Albinism, oculocutaneous
Albinism, oculocutaneous: An hereditary disorder characterized by deficiency of the pigment melanin in the eyes, skin and hair. The lack of eye pigment causes photophobia (sensitivity to light), nystagmus, and decreased visual acuity.
Oculocutaneous albinism is conventionally classified as to whether it is tyrosinase-negative or tyrosinase-positive. In the tyrosinase-negative class, there is absence of the enzyme tyrosinase. In the tyrosinase-positive class, tyrosinase is present but it cannot enter pigment cells to do its job and make melanin.
There are a number of specific types of oculocutaneous albinism, all of which are inherited in an autosomal recessive manner:
Type IA is characterized by absence of tyrosinase with complete absence of melanin, marked photophobia, and nystagmus. It is due to mutation of the tyrosinase gene (TYR) on chromosome 11q.
Type IB has reduced activity of tyrosinase. It has been called yellow albinism. The child looks “dead white” at birth and has the usual eye problems, but soon develops normal skin pigmentation and yellow hair.
Type II has normal tyrosinase activity. It is the most common type of oculocutaneous albinism. The hair darkens and freckles and nevi (moles) develop normally on the skin. The disorder is due to mutation of the oculocutaneous albinism gene (OCA2) on chromosome 15q.
Type III is characterized by absence of tyrosinase with the presence of pigmentation of the iris in the first decade of life. It is due to mutation of the tyrosine-related protein-1 gene (TYRP1) on chromosome 9p.
Type IV is characterized by normal tyrosinase and is due to mutation of the MATP gene on chromosome 5p. MATP stands for membrane-associated transporter protein.
Oculocutaneous albinism is also a feature of certain other genetic conditions including the Hermansky-Pudlak syndrome and Chediak-Higashi syndrome.
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