Microphthalmia

Other Names: Canine Congenital Eye Disease, Multiple ocular defects with microphthalmia, MAC
Affected Genes: RBP4
Inheritance: Complex Inheritance
Mutation: chr28:7830263-7830265: 3 bp deletion (del GAA)

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Common Symptoms

Microphthalmia is an inherited eye disease affecting the soft-coated wheaten terrier. 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.


Breed-Specific Information for the Soft Coated Wheaten Terrier

The Mutation of the RBP4 gene associated with congenital eye disease has been identified in soft-coated wheaten terrier. Although its overall frequency in this breed is unknown, approximately 22% out of 248 soft-coated wheaten terriers were carriers of the mutation and 3% were at-risk/affected. Not all dogs with two copies of the mutation will have clinical signs, but they can still pass the mutation on to their offspring. A unique feature of the RBP4 mutation in the soft-coated wheaten terrier is the maternal effect on inheritance. Clinical signs consistently manifest when both dam and offspring have two copies of the mutation. A Carrier puppy born to a dam with two copies of the RBP4 mutation may present with ocular abnormalities to a lesser degree. If the dam has only one copy, her offspring have grossly normal eye anatomy even if the offspring has two copies of the mutation.


Testing Tips

Genetic testing of the RBP4 gene in soft-coated wheaten terriers 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. Soft-coated wheaten terriers 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.


References

  • 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 [PubMed: 29847795]