Oculocutaneous albinism type 4 (ORPHA:79435)
A form of oculocutaneous albinism characterized by varying degrees of skin and hair hypopigmentation, numerous ocular changes and misrouting of the optic nerves at the chiasm.
Epidemiology
The estimated world prevalence of oculocutaneous albinism type 4 (OCA4) is 1/100,000 but it is more common in Japan.
Clinical description
Cutaneous hypopigmentation is often visible at birth and signs of nystagmus and strabismus present in the first year of life. Nystagmus is not always present at birth and can develop at 3 to 4 months of age. It can start off as rapid and slow down in later life and it is usually more noticeable when patients are tired, stressed, anxious or angry. Foveal hypoplasia is associated with a reduction in visual acuity. Alternating strabismus and reduced stereoscopic vision are also noted. Visual changes are not progressive and usually stabilize after childhood. A wide range of clinical phenotypes can be found with OCA4. Iris color can range from blue to brown. Photophobia is common. Hair color in newborns ranges from silvery white to light yellow. It may darken slightly with time (to light brown) but usually remains relatively unchanged. Skin color is usually creamy white. Over time, skin can become coarse, thickened and rough and solar keratoses are common in those who have not limited their exposure to the sun. Patients have an increased risk of developing basal and squamous cell carcinomas but melanomas are rare.
Etiology
OCA4 is caused by mutations in the membrane-associated transporter protein (MATP) gene, SLC45A2 (5p13.2), encoding a transporter protein which is thought to mediate melanin synthesis. Melanocyte cultures established from a mouse model of OCA4 have demonstrated that tyrosinase processing and trafficking is disrupted before delivery to melanosomes. Patients with OCA4 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 OCA4. 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 SLC45A2 gene can confirm diagnosis of OCA4 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 syndromes 1-7, Chediak-Higashi syndrome , Griscelli syndrome, and Waardenburg syndrome type II.
Antenatal diagnosis
Prenatal diagnosis is possible when a parent has been identified to have the disease causing mutation, but it is rarely performed.
Genetic counseling
OCA4 is inherited autosomal recessively and genetic counseling should be offered to 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
Ophthalmic care is ongoing and glasses or contact lenses can usually improve visual acuity. In some cases strabismus surgery may be performed. Patients must avoid exposure to sun as this can lead to pachydermia, premalignant lesions and an increased risk of developing skin cancer. A hat with a brim can protect the skin and eyes from the sun's harmful rays and patients should always wear sunscreen and cover exposed skin when outside. Dark glasses can be used to alleviate photophobia. Annual skin examinations should also be performed to identify any pre-cancerous or cancerous lesions.
Prognosis
The disease is not life threatening and usually remains stable after childhood. The medical and social consequences can however have an impact on a patient's daily life.

                            Last update: 
                            December 2020


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