DNA recipe for a pig
The bacon sandwich is a British institution, enjoyed by millions each weekend as they recover from the night before. But that’s not why scientists are so interested in pigs, well not all of them anyway. Our porcine friends have another – some would say more valuable – use, in medical research.
Almost unwittingly, the genetics of pigs has been of interest to humans for millennia; Europeans and Asians independently domesticated pigs about 10,000 years ago. Ever-since, we have been selecting the characteristics we like the most, in effect, genetically engineering the species to be tastier, produce more meat and be less susceptible to certain diseases. When comparing the genomes of ancient wild breeds and modern-day farm pigs, researchers have found that we’ve even inadvertently added a new bone to the spine, making the pig longer and thus produce more bacon.
That’s just one of the findings of the pig genome project, released last week as the first annotated sequenced pig genome was published. Only humans and mice have had their genomes mapped so fully.
For the past couple of decades scientists have been slowly teasing out the information held in the pig’s DNA, applying it to breed healthier and meatier pigs, and to try to create more faithful models of human disease.
Pigs are more expensive to keep than rodents, and they reproduce more slowly. But the similarities between pig and human anatomy and physiology can sometimes make them the most suitable animal to use in an experiment. UK Government statistics show that 4,340 scientific procedures were carried out on pigs in 2011. With similar-sized eyes to people, the pig became the first model for retinitis pigmentosa, a cause of blindness. And four years ago, researchers created a pig model of cystic fibrosis that, unlike mouse models, developed diseased airways resembling those suffered by people. Their notorious appetite has also made them valuable models of diabetes.
Such similarities also mean that pigs are the favoured animal donor of replacement heart valves for surgeons, and the possibility of whole organ transplants is no longer science fiction. Armed with their knowledge of the genome, scientists are trying to engineer pigs that could be the source of organs, including heart and liver, for human patients. Pig organs are roughly the right size, and researchers hope to create transgenic pigs carrying genes that deceive the recipient’s immune system to avoid organ rejection. Pig hearts have already been successfully transplanted into monkeys.
Back in the lab, pigs do have one very unfortunate feature – their size. At up to two metres long and weighing around 350kg, a normal farm pig is certainly not as convenient as a mouse. So in the late 1960s, going back on 10,000 years of farm husbandry, scientists began breeding smaller pigs, eventually creating the mini pig. At only 50cm or so tall, these miniature pigs weigh around 45kg and can be kept in reasonably small enclosures. They are now a commonly used laboratory animal around the world, replacing cats, dogs and monkeys in many experiments where larger mammals are needed.
By deciphering the pig genome, scientists can understand better how the pig’s genes are contributing to its growth, health and disease. It is the basic recipe for a pig. This is useful for farming but also allows scientists to manipulate the genome with great accuracy in experiments. By removing genes one by one, adding in new genes or mutating existing genes, scientists can understand their function and identify new drug targets.
In the draft pig genome paper the authors pointed to 112 gene variants that they had found which might be involved in human diseases. No doubt the genome has more secrets yet to reveal, and they won’t just make your breakfast taste better.