Equine Multiple Congenital Ocular Anomalies maps to a 4.9 megabase interval on horse chromosome 6

Abstract

Background: Equine Multiple Congenital Ocular Anomalies (MCOA) syndrome consists of a diverse set of abnormalities predominantly localized to the frontal part of the eye. The disease is in agreement with a codominant mode of inheritance in our horse material. Animals presumed to be heterozygous for the mutant allele have cysts originating from the temporal ciliary body, peripheral retina and/or iris. In contrast, animals predicted to be homozygous for the disease-causing allele possess a wide range of multiple abnormalities, including iridociliary and/or peripheral retinal cysts, iridocorneal angle abnormalities, cornea globosa, iris hypoplasia and congenital cataracts. MCOA is most common in the Rocky Mountain horse breed where it occurs at a high frequency among Silver colored horses. The Silver coat color is associated with mutations in PMEL17 that resides on ECA6q23. To map the MCOA locus we analyzed 11 genetic markers on ECA6q and herein describe a chromosome interval for the MCOA locus.

Results: We performed linkage analysis within 17 paternal half-sib families of the Rocky Mountain horse breed. More than half of the 131 offspring had the Cyst phenotype and about one third had MCOA. Segregation data were obtained by genotyping 10 microsatellite markers most of which are positioned on ECA6q22-23, as well as the missense mutation for the Silver phenotype in PMEL17. Significant linkage was found between the MCOA locus and eight of the genetic markers, where marker UPP5 (Theta = 0, z = 12.3), PMEL17ex11 (Theta = 0, z = 19.0) and UPP6 (Theta = 0, z = 17.5) showed complete linkage with the MCOA locus. DNA sequencing of PMEL17 in affected and healthy control individuals did not reveal any additional mutations than the two mutations associated with the Silver coat color.

Conclusion: The MCOA locus can with high confidence be positioned within a 4.9 megabase (Mb) interval on ECA6q. The genotype data on UPP5, PMEL17ex11 and UPP6 strongly support the hypothesis that horses with the Cyst phenotype are heterozygous for the mutant allele and that horses with the MCOA phenotype are homozygous for the mutant allele.

Figure 1

Clinical signs of MCOA syndrome. A. Oblique profile images of the lateral anterior segment of the right eye of a Rocky Mountain horse. A multiloculated cyst arising from the anterior ciliary body is present. B. Photograph of the right eye of a Rocky Mountain horse with ectropion uvea, dyscoria, cataract, and lens subluxation. The granula iridica is hypoplastic, the pupil is misshapen, and complete circumferential ectropion uvea is present. Nuclear cataract of the nuclear-cortical junction is present. Vitreous is present in the anterior chamber between the iris and lens secondary to posterior ventral lens subluxation. C. Profile photograph of a Rocky Mountain Horse with Cornea Globosa. Note the anterior protrusion of the cornea and large corneal diameter. D. Profile photograph of a Rocky Mountain Horse with a normal cornea. Note the normal corneal curvature and diameter.

Figure 2

A Silver colored Rocky Mountain Horse. The typical shiny white mane and tail as well as a slightly diluted body color with dapples is seen in this genetically black Silver colored horse. The phenotype is caused by dilution of eumelanin in the hair to white or grey. The dilution is most visible in the long hairs of the mane and tail. The horse has also been diagnosed with MCOA.

Figure 3

Pedigrees. Pedigrees of four of the half-sib families (sired by stallion 1–4) used in this study. Phenotype information along with genotype data on one genetic marker (PMEL17ex11) is shown for all individuals. Unaffected horses are indicated by open symbols, half filled symbols indicate horses affected with cysts, solid symbols indicate horses affected with MCOA and unexamined horses are indicated by symbols enclosing a question mark. We selected the SNP PMEL17ex11 to represent the near perfect correlation between phenotype and genotype of the three markers in our interval that show complete linkage with the MCOA locus. The only discrepancy (marked in red) is a healthy dam with a heterozygous genotype. However, since she has produced two MCOA affected offspring, this dam is most likely a case where the position and/or size of the cysts make them extremely difficult to detect. NG: Not Genotyped (as DNA was not available).

Figure 4

Positions of analyzed ECA6q markers. Ten microsatellite markers and one SNP marker (PMEL17ex11) spanning approximately 45.2 Mb were included in this study. Three markers showed complete linkage (recombination fraction = 0.00) with the MCOA locus; UPP5 (z = 10.84), PMEL17ex11 (z = 12.64), UPP6 (z = 11.14). Our result can position the MCOA susceptibility locus with high confidence to the 4.9 megabase interval between TKY412 and UPP7.