Researchers at Iowa State have discovered the “bubble boy syndrome,” otherwise known as SCID, in swine, according to a report in the Iowa State Daily at Iowa State University.
The National Human Genome Research Institute says SCID, or severe combined immunodeficiency, is usually characterized by a lack of the adaptive immune system, a crucial part of the immune system.
“The immune system has two major arms,” says Chris Tuggle, professor of animal science. “The innate immune system, which recognizes when there is a foreign object — such as a sliver — in the skin and goes to work immediately. There’s also the adaptive immune system, which responds very specifically to what type of pathogen is present in the body.”
SCID is a genetic defect found in humans, mice, dogs and horses, but this is the first time SCID has been found in swine.
Jack Dekkers, professor of animal science and one of the researchers on the project, was able to explain how this discovery was a happy accident.
“We were doing research on feed efficiency, developing a line of swine that grows faster with less feed, which is important for the swine industry because of the high cost of feed,” Dekkers says.
While testing how the current line held up to a normal disease challenge using facilities at Kansas State University, four piglets died early on in the process, causing some concern. When an autopsy was performed on these piglets, it was revealed they each had a very poorly developed immune system.
“It’s a finding that was totally unrelated to the [feed efficiency study],” Dekkers says, “but when we looked at it carefully, we knew it was something completely new and very important.”
After realizing the piglets were affected by SCID, the team set out to find the exact nature and cause of the immunodeficiency with the help of researchers at Kansas State University. Because the disease could only be passed down if both parents carried the gene, the search began for the recessive gene responsible for the lack of the piglets’ immune systems.
Emily Waide, graduate student studying quantitative genetics, has been with the project since October. “I have been using the genotypes of the piglets to try to find the gene that is responsible for the disease,” Waide says.
Because this is the first time SCID has been discovered in swine, there are some drawbacks due to rarity. Waide admits there are some difficulties that set her work apart from others. “It’s not like a lot of other data sets where there are 2,000 animals,” Waide says. “I only have 50 piglets to work with.”
More research has developed as the disease was found in more piglets. The researchers decided to treat the SCID piglets just as they would a human with the disease by giving the piglets a bone marrow transplant.
“Basically, we give them an immune system through the bone marrow transplantation procedure,” Dekkers says.
Matthew Ellinwood, associate professor of animal science with a background in veterinary medical genetics, had some experience with both SCID and bone marrow transplants in animals. He oversaw the bone marrow transplantation of the piglets. Ellinwood says he was excited to do the procedure because he had worked on transplants in SCID dogs.
“I had done bone marrow transplants before, and when Jack Dekkers came and asked how to do this, I said we would first need to destroy the immune system of the animals to be transplanted,” Ellinwood says. “[Dekkers] said, ‘Well I don’t think we need to do that ... because we have SCID pigs.'”
Ellinwood says for this procedure the swine would just have to receive the new marrow. Three SCID piglets from two litters born in January received the procedure. “We just got the data, and it looks like they are engrafting,” Ellinwood says. “They are all doing clinically well.”
This success gives the team hope for the future uses of this discovery, such as using these swine as a biomedical model for humans.
“This discovery is an incredibly important one,” Ellinwood says.
Jason Ross, assistant professor of animal science, has researched using animals as biomedical models. “The majority of human health research has been in rodent models, but they don’t always replicate the human conditions,” Ross says.
While it is more expensive to do research with swine instead of rodents, the similarity to humans is worth the extra money. Ross says rodents do not always demonstrate the same phenotype as humans despite similar genetic mutations such as cystic fibrosis, a genetic disorder that affects the lungs, pancreas, liver and intestines in humans for which there is no good rodent model.
Another issue with using rodents is their comparative size to humans. For example, retinitis pigmentosa, a type of progressive retinal deterioration that eventually leads to incurable blindness, would be challenging to study with the tiny size of rodent eyes.
A pig’s eye, which is very similar in size and function to that of a human eye, can make translational research more feasible. “It’s important to remember that there’s no ‘perfect’ model,” Ross says, “but you need to identify what will be the most useful model to use.”
There are currently no known limits to what this research could lead to.
“With the SCID piglets, we can better study how to treat people who have a weakened immune system from things like AIDS to radiation treatments,” Dekkers says. “SCID mice are extensively used in the study of many human diseases, including cancer. Availability of SCID pigs will make such research much more directly applicable to humans.”