Ocular melanosis in the Cairn Terrier: clinical description and investigation of mode of inheritance

Abstract

Ocular melanosis is an inherited, probably autosomal-dominant condition with a variable age of onset and rate of progression. It results in a thickening and pigmentation of the iris, release of pigmented material into the aqueous, pigment deposition in the sclera/episclera, and to a lesser extent posterior segment pigment deposition. Following extensive pigment deposition in the aqueous drainage pathways it can result in secondary glaucoma.

Exclusion of eleven candidate genes for ocular melanosis in cairn terriers

“…In addition, recombination hotspots are relatively rare within or close to genes [16]. Although the region of linkage disequilibrium becomes smaller with increasing numbers of meiotic events (generations) after the founder mutation occurred, the timing of the first descriptions of ocular melanosis in Cairn terriers and the increasing incidence of glaucoma in the breed [17], suggests that the mutant allele became relatively common in the breed within recent time (probably less than 10 generations ago [2] and unpublished results). Thus, by using two markers, the average recombination rate becomes a conservative estimate, because of the small probability of the occurrence of two hotspots within or near a gene that would lead to a false exclusion.…”

“…The pigmented cells result in a diffuse expansion of the root of the iris and are also shed into the aqueous humor. They accumulate in the anterior sclera which develops pigment plaques that progressively increase in size, possibly reaching that site via the conventional aqueous drainage pathways [1,2]. Eventually the aqueous drainage pathways become blocked with pigment results in secondary glaucoma and vision loss.…”

“…The pigmented cells appear to be primarily melanocytes although some admixture with melanophages (macrophages that have ingested melanosomes) does occur [3]. The pigmented cells also reach the posterior segment of the eye and over time can be seen to slowly migrate into and obscure the tapetum [2]. The posterior sclera also becomes pigmented and pigment can be found in the meninges around the optic nerve.…”

“…Pedigree analysis indicates that Cairn terrier OM is likely to be inherited as an autosomal dominant trait [2]. We tested the hypothesis that one of 11 candidate genes has suffered a mutation that causes OM phenotype by examination of allele sharing of genetic markers close to or within each gene.…”

Canine Secondary Glaucomas

“…17). 108,109 It is not clear whether these cells are melanocytes or melanophages (macrophages that have phagocytosed pigment granules), and underlying pathogenesis is also unclear. 110 Progression of disease is variable, but prognosis is poor for vision and globe retention in dogs that have developed glaucoma.…”