Report on horizontal gene transfer - Department of
Public Prosecution versus Gavin Harte and others, New Ross, Ireland
Mae-Wan Ho, March 22, 1999
Mae-Wan Ho, Reader in Biology at the Open University, B. Sc. (First
Class) 1964, and Ph. D. 1967, H K University; more than 30 years in
research and 25 years teaching experience; nearly 200 publications
covering human biochemical genetics, molecular genetics, evolution,
developmental biology, and biophysics. Awards include, Chan Kai Ming Prize
for Biological Sciences (HK) 1964: Fellow of the National Genetics
Foundation (USA) 1971-1974; Vida Sana Award (Spain) 1998; Guest of Honour
in Women of the Year Luncheon & Assembly (UK) 1998. From 1994,
Scientific Advisor to the Third World Network and other public interest
organizations on biotechnology and biosafety. Debated issues in the United
Nations, the World Bank, the European Parliament, in the UK, USA and many
other countries all over the world. Author of many papers and reports for
the public and for policy-makers, frequent broadcaster and public
lecturer. Recent publications relevant to genetic engineering: Genetic Engineering Dream or Nightmare? Mae-Wan Ho, Gateway Books,
Bath, UK, 1998; (revised, 2nd. edition, 1999). Gene Ecology and Gene Technology of Infectious Diseases, Mae-Wan Ho et
al, Microbial Ecology in Health and Disease 10, 33-59, 1998. Genetic Engineering and Infectious Diseases, Mae-Wan Ho et al,
Third World Network, Penang, 1998. Fatal Flaw in Food Safety Assessment: The FAO/WHO Joint Biotechnology &
Food Safety Report, Mae-Wan Ho and Ricarda A. Steinbrecher, Third
World Network, Penang, 1998. Fatal Flaw in Food Safety Assessment: The FAO/WHO Joint Biotechnology &
Food Safety Report, Mae-Wan Ho and Ricarda A. Steinbrecher, Environmental
and Nutritional Interactions 2, 51-84.
1. Horizontal gene transfer and genetic engineering (1)
1.1 Horizontal gene transfer refers to the transfer of genes or
genetic material directly from one individual to another by processes
similar to infection. It is distinct from the normal process of vertical
gene transfer - from parents to offspring - which occurs in reproduction.
Genetic engineering bypasses reproduction altogether by exploiting
horizontal gene transfer, so genes can be transferred between distant
species that would never interbreed in nature. For example, human genes
are transferred into pig, sheep, fish and bacteria. Toad genes are
transferred into potatoes. Completely new, exotic genes, can therefore be
introduced into food crops.
1.2 Natural agents exist which can transfer genes horizontally between
individuals. These are viruses, many of which cause diseases, and other
pieces of parasitic genetic material, called plasmids and transposons,
many of which carry and spread antibiotic and drug resistance genes. These
are able to get into cells and then make use of the cell's resources to
multiply many copies or to jump into (as well as out of ) the cell's
genome. The natural agents are limited by species barriers, so that for
example, pig viruses will infect pigs, but not human beings, and
cauliflower viruses will not attack tomatoes. However, genetic engineers
make artificial vectors (carriers of genes) by combining parts of
the most infectious natural agents, with their disease-causing functions
removed or disabled, and design them to overcome species barriers, so the
same vector may now transfer, say, human genes, which are spliced into the
vector, into the cells of all other mammals, or cells of plants.
1.3 Typically, foreign genes are introduced with strong genetic signals
- called promoters or enhancers - to boost the expression
of the genes to well above the normal level that most of the cell's own
genes are expressed. The most commonly used promoters are from plant
viruses which are related to animal viruses (see below). There will also
be selectable "marker genes" introduced along with the gene(s)
of interest, so that those cells that have successfully integrated the
foreign genes into their genome can be selected. The most commonly used
marker genes are antibiotic resistant genes originally isolated from
bacterial plasmids and transposons, which enable the cells to be selected
with antibiotics. These marker genes often remain in the genetically
1.4 One viral promoter which is in practically all transgenic plants is
from the cauliflower mosaic virus (CaMV), which is closely related to
human hepatitis B virus, and less so, to retroviruses such as the AIDS
virus. The CaMV promoter can drive the synthesis of these other
viruses;(2) it is active in most plants, in yeast, insects (3) and E.
coli.(4) As all genomes contain dormant viruses, there is a potential
for the CaMV promoter to reactivate them. Its strong promoter activity
causes introduced genes to be overexpressed, and may also have effects on
host genes far away from the site of foreign gene insertion. The promoter
from another virus - the figwort mosaic virus, is similar to CaMV in many
respects, and therefore equally hazardous. Recombination between the
figwort and CaMV promoters in the same plant is bound to take place with
untoward consequences for the crop plant, and also in creating new, broad
host range viruses.
