Horizontal gene transfer

A. What is HGT?
B. The extent of HGT in nature
C. Viral vectors of HGT
D. Bacterial vectors of HGT
E. References


A. What is HGT?

Horizontal gene transfer (HGT) is the stable transfer of DNA between sexually incompatible organisms. Genetic engineers do their own version of HGT: they build artificial DNA constructs and insert them into living cells.


B. The extent of HGT in nature

A computer search of scientific journals between 1993-6 came up with 67 papers giving direct or indirect evidence of horizontal gene transfers. Transfers have been identified between very different bacteria, between fungi, between bacteria and protozoa (single-cell organisms), between bacteria and higher plants and animals, between fungi and between insects.(1)

DNA transfer can involve both free DNA and DNA carried by a variety of vectors, such as viruses and bacteria.(3)

Many natural habitats have a high potential for DNA transfer; for example, plant roots, soil, and within animal intestines (3). It has been clearly demonstrated that DNA can survive intact in the soi1 (4), during animal feed processing (5), and in the digestive system from where it can enter micro-organisms(9) animal tissues(6). Bacterial uptake and expression of environmental DNA has been found to proceed within 1 minute (8)

Studies commissioned by the UK Food Standards Agency found that transgenic DNA transferred from GM food plants into microbes in the human intesting after a single meal.(9)


C. Viral vectors of HGT

Viruses have a proven ability to acquire genes from their hosts and to swap parts with other viruses. This can lead to very changeable and virulent strains. For example, the virus responsible for the 2009 H1N1 swine-flu epidemic is “a messy combination of sequences from bird, human and swine flu virus lineages from North America and Eurasia”(10)

Aware of the characteristics of viruses, and the known similarities between viral strains, one professor of plant genetics has outlined a cascade of DNA transfers triggered by the viral DNA which is now being used almost ubiquitously to genetically transform organisms for food production. He envisaged that the engineered viral DNA, being in an unusual and unstable form, would recombine with related viruses, such as Hepatitis B or Human Immuno-difficiency Virus, to ultimately create a supervirus propagated in plants, insects and humans. In such circumstances, human survival would be marginal.(7)


D. Bacterial vectors of HGT

Bacteria are well-known for their ability to swap genes. Rapid transfer of antibiotic resistance genes between bacteria in response to the frequent presence of antibiotic drugs in their environment has been the cause of the superbug epidemic in hospitals.

Besides rendering drugs ineffective, acquired genes can create pathogenic bacteria. The sequencing of the entire genome of E. coli 0157 revealed that 1,387 genes (out of a total of about 5,000) had been acquired by horizontal gene transfer. While this magnitude of foreign DNA 'absorption' may be rare, it warns us that there exist strains of microbes which possess elevated potential to incorporate foreign DNA. In the case of E. coli 0157, this potential has led to its extreme toxicity.(2)

Most engineered DNA constructs inserted into GM crop plants include sections homologous to bacterial DNA. It's known that DNA homology is an important factor in promoting HGT into bacteria.(11)

Newer techniques are being developed to avoid the genomic disruption caused by nuclear insertion of foreign DNA, and to enable a greater number of artificial genes to be used. The future of GM may be in gene-packed 'mini-chromosomes' which will lie outside the nucleus of the plant cells. Such artificial chromosomes have even more extensive similarities to bacteria and even more likelihood of promoting HGT.

A major way of inserting foreign genes into crop plants has been to use a common soil bacterium, Agrobacterium, as a delivery agent. In its native form, Agrobacterium, carries DNA which is pathogenic to plants. This DNA is separate from the main genome and is mobile. It is linked to invasive segments of DNA which insert it into the genome of a plant cell which, in nature, then grows into a gall (the plant equivalent of a tumour). Genetic engineers substitute their artificial DNA constructs for the genes for gall-formation in Agrobacterium. The GM bacteria then no longer cause galls, but ferry the artificial genes into plant cells.(12)

Besides the invasion of plant cells, Agrobacterium is also able to genetically transform other micro-organisms such as yeasts, fungi and pathogenic bacteria, and mammalian cells (including human). When newly transformed GM plants are grown, it has been found that 50% of them still harbour high levels of Agrobacterium after six months.(12)

In this soup of unnatural bacterial super-vectors and unnatural bacterial DNA we're creating in our environment, the potential for an escalating incidence of HGT and novel diseases (plant, animal and human) is inescapable.


E. References

1.Ho M.W., Department of Biology, UK Open University, Excerpt from a talk given at The National Council of Women of Great Britain Symposium on Food: Facts, Fallacies and Fears, 22 March 1996, Darlington
2.Perna N.T. et al. (2001) Genome sequence of enterohaemorrhagic Escherichia coll 0157:H7 Nature 409
3.Traavik T, Professor and Scientific Director at the Norwegian Institute of Gene Ecology, Department of Virology, University of Trompe, Norway, Evidence given at the trial of 28 Greenpeace activists against AgrEvo (92 references cited), April 2000
4.Gebhard F. and Smalla K. (1999) Monitoring field releases of genetically modified sugar beets for persistence of transgenic plant DNA and horizontal gene transfer, FEMS Microbiology Ecology, 28(3)
5.Forbes J.M. et al (1998) Effect of feed processing conditions on DNA fragmentation Final report on CS0116 to the ACNFP
6.Doerfler W. and Schubbert R. (1998) Uptake of foreign DNA from the environment: the gastrointestinal tract and the placenta as portals of entry. Wien Klin. Wochenschr. 110 (2)
7.Professor Joe Cummins, Department of Plant Sciences, University of Western Ontario, London, Ontario
8.Flint H. and Scott K. (2001), Dissemination of GM DNA and antibiotic resistance genes via rumen micro-organisms, Food Standards Agency Project G01011
9.Netherwood T. et al. (2001), Transgenes in genetically modified Soya survive passage through the human small bowel but are completely degraded in the colon, Food Standards Agency Project GO1008
10.Mae-Wan Ho and Joe Cummins, (2009) Fast-tracked Swine Flu Vaccine under Fire, Science in Society 43
11.Flint H. et al. (2001), Survival of ingested DNA in the gut and the potential for genetic transformation of resident bacteria, Food Standards Agency Project FSG01007
12.Mae-Wan Ho and Joe Cummins (2008), Agrobacterium & Morgellons Disease, a GM Connection?