Another success of CRISPR-Cas9 has just been announced with the production of pigs that are likely resistant to one of the scourges of porcine husbandry, Porcine Reproductive and Respiratory Syndrome (PRRS).
This viral disease infects thousands of pigs worldwide and is highly fatal, especially for pregnant sows or young pigs. As for the industry itself, hundreds of millions of dollars are lost annually in the US alone, with the worldwide total measuring in the multiple billions.
Now, in a study by researchers at the University of Edinburgh and in collaboration with the Pirbright Institute, a viral disease research center in the UK, and Genus PLC, a UK company focused on farm animal genetics, this may all be something in the past.
You Know What Time It Is
But first, let’s do what Bioscription is wont to do and discuss the history of PRRS and how it works, so a better understanding of how the scientists developed resistance in their pigs can be explained.
The first large-scale outbreak of the disease in North America and Europe happened almost simultaneously in 1980, presumably from the same group of infected shipments. It took some time before farmers and then scientists were able to determine that a virus was responsible for the condition, with it having the title “mystery swine disease” up until that point.
The general symptoms include normal flu-like conditions, along with increasingly severe respiratory distress, sometimes resulting in direct asphyxiation. In pregnant sows, the effects are more focused, often leading to forced abortion 30% of the time and still killing 80% of live born piglets eventually anyways.
This is also the same death rate for piglets at the same age that otherwise become infected from other pigs or their mother’s milk. For those that do survive, they have significantly reduced growth rates and weakened systems overall.
The Viral Components
The virus itself is a plus-stranded RNA virus that has an enclosed shell and lies within the Arteriviridae family, a particularly nasty group of viruses that have it out for various species. Other well-known diseases its siblings cause include equine arteritis and simian haemorrhagic fever.
Luckily (sort of, but not really), the cell types that it prefers to infect are fairly limited. Monocytes, also known as leukocytes or white blood cells, are its primary target, though it will also go after the cells that leukocytes differentiate into, namely macrophages.
On these cells, there is a specific receptor called CD163 that the virus latches onto. This receptor allows it to fuse its enclosed membrane with the host cell, injecting its viral material into the cell itself. Prior research has indicated that it is specifically subdomain 5 on the receptor that is interacting with the virus, so it is this that researchers aimed to change.
Do Your CRISPR Magic
Using CRISPR-Cas9, the scientists deleted the exon segment that coded for the subdomain on the CD163 gene, thus leaving the receptor itself intact but removing the interaction site. After this was performed on pig zygotes and they were brought to birth, two specific cell types were collected from them: pulmonary alveolar macrophages (PAMs) and peripheral blood monocytes (PBMCs).
These two cell types, as you might guess from their names, are two examples of cells that the virus goes after. With a followup in vitro (meaning an experiment taking place in a culture dish) test, the scientists were able to confirm that the cells were completely resistant to both genotypes of the virus and the various subtypes.
This was further confirmed by putting the cells under a microscope and showing that they lacked the subdomain point required for the virus to begin fusion of its membrane with the cell.
A Long-Running Series Of Experiments
The pigs themselves also grew up to be healthy, with no complications from the modification at all. This helped to confirm that removing the subdomain on the receptor did not damage their cells at all, a necessary precaution to make sure of from the experiment. Their eventual offspring were also confirmed to be healthy.
It should be noted that this is not the first experiment ever conducted to produce PRRS-resistant pigs, including with using CRISPR genome editing to accomplish it. An experiment in 2016 by Whitworth et al used a different method involving inserting a stop codon into the CD163 gene, thus stopping it from being read in the first place. But this stopped the functionality of the entire receptor.
Another experiment earlier this year replaced the subdomain section of the CD163 gene with the coding for the human version of the subdomain from the equivalent CD163-L1 gene. However, the results of this experiment created pigs that were resistant to genotype 1 of the virus and its subtypes, but not genotype 2.
Saving The Piglets
Due to this current experiment proving resistant to all virus types and keeping the functionality of the CD163 receptor, the authors feel that the specificity of their CRISPR-Cas9 usage is a better option for the creation of future PRRS-resistant pigs.
Also, though I find the distinction and argument pointless but it sadly is still relevant for legislative purposes around the world, this method produces a pig that is non-transgenic, as there is no addition of a stop codon or a human equivalent sequence, but instead just a removal of a small section of DNA.
With this new experiment and all the experiments preceding it, true production of disease-resistant pigs can now commence, saving the lives of thousands of pigs and especially piglets worldwide. And saving a significant chunk of change to boot.
Photo CCs: Sow and five piglets from Wikimedia Commons