![]() ![]() Another form of SCID is caused by a deficiency of the enzyme adenosine deaminase (ADA), normally produced by a gene on chromosome 20. The appropriate planning of breeding of carriers prevents the outcome of affected foals and decreases the incidence of the mutant gene in the population. The most common type of SCID is called XSCID because the mutated gene, which normally produces a receptor for activation signals on immune cells, is located on the X chromosome. The test should be performed in all Arabian and Arabian-crossbred horses used in reproduction. The definitive diagnosis of carriers and affected foals can be done by a DNA test (VetGen, Veterinary Genetic Services, Michigan, USA) of whole blood or cheek swab samples. The mode of inheritance of the genetic defect is an autosomal recessive trait. The DNA-PK enzyme defect results from a deletion mutation of the gene encoding the catalytic subunit. Deficiency of adenosine deaminase (ADA, EC3.5.4.4), a housekeeping enzyme of purine metabolism encoded by the Ada gene, is a cause of human severe combined immune deficiency (SCID). The disease is caused by the lack of activity of the enzyme DNA-dependent protein kinase (DNA-PK), which is required for gene rearrangement of the antigen-receptor on B and T lymphocytes. The poor B and T cell development results in lymphopenia (less than 1,000 cells/uL), marked serum IgM and IgA deficiency, and hypoplasia of lymphoid tissues (thymus, lymph node, spleen, mucosa-associated). This fatal disease was first described in the horse by McGuire and Poppie in 1973. Foals are normal at birth but soon develop fatal infections, particularly when circulating colostrum-derived antibody concentrations become low. adenovirus, coronavirus, Rhodococcus equi, Pneumocystis carinii, and/or Cryptosporidium parvum). This immunodeficiency may occur in Arabian foals (or breeds carrying Arab bloodlines), and manifests clinically by susceptibility to viral, bacterial, fungal, and protozoal organisms (e.g. This means that a child has to inherit the faulty gene from both parents to have the condition. ![]() It is inherited as an autosomal recessive condition. Severe combined immunodeficiency (SCID) is a fatal condition of both B (humoral) and T (cellular) cell dysfunction. ADA-SCID is a specific form of severe combined immunodeficiency (). ![]()
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