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Oculocutaneous albinism type 2
Oculocutaneous albinism type 2 (OCA2) is a type of OCA (see this term) and the most common form of OCA seen in the African population, characterized by variable hypopigmentation of the skin and hair, numerous characteristic ocular changes and misrouting of the optic nerves at the chiasm.
ORPHA:79432Classification level: Disorder
The prevalence of OCA2 is estimated at 1/38,000-1/40,000 in most populations throughout the world except in the African population that has a higher prevalence of 1/3,900-1/1,500.
Skin and hair pigmentation ranges from minimal to near normal. Most infants develop nystagmus before the age of 3-4 months, which may start off as rapid but usually slows down over time. Strabismus and visual inattention is also present in the first six months of life. Adult visual acuity usually ranges from 20/60-20/100 and does not worsen over time. Iris color ranges from blue to brown. Newborns all have pigmented hair ranging from light yellow to light brown and skin color is creamy white. Hair color may darken over time but does not change after adolescence. In Africans, a phenotype of light brown hair and skin and gray irises occurs, known as brown OCA (BOCA), which is part of the spectrum of OCA2. Patients of other ethnicities with BOCA have almost normal pigmentation. Exposure to sun can overtime lead to rough, coarse and thickened skin along with solar keratoses. Patients have an increased risk of developing basal and squamous cell carcinomas but melanomas are rare.
OCA2 is caused by a mutation in the OCA2 gene, encoding the OCA2 protein. The precise function of this protein is unknown, however, several studies have reported possible roles in the maintenance of proper intramelanosomal pH or the melanosomal structural matrix. Patients with OCA2 have melanocytes that still produce small amounts of melanin, but it is mostly yellow pheomelanin.
The characteristic clinical findings along with genetic testing are used to diagnose OCA2. Ophthalmologic examination reveals visualization of the choroidal blood vessels, reduced retinal pigment and foveal hypoplasia. Alternating strabismus, reduced stereoscopic vision, and an altered visual evoked potential (VEP) are associated with the characteristic misrouting of the optic nerves at the chiasm. Molecular genetic testing for a mutation in the OCA2 gene can confirm diagnosis of OCA2 and distinguish it from other forms of OCA.
Differential diagnoses include the other forms of OCA and X-linked recessive ocular albinism (XLOA) as well as syndromes with albinism as a feature such as Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, Griscelli syndrome, and Waardenburg syndrome type II (see these terms). The BOCA phenotype can also be seen in OCA3.
Prenatal testing is possible when the disease causing mutation in the parent is known.
OCA2 is inherited autosomal recessively and genetic counseling is possible.
The disease is not life threatening and stabilizes after childhood. However, the medical and social consequences can have an impact on patient's daily life.