Searching for a solution to PRRS via genetics

Interview with Dr Lucina Galina Pantoja, PIC

Porcine Reproductive and Respiratory Syndrome (PRRS) virus has been a troublesome pathogen for swine producers around the world for three decades. Various strategies have been introduced – from better biosecurity to vaccination – but the search continues for a solution to control the virus once and for all. A route through genetics might provide that solution, explains Dr Lucina Galina, technical project director at pig breeder PIC in the United States.

Scientist and veterinarian Dr Lucina Galina Pantoja has spent most of her professional career finding ways to limit the damage caused by respiratory diseases including Porcine Reproductive and Respiratory Syndrome (PRRS) in swine production. In that mission she is not alone. Nevertheless, 30 years of science hasn’t led to a final solution against the virus. Dr Galina says: “That is bizarre. Porcine Circovirus 2 (PCV2) entered the pig industry years later. And there has been a decent vaccine against that virus for years.”

These days she is employed by pig breeder PIC. She has been leading a project to breed genetically modified pigs, resistant to PRRS virus. That innovation could constitute the breakthrough to get rid of PRRS.

Can PIC guarantee that pigs without the fifth domain of the CD163 perform and behave like before?

“Based on internal research, pigs lacking the CD163 domain 5 (the PRRSv receptor) behave like conventional pigs based on production, reproduction and meat quality. Key performance indicators were compared between pigs carrying two versions of the CD163 gene lacking domain 5 (PRRS resistance is a recessive trait, so pigs need to be homozygous for the desirable gene) versus controls carrying one gene or none.

“Twenty production indicators were compared, including birth weight, weight at 140 days, carcass weight, mortalities and slaughter health checks. Reproductive performance indicators were also compared, including total pigs born, total born alive, total born dead, etc. Furthermore, over 97 meat quality and composition analysis traits were compared. The results indicated no statistically significant differences between the two groups. We have applied for FDA approval, which will have the final say on safety and efficacy in the United States.”

Are there no risks to human health by consuming pork of edited pigs?

“The safety of PIC’s PRRS-resistant pig is currently being evaluated by the United States Food and Drug Administration. As part of that process, PIC has submitted edible tissue compositional studies for FDA review. The CD163 protein has a proven track record of being safe for consumption in food derived from animals. In animals that inherited the genomic alteration, the CD163 protein is a shorter version of the unedited protein; the only difference is a single amino acid change. It is important to note that the pork is not from pigs directly edited but rather the offspring of edited pigs; animals edited were multiple generations in the past. It is expected that gene editing will have broad applications. At a high level, 3,500+ gene therapies are being developed for human medicine, half for cancer.”

Will this intervention even work for future types of PRRSv that arise after mutations or recombinants?

“Like other RNA viruses, PRRSv is known for constantly changing. Therefore, we must be vigilant and monitor resistance over time. There are currently two types of PRRSv, types 1 and 2, and new classifications further divide the virus into different lineages (L1-L9) and sub-lineages. We tested resistance by exposing pigs to types 1 and 2 and multiple lineages. Our studies to date have included representative isolates of 90%+ of the most dominant and contemporary lineages and sub-lineages diagnosed in the United States. In pigs carrying two versions of the resistant gene (homozygous resistant), we couldn’t detect PRRSv genomic material (using the PCR test) or an immune response against the virus (using the ELISA antibody test) up to 21 days post-infection.

“The oldest PRRSv isolate tested in our pig infection studies was recovered from pigs in 1997, and the most recent isolates were in 2022. We found similar results testing monocytic cells in laboratory conditions. What we know is that resistance has been effective against isolates that we have tested, which emerged and evolved for 25 years.”

When do you expect the FDA’s permission to sell PRRS-resistant pigs in the USA?

“PIC expects to hear a decision from the Food and Drug Administration in the first half of 2024. PIC is also seeking regulatory approvals and government determinations in several other countries. It is important to realise that even with FDA approval, the launch and commercialisation of the PRRS-resistant pigs will depend upon multiple factors, including approval in key export markets, etc.”

