HUMAN OCULOCUTANEOUS ALBINISM - FROM CLIN ICAL OBSERVATION TO MOLECULAR-BIOLOGY

Human oculocutaneous albinism (OCA) is a heritable metabolic defect tr ansmitted as an autosomal recessive trait and characterized by a hypop igmentation of skin, hair and eyes. This defect is mainly due to an al tered or absence of tyrosinase activity, the key enzyme of eu-and pheo -melanin synthesis. It is a seldom condition in white peoples but more frequent in Africans and in Afro-Americans. Albinos, especially in tr opical settings, have high prevalence of solar keratosis and squamous cell carcinoma. Ocular defects characteristics of OCA are photobia, ny stagmus and decreased visual acuity. Two forms of OCA have been distin guished in 1970 on the basis of their genetic, clinical, biochemical a nd ultrastructural characteristics: type I i. e. tyrosinase negative a nd type II i. e. tyrosinase positive. Actually 10 forms are described. Human tyrosinase gene has been mapped to chromosome 11 (q14-21) and c loned. It is formed by 5 exons. Several different human tyrosinase gen e mutations have been identified in patients with type I-A OCA. Non se nse, misense and frameshift mutations result in altered or absence of tyrosinase activity.