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B locus
B locus
The B or brown locus (b) is located on chromosome 4 of the mouse genome. This locus was identified as the tyrosinase-related protein 1 gene (Tyrp1) in work during the late 1980s and early 1990s.16,17 The wildtype allele produces black eumelanin, while the recessive allele (b) produces brown eumelanin, making a mouse harboring the Tyrp1b/b genotype cinnamon brown due to the occurrence of yellow-banded brown hairs. The effect on the melanosome of mice expressing the b allele versus the wildtype B allele is to change the nature of the eumelanin in the pigment granule, which is accompanied by a change in its size and shape. Brown granules are significantly smaller than black granules and display much less variation in size. The pigment granules found in the medulla of black-pigmented hairs are usually either long oval or oval in shape, while those in brown-pigmented hairs are round. Currently, there are over 60 targeted, spontaneous, and transgenic alleles of the Tyrp1 gene (http://www.informatics.jax.org/allele/ summary?markerId=MGI:98881, MGI accessed 27 June 2016). Some of the most common alleles that can be found in laboratory mice strains include light (Blt), cordovan (bc), and white-based brown (Bw).
although at a significantly reduced level. OCAIB is also known as yellow albinism.37,38 Patients with OCAIB are phenotypically similar at birth to type II oculocutaneous albinism (OCAII), yet rapidly develop normal skin pigmentation and yellow hair.37โ40 Interestingly, mutations in both TYR (R402Q) and MITF (1-bp deletion in exon 8) play roles in the autosomal recessive, digenic Waardenburg syndrome type 2 with ocular albinism (WS2-OA). Patients with WS2-OA experience reduced visual acuity, photophobia, nystagmus, strabismus, and albinotic fundus with foveal hypoplasia, along with deafness.41โ44
The TYRP1 gene in humans resides at chromosome location 9p23 and has been linked to a number of human pigmentation disorders. Several studies have identified mutations in the TYRP1 gene that reduce the stability of the transcript and significantly inhibit its catalytic activity causing type III oculocutaneous albinism (OCA3)18โ21 in either single patients or small cohorts of African Americans (106delT), southern Africans (368delA, S166X), Pakistani (R373Ter), and Asian Indian (1057delAACA) populations. Interestingly, Kenney etโฏal.22 identified a homozygous change in the N terminus of TYRP1 (R93C) that causes Melanesian blond hair. Similar to the OCA3 mutation, the R93C mutation affects the stability of the TYRP1 transcript and subsequently the catalytic activity of the protein.