Recent evidence confirms risks of horizontal gene transfer.

• Author: Ho, Mae-Wan

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 art 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. (1) 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. (2)

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. (3) 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 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 (GaMY) that infects Cruciferae plants.

Unfortunately, although the GaMY 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. (4)

In 1999, another problem with the GaMY 35S promoter was identified: it has a "recombination hotspot" where it tends to break and join up with other DNA. (5) 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...