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Origin of genetic diseases can be both environmental and hereditary. Although environmental factors are unpredictable and hard to control, hereditary genetic diseases are easily tracked. Hereditary diseases are genetic mutations which happened in germ cells and can therefore be passed onto offspring in dominant or recessive way. Frequently used terms in genetics, when taking about hereditary diseases, are explained in "Importance of genetic testing" section where the possibility of affected foals is represented as well. The majority of horse genetic mutations occur in autosomal, non-sex chromosomes, hence most mutations are described as autosomal dominant or autosomal recessive. Genetic diseases can affect physical appearance, performance, strength of a horse, and in some cases, be lethal. It is important to highlight that horses with inherited genetic disease will be affected or carriers for life and there is no cure but treatment to manage their illness.Our tests are performed using polymerase chain reaction (PCR) precisely set to amplify the gene of interest. Further analysis is performed with Sanger sequencing which makes genetic mutations easily trackable. Further text describes the most common genetic diseases in horses which can be easily tested using our services.
Polysaccharide storage myopathy (PSSM) is an abnormal accumulation of glycogen in muscles. It affects a broad range of horse breeds but it is the most common among Quarter Horse bloodlines such as Paint Horses and Appaloosas. The most frequent type of PSSM among Quarter horses is PSSM1, caused by an autosomal dominant mutation in glycogen synthase 1 (GYS1) gene. This mutation increases synthase activity of GYS1 which, as a consequence, has higher rate of glycogen production. The second type, PSSM2, is diagnosed when the GYS1 mutation is not present but the symptoms of PSSM are. However, the cause for PSSM2 still remains unknown.1 Symptoms commonly include sore muscles, lameness, sweating, muscle stiffness, cramping and reluctance to move. Horses diagnosed with PSSM can alleviate their symptoms with exercise and an adjusted diet with minimal carbohydrate intake where additional calories should be provided in the form of fat. There is no cure for PSSM. Besides, in rare cases horses with PSSM1 remain asymptomatic or start showing symptoms later in life which is why genetic testing before breeding is extremely important in order to avoid passing on the disease.2
Malignant hyperthermia (MH) is an autosomal dominant disease characterized by high body temperature, irregular heartbeats and muscle contraction. This condition is triggered when a horse is exposed to certain anesthesia drugs and during high stress or excitement. For now, MH has been reported in Quarter horses, Arabians, Thoroughbreds and ponies. It is caused by mutation in RyR1 gene which codes for ryanodine receptor 1. These receptors form ion channels in cell membrane responsible for calcium release in skeletal and cardiac muscles. Mutation in RyR1 gene leads to an uncontrolable and excessive release of calcium, which induces a hyper-metabolic state.3 Hyper-metabolic state can be fatal, considering that it can lead to rhabdomyolysis, respiratory and metabolic acidosis. Muscle contractions caused by this condition can lead to tying-up of a horse which becomes worse if, at the same time, the horse has GYS1 mutation responsible for PSSM1. (Tying-up is a condition that causes painful muscle cramping in the horse.) Since both diseases are dominant and only one affected parent is needed to pass the disease on to offspring, we recommend doing both tests to avoid the possibility of advanced tying-up.4
Hyperkalemic periodic paralysis (HYPP) is an autosomal co-dominant disease characterized by attacks of muscle tremors. HYPP occurs in Quarter Horses and their crossbreeds, American Paint Horses and Appaloosas. It is caused by a mutation in SCN4A gene which codes for a subunit of sodium channels. This mutation produces aberrant sodium channels which releases more sodium in muscle tissue and causes involuntary muscle contraction. Sodium channels are closely related to potassium channels. When potassium levels are high, sodium channels become more active.5Consequently, horses will experience symptoms of HYPP after fasting or eating food high in potassium which will cause hyperkalemia with potassium concentration levels > 5mmol /L. In addition to muscle tremors, HYPP can cause sudden paralysis and collapse which can lead to death if it is followed by cardiac arrest or respiratory failure. Symptoms of HYPP vary between horses: some of them will experience only a few episodes of collapse or muscle tremors in their lifetime while some will present symptoms more often than not. Also, since inheritance of this disease is co-dominant, homozygous horses show more severe symptoms than heterozygous ones. However, both genotypes will be carriers and affect offspring if breeding is not controlled according to genetic testing results.6
