Editing Genetic Health Defects

A dominant allele (En) is considered the determining genetic factor for the spotted phenotype, very variable in extent, attributed to the English spotting locus. The recessive wild-type allele (en) determines a self-colored phenotype [81]. Rabbits with the heterozygous genotype En/en have far larger patches of colored fur than rabbits with the homozygous En/En genotype [30]. Heterozygous En/en rabbits have the characteristic spotted patterns described in the English spot (or spotted), Butterfly, and Checkered Giant breeds [67]. This is a unique allelic combination that is needed to produce breed standards desired by fancy breeders. The patches of the spotted phenotypes can be of different colors according to the allelic combination at other loci. Other genes are probably involved in determining the degree of extension of the colored patterns versus the white areas [82]. Dominant homozygous En/En rabbits are affected by an underlying megacolon defect and are usually subvital compared to the animals with the other two genotypes, i.e., En/en and en/en [30,82,83,84,85,86,87]. The defect has incomplete penetrance, it is influenced by environmental factors (age, diet, stressors) and is recessive, as En/en rabbits are not affected [87]. Etiopathogenetic analyses of rabbits having this megacolon pointed out that abnormalities of the enteric nervous system throughout the colon might be involved in this defect [85,86,88].

Several genes that are implicated both in coat color spotted patterns and in similar disfunctions of the digestive tract have been described in rodents and other mammals. Two of these genes were also investigated in association studies with the coat color and megacolon phenotypes in rabbits. The endothelin receptor B (EDNRB) gene was first excluded to be involved in these traits in Checkered Giant rabbits [89]. Subsequently, the analysis of KIT gene markers in Checkered Giant families indicated that a synonymous polymorphism in exon 5 of this gene is associated with both traits [87]. The KIT gene encodes the mast/stem cell growth factor receptor that is mainly involved in the differentiation of melanoblasts and in regulating their migration from the neural crest along the dorsolateral pathway to reach the final destinations in the skin [90,91]. KIT gene expression in the gut musculature is prominent only in interstitial cells of Cajal (ICC) that are essential in gut motility [92]. KIT gene expression in cecum and colon specimens of En/En rabbits resulted very low compared to the expression in the same tissues observed in en/en rabbits. Reduced and altered c-kit immunolabelled ICC, together with neuronal and ICC abnormalities in cecum and colon tissues, were identified in En/En rabbits [87]. Therefore, in addition to the effect of the English spotting locus on a coat color spotted phenotype evidenced in Checkered Giant rabbits, the neuro-ICC changes in rabbits with En/En genotype are reminiscent of the human non-aganglionic megacolon [87]. Therefore, En/En rabbits might be considered as new animal models for this type of defect in humans [87]. The complete sequence characterization of the rabbit KIT gene and of the upstream and downstream regions is needed to identify the causative mutation(s) of the coat color pattern and of the megacolon defect described in this model. Other coat color patterns might have additional variants affecting the KIT gene.

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