Monday, August 31, 2009

What causes albinism? Are there any treatments for it?

Raymond Boissy, a dermatology professor at the University of Cincinnati College of Medicine, explains (as told to Coco Ballantyne): Albinism is a genetic disease causing partial or complete loss of pigmentation, or coloring, in the skin, eyes and hair. It arises from mutations affecting cells, called melanocytes, that produce the pigment melanin, which gives color to those body parts. In individuals with albinism, genetic alterations interfere with the melanocytes’ production of pigment or their ability to distribute it to keratinocytes, the major cell type of the skin’s outer layer.

The most common forms of albinism are oculocutaneous type 1 (OCA1) and type 2 (OCA2). Oculocutaneous means the disease affects the eyes (“oculo”) and skin (“cutaneous”). People with OCA1 have mutations in a gene called TYR that is responsible for production of the enzyme tyrosinase, used by cells to convert the amino acid tyrosine into pigment. OCA2, the most common form in Africa, results from a mutation in the OCA2 gene, which encodes the P protein—a protein whose role is not totally clear. This mutation is probably the oldest one causing albinism and, putatively, originated during humankind’s development in Africa.

Most people with OCA1 have white skin, white hair and pigmentless eyes. The iris, the colored part of the eye encircling the pupil, is pale, whereas the pupil itself may appear red. This redness results from light reflecting off blood vessels in the retina, the light-sensitive layer of tissue lining the back of the eyeball. Pupils ordinarily appear black because pigment molecules in the retina absorb light and prevent it from bouncing back out. People with OCA2 can make a small amount of pigment and thus may have somewhat less pronounced visual symptoms.

Individuals with albinism are often legally blind. Without melanin during the embryonic stage, the neuronal tracts leading from the eye to the visual cortex of the brain develop aberrantly, resulting in diminished depth perception. And in the absence of pigment in the eye, retinal photoreceptors can become overstimulated and send confusing messages to the brain, which often also produce a nystagmus, or fluttering of the eyes.

A dearth of skin pigment leaves people more susceptible to nonmelanoma skin cancers such as squamous cell carcinoma and basal cell carcinoma. Normally functioning melanocytes distribute pigment to keratinocytes to shield the nucleus and the DNA inside from the sun’s ultraviolet radiation. People with albinism may also experience premature skin aging, because UV-blocking melanin helps to prevent wrinkling and the loss of the skin’s elasticity.

Researchers such as geneticist Richard King of the University of Minnesota and cell biologist Vitali Alexeev of Thomas Jefferson University are working on gene therapies or drugs that would fix albinism-causing mutations. Scientists have had some success in correcting patches of depigmented skin and hair in mice, but they are a long way from translating this research to humans.

Source of Information : Scientific American(2009-06)

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