Gene-based medicine for equine muscle disorders

Researchers knew that the equine genome project eventually would lead to monumental discoveries and new methods of diagnosis and treatment of significant benefit to the horse.

Photo 1: This foal has congenital flexure deformities of both front limbs. The right front is more severely affected, and the contracture of the deep digital flexure tendon has positioned the foot on the toe in the characteristic "ballerina" posture. The left fore is not as severly affected but a credit card still slides easily under the heel of the foot.

Once the gene sequence, or DNA map, of the horse was completed, they could investigate abnormalities or "breaks" in that code. These abnormal genes could be identified and tests developed to look for their presence — all leading to better diagnosis.

Conditions that appeared clinically similar but which had different causes and etiologies could now potentially be distinguished from one another. Scientists hoped that further research would then lead to genetic modification therapy aimed at repairing those DNA abnormalities and allowing clinicians to treat and possibly cure some gene-based diseases.

Photo 2: PVC toe extensions were placed on the foal to attempt to stretch the tendons and return the limb to a more normal position.

While the veterinary community has not accomplished the repair-and-treatment phase just yet, the identification and diagnosis aspect of gene-based medicine is well under way.

Photo 3: The same foal 30 days later, after toe extension treatment.

One area that has benefited greatly from this is muscle disease in the horse. Muscle dysfunction — ranging from a mild, chronic stiffness or poor tone and development all the way to the massive contracture and severe spasms seen in full-blown rhabdomyolysis or tying-up cases observed in varying breeds and sports disciplines — is a common problem in performance horses.

Clinicians have long been frustrated with these cases because many of them appear outwardly similar — tight to spastic muscle. The Quarter Horse roping mare that displays a consistently stiff hind-limb gait with slightly elevated muscle enzymes (creatine phosphokinase (CPK) and aspartate transaminase (AST, also called serum glutamic oxaloacetic transaminase, or SGOT) looks very much like a Dutch Warm Blood gelding experiencing the same short-strided gait and muscle stiffness. The tied-up endurance horse that, at 35 miles of a 100-mile event, stands sweating profusely, unable to move and producing classic coffee-colored urine, presents as similar a clinical picture as the young Thoroughbred in race training that also ties up after a relatively short but intense training gallop.

Veterinarians looking at all of these horses with their similar variations of tying-up-like signs and investigating their histories, uses, diets and other management factors, generally have been unable to connect them.

Photo 4A: Sonny, an endurance horse, tied up after running 40 miles at an event. The horse is sweating, has mild colic and shows the characteristic swollen, hard muscles typical of this condition. The owners placed a blanket on the hard rump muscles to keep them warm prior to treatment with intravenous fluid.

PSSM tests developed

"One reason for this confusion regarding tying up may well be that there are several causes for muscle pain and cramping in the horse," explains Dr. Stephanie Valberg of the Department of Clinical and Population Sciences of the College of Veterinary Medicine, University of Minnesota.

Photo 4B: Sonny passed typical dark-brown, coffee-colored urine seen here. The dark urine cleared as the horse became hydrated, flushing the myoglobin muscle breakdown pigment from his system.

"Only with improved diagnostic procedures will specific etiologies for tying up in the horse be recognized," Valberg adds,

She, along with Dr. M. McCue and researchers at the Veterinary Diagnostic Laboratory at the University of Minnesota, took a step in that direction with the announcement that the gene mutation responsible for polysaccharide storage myopathy (PSSM) disease in the horse has been identified.

McCue, Valberg and their research group developed a genetic test for this mutation that is now commercially available through the University of Minnesota Diagnostic Lab.

The mutation is believed responsible for Type 1 or the classic form of PSSM, which accounts for greater than 90 percent of cases in some breeds. A second mutation (called MH) also was identified, which tends to make PSSM more severe in certain Quarter Horse and related breeds. There is a test available for MH as well, and veterinarians are encouraged to use it to determine the presence, likely severity and possible treatment options for PSSM in individual horses.

It is interesting that the research which produced these tests was able to determine that the two mutations appear to have originated prior to the foundation of modern breeds, which is why this disease occurs in several different but related horse breeds.

