Introduction
Piebaldism (MIM #172800) is a rare autosomal dominant disorder characterized by a congenital white forelock and leukoderma on the frontal scalp, forehead, ventral trunk, and extremities. This phenotype was induced by the abnormal migration of neural crest derived melanoblasts to the skin during embryogenesis or the local postnatal migration of skin melanoblasts and melanocytes(Alexeev & Yoon, 2006; Bondanza et al., 2007; Grichnik, 2006). Various “loss-of-function” mutations in c-KIT have been demonstrated in about 75% of the patients with piebaldism(Ezoe et al., 1995; Murakami et al., 2004; Murakami et al., 2005). Also, a satisfactory correlation has been described between the mutant genotype and clinical phenotypes: missense mutations in the extracellular ligand-binding domain and nonsense mutations preserved approximately 50% of the c-KIT function and induced mild piebaldism by haploinsufficiency; mutations in the tyrosine kinase domain preserved approximately 25% of the c-KIT function and induced severe piebaldism through a dominant-negative effect(Oiso, Fukai, Kawada, & Suzuki, 2013; Spritz, 1994; Ward, Moss, & Sanders, 1995).
Interestingly, piebaldism combined with congenital deafness was firstly reported in two Hopi Indian brothers in Arizona(Woolf, 1965). Until 1998, Spritz et al. firstly described the pathogenicity of heterozygous c-KIT (p.R796G) in piebaldism with congenital deafness(Spritz & Beighton, 1998). Recently, the homozygous deletion of exons 20 and 21 inc-KIT was reported to induce piebaldism and deafness in humans(Kilsby et al., 2013), and the heterozygous mutation of A608D was found in one adult patient with non-hereditary unilateral deafness(Hamadah et al., 2019). Furthermore, several KIT mutations (KITW-V , KIT Wads , and KITF856S) have been identified in mice with hypopigmentation and hearing loss(Hoshino et al., 2000; J. CABLE, 1994; Ruan, Zhang, & Gao, 2005). Nevertheless, the low and irregular coincidence of piebaldism with hearing loss seemed to be an expanded and occasional manifestation of c-KIT mutations. Thus, piebaldism was speculated as an inherited condition in an intermediate rather than a dominant manner(Hamadah et al., 2019; Spritz & Beighton, 1998).
In pigs, c-KIT is highly related with the dominant white color phenotype by two mutations: the copy number variations of the entire gene (~450 kbp) and a G to A substitution in the first nucleotide of intron 17(Giuffra et al., 2002; Johansson, Pielberg, Andersson, & Edfors-Lilja, 2005; Pielberg, Olsson, Syvanen, & Andersson, 2002). To date, six c-KIT alleles have been reported in pigs: i , the recessive allele in wild boar;IBe , the dominant allele for the belt phenotype in Hampshire pigs; IP , the half dominant allele for the patch phenotype in Pietrain; I(I1 , I2 , orI3 ), the completely dominant allele for white color in Landrace and large white pigs. The polymorphisms in the c-KIT sequence could discriminate the European from the Asian groups, the two main clusters produced during the domestication history(Niu, Shi, Xie, Liu, & Zhong, 2018). On the other hand, pigs exhibit more similar audiological characteristics to humans than rodents, including the cochlear structure, the range of audible frequency, and sensitivity to sound(Chen et al., 2016; Guo et al., 2015; Hai, Guo, et al., 2017), making it more advantageous in the research and treatment of hearing loss.
Previously, we reported the first large-scale ENU-mediated mutagenesis program in pigs in the world using Chinese Bama miniature pig and identified 36 dominant and 91 recessive novel strains(Hai, Cao, et al., 2017). One of the dominant inherited strain exhibited a severe phenotype of piebaldism and congenital profound hearing loss. The genome-wide association study (GWAS) and linkage analysis identified a de novo mutation inc-KIT (c. 2430T>A), which induced the missense mutation of Asp810Glu in c-KIT protein. Then, we described theIBe genotype of c-KIT in wild-type Bama pigs. In addition, the pathological results revealed the collapse of Reissner’s membrane and the absence of hair cells in the mutant inner ear. Finally, we reanalyzed the mutations in c-KIT gene to provide new clues for understanding the etiology and development of piebaldism with hearing loss.