Stephanie Valberg, D.V.M., Ph.D., a professor at the University of Minnesota, spoke about the genetic basis for different forms of tying-up. Horses possess 20,000 genes on 33 chromosomes. Through mutation, the genetic code of horses can be altered. Mutations are usually corrected before they are passed on to their descendants, though this is not always the case. Too, as horses age, DNA repair fails or becomes less efficient.
Following a discussion of recessive and dominant traits, Valberg introduced genetic testing, stating that expression of a gene mutation may not be identical among all affected horses. Environment and diet, for example, might change expression between two horses with the same gene mutation. Many diseases have complex genetic traits, where more than one gene is implicated in a genetic defect.
According to Valberg, genetic approaches to disease identification have been researched for glycogen branching enzyme deficiency (GBED), malignant hyperthermia (MH), and certain forms of tying-up.
Valberg explained the importance of genetic mapping of disease genes. Mapping uses the "position-dependent" approach, identifying the chromosomal region first. She implemented this approach with horses affected with polysaccharide storage myopathy (PSSM). She found one area on the chromosome that was similar on all genetic profiles. Identification of a sequence change must be followed up by a definite change in the gene's function, which Valberg and her coworkers discovered. The next step is to survey the population to find which breeds are affected. The researchers found many breeds that have the genetic disorder.
With regard to recurrent exertional rhabdomyolysis, another form of tying-up, most affected horses are high-strung fillies. Muscle biopsies and breeding trials revealed no sex predilection. Clinical signs, however, showed a sex predilection was verified. On a genetic level, a few genes have been identified. One segment is significantly associated with both sexes; another segment is associated with females only. Further research is ongoing.
Valberg emphasized the need for horse owners and breed associations to use the wealth of information provided by these genetic tests.