Myosin-Heavy Chain Myopathy (MYHM)
MYHM, IMM, Immune Mediated Myopathy
Myosin-Heavy Chain Myopathy (MYHM) which was previously referred to as Immune Mediated Myopathy (IMM) is an inherited muscle disorder that can have two different presentations. The first presentation known as IMM is an autoimmune disease that can be triggered by environmental factors such as an infection (Streptococcus) or certain vaccinations (influenza) and lead to severe muscle loss within days. Areas of muscle atrophy can include the gluteal and vertebral regions with additional symptoms including malaise, muscle stiffness, and overall weakness not associated with exercise. The second presentation known as “tying-up” presents with stiffness that can be accompanied by muscle swelling, muscle pain, and cramping in the back and haunches in the absence of exercise. The same mutation can lead to either or both conditions at different times in the horse’s life in which recovery is possible but can be followed by more frequent future episodes. In addition, horses with two copies of the mutation are thought to be at the highest risk for recurring episodes. Due to incomplete penetrance and the complex nature of this disease, it is also possible for horses that carry the mutation to not display any symptoms at all. Although there is no cure for this disorder, it can be managed through use of corticosteroids to treat the episodes and follow-up high protein diet to assist in regaining muscle mass.
Reading Your Results
A. (CLEAR/NORMAL):
These horses have two copies of the normal gene and will neither develop Myosin-Heavy Chain Myopathy nor pass this mutation to their offspring.
B. (CARRIER/NOT AFFECTED):
These horses have one copy of the normal gene and one copy of the mutation associated with this disease. They will develop Myosin-Heavy Chain Myopathy which typically results in muscle loss and “tying-up” and will, if bred, pass the mutation to 50% of its offspring, on average.
C. (AT RISK/AFFECTED):
These horses have two copies of the mutation associated with Myosin-Heavy Chain Myopathy which typically results in muscle loss and “tying-up”.
Additional Details
Inheritances
Autosomal Dominant with Incomplete Penetrance
Affected gene
MYH1
Chromosome
Ch. 11
Mutation
Chr11:52,993,878 T>C; p.E321G
Publication:
Finno CJ, Gianino G, Perumbakkam S, et al. A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skelet Muscle. 2018;8(1):7. [PubMed: 29510741].
