23 June 2015 | Column | By Bureau Report
Discussion about genetically-modified organisms (GMO) often focusses on the fear of Frankenstein-like creatures that might be created in the laboratory. GMOs are defined as ‘‘organisms whose genetic make-up or DNA has been altered in a way that does not occur naturally.’’ Critics argue that GMOs being unnatural could be toxic and could wipe out existing species. It is perhaps the term ‘altered genetic make-up’ that creates such anxiety since genetic change is often equated with mutations leading to cancer or inherited genetic disorders.
There is a deep rooted notion that the genetic make-up or DNA of an organism is pristine, unchanged throughout its life and that a genetic change always leads to a detrimental effect. In reality, the DNA of any organism in natural environment is vulnerable to change due to several innate and environmental factors. While most of the DNA sequence of an organism remains unchanged during its lifetime, all living organisms acquire small incremental changes in their DNA during their life cycle. Accumulation of such small changes, passed over generations followed by natural selection leads to diversity within a species and occasionally can give rise to a new species. Generation of diversity through changes in the genome is both inevitable and essential for generation of novel genes, evolution and finally survival of the species.
Since natural changes in DNA sequence in nature happen in a random manner, it is not possible to predict the outcome of these changes. If a change confers a beneficial trait, the organism propagates and if the trait is detrimental, it perishes. If the organism survives, the outcome of this natural process may or may not be beneficial to rest of the organisms. For example, while the rise of genes that confer insect-resistance in neem is a favourable trait for humans, it is detrimental to insects. Poisonous mushrooms or toxins in unripe lychees that could be fatal to children have arisen through completely natural process and are definitely not favourable genetic changes for humans. The outcome of genetic modifications that happen in nature remains unknown until the modified organism reproduces and reaches sufficient numbers. On the other hand, engineered GMOs are generated using the same process as that happens in nature but in stringently controlled experiments done in laboratory with focus on a particular desired outcome.
The genetic make-up of an organism can get modified in diverse ways. Random mutations in the genome can occur due to chemical mutagens in the environment, UV rays or due to accumulation of errors during normal cell division. Interestingly, organisms can also acquire foreign DNA from unrelated organism in a process known as horizontal gene transfer (HGT). HGT occurs when two unrelated organisms come in direct contact such as during symbiosis, parasitism or grafting. It can also occur when foreign DNA hitchhikes on vectors like viruses, bacteria or fungi that come on contact with multiple hosts.
Although a rare phenomenon, several instances of HGT have been reported. In reported instances, DNA has crossed not only species barrier but also kingdom barrier with DNA getting transferred from viruses to eukaryotes, fungi to plants and from bacteria to protozoa. The human genome itself carries almost 10% of foreign DNA from retroviruses, called as retroposons. GMOs are similar to natural organisms that received genes through HGT. Rice modified to synthesize provitamin A (β-carotene), for example, is similar to pea aphids that have received genes for carotenoid biosynthesis from fungi through HGT.
In contrast to the random way in which gene transfer occurs in nature, GMOs are generated in a precisely regulated manner and monitored at every step. The process is monitored to ensure that the right gene is introduced in the host, to control the number of copies of the gene introduced, the site in the genome where it integrates and the safety/toxicity issues that might be associated with the GM crop.
To generate GM crops, first the gene encoding beneficial trait to be introduced in a plant is isolated from a suitable host. The isolated sequence encodes information for synthesis of the protein responsible for the desired trait and is called the coding sequence. The second part of the gene is the promoter sequence which determines the pattern of expression of the gene of interest such as where and how much protein is synthesized. For example, in Golden rice which has been genetically engineered to express high levels of enzymes that make β-carotene, two genes involved in β-carotene biosynthesis have been introduced.
The gene for phytoene synthase is from daffodil and is expressed using glutelin promoter from rice to ensure expression of this transgene only in endosperm, the edible part of rice. The second gene encodes carotene desaturase enzyme from bacteria under viral 35S promoter and is expressed in all cells of the plant. These two enzymes together with a third enzyme produced in rice endosperm generate high amounts of β-carotene, giving bright yellow colour. Consumption of this rice is expected to reduce Vitamin A deficiency in malnourished children.
In addition to gene(s) conferring desirable properties, a gene called reporter with its own coding sequence and promoter is also transferred to the host. The reporter sequence encodes a trait whose expression can be checked easily. This gene is absent in the host and hence if this trait is observed in the host after genetic manipulation, it indicates that the host has been genetically modified.
The DNA consisting of above-mentioned genes is introduced in plant embryos using bacteria like Agrobacterium which infect plants or by using nanoparticles of gold or tungsten coated with the DNA. In these techniques, it is not possible to predict where the DNA will integrate in the host genome. Hence tests are carried out to ensure that the gene has integrated in a way that it does not interfere with functioning of host genes and an appropriate number of copies of gene of interest are integrated in the genome.
In this entire process about 10-20 kilo-basepairs of DNA is transferred to the host, depending on the number of genes transferred. This is a small amount of DNA compared to the size of plant genome (~1/40000th of the total rice genome which is 400-420 Mega-basepairs). More precise techniques of gene transfer such as use of zinc finger nucleases, TALE nucleases or CRISPR/Cas9 technology are being considered for genetic modification. These techniques have the ability to precisely modify a target sequence within an organism. It is now also possible to introduce genes in the DNA of chloroplast, rather than that of plant genome. Future technologies can therefore make the process of genetic modification precise and efficient.
Once the DNA is introduced, cells that have incorporated the gene of interest are selected using the marker as mentioned earlier. The marker is usually an antibiotic resistant gene that confers antibiotic resistance to the host. The cells are then cultured in the laboratory using tissue culture techniques to generate a callus that further develops into a plant that can be propagated.
These plants go through several generations in the laboratory to monitor levels of expression of the desired gene, to monitor the site in the genome where the gene has integrated and the tissue of plant in which it is expressed. Further tests are carried out to address safety issues that could be associated with GM plants. If the tests are satisfactory, the GM crop is approved by appropriate authorities for large-scale production.
Importantly, if upon long term use an undesirable effect of GM crop is observed, steps can be taken to further modify the plant to eliminate such an effect. If that is not possible, further use of the crop can be stopped. Just as naturally occurring genes can spread to neighbouring plants, whether beneficial or not, there is of course a potential for the transferred gene to spread in neighbouring plants. There are now precise techniques available to check this possibility. If the gene spreads and is found to be detrimental to other plants, further use of such GM crop can be prohibited. In this context, it is also important to follow guidelines set for use of GM crops, such as those for refuge areas.
While generating GM crops, scientists are applying the tools that nature has used over millions of years, but in a directed and regulated manner. The fear of GM crops is basically fear of the unknown. Incorporating and mimicking nature’s ways to improve human life is an approach that has been extensively used before, such as in the use of antibiotics and contraceptive pills, the use GM microbes to synthesize biochemicals like insulin, and for in vitro fertilisation to treat infertility.
Many lives would have been lost or made miserable if the experiments to develop these techniques had been banned owing to fear of the unknown. Used under appropriate regulations, GM crops have the potential to address problems like malnutrition, excessive pesticide usage and low agricultural productivity. The fear of unknown should not overshadow efforts to address problems faced by millions. The sooner this fear is overcome, the faster will be the progress in improving human life.