Congenital defects play a significant role in horse breeding, as they are associated with both economic losses, which are sometimes very high, and emotional losses. In order to prevent the production of diseased animals and to have a breeding influence on the further spread of defect alleles, the possibility of performing a genetic test is of great advantage.
Polysaccharide storage disease (PSSM) is a hereditary disease that can be manifested by painful muscle cramps, symptoms similar to cruciate rash, and even muscle wasting. It is caused by an abnormal accumulation of polysaccharides in the muscle fibers of the skeletal muscles. Particularly cold-blooded horse breeds such as the Rhenish-German Cold Blood, Belgian Cold Blood and Percheron as well as Quarter Horses and other warm-blooded horses and ponies can be affected.
Inheritance and development
PSSM is a glycogen storage disease caused by a point mutation in the gene GYS1 (glycogen synthase 1). This gene is responsible for the expression of glycogen synthase in skeletal muscle, an enzyme that stores sugar in the form of glycogen in the muscles. The mutation-induced exchange of the amino acids arginine to histidine causes an increase in the activity of the enzyme and thus in glycogen production. The consequences are an accumulation of glycogen and amylase-resistant inclusions in skeletal muscle (Larcher et al. 2008; McCue et al. 2008b). The disease is inherited in an autosomal dominant manner. This means that carriers of one copy of the mutated gene (N/ PSSM) are highly likely to develop the disease and pass the mutation to 50% of their offspring. Carriers of both copies (PSSM/ PSSM) are 100% likely to inherit the mutation to their offspring. In addition to this form caused by the GYS1 mutation, also called type 1 PSSM, there is another form (type 2) in which the mutation in GYS1 is undetectable despite a high storage of muscle glycogen in affected animals (McCue et al. 2008a; Baird et al. 2010). Detection of type 1 PSSM is possible via genetic testing. This requires a hair root sample or a blood sample containing the anticoagulant EDTA, from which DNA is isolated and tested according to standard procedures.
The provision of a genetic test is a great advantage for veterinarians, breeders and horse owners. It allows a quick and easy diagnosis with minimal stress to the horse. The foundation for this is already done by detecting the GYS1 mutation. But what exactly does a PSSM positive test result mean for my horse? What is the penetrance, i.e. the actual occurrence of the disease, if it has one or two defect alleles? The development of clinical symptoms is dependent on feeding, husbandry and especially on other genes. How strongly these factors act in detail and which interaction triggers an actual symptomatology remains to be clarified.
The research work of the Institute of Animal Breeding and Genetics of the University of Veterinary Medicine Hanover aims to verify the actual disease rate for PSSM positive animals with one and with two defect alleles and the influence of possible further genetic factors and thus to provide a basis for a reliable statement about the disease course of the individual animal and to give the breeder an aid for the breeding decision. Therefore, it is important that, if possible, all available samples (hair root sample or EDTA blood) from horses with symptoms typical of PSSM or GYS1-positive genetic test are sent to the Institute together with the completed recording sheet. In addition, the minimal intervention of taking a biopsy is necessary to confirm the diagnosis. In the future, this procedure should facilitate a clear diagnosis and enable a reliable statement by means of a genetic test.