QTL analysis of modifiers for pigmentary disorder in rats carrying Ednrb sl mutations.

Genetic studies on coat color mutations in mammals have a long history in biomedical research because of their viable and visible phenotypes. Today, a wealth of information about the pathways and genes involved in the pigmentation has been revealed. Nearly 130 genes with approximately 1000 different alleles have been detected to affect coat color1. Early in the 19th century, coat color mutation was used to prove Mendel’s laws2. Then, coat color mutations were used to generate different inbred lines of visible markers3,4. Melanin-based pigmentation is highly conserved across vertebrates5; thus, color mutations in mammals can provide models for some human diseases. A large number of diseases in humans are associated with pigmentary abnormalities, such as Waardenburg syndrome6, Hirschsprung’s disease7, oculocutaneous albinism I8, and piebaldism9. Moreover, the pigmentation system is a classical tool in ecological studies. Selective forces such as aposematism, crypsis, thermoregulation, and sexual signaling drive variation in the pigmentation pattern10. The endothelin3 (Edn3)/endothelin receptor B (Ednrb) ligand–receptor pair is involved in pigmentation11,12. Ednrb-deficient mice exhibit an almost completely white coat, and they develop megacolon11. Ednrbsl is a spontaneous null mutation characterized by deletion of 301 bp in the Ednrb gene in rats, resulting in Hirschsprung’s disease and pigmentary disorder13. In our previous study, we established two strains with different genetic background but carrying the same Ednrbsl mutation, namely, AGH-Ednrbsl and LEH-Ednrbsl,14; these two strains showed different pigment phenotype. AGH-Ednrbsl/sl rats showed almost no pigmentation all over the body, whereas a large pigmented spot appeared on the head of LEH-Ednrbsl/sl rats. Therefore, we hypothesized that modifier loci in the genetic background of LEH modulated the severity of the pigmentary disorder. In this study, we analyzed the difference in pigmentation between the Ednrbsl-mutated rats; we performed quantitative trait locus (QTL) analysis using the intercross descendants with varying severity of pigmentation disorder, and we tried to search the modifier gene(s) affecting the phenotype.