First the fish, in this story of a transporter protein, AIM1 (MATP), which regulates synthesis of the black/brown pigment, eumelanin. In 2001, Fukamachi and colleagues described positional cloning of the AIM1 gene in medaka (Oryzias latipes), a small fish bred and selected for pigmentation variants by the Japanese for centuries. The orange-red variant of medaka was known to be homozygous for the b allele, and the many different b locus mutations result in defects in the black pigment cells (melanophores), while other pigment cells of the fish (xanthophores, leukophores, iridophores) appear to be normal. Fukamachi and colleagues took advantage of the extreme polymorphism between two inbred medaka strains, as well as the high recombination frequency around the b locus, to identify the candidate gene AIM-1. This gene was highly homologous to a human gene that encodes a melanoma antigen, and contained mutations in those fish that exhibited the orange-red phenotype. The AIM-1 protein has 12 transmembrane domains, and the researchers speculated that it may transport precursors required for melanin biosynthesis.
B mutations lead to orange-red variants in medaka. From Fukamachi et al. (2001)
The medaka AIM-1 story was put to good use by Du and Fisher (2002), who showed that the mouse homolog of this gene maps to the underwhite locus, mutations in which cause reduced pigmentation in the eyes and fur. Distinct mutations in Aim-1 were discovered in three underwhite alleles; for example, the ww mutant mouse harbors a deletion that results in a frameshift and a premature stop codon. The researchers also examined the possibility that a pigment cell transcription factor, MITF, regulates expression of the AIM-1 gene. Although MITF increased AIM-1 expression in both melanoma cells and normal melanocytes, this transcription factor did not bind to consensus E-box repeats in the proximal promoter for the Aim-1 gene, in chromatin immunoprecipitation experiments.
American Cream Draft Horse, mare and foal
Mutations in Aim-1 seemed likely to cause reduced pigmentation in other mammals, and thus Mariat and colleagues (2003) investigated sequence changes in this gene (referred to as MATP) in cream-colored horses. Existence of a “cream” gene had been inferred from the ability of an equine locus to dilute bay to dun or buckskin, and chestnut to palomino; horses that are homozygous for “cream” may be nearly white, with pink skin and blue eyes. This cream dilution locus was known to reside on Equus caballus chromosome 21 (ECA21), and MYO10 and MATP were selected as candidate genes. DNA samples were obtained from 127 related and unrelated Connemara ponies of different colors (24 cream-colored ponies included), as well as from 14 Welsh and Barbe horses.
Mariat and colleagues confirmed localization of the cream locus to ECA21, using comparative mapping, and then identified microsatellite markers in BAC clones that contained the candidate genes. At this point, they were able to eliminate MYO10, and focus on the MATP gene. A mutation (allele A) in exon 2 of the MATP gene was present in cream horses, as compared to the sequence from bay individuals (allele G). All bay horses in the reference population were homozygous G/G, palomino and buckskin horses were heterozygous A/G, and cream horses homozygous A/A. Interestingly, the coat colors of two horses in the study had been misidentified prior to the genotype analysis: a bay horse that was actually a buckskin, and a cream horse that was a very pale palomino with brown eyes.
Du, J., and Fisher, D.E. (2002). Identification of Aim-1 as the underwhite mouse mutant and its transcriptional regulation by MITF. J. Biol. Chem. 277(1), 402-406.
Fukamachi, S., Shimada, A., and Shima, A. (2001). Mutations in the gene encoding B, a novel transporter protein, reduce melanin content in medaka. Nature Genetics 28, 381-385.
Mariat, D., Taourit, S., GuÃ©rin, G. (2003). . Genetics Selection Evolution, 35(1), 119-133. DOI: 10.1051/gse:2002039