Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus)

Abstract

Coat color genetics, when successfully adapted and applied to different mammalian species, provides a good demonstration of the powerful concept of comparative genetics. Using cross-species techniques, we have cloned, sequenced, and characterized equine melanocortin-1-receptor (MC1R) and agouti-signaling-protein (ASIP), and completed a partial sequence of tyrosinase-related protein 1 (TYRP1). The coding sequences and parts of the flanking regions of those genes were systematically analyzed in 40 horses and mutations typed in a total of 120 horses. Our panel represented 22 different horse breeds, including 11 different coat colors of Equus caballus. The comparison of a 1721-bp genomic fragment of MC1R among the 11 coat color phenotypes revealed no sequence difference apart from the known chestnut allele (C901T). In particular, no dominant black (ED) mutation was found. In a 4994-bp genomic fragment covering the three putative exons, two introns and parts of the 5'- and 3'-UTRs of ASIP, two intronic base substitutions (SNP-A845G and C2374A), a point mutation in the 3'-UTRs (A4734G), and an 11-bp deletion in exon 2 (ADEx2) were detected. The deletion was found to be homozygous and completely associated with horse recessive black coat color (Aa/Aa) in 24 black horses out of 9 different breeds from our panel. The frameshift initiated by ADEx2 is believed to alter the regular coding sequence, acting as a loss-of-function ASIP mutation. In TYRP1 a base substitution was detected in exon 2 (C189T), causing a threonine to methionine change of yet unknown function, and an SNP (A1188G) was found in intron 2.

Two cysteine substitutions in the MC1R generate the blue variant of the arctic fox (Alopex lagopus) and prevent expression of the white winter coat

“…Sequence comparison for MC1R regions that span amino acid 5 (amino acid 1-9 in the left box) and 280 (region 276-284, or 274-282 in mouse which has a two aa deletion). For the regions illustrated, additional species have been shown to be identical to species included in this figure [5,[10][11][12]16,18,25,26]. rable to the TM2/TM3 -mutations might be foreseen.…”

Analysis of agouti signaling protein (ASIP) gene polymorphisms and association with coat color in Tibetan sheep (Ovis aries)

“…Previous studies showed ASIP mutations related to animal coat color but mainly addressed the ASIP functional-deletion mutation and missense mutations. ASIP alleles causing coat-color variation have also been characterized in foxes, in which exon 2 is completely missing (Vage et al, 1997); rats, which have a 19-bp deletion within the coding DNA sequence (Kuramoto et al, 2001); cats, which have a 2-bp deletion in exon 2 (Eizirik et al, 2003); horses, which have an 11-bp deletion in exon 3 (Rieder et al., 2001); dogs, which have a missense mutation in exon 4 (Schmutz et al, 2007); and sheep, which have a 5-bp deletion in exon 2 and a missense mutation in exon 4 (Norris and Whan, 2008). The sheep ASIP exon 2 g.100-104 AGGAA deletion is a frameshift mutation that causes termination of the amino acids coding at locus 64 and results in the lack of a functional mature polypeptide signal domain (Royo et al, 2008;Gratten et al, 2010).…”

“…Therefore, we considered that Tibetan sheep could be an interesting model for identifying coat-color candidate genes. Although there are over 300 genes identified with known roles in mammalian pigmentation, a few key genes have been identified as major regulators of pigment production in domestic animals (Rieder et al, 2001;Liu et al, 2009;Suzuki, 2013).…”

“…The agouti signaling protein gene (ASIP) has been shown to be associated with coatcolor production in a variety of animals; previous studies mapped the coat color locus on chromosome 13 for cats (Eizirik et al, 2003), mice (Kuramoto et al, 2001), foxes (Vage et al, 1997), horse (Rieder et al, 2001), cattle (Girardot et al, 2005), pigs (Drogemuller et al, 2006), dogs (Kerns et al, 2004;Schmutz et al, 2007), sheep (Norris and Whan, 2008), and humans (Kwon et al, 1994). The ASIP gene encodes a signaling protein (ASIP) that can act as an antagonist of the alpha-melanocyte-stimulating hormone (a-MSH), preventing the binding of a-MSH to melanocortin 1 receptor, which induces a decrease in the levels of cyclic adenosine monophosphate (cAMP).…”

A genome-wide scan study identifies a single nucleotide substitution in ASIP associated with white versus non-white coat-colour variation in sheep (Ovis aries)

“…The ASIP gene has been well characterized in mice and humans (for example, Wilson et al, 1995;Klungland et al, 2000), and its Bos taurus cattle homologues were identified as the genomics of B. taurus developed (for example, see the review by Lauvergne et al (1989)). Several recognized natural patterns and many novel mutations in the ASIP gene, such as SNPs, insertion-deletion and tandem gene duplication, have been identified in mammals (for example, cattle (Dreger, 2008), dogs (Kerns et al, 2004), horses (Rieder et al, 2001), pigs (Mao et al, 2001), macaques (Nakayama et al, 2010), alpacas (Feeley et al, 2011) and rabbit (Fontanesi et al, 2010)). These include both spontaneous and induced mutations.…”