Glycogen branching enzyme deficiency (GBED) is an autosomal recessive disease which, when developed, usually results in late abortions or stillbirths. GBED is found in Quarter horses and their related bloodlines. Since it is a recessive disease, it requires for both parents to be carriers. It is caused by a mutation in glycogen branching enzyme 1 (GBE1) which results in improper glycogen storage and makes tissues unable to preserve glucose homeostasis. Foals who survive at birth usually live up to a few weeks, experiencing weakness and seizures followed by limb deformities or heart and respiratory failure. However, the majority of affected offspring will be stillbirths because of glycogen importance in proper growth of a developing fetus. Surviving GBED until adult age is not recorded so far since organs such as heart and brain can't rely on glycogen as a source of energy. Therefore, horses with GBED won't reach sexual maturity but they can be carriers and, if bred with another carrier, give affected offspring.7
Hereditary equine regional dermal asthenia (HERDA) is an autosomal recessive disease that results in abnormal collagen structure. It has been reported in breeds with Quarter horse lineage. Abnormal collagen is caused by a mutation in peptidyl-prolyl isomerase B (PPIB) gene. The protein synthesized from this mutant gene is not able to position procollagen in the appropriate configuration for collagen synthesis, which leads to the separation of the outer skin layer from the underlying layer. As a consequence, affected horses experience skin lesions, ulcers, swelling and abnormalities in eyes, heart and tendons. There is no treatment for those symptoms; regions with skin lesions usually leave scars or white hair. In addition, horses with HERDA need to have restricted exposure to sunlight. Symptoms of HERDA usually don't start at birth but rather later in life due to saddling and riding, accordingly, lesions usually appear on the dorsal side. Although symptoms of HERDA can range from mild to severe, horses affected by it are not able to ride or breed and are usually euthanized. On the other hand, horses which are carriers of HERDA don't experience any symptoms. Since HERDA can be similar to other horse skin diseases, which can be caused by bacteria or worms, genetic testing is the only reliable way to determine this condition.8
Cerebellar abiotrophy (CA) is an autosomal recessive disease that affects mostly Arabian horses and their breeds. Cases of CA have also been reported in the Oldenburg, Eriskay and Gottland pony. CA is a disease of progressive death of Purkinje neurons in the cerebellum causing horses to have tremors, lack of balance, startle and fall easily. Also, symptoms of ataxia vary in horses with CA, - some horses can experience exaggerated forelimb movement, dysmetria in thoracic limbs or difficulty to stand up from lying position. Symptoms usually start to appear between 6 weeks and 6 months of age. The mutation responsible for this disease is in target of EGR1 (TOE1) gene involved in cell - cycle regulation. Genetic testing made it possible for this mutation to be visible prior to breeding. Before sequencing methods, this diagnosis could be set only through histopathological examination. Since CA follows recessive mode of inheritance, symptoms are not present in carrier horses but only affected ones. Affected horses can usually be only pets, since their coordination is not good enough to be ridden.9
Severe combined immunodeficiency is an autosomal recessive disease and one of the most common genetic diseases among Arabian and part-Arabian horses. This disease causes complete absence of certain immune cells such as B lymphocytes and T lymphocytes which makes horse incapable of antigen-specific immunity. Horses affected with SCID usually don't live longer than 6 months due to their high suspectibility for infections. Horses with SCID generally start to show symptoms 2-3 months after birth considering that they still have passively transferred immunity from their mother. The most common symptoms are elevated temperature and respiratory problems although symptoms depend on the type of an infection.10 Since SCID leaves their immune system unprotected, horses are not able to defend against bacterial, fungal, viral or protozoic infections. Mutation that causes this disease is in the DNA - dependent protein kinase (DNA-PK) gene responsible for development of the immune system. There is no cure or treatment for SCID, genetic testing for DNA-PK mutation is the only way for prevention and precise diagnosis since SCID shares symptoms with many other diseases.11
Junctional epidermolysis bullosa (JEB1) is an autosomal recessive disease also known by the name Red foot disease and Hairless foal syndrome. It is characterized by the lack of intercellular membrane adhesion molecules responsible for skin adhesion. Consequently, horses diagnosed with JEB suffer from pathological desquamation.12 Symptoms of JEB are formation of splits and skin lesions often found on legs and pressure points sometimes followed by additional skin infections. There are two mutations in horses responsible for JEB, mutation in the gene for laminin subunit gamma 2 (LAMC2) and mutation in the gene for laminin subunit alpha 3 (LAMA3). Both mutations encompass subunits of laminin protein complex, glycoproteins responsible for complex interaction between cell membranes. Also, the type of mutation that will occur depends on horse breed. Mutation in LAMC2 is found in draft horses such as Belgian horse, Trait Breton and Trait Comtois and it is characterized as JEB1. On the other hand, mutation in LAMA3 gene is typical for American Saddlebred and it is characterized as JEB2.13 However, our service offers testing only for JEB1. There is no treatment for JEB, skin condition usually gets worse over time and horses soon die from infections caused by skin lesions or euthanasia is performed due to their extreme pain.
