Microphthalmia Soft Coated Wheaten Terrier (RBP4)
Microphthalmia Soft Coated Wheaten Terrier type is an inherited disorder of the eyes whose causative mutation has been identified in this dog breed. In humans, microphtalmia is part od spectrum of congenital eye diseases which are a known cause of childhood blindness. The disorder has been recognized in many dog breeds, however, until now the causative mutation has been confirmed only in Soft Coated Wheaten Terrier dog. Other breeds affected by microphthalmia include: Akita, Australian shepherd, Beagle, Border collie, Borzoi, Cavalier King Charles spaniel, Collie (rough and smooth), Dachshund, Dalmatian, Doberman pinscher, Great Dane, Old English sheepdog, Poodle, miniature, Poodle, standard, Poodle, toy, Portuguese water dog, Saint Bernard, Schnauzer, miniature, Shetland sheepdog, Labrador retriever, Lakeland terrier, Rottweiler, Samoyed, and Tibetan spaniel.
Characteristics and Symptoms
Affected dogs have small eyes which seems to be recessed in the eye socket, condition known as enophthalmos. Also, third eyelids may be prominent. These characteristics are often associated with other eye disorders, such as the abnormalities of the cornea, anterior chamber, lens or retina.
Other symptoms may appear, such as clumsy behaviour, anxiety, difficulties with coordination, asocial behaviour, excessive sleepiness, partial vision loss.
Microphthalmia Soft Coated Wheaten Terrier (RBP4) Genetics
Microphthalmia Soft Coated Wheaten Terrier (RBP4) is caused by the deletion mutation within the retinol-binding protein gene (RBP4). Role of the RBP4 is to carry vitamin A (retinol) from hepatic stores to peripheral tissues, including eye, where is participates in the synthesis of retinoic acid. Deficiency of vitamin A during the pregnancy is well recognized risk factor for ocular birth defects. Mentioned deletion mutation causes decreased vitamin A secretion from hepatocytes to serum.
The disorder is inherited ina n autosomal recessive pattern with penetrance dependent on maternal genotype. For the disorder to appear in puppies, puppies and mother must be homozygous for the mutation (affected). The dependence on maternal genotype arises from the obstructed transfer of retinol across the placenta.
Kaukonen, M., Woods, S., Ahonen, S., Lemberg, S., Hellman, M., Hytönen, M. K., … Lohi, H. (2018). Maternal Inheritance of a Recessive RBP4 Defect in Canine Congenital Eye Disease. Cell Reports, 23(9), 2643–2652. doi:10.1016/j.celrep.2018.04.118
Crook A et al. 2011. Canine Inherited Disorders Database (CIDD).