TABLE OF CONTENTS
1. gene manipulation / 2. gene editing / 3. genetically modified mosquitoes
Scroll down to a numbered question and click on it to find its answer.
1. How can geneticists manipulate a gene when it is so small, well beyond the reach of microscopes? . . . and then, for gene therapy, say, put the gene back into a person? –
Answer: What gets manipulated is not exactly “a gene,” but millions of copies of it. Genes, made of DNA, are extracted from a few drops of blood or almost any other body tissue. Small samples like these typically contain thousands or millions of cells, each cell containing a complete set of genes. The number of copies of a given gene, or any piece of DNA, can be further amplified in a test tube by the PCR method (polymerase chain reaction).
With the PCR method the copying process can also be influenced so as to get copies of the gene that are slightly different in sequence from the original ones. Normal gene copies can also be chemically linked to other pieces of DNA, modifying how the gene copies will be used when they are inserted back into cells. With such huge numbers of gene copies, a certain amount of sloppiness and inefficiency in the procedures is tolerable, as long as the “successful” DNA molecules one is interested in can be selected from the mixture. There are a number of clever ways of doing this.
Getting copies of a gene back into living cells can be accomplished in several ways. A couple of common methods are: (1) adding the DNA to a culture of cells in a flask; a few cells will absorb the DNA molecules; (2) using some chemical tricks to link up the human DNA (modified or unmodified) to the DNA of a virus (rendered harmless by genetic methods), and letting the virus infect the cells, carrying the new DNA into the cells that way.
All the methods are inefficient, but there are ways of afterwards detecting just which cells have taken up the DNA. Those cells can be selected, and the rest discarded. The successfully modified cells can be returned to the body by direct injection, or by transfusion into the blood stream. 17.v.14
2. What is “gene editing”? What is it good for?
Answer: Gene editing is a recent development in DNA technology that allows a specific gene (or a specified set of genes) to be removed, replaced, or modified. It is an advance over earlier methods, which were not so specific and often resulted in unpredictable changes in genes other than the intended one(s). It is also more widely applicable and more efficient than earlier methods.
The method of gene editing being most widely implemented is the CRISPR/Cas9 system. (This was first discovered in bacteria, where it evolved to inactivate the DNA of invading bacterial viruses.) Its essential component is the Cas9 protein, an enzyme that causes a break in any DNA molecule, like a pair of scissors cutting a piece of yarn. The Cas9 enzyme comes combined with a small molecule of “guide-RNA,” a molecular relative of DNA derived from the CRISPR part of the system.* The guide-RNA guides the Cas9 enzyme to a specific DNA sequence, which is then broken at that site by the enzyme.
* CRISPR stands for “Clustered Regularly Interspaced Short Palindromic Repeats,” part of the genetic system in bacteria that gives them a kind of immunity against viruses.
Artificial guide-RNA molecules of any desired sequence can be synthesized in the laboratory. These RNA molecules are combined with Cas9, and the combination can be introduced into a cell by any one of a variety of methods. It then causes a break in any pre-determined gene. The cell automatically attempts to repair the break, using DNA-repair molecules always present in cells. The repair is often successful, but sometimes it leads to a deletion of a small segment of DNA, making the broken gene useless even after the attempted repair; in this way the gene is effectively removed from the cell. If another loose piece of DNA is also present (introduced into the cell by the researcher), an imperfect repair sometimes leads to the insertion of a copy of that extra DNA.
The biologist can find the cells in which either of those two kinds of imperfect repairs have occurred, and can pick out the cells in which a bad DNA sequence has been removed, or in which a good new DNA sequence has been inserted in place of the old one.
Potential benefits of The CRISPR/Cas9 technology to human health include (1) gene therapy for genetic diseases such as muscular dystrophy, sickle-cell anemia, thalassemia, hemophilia, Huntington’s disease, and cystic fibrosis; (2) treatments for diseases caused by viruses such as HIV (the AIDS virus), hepatitis viruses, and the Zika virus; (3) curing diseases caused by parasites (e.g., leishmaniasis and malaria); (4) targeting the genes responsible for the growth of cancer cells; and (5) modifying the genes that influence heart disease, Alzheimer’s disease, infertility, aging, and other conditions. In addition, this new gene-editing technology can be used to increase crop yields, improve livestock, and decrease the impacts of insect pests.
Aside from its immediate practical applications in health and agriculture, the CRISPR/Cas9 technology will rapidly increase our knowledge about how evolution has produced Earth’s biological diversity, and tell us more about the role of genes in the lives of the microbial, plant, and animal species of the Earth.
