Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are born after the full 11-month gestation and externally appear normal, though they have all-white or nearly all-white coats and blue eyes. However, internally, these foals have a nonfunctioning colon. Within a few hours, signs of colic appear and affected foals die within a few days. Because the death is often painful, such foals often are humanely euthanized once identified. The disease is particularly devastating because foals are born seemingly healthy after being carried to full term.
The disease has a similar etiology to Hirschsprung's disease in humans. A mutation in the middle of the endothelin receptor type B (EDNRB) gene causes lethal white syndrome when homozygous. Carriers, who are heterozygous, that is, have one copy of the mutated allele, but themselves are healthy, can now be reliably identified with a DNA test. Both parents must be carriers of one copy of the LWS allele in order for an affected foal to be born.
Horses that are heterozygous for the gene that causes lethal white syndrome often exhibit a spotted coat color pattern commonly known as "frame" or "frame overo." Coat color alone does not always indicate the presence of LWS or carrier status, however. The frame pattern may be minimally expressed or masked by other spotting patterns. There also are different genetic mechanisms which produce healthy white foals and have no connection to LWS; another reason for genetic testing of potential breeding stock. Though there is no treatment or cure for LWS foals, a white foal without LWS that appears ill may have a treatable condition.
... Main article Lethal white syndrome See also Dominant white Foals which are homozygous for frame and thus have lethal white syndrome (LWS) are not albinos ... LWS foals are born almost or completely white with pink skin, but have blue eyes, not red ones ... The lethal trait is that the nerves of the foal's digestive system are undeveloped and the bowel cannot move food along ...
... From very early in research into its genetics, lethal white syndrome has been compared to Hirschsprung's disease (HSCHR) in humans, which is also caused by mutations on the EDNRB gene ... condition in humans is called Waardenburg-Shah syndrome ... The terms "piebald-lethal" and "spotting lethal" apply to similar conditions in mice and rats respectively, both caused by mutations on the EDNRB gene ...
... Overo refers to several genetically unrelated pinto coloration patterns of white-over-dark body markings in horses, and is a term used by the American Paint Horse Association to classify a set of pinto patterns that ... fall under the "overo" classification frame overo, sabino overo, and splash or splashed white overo ... associated with the frame overo pattern, which causes the controversial lethal white syndrome, and there is a DNA test for three mutations, SW-1, SW-2, and SW-3, associated with ...
... above, the frame gene is associated with a condition called lethal white syndrome or "lethal white overo" ... However, of the overo family of patterns, only frame is associated with lethal white ...
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