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[OCA2. 包括: 棕色OCA]
Oculocutaneous Albinism Type 2
[OCA2. Includes: Brown OCA]
Disease characteristics.Oculocutaneous albinism type 2 (OCA2) is characterized by hypopigmentation of the skin and hair and the characteristic ocular changes found in all types of albinism, including nystagmus; reduced iris pigment with iris translucency; reduced retinal pigment with visualization of the choroidal blood vessels on ophthalmoscopic examination; foveal hypoplasia associated with reduction in visual acuity; and misrouting of the optic nerves at the chiasm associated with alternating strabismus, reduced stereoscopic vision, and an altered visual evoked potential (VEP). Individuals with OCA2 are usually recognized within the first year of life because of the ocular features of nystagmus and strabismus. Vision is stable after early childhood and no major change or further reduction in vision occurs related to the albinism. The amount of cutaneous pigmentation in OCA2 ranges from minimal to near-normal. Newborns with OCA2 almost always have pigmented hair, with color ranging from light yellow to blond to brown. Hair color may darken with time, but does not vary significantly from childhood to adulthood. Brown OCA, initially identified in Africans and African Americans with light brown hair and skin, is part of the spectrum of OCA2.
Diagnosis/testing.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.
Management.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.
Genetic counseling.OCA2 is inherited in an autosomal recessive manner. The parents of a proband are obligate heterozygotes and therefore carry one mutant allele. Heterozygotes (carriers) are asymptomatic. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Prenatal testing is available on a limited basis.