechanisms of many other bacteria that cause diseases. 
A report submitted to MAFF in 1997 had indeed raised the possibility that Agrobacterium tumefaciens could be a vector for gene escape [15, 16]. The researchers found that it was extremely difficult to get rid of the Agrobacterium.
…transgenic plants created by the T-DNA vector system have a ready route for horizontal gene escape, via Agrobacterium, helped by the ordinary conjugative mechanisms of many other bacteria that cause diseases.
High rates of gene transfer are known to be associated with the plant root system and the germinating seed.  Agrobacterium could multiply and transfer transgenic DNA to other bacteria, as well as to the next crop plant. Agrobacterium was also found to transfer genes into several types of human cells , and in a manner similar to that which it uses to transform plant cells.
All the risks of horizontal gene transfer described above are real, and far outweigh any potential benefits that GM crops can offer. There is no case for allowing any commercial release of GM crops and food products.
The following experiments and tests should be done to address the risks of horizontal gene transfer:
1. Feeding experiments similar to those carried out by Dr. Arpad Pusztai’s team should be done, using well-characterized transgenic soya and/or maize meal feed, with full, adequate monitoring for transgenic DNA in the faeces, blood and blood cells, and post-mortem histological examinations that include tracking transfer of transgenic DNA into the genome of cells. As an added control, nontransgenic DNA from the same GM feed sample should also be monitored.
2. Feeding trials on human volunteers should be carried out using well-characterized transgenic soya and/or maize meal feed, with full, adequate monitoring for transgenic DNA in the faeces, blood and blood cells. Also as an added control, nontransgenic DNA from the same GM feed sample should also be monitored.
3. The stability of transgenic plants in successive generations should be systematically investigated, especially for those containing CaMV 35S promoter, using adequate quantitative molecular techniques.
4. Full molecular characterization of all transgenic lines must be carried out to establish uniformity and genetic stability of the insert(s).
5. All transgenic plants created by the Agrobacterium T-DNA vector system should be tested for the persistence of the bacteria and vectors. The soil in which they have been grown should also be monitored for gene escape to soil bacteria. And the potential for horizontal gene transfer to the next crop via the germinating seed and root system should be carefully monitored.
References and Notes
1. Ho MW, Horizontal Gene Transfer. The Hidden Hazards of Genetic Engineering, TWN Biotechnology Series, Third World Network, 2001 (available fom the ISIS online store http://www.i-sis.org.uk/onlinestore.php#books); also Mae-Wan Ho, Horizontal gene transfer and genetic engineering, SCOPES website, AAAS, 2000.
2. Ho MW, Briefing to the Rt. Hon. Michael Meacher, Minister for the Environment on the Special Safety Concerns of Transgenic Agriculture and Related Issues (http://www.i-sis.org.uk/meacher99.php). April 1999 , published in Seminario Internacional sobtre Direcito da Biodiversidade, Revista cej: Centro de estudos Judiciarios do Conselho da Justica Federal, Brasil, pp.120–6, 1999.
3. Science, News of the Week, 4 October 2002; see also Ho MW, Predicted hazard of gene therapy a reality, ISIS Report, October 2002 (http://www.i-sis.org.uk/PHGT.php).
4. Ho MW, GM maize approved on bad science in the UK, Science in Society 2002, 15 (http://www.i-sis.org.uk/isisnews/sis15.php), 10–25.
5. Kohli A., Griffiths S, Palacios N, Twyman R, Vain P, Laurie D and Christou P. Molecular characterization of transforming plasmid rearrangements in transgenic rice reveals a recombination hot spot in the CaMV 35S promoter and confirms the predominance of microhomology mediated recombination” Plant.J. 1999, 17,591–601.
6. Ho MW, Ryan A and Cummins J. Cauliflower mosaic viral promoter—a recipe for Disaster? Microbial Ecology in Health and Disease 1999 11, 194–7.
7. Ho MW, Ryan A. and Cummins J., Hazards of transgenic plants with the cauliflower mosaic viral promoter. Microbial Ecology in Health and Disease 2000, 12, 6–11.
8. Ho MW, Ryan A and Cummins J., CaMV35S promoter fragmentation hotspot confirmed and it is active in animals. Microbial Ecology in Health and Disease 2000, 12, 189.
9. Quist D. and Chapela IH., Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico. Nature 2001, 414, 541–3, 2001.
10. Ho MW, Astonishing denial of transgenic contamination, Science in Society 2002, 15, 13–14 (http://www.i-sis.org.uk/isisnews/sis15.php).
11.Netherwood T, Martin-Orue SM, O’Donnell AG, Gockling S, Gilbert HJ and Mathers JC., Transgenes in genetically modified Soya survive passage through the small bowel but are completely degraded in the colon. Technical report on the Food Standards Agency project G010008, Evaluating the risks associated with using GMOs in human foods–University of Newcastle.
12. Doerfler, W. and Schubbert, R. (1998). Uptake of foreign DNA from the environment: the gastroinestinal tract and the placenta as portals of entry, Wien Klin Wochenschr. 110, 40–44.p. 40.
13. Ferguson GC and Heinemann JA. Recent history of trans-kingdom conjugation. In Horizontal Gene Transfer 2nd ed. (ed. M Syvanen & CI Kado), pp 3–17, Academic Press, San Diego, 2002.
14. Ho MW. What’s unspeakable in horizontal gene transfer? Heredity (in press); Ho MW, Averting sense for nonsense, Science in Society 2002, 16, 29–30.
15. McNicole et al (1997) The Possibility of Agrobacterium as a Vehicle for Gene Escape. MAFF. R&D and Surveillance Report: 395 (http://www.i-sis.org.uk/isisnews/sis16.php).
16. Barrett et al (1997). A risk assessment study of plant genetic transformation using Agrobacterium and implications for analysis of trangenic plants. Plant Cell Tissue and Organ Culture 47: 135–144.
17. Sengelov G, Kristensen KJ, Sorensen AH, Kroer N, and Sorensen SJ. Effect of genomic location on horizontal transfer of a recombinant gene cassette between Pseudomonas strains in the rhizosphere and spermosphere of barley seedlings. Current Microbiology 2001, 42, 160–7.
18. Kunik T, Tzfira T, Kapulnik Y, Gafni Y, Dingwall C, and Citovsky V., Genetic transformation of HeLa cells by Agrobacterium. PNAS USA, 2001, 98, 1871–87; also, Common plant vector injects genes into human cells, ISIS News 2002, 11/12, p. 10 (http://www.i-sis.org.uk/isisnews/i-sisnews11.php).
This article can be found on the I-SIS website at http://www.i-sis.org.uk/FSAopenmeeting.php
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