| Contents |
|---|
| Lethal White Overo (LWO) |
| Summary |
| Understanding the Results |
| Disease Name and Genes |
| Inheritance |
Lethal White Overo (LWO)
Summary
Lethal White Overo (LWO), also known as Lethal White Foal Syndrome (LWFS) or Overo Lethal White Syndrome (OLWS), is an inherited condition associated with the Frame Overo patterning gene that affects coat color. Foals affected by Lethal White Overo have aganglionosis (lack of parasympathetic ganglion cells) in the distal small intestine and the large intestine, resulting in megacolon, an abnormal dilation of the colon due to paralysis of the peristaltic movements of the bowel. The condition is similar to the human inherited condition Hirschsprung disease, caused by mutations in the orthologous human gene. Affected foals dies shortly after birth due to intestinal blockage. Their coat color is entirely white. There is no effective treatment.
The perinatal lethality associated with Frame Overo is a recessive characteristic. The coat color phenotype (white spotting) is dominant. The Frame Overo variant is found in Paints, Pintos, Quarter Horses, Miniature Horses, and Thoroughbreds.
Date of Last Update: 08/02/2016
Results
Understanding the Results
Results of the genetic test for Lethal White Overo (LWO) are presented as shown below.
| Lethal White Overo (LWO) | ||
|---|---|---|
| n/n | Clear | This horse tested negative for frame Overo. |
| n/O | Carrier | Both the normal and mutant alleles are present. This horse is positive for the frame Overo mutation but will not develop symptoms. |
| O/O | Affected | This horse carries two copies of the frame Overo mutation and will die of the disease. |
Disease Name and Genes
Lethal White Overo (LWO) or Lethal White Foal Syndrome (LWFS) is caused by a TC to AG dinucleotide mutation of the endothelin-B receptor gene (EDNRB) that causes the replacement of Isoleucine (I) to Lysine (K) at amino acid position 118 of the protein (EDNRB-I118K).
Inheritance
Lethal White Overo (LWO) is a recessive lethal, with affected foals dying shortly after birth. It is caused by a recessive variant of the EDNRB gene (EDNRB-I118K), with the variant commonly abbreviated as O, and the dominant wild-type allele abbreviated as n. The EDNRB-I118K variant produces the frame Overo coat color pattern as a dominant effect.
Carriers of the recessive allele (n/O) have no symptoms of the disease. If two carriers are bred, each foal has a 25% chance of having two copies of the normal allele (n/n), a 50% chance of being a carrier (n/O), and a 25% chance of being affected (O/O).
If a carrier of the recessive allele (n/O) is bred to a normal horse (n/n), each foal has a 50% chance of having two copies of the normal allele (n/n) and a 50% chance of being a carrier (n/O).