In the Netherlands, for example, we have around 750,000 sows in production. How long does it take to transfer this herd from a conventional one into a PRRS-resistance sow herd?

“It will take multiple years to populate a given herd with PRRS-resistant pigs. Theoretically, if you were going to start populating a farm today, using only semen from sires that are homozygous resistant and with industry-standard replacement rates, by the end of the third year, you would produce 25% resistant pigs in the herd and by the end of the seventh year you would produce 70% resistant pigs. This timeline could be expedited by not only using semen from resistant boars but also using females carrying at least one version of the favorable gene, inseminating females with known genotypes, increasing the sow replacement rate, etc.”

Has PIC already discussed this project with the authorities of the European Union and the possibility of introducing it this side of the ocean?

“The European regulations on gene editing in plants and animals are rapidly evolving. The UK passed the Genetic Technology (Precision Breeding) Act for plants and animals in March 2023, which allows the use of gene editing technologies. The same year, the European Commission (EC) recognised the potential of new genomic techniques (NGTs), such as gene editing, to improve plant growth and contribute to sustainable food systems. Climate challenges and reduced yields due to crop failures have made it clear that global food systems are fragile and need more resilience.

“The current GMO legislation is unsuitable for NGTs, and the EC has proposed policy actions to exclude gene-edited crops from the GMO legislation and create separate pathways for NGT plants to enter the market. In a recent development, the European Food Safety Authority (EFSA) is expected to provide scientific opinion on animal NGTs in 2025, which is a key step in aligning plant and animal regulations. Breeding companies, including PIC, are actively involved in the continued evolution of the new European regulations.  While the regulatory landscape is changing in Europe, PIC is seeking regulatory approvals and determinations in multiple other countries; for example, Colombia recently indicated gene-edited pigs are considered regular pigs, not genetically modified organisms.”

The PIC approach

The PIC approach is built on years of research. A discovery in 2010 formed a basis for further research and development. In that year, it was revealed that Porcine Reproductive and Respiratory Syndrome (PRRS) virus manages to multiply. It needs the machinery of white blood cells, such as monocytes and macrophages, to make its own proteins.

Monocytes and macrophages re important in a pig’s immune system. When they encounter PRRS virus, it attaching itself to the fifth domain of the white blood cell’s protein CD163 – a protein which has a cleansing function. Normally a white blood cell will capture a virus within a vacuole to destroy it (a process called phagocytosis) – but in the case of PRRSv, the virus will attach itself to this CD163 protein. That way the virus can bring its genetic RNA material outside the vacuole close to the contents of the white blood cell – and thus multiply.

Seven years later, researchers from Scotland, UK, found a way to apply that knowledge. They did so by knocking out the so-called seventh “exon” from the pig gene that is responsible for creating CD163. Losing that piece of genetic material leads to pigs being born without this fifth domain in the CD163 protein. As a consequence, the PRRS virus will not be capable of attaching itself, nor multiplying – in other words, the pig will be resistant.

Is gene editing a potential solution against other viruses, like influenza?

“With a growing demand for animal protein, gene editing of farm animals could be a solution to increase health and productivity. Exploring new technologies is important for controlling infectious diseases, especially when current control options have limited success. Several examples of gene edits in animal agriculture can be found in the scientific literature, including hornless cattle (termed “polled”), slick hair coat cattle that are more tolerant to heat, red sea bream, and tiger puffer fish that grow bigger for the Japanese market, chicken more resistant to influenza, etc.

“It is important to note that the technology can benefit multiple stakeholders in the food chain. For example, the PRRS-resistant pig can provide benefits to the pig (improved welfare), producers (increased productivity), packing and processors (resilient supply chain, support of antibiotic policies), food services, retail, and consumers (lower environmental impact). In humans, the advantages are equally important. According to WHO, gene editing shows tremendous potential to help treat and prevent human diseases, including HIV, sickle-cell disease, and a variety of cancers.

“So, it will take time to explore gene editing as a tool to contain viruses and improve pig health, but we believe gene editing will be an important tool going forward.”

This interview was originally published at the Dutch trade journal Boerderij.
The interview was conducted by Kees van Dooren, senior reporter.