Incidence of the endothelin receptor B mutation that causes lethal white foal syndrome in white-patterned horses

Objective-To determine incidence of the IIe118Lys endothelin receptor B (ED NRB) mutation responsible for overo lethal white syndrome (OLWS) and its as sociation with specific types of white patterning. Animals-945 horses of white-patterned bloodlines and 55 solid-colored horse s of other breeds. Procedure-Horses were genotyped by use of allele-specific polymerase chain reaction to determine incidence of the IIe118Lys EDNRB mutation. Results-Genotypes detected were homozygous IIe118, homozygous Lys118, and h eterozygous. All foals with OLWS were homozygous for the IIe118Lys EDNRB mu tation, and adults that were homozygous were not found. White patterning wa s strongly associated with EDNRB genotype. Color patterns with highest inci dence (> 94%) of heterozygotes were frame overo, highly white calico overo, and frame blend overo. White-patterned bloodlines with lowest incidence of heterozygotes (< 21%) were tobiano, sabino, minimally white calico overo, splashed white overo, nonframe blend overo, and breeding-stock solid. The m utation was not detected in solid-colored horses from breeds without white patterning. Conclusions and Clinical Relevance-In homozygotes, the IIe118Lys EDNRB muta tion causes OLWS. In heterozygotes, the mutation is usually responsible for a frame overo phenotype. The frame pattern can be combined with other whit e patterns, making accurate estimation of EDNRB genotype by visual inspecti on difficult. Wide range of incidence of heterozygotes in various subtypes of white-patterned horses indicates different genetic control of these colo r patterns. Determination of EDNRB genotype by use or a DNA-based test is t he only way to determine with certainty whether white-patterned horses can produce a feat affected with OLWS.