A Universal Alphabet
What made the HYPP test possible, and what promises to unlock the doors to many other equine genetic disorders, is the breathtaking pace of human genetic research. "The human map is progressing at an incredible rate. That's going to be our encyclopedia. So much money and effort have been spent on finding these diseases in humans; now we just have to correlate them," says Cothran.
That correlation depends on finding "linkage groups"--pieces of DNA that are common to more than one species, rather like similar words in the Indo-European "family" of languages derived from common roots. By a happy circumstance of mammalian evolution, Cothran says, "there is a high degree of conservation of blocks of the genetic information, so if you know of a gene for whatever characteristic occurs in man, you know where to look for it in any other vertebrate."
The geneticist hopes to apply that model to several equine disorders he's studying, including epitheliogenesis imperfecta (EI), a fatal condition in which foals are born with large areas of skin, oral membranes and sometimes hooves missing. People suffer from a similar defect called epidermolysis bullosa. "Based upon the pathology of the disease, it appears that EI is equivalent to the disease that occurs in humans," says Cothran. "What we want to find is a marker for the gene so that carriers can be detected."
Correcting The Error
Finding a mutation is one thing; eliminating it is another matter entirely. Even if researchers manage to crack the equine genetic code and locate the errors responsible for inherited diseases, defects and deficiencies, breeders and breed registries are the ones who must put those discoveries to good use.
For breed registries, genetic defects pose a real quandary. How much pressure can they afford to apply to effectively blacklist a group of horses on the basis of one bad gene? How conclusive must the evidence be that this gene, and not another, is the cause of the disorder?
"Registries don't exist to tell people what to do," says Ralph Clark, executive director of the International Arabian Horse Association. If a CID test ever becomes available, "we would certainly recommend that people use it," says Clark; anything stronger than recommendation is "a ticklish subject."
Lisa Hale, DVM, has seen dozens of EI cases in her practice in Arthur, Ill., and says that breeder inaction is an important accomplice in the spread of a genetic disorder. "Everybody needs to be up front about it. If it's something that's hurting the breed and we stick our head in the sand, it's not going to go away."
The American Saddlebred Horse Association (ASHA) supported EI research by openly soliciting blood samples from afflicted foals for Cothran's research. "Until we have a diagnostic test, we can't label horses as being carriers," says ASHA executive director Pat Nichols. "This is going to be a moment of truth for our association once we have a diagnostic tool. There are no easy answers, but I feel that we're going to have full disclosure."
This article originally appeared in the December 1995 issue of EQUUS magazine.
