The cells of plants and animals carry their instructions in the form of DNA (Deoxyribonucleic acid), which is contained in genes. To make a protein, the sequence of genetic letters in each gene gets copied into matching strands of RNA (Ribonucleic acid) in a process called expression, which then float out of the nucleus to guide the protein-making machinery of the cell in a process called translation.
Until very recently, genetically engineered organisms were created by deletion of some of the genes in their genetic makeup or insertion of genes from external sources into their genetic makeup. This article is the third of a four-series feature on genetic engineering, after Genetic Engineering – Definition, History, Benefits and Risks, and, Genetic Engineering – The Great GMO Debate. Its aim is to highlight several advances in genetic engineering where organisms such as plants and animals have been modified without interfering with the composition and sequence of genes in their genetic makeup, which has not only been controversial, but is also very expensive and labor intensive.
For the last seventeen years multiple technologies for modifying gene structure have been developed, such as those used to silence genes, an example being RNA interference (RNAi). Companies have been modifying crops by spraying them with specially designed RNA instead of tampering with their genes. Bugs die when they eat the leaves of such crops that are coated with RNA. RNA interference (RNAi) based technologies have revolutionized mammalian biomedical research and enhanced the promises of gene therapy and are being increasingly used to produce drugs to combat conditions and diseases such as cancer, infectious diseases and neurodegenerative disorders.
Gene silencing is a way to temporarily switch off the activity of a gene by a mechanism other than genetic engineering. It offers control over genes without modifying a plant or animal’s genome – that is, without creating a GMO. When genes are silenced, their expression is reduced and its RNA is unable to make protein.
Genes can also be “knocked out”, meaning that they are completely erased from the organism’s genome and, thus have no expression. Gene silencing is considered a “gene knockdown” mechanism since the methods used to silence genes, such as RNAi or CRISPR, generally reduce the expression of a gene by at least 70% but do not completely eliminate it.
In the last five years since 2012, several other techniques for stable genetic modifications have also evolved, collectively called gene editing techniques. One such gene editing approach is based on the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and the CRISPR-associated protein 9 (CRISPR/Cas9) systems. Today, these systems that scientists have used to edit the genes of plants and animals with previously unparalleled precision have revolutionized the entire gene editing field. Using the CRISPR system for example, scientists have successfully inactivated an oxidase gene which leads cut mushrooms to brown upon exposure to air. CRISPR does not introduce new DNA from outside the organism; it just deletes already present genes, and for this reason, the USDA has already decided that CRISPR-edited genes do not fall under its regulation – calling into question whether these modified crops and animals can or will be grouped together with other genetically engineered crops and animals.
The major difference between these two powerful tools for gene-silencing are that: RNAi ‘knocks-down’ a gene, meaning that it reduces gene expression and so cells might still have some gene expression and contain the transcripts of the gene of interest; whereas CRSPR/Cas9 ‘knocks-out’ a gene, meaning that it turns off the gene entirely, resulting in no gene expression at all for the gene of interest. So, to completely silence a gene, CRISPR technology is better. Unlike RNAi, the CRISPR/Cas9 system is bestowed with the ability to introduce heritable precision insertions and deletions in plant and animal genomes.
The main advantage of the gene-silencing technology using RNA sprays is that it is faster and way cheaper than creating new GM crops or animals; because one gains control of a plant, or animal, with a spray which can change very rapidly, can be tested faster, experimented with faster, and brought to the market faster. This is unlike in the case of bringing a new GM crop to market, where public opposition, regulations, and the slow pace of plant breeding all make it very expensive with costs averaging at more than $100 million and takes too long, around 13 years.
Another advantage is that the gene-silencing effects of RNA interference last only a few days or weeks. That means you might spray on traits such as drought resistance in times of water shortage without affecting the plant’s performance in times of normal rainfall. The sprays are made from a ubiquitous molecule that degrades quickly in soil. They can be genetically precise enough to kill potato bugs but spare their ladybug cousins. Unlike RNA interference, conventional insecticides wipe out helpful insects along with the bad ones, an example being Imidacloprid, which is restricted in Europe for its suspected link to bee colony collapse.
A third advantage is the confirmation by the United States Environmental Protection Agency (EPA) that there is little evidence of risk from eating RNA; that consuming RNA molecules is safe for humans and animals.
EPA’s advisors have concerns that knowledge gaps make it difficult to predict exactly what problems might arise from RNA use. They contend that though RNA is natural, introducing large amounts of targeted RNA molecules into the environment is not. They recommend exploration of the potential for unintended ecological effects from RNA.
Beekeepers worry that pollinators could be hurt by unintended effects. They make the point that the genome of many insects aren’t known, so scientists can’t predict whether their genes will match an RNA target.
Being biopesticides, RNA treatments take longer to take effect. They do not knock bugs out instantly as a chemical neurotoxin does. Insects only start dying after four days, and some live two weeks.
The following are examples where gene-silencing has been used:
The gene which is vital for the survival of Colorado potato beetle, which is a voracious potato eater, was shut down using gene silencing.
By blocking a gene that makes tomatoes soft, the Flavr Savr tomato can now ripen longer on the vine.
The Hawaiian papaya plants are engineered to produce RNA that defends against ringspot virus.
Some corn plants have been genetically engineered, by incorporating an insecticidal RNA into its genetic makeup, to kill the western corn rootworm i.e. has been.
RNA has been introduced into sugar water that bees feed on in order to kill a parasitic mite that infests hives.
The gene that reacts to the “slicing” of the apple or the potato and in the process turning the slices brown has been silenced, the result being the non-browning apple and the non-browning, lower acrylamide potato.
A genetically modified banana has been engineered to produce higher levels of beta carotene, and hence vitamin A.
A tomato has been made purple by fortifying it with the same antioxidants found in blueberries.
A soybean has been created that produces a high-oleic acid that has the characteristic of olive oil.
The seed of Cameline, also known as false flax, has been spliced with genes that make Omega-3 fatty acids.
Oranges have been genetically engineered to withstand the ravages of citrus greening, a disease threatening to wipe out orange production in the USA.
Other articles in this series:
- Genetic Engineering – Definition, History, Benefits and Risks
- Genetic Engineering – The Great GMO Debate
- The Next Great GMO Debate
- Unnikrishnan Unniyampurath, Rajendra Pilankatta and Manoj N. Krishnan. RNA Interference in the Age of CRISPR: Will CRISPR Interfere with RNAi? International Journal of Molecular Sciences. 2016 Feb 26;17(3):291