Genetics of Aniridia
Simply put, genes hold the information to build and maintain an individual’s cells. All organisms have many genes corresponding to various biological traits, some of which are immediately visible, such as eye color or hair color. Minute changes in these genes (mutations) can have big consequences. In the vast majority of cases, aniridia is caused by a mutation in the PAX6 gene located on the short arm of chromosome 11 (11p13). If an individual has a normal PAX6 gene, another gene mutation may be mutated. Possible genes include: PITX, FOXC1, cypB, GDF6, or other. In relation to aniridia, most information and data is known about the PAX6 gene.
Inheritance and the Genetic Types of Aniridia
Every individual has two copies of every gene: one from their father and one from their mother. Unlike most genetic conditions, aniridia is dominant. It only takes one copy of the mutated gene to have the condition. Consequently, if a person with aniridia has a child, there is a 50% chance the child will have aniridia. There are four basic types of aniridia:
1. Familial aniridia (autosomal-dominant) is the most common form of aniridia. Familial aniridia is inherited directly from a parent who has aniridia. In familial aniridia, there is a mutation of the PAX6 gene.
2. Sporadic aniridia is the second most common form of aniridia. In this case, both parents have normal chromosomes and genes. However, in the affected child, a new change (mutation) of the PAX6 gene, whose cause is unknown, occurred before or soon after conception.
3. WAGR syndrome is a very rare form of sporadic aniridia (both parents have normal genes) where there is not only a mutation of the PAX6 gene but also involves a deletion of the neighboring genes on chromosome 11. In addition to the common traits of aniridia, children with WAGR have a high risk for developing Wilms’ timor and other medical complications such as genital abnormalities and learning and behavioral difficulties. Early diagnosis of WAGR is essential for monitoring and treating Wilm’s tumor and other complications.
4. Gillespie syndrome is an extremely rare (there are only a few documented cases) type of aniridia and is not thought to be associated with a PAX6 mutation. This type of aniridia is characterized with a particular appearance of the iris remnant (described as having a “scalloped” border) mental retardation, and cerebellar ataxia, (muscle incoordination).
Genetic Testing for Aniridia
Genetic testing for aniridia can be coordinated through genetic counselor. Genetic counselors are generally affiliated with universities and/or children’s hospitals. For more information on genetics and aniridia, please see Vision For Tomorrow’s webinar on the topic.