The New DNA Test for Severe Combined Immunodeficiency (SCID) in Arabian Horses
SCID has been of grave concern to Arabian horse breeders for the past several decades. It is inherited as an autosomal recessive disease, which means that one copy of the disease gene is inherited from a carrier stallion and another from a carrier mare. The foal which inherits two copies of the disease gene is affected with a lethal inability to fight infections, and dies within the first few months of life.
SCID disorders, which occur in humans, dogs, and mice as well as horses, are characterized by a lack of both humoral and cellular-mediated immunity. In the horse, SCID was first reported in Arabian foals in 1973 (McGuire and Poppie, 1973). In 1980, Perryman and Torbeck showed that SCID in Arabian horses was inherited as an autosomal recessive condition.
The critical defense molecules of the immune system are immunoglobulins and receptors on the T cells. The body normally generates a wide variety of these molecules which then recognize intruders, foreign and potentially hazardous agents such as bacteria and viruses. The specific mutation in SCID resides in one of the critical components of the system, an enzyme called DNA-protein kinase catalytic subunit, DNA-PKcs, which is responsible for the generation of the variety of the immune defense molecules (Shin et al. 1997).
While new studies using a DNA test for the specific defect will soon provide accurate and modern frequency data on this disease gene, studies from a little less than 20 years ago indicated that the frequency of affected Arabian foals was around 3% (Poppie and McGuire, 1977 and Studdert, 1978). This places the carrier frequency at about 28% and means that one out of every three or four adult Arabian horses is carrying the gene for this deadly disease. Another way of stating this is that 7-10% of all matings are between carriers, and thus this proportion of matings is at risk to produce a foal that will soon die from SCID.
The discovery of the genetic cause of SCID in Arabian horses has been achieved through the collaborative efforts of Dr. Lance Perryman and Dr. Katherine Meek at North Carolina State University and at the University of Texas Southwestern Medical Center at Dallas. VetGen LLC, an Ann Arbor, Michigan-based veterinary genetic testing company, has licensed the technology for the detection of this mutation. VetGen will offer a DNA test which will precisely identify carriers of the SCID disease gene. Any convenient source of DNA can be used. VetGen will supply mucosal brushes for those who wish to simply swab the inside of the horse's mouth. Mucosal cells which adhere to the brush provide the DNA. The brushes are mailed back to VetGen by regular mail, since the DNA is quite stable. Alternatively, a sample of blood can be drawn in EDTA or acid-citrate-dextrose "ACD" anticoagulant, and mailed to VetGen by regular mail in a padded envelope or container. The white cells in the blood provide the DNA.
It should be pointed out that continued breeding of horses that are carriers of the SCID gene is indeed now possible without the worry of producing SCID foals. For example, carrier stallions that possess highly desirable traits can now be selectively bred to clear (homozygous normal) mares (and vice versa). The resulting foals would have an equal chance of being a carrier or clear of SCID, but would definitely not be affected. The foals could be tested anytime after birth to determine their SCID genotype and future matings could be rationally planned.
VetGen's test has been designed to easily and reliably determine normal, affected and carrier horses. VetGen promises a fast turnaround time, two weeks or less after sample receipt until mailing of test results.
For further information on the Arabian horse SCID test, and how to order the test:
Contact VetGen at 3728 Plaza Drive, Suite 1,
Ann Arbor, Michigan;
(734) 669-8440; (800) 4-VETGEN;
Fax: (734) 669-8441.
- McGuire TC and Poppie MJ. Hypogammaglobulinemia and thymic hypoplasia in horses: A primary combined immunodeficiency disorder. Infection and Immunity 8: 272-277, 1973.
- Perryman LE and Torbeck RE. Combined immunodeficiency of Arabian horses: confirmation of autosomal recessive mode of inheritance. J Am Vet Med Assoc 76: 1250-1251, 1980.
- Poppie MJ and McGuire TC. Combined immunodeficiency in foals in Arabian breeding: evaluation of mode of inheritance and estimation of prevalence of affected foals and carrier mares and stallions. J Am Vet Med Assoc 170: 31-33, 1977.
- Shin EK, Perryman L, Meek K. A kinase negative mutation of DNA-PKcs in equine SCID results in defective coding and signal joint formation. The Journal of Immunology Vol. 158, No. 08 Issue of 04-15-97 pp 3565-3569
- Studdert MJ. Primary, severe, combined immunodeficiency disease of Arabian foals. Aust Vet J 54: 411-417, 1978.
- Weiler R, Leber R, Moore BB, VanDyk LF, Perryman LE, Meek K. Equine severe combined immunodeficiency: A defect in V(D)J recombination and DNA-dependent protein kinase activity. Proc Natl Acad Sci USA, 92: 1148-2249.