In the US, Appaloosa horses are culturally associated with the Native American tribes whose range stretched along the Palouse River in Washington and Idaho. However, spotted horses existed for hundreds of years in the Old World prior to their American debut, particularly in the Fergana region of the Central Asian steppes, and these leopard-patterned animals were brought by Spanish explorers to the New World, where they gave rise to the Appaloosa breed. Nine spotting patterns are recognized in Appaloosas: blanket, spotted blanket, leopard, few spot leopard, snowcap, snowflake, frosted, speckled, varnish roan, and mottled. In addition, this breed is characterized by striped hooves, white sclera of the eye, and mottling around the genitals, anus, and muzzle. Appaloosa spotting is controlled by an incompletely dominant allele of the leopard complex locus (LP), located on Equus caballus chromosome 1 (ECA1), and is likely to be influenced by modifying genes, which lead to the observed and highly valued variations in white spotting (Terry et al., 2004). However, the identity of the gene(s) that comprise the leopard complex remains elusive.
Spotted Blanket pattern (bay coat color)
Photo: Appaloosa Horse Club of the UK (www.aphcuk.org)
White spotting patterns similar in appearance to the appaloosa phenotype are present in several other mammals, including mice, cattle, swine, and humans, and this observation was a starting point for investigations of candidate genes. The KIT gene, expression of which is altered in Tobiano Paint horses, was one of the pigmentation genes examined first, in part because of the similarity between the rump white (Rw) coat color mutation in mice, and the blanketed appaloosa pattern. Rump white results from an inversion spanning 30 centiMorgans of mouse chromosome 5, which causes ectopic expression of KIT, as well as disruption of the gene encoding dipeptidyl aminopeptodase-like protein 6 (Hough et al., 1998). However, linkage analyses with 8 Appaloosa stallions, with their 52 solid and spotted offspring, eliminated KIT as a gene that contributes to appaloosa spotting, as part of the leopard complex (Terry et al., 2001). Similar linkage studies also eliminated two genes known to cause white spotting in mice, microphthalmia-associated transcription factor (ECA16) and mast cell growth factor (ECA28), as leopard complex genes (Terry et al., 2002).
Photo: Appaloosa Horse Club of the UK
Terry and colleagues mapped equine LP to ECA1 in 2004, and this assignment led to the suggestion that either the pink eyed dilution (p) gene or the transient receptor potential cation channel subfamily M, member 1 (TRPM1) gene was a positional candidate for the leopard complex. Pink eyed dilution is an old coat color variant of mouse fancy origin, and refers to the pink tint of the eye, as well as to reductions in black-brown (eumelanin) pigments. The yellow-red pheomelanin pigment is unaffected by the p mutation, and thus pink eyed dilution leads to orange, blue lilac, and champagne coat colors on the agouti, blue, and brown backgrounds, respectively (Grüneberg, 1952). Rinchik and colleagues (1993) identified the human homolog of p as the gene affected in Type II oculocutaneous albinism (OCA2), which encodes a putative tyrosine transporter in the melanosomal membrane. In Tanzania and other parts of Africa, mutations in the P gene are the most common cause of albinism, and OCA2 is associated with premature death, due to increased incidence of UV-induced squamous and basal cell carcinomas of the skin in affected individuals (Spritz et al., 1995).
Photo: Appaloosa Horse Club of the UK
A single nucleotide polymorphism (SNP) in the the equine OCA2 gene was recently eliminated as a cause for the appaloosa white spotting pattern (Bellone et al., 2006). OCA2 was mapped by fluorescence in situ hybridization (FISH) to the ECA1 interval in which LP resides, and thus a SNP in exon 7 of the gene was identified and examined for association with the appaloosa coat color pattern. Genomic DNA from the blood and hair samples of 47 Appaloosa, 42 Thoroughbred, and 43 Standardbred horses was analyzed for the SNP. Because LP acts in a dominant fashion, and because the SNP was present in 17 spotted Appaloosas and absent in 26 spotted Appaloosas, its association with the pattern was excluded. Identification of the gene(s) responsible for appaloosa white spotting may require genetic studies of other breeds that exhibit the leopard complex pattern, including the Pony of the Americas, the Spanish Mustang, and the Knapstrupper.
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Bellone, R., Lawson, S., Hunter, N., Archer, S., Bailey, E. (2006). Analysis of a SNP in exon 7 of equine OCA2 and its exclusion as a cause for appaloosa spotting. Animal Genetics, 37(5), 525-525. DOI: 10.1111/j.1365-2052.2006.01505.x