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Collie Eye Anomaly/Choroidal Hypoplasia

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Test Overview:

Collie Eye Anomaly is a developmental defect of the eyes that is inherited in a simple recessive manner. Expression is affected by several modifier genes, resulting in some variability in clinical disease, and there is some research suggesting the development of coloboma is polygenic (involves more than one gene). There is always (by definition) choroidal hypoplasia - a decrease in the development of the blood vessels at the back of the eye. This can be detected on eye examination by a veterinary eye specialist at 6-8 weeks of age. Later than this it may be difficult to detect changes because as the tapetum (a pigmented reflective section of the retina) develops it may hide any changes at the back of the eye – this is known as the “go normal” phenomenon, and does not mean that the animal is no longer affected, just that the changes can no longer be seen. As well as choroidal hypoplasia, affected dogs may also develop optic disc coloboma (which are developmental “holes” where the nerve supplying the eye enters) and retinal haemorrhages that may lead to retinal detachment. Retinal detachment occurs in around 4-5% of affected animals, and may be in localised areas, or detachment of the entire retina, which causes blindness. Intraocular haemorrhage can also occur in severe cases. Mild cases may just have choroidal hypoplasia, with little effect on vision. Coloboma is a more severe change, which may cause areas of reduced vision. Retinal haemorrhage and focal retinal detachment may cause blind spots, while complete retinal detachment results in a blind eye. This can occur in the puppy or happen spontaneously in an adult affected dog. A simple DNA test is now available for this disease, which has helped in reducing the prevalence of the disorder in the collie. Statistics from Optigen (the company that offer the DNA test for this condition) show the prevalence of this condition in the rough collie to be 72% and in the smooth collie it is 62%, although this is now dated somewhat. This includes affected and carrier animals. Closely related herding breeds such as the Border collie, Australian shepherd and Shetland sheepdog also have relatively high carrier rates of the condition. Several other breeds do carry the CEA mutation as well, and most likely had affected herding breeds in their breed ancestry. The condition has also been reported occasionally in mixed breeds.puppy is difficult as mildly affected parents may produce offspring that are severely affected.

Category:

Ophthalmologic - Associated with the eyes and associated structures

Gene:

Non-homologous end joining factor 1 (NHEJ1) on chromosome 37

Variant Detected:

Nucleotide Deletion 7799 base pair deletion in Intron 4 of the NHEJ1 gene

Severity:

Low-Moderate. This disease can cause some discomfort and/or dysfunction in the affected animal. It does not generally affect life expectancy.

Mode of Inheritance:

Autosomal Recessive with Incomplete Penetrance

Recommended Screening:

1. Check for this as part of routine puppy eye examination (with a veterinary eye specialist) at 6-8 weeks of age. 2. Screen all breeding animals via DNA test prior to breeding – e.g. at 1 year of age.

Research Citation(s):

Parker, HG, et al. Breed relationships facilitate fine-mapping studies: A 7.8-kb deletion cosegregates with Collie eye anomaly across multiple dog breeds. (2007) Genome Res, 17: 1562-1571

Associated Breed(s):

Australian Kelpie, Australian Shepherd, Bearded Collie , Border Collie, Boykin Spaniel, Collie Rough, Collie Smooth, Koolie , Lancashire Heeler, Miniature American Shepherd, Mixed Breed, Old English Sheepdog , Shetland Sheepdog, Smithfield, Whippet ,

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