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Aniridia

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Synonyms of Aniridia

Disorder Subdivisions

General Discussion

Aniridia is a rare genetic vision disorder characterized by abnormal development of the eye's iris. The iris is the circular colored membrane in the middle of the eyeball. It is perforated in the center by an opening known as the pupil, which regulates the amount of light that enters the eye. Aniridia is characterized by partial or complete absence of the iris. Various forms of aniridia have been identified. Each can be distinguished by accompanying symptoms.

Symptoms

Aniridia is marked by partial or complete absence of the eye's iris. Vision is preserved in some mild cases of aniridia. The iris fails to develop normally before birth in one or both eyes. At least four types of aniridia are thought to exist. One type is marked by incomplete expression of the disorder. Some people with this type of aniridia may be unaware of the eye problems because pupils appear normal and usually only one eye is affected with thinning of the iris. In a second type of aniridia, iris abnormalities may occur alone, or in combination with other disorders. Accompanying disorders may include cataracts (clouding of the crystalline lens of the eye), glaucoma (gradual loss of vision due to increased pressure inside the eyeball which may be accompanied by varying degrees of pain), or superficial clouding of the cornea (corneal pannus). Rapid involuntary movement of the eyeball (nystagmus), and underdevelopment of the fovea area of the retina (which controls acute vision) may also occur.

A third type of aniridia is associated with mental retardation, and a fourth type occurs in conjunction with Wilms' tumor, genitourinary abnormalities, and possible mental retardation. (For more information on Wilms' Tumor, please choose "Wilm" as your search term in the Rare Disease Database.)

Causes

Until recently, it was thought that two separate genes were responsible for two distinct forms of aniridia. However, a review of the evidence revealed that what was thought to be two forms is actually one and that one gene is responsible. It is known as the PAX6 gene, and it is located on the short arm (p) of chromosome 11 (11p13).

Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Pairs of human chromosomes are numbered from 1 through 22, and an additional 23rd pair of sex chromosomes which include one X and one Y chromosome in males and two X chromosomes in females. Each chromosome has a short arm designated "p" and a long arm designated "q". Chromosomes are further sub-divided into many bands that are numbered. For example, "chromosome 11p13" refers to band 13 on the short arm of chromosome 11. The numbered bands specify the location of the thousands of genes that are present on each chromosome.

The defect is thought to be inherited as an autosomal dominant trait, although some cases appear to be spontaneous genetic mutations. A second type of Aniridia associated with mental retardation and a third type occurring in conjunction with Wilms' Tumor are also thought to be inherited as autosomal dominant traits.

Genetic diseases are determined by two genes, one received from the father and one from the mother.

Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.

Affected Populations

All types of Aniridia affect males and females in equal numbers. This disorder is thought to occur in approximately one in 60,000 to 100,000 live births in the United States.

Related Disorders

Symptoms of the following disorders can be similar to those of Aniridia. Comparisons may be useful for a differential diagnosis:

Iridogoniodysgenesis is a genetic eye structure disorder present at birth. It is characterized by underdevelopment of the foundation substance (stroma) of the iris. Glaucoma also occurs followed by the iris changing to a lighter color. Glaucoma is a vision disorder marked by gradual loss of vision, and increased pressure inside the eyeball possibly accompanied by varying degrees of pain.

Rieger Syndrome, also known as Iridogonodysgenesis with somatic anomalies, is characterized by defective embryonic development of the middle layer of the cornea and iris sections of the eye. This is accompanied by abnormalities in the pupil, which is the opening that regulates the amount of light entering the eyeball. The edges of the cornea are clouded at birth and glaucoma also occurs.

Hereditary Juvenile Glaucoma is a genetic vision disorder which may be present at birth. However, onset of symptoms may occur later in childhood or adolescence. Other cases may represent an early onset of some other forms of glaucoma. Glaucoma is characterized by diminished clear vision accompanied by increased pressure with possible pain inside the eyeball in various degrees.

Standard Therapies

Treatment of aniridia is usually directed at improving and preserving vision. Drugs or surgery may be helpful for glaucoma and/or cataracts. Contact lenses may be beneficial in some cases. When a genetic cause cannot be identified, patients should be evaluated for the possibility of the development of Wilms' tumor. (For more information on this disorder, please choose "Wilm" as your search term in the Rare Disease Database.) Services that benefit vision impaired or mentally retarded individuals and their families may be of assistance in some cases. Genetic counseling is recommended. Other treatment is symptomatic and supportive.

Investigational Therapies

The Canadian Foundation for Aniridia Research (CFAR) maintains a close watch on developments involving this disorder. That organization's web site may be reached at
www.aniridia.org.

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:
www.centerwatch.com

Organizations related to Aniridia

References

TEXTBOOKS
Lauderdale JD. Aniridia. In: NORD Guide to Rare Disorders. Lippincott Williams & Wilkins. Philadelphia, PA. 2003:640.

Kanski JJ., ed. Clinical Ophthalmology, 4th ed. Woburn, MA: Butterworth-Heinemann; 1999:240-42.

REVIEW ARTICLES
Prosser J, et al. PAX6 mutations reviewed. Hum Mutat. 1998;11:93-108.

Dahl E, et al. Pax genes and organogenesis. Bioessays. 1997;19:755-65.

Gehring WJ. The master control gene for morphogenesis and evolution of the eye. Genes Cells. 1996;1:11-15.

JOURNAL ARTICLES
Laghmari M, et al. Bilateral congenital aniridia: 5 case reports. J Fr Ophthalmol. 2004;27:385-91.

Zumkeller W, Orth U, Gal A. Three novel PAX6 mutations in patients with aniridia. Mol Pathol. 2003;56:180-3.

Churchill AJ, et al. Prenatal diagnosis of aniridia. Ophthalmology. 2000;107:1153-56.

Chao LY, et al. Mutation in the PAX6 gene in twenty patients with aniridia. Hum Mutat. 2000;15:332-39.

Hartmann RW Jr, et al. Picture of the month. Congenital aniridia. Arch Pediatr Adolesc Med. 2000;154:525-26.

Tanzer DJ, et al. Black iris-diaphragm intraocular lens for aniridia and aphakia. J Cataract Refract Surg. 1999;25:1548-51.

Osher RH, et al. Cataract surgery combined with implantation of an artifical iris. J Cataract Refract Surg. 1999;25:1540-47.

Chen TC, et al. Goniosurgery for prevention of aniridic glaucoma. Arch Ophthalmol. 1999;117:1144-48.

FROM THE INTERNET
McKusick VA, ed. Online Mendelian Inheritance in Man (OMIM). Baltimore. MD: The Johns Hopkins University; Entry No:106200; Last Edit: 6/8/1994; Entry No: 106210; Last Update: 5/2/2000.

Report last updated: 2007/10/12 00:00:00 GMT+0