Oculocutaneous albinism type 2 (ORPHA:79432)
A form of oculocutaneous albinism characterized by variable hypopigmentation of the skin and hair, numerous characteristic ocular changes and misrouting of the optic nerves at the chiasm.
Epidemiology
The prevalence of Oculocutaneous albinism type 2 (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.
Clinical description
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.
Etiology
OCA2 is caused by a mutation in the OCA2 gene (15q12-q13), 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.
Diagnostic methods
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 diagnosis
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. The BOCA phenotype can also be seen in OCA3.
Antenatal diagnosis
Prenatal testing is possible when the disease causing mutation in the parent is known.
Genetic counseling
OCA2 is inherited autosomal recessively and genetic counseling is recommended for at-risk couples (both individuals are carriers of a disease-causing mutation), informing them of the 25% risk of having an affected child at each pregnancy.
Management and treatment
No curative treatment has been found for OCA to date. Protection from sunlight is imperative and patients should wear clothing and sunscreen on exposed skin to prevent burning and reduce the risk of skin cancer. Annual skin examinations should also be performed to identify any pre-cancerous or cancerous lesions. Periodical ophthalmologic examination is necessary and corrective lenses or glasses are given to improve visual acuity. Dark glasses may be needed to relieve photophobia. Strabismus surgery can be performed for functional or cosmetic reasons.
Prognosis
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.

                            Last update: 
                            December 2020


                            - Expert reviewer(s): 
                            Dr Masahiro HAYASHI  - Dr Tamio SUZUKI