Oculocutaneous albinism type 3
Overview
A rare inherited disorder characterized by slightly reduced pigmentation in the skin, eyes and hair (due to a genetic mutation of the TYRP1 gene). Type 3 is characterized by some pigmentation of the iris despite the complete absence of tyrosinase which is needed for the production of melanin which gives the skin, hair and eyes their color. Type 3 also has the milder eye problems than the other types.
Symptoms
* Reddish-brown skin * Ginger hair * Red hair * Hazel iris * Brown iris
Causes
Oculocutaneous albinism results from autosomal recessive inheritance; ocular albinism, from an X-linked recessive trait that causes hypopigmentation only in the iris and the ocular fundus. Normally, melanocytes synthesize melanin. Melanosomes, melanin-containing granules within melanocytes, diffuse and absorb the sun's ultraviolet light, thus protecting the skin and eyes from its dangerous effects. In tyrosinase-negative albinism (the most common type), melanosomes don't contain melanin because they lack tyrosinase, the enzyme that stimulates melanin production. In tyrosinase-positive albinism, melanosomes contain tyrosine, a tyrosinase substrate, but a defect in the tyrosine transport system impairs melanin production. In tyrosinase-variable albinism (rare), an unidentified enzyme defect probably impairs synthesis of a melanin precursor. Other rare forms of albinism are Chédiak-Higashi syndrome (tyrosine-negative albinism with hematologic and neurologic manifestations); Hermansky-Pudlak syndrome (tyrosinase-positive albinism with platelet dysfunction, bleeding abnormalities, and ceroidlike inclusions in many organs); and Cross-McKusick-Breen syndrome (tyrosinase-positive albinism with neurologic involvement). In the United States, both types of albinism are more common in Blacks than in Whites. Native Americans have a high incidence of the tyrosine-positive form.
Diagnosis
Diagnosis is based on clinical observation and the patient's family history. Microscopic examination of the skin and of hair follicles determines the amount of pigment present. Testing plucked hair roots for pigmentation when incubated in tyrosine distinguishes tyrosinase-negative albinism from tyrosinase-positive albinism. Tyrosinase-positive hair bulbs will develop color.