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Day Blindness (Achromatopsia; ACHM-2) Labrador Retriever Type
| Acronym: | ACHM-2 |
| Gene: | CNGA3 |
| Mutation: | c.1931_1933delTGG |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
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 retrievers and German shepherds as well. The disorder can vary among affected dogs by its severity, but a common trait of all affected ones is hemeralopia 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, research has revealed that the disorder is caused by abnormalities in the retina. The retina is the thin tissue, localized in the back of the eye that contains 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 cyclic 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 the 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 the only way to prevent it is to breed dogs that 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.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
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
Yeh, C., Y. et al. (2013.): Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genetics 14:27.