Canine Day Blindness (Achromatopsia)
Canine day blindness or achromatopsia (ACHM) is a congenital eye disorder. It was first identified in humans, but recently it has been recognized in Labrador retriever and German shepherds as well. The disorder can vary among affected dog by its severity, but common trait of all affected ones is the hemeralopa, or blindness in full sun. For this reason, it is also known as ”day blindness”, since the affected dog sees better in dim light. Bright light troubles the vision and causes the affected dog to see images blurry, which decreases with lowering the light brightness. Until now, researches have revealed that the disorder is caused by abnormalities in the retina.
Symptoms and Characteristics
Retina is the thin tissue, localized in the back of the eye that contain photoreceptors, light-sensitive cells. Signal transduction is important in retinal receptors and the cyclic nucleotide-gated (CNG) ion channels are the key mediators of it.
Genes CNGA3 and CNGB3 are encoding two structurally related subunits of cone cyclic nucleotide-gated channels. Genetic defects in these genes cause biosynthesis of abnormal subunits of the CNG channels, which leads to achromatopsia. The dysfunction of the cyclid nucleotide gated channel is particularly devastating, especially because they appear during the earliest ages. The ACHM manifests by cone photoreceptor dysfunction, severely reduced vision capability, pendular nystagmus, rod monochromacy reduced or complete color blindness and photophobia.
Canine day blindness or achromatopsia is caused by a mutation in CNGA3 gene. Identical mutation has been identified in human achromatopsia patients. Canine achromatopsia is inherited in an autosomal recessive pattern. Healthy parents of an affected puppy are obligate heterozygotes, and therefore carry one mutant allele. Heterozygotes have no symptoms. At conception, each cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Until now, no cure for achromatopsia has been found, and only way to prevent it, is to breed dogs which are not carriers of the mutation. Defected genes for autosomal recessive disease can be passed for many generations without affected individuals occurring until two carriers are bred to one another. The only way to find out if there is a chance of getting an affected puppy is to do genetic testing.
Tanaka N, Dutrow EV, Miyadera K, Delemotte L, MacDermaid CM, Reinstein SL, et al. (2015). Canine CNGA3 Gene Mutations Provide Novel Insights into Human Achromatopsia-Associated Channelopathies and Treatment. PloS ONE 10(9): e0138943. doi:10.1371/journal.pone.0138943