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☛ Muscle-wasting QH disorder caused by gene mutation





By Rick Dennis
April 13, 2018

Three years ago, I authored an article entitled “AQHA Genetic Pool Shrinks” regarding the shrinking genetic pool of certain equine disciplines e.g. reining, cutting and reined cow horses. The mutation was the result of breeding Quarter Horses within a specific and shrinking gene pool. The equine disease is called HERDA (Heredity equine regional dermal asthenia), a genetic skin disease predominately found in Quarter Horses in the particular lines of cutting horses. HERDA is characterized by hyperextensible skin, scarring and severe lesions along the back and body of affected horses.

Click for HERDA article>>

 Today, another result of the shrinking genetic bloodline pool in Quarter Horses, a new gene mutation discovery has been made with Quarter Horses –  specific to the reined cow horse and reining disciplines. This gene was also identified, via, an AQHA-funded research project.One of the researchers is “Stephanie Valberg, DVM, PHD, DACMIM, ACVSMR” of the Michigan State University, College of Veterinary Medicine. This new gene classification is identified as MYH1 (Myosin Heavy Chain 1 MYH1) and explained by the following definitions published in “Genetics Home Reference – Your Guide To Understanding Genetic Conditions.”

The DNA sequence of a gene can be altered in a number of ways.Gene mutations have varying effects on health depending on where they occur and whether they alter the function of essential proteins.  The types of mutations include:

     Missense Mutation: This type of mutation is a change in one DNA base pair that results in the substitution of one amino acid for another in the protein made by a gene, and

     Nonsense Mutation: This mutation is also a change in one DNA base pair instead of substituting one amino acid for another; however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.

Therefore, a missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses.

Click for Missense mutation article>>



According to an article appearing in “ dated March 7, 2018 – “Gene Mutation Linked To Wasting Disorder In Quarter Horses,” researchers have linked a mutation in the MYH1 gene to a muscle-wasting condition most commonly seen in Quarter Horses. Carrie Finno and her colleagues linked the missense mutation in the gene to immune mediated myositis (IMM), in which the immune system of horses attacks the skeletal muscles, causing rapid wasting of the muscle along the top line. The condition is rare in horses, but is the most common cause of rapid wasting of the top line in Paint and Quarter Horses.


The study team, writing in the journal “Skeletal Muscle,” said causes of autoimmune diseases such as IMM (immune-mediated myositis) are not well understood, but environmental stimuli, combined with a genetic predilection, appear to be important factors.


Research has shown that IMM is more likely to affect horses under 8 years old, or horses aged 17 and older. Because most horses affected by IMM are of Quarter Horse-related breeds, and since certain stallions appear to be over represented in the genetic lineage of Quarter Horses with the disorder, the researchers hypothesized that there was an underlying genetic variant that causes susceptibility to the conditions. IMM is normally treated with corticosteroids.


The study team was comprised of Carrie Finno, Giuliana Gianino, Sudeep Perumbakkam, Zoe Williams, Matthew Biordbari, Keri Gardner, Erin Burns, Sichong Peng, Sian Durward-Akhurst and Stephanie Valberg.  They are variously affiliated with the University of California, Davis; Michigan State University, and the University of Minnesota.

Click for “Horse Talk” article>>


Essentially, the cause of immune-mediated myositis (IMM), characterized by recurrent, rapid-onset muscle atrophy in Quarter Horses, is unknown. The histopathologic hallmark of IMM is lymphocytic infiltration of myofibers. The purpose of this study was to identify putative functional variants associated with equine IMM.


However, the study concluded that a mutation in MYH1 is highly associated with susceptibility to the IMM phenotype in Quarter Horse-related breeds. This is the first report of a mutation in MYH1 and the first link between a skeletal muscle myosin mutation and autoimmune disease.


