The intensity of the merle pattern is determined by the length of the poly(A) tail of a repeat element which has been inserted into the boundary of intron 10 and exon 11 of the PMEL17 locus in reverse orientation. This poly(A) tail behaves as a microsatellite, and due to replication slippage, longer and shorter alleles of it might be generated during cell divisions. The length of the poly(A) tail regulates the splicing mechanism. In the case of shorter tails, the removal of intron 10 takes place at the original splicing, resulting in a normal premelanosome protein (PMEL). Longer tails generate larger insertions, forcing splicing to a cryptic splice site, thereby coding for an abnormal PMEL protein, which is unable to form the normal fibrillar matrix of the eumelanosomes. Thus, eumelanin deposition ensuring the dark color formation is reduced. In summary, the longer the poly(A) tail, the lighter the coat color intensity of the melanocytes. These mutations can occur in the somatic cells and the resulting cell clones will shape the merle pattern of the coat. When they take place in the germ line, they occasionally produce offspring with unexpected color variations which are different from those of their parents.
Keywords: PMEL; coat color; dog; merle; mosaicism; short interspersed nuclear element (SINE); splicing.