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HEREDITARY MONOGENIC DISEASES

           

Oculocutaneous albinism type 2 (tyrosinase positive)

Oculocutaneous albinism type 2 (OCA2), is the most frequent form of OCA in the African population, and it is characterized by variable hypopigmentation of the skin and hair, numerous characteristic ocular changes and impaired opusation of the optic nerve in the chiasma.
The prevalence of OCA2 is estimated at 1/38,000 – 1/40,000 in most populations except in the African population, which has a prevalence of 1/3,900 – 1/1,500.
The pigmentation of the skin and hair can be variable, from mínimum pigmentation to almost normal. Most children develop nystagmus before 3-4 months, and strabismus and lack of visual attention can be observed during the first 6 months. Visual acuity in adults usually varies from 20/60-20/100 and does not get worse over time.
Phenotypes are usually characteristic: the color of the iris varies from blue to brown. All newborns have light yellow or light brown hair and creamy white skin. Hair color may become darker over time but it does not change after adolescence. In the African population, a phenotype with light brown hair and skin and gray iris is observed. This phenotype is known as brown OCA (BOCA), which is part of the OCA2 spectrum.
The skin can become hard and rough upone exposure to the sun. Patients have an increased risk of developing basal and squamous cell carcinomas, but melanomas are not common.
For OCA2 diagnosis both characteristic clinical symptoms examinations and genetic tests are used. Ophthalmologic examination reveals visualization of choroidal blood vessels, reduced retinal pigmentation and foveal hypoplasia. Alternating strabismus, reduced stereoscopic vision, and an altered visual evoked potential (VEP) are associated with the characteristic wrong accusation of the optic nerve in the chiasma.
Genetic tests for a mutation in the OCA2 gene can confirm the diagnosis. OCA2 is caused by a mutation in the OCA2 gene, which encodes the OCA2 protein. Melanocytes produce small amounts of melanin, but it is mainly yellow pheomelanin.
Genetic counseling is recommended since OCA2 is transmitted in an autosomal recessive manner.
The disease becomes stable after childhood. However, medical and social consequences can have an impact on the patient’s daily life.

GENE OR REGION STUDIED


  • OCA2