A recent article that appeared in the science journal Genomics, written by a group of scientists in America, reports significant progress in our understanding of the genetic cause of Collie Eye Anomaly (CEA), otherwise known as choroidal hypoplasia writes Dr Jeff Sampson.
CEA is an inherited eye condition that affects the development of the choroid and sclera, two membrane layers at the back of the eye that support the retina, in a number of breeds, including Rough, Smooth and Border Collies, the Australian Shepherd and the Lancashire Heeler. The clinical consequences can vary markedly in affected dogs; many exhibit no obvious clinical consequences and retain apparently normal vision throughout their life, others, unfortunately, can develop secondary retinal detachments, coloboma, intra-ocular bleeding and blindness. At present affected dogs are usually identified by ophthalmoscopic examination at around eight weeks of age.
Investigations into the mode of inheritance over the years have suggested that CEA is inherited as a single autosomal recessive condition. The article published by Lowe et al. in Genomics (vol. 82, pp 86-95 (2003)) provides confirmation that choroidal hypoplasia (CEA) has indeed an autosomal recessive mode of inheritance. However, the real significance of this paper lies in the fact that it locates the gene involved in CEA to a small region of chromosome 37, one of the smallest of all canine chromosomes. In order to understand the enormity of this discovery you could liken attempts to identify the gene involved in CEA to trying to find an incorrectly spelled word in the Complete Works of William Shakespeare. The work reported in this article is the equivalent of identifying the precise page where the spelt error occurs. I imagine that the group will now be feverishly scanning this region of chromosome 37 to see if there are any genes that are likely to be involved in the development of the eye.
Some years ago the UK breeds affected by CEA got together to set up the CEA Working Party with the intention of raising funds to support research into CEA, which they did very successfully. In addition breeders also set about the task of collecting the blood samples that would be required from affected dogs, their siblings, parents and grandparents. Samples were mostly stored at the Animal Health Trust (AHT) in Newmarket. However, progress in the UK was stalled when we got wind of the American work that ultimately led to the publication of the paper in Genomics.
Fortunately, the CEA Working Group persevered and now has a collaboration with Dr Cathryn Mellersh at the AHT. Cathryn will use the newly published information to test whether the mutation that is responsible for CEA in the UK population is also located on chromosome 37.
This is necessary because the vast majority of samples used by the American group will have come from North American bloodlines. Although we all expect the UK and the US populations to share the same mutation, it needs to be verified. In addition Cathryn will analyse samples from Lancashire Heelers, which was not one of the breeds studied by the Americans. Having verified that the same gene is responsible for CEA on both sides of the Atlantic, Cathryn will then be able to collaborate with the Americans in identifying the gene involved. Identifying the gene will bring us so much closer to characterising the exact mutation that causes CEA, an understanding of which will enable development of that elusive DNA test for CEA.
PS: An important postscript, at least for Border Collie breeders, is that Dr Alan Wilton’s group at the University of New South Wales, Australia, has recently identified the genetic mutation that causes ceroid lipofuscinosis (CL) in the Border Collie. To quote from Dr Wilton’s press release, "Cheap and effective methods of detecting asymptomatic carriers of the gene defect are currently being developed and will be available within 6 months". So, it appears that there will be two new DNA tests for the Border Collie in the foreseeable future – good news indeed.