Genetic Testing

Genetic Testing Information

The PHAA offers the following tests through Racing Australia and their contract Laboratory Massey University.

All testing requests for DNA must come through PHAA, no other parent validation results taken after 2012 will be accepted from other providers unless this horse is from overseas.

Genetic Testing Information 

OLWFS

OLWFS is a fatal disorder of the digestive system that is associated with a particular white pattern called “frame overo”

Full name

Overo lethal white foal syndrome

Breeds known to be affected

Many breeds, although it is found at its highest frequency in Paint breeds

Inheritance pattern

OLWFS is autosomal recessive. The white spotting pattern expressed by a heterozygous OLWFS horse is incomplete dominant.

Description

A single O allele causes the “frame” or “frame overo” spotting pattern. Expression of white in frame horses is variable, ranging from lots of white “framed” by the horse’s base colour, to minimal or just a few white hairs. It is possible, although unusual, for a horse with no white to carry Frame.

Overo Lethal White Foal Syndrome occurs when a horse is homozygous for the O mutation. These O/O foals are born almost or completely white, but do not have properly formed intestinal nerves and cannot pass faeces. They only survive a few days if not euthanised for compassionate reasons. Carrier horses (O/n) have no documented health issues. OLWS is associated with a two base pair change in the EDNRB gene. Because O can be minimally expressed, it is important to test any horse that might be a carrier even if it has little to no white on it, to prevent the birth of an affected foal.

Interpretation of results

n/n: Horse does not carry the mutation associated with OLWFS.

O/n: Horse has one copy of the mutation associated with OLWFS.

O/O: Horse has two copies of the mutation associated with OLWFS.

HERDA

HERDA is a skin disease found primarily in Quarter Horses and their derivatives

Full name

Hereditary equine regional dermal astheniaBreeds known to be affectedQuarter Horse and related breedsInheritance patternAutosomal recessive

Description

Hereditary equine regional dermal asthenia (HERDA) is characterised by hyper-extensible (fragile) skin which progresses to severe skin lesions, often on the horses back. The disorder affects the collagen that holds the skin in place, making it much easier to tear off than normal. Any rubbing, such as that caused by saddling, will cause the skin to split so affected horses are unable to be ridden. The lesions are painful and prone to infection. HERDA is associated with horses from the Poco Bueno sire line.HERDA is caused by a mutation in the PPIB gene and is recessive, so the horse must be homozygous (HERDA/HERDA) to be affected. If a horse is a carrier (n/HERDA), it will not show any clinical signs of HERDA. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.

Interpretation of results

n/n: Horse does not carry the mutation associated with HERDA.n/HERDA: Horse has one copy of the mutation associated with HERDA.

PSSM1

PSSM1 causes muscle cramping and tying up

Full name

Polysaccharide storage myopathy 1Breeds known to be affectedVarious, including Quarter Horse and Quarter Horse derivatives, and many draught breeds. PSSM1 has not been identified in purebred Arabians, Thoroughbreds or Standardbreds.

Inheritance pattern

Autosomal dominant

Description

PSSM1 causes a build-up of abnormal sugars in muscle. This is one of the causes of tying up, with clinical signs that include muscle twitches, stiffness, sweating, reluctance to move and painful cramps. Symptoms can vary widely in severity and age of onset. PSSM1 appears to be quite an old mutation so it is found in many breeds including Quarter Horses and draft breeds. It is a complex disorder that can often be controlled with changes to diet and exercise management.PSSM1 is associated with a mutation in the GYS1 gene and is inherited in a dominant fashion, so a horse only needs to carry one copy (PSSM1/n) to show symptoms. There is some evidence that homozygous horses (PSSM1/PSSM1) are more severely affected than heterozygotes. Please note that this test only detects this one specific type of tying up, and horses may still exhibit signs of tying up even if they are not positive for PSSM1.

Interpretation of results

n/n: Horse does not carry the mutation associated with PSSM1.

PSSM1/n: Horse has one copy of the mutation associated with PSSM1.

PSSM1/PSSM1: Horse has two copies of the mutation associated with PSSM1.

