In November 2015, scientists built a malaria-resistant mosquito by modifying its genes. Genetic engineering could be very promising for some of the most dangerous human diseases. With the recent health scare of the Zika pandemic, scientists are working around the clock to find a way to combat this disease.
Zika virus disease is caused by a virus transmitted by mosquitoes of the Aedes genus. The virus is found in parts of Africa as well as South America. Symptoms of Zika virus disease include fever, headache and muscle pain. Unlike most viruses, the Zika virus is able to cross the placenta in pregnant women which increases the chance that their unborn baby is infected with the virus. There is increasing evidence that babies born with Zika virus suffer from a condition called microcephaly or “small brain”. However, scientists are unsure if these babies’ microcephaly is the result of their mother’s Zika virus infection during pregnancy. The Public Health Agency of Canada is warning pregnant women to avoid travel in countries which currently have Zika outbreaks.
Right now, there is no treatment or vaccine for Zika. Scientists are currently looking at multiple ways to genetically engineer the DNA of mosquitoes that transmit this virus:
Oxitec, a UK based company, wants to release genetically modified mosquitoes into the wild. Oxitec has inserted a so-called ‘self-destruct’ gene into its mosquitoes. To keep these mosquitoes alive in the lab while they perform their experiments, Oxitec administers an antibiotic called tetracycline. Tetracycline acts as an antidote against this inserted ‘self-destruct’ gene, thus allowing the mosquitoes to survive. Without tetracycline, then, these self-destruct genes remain active and kill the mosquito.
This idea isn’t really new. In fact, it’s a spin-off of a birth-control method for flies that the US popularized and successfully used in the 30’s and 40’s to eliminate an insect called the screwworm. The technique was called “Sterilized Insect Technique” or SIT, for short. As the name suggests, scientists would sterilise male screwworms by irradiating them. This effectively kept tabs on the screwworm population until they had been completely eradicated.
As with most things related to technology, there is controversy around Oxitec’s idea. The most obvious concern is whether or not this will have unforeseen consequences on ecosystems as a whole.
Another concern has to do with how evolution works. Generally, natural selection will get rid of ‘bad’ genes – genes that don’t allow the organism to survive. This same concept can be applied to Oxitec’s mosquitoes, which may carry genes harmful enough to kill the mosquitoes before they have a chance to mate. This means that the gene won’t be passed onto their offspring.
Natural selection, therefore, may be a potential problem for Oxitec’s genetic modification of mosquitoes.
After its success in creating malaria-resistant mosquitoes, scientists are interested in a technology called “gene drive” to eliminate the Aedes mosquito that transmits the Zika virus. Gene drive allows scientists to introduce new genes across a population of mosquitoes as they reproduce.
Gene drive circumvents the normal rules of inheritance. Every organism has a pair of chromosomes – one from mom and one from dad. Normally, you have a 50% chance of getting any one of these chromosomes from either your mom or your dad. With gene drive, scientists insert a new gene into the mosquito’s genome. This gene then copies and inserts itself into the second chromosome, by cutting and attacking it. This means that offspring of the mosquito in question have a 100% chance of getting this gene and all the traits it confers.
One thing to keep in mind about mosquito-transmitted viruses is that only female mosquitoes can be disease carriers. Female mosquitoes require the iron from human blood to make their eggs, so they need to bite humans. Their male counterparts, on the other hand, get their nutrition by sipping on plant nectar. Since only female mosquitoes are disease carriers, scientists are trying to figure out ways they can change their sex so that they are harmless to humans.
Right now, scientists are interested in a gene-editing tool called CRISPR. CRISPR would allow scientists to insert ‘male genes’ into female mosquitoes.
Last year, Virginia Tech’s Zach Adelman and co-author Zhija Tu discovered the genes responsible for determining the male sex of mosquitoes. When they activated this gene in female embryos, they grew male genitalia!
A Final Word
While all three methods show promise for the future, WHO, has declared their support for Oxitec’s self-destructive mosquito. Since the mosquitoes die relatively quickly, they won’t survive long enough to potentially ‘harm’ the ecosystem. What’s more is that Oxitec can also insert a fluorescent marker into their genetically modified mosquitoes. This marker will be passed onto offspring so that Oxitec can monitor and track them.
Oxitec has conducted several trials with their self-destructive mosquitoes in the Cayman Islands and Brazil. The FDA has concluded that Oxitec’s mosquitoes will have no significant impact on the environment. We’ll have to wait to find out what the future holds.