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DNA in GM Food and Feed
by Dr. Mae-Wan Ho
“DNA is DNA is DNA,” said a proponent in a public debate in trying to convince the audience that there is no difference between genetically modified (GM) DNA and natural DNA. “DNA is taken up by cells because it is very nutritious!” “GM can happen in nature,” said another proponent. “Mother Nature got there first.”
So, why worry about GM contamination? Why bother setting contamination thresholds for food and feed? Why award patents for the GM DNA on grounds that it is an innovation? Why don’t biotech companies accept liabilities if there’s nothing to worry about?
As for GM happening in nature, so does death, but that doesn’t justify murder. Radioactive decay happens in nature too, but concentrated and speeded up, it becomes an atom bomb
Biotech companies are bypassing the strict environmental safety assessment required for growing GM crops in Europe by applying to import GM produce for food and processing only.
Is GM food safe?
There is both scientific and anecdotal evidence indicating it may not be; many species of animals were adversely affected after being fed different species of GM plants with a variety of GM inserts, suggesting that the common hazard may reside in the GM process itself, or the GM DNA.
How reliably can GM DNA be detected?
DNA can readily be isolated and quantified in bulk. But the method routinely used for detecting small or trace amounts of GM DNA is the polymerase chain reaction (PCR). This copies and amplifies a specific DNA sequence based on short “primers strings” of DNA that match the two ends of the sequence to be amplified, and can therefore bind to the ends to “prime” the replication of the sequence through typically 30 or more cycles, until it can be identified after staining with a fluorescent dye.
There are many technical difficulties associated with PCR amplification. Because of the small amount of the sample routinely used for analysis, it may not be representative of the sample, especially if the sample is non-homogeneous, such as the intestinal contents of a large animal. The primers may fail to hybridize to the correct sequence. The PCR itself may fail because inhibitors are present. Usually, the sequence amplified is a small fraction of the length of the entire GM insert, and will therefore not detect any other GM fragment present. If the target sequence itself is fragmented or rearranged, the PCR will also fail. For all those reasons, PCR will almost always underestimate the amount of GM DNA present, and a negative finding cannot be taken as evidence that GM DNA is absent. Consequently, the level of contamination is almost invariably underestimated.
…many species of animals were adversely affected after being fed different species of GM plants…
There is an urgent need to develop sensitive, standardized and validated quantitative PCR techniques to study the fate of GM DNA in food and feed. Regulatory authorities in Europe are already developing such techniques for determining GM contamination. One such technique has brought the limit of detection down to 10 copies of the transgene (the GM insert or a specific fragment of it).
In contrast, the limit of PCR detection in investigations on the fate of GM DNA in food and feed is extremely variable. In one study commissioned by the UK Food Standards Agency, the limit of detection varied over a thousand fold between samples, with some samples requiring more than 40,000 copies of the GM insert before a positive signal was registered. Such studies are highly misleading if taken at face value, given all the other limitations of the PCR technique.
Despite that, however, we already have answers to a number of key questions regarding the fate of DNA in food and feed:
1. Does DNA break down sufficiently during food processing?
The answer is no, not for most commercial processing. DNA was found to survive intact through grinding, milling or dry heating, and be incompletely degraded in silage. High temperatures (above 95° C) or steam under pressure were required to degrade the DNA completely.
“The results imply that stringent conditions are needed in the processing of GM plant tissues for feedstuffs to eliminate the possibility of transmission of transgenes,” the researchers warned.
They pointed out, for example, that the gene aad, conferring resistance to the antibiotics streptomycin and spectinomycin, is present in GM cottonseed approved for growth in the US and elsewhere. Streptomycin is mainly used as a second-line drug for tuberculosis. But it is in the treatment of gonorrhea that spectinomycin is most important. It is the drug of choice for treating strains of Neisseria gonorrhoea already resistant to penicillin and third generation cephalosporins, especially during pregnancy. The release of GM crops with the blaTEM gene for ampicillin resistance is also relevant here, because that’s where resistance to cephalosporins has evolved.
Another study found large DNA fragments in raw soymilk of about 2,000bp (base pairs, units of measurement for the length of DNA), which degraded somewhat after boiling, but large fragments were still present in tofu and highly processed soy protein. Heating in water under acid conditions was more effective in degrading DNA, but again, the breakdown was incomplete (fragments larger than 900bp remaining).
It is generally assumed, incorrectly, that DNA fragments less than 200bp pose no risk, because they are well below the size of genes. But that’s a mistake, as these fragments may be promoters (signals needed by genes to become expressed), and sequences of less than 10bp can be binding sites for proteins that boost transcription. The CaMV 35S promoter, for example, is known to contain a recombination hotspot, and is implicated in the instability of GM inserts.
2. Does DNA break down sufficiently rapidly in the gastrointestinal tract?
Although free DNA breaks down rapidly in the mouth of sheep and humans, this is not sufficiently rapid to prevent gene-transfer to bacteria inhabiting the mouth. DNA in GM food and feed will survive far longer. The researchers conclude: “DNA released from feed material within the mouth has potential to transform naturally competent oral bacteria.”
Several studies have now documented the survival of DNA in food throughout the gastrointestinal tract in pigs and mice, in the rumen of sheep and in the rumen and duodenum of cattle. The studies were variable in quality. Nevertheless they suggest that GM DNA can transfer to bacteria within the rumen and in the small intestine. In neither sheep nor cattle was feed DNA detected in the feces, suggesting that DNA breakdown may be complete by then.
