Equine Multiple Congenital Ocular Anomalies and Silver Coat Colour

Last changed: 11 December 2013

Equine Multiple Congenital Ocular Anomalies (MCOA) syndrome is a heritable eye disorder mainly affecting silver colored horses.

Clinically, the disease manifests in two distinct classes depending on the horse genotype. Horses homozygous for the mutant allele present with a wide range of ocular defects, such as iris stromal hypoplasia, abnormal pectinate ligaments, megaloglobus, iridociliary cysts and cataracts.

The phenotype of heterozygous horses is less severe and predominantly includes iridociliary cysts, which occasionally extend into the temporal retina. In order to determine the genetic cause of MCOA syndrome we sequenced the entire previously characterized 208 kilobase region on chromosome 6 in ten individuals; five MCOA affected horses from three different breeds, one horse with the intermediate Cyst phenotype and four unaffected controls from two different breeds. This was performed using Illumina TruSeq technology with paired-end reads.

Through the systematic exclusion of all polymorphisms barring two SNPs in PMEL, a missense mutation previously reported to be associated with the silver coat colour and a non-conserved intronic SNP, we establish that this gene is responsible for MCOA syndrome. Our finding, together with recent advances that show aberrant protein function due to the coding mutation, suggests that the missense mutation is causative and has pleiotrophic effect, causing both the horse silver coat color and MCOA syndrome.

Samples from silver colored horses needed

The study is in need of eye tissue from silver colored horses. We would therefore very much appreciate to be contacted before such a horse is culled, regardless of the cause, to get a tissue sample. All breeds are of interest and the horses do not need to have undergone an eye examination in advance. Because MCOA is a congenital disorder, we are interessted in eyes from aborted foals as well (both silver colored and not).

Contact regarding tissue samples:



Equine Multiple Congenital Ocular Anomalies and Silver Coat Colour Result from the Pleiotropic Effects of Mutant PMEL -  PLoS ONE

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BMC Vet Res. 2011 May 26;7:21. doi: 10.1186/1746-6148-7-21.
PMID: 21615885 [PubMed - indexed for MEDLINE] Free PMC Article

Equine Multiple Congenital Ocular Anomalies maps to a 4.9 megabase interval on horse chromosome 6.
Andersson LS, Juras R, Ramsey DT, Eason-Butler J, Ewart S, Cothran G, Lindgren G.
BMC Genet. 2008 Dec 19;9:88. doi: 10.1186/1471-2156-9-88.

Targeted analysis of four breeds narrows equine Multiple Congenital Ocular Anomalies locus to 208 kilobases.
Andersson LS, Lyberg K, Cothran G, Ramsey DT, Juras R, Mikko S, Ekesten B, Ewart S, Lindgren G.
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