Partial iris in a Dutch belted rabbit
Esther van Praag, Ph.D.
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Aniridia is a complex congenital defect that is characterized by the incomplete development of the colored part of the eye (iris). It is often accompanied by ophthalmologic complications. The degree of the defect varies from one individual to another, ranging from a partial iris to a total absence of the iris. The term "total absence" is, in fact, inappropriate because a rudimentary and non-functional residue of iris tissue is present even in the complete absence of the iris. It forms a very thin and barely visible collar around the pupil. Aniridia usually affects both eyes. The degree of damage may vary between one eye and the other.
This malformation of the iris is very rare. It has been observed in rodents, sheep, cows, horses, and humans. No case of aniridia seems to have been described in the rabbit so far. In any case, the frequency of aniridia is unknown in rabbits, but it is very rare (thanks to Michel Gruaz for informing me about this young rabbit). In humans, this malformation is observed in one in 50,000 to 100,000 people.
Dutch young rabbit with a rare eye anomaly : incomplete aniridia
Effects on vision and frequency
The complete or partial absence of the iris no longer makes it possible to filter light efficaciously. Indeed, the iris contains muscles that regulate the size of the pupil to limit the passage of light waves into the eye. If the iris is absent, even partially, the size of the pupil no longer varies according to the light brightness and remains dilated. In some cases, the pupil is so dilated that it is possible to distinguish the large circumference of the lens and the ciliary body that fixes the lens thanks to zonular fibers. As a consequence, an individual suffering from aniridia fears light and suffers from photophobia. When light is intense, the animal tries to look sideways or reacts strongly when there is a flash of light, e.g., from a camera, when taking a picture. The discomfort caused by the light can induce an involuntary movement of the eyes. Visual acuity can be affected. It varies from excellent to very bad depending on the degree of absence of the iris and the anomaly of other structures of the eye.
Congenital defect that is often inherited
Aniridia is generally a genetic malformation in vertebrates and in man, but it can also occur spontaneously or be caused by a penetrating wound. Family or lineage heredity is observed. The penetrance of this ocular disorder is variable. The transmission is usually of the autosomal dominant type. This means that when one parent is affected, descendants have 50% chance of having an aniridia. More rarely, transmission is autosomal recessive. In these individuals, aniridia is often accompanied by disturbances of balance and uncoordinated movements (ataxia). In about 1/3 of the cases, the parents do not carry the gene for aniridia. It is the result of a spontaneous mutation caused by the loss or insertion of one or more nucleotides in the gene responsible for the development of the eye, which causes this abnormality in a descendant.
Incomplete iris on its top part and very dilated pupil that does not respond to differences in brightness
Genetics of aniridia
In man and mammals, aniridia is linked to the PAX6 gene. This gene is activated at the beginning of embryogenesis and plays a major role in the morphogenesis of the eye. This gene plays the role of a "conductor". Its expression will cause a cascade of events on the 2000 to 3000 members of the orchestra, that is on the 2000 to 3000 genes involved in the formation of the different ocular structures, which lead to the formation of the eye. To achieve this, the PAX6 gene encodes for a protein of the same name. The latter will attach to the 2000 to 3000 genes located on different chromosomes and regulate their activities, either activating their expression or stopping it. Since there may be several copies of the same gene, the binding of the PAX6 protein with either of these copies promotes the expression of a large genetic diversity in the offspring.
The PAX6 protein thus controls the early initiation and formation of the many structures of the eye during the development of the embryo. A mutation in this gene will therefore have a major impact with many structural malformations. The most common are aniridia or a very thin cornea.
Eye complications due to aniridia
Aniridia is rarely the sole structural abnormality of the eye, although it is the most obvious. On the contrary, it is usually accompanied by other defects of the eye structure and the appearance of secondary visual complications with serious consequences in the young rabbit. This has been observed in both mice and humans.
The structural abnormalities of the eye that are associated with aniridia are numerous. A visible complication of aniridia, mentioned above, includes the dilatation and/or deformation of the pupil due to the absence of muscles in the partial or absent iris. Other complications include a thickening or vascularization of the cornea or the translucent anterior portion of the eyeball becoming opaque. Another complication is related to a reduction in the number of limbic stem cells in the periphery of the cornea and the conjunctiva, whose role is to renew the cornea. Their reduced number allows the connective membrane lining the inside of the eyelids to cover the cornea. The inability to repair this part of the eye after an injury results in scarring or vascularization with a more or less significant loss of vision. Aniridia may also be accompanied by early displacement or opacification of the lens and the onset of juvenile cataract. When the development of the structures of the corner of the eye is rudimentary, draining of tear fluid no longer occurs. The increase in pressure in the eye contributes to the development of juvenile glaucoma.
Aniridia may be accompanied by complications that are not related to the eye, but to the central nervous system, such as a difficulty in detecting odors.
Christine Goodhand - Michel Gruaz
Young dwarf rabbit with cataract caused by Encephalitozoon cuniculi, unrelated to aniridia (left), and young Japanese rabbit with juvenile glaucoma unrelated to aniridia (right)
Many thanks to Michel Gruaz (Switzerland) and Christine Goodhand (USA) for their photos and permission to use them for MediRabbit.