The only feeding trial in human volunteers was perhaps the most informative. After a single meal containing GM soya which contained some 3x1012 copies of the soya genome, the complete 2,266 bp epsps transgene was recovered from the colostomy bag in 6 out of 7 ileostomy subjects (who had their lower bowel surgically removed). This is a strong indication that DNA in food is not sufficiently rapidly broken down in transit through the gastrointestinal tract, confirming the results of an earlier experiment by the same research group.
No GM DNA was found in the feces of any of 12 healthy volunteers tested, suggesting that DNA has completely broken down, or all detectable fragments have passed into the bloodstream (see later) by the time food has passed through the body. This finding is in agreement with the results from ruminants.
In general, the studies report that GM DNA degrades to about the same extent and at about the same rate as natural plant DNA. However, no quantitative measurements have been made, and GM DNA was often compared with the much more abundant chloroplast DNA, which outnumbers the transgene by 10,000 to one.
3. Does GM DNA get taken up by bacteria and other micro-organisms?
The answer is yes. The evidence was reported in the human feeding trial mentioned. The transgene was not detected in the content of the colostomy bag from any subject before the GM meal. But after culturing the bacteria, low levels were detected in three subjects out of seven, calculated to be between 1 and 3 copies of the transgene per million bacteria.
According to the researchers, the three subjects already had the transgene transferred from GM soya before the feeding trial, probably by having eaten GM soya products unknowingly. No further transfer of GM DNA was detected from the single meal taken in the trial.
…DNA was able to cross the placenta and enter the cells of the fetus and the newborn..
Actually, GM DNA can already transfer to bacteria during food processing and storage. A plasmid was able to transform Escherichia coli in all 12 foods tested under conditions commonly found in processing and storage, with frequencies depending on the food and on temperature. Surprisingly, E. coli became transformed at temperatures below 5° C, i.e. under conditions of storage of perishable foods. In soy drink this condition resulted in frequencies higher than those at 37° C.
4. Do cells lining the gastrointestinal tract take up DNA?
The answer is yes. Food material can reach lymphocytes (certain white blood cells) entering the intestinal wall directly, through Peyer’s patches. And fragments of plant DNA were indeed detected in cows’ peripheral blood lymphocytes.
It is notable that in the human feeding trial, a human colon carcinoma cell line CaCo2 was directly transformed at a high frequency of 1 in 3,000 cells by an antibiotic resistance marker gene in a plasmid. This shows how readily mammalian cells can take up foreign DNA, as we have pointed out some years ago.
5. Does DNA pass through the gastrointestinal tract into the blood stream?
The answer is yes. As mentioned above, fragments of plant DNA were detected in a cow’s peripheral blood lymphocytes. However, attempts to amplify plant DNA fragments from blood have failed, most likely on account of the presence of inhibitors of the PCR amplification.
6. Does DNA get taken up by tissue cells?
The answer is yes, and this has been known since the mid 1990s. GM DNA and viral DNA fed to mice ended up in cells of several tissues, and when fed to pregnant mice, the DNA was able to cross the placenta and enter the cells of the fetus and the newborn. These results were confirmed in 2001, when soya DNA, too, was found taken into the tissue cells of a few animals. In general, abundant chloroplast sequences have been detected in the tissues of pigs and chickens but not single-gene DNA nor GM DNA. But rare events are most likely to go undetected, on account of the limitations of the PCR technique.
Recently, “spontaneous transgenesis”—the process of spontaneous uptake of foreign DNA resulting in gene expression—has been rediscovered by a team of researchers looking for new possibilities in gene therapy. They documented the phenomenon in several human B lymphocyte cell lines as well as peripheral blood B lymphocytes. The transgene in a plasmid was readily taken up and was found in many cell compartments including the nucleus, where gene transcription took place. The plasmid was not integrated into the genome, but the researchers say that its eventual integration cannot be ruled out.
Is GM DNA more likely to insert into genomes?
This is perhaps the most important question. There are reasons to believe GM DNA is more likely to insert in genomes after it is taken up into cells. Chief among those reasons is its sequence similarities (homologies) to a wide variety of genomes, especially those of viruses and bacteria. Such homologies are known to enhance horizontal gene transfer to bacteria up to a billion fold.
…GM DNA is more likely to insert in genomes after it is taken up into cells.
More significantly, the integration of non-homologous genetic material can occur at high frequencies when flanked by homologous sequences. “Homology-facilitated illegitimate recombination” increased the integration of foreign (non-homologous) DNA at least 105 fold when it was flanked on one side by a piece of DNA homologous to the recipient genome.
No experiment has yet been done to assess whether GM DNA is more likely to transfer horizontally than natural DNA. However, in the human feeding trial, where three ileostomy volunteers tested positive for the soya transgene in the bacteria cultured from their colostomy bag, the soya lectin gene Le was not detected in the bacterial cultures from any of the subjects.
The researchers found it necessary to remark, “Although the plant lectin gene was not detected in the microbial population, it is premature to conclude that the epsps transgene is more likely than endogenous plant genes to transfer into the microbial population.” But until this possibility has been adequately addressed, it cannot be ruled out.
Dr. Mae-Won Ho is director and co-founder of the Institute of Science in Society and scientific advisor to the Third World Network.
This article can be found on the I-SIS website at http://www.i-sis.org.uk/
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[26 jul 05]