How it Works: A Self-Limiting Gene

a pouring liquid

Over one million people die each year from a deadly animal. Can you guess the animal? I’ll give you a hint; the animal adds a buzz and a bite (or two) to otherwise delightful summer evenings. Do you know the answer?

The answer is…the mosquito! Mosquitoes cause over one million deaths each year when they transmit viruses such as Zika, Dengue, Yellow Fever, and Chikungunya or the parasite, Malaria.

The number of deaths caused by mosquitoes can be reduced when we prevent mosquito bites by using insect nets or repellents (like DEET and nootkatone). Alternatively, we can kill the mosquitoes by slapping them, using insecticides, or by using genetically modified mosquitoes that have been created by the company, Oxitec.

Oxitec, has genetically modified mosquitoes so that they contain a self-limiting gene. When these mosquitoes are released into the wild, they mate with wild mosquitos and the offspring never survive to adulthood.

But how does the self-limiting gene work?

First, let’s step back and consider that each mosquito is made up of many cells. In order for each cell to survive, it needs to make essential proteins. The proteins are made when genes (made of DNA) are converted into RNA and subsequently the RNA is converted into protein.

DNA → RNA → Protein … Happy cell

However, when the self-limiting gene (DNA) is made into a protein, the self-limiting protein is able to block the process of converting DNA into RNA.

DNA ✗ RNA ✗ Protein … dead cells, dead mosquito

Without RNA, proteins aren’t made, the cells die, and the mosquito dies too.

This is a pretty fascinating way to reduce the wild mosquito population. Yet, it leaves us asking, if self-limiting genes kill mosquitoes, how do the mosquitoes survive long enough to be released into the wild?

The answer is to control when the self-limiting gene is made into a protein by using the drug, tetracycline. When tetracycline is present in the factories at Oxitec, the self-limiting protein is not made and the mosquito can survive. However, when this mosquito is released into the wild, tetracycline is no longer available. Consequently, the self-limiting gene is made into a protein, blocks the conversion of DNA into RNA, and ultimately kills the mosquito.

To make sense of this, I find it easiest to consider a model. We can think of the liquid in this bottle as the self-limiting gene.

b bottle full of red liquid

And we can think of this beaker as a cell within the mosquito.

c bottle and beaker

Each cell of the mosquito has the self-limiting gene.

d beaker filled with red liquid

But in the presence of tetracycline (represented by saran wrap), the self-limiting gene is not made into a protein.

e beaker with wrap

When this mosquito is released into the wild, it mates with wild mosquitoes to make baby mosquitoes. All of the baby mosquitoes inherit the self-limiting gene. However, because tetracycline is not present in the wild, the self-limiting gene is made into a protein and kills all of the baby mosquitos.

d beaker filled with red liquid

When these genetically modified mosquitoes have been released into the wild, they can reduce local populations by up to 90%. With fewer mosquitoes, there is a lower chance for diseases to be transmitted from mosquitoes to humans. It will be exciting to see the impact of these genetically modified mosquitoes. In the meantime, you can learn more here:

Oxitec

 

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