As discussed in an earlier article, the selective breeding of the canine has greatly diminished the genetic diversity within each specific breed. In populations of low genetic diversity, recessive alleles are more likely to pair. Paired non-lethal recessives often generate phenotypes with decreased biological fitness. Recessive defective alleles are maintained in heterozygous (carrier) dogs within a population. Genetic testing can screen for these “hidden” alleles, and reduce the likelihood of breeding animals with defective pairings.
Eliminating all the carriers within a gene pool would unfortunately further reduce a breed’s genetic diversity. The remaining gene pool might have reduced biological fitness for other unforeseen traits. For example, a dog carrying genes asssociated with progressive retinal atrophy (PRA) might also carry associated genes promoting healthy hip conformations. The general goal of selective breeding is therefore to reduce but not necessarily eliminate non-lethal alleles.
Human genetic testing became routine in the 1960’s with the karyotyping of fetal cells following amniocentesis. Molecular biology techniques developed over the last 30 years have rapidly advanced and refined this genetic screening for numerous abnormalities. In 1999, the chromosomal location of genes associated with Progressive Rod Cone Degeneration (PRCD) was determined in dogs. The chromosomal location for PRCD is equivalent to that associated with human retinitis pigmentosa. Although the actual protein product(s) responsible for the PRCD have yet to be determined, genetic screening for PRCD is now common.
Genes or markers for different forms of retinal dysplasia and retinal degeneration have been determined. The Collie eye anomaly (CEA or choroidal hypoplasia) is a disorder associated with incomplete choroidal vasculature development. Approximately 25% of dogs afflicted with CEA develop retinal detachments, optic nerve colobomas, and/or abnormal vascularization with secondary hemorrhage. A recessive gene responsible for CEA can be screened for. A gene associated with lens luxation has also been recently discovered.
There are two common types of genetic screenings. The first are “marker-based” tests in which a disease is closely associated with a known, and sometimes seemingly unrelated, genetic sequence. For example, Multiple Congenital Ocular Anomalies (MCOA) is most typically found in silver-colored horses of the Rocky Mountain breed. The silver-color genes map to the same chromosomal location as those involved in MCOA. The silver color can therefore be used as a marker for genetic predisposition to MCOA. The specific genes responsible for MCOA are not known, but can thus be screened for indirectly.
The second common type of screening is “gene-specific.” Mutations in genes known to cause disease are directly screened for. The risk of error for true Mendelian disease traits (with complete penetrance) is extremely low when using these tests. The specificity and sensitivity of gene-specific tests can reach 100%. Very few “false positive” or “false negative” results are generated. However, laboratory error, mis-sampling, and/or misinterpretation of results can still invalidate even a gene-specific test.
Many breeders request their family veterinarian to perform genetic screening for ocular and other disorders. Generally, 2 or more mls of whole blood are collected in tubes with EDTA to prevent coagulation. In New York, Cornell’s OptiGen® genetic screening laboratory accepts mailed samples on weekdays, but not on weekends and holidays. Detailed packaging requirements, and the variety of available genetic screenings, can be found at www.optigen.com.
As with any clinical test, genetic screening results must be interpreted with regard to each individual patient for future breeding. The tests can provide accurate and meaningful information to breeders and other owners. If you have any questions about inheritable ocular disorders or their genetic screening, please feel free to consult with a veterinary ophthalmologist.
Figure 1: Retinal Degeneration and CEA found in different breeds for 2008 by OptiGen®.