Dorsum of filly with HERDA.
Findings from the study on the genetic cause of hereditary equine regional dermal asthenia, or HERDA, which primarily affects the American quarter horse breed, were reported last week in the scientific journal Genomics by veterinary geneticist Danika Bannasch and colleagues at the UC Davis School of Veterinary Medicine. A patent on the newly developed test is pending.
"Identification of the gene enabled us to develop a DNA screening test to help horse breeders avoid producing horses with this disease," Bannasch said.
"Equally important, the test should prevent the unnecessary destruction of young horses that actually have less serious skin irregularities, which can be mistaken for the early stages of HERDA."
This research represents the first time a scan of the whole horse genome has been used to identify a novel disease gene in the horse. These findings were presented to the horse genomics community at the Plant and Animal Genome XV Conference in January by Rob Tryon, the graduate student working on the project. Stephen White, a professor of veterinary dermatology at UC Davis, led the clinical investigations of HERDA.
HERDA causes skin along the saddle area of the horse's back to stretch and tear easily, making the horse unfit for riding. Researchers suspect that the disease may be related to a deficiency in the horse's wound healing process or a malfunction in its normal inflammatory response system. An earlier study of affected horses by the UC Davis group demonstrated that the disease cannot be definitively diagnosed by skin biopsy. Foals affected by the disease rarely show symptoms at birth, and most cases are not diagnosed until the horse is about two years old and being broke to ride.
Previous work by the UC Davis group showed that the disease has an autosomal recessive mode of inheritance and carriers show no signs of the disease. Breeders may never know that their horses are carriers of the disease because they usually sell their affected foals before the horses show symptoms. Since there is no available treatment, most horses diagnosed with the disease are euthanized because they cannot be ridden and are unsuitable for future breeding.
The UC Davis team members began their search for the mutation with the entire complement of horse genes, totaling about 25,000 genes. They were able to narrow the location to chromosome 1, the largest of the horse chromosomes, by identifying the section of the genome that was identical in horses affected by the disease. From there, they were able to zero in on a group of 20 genes in this region by comparing related regions in the genomes of humans, chimpanzees, mice, dogs and cows. The team performed a sequence analysis of those genes, and then identified the mutation.
Screening for this genetic mutation among the study's control group of unaffected horses suggests that 3.5 percent of American quarter horses are carriers. Although carriers do not develop symptoms, they can produce affected foals if bred to other carriers.
Funding for the research was provided by the American Quarter Horse Foundation and the Center for Equine Health at the UC Davis School of Veterinary Medicine.