Oculocutaneous albinism type 1B associated with a functionally significant tyrosinase gene polymorphism detected with Whole Exome Sequencing

Abstract

Background: Oculocutaneous albinism (OCA) is a Mendelian disorder characterized by hypopigmentation of the skin, hair, and eyes, hypoplastic fovea, and low vision, known to be caused by mutations in the Tyrosinase (TYR) gene. Among the known TYR variants, some reduce but do not completely eliminate tyrosinase activity, allowing residual production of melanin and resulting in a contradictory assignment as either pathogenic or benign, preventing a precise clinical diagnostic. Materials and Methods: In the present work, we performed Whole Exome Sequencing and subsequent Sanger sequencing in a young male clinically diagnosed with OCA. Results: Whole-exome sequencing analysis revealed the identification of two variants in trans in TYR. The first, corresponds to a known pathogenic variant G47D, while the second S192Y, was considered a polymorphism due to its relatively high frequency in the European population. Conclusion: The lack of other pathogenic variants in TYR, the reported reduced enzymatic activity (ca. 40% respect to wt) for S192Y, together with the structural in-silico analysis strongly suggest that both reported variants are jointly disease-causing and that S192Y should be considered as likely pathogenic, especially when it is found in trans with a null variant.