Microphthalmia is an inherited eye disease affecting dogs. Microphthalmia results from a vitamin A deficiency during gestation which produces under-developed eyes that are smaller than normal. In addition, the layer of tissue in the eye responsible for supplying blood and nutrients to the Retina, called the choroid layer, is affected. Dogs affected with Microphthalmia may also present with malformations of the optic nerve, known as a Coloboma and may have variable loss of vision. Eye abnormalities are evident as soon as the puppy’s eyes open. A veterinarian may observe these abnormalities during an initial eye exam and can assess for any vision loss.
Genetic testing of the RBP4 gene will reliably determine whether a dog is a genetic Carrier of microphthalmia. microphthalmia is inherited in an Autosomal Recessive manner in dogs with maternal effect. This means that in order to develop the disease dogs must not only receive two copies of the mutated gene (one from each parent) their dam must also have inherited two copies of the Mutation from her parents. In other words, a dog with two copies of the mutation will only develop clinical signs if the mother also has two copies of the mutation. In general, carrier dogs, and dogs with two copies of the mutation out of a carrier dam, do not have features of the disease. When breeding a female dog with two copies of the mutation with a carrier of the same mutation, there is a risk of having affected pups. Each pup born to this pairing has a 50% chance of inheriting two copies of the mutation and being affected by RBP4 gene mutation resulting in Microphthalmia. Reliable genetic testing is important for determining breeding practices. Because symptoms only appear if a dog’s dam had two copies of the mutation, genetic testing should be performed before breeding. To eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding known carriers to each other is not recommended. Dogs that do not carry the mutation have no increased risk of having affected pups.
There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.
Kaukonen M, Woods S, Ahonen S, Lemberg S, Hellman M, Hytonen MK, Permi P, Glaser T, Hannes L. Maternal Inheritance of a Recessive RBP4 Defect in Canine Congenital Eye Disease. Cell Reports. 2019 May 29, 23 (2643 – 2652). doi.org/10.1016/j.celrep.2018.04.118