Nevertheless, there are some social and ethical concerns. This new gene-editing method could also be used for “germ-line modification”; that is, altering the genes of egg and sperm cells, thus changing the genetic make-up of future generations without their consent. This might be good in some cases (reducing the incidence of some genetic diseases, for example), but could also have unpredictable consequences in other cases (attempting to make “designer babies” with higher IQs, lower aggression in males, or enhanced altruism, might have serious and undesirable side effects). Given the great complexity of the interactions of genes, proteins, and environmental factors, we don’t know nearly enough to plunge headlong into these possible new worlds. Greater knowledge of the interplay between genes and environment, and wider discussion by knowledgeable people of possible consequences of gene-editing technology is needed.
Further technical details can be found in the Wikipedia entries on “CRISPR,” “Cas9,” and “DNA repair (Double-strand breaks).”
Practical and ethical concerns about human germline modification are summarized by Eric Lander in his 2015 article, “Brave New Genome,” in The New England Journal of Medicine, available here: http://www.nejm.org/doi/full/10.1056/NEJMp1506446
1 August 2016
3. Are genetically modified mosquitoes safe and effective for mosquito control?
Answer: Yes, if the genetic modification is done right. Here are the facts about genetically modified Aedes aegypti. The wild mosquito species Aedes aegypti occurs throughout the tropical and subtropical regions of the world, and is a carrier of several viruses that can cause grave diseases in people: yellow fever, West Nile disease, dengue fever, chikungunya, and encephalitis. In some areas this mosquito carries the Zika virus, infections of which can cause serious fetal brain malformation. One way to lower the frequency of these diseases is to kill off mosquitoes that might be carrying the viruses. (Besides, for many of these viruses, there are currently no effective vaccines.)
Aedes mosquitoes have been genetically engineered in the laboratory. When males of the genetically modified strain (“GM mosquitoes”) are released, they mate with wild female mosquitoes, and all the offspring die.
A special piece of DNA, amounting to about 1/100,000th of the mosquito’s own DNA, was micro-injected into individual mosquito eggs. The mosquito larvae in which the procedure was successful were selected, raised to adulthood, and then mated with one another to produce large numbers of GM mosquitoes.
The inserted piece of DNA contains a “lethal gene” which when “on” interferes with the mosquito’s own genes, and the mosquito dies before becoming an adult. The inserted piece of DNA also contains another sequence of DNA making it sensitive to tetracycline, a common antibiotic. With tetracycline in the mosquito larvae’s food, the lethal gene is turned “off,” and the GM mosquitoes can be raised in the laboratory generation after generation. But when GM males are released into the wild, where there is no tetracycline, and mate with wild females, all their offspring die. (You’re not likely to see mosquito larvae coming out of their puddles and wriggling up to the pharmacist’s counter to have their tetracycline prescriptions filled!) Local decreases in the numbers of Aedes aegypti should reduce the chances of being bitten.
Is the method effective? Yes. In tests begun in 2010, and continuing, releases of millions of male GM mosquitoes into local neighborhoods in Brazil, Panama, and the Cayman Islands have, over a period of a few months, successfully reduced local wild Aedes aegypti populations to less than 10% of their original numbers. The release of male GM mosquitoes is more effective than other methods, such as trapping mosquitoes or spraying neighborhoods with insecticides. (In some places, the GM mosquito has been called “the friendly mosquito” by the locals.) Actual reduction in some of the viral diseases caused by mosquito bites is beginning to be seen, but more longer-term studies are still needed.
Is the method safe? Yes, both for people and for the environment. Male mosquitoes don’t bite, so the GM mosquitoes released into the wild would have no effect on people. And all the GM larvae produced in the wild, male and female, die before becoming adults, so the special DNA they carry has virtually no chance of coming in contact with people. Even if a female GM mosquito escapes or is inadvertently released from a laboratory, its bite does not transmit any mosquito genes or any harmful molecules it may contain as a result of being genetically engineered – so there would not be any consequences to people by that route.
The GM-mosquito releases appear not to have any environmental consequences that scientists can detect. The decreases in wild mosquito populations occur only locally, and increases in other mosquito species as a result has not been observed. (The effect of deleting Aedes aegypti from the menus of the dragonflies, small fish, and small birds that sometimes eat them is like having your local McDonald’s close down – you just drive to the next McDonald’s, or go to Burger King.) In the tests that have been carried out so far, there are no noticeable effects on other animals of eating GM mosquitoes – this is what would be expected, because animals that eat GM mosquitoes digest all the mosquito’s DNA molecules.
Each different application of a genetically modified organism needs to be judged for its own potential dangers and its own potential merits. In the case of this GM mosquito, after some years of testing, there appear to be no downsides – no detectable dangers to either people or the environment – and the program has the potential benefit of reducing the levels of some miserable viral diseases. The release of GM mosquitoes into local environments thus falls in the same category as other disease-reducing methods, such as adding disinfectants to water supplies, or recommending antibiotics and vaccinations against common diseases.
26 September 2016, revised 3 November 2016