The bovine viral diarrhea virus (BVDV) is one of the most dangerous pathogens affecting the health and wellbeing of cattle worldwide, costing the U.S. industry billions of dollars annually. First identified in the 1940s, BVDV can be disastrous to pregnant cows since it can infect developing calves, and often leads to spontaneous abortions and low birth rates.
While some BVDV-infected calves manage to survive, they remain infected for life, and shed massive amounts of virus to other cattle. Moreover, despite the fact that vaccines against the virus have been available for over half a century, they are not always effective in stopping transmission.
Over the past two decades, scientists have managed to identify the main cellular receptor (CD46) and the area where the virus binds to that receptor, causing infection in cattle. Now, a team of researchers led by USDA’s Agricultural Research Services (ARS) has used gene-editing technology to slightly alter CD46 so it would not bind the virus, yet retain all its normal functions in bovines.
After successfully testing this idea in laboratory cell cultures, the scientists collaborated with Acceligen – a company specializing in precision breeding technology aiming to improve animal welfare and resistance to disease – to genetically edit cattle skin cells in order to develop embryos carrying the altered gene. The team then transplanted the embryos into surrogate cows to assess whether this approach could reduce viral infection in live animals.
The technique proved to be highly successful and the first CD46 gene-edited calf, named Ginger, was born healthy on July 19, 2021. After keeping the calf under observation for a few months, the experts housed it for a week with a BVDV-infected dairy calf which was born shedding the virus.
Ginger’s cells displayed significantly reduced susceptibility to the virus, resulting in no observable adverse health effects. Although these results are highly promising, the scientists will continue to closely monitor Ginger’s health and ability to produce and raise her own calves.
This proof-of-concept study demonstrates the revolutionary possibilities of gene-editing technology in reducing the burden of BVDV in cattle.
At the same time, since BVDV infection puts calves at risk for secondary bacterial infections, using gene-editing technology to breed cattle resistant to this virus may also diminish the need for antibiotics in agriculture.
The research is published in the journal PNAS Nexus.
Gene editing is a group of technologies that gives scientists the ability to change an organism’s DNA. These technologies allow genetic material to be added, removed, or altered at particular locations in the genome. Several approaches to genome editing have been developed.
One of the most well-known is CRISPR-Cas9, which stands for “Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR-associated protein 9.” The CRISPR-Cas9 system has generated a lot of excitement in the scientific community because it is faster, cheaper, more accurate, and more efficient than other existing genome editing methods.
CRISPR-Cas9 was adapted from a naturally occurring genome editing system in bacteria. The bacteria capture snippets of DNA from invading viruses and use them to create DNA segments known as CRISPR arrays.
The CRISPR arrays allow the bacteria to “remember” the viruses (or closely related ones). If the viruses attack again, the bacteria produce RNA segments from the CRISPR arrays to target the viruses’ DNA. The bacteria then use Cas9 or a similar enzyme to cut the DNA apart, which disables the virus.
In the lab, scientists create a small piece of RNA with a short “guide” sequence that attaches (binds) to a specific target sequence of DNA in a genome. The RNA also binds to the Cas9 enzyme. As in bacteria, the modified RNA is used to recognize the DNA sequence, and the Cas9 enzyme cuts the DNA at the targeted location.
Although Cas9 is the enzyme that is used most often, other enzymes (like Cpf1) can also be used. Once the DNA is cut, researchers use the cell’s own DNA repair machinery to add or delete pieces of genetic material, or to make changes to the DNA by replacing an existing segment with a customized DNA sequence.
Gene editing is considered a type of genetic engineering. Other methods for genetic modification include gene targeting, zinc finger nucleases (ZFNs), and transcription activator-like effector nucleases (TALENs).
Applications of gene editing technologies are vast and include correcting genetic defects, treating and preventing the spread of diseases, and improving crops. However, it also raises ethical questions, particularly when it comes to editing the human genome. For example, there are concerns about its potential use in creating so-called “designer babies” with specified traits, such as intelligence or athletic ability.
There are also potential risks, such as off-target effects (unintended changes to DNA), and the long-term effects of gene editing are still largely unknown. It’s a rapidly evolving field with enormous potential, but it also requires careful regulation and oversight.