Albinism

Albinism is a recessive inherited defect in melanin metabolism in which pigment is absent from hair, skin, and eyes (oculocutaneous albinism) or just from the eyes (ocular albinism). Melanin is a dark biological pigment that is formed as an end product of the metabolism of the amino acid tyrosine. When human skin is exposed to sunlight it gradually darkens or tans due to an increase in melanin. Tanning helps protect the underlying skin layers from the sun's harmful ultraviolet rays.

The most common examples of albinism are the white rats, rabbits, and mice found at pet stores. The characteristic white coats and pink eyes of these albino animals contrast dramatically with the brown or gray fur and dark eyes of genetically normal rats, rabbits, and mice. Domestic white chickens, geese, and horses are partial albinos. They retain pigment in their eyes, legs, and feet.

In the past, albinos were often regarded with fear or awe. Sometimes they were killed at birth, although albino births were common enough in some groups not to cause any excitement. For example, among the San Blas Indians of Panama, one in approximately 130 births is an albino. In the mid-nineteenth century, albinos were exhibited in carnival sideshows. Whole families were displayed at times and were described as a unique race of night people. They were said to live underground and to come out only at night when the light was dim and would not hurt their eyes.

In humans, albinism is rare. One person in 17,000 has some type of albinism. Researchers have currently identified 10 different types of oculocutaneous albinism and five types of ocular albinism based on clinical appearance. Humans who have oculocutaneous albinism are unable to produce melanin; they have white, yellow, or yellow-brown hair, very light eyes (usually blue or grayish rather than pink), and very fair skin. The irises of their eyes may appear violet or pinkish because they have very little pigment and allow light to reflect back from the reddish retina in the back of the eye. People with albinism may also suffer from a variety optical disorders such as near- or far-sightedness, nystagmus (rapid irregular movement of the eyes back and forth), or strabismus (muscle imbalance of the eyes causing crossed eyes or lazy eye). They are very sensitive to bright light and sunburn easily. They must take great care to remain covered, wear a hat, and apply sunscreen anytime they are outdoors, since their skin is highly susceptible to pre-cancerous and cancerous growths. People with albinism often wear sunglasses or tinted lenses even indoors to reduce light intensity to a more bearable level.

In ocular albinism only the eyes lack melanin pigment, while the skin and hair show normal or near-normal color. People with this condition have a variety of eye disorders because the lack of pigment impairs normal eye development. They are extremely sensitive to bright light and especially to sunlight (photophobia). Treatment of ocular albinism includes the use of visual aids and sometimes surgery for strabismus.

Albinism occurs when melanocytes (melanin-producing cells) fail to produce melanin. This absence of melanin production happens primarily in two ways. In tyrosinase-negative An albino tiger cub and his sibling. © Tom Brakefield/Corbis. Reproduced by permission.

albinism (the most common form), the enzyme tyrosinase is missing from the melanocytes. Tyrosinase is a catalyst in the conversion of tyrosine to melanin. When the enzyme is missing no melanin is produced. In tyrosinase-positive albinism, a defect in the body's tyrosine transport system impairs melanin production. One in every 34,000 persons in the United States has tyrosinasenegative albinism. It is equally common among blacks and whites, while more blacks than whites are affected by tyrosinase-positive albinism. Native Americans have a high incidence to both forms of albinism.

Albinism cannot be cured, but people with this condition can expect to live a normal life span. Protection of the skin and eyes from sunlight is of primary importance for individuals with albinism. The gene carrying the defect that produces albinism is recessive, so both parents must carry this recessive gene in order to produce a child with the condition. When both parents carry the gene (and neither has albinism), there is a one in four chance with each pregnancy that their child will have albinism. The inheritance pattern of ocular albinism is somewhat different. This condition is X-linked, meaning that the recessive gene for ocular albinism is located on the X chromosome. X-linked ocular albinism appears almost exclusively in males who inherit the condition from their mothers. Recently, a blood test has been developed to identify carriers of the gene that causes tyrosinase-negative albinism, and a similar test can identify this condition in the fetus by amniocentesis.

Vitiglio is another pigmentation disorder that resembles partial albinism. In this condition the skin exhibits stark white patches resulting from the destruction or absence of melanocytes. About 1% of the U. S. population has this disorder and it primarily affects people between the ages of 10 and 30. Unlike albinism, the specific cause of vitiglio is not known, although there seems to be a hereditary component, since about 30% of those who have vitiglio have family members with the condition. A link also exists between vitiglio and several other disorders with which it is often associated, including thyroid dysfunction, Addison's disease, and diabetes. Chemicals such as phenols may also cause vitiglio.