1.5 The insertion of foreign genes into the host genome is not under the
control of the genetic engineer. It is entirely random, and gives rise to
correspondingly random, unintended effect, including toxins and allergens
in food plants, and cancer in mammalian cells.
1.6 Many of the dangers of genetic engineering are inherent to the
technology, in its ability to increase the potential for horizontal gene
transfer across unrelated species. Secondary, unintended horizontal gene
transfer can take place from the genetically engineered crops released
into the environment. The very cellular mechanisms that enable the foreign
genes to insert into the genome can also mobilize them to jump out again.
For example, the enzyme integrase, which catalyzes the integration
of viral DNA into the host genome, also functions as a disintegrase,
catalyzing the reverse reaction. These integrases are very widespread,
belonging to a superfamily of similar enzymes present in all genomes, from
viruses and bacteria to higher organisms.(5) The foreign genes can then
reinsert into another site in the genome, or else spread, uncontrollably
to other organisms.
2. Hazards of horizontal gene transfer from transgenic crops released
into the environment
2.1 The major hazards from secondary horizontal gene transfer are as
follows. Generation of new viral and bacterial pathogens from the viral and
bacterial genes introduced into the transgenic plants, Spread of drug and antibiotic resistance marker genes among pathogens, Random, secondary insertion of genes into cells of organisms interacting
with the crop plants, with harmful effects including cancer, Reactivation of dormant viruses that cause diseases, Ecological impacts due to the spread of the specific exotic genes
2.2 There has been an accelerated resurgence of drug and antibiotic
resistance diseases over the past 20 years, coinciding with the
development of genetic engineering biotechnology on commercial scales. A
group of scientists including myself have produced a report (6) on the
circumstantial, deductive and inductive evidence linking genetic
engineering biotechnology to the recent resurgence of infectious diseases
and demanding an independent public enquiry. There is overwhelming
evidence that horizontal gene transfer (and subsequent recombination) has
been responsible for creating new viral and bacterial pathogens and
spreading drug and antibiotic resistance genes. Many of the horizontal
gene transfer events have occurred very recently.
2.3 There is as yet no direct evidence that latent viruses can be
reactivated in transgenic plants, if only because the possibility has not
been investigated. However, plants engineered with genes from viruses to
resist virus attack actually show increased propensity to generate new,
often super-infectious viruses by horizontal gene transfer and
recombination with infecting viruses. (7) This suggests that other viral
genes engineered into transgenic plants may also take part in horizontal
gene transfer and recombination to generate new viruses.(8)
2.4 There is evidence that foreign genes introduced into plants behave
differently from the plants own genes. These foreign genes may be up to 30
times more likely to escape and spread than the plants own genes.(9) One
way this could happen is by secondary horizontal gene transfer via insects
visiting the plants for pollen and nectar.
2.5 Secondary horizontal tranfer of transgenes and antibiotic resistant
marker genes from genetically engineered crop plants into soil bacteria
and fungi have been documented in the laboratory.* Successful transfers of
marker genes to the soil bacterium Acinetobacter were obtained
using DNA extracted from homogenized plant leaf from a range of transgenic
plants: Solanum tuberosum (potato), Nicotiana tabacum
(tobacco), Beta vulgaris (sugar beet), Brassica napus and
Lycopersicon esculentum.(10) Despite the misleading title in one
of the publications,(11) a high "optimal" gene transfer
frequency of 6.2 x 10-2 was found in the
laboratory from transgenic potato to a bacterial pathogen. The authors
then proceeded to 'calculate' a frequency of 2.0 x 10-17
under extrapolated "natural conditions". The natural conditions,
are of course, largely unknown. There is no ground for assuming that such
horizontal gene transfer will not take place under natural conditions. On
the contrary, other recent findings suggest that genes can transfer
horizontally from transgenic plants to organisms in the soil, and in the
gut and other parts of organisms feeding on the transgenic plants.
