Cracking the equine genetic code

In the past 10 years there has been an explosion in our understanding of genetics in human disease. In particular, there is a greater appreciation of how even very small differences in a person’s genetic make-up can lead to changes in the risk of an individual suffering from a particular disease.

This understanding has come as a direct result of the human genome project, which was a worldwide effort to crack the entire human genetic code. This code breaking exercise has led to a revolution in understanding of genetic diseases in man. Virtually every day researchers are identifying major medical breakthroughs, which are occurring as a direct consequence of the genome project.

For more than a century, genetic diseases have been identified in both humans and animals. It has been appreciated for a long time how some serious diseases can occur when an individual inherits a certain gene from their parents. These types of genetic disease are fortunately very rare in the horse.

An example of such a disease is combined immunodeficiency (CID), which occurs in Arabian horses:

• A foal will inherit copies of an abnormal gene from the mare and stallion (whom are carriers of that particular gene).
• Only when two copies of the abnormal gene are carried by an individual will the disease occur.The parents will be normal since they each carry only one copy of the gene. An affected foal inherits two “bad” genes and consequently a lethal inability to fight infection, and will die within the first few months of life.How common this disease is in the UK is uncertain, but experience suggests that Arabian horses in the UK may be affected less than in some other countries. There are now genetic tests available, and it is possible to screen breeding animals to identify carriers. Ultimately, it should be possible to decrease the incidence of this disease and potentially eradicate it entirely by such an approach.Causes of CID
• There are many factors behind why a horse may suffer from a disease, including the way it is managed, how it has been managed in the past, and, finally, genetics.
• The change of a horse’s genetic code can frequently be very minor, with only one or two changes of information in many millions leading to an altered risk of a particular disease, as well as the degree to which an individual may be affected.
• Identifying the presence of the genes does not necessarily mean that an animal will suffer from a particular disease. What it might indicate is whether the animal is at an altered risk of suffering from a disease.
• However, there are also likely to be genetic reasons why a horse may be at altered risk of breaking its legs.
• Unfortunately, our understanding of such genetic variation is very much in its infancy and we have a long way to go before this technology can be applied to clinical cases.Understanding geneticsThe human genome project, which has allowed understanding of genetics in man to reach its current advanced state, was a vast multi-national project that cost a huge amount of money.While some human data can be applied to the horse, there is much basic work still to be done before we can really identify the role of complex genetics in horse diseases.By its very nature, such research is expensive and requires collaboration. Currently, there is work going on worldwide into the role of complex genetics in the horse, but compared with the scale of the human project the total effort is miniscule, so progress is never going to be speedy.Ultimately, it is likely that we will have a variety of genetic tests to identify animals of either decreased or increased risk of suffering from a particular condition.The question of whether such knowledge would be useful to the equine industry has still to be answered.But one of the most interesting and controversial areas where complex genetic analysis may play a role is in identifying animals which could perform better. It may not be useful to know that a horse has a somewhat decreased risk of suffering from a particular disease.However in competition, where small improvements may be enough to win, identification of complex genetic traits that improve performance by even a fraction could endow huge advantages to those interested in selecting competition horses.This type of work is very new and is bound to create debate, but in the future such tests may become available.There is no doubt that over the next decade there will be major advances in the analysis of complex genetic traits in the horse. Then the debate can start in earnest about how we can use this technology for the benefit of the horse.

This article first appeared in Horse & Hound (10 June)