oculocutaneous albinism(redirected from oca)
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a disorder characterized by deficiency of pigment in skin, hair, and eyes, photophobia, nystagmus, and decreased visual acuity; there are two types: tyrosinase negative [MIM*203100] in which there is absence of tyrosinase, and tyrosinase positive [MIM*203200] in which normal tyrosinase cannot enter pigment cells; the compound heterozygote is normal so the two forms are not allelic. There are several forms of autosomal recessive inheritance: type IA is characterized by absence of tyrosinase with life-long complete absence of melanin, marked photophobia, and nystagmus, caused by mutation in the tyrosinase gene (TYR) on chromosome 11q. Type II has normal tyrosinase activity and is the most common; hair color darkens and nevi and freckles develop; caused by mutation in the oculocutaneous abinism gene (OCA2) on 15q. Type III is characterized by absent tyrosinase but pigmentation of the iris in the first decade; caused by mutation in the tyrosine-related protein-1gene (TYRP1) on 9p. Type IV is found in Africans with normal tyrosinase and type V is associated with red hair. Type VI is synonymous to Hermansky-Pudlak syndrome [MIM*203300], with low to absent tyrosinase and hemorrhage due to platelet deficiency, caused by mutation in the Hermansky-Pudlak gene (HPS) on 10q.
Congenital anomaly due to a defect of melanin production as a result of one of several possible genetic defects. Oculocutaneous albinism type 1(OCA1) is due to a genetic defect in tyrosinase, the enzyme that metabolizes the amino acid tyrosine, which is essential for its conversion to melanin (formerly called tyrosinase-negative albinism). It is an autosomal recessive condition, which affects the skin, hair and eyes. The iris is a pale colour, the fundus and the pupil are reddish and the eye transilluminates markedly. There is poor visual acuity, photophobia, nystagmus and strabismus. Oculocutaneous type 2 (OCA2) is caused by a mutation of the OCA2 ('P') gene resulting in variable amounts of melanin synthesis. The hypopigmentation of the eyes, skin and hair varies from fair to normal (formerly called tyrosinase-positive albinism). It may be associated with the Hermansky-Pudlak syndrome in which there is albinism and easy bruising or bleeding. The other type of albinism is ocular albinism type 1 (OA1). It is inherited either as an X-linked or less commonly as an autosomal recessive trait. It affects mainly the eyes and in most instances males only and the skin colour is usually normal. Management involves full correction, possibly with tinted lenses. Surgery may be required for strabismus. See ocular fundus; inheritance; trans-illumination.