Albinism presents itself in congenital hypopigmentation that can affect skin, hair and eyes (Oculocutaneous albinism or OCA), or can be confined mainly to the eyes (Ocular albinism or OA).
The incidence is 1/ 20000 births but is 1/ 12000 among African Americans in the U.S. and in certain regions such as Niger is around 1/1000.
It is characterized by a defect in the biosynthesis and distribution of melanin, which normally determines the visible pigmentation of skin, hair and eyes.
Normal pigmentation requires, during growth, several critical steps in which a large number of genes are involved in the process, both directly and indirectly.
Melanin is produced by melanocytes, which represents a relatively small subpopulation of cells and this synthesis takes place within a membrane-bound intracellular organelle, called the melanosome.
Melanocytes are present:
» in the skin
» in the mucous membranes
» in the nervous system (pia-arachnoid)
» in the eye (urea and retinal pigment epithelium)
» in the inner ear
There are two types of melanin produced by melanocytes: one is call eumelanin and is black or brown, the other one is called phaeomelanin, and is yellow or red.
The different skin colour in different races is not due to a different number of melanocytes but to a different ability to produce the two types of melanin.
Melanin is synthesized by melanocytes starting from the tyrosinase using a single enzyme: tyrosinase. The tyrosinase gene family currently has three members: the tyrosinase (TYR) in chromosome 11q14-21, the tyrosinase-related protein 1 (TYRP1, also known as TRP1 e gp75) in chromosome 9p23 and the tyrosinase-related protein 2 (DCT, also known as TRP2, TYRP2 and Tyrp2) in chromosome 13q31-32.