Abstract
BackgroundThe antisense insertion of a canine short interspersed element (SINEC_Cf) in the pigmentation gene PMEL (or SILV) causes a coat pattern phenotype in dogs termed merle. Merle is a semi-dominant trait characterized by patches of full pigmentation on a diluted background. The oligo(dT) tract of the Merle retrotransposon is long and uninterrupted and is prone to dramatic truncation. Phenotypically wild-type individuals carrying shorter oligo(dT) lengths of the Merle allele have been previously described and termed cryptic merles. Two additional coat patterns, dilute merle (uniform, steely-grey coat) and harlequin merle (white background with black patches), also appear in breeds segregating the Merle allele.ResultsSequencing of all PMEL exons in a dilute and a harlequin merle reveals that variation exists solely within the oligo(dT) tract of the SINEC_Cf insertion. In fragment analyses from 259 dogs heterozygous for Merle, we observed a spectrum of oligo(dT) lengths spanning 25 to 105 base pairs (bp), with ranges that correspond to the four varieties of the merle phenotype: cryptic (25–55 bp), dilute (66–74 bp), standard (78–86 bp), and harlequin (81–105 bp). Somatic contractions of the oligo(dT) were observed in 43% of standard and 51% of harlequin merle dogs. A small proportion (4.6%) of the study cohort inherited de novo contractions or expansions of the Merle allele that resulted in dilute or harlequin coat patterns, respectively.ConclusionsThe phenotypic consequence of the Merle SINE insertion directly depends upon oligo(dT) length. In transcription, we propose that the use of an alternative splice site increases with oligo(dT) length, resulting in insufficient PMEL and a pigment dilution spectrum, from dark grey to complete hypopigmentation. We further propose that during replication, contractions and expansions increase in frequency with oligo(dT) length, causing coat variegation (somatic events in melanocytes) and the spontaneous appearance of varieties of the merle phenotype (germline events).
Highlights
The antisense insertion of a canine short interspersed element (SINEC_Cf) in the pigmentation gene PMEL causes a coat pattern phenotype in dogs termed merle
Exonization of the Merle Short interspersed element (SINE) occurs adjacent to the transmembrane domain (TMD), suggesting that the white fur of harlequin dogs may result from a similar mechanism whereby the production of abnormal protein negatively impacts melanocyte viability
We propose that alternative splicing resulting in SINE exonization and mutant transcripts from the Merle allele causes a reduction of PMEL fibers and the steel-grey fur color of dilute merles
Summary
The antisense insertion of a canine short interspersed element (SINEC_Cf) in the pigmentation gene PMEL (or SILV) causes a coat pattern phenotype in dogs termed merle. Merle is a semi-dominant trait characterized by patches of full pigmentation on a diluted background. Domesticated dogs (Canis lupus familiaris) present with a unique pattern of pigmentation termed merle. Merle coats have two features: 1) a light, diluted base color and 2) random patches of fully pigmented fur. Together, these two characteristics constitute the standard merle phenotype. These two characteristics constitute the standard merle phenotype In some breeds, this phenotype is highly desirable because each dog possesses a unique combination of pigment intensity and spot distribution. Homozygosity for Merle causes severe hypopigmentation with patches of merling and is associated with visual and auditory defects [1, 2]
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