title: Multiple congenital ocular anomalies in Icelandic horses.
abstract: BACKGROUND
  Multiple congenital ocular anomalies (MCOA) syndrome is a hereditary congenital eye defect that was first described in Silver colored Rocky Mountain horses. The mutation causing this disease is located within a defined chromosomal interval, which also contains the gene and mutation that is associated with the Silver coat color (PMEL17, exon 11). Horses that are homozygous for the disease-causing allele have multiple defects (MCOA-phenotype), whilst the heterozygous horses predominantly have cysts of the iris, ciliary body or retina (Cyst-phenotype). It has been argued that these ocular defects are caused by a recent mutation that is restricted to horses that are related to the Rocky Mountain Horse breed. For that reason we have examined another horse breed, the Icelandic horse, which is historically quite divergent from Rocky Mountain horses.
  
  
  RESULTS
  We examined 24 Icelandic horses and established that the MCOA syndrome is present in this breed. Four of these horses were categorised as having the MCOA-phenotype and were genotyped as being homozygous for the PMEL17 mutation. The most common clinical signs included megaloglobus, iris stromal hypoplasia, abnormal pectinate ligaments, iridociliary cysts occasionally extending into the peripheral retina and cataracts. The cysts and pectinate ligament abnormalities were observed in the temporal quadrant of the eyes. Fourteen horses were heterozygous for the PMEL17 mutation and were characterized as having the Cyst-phenotype with cysts and occasionally curvilinear streaks in the peripheral retina. Three additional horses were genotyped as PMEL17 heterozygotes, but in these horses we were unable to detect cysts or other forms of anomalies.One eye of a severely vision-impaired 18 month-old stallion, homozygous for the PMEL17 mutation was examined by light microscopy. Redundant duplication of non-pigmented ciliary body epithelium, sometimes forming cysts bulging into the posterior chamber and localized areas of atrophy in the peripheral retina were seen.
  
  
  CONCLUSIONS
  The MCOA syndrome is segregating with the PMEL17 mutation in the Icelandic Horse population. This needs to be taken into consideration in breeding decisions and highlights the fact that MCOA syndrome is present in a breed that are more ancient and not closely related to the Rocky Mountain Horse breed.
authors: Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, Box 597, SE-751 24 Uppsala, Sweden. Lisa.Andersson@slu.se
authors: Lisa S
authors: LS
authors: Andersson
authors: Jeanette
authors: J
authors: Axelsson
authors: Richard R
authors: RR
authors: Dubielzig
authors: Gabriella
authors: G
authors: Lindgren
authors: Björn
authors: B
authors: Ekesten
references: Can Vet J. 2008 Jul;49(7):675-81
references: 18827844
references: J Hered. 2000 Mar-Apr;91(2):93-8
references: 10768120
references: Vet Ophthalmol. 1999;2(1):47-59
references: 11397242
references: BMC Genet. 2008 Dec 19;9:88
references: 19099555
references: Science. 2009 Apr 24;324(5926):485
references: 19390039
references: BMC Genet. 2006 Oct 09;7:46
references: 17029645