MIM (PSSM2) Genetic Testing – What’s the Deal with it?

In the last two decades, modern molecular biology methods, including DNA sequencing – perhaps one should say mainly DNA sequencing – have turned the world of human diagnostics upside down. There is so much new information about the genetic makeup of humans, causes of features, diseases, and more and news is coming in daily.

In veterinary medicine, mainly because of the lack of systematic public funding originating in inevitable and understandable priorities of our societies, the situation is different and the progress much slower. Unfortunately, even our understanding regarding the causes of some serious diseases is limited and kind of waiting for someone to dig in. Take MIM as an example.

Muscle Integrity Myopathy (MIM), also known as Type 2 Polysaccharide Storage Myopathy (PSSM2), is a term used to describe a group of myopathies in horses. MIM is a newer term than PSSM2, which is a name that was established in connection with the Type 1 Polysaccharide Storage Myopathy (PSSM1, or just PSSM).

PSSM1 is a genetic disease resulting in an abnormal accumulation of glycogen in muscle cells. There is a well-known genetic mutation (GYS1) associated with this disease, and a genetic test is a validated method for determining diagnosis.

MIM-positive horses do not carry the mutation linked with PSSM1, but they show similar symptoms and their muscle biopsies are abnormal. Unlike with PSSM1, there aren’t any universally accepted validated genetic markers linked with MIM, as there hasn’t been found a link between the results of genetic testing and histopathological findings in muscle biopsies of affected horses.

So far, it’s not clear what the cause of myopathic episodes in these horses is, and there is no confirmation that the tested genetic markers are associated with the manifestation of this disease. If you want an answer to whether a particular horse suffers from MIM, you can flip a coin, and you will get about as accurate as you would with genetic testing, as of now.

MIM diagnosis includes evaluating clinical symptoms, ruling out other possible causes, and performing a muscle biopsy. The exact causes of MIM are currently unknown, thought to be multifactorial, and although we do not know their form – they are possibly hidden in the equine genome. If true, only systematic analysis of horse genomes is capable of finding them and although it is a long road to go, it is a road worth the effort. Moreover, this systematic approach by whole genome sequencing has a potential to reveal many more than only the MIM secret. Would you join EquineTest on it?