MH

MH is a muscle disorder that causes an increased rate of metabolic activity usually associated with administration of certain types of anaesthetic gases

Full name

Malignant hyperthermiaBreeds known to be affectedQuarter Horse and related breeds

Inheritance pattern

Autosomal dominant

Description

MH is a muscle disorder that may only become apparent if the horse is subjected to an extreme stress or exposed to a halogenated anaesthetic. When exposed, the mutation triggers the release of excess calcium in skeletal muscle cells causing a high temperature (hence the name), increased heart rate and blood pressure, sweating and muscle rigidity. MH is frequently fatal. MH sometimes occurs in horses which are also positive for PSSM1, causing them to have more severe tying up symptoms.MH is rare and only found in some Quarter Horse and paint families; however, because it is potentially fatal it is recommended all possible carriers be tested before undergoing anaesthesia. MH is associated with a mutation in the RyR1 gene and is a dominant trait, meaning a horse only needs 1 copy of the mutation (MH/n) to be affected.

Interpretation of results

n/n: Horse does not carry the mutation associated with MH.

MH/n: Horse has one copy of the mutation associated with MH.

MH/MH: Horse has two copies of the mutation associated with MH.

GBED

GBED is a disorder that prevents the storage of glycogen in muscles and other tissues, causing late term abortion, stillbirths and death in affected foals

Full name

Glycogen branching enzyme deficiency

Breeds known to be affected

Quarter Horse and related breeds

Inheritance pattern

Autosomal recessive

Description

GBED is a lethal storage myopathy caused by a mutation in the GBE1 gene that prevents the animal from properly storing glucose. The horse will eventually run out of stored energy, which will damage its organs. The symptoms observed are associated with the lack of energy preventing the organs from working correctly, and may include general weakness, failure to thrive, low body temperature, seizures and difficulty rising.GBED is always fatal, with most affected foals dying before the age of 8 weeks. GBED can also cause foetuses to be aborted or born prematurely. It is inherited as a recessive trait, so only homozygous (GBED/GBED) horses are affected. If a horse is a carrier (n/GBED), it will not show any clinical signs of GBED. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.

Interpretation of results

n/n: Horse does not carry the mutation associated with GBED.

n/GBED: Horse has one copy of the mutation associated with GBED.

GBED/GBED: Horse has two copies of the mutation associated with GBED.

MYHM (IMM)

MYHM (previously called IMM) is a muscle disease found in Quarter Horses

Full name

Myosin-heavy chain myopathy

Alternate names

Immune mediated myositis (IMM)

Breeds known to be affected

Quarter Horses and related breeds

Inheritance pattern

Autosomal codominant

Description

MYHM can cause two different types of disease. One is muscle weakness and stiffness, followed by a rapid, significant loss of muscle, particularly from the topline of the horse. This is often associated with an infection or vaccination, particularly with Strangles or other respiratory viruses. This syndrome was originally named “immune mediated myositis” or IMM. A mutation associated with IMM was identified by researchers at UC Davis and Michigan State University in the Myosin Heavy Chain 1 (MYH1) gene.

It was later found that horses carrying this mutation were also susceptible to non-exertional rhabdomyolysis. This manifests as stiffness, firm muscles, short stride and often is accompanied by dark coloured urine. Unlike many other forms of tying up, it is not associated with exercise. The dark urine is an indicator that muscle damage has occurred. The identification of this second set of clinical signs associated with the mutation caused IMM to be renamed Myosin-heavy chain myopathy.

MYHM is co-dominant, meaning that the action of the variant is independent of the second variant. If a horse has one copy of the MYHM mutation it can be affected with MYHM. MYHM has ‘incomplete penetrance’ – so not every horse carrying this mutation will show the same severity of symptoms. It is thought that if a horse has two copies it is more likely to be more severely affected.

Interpretation of results

n/n: Horse does not carry the mutation associated with MYHM.

MYHM/n: Horse has one copy of the mutation associated with MYHM.

MYHM/MYHM: Horse has two copies of the mutation associated with MYHM. It will pass MYHM on to all its offspring and is likely to be more severely affected than a horse with only one copy.