Definitions

Aniridia

Aniridia

Aniridia is a rare congenital condition characterized by the underdevelopment of the eye's iris. This usually occurs in both eyes. It is associated with poor development of the retina at the back of the eye preventing normal vision development.

Types

Aniridia may be broadly divided into hereditary and sporadic forms. Hereditary aniridia is usually transmitted in an autosomal dominant manner (each offspring has a 50% chance of being affected), although rarer autosomal recessive forms (such as Gillespie syndrome) have also been reported. Sporadic aniridia mutations may affect the WT1 region adjacent to the AN2 aniridia region, causing a kidney cancer called nephroblastoma (Wilms tumor). These patients often also have genitourinary abnormalities and mental retardation (WAGR syndrome).

The AN2 region of the short arm of chromosome 11 (11p13) includes the PAX6 gene (named for its PAired boX status), whose gene product helps regulate a cascade of other genetic processes involved in the development of the eye (as well as other nonocular structures). This PAX6 gene is around 95% similar to the pax gene found in zebrafish, a creature which diverged from the human ancestry around 400 million years ago. Thus, the PAX6 gene constitutes an important evolutionary link to mankind's distant ancestors.

Defects in the PAX6 gene cause aniridia-like ocular defects in mice (as well as Drosophilia). Aniridia is a heterozygotic disease, meaning that only one of the two chromosome 11 copies is affected. When both copies are altered (homozygous condition), the result is a uniformly fatal condition with near complete failure of entire eye formation. In 2001, two cases of homozygous Aniridia patients were reported; the foetuses died prior to birth and had severe brain damage. In mice, homozygous Small eye defect (mouse Pax-6) led to loss of eyes, nose and the foetuses suffered severe brain damage.

Several different mutations may affect the PAX6 gene. Some mutations appear to inhibit gene function more than others, with subsequent variability in the severity of the disease. Thus, some aniridic individuals are only missing a relatively small amount of iris, do not have foveal hypoplasia, and retain relatively normal vision. Presumably, the genetic defect in these individuals causes less "heterozygous insufficiency," meaning they retain enough gene function to yield a milder phenotype.

  • AN1
  • AN2
  • Aniridia and absent patella
  • Aniridia, microcornea, and spontaneously reabsorbed cataract
  • Aniridia, cerebellar ataxia, and mental deficiency (Gillespie syndrome)

References

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