PSSM has been identified in Quarter Horses, American Paint Horses, Appaloosas, Quarter Horse crosses, Warm Bloods, Draft breeds and Draft crosses. Pedigree analyses of Quarter Horses, American Paint Horses and Appaloosas with PSSM tend to support a familial basis for the disease. "Line breeding of these breeds to two popular related stallions may be responsible for the increasing incidence of this condition," according to Valberg. Warm Bloods, Draft and Draft crosses likely represent a similar and possibly related etiology for PSSM but that connection has yet to be established.

CFS research advances

Another condition or disease that has benefited from gene-based medicine and information provided by the horse genome project is contracted foal syndrome (CFS), the most common equine congenital disorder diagnosed at the University of Kentucky's Livestock Disease Diagnostic Center (LDDC).

Foals born with this condition have varying degrees of limb contractures that may be mild to severe and may affect only one limb or all four. Mild cases of CFS can be resolved with splinting (of many types and designs), surgery or other therapies, while severely affected foals may be unable to stand or walk and often are euthanized because of an inability to properly nurse and because of poor long-term prognosis.

CFS in horses closely resembles a condition in humans called distal arthrogryposis (DA). Children born with DA exhibit one of several muscle-contracture syndromes that result in varying degrees of clenched fists and/or clubbed feet.

The skeletal anatomy of the equine lower limb is comparable to the bones of the palm and fingers or the foot and toes, making CFS and DA very similar and offering researchers an opportunity to learn from and possibly help both conditions. There are other similarities between DA and CFS as well.

Children with DA may be born with other congenital abnormalities such as scoliosis, facial muscle contracture and smaller mouth size. CSF foals may present with torticollis (neck flexion), facial bone deviation (wry nose) and scoliosis.

Researchers know that DA in humans is caused by one or more mutations to genes that regulate skeletal muscle contracture. The production of abnormal muscle proteins caused by these mutated genes results in skeletal muscle contracture, which in turn causes finger and toe ligament contracture during fetal development. As with CFS, some children born with DA respond to splinting and surgery, and some do not.

Because muscle, tendons and ligaments in CFS-affected foals appear histologically normal, a biochemical or other basis for this condition has been sought by researchers, and the connection with DA may provide a gene-based link to abnormal muscle contracture.

The mode of inheritance for CFS appears to be in a dominant fashion, with breeding records showing individual mares bred to different stallions still producing high numbers of affected offspring. The inheritance of DA in humans does not appear to be direct, with more variability noted and with a high incidence of spontaneous gene mutation seen.

Researchers at the Maxwell H. Gluck Equine Research Center at the University of Kentucky have been greatly assisted by the equine genome project. Dr. Teri Lear and Dr. Ernie Bailey note that the availability of the horse genome sequences has helped in their study of CFS.

"The DNA sequence information for any horse gene of interest can be downloaded from Internet databases," Lear and Bailey explain. "The necessary tools can be prepared for sequencing candidate genes from affected and non-affected individuals. Based on the gene mutations causing DA in humans, we have selected and are currently sequencing candidate genes in an effort to identify mutations that cause CFS."

Future genome applications

Additional research remains to be done, however, and the goal — much like that with the identification of a PSSM-causing gene mutation — is to produce a diagnostic test that will identify at-risk individuals and allow farm/breeding managers to make informed mating decisions that might help reduce the incidence of CFS.

Additionally, genetic testing may help to identify CFS foals that will respond to treatment and those that will not.

Veterinarians, owners and breeders are encouraged to help this effort by providing research samples and genetic information from stallions and dams that produce CFS foals.

All information is kept confidential, including horse and farm names.

The equine genome has provided much information, and the development of gene-based medicine has provided another step in the understanding, identification, diagnosis and maybe even treatment of many equine muscle-based diseases.

Participation of veterinarians and horse owners is still crucial for the furthering of this important research and the development of an understanding of still other equine conditions and disorders.

Marcella is an equine practitioner in Canton, Ga.