2.6 There is now abundant evidence that the genetic material, DNA,
released from dead and live cells, are not readily broken down as
previously supposed, but are rapidly adsorbed onto clay, sand and humic
acid particles where they retain the ability to infect (transform) a range
of organisms in the soil. (12) That means transgenes and marker genes will
be able to spread to bacteria and viruses with the potential of creating
new pathogens and spreading antibiotic resistance genes among the
2.7 There is also recent evidence that DNA is not broken down rapidly in
the gut as previously supposed.(13)That means genes can spread from
ingested transgenic plant material to bacteria in the gut and also to
cells of the organism ingesting the material.
2.8 Horizontal gene transfer between bacteria in the human gut has been
demonstrated since the 1970s and similar transfers in the gut of chicken
and mice in the early 1990s.(14) This is confirmed in new research showing
that antibiotic resistant marker genes from genetically engineered
bacteria can be transferred to indigenous bacteria at a substantial
frequency of 10-7 in an artificial gut. (15)
2.9 Mammalian cells are known to take up foreign DNA by many mechanisms.
Studies since the 1970s have documented the ability of bacterial plasmids
carrying a mammalian SV40 virus to infect cultured cells which then
proceeded to make the virus. Similarly, bacterial viruses and baculovirus
(of insects) can also be taken up by mammalian cells. Baculovirus is so
good at gaining access that it is being engineered as a vector for human
gene therapy, at the same time that it is being engineered to control
insect pests in agriculture.(16)
2.10 Viral and plasmid DNA fed to mice has been found to resist
digestion in the gut. Large fragments passed into the bloodstream and into
white blood cells, spleen and liver cells. In some instances, the viral
DNA was found attached to mouse DNA and E. coli DNA, suggesting
that it has integrated into the mouse cell genome and the bacterial cell
genome respectively.(17) When fed to pregnant mice, large fragments of the
DNA are found in the nucleus of cells of the foetus and the newborn.(18)
2.11 Viral DNA is now known to be more infectious than the intact virus,
which has a protein coat wrapped around the DNA. For example, intact human
polyoma virus injected into rabbits had no effect, whereas, injection of
the naked viral DNA gave a full-blown infection.(19) Viral DNA is in
practically all transgenic plants especially in the form of viral
promoters, which, if integrated into mammalian cells may reactivate
dormant viruses or cause cancer.
2.13 Recent revelations on the results of feeding experiments performed
by Dr. Arpad Puztais group in the Rowett Institute suggests that
transgenic potatoes are toxic to rats, and that most of the toxicity is
due to the genetic engineering process,(20) which is common to practically
all transgenic plants. This same kind of transgenic potatoes was earlier
shown to harm ladybirds fed on aphids that have eaten the transgenic
potatoes.(21) This should raise serious concerns on the continued release
of transgenic plants into the environment, whether as field trials or as
commercially grown crops.
3. Specific hazards from the transgenic sugar beet released in the
3.1 I understand that genes from three bacteria and two viral promoters
(from CaMV and figwort mosaic virus) known to be potentially hazardous,
have been introduced into the sugar beet at the trial site in Arthurstown,
3.2 As the insertion of these genes is a random process, and as the
introduced genes will interact with host genes, there are bound to be
unpredictable, unintended effects on the physiology and biochemistry of
the sugar beet plant. These effects can spread very far away from the site
of insertion.(22)The ecological impacts on other organisms (including
insects, birds and small mammals) interacting with the transgenic sugar
beet plants are largely unknown and even more unpredictable. The recent
revelation that much of the toxicity of transgenic foods may be inherent
to the genetic engineering technology itself suggests that the ecological
impacts of the transgenic sugar beet could be devastating.
3.3 The viral promoters may reactivate dormant viruses in the transgenic
sugar beet plants or create new viruses by horizontal gene transfer and
recombination. These viruses could then be spread by insects and
pollinators to many other plants.
3.4 Horizontal transfer of genes to bacteria and fungi in the soil may
occur when the transgenic sugar beet plant material decomposes, or simply
from exudates from the plant-roots. Transfer may occur to pollinators and
other insects feeding on the plant, and via the insects, the genes may be
transferred to unrelated species of higher plants. Transfer may also occur
to gut bacteria or cells of small mammals feeding on the plants. These
transfers may reactivate dormant viruses or create new viruses in all the
species concerned, as the CaMV and figwort mosaic viral promoter are both
active in all the species. In addition, each of these transfers will have
its own ecological consequences.
