The Influence of Genetic Deficits in Aggrecan Structure (Cartilage) on Race Track Breakdown

Racetrack breakdowns are tragic for the horse and all involved. In some instances, the breakdowns can be life threatening for horse and rider. One possible source of racetrack breakdowns is failure of cartilage to provide sufficient cushion for joints. The biomechanical stresses in horses are probably greater than in any other mammalian system under any athletic activity. Aggrecan is a protein which makes up part of the extracellular matrix for cartilage and other connective tissues. The CS1 unit of aggrecan is a portion that binds chains of chondroitin sulfate. Chondroitin sulfate chains associate with water and are responsible for a large part of the cushioning aspect of cartilage. Variation in the number of CS1 units has been seen in humans, ranging from 13 to 34 chondroitin sulfate binding domains. Variation in the CS1 subunit of aggrecan has also been associated with degeneration of lumbar disks of people in several studies. While horses do not typically experience problems with lumbar disks, they do suffer injuries to articular cartilage, of which Aggrecan is a key component. We are proposing to investigate the extent of variation found among Throughbred horses then investigate whether decreased size of the aggrecan CS1 subunit is associated with fatal breakdowns in horses. We will test 200 randomly selected Thoroughbred horses for length variation. Then we will test DNA from approximately 90 horses which experience fatal race track injuries since 2009. We will compare the distribution among the two groups. If variation in the size of aggrecan is found to have an impact, there are tthree possible solutions: 1) increase the density of aggrecan molecules or length of chondroitin sulfate side chains through some form of management, 2) manage horses with predisposition to cartilage degeneration in a fashion to avoid breakdowns, and 3) avoid using horses for racing with aggrecan deficits. The immediate contribution of this work will be to determine the extent of variation among Thoroughbreds, provide preliminary data on its impact on fatal racetrack breakdowns and provide a basis for further investigations into the primary causes of cartilage defects among horses.