Aniridia is characterized by complete or partial iris hypoplasia and can result in a range from readily visible, almost complete absence of the iris, through enlargement and irregularity of the pupil mimicking a coloboma, to small slit-like defects in the anterior layer seen only with a slit-lamp. Although the phenotype can be variable within a family, individuals usually show little difference between the two eyes.
Aniridia may be caused by heterozygous mutations in PAX6. PAX6 encodes a transcription factor involved in islet cell differentiation and function and members of families carrying PAX6 mutations also exhibit impaired glucose tolerance and diabetes later in life. PAX6 haploinsufficiency through loss of function mutations result in classic aniridia, while PAX6 missense mutations typically produce atypical or variable-phenotype aniridia. PAX6 mutations have also been described in a number of other ocular developmental disorders including Aniridia, Cerebellar Ataxia and Mental retardation, Foveal Hypoplasia and Presenile Cataract syndrome and Peters anomaly.