Testing Result Definitions:

  1. Heterozygous – genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive), such as “Rr:” or “Ss”.Alternatively, a heterozygote for gene “R” is assumed to be “Rr”.  The capital letter is usually written first. Dr. Valberg has stated that in the horses tested by him, the mutated gene was found more often in the reining and working cow horse disciplines.
  2. 21 percent of the 37 reining stallions tested heterozygous for the mutation; one was homozygous.
  3. 17 percent of the 41 working cow horse stallions tested heterozygous; none were homozygous.
  4. 16 percent of the 50 halter stallions were heterozygous. None were homozygous.

What is the difference between homozygous and heterozygous?

Humans and animals contain two copies of each gene, one from the father and one from the mother, which sometimes are referred to as the alleles of a gene. If a mutation occurs in just one copy of the gene then that individual is considered heterozygous. On the other hand if both copies of a gene are mutated then that individual is homozygous genotype.

Majority of hereditary disorders are harmful if both copies or alleles of a gene are affected, which means protein products from both genes may fail to operate properly. In such cases immediate medical attention is needed so the function of a defected protein can be restored through medication. In heterozygous genotypes one copy of the gene is healthy and can produce fine proteins thus these individuals are usually not affected and are considered just carriers. However in a few hereditary disorders heterozygous individuals may suffer from a milder version of the disease.

Testing designations:

What represented heterozygous? “Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lower case letter (representing the recessive allele), such as “Rr” or “Ss”. Alternatively, a heterozygote for gene “R” is assumed to be “Rr”. The capital letter is usually written first.”





In the article I authored three years ago, with specific interest in the shrinking genetic pool of certain equine horse disciplines, (e.g. reining, cutting, reined cow horse, etc.), as well as the resultant genetic mutationsemerging from breeding Quarter Horses within a specific and shrinking gene pool, I talked about equine diseases emerging from a declining gene pool called HERDA or Hereditary equine regional dermal asthenia. HERDA is a genetic skin disease predominantly found in the American Quarter Horse. Within the breed, the disease is prevalent in particular lines of cutting horses. HERDA is characterized by hyperextensible skin, scarring and severe lesions along the back of affected horses.


Affected foals rarely show symptoms at birth. The condition typically occurs by the age of two, most notably when the horse is first being broke to saddle. There is no cure and the majority of diagnosed horses are euthanized because they are unable to be ridden and are inappropriate for future breeding. HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions. Research carried out in Dr. Danika Bannasch’s laboratory at the University of California, Davis, has identified the gene and mutation associated with HERDA.


The diagnostic DNA test for HERDA that has been developed allows identification of horses that are affected or that carry the specific mutation. Other skin conditions can mimic the symptoms of HERDA. The DNA test will assist veterinarians to make the correct diagnosis. For horse breeders, identification of carriers is critical for the selection of mating pairs. Breedings of carrier horses have a 25 percent chance of producing an affected foal. Breedings between normal and carrier horses will not produce a HERDA foal although 50 percent of the foals are expected to be carriers.


As a result of this American Quarter Horse Association’sfunded research, AQHA’s research team developed the 5-panel test.  The following test result designation for the HERDA gene was adopted by AQHA:

Results reported as:


N/N            Normal – horse does not have the HERDA gene.

N/HRD       Carrier – horse carries one copy of the HERDA gene.

HRD/HRD   Affected – horse has two copies of the HERDA gene.


One interesting fact of this study concluded the HERDA gene is more prevalent in the cutting horse line.In order to increase the odds of successful probability is to breed to a stallion with an N/HRD designation to a breeding mare with the N/N – Normal designation. This will afford the breeder a 50 percent probability of an unaffected foal. However, this isn’t always a certainty due to the recessive gene factor. Therefore, the only way to determine the correct breeding match-up is to 5-panel test your mare through the AQHA before breeding.



 I’ve learned that Dr. Valberg provided a presentation at the AQHA Convention recently held in Jacksonville, Florida. The AQHA Stud Book and Registration Committee referred these findings to the Executive Committee for the commission of a study. It is unclear whether or not the MYH1 mutation will be a part of the 5-panel genetic tests in the future.

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  1. “Wow”

  2. Thank you Rick for your immense experience with horses.

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