Cone Rod Dystrophy 2 (crd2) in the American Pit Bull Terrier   

June 16, 2010

Announcing the crd2 mutation DNA test in the American Pit Bull Terrier (APBT): OptiGen is pleased to provide a new DNA test that detects the presence of a mutation causing an inherited eye disease, Cone Rod Dystrophy 2 (crd2), in the American Pit Bull Terrier.  

What is crd2? crd2 is one of several cone rod dystrophies that have been recognized in multiple breeds of dogs.  All forms of CRD are characterized by the initial loss of cones, the cells in the retina that are responsible for vision in bright light/daylight, followed by the degeneration of rods, the retinal cells that operate during night vision. Clinical symptoms of crd2 are typically evident early in a pup’s life and the severe retinal degeneration that characterizes this disease usually results in complete blindness by 1 year of age.

What breeds of dog carry crd2? To date, the mutation causing crd2 has only been observed in one breed of dog, the APBT.  Researchers were somewhat surprised to learn that this mutation was not the cause of a very similar disease that occurs in a closely related breed, the American Staffordshire Terrier (Am.Staff.).  The Am.Staff.’s disease has been termed Cone Rod Dystrophy 1 (crd1) and is currently being molecularly characterized.  Because Pit Bulls and Am. Staffs share many common ancestors and interbreeding among these and other bull-type dogs is quite common, OptiGen will accept samples from any of the “bull” breeds for crd2 testing. It should be emphasized, however, that there is currently no evidence to suggest that crd2 is present in any breed besides the APBT.  Owners of pedigreed dogs that have been diagnosed by a veterinary ophthalmologist as having retinal degenerations (e.g. PRA or CRD) are encouraged to contact OptiGen to inquire about our Free DNA testing/Research program (http://www.optigen.com/opt9_research.html).

How is crd2 inherited? How should I use the DNA test? crd2 is inherited in an autosomal recessive mode, meaning that offspring need to inherit the mutation from both parents (i.e. have two copies of/be homozygous for the mutation) for disease to occur. Carriers of one copy of the mutation (heterozygotes) will not show disease but they can pass the mutation on to their offspring.  The great value of the DNA test is that it will allow detection of Carriers.  In order to avoid producing crd2-affected offspring, carriers of the mutation should never be bred to other cd2-carriers.  In other words, at least one of any breeding pair should be homozygous Normal/Clear of crd2 to ensure that no crd2-Affected offspring are produced. (See chart below.)

This table highlights in yellow the desirable breedings that will NOT produce crd2 -affected pups. These breedings include at least one parent proven "Normal/Clear" by the OptiGen crd2 test. All other breedings are at risk of producing crd2-affected pups. With the aid of DNA testing, all dogs, even those carrying the crd2 mutation, can be bred with complete confidence that no crd2-affected pups will be produced. It isn't necessary-or even desirable-to remove any dog from the breeding population. Rather, when choosing pups to retain as potential breeding stock, one may select for dogs proven "Normal/Clear" by the OptiGen crd2 test, and select against dogs proven to be carriers. In this way, breeders can retain desirable traits in their lines while moving away from disease.  Pups can be tested to distinguish carriers from Normals as soon as they are old enough to have a small blood sample or non-contaminated cheek swab collected.

How do I get my dog’s DNA tested for crd2? To test your dog’s DNA for  crd2 please see OptiGen’s Instructions & Information webpage (http://www.optigen.com/opt9_info.html).  Complete an order form and send either a cheek swab or whole unclotted blood in EDTA to OptiGen.  Tests are typically reported within two weeks of OptiGen’s receiving the sample.  Contact genetest@optigen.com with any questions.

Research leading to the discovery of the crd2 mutation was conducted at Cornell University and the University of Pennsylvania, with collaboration by OptiGen.