3.5 Horizontal gene transfer to soil fungal and bacterial pathogens will
render them resistant to Roundup (Roundup resistance being due to two
bacterial genes introduced, CP4 EPSPS and gox). This can
lead to an ecological disaster as it is already known that beneficial
organisms such as earthworms and mycorrhizal fungi and other
microorganisms involved in nutrient recycling in the soil are susceptible
to glyphosate (the active ingredient in Roundup herbicide). Glyphosate is
so generally toxic that it is being considered for use as an
3.6 Horizontal transfer of Roundup resistance genes to other species of
higher plants will create weeds and superweeds resistant to Roundup.
3.7 Horizontal transfer of Roundup resistance genes to gut bacteria will
create a reservoir of glyphosate resistant bacteria which may pass the
genes onto pathogenic bacteria.
3.8 Horizontal gene transfer to insects, bird, and small mammals will
cause many harmful effects including cancer, resulting in a loss of
See Ho, M.W. (1998, 1999). Genetic
Engineering Dream or Nightmare? The Brave New World of Bad Science
and Big Business, Gateway Books, Bath; Ho, M.W., Traavik, T.,
Olsvik, R., Tappeser, B., Howard, V., von Weizsacker, C. and McGavin, G.
(1998a). Gene Technology and Gene Ecology of Infectious Diseases. Microbial
Ecology in Health and Disease 10, 33-59; also Ho, M.W., Meyer, H.
and Cummins, J. (1998b). The biotechnology bubble. The Ecologist
28(3), 146-153, and references therein.
Xiong, Y. and Eikbush, T. (1990). Origin and evolution of
retroelements based upon the reverse transriptase sequences. The Embo
Journal 9, 3363-72; see also Cummins, J. (1998). A virus promoter used
in the majority of genetically engineerd crops. Available from the
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Smerdon, G., Aves, S. and Walton, E. (1995). Production of human
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Scoulten, A., Usmany, M., Belsham, G., KlingeRoode, E., Maule, A.,
Vanlent, M. and Zuideman, D. (1990). Expression of Cauliflower Mosaic
Virus Gene I. Virology 179, 312-20.
Assad, F.F. and Signer, E.R. (1990). Cauliflower mosaic virus P35S
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Asant-Appiah, E. and Skalka, A.M. (1997). Molecular mechanisms in
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Ho et al, 1998a (see note 1).
Vaden V.S. and Melcher, U. (1990). Recombination sites in cauliflower
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threat to farms. The Independent on Sunday, 21 March.
This possibility has been suggested by Cummins since 1994 (see
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Bergelson, J., Purrington,c.B. and Wichmann, G. (1998). Promiscuity
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Hoffman, T., Golz, C. & Schieder, O. (1994). Foreign DNA
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after co-culture with transgenic higher plants. Current Genetics
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Horizontal gene-transfer from a transgenic potato line to a bacterial
pathogen (Erwinia-chrysanthem) occurs, if at all, at an
extremely low-frequency. Bio/Techology 13: 1094-1098;
Gebhard, F. and Smalla, K. (1998). Transformation of Acinetobacter
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Schlutter et al, 1995 ( note 9).
Reviewed by Lorenz, M.G. and Wackernagel, W. (1994). Bacterial gene
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See Ho et al, 1998a (note 1) and references therein.
See Ho et al, 1998a (note 1) and references therein.
See MacKenzie, D. (1999). Gut reaction. New Scientist, 30
See Ho etal, 1998a (note 1).
Schubbert, R., Lettmann, C. & Doerfler, W. (1994). Ingested
foreign (phage M13) DNA survives transiently in the gastrointestinal
tract and enters the bloodstream of mice. Mol. Gen. Genet. 242:
495-504; Schubbert, R., Renz, D., Schmitz, B. and Doerfler, W. (1997).
Foreign (M13) DNA ingested by mice reaches peripheral leukocytes, spleen
and liver via the intestinal wall mucosa and can be covalently linked to
mouse DNA. Proc. Natl. Acad. Sci. USA 94, 961-6.
Doerfler, W., Schubbert, R., Heller, H., Hertz, J., Remus, R.,
Schrier. J., Kämmer, C., Hilger-Eversheim, K., Gerhardt, U.,
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See Traavik, T. (1995). Too Early May Be Too Late. Ecological
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Research, Production and Therapy (Norwegian), Report for the
Directorate for Nature Research Tungasletta 2, 7005 Trondheim. English
translation, 1999; Ho et al, 1998 (see note1); also Ho, 1999
Chapter 10 ( note 1).
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