Foal immunodeficiency syndrome (FIS) is an autosomal recessive disease found in Fell pony and Dale pony breeds as well as Gypsy horses. Usually within first 6 weeks horses with FIS develop fatal anemia and broad range of infections due to compromised immune system. Besides anemia, symptoms include low number of erythrocytes and B lymphocytes, diarrhea, inability to suckle, decreased appetite, lethargy and weakness, symptoms depend on the type of infection foal gets. In contrast to SCIF, FIS affected foals still have optimal number of T lymphocytes. FIS is caused by mutation in the sodium/myo-inositol cotransporter gene (SLC5A3) neccessary for transport of sodium ions and an osmolyte myo - inositol, an important growth-promoting factor.14 This cotransporter has an important function in response to osmotic stress by preventing high accumulation of myo-inositol and therefore preventing imbalance of cellular function. Treatment can be made depending on the type of infection but it only shortly prolongs foals life, they usually die or are euthanized within first 3 months. Study in 2012. showed high 54% of SLC5A3 mutation carriers among Fell ponies foals so genetic testing is highly reccomended in this breed since developing FIS has fatal outcome. 15
As previously mentioned, hereditary diseases can be passed on from parents to offspring in dominant and recessive way. Inherited features come as a product of 2 genes, one gene from each parent. Consequently, horse is heterozygous if it has 2 different genes, in case of hereditary genetic diseases meaning one healthy and one mutant gene for specific feature, or homozygous with two identical genes, whether healthy or mutant. Dominant diseases need only one gene mutated in order to produce affected offspring. Percentage of affected offspring depends on parents genotype. If both parents have only one mutant gene, chances of offspring to be affected are 75%. If only one parent has one mutant gene chances are 50%. Possible combinations of genotypes where parents are heterozygous are presented in Table 1.
Table 1. Inheritance of dominant diseases. Letters in black represent genotype of two parents. "A" presents healthy, wild type gene, while "a" presents mutant gene. Only one mutant gene is enough for disease development (shown in red) and only if both genes are wild type will offspring be healthy (shown in green).
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
On the other hand, recessive diseases are developed only if genes from both parents are mutant. Accordingly, if both parents are carriers, 25% of foals will be affected while 50% will be carriers of mutant gene. Inheritance of recessive disease is shown in Table 2.
Table 2. Inheritance of recessive diseases. Letters in black represent genotype of two parents. "A" presents healthy, wild type gene, while "a" presents mutant gene. Both mutant genes need to be present for recessive disease to be visible.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
In addition, HYPP is mentioned as an example of co-dominant disease. This type of inheritance is the same as the one for dominant disease with exception of less severe symptoms for heterozygous than homozygous mutants.
Tests for all listed diseases are available in our service, we suggest covering all tests that corelate with the same symptoms such as PSSM, MH and HERDA that share uncontrollable muscle movement and are available in our panel.
In order to perform genetic testing, biological sample needs to be taken from an animal of interest. For genetic testing of horses, biological sample can be hair or blood taken by a veterinarian. Blood should be non-coagulated and in the test tube containing EDTA. If sample is hair, it is important to capture the hair follicules since it is where cells for the DNA isolation are taken. Also, 20-30 hairs should be collected to have a proper biological sample. Gloves should be worn while taking the sample and the process needs to be carefully done with the minimum risk of contamination with foreign DNA. If sample was properly taken, results of genetic testing are sent within 2 weeks, otherwise, new sample will need to be taken. Sample should be put in an envelope together with an appropriate order form filled (https://www.animalabs.com/2021/order-forms/).