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Why genetic engineering is hazardous

The dangers of genetic engineering, argues Dr Mae-Wan Ho, are inherent in the technology itself. In the two pieces below, she outlines the dangers of releasing genetically modified organisms into the environment and elucidates the possible links between genetic engineering and the resurgence of infectious diseases.


1. Horizontal gene transfer and genetic engineering

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 engineered organisms.

CaMV promoter

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; it is active in most plants, in yeast, insects and E. coli. 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 mobilise them to jump out again. For example, the enzyme integrase, which catalyses the integration of viral DNA into the host genome, also functions as a disintegrase, catalysing 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. 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 introduced.

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 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. This suggests that other viral genes engineered into transgenic plants may also take part in horizontal gene transfer and recombination to generate new viruses.

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. 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 homogenised plant leaf from a range of transgenic plants: Solanum tuberosum (potato), Nicotiana tabacum (tobacco), Beta vulgaris (sugar beet), Brassica napus and Lycopersicon esculentum. Despite the misleading title in one of the publications, 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, is not readily broken down as previously supposed, but is rapidly adsorbed onto clay, sand and humic acid particles where they retain the ability to infect (transform) a range of organisms in the soil. 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 pathogens.

2.7 There is also recent evidence that DNA is not broken down rapidly in the gut as previously supposed. 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. 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.

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.

Digestion

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. When fed to pregnant mice, large fragments of the DNA are found in the nucleus of cells of the foetus and the newborn.

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. 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 Pusztai's 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, 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. This should raise serious concerns on the continued release of transgenic plants into the environment, whether as field trials or as commercially grown crops.

Genetic engineering and the resurgence of infectious diseases

IT is argued that horizontal gene transfer has always operated in nature, and therefore, genetic engineering cannot be said to pose any new threat. However, horizontal gene transfer has been relatively rare in our evolutionary past, both because natural species barriers prevent gene exchange, especially between distant species, and because there are mechanisms which inactivate or break down foreign DNA.

Genetic engineering greatly accelerates the rate of horizontal gene transfer as well as enlarging its cope. It creates large numbers of arbitrary combinations of genes from different species and their pathogens, and uses increasingly sophisticated means to overcome species barriers. It is foolhardy to be complacent about releasing great quantities of such arbitrary combination of viral and bacterial genes (used in genetic engineering) into the environment without expecting the worst. The following summarises the evidence of possible links between genetic engineering biotechnology and the recent resurgence of infectious diseases

Inductive

1. Horizontal gene transfer (and consequent recombination) is found to be responsible for the spread of virulence and antibiotic resistance.

2. Direct experimental evidence of horizontal gene transfer, some between phylogenetically distant species, has been obtained in all natural environments as well as in the gastrointestinal tract. These were all accomplished using artificially constructed vectors.

3. Genetic engineering biotech-nology makes extensive use of antibiotic resistance genes as selectable markers, thereby increasing the spread of antibiotic resistance genes.

4. The presence of antibiotics increases the frequency of horizontal gene transfer 10 to 10000 fold, thereby encouraging the horizontal transfer and recombination of virulence as well as antibiotic resistance genes.

5. 'Crippled' strains of bacteria can survive in the environment to exchange genes with bacteria in the environment.

6. DNA released from dead cells (as well as live cells) are not readily broken down in the general environment, nor in the gastrointestinal tract, where they may retain the ability to transform other bacteria.

7. Some viral DNA is more infectious than the virus itself.

8. Routine chemical inactivation of genetically engineered microorganisms prior to disposal in the general environment may be ineffective, leaving a substantial proportion of viruses and bacteria in an infective state.

9. Current legal limits of 'tolerated releases' of GEMs from contained use vastly exceed the minimal infective dose of pathogens and potential pathogens.

10. Non-pathogens are transformed into pathogens by horizontal gene transfer of unit blocks of virulence genes.

11. Horizontal gene transfers are bi-directional. Released non-pathogens can be readily converted into pathogens in one step, by acquiring unit-blocks of virulence genes.

Deductive

1. Genetic engineering is based on facilitating horizontal gene transfer between distant species by constructing vectors that break down species barriers.

2. The artificial vectors constructed for genetic engineering are chimaeric combinations of viral pathogens and other invasive genetic elements that can generate new cross-species viral pathogens.

3. The artificial vectors constructed for genetic engineering are inherently unstable and prone to recombination, thereby enhancing horizontal gene transfer and recombination.

4. Shuttle vectors made by genetic engineering are essentially unstoppable, as they contain signals for transfer and replication in different species; and helper functions for mobilisation and transfer can be supplied by viruses, plasmids and transposons which occur naturally in bacteria in all environments.

Circumstantial

1. The accelerated emergence of infectious diseases and of drug and antibiotic resistance coincide with the development of commercial genetic engineering biotech-nology.

2. Many horizontal gene transfer events responsible for the spread of virulence and antibiotic resistance are recent, based on the high degree (>99%) of homologies in sequences of genes found in unrelated species.

The totality of evidence is sufficiently compelling, especially in view of the precautionary principle, to warrant, at the very least, an independent, full public enquiry into genetic engineering biotechnology and the etiology of infectious diseases.

In addition, we urgently need research directed at understanding general mechanisms for horizontal gene transfer, which aim to strengthen the barriers against the transfer of recombinant DNA, and which can form the basis for scientific risk assessment. Such research must be carried out by independent research groups dedicated to the task, and not left in the hands of those who are involved in commercial exploitation of genetic engineering biotechnology. - Mae-Wan Ho

3. Specific hazards from the transgenic sugar beet released in the field-trial

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, New Ross.

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. 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 antimicrobial.

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, birds, and small mammals will cause many harmful effects including cancer, resulting in a loss of biodiversity. (Third World Resurgence No. 104/105, April/May 1999)

The above article is an edited version of a report on horizontal gene transfer presented by the author as evidence in the Irish case of DPP vs. Gavin, Harte and others.

Dr Mae-Wan Ho is a Reader in Biology at the Open University, UK, and a Fellow of the US National Genetics Foundation.

 


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