Pacing horses owe ability to gene mutation

A study published in Nature today has linked a single gene primarily responsible for the ability to pace in horses with the discovery of a new type of nerve cell in mice, resulting in a breakthrough for our understanding of the neural networks underlying locomotion in vertebrates.

Two groups associated with the Science for Life Laboratory at Uppsala University combined data from their independent work and came to the conclusion that the gene DMRT3, which is expressed as a protein in a certain type of spinal cord neuron, is the location of a mutation which influences gait in horses.

A single base change in the DMRT3 gene was found to be associated with the ability of some Icelandic horses to perform an ambling gait called the tölt and a flying pace as well as the walk, trot, canter and gallop exhibited by other horses. Pacing involves the legs on the same side moving forward together rather than diagonally opposite legs moving in synchrony. The mutation has since been found in other breeds, such as the Tennessee Walking Horse, the South American Paso Fino and horses bred for harness racing, in which the horse is required to pace.

Professor Leif Andersson, from Uppsala University and the Swedish University of Agricultural Sciences, who was primarily involved in the work on horses spoke to ScienceOmega.com to explain more about the research from his perspective and describe the wider implications for our knowledge of the neural systems involved in locomotion…

What led you and your team to conduct your part of the study?
For a long time we’ve been aware of this very interesting variation in Icelandic horses. You can divide Icelandic horses into those that can pace and those that cannot pace. It’s a very clear distinction between the two groups and of course, as a geneticist, I was interested in trying to understand where that difference arises. For us, it looked like a unique opportunity to learn something about the genetics of variation in locomotion.

Were you surprised to find that such a small genetic variation coded for the trait?
We conducted a genome-wide association analysis on 70 Icelandic horses, 40 of which were pacers and 30 which were non-pacers. We screened the genomes and were amazed to find that there was one single region which showed a highly significant association with the trait.

It was a single locus which explained the most potent difference between pacers and non-pacers. You could make a comparison with human height, for instance. In the most recent study where 300,000 or so people were analysed, around 700 loci in the genome were found to influence height in humans. You would think that gait in horses could be at least as complex a trait as human height, yet we found one main locus which expresses the most important part of that difference.

It is important to stress that this is probably not the only factor – there will be others which contribute – but this seems to be the dominating factor in determining whether a horse can pace or not.

Can some horses be taught to pace?
Our view is that you cannot take an ordinary horse and train it to pace; the horse needs the genetic predisposition to do so. It is uncertain whether all horses that have this mutation can pace. The mutation is described as ‘permissive’; it is required, but possibly not sufficient for expression of the characteristic. All of the Icelandic horses we tested that could pace carried this mutation without exception. We have not yet found a horse that does not have this mutation and can pace.

Are horses unusual in the variety of distinct gaits that they can perform?
This is where the clue to this whole study lies. The gait of the horse is important to the ways in which we can use it. The driving force behind this would have been that humans thousands of years ago or more noticed there were some horses with a different gait which made riding more smooth and easy. That was an advantage in a time when they were no cars and people were spending days and days on horseback. If it produced a smoother ride, that trait would be favoured.

In this study we observed that the mutation is present in a number of breeds in Europe and America. We expect to find it also in Asia, and that is what we are focusing on in the follow-up work.

What are the implications of this discovery for the way that we understand the relationship between horse gaits and the neural circuitry involved in locomotion?
In this case, using typical positional cloning, we were able to identify this gene mutation purely based on a genetic study. We could claim that this must be the genetic change underlying the difference in locomotion in horses. It follows to ask what the function of a gene that controls gait in horses is.

I was well aware of my colleague here at Uppsala, Professor Klas Kullander, and his work on spinal cord function and the coordination of movement by neurons. I told him that we had made an important discovery regarding locomotion in horses and which gene we had identified. He was amazed because they had been studying, from a completely different approach, those genes with an interesting expression pattern in the spinal cord. This gene, DMRT3, was one of the hits they got with a transcription factor which marks certain neurons. The position of expression of those neurons seems perfect for it to be involved in regulating locomotion and coordination.

They had suspected that these neurons and this gene were important for gait and our data confirmed it. For our data, this provided an obvious mechanism to explain why a mutation on this gene would have this effect. When you have found a mutation on a gene controlling gait you wouldn’t expect it to be expressed in the skin or eyes, for example. The fact that this gene was expressed in this position made us 100 per cent sure that it was the correct genetic identification.

How did removal of the DMRT3 gene manifest in mice?
The knockout mouse was developed by another group several years ago. It’s not unusual for a group to make a knockout mouse because of a transcription factor that looks like it may be interesting. Not much happened, so they were disappointed of course and almost didn’t publish the paper.

Although the neural circuits looked to be very chaotic in the newborn mice, after a few days they developed compensatory mechanisms. If you observed them running around the cage you couldn’t really tell the difference, but under more rigorous testing it became evident that there was a defect in their locomotion. That was the final confirmation of the identification of DMRT3 as a crucial gene for controlling coordination of movements in vertebrates.

Does the research increase our understanding of locomotion in mammals and vertebrates more generally?
Yes. We have identified a subset of neurons that apparently has a crucial role for controlling coordination of movement. The characterisation of these neurons is very interesting. They are interneurons, which means they cross the midline of the spinal cord connecting the right side with the left side.

The horse data suggests that they must also coordinate the hind legs with the fore legs. What our colleagues have shown is that these are inhibitory neurons connecting directly to motor neurons – the neurons which regulate muscle contractions. What you could foresee with the horse is that when it moves its left hind leg, the signal inhibits the movement of the left foreleg, resulting in the normal trot. With the mutation, this strict regulation is loosened, allowing the horse to move the leg on the same side. This is why we don’t believe it is possible to train a wild-type horse; it needs this mutation to have the effect.

It is interesting think about the movement of human arms in synchrony with legs. If you walk and let your arms swing, your right arm follows your left leg and vice versa. You can try to pace, and I can pace if I think about it, but you feel how your neurons don’t like that – they promote the contra-movement of arms and legs. It’s fascinating to think how sophisticated the regulation system must be for you to be able to walk and run and for your muscles to compensate so that you don’t fall.