Tiger Eye (SLC24A5-linked)

Gene: SLC24A5

Transmission: Autosomal recessive

For an autosomal recessive genetic disease an animal must have two copies of the mutation in question to be at risk of developing the disease.  Both parents of an affected animal must be carriers of at least one copy of the mutation.  Animals that have only one copy of the mutation are not at risk of developing the disease but are carrier animals that can pass the mutation on to future generations.

Mutations:

Tiger Eye mutation 1: Substitution, SLC24A5 gene: c.272A>T, p.(Phe91Tyr), exon2, Chr.1.

Tiger Eye mutation 2: Deletion, SLC24A5 gene: c.875-340_1081+82del., deletion 628 bp including exon7, Chr.1.

Medical system:

Breed: Paso Fino (Puerto Rica)

Age of onset of symptoms: In the young foal.

Paso Fino horses are descendants of Spanish horses brought to the Caribbean during the European colonization of the Americas.  The Puerto Rican Paso Fino horse can have a phenotype described as “tiger eye”, where the iris has a bright yellow, amber or orange coloration.  The tiger eye phenotype is not associated with ocular defects or vision problems, nor with coat color dilutions.  Thus, it can be considered a genetic trait rather than a genetic disease.  By pedigree analysis over 10 generations, the tiger eye phenotype displays autosomal recessive inheritance with a probable common ancestor.

Initial molecular studies of the tiger eye phenotype identified an associated region on chromosome 1 which included the SLC24A5 gene.  The SLC24A5 gene codes for a sodium-potassium ion exchange protein expressed in melanocytes, known to be important for melanosome maturation and melanin production.  In humans, mutations in the SLC24A5 gene are responsible for skin color variations and for some forms of oculocutaneous albinism.  The SLC24A5 gene was sequenced in tiger eye horses, and two mutations were identified to account for the phenotype, either in homozygous (1/1) or compound heterozygous (1/2) forms.  As the tiger eye phenotype is striking and does not have associated deleterious health effects, it is maintained within the Paso Fino population by breeders.

An additional mutation in the equine SLC24A5 gene has been described and is thought to be responsible for one form of dilution of black horse pigmentation.

References:

OMIA link: [2124-9796]

Mack M, Kowalski E, Grahn R, et al. (2017) Two variants in SLC24A5 are associated with “Tiger-Eye” iris pigmentation in Puerto Rican Paso Fino horses. G3 (Bethesda) 7:2799-2806.  [pm/28655738]

Kreuzer S, Reissmann M, Brockmann GA. (2010) A mutation in the equine SLC24A5 gene is associated with a dilution of black horses. Proceedings of the 9th World Congress on Genetics Applied to Livestock Production Leipzig (Germany). August 1-6. Horse Breeding. [Poster 0516]

With contributions by: Annie-Jade Bourgeois and Frédérike Desrosiers, Class of 2030, Faculty of Veterinary Medicine, University of Montreal.  (Translation: DWS).