The horse homolog of congenital aniridia conforms to codominant inheritance

Anterior segment dysgenesis syndrome occurs frequently in Rocky Mountain horses and has two distinct ocular phenotypes: (1) large cysts originating from the temporal ciliary body or peripheral retina and (2) multiple anterior segment anomalies including ciliary cysts, iris hypoplasia, iridocorneal adhesions and opacification, nuclear cataract, and megalocornea. To determine if anterior segment dysgenesis syndrome is heritable in horses we performed ophthalmic examinations and collected pedigree information on horses (n = 516) in an extended Rocky Mountain horse pedigree. Logistic regressive segregation analysis of a subset of animals (n = 337) in which the ocular phenotypes of progeny and both parents were known indicated that the codominant inheritance model best fit the data. This model predicted cyst phenotype expression in heterozygous animals and multiple anterior segment anomalies in homozygous animals. Several cases of nonpenetrance of the cyst phenotype were detected in one lineage. The close resemblance between the inheritance and lesions observed in Small eye mice and rats, humans with congenital aniridia or anterior segment malformation, and horses with anterior segment dysgenesis syndrome supported the conclusion that anterior segment dysgenesis syndrome in the horse may be homologous to similar ophthalmic anomalies in other species.