Oculocutaneous albinism type 2
Overview
A rare inherited disorder characterized by reduced pigmentation in the skin, eyes and hair. Type 2 has normal levels of active tyrosinase which is needed for the production of melanin which gives the skin, hair and eyes their color - the fairness is caused by a different genetic mutation (OCA2 gene).
Symptoms
* Creamy white skin * Light yellow hair * Blond hair * Light brown hair * Light-colored irises
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
The diagnosis of OCA2 is based on the clinical findings. The gene OCA2 (previously called the P gene) is the only gene known to be associated with oculocutaneous albinism type 2. Testing for the 2.7-kb deletion found in individuals of African heritage is available on a clinical basis. Sequence analysis and mutation scanning of the OCA2 gene are available clinically.
Treatment
Correction of refractive error with spectacles or contact lenses may improve visual acuity; strabismus surgery can be considered for cosmetic reasons. Hats with brims often reduce photophobia. Protection from sun exposure with body-covering clothing and sunscreens prevents burning, skin damage, and skin cancer; prolonged sun exposure should be avoided. Skin cancer is treated as for the general population.