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Incidence

OCA affects approximately 1 in 15,000 - 20,000 live births.

Genetic inheritance

Autosomal recessive. Albinism exists in all racial groups.

Pathophysiology

The common denominator linking all forms of OCA is a congenital melanin biosynthesis defect, which is initiated by the catalytic oxidation of tyrosine to dopa (3,4-dihydroxyphenylalanine) by tyrosinase. This reaction requires dopa as a cofactor. Tyrosinase then further catalyzes the dehydrogenation of dopa to dopaquinone, which via several steps eventually gets converted to melanin, which consists of at least three basic types: dark brown/black insoluble eumelanin, light red/yellow, alkali-soluble sulfur-containing pheomelanin, and neuromelanin (whose function is not well understood). Skin pigmentation is the result of accumulation of melanin granules in keratinocytes, where they absorb ultraviolet radiation (UVR) and block it from penetrating into the deeper skin layers where cell proliferation occurs. Melanin biosynthesis takes place in cellular organelles called melanosomes inside the melanocytes in the lowest layer of epidermis and in the dermis. Melanocytic dendrites then allow for transport of the melanosomes to keratinocytes. Upon exposure to UVR keratinocytes secrete α-melanocyte stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH), which stimulates the melanocytes to increase melanin synthesis. Albinos are either tyrosinase-positive or negative, and the subtypes can be identified by clinical and biochemical methods. The degree of melanin depletion and the different mutations are used to classify the different subtypes of the disease.

Diagnosis

Both, ocular and cutaneous tissues are affected in OCA, with diffuse leukodermia and variable pilosity disorders. The disease is potentially severe because patients may develop cutaneous carcinoma or melanoma with a more or less early onset. Hypoplasia of the fovea, translucent iris, photophobia, nystagmus, decreased visual acuity, and disturbed binocular vision due to misrouting of optic nerve fibers are crucial to the diagnosis.

Clinical aspects

Patients with OCA have a normal development, lifespan, fertility, and intelligence (an association with higher intellect may exist). In OCA, pigmentation of skin, hair, and eyes is decreased or absent, whereas in OA the disease is limited to the eyes. Light-skin colored people are at a significantly higher risk of developing skin cancer (basal cell carcinoma, squamous cell carcinoma, melanoma) when compared to individuals with dark skin. Hence there is an inverse relationship between cutaneous melanin content and incidence of skin cancer. These protective effects of melanin are mainly due to eumelanin that also scavenges UVR-induced free radicals. This fact puts patients with albinism at an even higher risk for skin cancers. Paradoxically, the presence of pheomelanin has been implicated in the pathogenesis of melanoma. One hypothesis how pheomelanin promotes carcinogenesis may be related to the increased production of free radicals that is associated with its synthesis, which requires large amounts of antioxidants, thereby depleting free radical scavengers, such as glutathione and rendering melanocytes more vulnerable to free radicals-related damage. Ocular changes are found in ...

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