Research is making it easier to find a marker linked to the gene and develop tests to identify this disease in carrier horses.

Question:

Could you provide an update on anterior segment dysgenesis? Is it carried with the silver gene, and are all homozygous ASD horses also homozygous for silver?

Answer:

Your question is very timely, as more and more information is being discovered about the genetics of this syndrome.

Multiple congenital ocular anomalies is the new name for anterior segment dysgenesis. The disease was renamed because ASD involves a specific pattern of cornea and lens abnormalities (seen most often in humans and rarely in dogs) not identified in horses with MCOA. MCOA is seen most commonly in Rocky Mountain horses but can also be seen in other breeds. The most common ocular signs are cysts of the iris and ciliary body (the ocular structure right behind the eye that produces the aqueous humor within the eye), small pupil size, resistance of the pupil to dilation, prominent eye position/shape and retinal abnormalities (including retinal detachment). Horses are generally considered to fit into one of three categories: unaffected, horses with only cysts and horses with cysts and other abnormalities. A recent scientific publication journal (BMC Genetics) has demonstrated that the gene responsible for the development of cysts alone or the development of MCOA is on equine chromosome 6q and is linked to the markers UPP5, PMEL117ex11 and UPP6. This chromosome region is the same one where the gene for the silver coat color (PMEL117) resides.

While finding genes can be really difficult, finding a marker that is linked to the gene is often easier and makes developing a test to identify the disease in carrier horses much simpler. Interestingly, mutations in the PMEL117 gene results in the silver coat color that is so frequently (but not always) associated with cysts and MCOA, however the researchers do not believe that the gene that causes the coat color silver is the same gene that causes MCOA. Not all horses with MCOA are silver in color (and thus homozygous for the silver gene). I hope we will see more information about the genetics of this disease published soon, followed by a genetic test that will allow breeders to make more informed breeding decisions.

– Dr. Amber Labelle, member of the American Association of Equine Practitioners

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