Risk Factors
Tyrosine, an amino acid, is normally converted by the body to a variety of pigments called "melanins," which give an organism its characteristic colors in areas such as the skin, hair, and eyes. Albinism results when the body is unable to produce melanin because of defects in the metabolism of tyrosine. Those with albinism can be divided into two subgroups: tyrosinase-negative (those who lack the enzyme tyrosinase) and tyrosinase-positive (those in whom tyrosinase is present but inactive). The most serious case is that of complete albinism or tyrosinase-negative oculocutaneous albinism, in which there is a total absence of pigment.
Etiology and Genetics
Albinism appears in various forms and may be passed to offspring through autosomal recessive, autosomal dominant, or X-linked modes of inheritance. In the autosomal recessive case, both parents of a child with autosomal recessive albinism are carriers—that is, they each have one copy of the recessive form of the gene and are therefore not albino themselves. When both parents are carriers, there is a one-in-four chance that the child will inherit the condition. On the other hand, X-linked albinism occurs almost exclusively in males, and mothers who carry the gene will pass it on 50 percent of the time.
Symptoms
People with this condition have white hair, colorless skin, red irises, and serious vision defects. The red irises are caused by the lack of pigmentation in the retina and subsequent light reflection from the blood present in the retina. These people also display rapid eye movements (nystagmus) and suffer from photophobia, decreased visual acuity, and in the long run, functional blindness. People with this disorder sunburn easily, since their skin does not tan. Partial albinos have a condition known as "piebaldism," characterized by the patchy absence of skin pigment in places such as the hair, the forehead, the elbows, and the knees.
Ocular albinism is inherited and involves the lack of melanin only in the eye, while the rest of the body shows normal or near-normal coloration. This condition reduces visual acuity from 20/60 to 20/400, with African Americans occasionally showing acuity as good as 20/25. Other problems include strabismus (crossed eyes or “lazy eye”), sensitivity to brightness, and nystagmus.
Several complex diseases are associated with albinism. Waardenburg syndrome
is identified by the presence of a white forelock (a lock of hair that grows on the forehead) or the absence of pigment in one or both irises, Chediak-Higashi syndrome
is characterized by a partial lack of pigmentation of the skin, and tuberous sclerosis
patients have only small, localized depigmented areas. A more serious case is the Hermansky-Pudlak syndrome, a disorder that includes bleeding.
Screening and Diagnosis
A physical examination will reveal the nature and extent of albinism. In ocular albinism, the color of the iris may be any of the normal colors, but an optician can easily detect the condition by shining a light from the side of the eye.
Treatment and Therapy
Albinism can affect an individual’s lifestyle. Treatment of the disease involves reduction of the discomfort the sun creates. Thus, photophobia may be relieved by sunglasses that filter ultraviolet light, while sunburn may be reduced by the use of sun protection factor (SPF) sunscreens and by covering the skin with clothing.
In ocular albinos, the light shines through the iris because of the absence of the light-absorbing pigment. Children with this condition have difficulty reading what is on a blackboard unless they are very close to it. Surgery and the application of optical aids appear to have had positive results in correcting such problems.
Prevention and Outcomes
Albinism has long been studied in humans and captive animals. Since albinism is basically an inherited condition, genetic counseling is of great value to individuals with a family history of albinism. Albinism has not been found to affect expected life span among humans. However, albino humans are susceptible to sunburns and skin cancer.
Albinism has also been detected in wild animals, but such animals often have little chance of survival because they cannot develop normal camouflage colors, important for protection from predators. Animals in which albinism has been recorded include deer, giraffes, squirrels, frogs, parrots, robins, turtles, trout, and lobsters. Partial albinism has also been reported in wildlife. In other cases, such as the black panther of Asia, too much melanin is formed and the disorder is called "melanism."
Albinism has also been observed in plants, but their life span rarely goes beyond seedline state, because without the green pigment chlorophyll, they cannot obtain energy using photosynthesis. A few species of plants, such as Indian pipes (Monotropa), are normally albino and obtain their energy and nutrition from decaying material in the soil.
Bibliography
Alan, Rick. "Albinism." Health Library. EBSCO Information Services, 14 Jan. 2014. Web. 14 July 2014.
"Albinism." Medline Plus. US National Library of Medicine, 29 Oct. 2013. Web. 14 July 2014.
Gahl, William A., et al. “Genetic Defects and Clinical Characteristics of Patients with a Form of Oculocutaneous Albinism (Hermansky-Pudlak Syndrome).” New England Journal of Medicine 338.18 (1998): 125. Print.
Gershoni-Baruch, R., et al. “Dopa Reaction Test in Hair Bulbs of Fetuses and Its Application to the Prenatal Diagnosis of Albinism.” Journal of the American Academy of Dermatology 24.2 (1991): 220–22. Print.
Lewis, Richard Alan. "Oculocutaneous Albinism Type 1." GeneReviews. U of Washington, Seattle, 16 May 2013. Web. 14 July 2014.
King, Richard A., et al. “Albinism.” The Metabolic and Molecular Bases of Inherited Disease. Ed. C. R. Scriver et al. 7th ed. New York: McGraw, 1995. Print.
King, Richard A., and C. Gail Summers. “Albinism: Ocular and Oculocutaneous and Hermansky-Pudlak Syndrome.” Management of Genetic Syndromes. Ed. Suzanne B. Cassidy and Judith E. Allanson. 3rd ed. Hoboken: Wiley, 2010. 53–68. Print.
Pollier, Pascale. Journal of Audiovisual Media in Medicine 24.3 (2001): 127. Print.
Salway, Jack G. “Amino Acid Disorders: Maple Syrup Urine Disease, Homocystinuria, Alkaptonuria, and Albinism.” Medical Biochemistry at a Glance. 3rd ed. Malden: Blackwell, 2012. 102–4. Print.
Scriver, Charles, et al., eds. The Metabolic and Molecular Bases of Inherited Disease. 8th ed. 4 vols. New York: McGraw, 2001. Print.
Tomita, Yasuchi. “Molecular Bases of Congenital Hypopigmentary Disorders in Humans and Oculocutaneous Albinism 1 in Japan.” Pigment Cell Research 13.5 (2000): 130. Print.
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