Severe Combined Immunodeficiency (SCID)
Gene or Region: DNAPK
Reference Variant: TCTCA
Mutant Variant: -
Affected Breeds: Arabian
Research Confidence: High - Findings reproduced in multiple studies
Explanation of Results: scid/scid = homozygous for Severe Combined Immunodeficiency, trait expressed scid/n = heterozygous for Severe Combined Immunodeficiency, carrier n/n = no variant detected
General Information
Severe combined immunodeficiency disease is a condition in which the foal has essentially no immune system, due to a complete absence of certain types of immune cells, specifically B and T lymphocytes. Affected foals are unable to mount an effective immune response after infection and are unable to recover from infections that unaffected foals would easily resist. The disease is universally fatal in horses, usually within 4-6 months of life. It is a recessive trait, meaning that it requires two mutant alleles to manifest; animals carrying just one copy of the mutant allele are completely healthy and are “carriers”, meaning that they can pass the mutation to their offspring. If two carrier horses mate, there is a 25% chance that the foal will have SCID. It has only been detected in horses of Arabian descent, and studies have determined that ~8% of Arabian horses are carriers of the SCID allele.
The particular mutation assessed is a five-nucleotide deletion within the DNAPK gene, leading to a mutant form of the DNA-PK protein that is non-functional. DNA-PK is involved in a process called V(D)J recombination, which leads the development of mature B and T lymphocytes. All known cases of horse SCID arise from this mutation.
Publications
Ding et al., DNA-PKcs mutations in dogs and horses: allele frequency and association with neoplasia.” (2002) Gene 283: 263-9. PMID: 11867233
Bernoco, D and Bailey, E, “Frequency of the SCID gene among Arabian horses in the USA.” (1998) Animal Genetics 29: 41-42. PMID: 9682449
Shin et al., “A Kinase-Negative Mutation of DNA-PKcs in Equine SCID Results in Defective Coding and Signal Joint Formation.” (1997) Journal of Immunology 158: 3565-9. PMID: 9103416
More Horse Health
Polysaccharide Storage Myopathy
Polysaccharide Storage Myopathy (PSSM) is a glycogen storage disorder in which affected horses have chronic episodes of exertional rhabdomyolysis (“tying-up”). Acute symptoms are an unwillingness to move and muscle damage following exercise. Moderate symptoms can usually be managed through a low-sugar/starch diet and exercise.
Recurrent Laryngeal Neuropathy
Recurrent Laryngeal Neuropathy (RLN) is a disease that results in loss of the neurons that open the larynx, significantly affecting performance in Thoroughbreds and other sport horses. It is an important issue for horses in competitive events because the resultant paralysis of the larynx leads to obstruction of air flow during intense exercise, creating the abnormal inspiratory noise that gives RLN its common name: “roaring”.
West Nile Virus Symptom Susceptibility / Risk
West Nile Virus (WNV) is a mosquito-borne virus that can infect multiple types of mammals, including horses. Infection can lead to a series of severe symptoms, most commonly encephalitis – acute inflammation of the brain. Severe neurological symptoms develop in ~8% of exposed horses, and once symptoms manifest, the fatality rate ranges from 20-40% and is higher in older horses.