Cone Degeneration in several dog breeds

The cones are receptor cells in the retina sensitive to light and are important for the high visual acuity at daylight and for colour distinguishing.

The congenital achromatopsia, so-called day blindness, is a rare vision disorder leading to retinal cone degeneration, while the rod receptors enabling high contrast vision and vision at low light remain intact.  It is characterized by reduced visual acuity, photophobia (photodysphoria), nystagmus (uncontrollable movement of eyes) and total colour blindness. The affected dogs are not able to localize the approaching objects at daylight, if they do not hear or smell them.  This disease can already be observed in puppies from 8 to 12 weeks of age.

The visual impairment is noticeable in bright daylight and the vision in dim light and at night is substantially better.  It is important to realize that the dogs do not become blind, but the symptoms of the disease remain the same during their lifetime.

This disease is caused by complete deletion of the CFA29 chromosome that includes the CNGB3-gene, coding for key components of the cone phototransduction cascade. The mutations in these gene cause total loss or substantial worsening of cone photoreceptor function.

The mutation leading to achromatopsia occurs in several canine breeds, in particular of Arctic line such as Siberian Husky or Alaskan malamute.  This defect origins from only one family mutation (from one common ancestor) and spread long before the individual canine breeds were distinguished. Currently, this disease is also known in Australian Shepherds, Miniature Australian Shepherds, German Shorthaired Pointers and Samoyed dogs . Genomia laboratory offers genetic testing for the canine breeds mentioned herein.

CD is a recessively inherited disease. The disease develops in dogs which inherit the mutated gene from each parent. These dogs are designated as P/P (positive/positive). The carriers of the mutated gene are designated as N/P (negative/positive). The carriers inherited the mutated gene from one parent only and are without clinical signs. However, they pass the disease on to their offspring.  When mating two heterozygotes (N/P), there will be theoretically 25% of the offspring healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will be affected by CD.  Mating one healthy dog (N/N) with a carrier of this mutation (N/P) will theoretically produce 50% carriers and 50% healthy offspring.  If a carrier (N/P) is mated with an affected dog (P/P), there will be theoretically 50% affected dogs and 50% carriers.

.

References:

Yeh et al.: Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genetics 2013 14:27.