oculocutaneous albinism

(OCA) albinism characterized by partial or total absence of melanin from the melanocytes of the skin, hair, and eyes, with additional ocular defects including hypoplastic fovea, photophobia, nystagmus, and decreased visual acuity. There are a number of types, varying in incidence and in genetic, biochemical, and clinical characteristics. Inheritance is autosomal recessive, except for a few families which appear to exhibit an autosomal dominant pattern, which may be due to incomplete penetrance or quasidominance.

     type 1a (OCA1A) a type caused by mutations in the TYR (tyrosinase) gene (locus: 11q14-q21) that result in a complete absence of tyrosinase activity and consequent inability of cells to synthesize melanin. It is characterized by absence of pigment in hair, skin, and eyes. Signs include white hair throughout life, skin that is pink and highly susceptible to neoplasias, absence of pigmented nevi or freckles, gray to blue eyes with prominent red reflexes from the fundi. Called also tyrosinase-negative a.

     type 1b (OCA1B) a type caused by mutations in the TYR (tyrosinase) gene (locus: 11q14-q21) that lead to reduced tyrosinase activity, characterized by yellow hair, fair skin, and severe ocular abnormalities. The hair and skin are white at birth but become pigmented in infancy. The eyes are blue at birth and darken with age; from the age of 3 years, transillumination of the iris reveals a cartwheel effect. Pigmentation of the fundus is slight or absent and the macular reflex is absent or minimal. Called also yellow mutant a.

     type 2 (OCA2) the most common type of oculocutaneous albinism, caused by a mutation in the P (or OCA2) gene (locus: 15q11.2-q12) which encodes a membrane protein involved in regulating melanosomal pH. It is characterized by reduced, but usually visible, pigmentation of hair, skin, and eyes; the phenotype is highly variable, ranging from nearly normal pigmentation to almost none; pigmentation increases with age, and pigmented nevi may occur. The eyes usually develop some pigment, and visual acuity improves from infancy through adolescence. Newborns usually have pigmented hair but occasionally it is white, so that affected infants resemble those with OCA1A. Called also tyrosine-positive a.

     type 3 (OCA3) a type caused by a mutation in the TYRP1 gene (locus: 9p23), which encodes a protein involved in melanin synthesis. It occurs in persons of black African descent and is characterized by red to reddish brown coloration of the skin, hair, and iris; photophobia and nystagmus are mild and visual acuity is normal or nearly normal. Called also red or rufous a. and xanthism.

     type 4 (OCA4) a type caused by mutations in the MATP gene (locus: 5p13.3), which encodes a transporter that mediates melanin synthesis; it is rare worldwide but is one of the most common forms of albinism in Japan. The phenotype is highly variable and resembles that of OCA2.