Potential of gene-based testing in Thoroughbreds to reduce disease highlighted

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Genetics and genomics in Thoroughbreds have far-reaching potential beyond that of breeding and selecting faster horses, a researcher in the field of molecular genetics says.

Dr Brandon Velie, from the Department of Animal Breeding and Genetics at the Swedish University of Agricultural Sciences, says understanding diseases caused by a single gene as well as complex diseases caused by multiple genes and the environment can lead to early diagnosis and targeted treatments.

“While the list of reasons a racehorse never reaches its potential may seem endless, genetics and genomics provide an opportunity to cross certain disorders off that list, thereby helping to eliminate or reduce the occurence of those diseases,” Velie writes in the latest issue of Equine Disease Quarterly.

Genetics refers to the study of genes and the way traits or conditions are passed down from one generation to another. Genomics, on the other hand, describes the study of all genes (the genome), including interactions of genes with each other and the environment.

“Although much of the genetic and genomic research done in Thoroughbreds is applied to racing performance, the full breadth of application for genetic and genomic research goes beyond that of faster horses,” he says.

“Genetics and genomics allow for a more complete understanding of both simple and complex
diseases.

“From a genetic perspective, ‘simple’ is a term used to describe a disease that follows a single gene pattern of inheritance. These diseases are controlled by one gene, with other genes and outside factors having very little influence (that is, the presence of the gene equals disease).”

Diseases inherited in this way are typically qualitative, where an animal either has the disease or it doesn’t (for example, lethal white foal syndrome).

“Complex diseases, on the other hand, are usually controlled by not one but many different genes and are often affected by environmental factors, such as nutrition and living conditions (for example, Cervical Vertebral Stenotic Myelopathy, also known as Wobbler Syndrome).

“This combination of both genetic and environmental factors results in complex or ‘multifactorial’ diseases.

Basically, three different scenarios determine the manifestation of a complex disease:

  • No genetic predisposition + environmental triggers = no clinical signs of disease;
  • Genetic predisposition + no environmental triggers = no clinical signs of disease;
  • Genetic predisposition + environmental triggers = highly varied clinical signs.

“As a result, complex genetic diseases can be extremely difficult to diagnose early and/or prevent
using traditional methods such as pedigree analyses and veterinary evaluations.

“In some instances, a simple disease may even be classified as complex based on the inability of epidemiological studies and pedigree analyses to find common factors among cases.

“Hydrocephalus, for example, is a developmental disorder that often results in stillbirth of foals and
dystocia [difficult birth] in dams.

“Possible causes of the defect in horses could not be proven based on field data and pedigree analyses suggested the disorder to be complex.”

With this in mind, a genome scan of 82 horses, comprising 13 cases and 69 controls, was performed
and a small section of the genome was identified as containing the cause of the disorder.

Genomic sequencing was then performed on 10 horses (four cases and six controls), and the genetic cause of the disorder was pinpointed.

“Although previously believed to be a complex disease, genetic and genomic methods were able to prove that the disorder was in fact simple, leading to the development of a genetic test that can help breeders avoid the disorder.

“It is,” he continues, “important to note the difference between a genetic test for a simple disease, such as hydrocephalus, and a genetic test for a complex disease Control (for example, osteochondrosis).

“Genetic tests for simple diseases can confirm or rule out a genetic condition; however, genetic tests for complex diseases only help to determine an individual’s chance of developing a genetic disorder − an important distinction when genetic tests are used to help make breeding decisions.”

Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.

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