Testing of dogs: CEA*

CEA - Collie Eye Anomaly

* The tests are performed by the partner laboratory. Genomia guarantees the quality of its partner's services.

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Collie Eye Anomaly (CEA) is an inherited eye defect. The most often affected breeds include Rough Collie, Smooth Collie, Border Collie, Shetland Sheepdog, Nova Scotia Duck Tolling Retriever and Australian Sheepdog.

CEA is caused by an abnormal development of the eyeball, starting at about day 30 of the embryonic development. Postnatally, different defects are found depending on the severity of disturbances in three layers of the eye.

In case of mesodermal defect choroid hypoplasia develops, which is considered the primary characteristic of CEA. In this case we talk about "mid-severe CEA". When examined by an ophthalmoscope, little or no retinal pigment displays underlying blood vessels and sclera. The number of blood vessels is reduced and they can have abnormal shapes. After three months of age, retinal pigment can cover the defective choroid. That is why it is necessary to perform clinical examination of puppies in at early age. Animals affected by mid-severe CEA won´t go blind in most cases, but their offspring can develop severe forms of the disease.

In case of ectodermal involvement, tortuosity of blood vessels, retinal folds and detachment, and/or intraocular bleeding can be observed. Colobomas (clefts) of the optic nerve and the eyeball can also be present, resulting in missing parts of iris or retina. When only iris is affected, there are no blind spots, but retinal detachment can cause the blindness of the eye. In this case we talk about "severe form of CEA", which is present in about 25 % of animals suffering from CEA. Defects of choroid and colobomas are distinguishable in 8 - 12 weeks. They affect mostly both eyes, but the types of defect can combine and their intensity can differ. The symptoms of CEA can also change with the age of the animal. Small lesions of choroid hypoplasia can be covered by retinal pigment (about 30 %), making clinical diagnosis difficult. Intraocular bleeding a retinal detachment increases in severity with age, tortuosity of blood vessels and colobomas tend to be static.

In all CEA affected animals, homozygous deletion of 7.8 kbp in intron 4 of NJEH1 gene was found. This mutation was not observed in any healthy individual. Comparative analysis od NJEH1 region in the genomes of dog, human, mouse and rat discovered conserved binding sites for several DNA-binding proteins in this location (Parker et al. 2007).

CEA is inherited as an autosomal recessive trait. That means the disease affects dogs with P/P genotype only.  The dogs with P/N genotype are considered carriers of the disease (heterozygotes). In offspring of two heterozygous animals following genotype distribution can be expected: 25 % N/N (healthy non-carriers), 25 % P/P (affected), and 50 % N/P (healthy carriers). Because of high risk of producing affected offspring, mating of two N/P animals (carriers) can not be recommended.

Clinical diagnosis of CEA can be difficult. DNA test is an advisable alternative to the ophthalmologic examination. The disease cannot be cured, but it is possible to eliminate it through genetic testing of litters and proper choice of parents.  The test can be performed only once in life of the animal, because the genotype does not change with age. DNA sample can be obtained from buccal swabs (non-invasive method suitable for owners) or from blood samples taken by a veterinarian.

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References:
Staňa P. (2001) Anomálie oka kólií. Pes přítel člověka. 12, 46, 6 - 7.
Eva Chmelíková, Vilma Lánská, Markéta Sedmíková, Helena Härtlová, Jaroslav Petr (2006) Strašák zvaný CEA. Planeta zvířat. 2, 2006
Parker, H. G.; Kukekova, A. V.; Akey, D. T.; Goldstein, O.; Kirkness, E. F.; Baysac, K. C.; Mosher, D. S.; Aguirre, G. D.; Acland, G. M.; Ostrander, E. A. : Breed relationships facilitate fine-mapping studies: a 7.8-kb deletion cosegregates with Collie eye anomaly across multiple dog breeds. Genome Res. 17: 1562-1571, 2007. PubMed ID : 17916641