About PSSM type 2/MIM

PSSM type 2 or PSSM2 is a genetic muscle problem that occurs in a wide variety of horse breeds.

Horses that have PSSM2 have similar symptoms to horses that have PSSM1, but they do not have the GYS1 mutation specific to PSSM1. Until fairly recently, PSSM2 could only be diagnosed by taking a biopsy of the skeletal muscle of the hamstring (M. Semimembranosis), but by now several genes have been identified that could be responsible for at least some of the symptoms of PSSM2, making genetic testing possible.
PSSM2/MIM has been found to not be a sugar storage problem, though glycogen pooling can sometimes be seen on biopsies. Most likely, this is a by-product of an underlying mechanism of damaged muscle tissue.

EquiSeq, a company based in the USA, has identified several genes that can cause PSSM2 symptoms, which had added genetic testing as an option in diagnosing PSSM2/MIM. EquiSeq’s findings have not been peer reviewed yet and can therefore not be considered academically validated, but they do already offer the possibility to test for the genes that they have identified. Horse owners have reported that adjusting management according to the outcome of the EquiSeq tests have offered a considerable improvement of life quality or a return to or improvement of performance. This makes testing for these genes worth considering.

The EquiSeq panel has been renamed Muscle Integrity Myopathy or MIM panel, which more accurately describes the underlying mechanism.

Dr Valberg, who discovered the GYS1 gene that causes PSSM type 1, has published her findings on the MYH1 gene, another gene that causes muscle issues (2018). In its acute form, this gene can be responsible for rapidly occurring atrophy of a horse’s topline. In milder cases, it can express itself in more general PSSM2 type symptoms. The disease seems to be triggered by illness or inflammation in the body and has therefore been dubbed Immune Mediated Myositis (IMM). This variant is specific to Quarter Horses and related breeds, reining and reined cow horses being most often affected.